Reviews in Nanoscience and Nanotechnology Vol. 5, pp. 1–27, 2016 (www.aspbs.com/rnn)
Copyright © 2016 by American Scientific Publishers All rights reserved. Printed in the United States of America
Rare Earth-Doped Zinc Oxide Nanostructures: A Review Daksh Daksh∗ and Yadvendra Kumar Agrawal Institute of Research and Development, Gujarat Forensic Sciences University, Sector-9, Gandhinagar 382007, Gujarat, India
The emerging strategies for the use of highly modified and sophisticated nano systems or devices are rapidly changing and demanding. New goals for providing better solutions with the help of nanotechnology have emerged from the electronics industry. Nano-electronics has focused on the structural, optical, magnetic, and photoluminescence properties of nanostructures for developing nano-electronic and optoelectronic devices. These properties play a vital role at the nano-scale level in comparison of bulk compounds. The use of semiconductor materials has always been in demand. Here, we are focusing on the unique and distinct semiconducting properties of rare earth-doped ZnO nanostructures and their applications in various emerging fields of applied sciences or in the industrial applications. This review provides an overall study of the synthesis of rare earth-doped ZnO nanostructures, and their properties and applications in different fields of industry. KEYWORDS: ZnO Nanostructures, Rare Earth-Doped ZnO, Nano-Electronics, Optoelectronic Devices.
Delivered by Ingenta to: Wei Chen IP: 129.107.80.80 On: Wed, 14 Sep 2016 16:57:39 CONTENTS 1. INTRODUCTION Copyright: American Scientific Publishers 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Nanotechnology can be defined as an emerging science 2. Fundamentals and Applications of ZnO Nanostructures . . . . . 2 and technology of the future having great impact over 3. Synthesis of ZnO Nanostructures . . . . . . . . . . . . . . . . . . . 3 other technologies due to the changes in the properties of 4. Rare Earth Metals and Need for Doping . . . . . . . . . . . . . . . 5 particles or materials conducted at the nanometer scale. 5. Methods for Synthesizing Rare Earth-Doped When the technology is applied to engineering methods to ZnO Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 provide industrial applications at nanometre scale, it can 5.1. Co-Precipitation Method . . . . . . . . . . . . . . . . . . . . . . 6 5.2. Solvothermal and Hydrothermal Method . . . . . . . . . . . . 7 be termed as nanoengineering. The field of nanoengi5.3. Sol–Gel Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 neering deals with interdisciplinary aspects of chemical, 5.4. Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 physical, biological, and engineered methods for providing 6. Rare Earth Metals Used for Doping of ZnO . . . . . . . . . . . . 7 advanced techniques and methods. It can be used for char6.1. Erbium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 acterization of structural, optical, mechanical, magnetic 6.2. Europium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 and electrical properties of the nanoparticles.1 2 The stud6.3. Gadolinium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 ies show that there is a drastic change in properties at the 6.4. Neodymium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7. Other Miscellaneous Rare Earth Metals . . . . . . . . . . . . . . . 15 nano level in comparison to bulk crystals. The properties 7.1. Cerium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 of nanoparticles depend on the manufacturing of nanopar7.2. Dysprosium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 ticles whether we are using a Top-Down or Bottom-Up 7.3. Holmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Approach.3 For providing better platforms, nanostructures 7.4. Praseodymium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 also can be formed with the help of fabricating devices 7.5. Samarium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 to use them in developing micro-electromechanical to 7.6. Ytterbium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 nano-electromechanical devices. In the field of nanoelec7.7. Terbium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8. Applications of Rare Earth-Doped ZnO Nanostructures . . . . . 22 tronics, nanoparticles provide a new path for the devel9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 opment of new technologies by enhancing conducting References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 properties of nanoparticles at nano level. The development of single electron transistors, phototransistors, and NEMS devices are the applications of nanoelectronics. The ∗ Author to whom correspondence should be addressed. conducting properties of nanoparticles can be enhanced Email:
[email protected] with the help of semiconductor properties of the particles Received: 13 March 2016 Accepted: 17 May 2016 present at nano scale.4 5 The semiconducting properties Rev. Nanosci. Nanotechnol. 2016, Vol. 5, No. 1
2157-9369/2016/5/001/027
doi:10.1166/rnn.2016.1071
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
Daksh and Agrawal
can be manipulated into providing defect with the help of In the applications of ZnO NPs, controlling the size atomic/molecular manipulations through nanotechnology. and shape of growing nanoparticles is the main probThe uses of semiconductors as nano-materials are drastilem. There is evidence that the shape and size of the cally trending towards the future applications.6 To provide ZnO NPs influence their physical properties. Simplicity, better results and applications, the enhancing of properhigh speed and low cost of their production are some ties of nanoparticles seems to be a common problem from other aspects that must be considered. ZnO NPs can be initial synthesis to final stages. Here, we use zinc oxide prepared by several methods, resulting in nanostructures nanoparticles as they reflect semiconducting properties at of different shapes. The most popular methods are wet the nano level.7 8 chemical methods performed in water, organic solvents, The nanoparticles of zinc oxide (ZnO NPs) are one of and ionic liquids or micro-emulsions. Sonochemicalthe most studied materials. They are generally of very and microwave-assisted synthesis of ZnO NPs also was low toxicity. However, they may pose a significant risk reported. Various methods such as electrochemical deposito the environment at higher concentrations. On the other tion, evaporation-condensation and chemical vapor deposihand, ZnO NPs possess several favourable properties such tion have been used to produce ZnO nanostructures. These as good transparency, strong room-temperature luminesare expensive and require specific equipment for obtaincence, high electron mobility, wide band gap, chemical ing nanostructures of desirable morphology. The rare earth and photochemical stability, etc. These properties make metals have distinct and unique properties, such as high them an interesting material for using in new light-emitting luster, high conductivity and high refractive index. Here, devices, solar cells, bio-sensors and as photocatalysts. Zinc we are using different rare earth metals for adding defect oxide (ZnO) nanoparticles are an emerging material for in ZnO nanoparticles through doping and providing the next-generation short wavelength optoelectronic devices comparative studies of their enhanced properties and future due to their large band gap of 3.37 eV and exciton bindapplications.9 10 11–13 ing energy of 60 MeV. High quality ZnO nanostructures are relatively easy to synthesize and have potential appli2. FUNDAMENTALS AND APPLICATIONS OF cations in optoelectronics, energy generation, bio-sensing ZnO NANOSTRUCTURES and many industrial applications with the Delivered help of fabricaby Ingenta to: Wei Chen Here, we 2016 describe the synthesis of ZnO NPs through we129.107.80.80 are trying to explain tion technologies.1–3 In this paper,IP: On: Wed, 14 Sep 16:57:39 American Scientific Publishers the different modes used to develop nanostructures with the properties of ZnO nanoparticles Copyright: and also providing novel properties. The properties depend on size effect and ways to enhance their properties. The ZnO nanoparticles dimensional structures in 3-D space. The effect of van are doped with rare earth metals to enhance their characder Waal’s forces and intermolecular stretching’s may lead teristics and properties.
Daksh Daksh was born in 1990. He obtained his B.Tech. (Hons.) in Computer Science and Engineering from Lovely Professional University, India in 2012. He worked 20 months from 2012–2014 in the field of computer science as Computer Engineer in AONHewitt Pvt. Ltd., gaining exposure to mainframes and other IBM technologies and different softwares. Currently, he is enrolled in Institute of Research and Development, Gujarat Forensic Sciences University, India for MS (Forensic Nanotechnology) degree since 2014.
Yadvendra Kumar Agrawal, M.Sc.-Ph.D., D.Sc. (USA) in Pharmaceutical Science, D.Sc. (India), F.R.S.C. (UK), C.Chem. (UK), F.I.C., F.S. (Switz), F.A.Sc. (USA), F.G.S.A., is the Director for the Institute of Research and Development—Gujarat Forensic Sciences University. Formerly, he has worked as the Director—Nirma Institute of Science and Technology—Nirma University—Ahmedabad; Director—School of Sciences—Gujarat University—Ahmedabad; and Professor and Head—Pharmacy Department—Faculty of Technology and Engineering—M.S. University—Baroda. He has more than 45 years of teaching and research experience and has published more than 500 research papers, in National and International Journals of repute. He has 5 patents, guided 117 Ph.D. Students (in Pharmacy, Chemistry, Engineering and Biosciences) and also 30 M.Pharm.; 5 M.Phil. and 5 M.E. students, during the course of his career. 2
Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
Daksh and Agrawal
Rare Earth-Doped Zinc Oxide Nanostructures: A Review
to increases in the size of nanoparticles. The surface and quantum confinement are the important factors for determining the unique structural, mechanical, electrical and optical properties of nanoparticles.5 6 Due to these properties, ZnO NPs are widely used in many applications as NEMS systems, single electron transistors, engineered solar cells, OLED’s, laser diodes and many other optoelectronic devices.12 13 The study of ZnO nanostructures in different structural forms as nanotubes, nanorods, nanocables and nanowires generates interest in research and industrial applications. ZnO is a II–VI inorganic compound semiconductor and possesses a wurtzite structure in the hexagonal unit cell. ZnO has a wide band gap semiconductor with large exciton binding energy of 60 MeV and due to the wide band gap, ZnO is advantageous over other band gap semiconductors.14–16 The wide optical band gap has an effect on the physical properties of the particles, such as optical, refractive index, conductivity, and luminescence properties.17–20 Due to the wide optical band gap, ZnO may be used in photo-voltaic applications. This why ZnO NPs often are the subject of special interest in research due to their low cost, efficiency, and environment-friendly nature.
electronic states in semiconductor quantum particles. Thus, capping agents play a vital role in controlled growth of nanoparticles and enhance the properties of nanoparticles by capping them, which can be used in many applications, such as targeted drug delivery in nanomedicine,23 25–27 and in semiconductors as enhancing photoluminescence properties.28–31 Hong et al. showed the formation of ZnO NPs using the co-precipitation method by using zinc acetate (Zn(CH3 COO)2 · H2 O) and ammonium carbonate ((NH4 2 CO3 .41 They reported ZnO nanoparticles of 30–40 nm by using heterogeneous azeotropic distillation, which reduces agglomeration. The morphology of ZnO is a key factor for observing the differences between the properties of nanomaterials in comparison to bulk compound.38–43 With different morphologies and sizes, ZnO materials can be used in many applications. The luminescence property of ZnO is morphology-dependent, and the relative intensity of the luminescence is the greatest for nanowire and the least for nanoparticle (nanowire > nanopowder > nanoneedle > nanoparticle). There is a need for obtaining uniform ZnO nanostructures with distinct morphology, controlled growth, and crystalline structures.41 Small changes in synthesis parameters and in key factors may result in the formation 3. SYNTHESIS OF ZnO NANOSTRUCTURES of nanostructures with different morphologies and propDelivered by of Ingenta to: Wei Chen There are several methods reported for the synthesis So, 2016 there 16:57:39 is a need for characterization of forming IP: 129.107.80.80 On: Wed,erties. 14 Sep ZnO nanostructures having different Copyright: structures, American shapes, Scientific ZnO nanostructures Publishersthrough different techniques for getand morphologies. Basically, the methods are chemical, ting unique morphology.47–52 physical, or mechanical, such as a co-precipitation method, Searson et al. showed that the rate constant for coarsensol–gel process, hydrothermal synthesis, etc. ing is determined by the solvent viscosity, surface energy, Out of these methods, the co-precipitation method is and the bulk solubility of ZnO in the solvent.51 The synsimple to implement, economically effective, and high thesis of ZnO quantum particles by co-precipitation from yielding method for obtaining nanoparticles in the lab a series of n-alkanols from ethanol to 1-hexanol alcohols results in stable colloids of nanometer-sized partiat desired room temperature.21–24 This method is steady, cles. For ethanol and isopropanol, nucleation and growth fast, and spontaneous for the controlled growth of ZnO are retarded compared with longer chain length alcohols NPs by controlling the factors of temperature, pH, and where nucleation and growth are fast. When the super satthe effect of solvent and reacting time of precursors to uration has been depleted and nucleation and growth are form co-precipitation. The co-precipitation method can be completed, the average particle size continues to increase described as when an inorganic metal compound dissolved due to diffusion limited coarsening. They illustrate that the in solvent is hydrolyzed by using a base solution of NaOH solvent is an important parameter in controlling particle or NH4 OH, then these compounds condense to form a size. ZnO nanostructures represent the wurtzite structure. metal oxide precipitate by increasing the concentrations of It is considered a most thermodynamically stable phase OH− ions. The precipitate obtained is then washed and and to have hexagonal lattice. The structure of ZnO can dried to collect the crystalline metal oxide powder form. be described as alternating planes in z-axis stacked with The overall reaction for the synthesis of ZnO nanoparticles tetrahedral coordinated O2− and Zn2+ ions. Due to tetrafrom Zn(II) acetate can be written as: hedral representation, ZnO shows non-central symmetric structure and the bonding of Zn–O also shows strong ionic ZnCH3 CO2 2 + 2NaOH → ZnO + 2NaCH3 CO2 2 + H2 O character. Hence, ZnO lies in between ionic and covalent This process is advantageous over other processes as it compounds.55–58 is economical and widely used to synthesize a range of Thus, due to their unique properties ZnO NPs are used single and multi-oxide nanomaterials. But there is one disin many applications. ZnO Nanoparticles are used in the advantage of agglomeration of nanoparticles, which can field of electronics for making varistor material for piezobe solved by using capping agents. These capping agents electric transducers59 and devices, surface acoustic wave devices, electrode of solar cells,60–62 etc. can neutralize the nanoparticles in the solution and surface Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
Daksh and Agrawal
Table I. Various methods for synthesis of ZnO nanostructures. Synthesis method of ZnO Mechanochemical process
Co-precipitation process
Chemical reagents Zinc chloride, Na2 CO3 , NaCl, ethanol Zinc chloride, Na2 CO3 , NaCl, ethanol Zinc chloride, Na2 CO3 , NaCl, ethanol Zinc chloride, Na2 CO3 , NaCl, ethanol
Temp-350 –450 C Calcination-1 hr. Temp-400 C Calcination-0.5 hr.
Zinc nitrate, NaOH, Ionised water Zinc nitrate, NaOH, Ionised water Zinc acetate, KOH, Ionised water ZnSO4 , NH4 HCO3 , ethanol
Temp-100 C Stirring overnight at room temp. Temp-100 C Cacination-2 hrs.
Zinc acetate, NaOH, Ionised water/Alcoholic media ZnSO4 , NH4 HCO3 , ethanol Sol–gel method
Hydrothermal process
Temp-600 C Calcination-1 hr. Temp-400 –800 C
Temp-20 –80 C Temp-100 C Overnight drying Cacination-2 hrs. Temp-75 C Overnight drying at room temp. Temp-25 C Drying-80 C Cacination-1 hr.
Zinc acetate, NaOH, HMTA
Temp-100 –220 C for 5–10 hrs. and HMTA be of 0–200 ppm Temp-100 –220 C for 5–10 hrs.
Reference
Hexagonal and regular shape of particles, particle size of 51 nm Hexagonal and regular shape of particles, particle size of 27–56 nm Hexagonal structure, particle size of 21–25 nm Hexagonal structure, particle size of 18–35 nm
[34]
Particles of spherical size of 40 nm
[38]
Particle size of 50 nm applications: Gas sensing applications Particle size of 160–500 nm
[37]
Wurtize structure, particle size of 12 nm
[39]
Hexagonal structure, flower shape, particle size of 500–800 nm. Applications: anti-microbial activity Hexagonal structure, flower-like and rod-like shape, size of 15–25 nm
[43]
Temp-60 C Drying 24 hrs., Uniform, wurtize structure particle size 60–80 C and calcinations at of 100 nm Application: as 500 C neuro-toxic agent Wurtize structure, spherically shaped Zinc acetate, oxalic acid, Temp-50 C, time 60 min Dried C Ingenta for 20 hrs. to: Wei Chenparticles, size of 50–200 nm at 80by ethanol Delivered C Sep 2016 16:57:39 calcination at 650 14 IP: 129.107.80.80 On: Wed, Hexagonal wurtize structure and Zinc accetate, Synthesis at room temp. and Publishers Copyright: American Scientific nanotubes of 70 nm diethanolamine, ethanol annealing: 500 C for 2 hrs.
[33] [30] [31]
[35]
[42] [46]
[47]
[49]
Spherical shaped structures having particle size of 55–110 nm Different types of structures are obtained: bullet like, rod like, sheet of size 50–200 nm Hexagonal wurtize structure of size greater than 100 nm Crystallite size of 9–31 nm and particle diameter of 40–200 nm
[51] [50]
Zinc acetate, Zinc nitrate, LiOH, KOH, NH4 OH Zinc acetylacetonate, n-tuboxyethanol and zinc oximate Zinc nitrate, deionized water, HMT
Temp-120 –250 C for 10–48 hrs. Microwave heating of 800 W for 4 min and drying at 75 C Microwave heating at 90 C for 2 min and dried at 60 C, 2 hrs.
Hexagonal wurtize structure, nanorod and nanowire shape of 280 nm diameter. Applications: nano-optoelectronic devices
[57]
Diethylzinc, oxygen
Using helium as carrier gas
[67]
Zinc acetate
Pyrolysis or thermal deposition around 800 C Temp.-140 C for 15 hrs. and dried at 60 C
Hexagonal wurtize structure and size of 9 nm Hexagonal wurtize structure and size of 20–30 nm Hexagonal wurtize structure and varying size of 50–200 nm
Zinc nitrate, NaOH, heptanes, hexanol, triton X-100, PEG Zinc acetate, NaOH, decane, water, ethanol
Temp.-90 C for 2 hrs.
They are used in photo-voltaic and thin film transistor applications, photo detectors, laser diodes, flat panel displays, nano-motors, optical waveguides and fabricating LEDs for getting different colors.62 ZnO Nanoparticles are used in glass and rubber industry for removing and 4
Properties
Zinc acetate, oxalic acid, ethanol
Zinc chloride, NaOH
Other methods
Synthesis conditions
Spherical and hexagonal particles, size of 100–230 nm
[52] [56]
[65] [61]
[58]
reducing heat and thermal expansion.58 60 They are used in the paints and lubricants industry for removal of corrosion that wears on engines. In the field of pharmaceutical sciences, they are used for developing the antiseptic healing creams, lotions, and cosmetic creams.61–67 They can be Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
or germanium to make it a good combination between electron and hole pair.73 Due to this, researchers have developed several applications of doped-silicon materials, which is now the basis of the largest global electronics industry. It is well known that the incorporation of defects or impurities into the semiconductor lattices is the primary means of controlling electrical conductivity, optical, luminescent, magnetic, and other physical properties.73 74 The pure ZnO is an insulator and the conductivity of ZnO can be observed over 10 orders of magnitude with the small change in the concentrations of native or non-native defects. At this magnitude, the electrical devices would not operate without having impurities. So, for using ZnO in different devices, the doping is required. In ZnO nanostructures, there are two types of defects: intrinsic defects and extrinsic defects. The intrinsic or native point defects concerned with ZnO NPs are the interstitial of zinc and oxygen and vacancies at the bonding between zinc and oxygen. The oxygen vacancies and zinc interstitials are dominant native or relevant donor. And in this, the formation of donor levels may probable, if the Fermi energy band is equal to the valence band.74 The main native acceptor in ZnO is the zinc vacancies. Its transition level energy is with 0.11 eV or 0.8 eV above EV depending on its transition state. Among the native pointChen defects of ZnO NPs, oxygen anti-sites have Delivered by Ingenta to: Wei IP: 129.107.80.80 On: Wed,the14highest Sep 2016 16:57:39 defect formation energy even for oxygen rich Copyright: Scientific Fig. 1. Morphologies of zinc oxide nanostructures. Reprinted American with perconditionsPublishers and may be considered as deep acceptors. mission from [67], T. Xu, et al., Growth and structure of pure ZnO Extrinsic defects are classified into two types: n-type micro/nanocombs. J. Nanomater. 2012, 797935 (2011). © 2011, Hindawi and p-type. The n-type ZnO are obtained by doping with Publishing. Group III elements as Al, In, etc., transition metal elements as Pb, Mn, Fe, Ni, Co,75–77 and rare earth metals as Eu, Y, used with fertilizers for providing a micronutrient source Gd,77–79 etc. These elements incorporate on the zinc latof zinc.64 65 They are used in cigarette filters for reductice site and become shallower effective mass donors. The ing the amounts of HCN and H2 S in the smoke. As ZnO free electron concentration of n-type ZnO can be formed Nanoparticles show high sensitivity at room temperatures, with the help of hydrogen. The formation of p-type ZnO they are used in sensing applications, for example, gas for practical as well as industrial applications has proven sensing and detection of hazardous compounds.11 12 66 67 very difficult and become the bottleneck in the development of ZnO-based devices due to the asymmetric doping limitations in ZnO nanostructures.80 4. RARE EARTH METALS AND NEED FOR DOPING For doping of ZnO, rare earth elements are more likely to be considered as dopants due to their optical and high For using ZnO NPs in advanced and sophisticated indusconductivity properties.73 75–77 Rare earth metals doping trial applications, numerous studies are taking place by for providing wide bandgap semiconductors continues to researchers to find the optimum solution for enhancing the be of interest for display applications involving UV, visperformance properties of ZnO without changing its physible, and infrared light emission. Wide band gap exhibits iochemical properties. Thus, modifications are taking place less thermal quenching of emissions than narrow gap semifor improving its performance properties.68–70 For enhancconductors. The studies of green luminescence from Tbing the properties of ZnO nanostructures, we have to add doped ZnO and red luminescence from Eu-doped ZnO impurity or defect at the time of synthesis of nanoparsuggest that these materials may prove useful in optoelecticles. The addition of foreign atoms or impurities to a tronic applications.81 82 The Gd-doped ZnO NPs shows compound by creating defect for enhancing its physical the ferromagnetism which can be applied in spintronic properties is termed as doping. Here, doping is required to applications. modify the physical properties of nanostructures.71 72 For Thus we can say that the properties of ZnO nanoexample, pure silicon has very poor electrical conductance structures depend on their crystal structure, morphology, properties, but it can be doped with phosphorus, boron, Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
Daksh and Agrawal
Fig. 2. Morphologies of zinc oxide nanostructures. Reprinted with permission from [27], X. Guo, et al., Synthesis of ZnO nanoflowers and their Delivered Ingenta to:OSA WeiPublishing. Chen wettabilities and photocatalytic properties. Opt. Express 18, 18401by (2010). © 2010,
IP: 129.107.80.80 On: Wed, 14 Sep 2016 16:57:39 Copyright: American Scientific Publishers The methods for synthesizing RE-doped ZnO nanosize, and surface defects. It has been extensively structures are chemical, physical, or mechanical as proven that modifications of ZnO Nanostructures such as co-precipitation method, sol–gel process, hydrothermal doping of transition metals or rare earth metals could synthesis, etc. providing different structures, shapes, and improve their properties.77 The photocatalytic properties of sizes. ZnO NPs were significantly increased and improved when modified with the incorporation of dopant ions. The dop5.1. Co-Precipitation Method ing of metal ions in ZnO nanostructures can lead to effects such as an enhancement/decrease in fluorescence and conCo-precipitation takes place when any solution is supersaturated to form precipitates with another substance. In trolling concentration of surface defects.74 Moreover, ZnOthis method, an inorganic metal salt is dissolved in solvent doped samples with a high concentration of oxygen defects and these formed species are hydrolyzed by adding a basic will exhibit excellent photodegradation of organic pollusolution example NaOH or KOH. By the action of cationic tants. The energy level or charge transfer between states solution, solutions are condensed with each other to form helps in determining the intrinsic ferromagnetic properties a metal hydroxide precipitate. The formed precipitate has of rare earth-doped ZnO.82 83 undergone several processes, such as filtration, annealing and drying, to collect nanopowders of crystalline metal 5. METHODS FOR SYNTHESIZING RARE oxides.78–82 The main advantage of using this method is EARTH-DOPED ZnO NANOSTRUCTURES that it is very economical and metal oxide nanopowders Rare earth-doped ZnO nanostructures are synthesized can easily be synthesized. The co-precipitation process can and studied with different techniques and methods to be affected by several parameters such as pH, temperature, obtain a good size yield with specific morphologies. concentration of the solutes, annealing and effect of cataUntil now, there are continuous studies on RE-doped lyst, reaction time, and drying for obtaining metal oxides ZnO for determining their different characteristics and nanostructures of different morphologies.51 78 83 85 A drawmorphologies.82–84 In this paper, we are trying to focus back of using this process is its inability to control the on the important aspects of RE-doped ZnO nanostructures growth and size of nanoparticles and agglomeration. But having different morphologies and properties and also that limitation can be resolved with the help of a capdiscuss their future aspects of applications in different ping agent, such that it is layered down over the formed fields. nanoparticles to stop the process of agglomeration.51 78 82 78 81
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The co-precipitation method is useful for creating defect because it uses three mechanisms—inclusion, occlusion and adsorption. Thus an impurity can be added to the formed crystalline lattice nanostructures or to the surface of the crystalline nanostructures by the action of these mechanisms. It can also be used for synthesizing magnetic nanoparticles of different morphologies.78 85
Rare Earth-Doped Zinc Oxide Nanostructures: A Review
6. RARE EARTH METALS USED FOR DOPING OF ZnO
6.1. Erbium Many researchers have reported the formation of Er-doped ZnO nanostructures and reported their unique and distinct properties and applications. Mohamed et al. showed that in the absence of any surfactants or structure directing reagents,79 Er-doped spherical-like ZnO hierarchical nano5.2. Solvothermal and Hydrothermal Method structures were successfully obtained by a facile chemical solution route. Er doping increased the visible light The term hydrothermal was first coined by British geolabsorption ability of Er/ZnO and a red shift for Er/ZnO, ogist Sir Roderick Murchison for describing the effect of appeared when compared with pure ZnO. The photocatpressure and temperature on the water at elevation while alytic results showed that doping of Er into ZnO can sigexplaining the information about the formation of rocks nificantly improve the photocatalytic efficiency of ZnO and minerals from the earth’s crust. Hence, the method under visible light irradiation. The obtained ZnO products for synthesizing single crystals under high temperature contain very developed free surfaces and grain boundaries, and pressure conditions depending upon the solubility of which can show ferromagnetic behavior. Zhang et al. fabsolutes in the solvent may be termed as a hydrothermal 86 ricated the erbium (Er)-doped ZnO nanocrystals (NCs).80 method. The only difference between hydrothermal and The enhanced defects or vacancies resulted in high activity solvothermal method is the precursor solution is not aquesurfaces, which could enhance sensing properties of the ous while using solvothermal method. Autoclave has to ZnO-based nanostructures. Furthermore, sensing experibe used for crystal growth because it maintains a temperments confirmed that the sensing performance of Er-doped ature gradient between opposite ends of the chamber. At ZnO NCs was improved, almost doubling that of the pure the hotter end, the solutes dissolve and at cooler end, crysZnO NCs. Therefore, the high sensibility could be realized tal growth takes place. The temperature and pressure play by doping Er3+ ions into the ZnO NCs. a vital role in the growth of nanostructures in this proThangavel et al. prepared Erbium (Er)-doped zinc oxide cess. Hydrothermal method has an advantage that itbyhas Delivered Ingenta to: Wei Chen (ZnO) sol–gel films on glass substrate using the spin IP: 129.107.80.80 2016thin 16:57:39 the ability to obtain crystalline phases which are notOn: sta-Wed, 14 Sep 81 The Er-doped ZnO films were formed to coating method. Copyright: American Scientific Publishers ble at melting points. That’s why, by using this method, retain the hexagonal wurtzite structure. The modification we can obtain nanostructures of high quality and single of localized levels on doping had enhanced green luminescrystalline structures of different morphologies. Hydrothercence. The increase in exciton binding energy on doping mal synthesis provides a synthesis of pure and doped ZnO substantiates the modified localized levels. The presence nanostructures that can be used in many industrial appliof oxygen related defects might be responsible for the cations, mainly in optoelectronic applications.86–89 appearance of a near infra-red emission peak in doped ZnO. All the doped films presented higher exciton binding 5.3. Sol–Gel Method energy than the undoped ZnO. The fundamental absorpSol–gel method is the process for obtaining crystalline tion edge at 360 nm could be ascribed to the red shift on materials from a very tiny molecule.81 This method can be Er doping.82 The average transmission of the films was used for the fabrication purposes, mainly of metal oxides, above 80% in the visible region. The theoretical and expersuch as silicon. In this process, the monomers are conimental values of refractive index nearly coincide with the verted into colloidal solutions acting as precursors for the films with higher value of refractive index. The optical gel formation using discrete particles.81 82 91 Further, using conductivity of the EZ films was mainly in the UV region, filtration and drying methods, the micro or nanostructures which emphasizes that it can be used in the UV detector can be obtained. There is one advantage of this method applications. that the synthesis can be done at low temperature. Due Zamiri et al. have explained the optical and dielectric to their numerous properties, the sol–gel methods can be properties of Er-doped ZnO nanoplates.83 They have synapplied in many applications such as protective coatings thesized the nanoplates and carefully studied the dielectric for the formation of thin films and fibers, and biomedical properties. Scanning electron microscopy (SEM) reveals and optomechanical applications.81 82 the growth of nanoplate-like structures in the prepared samples. Further, the presence of Er ions doping in ZnO 5.4. Other Methods matrix has been confirmed by EDS measurement. A sharp Several different methods also exist for obtaining nanoand strong peak at around 435 cm−1 was observed in structures of different sizes and shapes. Those are paralysis Raman spectra of nanoplates. The dielectric properties spray method,34 84 sonochemical method,38 101 combustion of pure and Er-doped ZnO nanoplates follow a behavior method,90 94 thermal evaporation,89 microwave assisted,92 based on the Maxwell–Wagner model and the AC conductivity tends to decrease by Er doping. Liauet et al. showed and electrospinning method,96 105 etc. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
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Table II. Schematic representation of different methods for synthesizing Er-doped ZnO nanostructures. Methods Chemical solution route
Chemical reagents
Synthesis conditions
Properties and applications
References
Spherical like structures having particle size of 15–50 nm and diameter of appx. 850 nm. Used in photocatlytic activity Nanocrystals are obtained having particle size of 23–30 nm. Applications: shows luminescence and used in sensing applications mainly in gas sensing ex: ethanol, acetone
[79]
ZnA, ErCl2 , dist. Water
Room temp.-25 –35 C
Zn(NO3 2 , Er(NO3 3 , octanol
Solvothermal conditions, Temp-200 C for 10 hrs. and washed with ethanol
Sol–gel method
ZnA, ErCl2 , MEA
Stirring with temp.-60 C, for 1 hr. Annealing at 500–600 C for 1 hr.
Sphere like structures obtained on crystalline surfaces having size of 27.44 nm and 29.28 nm. High optical properties with high refractive index. Applications: It can be used in UV detectors
[81, 82]
Co-precipitation method
ZnA, ErA, ethanol
Temp.-60–80 C. Stirring overnight
XRD shows hexagonal wurtzite structure. SEM shows nanoplates like structure having size of 20–30 nm. It shows optical and dielectric properties
[78, 83]
[80]
This prevents observation of the sharp 5 D0 –7 F2 transition of the Eu3+ ion at around 615 nm. The Eu must be incorporated during growth in order to see this emission. So, Eu2 O3 is formed on the surface of the ZnO nanowire forming a coaxial nanowire heterostructure system. Cao et al. showed that the Eu-doped ZnO microrods with wurtzite Delivered by Ingenta to: Weiand Chen structure [0001] growth direction have been prepared 6.2. Europium IP: 129.107.80.80 On: Wed, 14 Sep 2016 16:57:39 on Si substrate by a simple hydrothermal method.86 The The study of Eu in ZnO has attracted Copyright: interest for American applica- Scientific Publishers observed results indicate that Eu3+ ions was located on the tions including visible red lasing, which was hampered by 3+ surface sites of ZnO microrods. Additionally, the characterinefficient energy transfer from the ZnO host to the Eu istic intra-4f transitions of Eu3+ ions and room temperature ions. When the red emission is dominant, it has generally energy transfer from ZnO host to Eu3+ were observed, and been observed that defect states are involved in the energy 85 86 it was considered that surface defects may act as a step in transfer process. It is clearly of interest to examine the process of energy transfer. This work has proved the the properties of Eu-doped ZnO nanowires for potential possibility of incorporating Eu3+ ions into ZnO microrods interest in very low threshold emitters. with high crystalline and realized the energy transfer from Pearton et al. showed that the green luminescence of the ZnO nanowires with Eu diffusion process observed at about 515 nm is attributed to Eu.85 The thermal annealing of diffusion process caused a red shift of the NBE emission from the nanowires. There exists Eu2 O3 formed on the surface of ZnO nanowires after the Eu diffusion process. the deposition of Er-doped ZnO (EZO) on Si substrate at 500 C by reactive ion beam sputtering utilizing a capillaritron ion source at various oxygen partial flow rates.84 All the EZO films exhibit a preferred (002) growth direction and emission at 984 nm is recorded.
Fig. 3. SEM images of Er-doped ZnO. Reprinted with permission from [72], K.-S. Yu, et al., Synthesis, characterization, and photocatalysis of ZnO and Er-doped ZnO. J. Nanomater. 2013, 372951 (2013). © 2013, Hindawi Publishing.
8
Fig. 4. UV-Vis spectra for ZnO and the Er-doped ZnO nanocomposites. Reprinted with permission from [72], K.-S. Yu, et al., Synthesis, characterization, and photocatalysis of ZnO and Er-doped ZnO. J. Nanomater. 2013, 372951 (2013). © 2013, Hindawi Publishing. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
Fig. 5. PL spectra of the Er-doped ZnO nanocomposites with different Er concentrations. Reprinted with permission from [72], K.-S. Yu, et al., Synthesis, characterization, and photocatalysis of ZnO and Er-doped ZnO. J. Nanomater. 2013, 372951 (2013). © 2013, Hindawi Publishing.
ZnO host to Eu3+ ions, which would be helpful in optoelectronic and full color display device applications.87 Sudarsana et al. showed that the Nanopowders of ZnO: Eu3+ was successfully prepared by a simple solution Table III.
combustion method.88 Average ZnO: Eu3+ particle sizes evaluated have approximate values to ∼50 nm. Room temperature PL spectra studies showed strong emission bands at ∼618 nm when excited with 515 nm wavelength indicating an electric dipole transition (5 D0 –7 F2 and implying that Eu3+ ions was not in a symmetry center. Ishizumi et al. have fabricated the ZnO:Eu nanostructures by the reverse micelle method89 and have studied their PL properties. It was found that the intensities of the exciton, defect, and Eu3+ PL are sensitive to the surrounding atmosphere. In the nitrogen gas atmosphere, the exciton and Eu3+ PL are strong. In the oxygen gas atmosphere, on the other hand, the defect and Eu3+ PL are strong. These findings show that the nitrogen molecules adsorbed on the NC surfaces passivate the non-radiative centers and that the oxygen molecules produce the localized states which cause the defect PL. It is suggested that the localized states improve the energy transfer from the ZnO host NCs to the Eu3+ ions.90 Reddy et al. have prepared ZnO:Eu nanopowders by auto ignition based low temperature solution combustion method using ODH fuel.90 Powder X-ray diffraction (PXRD) patterns confirm the nanosized particles that
Schematic representation of different methods for synthesizing Eu-doped ZnO nanostructures.
Methods Diffusion process
Delivered by Ingenta to: Wei Chen Properties and applications Sep 2016 16:57:39 Copyright: American Scientific Publishers Temp.-900 C under vaccum Single crystal Eu-doped ZnO nanowire
Chemical reagents Synthesis IP: 129.107.80.80 On:conditions Wed, 14 ZnA, Eu, O2
and annealing for 1 hr.
References [85]
Zinc nitrate hexahydrate, methenamine, Eu(NO3 3 · 6H2 O
Temp.-95 C for 24 hrs and annealing at 400 C
diameter appx. 200 nm. It shows green luminescence at 515 nm Wurtzite structure, microrods of Eu-doped ZnO diameter of 2–3 m. Applications: optoelectronic and full colour display device
ZnA, Eu2 O3 , methanol, NaOH
Temp.-150 C for 12 hrs. under stirring and dried at 60 C
Wurtzite structure NPs size of 9–12 nm. It shows red PL emissions
[95]
ZnO, Eu2 O3 , nitric acid, water, NH4 OH
Nanoparticles obtained of size ∼50 nm. It indicates an electric dipole transition and high energy transfer between states Hexagonal wurtzite structure having size of 35–39 nm. PL emission shows excitation for red, blue and green colours
[87]
Zinc nitrate, ODH, water, Europium nitrate
Temp.-350 C for boiling, pH = 5, Annealed at 550 C Temp.-300 C, Stirring for 5 mins
Zinc nitrate, Europium nitrate, urea
Temp.-350 C, Annealed at 550 C
Nanoparticles obtained of size ∼28 nm NPs can apply in technological applications
[94]
ZnA, NaBH4 , Europium nitrate, water
Stirring for 4 hrs. at 60 C, dried at 200 C
[90]
ZnA, Eu(CF3 COO)3 · 3H2 O, ethanol, NaOH
Stirring for 2 hrs. at room temp, dried under vaccum for 24 hrs at 60 C
Nanoparticles formed of size 1–60 nm and show PL properties. It shows red PL applied in optoelectronic and display applications Quasi-spherical crystalline structures having size of 5.67 nm. Applications: photocatalytic applications
Reverse micelle method
Zn(CH3 COO)2 · 2H2 O, Eu(CH3 COO)3 · 4H2 O, CTAB, butanol and octane
Temp.-200 C for 10 hrs.
Nanocrystals of Eu-doped ZnO of size ∼50 nm, having high Photoluminescence properties
[89]
Microwave assisted method
ZnA, NaOH, Europium nitrate, water, ethanol, PVA
Constant stirring, room temp, pH-10, annealed at 200 C for 2 hrs.
Hexagonal crystal structure nanorods of size 25 nm. Applications: photocatalytic applications
[91]
Hydrothermal process
Combustion method
Co-precipitation method
Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
[86]
[88]
[92]
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exhibit hexagonal wurtzite structure without secondary phase. The crystalline size estimated from Scherrer’s formula was found to be in the range of 35–39 nm. A blue shift of absorption edge with an increase in band gap is observed for Eu-doped ZnO samples. Upon 254 nm excitation, ZnO:Eu nanopowders show peaks in regions blue (420–484 nm), green (528 nm), and red (600 nm), which corresponds to both Eu2+ and Eu3+ ions. With the increase of excitation wavelength to 488 nm, the emission peaks at 586 nm and 652 nm are observed which are due to 0 1 0 3 3+ ions, respec5 D → 7 F and 5 D → 7 F transitions of Eu tively. The electron paramagnetic resonance (EPR) spectrum exhibits a broad resonance signal at g = 4195 which is attributed to Eu2+ ions. The kinetic analysis of the experimental TL glow curve was carried out using Chen’s peak shape method and the average activation energy was found to be in the range 0.21–1.26 eV. Shahroosvand et al. have investigated the effect of the Fig. 7. Photodegradation efficiency of the MB aqueous solutions of different percentages of Eu-doped ZnO under UV light within 300 min. coupling structure of Eu in ZnO on the photoluminesReprinted with permission from [126], A. Phuruangrat, et al., Synthecent characteristics.91 They determined the optimum consis and characterization of europium-doped zinc oxide photocatalyst. ditions toward enhanced red emission. The analysis of J. Nanomater. 2014, 367529 (2014). © 2014, Hindawi Publishing. X-ray diffraction and photoluminescence spectra measurements indicate that, for Eu3+ -doped ZnO synthesized in spectroscopy (XPS), scanning electron microscopy (SEM), different pathway reactions, Eu exists in the host lattice and transmission electron microscopy (TEM). The averand expands the region of emission of the ZnO host. Euage crystallite size and band gap energy of Eu-doped ZnO doped ZnO nanoparticles were synthesizedDelivered by using aby simIngenta to: Wei varied withChen the Eu content. The XRD pattern of Eu-doped ple solution-based synthetic method and their role inOn: theWed, 14 Sep 2016 16:57:39 IP: 129.107.80.80 ZnO indicated hexagonal crystal structure with an avermodification of red photoluminescence was studied. The Scientific Publishers Copyright: American age crystallite size of 25 nm. The presence of europium average particle sizes were calculated to be about 1–60 nm with trivalent state and its doping successfully into the 90 91 From the PL spectra analysis, by different methods. crystal lattice of ZnO matrix was confirmed by XPS techthere is a correlation between the synthesis methods and nique. The Eu doping in ZnO matrix acted as an electronic the characteristics of the red photoluminescence, which scavenger, which prevented the recombination of electron– suggests that the intrinsic defects can assist efficient energy hole pairs on the surface of ZnO nanorods and thereby 3+ transfer from the ZnO host to the Eu ions. Three peaks improved the charge transfer process. Through study5 7 in the region of D0 – F1 were observed, which indicate ing degradation efficiency, it was found that Eu-doped 3+ that the Eu is located on the surface lattice sites of the ZnO nanorods prepared by the microwave assisted method ZnO nanomaterials. played a key role in the photocatalytic performance. The Garadkar et al. studied doped ZnO nanorods as a phophotocatalytic activity of Eu-doped ZnO nanorods was tocatalyst with different Eu contents prepared by the evaluated for methyl orange degradation. The photodegramicrowave assisted method.92 They were characterized dation efficiency was increased with increase of catalyst by means of X-ray diffraction (XRD), energy-dispersive loading up to 3 g/dm3 .93 The maximum ∼91% photocatX-ray spectroscopy (EDS), UV-Vis spectroscopy, surface alytic degradation of methyl orange was obtained under area Brunauer-Emmett-Teller (BET), X-ray photoelectron UV light (365 nm) within 180 min, when the pH was adjusted to 7, at 3 g/dm3 loading of 0.2 mol.% Eu-doped ZnO nanorods.90–93 Franco, Jr. et al. studied that the photoluminescence and dielectric properties of nanoparticulate powders of un-doped ZnO and Eu-doped ZnO synthesized by the combustion reaction method.94 The photoluminescence emission spectra of Eu-doped ZnO nanocrystalline powders showed three intense band emissions in 578 nm, 590 nm and 612 nm attributed to 5 D0 –7 F0 , 5 D0 –7 F1 Fig. 6. SEM images of Eu-doped ZnO. Reprinted with permission from and 5 D0 –7 F2 transitions of Eu3+ ions when excited with [126], A. Phuruangrat, et al., Synthesis and characterization of europium394 nm wavelength, respectively, originated from intra-4f doped zinc oxide photocatalyst. J. Nanomater. 2014, 367529 (2014). transition of Eu3+ ions. The emission intensity becomes © 2014, Hindawi Publishing. 10
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
stronger with increasing the Eu3+ doping and the highThey are used for the fabrication and growth of thin films over substrates, which can be used in the semiconductor est intensity peak was at 612 nm, red emission, attributed industry. to 5 D0 –7 F2 electric dipole transitions. The static dielectric constant, , increased with Eu-doping concentration. However, it was comparable to those values for bulk ZnO. 6.3. Gadolinium Thus, it is concluded that the combustion reaction method ZnO based materials have considerable potential in transis an alternative method for the synthesis of Eu-doped parent conductive oxides (TCOs) due to their wide band nanoparticles with optical and dielectric qualities suitable gap (3.37 eV) and transparency under visible light. The for technological applications. presence of intrinsic defects, including oxygen vacancies Najafi et al. have fabricated Eu-doped ZnO through or interstitial zinc atoms, causes ZnO to exhibit n-type CVD method.95 The PL spectra analysis demonstrated a conduction.2–5 ZnO NPs can also be applied to light emitcorrelation between defect states and energy transfer from ting devices due to its large binding energy of 60 MeV. the ZnO host to the Eu3+ ions, which consequently led Wu et al. have studied the electronic and optical propto strong red emission. PL results also revealed in various erties of Gd-doped ZnO. The lattice constants and band synthesis temperatures and different gas ratio, the Eu3+ gap of ZnO calculated in this study are in agreement with ions take a site with diverse symmetry in the ZnO host, experimental values.98 The donor concentration and optiwhich have a direct effect on the red emission intensity cal band gap of Gd-doped ZnO increase with an increase and the ratio of (5 D0 /7 F2 /I (5 D0 /7 F1 . After annealing, in Gd content which enhances conductivity and transboth defects and Eu3+ related red emissions were conmittance, respectively. However, electrical conductivity is siderably quenched. A decrease in the red emissions was reduced when localized states close to the Fermi level and correlated with a corresponding decrease in oxygen defihigher scattering probability of free electrons occurs with ciency. Eu-doped ZnO Nanorods samples indicated that high Gd concentration. Following the incorporation of Gd increasing surface area has a direct effect on producing into ZnO, the average transmittance of light in both the intense red emission from Eu sites. Dan-Dan et al. studvisible and UV ranges exceeds that of ZnO. However, the ied and explained the red emission given by Eu-doped stronger and wider donor states obtained from high doping nanostructures.96 They obtained Eu-dopedDelivered nanostructures levels significantly by Ingenta to: Wei Chen decreases the average transmittance. as like nanosheets with the helpIP:of129.107.80.80 pyrolyzing method. selecting suitable doping level is crucial to optiOn: Wed,Thus, 14 Sep 2016 a16:57:39 Copyright:and American Publishers Assadi et al. explained the electronic, structural, mag- Scientific mizing the photoelectric performance of Gd-doped ZnO. netic properties of Eu-doped ZnO NPs.97 Beside these, Lu et al. have reported the development of Gd-doped there are several other applications of Eu-doped ZnO NPs. ZnO quantum dots (QDs) as dual modal fluorescence and Table IV. Schematic representation of different methods for synthesizing Gd-doped ZnO nanostructures. Methods Simple chemical method
Chemical reagents
Synthesis conditions
Properties and applications
References
Nanoparticles formed of size 3–5 nm. Applications: Fluorescence imaging and MR Imaging Nanoparticles formed of size 9.3 nm and shows ferromagnetism. Applications: optoelectronic and spintronic devices, Photocatalytic applications
[99]
ZnA dehydrate, TMAH, GdA hydrate, ethanol, oleic acid Zinc acetate, Gadolinium chloride, NaOH, ethanol
Constant stirring at room temp.
ZnAdihydrate, GdNhexahydrate, water, ethanol, acetone
Temp.-60–200 C for drying
Nanoparticles formed of size 2–5 nm and band gap of 2.97 eV and enhancing photocatalytic degradation and magnetic properties. Applications: nanoscale spin based devices
[106]
Thermal evaporation method Sol–gel method
ZnAdihydrate, GdNhexahydrate, O2 ZnA dehydrate, GdA hydrate, ethanol, NÅH
Temp.-500 C for 2 hrs.
Nanoparticles formed of size 10 nm. Applications: optoelectronic devices Nanoparticles formed of size 8.83–21.85 nm. Applications: Photocatalytic applications
[100]
Sonochemical synthesis method
ZnA dehydrate, GdNhexahydrate, PVP/CTAB, ammonia
Sonication 30 mins and dried at 320 C
Hexagonal wurtzite structure of size 10–70 nm
[102]
Electrospinning calcinations method
ZnA dehydrate, GdNhexahydrate, PAN, N ,N -dimethyl formamide
Stirred at 35 C for 3 hrs., Voltage-22.5 kV, Calcinated at 500 C
Nanorods composed of NPs formed size of 73 nm. Applications: Photocatalytic applications
[103]
Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
Temp.-80 C for 6 hrs., stirring at 40 C for 5 hrs, dried at 200 C for 12 hrs.
Stirring at 80 C for 6 hrs., pH 8–11
[105]
[101]
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magnetic resonance imaging nanoprobes.99 They are fabricated in a simple, versatile, and environmentally friendly method, not only decreasing the difficulty and complexity, but also avoiding the increase of particle size brought about by the silica coating procedure. They have presented a facile strategy for the fabrication of novel MRI-FI nanoprobes named as Gd-doped ZnO QDs with reduced size. Such nanoprobes exhibited significantly enhanced yellow emission due to the Gd doping. The utility of Gddoped ZnO QDs as optical imaging agents has been clearly demonstrated. HeLa cells could be successfully imaged in a short time and retained high viability even at high dose of QDs, revealing the weak toxicity of Gd-doped ZnO QDs. Furthermore, Gd-doped ZnO QDs showed contrast enhancement in MRI due to their effect on the T1 relaxation times. These properties allowed Gd-doped ZnO QDs Fig. 9. PL Spectra of undoped and Gd-doped ZnO. Reprinted with to function as effective dual modal imaging nanoprobes permission from [127], O. Oprea, et al., Photoluminescence, magnetic properties and photocatalytic activity of Gd3+ doped ZnO nanoparticles. and thus, these nanoprobes would find a broad range of Digest J. Nanomater. Biostruc. 7, 1757 (2012). © 2012, National Institute applications in the biomedical field. Ma et al. have preR&D Materials Physics. 100 sented a study of the light emission properties, from UV to blue spectral region, of Gd-doped ZnO nanocrystals implantation in up to 12% of the near-surface atoms. The fabricated by means of a thermal evaporation vapor phase carrier concentration of implanted films is up to several deposition process. The pure ZnO nanocrystals exhiborders of magnitude larger compared with unimplanted ited a strong and predominant UV emission peaking at ZnO. Annealing leads to the diffusion of ions, which most 375 nm. Besides the UV emission of ZnO nanocrystals, likely reduces the number of Gd ions at substitutional lattwo strong blue emissions, located at 432 Delivered nm and 397bynm, Ingenta Wei The Chenimplantation induced disorder was found ticeto:sites. IP: 129.107.80.80 are observed for the sample doped with 5% Gd.On: TheWed, 14 Sep 2016 16:57:39 around 575 cm−1 , which is a common feature of ion Copyright: strong blue emissions are mainly induced by theAmerican impu- Scientific Publishers implantation induced crystalline defects. Annealing did not rity levels of Gd introduced into the band gap of the make a significant difference in the carrier concentration ZnO nanocrystals. The UV emission of ZnO decreases as in contrast to significant impact on the observed ferromagthe doping concentration of Gd increases, and the blue netism. Niaz et al. have studied the effect of gadolinium emission is replaced by a broad defect emission due to (Gd3+ doping on the structural and physical properties the greater number of defects and impurities, as well as of ZnO nanoparticles. The sol–gel method was utilized to Gd2 O3 on the surface.98–100 For higher doping concentrasynthesize ZnO and Gd-doped ZnO nanoparticles. These tions, both the UV and blue emissions decrease, while a powders have been indexed as a wurtzite structure with large, broad defect emission appears due to the large numthe smallest average crystallite size of about 14.46 nm bers of defects, impurities and the excess Gd2 O3 produced. and grain size of 76 nm.101 The Gd doping significantly This study outlines a method for fabrication of Gd-doped affects the particle size, the particle size of Gd-doped ZnO ZnO optoelectronic devices.100 is much smaller when compared with that of undoped Kennedy et al. have reported the structural and magZnO. The photocatalytic activity of undoped and doped netic properties of ZnO single crystals implanted with ZnO nanocrystalline powders has been evaluated by monGd ions at 40 keV.101 Ion beam analysis shows that Gd itoring the photo-bleaching of the aqueous solutions of ions mostly occupy the substitutional lattice sites after methyl blue (MB) dye under sunlight.102 The photocatalytic activity of the 3 wt.% Gd-doped ZnO NPs are higher when compared with undoped ZnO NPs. This reduction is important for ZnO nanoparticles to be used as UV shielding agents to protect organic substrates. The results showed the increase in current with Gd loading and the gadolinium incorporation affect the resistivity of ZnO nanocrystalline powders. The Gd-doped ZnO nanocrystalline powder has a lower resistivity than the other nanocrystalline powders.102 Sheikh et al. have reported the effect of Gd doping Fig. 8. SEM images of Gd-doped ZnO. Reprinted with permission from and nature of surfactants on crystalline size, morphology, [103], H. Khajuria, et al., Surfactant assisted sonochemical synthesis and band gap of ZnO nanoparticles.103 The microstrucand characterization of gadolinium doped zinc oxide nanoparticles. Acta ture analysis showed the different morphology for doped Chim. Slov. 62 (2015). © 2015, Slovenian Chemical Society. 12
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ZnO samples prepared in CTAB and PVP. The effect of the surfactants on doping clearly showed the higher amount of dopant inserted in the ZnO lattice in PVP compared with CTAB. UV-Visible spectroscopy indicated that the band gap for Gd-doped ZnO nanoparticles decreases with an increase in Gd-doping compared with pure ZnO.103 Chen et al. have synthesized Gd-doped ZnO nanorods via an electrospinning combined with calcination route.104 The photocatalytic activity of Gd-doped ZnO is affected by the cooperative effects of residual carbon, crystalline, and nanoparticle size. It is revealed that the Gd-doped ZnO catalyst calcinated at 500 C for a time of 7 hours showed the best photodegradation activity. A short calcination time of 1 hour gives rise to large amounts of residual carbon and low crystallinity of ZnO, which are adverse for charge separation. By increasing the calcination time up to 3 hours or 5 hours, suitable amounts of residual carbon that favors charge transfer play a greater role, in Fig. 10. UV-Vis spectra of undoped and Gd-doped ZnO nanoparticomparison to crystalline. When the calcination time is cles. Reprinted with permission from [103], H. Khajuria, et al., Surfacfurther prolonged to 9 hours, a growth of nanoparticle size tant assisted sonochemical synthesis and characterization of gadolinium leads to a decreased photocatalytic performance.104 Thus, doped zinc oxide nanoparticles. Acta Chim. Slov. 62 (2015). © 2015, the efficiency of degradation activity is determined by the Slovenian Chemical Society. combinational effects of residual carbon, crystalline activity, and nanoparticle size.105 doping. These nanoprobes would find a broad range of Varughese et al. have synthesized Gadolinium-doped Delivered by Ingenta to: Wei Chen applications in the biomedical field.99 IP: 129.107.80.80 2016 16:57:39 ZnO nanoparticles by co-precipitation method.106 On: TheWed, 14 MaSep et al. studied the optical properties of Gd-doped Copyright: American Publishers absorption spectra have been obtained by UV-Vis spec- Scientific ZnO nanocrystals fabricated through thermal evaporation trometer to find the optical band gap and the obtained valvapor phase deposition process.110 They showed the UV ues have been found to be in the range 2.94 eV. It was emission of ZnO decreases as the doping concentration found that energy band gap Eg decreases with doping of of Gd increases and the blue emission is replaced by Gd. It is clear that as temperature increases, particle size a broad defect emission due to the greater number of also increases. The change in particle size causes large defects. The fabrication technique that they discussed can variation in the physical properties.105 be used for fabrication of optoelectronic devices.108–111 Baturay et al. represented the Gd-doped ZnO films Ma presented the ferromagnetic properties of Gd-doped deposited on p-type Si using spin coating.107 They ZnO nanowires.111 The magnetic properties of ZnO:Gd observed that the Gd doping had a strong effect on the are a function of external magnetic field and temperaperformance of the Gd-doped ZnO p-Si heterojunctions ture. They calculated the average moment as 3278 B. formed with 1% Gd-doped ZnO. These devices had the Noel et al. reported the optical and structural properties of strongest rectifying, highest barrier height, and lowest Gd-doped ZnO nanoparticles through sol–gel method.112 series resistance. So, the devices formed using 1% GdThey successfully synthesized the varying sized nanopardoped ZnO thin films exhibited the highest barrier height, ticles, from nano to micrometers, by doping and discussed largest open current voltage VOC , and largest short cirtheir ethanol sensing properties. Ungureanu et al. synthecuit current ISC , as well as the smallest ideality facsized the Gd-doped ZnO thin films.113 They had observed 108 tor and the smallest series resistance. Khataee et al. that there was no Hall effect in the formed thin films but reported the synthesis of Gd-doped ZnO NPs through showed paramagnetic response. So, the magnetic propersonochemical process. They observed the degrading activities of these RE-doped ZnO can be checked and observed ties of the formed NPs for organic pollutants. They showed for the development of better optoelectronic devices. The the application of Gd-doped ZnO nanoparticles can be a UV Absorption spectra show a red shift towards 200 nm promising and efficient approach for the sonocatalysis of due to doping with gadolinium. The gadolinium-doped coloured effluents with high reusability potential.109 Liu ZnO is highly effective and can significantly enhance the et al. reported the development of Gd-doped ZnO quanphoto catalytic degradation and magnetic properties. Doptum dots (QDs) as dual modal fluorescence and magnetic ing with gadolinium nitrate decreases the band gap energy resonance imaging nanoprobes. Such nanoprobes exhibof ZnO. Thus, these studies enable the use of magnetic ited significantly enhanced yellow emission due to the Gd ZnO nanowires as nanoscale spin-based devices.106 113 Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Table V. Schematic representation of different methods for synthesizing Nd-doped ZnO nanostructures. Methods Co-precipitation methods
Sol–gel method
Hydrothermal method
Chemical reagents
Synthesis conditions
Properties and applications
References
Zinc nitrate, neodymium nitrate, NaOH, water and ethanol Zinc acetate, neodymium chloride, NaOH, ethanol
Stirring at room temp. for 30 min, then temp.-80 C for 5 hrs. and drying at 120 C for 2 hrs. Stirring at 40 C for 5 hrs., Centrifuge at 6000 rpm dried at 100 C for 12 hrs.
The Hexagonal wurtzite structure having size of 29–35 nm and shows ferromagnetism Nanoparticles formed of size 9–16 nm and enhances PL and ferromagnetic properties and shows green emission in spectra. Applications: Photocatalytic applications
[118]
Zinc acetate, neodymium nitrate, CTAB, ammonium bromide,
Stirring and aged at room temp. for 48 hrs. then dried at 60 C for 12 hrs.
Nanoparticles formed of size 22–33 nm. Applications: Photocatalytic applications
[119]
Zinc acetate, neodymium nitrate, water, ethylene glycol, monomethyl ether Zinc acetate, neodymium nitrate, water, HTMA
Stirring for 8 hours over a hot plate at 60 C and dried at 120 C Constant stirring for 1 hr., then heated at 95 C for 16 hrs.
Nanoparticles formed of size 20–50 nm and shows paramagnetic nature with band gap of 3.26 eV Hexagonal wurtzite structure having size of ∼200 nm. Applications: Photocatalytic applications and electronic devices
[120]
[117]
[116]
6.4. Neodymium of Nd-doped ZnO NPs are originated from the oxygen vacancies and the diamagnetic behaviour of undoped ZnO Neodymium-doped ZnO NPs have been found to be was changed into the weak ferromagnetic nature for Ndimproving the photocatalytic activity of zinc oxides comdoped ZnO. pared with undoped ZnO NPs. Kumar et al. have preAbd-Lefdil et al. showed surface roughness increased pared nanostructured Nd-doped ZnO thin films deposited Delivered by 114 Ingenta to: Nd Weidoping. Chen 116 The photoluminescence measurement with on a glass substrate by a sol–gel spin coating technique. IP: 129.107.80.80 On: Wed,has 14 shown Sep 2016 16:57:39 a decrease in the band gap with Nd doping, All the prepared films possess a hexagonal wurtzite struc- Scientific Publishers Copyright: American but no emission in the visible range. Neodymium-doped ture. The optical studies confirm that Nd ion doping in zinc oxide (NZO) nanofilms are highly transparent in the ZnO leads to reduction in band gap and exhibits higher visible region. The lowest electrical resistivity value of absorption in the visible region. Nd doping in ZnO films about 4.0 10−2 cm was obtained for 1% Nd effective was found to have enhanced photocatalytic activity and doping. Jheng et al. have studied optical properties and antibacterial activity compared with pure ZnO under visiphotocatalytic activity of Nd-doped ZnO.117 The photoble light irradiation, which is attributed to the high charge separation efficiency and ROS generation ability. The luminescence (PL) spectrum demonstrated a strong and influence of Nd doping on the photocatalytic activity of ZnO for the degradation of methylene blue dye was studied under visible light illumination. The decrease in grain size and light absorption over an extended visible region by Nd ion doping in ZnO nanofilms contributed equally to improve the photocatalytic activity. The optical density measurement of the Nd-doped ZnO nanofilms against Escherichia coli and Staphylococcus aureus bacteria showed better antibacterial activity at a higher level of Nd doping in ZnO. These results suggest that the Nd-doped ZnO nanofilm may be a potential photocatalyst in the degradation of organic contaminations and excellent antibacterial agent for practical applications.114 Vijayaprasath et al. have synthesized neodymium-doped ZnO nanoparticles by a co-precipitation method with improved magnetic property.115 The hexagonal wurtzite structure was identified for undoped and Nd-doped ZnO. The photoluminescence (PL) measurements revealed broad Fig. 11. TEM micrograph of Zn1−x Ndx O nanoparticles. Reprinted with emission that was composed of different bands due to zinc permission from [125], J. H. Zheng, et al., Optical and magnetic propand oxygen vacancies. The result of PL and the vibrational erties of Nd-doped ZnO nanoparticles. Cryst. Res. Technol. 1 (2012). © 2012, John Wiley and Sons. sample magnetometer (VSM) showed that ferromagnetism 14
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Fig. 12. PL spectra of Zn1−x Ndx O alloys with different Nd doping concentration measured at room temperature. Reprinted with permission from [125], J. H. Zheng, et al., Optical and magnetic properties of Nddoped ZnO nanoparticles. Cryst. Res. Technol. 1 (2012). © 2012, John Wiley and Sons.
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Cao et al. have synthesized Nd-doped ZnO nanorods and studied the gas sensing properties of ethanol.122 The results indicated that the sensor based on 2% Nddoped ZnO nanorods presented much higher sensitivity, better selectivity, and shorter response–recovery time to 100 ppm ethanol vapor than the pure ZnO nanorods sensor. The promoting effect on gas-sensing properties of ZnO nanorods seem to result from the catalytic oxidation of ethanol vapor. Yayapao et al. also presented the Nddoped ZnO nanoneedles and observed their photocatalysis properties.123 The photocatalytic efficiencies of the 0, 0.5, and 1% Nd-doped ZnO nanoneedles were evaluated by the degradation of methylene blue under UV light. In their research, they confirmed that 1% Nd-doped ZnO has the best photocatalytic performance with 2.5 times of the undoped ZnO having the highest efficiency of 92% for 300 min.123 124
7. OTHER MISCELLANEOUS RARE EARTH METALS
7.1. Cerium broad peak in the visible light region, and the intensity of Ahmad et al. synthesized the Ce-doped ZnO nanopowvisible light emission was enhanced by Nd doping. The ders photocatalysts.130 Their photoluminescence and optiphotocatalytic activity was evaluated by the degradation of cal properties were also discussed. They found that the methyl orange solution.117 118 It is shown that doping of Nd Ce-doped ZnO NPs had stronger light absorption in the into ZnO induces an increase in the photocatalytic activity Delivered by Ingenta to: Wei visible lightChen region. These photocatalysts showed enhanced and it reaches to optimum at 3%IP:(mole fraction) doping 129.107.80.80 On: Wed, 14 Sep 2016 16:57:39 118–120 photocatalytic activity under UV and invisible light irradiconcentration. The intense visibleCopyright: light emission and Scientific Publishers American ation for dyes. the enhanced photocatalytic activity were explained by the Chang et al. synthesized Ce-doped ZnO nanorods using increase in electron hole pairs and induced defects, such the hydrothermal process.131 The formed nanorods were as antisite oxygen OZn and interstitial oxygen Oi due to used for photocatalytic degradation activities. With the the doping effect of Nd.121 122 The improved photocatalytic help of cerium ions, excellent catalytic activity can be activity can be attributed to the increased electrons and achieved due to increased surface oxygen vacancies and hole pairs, and the introduction of intrinsic defects by Nd 117–121 doping.
Fig. 13. Magnetization versus magnetic field (M–H ) curves for Zn1−x Ndx O alloys with different Nd doping concentration measured at room temperature. Reprinted with permission from [125], J. H. Zheng, et al., Optical and magnetic properties of Nd-doped ZnO nanoparticles. Cryst. Res. Technol. 1 (2012). © 2012, John Wiley and Sons. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
Fig. 14. Photoluminescence emission spectrum (A) pure ZnO (B) Ce (5%) doped ZnO (C) Ce (10%) doped ZnO and (D) Ce (15%) doped ZnO. Reprinted with permission from [128], T. Marimuthu, et al., Structural, functional and optical studies on Ce doped ZnO nanoparticles. J. NanoSci. NanoTech. 2, 62 (2014). © 2014, IndiaScienceTech.
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compared with the pure ZnO nanomaterials, leading to extended electron–hole stability and increased photocatalytic efficiency. Mahmoud studied and observed the optical properties of Ce-doped ZnO nanoplatelets.136 These doped nanoplatelets exhibit a blue shift and weak ultraviolet emission peak as compared with pure ZnO. These nanoplatelets showed a great promise for applications to the optoelectronic devices. Sinha et al. synthesized Ce-doped ZnO NPs using wet chemical solution route and observed their length of 80–120 nm and 16–20 nm in diameter.137 There is a reduction in their band gap from 3.22 eV to 3.08 eV due to doping with Ce ions. The doping with Ce ions resulted in the broadening of the green emission photoluminescence band. An enhanced ionic behaviour resulted Fig. 15. UV-Visible Diffuse reflectance spectrum pure and Cein a increase in dielectric constant, piezoelectric coeffidoped ZnO nanoparticles. Reprinted with permission from [129], cient and ferroelectric properties. Thus, the nanoparticles C. Jayachandraiah, et al., Ce induced structural and optical properties showed the rectifying nature of the I –V characteristics. of Ce-doped Zno nanoparticles. Int. J. ChemTech. Res. 6, 3378 (2014). Wan et al. successfully synthesized Ce-doped ZnO nano© 2014, Sphinx Knowledge House. fibers using electro-spinning method.138 They investigated the acetone sensing properties of these doped nanofibers. band gap modifications. The doping of Ce leads to the The high response and quick response/recovery time can red shift in the absorption band. Thus, Ce-doped ZnO be attributed to the large surface area and the enhancenanorods had shown photocatalytic degradation activities ment of the adsorption ability of surfaces in Ce-doped ZnO under UV as well as light conditions. nanofibers. The prominent sensing properties are due to the Dar et al. have reported the synthesis of Ce-doped ZnO 1Dto: structure of ZnO nanofibers and the promoting effects by of Ingenta Wei Chen nanorods used as a chemical sensor for Delivered the detection IP: 129.107.80.80 2016The 16:57:39 CeSep doping. results demonstrate that Ce-doped ZnO nanorods On: wereWed,of14 hazardous chemicals.132 The synthesized Copyright: American Scientific Publishers nanofibers can be used as a promising material for selecused as an effective electron mediator for the fabrication of tive detection of acetone. an efficient hydroquinone chemical sensor which exhibits Yang et al. have prepared Ce-doped ZnO nanorods with a high and a reproducible sensitivity with detection limits the help of sol–gel method at low temperature growth of ∼10 nM. Faisal et al. also synthesized Ce-doped ZnO and high temperature growth.139 140 The results showed nanorods and reported their photocatalytic properties.133 the blue shift in the UV region and intensity of peaks Optical properties indicate that the Ce doping can shift is decreased. The PL spectra showed a strong UV emisthe absorption edge of ZnO to the visible range and sion band at 377 nm. The facile, reproducible, and effecreduce the band gap. Here, Ce acts as an electron resertive route presented here provide a useful method for voir and inhibits the photogenerated electrons–hole pair the RE3+ -doped ZnO crystals. The high crystal quality of recombination and helps in the improvement of photothe Ce-doped nanorods and good optical properties under catalytic performance of Ce-doped samples. Karunakaran room temperature make it a suitable candidate for the optiet al. have studied the antimicrobial Ce-doped ZnO NPs cal applications in the future.140 Yousefi et al. have prefor photocatalytic detoxification of cyanide.134 With the pared Ce-doped ZnO nanocomposite thin films by using help of doping, they reduced the intragranular resistance the sol–gel method of optimum annealing temperature of and recombination of the photogenerated electron–hole 500 C.141 It was also observed that adding cerium to pairs. These NPs also showed antibacterial activities when ZnO nanocomposite thin films resulted in the enhancement tested for E. coli bacteria. The photo detoxification folof the photoresponsivity of the layers. Among different lows Langmuir-Hinshelwood kinetics. The kinetic paramCe/Zn ratios examined, the optimum doping concentraeters deduced from the data fit are compared with those of tion demonstrated the highest photocurrent density due to the bare oxides and it provides information for the shifting its action as an electron trap in the photoelectrochemiof optical absorption to the visible region. cal (PEC) reaction compared with other investigated ratios Kumar et al. synthesized Ce-doped ZnO NPs with under similar experimental conditions. the help of solution combustion method and observed their photocataytic activities.135 By finding an optimum 7.2. Dysprosium concentration of the Ce doping, they got the photocatAjimsha et al. prepared Dy-doped ZnO thin films by alytic degradation up to 99.5% in only 70 minutes. The using buffer assisted pulsed laser deposition and the increased photocatalytic performance in Ce-doped ZnO band gap and carrier concentration of Dy:ZnO thin films might be due to the widening of band gap energy of ZnO 16
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Fig. 16. SEM images of Dy-doped ZnO nanoparticles. Reprinted with permission from [142], A. Khataee, et al., Synthesis and characterization of dysprosium-doped ZnO nanoparticles for photocatalysis of a textile dye under visible light irradiation. Ind. Eng. Chem. Res. (2014). © 2014, ACS Publications.
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Wu et al. synthesized the Dy-doped ZnO nanowires using sol–gel template technique having a diameter of 70 nm.145 The PL of the Dy-doped ZnO nanowires show an NBE band at 384 nm and a weak peak at 575 nm, which can be attributed to 4 F9/2 → 6 H13/2 transition of the Dy3+ ion. Yayapao et al. have prepared Dy-doped ZnO nanostructures using sonochemical method.146 They also investigated optical and photocatalytic properties. The results defined that the synthesized 3% Dy-doped ZnO has an excellent optical property and higher photocatalytic activity than that of ZnO for degradation of methylene blue under UV radiation.
initially increased, then decreased with increase of Dy concentration.143 The blue shift of the band gap of 7.3. Holmium Dy:ZnO thin films with increasing carrier concentration Khataee et al. synthesized Ho-doped ZnO NPs using sonowas attributed to the competing effects of Burstein-Moss chemical technique.148 These NPs were applied as a sonoshift and band gap narrowing. A bright room temperature catalyst for sonocatalytic degradation of an azo dye in photoluminescence observed at 575 nm in the Dy:ZnO aqueous solution. The results showed that the degradation thin films with maximum intensity at ∼0.45% of Dy efficiency (DE) of the doped sonocatalysis was higher than doping attributed to the intra-band transitions of Dy3+ that with sonolysis alone and with undoped sonocatalyin ZnO. The high transparency and high conductivity sis. The DE % was 54.89%, 62.9%, and 88.3% in sonolof these films make them suitable for transparent conysis and sonocatalysis with undoped and Ho-doped ZnO ducting electrode applications, as well as light emitting nanoparticles. The degradation efficiency was affected by and luminescent device applications. Jayachandraiah et al. the dopant content, and 4% Ho-doped ZnO nanoparticles have prepared Dy-doped ZnO nanoparticles usingbycoDelivered Ingenta to: Wei Chen IP: 129.107.80.80 On:aWed, 14 Sep 2016 16:57:39 indicated precipitation method.144 UV-Vis measurements Copyright: American Scientific Publishers blue shift optical band gap with Dy doping. PL spectrum demonstrated immense enhanced green emission which attributes to the increase in defect concentration. Dy-doped ZnO nanoparticles showed that the intensity increased by three times in the green region at 558 nm and PL depends on the dopant concentration and excitation wavelength. The emissions intensity could be tuned by varying the Dy concentration and excitation wavelength for the application of better optoelectronic devices.
Fig. 17. Varying photocatalytic decolorization efficiency versus initial dye concentration of Dy-Doped ZnO. Reprinted with permission from [142], A. Khataee, et al., Synthesis and characterization of dysprosiumdoped ZnO nanoparticles for photocatalysis of a textile dye under visible light irradiation. Ind. Eng. Chem. Res. (2014). © 2014, ACS Publications. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
Fig. 18. Room temperature upconverted luminescence of 19ZnO– 80TeO2 –1Ho2 O3 glass upon excitation at 646 or 754 nm. Reprinted with permission from [147], J. C. Boyer, et al., Optical transitions and upconversion properties of Ho3+ doped ZnOÀTeO2 glass. J. Appl. Phys. 93, 9460 (2003). © 2003, AIP Publishing LLC.
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Fig. 19. Diffuse reflectance spectra of undoped and Ho-doped ZnO nanoparticles. Reprinted with permission from [150], Shubra Singh, et al., Synthesis and comparative study of Ho and Y doped ZnO nanoparticles. Mater. Lett. 65, 2930 (2011). © 2011, Elsevier BV.
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Fig. 20. Room temperature emission spectra under He–Cd 325 nm laser excitation of ZnO:Pr (0.9%) films prepared at different deposition temperatures and after annealing. Reprinted with permission from [151], M. Balestrieri, et al., Luminescent properties and energy transfer in Pr3+ doped and Pr3+ –Yb3+ Co-doped ZnO thin films. J. Phys. Chem. C 118, 13775 (2014). © 2014, ACS Publications.
with increasing the concentration of acetic acid. Pr-doped was chosen as the most effective catalyst for the remaining ZnO nanofibers show a high selectivity among acetic acid, experiments. Phuruangrat et al. prepared Ho-doped ZnO acetone and methanol. Wang et al. have studied Pr-doped nanostructures using the sonochemical method.149 The HoZnO NPs prepared by the sol–gel method having crysdoped ZnO exhibited a relatively higherDelivered photocatalytic by Ingenta to: Wei talline size Chen of 25 to 29 nm.154 Pr ions lead to a high breakIP:degradation 129.107.80.80 On: Wed,down 14 Sep 2016 16:57:39 activity than the pure ZnO in the of methyvoltage, which is appreciable for the fabrication of Copyright: American Publishers lene blue under UV light. It was found that 98.26% of Scientific voltage switching devices in the near future. MB were degraded after 300 min UV light irradiation in the presence of 3% Ho-doped ZnO. It presents that the 7.5. Samarium Ho-doped ZnO nanorods are suitable candidates for phoLin et al. synthesized Sm-doped ZnO nanostructures tocatalyst materials in future applications. using the hydrothermal method at a low temperature of Singh et al. synthesized Ho and Y-doped ZnO NPs using 90 C.155 Photoluminescence spectrum of Sm-doped ZnO wet chemical synthesis technique.150 Magnetic measurements confirmed that Ho-doped ZnO samples exhibit hysteresis at low temperature. Optical studies reveal that the near band edge position shifts towards the lower wavelength side for the as synthesized nanoparticles. It was found that though both ions have the same ionic radii, the morphological and electrical properties are completely different in addition to the expected difference in their magnetic properties. 7.4. Praseodymium Ilanchezhiyan et al. studied the doping effect of Pr on ZnO nanorods using the aqueous solution route.152 The PL measurements showed a significant reduction in the optical band gap of the material, resulting from the newly formed molecular orbital states (due to trapping level and the presence of a large number of defects in them). Wang et al. synthesized Pr-doped ZnO nanostructures by eletrospinning method and used them as a static gas sensor device.153 Pr-doped ZnO nanostructure exhibit improved acetic acid sensing properties at 380 C. The response time of the pure ZnO sensor increases at first and then decrease 18
Fig. 21. Low-temperature excitation spectra of the main emission line of Pr3+ in ZnO before and after annealing. Reprinted with permission from [151], M. Balestrieri, et al., Luminescent properties and energy transfer in Pr3+ -doped and Pr3+ –Yb3+ Co-doped ZnO thin films. J. Phys. Chem. C 118, 13775 (2014). © 2014, ACS Publications. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Fig. 22. UV-Visible Diffuse reflectance spectrum pure and Sm-doped ZnO nanoparticles. Reprinted with permission from [159], H. Y. He, et al., Sm-doping effect on optical and electrical properties of ZnO films. J. Nanostruct. Chem. 5, 169 (2015). © 2015, Springer.
Rare Earth-Doped Zinc Oxide Nanostructures: A Review
clusters may lead to the high paramagnetic signal at low temperature. Sin et al. have prepared and studied the photocatalytic properties of Sm-doped ZnO nanorods using the solvothermal method.157 The investigation of photocatalytic ability showed that Sm/ZNRs were differently affected by Sm doping content in the catalyst, Sm/ZNRs amount, initial substrate concentration and solution pH. The synthesized Sm-doped ZnO nanorods could be easily recycled without any significant loss of the photocatalytic activity, which was favorable for the potential practical applications. Tsuji et al. studied the photoluminescence properties of Sm-doped ZnO using the CVD technique.158 In the photoluminescence (PL) measurements of annealed ZnO:Sm, sharp emission lines from intra-4f transitions in Sm3+ ions were observed at room temperature under the excitation energy above the band gap energy of ZnO (indirect excitation). In the dependence of the PL intensity at 77 K on Sm concentration, the Sm3+ PL intensity was the largest at Sm concentration of 0.4%.
nanorods shows a slightly red-shifted decrease of UV emission and an enhancement of photoluminescence per7.6. Ytterbium formance of green-yellow visible emission of the SmJiang et al. prepared microstructure of Yb and Li-codoped ZnO nanorods. Room temperature ferromagnetism doped ZnO and observed them in the enhanced inferred is also observed from magnetization curves of both ZnO emission regions.161 According to the microstructure analand Sm-doped ZnO nanorods. The increase of the saturaDelivered by Ingenta to:these Wei Chen ysis, regions are not uniformly distributed on the tion magnetization induced by theIP:Sm doping in the On: ZnOWed,ZnO 129.107.80.80 14 Sep 2016 surface, and16:57:39 the density is very low. Otal et al. synCopyright: PublishersZnO using wet chemistry route and nanorods reveals an association with the increase American of oxy- Scientific thesized Yb-doped gen vacancies and oxygen interstitials. Piao et al. have also studied the ferromagnetism properties of Sm-doped ZnO nanorods using the hydrothermal method.156 They observed the Ferromagnetic coupling in all the samples and for high doping concentration, the segregation or formation of small clusters may be attributed to the observed ferromagnetic signal except Sm substitution induced ferromagnetic ordering. The precipitation or
Fig. 23. Photoluminescence spectra of the ZnO: Sm films. Reprinted with permission from [159], H. Y. He, et al., Sm-doping effect on optical and electrical properties of ZnO films. J. Nanostruct. Chem. 5, 169 (2015). © 2015, Springer. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
Fig. 24. CL spectra of ZnO nanoparticles doped with different Yb contents: (a) undoped, (b) 2% Yb-doped, and (c) 5% Yb-doped. Reprinted with permission from [160], A. Susarrey-Arce, et al., Cathodoluminescence quenching in Yb-doped ZnO nanostructures. J. Nano Res. 5, 177 (2009). © 2009.
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Here, we are providing information about the observed PL and optical studies. Liu et al. have synthesized terbium-doped zinc oxide nanocrystals and studied their structural and optical properties.165 Tb3+ ions show green luminescence due to 4f –4f absorption transitions. Liu et al. also studied Tbdoped ZnO NPs synthesized using wet chemical route and observed their PL properties.166 Relaxation of carriers from excited states of ZnO hosts to rare earth dopants is due to the emission intensity of Tb3+ centers increases with increases in the Tb content at the expense of the emission from surface defect states in the ZnO matrix. Jia et al. prepared Tb-doped ZnO nanocrystals and studied their photoluminescence properties.167 They observed that the strong Fig. 25. CL spectra of undoped, 2% Yb-doped, and 10% Yb-doped UV emission peak of Tb-doped ZnO photoluminescence ZnO. Reprinted with permission from [160], A. Susarrey-Arce, et al., Cathodoluminescence quenching in Yb-doped ZnO nanostructures. spectra exhibited a red shift compared with pure ZnO, J. Nano Res. 5, 177 (2009). © 2009. which was attributed to the defects and the formation of shallow energy level caused by the incorporation of Tb dopants. 162 studied their wavelet analysis. So, their studies sugWu et al. have synthesized Tb-doped ZnO nanocrysgest that Lanthanides doped ZnO is an interesting material talline films and observed their room temperature for optical and electrical applications. Zamiri et al. have ferromagnetism properties.168 They suggest that the ferroalso synthesized Yb-doped ZnO nanostructures through magnetism is caused by the incorporation of Tb into the wet precipitation method and studied their structural and ZnO lattice. The saturation magnetization of the films is optical properties.163 This morphological change could be about 0.38 mb/Tb. Electrical property investigation proves explained by a decrease in the ZnO crystal growth rate thatto:the carriers Delivered by 3+ Ingenta Wei Chenof the films are strongly localized, which along the vertical direction due IP: to incorporation of On: Er Wed,suggests that the16:57:39 ferromagnetism in the film may be caused 129.107.80.80 14 Sep 2016 and Yb3+ ions in its crystal structure. Copyright: The band gap value Scientific American Publishers by the defects in the films and ferromagnetism may be of pure ZnO has also changed due to defects produced related to the defects in the films. Ziani et al. observed with rare earth doping. the annealing effect of PL of Tb-doped ZnO films synthesized using radio frequency magnetron sputtering at low 7.7. Terbium temperature.171 The result suggests that the anneal treatThe Photoluminescence emission spectra of terbium ions ment favors an optimum distribution of the ions in the were observed and their behavior has to be explored such matrix and there is no relationship between the stress state that their PL properties should be clearly understood.164 of the film and the Tb emission.
Fig. 26. The Tb L3-edge XANES spectra of all the films as well as those of standard Tb oxides. Reprinted with permission from [169], W. Q. Zou, et al., Ferromagnetism in Tb doped ZnO nanocrystalline films. J. App. Phys. 111, 113704 (2012). © 2012, AIP Publishing LLC.
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Fig. 27. Absorption spectra of RE-ZnO core–shell nanocrystals, REZnO nanosheets, and undoped ZnO nanosheets. Reprinted with permission from [170], S. Ji, et al., Synthesis of rare earth ions-doped ZnO nanostructures with efficient host-guest energy transfer. J. Phys. Chem. C 113, 16439 (2009). © 2012, ACS Publications. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Fig. 28. PLE spectra monitoring: (a) 542 nm emission of (4% Tb)-ZnO, (b) 572 nm emission of (4% Dy)-ZnO, and (c) 552 nm emission of (4% Er)ZnO nanocrystals. Reprinted with permission from [170], S. Ji, et al., Synthesis of rare earth ions-doped ZnO nanostructures with efficient host-guest energy transfer. J. Phys. Chem. C 113, 16439 (2009). © 2009, ACS Publications.
Urbieta et al. synthesized Tb-doped ZnO nanostructures matches with possible resonances in a hexagonal cavity, using a catalyst free vapor-solid method.172 They found indicating that the wires act as optical resonators. that the luminescence of the structures can be controlled Hastir et al. synthesized Tb-doped ZnO NPs using through the growth conditions and the nanostructures can co-precipitation route and observed their gas sensing act not only as luminescence emitters but also as wavegproperties.173 Photoluminescence emissions indicated an increase in concentration of oxygen vacancies with uides, opening the possibility of their use in a wide variety of optical nanodevices. Luminescence spectra of nanobelts introduction of dopant. Tb-doped ZnO sensors have exceptionally high sensing response and temperature and wires show the Tb3+ emission lines. In addition, waveguiding behavior has been observed for both ZnO dependent selectivity towards ethanol and acetone. Sensors and Tb3+ luminescence bands in the microwires. On the were able to detect even low concentrations (∼10 ppm) other hand, resonances have been observed in the specof these vapours. The temperature dependent selectivity of tra recorded on tapered nanowire, and the Delivered peak separation terbium-doped ZnO depends on target gas which may be by Ingenta to: Wei Chen IP: 129.107.80.80 On: Wed, 14 Sep 2016 16:57:39 Copyright: American Scientific Publishers
Fig. 29. PL spectra of (a) (4% Tb)-ZnO core–shell nanocrystals under 278, 368, and 487 nm excitations, (b) (4% Dy)-ZnO core–shell nanocrystals under 280, 365, and 451 nm excitations, and (c) (4% Er)-ZnO core–shell nanocrystals under 287, 363, and 457 nm excitations. Reprinted with permission from [170], S. Ji, et al., Synthesis of rare earth ions-doped ZnO nanostructures with efficient host-guest energy transfer. J. Phys. Chem. C 113, 16439 (2009). © 2009, ACS Publications.
Fig. 30. PL (solid line) and PLE (dotted line) spectra of (a) (4% Tb)-ZnO, (b) (4% Dy)-ZnO, and (c) (4% Er)-ZnO nanosheets. Reprinted with permission from [170], S. Ji, et al., Synthesis of rare earth ions-doped ZnO nanostructures with efficient host-guest energy transfer. J. Phys. Chem. C 113, 16439 (2009). © 2009, ACS Publications. Rev. Nanosci. Nanotechnol., 5, 1–27, 2016
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Table VI. Schematic representation of applications of RE-doped ZnO nanostructures. Sr. no. 1.
2. 3. 4. 5. 6. 7. 8. 9. 10.
Different fields Electronics and semiconductor industry Sensors Rubber and Paint industry Pharmaceutical and cosmetic industry Textile industry Photocatalysis Nanomedicine Computing Spintronics Miscellaneous
Applications
References
Photodetectors, transistors, LEDs, laser diodes, flat panel displays, transparent electrodes, nano-generators, solar cells, optical waveguides, PZT transducers, surface acoustic wave devices and optoelectronic devices, etc. Gas sensing and detection of different compounds, UV detectors. Stabilization of latex, fire resistance, provides tensile strength, lubricating applications. Antiseptic healing creams, suntan lotions, source of micronutrient zinc.
[79 86 106 108 109 114]
Removes UV radiation. Photocatalytic degradation of various organic and inorganic dyes. Target and control release of drug, antibacterial and anticancer activities. Energy as well as memory storage devices, optical imaging. Solid state devices, electro-magnetic devices and logic based devices. Printing inks, fire resistant materials, artificial fertilizer, latent fingerprint analysis, cigarette filters, biosensors, etc.
[80 82 97] [57 58 96] [64 90 116] [58 91] [83 93 102 104 107 110] [99 105 115 116] [100 113 114] [97 98 109 114] [51–62, 84–90, 112, 116–118]
ascribed to interaction of target gas molecules and doped photocatalysis. They show photocatalytic degradation of metal oxide surface at optimum operating temperature. various anionic and cationic dyes.117–122 With this, they The enhanced sensing response has been attributed to an are used for regulated and controlled drug release over increase in oxygen vacancies, reduction in particle size, sites and can be helpful in anticancer and antibacterial large structural disorders, and high surface basicity. activities.172–175 As a micronutrient, zinc can also be used as a supplement for plants and animals. The RE doped From the above discussion and analysis, we have found ZnO Nanoparticles have several other applications in printthat the rare earth-doped ZnO nanostructures have shown ing inks, fire resistant materials, artificial fertilizer, finoptical, structural, and ferromagnetism properties that can Delivered by Ingenta to: Wei Chen cigarette filters and bio-sensors for the be used in many applications, IP: such as photocatalysis, 129.107.80.80 On: Wed,gerprint 14 Sepanalysis, 2016 16:57:39 detectionPublishers of various enzymes and other bio-molecules. The optoelectronic devices, fingerprint analysis and in phar- Scientific Copyright: American applications of RE-doped ZnO Nanostructures are illusmaceutical and health industries. With the help of doptrated in the below table. ing the ZnO nanostructures presented the optimum energy band gap which can be applied in forming the semiconducting materials and devices. Here, the authors tried to 9. CONCLUSION represent brief information about the synthesis and appliIn this review, authors mainly concentrate on the semiconcations of various rare earth-doped ZnO nanostructures. ducting properties of rare earth-doped ZnO nanostructures This clearly represents a systematic view for selecting rare for providing a better approach in the future applications. earth metals for doping having characteristic features and The rare earth-doped ZnO nanostructures have shown properties. enhanced optical, structural, and photoluminescence properties compared with undoped ZnO nanostructures and 8. APPLICATIONS OF RARE EARTH-DOPED ZnO bulk crystals. With the help of these properties, these ZnO NANOSTRUCTURES nanostructures can be used in different and sophisticated Due to their unique and versatile features and propapplications in various applied fields of technology. This erties, the rare earth-doped ZnO nanostructures can provides a solution by developing new systems and techbe used in various industrial applications.174 175 The niques in micro as well as the nano scale level. With a RE-doped ZnO nanostructures have major applicawide range of applications, the modifications of electronic tions in various fields compared with undoped ZnO band and band gap engineering can be done by adding nanostructures.175–177 They are used in the field of elecimpurity through atoms. Thus, changing the doping level tronics and semiconductor industry for developing sinof impurity atoms has shown greater influence on the band gle electron transistors, photodetectors, fabricated LEDs, gap energies of formed doped nanostructures. This signiflaser diodes, flat panel displays, transparent electrodes, icance leads rare earth-doped ZnO nanostructures toward nano-generators, solar cells, optical waveguides, PZT optoelectronic devices, spintronic device applications, fabtransducers, surface acoustic wave devices, optoelectronic rication of thin films, and sophisticated devices, such as devices,3–5 85–90 95–100 104–110 121–140 150–154 170–175 etc. They MEMS or NEMS. Thus, this paper reports about the variare used in the field of spintronics for making solid ous synthesizing methods of ZnO nanostructures and rare state devices for memory storage and electro-magnetic earth-doped ZnO nanostructures, their properties, and uses devices. They are used in sensing applications and for in the different fields of future applications. 22
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Rare Earth-Doped Zinc Oxide Nanostructures: A Review
References and Notes
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