Article pubs.acs.org/crystal
Epitaxial-like Growth of Co3O4/ZnO Quasi-1D Nanocomposites Daniela Bekermann,† Alberto Gasparotto,*,† Davide Barreca,§ Chiara Maccato,† Marco Rossi,∥ Roberto Matassa,∥ Ilaria Cianchetta,# Silvia Orlanducci,# Marko Kete,‡ and Urška Lavrenčič Štangar‡ †
Department of Chemistry, Padova University and INSTM, Via Marzolo 1, 35131 Padova, Italy CNR-ISTM and INSTM, Department of Chemistry, Padova University, Via Marzolo 1, 35131 Padova, Italy ∥ Department of Basic and Applied Sciences for Engineering (FASE), Research Center for Nanotechnologies Applied to the Engineering, University of Rome ‘La Sapienza’, Via Scarpa 14, 00161 Rome, Italy # Department of Chemical Science and Technology and MINAS Lab, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica, 00133 Rome, Italy ‡ Laboratory for Environmental Research, University of Nova Gorica, Vipavska 13, 5001 Nova Gorica, Slovenia §
S Supporting Information *
ABSTRACT: The development of quasi-1D Co3O4/ZnO nanocomposites by a two-step plasma enhanced-chemical vapor deposition (PE-CVD) process is presented. Arrays of ⟨001⟩ oriented ZnO nanorods were first grown on Si(100) and subsequently used as templates for the PE-CVD of Co3O4, whose amount was tailored as a function of deposition time. The obtained composites were thoroughly characterized by means of a multitechnique approach, involving field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), micro-Raman and Fourier-transform infrared (FT-IR) spectroscopies, X-ray photoelectron and X-ray excited Auger electron spectroscopies (XPS, XE-AES), glancing incidence X-ray diffraction (GIXRD), and reflection high energy electron diffraction (RHEED). The use of moderate deposition temperatures (≤300 °C), together with the unique activation provided by nonequilibrium plasmas, prevented undesired solidstate reactions between the two oxides and promoted Co3O4 growth on the tips of vertically aligned ZnO nanostructures. In particular, the resulting quasi-1D Co3O4/ZnO composites were characterized by an interface epitaxial-like relationship, an important issue for the development of semiconductor-based functional nanosystems. Photoinduced hydrophilic (PH) and photocatalytic (PC) performances of the present nanocomposites were preliminarily investigated, showing attractive results toward the possible fabrication of advanced smart materials.
1. INTRODUCTION
functional properties. In particular, structural control of the interface between the composite constituents is one of the critical factors affecting the ultimate behavior of the resulting heterostructures. For many applications, an epitaxial relationship is highly demanded and extended/point defects, impurities incorporation, or compositional gradients need to be suppressed in order to obtain an efficient communication (e.g., charge separation and transportation) between materials at the heterointerface.4,5,11−14,20,21 Although interfacial quality is directly dependent on the concurrence of various parameters, such as strain, surface energy, and solubility,3,22 1D axial heterostructures are intrinsically more favorable than other architectures to obtain a sharp, well-defined interface. In fact, thanks to their low-sized diameters, axial heterostructures can
The importance of one-dimensional (1D) inorganic nanostructures is widely recognized and documented, due to their key role in the understanding of nanoscale effects, as well as in the development of novel functional materials and devices.1−9 In recent years, the focus of the scientific community has been increasingly shifted toward 1D nanoheterostructures, joining the inherent properties of anisotropic systems (such as wirelike connectivity, high aspect ratio, and quantum size effects arising from radial confinement), with unique phenomena originating from the coupling of two (or more) different materials.1−4,10−18 To date, various 1D nanocomposites have been reported, including core/shell, axial, and branched systems as well as hierarchical structures, and their attractive properties have been demonstrated, especially in the fields of photonics and electronics.1−8,11−14,17,19 Nevertheless, the elucidation of material nucleation and growth is still challenging, not only for fundamental studies but also for the precise design of advanced © 2012 American Chemical Society
Received: July 30, 2012 Revised: September 7, 2012 Published: September 10, 2012 5118
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
Crystal Growth & Design
Article
Figure 1. Cross-sectional (a−d), plane-view (e−h), FE-SEM micrographs and EDXS line-scans (i−l) of Co3O4/ZnO specimens obtained with different cobalt oxide deposition times. The abscissa values of EDXS line-scans increase from the deposit/substrate interface to the sample surface along the line marked in cross-sectional FE-SEM images.
easily accommodate strain through lateral relaxation, allowing, thus, the occurrence of heteroepitaxy, even in the case of high lattice mismatch.5,12,13,23 This advantage has been exploited to develop several 1D nanomaterials, most of which are based on II−VI and III−V semiconductors (SCs) and whose applications encompass diodes, photonic crystals, and single electron transistors.2−6,20−22 Methods used for the fabrication of such systems normally involve either a metal particle-seeded growth or the use of surfactants, that often result in impurity incorporation degrading device performances.3,13,17,22 As a consequence, the development of catalyst-free preparation routes toward 1D nanocomposites is highly required to overcome the above limitations.4,6,20,23
Among 1D architectures, oxide-based axial heterostructures have been scarcely investigated, although their high stability and wide range of chemical, structural, and electrical properties hold a significant promise for technological applications.1,7,24 In this context, combination of p-type with n-type SC oxides, such as p-Co3O4 (EG ≈ 2.1 eV) and n-ZnO (EG ≈ 3.4 eV), results in the build-up of an inner electric field at the p/n junction interface that can be advantageous for photocatalytic, optoelectronic, and gas sensing applications.10,19,21,25−27 Recently, we have fabricated nanocomposite gas sensors by dispersing p-type Co3O4 nanoparticles over alumina-supported n-type ZnO.10 In this work, we focus on the epitaxial-like growth of quasi1D Co3O4/ZnO in view of potential light-assisted functional 5119
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
Crystal Growth & Design
Article
PC measurements were based on the photocatalytic oxidation of terephthalic acid (TPA) to highly fluorescent hydroxyterephthalic acid (HTPA).30−32 Each specimen was coated by a transparent solid layer comprising TPA and subsequently irradiated under the same conditions adopted for PH tests. The reaction kinetics were studied by high performance liquid chromatography with fluorescence detection (HPLC-FLD, HP 1100 series chromatograph), monitoring HTPA concentration as a function of illumination time. PC tests were repeated twice on each specimen, cleaning the sample surface in an ultrasonic bath (CH3CH2OH/H2O) for 15 min, followed by a 3 h preirradiation between consecutive measurements.
applications. To control nanocomposite properties and interfacial quality, we exploit the unique gas-phase and surface chemistry of cold plasmas.28 Specifically, the adopted strategy consists in a two-step PE-CVD process based on the initial growth of oriented ZnO arrays on Si(100) substrates, followed by Co3O4 deposition. The morphology, chemical composition, and structure of the obtained systems were investigated by means of an integrated multitechnique approach and systematically studied as a function of Co3O4 content, that was tailored by controlled variations of the corresponding deposition time. In particular, the potential of the RHEED technique is used to elucidate the system structural properties and the interfacial relationships between ZnO and Co3O4. The PH and PC performances of the obtained systems are also preliminarily investigated, in an attempt to provide an insight into the interrelations between material features and their functional behavior.
3. RESULTS AND DISCUSSION 3.1. Morphology and Composition. Based on our previous works,29,30,33 zinc oxide deposition on Si(100) was performed under optimized conditions, yielding quasi-1D ZnO arrays (column length ≈ 120 nm, diameter ≈ 20 nm) aligned perpendicularly to the substrate surface. Upon Co 3 O 4 deposition, FE-SEM cross-sectional images (Figure 1a−d) clearly evidenced the formation of a double-layer structure with an overall thickness progressively increasing from 145 (sample1) to 215 nm (sample4), in line with the higher Co3O4 loading for longer deposition times. Interestingly, Co3O4 grew, continuing the quasi-1D ZnO morphology, producing axial Co3O4/ZnO nanocomposites whose structural features are described in detail in section 3.2. The system quasi1D growth was promoted by the anisotropic zinc oxide wurtzite structure, driving the preferential nucleation of Co 3 O 4 crystallites on the (001) facets of ZnO nanorod tips.15,22 Further effects contributing to the formation of the observed heterostructures are related to plasma-assisted phenomena. In particular, the latter include the role of the sheath electric field perpendicular to the substrate surface that promotes a directional growth along the field lines, as well as the occurrence of anisotropic etching effects minimizing the competitive lateral growth.17,29 For the lowest Co3O4 deposition time (sample1), plane-view FE-SEM micrographs evidenced a morphology dominated by quasi-spherical topped nanoparticles (NPs) with an average diameter (ϕ) of 20 nm (Figure 1e). Upon progressively increasing the Co3O4 deposition time (i.e., loading, see Figure 1f−h), NP sizes underwent a parallel increase and, finally, faceted nanoparticles with a well-evident triangular shape (ϕ = 30 nm) were observed for sample4. Such a morphology corresponds to the preferential exposure of low-energy {111} surface planes, already reported for face-centered cubic systems such as Co3O4.34 Parts i−l of Figure 1 show the Zn Kα1 and Co Kα1 EDXS line-scan profiles for samples1−4 along the cross sections displayed in parts a−d. As can be observed, the Co signal had a maximum in the outermost sample region and underwent a progressive intensity reduction upon going toward the substrate, whereas the Zn line followed an opposite trend. Furthermore, the thickness of the cobalt-rich region increased as a function of Co3O4 deposition time. Taken together, the above FE-SEM and EDXS data supported the presence of the two separate oxide phases. In order to further elucidate this issue, the system composition was investigated by Raman spectroscopy, as well as FT-IR and XPS/XE-AES analyses (see also Supporting Information, SI). Representative Raman spectra are reported in Figure 2 for sample3 and sample4. The observed bands centered at 488 (Eg), 522 (F2 g), 620 (F2 g) (the latter detectable only for sample4), and 695 (A1 g) cm−1 are in agreement with literature
2. EXPERIMENTAL SECTION Growth processes were performed using a two-electrode custom-built PE-CVD apparatus, adopting Zn(ketoimi)2 [ketoimi = CH3O(CH2)3NC(CH3)CHC(CH3)O] and Co(dpm)2 [dpm = (CH3)3CC(O)CHC(O)C(CH3)3] as zinc and cobalt precursors, respectively.25,29 p-type Si(100) (MEMC, Merano, Italy) substrates were cleaned before each deposition following a previously reported literature procedure.29 Electronic grade Ar and O2 were used as plasma sources. The precursors were placed in an external vessel heated by an oil bath and transported toward the deposition zone by an Ar flow (rate = 60 sccm) through heated gas lines. Zn(ketoimi)2 and Co(dpm)2 vaporization temperatures were 150 and 100 °C, respectively. Two independent gas-lines were used to introduce Ar (rate = 15 sccm) and O2 (rate = 20 sccm) directly into the reactor. For all experiments, the interelectrode distance, total pressure, and RF-power were set at 6 cm, 1.0 mbar, and 20 W, respectively. For ZnO deposition, growth temperature and process duration were fixed at 300 °C and 60 min. Subsequently, Co3O4 was deposited over ZnO at a temperature of 200 °C, tailoring its amount as a function of the process duration (10−120 min). In the following, samples will be labeled according to the Co3O4 deposition time as follows: sample1 (10 min), sample2 (30 min), sample3 (60 min), and sample4 (120 min). FE-SEM and EDXS analyses were carried out by means of a Zeiss SUPRA 40VP instrument equipped with an Oxford INCA x-sight Xray detector, using a primary beam acceleration voltage of 20.0 kV. Raman analyses were performed using a micro-Raman apparatus composed by a 1800 gr/mm grating spectrometer (iHR550 - HORIBA JOBIN YVON) coupled with an Ar laser (excitation wavelength = 514 nm) and a liquid-nitrogen cooled CCD. Spectral resolution was lower than 1 cm−1. GIXRD patterns were recorded by means of a Bruker D8 Advance diffractometer equipped with a Göbel mirror and a Cu Kα source (40 kV, 40 mA), at a fixed incidence angle of 3.0°. RHEED measurements were performed on an electron optics column AEI EM6G equipped with a high resolution diffraction stage, operating at a fixed primary beam voltage of 60.0 kV. By rotating samples around the normal to the surface, diffraction patterns could be observed (if present) in different azimuthal directions. In addition, the capability of tilting specimens from glancing conditions up to a maximum value of 4.5° enabled obtainment of information on the composite structure at different sampling depths. PH properties were studied by monitoring the evolution of water contact angle (WCA) as a function of UV irradiation time. After storage in the dark, Co3O4/ZnO samples were irradiated in a photochamber equipped with UV lamps (CLEO 20 W, 438 × 26 mm2, Philips; broad maximum at 355 nm; photon irradiance: 5.7 × 10−9 einstein cm−2 s−1). WCA values were determined ex-situ with a contact angle meter (CAM-100, KSV Instrument, Ltd.). Three repetitions of WCA measurements were performed on each sample, thus enabling obtainment of average values and the pertaining uncertainties. 5120
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
Crystal Growth & Design
Article
technique, possessing a high surface sensitivity,50,51 has been used to investigate the Co3O4/ZnO interface. For all the analyzed specimens, the most intense peak of GIXRD patterns (Figure 3), located at 2θ = 34.4° (fwhm =
Figure 2. Raman spectra of Co3O4/ZnO specimens obtained with a cobalt oxide deposition time of 60 min (sample3) and 120 min (sample4). Figure 3. GIXRD patterns of samples1−4.
data for spinel-type Co3O4, with Co(II) and Co(III) centers characterized by a tetrahedral (A sites) and octahedral (B sites) coordination, respectively.35−38 It is worth noting that the band located at 522 cm−1 is partially due to the first-order Raman signal of the Si substrate, whose contribution to the overall peak intensity is less marked for sample4, corresponding to the highest Co3O4 deposition time. Based on the measured peak positions, doping of cobalt oxide with Zn(II) species can be ruled out, since no significant blue shift was observed, at variance with Zn:Co3O4 systems.35 In addition, the relatively narrow full width at half-maximum (fwhm) values of the bands (in the range 8−13 cm−1) suggested a high Co3O4 crystalline quality, with no appreciable stress effects.36 Complementary information on the system composition was gained by FT-IR analysis (Figure SI1 of the Supporting Information). For all specimens, the E1 transverse optical mode of ZnO was present at 410 cm−1.39,40 Furthermore, two other bands at v1 = 560 and v2 = 660 cm−1 were observed and attributed to Co3O4 stretching modes. Whereas the former was associated to B−O vibrations in the spinel lattice, the latter could be ascribed to ABO3 vibration modes.34,38,41−44 Notably, the intensity of bands v1 and v2 progressively increased as a function of Co3O4 deposition time, in line with the parallel increase of Co3O4 loading. As a matter of fact, FT-IR data confirmed the copresence of ZnO and Co3O4 and enabled ruling out the formation of Co−Zn−O ternary phases in the synthesized systems. XPS and XE-AES analyses (see Figure SI2 of the Supporting Information and related discussion) further supported the formation of biphasic composites under the adopted processing conditions. 3.2. Structure and Heteroepitaxy. The structure of Co3O4/ZnO samples was investigated by the complementary use of GIXRD and RHEED. In this regard, it is worth recalling that the structural information provided by GIXRD is averaged over a relatively large volume of interaction. On the other hand, electron diffraction-based techniques are a useful tool for the investigation of fine structural features,45 since they are able to reveal even the presence of minority phases, enabling a careful structural characterization of nanoparticles and nanoassemblies.46−49 In particular, in the present work, the RHEED
0.3°), was attributed to the (002) wurtzite reflection. This observation indicated the formation of crystalline ZnO with a strong ⟨001⟩ preferential orientation (PDF#00-036-01451), in line with the quasi-1D morphology evidenced by crosssectional FE-SEM micrographs (see Figure 1a−d). Despite no other peaks were detected for sample1, the patterns of samples2−4 revealed two additional diffraction signals at 2θ = 19.0° and 36.8°, corresponding to the (111) and (311) planes of Co3O4, respectively (PDF#01-071-0816). While the (002) wurtzite peak showed a similar intensity for all specimens (as expected based on the constant ZnO growth parameters), the intensity of the Co3O4 reflections underwent an enhancement upon increasing the corresponding deposition time. Interestingly, the intensity ratio between the (111) and (311) Co3O4 peaks was systematically higher than the expected reference value (PDF#01-071-0816), suggesting an anisotropic cobalt oxide growth along the ⟨111⟩ direction. It is worthwhile to notice that no additional reflections or peak shifts suggesting the presence of ternary phases were ever detected. In fact, the formation of Co−Zn−O ternary systems typically occurs under harsher conditions than the ones adopted in the present work.26,52,53 A deeper insight into the nanocomposite structural features was gained by RHEED analyses. In general, all the signals of the recorded electron diffraction (ED) patterns could be assigned to hexagonal ZnO and/or cubic Co3O4, according to the reference values reported in Table SI1 of the Supporting Information.54,55 Representative RHEED patterns are displayed in Figure 4. For sample3 (Figure 4a), both ZnO and Co3O4 reflections were observed. In fact, for this specimen, the thickness of the Co3O4 layer (≈ 60 nm) is sufficiently thin to enable the impinging electron penetration down to the ZnO matrix. Under these conditions, diffraction arcs from both the outermost Co3O4 crystallites and the underlying ZnO rods are, hence, superimposed in the ED pattern. The occurrence of diffraction arcs, instead of the Debye rings expected for an “ideal” polycrystalline material, resulted from a strong preferential orientation, in agreement with GIXRD 5121
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
Crystal Growth & Design
Article
its preferential growth direction (Figure 5b), showing a structural similarity to the (001) planes of the ZnO phase. This feature might justify the observed growth of the present composite materials, in agreement with the findings of Lee et al.21 3.3. PH and PC Performances. The obtainment of quasi1D Co3O4/ZnO arrays with reduced lateral size, well-defined interface, and morphological features tunable as a function of the synthesis conditions is extremely attractive from a technological point of view. In particular, heteroepitaxy of axial nanostructures between p- and n-type metal oxides is considered favorable for the exploitation of their light-activated performances in photocatalysis, as well as for antifogging and self-cleaning applications. In fact, one of the major drawbacks in this field is the fast recombination rate of photogenerated electron−hole pairs, a phenomenon that can be minimized by taking advantage of p/n junction effects to achieve an enhanced charge carrier separation. In particular, upon the formation of p/n junctions between p-type Co3O4 and n-type ZnO, an inner electric field will be formed at the interface. Under equilibrium conditions, the field renders p-type Co3O4 and n-type ZnO negatively and positively charged. Under irradiation, photogenerated electrons and holes are separated by the above field, flowing into the n- and p-semiconductors, respectively. As a consequence, their recombination is hindered, resulting in improved material performances.16,18,26 Based on these considerations and due to the scarcity of literature reports on the light-activated properties of p-Co3O4/ n-ZnO systems,19 the PH and PC properties of the present composites were preliminarily investigated. More specifically, PH tests were carried out by measuring the evolution of the water contact angle as a function of UV irradiation time (Figure 6a). In the absence of UV light, the WCA was close to 120°, indicating an initially hydrophobic state. This phenomenon can be traced back to the relatively rough composite surface,9,56 as well as to the presence of aliphatic carbon species in the outermost sample layers due to air exposure.30,57 Upon 5 h of illumination, the WCA underwent a progressive decrease to a value of 50°, that remained constant up to 30 h of irradiation. This effect, pointing to an enhanced surface wettability, originates from the formation of oxygen vacancies by the reaction of photogenerated holes with lattice O, ultimately resulting in the dissociative chemisorption of H2O.9,30,56 Based on the procedure previously proposed by some of us,31 the self-cleaning activity of Co3O4/ZnO was tested by
Figure 4. RHEED patterns of selected Co3O4/ZnO nanocomposites. (a) sample3: diffraction signals are located on the grey and white rings, corresponding to the calculated Debye rings for cubic Co3O4 and hexagonal ZnO phases, respectively. (b) sample4: experimental diffraction arcs are fully overlapped with the calculated ones for Co3O4.
results. Whereas ZnO was characterized by a preferential growth along the ⟨001⟩ direction, the cubic Co3O4 phase presented a ⟨111⟩ preferential orientation aligned to the ⟨001⟩ one of the ZnO matrix. For sample4 (Figure 4b), RHEED analysis revealed the sole presence of the Co3O4 phase, even upon increasing the electron beam incidence angle up to 4.5° to achieve a higher sampling depth. This finding can be reasonably ascribed to the increased thickness of the cobalt oxide layer (≈ 100 nm for a Co3O4 deposition time of 120 min). Moreover, the arcs in Figure 4b were characterized by a reduced angular broadening with respect to those of Figure 4a, indicating a higher degree of ⟨111⟩ preferential orientation for longer cobalt oxide deposition times. The three-dimensional crystal structures for the ZnO and Co3O4 phases viewed along the ⟨001⟩ and ⟨111⟩ directions are displayed in Figure 5. ⟨001⟩ zinc oxide nanorods grown perpendicularly to the Si substrate expose a surface with hexagonal symmetry (Figure 5a). Interestingly, even Co3O4 exhibits a hexagonal symmetry along the ⟨111⟩ axis, i.e. along
Figure 5. Sketch of the atomic structure of the Co3O4/ZnO interface viewed along the ⟨001⟩ZnO/⟨111⟩Co3O4 direction. Color code: O = red, Zn = gray, Co = blue. 5122
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
Crystal Growth & Design
■
REFERENCES
(1) Cheng, K.-W.; Lin, Y.-T.; Chen, C.-Y.; Hsiung, C.-P.; Gan, J.-Y.; Yeh, J.-W.; Hsieh, C.-H.; Chou, L.-J. Appl. Phys. Lett. 2006, 88, 043115. (2) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (3) Ouyang, L.; Maher, K. N.; Yu, C. L.; McCarty, J.; Park, H. J. Am. Chem. Soc. 2007, 129, 133. (4) Shapiro, J. N.; Lin, A.; Wong, P. S.; Scofield, A. C.; Tu, C.; Senanayake, P. N.; Mariani, G.; Liang, B. L.; Huffaker, D. L. Appl. Phys. Lett. 2010, 97, 243102. (5) Boulanger, J. P.; LaPierre, R. R. J. Cryst. Growth 2011, 332, 21. (6) Scofield, A. C.; Shapiro, J. N.; Lin, A.; Williams, A. D.; Wong, P.S.; Liang, B. L.; Huffaker, D. L. Nano Lett. 2011, 11, 2242. (7) Tak, Y.; Yong, K. J. Phys. Chem. C 2008, 112, 74. (8) Hayden, O.; Agarwal, R.; Lu, W. Nano Today 2008, 3, 12. (9) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (10) Bekermann, D.; Gasparotto, A.; Barreca, D.; Maccato, C.; Comini, E.; Sada, C.; Sberveglieri, G.; Devi, A.; Fischer, R. A. ACS Appl. Mater. Interfaces 2012, 4, 928. (11) Cheng, C.; Tay, Y. Y.; Hng, H. H.; Fan, H. J. J. Mater. Res. 2011, 26, 2254. (12) Dick, K. A.; Kodambaka, S.; Reuter, M. C.; Deppert, K.; Samuelson, L.; Seifert, W.; Wallenberg, L. R.; Ross, F. M. Nano Lett. 2007, 7, 1817. (13) Johansson, J.; Dick, K. A. CrystEngComm 2011, 13, 7175. (14) Lensch-Falk, J. L.; Hemesath, E. R.; Lauhon, L. J. Nano Lett. 2008, 8, 2669. (15) Sadtler, B.; Demchenko, D. O.; Zheng, H.; Hughes, S. M.; Merkle, M. G.; Dahmen, U.; Wang, L.-W.; Alivisatos, A. P. J. Am. Chem. Soc. 2009, 131, 5285. (16) Shifu, C.; Wei, Z.; Wei, L.; Huaye, Z.; Xiaoling, Y. Chem. Eng. J. 2009, 155, 466. (17) Gasparotto, A.; Barreca, D.; Maccato, C.; Tondello, E. Nanoscale 2012, 4, 2813. (18) Zhang, Z. Y.; Shao, C. L.; Li, X. H.; Wang, C. H.; Zhang, M. Y.; Liu, Y. C. ACS Appl. Mater. Interfaces 2010, 2, 2915. (19) Kanjwal, M. A.; Sheikh, F. A.; Barakat, N. A. M.; Chronakis, I. S.; Kim, H. Y. Appl. Surf. Sci. 2011, 257, 7975. (20) Hiruma, K.; Tomioka, K.; Mohan, P.; Yang, L.; Noborisaka, J.; Hua, B.; Hayashida, A.; Fujisawa, S.; Hara, S.; Motohisa, J.; Fukui, T. J. Nanotechnology 2012, 169284. (21) Lee, T. I.; Lee, S. H.; Kim, Y.-D.; Jang, W. S.; Oh, J. Y.; Baik, H. K.; Stampfl, C.; Soon, A.; Myoung, J. M. Nano Lett. 2012, 12, 68. (22) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Nano Lett. 2005, 5, 445. (23) Shin, J. C.; Kim, K. H.; Yu, K. J.; Hu, H.; Yin, L.; Ning, C.-Z.; Rogers, J. A.; Zuo, J.-M.; Li, X. Nano Lett. 2011, 11, 4831. (24) Lu, J. G.; Chang, P.; Fan, Z. Mater. Sci. Eng., R 2006, 52, 49. (25) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo, C.; Armelao, L.; Tondello, E. Chem. Mater. 2001, 13, 588. (26) Zhuge, L. J.; Wu, X. M.; Wu, Z. F.; Yang, X. M.; Chen, X. M.; Chen, Q. Mater. Chem. Phys. 2010, 120, 480. (27) Bekermann, D.; Barreca, D.; Gasparotto, A.; Maccato, C. CrystEngComm 2012, 14, 6347.
monitoring the photocatalytic oxidation of TPA to HTPA. To this aim, Figure 6b displays the HTPA concentration as a function of UV irradiation time for a representative specimen. The obtained data points could be fitted according to the model introduced in refs 30 and 31, assuming zero-order kinetics for HTPA formation (rate constant = k1) and pseudofirst-order kinetics for consequent HTPA degradation (rate constant = k2). Under these conditions, a value of k1 = 5 × 10−9 mol L−1 min−1 was obtained. Notably, this result compares favorably to that of the benchmark Pilkington Active glass material (k1 = 2.5 × 10−9 mol L−1 min−1 under the same experimental conditions). This behavior can be traced back to an efficient p-Co3O4/n-ZnO interfacial coupling, promoting a beneficial separation and an increased lifetime of photogenerated electrons and holes.16,18 As a whole, the present findings highlight the potential of Co3O4/ZnO nanocomposites for self-cleaning and antifogging applications.30,56,58
4. CONCLUSIONS The present work has focused on the development of a twostep PE-CVD approach to quasi-1D Co3O4/ZnO nanocomposites consisting in the initial growth of zinc oxide nanorods on Si(100) followed by cobalt oxide deposition. Thanks to the unique plasma activation, ⟨111⟩ oriented Co3O4 grew on the tips of the underlying quasi-1D ⟨001⟩ ZnO arrays under relatively mild processing conditions and without the need of any catalyst. An epitaxial-like relationship with a welldefined interface between the two single-phase oxides was obtained. Tailoring of cobalt oxide deposition time also resulted in a system morphology tunable as a function of the synthesis conditions. As demonstrated by preliminary PH and PC tests, the obtained nanoarchitectures displayed promising properties for the development of functional materials for light-activated applications. Further studies in this direction, with particular attention to the role of Co3O4 loading on the systems performances, are already underway. ASSOCIATED CONTENT
S Supporting Information *
FT-IR, XPS/XE-AES spectra and structural data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
ACKNOWLEDGMENTS
The research leading to these results has received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under Grant Agreement No. ENHANCE238409, as well as from the Padova University ex-60% 2012 (No. 60A03-5517) and PRAT 2010 (No. CPDA102579) projects. K. Xu, M. Banerjee, Prof. A. Devi, and Prof. R. A. Fischer (Ruhr-University Bochum, Germany) are gratefully acknowledged for assistance in Zn precursor synthesis, XPS and XE-AES analyses, and scientific support.
Figure 6. (a) Water contact angle and (b) HTPA concentration as a function of UV irradiation time obtained from sample1.
■
■
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 5123
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
Crystal Growth & Design
Article
(28) Gasparotto, A.; Barreca, D.; Bekermann, D.; Devi, A.; Fischer, R. A.; Maccato, C.; Tondello, E. J. Nanosci. Nanotechnol. 2011, 11, 8206. (29) Bekermann, D.; Gasparotto, A.; Barreca, D.; Bovo, L.; Devi, A.; Fischer, R. A.; Lebedev, O. I.; Maccato, C.; Tondello, E.; Van Tendeloo, G. Cryst. Growth Des. 2010, 10, 2011. (30) Bekermann, D.; Gasparotto, A.; Barreca, D.; Devi, A.; Fischer, R. A.; Kete, M.; Lavrenčič Štangar, U.; Lebedev, O. I.; Maccato, C.; Tondello, E.; Van Tendeloo, G. ChemPhysChem 2010, 11, 2337. (31) Č ernigoj, U.; Kete, M.; Lavrenčič Štangar, U. Catal. Today 2010, 151, 46. (32) Lavrenčič Štangar, U.; Kete, M.; Č ernigoj, U.; Ducman, V. Adv. Sci. Technol. 2010, 68, 126. (33) Barreca, D.; Bekermann, D.; Devi, A.; Fischer, R. A.; Gasparotto, A.; Maccato, C.; Tondello, E.; Rossi, M.; Orlanducci, S.; Terranova, M. L. Chem. Phys. Lett. 2010, 500, 287. (34) Barreca, D.; Gasparotto, A.; Lebedev, O. I.; Maccato, C.; Pozza, A.; Tondello, E.; Turner, S.; Van Tendeloo, G. CrystEngComm 2010, 12, 2185. (35) Rubio-Marcos, F.; Calvino-Casilda, V.; Bañ ares, M. A.; Fernandez, J. F. J. Catal. 2010, 275, 288. (36) Bhargava, R.; Sharma, P. K.; Kumar, S.; Pandey, A. V.; Kumar, N. J. Raman Spectrosc. 2011, 42, 1802. (37) Hadjievl, V. G.; Ilievl, M. N.; Vergilov, I. V. J. Phys. C: Solid State Phys. 1988, 21, L199. (38) Agawane, S. M.; Nagarkar, J. M. Catal. Sci. Technol. 2012, 2, 1324. (39) Morkoç, H.; Ö zgür, Ü . Zinc Oxide: Fundamentals, Materials and Device Technology; Wiley-VCH: Weinheim, 2009. (40) Zou, J. P.; Le, R. P.; Musa, I.; Yang, S. H.; Dan, Y.; That, C. T.; Nguyen, T. P. Thin Solid Films 2011, 519, 3997. (41) Li, W.-H. Mater. Lett. 2008, 62, 4149. (42) Jiu, J.; Ge, Y.; Li, X.; Nie, L. Mater. Lett. 2002, 54, 260. (43) Jiang, J.; Li, L. Mater. Lett. 2007, 61, 4894. (44) Lu, X. H.; Xia, Q. H.; Fang, S. Y.; Xie, B.; Qi, B.; Tang, Z. R. Catal. Lett. 2009, 131, 517. (45) Engler, A. O.; Randle, V. Introduction to Texture Analysis: Macrotexture, Microtexture, and Orientation Mapping, 2nd ed.; CRC Press: Boca Raton, FL, 2009. (46) Cowley, J. M. Micron 2004, 35, 345. (47) Serra, A.; Manno, D.; Filippo, E.; Tepore, A.; Terranova, M. L.; Orlanducci, S.; Rossi, M. Nano Lett. 2008, 8, 968. (48) Terranova, M. L.; Manno, D.; Rossi, M.; Serra, A.; Filippo, E.; Orlanducci, S.; Tamburri, E. Cryst. Growth Des. 2009, 9, 1245. (49) Toschi, F.; Orlanducci, S.; Guglielmotti, V.; Cianchetta, I.; Magni, C.; Terranova, M. L.; Pasquali, M.; Tamburri, E.; Matassa, R.; Rossi, M. Chem. Phys. Lett. 2012, 539−540, 94. (50) Rossi, M.; Vitali, G.; Karpuzov, D.; Kalitzova, M.; Budinov, H. J. Mater. Sci. 1991, 26, 3337. (51) Tang, F.; Parker, T.; Wang, G. C.; Lu, T. M. J. Phys. D: Appl. Phys. 2007, 40, R427. (52) Wei, L.; Li, Z. H.; Zhang, W. F. Appl. Surf. Sci. 2009, 255, 4992. (53) Sudakar, C.; Kharel, P.; Lawes, G.; Suryanarayanan, R.; Naik, R. Appl. Phys. Lett. 2008, 92, 062501. (54) Liu, X.; Prewitt, C. T. Phys. Chem. Minerals 1990, 17, 168. High-temperature X-ray dif f raction study of Co3O4: transition f rom normal to disordered spinel. Sample: RUN II, T = 1073 K. (55) McMurdie, H.; Morris, M.; Evans, E.; Paretzkin, B.; Wong-Ng, W.; Ettlinger, L.; Hubbard, C. Powder Diffr. 1986, 1, 76. (56) Barreca, D.; Gasparotto, A.; Maccato, C.; Tondello, E.; Lavrenčič Štangar, U.; Patil, S. R. Surf. Coat. Technol. 2009, 203, 2041. (57) Barreca, D.; Gasparotto, A.; Maccato, C.; Maragno, C.; Tondello, E. Surf. Sci. Spectra 2007, 14, 19. (58) Mills, A.; Lepre, A.; Elliott, N.; Bhopal, S.; Parkin, I. P.; O’Neill, S. A. J. Photochem. Photobiol., A 2003, 160, 213.
5124
dx.doi.org/10.1021/cg301083g | Cryst. Growth Des. 2012, 12, 5118−5124
