Materials Chemistry and Physics 161 (2015) 219e227
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In-situ synthesis and attachment of colloidal ZnO nanoparticles inside porous carbon structures Adrian Mihail Motoc a, *, Ioan Albert Tudor a, Mirela Petriceanu a, Viorel Badilita a, Elena Palomo del Barrio b, Prasanta Jana c, Vanessa Fierro c, Alain Celzard c, Radu Robert Piticescu a a b c
National R&D Institute for Non-Ferrous and Rare Metals, 102 Biruintei blvd., Pantelimon, Ilfov 077145, Romania CNRS-Institute of Mechanical Engineering of Bordeaux I2M, Esplanade des Arts et Mettiers, Talance, France CNRS - Institut Jean Lamour I2J, Rond-point Marguerite de Lorraine, Nancy, France
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Functionalization of carbon foams with flower-like zinc oxide. Zinc carbonate hydroxide: attachment to carbon foam and transformation to ZnO. Evolution of zeta potential of complex zinc carbonate hydroxide hydrate particles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2014 Received in revised form 12 May 2015 Accepted 14 May 2015 Available online 19 May 2015
A simple hydrothermal impregnation process enabling functionalization of carbon foams by attachment of nanostructured ZnO with flower-like morphology to control the interface interactions with some sugar alcohols based phase-change materials for thermal energy storage applications was developed. The process involves decomposition of urea in hydrothermal solution, formation of Zinc Carbonate Hydroxide Hydrate (ZCHH) as major solid phase and zinc hydroxide as minor phase, attachment of zinc carbonate hydroxides and zinc hydroxides to the carbon substrate and their decomposition on the inner walls of carbon foam by thermal treatment at temperatures in the range 250e400 C producing ZnO nanoparticles. The zeta potential of complex ZCHH particles with and without dispersants adsorbed on particles' surfaces was studied. The results show that, during hydrothermal synthesis process, negative charges of hydroxylated zinc species are formed on the surface of tetrahedral ZHCC crystal habit that enable interactions with positive charges from the inner surface of the carbon foams. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Differential scanning calorimetry (DSC) Electron microscopy (SEM)
1. Introduction The application of new phase change materials (PCMs) for
* Corresponding author. E-mail addresses:
[email protected] (A.M. Motoc),
[email protected] (I.A. Tudor),
[email protected] (M. Petriceanu),
[email protected] (V. Badilita), elena. palomo@trefle.u-bordeaux.fr (E. Palomo del Barrio),
[email protected] (P. Jana),
[email protected] (V. Fierro),
[email protected] (A. Celzard),
[email protected] (R.R. Piticescu). http://dx.doi.org/10.1016/j.matchemphys.2015.05.039 0254-0584/© 2015 Elsevier B.V. All rights reserved.
thermal energy seasonal storage in the range of medium temperatures has been proposed recently for space heating during winter using the energy collected during summer by thermal solar collectors. Different organic and inorganic PCMs have been already studied for their high thermal energy density [1]. In particular, PCMs based on sugar alcohols and their molecular mixtures have been proposed due to some expected advantages, such as: low cost, environmental free and safe (neither toxic nor corrosive) products, adjustable melting point in the temperature range from 70 C to 180 C, energy density expected to be greater than 200 kWh/m3,
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long-term storage achievable with significant reduction of the thermal losses, and storage heat release at relatively high temperature with, consequently, reduction of power requirements [2]. As most PCMs, sugar alcohols impose a principal heat transfer problem on the storage design associated with low thermal conductivity (typically less than 1 W/m/K) and the formation of solid deposits on the heat transfer surfaces. This leads to reduced charging/discharging power, exit temperature and usable heat capacity, and may result in expensive storage heat exchangers. The utilization of nanomaterials in thermal energy storage technologies become more and more important as a radical solution for improving the performances of thermal systems and overcoming the aforementioned problems of heat transfer [3]. Enhanced thermal transfer properties are the result of very large specific surface area brought by the utilization of nanosystems. Such nanosystems are based on nanofluids obtained by nanoencapsulation of PCMs or nanocomposites combining PCMs with a non-active, highly conductive phase, mainly graphite foams [4]. In order to be used as matrix, carbon surfaces must be treated to avoid significant sugar alcohol undercooling reduction. Consequently, functionalizing carbon surfaces is required to reduce as much as possible the probability of heterogeneous nucleation. The attachment of nanoparticles has been proposed recently as one method for potential development of super-hydrophobic surfaces [5]. Nanostructured zinc oxide coating, developing multi-textured flower-like habit on the carbon surface, is a good candidate for drastic reduction of contact angle and surface energy [6,7]. Different methods have been proposed to develop functionalized carbon surfaces by incorporating a secondary phase within carbon frameworks. These methods may be classified in three major groups: impregnation, metal transfer reactions and composite foams, and have been revised in [8]. The impregnation methods are widely used, being easier to be carried out and less expensive for many metal oxides. Solution growth of zinc oxide on different substrates has been intensively studied mainly as one-dimensional (1D) nanostructures for developing new functional properties for application in electronics, optoelectronics, spintronics, electrochemical or electromechanical devices [9]. Hydrothermal method for growing ZnO single-crystals on modified well-aligned carbon nanotube arrays was developed. The pre-deposited ZnO grains on the CNTs served as the nucleation sites for the growth of ZnO nanowires [10]. However, only a few studies regarding the growth of ZnO on carbon materials were reported in the literature [11]. It is well established that hexagonal ZnO (wurtzite) single crystals grow along the c axis and therefore (0001) surfaces are highly polar due to Zn2þ- and O2-terminated surfaces, while (001) surfaces are nonpolar [12]. Therefore, it is possible to explore the growth of nonpolar side surfaces of the (101) crystalline faces in view of controlling the hydrophobicity of carbonaceous substrates. The aim of the research work presented here was to develop a simple hydrothermal impregnation process enabling functionalization of carbon foams by attachment of nanostructured ZnO with flower-like morphology, and thereby control their surface properties to avoid the crystallization of some sugar alcohols selected PCMs (D-Mannitol, Erythritol, Xylitol) during thermal cycles in a thermal energy storage system. The hydrothermal method was proposed for its advantages in producing homogeneous nucleation in one-step at moderate temperatures and pressures without further thermal treatment in a closed vessel. The process is therefore economically competitive and environment-friendly [13]. Usually, the growth of ZnO is carried out in alkaline KOH solutions to provide the supersaturation for dissolution e reprecipitation mechanism [14]. The nucleation involves the formation of Zn(OH)2 and ZnðOHÞ4 2 species that can be precipitated at low
temperature, and therefore is not suitable for the proposed application because it involves a heterogeneous nucleation that can block the carbon foams' pores. A homogeneous precipitation method was proposed based on the utilization of urea as mineralizing agent [15]. The main process steps are the decomposition of urea with the formation of CO2 and NH3$H2O, hydrolysis of Zn (II) ions in the presence of CO3 2 ions formed during decomposition of urea with the formation of zinc carbonate hydroxide hydrate Zn4CO3(OH)6$H2O precipitate and transformation of ZnO by calcination at 500 C. The orientation and growth mechanism of ZnO nanocrystals is discussed, showing a preferential growth along caxis. A lot of works have been done for the analytical characterization of ZnO nanoparticles. Recently ZnO was suggested as a reference material for the determination of the size and size distribution of nanoparticles [16]. The hydrodynamic diameter and zeta potential of commercially available nanoparticles Zincox™ were analyzed in detail in [17], with respect to the importance of ionic strength in controlling their dispersibility and dosage during bio-toxicological tests. It was also shown that isothermal adsorption of different dispersants such as commercial ammonium salts of poly-acrylic acid onto zinc oxide surface at controlled pH strongly influences the determination of zeta potential of ZnO nanoparticles with different impurities [18]. In order to be applied for macro-porous carbon foams functionalization, the hydrothermal process must fulfill the following requirements: (i) nucleation and growth of ZnO nanoparticles must take place in-situ inside carbon foams' macro-pores; (ii) chemical compatibility between the carbon foam skeleton and the kind of solution at hydrothermal parameters used in the synthesis (temperature, pressure, pH) has to be ensured; and (iii) a sufficient adhesion between the new created ZnO nanoparticles and carbon surfaces needs to be produced. Considering these constraints, the aim of this paper was to study the grain sizes and zeta potential of colloidal ZnO precursors formed during hydrothermal impregnation using urea as mineralizing agent, in order to produce ZnO coatings with flower-like morphology onto the inner porosity of carbon foams to be used as porous matrices for hosting phase-change materials. 2. Materials and methods 2.1. Synthesis of macro-porous carbon foams Macro-porous carbon foams presenting both high porosity (in the range 86e88%) and optimal thermal conductivity for the seasonal thermal storage (in the range 3e10 W/m/K) have been synthesized according to a method described elsewhere [19]. Briefly, a homogeneous mixture of water, sucrose, graphite powder and nickel nitrate was prepared and heated at 120 C for 48 h for spontaneous foaming by polymerization of sugar and simultaneous release of steam. The “green” foam was subsequently pyrolyzed at 900 C under nitrogen for getting a carbon-graphite composite foam. In such process, graphite significantly improved the thermal conductivity of the resultant material, whereas nickel nitrate served as a catalyst for foaming but also for partial graphitization of the sucrose-derived carbon, further increasing the thermal conductivity. The composition of the initial mixture was optimized for obtaining as high porosity and conductivity as possible, a difficult challenge as these two characteristics vary in opposite directions. 2.2. Attachment of ZnO nanoparticles onto the inner surface of carbon foams Selected carbon foam samples have been surface-treated by
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boiling them in 1 M nitric acid solution for 1 h to create additional surface functional groups and therefore improve the surface reactivity for attachment of ZnO nanoparticles [8]. Zinc nitrate tetrahydrate Zn(NO3)2$4H2O p.a. was weighed and dissolved in distilled water to obtain 0.1 M Zn(II) nitrate solution. Analytical grade urea powder NH2CONH2 was dissolved in distilled water to obtain a 0.2 M urea solution. The two solutions were then mixed by stirring. The nitric acid-treated carbon foam samples have been immersed in a vertical autoclave containing the mixed zinc nitrate solution and urea with stoichiometric ratio between urea and Zn (II). After closing the autoclave, the immersed samples have been hydrothermally treated for 2 h in TEFLON vessels at a maximum temperature of 250 C and corresponding autogenic pressure (the gas pressure over the solution). After completing the experimental program, the samples were extracted from the autoclave and rinsed gently with distilled water. The samples were cut into more parts for chemical, structural and thermal characterizations. The white zinc carbonate hydroxide hydrate powder precursor collected on the bottom of the autoclave was also separated by filtering, washed 3 times with distilled water and characterized by chemical analysis, XRD and DSC-TG. Thermal treatment of impregnated carbon foams and collected powders was carried out in a digital chamber furnace for 2 h at 500 C.
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system. Thermal characterization of carbon foams prior and after ZnO attachment and of the precipitated powders was done by differential scanning calorimetry and thermogravimetry (DCS-TG) with the help of a SETARAM Setsys equipment working from room temperature up to 1600 C. 2.4. Size distribution and zeta potential of precursor powders Particle sizes and zeta surface potential were analyzed by dynamic light scattering with a Zetasizer ZS90 equipment (Malvern Instruments Ltd), endowed with automatic titration system. The particles have been ultrasonicated for 3 h in 103 M NaCl solution, either pure or with the addition of sodium salt of polyacrylic acid (PAAS, molecular weight 25,000) and polyethyleneimide (PEI, molecular weight 1200) as dispersants. The total volume of the sample was kept constant at 100 ml. The weight ratio of PAAS and PEI was kept constant at 103 mg/g of dispersed powder. The influence of the pH was studied by auto-titration with HNO3 and NaOH using three bottles with three different concentrations (0.2 M, 0.3 M and 0.5 M). 3. Results and discussions
2.3. Chemical and structure characterization
3.1. Mechanism of ZnO attachment to the carbon surface
Pristine parallelepiped carbon foam samples whose dimensions were accurately measured with an electronic caliper were first weighed, and the corresponding bulk density, rb, of the blocks was simply calculated as the mass to volume ratio. A few samples were also finely ground in an agate mortar, and the resultant powder was analyzed by a helium pycnometer for obtaining the skeletal density, rs. The overall porosity, F, was subsequently calculated as F ¼ 1 rb/rs. The microstructure has been analyzed by optical microscopy. The thermal conductivity was measured by the transient plane source technique using a ThermoConcept Hot Disk TPS 2500 apparatus. The method is based on a transiently heated plane sensor, used both as a heat source and as a dynamic temperature sensor [20]. It consists of an electrically conducting pattern in the shape of a double spiral, which has been etched out of a thin nickel foil and sandwiched between two thin sheets of Kapton®. The plane sensor was fitted between two identical parallelepiped pieces of porous monolith, each one with a smooth, flat, surface facing the sensor. At the end, the thermal conductivity was calculated with the Hot Disk 6.1 software. Zn content in the precipitated powder and in carbon foams was quantitatively analyzed by direct coupled plasma spectrometry (DCP-Spectroflame) according to ASTM E 1097/1993, and the ignition losses in the precipitated powders were determined according to SR ISO 60606-2000 standard. XRD structural characterization was performed on BRUKER D8 ADVANCE diffractometer using the Bragg-Brentano diffraction method, QeQ coupled in vertical configuration, with the following parameters: CuKa radiation, 2Q region 4e74 , 2Q step 0.02 , time/ step 3 sec. Data acquisition and processing were performed using the software DIFFRACplus BASIC Evaluation Package, version EVA12 Release 2006 (Bruker AXS) and ICDD PDF-2 Release 2006 database. The samples subjected to X-ray diffraction were micronized in a mortar with an agate pestle. Optical microscopy was performed using a Zeiss AXIOIMAGER A1m microscope. The impregnated foams have been sliced at parallel faces prior to visual inspection in polarized light. The same samples were used to analyze the morphology of the attached ZnO nanoparticles on carbon surfaces by SEM using a HITACHI S2600N
According to the thermodynamic predictions performed using the HSC Chemistry v.7.1 software and database (Outotec Research Centre, Finland), and according to the Pourbaix diagram for the system ZneCeNeH2O (Fig. 1) with initial Zn(NO3)2$4H2O concentration of 0.01 M at ionic strength I ¼ 0.03 at 250 C and corresponding autogenic pressure of 39.23 bar, a pH higher than 8.0 is needed to ensure complete precipitation of Zn ions with the expected formation in the stability domain of water of hydrozincite Zn5(OH)6(CO3)2$2H2O as major phase and hydrated zinc oxide, denoted as ZnO(a). The equilibrium composition calculated with the same software is presented in Fig. 2, showing that hydrozincite decomposes with the formation of ZnO as solid phase, the decomposition being practically total at around 400 C. It is therefore expected during hydrothermal synthesis of ZnO using urea as mineralizing agent to have in the first stage the formation of hydroxyzincite as solid precursor that is transformed in ZnO by thermal treatment. These processes should take place insitu onto the inner surface of carbon foams. The XRD analysis of the powder collected on the bottom of the autoclave during hydrothermal (HT) synthesis and attachment of ZnO nanoparticles is presented in the bottom line of Fig. 3. It mostly confirms the thermodynamic predictions, showing the presence of Zinc Carbonate Hydroxide Hydrate Zn4CO3(OH)6$H2O as major phase, Zinc Carbonate Hydroxide Zn5(OH)6(CO3)2 and Zinc Hydroxide - Zn(OH)2 as minor crystalline phases. The XRD spectra of powders after thermal treatment (TT) during DSC measurements are presented in the top line of Fig. 3 and shows the transformation to ZnO zincite as the unique crystalline phase formed. Details of the phase composition are presented in Table 1. The DSC-TG spectrum in Fig. 4 presents a sharp endothermal peak at 267.8 C due to decomposition of hydroxy-carbonates with the formation of ZnO with an enthalpy of ~88 J/g (7.16 kJ/mol ZnO) and a total mass loss of ~7.3%. The nucleation of zinc hydroxy carbonates on the surface of the carbon foams is demonstrated in the XRD spectrum of Fig. 5 for powders prepared by milling the hydrothermally impregnated foam. The XRD pattern shows the presence of the same crystalline phases as those observed in the powder collected from the bottom
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Fig. 1. Calculated Pourbaix diagram of the system ZneCeNeH2O.
Fig. 2. Calculated equilibrium composition during thermal treatment of hydroxizincite.
of the autoclave vessel together with the phases detected from the initial carbon foam (see again Table 1). The carbon foam samples used in the present study had a thermal conductivity of 0.31 W/m/K, a bulk density of 0.32 g/cm3, total porosity of 86%, and a mean pore size around 803 mm. Optical micrographs of Fig. 6 show the presence of ZnO coating on impregnated carbon foams after thermal treatment at 500 C. SEM micrographs of Fig. 7 show the formation of ZnO nanopowders with flower-like structure after thermal treatment of impregnated carbon foams for 2 h at 500 C.
XRD, thermal analysis and microscopy results all confirmed that attachment of ZnO nanoparticles onto pore surfaces of carbon foams by hydrothermal impregnation took place by the following main steps: - Decomposition of urea in hydrothermal solution first occurs according to:
COðNH2 Þ2 þ 3H2 O ¼ CO2 ðgÞ þ 2OHðÞ þ 2NH4 ðþÞ
(1)
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Fig. 3. XRD phase analysis of hydrozincite powder obtained after hydrothermal synthesis with urea.
Table 1 XRD Phase analysis of precipitated ZnO precursor powders and hydrothermally impregnated carbon foams. Phase/legend in XRD pattern from Fig. 3
Formula
pdf reference
Content (weight %) Powder
Impregnated foam
Zinc Carbonate Hydroxide Hydrate/ZCHH Zinc Carbonate Hydroxide/ ZCH Zinc Hydroxide/ZH Amorphous Carbon/C Quartz/Q Sodium Calcium Silicate/SCS Calcium Aluminum Iron Oxide/CAIO
Zn4CO3(OH)6$H2O Zn5(OH)6(CO3)2
00-011-0287(Q) 00-054-0047(Q)
t.p., major t.p.
p. p.
Zn(OH)2
00-020-1435 (I)
C SiO2 Na15.6Ca3.84(Si12O36) Ca((Al1.817Fe0.183)O4)
00-041-1487 (I) 01-079-1910 01-075-1332 01-070-7252
1-p e e e e e
n.d. t.p, major p. ~1 Presumptive Presumptive
Notes: p e phase content from 1 to a few percents; t.p.-phase content at the level of tens of percents; Presumptive: phases with similar formula may exist as mixtures coming from the carbon foam synthesis process.
Fig. 4. DSC-TG curves of hydrozinicite powder obtained after hydrothermal synthesis with urea.
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Fig. 5. XRD phase analysis of hydrothermally impregnated carbon foams.
Fig. 6. a) General aspect of carbon foam before (left) and after hydrothermal impregnation (right); b) optical micrograph of impregnated carbon foam showing the presence of white ZnO particles attached inside macropores.
- Formation of Zinc Carbonate Hydroxide Hydrate as major solid phase and zinc hydroxide as minor phase as revealed by XRD spectra takes place according to the general reactions: ðþ2Þ
5Zn
ðÞ
þ2CO2 þ10OH
¼ Zn5 ðCO3
x1 Zn4 CO3 ðOHÞ6 $H2 O þ x2 Zn5 ðCO3
2 ðOHÞ6 þ2H2 O
2 ðOHÞ6 þx3 ZnðOHÞ2
(2)
Znðþ2Þ þ2OHðÞ ¼ ZnðOHÞ2
(3)
- Attachment of zinc carbonate hydroxides and zinc hydroxides to the carbon substrate:
= C foam substrate
(4)
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225
Fig. 7. SEM micrographs of impregnated carbon foams at magnifications: (a) 10,000 showing the coverage of the carbon foam surface; (b) 200,000 showing the flower-like growth of ZnO nanocrystallites.
Table 2 Mean hydrodynamic diameters (dh) and zeta potential (z) vs. dispersion conditions in water. Dispersion concentration, M
101 102 103
NaCl þ PAAS
NaCl
NaCl þ PEI
dh (mm)
z (mV)
dh (mm)
z (mV)
dh (mm)
z (mV)
3.774 5.967 2.518
36.6 ± 8.37 1.88 9.12
1.796 2.901 1.690
94.8 ± 12.6 119 ± 19.5 101 ± 22.5
1.700 4.109 0.0987
0.889 48 11.6
where x1, x2 and x3 are the wt.% of ZCHH, ZCH and ZH, respectively. - Decomposition of zinc carbonate hydroxide complex deposited on the walls of carbon foam to ZnO by thermal treatment at temperatures in the range 250e400 C produces zinc oxide nanoparticles:
x1 Zn4 CO3 ðOHÞ6 $H2 O þ x2 Zn5 ðCO3
2 ðOHÞ6 þx3 ZnðOHÞ2 ¼
hydrodynamic diameters and zeta potential for zinc carbonate hydroxide hydrate powders dispersed at three different concentrations of 0.1 M, 0.01 M and 0.001 M. The reports of the Malvern Zetasizer ZS90 (Fig. 8) show good qualitative results for mean zeta potential measurements, but the results of mean hydrodynamic diameters are affected by the polydispersity of samples, with the presence of some large sedimented particles. The polydispersity index (defined according to
ð4x1 þ5x2 þx3 ÞZnO þ ðx1 þ2x2 ÞCO2 ðgÞ þ ð4x1 þ3x2 þx3 ÞH2 O
Zinc Carbonate Hydroxide Hydrate nucleates spontaneously from solution to multi-nuclei aggregates. These serve as sites for the growth of 1D ZnO along [001] direction forming flower-like nanostructures composed of hexagonal pyramids as observed in SEM micrographs of Fig. 7. 3.2. Study of the grain sizes and zeta potential of colloidal zinc carbonate hydroxide precursors formed during hydrothermal impregnation of carbon foams Considering the suggested attachment mechanism, the grain sizes and zeta potential of zinc hydroxide carbonate particles formed during hydrothermal synthesis using urea as mineralizing agent inside pores play an important role in controlling the pores impregnation and adhesion of ZnO coatings with flower-like morphology onto the carbon surfaces. Table 2 shows the values of
(5)
the ISO standard documents 13321:1996 E and ISO 22412:2008) is higher than 0.7 for samples dispersed in NaCl and NaCl þ PEI respectively. For this reason, for the studied samples the errors for zeta potential measurements could not be taken into account. The mean particle sizes of zinc carbonate hydroxide hydrate is significantly lower than the mean sizes of the cells in carbon foams (around 800 mm), and their attachment inside the porosity is therefore quite feasible. All samples have bimodal size distribution except for the sample with concentration 103 M dispersed in the presence of PEI as surfactant, showing a trimodal distribution. It may be assumed that due to both low concentration of the dispersion and effect of PEI dispersant, the hydrodynamic diameters of each phase from the complex structure of the precipitate formed from the two carbonates (ZCHH, ZCH) and hydroxide (ZH) according to Eq. (4) could be revealed. The evolution of zeta potential of zinc hydroxide carbonate particles with solution pH dispersed at three different
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Fig. 8. Histogram showing the size distribution of 0.001 M Zinc carbonate hydroxide hydrate complex dispersed in NaCl solution.
concentrations is presented in Fig. 9aec. Particles at 0.1 M concentration dispersed in NaCl solutions have a positive zeta potential in the pH range 1e10, where a first isoelectric point (IEP) is observed. Decreasing the dispersion concentration to 102 M and 103 M, the IEP was shifted toward acidic pH values (3.8 and 3, respectively). Addition of PAAS shifted the IEP to acidic pH value (2.96 for 0.1 M, 2.61 for 0.01 M and 3.08 for 0.001 M concentration of the dispersed particles). Particles dispersions with 0.1 M and 0.01 M concentration in the presence of PEI showed positive zeta potential over the whole pH range, whereas the IEP was observed at pH ¼ 9.8 for particles dispersed at 0.001 M concentration. Considering the HelmholzeSmoluchowski equation [21]:
yc ¼
zεH 4ph
(6)
where nc is the electrophoretic mobility, z is the zeta potential, ε is the dielectric constant of the solvent and h is the viscosity of the dispersion media, it may be concluded that dispersions with high z have higher mobility and can be easily attached inside the carbon pores structures. At the IEP there are no repulsive forces between dispersed particles, which tend to agglomerate and form aggregates with large particle sizes. When the pH is changed from that of the IEP, the hydrodynamic radii of dispersed particles decrease and particle surface charge increases. In the alkaline pH produced by the decomposition of urea in hydrothermal solutions, the decrease of zinc carbonate hydroxide hydrate particles concentration in suspension lead to higher negative zeta potential and higher surface charges. The addition of PAAS increases the negative values of zeta potential and the addition of PEI induces positive values of zeta potential. These results suggest that zinc carbonate hydrate hydroxide particles formed during hydrothermal in-situ reactions between Zn(II) precursor and urea have negative surface charges indicating the presence of hydroxylated zinc species such as ZnðOHÞ4 2 on the surfaces of tetrahedral ZHCC crystal habit [15,17]. Electrostatic interactions between surface charges of ZCHH particles and dispersants may be assumed according to Fig. 10. The attachment of ZnO particles on the inner surfaces of carbon porous structures may be therefore explained considering the interactions between the partial negative charges of ZCHH crystals and the positive charges of the internal pores surfaces of the carbon structure provided by additives, impurities and surface treatment prior to hydrothermal treatment. Investigations on detailed
Fig. 9. Evolution of zeta potential of zinc carbonate hydroxide hydrate dispersed in NaCl solutions and the effect of PAAS and PEI dispersants for a) 0.1 M; b) 0.01 M and c) 0.001 M particles concentration.
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Fig. 10. Electrostatic interactions between ZCHH surface charges and PAAS/PEI dispersants.
attachment mechanisms by spectroscopy and high resolution microscopy studies and on the interactions with sugar alcohols are presently in progress. 4. Conclusions ZnO nanoparticles with flower-like morphology were deposited on the inner porosity of specially designed carbon foams by an insitu hydrothermal process using urea as mineralizing agent. The process parameters (temperature, pH, Zn concentration, urea: Zn(II) ratio) were selected to trigger homogeneous nucleation and growth of ZnO inside carbon foams pores without affecting the open porosity and producing a continuous adherent film. The process involves the formation of Zinc Carbonate Hydroxide Hydrate as major solid phase and zinc hydroxide as minor phase, attachment of zinc carbonate hydroxides and zinc hydroxides to the carbon substrate and their decomposition by thermal treatment at temperatures in the range 250e400 C producing ZnO nanoparticles. The mechanism confirms thermodynamic predictions based on Pourbaix diagrams and equilibrium composition calculations. The zeta potential of complex ZCHH particles with PAAS and PEI dispersants adsorbed on particles surfaces show that during hydrothermal synthesis negative charges of hydroxylated zinc species are formed on the surfaces of tetrahedral ZHCC crystal habit that enable interactions with positive charges from the inner surface of the carbon foams. Acknowledgment We gratefully mention the financial support from European Commission in the frame of FP7-Energy -2011 grant no. 296006 “Sugar Alcohol Based Materials for Seasonal Energy Storage Applications”. The Romanian participants also acknowledge the cofinancing from Romanian Executive Unit for Financing Scientific Research,(175EU) Development and Innovation - UEFISCDI.
We also thank Dr. Eugeniu Vasile from 3 MN Centre of the University POLITEHNICA Bucharest for SEM characterization. References [1] H. Mehling, L.F. Cabeza, Heat and Cold Storage with PCM, Springer-Verlag Berlin Heidelberg, 2008, ISBN 978 3 540 68556 2. [2] E. Palomo del Barrio, Newsletter of the International Energy Agency Solar Heating and Cooling Programme - SolarUpdate, vol. 56, June 2012. [3] H.H. Al-Kayiem, S.C. Lin, A. Lukmon, Nanosci. Nanotechnol.-Asia 3 (1) (2013) 1e11. [4] X. Py, S. Olives, S. Mauran, Int. J. Heat Mass Transfer 44 (2001) 2727e2737. re , Nat. Mater. 2 (7) (2003) 457e460. [5] A. Lafuma, D. Que [6] E. Celia, T. Darmanin, E.T. de Givenchy, S. Amigoni, F. Guittard, J. Colloids Interface Sci. 402 (2013) 1e18. [7] M. Nosonovsky, B. Bhushan, Curr. Opin. Colloid Interface Sci. 14 (4) (2009) 270e280. [8] A. Stein, Z. Wang, M.A. Fierke, Adv. Mater. 21 (2009) 265e293. [9] S. Xu, Z.L. Wang, Nano Res. ISSN: 1998-0124 (2011), http://dx.doi.org/ 10.1007/s12274-011-0160-7. [10] W.D. Zhang, Nanotechnology 17 (2006) 1036e1040. [11] T. Hamada, E. Fujii, D. Chu, K. Kato, Y. Masuda, J. Cryst. Growth 314 (2011) 180e184. [12] R.A. Laudise, A.A. Ballman, J. Phys. Chem. 64 (1960) 688e691. [13] K. Byrappa, M. Yoshimura, Handbook of Hydrothermal Technology, Chapter 1, Elsevier, 2013, ISBN 978-0-12-375090-7, pp. 1e49. [14] L.N. Demianets, D.V. Kostomarov, I.P. Kuzmina, S.V. Pushenko, Crystallogr. Rep. 47 (2002) S86eS89. [15] Y. Liu, J. Zhan, A. Larbot, M. Persin, J. Mater. Process. Technol. 189 (1e3) (2007) 379e383. [16] O.E.C.D. Environment, Series on the Safety of Manufactured Nanomaterials, No. 41, ENV/JM/MONO, Health and Safety Publications, 2014, p. 15. [17] A. Bragaru, M. Kusko, E. Vasile, M. Simion, M. Danila, T. Ignat, I. Mihalache, R. Pascu, F. Craciunoiu, Nanopart. Res. 15 (2013) 1352e1367, http:// dx.doi.org/10.1007/s11051-012-1352-0. [18] A. Degen, M. Kosec, J. Am. Ceram. Soc. 86 (12) (2003) 2001e2010. [19] A. Celzard, P. Jana, V. Fierro, French Patent FR2013/059063; European Patent es poreuses pour le stockage d'e nergie PCT/EP2014/069800. Matrices carbone thermique. [20] S.E. Gustafsson, Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials, Rev. Sci. Instrum. 62 (3) (1991) 797e804. [21] M. Kosmulkowski, J. Colloid Interface 337 (2009) 439e448, http://dx.doi.org/ 10.1002/sml.200700595.
