Size and Morphology Dependence of ZnO Nanoparticles Synthesized

ARTICLE pubs.acs.org/crystal

Size and Morphology Dependence of ZnO Nanoparticles Synthesized by a Fast Continuous Flow Hydrothermal Method Martin Søndergaard, Espen D. Bøjesen, Mogens Christensen,* and Bo B. Iversen Centre for Materials Crystallography, Department of Chemistry and iNANO, University of Aarhus, DK-8000 Aarhus C, Denmark

bS Supporting Information ABSTRACT: We have investigated the formation of ZnO nanoparticles using a fast continuous flow hydrothermal synthesis method. The synthesis conditions have been varied with respect to temperature, pH, and concentration of the Zn(NO3)2 3 4H2O + NaOH aqueous precursor. The different conditions affect the size, morphology, and crystallinity of the produced ZnO nanoparticles. The nanoparticles were investigated by Rietveld refinement of powder X-ray diffraction data, transmission electron microscopy, and scanning electron microscopy. The particle size and morphology are highly temperature dependent: Anisotropic particles of a maximum length of 1 μm are produced at a reactor temperature of 122 °C, while isotropic particles of around 25 nm are produced at temperatures above 200 °C. The crystallinity of the particles increases from 90(1)% at 122 200 °C to 99(1)% at 390 °C. Variation of the pH of the precursor results in different morphologies: (1) acidic conditions produce large rods, (2) neutral conditions give isotropic particles, and (3) alkaline conditions result in large plates. Finally, it was found that the particle size increases with the precursor concentration.

1. INTRODUCTION Zinc oxide (ZnO) is a biofriendly, cheap, wide band gap II VI semiconductor, and it has found its way into a range of applications in everyday products as well as high tech applications. The applications include piezoelectric sensors and mechanical actuators,1 3 solar cells,4,5 varistors,6,7 and thermoelectrics.8 Furthermore, light emitting diodes (LEDs)9,10 and lasers11 are potential applications if reliable p-type doping can be achieved. ZnO has one of the richest families of nanostructures,2,12 and a vast amount of synthesis techniques, such as hydrothermal,13,14 sol gel,15 RF magnetron sputtering,16 chemical vapor synthesis,17 mechanochemical,18,19 laser/hydrothermal hybrid,20 pulsed laser deposition,21,22 and sonochemical and microwaveassisted,23 have been employed to produce ZnO nanostructures. ZnO is most commonly found in the wurtzite crystal structure, which is non-centrosymmetric leading to pyro- and piezoelectric properties. Zinc oxide has polar crystal faces, and this is important for the growth habit of ZnO crystals. Considering the many possible technological applications of ZnO, the ability to understand and control the growth behavior is paramount for tailoring materials with specific properties. Previous studies on growth habits of ZnO under hydrothermal conditions have established that temperature, concentration, pH, and type of base are all influencing factors on the resulting particle size and morphology.2,24 29 Continuous flow hydrothermal synthesis of ZnO is simple, fast, flexible, scalable, and even green as it is free of organic solvents.30,31 Other research groups have used similar hydrothermal r 2011 American Chemical Society

flow synthesis setups, but they have mostly investigated the synthesis around and above the critical point of water (374 °C, 22 MPa).27,28,32,33 Compared to this, we investigate a more extensive synthesis temperature range (20 390 °C) and report the onset of formation of ZnO nanoparticles from c. 75 °C. Furthermore, the changes in crystallinity with temperature and the development of size and morphology of the particles are investigated. Finally, results of variations of precursor pH and concentration are discussed.

2. EXPERIMENTAL SECTION 2.1. Synthesis. All syntheses presented in this work were carried out in a specially designed supercritical fluid synthesis apparatus, Figure 1. In a typical synthesis, analytical grade Zn(NO3)2 3 4H2O was dissolved in deionized water under magnetic stirring and neutralized with a NaOH solution to form a gel. To avoid aging of the precursor, the gel was prepared immediately before commencing synthesis. The cold suspension was pumped into the apparatus using a high pressure pump with a flow rate of 10 mL/min. In the synthesis, the gel precursor was mixed with a preheated stream of deionized water at the mixing point, where the temperature was observed with a thermocouple, (Tmix). Before mixing, the water was pumped through a preheater (Tpre) and a main solvent heater (Tsol) at a flow of 10 mL/min. After the mixing point, the mixture flowed through the heated reactor tube (Trea) and was Received: May 11, 2011 Revised: July 2, 2011 Published: July 15, 2011 4027

dx.doi.org/10.1021/cg200596c | Cryst. Growth Des. 2011, 11, 4027–4033

Crystal Growth & Design

ARTICLE

Figure 1. Schematic of the hydrothermal flow synthesis setup. quenched immediately after leaving the reactor area using a water jacket. Finally the product exited the apparatus through a pressure relief valve. The pressure in the apparatus was kept at 260 ((10) bar in all syntheses. Different synthesis conditions were employed as described below. The collected products were in all cases centrifuged, washed with deionized water twice and once in ethanol, and dried in air. A flow rate of 10 mL/ min and a precursor concentration of 0.05 mol/L resulted in the synthesis of approximately 2.4 g/h of ZnO. Temperature Dependence. The precursor concentration was 0.05 mol/L of Zn(NO3)2 3 4H2O with pH adjusted to neutral using NaOH. The heat controller temperatures Tpre, Tsol, and Trea were varied over a wide range, so that Tmix was in the range from 20 to 290 °C, while the reactor temperatures were varied between 20 and 390 °C. Generally, Tmix fluctuated in a temperature interval of 10 20 °C around the reactor temperature (see Supporting Information). pH Modification. In the investigation of pH, the mixing temperature Tmix was held at 260 290 °C, while the reactor (Trea) was set to a temperature of 300 °C. The Zn concentration in the precursor was 0.05 mol/L. The synthesis was performed at acidic, neutral, and alkaline conditions at pH values of 6.2, 7.0, and 12.0. The pH of 6.2 corresponds to mixing equivalent volumes of 0.1 mol/L Zn(NO3)2 3 4H2O and 0.1 mol/L NaOH solutions. Precursor Concentration. The temperature setting of the pH study was adopted from the precursor concentration study. All precursors were adjusted with NaOH to pH values of 7. The different concentrations of Zn in the precursors were 0.5, 0.05, and 0.005 mol/L. Subsequent Neutralization. For comparison, syntheses were carried out with an aqueous precursor of 0.05 mol/L pure Zn(NO3)2 3 4H2O. In these syntheses, the reaction product was neutralized in a 0.1 mol/L NaOH solution at the exit of the apparatus. Different temperatures of Trea = 300, 350, and 400 °C were employed. 2.2. Structural Characterization. Powder X-ray diffraction (PXRD) patterns were collected on a STOE STADI P diffractometer equipped with an imaging plate position sensitive detector and using a Cu KR1 source in transmission geometry. In order to extract crystallographic information and the size and morphologies of the crystallites, the PXRD data were Rietveld refined in a similar fashion as described in detail by Lock et al34 using the Fullprof Suite software.35 The method is based on a general phenomenological model that uses an anisotropic size model function. The Scherrer formula36 is utilized with an assumption made that the size broadening can be written as a linear combination of

Figure 2. PXRD of products synthesized at different temperatures: Reactor (left) and mixing temperatures (right).

Figure 3. Sizes of crystallites in the crystallographic a- and c-directions as determined by Rietveld refinement of PXRD data. 4028

dx.doi.org/10.1021/cg200596c |Cryst. Growth Des. 2011, 11, 4027–4033

Crystal Growth & Design spherical harmonics, as proposed by Popa.37 In all refinements, potential profile broadening due to strain and defects was ignored. For the determination of crystallinity, silicon was used as an internal reference

ARTICLE

(see Supporting Information for details). Scanning electron microscopy (SEM) images were recorded using a Nova 600 Nano SEM from FEI and transmission electron microscopy (TEM) images were collected using a Phillips CM20 200 kV TEM. PXRD refinements provide the average crystallite domain size of the particles in the sampled volume,38,39 whereas SEM and TEM techniques allow direct observation of individual particles.

3. RESULTS AND DISCUSSION

Figure 4. PXRD pattern of ZnO (red) synthesized at 300 °C with hklindices and calculated pattern (black) and the difference between calculated and observed pattern (blue).

Figure 5. Crystallinity of ZnO products from different precursors determined with an internal Si-standard as reference. The hollow bars are the gel precursors of Zn(NO3)2 3 4H2O + NaOH.

Temperature Dependence. The powder diffraction patterns for the products prepared at different mixing and reactor temperatures are shown in Figure 2. At reactor temperatures between 20 and 65 °C, no ZnO is observed in the PXRD pattern and only the precursor phase is recorded. From approximately 75 °C, the PXRD data reveal the production of ZnO in the wurtzite structure together with a minor phase of the precursor, and at Tmix = 84 100 °C and Trea = 122 °C the sample is phase pure. The formation of ZnO at elevated temperature presumably is due to the lowering of the dielectric constant of water from εr ≈ 86 at 25 °C to εr ≈ 68 at 75 °C and 260 bar40,41 (slightly increased with increased pressure) and the increased reaction speed. The crystal domain sizes of the produced ZnO nanoparticles extracted from Rietveld refinement of the PXRD data are given along the a- and c-directions as a function of reactor temperature in Figure 3. In the Rietveld refinements, the structural reliability factors related to the intensity, RB, were generally below 5% (see Supporting Information). An example of a PXRD pattern and corresponding Rietveld refinement is presented in Figure 4. At low temperatures, the particle sizes are >50 nm. Such large particles cannot be reliably refined using the present PXRD data because of the substantial instrumental broadening of the peak profile.36,37,42 Particles sizes exceeding 50 nm are therefore not fully reliable. Nevertheless, trends can still be seen, and it is observed that larger particles are synthesized at lower temperatures. The sizes of the particles as well as the anisotropy, i.e., the ratios between the particle lengths in the a- and c-directions, are largest at the low temperatures of 122 155 °C. The morphology of the particles turns more isotropic at elevated temperatures. At 210 °C a minimum in particle size of 20 and 25 nm is observed, while at higher temperatures, 240 400 °C, the

Figure 6. Average morphology of particles synthesized at different reactor temperatures as determined by Rietveld refinement of PXRD data. 4029



ARTICLE

Figure 7. (a) SEM images and (b) TEM images of particles synthesized at different temperatures.

particles grow slightly larger with almost unchanged morphology. Even though the supersaturation increases at temperatures above 210 °C, the diffusion and growth rate also increase. The crystallinity of selected samples is shown in Figure 5 and is seen to increase with the temperature of the reactor from 91(1)% at 122 °C to 99(1)% at 390 °C. The effect of the reaction temperature on the average morphology as determined by spherical harmonic functions in Rietveld refinements is shown in Figure 6. A definite trend is seen: at low temperatures the particles appear to have an elongated diamond shape, whereas higher temperatures lead to a more isotropic crystallite domain shape. At around 210 °C and up to 390 °C, the morphology does not change significantly. The sample synthesized at 90 °C deviates from the general trend, but the refinement of these data is less reliable as the sample was not phase pure.

Representative SEM and TEM images at various temperatures are shown in Figure 7, panels a and b, respectively. These confirm the trends observed in sizes and shapes from the refinement of the PXRD data. The batches synthesized at temperatures of 90 to 140 °C show particles of several hundred nanometers and up to around 1 μm at 122 °C. It can be seen from the SEM and TEM images that the products do not have a homogeneous appearance, and especially the particles synthesized below 300 °C exhibit wide size distributions. The wide size distribution could be due to the presence of different zinc hydroxide ionic complexes,43,44 which form different types of ZnO particles. Clearly the growth habits depend on the temperature. The observed smaller particle sizes at higher temperatures can be explained as follows. As the reaction temperature increases the dielectric constant of water decreases and a higher degree of supersaturation is achieved; thus, more nucleation sites will be 4030



ARTICLE

Figure 8. SEM images of particles synthesized at acidic, neutral, and alkaline conditions.

available giving a fast depletion of the precursor. The fewer nucleation sites at lower temperatures and lower supersaturation favor growth by diffusion on steps and kinks and possibly by Ostwald ripening, where smaller particles dissolve and grow onto larger particles.45,46 The drop in size observed when going from 300 to 340 °C might originate in an effect similar to what Toft et al. observed when studying synthesis of anatase nanoparticles in supercritical isopropanol/water.47 In their study, a drop in particle size was noted at temperatures of about 30 40 deg below the critical point of isopropanol. Below 210 °C, the dominant particle appearance is large double-cone structures with rough surfaces indicative of diffusion controlled growth (see Figure 7). Electron-diffraction reveals that the particles are single crystals. Most of the particles have a double-cone structure with a line down the middle indicative of a twinned crystal grown from a common nucleation site. Other groups have also observed starlike ZnO structures and fourlings (see McBride et al.29 and references therein). Our double-coned structures were only observed in minute amounts at reactor temperatures of 210 °C and above, due to the higher supersaturation and much faster formation of ZnO at these temperatures. The observed morphologies of the particles synthesized at 210 °C and above resemble that of the theoretical growth form of ZnO from the periodic bond chain (PBC) model.26

pH Modification. At acidic conditions, the SEM images reveal the appearance of very large rods of different dimensions with pointed tips and flat ends along with the smaller particles also observed at neutral pH values, Figure 8. At alkaline conditions, flat nanoplates seem to be the most favored morphology even though a few rods and particles also appear. The plates grow to around 100 nm under alkaline conditions. The observation of these larger structures might be explained by a change in the precursor at higher pH values. According to Yamabi et al.43 and McBride et al.,29 ZnO should be more stable at elevated pH (below 13.5), and semicrystalline ZnO is already formed before the hydrothermal processing. Therefore, the individual plates grow larger than the particles at neutral conditions. Neutral conditions will also give a larger supersaturation than alkaline or acidic conditions, and as a result smaller particles will be synthesized due to the fewer Zn(OH)x ions available as building blocks for the growth. Several studies have investigated the effect of the alkali cation from MOH (M = Li, Na, K) on the growth properties of ZnO nanoparticles.27,48 The change of favored morphology on changing the pH of the precursor is in accordance with the inhibition effect of the Na+-ions on the polar (001) face (c-axis) of the ZnO crystals and the shielding effect of the OH ions on the growth units.26,27,48 Liu et al.14 have demonstrated growth of ZnO 4031



ARTICLE

Table 1. Dimensions of Particles Synthesized at Different Concentrations As Determined by PXRD size a-direction

size c-direction

concentration (mol/L)

(nm)

(nm)

ratio c/a

0.005 0.05

29.0(2) 32.0(2)

33.6(3) 38.0(5)

1.16(2) 1.19(5)

0.5

47.1(3)

59.2(9)

1.26(11)

Crystallite sizes of the different concentration were extracted through Rietveld refinements of powder diffraction data.

nanorods from Zn(NO3)2 and NaOH under alkaline conditions (Zn2+/OH = 1:20) in the presence of ethylenediamine. While ethylenediamine promotes growth in the c-direction the addition of citrate ions inhibits the c-direction growth and forces the ZnO structures into plates.49 Li et al.26 describe the formation of pencil-shaped ZnO nanorods in a hydrothermal reaction as the idealized growth habit. This implies that the growth velocity is highest in the (001)-direction by default. For longer reaction times, they observe ZnO rods in neutral medium that changes to more spherical morphologies in an alkaline medium. Precursor Concentration. The relation between the particle size determined by PXRD and precursor concentration is shown in Table 1. Increasing the precursor concentration increased the particle size dimensions a and c from 29 and 34 nm at 0.005 mol/ L over 32 and 38 nm at 0.05 mol/L to 47 and 59 nm at 0.5 mol/L. Within the uncertainties, the ratio c/a did not increase significantly upon increasing concentration. The increase in crystallite size with increased precursor concentration can be attributed to the larger amount of precursor remaining for crystal growth after the initial nucleation burst. SEM images show small isotropic particles with a few rods in the concentrated sample (see Supporting Information). These were completely absent in the diluted samples. The observations of rods in the more concentrated sample could be due to the presence of higher pH gradients within the precursor. Subsequent Neutralization. In our setup, we were not able to produce ZnO without any base. We believe that any formed ZnO is dissolved in the acidic conditions from the nitric precursor. When the base is added immediately after the reaction and quenching of the aqueous precursor of Zn(NO3)2, the crystallinity of the formed ZnO nanoparticles is very low (51 57%), Figure 5, and the particles seem slightly smaller (≈20 nm). This indicates that the ZnO particles in these cases are either partly dissolved or maybe even not formed before they are neutralized after exit of the apparatus. In the latter case, they do not mature and increase their crystallinity in the heated reactor tube. Other groups have synthesized ZnO nanoparticles in setups similar to ours,27,28,32,33 and in these reports the addition of base is varied in the synthesis. The base can be added prior to the heating, prior to the quenching of the reaction, or after the reaction and quenching. Ohara et al.32 succeeded in synthesizing ZnO in a similar setup from zinc nitrate in water as precursor without addition of base. Sue et al.28 synthesized ZnO nanorods when first heating a Zn(NO3)2 aqueous solution prior to mixing with a KOH solution, whereas isotropic nanoparticles were obtained when a KOH solution was heated before mixing with Zn(NO3)2 solution. The results are similar to ours at different pH values. Some of the particles made by other groups appear more homogeneous and with smaller size distributions than the present results. This could be due to

differences in the choice of precursors, reaction times, pump frequencies, and volumes. For the controlled synthesis of nanoparticles, a narrow size distribution is typically desired. However, in thermoelectric applications a broader size distribution can be beneficial due to the enhanced scattering of phonons with different wavelengths.50 Smaller particles with larger surface areas are advantageous for example for photocatalysis and sensor devices. On the other hand, a too large surface area may become a problem in LEDs and solar cell devices if the charge carrier lifetime is reduced due to increased surface recombination rates. We are currently investigating the relation between the different sizes, morphologies, and crystallinities of our synthesized nanoparticles, with respect to their photocatalytic activities. Investigations of residence time in the reactor would be highly interesting, and experiments to clarify the effect of this should be done in the future.

4. CONCLUSION We have shown the potential for tuning size, shape, and crystallinity of ZnO nanoparticles by varying temperature, pH, concentration, and precursor in a fast continuous flow hydrothermal synthesis. With the present synthesis method, it was possible to produce small particles with a very high crystallinity at relatively low temperatures in a matter of seconds. Double cone shaped particles of maximum size 1 μm were produced at a reactor temperature of 122 °C, while isotropic particles of sizes down to 20 25 nm were produced at temperatures above 210 °C. The crystallinity of the particles increased with temperature and reaches 99% at 390 °C. Different NaOH concentrations of the precursor resulted in (1) large rods at acidic conditions, (2) smaller isotropic particles at neutral conditions, and (3) large plates at alkaline conditions. The particle size increased with the precursor concentration. Finally, neutralization of aqueous Zn(NO3)2 with NaOH after the exit of the synthesis apparatus resulted in ZnO with very low crystallinity (51 57%). Comparing the present results with the literature gives an indication that it is not only the synthesis conditions (temperature, ions in the solution, concentration, pH, type of base) but also the synthesis method itself that give rise to the different ZnO structures. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional description of methods and results are available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Danish National Research Foundation (Center for Materials Crystallography) and the Danish Strategic Research Council (Center for Energy Materials and OTE-POWER). ’ REFERENCES (1) Look, D. C. Mater. Sci. Eng., B 2001, 80, 383–387. (2) Wang, Z. L. J. Phys.: Condens. Mat. 2004, 16, 829–858. 4032


Crystal Growth & Design (3) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 103. (4) Gratzel, M. Nature 2001, 414, 338–344. (5) Keis, K.; Magnusson, E.; Lindstr€om, H.; Lindquist, S.-E.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2002, 73, 51–58. (6) Pillai, S. C.; Kelly, J. M.; McCormack, D. E.; O’Brien, P.; Ramesh, R. J. Mater. Chem. 2003, 13, 2586–2590. (7) Emtage, P. R. J. Appl. Phys. 1977, 48, 4372–4384. (8) Ohtaki, M.; Araki, K.; Yamamoto, K. J. Electron. Mater. 2009, 38, 1234–1238. (9) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Nat. Mater. 2005, 4, 42–46. (10) Willander, M.; Nur, O.; Zhao, Q. X.; Yang, L. L.; Lorenz, M.; Cao, B. Q.; Perez, J. Z.; Czekalla, C.; Zimmermann, G.; Grundmann, M.; Bakin, A.; Behrends, A.; Al-Suleiman, M.; El-Shaer, A.; Mofor, A. C.; Postels, B.; Waag, A.; Boukos, N.; Travlos, A.; Kwack, H. S.; Guinard, J.; Dang, D. L. Nanotechnology 2009, 20, 332001. (11) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897–1899. (12) Willander, M.; Klason, P.; Yang, L. L.; Al-Hilli, S. M.; Zhao, Q. X.; Nur, O. Phys. Status Solidi C 2008, 5, 3076–3083. (13) Sakagami, N.; Wada, M. Rec. Electr. Commun. Eng. Conversazione Tokohu Univ. 1971, 40, 221–225. (14) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430–4431. (15) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826–2833. (16) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Nunes, J. G. Appl. Phys. Lett. 2003, 82, 1117–1119. (17) Polarz, S.; Roy, A.; Merz, M.; Halm, S.; Schroder, D.; Schneider, L.; Bacher, G.; Kruis, F. E.; Driess, M. Small 2005, 1, 540–552. (18) Tsuzuki, T.; McCormick, P. G. Scripta Mater. 2001, 44, 1731–1734. (19) Liu, L.; Zhang, L.; Jia, D. Nanotechnology 2006, 17, 2266–2270. (20) Jayawardena, K. D. G. I.; Fryar, J.; Silva, S. R. P.; Henley, S. J. J. Phys. Chem. C 2010, 114, 12931–12937. (21) Kaidashev, E. M.; Lorenz, M.; von Wenckstern, H.; Rahm, A.; Semmelhack, H. C.; Han, K. H.; Benndorf, G.; Bundesmann, C.; Hochmuth, H.; Grundmann, M. Appl. Phys. Lett. 2003, 82, 3901–3903. (22) Kawakami, M.; Hartanto, A. B.; Nakata, Y.; Okada, T. Jpn. J. Appl. Phys. Part 2 - Lett. 2003, 42, L33–L35. (23) Hu, X.-L.; Zhu, Y.-J.; Wang, S.-W. Mater. Chem. Phys. 2004, 88, 421–426. (24) Laudise, R. A.; Kolb, E. D.; Caporaso, A. J. J. Am. Ceram. Soc. 1964, 47, 9–12. (25) Laudise, R. J. Am. Ceram. Soc. 1966, 49, 302–305. (26) Li, W.-J.; Shi, E.-W.; Zhong, W.-Z.; Yin, Z.-W. J. Cryst. Growth 1999, 203, 186–196. (27) Sue, K.; Kimura, K.; Murata, K.; Arai, K. J. Supercrit. Fluids 2004, 30, 325–331. (28) Sue, K. W.; Kimura, K.; Yamamoto, M.; Arai, K. Mater. Lett. 2004, 58, 3350–3352. (29) McBride, R. A.; Kelly, J. M.; McCormack, D. E. J. Mater. Chem. 2003, 13, 1196–1201. (30) Adschiri, T.; Kanazawa, K.; Arai, K. J. Am. Ceram. Soc. 1992, 75, 1019–1022. (31) Hald, P.; Becker, J.; Bremholm, M.; Pedersen, J. S.; Chevallier, J.; Iversen, S. B.; Iversen, B. B. J. Solid State Chem. 2006, 179, 2674–2680. (32) Ohara, S.; Mousavand, T.; Umetsu, M.; Takami, S.; Adschiri, T.; Kuroki, Y.; Takata, M. Solid State Ionics 2004, 172, 261–264. (33) Ohara, S.; Mousavand, T.; Sasaki, T.; Umetsu, M.; Naka, T.; Adschiri, T. J. Mater. Sci. 2008, 43, 2393–2396. (34) Lock, N.; Hald, P.; Christensen, M.; Birkedal, H.; Iversen, B. B. J. Appl. Crystallogr. 2010, 43, 858–866. (35) Rodriguezcarvajal, J. Physica B 1993, 192, 55–69. (36) Warren, B. E. X-ray Diffraction; Dover Publications: New York, 1990.

ARTICLE

(37) Popa, N. C. J. Appl. Crystallogr. 1998, 31, 176–180. (38) Dinnebier, R. E.; Billinge, S. J. L. Powder Diffraction: Theory and Practice; Cambridge: Royal Society of Chemistry.: Cambridge, UK, 2008. (39) Scardi, P.; Leoni, M. Acta Crystallogr. A 2001, 57, 604–613. (40) Floriano, W. B.; Nascimento, M. A. C. Braz. J. Phys. 2004, 34, 38–41. (41) Uematsu, M.; Franck, E. U. J. Phys. Chem. Ref. Data 1980, 9, 1291–1306. (42) Jarvinen, M. J. Appl. Crystallogr. 1993, 26, 525–531. (43) Yamabi, S.; Imai, H. J. Mater. Chem. 2002, 12, 3773–3778. (44) Rayner-Canham, G.; Overton, T. Descriptive Inorganic Chemistry, 4th ed.; W. H. Freeman and Company: New York, 2006. (45) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35–50. (46) Kahlweit, M. Adv. Colloid Interface Sci. 1975, 5, 1–35. (47) Toft, L. L.; Aarup, D. F.; Bremholm, M.; Hald, P.; Iversen, B. B. J. Solid State Chem. 2009, 182, 491–495. (48) Santra, P. K.; Mukherjee, S.; Sarma, D. D. J. Phys. Chem. C 2010, 114, 22113–22118. (49) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954–12955. (50) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Adv. Mater. 2010, 22, 3970–3980.

4033


Size and Morphology Dependence of ZnO Nanoparticles Synthesized

Recommend Documents