In Situ Powder Diffraction Study of the Hydrothermal Synthesis of ZnO

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In Situ Powder Diffraction Study of the Hydrothermal Synthesis of ZnO Nanoparticles Espen D. Bøjesen,† Kirsten M. Ø. Jensen,† Christoffer Tyrsted,† Nina Lock,†,‡ Mogens Christensen,† and Bo B. Iversen*,† †

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark Department of Inorganic Chemistry, Georg-August-Universität Göttingen, D-37077 Göttingen, Germany

‡

S Supporting Information *

ABSTRACT: ZnO is one of the most widely applied nanomaterials, and the vast majority of ZnO nanoparticles are synthesized from aqueous solutions of inexpensive salts. Here we present the first in situ powder X-ray diffraction (PXRD) study of the formation of ZnO nanoparticles under hydrothermal conditions. An aqueous Zn(NO3)2 and NaOH precursor gel was studied at three different temperatures (150, 200, and 250 °C) by in situ PXRD, and Rietveld refinements of the data were used to extract crystal structure and nanostructural information. Interesting trends in the evolution of crystallite size and twin faulting with temperature were established. The morphology of the synthesized crystallites depends on temperature and reaction time; at high temperatures and long reaction times almost isotropic crystallites are formed, while the anisotropy increases with shorter synthesis times and lower temperatures. Furthermore, the twin fault probability decreases with reaction time and increasing reaction temperature. Under the present synthesis conditions, a minimum ZnO crystallite size of around 14 nm in the a-direction and 22 nm in the c-direction is observed.

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INTRODUCTION Zinc oxide (ZnO) is a wide band gap semiconducting ceramic material which, despite its very simple wurtzite crystal structure,1,2 has a remarkably broad field of applications. In terms of quantity, it is one of the most widely applied nanomaterials, for example, filler material in rubbers, varistors, photovoltaics, catalysis, etc.3,4 Furthermore, given the noncentrosymmetric crystal structure of wurtzite, ZnO also exhibits ferroelectric and piezoelectric properties.4 In numerous instances it has been found that the properties of nanosized ZnO particles surpass the properties of the bulk material.5−7 Thus, a plethora of methods for the synthesis of nano- and microsized ZnO particles have been devised, ranging from spray pyrolysis over various colloidal methods to microwave and surfactant assisted hydro- and solvothermal syntheses.8−13 It has been suggested that ZnO has the richest family of nanostructures among all materials, and it has been shown that the size and shape of ZnO nanoparticles heavily influence its physical properties.6,7,14,15 This makes size and shape control during synthesis of paramount importance. One of the most promising methods for controlling size and morphology, while at the same time producing appreciable quantities of nanoparticles, has been the use of sub- and supercritical continuous flow synthesis reactors.16−22 Hydrothermal flow synthesis methods achieve very high heating rates, and this leads to fast supersaturation of reactive species, which © 2014 American Chemical Society

in turn causes a nucleation burst followed by a depletion of the amount of precursor left for particle growth.23,24 However, the hydrothermal process is basically a black box system and determining the parameters governing the nucleation and growth of nanoparticles can involve extensive trial-and-error studies. In the quest for a better comprehension of the processes that occur during the initial stages of oxide nanoparticle formation under sub- and supercritical conditions, in situ powder X-ray diffraction (PXRD), at times combined with other techniques, has proven to be an extremely valuable tool.25−43 Using this method, phase transformations, intermediate phases, and crystallite growth can be followed in real time. As a means to study the formation of ZnO in situ, several characterization techniques have been applied in previous studies. These include hyper-Rayleigh scattering,44 small-angle X-ray scattering,45 UV/vis spectroscopy,46−48 and extended X-ray absorption fine structure (EXAFS).45 All of these techniques are promising tools for investigations of the particle growth during synthesis, but nevertheless every technique has it strengths and weaknesses, as pointed out in the recent review by Ludi et al.49 Considering the aforementioned methods, only EXAFS Received: January 12, 2014 Revised: April 5, 2014 Published: April 16, 2014 2803

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were sequentially Rietveld refined using the Fullprof Suite software.54 The instrumental resolution function was determined by LeBail fitting of data collected on the LaB6 standard.55 The crystallite size and morphology were refined using a method based on a general phenomenological model using an anisotropic size function. In this method the Scherrer formula is utilized assuming that the size broadening can be described with a linear combination of spherical harmonics, as proposed by Popa.56−59 The method has been successfully applied in several previous studies on anisotropic crystallites.16,30 In all refinements, the profile broadening was ascribed to size effects and stacking faults, while microstrain was ignored. Rietveld refinements including both an anisotropic size contribution and either an isotropic or anisotropic strain contribution (Stephens model60) were performed. No significant improvements of the fit (neither visual nor improved R-values) were obtained when allowing for microstrain. Thus, to minimize the number of refined parameters, and since the size contribution is dominating, microstrain was omitted from the model. More details on the refinement strategy can be found in the Supporting Information. To complement the in situ investigations, PXRD patterns were collected on the coprecipitated precursor samples using a Rigaku Smart Lab diffractometer configured with a Cu Kα source and parallel beam optics. The data were Rietveld refined using the FullProf software suite (see Supporting Information). Additional single-frame Rietveld refinements were performed using the MAUD software to assess the twin faulting in the crystallites.61 The method applied to extract twin fault probabilities is based on Warren’s model for twin faulting in hexagonal systems56 and has previously been successfully applied to describe twin boundary concentrations in ZnO.62

provides atomic scale structural information, albeit on the local structure level, on the formation of crystallites.45,49 The importance of structural information during ZnO formation was highlighted in the study by Brioris et al.,45 where it was established that at certain synthesis conditions, even after long reaction times, several different crystalline phases were present in the synthesis product. In situ PXRD is in this respect a superior technique to extract information on the crystal structural and morphological changes during the formation of ZnO nanoparticles. To the best of our knowledge, no in situ PXRD studies of hydrothermal ZnO synthesis have been performed. The in situ studies on ZnO mentioned above all focused on relatively slow,50 low temperature synthesis processes using organic solvents and organic derived zinc precursors such as acetate or alkoxide compounds. This is irrespective of the fact that the vast majority of ZnO nanostructures are synthesized under hydrothermal conditions using zinc salts and alkali additives. Using these methods large quantities of highly crystalline ZnO particles can easily be produced in an environmentally benign way.16,17,20,22 In the present study we shed light on the conditions governing the growth of ZnO from aqueous solutions of inexpensive salts. The synthesis temperature is among the most influential parameters affecting the size, shape, and crystallinity of ZnO particles synthesized using hydrothermal methods, and in the present work we therefore focus on temperature and the implications it has on the synthesis.16

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RESULTS AND DISCUSSION Nanocrystal Formation. In all experiments a crystalline phase, Zn5(OH)8(NO3)2·2H2O was identified in the precursor mixture.63 It crystallizes in the monoclinic space group C2/m and has a unit cell of a = 19.480 Å, b = 6.238 Å, c = 5.517 Å, β = 93.28°. The Zn5(OH)8(NO3)2·2H2O structure is shown in Figure 1 together with the ZnO structure. The Zn5(OH)8(NO3)2·2H2O phase consists of layers of edge sharing Zn(OH)64− octahedra, and in addition tetrahedrally coordinated Zn2+ ions are present in the structure. The apexes of the tetrahedra consist of water molecules, pointing toward the interlayer space, where nitrate ions are located. Nitrate is easily exchanged with other anions.64 Contrary to previous reports no intermediate crystalline Zn(OH)2 or other crystalline phases formed during the present syntheses of ZnO at the applied temperatures and heating rates.16,21 Figure 2 shows a plot of the PXRD patterns as a function of time, demonstrating the change from Zn5(OH)8(NO3)2·2H2O to ZnO. The figure also shows the refined weight fractions for ZnO and Zn5(OH)8(NO3)2·2H2O as a function of time. It is evident that the rate of depletion of crystalline precursor and conversion to pristine ZnO is highly temperature dependent. Figure 3 shows the Rietveld refined PXRD patterns, and accompanying fits, representing three different refinement (time) regimes. At first, the only crystalline phase present is Zn5(OH)8(NO3)2·2H2O, which coexists with an amorphous phase and water giving a high background. After 10 s of heating (200 °C), it is necessary to include both crystalline ZnO and Zn5(OH)8(NO3)2·2H2O in the model to properly fit the data. After disappearance of the crystalline precursor phase, the refinements are conducted with ZnO as the only crystalline phase. The present data show that no other crystalline intermediates are present when using the hydrothermal approach, contrary to what is observed during thermal decomposition of Zn5(OH)8(NO3)2·2H2O in air, where Zn3(OH)4(NO3)2 and

EXPERIMENTAL SECTION

In Situ Measurements. The in situ X-ray measurements were carried out at beamline I711, MAX-II, MAX-lab, Sweden. The wavelength used was 1.006(9) Å, and the diffraction data were collected using an Agilent Technologies Titan S2 CCD 2D detector. A detailed description of the experimental setup utilized can be found in the paper by Becker et al.51 The reactor itself consists of a single crystal sapphire capillary with inner and outer diameters of 0.7 and 1.35 mm, respectively. The capillary is pressurized with water, while the heating is achieved by a jet of hot air. Due to the small volume of the capillary, fast heating rates can be achieved, thus to some extent mimicking the conditions found in hydrothermal flow reactors used for large scale synthesis. For the in situ experiments all precursor mixtures were prepared prior to injection into the reactor tube. For all experiments 3.0 mL of a 1.0 M Zn(NO3)2·6H2O (98.5% Sigma-Aldrich) aqueous solution were mixed with 1.5 mL of H2O. This 4.5 mL solution was then mixed with 1.5 mL of 3.2 M aqueous NaOH resulting in a turbid gel for which the viscosity and appearance altered during stirring. At first, the gel became increasingly viscous reverting back to a less viscous white gel upon further stirring. All precursors were stirred an equal amount of time (2 min) before being injected into the sapphire tube. A maximum of 10 min elapsed between mixing and initiation of the heating in all instances. The present method was chosen to prevent aging of the gel, which could possibly cause precipitation of ZnO even at room temperature. According to the literature the precursor mixture having a molar Zn/OH ratio of 1.0:1.6 should be stable for at least 1 h.52 Experiments were carried out at three different temperatures 150, 200, and 250 °C. Experiments performed at 350 °C instantaneously led to formation of very large crystallites, resulting in a poor powder average. In addition, the precursor phase was produced by coprecipitation and characterized ex situ (see Supporting Information). The experimental method for the ex situ study has previously been described by Søndergaard et al.16 Crystal Structure and Nanostructural Characterization. The in situ PXRD data were integrated using the Fit2D software.53 Wavelength calibration and detector distance corrections were performed using a NIST 660a LaB6 standard. The integrated data 2804

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Figure 3. Rietveld refinement of in situ PXRD patterns after different reaction times. (Top) No heat is applied and the only crystalline phase is Zn5OH8(NO3)2·2H2O. (Middle) After 10 s of heating both crystalline ZnO and Zn5OH8(NO3)2·2H2O are present. (Bottom) After 20 s of heating the only crystalline phase present is pristine ZnO.

Figure 1. (a) The ZnO wurtzite structure can be described as an alternating stacking of planes (along the crystallographic c-axis) of tetrahedrally coordinated oxide ions and zinc(II) cations. (b) The Zn5(OH)8(NO3)2·2H2O structure consist of infinite brucite (Mg(OH)2) like layers of octahedrally coordinated zinc atoms (yellow). In these layers, one-quarter of the zinc atoms sites are vacant, effectively producing some tetrahedrally coordinated Zn atoms (green). These tetrahedra have a base consisting of OH, and at the apex a water molecule is located. The nitrate anions are situated between the brucite-like layers, and these nitrate ions act as linkers between the layers. Two oxygen atoms of each nitrate group are hydrogen bonded with the apical water molecule, while the third oxygen binds two hydroxide ions of the opposite sheet.65

an anhydrous zinc nitrate are formed as intermediates.65 This supports the notion that in the hydrothermal process different mechanisms are at work than just a thermal decomposition. Another evidence of a different conversion mechanism is the significantly lower temperatures needed for full conversion of freshly prepared Zn5(OH)8(NO3)2·2H2O to ZnO under hydrothermal conditions compared with solid state reactions. Temperatures in the region of 160−260 °C are necessary for full conversion to occur when calcining a dry sample in air.65

Figure 2. (Left) Example of time-resolved PXRD patterns. The initial crystalline precursor phase Zn5(OH)8(NO3)2·2H2O disappears shortly after heating is commenced. (Right) Refined weight fraction as a function of time. Estimated standard deviations are 1−2%. Dashed lines indicate the refined ZnO weight fraction, and solid lines the refined weight fraction. 2805

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Figure 4. Growth curves for the three different synthesis temperatures with the a-direction shown to the left and the c-direction in the middle. The figure to the right shows the crystallite shape obtained from the Rietveld refinements (using two spherical harmonics) at the end of reaction for the 150 °C synthesis.

Evolution of Crystallite Size and Morphology. In Figure 4 the evolution of the ZnO crystallite size along the aand c-directions is shown. The influence of temperature on the initial crystallite size, growth rate, and duration of growth is evident. The crystallite size along the c-direction is larger than along the a-direction throughout the entire synthesis time for all samples. At both 150 and 200 °C, significant growth is only seen in the initial stages of the synthesis before a constant size is reached. The synthesis at 250 °C shows the most pronounced initial and continuous growth. At 250 °C, an initial fast growth is followed by an extended period of time with slower but still significant growth. The growth curves indicate that the synthesis temperature has a great impact on the initial growth of the ZnO crystallites. This has a strong impact on the final crystallite size that is obtained for synthesis methods relying on short reaction times such as hydrothermal flow synthesis. In Figure 5 the aspect ratio, i.e., the size along the c-direction divided by the size along the a-direction, is shown as a function

difference in growth rate along the different crystallographic directions in ZnO becomes less significant, as discussed in our previous study16 and later in this paper, leading to more isotropic crystallites. A more detailed study on the initial nucleation is ongoing, and it will hopefully contribute to the understanding of morphology evolution during synthesis for this system. Furthermore, a difference in surface absorption affinities of nitrate and sodium ions to various growing facets might influence the observed growth,48 and they may change with time and temperature. Figure 6 shows a plot of the unit cell dimensions as a function of nanocrystal volume. The unit cell dimensions

Figure 6. Relative change of the unit cell parameters as a function of crystallite volume.

decrease in all crystallographic directions with increasing crystallite size, but the change is most pronounced for the caxis corroborating the observation that growth is more pronounced along the crystallographic c-direction. The trend of relaxing unit cell with increasing crystallite size has been observed for many other simple oxide systems and is often explained by reduced defect concentration or surface relaxation.31,66 Planar Faulting. Various forms of planar faults can occur in hexagonal close packed systems. These include: twin faulting, intrinsic deformation faulting, and growth faulting/extrinsic deformation faulting. Each type of faulting is known to affect the PXRD pattern of a given sample in several ways.56 Most prominently the faulting can introduce hkl dependent profile asymmetry and broadening. Various approaches on how to model these effects exist,67−69 and it is generally accepted that methods based on integral breadth analysis, such as it is implemented in MAUD, are valid to investigate trends within a

Figure 5. Aspect ratio as a function of time. Values obtained from a flow study are marked by arrows.16 These values are found by interpolation between the values obtained at the synthesis temperatures used in the flow study. The uncertainties for those values are on the order of 3−7%.

of reaction time. For all temperatures, the aspect ratio decreases fast and it levels out to a value that depends on the temperature. Thus, a clear relation between the aspect ratio and temperature and reaction time exists; higher temperatures as well as prolonged reaction times lead to more isotropic crystallites. This behavior indicates that the initial nucleation mechanism and thus possibly the morphology of the precursor particles play a role in the formation of initially very anisotropic crystallites. With increasing reaction time and temperature the 2806

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worst fit was found with edf only or no faulting. On the basis of the refined values the dominant fault seemed to be twin faulting. In order to minimize the number of refined parameters and since this is a study on trends and not absolute values, it was chosen to only allow twin faulting in the refinement model. Support of this choice is provided by many reports of twin faulted ZnO in literature11,72,73and by the electron microscopy pictures shown by Søndergaard et al.16 It has to be mentioned that double deformation faults and other multiple faulting will give rise to similar effects on the line profile as twin faulting.56 Figure 8 illustrates the improved fit when including twin faulting; especially the fit to the (102) and (101) peaks are improved significantly when allowing for twin faulting. These two peaks are affected by faulting since it occurs along the ⟨001⟩ direction, and they fall within two different subgroups of reflections as defined by Warren 56 and applied most prominently by Langford et al.71 According to this classification the (102) peak should be affected more by stacking faults than the (101) peak, which fits well with our observations. Planar faulting can influence the physical properties of ZnO,74 and it is therefore important to describe this aspect of the crystal structure and if possible control the degree of faulting in the final product. Equally for all the results presented, the absolute values have to be considered as rough estimates due to the relatively low qrange and limited instrumental resolution. The estimated twin fault probabilities as a function of reaction time are shown on Figure 9. A trend toward lower twin fault probability with increasing reaction time is evident for all samples. Furthermore, the high temperature synthesis exhibits significantly lower twin fault probabilities than the 200 and 150 °C syntheses, and it does so faster. The high twin fault density at the onset of the reaction (Figure 9) could indicate that oriented attachment plays an important role in the early stages of the crystallite formation.72,75 The 250 °C synthesized sample shows lower twin fault densities in comparison with the samples from the two syntheses at 150 and 200 °C. This observation agrees well with TEM and SEM pictures from the ex situ flow synthesis study.16 Comparison with Ex Situ Hydrothermal Flow Synthesis. Good agreement is observed when comparing trends from the in situ study with results from continuous flow hydrothermal synthesis of ZnO particles.16 The flow synthesis procedure is different from the in situ batch reactor method in a number of ways. This includes different metal salt concentrations (0.05 M ex situ vs 0.5 M in situ), difference in the mixing of the precursor slurry with the preheated water, difference in the heating method, and other factors. Nevertheless, it is still possible to explain some of the results of the flow study since the in situ system can be considered a model system for the flow reactor. By transferring the knowledge gained, it may be possible to improve the flow synthesis procedure and thereby produce large quantities of material useful for actual applications. For the flow synthesis study a minimum size was found at 210 °C, with larger crystallites forming at both higher and lower temperatures.16 In Figure 10 we have replotted the size versus temperature data from that study since we discovered an error in the estimation of the estimated standard uncertainties of the data points. The aspect ratio was found to decrease with increasing temperature, and a correlation between unit cell parameters and crystallite size was established as also observed

certain system, but that they are not optimal for extracting absolute values.68 In this work we have chosen to use the Warren model as it is implemented in the MAUD61 software package, due to its previous successful use in other studies.62 It has been previously shown that even though ZnO has a AαBβ type stacking (Roman letters identifying oxygen layers, while the Greek letters signify zinc layers along ⟨001⟩),70 the Warren model for faulting in hcp structures, or modified versions hereof, is applicable to some degree.62,71 Twin faulting in hexagonal closed packed structures occurs when, instead of the usual ABABAB stacking of anions, a stacking sequence of ABABCBABA occurs. Intrinsic deformation faulting (idf) manifests itself in an ABABCACA layer sequence, while extrinsic deformation faulting (edf) has an ABABCBCB sequence. The different stacking sequences are illustrated in Figure 7.

Figure 7. (a) Illustration of a normal hcp stacking of ions (ABABABA). (b) Illustration of a twin fault in a hcp array (ABABCBABA). (c) Illustration of an intrinsic deformation fault in a hcp array (ABABCACA). (d) Illustration of an extrinsic deformation fault in a hcp structure (ABABCBCB).

In separate refinements it was found that discrepancies between the model profile and the observed data could be minimized by allowing for a certain amount of faulting. Different refinement strategies using the Warren model as implemented in MAUD61 were tested: idf only, twin faulting and idf, edf only, and finally only twin faulting. The best fit was found when allowing for both idf and twin faulting, while the 2807

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Figure 8. Rietveld fit of PXRD data collected after 15 s of heating at 200 °C. (Left) Twin faulting is excluded in the model. (Right) Twin faulting is included in the model.

same time to an increase in growth rate, as is evident from the in situ growth curves. For the in situ synthesis performed at 350 °C, micrometersized crystallites were formed almost instantly. This supports the argument that the main factors preventing increased crystallite growth in the flow process at high temperatures are (i) a short retention time (around 10 s at 350 °C), (ii) very high heating rates, and (iii) lower concentrations which lead to fast depletion of precursor material left for crystallite growth. On the basis of the growth curves, in situ as well as ex situ, it can similarly be deduced that when using Zn(NO3)2 and alkali hydroxide mixtures as precursors combined with hydrothermal treatment, it appears impossible to synthesize pristine ZnO with crystallite dimensions smaller than 14 nm along the adirection and 22 nm along the c-direction. This agrees with the literature where there are no records of crystallite sizes smaller than 16 nm when using the present approach.17−20,76,77 Thus, we suggest that even at extreme heating rates, utilizing supercritical conditions and short retention times, one cannot avoid the large initial crystallite size. The explanation for this intrinsic minimum size and an explanation of a possible nucleation mechanism are currently being investigated using total X-ray scattering techniques.25,26 The trend toward more isotropic crystallite shape with increasing temperature can be explained when considering the typical growth habits of ZnO observed under hydrothermal conditions. The polar crystal structure of ZnO influences the growth of the crystallites and forms the basis for the anisotropic crystallite shape often observed. The growth along the [001] direction is generally faster than along all of the other directions.78 TEM pictures of the products from flow synthesis suggest the possibility of twinned crystallites having the (001̅) as composition plane leading to exposed fast growing {001} facets.11,16,78 With increasing temperature, the difference in surface energy of the various facets becomes less important, and this allows for more isotropic crystallite shapes.78,79 The high twin fault density determined in the in situ study (see Figures 7 and 8) serves to support the subtle difference in crystallinity found in the flow study.16 In the flow study the experimentally determined crystallinity, calculated from diffraction patterns of samples including an internal 100% crystalline Si standard, was found to increase with increasing synthesis temperature (90(1)% at 121 and 200 °C, 96(1)% at 300 °C and 99(1)% at 390 °C). This can be explained by considering that the higher temperature provides more energy for rearrangement of the atomic layers, and this leads to a lowering in twin faulting. Overall, the in situ information

Figure 9. Plot of the twin fault probability as a function of synthesis time. Open symbols indicate 10 and 15 s reaction time, while the first closed symbols are at 20 s. This is the first point in time where a reliable refinement including twin faulting was possible for the 150 °C synthesis. The lines are meant to be a guide for the eye, and they represent a fit using a combination of two decaying exponential functions.

Figure 10. Sizes of crystallites in the crystallographic a- and cdirections as determined by Rietveld refinement of PXRD data (measured on STOE STADI P diffractometer), from the flow synthesis study.16 The uncertainties on the values for the c-direction are smaller than in the original paper by Søndergaard et al. This is due to an error in unit conversion from Å to nm in the propagation of error calculations in the original figure from the paper.16

in the in situ studies. In the flow reactor the residence time is calculated from the flow rate, reactor length, and the density of water at various temperatures. Considering that the residence time within the flow reactor was 8−30 s (longest at lower temperatures and shortest at higher temperatures), it is possible to explain the subtle, but significant, increase in crystallite size observed in the flow study when the synthesis temperature was increased from 210 to 240 °C. Increasing the synthesis temperature leads to a decrease in residence time, but at the 2808

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(6) Søndergaard, M.; Bøjesen, E. D.; Borup, K. A.; Christensen, S.; Christensen, M.; Iversen, B. B. Acta Mater. 2013, 61 (9), 3314−3323. (7) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16 (25), R829−R858. (8) Height, M. J.; Madler, L.; Pratsinis, S. E.; Krumeich, F. Chem. Mater. 2006, 18 (2), 572−578. (9) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102 (40), 7770−7775. (10) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91 (14), 3789−3798. (11) Chen, W.; Yu, D. D.; Ruan, H.; Li, D. Z.; Hu, Y.; Chen, Y. B.; He, Y. H.; Fu, X. Z.; Shao, Y. J. Am. Ceram. Soc. 2012, 95 (7), 2322− 2329. (12) Kunjara Na Ayudhya, S.; Tonto, P.; Mekasuwandumrong, O.; Pavarajarn, V.; Praserthdam, P. Cryst. Growth Des. 2006, 6 (11), 2446− 2450. (13) Baruah, S.; Dutta, J. Sci. Technol. Adv. Mater. 2009, 10 (1), 013001. (14) Willander, M.; Klason, P.; Yang, L. L.; Al-Hilli, S. M.; Zhao, Q. X.; Nur, O. Phys. Status Solidi C 2008, 5 (9), 3076−3083. (15) 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 (33), No. 332001. (16) Sondergaard, M.; Bojesen, E. D.; Christensen, M.; Iversen, B. B. Cryst. Growth Des. 2011, 11 (9), 4027−4033. (17) Ohara, S.; Mousavand, T.; Sasaki, T.; Umetsu, M.; Naka, T.; Adschiri, T. J. Mater. Sci. 2008, 43 (7), 2393−2396. (18) Ohara, S.; Mousavand, T.; Umetsu, M.; Takami, S.; Adschiri, T.; Kuroki, Y.; Takata, M. Solid State Ionics 2004, 172 (1−4), 261−264. (19) Mao, Z. G.; Li, S. N.; Leng, Y. P.; Zhao, Y. P. J. Supercrit. Fluids 2013, 79, 268−273. (20) Sue, K. W.; Kimura, K.; Yamamoto, M.; Arai, K. Mater. Lett. 2004, 58 (26), 3350−3352. (21) Roig, Y.; Marre, S.; Cardinal, T.; Aymonier, C. Angew. Chem., Int. Ed. Engl. 2011, 50 (50), 12071−12074. (22) Shi, L.; Naik, A. J.; Goodall, J. B.; Tighe, C.; Gruar, R.; Binions, R.; Parkin, I.; Darr, J. Langmuir 2013, 29 (33), 10603−9. (23) Adschiri, T.; Kanazawa, K.; Arai, K. J. Am. Ceram. Soc. 1992, 75 (4), 1019−1022. (24) Aymonier, C.; Loppinet-Serani, A.; Reveron, H.; Garrabos, Y.; Cansell, F. J. Supercrit. Fluids 2006, 38 (2), 242−251. (25) Jensen, K. M. O.; Christensen, M.; Juhas, P.; Tyrsted, C.; Bojesen, E. D.; Lock, N.; Billinge, S. J. L.; Iversen, B. B. J. Am. Chem. Soc. 2012, 134 (15), 6785−6792. (26) Tyrsted, C.; Ørnsbjerg Jensen, K. M.; Bøjesen, E. D.; Lock, N.; Christensen, M.; Billinge, S. J. L.; Brummerstedt Iversen, B. Angew. Chem., Int. Ed. Engl. 2012, 51 (36), 9030−9033. (27) Mi, J.-L.; Clausen, C.; Bremholm, M.; Lock, N.; Jensen, K. M. Ø.; Christensen, M.; Iversen, B. B. Cryst. Growth Des. 2012, 12 (12), 6092−6097. (28) Tyrsted, C.; Pauw, B. R.; Jensen, K. M. Ø.; Becker, J.; Christensen, M.; Iversen, B. B. Chem.Eur. J. 2012, 18 (18), 5759− 5766. (29) Laumann, A.; Ørnsbjerg Jensen, K. M.; Tyrsted, C.; Bremholm, M.; Fehr, K. T.; Holzapfel, M.; Iversen, B. B. Eur. J. Inorg. Chem. 2011, 2011 (14), 2221−2226. (30) Jensen, K. M. Ø.; Christensen, M.; Tyrsted, C.; Bremholm, M.; Iversen, B. B. Cryst. Growth Des. 2011, 11 (3), 753−758. (31) Lock, N.; Christensen, M.; Jensen, K. M. Ø.; Iversen, B. B. Angew. Chem., Int. Ed. Engl. 2011, 50 (31), 7045−7047. (32) Mi, J. L.; Christensen, M.; Tyrsted, C.; Jensen, K. O.; Becker, J.; Hald, P.; Iversen, B. B. J. Phys. Chem. C 2010, 114 (28), 12133−12138. (33) Walton, R. I.; O’Hare, D. Chem. Commun. 2000, 23, 2283− 2291. (34) Walton, R. I.; Millange, F.; O’Hare, D.; Davies, A. T.; Sankar, G.; Catlow, C. R. A. J. Phys. Chem. B 2000, 105 (1), 83−90. (35) Beale, A. M.; Reilly, L. M.; Sankar, G. Appl. Catal., A 2007, 325 (2), 290−295.

suggests that it is possible to produce highly crystalline ZnO nanoparticles with a certain aspect ratio, size, and twin fault density by changing the residence time and/or temperature in the flow synthesis process.

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CONCLUSION We have used in situ PXRD in combination with Rietveld refinement to extract information on the formation and growth of ZnO nanoparticles during hydrothermal synthesis. Low synthesis temperatures lead to relatively large initial crystals (23−32 nm) and limited growth, whereas higher temperatures lead to smaller initial crystallites (15−25 nm) and a prolonged growth period. The aspect ratio of the crystallites decreases fast during synthesis, and the final constant value depends on the temperature. Thus, reaction temperature and time are effective synthesis parameters for controlling the aspect ratio. In general higher temperatures as well as prolonged reaction times lead to more isotropic crystallites. High twin fault probabilities were observed in the early stages of synthesis, but these diminished with synthesis time and increasing temperature. The in situ results also allow rationalization of results obtained previously in a hydrothermal flow synthesis study. The results allow explaining ex situ observations in a hydrothermal flow synthesis study. The present study exemplifies how in situ PXRD studies of hydrothermal synthesis can be an important technique for understanding crystal formation and growth, and the in situ information provides guidance on how to synthesize nanocrystals with specific dimensions and crystallinity.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information on the data reduction, Rietveld refinement of the precursor, description of the particle size determination, and representative Rietveld refinements of the in situ data. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), the Danish Research Council for Nature and Universe (DanScatt), and the Danish Council for Strategic Research (Contract No. 10-093971, OTE-POWER). The authors are grateful for the beamtime obtained at the beamline I711, MAXlab synchrotron radiation source, Lund University, Sweden.

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