ARTICLE pubs.acs.org/JPCC
Tuning of the Crystallite and Particle Sizes of ZnO Nanocrystalline Materials in Solvothermal Synthesis and Their Photocatalytic Activity for Dye Degradation John Becker,† Krishna Reddy Raghupathi,† Jordan St. Pierre,† Dan Zhao,*,‡ and Ranjit T. Koodali*,† † ‡
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States Department of Chemical Engineering, Ningbo University of Technology, Ningbo 315211, China
bS Supporting Information ABSTRACT: ZnO nanocrystalline materials with different crystallite and particle sizes were synthesized by the solvothermal approach. The structure and optical properties were investigated using powder X-ray diffraction (XRD), nitrogen adsorption isotherms, UVvis diffuse reflectance spectroscopy (DRS), transmission and scanning electron microscopy (TEM), and room temperature photoluminescence. It was found that the crystallite and particle sizes of ZnO nanocrystalline could be controlled by changing the solvent during the solvothermal synthesis. The photocatalytic activity was evaluated by the degradation of dye Rhodamine B (RhB) under visible irradiation. The individual effect of crystallite size and particle size on the photocatalytic activity of ZnO was studied using the ZnO nanocrystallites prepared in this work.
’ INTRODUCTION ZnO, an IIVI semiconductor with noncentrosymmetric wurtzite crystal structure, a direct band gap of 3.37 eV, and a large excitation binding energy of 60 meV, has been extensively investigated because of its potential applications in piezoelectric devices, transistors, photodiodes, and photocatalysis.15 The unique antibacterial function of ZnO nanostructures both in the dark and under solar irradiation has also attracted great interest.6,7 In the field of photocatalysis, ZnO is usually believed to be an alternative photocatalyst material to TiO2, since they have similar band gaps and similar photocatalytic mechanisms.4,5 In addition, it was reported in several works that ZnO exhibited better activity than TiO2 for the photocatalytic degradation of environmental pollutants, especially for the decomposition of dyes under visible irradiation.810 The structure of nanomaterial, including morphology, particle size, and two-dimensional and three-dimensional architectures, can play important roles in determining the electrical, optical, and catalytic properties. A large volume of work have been done to elucidate the structureproperty relationship in heterogeneous catalysis and to provide useful information for the design and building of efficient nanostructured catalysts. The morphology, particle size, crystal orientation, crystallinity, and oxygen defects are some factors that influence the photocatalytic performance and stability of ZnO photocatalysts. ZnO nanostructure with different three-dimensional architectures, such as microscale rods, tubes, plates, porous hollow microspheres, and flowerlike hierarchical micro/nanoarchitecture, were fabricated by chemical vapor deposition, thermal evaporation, and r 2011 American Chemical Society
wet-chemical routes including solgel processes and solvothermal (hydrothermal) methods.1119 The enhanced photocatalytic activity of the nanomaterials compared to commercial ZnO were ascribed to the larger surface area, increased oxygen vacancy, and the facilitation of diffusion and mass transportation of the reactant molecules. The orientation dependence of photoactivity of ZnO was studied by some researchers using ZnO single crystal and ZnO nanocrystals with different shapes.2022 Kislov et al. studied the photocatalytic destruction of methyl orange over different single crystal ZnO surfaces and found that nonpolar ZnO(1010) surface shows the best efficiency and the activity of polar ZnO(0001)-Zn is higher than ZnO(0001)-O surface.21 Platelike and rodlike ZnO nanocrystals with different ratio of polar to nonpolar faces were prepared by Choy et al. and Tsang et al. by morphology-controlled synthesis.20,22 However, their results show polar (001) and (001) faces are more active than the nonpolar faces for photocatalytic H2O2 generation and methylene blue decomposition. The effect of crystallite size and particle size on the photocatalytic performance of semiconductor photocatalyst has been attributed to their influence on the charge carrier recombination rate and specific surface area. It was found that ZnO nanoparticles prepared by heat treatment of ZnCO3 show best photocatalytic activity with the particle size of 33 nm over the range 2857 nm.23 However, in order to control the particle size, the ZnO nanostructures were synthesized with Received: April 26, 2011 Revised: June 14, 2011 Published: June 15, 2011 13844
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The Journal of Physical Chemistry C different methods or sintered at different temperatures in these studies, which unavoidably alters the morphology and crystallinity.24,25 Synthesis methods need to be developed to fabricate ZnO nanostructures with various sizes but with similar morphologies and crystallinities to clearly reveal the effect of crystallite size and particle size on the photocatalytic activity of ZnO. Solvothermal (hydrothermal) methods have been widely used for the fabrication of nanostructure materials because of the mild synthesis conditions and the simple control of size and morphology of nanostructure materials by using different solvents. It was reported that crystalline ZnO had different growth habit in different solvents so that ZnO with various morphologies could be synthesized via the solvothermal method.26 The solubility of the precursor, polarity, and saturated vapor pressure of the solvents influence the nucleation and preferential growth of polar ZnO crystals and result in ZnO nanostructure with different morphologies. In this work, instead of using the solvents with great disparity in polarity and saturated vapor pressure, three kinds of simple aliphatic alcohol, methanol, ethanol, and propanol were used as the solvents in the solvothermal synthesis. ZnO nanoparticles with different crystallite and particle sizes could be prepared. The structure and optical properties of ZnO were investigated by powder X-ray diffraction (XRD), nitrogen adsorption isotherms, UVvis diffuse reflectance spectroscopy (DRS), transmission and scanning electron microscopy (TEM), and room temperature photoluminescence. The individual effect of crystallite size and particle size on the photocatalytic activity of ZnO was studied using the particulate ZnO nanostructures with various sizes but with the similar morphologies and crystallinities.
’ EXPERIMENTAL SECTION Materials. Zinc nitrate hexahydrate (98%), tetramethylammonium hydroxide (TMAOH, 25 wt % in water), tetraethylammonium hydroxide (TEAOH, 25 wt % in water), methanol (99.9%), and 1-propanol (99.9%) were obtained from Acros. Ethanol (ACS/USP grade, anhydrous) was purchased from PharmoAAPER. Deionized water was used throughout this study. Synthesis of ZnO. The synthesis was carried out through a solvothermal method. In a typical procedure, zinc nitrate was dissolved in solvent (methanol, ethanol, or propanol) and mixed with the base (TMAOH or TEAOH) from a buret at the rate of 5 cm3 min1. The [base]/[Zn2+] ratio was varied from 5 to 15 by changing the concentrations of zinc nitrate solution accordingly. The reaction mixture was stirred for 2 h, sonicated for 30 min at room temperature, and then transferred into a Teflon-lined autoclave. The autoclave was sealed and maintained at 80 °C for 24 h followed by water cooling to room temperature. The white precipitate was filtered and dried in a static air oven at 80 °C for 12 h. The sample was then calcined at 400 °C for 6 h in static air at a heating rate of 3 °C/min. Structure Characterization. X-ray diffraction (XRD) measurements were performed at room temperature using a Scintag Pad V or Rigaku Ultima IV X-ray diffractometer with Cu KR radiation of wavelength 1.540 806 Å. The diffractometer was operated at 40 kV and 44 mA and scanned with a step size of 0.02° at a scan speed of 1°/min in the range of 2θ = 24°75°. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 instrument operating at 120 kV. For the sample preparation for the TEM study, a suspension of ZnO in ethanol was prepared, and the suspension was sonicated for 30 min. One drop of the suspension was placed on the copper
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grid coated containing a carbon film and allowed to dry overnight. SEM images were typically recorded at 20 kV and 2 106 Torr using the FEI 450 instrument. The powders were gently smeared on the double-coated tape fixed on top of the aluminum sample holder. Excess particles that did not adhere to the tape were removed by tapping. Nitrogen adsorption measurements were carried out at 77 K using a Quantachrome Nova 2200e gas adsorption analyzer. The samples were outgassed for at least 1 h at 100 °C prior to the adsorption measurements. The surface area of the ZnO nanocrystallites were calculated by applying the BrunauerEmmettTeller (BET) equation in the relative pressure (P/P0) range 0.050.30. The UVvis diffuse reflectance spectra were recorded by Cary 100 Bio UVvis spectrophotometer with praying mantis diffuse reflection accessory (Harrick Scientific). The room temperature fluorescence emission measurements were performed using a Spex FluoroMax fluorometer. The excitation wavelength used was 325 nm, and the emission was monitored in the range 350650 nm. Photocatalytic Degradation of Rhodamine B. The light source used was a 500 W xenon lamp, Newport Inc. To ensure illumination by only visible light, a cutoff filter was used to completely eliminate any radiation at wavelength below 420 nm. 20 mL of Rhodamine B (RhB) solution and 10 mg of ZnO catalyst powders were placed in a double-walled Pyrex vessel. Water was circulated to the Pyrex vessel in order to maintain a constant temperature of 25 ( 2 °C during the photocatalytic reactions. Prior to irradiation, the suspensions were magnetically stirred in the dark for ca. 30 min to ensure the establishment of an adsorption/desorption equilibrium. At a given time, 3 mL aliquots were collected and centrifuged to remove the solid catalyst particles. The concentration of the filtrate was analyzed using a UVvis spectrophotometer (Cary 100 Bio). The degradation of RhB was followed by monitoring its disappearance at 544 nm. The photodegradation rate was calculated assuming pseudo-firstorder kinetics, and the rate constant (k) was calculated from the first-order plot using the equation ln(C0/C) = kt, where C0 is the initial concentration of RhB and C is the concentration of RhB at any given time t. The concentrations of RhB were determined from the calibration plot made previously.
’ RESULTS AND DISCUSSION XRD patterns of the ZnO nanostructures synthesized with TMAOH and TEAOH (hydrolysis ratio of 5) are shown in Figure 1A,B. All the diffraction peaks can be indexed as the hexagonal wurtzite ZnO (JCPDS No. 36-1451), and no other crystallite phases were observed for any of these samples at these hydrolysis ratios. For the ZnO synthesized with TMAOH, the average crystallite size estimated from the broadening of the diffraction peaks by Scherrer formula was about 4.8 nm when methanol was used as solvent and 4.1 nm when ethanol was used as solvent (entries 1 and 2 in Table 1). However, the average crystallite size increased to 14.7 nm when propanol was used as solvent (entry 3 in Table 1). In case of TEAOH as the base, the average crystallite sizes were found to increase from 13.5 nm, to 17.5 nm, and to 31.4 nm when methanol, ethanol, and propanol were used as solvents (entries 46 in Table 1). This clearly indicates that the crystallite sizes could be controlled well by merely changing the solvent in the solvothermal synthesis. The “solvent effect” may be partially understood by comparing the dielectric constants (ε) of water, methanol, ethanol, and propanol, which are 80, 32, 25, and 21, respectively, at 20 °C. In 13845
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Figure 1. XRD patterns: (A) (a) ZnO-5M-TMAOH, (b) ZnO-5E-TMAOH, and (c) ZnO-5P-TMAOH; (B) (d) ZnO-5M-TEAOH, (e) ZnO-5ETEAOH, and (f) ZnO-5P-TEAOH. M, E, and P [base]/[Zn2+] ratio, i.e., 10 and 15. M refers to methanol, ethanol, and 1-proponal used as solvents; TMAOH and TEAOH refer to tetramethylammonium hydroxide and tetraethylammonium hydroxide employed as base.
Table 1. Summary of Physicochemical Properties of ZnO material
solvent
base
crystallite sizea (nm)
surface areab (m2 g1)
average particle diameterc (nm)
band-gap energyd (eV)
ZnO-M-TMAOH ZnO-E-TMAOH
methanol ethanol
TMAOH TMAOH
4.8 4.1
56.0 35.2
19.8 30.4
3.29 3.23
ZnO-P-TMAOH
propanol
TMAOH
14.7
17.4
61.7
3.15
ZnO-M-TEAOH
methanol
TEAOH
13.5
16.5
64.9
3.18
ZnO-E-TEAOH
ethanol
TEAOH
17.5
14.3
74.9
3.18
ZnO-P-TEAOH
propanol
TEAOH
31.4
12.0
89.6
3.18
a
Estimated from broadening of the diffraction peaks by Scherrer formula. b Determined by applying BrunauerEmmettTeller (BET) equation to a relative pressure (P/P0) range of 0.050.35 in the adsorption isotherm. c Calculated by using the equation particle diameter D = 6/Ssp Fa. d Estimated from the UVvis absorption spectra using the KubelkaMunk function.
general, a solvent with lower dielectric constant leads to larger crystallite or particle sizes, with the exception of ZnO prepared using ethanol and TMAOH. Solvents play an important role in the growth behavior of nanostructured oxides. Solvents with lower dielectric constants tend to induce faster and uncontrolled precipitation kinetics. We postulate that use of solvents with lower dielectric constant leads to supersaturation of the Zn2+ ion due to the lower solubility of the zinc salts. This provides the driving force for the nucleation and growth of ZnO nanoparticles, i.e., a shortened nucleation time and higher solid particle growth. Also, the dielectric constant of the medium can affect the interparticle forces and hence the growth and size of the resulting particles. Other important factors that could contribute to differences in the crystallite and particle size are coordination sphere, bonding, size of the solvation sphere, and viscosity. However, a detailed study needs to be conducted to examine the role of alcohols in modulating the crystallite or particle size and is worthy of a separate investigation and thus beyond the purview of this work. We also attempted to tune the crystallite sizes by changing the ratio of [TMAOH]/[Zn2+] in the solvothermal synthesis. The XRD patterns and physicochemical properties of ZnO samples synthesized with [TMAOH]/[Zn2+] ratios of 5, 10, and 15 are shown in Figure 1S and Table 1S (Supporting Information). These three samples were found to have similar crystallite sizes of
34 nm. New diffraction peaks (that could not be indexed confidently despite our best efforts) located at 27.0° and 28.6° were observed for the sample synthesized with [TMAOH]/[Zn2+] ratio of 15, indicating the formation of unknown impurity phase(s). Additionally, we found that the yields of ZnO progressively decreased with increase in ratio of [base]/[Zn2+]. We also attempted to prepare ZnO at low [base]/[Zn2+] ratios of 1 and 2. However, pure ZnO phase was not obtained. This is indicated from the following equations: Zn2+ + 4OH f ZnO2 2 + 2H2 O ZnO2
2
+ H2 O f ZnO + 2OH
ðiÞ ðiiÞ
It can be inferred that ratios of [OH]/[Zn2+] of at least four are required for the formation of ZnO. Our powder XRD’s of materials synthesized at ratios of [OH]/[Zn2+] = 1 indicate the presence of not only ZnO but also additional phases due to zinc tetrahydroxide bis(nitrate) and zinc nitrate dihydrate. Overall, the XRD results suggest that highly crystalline and pure ZnO nanomaterials could be obtained when the ratio of [OH]/[Zn2+] was kept at 5 and 10. However, we decided to pursue the photocatalytic activity studies primarily with ZnO prepared using ratio of [OH]/[Zn2+] = 5 since higher yields (>90%) of ZnO could be obtained. 13846
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Figure 2. TEM images of ZnO-10M-TMAOH. The left panel shows the TEM image (the bar scale corresponds to 50 nm). The right panel shows the high-resolution TEM image (the bar scale corresponds to 5 nm and the distance between the fringes is 0.25 nm as indicated by the mark).
Figure 4. Nitrogen adsorptiondesorption isotherm of ZnO-5MTMAOH. The triangles represent adsorption isotherm whereas the squares represent the desorption isotherm. The inset shows the pore size distribution.
Figure 3. SEM image of ZnO-5M-TMAOH.
Further morphologic and structural characterization of the ZnO nanostructures was performed by transmission and scanning electron microscopy. Figure 2 shows the TEM micrographs of ZnO-10M-TMAOH as an example. It is found that single crystalline ZnO nanoparticles were obtained via the solvothermal synthesis when simple alcohols—methanol, ethanol, and propanol—were used as solvents. The primary ZnO nanoparticles are inclined to agglomerate as seen in Figure 2A. The high-resolution TEM shown in Figure 2B show 2D lattice fringes and indicate the high crystallinity of the ZnO nanoparticles, and the distance between two fringes was estimated to be 0.25 nm consistent with (d101) reflection of ZnO (JCPDS 36-1451). A representative scanning electron microscopic image of ZnO synthesized with TMAOH as base ([OH]/[Zn2+] = 5) and methanol as solvent is shown in Figure 3. The SEM image indicates that the primary ZnO nanoparticles aggregate to form larger secondary particles that vary in sizes and shapes significantly. According to Figure 3, the aggregates vary in sizes from less than 1 μm to as large as 60 μm with no uniform morphologies noticed. Our SEM studies with other ZnO nanoparticles also indicate no predisposition toward the formation of any distinct morphologies and significant aggregation of the primary ZnO nanocrystallites to form secondary aggregates that range from 1 to 60 μm. Nitrogen physisorption studies of ZnO nanoparticles were also conducted to determine the textural properties (specific
surface areas, pore volumes, and pore diameters) of the ZnO nanopowders. A type IV isotherm according to the IUPAC classification with H3 hysteresis was observed for all the samples, which is typical of mesoporous materials. Nitrogen adsorptiondesorption isotherm of ZnO synthesized with TMAOH as base ([OH]/[Zn2+] = 5) and methanol as solvent is shown in Figure 4 as an example. Monolayer and multilayer adsorption occurs in the low pressure range with relative pressure (P/P0) lower than 0.5. The further increase of pressure led to a sharp increase in the volume of nitrogen adsorbed due to its condensation within the pores. Hysteresis occurs when the pore filling and the pore emptying mechanisms are different due to pore connectivity and network effects. The specific surface area of this sample was calculated to be 56 m2 g1 by the BET equation. The inset of Figure 4 shows the pore size distribution estimated using the BarrettJoynerHalenda (BJH) equation to the desorption isotherm. Most of the pores are in the range 15100 nm with a maximum at 26 nm, which suggest that the pores are fairly broad and the ZnO nanostructures possess pores hierarchical set of pores, in both meso- and macropore range. The particle sizes of the ZnO nanomaterials were determined by using the equation particle diameter D = 6/Ssp Fa, where Ssp is the specific surface area and Fa is the density of ZnO = 5.6 g/mL. This method of calculation of average particles sizes from surface area measurements does not indicate the variability in particle size distribution and thus should be used only as a rough guide for comparison purposes and in understanding relative trends within a set of samples. The average particle diameter (BET particle size) calculated from the surface area measurements and the particle sizes from TEM studies were found to be consistent. The results of surface area and the average particle size of ZnO nanostructures are listed in Table 1. The BET particle size was significantly larger than the crystallite size estimated from XRD due to the aggregation of the ZnO nanocrystallites to form primary particles. It is found that the specific surface area and hence the average particle diameter of ZnO varied when different solvents were used. The crystallite sizes calculated from powder XRD and the particle sizes calculated from the physisorption experiments 13847
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Figure 5. Diffuse reflectance spectra: (A) (a) ZnO-M-TMAOH, (b) ZnO-E-TMAOH, and (c) ZnO-P-TMAOH; (B) (d) ZnO-M-TEAOH, (e) ZnOE-TEAOH, and (f) ZnO-P-TEAOH. The inset shows the plots of transformed KubelkaMunk function versus the wavelength.
Figure 6. Room temperature photoluminescence spectra: (A) (a) ZnO-M-TMAOH, (b) ZnO-E-TMAOH, and (c) ZnO-P-TMAOH; (B) (d) ZnOM-TEAOH, (e) ZnO-E-TEAOH, and (f) ZnO-P-TEAOH.
show a good correlation for the ZnO synthesized with both TMAOH and TEAOH, in the fact that the larger particles are composed of larger crystallites. ZnO synthesized with TMAOH and methanol have a crystallite size of 4.8 nm and average particle size of 19.8 nm, while the sample synthesized with the same base and propanol shows a much larger crystallite size of 14.7 nm and much larger average particle size of 61.7 nm. The UVvis absorption spectra of ZnO nanostructures are shown in Figure 5. ZnO samples show a strong absorption in the ultraviolet region less than 400 nm. The band gap energies were estimated from the plots of the transformed KubelkaMunk function versus the wavelength and listed in Table 1. The three ZnO samples synthesized with TEAOH and the ZnO synthesized with TMAOH and propanol have an optical band gap of about 3.2 eV (3.18 and 3.15 eV, respectively), which is consistent with the value reported for ZnO material. The ZnO samples synthesized with TMAOH and methanol or ethanol with crystallite
size of 4.8 and 4.1 nm show a slightly larger band gap due to quantum size effect due to their relatively smaller crystallite sizes. Information on the crystal quality and structure defects of ZnO nanostructures can be revealed by photoluminescence (PL) studies. The room temperature photoluminescence emission spectra of ZnO synthesized with TMAOH and TEAOH are shown in parts A and B of Figure 6, respectively. The ZnO samples displayed a broad absorption in both the UV and visible region ranging from 370 to 600 nm. The UV emission at 390 nm is the characteristic near-band-edge emission due to the recombination of free photogenerated electrons and holes. The shoulders and weak peaks in the range 420480 nm are attributed to the band-edge free excitons and bound excitons. Thus, the luminescence in the visible range of ZnO is due to various intrinsic and extrinsic structure defects. It has to be noted that although the UVvis spectrophotometry studies indicate shift in the onset of absorption, the photoluminescence studies do not clearly indicate any 13848
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Figure 7. Photocatalytic degradation kinetics of RhB (left) and rate constants (right) with ZnO synthesized with different solvents.
shift due to the presence of several peaks arising due to the intrinsic and extrinsic defects in ZnO. It is reported that the yellow emission is associated with excess oxygen, while the green emission (obtained at ∼556 nm in this study) is associated with oxygen vacancies and other vacancy-related defects.17,2628 The three ZnO samples synthesized with TEAOH were found to show higher UV emission (indicated by the steep increase, see Figure 6B in comparison with Figure 6A) than the samples synthesized with TMAOH, which is an indicative of higher crystallinity of these samples.29 As an advanced oxidation processes, semiconductor photocatalysis has been extensively studied for the degradation of toxic pollutants in the environment. Industrial dye pollutants have caused significant environmental problems globally.30,31 The degradation and mineralization of dyes using nanostructured semiconductor under visible light has attracted great interest in recent years.32 The mechanism of photocatalytic degradation of dye molecules under visible light excitation is different from the traditional mechanism under UV irradiation. In these systems, the dye, rather than the semiconductor, is subject to the visible light excitation. The excited dye molecule then transfers an electron into the conduction band of semiconductors like TiO2 or ZnO, leading to the formation of a cationic radical of the dye. The injected electron then reacts with the oxygen adsorbed on the surface of photocatalyst nanoparticle and generates a series of reactive oxygen species (ROS) such as O2•, •OH, OH, and H2O2. The subsequent radical chain reactions involving ROS and the dye molecule lead to the degradation and mineralization of the dye pollutants.3336 In this work, the activities of ZnO samples synthesized with different solvents were evaluated for the photocatalytic degradation of Rhodamine B dye (RhB) under visible irradiation. No significant decrease in the concentration was observed with ZnO in the dark, indicating negligible adsorption and in the absence of ZnO under visible light illumination. Figure 7A,B compares the kinetics of photodegradation of dye RhB in the presence of ZnO photocatalysts. Under visible irradiation, all the systems exhibited notable degradation of RhB. The concentration of RhB tends to decrease with irradiation time, and the kinetics were fitted by the pseudo-first-order process. The decay rate constants obtained were 0.210 ( 0.01, 0.154 ( 0.004, and 0.148 ( 0.008 h1 for
ZnO synthesized with methanol, ethanol, and propanol as solvent and TMAOH as base. It is found that the ZnO, which was synthesized with methanol, has smaller crystallite size and particle size and shows higher photocatalytic activity compared with the other two samples. As mentioned above, the three ZnO samples were synthesized under the same experimental conditions excepting under different solvents. Similar morphology and crystallinity were observed for all of them by TEM and photoluminescence studies. Thus, the results clearly reveal the individual effect of crystallite size and particle size on the photocatalytic activity of ZnO since the morphologies (shapes) and crystallinities are similar in the samples prepared using TMAOH. The higher activity of ZnO-M-TMAOH is clearly attributed to crystallite or particle size effects. It is found that smaller particle size rather than the crystallite size favor higher photocatalytic activity when the rate constants of ZnO-M-TMAOH and ZnOE-TMAOH were compared. In addition, the rate constant only decreases marginally when the particle size increases from ∼30 nm for the sample ZnO-E-TMAOH to ∼62 nm for ZnOP-TMAOH. The higher photocatalytic activity of smaller ZnO nanoparticles can be explained by the larger surface area and more positive conduction band, which facilitate the injection of electron from the excited dye molecule to ZnO conduction band and the formation of active oxygen species. The same activity trend (methanol > ethanol > 1-propanol) was found for the three ZnO samples synthesized with TEAOH. The decay rate constants obtained were 0.259 ( 0.005, 0.174 ( 0.003, and 0.150 ( 0.006 h1 for ZnO synthesized with methanol, ethanol, and propanol as solvent. In addition, the photocatalytic activity of ZnO synthesized with the same solvent but two different kinds of base are also compared in Figure 7B. The three ZnO samples synthesized with TEAOH show larger crystallite and particle size (Table 1) but higher activity than the corresponding samples synthesized with TMAOH. The better performance of ZnO samples synthesized with TEAOH could be ascribed to their higher crystallinities as indicated by the PL results, which reduced the capture of electrons by the defect sites and resulted in the high efficiency of electron transfer in the bulk of nanoparticles. In comparing the photocatalytic activity of samples prepared using TMAOH with TEAOH, it is clear that the crystallinity seems to be the overriding factor compared to 13849
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The Journal of Physical Chemistry C the surface area (and hence particle size). ZnO samples prepared with TEAOH are more crystalline (larger crystallite and particles sizes and hence smaller surface areas) and exhibit higher photocatalytic activity for the degradation of RhB in general compared with samples prepared using TMAOH as the base. However, when the particle size or the crystallite sizes increase beyond a certain size, the photocatalytic activity tends to decrease and their activity approaches the activity of samples prepared using TMAOH. The resulting photocatalytic activity is a balance of these competing factors.
’ CONCLUSIONS ZnO nanocrystalline were synthesized by solvothermal approach. XRD, TEM, SEM, and nitrogen adsorption isotherms demonstrated that the crystallite size and particle size of ZnO nanocrystalline were successfully controlled by using methanol, ethanol, or propanol as solvent in the solvothermal synthesis. All the ZnO showed significant activity for the photocatalytic degradation of dye RhB under visible irradiation. The individual effect of crystallite size and particle size on the photocatalytic activity was revealed using the ZnO nanocrystalline prepared in this work, which have the same morphology and crystallinity. The ZnO sample with smaller crystallite size and particle size showed higher photocatalytic activity among a subset of samples. When the ZnO samples synthesized with the same solvent but different bases were compared, it is found that crystallinity is the most important factor that would influence the photocatalytic performance. In general, ZnO nanocrystalline with higher crystallinity exhibited larger rate constants for the degradation of RhB. ’ ASSOCIATED CONTENT
bS
Supporting Information. XRD patterns and physicochemical properties of ZnO samples synthesized with [TMAOH]/ [Zn2+] ratios of 5, 10, and 15 (Figure 1S and Table 1S). This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 1-605-677-6189. Fax: 1-605-677-6397. E-mail: zhaodaniccas@ yahoo.cn (D.Z.),
[email protected] (R.T.K.).
’ ACKNOWLEDGMENT This work was supported by NSF-CHE-0722632, NSF-CHE0532242, NSF-CHE-0840507, NSF-EPS-0903804, State of SD supported 2010 Center-CRDLM, and DOE-DE-EE0000270. We are thankful to Ms. S. Chadima for help with XRD, Dr. C. Lin for TEM, and Mr. T. S. Remund for SEM studies. ’ REFERENCES
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dx.doi.org/10.1021/jp2038653 |J. Phys. Chem. C 2011, 115, 13844–13850
