Transformation Process and Photocatalytic Activities of Hydrothermally

Feb 28, 2008 - ... phase transformation probably evolved via a “dissolution−recrystallization” mechanism and accompanying the “Ostwald ripenin...
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J. Phys. Chem. C 2008, 112, 4159-4167

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Transformation Process and Photocatalytic Activities of Hydrothermally Synthesized Zn2SnO4 Nanocrystals Jia Zeng,† MuDi Xin,† KunWei Li,† Hao Wang,*,† Hui Yan,† and WenJun Zhang‡ The College of Materials Science and Engineering, Beijing UniVersity of Technology, Beijing, 100022, P.R. China, and Department of Physics and Materials Science, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong ReceiVed: December 2, 2007; In Final Form: January 16, 2008

In this study, a systematic approach was applied to the hydrothermal synthesis of Zn2SnO4 (ZTO) nanocrystals to gain insight into the fundamental factors controlling phase composition, particle size, crystal morphology and photocatalytic activity. The influence of various operating conditions, such as reaction temperature, alkaline concentration, duration time, and additive surfactants on the treatment process were investigated. By combining the results of X-ray diffraction (XRD), electron microscopy (SEM/TEM/ED/HRTEM), Raman and FT-IR spectroscopy, a complete structural and morphological characterization of the products was performed. The results indicated that the phase transformation probably evolved via a “dissolution-recrystallization” mechanism and accompanying the “Ostwald ripening” process. Furthermore, a correlation between the photocatalytic activity in the UV photodegradation of MB solutions and the particle properties was established.

1. Introduction Zn2SnO4 (ZTO) is an important semiconducting material with a typical inverse spinel structure (space group Fd3m) and a band gap of 3.6 eV.1,2 Because of its high electron mobility, high electrical conductivity, and low visible absorption, ZTO has promising applications in photoelectrical devices, chemical sensors, functional coatings, and transparent conducting electrodes.3-8 The application of ZTO as anode in Li-ion batteries,9-11 as photocatalysts for degradating organic pollutants in aqueous solutions,12,13 and as working electrodes in dye sensitized solar cells (DSSC) has also been demonstrated.14,15 The great application potentials of ZTO have stimulated broad research interests on the synthesis.5,7,9-21 ZTO is usually synthesized by a high-temperature solid-state reaction between powdery ZnO and SnO2. To avoid evaporation of ZnO during synthesis, different heat-treatment programs have been proposed.17 ZTO was also prepared through a thermal evaporation method by heating metal or metal oxide powder at high temperatures.19-21 Moreover, coprecipitation of Zn and Sn hydroxides with alkali from an aqueous solution was used, and then the precipitate was converted to ZTO after drying and calcination in air.12,16 The first work of hydrothermal synthesis of ZTO was proposed by Fang et al.,18 which reported several experimental parameters for the effect of phase formation. An outstanding advantage of such a hydrothermal synthesis is that the reaction temperature required to produce crystalline ZTO is much lower than those in other methods. Recently, ZTO was synthesized by the reaction of inorganic salt with concentrate aqueous sodium hydroxide (NaOH), ammonia (NH3‚H2O), or hydrazine hydrate (N2H4‚H2O) solution under hydrothermal conditions, yielding cube-shaped, spherical, or rodlike ZTO nanocrystals.10,11,13-15 * Corresponding author. Tel: 86-10-67392733. Fax: 86-10-67392445. E-mail address: [email protected]. † Beijing University of Technology. ‡ City University of Hong Kong.

In the past decade, solution synthesis has been demonstrated to be an effective approach to achieve a variety of nanostructures including zero-dimensional nanoparticles/quantum dots, onedimensional nanorods/nanowires/nanotubes, two-dimensional nanoplates/nanosheets, and three-dimensional nanocubes/ nanocages.22-28 In the reaction, the chemicophysical parameters of the system, such as temperature, concentration of alkaline, reaction duration, and surfactants can be varied easily to control the kinetics and thermodynamics in the nucleation and growth of nanocrystals.29 Although the hydrothermal syntheses of ZTO nanostructuresinvolvingalkalinesolutionhasbeenreported,10,11,13-15,18 the attention was focused on the final products when the reaction reached equilibrium, and the detailed evolution of these nanostructures during the reaction remains unexplored. Consequently, the reaction kinetics cannot be deliberately controlled to achieve delicate new structures with desired morphologies. Although crystal formation in such solution-based synthesis is chaotic, a few works were proposed to rationalize the mechanism of such processes. This urges us to understand the “magic” type of synthesis and to elucidate the corresponding formation mechanism. In this work, we investigate the structure and morphology evolution of ZTO nanostructures during the reaction in aqueous sodium hydroxide solution under hydrothermal conditions. The synthesis is relatively simple, in terms of both source composition and procedures, which thus enables us to elucidate the conversion mechanism of the process. By monitoring the structure evolution during the reaction process with comprehensive techniques including electron microscopy (TEM/SEM), X-ray diffraction (XRD), Raman spectroscopy, and FT-IR spectroscopy, we proposed the formation mechanism of ZTO nanostructures in the hydrothermal reactions. Furthermore, it is known that the phase composition, particle size, and morphology play crucial roles in determining the efficiency in their photocatalytic properties. Therefore, another aim of this work is to investigate the evolution of photocatalytic activity of ZTO nanocrystals because the direct solar-to-chemical conversion/

10.1021/jp7113797 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008

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Figure 1. TEM images of the various products prepared by hydrothermal treatment at 200 °C for 20 h with CA ) 0 M (a), CA ) 1 M (b), and CA ) 2 M (c).

degradation of organic pollutants by semiconductor-based catalysts is an important issue in environmental science.30-38 2. Experiments 2.1. Materials Preparation. All reagents are commercially available in analytical grade, and further purification is required. ZnSO4‚6H2O (10 mM) and SnCl4‚5H2O (5 mM) were put into a beaker and then dissolved by deionized water. Acting as a mineralizer, NaOH solution was dropped to the above solution to form a white precipitation mixture. After 15 min of magnetic stirring, the mixture was transferred into a Teflon-lined stainlesssteel autoclave (50 mL capacity), which was filled with the mixture to 80% of the total volume, and was kept at a temperature between 120 and 250 °C for a designed period of time in an electric oven. Then the autoclave was cooled to ambient temperature naturally. The resulting precipitates were collected by centrifugation at 3000 rpm for several times, washed with deionized water and ethanol thoroughly, and dried at 80 °C in the oven for 5 h before further characterization. 2.2. Materials Characterization. X-ray Diffraction. Crystallographic phases and purity information of the prepared samples were investigated by X-ray diffraction at room temperature with a Bruker AXS D8 ADVANCE diffractometer using Cu Ka1 radiation (λ ) 1.541 Å). The accelerating voltage, emission current, and scanning speed were 40 kV, 40 mA, and 0.2 deg/ s, respectively. Raman and FT-IR Spectroscopy. Raman spectra were obtained on a Renishaw Invia Raman spectrometer with a solidstate laser (excitation at 785 nm) at room temperature. The laser power was kept low enough to avoid heating of the samples by optical filtering and/or defocusing the laser beam at the sample surface. FT-IR spectra were recorded on a Perkin-Elmer Spectrum One FT-IR machine by using KBr for diluting (sample: KBr ) 0.5:100). UV-Visible Spectroscopy. Shimadzu UV-3101PC ultravioletvisible-near-infrared (UV-vis-NIR) spectrophotometer was used to investigate the optical absorption of powder and solution. UV-visible absorption spectra of powder samples were recorded using pure BaSO4 pellet as the reference. BET Measurement. The specific surface areas of the powders were determined by using a Micromeritics ASAP 2020 specific surface area and porosity analyzer in the method of BrunauerEmmett-Teller (BET) nitrogen adsorption and desorption. Electron Microscopy. The size and morphology of solid powder samples of precursor and resulting ZTO nanocrystals were characterized using a SEM (Hitachi SEM S-3500N). TEM and SAED images were taken at an accelerating voltage of 120 kV on a Philips Tecnai-12 instrument. HRTEM images were measured with a CM200 FEG high-resolution transmission electron microscope operating at 200 kV. Specimens for the

TEM studies were prepared by depositing a drop of these aqueous suspension samples onto a carbon-coated, holey film supported on a copper grid for the above measurements. Prior to deposition, solutions containing samples of were sonicated in ethanol for 15 min to ensure adequate dispersion in solution. 2.3. Photocatalytic Studies. In a typical experiment, 0.1 g of sample was dispersed in 100 mL of MB solutions. Prior to irradiation, the suspensions were stirred magnetically in a dark condition for 15 min to establish adsorption/desorption equilibrium. The suspensions were then irradiated under a UV lamp at a 12 cm separation distance. A 400 W high-pressure Hg lamp (Institute of Electric Light Source, Beijing) with a maximum emission at about 365 nm was used as the light source. Analogous control experiments were performed either without ZTO (blank) or with commercial Degassa P-25 TiO2. Degradation was monitored by taking aliquots at given irradiation time intervals. The suspension including the photocatalyst and MB were centrifuged, and the absorption spectra of the samples were recorded by measuring the absorbance at 663 nm with the UVvisible (Shimadzu UV-vis 3010) spectrometer. The concentrations of MB were calculated using the Beer-Lambert law A ) ecl, where A is the absorbance at λmax of MB, e is the molar absorptivity of MB (79.5l × 103 L/mol cm),37 and l is the sample cell length (1 cm). MB degradation was expressed as C/C0 versus UV-irradiation time, where C0 was the initial concentration. 3. Results and Discussion Several decadal samples corresponding to different experimental conditions (CA: alkaline concentration; TR: reaction temperature; TD: duration time; SA: additive surfactants) were synthesized and fully characterized. Hydrothermal treatments were mainly carried out at 120 and 250 °C from 0 to 20 h. Some experiments were also performed at lower temperature or for a shorter time. Some preparations were repeated two or three times to evaluate the reproducibility of the process. Otherwise, the gist of these discussion in the results reported of the current work has a twofold objective: (i) to provide a rationale for explaining the evolution of phase composition, particle size, and crystal morphology during the hydrothermal synthesis of ZTO as a function of TR, CA, TD, and SA, and (ii) to understand the correlations between powder properties and photocatalytic activities. 3.1. Powder Characterization by XRD/SEM/TEM/SAED/ Raman and FT-IR Spectroscopy. 3.1.1. Effect of Alkaline Concentration (CA). Figure 1 provides a series of TEM images that summarize the results of the optimum alkaline concentration (CA) for the synthesis of ZTO crystals. As can be seen clearly from these TEM images of the hydrothermal reaction products, after 20 h of treatment at 200 °C, three kinds of materials were

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Figure 2. X-ray diffraction (XRD) patterns of the as-prepared samples. From the bottom: trace a is for the SnO2 in Figure 1a, trace b is for the ZTO in Figure 1b, and trace c is for ZnO in Figure 1c.

formed in the presence of different alkaline concentrations. We also observed that the effect of phase structure of these products with increasing CA as illustrated by the XRD patterns (Figure 2). According to the XRD traces, when no NaOH solutions were used, tetragonal-phase SnO2 (JCPDS 72-1147, Figure 2a) was obtained; while pure-phase ZTO with an inverse spinel structure (JCPDS 24-1470, Figure 2b) could be obtained using a 1 M NaOH solution. However, when the concentration of NaOH was increased to 2 M, all of the diffraction peaks (as shown in Figure 2c) were attributed to hexagonal-phase ZnO (JCPDS 79-2205). It is well known that the mineralizers such as MOH (M ) K, Na, etc.) play a fundamental function in the hydrothermal process.29,32,33 In our experiments, at the beginning of experimental stage, ZnSO4 and SnCl4 were dissolved and ionized in water. When there was no NaOH in the solution, owing to the stronger hydrolysis effect of Sn4+ ions, eq 1 occurred (pH ≈ 1) and resulted in the formation of colloidal H2SnO3; then, these reactants were sealed in the autoclave and heated, SnO2 deposits (Figure 1a) were formed according to eq 2, and Zn2+ ions were still kept in the solution and washed away after reaction. Furthermore, OH- would be counteracted thoroughly by H+ via eq 3. Sn4+ + 3H2O T H2SnO3 + 4H+

(1)

H2SnO3 f SnO2 V +H2O

(2)

H+ + OH- f H2O

(3)

When the NaOH concentration came up to 1 M, in the appropriate alkaline conditions, ZnSn(OH)6 phase will produce first (shown in eq 4), and the additional Zn2+ will react with OH- and produce Zn(OH)42- phase. With the increase of temperature and duration time, Zn2SnO4 phase will produce via eq 6, which is due to the fact that the solubility of ZTO was lower than that of other compounds under this condition. Zn2+ + Sn4+ + 6OH- f ZnSn(OH)6 V

(4)

Zn2+ + 4OH- f Zn(OH)42-

(5)

ZnSn(OH)6 + Zn(OH)42- f Zn2SnO4 V + 4H2O + 2OH- (6) With further increasing concentrations of NaOH, eq 4 is restrained. It can be confirmed by the experimental results that

Figure 3. TEM images of the products of hydrothermal reaction at various temperatures with 1 M NaOH for 20 h. Image a is the mixture products prepared by the hydrothermal treatment at 180 °C, images b, c, and d are the ZTO product prepared at 200, 220, and 250 °C, respectively.

Figure 4. XRD patterns of temperature series products synthesized for 20 h with the same alkaline concentration (1 M) at 120 °C (a), 160 °C (b), 180 °C (c), 200 °C (d), 220 °C (e), and 250 °C (f).

ZnO precipitates (Figure 2c) can be found via eq 7 when the concentration of NaOH attained is 2.0 M. In the meantime, Sn species were washed away after hydrothermal reaction according to eq 8. Zn(OH)42- f ZnO V +H2O + 2OH-

(7)

Sn4+ + 6OH- f Sn(OH)62-

(8)

Otherwise, when the alkaline content was too high, Zn2+ and Sn4+ would react with OH- and produce Zn(OH)42- and Sn(OH)62- phase, respectively. Thereby, there is no precipitate in the Teflon-lined stainless-steel autoclave. 3.1.2. Effect of Reaction Temperature (TR). The temperature effect on the crystalline phase was demonstrated through the changes of the TEM images in Figure 3 and XRD patterns in Figure 4. From 120 to 160 °C, all crystal diffraction peaks were ascribed to ZnSn(OH)6. By increasing the reaction temperature to 180 °C, the mixture phase of cubic ZnSn(OH)6 and ZTO particles were obtained (as shown in Figure 3a and Figure 4c).

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Figure 5. XRD patterns of phase transition process from the hydroxide precursor to ZTO for different duration times (temp: 200 °C; CNaOH: 1 M).

At the temperature extent from 200 to 250 °C, the relative lower Th (200 °C) results in well-defined ZTO nanocrystals with a size of around 150 nm (Figure 3b). When Th is increased to 250 °C, the size of ZTO nanoparticles decreased to several tens of nanometers. As we know, if the rate of crystal nucleation is greater than that of crystal growth, then the crystal size will be small.29 Therefore, at relatively high temperatures, this condition is presumably more favorable to particle nucleation than growth, resulting in smaller particle size of ZTO crystals. Scherrer line width analysis of all of the shown peaks yields particle diameters that is consistent with the TEM images. 3.1.3. Effect of Duration Time (TD). To further study the kinetics formation of ZTO, through the time-dependent experiments, we could clearly observe the evolution of structure and morphology of the products that formed as a function of reaction temperature at 200 °C. Figure 5 shows the XRD patterns of the as-prepared samples at different TD values. The resultant structures of short time hydrothermal treatment appear to be intermediates of an incomplete phase conversion reaction. In these intermediate products, ZnSn(OH)6 coexists with ZTO but in different configurations. When the reaction time reached 2 h, the XRD pattern showed only ZnSn(OH)6 phase. After 4 h, the ZnSn(OH)6 was less crystallized than the one with 2 h reaction time, and some weak diffraction peak of ZTO appeared. When the reaction time increase to more than 6 h, all of the ZTO crystal phases were very clear. It was obvious that the intensity of ZTO peaks increased with TD. The optimum XRD pattern was obtained for 20 h hydrothermal treatment. In the XRD patterns with different reaction stages, we could know the crystallization state of the prepared ZTO, which is a good assistance to observe the growing process of the ZTO nanocrystals. Figure 6 shows the SEM and TEM observation of different TD values, respectively. The coexistence of ZTO microcrystals and ZnSn(OH)6 cubes in the intermediate resembles a mechanically blended mixture. In contrast, within the intermediate phase transition to ZTO, small cubic ZnSn(OH)6 with a size about 200 nm over the outer surface of ZTO microcrystal are observed (shown in Figure 6a-c). When the TD reached 12 h, the small particles began to grow up, while increasing the TD to 20 h, the particle size also grew gradually with the size of about 100150 nm (Figure 7e ). Selected area electron diffraction (SAED) patterns (Figure 6d and e) can be indexed to the reflection of ZTO polycrystalline in structure. Furthermore, Raman spectra of samples synthesized by the different duration time excited by a red laser (785 nm) are shown in Figure 7A, while the IR spectrum of the series powders diluted with KBr are presented in Figure 7B. As shown in Figure

Figure 6. SEM patterns and TEM images of time series (temp: 200 °C; CNaOH: 1 M) products from the precursor by the phase conversion reactions of (a) 2 h, (b) 4 h, (c) 5 h, (d) 12 h, and (e) 20 h.

7, the shift of Raman and FT-IR spectra bands changed remarkably with the TD extending. The four Raman modes at 297, 374, 438, and 608 cm-1 for ZnSn(OH)6 arise from the breathing vibration of the long M-OH bonds and M-OH-M (bridging OH group) bending modes. With the increase of TD, the above four Raman modes were slowly shifted to two new peaks at 538 and 677 cm-1. The broad peak at 677 cm-1 corresponds to the typical Raman shift of ZTO and is assigned to stretching vibrations of short M-O bonds in the MO6 octahedron that stick out into the structure spaces. The peak at 538 cm-1 is associated with internal vibrations of oxygen tetrahedron. Moreover, in the FT-IR spectra as shown in Figure 7B, no change of absorption peaks at 1626 and 3400 cm-1 are ascribed to the vibration of absorptive water, while the

Hydrothermally Synthesized Zn2SnO4 Nanocrystals

Figure 7. Raman spectra (A) and FT-IR spectra (B) of the resultant products treated by different duration times: (a) 2 h, (b) 4 h, (c) 5 h, (d) 12 h, and (e) 20 h.

broadband at 3100-3240 cm-1 is assigned to the bending vibration and stretching vibration modes of the hydroxyl group in the structure of ZnSn(OH)6. However, the presence of peaks at 777, 847, 1175, and 2298 cm-1, which may be also attributed to vibrations of M-OH or M-OH-M groups for ZnSn(OH)6, and the absorption peak at 538, 574, and 1113 cm-1 are due to vibration of M-O or M-O-M groups for ZTO. Those spectroscopy evolution were attributed to conversion of local structure of the obtained products, which were in accord with XRD results (as shown in Figure 5). Thereby, it further testified the phase conversion process. 3.2. Transformation Mechanism of ZTO Nanocrystals. It is well known that the precipitation or hydrothermal synthesis involves chemical reactions between ions or molecular species in aqueous solution and one or more solid phases. The chemical composition as well as the properties of the solid phases and the phase morphology can be controlled by changing the physical and chemical variables of the given system.22-29,32,33,38-43 On the basis of the experimental results (i.e., XRD patterns, TEM/SEM/SAED images, Raman and FT-IR spectroscopy), we propose that the crystal-phase transformation from the hydroxide precursor to ZTO nanocrystals in the congruent alkaline condition was a dynamic process, which occurs under the influence of the different factors, such as TR, CA, and TD, and

J. Phys. Chem. C, Vol. 112, No. 11, 2008 4163 so forth. The probable conversion process was elucidated as a two-step stage: formation of ZTO nucleus from ZnSn(OH)6 nanocubes according to the “dissolution-recrystallization” mechanism, and growth of stable ZTO nanocrystals through an “Ostwald ripening” process. As we know, solubility plays an important role in determining the growth of nanocrystals.29,32,40 In the present hydrothermal process, initially, during the mixing of the reactants, massive precipitation of ZnSn(OH)6 nuclei formed quickly, followed by the growth of the nuclei into cube-shaped crystals. With the temperature and pressure increasing steadily, the solubility of many oxides increases in water.40 Thereby, because of the large solubility and metastability of ZnSn(OH)6 compared with ZTO, this metastable intermediate phase decomposed and recrystallized to form ZTO nuclei according to the “dissolutionrecrystallization” mechanism. Then, the formation of ZTO tiny crystalline nuclei in a supersaturated medium occurs first, followed by the growth of larger crystals from smaller crystals due to the fact that the smaller particles have larger solubility than the larger ones, according to the well-known GibbsThomson law.29 Thereby, the classical Ostwald ripening process was accompanied. On the basis of the foregoing discussion, we believed that the transformation process shown in Scheme 1 described the hypothesis for the formation of ZTO nanocrystals. The evolution of phase composition, crystal morphology, and size during the hydrothermal treatment is controlled, at least, by several driving forces (i.e., TR, CA, and TD), which act simultaneously: the phase transformation from ZnSn(OH)6 to thermodynamically more stable ZTO crystals when the reduction of the solid-liquid interface and overall free energy by crystal growth. How far these phase conversions proceed along a series of increasingly stable intermediates depends on the solubility of the product and on the free energies of activation of their conversions, all of which are strongly influenced by the experimental details.29 Although the true situation is rather more complex and the oversimplification conversion process in the above analysis is somewhat speculative, it can explain the experimental phenomenon well, such as the formation of ZnSn(OH)6 phase and other precipitation phase in the products (results of XRD), the effects of concentration of OH- on precipitation phase, and the reaction time for the phase transformation process. 3.3. Compared Experiments by Addition of Surfactants (SA). TEM images and XRD patterns of ZTO products obtained by adding citrate (0.1 M) and CTAB (0.05 M) were shown in Figure 8. As can be seen from Figure 8a, when citrate was used, the particles size was ultrathin spherical particles with diameters of several to 10 nanometers; however, as illustrated in Figure 8b, using CTAB as the additive agent, the obtained ZTO crystallites become urchin-like shapes. Figure 8c and d shows the representative high-resolution TEM images of the tips of ZTO nanocrystals. HRTEM shows the clear lattice fringes, which indicates that the tip of the urchin-like ZTO is structurally uniform and well-crystallized. The adjacent lattice spacing is 2.68 Å, confirming that [311] is the preferred growth direction.

SCHEME 1: Schematic Illustration of the Transformation Process for ZTO Nanocrystals

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Figure 9. UV-vis diffuse reflectance spectra of three kinds of ZTO samples synthesized with at 200 °C for 20 h hydrothermal treatment.

Figure 8. TEM images of the synthesized ZTO products reacted with different additive surfactants. (a) Monodispersed ZTO nanoparticles obtained with addition of citrate. (b) Urchin-like ZTO nanocrystals obtained with CTAB as the surfactants. Images c and d are the HRTEM of the tip of urchin-like ZTO. (e) XRD patterns of ZTO products of all of the same parameters (1 M NaOH, 200 °C, 20 h) except for various additive agents.

Moreover, the XRD patterns in Figure 8e are further testifying these results. It was widely accepted that surfactants are very important in the wet synthesis of nanocrystals during nucleation, growth, and stabilization of the particles, and it is also important in the postsynthesis process when agglomeration and stabilization in organic solvents are concerned.22,27,29,41-43 Compared with the foregoing experiments, surfactant citrate molecules adhere to the surface of particles that serve as a protective layer to confine the growth of ZTO particles. Therefore, particles with obtuse spherical morphology are obtained (as shown in Figure 8a). However, with CTAB as a typical long-chain cationic surfactant, the stabilization and formation of different nanocrystalline shapes during nanocrystal synthesis is possible. Previous studies have postulated that nanorod growth involves preferential adsorption of CTAB along the long-axis crystal faces of the growing nanorod, rather than CTAB functioning as a “soft structural template”.41-43 Although the detailed mechanism of the cooperation was still unclear, it was firmly believed that different kinds of surfactants may result in different kinds of morphologies of the products via affecting the surface energy and developing orientation of crystal facets and through affecting the polarity and Van der Waals forces between the long-chain molecules and crystal facets.27 As shown the HRTEM images in Figure 8c and d, [311] is the preferred growth direction of rodlike ZTO, suggesting that CTAB molecules bind more weakly to the [311] edges than other end faces. In these cases, of all of the reactant parameters, we propose that the selected additive agent plays an important role in the particle size and morphology, and further research works need to be done to investigate the details of the mechanism for the formation of the rodlike ZTO nanocrystals in the presence of CTAB. 3.4. UV-Vis DRS and Band Gap Calculated. UV-vis absorption measurement is one of the most important methods to reveal the energy structures and optical properties of

semiconductor nanocrystals, and the migration of the lightinduced electrons and holes are the key factors controlling a photocatalytic reaction, which is relevant to the electronic structure characteristics of the material.44 The photoabsorption ability of various ZTO samples (sample A: citrate as the additive agent; sample B: none; sample C: CTAB as the additive surfactant) was detected by UV-vis DRS, as shown in Figure 9. All optical absorption of the nanomaterials was nearly the same. It was shown that ZTO presented strong photoabsorption properties in the UV light region around 320 nm. The steep shape of the spectra indicated that the light absorption was not due to the transition from the impurity level but was due to the band gap transition. For semiconductors, the absorption of photons with energy similar to that of the band gap, hλ ≈ Eg, leads to an optical transition producing an electron in the conduction band and a hole in the valence band (exciton). Such an electronic transition is subject to the selection rule such that the wave vector (k) must be conserved (i.e., k ) 0).45 Semiconductors such as ZTO, in which the wave vector is conserved for optical transitions, are known as direct band gap semiconductors. It is known that the relationship between the adsorption coefficient (a) near the absorption edge and the optical band gap (Eg) for direct interband transitions obeys the following formula46

(ahV)2 ) A(hV - Eg) where A is the parameter that relates to the effective masses associated with the valence and conduction bands and hV is the photo energy. Hence, the optical band gap for the absorption edge can be obtained by extrapolating the linear portion of the polt (ahV)2 - hV to a ) 0 in Figure 10. The dashed lines display the linear fit and corresponding extrapolation. From Figure 10, these three values of ZTO nanocrystals are apparently greater than the value for bulk ZTO (3.6 eV), which is also blue-shifted 0.13, 0.09, and 0.12 eV for the samples A, B, and C, respectively. We proposed that the increase in the band gap of the ZTO nanostructures is indicative of quantum confinement effects arising from the small size regime47 because the size of nanostructures is smaller in comparison with the bulk materials, as indicated by SEM and TEM observations. 3.5. Photocatalytic Activity Studies. Temporal evolution of the spectral changes taking place during the photodegradation of MB mediated by typical ZTO products (sample B) under ultraviolet light is displayed in Figure 11. Total concentrations of all MB species were simply determined by the maximum absorption measurement because the molar extinction coefficient

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Figure 10. (ahV)2 vs hV curves of the ZTO direct transition semiconductors.

Figure 12. Pseudo-first-order plots comparing direct light photolysis in the absence of catalyst and photocatalytic mineralization in the presence of various ZTO samples, P25 TiO2 as the reference. (Both experiments were performed at initial MB concentrations of 1 × 10-5 M, and a catalyst concentration of 1 g/L was used. C0 is the initial MB concentration, and C is the final MB concentration at a given elapsed time.)

order linear fit, the determined reaction rate constant k for MB degradation was 0.07376, 0.06061, 0.05581, and 0.05283 min-1, respectively (as shown in Figure 13). In contrast, the constant k of the blank sample was only 0.00252 min-1. For the photocatalysts, the detailed mechanism of the photomineralization process has been discussed previously,31,34-36,38 and a general simplified scheme of them is summarized briefly as follows:

Figure 11. UV-visible spectral changes of MB (1 × M) in aqueous ZTO (sample B) dispersions as a function of irradiation time from 0 to 50 min. (The inserted pattern is the concentration changes of MB over sample B, Amax ) 663 nm.) 10-5

range.37

Emax of different MB species were in a narrow As we can see from Figure 11, in the photodegradation process, the intensity of major absorption band decreased step by step at a given elapsed time (i.e., 10 min). The inserted pattern in Figure 11 indicates the concentration changes of MB with increasing the treatment time. It was almost decolorized ultimately when the irradiation time was elongated to 50 min. Moreover, under the same experimental condition with different samples, the dye was decolored in a similar manner with the color of the dispersion changing from initial blue to a light white. Most studies on MB decomposition investigated some factors that might influence the degradation rate of the dye and proposed some kinetic models to explain their results. On the basis of those experimental results, Xu et al.34 proposed that the photodegradation of MB obeyed the pseudo-first-order kinetics law and hence the rate for degradation, k, was obtained from the first-order plot according to eq 9

()

ln

C ) kt C0

(9)

where C0 is the initial concentration, C is the concentration after a time (t) of the MB degradation, and k is the first-order rate constant. As shown in Figure 13, among the photocatalysts tested, sample A was the most effective sample, followed by sample C, which outperformed P25 and sample B. Via the first-

ZTO f e- + h+

(10)

e- + h+ f energy

(11)

h+ + H2O f H+ + OH•

(12)

h+ + OH- f OH•

(13)

e- + O2 f O•2-

(14)

O°2- + H+ f HO•2

(15)

(OH°, O°2-, HO°2) + MB f degradation products

(16)

It is well-established that conduction band electrons (e-) and valence band holes (h+) are generated when the aqueous ZTO suspension is irradiated by light energy greater that its band gap energy (eq 10). Meanwhile, charge carriers can be recombined in the lattice site of the crystal, with energy depletion (eq 11). Both electron and hole may, in principle, react and generate some radicals (i.e., OH•, O•2-, and HO•2) adsorbed at the surface of ZTO (eqs 12-15). The resulting radicals, being very strong oxidizing agents, can oxidize most of the azo dyes to the mineralization end-products. Nevertheless, a limited number of stages can be identified and discussed.38 The whole process (eq 16) will be promoted if (i) the dye molecules, (ii) H2O, (iii) OH-, and (iv) O2 are efficiently adsorbed on the surface of catalysts, (v) electron-hole pairs and (vi) radicals are efficiently produced on the surface of catalysts, and (vii) electron-hole recombination is inhibited. Each stage may give a significant contribution to the final process (eq 16), and some speculation on the affected factors can be discussed. First, in our experiments, the small difference between band gaps of various ZTO samples should not exert any significant influence on the photocatalytic activity (shown in Figure 10);

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Figure 13. Reaction rate constant for MB degradation for the photocatalysts.

second, the adsorption stages are expected to be strongly affected by the surface state of catalysts, because all of the photocatalytic tests were performed with the same kind and amount of MB (1 × 10-5 M), volume of solution, and ambient temperature (25 ( 2 °C); Finally, electron-hole recombination is often considered one of the predominating factors.30 Recombination may occur on the surface or in the bulk and is in general catalyzed by impurities, defects, or all factors that introduce bulk or surface imperfections into the crystal.48 According to these arguments, crystalline particles showed enhanced photoactivity, which contrasted to powders with poor crystallinity.38 Herein, as a result of these considerations, adsorption stage and recombination stage, associated to the surface properties of the catalysts, may be considered of primary relevance in determining the rate of MB photodegradation and may be suitably related to the crystallite size and morphology of the powdered samples. In this work, all ZTO samples investigated in the present study are well-crystallized as a consequence of the coarsening and ripening processes occurring under hydrothermal conditions. XRD/TEM/HRTEM/SEM/UV-vis DRS indicated that the samples prepared by different additive surfactants have the same phase structure (Figure 8e) and nearly the same band gap (Figure 10), as well as a relatively well-crystallized structure (Figure 8c and d), but particle size and morphology of the three kinds of samples tends to differ obviously, which leads to the difference photocatalytic activity. As we know, ultrafine catalyst powders should exhibit superior activity because of their lower particle size with large surface-to-volume ratios, which can increase the number of active surface sites where the photogenerated charge carriers are able to react with absorbed molecules to form hydroxyl and superoxide radicals;34 otherwise, the relatively small size can quicken the rate of interfacial charge transfer and inhibit high rate of charge carrier recombination.38 In agreement with our results, sample A (BET: 117.63 m2/g) which has larger specific surface areas, and sample C (BET: 49.36 m2/g), which has a higher aspect ratio (length/width) than sample B (BET: 28.57 m2/g), should present higher spatial separation between oxidation and reduction sites with lower probability of recombination, which leads to enhancement of the photoconversion efficiency and, finally, to the improvement of photocatalytic activity. Therefore, the above considerations suggest that the overall efficiency of the photocatalytic degradation process is determined by the combined effect of stages i-iii, vi, and vii. 4. Conclusions In summary, a systematic approach was applied to the hydrothermal synthesis of ZTO nanocrystals to gain insight into

Zeng et al. the fundamental factors (i.e., CA, TR, TD, and SA) controlling phase composition, particle size, crystal morphology, and photocatalytic activity. The intermediate and final products during the hydrothermal process were characterized by XRD, SEM, TEM, SAED, HRTEM, and Raman and FT-IR spectroscopy; the results indicated that the phase transformation probably evolved via a “dissolution-recrystallization” mechanism and accompanying the “Ostwald ripening” process. Especially, control of the particle size and morphology was attained by adding the different surfactants, SA. Upon the addition of citrate in a stable dispersion solution by a hydrothermal process, ultrathin ZTO nanoparticles will be obtained, while urchin-like ZTO nanocrystals will be obtained when CTAB is used as the surfactant. Of all of the same parameters in the process of photocatalystic experiments, we deduced that the lower particle size of photocatalysts with bigger surface-to-volume ratios and larger specific surface areas can enhance the photocatalytic activity because they can increase the number of active surface sites or strengthen the spatial separation of photogenerated charge carriers. In addition, because the large majority of the operative cost of an industrial photominaralization unit can be ascribed to the energy supplied to the UV source, an increase of the mineralization rate corresponds to a decrease of the operating costs, with a significant advantage coming from the use of a well-optimized photocatalyst powder. Thereby, the simple, lowcost, and reproducible synthesis pathway reported herein makes it of potential interest for practical exploitation. Acknowledgment. This work is supported by the Scientific and Technological Development Project of the Beijing Education Committee (no. KM200710005029). References and Notes (1) Hahn, T. International Tables for Crystallography, 3rd ed.; Kluwer Academic, Dordrecht, 1992; Vol. A. (2) Coutts, T. J.; Young, D. L.; Li, X.; Mulligan, W. P.; Wu, X. J. Vac. Sci. Technol., A 2000, 18, 2646. (3) Jackson, W. B.; Hoffman, R. L.; Herman, G. S. Appl. Phys. Lett. 2005, 87, 193503. (4) Peiteado, M.; Iglesias, Y.; Fernandez, J. F.; De Frutos, J.; Caballero, A. C. Mater. Chem. Phys. 2007, 101, 1. (5) Fu, G.; Chen, H.; Chen, Z. X.; Zhang, J. X.; Kohler, H. Sens. Actuators, B 2002, 81, 308. (6) Niranjan, R. S.; Hwang, Y. K.; Kim, D. K.; Jhung, S. H.; Chang, J. S.; Mulla, I. S. Mater. Chem. Phys. 2005, 92, 384. (7) Minami, T.; Takata, S.; Sato, H.; Sonohara, H. J. Vac. Sci. Technol., A 1995, 13, 1095. (8) Gorrn, P.; Holzer, P.; Riedl, T.; Kowalsky, W.; Wang, J.; Weimann, T.; Hinze, P.; Kipp, S. Appl. Phys. Lett. 2007, 90, 063502. (9) Belliard, F.; Connor, P. A.; Irvine, J. T. S. Solid State Ionics 2000, 135, 163. (10) Zhu, H. L.; Yang, D. R.; Yu, G. X.; Zhang, H.; Jin, D. L.; Yao, K. H. J. Phys. Chem. B 2006, 110, 7631. (11) Rong, A.; Gao, X. P.; Li, G. R.; Yan, T. Y.; Zhu, H. Y.; Qu, J. Q.; Song, D. Y. J. Phys. Chem. B 2006, 110, 14754. (12) Wang, C.; Wang, X. M.; Zhao, J. C.; Mai, B. X.; Sheng, G. Y.; Peng, P. A.; Fu, J. M. J. Mater. Sci. 2002, 37, 2989. (13) Wang, W. W.; Zhu, Y. J.; Yang, L. X. AdV. Funct. Mater. 2007, 17, 59. (14) Tan, B.; Toman, E.; Li, Y. G.; Wu, Y. Y. J. Am. Chem. Soc. 2007, 129, 4162. (15) Villarreal, T. L.; Boschloo, G.; Hagfeldt, A. J. Phys. Chem. C 2007, 111, 5549. (16) Wang, S. M.; Yang, Z. S.; Lu, M. K.; Zhou, Y. Y.; Zhou, G. J.; Qiu, Z. F.; Wang, S. F.; Zhang, H. P.; Zhang, A. Y. Mater. Lett. 2007, 61, 3005. (17) Palmer, G. B.; Poeppelmeier, K. R.; Mason, T. O. J. Solid State Chem. 1997, 134, 192. (18) Fang, J.; Huang, A. H.; Zhu, P. X.; Xu, N. S.; Xie, J. Q.; Chi, J. S.; Feng, S. H.; Xu, R. R.; Wu, M. M. Mater. Res. Bull. 2001, 36, 1391.

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