Organic Additives-Free Hydrothermal Synthesis and Visible-Light

Mar 14, 2014 - Jiangsu Dragon Photoelectric Technology Co., LTD, Zhenjiang, Jiangsu 212000, P. R. China. ⊥. State Key Laboratory of Environmental ...
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Organic Additives-Free Hydrothermal Synthesis and Visible-LightDriven Photodegradation of Tetracycline of WO3 Nanosheets Gehong Zhang,†,‡ Weisheng Guan,‡ Hao Shen,§ Xian Zhang,⊥ Weiqiang Fan,† Changyu Lu,‡ Hongye Bai,† Lisong Xiao,† Wei Gu,† and Weidong Shi*,† †

School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang, Jiangsu 212013, P. R. China School of Environmental Science and Engineering, Chang’an University, Yanta Road 126, Xi’an, Shaanxi 710054, P. R. China § Jiangsu Dragon Photoelectric Technology Co., LTD, Zhenjiang, Jiangsu 212000, P. R. China ⊥ State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, P. R. China ‡

ABSTRACT: In our work, we have successfully synthesized WO3 nanosheets via a simple, fast, and organic additives-free hydrothermal method. The obtained products are 200−500 nm-sized with square corners and high crystallinity. A possible formation mechanism of WO3 nanosheets is discussed in this study based on the experimental results and our understanding. WO3 nanosheets modified with Pt exhibit enhanced photocatalytic activity of tetracycline compared with bare WO3 nanosheets under visible-light irradiation (λ > 420 nm). Furthermore, the effects of Pt as cocatalyst can be clearly drawn, and the degradation ratio of 2% Pt/WO3 nanosheets is over 3 times that of bare WO3 nanosheets, which is reasonable to expect the obvious Ptdependent activity. The addition of 10 mL of iso-propanol leads to a considerable decreased degradation ratio (DR) of tetracycline (TC) on 2% Pt/WO3 photocatalysts, further indicating the ·OH radicals do exist in the process of the photocatalytic oxidation (PCO) of TC. to find a single-phase visible-light-driven photocatalyst with high activity for TC degradation is of utmost importance. Although many visible-light-driven photocatalysts are found to be active in degradation of organic pollutants, few show activities on the degradation of TC, because their VB are too high to produce ·OH radicals (2.38 V vs NHE).13 As we know, the VB of WO3 is lower than the redox potential of E° (·OH/ OH−), indicating that WO3 possesses the ability to produce · OH under visible light. For instance, the enhanced photocatalytic degradation of benzene from ultrasmall WO3 nanocrystals was recently reported by Tanaka et al.14 Thus, the visible-light-driven photocatalytic degradation of TC, which contains four paralleled benzene rings, by WO3 photocatalysts can be expected. To enhance the photocatalytic performance of WO3 catalysts, nanostructuring strategy15,16 has been frequently adopted because reducing the particle size into nanoscale can facilitate the migration of charge-carriers by increasing the thermodynamic driving force via the so-called quantum-size effect and shortening the distance that charge-carriers migrate from deep bulk to reaction active sites.13,14Among them, 2D sheet structure shows supreme advantages on photocatalysis for the huge surface area compared with the 1D and 3D counterparts. 2D WO 3 nanosheets have already been synthesized through the hydrothermal method and high temperature reaction.13,16 However, all of them require the addition of organic additives or the prolonged crystallization

1. INTRODUCTION Tetracycline (TC), a very famous broad-spectrum antibacterial agent widely used for bacterial infection treatment, is extensively found in the aquatic environment, which poses serious threats to the ecosystem and human health by inducing proliferation of bacterial drug resistance. For example, after years of abuse, most Gram-positive bacteria such as Staphylococcus aureus and Gram-negative bacteria such as Enterobacter bacilli show resistance to TC. The removal of TC from the environment has become a mandatory issue. Unfortunately, the conventional degradation processes are often constrained by the low efficiency and high cost. Recently, the photocatalytic oxidation (PCO) has been established to be one of the most promising technologies for environment remediation1−6 and provides a good tool for the transformation and degradation of TC.7−11 The process of PCO is initiated by the light irradiation to excite electrons from photocatalyst to its conduction band (CB) and leave positive holes on the valence band (VB).1 For now, the photocatalytic degradation of TC is reported mostly on the UV-driven photocatalysts such as TiO2 and ZnO,7−9 because UV-driven photocatalysts with large band gap energy usually have lower VB to produce ·OH radicals (2.38 V vs NHE). However, the ultraviolet region accounts for merely about 4% of the incoming solar energy, which makes the efficiently practical application of PCO in TC treatment an impossible. Efforts have been made by Wang et al.12 to realize the photodegradation of TC under visible light by doping C, N, and S into TiO2 photocatalysts, but the internal defects and discrete energy bands caused by ions doping still vastly constrain the efficiency and practical use of PCO for TC. Thus, © 2014 American Chemical Society

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were collected on an S-4800 field emission SEM (SEM, Hitachi, Japan). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were collected on an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.), using a 200 KV accelerating voltage. UV−vis diffused reflectance spectra of the samples were obtained from a UV−vis spectrophotometer (UV2550, Shimadzu, Japan); BaSO4 was used as a reflectance standard. The photoluminescence properties of the obtained nanosheets were measured on a Perkin-Elmer LS 55 luminescence spectrometer. Total organic carbon (TOC) analyses were conducted on a multi N/C 2100 (Analytik Jena AG, Germany) TOC analyzer. 2.4. Photocatalytic Degradation of TC. The photodegradative reaction for TC was carried out at 298 K in a photochemical reactor under visible light. The photochemical reactor contains 0.1 g of WO3 catalyst and 100 mL of a 20 mg/ L TC solution. To determine the initial absorbency of samples, the reactor was kept in darkness for 30 min to reach absorption equilibrium. The photochemical reactor was irradiated with a 250 W xenon lamp which was located with a distance of 8 cm at one side of the containing solution. UV lights with wavelengths less than 420 nm were removed by a UV-cutoff filter in the visible-light-driven TC degradation experiment. The sampling analysis was conducted in a 10 min interval. The photocatalytic degradation ratio (DR) was calculated by the following formula:

process from several to tens of days, which goes against the principles of large-scale production and green chemistry. In addition, WO3 takes the nature of narrow band gap and the suitable VB for PCO, but the low photodegradation ratio limits its practical application. Therefore, further attempts have been made to improve the photocatalytic activity of WO3 through introduction of a cocatalyst such as Pt, since Pt commonly serves as a pool for electrons when a semiconductor catalyst is irradiated by light, which could depress the recombination of electrons and holes.17 Herein, we first report a very fast organic additives-free hydrothermal synthetic approach of the WO3 nanosheets. Through adjusting the parameter of time, we propose the growth mechanism of WO3 nanosheets. Furthermore, we also reveal that the depositing quantity of Pt nanoparticles takes an important role on the photocatalytic activity of WO3 nanosheets. Under visible-light irradiation, the as-prepared 2% Pt/ WO3 nanosheets (72.82%) show higher activity of photocatalytic degradation of TC than WO3 nanosheets (19.69%).

2. EXPERIMENTS 2.1. Chemicals. Sodium tungsten oxide was purchased from Aladdin (China). Nitric acid was purchased from Sinopharm (China). All reagents were of analytical grade without further purification, and the deionized water was used in all experiments. 2.2. Catalysts Synthesis. WO3 square nanosheets were prepared by a fast organic additives-free hydrothermal method. In a typical synthesis procedure, 5 mL of 65% nitric acid was first dropped slowly into 25 mL of deionized water solution and stirred for 10 min. 1.5 mmol of Na2WO4·2H2O was dissolved homogeneously in 10 mL of deionized water to form a transparent solution and was added into the above nitric acid solution. Then, a precipitate appeared with its color gradually turning from white to light yellow. After stirring for another 30 min, the above suspension was transferred into a Teflon-lined stainless steel of 50 mL capacity. The hydrothermal route was carried out at 180 °C for 3 h. After reaction under selfgenerated pressure and cooling down to room temperature, the final yellow products were collected by centrifugation, washed with the deionized water and ethanol for several times, and dried in vacuum at 60 °C for 12 h. The deposition of platinum onto the surface of WO3 was performed using a photodeposition method.18−25 The Ptloaded WO3 sample was prepared with chloroplatinic acid (H2PtCl6·6H2O) as follows: the obtained WO3 (0.3g) was added into 100 mL of deionized water by magnetic stirring. Then, 2 mL of H2PtCl6 (1.4 g/L) was added to the above solution. The loading amount is 1% Pt/WO3. The loading amounts of 2% Pt/WO3 and 3% Pt/WO3 nanosheets were prepared with 4 and 6 mL of H2PtCl6 (1.4 g/L). Finally, the photodeposition step was carried out in a 250 mL beaker under the irradiation of two 125 W ultraviolet lamps for 30 min. The reference sample bulk WO3 powder was prepared through the high-temperature solid-state reaction, by annealing the precursor precipitation in the synthesis of WO3 nanosheets at 800 °C for 4 h. 2.3. Characterization. The crystal structure of samples was determined by the X-ray diffraction (XRD) method using Cu Kα radiation (λ = 1.54178 Å). The chemical composition of the samples was determined by scanning electron microscope-X-ray energy dispersion spectra (SEM-EDX) with an accelerating voltage of 25 KV. Scanning electron microscopy (SEM) images

⎛ A ⎞ DR = ⎜1 − i ⎟ × 100% A0 ⎠ ⎝

A0 is the initial absorbance of TC that reached absorption equilibrium, while Ai is the absorbance after the sampling analysis. The absorbance of TC was measured by a UV−vis spectrophotometer with the maximum absorption wavelength at 355 nm. 2.5. Kinetics of Photocatalytic Degradation of TC. According to the Langmuir−Hinshelwood kinetics model,26−28 the photocatalytic process of TC can be expressed as the following apparent first-order kinetics equation (eq 1). ⎛C ⎞ ln⎜ 0 ⎟ = K app·t ⎝C⎠

(1)

where C is the concentration of solute remaining in the solution at irradiation time of t and C0 is the initial concentration at t = 0. Kapp denotes the degradation rate constant. The apparent rate constant (Kapp) has been chosen as the basic kinetic parameter for the different photocatalysts since it enables one to determine a photocatalytic activity independent of the previous adsorption period in the dark and the concentration of solute remaining in the solution.28

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. The morphology and crystalline structure of the as-obtained WO3 (3 h) nanosheets are visualized by SEM, TEM, and HRTEM images in Figure 1. In the low-magnified SEM image (Figure 1a), the square sheetlike morphology of the products can be clearly observed, and the inset reveals the thickness of the sheet structure (ca. 100 nm) is within nanoscale level. According to the high-magnified SEM image (Figure 1b), the side length of a single WO3 sheet is about 300 nm, and the square character of the WO3 sheet can 5444

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structure with the reaction time up to 3 h (Figure 1a,b). The crystallographic phase purity and elementary composition of the as-synthesized WO3 nanosheets were characterized by XRD analysis. As shown in Figure 2d, all characteristic peaks of WO3 nanosheets can be readily indexed to the monoclinic phase of WO3, which is in good agreement with the standard card (JCPDS No. 43-1035). 3.2. Crystallization Process. To further figure out the crystallization process of WO3 nanosheets, a series of timedependent experiments were carried out. In these experiments, only the reaction time was changed without altering any other reaction conditions. Figure 3 shows XRD patterns of the obtained samples at different reaction times (from 0 h, 1 h, and 2 h). It can be seen that the precursor before the hydrothermal reaction is H2W4O13, which corresponds to the standard card No. 01-0583. After a 1 h hydrothermal reaction, very weak characteristic peaks of monoclinic WO3 emerge. With the reaction time prolonged to 2 h, the intensity of WO3 peaks is slightly enhanced, but the H2W4O13 composition is still predominant in the samples. Thus, the major composition of these sheet structures is H2W4O13 rather than WO3. A sharp phase change is observed after a 3 h hydrothermal reaction in which the H2W4O13 phase is entirely vanished in the XRD pattern, leaving the sharp and strong peaks of the monoclinic WO3 phase only (Figure 2d). From the XRD patterns, we can conclude that a very important and fast chemical transformation occurs in the reaction time between 2 and 3 h. The composition of square sheets is pure monoclinic WO3. Compared with the sample after a 2 h reaction, the chemical composition of the sample is totally changed but the sheet structure remains except the appearance of the square character, which means the sharp phase transformation from H2W4O13 to WO3 is from the in situ reaction with these sheet structures themselves as templates. It is very interesting that we did not add any additive in the precursor as template, but hard templates were formed during the hydrothermal reaction itself. Figure 3b shows the EDX image of the as-prepared WO3 nanosheets, in which the signals of W and O can be clearly observed. The signal of Au is ascribed to the substrate of the SEM-EDX analysis itself. No other signals can be observed in the spectrum, indicating the elementary composition of our WO3 nanosheets is pure. On the basis of the above analyses, we propose a mechanism process of the WO3 nanosheets as shown in Scheme 1. First, Na2WO4 and HNO3 react at ambient temperature to form H2W4O13 sheets as precursor (reaction 2). Then, these H2W4O13 sheets integrate with each other to form the thicker sheet structure in the hydrothermal reaction process; as the reaction goes on, these mutilated sheet structures tend to be more complete. During this process, a very small amount of WO3 is formed from the phase transformation, and the content of WO3 is increasing as the reaction time is prolonged, but the predominant composition of the sheet structure is still H2W4O13. However, a very sharp and fast in situ phase transformation reaction occurs with the sheet structure itself as template after 2 h (reaction 3). Through the in situ phase transformation, complete WO3 nanosheets with square corners are formed. Possible reaction formulas are as follows:

Figure 1. SEM images of WO3 nanosheets in (a) low and (b) high magnifications with the magnified specific area (inset of a); (c) TEM image and (d) the representative HRTEM image of an individual nanosheet with the corresponding SAED image (inset of d).

be found. The TEM image (Figure 1c) of an individual WO3 sheet suggests that the angle of the corner is nearly 90°, which further confirms the square character of the obtained WO3 sheets. The representative HRTEM image (Figure 1d) of an individual WO3 sheet shows clear lattice fringes on the edge of the sheet, and the interplanar spacing d = 0.37 nm corresponds to the (020) plane of the monoclinic phase of WO3, which is in good agreement with the XRD results. Furthermore, the largerange continuous crystal lattices and the well-arranged SAED image (inset) reveal the single-crystalline nature of the asobtained WO3 nanosheets. To confirm the mechanism of the formation of WO3 nanosheets, time-dependent SEM analyses are conducted. As shown in Figure 2, the yellowish precursor (Figure 2a) is

Figure 2. SEM images of obtained samples with different hydrothermal reaction times of (a) 0 h, (b) 1 h, and (c) 2 h and (d) XRD patterns of WO3 nanosheets.

composed of thin mutilated nanosheets. After a 1 h hydrothermal reaction (Figure 2b), the mutilated sheets begin to be the thicker structures. From the ragged surface of these thicker mutilated sheets, we can easily assume that the thicker structure is assembled by the thiner nanosheets. After a 2 h reaction (Figure 2c), the sheet structures are more complete, but the square character of these sheet structures disappears. Moreover, the final product exhibits the complete square-sheet

4Na 2WO4 + 8HNO3 → H 2W4O13 + 8NaNO3 + 3H 2O (2)

H 2W4O13 → 4WO3 + H 2O 5445

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Figure 3. (a) XRD patterns of obtained samples with different hydrothermal reaction times and (b) SEM-EDX spectrum of WO3 nanosheets.

dense. The sizes of the QDs were close to 5 nm. Further characterization (Figure 4d) shows the SEM-EDX image of the as-prepared Pt/WO3 nanosheets, which proves the successful photodeposition of Pt on the WO3 nanosheets. In the SEM image (Figure 4e,f), bulk WO3 powder is in microscale size. Bulk WO3 powder has an irregular surface, and the size is not uniform with 1−3 μm. 3.4. Photocatalytic Degradation of TC. To probe the potential application of WO3 nanosheets in photocatalytic degradation of TC, we evaluated the photocatalytic degradation of TC by WO3 nanosheets in relation to Pt/WO3 nanosheets under visible light. As shown in Figure 5, the DRs of bulk WO3 powder, pure WO3, and 1% Pt/WO3, 2% Pt/WO3, and 3% Pt/ WO3 nanosheets for 60 min are 2.86%, 19.69%, 58.01%, 72.82%, and 64.46%, respectively. Compared with bare WO3, the photodegradation with bulk WO3 powder is merely lower. Compared with bare WO3, the photodegradation significantly increased by further loading Pt. Moreover, the photodegradation in the presence of 2% Pt/WO3 nanosheets was conversely higher as compared to 1% Pt/WO3 and 3% Pt/WO3 nanosheets. Therefore, the effects of Pt as cocatalyst can be clearly drawn, for the DR of 2% Pt/WO3 nanosheets is over 3 times that of pure WO3 nanosheets, which is reasonable to expect the obvious Pt-dependent activity. As far as we know, this work is the only reported visible-light-driven photocatalyst for TC degradation by Pt/WO3 until now, so the Pt/WO3 as a visible light photocatalyst for antibiotic water treatment are efficient and promising. 3.5. The Kinetic Study of Photocatalytic Degradation of TC. In order to further illustrate the photocatalytic reaction, the kinetic behavior is discussed. The photodegradation reaction kinetics of TC can be described by a Langmuir− Hinshelwood model according the report.29 The decomposition of TC approximated the first order kinetic. The variations in ln (C0/C) as a function of irradiation times are given in Figure 6. The calculated apparent rate constant Kapp values for WO3, 1% Pt/WO3, 2% Pt/WO3, and 3% Pt/ WO3 in visible light are given in Table 1. 3.6. UV−Vis Absorption Spectra and Photoluminescence Spectra. We have studied the UV−vis diffused reflection spectra. As shown in Figure 7, the prepared WO3 nanosheets have a strong absorption at ca. 460 nm, and the Pt/ WO3 nanosheets show similar absorption spectra. However, the Pt/WO3 nanosheets still have obvious absorption after 460 nm. We can also draw from the color contrast of samples (inset)

Scheme 1. Possible Formation Mechanism of the WO3 Nanosheets

3.3. Characterization of Pt/WO3 and Bulk WO3. In the SEM image (Figure 4a), the square sheet-like morphology of

Figure 4. (a) SEM image, (b, c) TEM images, (d) SEM-EDX spectrum of Pt/WO3 nanosheets, and (e) low and (f) high magnification SEM images of bulk WO3.

the Pt/WO3 can be clearly observed. The morphology of Pt/ WO3 has not changed compared with square WO3. Figure 4a does not clearly represent the existence of Pt particles, which may have been due to the small amount of Pt. Figure 4b,c shows the TEM images of Pt/WO3, which represent the existence of Pt particles. The as-prepared particles were quantum dots (QDs). The QDs are spherically shaped and 5446

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Figure 5. Photocatalytic degradation ratios of TC with different samples: (a) bulk WO3 powder, (b) WO3 nanosheets, (c) 1% Pt/WO3 nanosheets, (d) 2% Pt/WO3 nanosheets, and (e) 3% Pt/WO3 nanosheets under visible light for 60 min.

can serve as a pool for electrons, which leads to the effective excited electron consumption.17 To confirm the validity of our experiment conclusions, photoluminescence (PL) analyses were further conducted. As shown in Figure 8, the PL emission

Figure 6. Kinetics of the photocatalytic degradation of (a) WO3 nanosheets, (b) 1% Pt/WO3 nanosheets, (c) 2% Pt/WO3 nanosheets, and (d) 3% Pt/WO3 nanosheets under visible light for 60 min.

Table 1. Apparent Rate Constant Values for PhotoDegradation of the TC Solution over Different Photocatalysts in 60 min under Visible Light Irradiation samples kapp (min−1)

WO3 0.0033

1% Pt-WO3 0.0142

2% Pt-WO3 0.0202

Figure 8. PL spectra different samples: (a) WO3 nanosheets and (b) 2% Pt/WO3 nanosheets.

3% Pt-WO3 0.0172

intensity of WO3 nanosheets is higher than that of 3% Pt/WO3 nanosheets. The excitation wavelength of the PL is 360 nm, and emission wavelength of the PL is 469 nm. It is well acknowledged that the PL emission intensity of a semiconductor is proportional to the opportunity of the recombination of photoinduced electron−hole pairs. Therefore, the lower PL emission intensity of 2% Pt/WO3 nanosheets suggests that the Pt/WO3 nanosheets have less of an opportunity for electron−hole pairs’ recombination and facilitates the migration of charge-carriers more effectively than the WO3 nanosheets. This result fully coincides with the points we proposed. In addition, the loading quantity of Pt on WO3 was 2%, which was the optimal loading quantity for photocatalytic oxidation as shown in Figure 5. It might be due to the increase of Pt on the surface of WO3 which could well respond to visible light with a loading amount from 1% to 2%. However, the photocatalytic ratio of Pt/WO3 decreased with a further increase in the amount of Pt (>2%), because the excessive Pt led to WO3 being covered, which causes the reduction of contact area between WO3 and water. This tentatively suggests that the optimal condition for Pt/WO3 is reached at 2% (Pt). In other words, platinum favors this electron transfer process to improve the pairs’ separation.

Figure 7. UV−vis absorption spectra with color contrast in photos (inset) of different samples: (a) WO3 nanosheets, (b) 1% Pt/WO3 nanosheets, (c) 2% Pt/WO3 nanosheets, and (d) 3% Pt/WO3 nanosheets.

that the Pt/WO3 nanosheets were deeper in color than WO3 nanosheets. Pt decorated WO3 nanosheets show the higher photo degradation ratio for TC in our study, since Pt on the surface 5447

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Figure 9. Photocatalytic degradation ratios of TC using different radical scavengers over 2% Pt/WO3 nanosheets: (a) reaction in the absence of radical scavengers, (b) reaction with iso-propanol for ·OH, (c) reaction with TEA as a scavenger for h+, and (d) reaction with AgNO3 as a scavenger for e− under visible light irradiation for 60 min.

Furthermore, the PCO process on WO3 photocatalysts can be classified as the ·OH radicals oxidation. Organic compound degradation during the photocatalytic process resulted from the oxidation reaction of the organic compound with ·OH radicals or holes. When WO3 nanosheets are used as the photocatalyst, the ·OH radicals and radical anions are the oxidizing species for organic compound oxidation.30 However, the excited electrons from WO3 nanosheets can not reduce O2, so the photocatalytic activity of WO3 nanosheets is lower. When Pt is used as the cocatalyst, the excited electrons can react with O2 to produce · OH radicals.30 There is an enhancement of photocatalytic activity in Pt-loaded WO3. A series of active species trapping experiments was conducted to further investigate the photocatalytic oxidation mechanism of TC. Figure 9 shows the results of adding different radical scavengers over the 2% Pt/WO3 photocatalyst reaction system under visible-light irradiation. When 10 mL of iso-propanol (IPA)31 for ·OH is added into the reaction system, the photo degradation is significantly inhibited (entry b in Figure 9) compared to the reaction in the absence of radical scavengers (entry a in Figure 9). We can easily find that the addition of iso-propanol in the catalytic system leads to a 50% decrease to the photocatalytic degradation rate of TC over 2% Pt/WO3. A similar and obvious inhibition phenomenon for the photocatalytic reaction is also observed with the triethanolamine (TEA)32−36 scavenger for h+ (entry c in Figure 9). Therefore, it can be concluded that ·OH and h+ are the main active species of Pt/WO3 in aqueous solution under visible light irradiation. On the contrary, the photocatalytic degradation of TC obviously increased with the addition of AgNO3 for e− (entry d in Figure 9).32−36 The increase suggests that the scavenger of e− has less of an opportunity for electron−hole pairs’ recombination and facilitates the production of more holes. This result fully coincides with the points we proposed, further indicating the ·OH radicals and h+ do exist in the process of PCO of TC. To further demonstrate photocatalytic properties of the photocatalysts, TOC analyses were conducted. Figure 10 showed the removal rate of TOC with 2% Pt/WO 3 photocatalysts on the degradation of TC under visible irradiation. In the degradation of TC after 60 min, the removal rate of TOC reached 49.66%, which was lower than photocatalytic degradation ratio (Figure 5) . This is reasonable because the degradation curve data were measured after the photocatalyst separation step by centrifugation.37 Similar trends of TOC removal and the degradation curves indicate that our experiments of photo degradation successfully and correctly evaluated the photocatalytic activity of our photocatalysts.

Figure 10. The TOC removal curves of 2% Pt/WO3 nanosheets under visible light for 60 min.

Moreover, the reduced TOC removal rate suggests that our photocatalysts have huge potential in environmental applications such as the mineralization of TC.38 It can be concluded that the TC molecules can be mineralized by photocatalysts. From the TOC removal ratio, it is also inferred that there were a lot of intermediate products generated in the solution. On the basis of the experimental results, we have proposed a possible mechanistic pathway. As depicted in Scheme 2, the photogenerated holes from WO3 nanosheets reacted with water and OH− to produce ·OH radicals. These as-produced ·OH radicals can then degrade the TC molecules into CO2 and H2O. When Pt is used as the cocatalyst, the excited electrons can react with O2 producing ·OH radicals.17,25 Therefore, the presence of ·OH radicals produced from the reaction between Scheme 2. Mechanistic Pathway of Electrons and Holes under Visible Light Illumination on Pt/WO3 Photocatalysts

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the photocatalyst and absorbed water controlled the photodegradation rate. This implies that Pt contributes to the improved photocatalytic activity of WO3. Meanwhile, holes can directly degrade the TC molecules into CO2 and H2O. That is to say, it can be concluded that ·OH and h+ are the main active species of Pt/WO3 in aqueous solution under visible light irradiation.

4. CONCLUSION In this report, a fast and convenient hydrothermal synthetic method for the preparation of WO3 nanosheets has been proposed. The possible formation mechanism of the WO3 nanosheets is discussed. During the crystalline process, a very interesting in situ phase transformation reaction was observed. Although we did not add any additives in the precursor, hard templates were formed during the hydrothermal reaction itself. The as-obtained WO3 nanosheets exhibit visible-light-driven activity for photocatalytic degradation of TC. Significant enhancement of activity has been clearly found on Pt/WO3 nanosheets compared with WO3 nanosheets. Our work highlights the facile and fast hydrothermal synthesis of WO3 photocatalysts which exhibits visible-light-driven activity for photocatalytic degradation of TC.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 511 8879 0187. Fax: +86 511 8879 1108. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for National Natural Science Foundation of China (21276116, 21301076, 21303074, and 21201085), the Doctor Found at ion (Grant No. 20110205110014), Natural Science Foundation of Jiangsu Province (BK20131257, BK2012294), Special Financial Grant from the China Postdoctoral Science Foundation (2013T60501), Open Project of State Key Laboratory of Rare Earth Resource Utilizations (RERU2014010), Program for New Century Excellent Talents in University (NCET-130835), and National Training Programs of Innovation and Entrepreneurship for Undergraduates (201310710055).

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