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Highly crystalline multi-layered WO3 sheets for photodegradation of Congo red under visible light irradiation
Accepted Manuscript Title: Highly crystalline multi-layered WO3 sheets for photodegradation of Congo red under visible light irradiation Author: S.V. Prabhakar Vattikuti Chan Byon Ich-Long Ngo PII: DOI: Reference:
S0025-5408(16)30500-1 http://dx.doi.org/doi:10.1016/j.materresbull.2016.08.008 MRB 8888
To appear in:
MRB
Received date: Revised date: Accepted date:
26-4-2016 1-8-2016 5-8-2016
Please cite this article as: S.V.Prabhakar Vattikuti, Chan Byon, Ich-Long Ngo, Highly crystalline multi-layered WO3 sheets for photodegradation of Congo red under visible light irradiation, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly crystalline multi-layered WO3 sheets for photodegradation of Congo red under visible light irradiation S.V. Prabhakar Vattikuti1*, Chan Byon1* and Ich-Long Ngo1 1
School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea, 712-749
*Corresponding author Address: Dr. S.V.Prabhakar Vattikuti, School of Mechanical Engineering Yeungnam University 214-1 Dae-dong Gyeongsan-si, Gyeongsangbuk-do (712-749, Republic of Korea) Mobile: +82-(0)53-810-2452 Fax: +82-53-810-4627 *Corresponding author E-mail: [email protected] (S.V.Prabhakar Vattikuti) [email protected] (Chan Byon) Graphical abstract
Highlights:
WO3 multi- layered sheets were successfully synthesized by simple thermal decomposition method
Phase transformation of sintered WO3 occurred from monoclinic to orthorhombic structure
Few-to-multi layered WO3 sheets are formed by the oriented attachment (OA)-self-assembly (SA)
Photodegradation mechanism of WO3 sheets is elucidated and discussed
Synthetic procedure may open up an opportunity to tailor the morphologies of semiconductors
Abstract We report a simple synthetic method for few- to multi-layered tungsten oxide (WO3) sheets through complexation of a sodium tungstate dihydrate precursor by 1-propanol, sulfuric acid and urea. The formation of tungsten oxo-clusters and their organized transformation into few-tomulti-layered WO3 sheets occurs during the hydrolysis stage, and crystallization occurs during the post-sintering process. XRD results exhibited a phase transformation from monoclinic to orthorhombic β-WO3 due to post-sintering effects. The as-prepared material appears as selforganized few- to multi-layered WO3 sheets. The as-prepared and sintered samples are used for the photodegradation of Congo red (CR) under visible light irradiation. A sample sintered at 300oC (S-300) shows much higher photocatalytic activity than the as-prepared WO3 and those sintered at 180oC and 400oC. The S-300 photocatalyst has the optimal performance in the removal of CR, which is due to high specific surface area and patches of crystalline sites on the photocatalyst surface. These sites can suppress the recombination of electrons and holes by accelerating the charge carrier transformation within the photocatalyst.
Keywords: Semiconductor; Self-organized sheets; Photocatalytic activity; Congo red; Postsintering.
1. Introduction Sunlight-driven photocatalysts are in demand for development of eco-friendly materials due to environmental impact and energy issues [1, 2]. Semiconductor photocatalysis has arisen as one of the most promising technologies due to better utilization of solar energy [3-5]. Better photocatalyst performance strongly depends on the absorption capacity toward sunlight and the effectiveness of photoinduced reactions of the charge carriers [3]. However, various traditional photocatalysts are more active under UV light irradiation due to their wide band gap and high recombination effect of photoinduced electron and holes. This is especially true for multi-metal oxides, oxynitrides, and sulfides [4-6]. However, other issues need to be addressed, including stability, reusability, and scalability. Recently, nanosized WO3 materials have been broadly used for photocatalysts, solar cells, and gas sensors [7]. WO3 is an n-type semiconductor and has a band gap of around 2.5–3.5 eV. Layered WO3 has unique interlayer chemistry and high applicability for exfoliation/intercalation reactions [3, 8-10]. Furthermore, it can undergo exfoliation to produce single- or multi-layer sheets with unique 2D morphology, which are attractive as building blocks for fabrication of nanodevices [11]. Another important application of WO3 is as a photocatalyst for degradation of organic pollutants. But pure WO3 photocatalyst shows lower visible light energy conversion efficiency due to its low conduction band level and high recombination rate of the photoinduced electron-hole pairs [9, 10]. Hence, many attempts have been made to improve the photocatalytic activity of WO3, including controlled morphology, heterogeneous systems, compositing with other materials, and doping with other metals [12-15]. Different factors improve photocatalytic activity, such as high surface area, coupling of different transition metals to reduce the band gap, and controlled morphology [16-17]. Recently, 2D and 3D WO3 nanostructures have attracted significant attention due to their unique physico-chemical properties for potential applications [18-20]. Huang et al. reported that a single hexagonal WO3 nanowire is worked as sensing element for UV photodectors, biological sensors and optoelectronic devices, which exhibits excellent sensitivity and reversibility under ultraviolet region [10]. To improve the photoactivity of the WO3 material is required to obtain a high surface area [21, 22]. Domene et al. [23] have reported the high photocatalytic performance of anodized WO3 nanoplates of globular clusters and monoclinic WO3 nanoplates reported by Zhang et al. [24] were exhibited enhanced photocatalytic activity under the sun light irradiation, which may be ascribed to its high BET surface area. However, the photocatalytic performance of single-phase semiconductors is limited due to high recombination rate of photo-generated electron-hole pairs. Many strategies have been dedicated to improve the efficiency of photocatalytic activity under visible light irradiation, including preparation of nanocomposites or doping of noble metal on to semiconductor or preparation of semiconductors with various morphological structures or control crystalline structures, etc. [25, 26]. To date, many efforts have been devoted to synthesizing WO3 with different controlled morphologies, such as
nanofibers [27], nanoflowers [28], nanosheets [29], and nanoplatelets [30]. However, the methods often require multiple steps, expensive templates, costly processes, and high temperatures and pressures. Finding a simple synthetic method that is economical and reliable to obtain these morphologies is a considerable challenge. We report the one-step synthesis of WO3 by a thermal hydrolysis process using 1propanol, sulfuric acid and urea. The as-prepared WO3 samples were sintered at 180°C, 300°C, or 400°C in order to evaluate the morphology features. Different content of WO3 photocatalyst were used to evaluate the photocatalytic degradation of Congo red (CR) under visible light exposure. The results show that the S-300 sample showed higher photocatalytic activity for degradation of CR than the as-prepared WO3, S-180, and S-400. The as-prepared WO3 and S-300 samples also exhibit good stability and reusability after four successive recycles. A possible photodegradation mechanism of the S-300 sample is discussed based on the obtained results. 2. Experimental details 2.1 Preparation of WO3 sheets All chemical reagents were used as obtained. To get the WO3 sheets, sodium tungstate dihydrate (Na2WO4·2H2O, Aldrich), hydrochloric acid (HCl), sulfuric acid, 1-propanol, urea, and absolute ethanol were used as starting materials. First, 7.75 g of Na2WO4·2H2O and 15 mL of 1propanol were dissolved in 25 ml of distilled water and stirred continuously for 10 min to get a homogeneous solution. Then, 12.5 ml of ethanol and 2.78 mL of sulfuric acid were added to the resulting solution and magnetically stirred for 30 min at 80°C on a hot plate to form a homogeneous solution. Then, 0.58 g of urea was added. The pH of the resulting solution was adjusted to 3 by adding a sufficient amount of hydrochloric acid (HCl). After stirring at 80°C for 1 h, the obtained solution (white-yellow color) was thermal decomposed at 140°C for 3 h. The resulting yellow precipitates were collected through centrifugation at 5000 rpm for 30 min and then filtered and washed three times with distilled water and ethanol. The as-prepared precipitates were dried under vacuum at 100°C for 6 h and then sintered at 180°C, 300°C, and 400°C for 2 h in a muffle furnace. Hereafter, the sintered samples are denoted as S-180, S-300, and S-400, respectively. 2.2 Characterization of WO3 sheets The crystal structures of the samples were studied by wide-angle powder X-ray diffraction (XRD) on a PANalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation (0.154056 nm) at 40 kV and 30 mA. The surface morphology of synthesized samples were characterized using a scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, Hitachi H-7000), and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F 20 s-twin TEM). Elemental analyses were performed through energy-dispersive X-ray spectroscopy (EDX) with an X-ray analyzer integrated into the SEM instrument. Fourier transform infrared spectroscopy (FTIR, Avatar 370) with a spectral range of 4000–400 cm−1 was used for the phase purity test. Thermal stability analysis was performed with a Shimadzu TGA/DTA instrument (DTG-60/60H thermal analyzer).
The optical properties of the samples were tested with a Cary 5000 UV-Vis-NIR spectrophotometer. Raman measurements were taken with a Renishaw instrument with λ=532 nm and an Ar ion laser. High-resolution X-ray photoelectron spectroscopy (HRXPS) was carried out using a Thermo Scientific K-alpha instrument. N2 adsorption–desorption isotherms, specific surface area, pore volume, and pore size were examined using a Micrometrics ASAP 2000 adsorption analyzer, the Brunauer-Emmett-Teller (BET) method, and Barrett-Joyner-Halenda (BJH) desorption, respectively. 2.3 Photocatalytic test CR was used as a target pollutant for assessing the photocatalytic activity of the WO3 photocatalyst. Visible light irradiation was provided by a 300W xenon lamp with a wavelength of >420 nm, which was surrounded by a quartz vessel. The light source was completely surrounded by a suspension of the WO3 photocatalyst and aqueous CR (100 mL, 15 mg L-1). Before irradiation, the suspension was stirred in the dark for 30 min to obtain adsorption– desorption equilibrium between the organic pollutant molecules and the photocatalyst. During light irradiation, samples of the reaction solution were collected at particular intervals and studied for their absorption properties using an optical spectrophotometer. 3. Results and discussion 3.1 Structural characterization of WO3 photocatalyst The crystal structure of WO3 sheets was characterized by XRD, as shown in Fig. 1. The XRD pattern of the as-prepared sample matches well with monoclinic (JCPDS No. 83-0950). In the case of the sintered samples, peaks appeared at 23.11°, 23.58°, 24.36°, 33.26°, 33.56°, 34.16°, 49.93°, 50.33°, 50.72°, 53.46°, 54.77°, 55.93°, 56.08°, 60.39°, 61.66°, and 62.26°, which indicate the structural transformation from monoclinic to orthorhombic β-WO3 structure (at and above 180°C). The patterns of all sintered samples match well with orthorhombic tungsten oxide with lattice constants of a= 7.36 Å, b= 7.573 Å, and c= 7.762 Å (JCPDS No. 89-4479). This transformation leads to increasing packing density, which is favorable for increasing lattice sites [31, 32]. The broad and sharp reflection peaks indicated that the sintered samples were crystalline. The relative intensities of the (002) and (200) peaks increase with the sintered temperature, which reflects the increased number of stacking layers (i.e., sheet thickness), which is supported by the SEM images in Fig. 3. Fig. 2 shows the morphology by SEM and TEM images and SEM-EDX spectrum of the as-prepared WO3 sample. Randomly oriented sheet-like structures were observed with thickness of a few tens nanometers and widths of a few micrometers (Fig. 2a). The high-magnification SEM image of the as-prepared WO3 sample (Fig. 2b) shows an irregular sheet-like morphology with several ridges at the surface that could be observed clearly. Also, the TEM images of the asprepared WO3 sample also clearly showed sheet-like morphology (Fig.2c). The EDX spectrum (Fig. 2d) revealed the presence of W and O, and no other impurity peaks were observed. This confirms the pure nature of the as-prepared WO3.
The influence of sintering temperature on the morphologies of S-180, S-300 and S-400 samples are shown in Fig.3. All sintered samples show well-ordered sheet-like microstructures. Sample sintered at 300°C is compact aggregates of leaf-like shapes with tiny sheet-like structures has been indicated by circles (Fig.3 d, f). As can be seen raising sintering temperature from 180 to 300°C is enough to limit the formation of leaf-like sheets morphology. In the case of S-400 sample, high temperature destroys the leaf shape morphology and increase the intensity of tiny sheets on the surface. Fig. 4 shows the TEM images and selected area electron diffraction (SAED) patterns of the samples. The sintered samples showed multi-layered sheets morphology, as shown in Fig.4ac. The SAED pattern confirmed the existence of crystal fringes over the entire surface of the WO3 sheet (S-300) in HRTEM image. The occurrence of a halo-like SAED pattern and the dot pattern of the as-prepared WO3 sample (Fig.4d) confirmed that the nanostructure was nanocrystalline. This shows good agreement with the XRD results. The resolved spacing between two parallel, adjacent fringes was 0.372 nm, which matches the (020) lattice plane group of orthorhombic WO3. The vibrational modes of the chemical groups in the WO3 sheets at different sintered samples were examined by FTIR spectroscopy, as shown in Fig. 5. For the as-prepared sample, broad bands between 600 and 750 cm-1 are ascribed to the W-O-W bonding modes of WO3 corner-distributed species [33]. The peaks between 850 and 1000 cm-1 are attributed to the stretching mode of O-W-O or terminal vibrational W=O modes of WO3. The absorption bands of the as-prepared sample at 1414, 1624, 3117, 3245, and 3401 cm-1 are assigned to the hydroxyl groups of (δ(W-OH-H2O)) [34] and vibrational stretching modes of H2O molecules and -OH groups, respectively [35]. In the case of sintered samples, as temperature increases all bands turn weak and narrow. In the wavenumber range between 900 and 1200 cm-1, the as-prepared sample has a broad shoulder compared to the sintered samples, which means increasing temperature gradually makes the broad bands weaker and shortens the W=O bond length [36]. At wavenumber above 1600 cm-1, all bands disappeared due to the lower number of water molecules. The TG-DTA analysis results of the as-prepared sample are shown in Fig.6. The composition mainly results in two significant mass loss stages when the sample is heated from room temperature to 900°C. Exothermic reactions occur during the oxidative thermal decomposition of organic material. The as-prepared sample shows remarkable weight loss of about 16% from room temperature to 220°C, which is due to the loss of crystal water attached to the sample. The weight loss above 660°C is attributed to the decomposition of oxygencontaining groups, breakage of the organic links, and thermal degradation. The total mass loss was 44.9% at 900°C. H2WO4 gradually starts to form when the 1-propanol and sulfuric acid are added to Na2WO4·2H2O [37]. With longer reaction time, the H2WO4 is decomposed to form a WO3 nucleus due to the much lower solubility of WO3 than the tungsten precursors [38]. Therefore, the growth mechanism of the WO3 sheets can be understood as follows [37, 38]: WO42−+ 2H+ → H2WO4 H2WO4 → WO3 + H2O
(1) (2)
During the thermal decomposition process, the WO3 nanoparticles are transformed to nanosheets in the presence of urea, which affect the growth orientation and favor the formation
of faceted sheet-like structures. With further aggregation and growth, a few layers of WO3 nanosheets are assembled and stacked. The urea plays a critical role in the formation of the faceted WO3 sheets. The survey spectrum of XPS shows two distinct peaks corresponding to tungsten and oxygen, as shown in Fig.7a. A weak carbon peak is observed due to absorbed carbon during the XPS measurement. The fine spectrum of W consisting of split doublet peaks is shown in Fig.7b Two major peaks are well separated without any shoulders at ca. 35.57 and 37.68 eV are ascribed to W4f7/2 and W4f5/2, respectively, which have a full width at half-maximum of 1.78 eV, an intensity ratio of 0.77 and ΔE of 2.1 eV, might be associated with the W 6+ oxidation state of W atoms and are consistent with XPS data reported in the literature [39, 40]. The O1s peaks at 531.46 eV and 533.35 are assigned to the oxygen and WO3(H2O)n phases at the surface of the asprepared WO3 sample (Fig.7c). However, chemisorbed water decomposes under sintering temperatures, and oxidized forms can be easily absorbed on the surface of the sample. No surfactant decomposition peaks were observed in FTIR analysis, and the presence of water molecules in the as-prepared sample is more obvious. This synthetic process allows the production of WO3 without impurities, as confirmed by XPS and XRD results. Fig.8 shows the BET specific surface area and pore size of the as-prepared WO3 and S300 samples. The N2 adsorption-desorption isotherms of both samples show type IV isotherms with an H4 hysteresis loop. The specific surface area, pore volume and pore diameter of the S300 sample are 67.17 m2g-1, 0.31 cm3g-1, and 21.18 nm, respectively. For the as-prepared WO3 sample, the specific surface area, pore volume, and pore diameter are 51.32 m 2g-1, 0.19 cm3g-1, and 17.84 nm, respectively, which are lower than the sintered samples. The nitrogen adsorption capability was enhanced for the sintered sample. These results show that the sintering temperature significantly improves the surface area and pore structure. The light absorbance properties of the as-prepared WO3 and S-300 samples were measured using a UV-Vis-NIR spectrophotometer, as shown in Fig.9a. Compared with the asprepared WO3 sample, broad absorption was observed in the visible light region for S-300 sample, which is in good agreement with data reported in the literature [41]. This can be attributed to the surface modification of WO3. Therefore, when the intensity of the absorption edge for S-300 sample has been increased compared with the as-prepared WO3 sample. Correspondingly, the band gaps of the as-prepared WO3 and S-300 samples are 2.61 and 2.55 eV, (Fig.9b) respectively. The photocatalytic performance of the as-prepared and sintered samples was studied by the degradation of CR in aqueous solution under xenon lamp irradiation. Fig.10 shows the difference in the absorption spectrum of CR with respect to light exposure time using 50 mgL-1 of the S-300 sample. Intense absorption peaks are obtained at 237 nm, 344 nm, and 495 nm respectively, which gradually decrease with increasing irradiation time. More than 86% of the initial concentration of CR was degraded within 60 min with S-300. On the other hand, 77% and 80% and 54.2% degradation was recorded with S-180, S-400 and the as-prepared WO3 sample. Fig.11 shows the photocatalytic degradation results without any WO3 photocatalyst (a blank test), with the as-prepared WO3 sample, and with the sintered samples. Blank test shows that a negligible amount (i.e. < 1.4%) of CR is degraded after 60 min of irradiation (Fig.11). The as-prepared WO3 sample shows lower photocatalytic performance than the sintered samples. The order of degradation rates is as follows: as-prepared sample < S-180 < S-400 < S-300. The S-300 showed higher photocatalytic performance than other samples. This may be due to the high absorption capacity, high surface area, active sites, band gap energy, and moderate crystallinity.
Hence, the sintering process significantly improves the crystallinity of the samples, which enhances the photocatalytic efficiency. The semilog curve of the photocatalytic degradation of CR with the different samples is shown in Fig.12. All samples followed a pseudo-first-order reaction, which is indicated by the straight line of the kinetic fitting of the curve with respect to irradiation time. Generally, the kinetic rate constant is estimated using the following equation: ln (C/C0) = kt
(3)
Where C is the concentration at time t, C0 is the initial concentration, k is the rate constant, and t is the irradiation time. The CR degradation rate constants are 0.0078, 0.0129, 0.0163, and 0.0151 min-1 respectively for as-prepared WO3, S-180, S-300, and S-400 samples. The low degradation rate of the as-prepared WO3 sample is may be due to the poor crystallinity, as demonstrated by the XRD results. The purity of WO3 is expected to influence the optical property which is an important property for enhancing the charge carrier separation process. The crystallinity eliminates the surface defects, which provide recombination centers for photoinduced electron-hole pairs and it could be affect the photocatalytic performance [4, 6, 9]. Thus, the enhanced photocatalytic performance of the S-300 sample can be explained as follows. Under the visible light irradiation, the electrons transitioned from the valence band to the conduction band of WO3 and created hydroxyl radicals (·OH), which are responsible for the degradation of CR molecules. WO3 has large electron storage capacity, which is favorable for suppressing the recombination effect and enhancing the photocatalytic performance. The cycle stability and reusability of the as-prepared WO3 and S-300 samples were investigated, as shown in Fig.13. The samples were collected after each cycle of degradation experiments and reused four times. As a result, the photocatalytic degradation of CR decreased slightly after four cycles. For the as-prepared sample, only 59.3% of CR can be removed in the four cycles, decreasing to approximately 5.1%. In contrast, for the S-300 sample, there was only a slight drop in efficiency from 86% to 82.7%. This shows that the S-300 sample has better stability and reusability than the as-prepared WO3. This may be due to too many layers of the sheets or from breaking, which decreases the ability to capture light in the recycling process. The photoluminescence spectra of as-prepared and S-300 samples are shown in Fig.14. The enhancement of photocatalytic performance is often related with a diminished recombination rate of the photoinduced carriers that corresponds to a low PL intensity [28, 41]. The PL spectra asprepared and S-300 samples were measured with an excitation wavelength of 284 nm. It can be seen that the as-prepared WO3 and S-300 samples exhibited a strong emission peak at 474 nm. However, the peak intensity of the S-300 lower than that of as-prepared WO3 sample, indicating the S-300 sample can significantly suppress the recombination of electron and holes. From the PL studies, S-300 photocatalyst exhibited the lowest PL intensity that correlated to its superior photocatalytic activity over the as-prepared WO3 photocatalyst activity. We proposed the possible photocatalytic mechanism for degradation of CR with presence of WO3 photocatalyst, as shown in Scheme 1. Based on the optical absorption results, the WO3 can be easily excited to generate electrons and holes under visible light irradiation due to high absorption capacity in between the conduction band and valence band. The photocatalytic reactions were originated when WO3 photocatalyst absorbs photons with energies larger than that of its band gap energy (2.54 eV) from the illumination. Under
visible light irradiation, electrons and holes are generated simultaneously and accumulate on the WO3 due to the van der Waals forces of the WO3 surface. Under visible light illumination, the photoinduced electrons are directly transferred from the valence band (VB) to the conduction band (CB) and are ultimately scavenged by the oxygen in the water, forming hydroxyl radicals to degrade the CR [16, 23]. In the meantime, the remaining holes in the VB of WO3 can be consumed directly by the CR or react with water or hydroxyl to form hydroxyl free radicals (·OH), which are a strong oxidant for CR degradation [27, 28]. The photocatalytic activity is enhanced with help of more photoinduced holes, which can speed up the photocatalytic degradation of CR. This is responsible for the photocatalytic activity of WO3. The following photoreactions are proposed for the effective separation of electrons and holes of WO3 under visible light irradiation: WO3 + hν → WO3 (e¯ + h+)
(4)
e¯ + O2 → ˙O2¯
(5)
h+ + OH¯ → ˙OH
(6)
CR + ˙O2¯/ ˙OH → products
(7)
CR + h+ → CO2 + H2O + by products
(8)
In the photocatalytic process, photo-induced electron-holes obtainable over a WO3 and react with O2 and OH¯ species and ·O2¯ and ·OH¯ oxidants are formed [23, 27, 28]. These radicals can oxidize CR to intermediates and finally mineralized to H2O and H2O2 [21, 26-28]. The large number of main reactive species such as h+, O2 and OH¯ are actively involved in photocatalytic reaction and responsible for degradation of CR. On the basis of the above results, the possible improvement in photodegradation mechanism for WO3 sheets (S-300) photocatalyst can be proposed by following reasons: (i) sintering process is beneficial to the enhancement of light absorption capacity; (ii) large surface area can improve the photocatalytic activity; (iii) moderate crystallinity; and (iv) increased reaction sites exposed by formation of tiny sheet on their surface. Therefore these are very important causes for the enhancement of photocatalytic activity.
4. Conclusions In summary, we have successfully synthesized a novel photocatalyst of few- to multilayered WO3 sheets via thermal decomposition. The influence of the sintering temperature on the morphology has been studied. The S-300 sample shows higher photocatalytic performance than the as-prepared WO3, S-180 and S-400 samples, and S-300 sample has optimal performance for the removal of CR. The improved performance can be attributed to the increased surface area and charge separation due to the tiny sheets grown on the surface with sintering treatment. Also, the recycling and reusability tests showed that the S-300 sample is a photo-stable photocatalyst. This work is likely to offer useful information about the fabrication and temperature effects of
WO3 photocatalyst with enhanced photocatalytic activity for the degradation of organic pollutants under ambient conditions. Acknowledgement This work was conducted under the framework of the National Research Foundation of Korea (NRF) and funded by the Ministry of Science, ICT, and Future Planning (2014R1A2A2A01007081).
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Fig.1 XRD patterns of (a) as-prepared sample and sintered samples (b) S-180, (c) S-300 and (d) S-400 of WO3 sheets
Fig.2 (a, b) SEM images, (c) TEM image, (and d) EDX spectrum of as-prepared WO3 sample
Fig.3 SEM images of (a, b) S-180, (c, d) S-300, and (e, f) S-400 samples
Fig.4 TEM images of (a) S-180, (b) S-300, and (c) S-400 samples and (d) SAED pattern of S300 sample
Fig.5 FTIR spectrum of (a) as-prepared WO3 sheets, (b) S-180, (c) S-300 and (d) S-400 samples
Fig.6 TG-DTA curves of as-prepared WO3 sample
Fig. 7 High resolution X-ray photoelectron spectra of as-prepared WO3 sample: (a) survey, (b) W4f and (c) O1s
Fig.8 N2 adsorption/desorption isotherm of (a) as-prepared and (b) S-300 sample; inset: Barrett– Joyner–Halenda (BJH) pore size distribution data of the same samples
Fig.9 (a) UV–vis spectra and (b) Tauc plots of as-prepared WO3 and S-300 samples.
Fig.10 Time-dependent visible light absorbance spectra of the CR solution along with S-300 photocatalyst taken at different times
Fig.11 Photocatalytic degradation of CR under visible light irradiation in presence of S-300 photocatalyst
Fig.12 The kinetic plot of photocatalytic degradation of CR with S-300 photocatalyst under visible light irradiation
Fig.13 Recycling and reusability performance test of as-prepared WO3 and S-300 photocatalyst
Fi.14 Photoluminescence spectra of as-prepared WO3 and S-300 photocatalyst
Scheme 1 Schematic diagram illustrating the proposed mechanism for the photocatalytic activity of the WO3 sheets
S0025-5408(16)30500-1 http://dx.doi.org/doi:10.1016/j.materresbull.2016.08.008 MRB 8888
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Please cite this article as: S.V.Prabhakar Vattikuti, Chan Byon, Ich-Long Ngo, Highly crystalline multi-layered WO3 sheets for photodegradation of Congo red under visible light irradiation, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly crystalline multi-layered WO3 sheets for photodegradation of Congo red under visible light irradiation S.V. Prabhakar Vattikuti1*, Chan Byon1* and Ich-Long Ngo1 1
School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea, 712-749
*Corresponding author Address: Dr. S.V.Prabhakar Vattikuti, School of Mechanical Engineering Yeungnam University 214-1 Dae-dong Gyeongsan-si, Gyeongsangbuk-do (712-749, Republic of Korea) Mobile: +82-(0)53-810-2452 Fax: +82-53-810-4627 *Corresponding author E-mail: [email protected] (S.V.Prabhakar Vattikuti) [email protected] (Chan Byon) Graphical abstract
Highlights:
WO3 multi- layered sheets were successfully synthesized by simple thermal decomposition method
Phase transformation of sintered WO3 occurred from monoclinic to orthorhombic structure
Few-to-multi layered WO3 sheets are formed by the oriented attachment (OA)-self-assembly (SA)
Photodegradation mechanism of WO3 sheets is elucidated and discussed
Synthetic procedure may open up an opportunity to tailor the morphologies of semiconductors
Abstract We report a simple synthetic method for few- to multi-layered tungsten oxide (WO3) sheets through complexation of a sodium tungstate dihydrate precursor by 1-propanol, sulfuric acid and urea. The formation of tungsten oxo-clusters and their organized transformation into few-tomulti-layered WO3 sheets occurs during the hydrolysis stage, and crystallization occurs during the post-sintering process. XRD results exhibited a phase transformation from monoclinic to orthorhombic β-WO3 due to post-sintering effects. The as-prepared material appears as selforganized few- to multi-layered WO3 sheets. The as-prepared and sintered samples are used for the photodegradation of Congo red (CR) under visible light irradiation. A sample sintered at 300oC (S-300) shows much higher photocatalytic activity than the as-prepared WO3 and those sintered at 180oC and 400oC. The S-300 photocatalyst has the optimal performance in the removal of CR, which is due to high specific surface area and patches of crystalline sites on the photocatalyst surface. These sites can suppress the recombination of electrons and holes by accelerating the charge carrier transformation within the photocatalyst.
Keywords: Semiconductor; Self-organized sheets; Photocatalytic activity; Congo red; Postsintering.
1. Introduction Sunlight-driven photocatalysts are in demand for development of eco-friendly materials due to environmental impact and energy issues [1, 2]. Semiconductor photocatalysis has arisen as one of the most promising technologies due to better utilization of solar energy [3-5]. Better photocatalyst performance strongly depends on the absorption capacity toward sunlight and the effectiveness of photoinduced reactions of the charge carriers [3]. However, various traditional photocatalysts are more active under UV light irradiation due to their wide band gap and high recombination effect of photoinduced electron and holes. This is especially true for multi-metal oxides, oxynitrides, and sulfides [4-6]. However, other issues need to be addressed, including stability, reusability, and scalability. Recently, nanosized WO3 materials have been broadly used for photocatalysts, solar cells, and gas sensors [7]. WO3 is an n-type semiconductor and has a band gap of around 2.5–3.5 eV. Layered WO3 has unique interlayer chemistry and high applicability for exfoliation/intercalation reactions [3, 8-10]. Furthermore, it can undergo exfoliation to produce single- or multi-layer sheets with unique 2D morphology, which are attractive as building blocks for fabrication of nanodevices [11]. Another important application of WO3 is as a photocatalyst for degradation of organic pollutants. But pure WO3 photocatalyst shows lower visible light energy conversion efficiency due to its low conduction band level and high recombination rate of the photoinduced electron-hole pairs [9, 10]. Hence, many attempts have been made to improve the photocatalytic activity of WO3, including controlled morphology, heterogeneous systems, compositing with other materials, and doping with other metals [12-15]. Different factors improve photocatalytic activity, such as high surface area, coupling of different transition metals to reduce the band gap, and controlled morphology [16-17]. Recently, 2D and 3D WO3 nanostructures have attracted significant attention due to their unique physico-chemical properties for potential applications [18-20]. Huang et al. reported that a single hexagonal WO3 nanowire is worked as sensing element for UV photodectors, biological sensors and optoelectronic devices, which exhibits excellent sensitivity and reversibility under ultraviolet region [10]. To improve the photoactivity of the WO3 material is required to obtain a high surface area [21, 22]. Domene et al. [23] have reported the high photocatalytic performance of anodized WO3 nanoplates of globular clusters and monoclinic WO3 nanoplates reported by Zhang et al. [24] were exhibited enhanced photocatalytic activity under the sun light irradiation, which may be ascribed to its high BET surface area. However, the photocatalytic performance of single-phase semiconductors is limited due to high recombination rate of photo-generated electron-hole pairs. Many strategies have been dedicated to improve the efficiency of photocatalytic activity under visible light irradiation, including preparation of nanocomposites or doping of noble metal on to semiconductor or preparation of semiconductors with various morphological structures or control crystalline structures, etc. [25, 26]. To date, many efforts have been devoted to synthesizing WO3 with different controlled morphologies, such as
nanofibers [27], nanoflowers [28], nanosheets [29], and nanoplatelets [30]. However, the methods often require multiple steps, expensive templates, costly processes, and high temperatures and pressures. Finding a simple synthetic method that is economical and reliable to obtain these morphologies is a considerable challenge. We report the one-step synthesis of WO3 by a thermal hydrolysis process using 1propanol, sulfuric acid and urea. The as-prepared WO3 samples were sintered at 180°C, 300°C, or 400°C in order to evaluate the morphology features. Different content of WO3 photocatalyst were used to evaluate the photocatalytic degradation of Congo red (CR) under visible light exposure. The results show that the S-300 sample showed higher photocatalytic activity for degradation of CR than the as-prepared WO3, S-180, and S-400. The as-prepared WO3 and S-300 samples also exhibit good stability and reusability after four successive recycles. A possible photodegradation mechanism of the S-300 sample is discussed based on the obtained results. 2. Experimental details 2.1 Preparation of WO3 sheets All chemical reagents were used as obtained. To get the WO3 sheets, sodium tungstate dihydrate (Na2WO4·2H2O, Aldrich), hydrochloric acid (HCl), sulfuric acid, 1-propanol, urea, and absolute ethanol were used as starting materials. First, 7.75 g of Na2WO4·2H2O and 15 mL of 1propanol were dissolved in 25 ml of distilled water and stirred continuously for 10 min to get a homogeneous solution. Then, 12.5 ml of ethanol and 2.78 mL of sulfuric acid were added to the resulting solution and magnetically stirred for 30 min at 80°C on a hot plate to form a homogeneous solution. Then, 0.58 g of urea was added. The pH of the resulting solution was adjusted to 3 by adding a sufficient amount of hydrochloric acid (HCl). After stirring at 80°C for 1 h, the obtained solution (white-yellow color) was thermal decomposed at 140°C for 3 h. The resulting yellow precipitates were collected through centrifugation at 5000 rpm for 30 min and then filtered and washed three times with distilled water and ethanol. The as-prepared precipitates were dried under vacuum at 100°C for 6 h and then sintered at 180°C, 300°C, and 400°C for 2 h in a muffle furnace. Hereafter, the sintered samples are denoted as S-180, S-300, and S-400, respectively. 2.2 Characterization of WO3 sheets The crystal structures of the samples were studied by wide-angle powder X-ray diffraction (XRD) on a PANalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation (0.154056 nm) at 40 kV and 30 mA. The surface morphology of synthesized samples were characterized using a scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, Hitachi H-7000), and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F 20 s-twin TEM). Elemental analyses were performed through energy-dispersive X-ray spectroscopy (EDX) with an X-ray analyzer integrated into the SEM instrument. Fourier transform infrared spectroscopy (FTIR, Avatar 370) with a spectral range of 4000–400 cm−1 was used for the phase purity test. Thermal stability analysis was performed with a Shimadzu TGA/DTA instrument (DTG-60/60H thermal analyzer).
The optical properties of the samples were tested with a Cary 5000 UV-Vis-NIR spectrophotometer. Raman measurements were taken with a Renishaw instrument with λ=532 nm and an Ar ion laser. High-resolution X-ray photoelectron spectroscopy (HRXPS) was carried out using a Thermo Scientific K-alpha instrument. N2 adsorption–desorption isotherms, specific surface area, pore volume, and pore size were examined using a Micrometrics ASAP 2000 adsorption analyzer, the Brunauer-Emmett-Teller (BET) method, and Barrett-Joyner-Halenda (BJH) desorption, respectively. 2.3 Photocatalytic test CR was used as a target pollutant for assessing the photocatalytic activity of the WO3 photocatalyst. Visible light irradiation was provided by a 300W xenon lamp with a wavelength of >420 nm, which was surrounded by a quartz vessel. The light source was completely surrounded by a suspension of the WO3 photocatalyst and aqueous CR (100 mL, 15 mg L-1). Before irradiation, the suspension was stirred in the dark for 30 min to obtain adsorption– desorption equilibrium between the organic pollutant molecules and the photocatalyst. During light irradiation, samples of the reaction solution were collected at particular intervals and studied for their absorption properties using an optical spectrophotometer. 3. Results and discussion 3.1 Structural characterization of WO3 photocatalyst The crystal structure of WO3 sheets was characterized by XRD, as shown in Fig. 1. The XRD pattern of the as-prepared sample matches well with monoclinic (JCPDS No. 83-0950). In the case of the sintered samples, peaks appeared at 23.11°, 23.58°, 24.36°, 33.26°, 33.56°, 34.16°, 49.93°, 50.33°, 50.72°, 53.46°, 54.77°, 55.93°, 56.08°, 60.39°, 61.66°, and 62.26°, which indicate the structural transformation from monoclinic to orthorhombic β-WO3 structure (at and above 180°C). The patterns of all sintered samples match well with orthorhombic tungsten oxide with lattice constants of a= 7.36 Å, b= 7.573 Å, and c= 7.762 Å (JCPDS No. 89-4479). This transformation leads to increasing packing density, which is favorable for increasing lattice sites [31, 32]. The broad and sharp reflection peaks indicated that the sintered samples were crystalline. The relative intensities of the (002) and (200) peaks increase with the sintered temperature, which reflects the increased number of stacking layers (i.e., sheet thickness), which is supported by the SEM images in Fig. 3. Fig. 2 shows the morphology by SEM and TEM images and SEM-EDX spectrum of the as-prepared WO3 sample. Randomly oriented sheet-like structures were observed with thickness of a few tens nanometers and widths of a few micrometers (Fig. 2a). The high-magnification SEM image of the as-prepared WO3 sample (Fig. 2b) shows an irregular sheet-like morphology with several ridges at the surface that could be observed clearly. Also, the TEM images of the asprepared WO3 sample also clearly showed sheet-like morphology (Fig.2c). The EDX spectrum (Fig. 2d) revealed the presence of W and O, and no other impurity peaks were observed. This confirms the pure nature of the as-prepared WO3.
The influence of sintering temperature on the morphologies of S-180, S-300 and S-400 samples are shown in Fig.3. All sintered samples show well-ordered sheet-like microstructures. Sample sintered at 300°C is compact aggregates of leaf-like shapes with tiny sheet-like structures has been indicated by circles (Fig.3 d, f). As can be seen raising sintering temperature from 180 to 300°C is enough to limit the formation of leaf-like sheets morphology. In the case of S-400 sample, high temperature destroys the leaf shape morphology and increase the intensity of tiny sheets on the surface. Fig. 4 shows the TEM images and selected area electron diffraction (SAED) patterns of the samples. The sintered samples showed multi-layered sheets morphology, as shown in Fig.4ac. The SAED pattern confirmed the existence of crystal fringes over the entire surface of the WO3 sheet (S-300) in HRTEM image. The occurrence of a halo-like SAED pattern and the dot pattern of the as-prepared WO3 sample (Fig.4d) confirmed that the nanostructure was nanocrystalline. This shows good agreement with the XRD results. The resolved spacing between two parallel, adjacent fringes was 0.372 nm, which matches the (020) lattice plane group of orthorhombic WO3. The vibrational modes of the chemical groups in the WO3 sheets at different sintered samples were examined by FTIR spectroscopy, as shown in Fig. 5. For the as-prepared sample, broad bands between 600 and 750 cm-1 are ascribed to the W-O-W bonding modes of WO3 corner-distributed species [33]. The peaks between 850 and 1000 cm-1 are attributed to the stretching mode of O-W-O or terminal vibrational W=O modes of WO3. The absorption bands of the as-prepared sample at 1414, 1624, 3117, 3245, and 3401 cm-1 are assigned to the hydroxyl groups of (δ(W-OH-H2O)) [34] and vibrational stretching modes of H2O molecules and -OH groups, respectively [35]. In the case of sintered samples, as temperature increases all bands turn weak and narrow. In the wavenumber range between 900 and 1200 cm-1, the as-prepared sample has a broad shoulder compared to the sintered samples, which means increasing temperature gradually makes the broad bands weaker and shortens the W=O bond length [36]. At wavenumber above 1600 cm-1, all bands disappeared due to the lower number of water molecules. The TG-DTA analysis results of the as-prepared sample are shown in Fig.6. The composition mainly results in two significant mass loss stages when the sample is heated from room temperature to 900°C. Exothermic reactions occur during the oxidative thermal decomposition of organic material. The as-prepared sample shows remarkable weight loss of about 16% from room temperature to 220°C, which is due to the loss of crystal water attached to the sample. The weight loss above 660°C is attributed to the decomposition of oxygencontaining groups, breakage of the organic links, and thermal degradation. The total mass loss was 44.9% at 900°C. H2WO4 gradually starts to form when the 1-propanol and sulfuric acid are added to Na2WO4·2H2O [37]. With longer reaction time, the H2WO4 is decomposed to form a WO3 nucleus due to the much lower solubility of WO3 than the tungsten precursors [38]. Therefore, the growth mechanism of the WO3 sheets can be understood as follows [37, 38]: WO42−+ 2H+ → H2WO4 H2WO4 → WO3 + H2O
(1) (2)
During the thermal decomposition process, the WO3 nanoparticles are transformed to nanosheets in the presence of urea, which affect the growth orientation and favor the formation
of faceted sheet-like structures. With further aggregation and growth, a few layers of WO3 nanosheets are assembled and stacked. The urea plays a critical role in the formation of the faceted WO3 sheets. The survey spectrum of XPS shows two distinct peaks corresponding to tungsten and oxygen, as shown in Fig.7a. A weak carbon peak is observed due to absorbed carbon during the XPS measurement. The fine spectrum of W consisting of split doublet peaks is shown in Fig.7b Two major peaks are well separated without any shoulders at ca. 35.57 and 37.68 eV are ascribed to W4f7/2 and W4f5/2, respectively, which have a full width at half-maximum of 1.78 eV, an intensity ratio of 0.77 and ΔE of 2.1 eV, might be associated with the W 6+ oxidation state of W atoms and are consistent with XPS data reported in the literature [39, 40]. The O1s peaks at 531.46 eV and 533.35 are assigned to the oxygen and WO3(H2O)n phases at the surface of the asprepared WO3 sample (Fig.7c). However, chemisorbed water decomposes under sintering temperatures, and oxidized forms can be easily absorbed on the surface of the sample. No surfactant decomposition peaks were observed in FTIR analysis, and the presence of water molecules in the as-prepared sample is more obvious. This synthetic process allows the production of WO3 without impurities, as confirmed by XPS and XRD results. Fig.8 shows the BET specific surface area and pore size of the as-prepared WO3 and S300 samples. The N2 adsorption-desorption isotherms of both samples show type IV isotherms with an H4 hysteresis loop. The specific surface area, pore volume and pore diameter of the S300 sample are 67.17 m2g-1, 0.31 cm3g-1, and 21.18 nm, respectively. For the as-prepared WO3 sample, the specific surface area, pore volume, and pore diameter are 51.32 m 2g-1, 0.19 cm3g-1, and 17.84 nm, respectively, which are lower than the sintered samples. The nitrogen adsorption capability was enhanced for the sintered sample. These results show that the sintering temperature significantly improves the surface area and pore structure. The light absorbance properties of the as-prepared WO3 and S-300 samples were measured using a UV-Vis-NIR spectrophotometer, as shown in Fig.9a. Compared with the asprepared WO3 sample, broad absorption was observed in the visible light region for S-300 sample, which is in good agreement with data reported in the literature [41]. This can be attributed to the surface modification of WO3. Therefore, when the intensity of the absorption edge for S-300 sample has been increased compared with the as-prepared WO3 sample. Correspondingly, the band gaps of the as-prepared WO3 and S-300 samples are 2.61 and 2.55 eV, (Fig.9b) respectively. The photocatalytic performance of the as-prepared and sintered samples was studied by the degradation of CR in aqueous solution under xenon lamp irradiation. Fig.10 shows the difference in the absorption spectrum of CR with respect to light exposure time using 50 mgL-1 of the S-300 sample. Intense absorption peaks are obtained at 237 nm, 344 nm, and 495 nm respectively, which gradually decrease with increasing irradiation time. More than 86% of the initial concentration of CR was degraded within 60 min with S-300. On the other hand, 77% and 80% and 54.2% degradation was recorded with S-180, S-400 and the as-prepared WO3 sample. Fig.11 shows the photocatalytic degradation results without any WO3 photocatalyst (a blank test), with the as-prepared WO3 sample, and with the sintered samples. Blank test shows that a negligible amount (i.e. < 1.4%) of CR is degraded after 60 min of irradiation (Fig.11). The as-prepared WO3 sample shows lower photocatalytic performance than the sintered samples. The order of degradation rates is as follows: as-prepared sample < S-180 < S-400 < S-300. The S-300 showed higher photocatalytic performance than other samples. This may be due to the high absorption capacity, high surface area, active sites, band gap energy, and moderate crystallinity.
Hence, the sintering process significantly improves the crystallinity of the samples, which enhances the photocatalytic efficiency. The semilog curve of the photocatalytic degradation of CR with the different samples is shown in Fig.12. All samples followed a pseudo-first-order reaction, which is indicated by the straight line of the kinetic fitting of the curve with respect to irradiation time. Generally, the kinetic rate constant is estimated using the following equation: ln (C/C0) = kt
(3)
Where C is the concentration at time t, C0 is the initial concentration, k is the rate constant, and t is the irradiation time. The CR degradation rate constants are 0.0078, 0.0129, 0.0163, and 0.0151 min-1 respectively for as-prepared WO3, S-180, S-300, and S-400 samples. The low degradation rate of the as-prepared WO3 sample is may be due to the poor crystallinity, as demonstrated by the XRD results. The purity of WO3 is expected to influence the optical property which is an important property for enhancing the charge carrier separation process. The crystallinity eliminates the surface defects, which provide recombination centers for photoinduced electron-hole pairs and it could be affect the photocatalytic performance [4, 6, 9]. Thus, the enhanced photocatalytic performance of the S-300 sample can be explained as follows. Under the visible light irradiation, the electrons transitioned from the valence band to the conduction band of WO3 and created hydroxyl radicals (·OH), which are responsible for the degradation of CR molecules. WO3 has large electron storage capacity, which is favorable for suppressing the recombination effect and enhancing the photocatalytic performance. The cycle stability and reusability of the as-prepared WO3 and S-300 samples were investigated, as shown in Fig.13. The samples were collected after each cycle of degradation experiments and reused four times. As a result, the photocatalytic degradation of CR decreased slightly after four cycles. For the as-prepared sample, only 59.3% of CR can be removed in the four cycles, decreasing to approximately 5.1%. In contrast, for the S-300 sample, there was only a slight drop in efficiency from 86% to 82.7%. This shows that the S-300 sample has better stability and reusability than the as-prepared WO3. This may be due to too many layers of the sheets or from breaking, which decreases the ability to capture light in the recycling process. The photoluminescence spectra of as-prepared and S-300 samples are shown in Fig.14. The enhancement of photocatalytic performance is often related with a diminished recombination rate of the photoinduced carriers that corresponds to a low PL intensity [28, 41]. The PL spectra asprepared and S-300 samples were measured with an excitation wavelength of 284 nm. It can be seen that the as-prepared WO3 and S-300 samples exhibited a strong emission peak at 474 nm. However, the peak intensity of the S-300 lower than that of as-prepared WO3 sample, indicating the S-300 sample can significantly suppress the recombination of electron and holes. From the PL studies, S-300 photocatalyst exhibited the lowest PL intensity that correlated to its superior photocatalytic activity over the as-prepared WO3 photocatalyst activity. We proposed the possible photocatalytic mechanism for degradation of CR with presence of WO3 photocatalyst, as shown in Scheme 1. Based on the optical absorption results, the WO3 can be easily excited to generate electrons and holes under visible light irradiation due to high absorption capacity in between the conduction band and valence band. The photocatalytic reactions were originated when WO3 photocatalyst absorbs photons with energies larger than that of its band gap energy (2.54 eV) from the illumination. Under
visible light irradiation, electrons and holes are generated simultaneously and accumulate on the WO3 due to the van der Waals forces of the WO3 surface. Under visible light illumination, the photoinduced electrons are directly transferred from the valence band (VB) to the conduction band (CB) and are ultimately scavenged by the oxygen in the water, forming hydroxyl radicals to degrade the CR [16, 23]. In the meantime, the remaining holes in the VB of WO3 can be consumed directly by the CR or react with water or hydroxyl to form hydroxyl free radicals (·OH), which are a strong oxidant for CR degradation [27, 28]. The photocatalytic activity is enhanced with help of more photoinduced holes, which can speed up the photocatalytic degradation of CR. This is responsible for the photocatalytic activity of WO3. The following photoreactions are proposed for the effective separation of electrons and holes of WO3 under visible light irradiation: WO3 + hν → WO3 (e¯ + h+)
(4)
e¯ + O2 → ˙O2¯
(5)
h+ + OH¯ → ˙OH
(6)
CR + ˙O2¯/ ˙OH → products
(7)
CR + h+ → CO2 + H2O + by products
(8)
In the photocatalytic process, photo-induced electron-holes obtainable over a WO3 and react with O2 and OH¯ species and ·O2¯ and ·OH¯ oxidants are formed [23, 27, 28]. These radicals can oxidize CR to intermediates and finally mineralized to H2O and H2O2 [21, 26-28]. The large number of main reactive species such as h+, O2 and OH¯ are actively involved in photocatalytic reaction and responsible for degradation of CR. On the basis of the above results, the possible improvement in photodegradation mechanism for WO3 sheets (S-300) photocatalyst can be proposed by following reasons: (i) sintering process is beneficial to the enhancement of light absorption capacity; (ii) large surface area can improve the photocatalytic activity; (iii) moderate crystallinity; and (iv) increased reaction sites exposed by formation of tiny sheet on their surface. Therefore these are very important causes for the enhancement of photocatalytic activity.
4. Conclusions In summary, we have successfully synthesized a novel photocatalyst of few- to multilayered WO3 sheets via thermal decomposition. The influence of the sintering temperature on the morphology has been studied. The S-300 sample shows higher photocatalytic performance than the as-prepared WO3, S-180 and S-400 samples, and S-300 sample has optimal performance for the removal of CR. The improved performance can be attributed to the increased surface area and charge separation due to the tiny sheets grown on the surface with sintering treatment. Also, the recycling and reusability tests showed that the S-300 sample is a photo-stable photocatalyst. This work is likely to offer useful information about the fabrication and temperature effects of
WO3 photocatalyst with enhanced photocatalytic activity for the degradation of organic pollutants under ambient conditions. Acknowledgement This work was conducted under the framework of the National Research Foundation of Korea (NRF) and funded by the Ministry of Science, ICT, and Future Planning (2014R1A2A2A01007081).
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Fig.1 XRD patterns of (a) as-prepared sample and sintered samples (b) S-180, (c) S-300 and (d) S-400 of WO3 sheets
Fig.2 (a, b) SEM images, (c) TEM image, (and d) EDX spectrum of as-prepared WO3 sample
Fig.3 SEM images of (a, b) S-180, (c, d) S-300, and (e, f) S-400 samples
Fig.4 TEM images of (a) S-180, (b) S-300, and (c) S-400 samples and (d) SAED pattern of S300 sample
Fig.5 FTIR spectrum of (a) as-prepared WO3 sheets, (b) S-180, (c) S-300 and (d) S-400 samples
Fig.6 TG-DTA curves of as-prepared WO3 sample
Fig. 7 High resolution X-ray photoelectron spectra of as-prepared WO3 sample: (a) survey, (b) W4f and (c) O1s
Fig.8 N2 adsorption/desorption isotherm of (a) as-prepared and (b) S-300 sample; inset: Barrett– Joyner–Halenda (BJH) pore size distribution data of the same samples
Fig.9 (a) UV–vis spectra and (b) Tauc plots of as-prepared WO3 and S-300 samples.
Fig.10 Time-dependent visible light absorbance spectra of the CR solution along with S-300 photocatalyst taken at different times
Fig.11 Photocatalytic degradation of CR under visible light irradiation in presence of S-300 photocatalyst
Fig.12 The kinetic plot of photocatalytic degradation of CR with S-300 photocatalyst under visible light irradiation
Fig.13 Recycling and reusability performance test of as-prepared WO3 and S-300 photocatalyst
Fi.14 Photoluminescence spectra of as-prepared WO3 and S-300 photocatalyst
Scheme 1 Schematic diagram illustrating the proposed mechanism for the photocatalytic activity of the WO3 sheets