Subscriber access provided by INDIAN INST OF TECH KHARAGPUR Article Crystal Phase and Size-controlled Synthesis of Tungsten Trioxide Hydrate Nanoplates at Room Temperature: Enhanced Cr(VI) Photoreduction and Methylene Blue Adsorption Properties Arpan Kumar Nayak, Seungwon Lee, Young In Choi, Hee Jung Yoon, Youngku Sohn, and Debabrata Pradhan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b03084 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering Crystal Phase and Size-controlled Synthesis of Tungsten Trioxide Hydrate Nanoplates at Room Temperature: Enhanced Cr(VI) Photoreduction and Methylene Blue Adsorption Properties Arpan Kumar Nayak,1 Seungwon Lee,2 Young In Choi,2 Hee Jung Yoon,2 Youngku Sohn,2,* Debabrata Pradhan1,* 1 Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, W.B., India 2 Department of Chemistry, Yeungnam University, Gyeongsan 38541, Republic of Korea ABSTRACT Controlling the crystal phase of a material using solution-based method is a challenging task and has significant consequence to the material’s properties. Herein we report the phase and sizecontrolled synthesis of tungsten oxide hydrates at room temperature via a simple precipitation method. In the absence and presence of oxalic acid, orthorhombic WO3⋅H2O and monoclinic WO3⋅2H2O nanoplates of size in the range of 200−600 (thickness <50 nm) and 40−200 nm (thickness <20 nm), were respectively synthesized. Oxalic acid is found to play the central role in the phase transition due to its chelating nature that facilitates bonding of oxalate ions to tungsten cations leading to formation of WO3⋅2H2O. Upon annealing at 400°C for 2h under air, both WO3⋅H2O and WO3⋅2H2O nanoplates were converted to monoclinic WO3 nanoplates. These nanoplates were demonstrated to be highly efficient for the photocatalytic detoxification of toxic Cr(VI) in the acidic pH under the visible light irradiation. The best Cr(VI) reduction performance was obtained with WO3⋅2H2O nanoplates due to its smaller band gap and larger effective surface area. In addition, a lower pH value is found to facilitates the Cr(VI) reduction. Furthermore, highly concentrated methylene blue was efficiently removed (>95%) by adsorption on the nanoplates within a minute, suggesting the importance and potential of a material that can be synthesized at room temperature. Keywords: WO3⋅H2O; WO3⋅2H2O; WO3; nanoplates; phase control; Cr (VI) detoxification; adsorption * E-Mail: deb@matsc.iitkgp.ernet.in and youngkusohn@ynu.ac.kr 1 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1. INTRODUCTION Two common existing oxidation states of chromium (Cr) compounds are Cr (III) and Cr (VI).1 The hexavalent Cr(VI) is highly toxic, mutagenic, and carcinogenic, whereas Cr(III) is less toxic, also considered as human nutrient, and can be readily precipitated out as Cr(OH)3.2,3 The main sources of Cr(VI) contaminations in the ground water are leather tanning, metal finishing, chrome plating, textile production, and dyeing. 4 Cr(VI) is also used in paper, petroleum, and cement industries.2 The highly toxic and hazardous Cr(VI) is an environmental concern and its presence in drinking water enhances the risk of bladder, liver, kidney, and skin cancer via chronic inhalation.2, 5 The remediation of Cr(VI) contaminations are thus of prevailing importance in the wastewater treatment.6 Thus, efficient technologies are indispensable for the reduction of Cr(VI) to Cr(III). For the removal of Cr(VI) from water/wastewater, various treatments have been reported including microbial reduction, 7 adsorption technique, 8 and photocatalytic reduction.9,10,11,12 Among these, photocatalytic reduction of Cr(VI) is believed to be the most cost-effective, easy, and efficient for treating toxic Cr(VI). In the photocatalytic Cr(VI) reduction process, several metal and metal oxides such as Pd nanoparticles,2,9 TiO2 nanosheets,10 WO3-doped TiO2 nanosheets,11 and more complex NH2-mediated zirconium metal organic framework12 have been employed as catalyst. In the past, wide band gap materials (e.g., TiO2, SnO2, and In2O3) were mostly used for the photocatalytic degradation of organic contaminants under ultra violet (UV) light irradiation.13,14 However, for the practical applications, lower band gap visible light photocatalysts are desirable. Thus, the current research is directed toward reducing the band gap of wide band gap semiconductor and/or explore suitable lower band gap materials.15,16,17 In such aspect, WO3 is one the potential visible light photocatalysts due to its tunable band gap (2.4−2.8 eV) in the visible region along with its high stability and non toxicity.18,19,20 Various nanostructures of WO3 such as nanowires,21,22 nanorods,23 nanoplates,24 and hierarchically flower, 25 have been prepared for different applications. All these WO3 nanostructures were synthesized by hydrothermal technique. However, there are only a few reports on hydrated tungsten oxide (WO3·nH2O), particularly with n values 0.33 or 1.24,26,27 On the other hand, WO3·2H2O is seldom reported.28,29 Kalantar-zadeh et al. obtained WO3·2H2O nanoplatelets by placing pieces of tungsten foils in 0.5 M HNO3 and heating at 80°C for 6h.28 Liang et al. synthesized WO3·2H2O ultrathin nanosheets using two step process: first tungstic acid dispersed in a mixture of dodecylamine and heptanes, and then continuously stirred for 48 h 2 ACS Paragon Plus Environment Page 2 of 28 Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering at room temperature to get white solid power of tungstate-based inorganic–organic nanohybrids; in the second step, tungstate-based inorganic–organic nanohybrids were dispersed in nitric acid and stirred for 48h at 10°C and ultrasonicated in ice water for 6h.29 Here we demonstrate the synthesis of WO3·2H2O nanoplates, in addition to WO3·H2O, by a simple single-step solutionbased precipitation technique at room temperature. Methylene blue (MB) is an important cationic hazardous organic dye extensively used in the textile industries. MB is not only harmful for the environment but also for the aquatic lives. MB has been widely used to examine the adsorption and photocatalytic performance of a catalyst. Adsorption technique is considered to be much simpler than that of other dye removal techniques. 30 , 31 This method is cost-effective, highly efficient, and facile to remove highly concentrated dyes in a short duration. Removing organics from wastewater has been extensively studied by adsorption/photocatalytic method using adsorbents/photocatalysts.32 It has commonly been believed that the catalytic performance and adsorption ability are mainly dependent on the exposed crystal facets, morphology, and size of catalysts.33 Numerous efforts have been made to control the morphology and thus exposed facets of crystals. Liu et al. reported the hollow TiO2 microspheres with {001} exposed facets that show good photocatalytic performance.34 Zhu et al. reported improved adsorption property of hexagonal phase WO3 nanorod grown along [110].35 Similar facets-depended photocatalytic properties are reported by other researchers for different materials.36,37,38 Herein, for the first time, we demonstrate the size and crystal phase-controlled synthesis of hydrated tungsten oxides i.e. orthorhombic WO3·H2O and monoclinic WO3·2H2O nanoplates, which were subsequently annealed under air to obtain monoclinic WO3 nanoplates. The importance of present work lies in (i) room temperature synthesis and (ii) demonstrating oxalic acid as a phase and size controlling agent for the synthesis of hydrated tungsten oxide (WO3·2H2O). The as-synthesized hydrated WO3 and WO3 nanoplates were then studied for the photocatalytic degradation of Cr(VI) under visible light irradiation and adsorption of MB. To the best of our knowledge, there is no report on Cr(VI) reduction over WO3⋅H2O or WO3·2H2O nanoplates under UV or visible light. In addition, there is no literature on the adsorption of an organic dye on either WO3·H2O or WO3·2H2O although adsorption of MB on hexagonal phase WO3 has been reported.35,39 Thus the present study demonstrates the further applicability of 3 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 hydrated tungsten oxides as visible light photocatalyst for the Cr(VI) reduction and as adsorbent for the treatment of contaminated water. 2. EXPERIMENTAL SECTION Chemicals. Sodium tungstate dihydrate (Na2WO4·2H2O) from Sigma-Aldrich; hydrochloric acid (HCl, ~35% v/v) from OCI Korea; oxalic acid (C2H2O4) from Samchun, Korea; and ethanol (C2H5OH) from J. T. Baker. All the above reagents were analytical grade and used without further purification. Synthesis of WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates. WO3⋅H2O and WO3·2H2O nanoplates were synthesized by a precipitation method at room temperature. The synthesis process is presented in Scheme 1. In a typical synthesis process, 1.6 g sodium tungstate dihydrate (0.12 M) was dissolved in 40 mL deionized water and stirred for a while at room temperature. Then, 4 mL of concentrated HCl (35%, 11.65 M) was added drop wise to the above transparent solution while stirring. The resulting solution turned pale turbid yellow color. The solution was stirred for 1h and WO3·H2O precipitate was collected by centrifuging. For the synthesis of WO3·2H2O, after adding 4 mL of HCl, the solution was stirred for 5 min. Then 1 g of oxalic acid (0.2 M) was added, which turns the pale yellowish solution to transparent within 2 min. Upon continuous stirring the above solution for 1h, the transparent solution turned turbid pale yellow again and the precipitate was formed after 4h of stirring. The final precipitate was washed by ethanol and distilled water, and dried overnight in an oven at 60°C. The final powder product was WO3·2H2O as confirmed by X-ray diffraction (XRD) measurement. The as-synthesized products were annealed for 2h at 400°C (heating rate 10°C/min) under air in a muffle furnace to obtain WO3. 4 ACS Paragon Plus Environment Page 4 of 28 Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering Scheme 1. Room temperature synthesis process of hydrated WO3 (WO3⋅H2O and WO3⋅2H2O) Characterization. The surface morphology of the as-synthesized samples was examined using a field emission scanning electron microscope (FESEM, Hitachi SE-4800). The powder XRD patterns were collected by a PANalytical X’Pert Pro MPD diffractometer with Cu Kα radiation (40 kV and 30 mA). For the detailed microstructure analysis, transmission electron microscope (TEM) and high resolution TEM (HRTEM) images were obtained for the sample placed on a carbon-coated Cu grid. The TEM/HRTEM images and selected area electron diffraction (SAED) patterns were obtained using a FEI Tecnai G2 F20 TEM operated at 200 kV. The UV–vis absorption characteristics (and band gaps) were measured using a Scinco Neosys 2000 UV–vis absorption spectrophotometer for the powder samples. The Raman scattering study was performed with a Raman Spectrophotometer (HORIBA Jobin Yvon, MODEL T64000) at an excitation wavelength of 514 nm at 40 mW. Fourier-transform infrared (FTIR) measurements were performed using a Thermo Scientific Nicolet iS10 spectrometer. The thermo gravimetric analysis (TGA) was carried out with a Perkin Elmer Pyris Diamond TG-DTA under air atmosphere. The effective Brunauer–Emmett–Teller (BET) surface area of the as-synthesized samples was measured with an Autosorb iQ2 BET surface analyzer (Quantachrome, USA). Photocatalytic and adsorption study of WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates. The photocatalytic Cr(VI) reduction in the absence and presence of the as-synthesized nanoplates 5 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 catalyst was carried out under visible light irradiation. For the photocatalytic study, 40 mg of a catalyst was dispersed in a 40 mL aqueous potassium dichromate solution (5 × 10−4 mol/L) followed by addition of citric acid to adjust the pH value, and sonicated for 1h under dark for a complete adsorption-desorption equilibrium. Then, the solution was placed under a halogen lamp (500 W). The Cr(VI) concentration as a function of irradiation duration was measured using UVvis absorption spectrophotometer (Jasco V-530). The adsorption tests of the as-prepared WO3⋅H2O, WO3·2H2O, and WO3 nanoplates were performed with various concentration of methylene blue (MB) solution in the absence of light. 80 mg of the powder sample was dispersed in a 40 mL aqueous MB solution (5 − 400 mg/L) and stirred for different durations. The decrease in MB concentration in the solution due to its adsorption on WO3-based samples after 1 min was measured using a UV-Vis absorption spectrophotometer. 3. RESULTS AND DISCUSSION Morphology. The morphology of the as-synthesized samples was examined by FESEM as shown in Figure 1. Figure 1a and 1b show the FESEM images of WO3⋅H2O nanoplates (confirmed by XRD, discussed later) at different magnifications synthesized using sodium tungstate dihydrate and HCl at room temperature (~22 °C) in 1h. These nanoplates were found to be stacked and of irregular shape with 200−600 nm length/width and <50 nm thickness (measured from the magnified images). Upon adding oxalic acid in the synthesis of tungsten oxide hydrate keeping other parameters fixed, nanoplates of WO3⋅2H2O (confirmed by XRD) with reduced size (40−200 nm length/width) and thickness (<20 nm) were produced as shown in Figure 1c and 1d. Moreover, these nanoplates were found to be highly uniform, square shaped, and stacked. Upon annealing at 400°C for 2h in air, the morphology of WO3⋅H2O nanoplates (Figure 1a,b) showed no critical change as shown in the Figure 1e and 1f. However, WO3⋅2H2O nanoplates (Figure 1c,d) not only lost their square shape but also appear to be porous as shown in Figure 1g and 1h. This is believed to be due to the loss of two water molecules upon calcination. The colors of the respective powder samples before and after annealing are shown as the inset digital photographs of the corresponding FESEM images. It is important to note that Huang et al. also reported square slab-like WO3⋅H2O structure, however with a much bigger size (2 µm long/wide) at 70°C for 10h. 40 Recently, Guo et al. synthesized WO3⋅H2O nanoplates by 6 ACS Paragon Plus Environment Page 6 of 28 Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering hydrothermal method at 70°C for 10h using sodium tungstate, citric acid, and HCl.27 The formation of much smaller dimension nanoplates in the present work is attributed to lower synthesis temperature and shorter reaction duration. Moreover, Liang et al. obtained ultrathin WO3⋅2H2O nanosheets using a two-step exfoliation technique at low temperature.29 Figure 1. FESEM images of the room temperature synthesized (a,b) WO3⋅H2O and (c,d) WO3·2H2O without and with oxalic acid, respectively, along with sodium tungstate dihydrate and HCl. The corresponding FESEM images of (e,f) WO3 obtained by post-annealing of WO3⋅H2O, and (g,h) WO3 obtained by post-annealing of WO3⋅2H2O under air at 400°C for 2h. Insets show the digital photographs of the corresponding powder samples. 7 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Structural and microstructural property. The structural properties of the as-synthesized and annealed nanoplates were examined by the powder XRD. Figure 2a and 2b show the XRD patterns of the nanoplates synthesized at room temperature without and with oxalic acid, respectively, keeping other synthesis parameters fixed [0.12 M sodium tungstate dihydrate, 4 mL HCl (35%) at room temperature]. The XRD pattern (Figure 2a) of the sample prepared without oxalic acid is found to match orthorhombic (Pmnb) WO3⋅H2O (a = 5.249 Å, b = 10.711 Å, c = 5.133 Å, JCPDS: 01-084-0886), with intense diffraction intensities from the (111), (020), and (131) planes. The XRD pattern of the powder (Figure 1c,d) prepared with oxalic acid is shown in Figure 2b that match monoclinic (P2/m) WO3⋅2H2O (a = 7.50 Å, b = 6.93 Å, c = 3.70 Å, JCPDS:00-018-1420) with the strongest (010) diffraction plane. Figure 2c and 2d show the XRD patterns of WO3 that match monoclinic (P21/n) WO3 (a = 7.3013 Å, b = 7.5389 Å, c = 7.6893 Å, JCPDS:01-083-0951) with the strongest (002) diffraction plane, obtained by annealing WO3⋅H2O and WO3⋅2H2O nanoplates at 400°C for 2h, respectively. It is to be noted that the lower intensity diffraction peaks are not assigned in the Figure 2. Moreover, no characteristic diffraction peaks from possible impurity were detected indicating the phase pure products. The average crystallite sizes were estimated to be 84.2 nm (for WO3·H2O), 40.5 nm (for WO3·2H2O), 117.4 nm (for WO3 obtained by annealing WO3·H2O) and 71.4 nm (for WO3 obtained by annealing WO3·2H2O) from the most intense XRD peak width of the corresponding pattern using the Scherrer equation. 8 ACS Paragon Plus Environment Page 8 of 28 (420) (400) (222) (002) (120) (020) (112) (022) (200) (202) (d) Monoclinic WO3 (c) 20 30 (031) Monoclinic WO3⋅2H2O (202) (222) (311) (113) (230) (020) (201) (011) (220) (221) (101) (001) (200) 10 (131) (a) (010) (b) Monoclinic WO3 (020) Intensity (a.u.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering (111) Page 9 of 28 40 50 Orthorhombic WO3⋅H2O 60 70 80 2θ (degree) Figure 2. Powder XRD patterns of WO3⋅H2O and WO3⋅2H2O nanoplates synthesized at room temperature (a) without oxalic acid and (b) with oxalic acid, respectively. (c) and (d) show the powder XRD patterns of WO3 nanoplates obtained by annealing WO3⋅H2O and WO3⋅2H2O under air at 400°C for 2h, respectively. The Raman spectra were collected to further analyze the particle size, crystallinity, and phase of the as-synthesized samples. Three major peaks were found in the range of 500 to 1000 cm−1 in the Raman spectra as shown in Figure 3. The former two peaks below 900 cm−1 arises due to W−O−W stretching vibrations and the peak at ~945 cm−1 corresponds to symmetric stretching mode of the terminal W=O bonds (Table 1).28,41 The peaks at 192 cm−1 and 256 cm−1 correspond to the W−O−W bending vibrations of respective samples. All types of tungsten hydrates and oxides show these two vibrations albeit different intensities.28,41 Figure 3a shows the Raman spectrum of WO3⋅H2O nanoplates. The position of Raman shifts and intensity of different bond vibrations were found drastically changed for WO3⋅2H2O nanoplates as shown in Figure 3b. A sharp peak at ~945 cm−1 was obtained for W=O stretching vibrations and the first W−O−W stretching vibration [ν1(W−O−W)] was shifted to a lower wavenumber (~645 cm−1) indicating WO3⋅2H2O phase.28,42 A higher intensity of Iν(W=O) further indicates smaller particle 9 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering size,41 correlating our microscopic investigations. The intensity ratio of Iν2(W−O−W)/Iν(W=O) is known to be lower for the WO3⋅xH2O with x > 1 and become higher for x < 1, as also revealed here, confirming the earlier report.41 The Iν2(W−O−W)/Iν(W=O) ratio is further improved with dehydration in the order of WO3⋅2H2O < WO3⋅H2O < WO3 (obtained by annealing WO3⋅2H2O) < WO3 (obtained by annealing WO3⋅H2O). ν2(W-O-W) (d) Intensity (a.u.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 10 of 28 ν1(W-O-W) ν(W=O) WO3 (c) WO3 (b) WO3⋅2H2O (a) WO3⋅H2O 300 600 900 1200 1500 -1 Raman Shifts (cm ) Figure 3. Raman spectra of WO3⋅H2O and WO3⋅2H2O nanoplates synthesized at room temperature (a) without oxalic acid and (b) with oxalic acid, respectively. (c) and (d) show the Raman spectra of WO3 nanoplates obtained by annealing WO3⋅H2O and WO3⋅2H2O under air at 400°C for 2h, respectively. 10 ACS Paragon Plus Environment Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering Table 1. Characteristics IR and Raman frequencies (in cm−1) of WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates ν(OH) WO3⋅H2O IR Raman 3388 WO3⋅2H2O IR Raman 3524 3370 3150 WO3 IR Raman 1730 ν(C=O) δ (OH) 1618 ν(W=O) 942 950 918 948 955 980 ν(O−W−O) 650 590 800 700 660 590 820 643 805 683 598 800 698 ν(W−O−W) 1595 1406 255 1622 232 255 Figure 4 displays the FTIR spectra of the as-synthesized WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates. The as-synthesized WO3⋅H2O (Figure 4a) and WO3⋅2H2O nanoplates (Figure 4b) show major O−H stretching vibrations of water molecules in the frequency range of 3500−3150 cm−1. The O−H vibration peaks were significantly reduced for the WO3 nanoplates (Figure 4c,d), as expected. The different bond vibrations at respective IR frequencies are presented in Table 1 and well-matched to the literature values.43,44,45 The only additional IR peaks at 1730 cm−1 and 1103 cm−1 for WO3⋅2H2O nanoplates (synthesized with oxalic acid) are assigned to C=O and C– O stretching modes, respectively. This confirms that oxalic acid was adsorbed on the surface of WO3⋅2H2O nanoplates thereby restricting growth to produce uniform nanoplates of smaller dimensions. The FTIR peak at ~1620 is assigned to the O−H bending vibration and the intensity of same is found to be drastically reduced for WO3.43 A minor peak at ~1622 for WO3 is due to the surface O−H bonds. 11 ACS Paragon Plus Environment Page 12 of 28 WO3⋅2H2O 918 955 1040 1622 1103 (b) 1406 WO3(no OA) 1730 1595 (c) 3150 WO3 (OA) 3524 3370 (d) WO3⋅H2O 650 590 942 660 1618 (a) 3388 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Transmittance (%) ACS Sustainable Chemistry & Engineering 3000 2000 1000 −1 Wave number (cm ) Figure 4. FTIR spectra of WO3⋅H2O and WO3⋅2H2O nanoplates synthesized at room temperature (a) without oxalic acid and (b) with oxalic acid, respectively. (c) and (d) show the FTIR spectra of WO3 nanoplates obtained by annealing WO3⋅H2O and WO3⋅2H2O under air at 400°C for 2h, respectively. The microstructures of the as-synthesized WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates were examined by TEM and HRTEM as shown in Figure 5. Figure 5a shows a TEM image of orthorhombic WO3⋅H2O nanoplates prepared without adding oxalic acid. The shape of these nanoplates was found to be non-uniform and stacked. Upon adding oxalic acid as a shape and size controlling agent, keeping other parameters fixed, the monoclinic WO3⋅2H2O nanoplates were found to be formed with square shape as shown in Figure 5b, in good accord to the FESEM images (Figure 1c,d). The square marked portion of Figure 5b was magnified to obtain a lattice image of monoclinic WO3⋅2H2O nanoplate as shown in Figure 5c, which shows continuous lattices with a spacing of 3.8 Å corresponding to the (200) plane. The SAED pattern as shown in the inset of Figure 5c confirms the single crystalline nature of monoclinic WO3⋅2H2O nanoplates. Figure 5d represents a TEM image of the post-calcined orthorhombic WO3⋅H2O (Figure 5a). The morphology remains same with the formation of voids (as depicted in the inset Figure) and the nanoplates were found to be stacked. The TEM image of WO3 nanoplates obtained by annealing 12 ACS Paragon Plus Environment Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering monoclinic WO3⋅2H2O (Figure 5b, with oxalic acid) under air is shown in Figure 5e. These annealed nanoplates appear to be very thin with voids in accordance to the FESEM images (Figure 1g,h). Figure 5f shows an HRTEM image of a part of nanoplate with lattice spacing 3.85 Å corresponding to the (002) plane of monoclinic WO3,23,46 which shows the maximum XRD intensity (Figure 2d). The SAED pattern (inset Figure 5f) further confirms the single crystalline nature of the WO3 nanoplates. The size and shape of these nanoplates analyzed by the TEM are found to correlate the SEM study. Figure 5. TEM image of (a) WO3⋅H2O and (b) WO3⋅2H2O nanoplates prepared without and with oxalic acid, respectively. (c) Magnified image of square marked portion of b, the inset shows the corresponding SAED pattern. (d) and (e) show the TEM image of WO3 nanoplates obtained by annealing WO3⋅H2O and WO3⋅2H2O nanoplates, respectively. Insets of (d) and (e) show the corresponding magnified images. (f) HRTEM image WO3 shown in (e) and inset shows the corresponding SAED pattern. Thermal Analysis. To examine the effect of temperature on the crystal phase transformation, thermogravimetry (TG) measurement was carried out on orthorhombic WO3⋅H2O and monoclinic WO3⋅2H2O nanoplates in air atmosphere with temperature ramping from 34°C to 800°C, as shown in Figure 6. The weight loss was initially slow for WO3⋅H2O and up to 5% (at 400°C) and 7.2% (at 660°C), and then was becoming stable, ascribing loss of one water 13 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering molecule. On the other hand, the gradual weight loss was observed from room temperature to 400°C for WO3⋅2H2O and a total weight loss of approximately 13% was observed at 400°C confirming loss of two water molecules from WO3⋅2H2O to produce WO3. The thermal analysis matched well to the crystal phase confirmed in the XRD upon annealing the hydrated WO3 (WO3⋅H2O and WO3⋅2H2O) to WO3. It is also to be noted that the water losses occurred at a lower temperature for WO3⋅2H2O than that of WO3⋅H2O suggesting water between WO3 layers is bonded weakly in the former than the latter. This correlates the previous result on the water loss at a lower temperature (<200°C) with hydrate rich WO3 i.e. WO3·1.5H2O.47 100 Weight (%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 14 of 28 (a) WO3⋅H2O 95 90 (b) WO3⋅2H2O 85 0 200 400 600 800 Temperature (°C) Figure 6. Thermogravimetry plots of (a) WO3⋅H2O and (b) WO3⋅2H2O nanoplates synthesized at room temperature in the absence and presence of oxalic acid, respectively. Optical properties. Figure 7A shows the UV-vis absorption spectra of WO3⋅H2O, WO3⋅2H2O, and WO3 powder samples. An abrupt decrease in absorbance was commonly found in the wavelength range of 400−550 nm suggesting a band gap in the visible region of the electromagnetic spectrum. Considering the indirect band of WO3,48,49 the band gap of the assynthesized samples was measured from the zero-crossing values obtained by extrapolation of a linear fit to the rising edges of the respective (αhν)1/2 vs. photon energy (hν) plots as shown in Figure 7B. The optical band gap of the orthorhombic WO3⋅H2O and monoclinic WO3⋅2H2O nanoplates synthesized in the absence and presence of oxalic acid was measured to be 2.37 eV 14 ACS Paragon Plus Environment Page 15 of 28 and 2.18 eV, respectively. The corresponding annealed sample (monoclinic WO3 nanoplates) showed a higher band gap of 2.48 eV. 18 (A) (a) WO3.H2O 2.0 (b) WO3.2H2O (B) (a) WO3⋅H2O (c) WO3 (no OA) 0.8 (d) WO3 (OA) 0.6 1.0 2.18 eV 0.5 (a) (d) (c) 0.2 400 (c) WO3 (d) WO3 (b) 0.4 (b) WO3⋅2H2O 1.5 1/2 1.0 (αhν) Absorbance (%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering (b) (a) (c,d) 600 0.0 800 2.0 Wavelength (nm) 2.37 eV 2.48 eV 3.0 4.0 hν (eV) Figure 7. (A) UV-vis absorption spectra of WO3⋅H2O, WO3.2H2O, and WO3 nanoplates. (B) The plots of (αhν)1/2 vs. photon energy, hν, to measure the band gaps of corresponding samples. Growth Mechanism. To understand and propose a plausible mechanism on the formation of WO3⋅H2O and WO3⋅2H2O nanoplates at room temperature, further experiments were performed by varying the HCl and oxalic acid content in the reaction medium. Upon adding 2 mL HCl (35% v/v) (11.65 M) in the sodium tungstate aqueous solution, followed by 1h stirring, we obtained 200−600 nm long/wide WO3⋅H2O nanoplates with irregular shape and all the nanoplates were stacked each other. Equations 1−3 show the probable chemical reactions to obtain precipitate of WO3⋅H2O nanoplates. By increasing the HCl contents (4 mL and 8 mL), we obtained orthorhombic WO3⋅H2O nanoplates with same morphology, suggesting availability of enough H+ ions to precipitate WO3⋅H2O. However, upon adding 4 mL HCl at lower concentrations (0.1 M, 1 M, and 3 M), no precipitate was obtained at room temperature. Huang et al. reported square platelet, square slab, and microflower-like morphologies by adding 1 mL, 2 mL, and 5 mL of 10 M HCl at 70°C, respectively, suggesting agglomeration of nanoplates at higher HCl contents.40 This was ascribed to increase in the nuclei formation at a higher HCl content. Yang et al. reported WO3⋅H2O nanoplates (using 3 mL HCl and ammonium oxalate as capping agent) of much larger dimensions (>750 nm length/width and >250 nm thickness) in the 15 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 16 of 28 temperature range of 80−120°C by the hydrothermal process.50 They obtained no product below 80°C. On the other hand, the present work suggests that an optimum HCl content can successfully produce much smaller size WO3⋅H2O nanoplates (<600 nm length/width and <50 nm thickness) even at room temperature. Furthermore, by adding 1 g of oxalic acid to the solution of sodium tungstate and 4 mL HCl, even smaller size uniform square-shaped nanoplates (<200 nm length/width and <20 nm thickness) of monoclinic WO3⋅2H2O (equation 4) were obtained. Oxalic acid is demonstrated here as size and crystal-phase controlling agent for hydrated tungsten oxide synthesis. Oxalic acid is not only a reducing agent but also act as a chelating agent for the metal cations and forms bond through oxalate ions (C2O42− ions). It is believed that two negatively charged oxygen atoms of C2O42− form bonds with tungsten atoms of WO3. This would facilitate tungsten trioxide di-hydride formation as observed in the present study. The chelating nature of oxalic acid is known to decrease the particle size by increasing its concentration.51 Similar observation is also made in the present study i.e. synthesis in presence of oxalic acid produced WO3⋅2H2O nanoplates of smaller and uniform size. The chelating of oxalate on WO3 is also confirmed by FTIR analysis. The FTIR spectrum of WO3⋅2H2O nanoplates (Figure 4b) show clear stretching vibration peak at 1730 cm−1 for C=O confirming adsorption/capping of oxalic acid on the surface, leading to smaller dimension nanoplates. However, by increasing the oxalic acid content twice (i.e. 2 g), much less quantity of precipitate was formed and it took much longer duration of 48h to obtain the precipitate. This is due to the rapid capping of tungsten oxide nuclei by oxalic acid and impeding further growth. 52 This suggests that the amount of oxalic acid also plays an important role. As expected, by annealing the WO3⋅H2O and WO3⋅2H2O nanoplates at 400°C for 2h under air, monoclinic WO3 was obtained. Scheme 2 shows the schematics on the formation of different products by varying the reaction parameters. Na2WO4⋅2H2O → 2Na+ + WO42− + 2H2O (eq. 1) 2Na+ + WO42− + 8HCl → WCl6 + 4H2O + 2NaCl (eq. 2) WCl6 + 4H2O → WO3⋅H2O + 6HCl (eq. 3) WCl6 + 4H2O + C2H2O4 → WO3⋅2H2O + 6HCl + CO2 + CO (eq. 4) 16 ACS Paragon Plus Environment Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering Scheme 2. Schematic illustrations of different crystal structures obtained in the present work Photoreduction of Cr(VI) under visible light. Cr(VI) is extremely harmful to the living organisms and, thus, it must be rapidly detoxified before being discharged into the surroundings. The as-synthesized WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates were employed as catalysts to reduce Cr(VI) in acidic condition by adding citric acid under visible light. Citric acid was considered here because it is a relatively weak acid and acts both as chelating agents and electron donors. In particular, the electron donating (holes scavenger) efficiency of citric acid is higher than several other organic acids such as acetic acid, formic acid, mandelic acid, and salicyclic acid.53 Figure 8a shows the UV-vis absorption spectra of aqueous potassium dichromate solution indicating the reduction of Cr(VI) to Cr(III) on the WO3⋅2H2O nanoplates at pH =1 (adjusted with citric acid) under visible light as a function of irradiation time. Prior to the photoirradiation, the catalyst and citric acid were added to the aqueous Cr(VI) solution and stirred under dark for 30 min for complete adsorption-desorption equilibrium of Cr(VI) on the catalyst surface. Inset of Figure 8a displays the color change of Cr(VI) solution from yellow to transparent with increasing the irradiation duration. The formation of Cr(III) was confirmed by treating the transparent solution (obtained after 60 min) with NaOH that forms a cyan color solution due to formation of hexahydroxochromate. 54 Figure S1 [Supporting Information (SI)] shows the Cr(VI) photoreduction activity with WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates as measured by UV-vis absorption spectra. Figure 8b represents the reduction behavior of Cr(VI) at different conditions. There was no Cr(VI) reduction in the absence of either citric acid or catalyst confirming the role 17 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 18 of 28 of both. The photocatalytic reduction efficiency was found to be in order of WO3⋅2H2O nanoplates at pH = 1 (96.19%) in 60 min > WO3⋅2H2O nanoplates at pH = 3 (96%) in 90 min > WO3 (with oxalic acid) nanoplates at pH = 3 (77.14%) in 90 min > orthorhombic WO3⋅H2O nanoplates at pH = 3 (54.29%) in 90 min > WO3 nanoplates at pH = 3 (40%) in 90 min irradiation. The photoreduction activity of the monoclinic WO3⋅2H2O nanoplates is found to be the highest among the studied samples. This is ascribed to smaller size and lower band gap of WO3⋅2H2O nanoplates. As the effective surface area plays an important role in the heterogeneous catalysis, we have measured the effective BET surface area and porosity of the as-synthesized samples from the type (IV) nitrogen adsorption-desorption isotherms (Figure S2, SI). The measured effective surface area were 43.0 m2/g for WO3·H2O, 45.9 m2/g for WO3·2H2O, 38.6 m2/g WO3 (obtained by annealing WO3·H2O), and 45 m2/g for WO3 (obtained by annealing WO3·2H2O). No significant difference in the effective surface area was obtained. A slightly larger surface area of the samples prepared with oxalic acid was due to their smaller sizes. The average pore size (total pore volume) was estimated to be in the range of 20 to 40 nm (0.1−0.3 cm3/g) suggesting mesoporous nature of the samples. We further plotted the Cr(VI) reduction in mol per unit surface area as a function of irradiation time (Figure S3, SI). Similar catalytic Cr(VI) reduction trend was obtained as shown in Figure 8b. The mechanism of Cr(VI) reduction can be ascribed to the role of both the catalyst and citric acid. Upon light irradiation, free electrons and holes are created on the catalyst surface (equation 5). The free holes can be scavenged by the citric acid and surface electrons would take part in the reduction of Cr(VI) to Cr(III) as shown in equation 6.11 It must be noted that neither under dark nor under visible light irradiation, Cr(VI) reduction occurred in presence of either citric acid or catalyst. This is believed to be due to fast electron-hole recombination in the catalyst under light and poor electron donating ability of citric acid directly to Cr(VI), emphasizing the need of both for the efficient Cr(VI) reduction. Semiconductor (WO3 or WO3⋅2H2O) + hν → h+ + e− (eq. 5) ν +Catalysts 4Cr2O72− + C6H5O73− + 41H+ + 6ecatalysts− h  → 8Cr3+ + 6CO2 + 23H2O (eq. 6) The Cr(VI) photoreduction activity of WO3⋅2H2O demonstrated here is found to be comparable to Bi2S3/Bi2WO655 and superior to several other materials reported earlier.56,57,58,59 In 18 ACS Paragon Plus Environment Page 19 of 28 particular, Qin et al. reported <50% reduction of Cr(VI) in 3h with Cu2O crystals of diverse shapes.56 A faster reduction (complete reduction in 2.5h) was reported by Li et al. using Cu2O crystals under visible light irradiation that has been attributed to easy formation of electrons and holes.59 It is to be noted that there are no previous reports on Cr(VI) reduction with WO3⋅H2O, WO3⋅2H2O, and WO3. 1.2 (a) 1.2 Photo reduction of Cr(VI) Irradiation time 0.8 0.4 WO3⋅2H2O (pH-6) CA (pH-3) CA+WO3⋅H2O (pH-3) 0.8 0 min 15 min 30 min 45 min 60 min 0.6 (b) 1.0 pH - 1 1.0 C/C0 Absorbance (%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering CA+WO3⋅2H2O (pH-3) CA+WO3 (pH-3) 0.6 CA+WO3 (OA) (pH-3) CA+WO3⋅2H2O (pH-1) 0.4 0.2 0.2 0.0 0.0 300 350 400 450 500 550 0 Wavelength (nm) 20 40 60 80 100 120 140 160 Irradiation Time (min) Figure 8. (a) UV-vis absorption spectra of dichromate solution as a function of photoirradiation time with WO3⋅2H2O nanoplates at pH = 1. (b) photocatalytic reduction (C/C0) of Cr(VI) with different catalyst at selected pH as a function of photoirradiation durations. CA and OA stand for critic acid and oxalic acid, respectively. Adsorption of methylene blue. The adsorption performances of WO3⋅H2O, WO3⋅2H2O, and WO3 nanoplates were examined with MB solution (5−400 mg/L) using UV-vis absorption spectroscopy. Figure 9a shows typical UV-vis absorption spectra of MB solution (50 mg/L) in the presence of WO3⋅2H2O nanoplates under dark condition indicating complete removal of MB within a minute. Figure 9b shows the adsorption efficiency of MB at different concentrations in the presence of orthorhombic WO3⋅H2O, monoclinic WO3⋅2H2O, and monoclinic WO3 catalysts in 1 min. Figures S4−S7 show the UV-vis absorption spectra of MB solution at different concentrations before and after 1 min of adding WO3 based nanoplates synthesized in the present work. It must be noted that there was no change in MB concentration in the absence of catalyst under dark. The concentration of MB was found to be decreased instantly upon adding the 19 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 20 of 28 catalyst due to the adsorption of MB (adsorbate) on the surface of catalyst (adsorbent). The adsorption efficiency [(C0 − C)/C0 × 100] was found to be >85% for all the samples. Here, C0 and C are the dye concentrations before and after 1 min of adsorption, respectively. The amount of MB adsorption at different initial concentrations by different samples is also presented per unit area (Figure S8). The adsorption performance was not markedly changed with increasing the initial MB concentration up to 100 mg/L. However, upon further increasing the MB concentration, the adsorption efficiency was decreased to 80.0% (150 mg/L), 76.0% (200 mg/L), and 69.9% (400 mg/L) in presence of WO3⋅2H2O nanoplates. This suggests 100 mg/L is the optimum initial MB concentration for the maximum adsorption efficiency. To the best of our knowledge, there is no report on the adsorption of MB on either orthorhombic WO3⋅H2O or monoclinic WO3 ⋅2H2O. However, there is a report on the adsorption of MB on hexagonal WO3 with exposed facets along [110] axis.35 Although, all the samples show >85% adsorption at all the MB concentration up to 100 mg/L studied in this work, the samples synthesized with oxalic acid showed slightly better adsorption performances than those synthesized without oxalic acid, which is attributed to smaller size of the former. No adsorption isotherm mechanism, i.e. adsorption as a function of duration, was studied here due to the fast adsorption (in less than 1 min). Moreover, the cyclic stability of the as-synthesized WO3⋅2H2O nanoplates was studied. After MB adsorption, the color of WO3⋅2H2O nanoplates powder turns blue from pale yellow indicating adsorption. To activate the surface of the catalyst, the adsorbed MB must be removed from the catalyst surface either by annealing or washing/dissolving by a reagent. Here adsorbed MB was removed by annealing WO3⋅2H2O nanoplates at 300°C for 10 min under air. Upon annealing, MB was not only decomposed and removed from the surface but also WO3⋅2H2O was phase converted to WO3 as expected. The same WO3 powder was then used for MB adsorption in the subsequent cycle, which shows slightly lower adsorption efficiency of 92%, same as that observed for WO3 obtained by annealing WO3⋅H2O and WO3⋅2H2O nanoplates. This decrease in MB adsorption was due to phase conversion. The WO3 powder was further annealed at 300°C for 10 min under air and used for MB adsorption for 15 more cycles. The MB adsorption efficiency was found to be almost same i.e. 92% after 15 cycles (as shown in Figure 10) suggesting the stability of WO3 for practical applications. 20 ACS Paragon Plus Environment Page 21 of 28 (a) MB Dye (50 mg/L) Absorbance (%) 3.0 0 min 1 min 2.0 1.0 0 min 1 min 0.0 300 400 500 600 700 800 Wavelength (nm) Figure 9. (a) Typical UV-vis absorption spectra of MB solution (50 mg/L) before and after adding WO3.2H2O nanoplates and keeping for 1 min. (b) The MB (5, 10, 25, 50, 100 mg/L) adsorption performance of WO3⋅H2O, WO3.2H2O, WO3 (without oxalic acid) and WO3 (with oxalic acid) nanoplates in 1 min 100 85 80 (WO3) 92.3 % 90 (WO3) 92.33 % 95 (WO3) 92.5 % MB adsorption (100 mg/L) for 1 min (WO3⋅2H2O) 96.88 % (WO3) 92.9 % Adsorption efficiency (%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering 75 70 0 5 10 15 No. of cycles (Counts) Figure 10. The cyclic stability of the as-synthesized catalyst. 4. CONCLUSIONS In summary, we have successfully synthesized WO3⋅H2O and WO3⋅2H2O nanoplates by a precipitation method at room temperature. Oxalic acid was found to play an important role in controlling the crystal phase and size of the nanoplates. Orthorhombic (WO3⋅H2O) and 21 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 22 of 28 monoclinic (WO3⋅2H2O) crystal phases of nanoplates were obtained without and with oxalic acid, respectively. The chelating nature of oxalic acid is ascribed for the phase conversion of WO3. Upon annealing hydrated WO3 at 400°C for 2h under air, monoclinic WO3 nanoplates were obtained. The photocatalytic detoxification of Cr(VI) was found to be extremely efficient with WO3⋅2H2O nanoplates under visible light in the acid medium. This was due to a smaller band gap of the WO3⋅2H2O nanoplates and larger effective surface area. A lower pH value was also found to promote Cr(VI) reduction at a faster rate by providing enough H+ ions needed for Cr(VI) reduction. In addition, the as-synthesized samples were used for MB adsorption. All the samples showed very high adsorption capacity for MB. The WO3 nanoplates (prepared with oxalic acid and post calcination) showed the highest MB adsorption performance (MB concentration of 100 mg/L decreases 96.88% in 1 min), plausibly due to smaller size. The present study demonstrates the potential of room temperature synthesized tungsten oxide hydrate nanoplates for the detoxification of Cr(VI) and removal of MB from contaminated water. ASSOCIATED CONTENTS Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. UV-Vis absorption spectra for Cr(VI) photoreduction and MB adsorption in presence of different catalysts. N2 adsorption/desorption isotherms. AUTHOR INFORMATION Corresponding Author * E-Mail: deb@matsc.iitkgp.ernet.in and youngkusohn@ynu.ac.kr Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by Science and Engineering Research Board (SERB), Department of Science and Technology, New Delhi, India through the grant SB/S1/IC-15/2013 and National Research Foundation of Korea, MEST (NRF-2012R1A1A4A01005645). 22 ACS Paragon Plus Environment Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Sustainable Chemistry & Engineering REFERENCES 1 Schrank, S. G.; José, H. J.; Moreira, R. F. P. M. Simultaneous Photocatalytic Cr(VI) Reduction and Dye Oxidation in a TiO2 Slurry Reactor. J. Photochem. Photobiol., A 2002, 147, 71–76. 2 Dandapat, A.; Jana, D.; De, G. Pd Nanoparticles Supported Mesoporous γ−Al2O3 Film as a Reusable Catalyst for Reduction of toxic Cr VI to Cr III in Aqueous Solution. Appl. Catal., A 2011, 396, 34−39. 3 Ke, Z.; Huang, Q.; Zhang, H.; Yu, Z. 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Dalton Trans. 2011, 40, 6745−6750. 27 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Table of Content Graphics Room Temperature Crystal Phase and Size-controlled Synthesis of Tungsten Trioxide Hydrate Nanoplates: Enhanced Cr(VI) Photoreduction and Fast Methylene Blue Adsorption Arpan Kumar Nayak,1 Seungwon Lee,2 Young In Choi,2 Hee Jung Yoon,2 Youngku Sohn,2,* Debabrata Pradhan1,* 1 Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, W.B., India 2 Department of Chemistry, Yeungnam University, Gyeongsan 38541, Republic of Korea The room temperature synthesized WO3-based nanoplates are demonstrated as efficient catalysts for Cr(VI) reduction and methylene blue adsorption. 28 ACS Paragon Plus Environment Page 28 of 28