Hydrothermal Synthesis of Co-Exposed-Faceted WO3 Nanocrystals with Enhanced Photocatalytic Performance

In this paper, rod-shaped, cuboid-shaped, and irregular WO3 nanocrystals with different co-exposed crystal facets were prepared for the first time by a simple hydrothermal treatment of tungstic acid colloidal suspension with desired pH values. The crystal structure, morphology, specific surface area, pore size distribution, chemical composition, electronic states of the elements, optical properties, and charge migration behavior of as-obtained WO3 products were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), fully automatic specific surface area and porosity analyzer, UV–vis absorption spectra, photoluminescence (PL) spectra, and electrochemical impedance spectroscopy (EIS). The photocatalytic performances of the synthesized pHx-WO3 nanocrystals (x = 0.0, 1.5, 3.0, 5.0, and 7.0) were evaluated and compared with the commercial WO3 (CM-WO3) nanocrystals. The pH7.0-WO3 nanocrystals with co-exposed {202} and {020} facets exhibited highest photocatalytic activity for the degradation of methylene blue solution, which can be attributed to the synergistic effects of the largest specific surface area, the weakest luminescence peak intensity and the smallest arc radius diameter.


Introduction
Transition metal oxide nanocrystals with tailored shapes and reactive facets have sparked intense research interest over the past two decades due to their many intrinsic morphology and crystal plane-dependent properties [1]. Among many transition metal oxides, tungsten trioxide (WO 3 ) is a typical narrow band gap (2.4~2.8 eV) n-type 5d 0 transition metal oxide semiconductor, which plays a key role in many applications such as gas sensors, photoelectrochemical water splitting, electrochromic and photochromic devices, and photocatalytic systems [2,3]. Generally, WO 3 nanocrystals are formed by sharing the corners and edges of WO 6 octahedra, which exists in five polymorphs namely ε-WO 3 (monoclinic II, space group Pc, stable temperature <−43 • C), δ-WO 3 (triclinic, space group P − 1, stable temperature −43~17 • C), γ-WO 3 (monoclinic I, space group P2 1 /n, 17~330 • C), β-WO 3 (orthorhombic, space group Pmnb, transition temperature 330~740 • C), and α-WO 3 (tetragonal, space group P4/nmm, stable temperature > 740 • C) [4,5]. Among the five different crystalline phases of WO 3 , the γ-WO 3 is the most thermodynamically stable phase at room temperature [5]. Therefore, the commonly mentioned WO 3 refers specifically to γ-WO 3 . WO 3 has been considered a promising visible-light-driven photocatalyst not only due to its high hole mobility and moderate hole diffusion length, but also due to its photosensitivity, inherently good electron transport properties, resistibility to photocorrosion, and low cost [6,7]. However, the low conduction band level of WO 3 inhibits its ability to react with electron acceptors and increases the recombination of photogenerated electron-hole pairs, resulting in poor photocatalytic activity for the degradation of organic pollutants [8,9]. Therefore, strenuous efforts have been made to improve the photocatalytic activity of WO 3 materials, such as controlling the particle size, crystal structure, crystal morphology, crystal surface exposure, crystal composition, etc. [10][11][12]. In particular, the morphology and exposed crystal surface of WO 3 material have an important influence on photocatalytic performance. In view of this, extensive research work has been carried out to synthesize numerous morphological WO 3 materials with specific exposed crystal surfaces via different methods. For instance, Xie et al. [13] synthesized a quasicubic-like monoclinic WO 3 crystal with co-exposed {002}, {200} and {020} facets, and a rectangular sheet-like monoclinic WO 3 crystal with predominant {002} facet via a simple solvothermal synthesis method. Han et al. [14] synthesized monodisperse triclinic WO 3 nanoparticles with co-exposed {001}, {100} and {010} facets via a simple hydrothermal method. D'Arienzo et al. [15] synthesized WO 3 nanocrystals with tailored morphology (rectangular nanocrystals, square-like platelets, and rectangular platelets) and definite prominent surfaces (co-exposed high-energy {020} and {002} facets) via a simple hydrothermal synthesis method. Dirany et al. [16] synthesized a well-crystallized orthorhombic quadrangular WO 3 nanoplates with dominant exposed {020} facets via free template aqueous mineralization processes. Bu et al. [17] fabricated a well-defined hierarchical WO 3 nanoflower-like thin film photoanode composed of WO 3 nanoflakes with mismatched {002} and {020} facets exposed via a complex template assistant method.

Synthesis of WO 3 Nanocrystals
The WO 3 nanocrystals were synthesized by a mild hydrothermal method. Briefly, 20.0 g of the white Na 2 WO 4 ·2H 2 O powders were dissolved in 2.0 L of 1.0 mol/L HNO 3 with stirring for 3 days at room temperature, while the HNO 3 solution was replaced daily with a fresh solution of equal volume and equal concentration, to prepare the white tungstic acid monohydrate (H 2 WO 4 ·H 2 O) powders. 14.0 g of H 2 WO 4 ·H 2 O and 18.2 g of TMAOH were dissolved in 140 mL of deionized water with stirring for 15 min and then transferred into two 100 mL of Teflon-lined autoclave on average. After adequate sealing, the two autoclaves were fixed in a homogeneous reactor and heated at 85 • C for 24 h with constant stirring to prepare a TMA + -intercalated tungstic acid compound and then cooled to ambient temperature. The above compound was dispersed in 500 mL of deionized water and stirred at room temperature for 3 days to obtain a white tungstic acid colloidal suspension. An amount of 65 mL of the precursor colloidal suspension was transferred into a 100 mL Teflon-lined autoclave, and then adjusted to the set pH value (pH = 0.0, 1.5, 3.0, 5.0, and 7.0) under stirring conditions. After the autoclaves were tightly sealed, they were put into the constant temperature blast drying oven for a reaction at 180 • C for 24 h. Yellow WO 3 products (pH0.0-WO 3 , pH1.5-WO 3 , pH3.0-WO 3 , pH5.0-WO 3 , and pH7.0-WO 3 ) were obtained by centrifuge after multiple times washing with deionized water and drying at room temperature for longer than 24 h, and then calcined in a high-temperature box furnace at 500 • C for 4 h.

Sample Characterization
Powder X-ray diffraction analysis was carried out using an XRD-6100 (Shimadzu, Kyoto, Japan) with monochromated Cu Kα radiation (λ = 0.15406 nm). The morphology of the precursor Na 2 WO 4 ·2H 2 O, H 2 WO 4 ·H 2 O, and the synthesized WO 3 samples were observed using a field emission scanning electron microscopy (FESEM, Hitachi SU8100, Tokyo, Japan). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by using an FEI TALO F200S (Portland, OR, USA) at an operating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a K-Alpha instrument (Thermo Fisher Scientific, New York, NY, USA) and calibrated by the binding energy of C 1s 284.6 eV. The Brunauer-Emmett-Teller surface areas were obtained by using a micromeritics ASAP 2020 nitrogen adsorption instrument (Micromeritics Instrument Corp., Atlanta, GA, USA) at 77 K. Ultraviolet-visible (UV-Vis) absorption spectra of the WO 3 samples were recorded with a UV-Vis spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan). Photoluminescence analysis of the WO 3 samples was measured on a fluorescence spectrometer (PL, HORIBA Fluoromax-4, HORIBA Instruments Inc., Kyoto, Japan) and the emission spectrum were excited at a wavelength of 325 nm. The electrical measurements were performed using electrochemical impedance spectroscopy (EIS, CHI600E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) with indium-tin oxide glass, platinum plate (opening area: 1 cm 2 ), and Ag/AgCl (saturated KCl solution) as the working electrode, counter electrode, and reference electrode, respectively, under the irradiation of a 300 W xenon lamp. The EIS were performed in a 0.2 mol/L Na 2 SO 4 solution with a frequency range from 0.01 Hz to 100 kHz under open circuit potential conditions. The working electrode was prepared by a simple method as follows: 25 mg of WO 3 was dispersed in 1 mL of 5% polyvinylidene fluoride (PVDF) solution and stirred for 60 min to form uniform WO 3 slurry. Then, 20 µL of the slurry was dripped on the ITO glass with a 3 cm × 1 cm area and dried at 80 • C for 12 h.

Photocatalytic Experiments
Photocatalytic activities of the synthesized pHx-WO 3 samples were evaluated for the decolorization of methylene blue (MB) aqueous solution with a 175 W low-pressure mercury lamp at ambient temperature. Typically, 150 mg of the WO 3 sample (pH0.0-WO 3 , pH1.5-WO 3 , pH3.0-WO 3 , pH5.0-WO 3 , pH7.0-WO 3 , and CM-WO 3 ) was dispersed in 150 mL of 15 mg/L MB aqueous solution (4.07 × 10 −5 mol/L). Before the sample was exposed to ultraviolet-visible light irradiation, the suspension was magnetically stirred in the dark for 2 h to ensure the adsorption-desorption equilibrium of the MB dye on the surfaces of WO 3 sample. Then, at given time of irradiation, 3 mL of suspension was taken out and the WO 3 sample was immediately centrifuged to analyze the supernatant liquor by an ultraviolet-visible spectrophotometer (TU 1901, Beijing Purkinje General Instrument Co., Ltd., Beijing, China).

XRD Analysis
The commercial precursor, orthorhombic Na2WO4·2H2O, is confirmed by the powder X-ray diffraction (XRD) pattern (JCPDS no. 47-0064, a = 10.592, b = 13.858, and c = 8.479), as shown in Figure 1a. The existence of the main diffraction peaks of (020), (040), and (060), and the corresponding d values are 0.685, 0.344, and 0.229 nm, respectively, indicate that the precursor Na2WO4·2H2O belongs to layered compounds. After the precursor Na2WO4·2H2O was treated with 1.0 mol/L HNO3 for 3 days at room temperature, monoclinic H2WO4·H 2O (JCPDS no. 18-1420, a = 0.750, b = 0.693, c = 0.370 nm, and β = 90.5°) were obtained (Figure 1b). The diffraction peaks of H2WO4·H 2O at 2θ = 12.92°, 25.90°, and 39.28° correspond to the (010), (020), and (030) crystal planes and the crystal plane spacing is 0.685, 0.344, and 0.229 nm, respectively, indicating that the H2WO4·H 2O sample also has a layered structure.   (14)) with the standard card JCPDS no. 43-1035, indicating that the samples have high crystallographic purity. The strong and sharp reflection peaks and very horizontal baselines in Figure 2a-e indicate the high crystallinity and purity in the as-synthesized WO3 powder samples [3]. The relative intensities of the (002), (020), and (200) diffraction peaks of the as-synthesized WO3 powder samples are different, indicating that different controlling agents may adjust the different exposed facets [2].   Figure 2a-e indicate the high crystallinity and purity in the as-synthesized WO 3 powder samples [3]. The relative intensities of the (002), (020), and (200) diffraction peaks of the as-synthesized WO 3 powder samples are different, indicating that different controlling agents may adjust the different exposed facets [2].

FESEM and TEM Analysis
The morphology of the Na 2 WO 4 , H 2 WO 4 , and the as-synthesized pHx-WO 3 samples were characterized by FESEM. Figure 3a illustrates a typical FESEM image of Na 2 WO 4 sample. It can be seen that most of the samples exhibit square rod-like morphology with wide size distributions (length ranges from hundreds of nanometers to several micrometers and width ranges from 0.1 to 0.5 µm). After the ion exchange reaction, the obtained H 2 WO 4 sample exhibits irregular rod-like morphology with wide size distributions (length ranges from 0.1 to 1.5 µm and width ranges from 0.1 to 0.5 µm) (Figure 3b), which is probably caused by the fracture of the rod-like Na 2 WO 4 particles under the condition of intense agitation. Figure 3c shows a typical FESEM image of pH0.0-WO 3 product prepared by hydrothermal treatment of TMA + -intercalated tungstic acid colloidal suspension in pH value of 0.0. Most of the products exhibit irregular rod-like morphology with wide size distributions (length is from 0.1 to 1.5 µm and width is from 30 to 230 nm). Figure 3d shows that when the pH value of colloidal suspension is 1.5, the pH1.5-WO 3 product exhibits a discordant rod-like shape with a length of about 0.2~2.2 µm and a width of about 50~30 nm, and several irregular particles. Some discordant rod-like nanoparticles with 70~350 nm in length and 30~70 nm in width and many irregular nanoparticles with wide size distributions are observed when the pH value of colloidal suspension is 3.0, as shown in Figure 3e,f. As shown in Figure 3g,h, the pH5.0-WO 3 and pH7.0-WO 3 samples consist of large amounts of cuboid, spherical and irregular nanoparticles with an average size of 34.8 nm and 37.9 nm, respectively. CM-WO 3 sample consists of large amounts of irregular nanoparticles with an average size of 137 nm, as shown in Figure 3i.

FESEM and TEM Analysis
The morphology of the Na2WO4, H2WO4, and the as-synthesized pHx-WO3 samples were characterized by FESEM. Figure 3a illustrates a typical FESEM image of Na2WO4 sample. It can be seen that most of the samples exhibit square rod-like morphology with wide size distributions (length ranges from hundreds of nanometers to several micrometers and width ranges from 0.1 to 0.5 µ m). After the ion exchange reaction, the obtained H2WO4 sample exhibits irregular rod-like morphology with wide size distributions (length ranges from 0.1 to 1.5 µ m and width ranges from 0.1 to 0.5 µ m) (Figure 3b), which is probably caused by the fracture of the rod-like Na2WO4 particles under the condition of intense agitation. Figure 3c shows a typical FESEM image of pH0.0-WO3 product prepared by hydrothermal treatment of TMA + -intercalated tungstic acid colloidal suspension in pH value of 0.0. Most of the products exhibit irregular rod-like morphology with wide size distributions (length is from 0.1 to 1.5 µ m and width is from 30 to 230 nm). Figure 3d shows that when the pH value of colloidal suspension is 1.5, the pH1.5-WO3 product exhibits a discordant rod-like shape with a length of about 0.2~2.2 µ m and a width of about 50~30 nm, and several irregular particles. Some discordant rod-like nanoparticles with 70~350 nm in length and 30~70 nm in width and many irregular nanoparticles with wide size distributions are observed when the pH value of colloidal suspension is 3.0, as shown in Figure 3e,f. As shown in Figure 3g,h, the pH5.0-WO3 and pH7.0-WO3 samples consist of large amounts of cuboid, spherical and irregular nanoparticles with an average size of 34.8 nm and 37.9 nm, respectively. CM-WO3 sample consists of large amounts of irregular nanoparticles with an average and 0.384 (or 0.385) nm, respectively, and the (020) and (002) crystal planes parallel to the two sets of surfaces of rod-shaped (or cuboid-shaped) WO 3 crystals, respectively, indicating that the co-exposed facets of pH1.5-WO 3 crystals are {002}, {020} and {200} facets. Rodshaped WO 3 crystals with a length of 240-540 nm and a width of 40-110 nm were observed in the TEM image of pH3.0-WO 3 crystals (Figure 4g). Figure 4h shows the HRTEM image of an individual pH3.0-WO 3 nanorod taken from the marked area of the TEM images, revealing that the nanorod possesses a single-crystal structure and the lattice spacing of around 0.379 and 0.389 nm along the horizontal axis and longitudinal axis direction correspond to the d spacing of WO 3 (020) and (002) crystal planes. The above analysis results further indicate that the co-exposed facets of pH3.0-WO 3    The pH5.0-WO 3 sample consists of a large amount of irregular morphology nanocrystals with a size of 18~106 nm, as shown in Figure 5a. The corresponding HRTEM images (Figure 5b,c) show that the distances of the visible lattice fringes over a large area were measured to be 0.525, 0.389, and 0.375 (or 0.378) nm, which are in agreement with the lattice spacing of (110), (002) and (020) atomic planes of the monoclinic structure of WO 3 . In addition, the (110) and (002) crystal planes parallel to the sides of the irregular nanocrystal with an interfacial angle of 89.3 • , indicating that the irregular nanocrystal with co-exposed of {110} and {002} facets on its sides, as shown in Figure 5b. Irregular-shaped WO 3 nanocrystals with a size of 18-86 nm were observed in the TEM image of the pH7.0-WO 3 sample, as shown in Figure 5d. The HRTEM images (Figure 5e,f) show that the distances of the visible lattice fringes over a large area were measured to be 0.263 and 0.375 nm, which are in agreement with the lattice spacing of (202) and (020) atomic planes of the monoclinic structure of WO 3 . Furthermore, the interfacial angle between (202) and (020) crystal planes is 90 • , which is consistent with the theoretical value. Since the crystal planes perpendicular to (202) and (020) at the same time cannot be determined, there are no specific exposed crystal facets on the base surface of the pH7.0-WO 3 nanocrystals with irregular morphology. The (202) and (020) crystal planes are parallel to the sides of the irregular nanocrystal, so the {202} and {020} crystal facets are co-exposed on the sides. Compressed hexagonal prismatic and irregular-shaped WO 3 nanocrystals with a size of 55-730 nm were observed in the CM-WO 3 sample, as shown in Figure 5g. The lattice spacings of 0.375 and 0.383 with an interfacial angle of 90 • can be indexed to the (020) and (002) crystal planes, respectively ( Figure 5h). Furthermore, the (020) and (002) crystal planes are parallel to the sides of the compressed hexagonal prism, indicating that the co-exposed crystal facets of the com-  The pH5.0-WO3 sample consists of a large amount of irregular morphology nanocrystals with a size of 18~106 nm, as shown in Figure 5a. The corresponding HRTEM images (Figure 5b,c) show that the distances of the visible lattice fringes over a large area were measured to be 0.525, 0.389, and 0.375 (or 0.378) nm, which are in agreement with the lattice spacing of (110), (002) and (020) atomic planes of the monoclinic structure of WO3. In addition, the (110) and (002) crystal planes parallel to the sides of the irregular

X-ray Photoelectron Spectroscopy Analysis
To complete the previous analysis, X-ray photoelectron spectroscopy (XPS) analysis reveals the surface chemical composition and electronic states of the elements of the

X-ray Photoelectron Spectroscopy Analysis
To complete the previous analysis, X-ray photoelectron spectroscopy (XPS) analysis reveals the surface chemical composition and electronic states of the elements of the Na 2 WO 4 , H 2 WO 4 , as-prepared pHx-WO 3 and CM-WO 3 samples (Figure 7). The C 1s peak at 284.88 eV observed in the survey scan is due to carbon contamination used to calibrate the binding energy [22]. The full wide-scan spectra of the Na 2 WO 4 , H 2 WO 4 , as-prepared pHx-WO 3, and CM-WO 3 samples are presented in Figure 7a, from which we observe clearly characteristic peaks of Na (exists only in Na 2 WO 4 ), W, O and C elements. The high-resolution W 4f spectrum in Figure 4b, displays two peaks with binding energy values of 37.08~38.08 and 34.98~35.88 eV for W 4f 5/2 and W 4f 7/2 indicating the W(VI) oxidation state of Na 2 WO 4 , H 2 WO 4 , as-prepared pHx-WO 3 and CM-WO 3 samples [23]. The O 1s peak (Figure 4c) at 530.18~530.68 eV matches well with oxygen species in the Na 2 WO 4 , H 2 WO 4 , as-prepared pHx-WO 3, and CM-WO 3 samples, which can be assigned to typical surface lattice oxygen [23]. The Na 1s peak (Figure 4d) at 1071.08 eV matches well with sodium species in the Na 2 WO 4 sample. No peak of Na 1s was observed in H 2 WO 4 , further indicating that Na + ions in Na 2 WO 4 were well displaced by H + ions. values of 37.08~38.08 and 34.98~35.88 eV for W 4f5/2 and W 4f7/2 indicating the W(VI) oxidation state of Na2WO4, H2WO4, as-prepared pHx-WO3 and CM-WO3 samples [23]. The O 1s peak (Figure 4c) at 530.18~530.68 eV matches well with oxygen species in the Na2WO4, H2WO4, as-prepared pHx-WO3, and CM-WO3 samples, which can be assigned to typical surface lattice oxygen [23]. The Na 1s peak (Figure 4d) at 1071.08 eV matches well with sodium species in the Na2WO4 sample. No peak of Na 1s was observed in H2WO4, further indicating that Na + ions in Na2WO4 were well displaced by H + ions.

Optical Properties
The comparison of the UV-vis absorption spectra and PL spectra of the as-prepared pHx-WO3 and the CM-WO3 samples are presented in Figure 8. As demonstrated in Figure 8a, the as-prepared pHx-WO3 and the CM-WO3 samples have similar adsorption spectra, and the absorption edge is at ~475 nm. The light adsorption intensity of the

Optical Properties
The comparison of the UV-vis absorption spectra and PL spectra of the as-prepared pHx-WO 3 and the CM-WO 3 samples are presented in Figure 8. As demonstrated in Figure 8a, the as-prepared pHx-WO 3 and the CM-WO 3 samples have similar adsorption spectra, and the absorption edge is at~475 nm. The light adsorption intensity of the as-prepared pHx-WO 3 and the CM-WO 3 samples decreases in the order of pH3.0-WO 3 > pH1.5-WO 3 > pH0.0-WO 3 > pH5.0-WO 3 > CM-WO 3 > pH7.0-WO 3 . Using the concept of the edge at the intersection of wavelength through extrapolation of the horizontal and sharply rising portions of the curves, the absorption peak for pH0.0-WO 3 , pH1.5-WO 3 , pH3.0-WO 3 , pH5.0-WO 3 , pH7.0-WO 3 , and CM-WO 3 was determined at 466, 470, 475, 476, 475, and 476 nm, respectively [24,25]. Using the formula Band gap = 1240/Wave length [24,25], 2.66, 2.64, 2.61, 2.60, 2.61, and 2.60 eV was calculated as band gap energy for pH0.0-WO 3 , pH1.5-WO 3 , pH3.0-WO 3 , pH5.0-WO 3 , pH7.0-WO 3 , and CM-WO 3 samples, respectively, which are consistent with the energy gap of monoclinic WO 3 (2.6-2.8 eV), indicating that the pH value had little effect on the band gap of WO 3 [26]. The photocatalytic activity is related to the charge migration and the recombination rate of photogenerated carriers [9]. Therefore, in order to determine the charge migration and degree of recombination of electron-hole pairs, that is, the separation ability of electron-hole pairs, the photoluminescence (PL) spectra of the as-prepared pHx-WO 3 and the CM-WO 3 samples was studied as shown in Figure 8b. As shown in Figure 8b, the photoluminescence intensity of the as-prepared pHx-WO 3 and the CM-WO 3 samples decreases in the order of CM-WO 3 > pH5.0-WO 3 > pH0.0-WO 3 > pH3.0-WO 3 > pH1.5-WO 3 > pH7.0-WO 3 . Generally speaking, the recombination rate of electron-hole pairs is related to the intensity of the luminescence peak. The sample with strong luminescence peak intensity indicates the faster recombination rate of electron-hole pairs, whereas the sample with weak luminescence peak intensity indicates the lower recombination rate of electron-hole pairs. Therefore, the high intensity of the luminescence peaks for CM-WO 3 corresponds to higher charge carrier recombination. Among all the WO 3 samples, the pH7.0-WO 3 exhibits the weakest luminescence peak intensity, indicating the lowest charge carrier recombination rate in pH7.0-WO 3 , that is, the pH7.0-WO 3 offers an excellent photocatalytic activity. [24,25], 2.66, 2.64, 2.61, 2.60, 2.61, and 2.60 eV was calculated as band gap energy for pH0.0-WO3, pH1.5-WO3, pH3.0-WO3, pH5.0-WO3, pH7.0-WO3, and CM-WO3 samples, respectively, which are consistent with the energy gap of monoclinic WO3 (2.6-2.8 eV), indicating that the pH value had little effect on the band gap of WO3 [26]. The photocatalytic activity is related to the charge migration and the recombination rate of photogenerated carriers [9]. Therefore, in order to determine the charge migration and degree of recombination of electron-hole pairs, that is, the separation ability of electron-hole pairs, the photoluminescence (PL) spectra of the as-prepared pHx-WO3 and the CM-WO3 samples was studied as shown in Figure 8b. As shown in Figure 8b, the photoluminescence intensity of the as-prepared pHx-WO3 and the CM-WO3 samples decreases in the order of CM-WO3 > pH5.0-WO3 > pH0.0-WO3 > pH3.0-WO3 > pH1.5-WO3 > pH7.0-WO3. Generally speaking, the recombination rate of electron-hole pairs is related to the intensity of the luminescence peak. The sample with strong luminescence peak intensity indicates the faster recombination rate of electron-hole pairs, whereas the sample with weak luminescence peak intensity indicates the lower recombination rate of electron-hole pairs. Therefore, the high intensity of the luminescence peaks for CM-WO3 corresponds to higher charge carrier recombination. Among all the WO3 samples, the pH7.0-WO3 exhibits the weakest luminescence peak intensity, indicating the lowest charge carrier recombination rate in pH7.0-WO3, that is, the pH7.0-WO3 offers an excellent photocatalytic activity.

Electrochemical Impedance Spectra Analysis
Electrochemical impedance spectra (EIS) measurement was employed to investigate the charge migration behavior between the photoinduced electrons and holes [27]. The EIS Nyquist plots of electrodes based on as-prepared pHx-WO3 and the CM-WO3 are shown in Figure 9. The ideal Nyquist plot will show three semicircles in the high-frequency, medium-frequency, and low-frequency ranges, corresponding to the charge transfer at the counter electrode-electrolyte interface, the charge transfer at the oxide-electrolyte interface (Rct), and the diffusion of ions through the electrolyte, respectively [28]. Under illumination, the photoinduced electron-hole pairs are separated by the applied potential, resulting in the reduction of the Rct and the enhancement of the electronic conductivity of the WO3 electrode [29]. The diameter of arc radius on the EIS

Electrochemical Impedance Spectra Analysis
Electrochemical impedance spectra (EIS) measurement was employed to investigate the charge migration behavior between the photoinduced electrons and holes [27]. The EIS Nyquist plots of electrodes based on as-prepared pHx-WO 3 and the CM-WO 3 are shown in Figure 9. The ideal Nyquist plot will show three semicircles in the high-frequency, medium-frequency, and low-frequency ranges, corresponding to the charge transfer at the counter electrode-electrolyte interface, the charge transfer at the oxide-electrolyte interface (R ct ), and the diffusion of ions through the electrolyte, respectively [28]. Under illumination, the photoinduced electron-hole pairs are separated by the applied potential, resulting in the reduction of the R ct and the enhancement of the electronic conductivity of the WO 3 electrode [29]. The diameter of arc radius on the EIS Nyquist plot of the pH7.0-WO 3 is smaller than that of pH0.0-WO 3 , pH1.5-WO 3 , pH3.0-WO 3 , pH5.0-WO 3 , and CM-WO 3 , indicating that the pH7.0-WO 3 has the lowest electric resistance and the highest conductivity. As we all know that the rapid separation of photoexcited electronhole pairs is essential to improving photocatalytic activity [30]. The high conductivity of pH7.0-WO 3 is also conducive to the transfer of electrons, thus promoting an effective charge separation [30]. The Nyquist plots of pH0.0-WO 3 , pH1.5-WO 3 , pH5.0-WO 3 , and CM-WO 3 have similar trends, and no typical semicircle is observed at high frequency, indicating the lower electron-hole separation efficiency. The diameter of arc radius on the EIS Nyquist plot of the pH3.0-WO 3 is bigger than that of other WO 3 , suggesting that the pH3.0-WO 3 has the highest electric resistance. No typical semicircle is observed on the EIS Nyquist plot of pH3.0-WO 3 , indicating that the charge separation efficiency of pH3.0-WO 3 is also very low and lower than that of other WO 3 samples. pH1.5-WO3, pH5.0-WO3, and CM-WO3 have similar trends, and no typical semicircle is observed at high frequency, indicating the lower electron-hole separation efficiency. The diameter of arc radius on the EIS Nyquist plot of the pH3.0-WO3 is bigger than that of other WO3, suggesting that the pH3.0-WO3 has the highest electric resistance. No typical semicircle is observed on the EIS Nyquist plot of pH3.0-WO3, indicating that the charge separation efficiency of pH3.0-WO3 is also very low and lower than that of other WO3 samples. Figure 9. Electrochemical impedance spectra Nyquist plots of the as-prepared pHx-WO3 and the CM-WO3 samples.
The particle size of the crystal is inversely proportional to the specific surface area. Generally speaking, in photochemical reaction, a smaller particle size (enhancing redox capacity) is conducive to the acceleration of the migration rate of photogenerated electrons and holes and the deceleration of the recombination rate [33], and a larger specific surface area (providing more adsorption sites) is conducive to the adsorption of dye molecules on the surface of the catalyst [34], thereby improving the photocatalytic activity of the catalyst. Based on the above discussion, the specific surface area ranks in the order of pH7.0-WO 3 (14.3 m 2 /g) > pH5.0-WO 3 (13.7 m 2 /g) > pH3.0-WO 3 (7.7 m 2 /g) > pH0.0-WO 3 (6.4 m 2 /g) > pH1.5-WO 3 (5.4 m 2 /g) > CM-WO 3 (2.6 m 2 /g). The increasing order of photocatalytic activity is blank < CM-WO 3 < pH0.0-WO 3 < pH1.5-WO 3 < pH3.0-WO 3 < pH5.0-WO 3 < pH7.0-WO 3 , which is almost the same as that of specific surface area. Moreover, according to the previous PL and EIS analysis, pH7.0-WO 3 has the weakest luminescence peak intensity and the smallest arc radius diameter, indicating that the pH7.0-WO 3 has the highest separation and transfer efficiency and the lowest recombination rate of photoinduced electron-hole pairs.
Multiple use evaluations of a photocatalyst can predict its long-term performance and economic viability. The reusability of pH5.0-WO 3 and pH7.0-WO 3 for MB photocatalytic degradation efficiency was examined, as shown in Figure 11. Filtered the methylene blue solution containing solid catalyst after illumination to re-obtain pH5.0-WO 3 or pH7.0-WO 3 solid, and then dry it naturally for subsequent use. For pH5.0-WO 3 , 90.5% MB degradation is found in the first run which decreases to 88.5% and 85.8% in the second and third run, respectively. For pH7.0-WO 3 , 95.0% MB degradation is found in the first run which decreases to 92.7% and 89.6% in the second and third run, respectively. The photocatalytic activity of pH5.0-WO 3 and pH7.0-WO 3 decreased only 4.7% and 5.4% for MB, respectively, after three consecutive cycles, indicating that the pH5.0-WO 3 and pH7.0-WO 3 possessed good reusability.
Multiple use evaluations of a photocatalyst can predict its long-term performance and economic viability. The reusability of pH5.0-WO3 and pH7.0-WO3 for MB photocatalytic degradation efficiency was examined, as shown in Figure 11. Filtered the methylene blue solution containing solid catalyst after illumination to re-obtain pH5.0-WO3 or pH7.0-WO3 solid, and then dry it naturally for subsequent use. For pH5.0-WO3, 90.5% MB degradation is found in the first run which decreases to 88.5% and 85.8% in the second and third run, respectively. For pH7.0-WO3, 95.0% MB degradation is found in the first run which decreases to 92.7% and 89.6% in the second and third run, respectively. The photocatalytic activity of pH5.0-WO3 and pH7.0-WO3 decreased only 4.7% and 5.4% for MB, respectively, after three consecutive cycles, indicating that the pH5.0-WO3 and pH7.0-WO3 possessed good reusability. Figure 11. Cyclic degradation curves of pH5.0-WO3 and pH7.0-WO3 for methylene blue solution. Figure 12 is the XRD patterns of WO3 samples separated from methylene blue solution after photocatalytic degradation. It can be seen from Figure 12 that the crystal structure of pHx-WO3 and CM-WO3 has not changed after the photocatalytic reaction, indicating that they are stable during the photocatalytic reaction. Compared with Figure  2, only the crystallinity is reduced, which is caused by the dispersion of the agglomerated particles under vigorous stirring conditions.  Figure 12 is the XRD patterns of WO 3 samples separated from methylene blue solution after photocatalytic degradation. It can be seen from Figure 12 that the crystal structure of pHx-WO 3 and CM-WO 3 has not changed after the photocatalytic reaction, indicating that they are stable during the photocatalytic reaction. Compared with Figure 2, only the crystallinity is reduced, which is caused by the dispersion of the agglomerated particles under vigorous stirring conditions.

Conclusions
In summary, rod-shaped WO 3 nanocrystals with co-exposed {002}, {020} and {200} facets, cuboid-shaped WO 3 nanocrystals with co-exposed {020} and {200} facets, and irreg-ular WO 3 nanocrystals with co-exposed {110} and {002} facets (or {202) and {020} crystal facets) were synthesized by a simple hydrothermal treatment of white tungstic acid colloidal suspension with desired pH values. The crystal structure, morphology, specific surface area, pore size distribution, chemical composition, electronic states of the elements, optical properties, and charge migration behavior of as-obtained WO 3 products were characterized by XRD, FESEM, TEM, HRTEM, XPS, fully automatic specific surface area and porosity analyzer, UV-vis absorption spectra, PL spectra, and EIS. Photocatalytic degradation of MB performance of the as-obtained WO 3 nanocrystals was investigated under ultraviolet irradiation. The increasing order of photocatalytic activity is blank < CM-WO 3 < pH0.0-WO 3 < pH1.5-WO 3 < pH3.0-WO 3 < pH5.0-WO 3 < pH7.0-WO 3 . The highest photocatalytic activity of pH7.0-WO 3 could be attributed to the synergistic effects of the largest specific surface area, the weakest luminescence peak intensity, and the smallest arc radius diameter in comparison with CM-WO 3 and other pHx-WO 3 nanocrystals.

Conflicts of Interest:
The authors declare no conflict of interest.