Cs-Doped WO3 with Enhanced Conduction Band for Efficient Photocatalytic Oxygen Evolution Reaction Driven by Long-Wavelength Visible Light

Cesium doped WO3 (Cs-WO3) photocatalyst with high and stable oxidation activity was successfully synthesized by a one-step hydrothermal method using Cs2CO3 as the doped metal ion source and tungstic acid (H2WO4) as the tungsten source. A series of analytical characterization tools and oxygen precipitation activity tests were used to compare the effects of different additions of Cs2CO3 on the crystal structure and microscopic morphologies. The UV–visible diffuse reflectance spectra (DRS) of Cs-doped material exhibited a significant red shift in the absorption edge with new shoulders appearing at 440–520 nm. The formation of an oxygen vacancy was confirmed in Cs-WO3 by the EPR signal, which can effectively regulate the electronic structure of the catalyst surface and contribute to improving the activity of the oxygen evolution reaction (OER). The photocatalytic OER results showed that the Cs-WO3-0.1 exhibited the optimal oxygen precipitation activity, reaching 58.28 µmol at 6 h, which was greater than six times higher than that of WO3-0 (9.76 μmol). It can be attributed to the synergistic effect of the increase in the conduction band position of Cs-WO3-0.1 (0.11 V) and oxygen vacancies compared to WO3-0, which accelerate the electron conduction rate and slow down the rapid compounding of photogenerated electrons–holes, improving the water-catalytic oxygen precipitation activity of WO3.


Introduction
Hydrogen energy converted from abundant solar energy is believed to be a sustainable green fuel to replace traditional fossil fuel due to its being pollution-free and low-cost [1].Hydrogen production and oxygen production can be generated simultaneously by photocatalytic water splitting, which results from a hydrogen evolution reaction (HER) and OER [2].Compared with the HER, the OER, as a four-electron transfer reaction, requires higher photon energy to overcome the reaction kinetic barriers and it is considered to be the rate-determining step in the entire water splitting process [3,4].Currently, IrO 2 and RuO 2 have been widely used as photocatalysts for the OER due to their high catalytic activities [5,6].However, these noble metals are scarce and costly, limiting their large-scale application for water splitting.Therefore, the development of an advanced photocatalyst based on low-cost and earth-abundant materials with highly efficient acceleration of the reaction dynamic and lowering of the energy barrier of OER is core issue for photocatalytic water splitting.
In recent years, some nanostructured semiconductor photocatalysts with lower OER overpotentials, including WO 3 , Fe 2 O 3 , BiVO 4 , TaON, BaZrO 3 -BaTaO 2 N and so on, have been explored [7][8][9][10][11].Among these compounds, WO 3 has attracted immense attention owning to its suitable bandgap (2.4-2.8 eV) [12,13], earth abundance, good chemical stability in strongly acidic media and its thermodynamically suitable valence band positions for OER.Nevertheless, WO 3 suffers from limited visible-light response capability (λ < 460 nm), resulting in poor OER kinetics.Therefore, it is urgent to develop a WO 3 photocatalyst with efficient light absorption at longer wavelengths in the visible region.Nonmetal element doping as one of the most effective strategies to improve the visible-light absorption of WO 3 has been confirmed due to the formation of an intermediate level between the conduction band (CB) and valence band (VB) of WO 3 , resulting in the decrease in the band gap [14,15].Also, metal element doping has already been used as a strategy to promote the visible-light absorption of WO 3 because the conduction band position of WO 3 can be adjusted to narrow the bandgap [16].
Herein, we report the first synthesis of Cs-doped WO 3 (Cs-WO 3 ) photocatalysts with Cs + doping into the WO 3 lattice through the hydrothermal method using cesium carbonate (Cs 2 CO 3 ) as the Cs ion source.The Cs-WO 3 was responsive to visible light of λ ≤ 520 nm, which give a red shift of 80 nm compared with that of pristine WO 3 (440 nm).The photocatalytic activities of Cs-WO 3 for OER were significantly enhanced compared with pristine WO 3 , because the CB potential of WO 3 is significantly increased through cesium doping, while introducing more lattice defects and oxygen vacancies, thereby improving the conductivity of the semiconductor.In particular, there is an extremely significant mutually dependent relation between the addition of the Cs source and the photocatalytic activities of Cs-WO 3 for OER.

Characterization and Influencing Factors of Structure of Cs-WO 3
The results of the influencing factors on the structure of Cs-WO 3 -0.1 are shown in Figure 1. Figure 1A exhibits the XRD patterns of the samples prepared at different solvothermal temperatures from 120 to 180 • C. The XRD pattern prepared at 120 • C agreed with that of orthorhombic H 2 WO 4 (JCPDS no.01-084-0886).When the solvothermal temperature was 150 • C, it can be clearly observed that the peaks at 2θ = 15.0 • , 24.6 • , 28.9  210), (311), ( 222), ( 422) and (511) plane, respectively, which can be assigned to a cubic WO 3 •0.5H 2 O crystalline phase (PDF # 01-084-1851).The peaks intensities of the Cs-WO 3 -0.1 decreased when increasing the temperature to 180 • C, and meanwhile the peaks at 27.2 • and 27.8 • assigned to the Cs 2 W 6 O 19 compound (PDF # 01-045-0522) were formed.The XRD patterns of Cs-WO 3 -0.1 prepared at 150 • C for different reaction times of 12~36 h are exhibited in Figure 1B.The peak intensities of Cs-WO 3 -0.1 increased with the reaction time from 12 to 24 h, and thereby decreased above 24 h due to the formation of the Cs 2 W 6 O 19 compound.The XRD results suggest that the optimum preparation conditions of Cs-WO 3 -0.1 are a solvothermal temperature of 150 • C and a reaction time of 24 h, which were employed in our experiments.As shown in Figure 1C, the XRD patterns of the WO 3 -0 and Cs-WO 3 samples show the characteristic peaks of the cubic WO 3 •0.5H 2 O crystalline phase (PDF # 01-084-1851).However, the peaks of the Cs 2 W 6 O 19 compound began to be seen in the Cs-WO 3 -0.5 alongside the main peaks of the cubic phase WO 3 •0.5H 2 O while overdoping with Cs + ions.In Figure 1B,C, obvious shifts of XRD peaks are observed, which are ascribed to the existence of structural strain [34].
Additionally, compared with the WO 3 -0, the crystallinity of the Cs-WO 3 samples decreased with increasing the doping concentration of Cs ions.This may be due to the disruption of the crystal structure of WO 3 , when the Cs ions with larger ionic radii (1.67 Å) are doped into the WO 3 lattice, resulting in the formation of a distorted structure.
Molecules 2024, 29, 3126 3 of 13 1851).However, the peaks of the Cs2W6O19 compound began to be seen in the Cs-WO3-0.5alongside the main peaks of the cubic phase WO3•0.5H2O while overdoping with Cs + ions.
In Figure 1B,C, obvious shifts of XRD peaks are observed, which are ascribed to the existence of structural strain [34].Additionally, compared with the WO3-0, the crystallinity of the Cs-WO3 samples decreased with increasing the doping concentration of Cs ions.This may be due to the disruption of the crystal structure of WO3, when the Cs ions with larger ionic radii (1.67 Å) are doped into the WO3 lattice, resulting in the formation of a distorted structure.The average crystal grain size was calculated from the (311) peak (2θ = 28.9˚) of cubic WO3•0.5H2Oaccording to Scherrer's equation [35], as shown in Table 1.The crystallite diameter of the WO3-0 (23.6 nm) was larger than those of Cs-WO3-0.1 (22.6 nm), Cs-WO3-0.3(20.9 nm) and Cs-WO3-0.5 (19.1 nm).The calculated lattice parameters and average crystalline size of WO3 were decreased (Table 1) by Cs doping.The results indicate that Cs-doped WO3 has a strong restraining function with the increase in crystal size owning to the dopant cations Cs + preventing the growth of crystal grains in the nanoparticles [36]. (a) The content of Cs + was analyzed according to the approach we reported previously. (b) The crystallite diameters were calculated from XRD data according to the Scherrer equation and expressed as average values calculated based on the (311) peak. (c) The surface areas were provided from N2 sorption isotherms.
As shown in Figure 2a, the elliptical-like WO3-0 is composed of nanosheets and nanoparticles with a major axis of about 55 µm and a semimajor axis of 40 µm.Notably, the Cs-WO3-0.1 is made up of a microsphere of about 41 µm in diameter which mainly The average crystal grain size was calculated from the (311) peak (2θ = 28.9• ) of cubic WO 3 •0.5H 2 O according to Scherrer's equation [35], as shown in Table 1.The crystallite diameter of the WO 3 -0 (23.6 nm) was larger than those of Cs-WO 3 -0.1 (22.6 nm), Cs-WO 3 -0.3(20.9 nm) and Cs-WO 3 -0.5 (19.1 nm).The calculated lattice parameters and average crystalline size of WO 3 were decreased (Table 1) by Cs doping.The results indicate that Cs-doped WO 3 has a strong restraining function with the increase in crystal size owning to the dopant cations Cs + preventing the growth of crystal grains in the nanoparticles [36]. (a) The content of Cs + was analyzed according to the approach we reported previously. (b) The crystallite diameters were calculated from XRD data according to the Scherrer equation and expressed as average values calculated based on the (311) peak. (c) The surface areas were provided from N 2 sorption isotherms.
As shown in Figure 2a, the elliptical-like WO 3 -0 is composed of nanosheets and nanoparticles with a major axis of about 55 µm and a semimajor axis of 40 µm.Notably, the Cs-WO 3 -0.1 is made up of a microsphere of about 41 µm in diameter which mainly consists of nanoparticles and nanorods (Figure 2b).The diameter of the microsphere (43 µm) increased and the amount of nanorods gradually decreased with the increasing doping amount of Cs + (Figure 2c).A smooth microsphere with a diameter of 45 µm was formed for the Cs-WO 3 -0.5 (Figure 2d), suggesting that the formation of micrometer spheres mainly relies on self-assembly effects.As shown in Table 1, it was found that the specific surface area of the sample strongly depends on its morphology.The Cs-WO 3 -0.1 possessed a higher specific surface area (16.1 m 2 /g) relative to those of WO 3 -0 (9.2 m 2 /g) and Cs-WO 3 -0.3(10.6 m 2 /g) owing to the formation of nanorods on the surface of the microsphere, which is beneficial to form more accessible active sites for the OER.
µm) increased and the amount of nanorods gradually decreased with the increa doping amount of Cs + (Figure 2c).A smooth microsphere with a diameter of 45 µm formed for the Cs-WO3-0.5(Figure 2d), suggesting that the formation of microm spheres mainly relies on self-assembly effects.As shown in Table 1, it was found tha specific surface area of the sample strongly depends on its morphology.The Cs-WO possessed a higher specific surface area (16.1 m 2 /g) relative to those of WO3-0 (9.2 m and Cs-WO3-0.3(10.6 m 2 /g) owing to the formation of nanorods on the surface o microsphere, which is beneficial to form more accessible active sites for the OER.doping amount of Cs + (Figure 2c).A smooth microsphere with a diameter of 45 µm was formed for the Cs-WO3-0.5(Figure 2d), suggesting that the formation of micrometer spheres mainly relies on self-assembly effects.As shown in Table 1, it was found that the specific surface area of the sample strongly depends on its morphology.The Cs-WO3-0.1 possessed a higher specific surface area (16.1 m 2 /g) relative to those of WO3-0 (9.2 m 2 /g) and Cs-WO3-0.3(10.6 m 2 /g) owing to the formation of nanorods on the surface of the microsphere, which is beneficial to form more accessible active sites for the OER.The chemical composition and valence states of Cs-WO 3 samples were investigated by XPS.The spectra were calibrated with the C 1s peak as reference.As shown in Figure 4A, the XPS survey spectrum of WO 3 -0 (a) depicts that no other impurity phases were detected except W and O elements.W, O and Cs elements were co-present in the Cs-WO 3 -0.1 (b) and Cs-WO 3 -0.3(c).In Figure 4B, the high-resolution XPS spectrum of W 4f exhibits two peaks at 37.7 eV and 35.5 eV that are, respectively, ascribed to the spin-orbit doublet of W 4f 5/2 and W 4f 7/2 , respectively, for a W 6+ state in WO 3 [37].The XPS spectra of W 4f for Cs-WO 3 -0.1 and Cs-WO 3 -0.3(Figure 3b) exhibit two characteristic peaks at 38.1 eV and 35.9 eV, corresponding to 4f 5/2 and W 4f 7/2 of the WO 3 lattice, respectively.As shown in Figure 3c, the binding energy at 531.5 eV and 530.4 eV in the XPS spectrum of O 1s for the WO 3 -0 can be assigned to the H 2 O and lattice oxygen, respectively [38].The main peaks in the high-resolution O 2p spectra for the Cs-WO 3 -0.1 and Cs-WO 3 -0.3located at 530.8 eV can be assigned to the lattice oxygen of the W-O bond in the crystalline WO 3 .The banding energies at 531.9 eV correspond to the adsorbed oxygen ions and hydroxyl groups on the surface, respectively.The ratios of adsorbed O increased to 12.7% and 13.5% in Cs-WO 3 -0.1 and Cs-WO 3 -0.3due to the surface oxygen vacancies, while it was 10.3% in WO 3 .This can serve as indirect evidence for the presence of oxygen vacancies.The high-resolution XPS spectra of the Cs element for both Cs-WO 3 -0.1 and Cs-WO 3 -0.3(Figure 3d) exhibited two peaks at 738.4 eV and 724.7 eV, which are assigned to the spin orbits of Cs 3d 3/2 and Cs 3d 5/2 , respectively [32,39].Notably, the positive shifts of 0.4 eV and 0.8 eV in W 4f and O 1s for Cs-WO 3 can be seen after Cs + doping.This is due to the occurrence of ion exchange on the WO 3 surface, which also confirmed that the Cs was successfully introduced into the lattice of WO 3 and formed W-Cs bonds in the doped samples The positive shifts improve the PEC performance of WO 3 .Furthermore, it can be seen that the peak intensities of W 4f and O 1s for Cs-WO 3 decreased with increasing the doped contents of Cs element because of the replacement of W 6+ by Cs + leading to the contamination of oxygen.Generally, the larger the ionic radius is, the more difficult it is for doping to occur due to the requirement of high formation energy.Therefore, the replacement of W 6+ by Cs + is more favorable than replacing O 2 -with Cs + .
of W 4f5/2 and W 4f7/2, respectively, for a W 6+ state in WO3 [37].The XPS spectra of W 4f Cs-WO3-0.1 and Cs-WO3-0.3(Figure 3b) exhibit two characteristic peaks at 38.1 eV a 35.9 eV, corresponding to 4f5/2 and W 4f7/2 of the WO3 lattice, respectively.As shown Figure 3c, the binding energy at 531.5 eV and 530.4 eV in the XPS spectrum of O 1s for WO3-0 can be assigned to the H2O and lattice oxygen, respectively [38].The main peak the high-resolution O 2p spectra for the Cs-WO3-0.1 and Cs-WO3-0.3located at 530.8 can be assigned to the lattice oxygen of the W-O bond in the crystalline WO3.The band energies at 531.9 eV correspond to the adsorbed oxygen ions and hydroxyl groups on surface, respectively.The ratios of adsorbed O increased to 12.7% and 13.5% in Cs-W 0.1 and Cs-WO3-0.3due to the surface oxygen vacancies, while it was 10.3% in WO3.T can serve as indirect evidence for the presence of oxygen vacancies.The high-resolut XPS spectra of the Cs element for both Cs-WO3-0.1 and Cs-WO3-0.3(Figure 3d) exhibi two peaks at 738.4 eV and 724.7 eV, which are assigned to the spin orbits of Cs 3d3/2 a Cs 3d5/2, respectively [32,39].Notably, the positive shifts of 0.4 eV and 0.8 eV in W 4f a O 1s for Cs-WO3 can be seen after Cs + doping.This is due to the occurrence of ion exchan on the WO3 surface, which also confirmed that the Cs was successfully introduced i the lattice of WO3 and formed W-Cs bonds in the doped samples The positive sh improve the PEC performance of WO3.Furthermore, it can be seen that the p intensities of W 4f and O 1s for Cs-WO3 decreased with increasing the doped contents Cs element because of the replacement of W 6+ by Cs + leading to the contamination oxygen.Generally, the larger the ionic radius is, the more difficult it is for doping to oc due to the requirement of high formation energy.Therefore, the replacement of W 6+ Cs + is more favorable than replacing O 2-with Cs + .In situ electron paramagnetic resonance (EPR) measurements were carried out further prove the existence of surface oxygen vacancies and investigate their propert No signal was detected for the WO3-0; however, Cs-WO3 samples exhibited relativ stronger EPR peaks intensity at g ≈ 2.002 under the same conditions, as shown in Fig 5 .The higher the doped amounts of Cs, the stronger the signal intensity.These resu agreed with results reported previously, confirming the presence of surface oxyg In situ electron paramagnetic resonance (EPR) measurements were carried out to further prove the existence of surface oxygen vacancies and investigate their properties.No signal was detected for the WO 3 -0; however, Cs-WO 3 samples exhibited relatively stronger EPR peaks intensity at g ≈ 2.002 under the same conditions, as shown in Figure 5.The higher the doped amounts of Cs, the stronger the signal intensity.These results agreed with results reported previously, confirming the presence of surface oxygen vacancies on Cs-WO 3 samples [40,41].The oxygen vacancies may be beneficial to improve the photocatalytic performance of Cs-WO 3 .

The Optical Properties of Cs-WO3
Figure 6 shows the UV-visible DRS spectra in the range from 300 to 700 nm and plots of WO3-0 and Cs-WO3 samples, respectively.As shown in Figure 6A, WO3-0 can absorb light below 440 nm.However, a significant red shift in the absorption edge new shoulders appearing at 440-520 nm can be seen for Cs-WO3 samples.Absorp above 600 nm was observed for Cs-WO3 samples because of the formation of an oxy defect caused by doping [42], in contrast to negligible absorption for neat WO3, whi consist with the results of XPS and EPR.Furthermore, Figure 6B shows the Tauc p based on DRS data, in which two different slopes are observed for the Tauc plots o WO3 samples due to the appearance of the new shoulders.Therefore, these estim values of band energies for Cs-WO3-0.1 (2.38 eV) and Cs-WO3-0.3(2.47 eV) were obta from the slopes, which decreased by 0.43 eV and 0.34 eV, respectively, compared to eV of WO3-0.

The Optical Properties of Cs-WO 3
Figure 6 shows the UV-visible DRS spectra in the range from 300 to 700 nm and Tauc plots of WO 3 -0 and Cs-WO 3 samples, respectively.As shown in Figure 6A, WO 3 -0 can only absorb light below 440 nm.However, a significant red shift in the absorption edge with new shoulders appearing at 440-520 nm can be seen for Cs-WO 3 samples.Absorption above 600 nm was observed for Cs-WO 3 samples because of the formation of an oxygen defect caused by doping [42], in contrast to negligible absorption for neat WO 3 , which is consist with the results of XPS and EPR.Furthermore, Figure 6B shows the Tauc plots based on DRS data, in which two different slopes are observed for the Tauc plots of Cs-WO 3 samples due to the appearance of the new shoulders.Therefore, these estimated values of band energies for Cs-WO 3 -0.1 (2.38 eV) and Cs-WO 3 -0.3(2.47 eV) were obtained from the slopes, which decreased by 0.43 eV and 0.34 eV, respectively, compared to 2.81 eV of WO 3 -0.
vacancies on Cs-WO3 samples [40,41].The oxygen vacancies may be beneficial to impro the photocatalytic performance of Cs-WO3.

The Optical Properties of Cs-WO3
Figure 6 shows the UV-visible DRS spectra in the range from 300 to 700 nm and Ta plots of WO3-0 and Cs-WO3 samples, respectively.As shown in Figure 6A, WO3-0 can o absorb light below 440 nm.However, a significant red shift in the absorption edge w new shoulders appearing at 440-520 nm can be seen for Cs-WO3 samples.Absorpt above 600 nm was observed for Cs-WO3 samples because of the formation of an oxyg defect caused by doping [42], in contrast to negligible absorption for neat WO3, which consist with the results of XPS and EPR.Furthermore, Figure 6B shows the Tauc pl based on DRS data, in which two different slopes are observed for the Tauc plots of WO3 samples due to the appearance of the new shoulders.Therefore, these estima values of band energies for Cs-WO3-0.1 (2.38 eV) and Cs-WO3-0.3(2.47 eV) were obtain from the slopes, which decreased by 0.43 eV and 0.34 eV, respectively, compared to 2 eV of WO3-0.

Photocatalytic Activity
The photocatalytic O 2 evolution activities over WO 3 -0, Cs-WO 3 -0.1 and Cs-WO 3 -0.3 were carried out under visible-light irradiation in Fe 2 (SO 4 ) 3 solution; the results are shown in Figure 7.In Figure 7A, the WO 3 -0 generates only 9.76 µmol of O 2 evolution due to the high recombination rate of photogenerated carriers.It is noted that the activities for O 2 evolution are drastically improved after Cs doping.The Cs-WO 3 -0.1 shows a higher amount of O 2 evolution (58.28 µmol), which is an increase of about 6 times and 1.2 times (47.68 µmol) compared with WO 3 -0 and Cs-WO 3 -0.3.It can be explained by the formation of oxygen vacancies, leading to the acceleration of electron and hole transport rates caused by Cs doping.However, the photocatalytic activity decreased with further increases in doping concentration, demonstrating that a higher Cs doping content could also form the recombination centers for the photogenerated carriers caused by excessive dopants.As shown in Figure 7B, the stability test of photocatalytic O 2 evolution activity for Cs-WO 3 -0.1 was performed repeatedly for four cycles.The results suggest that the Cs-WO 3 -0.1 can be used as an efficient and stable visible-light excited photocatalyst for photocatalytic O 2 evolution.The small decrease in the O 2 production rate during the recycling reaction is mainly attributing to the small loss of catalysts during the photocatalytic process at a low dosage of the catalysts, and the powdered catalyst in aqueous solution can be dispersed easily to be taken away from the photocatalytic system during the real-time sampling procedure.

Photocatalytic Activity
The photocatalytic O2 evolution activities over WO3-0, Cs-WO3-0.1 and Cs-WO3-0.3were carried out under visible-light irradiation in Fe2(SO4)3 solution; the results are shown in Figure 7.In Figure 7A, the WO3-0 generates only 9.76 µmol of O2 evolution due to the high recombination rate of photogenerated carriers.It is noted that the activities for O2 evolution are drastically improved after Cs doping.The Cs-WO3-0.1 shows a higher amount of O2 evolution (58.28 µmol), which is an increase of about 6 times and 1.2 times (47.68 µmol) compared with WO3-0 and Cs-WO3-0.3.It can be explained by the formation of oxygen vacancies, leading to the acceleration of electron and hole transport rates caused by Cs doping.However, the photocatalytic activity decreased with further increases in doping concentration, demonstrating that a higher Cs doping content could also form the recombination centers for the photogenerated carriers caused by excessive dopants.As shown in Figure 7B, the stability test of photocatalytic O2 evolution activity for Cs-WO3-0.1 was performed repeatedly for four cycles.The results suggest that the Cs-WO3-0.1 can be used as an efficient and stable visible-light excited photocatalyst for photocatalytic O2 evolution.The small decrease in the O2 production rate during the recycling reaction is mainly attributing to the small loss of catalysts during the photocatalytic process at a low dosage of the catalysts, and the powdered catalyst in aqueous solution can be dispersed easily to be taken away from the photocatalytic system during the real-time sampling procedure.

Photoelectrocatalytic Properties
The linear sweep voltammograms (LSVs) for these electrodes were taken with chopped visible-light irradiation to investigate their photoelectrocatalytic performances, as shown in Figure 8.The photoanodic currents of these electrodes were observed above 0.1 V vs. Ag/AgCl based on water oxidation.For the WO3-0 electrode, the photocurrent of 0.06 mA cm −2 at 1.0 V was hard to observe; however, the PEC water oxidation performance of Cs-WO3 electrodes was significantly enhanced.The highest photocurrent of 2.12 mA cm −2 for Cs-WO3-0.1 was generated, which was about two times higher than that of Cs-WO3-0.3(1.1 mA cm −2 ).

Photoelectrocatalytic Properties
The linear sweep voltammograms (LSVs) for these electrodes were taken with chopped visible-light irradiation to investigate their photoelectrocatalytic performances, as shown in Figure 8.The photoanodic currents of these electrodes were observed above 0.1 V vs. Ag/AgCl based on water oxidation.For the WO 3 -0 electrode, the photocurrent of 0.06 mA cm −2 at 1.0 V was hard to observe; however, the PEC water oxidation performance of Cs-WO 3 electrodes was significantly enhanced.The highest photocurrent of 2.12 mA cm −2 for Cs-WO 3 -0.1 was generated, which was about two times higher than that of Cs-WO 3 -0.3(1.1 mA cm −2 ).Mott-Schottky plots (Figure 9A) from alternating-current impedance me were taken to reveal the relative positions of the valence band (VB) and cond (CB) in WO3-0 and Cs-WO3.As a result, the flat band (EFB) potentials (vs.A WO3-0, Cs-WO3-0.1 and Cs-WO3-0.3were 0.46 eV, 0.01 eV and 0.23 eV, corre 0.66, 0.21, and 0.43 eV (vs.NHE), respectively.The CB potential of the sem material was 0.1-0.3eV lower than that of the EFB (vs.NHE) [43], so the CB va 0, Cs-WO3-0.1 and Cs-WO3-0.3were calculated as 0.56, 0.11 and 0.33 eV respectively.The VB values of WO3-0, Cs-WO3-0.1 and Cs-WO3-0.3were estim eV, 2.49 eV, and 2.8 eV, respectively, according to the formula of Eg = Moreover, the donor carrier densities (ND [cm −3 ]) were provided from the x-in the slopes of the straight line [45] (Table 2).The ND values of the Cs-WO3 were those for WO3-0.In particular, the highest ND value for Cs-WO3-0.1 (3.46 × 1 calculated, which was 1.5 and 1.1 times higher than those of WO3-0 (2.26 × 1 Cs-WO3-0.3(3.13 × 10 19 cm −3 ).The negative shift in the EFB potential and the in ND are beneficial to enhance the photocatalytic activity and photoele performance for the OER.The Tafel plots are useful to investigate the reaction kinetics of the OER. Figure 9B, the Tafel slopes of the Cs-WO3 prominently decrease compared w Mott-Schottky plots (Figure 9A) from alternating-current impedance measurements were taken to reveal the relative positions of the valence band (VB) and conduction band (CB) in WO 3 -0 and Cs-WO 3 .As a result, the flat band (E FB ) potentials (vs.Ag/AgCl) of WO 3 -0, Cs-WO 3 -0.1 and Cs-WO 3 -0.3 were 0.46 eV, 0.01 eV and 0.23 eV, corresponding to 0.66, 0.21, and 0.43 eV (vs.NHE), respectively.The CB potential of the semiconductor material was 0.1-0.3eV lower than that of the E FB (vs.NHE) [43], so the CB values of WO 3 -0, Cs-WO 3 -0.1 and Cs-WO 3 -0.3 were calculated as 0.56, 0.11 and 0.33 eV (vs.NHE), respectively.The VB values of WO 3 -0, Cs-WO 3 -0.1 and Cs-WO 3 -0.3 were estimated as 3.37 eV, 2.49 eV, and 2.8 eV, respectively, according to the formula of E g = E VB -E CB [44].Moreover, the donor carrier densities (N D [cm −3 ]) were provided from the x-intercept and the slopes of the straight line [45] (Table 2).The N D values of the Cs-WO 3 were higher than those for WO 3 -0.In particular, the highest N D value for Cs-WO 3 -0.1 (3.46 × 10 19 cm −3 ) was calculated, which was 1.5 and 1.1 times higher than those of WO 3 -0 (2.26 × 10 19 cm −3 ) and Cs-WO 3 -0.3(3.13 × 10 19 cm −3 ).The negative shift in the E FB potential and the increase in the N D are beneficial to enhance the photocatalytic activity and photoelectrocatalytic performance for the OER.charges.This is mainly due to the n-type doping of WO3 by cesium doping, which injec electrons into the Fermi level, enhances the CB potential and also increases lattice defec and oxygen vacancies in WO3, thereby improving the conductivity of the WO3.The Taf and electrochemical impedance results provide favorable evidence for the improvemen of photocatalytic activity for the OER.The energy positions were investigated to elucidate Cs-doping effects on the ban energy of WO3. Figure 10 shows energy positions for WO3-0, Cs-WO3-0.1 and Cs-WO3-0.It has been well documented that the VB of Cs-WO3 consists of the hybridization betwee O2p and Cs 4s, and the CB is from W 5d electronic components.The enhanced optic absorption is illustrated for the contribution from the Cs 4s hybridization in VB.The Tafel plots are useful to investigate the reaction kinetics of the OER.As shown in Figure 9B, the Tafel slopes of the Cs-WO 3 prominently decrease compared with those of WO 3 -0.Cs-WO 3 -0.1 possesses a lower Tafel slope of 16.46 mVdec −1 than Cs-WO 3 -0.3(32.78 mVdec −1 ) and WO 3 -0 (48.54 mVdec −1 ), indicating that Cs doping gives a faster kinetic response in the OER and makes the Cs-WO 3 catalysts have higher photocatalytic activities for the OER.
The electrochemical impedance was utilized to give an insight into the kinetics of the charge transfer process and to evaluate its effect on photocatalytic O 2 evolution activity.As seen in the results of the Nyquist plots in Figure 9C, Cs-WO 3 -0.1 exhibited smaller semicircles than WO 3 -0 and Cs-WO 3 -0.3.As is well known, the diameter of the semicircle in the Nyquist plot corresponds to the impedance of the electrode, and the larger the radius, the larger the impedance [46].This result indicates that the Cs-WO 3 -0.1 has a lower charge transfer resistance and higher separation efficiency for photogenerated electron-hole pairs than other electrodes and inhibits the recombination of photogenerated charges.This is mainly due to the n-type doping of WO 3 by cesium doping, which injects electrons into the Fermi level, enhances the CB potential and also increases lattice defects and oxygen vacancies in WO 3 , thereby improving the conductivity of the WO 3 .The Tafel and electrochemical impedance results provide favorable evidence for the improvement of photocatalytic activity for the OER.
The energy positions were investigated to elucidate Cs-doping effects on the band energy of WO 3 .Figure 10 shows energy positions for WO 3 -0, Cs-WO 3 -0.1 and Cs-WO 3 -0.3.It has been well documented that the VB of Cs-WO 3 consists of the hybridization between O2p and Cs 4s, and the CB is from W 5d electronic components.The enhanced optical absorption is illustrated for the contribution from the Cs 4s hybridization in VB.
The energy positions were investigated to elucidate Cs-doping effects on the band energy of WO3. Figure 10 shows energy positions for WO3-0, Cs-WO3-0.1 and Cs-WO3-0.3.It has been well documented that the VB of Cs-WO3 consists of the hybridization between O2p and Cs 4s, and the CB is from W 5d electronic components.The enhanced optical absorption is illustrated for the contribution from the Cs 4s hybridization in VB.

Synthesis of WO 3 Powders
Typically, 1.0 g H 2 WO 4 (4.0 mmol) was dissolved into the H 2 O 2 (20 mL) under vigorous stirring at room temperature, forming the pale-yellow solution A. Cs 2 CO 3 (0.33 g) was dissolved in water to form the B solution.The B solution was added into the A solution dropwise to form the C solution.Then, the C solution was transferred to Teflon-lined stainless-steel autoclaves (reactor volume: 50 mL) at 120-150 • C in 12-36 h for hydrothermal reaction.The 0.1 mol% Cs-doped WO 3 (Cs-WO 3 -0.1)powder was obtained after centrifugation, washed repeatedly with ethanol, and air-dried.The Cs-WO 3 -0.3 and Cs-WO 3 -0.5 were prepared in the same manner by changing the Cs 2 CO 3 amounts of 1.0 g and 1.63 g, respectively.A pure WO 3 sample denoted as WO 3 -0 was prepared in the same manner without the addition of Cs 2 CO 3 .

Fabrication of Electrodes
In the typical procedure, 0.4 g of powder (Cs-WO 3 -0.1 and Cs-WO 3 -0.3)was mixed in the PEG (0.4 mL) with slow stirring until there were no bubbles; a smooth paste was formed.The resulting paste was squeezed over a clean FTO glass substrate by a doctorblade coater and dried under a 100 W infrared lamp.After repeating the procedure twice, the Cs-WO 3 -0.1 and Cs-WO 3 -0.3electrodes were prepared.The pure WO 3 electrode was fabricated by the same method using a precursor prepared without the addition of Cs 2 CO 3 .

Characterization of the Photocatalysts
The crystal structure of the samples was analyzed using an X-ray diffractometer (XRD-6000, Japan) and a scanning speed (4 • C/min).A UV-2700 UV-Vis spectrophotometer (Shimadzu, International Trade (Shanghai) Co., Ltd., Shanghai, China) was used to examine the absorption spectra of solid powder samples.Using a thermal field emission scanning electron microscope, the surface morphology of the samples was examined (SIGMA:500, Jena, Germany).The Energy Dispersive X-ray Spectroscopic (EDS) data were taken using electron probe microanalysis (JEOL JED-2300, Tokyo, Japan) operated at an accelerating voltage of 10 kV.Elemental and valence analyses of the samples were performed using an ES-CALAB Xi X-ray photoelectron spectrometer (manufactured by Thermo Fisher Scientific (China) Co., Ltd., Shanghai, China) and calibrated by the C 1 s peak appearing at 284.2 eV.

Photocatalytic Activity Measurement
Photocatalytic experiments were conducted in a quartz glass reactor ca.40 cm 3 , and 10 mg of catalyst was suspended in Fe 2 (SO 4 ) 3 •9H 2 O (2.1 mM, 30 mL) solution.Then, the system was degassed by bubbling Ar gas to remove oxygen.Under a 300 W xenon lamp, the photocatalytic process was carried out under continuous stirring to ensure the catalyst dispersed in the solution well.The evolution amount of oxygen was detected by gas chromatography with a TCD detector (Shimadzu GC-8A with a TCD, 5 A column, Ar as carrier).

Photoelectrocatalytic Property Measurement
The photoelectric properties of the samples were tested on a Chenhua CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) using a three-electrode system: the FTO photoelectrode as the working electrode (with an approximate working area of 1 cm 2 ), platinum wire as the counter electrode, and saturated AgCl electrode (Ag/AgCl) as the reference electrode.
For photovoltaic performance testing, a 300 W xenon lamp was used (Optical Module X; Ushio Inc., Tokyo, Japan) to simulate sunlight and its light intensity was adjusted to 100 mW cm −2 .The linear sweep voltammograms (LSVs) were measured at a scan rate of 5 mV s −1 .Light was irradiated from the back side of the working electrode using a 300 W xenon lamp with a UV-cut filter (λ ≥ 420 nm).Electrochemical impedance spectra were measured at an applied potential of 0.68 V vs. Ag/AgCl (1.23 V vs. RHE) in a frequency range of 10 mHz to 20 kHz (amplitude of 50 mV).Light irradiation (λ ≥ 420 nm) was conducted with the electrode with a 300 W xenon lamp (Ushio Inc., Tokyo, Japan, Optical ModuleX).

Conclusions
The Cs-doped WO 3 with spatial charge separation was synthesized using a hydrothermal approach, which exhibited a significant enhancement of the photocatalytic performance for water spitting to boost the production of oxygen.The addition of Cs dependence on the physiochemical properties and the performance of the photocatalytic OER of the WO 3 -0 and Cs-WO 3 catalysts were investigated to characterize Cs doped into the WO 3 lattice and reveal the mechanism of superior performance for the photocatalytic OER of Cs-WO 3 .The Cs doping is responsible for the significant red shift in the absorption edge, with a new shoulder appearing at 440-520 nm compared to that in WO 3 -0.The Cs-WO 3 catalyst is able to utilize visible light at longer wavelengths below 520 nm for photocatalytic OER, in contrast to utilization below 440 nm for the WO 3 -0 catalyst.These results demonstrate that Cs doping is an effective strategy for improving the photocatalytic performance of WO 3 photocatalysts for the OER, and thus it is expected to be applied for photocatalytic OERs as artificial photosynthesis to improve the solar energy conversion efficiency.

Figure 1 .
Figure 1.The XRD patterns of Cs-WO3-0.1 samples prepared at (A) different temperatures and (B) different reaction times.(C) The XRD patterns of the pure WO3 and Cs-WO3 samples prepared with different doping concentrations of Cs + ions.

Figure 1 .
Figure 1.The XRD patterns of Cs-WO 3 -0.1 samples prepared at (A) different temperatures and (B) different reaction times.(C) The XRD patterns of the pure WO 3 and Cs-WO 3 samples prepared with different doping concentrations of Cs + ions.

Figure 2 .
Figure 2. The SEM images of (a) WO 3 -0, (b) Cs-WO 3 -0.1,(c) Cs-WO 3 -0.3,(d) Cs-WO 3 -0.5.The elemental maps of the EDX for the Cs-WO 3 -0.1 are shown in Figure 3.The mapping signals of the W and O (Figure 3c,d) and of the Cs (Figure 3e) were detected.The results showed a uniform distribution of Cs, O and W, in good chemical agreement with Cs-WO 3 .

Table 2 .
Summary of optical and electrochemical properties and energies of band various WO3 samples.

Table 2 .
Summary of optical and electrochemical properties and energies of band structures of various WO 3 samples.