Construction of 1D/2D W18O49/Porous g-C3N4 S-Scheme Heterojunction with Enhanced Photocatalytic H2 Evolution

Yue Huang , Feifei Mei , Jinfeng Zhang , Kai Dai , Graham Dawson

1 Anhui Province Key Laboratory of Pollutant Sensitive Materials and Environmental Remediation, Huaibei Normal University,Huaibei 235000, Anhui Province, China.

2 Department of Chemistry, Xi’an Jiaotong Liverpool University, Suzhou 215123, Jiangsu Province, China.

Abstract: Photocatalytic hydrogen production is an effective strategy for addressing energy shortage and converting solar energy into chemical energy. Exploring effective strategies to improve photocatalytic H2 production is a key challenge in the field of energy conversion. There are numerous oxygen vacancies on the surface of non-stoichiometric W18O49 (WO),which result in suitable light absorption performance, but the hydrogen evolution effect is not ideal because the band potential does not reach the hydrogen evolution potential. A suitable heterojunction is constructed to optimize defects such as high carrier recombination rate and low photocatalytic performance in a semiconductor. Herein, 2D porous carbon nitride (PCN) is synthesized, followed by the in situ growth of 1D WO on the PCN to realize a step-scheme (S-scheme)heterojunction. When WO and PCN are composited, the difference between the Fermi levels of WO and PCN leads to electron migration, which balances the Fermi levels of WO and PCN. Electron transfer leads to the formation of an interfacial electric field and bends the energy bands of WO and PCN, thereby resulting in the recombination of unused electrons and holes while leaving used electrons and holes, which can accelerate the separation and charge transfer at the interface and endow the WO/PCN system with better redox capabilities. In addition, PCN with a porous structure provides more catalytic active sites. The photocatalytic performance of the sample can be investigated using the amount of hydrogen released. Compared to WO and PCN, 20%WO/PCN composite has a higher H2 production rate (1700 μmol?g-1?h-1), which is 56 times greater than that of PCN (30 μmol?g-1?h-1). This study shows the possibility of the application of S-scheme heterojunction in the field of photocatalytic H2 production.

Key Words: S-scheme; Photocatalytic H2 production; W18O49; Porous carbon nitride; Heterojunction

1 Introduction

Nowadays, our main energy source is fossil fuel, and its persistent combustion has caused a worldwide energy crisis and environment pollution, so looking for a clean energy source is an urgent matter for humanity1-6. Solar energy is an inexhaustible energy source, so using photocatalytic technology to convert solar energy into clean energy is a feasible strategy7,8, and developing H2production is one of the most investigated strategies to solve energy problems9-11. Since Fujishima reported on TiO2, it has been gradually studied by scientific researchers as a stable photocatalyst12-16. However, the wide band gap of TiO2(3.20 eV) causes it to be excited only by ultraviolet light, and ultraviolet light only accounts for 5%of the spectrum, which greatly reduces the utilization of solar energy17-19. In addition, it is known to all that other challenges photocatalysts encounter are low light absorption and photocatalytic efficiency. There is an urgent need to develop semiconductor photocatalysts with narrow band gaps.

Blue W18O49(WO) is a non-stoichiometric ratio of tungsten oxide with a bandgap of 2.66 eV. Due to the large number of oxygen vacancies on its surface, it exhibits strong light absorption under sunlight20-22. Nevertheless, WO cannot be used alone for photocatalytic H2evolution because its conduction band (CB) potential is more positive than H+/H2redox potential. Xiong’s research team used Mo doping to refine the defect state in WO for improving its photocatalytic nitrogen fixation activity23. Cheng’s group made the CB of WO more negative and enhanced its CO2reduction ability by doping Cu+in WO24. It can be seen that the photogenerated charge separation and transport efficiency of WO can be effectively improved by doping. In addition, constructing heterojunction with band gap-matched semiconductor can also overcome these limitations and get an effective strategy for the evolution of H2driven by sunlight. Luet al.loaded WO quantum dots on CdS nanorods to effectively separate the photogenerated carriers on the bulk and surface, thereby enhancing its photocatalytic H2production performance and stability25. Since Prof. Wang first reported on graphite carbon nitride (CN) in 2009, CN has been widely studied in photocatalytic systems26. As a reduction semiconductor, CN has a narrow band gap (～2.7 eV) and a negative CB potential27,28. Using oxidation semiconductors to construct heterojunctions with CN can improve the redox capability of the entire system3,29. At the same time, it promotes the separation and transportation of photoexcited charges. For example, Bi2O3QDs/CN30and BiOBr/CN31are prepared. However, how to construct and effectively use reduction photocatalysts and oxidation photocatalysts is still a challenge.

Step-scheme (S-scheme) photocatalysts were proposed by Prof. Yu, it can solve the problem of low redox potential of photocatalytic system32-34. S-scheme heterojunction generally contains two semiconductors35-37. One is an oxidation semiconductor and the other is a reduction semiconductor. The Fermi level of reduction semiconductor is higher than that of oxidation semiconductor. When two n-type semiconductors are combined, the electrons will move spontaneously due to the difference in the position of the Fermi level, and the electrons on the photocatalyst with higher Fermi level will flow to the photocatalyst with lower Fermi level until the Fermi level is balanced. Due to the transfer of electrons, an interfacial electric field is generated, the surface of reduction semiconductor is positively charged, and the surface of the oxidation semiconductor is negatively charged. The energy band of a semiconductor with a high Fermi level is bent upward due to the formation of an electron-loss layer, and the energy band of a semiconductor with a low Fermi level is bent downward due to the formation of an electron-rich layer. After being excited by light, electron-hole pairs are generated, the electrons on the CB of the semiconductor with low Fermi level recombine with the holes on the VB of the semiconductor with high Fermi level under the promotion of the built-in electric field. Finally, holes and electrons with strong oxidation and reduction capabilities are left in the system for photocatalytic reactions. The special structure of the S-scheme heterojunction allows the photogenerated electron-hole pairs to be separated spatially, and effectively improves the separation of carriers and the transmission efficiency38,39.

Herein, 2D porous CN (PCN) is first prepared by thermal condensation polymerization, and then 1D WO was successfully grown on PCN by a solvothermal method to construct 1D/2D WO/PCN S-scheme heterojunction, which effectively increases the absorption of the photon energy in the full optical spectrum.Simultaneously, the active charge carriers are generated at the appropriate energy level to participate in the evolution of H240-44.Compared with pure PCN, the construction of WO/PCN Sscheme heterojunction effectively improves the separation and transport efficiency of charges, thereby the photocatalytic H2production activity is enhanced. Our work will provide a feasible strategy for practical application of WO in the field of photocatalytic H2production.

2 Experimental

2.1 Materials

Urea (AR 99%, CH4N2O), thiourea (AR 99%, CH4N2S),tungsten hexachloride (AR 99%, WCl6) and ethanol absolute(AR 99%) were purchased from Sinopharm Chemical Reagent Co. Ltd (China). The purity of all experimental reagents is analytical grade purity.

2.2 Fabrication of PCN

The ratio of 3 : 1 CH4N2O and CH4N2S is fully ground for 30 min, then transferred to a 30 mL crucible, and heated to 550 °C for 120 min in muffle furnace. Finally, a light yellow PCN is obtained after grinding.

2.3 Preparation of WO/PCN composites

Place the obtained PCN in 20 mL of absolute ethanol and peel off with an ultrasonic probe for 30 min. The right amount of WCl6is dissolved in 12 mL of ethanol absolute under stirring.After stirring for 30 min, WCl6solution was added to the PCN suspension drop by drop. After further stirred for 3 h, the suspension was placed in an autoclave and heated to 180 °C for 24 h. Finally, the WO/PCN composites were obtained after washing and drying. The synthesis of WO is similar to above method, except that there is no PCN added.

2.4 Characterization

Microscopic imaging of the samples surface is characterized by scanning electron microscope (SEM, HITACHI Regulus 8220, Japan) and transmission electron microscope (TEM, JEOL JEM-2010, Japan). The Brunauer-Emmett-Teller (BET) specific surface area values were recorded by Micromeritics ASAP 2060(USA). XRD (Panalytical Empyrean diffractometer,Netherlands) was used to analyze the composition and crystal orientation of the samples. UV-Vis diffuse reflectance spectroscopy (DRS, PerkinElmer Lambda 950, USA) can measure the band gap and absorption band edge of the samples.X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA) determines the chemical composition of the sample.Photoluminescence spectra (PL, FLS920, U.K.) were utilized to test optical performance of the catalysts. The electrochemical properties were tested on Chenhua CHI-660D system (China)with three-electrodes. The electrolyte solution is 1.0 mol·L-1Na2SO4.

2.5 Computational detail

The CASTEP module of Materials Studio software can realize first principles density functional theory (DFT) calculation. In generalized gradient approximation, the exchange-correlation function in the form of Perdew-Burke-Ernzerhof is used to calculate the PCN (001) and WO (010) surfaces. The cut-off energy is 320 eV. A 3 × 3 × 1 Monkhorst-Pack grids is considered for geometric optimization of PCN. A 1 × 1 × 1 Monkhorst-Pack grids is utilized for geometric optimization of WO. The total energy of convergence criterion for geometric optimization is 5.0 × 10-6eV·atom-1. The maximum force is 0.01 eV·?-1(1 ? = 0.1 nm), the maximum stress and maximum displacement are 0.02 GPa and 5.0 × 10-4?, respectively.

2.6 Measurement of activity

Disperse photocatalyst (20 mg) in triethanolamine (TEOA)aqueous solution (50 mL, 10 mg·L-1), then add 30 μL of chloroplatinic acid and sonicate for 30 min. After sealing,exclude air with N2and irradiate with 300 W Xenon lamp (λ＞420 nm). Under light irradiation, 1 mL of mixed gas was sampled every 60 min and measured by a gas chromatograph(GC-7900).

3 Results and discussion

3.1 Synthetic route

The WO/PCN composites were obtained by solvothermal method. The specific preparation is shown in Fig. 1. In step 1,the PCN was calcined in a muffle furnace. CH4N2O and CH4N2S with a mass ratio of 3 : 1 are thoroughly mixed and ground for 30 min, and then the mixture was put into a crucible in a muffle furnace and heated to 550 °C for 2 h. In step 2, PCN was ultrasonically dispersed in 20 mL ethanol solution for 30 min.WCl6was dispersed in 12 mL of ethanol and added dropwise to the PCN suspension. After fully stirring, the suspension was heated at 180 °C for 24 h in an autoclave. Finally, after washing and drying, WO/PCN composite materials are obtained.

Fig. 1 Schematic diagram of the formation process of WO/PCN composite materials.

Fig. 2 XRD patterns of WO, WO/PCN and PCN.

3.2 Phase and microscopic morphology analysis

The crystal structure of WO, PCN and WO/PCN can be measured by XRD. The XRD peaks of monoclinic WO (JCPDS No. 71-2450) are all well directed, the typical 23.2° diffraction peak corresponds to the (010) plane of WO (Fig. 2). PCN has a peak around 27°, which corresponds to (002) crystal facet of CN(JCPDS No. 87-1526). The characteristic peaks of WO and PCN can be obviously seen in WO/PCN, and as the ratio of WO increases in the composite, the peak intensity also increases. The presence of sharp and obvious characteristic peaks indicates that WO and PCN are combined. In WO/PCN, there are no other characteristic peaks, indicating that no other impurities are mixed.

The microscopic morphology of the samples can be seen with TEM and SEM. In Fig. 3a, the PCN is a sheet-like structure with holes. WO is a nanorod-like structure (Fig. 3b)45,46. It can be found that WO is attached to the surface of PCN (Fig. 3c),indicating that WO and PCN are composite rather than mechanically mixed. The HRTEM image in Fig. 3d shows the structure of WO/PCN. Two different lattice fringes can be investigated from HRTEM image. The lattice spacing value of WO is 0.323 nm, which is (203) crystal plane. The lattice spacing value of PCN is 0.337 nm, which points to the (002) crystal facet.From Fig. 3e and f taken by the SEM, WO is a rod-like structure,which further shows that WO is deposited on the sheet-like PCN.This shows that there is a close contact between WO and PCN.

3.3 XPS and elemental analyses

The chemical state of the element is characterized by XPS.Fig. 4a is the XPS full spectrum of 20%WO/PCN, which shows that there are C 1s, W 4f, O 1sand N 1selements, without other impurities. The N 1shigh-resolution XPS of 20%WO/PCN (Fig.4b) contains two main peaks at 399.1 and 401.2 eV,corresponding to the C＝N―C and bridging nitrogen atoms(H―N―(C)2), respectively. In Fig. 4c, the O 1shigh-resolution XPS shows two peaks at 530.2 and 531.6 eV. The peak at 530.2 eV may be generated by lattice O atoms, and the peak at 531.6 eV belongs to the absorbed water molecules. The C 1sspectrum(Fig. 4d) has two characteristic peaks at 288.5 and 284.9 eV,respectively. The peak at 288.5 eV can be attributed to the N―C＝N bonding carbon in PCN and the peak of C―C bond at 284.9 eV47. The W 4fspectrum (Fig. 4e) has four characteristic peaks. Peaks at 37.4 and 35.3 eV are characteristic peaks of W6+, and the two peaks at 36.9 and 33.8 eV are characteristic peaks of W5+48.

3.4 Specific surface area analysis

Fig. 3 TEM images of (a) PCN, (b) WO and (c) WO/PCN, (d) HRTEM image, SEM images of (e) WO and (f) WO/PCN.

Fig. 4 (a) XPS full spectrum of 20%WO/PCN, 20%WO/PCN high-resolution XPS of (b) N 1s, (c) O 1s, (d) C 1s and (e) W 4f.

Fig. 5 (a) N2 adsorption-desorption isotherms curves and (b) SBET of WO, 20%WO/PCN and PCN.

To a certain extent, the BET surface area values (SBET) of the catalyst has a certain influence on the photocatalytic activity. It has been deeply explored. Fig. 5a displays N2adsorptiondesorption curves of WO, PCN and 20% WO/PCN. According to Brunauer-Deming-Deming-Teller classification, the photocatalyst isotherm is type-IV and the hysteresis loop is type-H3. Fig. 5b isSBET. TheSBETvalues of WO, 20%WO/PCN and PCN are 93.6, 65.4 and 42.8 m2·g-1, respectively. After adding WO,SBETof 20%WO/PCN is much larger than that of PCN,which will improve the photocatalytic performance. TheSBETof the catalyst is an index to measure the performance of the catalyst. To a certain extent, the larger theSBET, the higher the reaction activity. However, theSBETis large due to the small pore diameter of the catalyst, which leads to an increase in internal diffusion resistance. The diffusion rate of the carrier decreases,and the contact time with the active site of the catalyst is reduced,thus the catalytic activity is poorer.

3.5 Optical property analysis

The optical properties of catalysts were explored by UV-Vis DRS. As shown in Fig. 6a, the intrinsic absorption band edge of PCN is 463 nm. In contrast, 1D WO has a larger absorption range. The intrinsic absorption band edge of WO is about 508 nm. When WO is composited with PCN, a significant red shift can be seen at the edge of the absorption band. The band gaps of PCN and WO can be obtained by the following formula49-51:

(αhv)1/n=A(hv-Eg) (1)

wherehis Planck's constant,αis absorption coefficient,nis directly related to semiconductor type. For direct-gap semiconductor and indirect-gap semiconductor, the value ofnis 1/2 and 2, respectively. WO and PCN are direct-gap semiconductors, wherenis 1/2. It is calculated that the energy band gap (Eg) of PCN is 2.85 eV, and theEgof WO is 2.66 eV(Fig. 6b). The CB position (ECB) and VB position (EVB) of WO and PCN are based on the following formula52:

EVB=X-Ee+ 0.5Eg(2)

ECB=EVB-Eg(3)

Where:Xis the electronegativity of the semiconductor.Eeis the energy of free electrons in hydrogen scale and its value is 4.5 eV. The electronegativity of WO is 6.49 eV, and theECBof WO is 0.66 eV and theEVBis 3.32 eV by calculation. The electronegativity of PCN is 4.64 eV. TheECBof PCN is -1.29 eV, while theEVBis 1.56 eV.

Fig. 6 (a) UV-Vis DRS spectra of all samples; (b) the relationship between WO and PCN (ahv)2 and energy (hv).

Fig. 7 PL spectra of WO, 20%WO/PCN and PCN.

The PL spectra of 20%WO/PCN and PCN at excitation wavelengths of 270 nm are shown in the Fig. 7. The PL spectrum of the PCN nanosheets at room temperature shows a clear em ission peak at about 449 nm. Different semiconductors exhibit different PL responses, the peak intensity of WO is very small,but this does not mean that the charge separation rate of WO is better53,54. When WO nanorods are attached to the surface of PCN, it can be found that the intensity of the emission peak is significantly lower than that of pure PCN. Due to the rapid recombination of photoexcited carriers in PCN, their surface catalytic activity is relatively low. After the recombination of WO and PCN, the charge transfer at the interface inhibits the recombination of photogenerated carriers, resulting in a decrease in peak intensity.

Fig. 8 (a) Compare the H2 production rates of all samples; (b) cycling experiments of WO/PCN and PCN.

3.6 Photocatalytic H2 evolution performance

From the performance tests, we can find that the H2production rate of PCN and 20%WO/PCN are 30 and 1700 μmol·g-1·h-1respectively (Fig. 8a). It can be seen that WO exhibit very low H2evolution performance, and H2production performance of 20%WO/PCN is 56 times higher than that of PCN. In 10%WO/PCN, because the content of WO is relatively low, the area of S-scheme heterojunctions is relatively small,leading to poor photocatalytic performance. And in 30%WO/PCN, a large amount of WO will cover the photocatalytic active sites, resulting in poor photocatalytic performance. The H2productivity of WO and PCN mechanical mixing is 177 μmol·g-1·h-1. The performance of mechanical mixing of WO and PCN is far worse than that of 20%WO/PCN.Thein situgrowth of WO on PCN to form S-scheme heterojunction, which not only increases visible light response,but also inhibits light corrosion to improve the carrier transfer efficiency. The formation of heterojunction in WO/PCN system promotes the separation and transportation of photogenerated charges, thereby enhancing the photocatalytic performance of H2production. After 4 cycles (Fig. 8b), 20%WO/PCN can maintain good photocatalytic activity, showing that 20%WO/PCN composites have better stability after forming an S-scheme heterojunction.

3.7 Electrochemical analysis

Fig. 9 (a) Transient photocurrent responses and (b) EIS of PCN, WO and 20%WO/PCN.

Fig. 10 Calculated Fermi levels of (a) PCN and (b) WO.

The electrochemical properties of synthesized catalysts are exhibited in Fig. 9. Transient photocurrent and electrochemical impedance spectra (EIS) are used to characterize the charge separation and transfer efficiency of PCN, WO and 20%WO/PCN. All samples show fast and stable photocurrent response, but compared to PCN and WO, 20%WO/PCN composites showed the highest photocurrent density (Fig. 9a).This indicates that 20%WO/PCN has higher light trapping and low photo-generated charge recombination efficiency. EIS is an effective means to characterize electron transfer efficiency (Fig.9b). 20%WO/PCN exhibits the smallest arc, WO exhibits the largest arc radius, which means that 20%WO/PCN has the highest electron mobility, and WO has a higher carrier recombination rate and poor conductivity, which is not conducive to charge transfer. The charge separation and increase in transport efficiency seen in 20%WO/PCN are ascribed to the formation of S-scheme heterojunction, which separates photogenerated hole and electron pairs spatially, thereby improving the H2production performance.

3.8 DFT calculation

The charge transfer between PCN and WO can be studied based on DFT calculations55. Fig. 10 shows the optimized structure of PCN on (001) and WO on (010) surface. In order to further study the charge transfer, the work functions of PCN and WO on (001) and (010) surfaces were calculated by optimizing the structure. The calculated work function of the PCN (001)surface is 4.32 eV (Fig. 10a), and the calculated work function of the WO (010) surface is 4.65 eV (Fig. 10b). It shows that the Fermi level of PCN is higher than that of WO. When WO and PCN form close contact, electrons will migrate from PCN to WO until the Fermi level is at the same level. A built-in electric field from PCN to WO is formed at the contact interface of WO and PCN.

3.9 Photocatalytic mechanism

Fig. 11 The possible S-scheme mechanism of WO/PCN.

According to the experimental results and the above analysis,the possible S-scheme mechanism of the H2production process is proposed from the perspective of energy band theory and is shown in Fig. 1156-60. As shown in Fig. 11a, WO and PCN are oxidation semiconductor and reduction semiconductor,respectively. PCN has a higher Fermi level than WO. When WO and PCN are in close contact (Fig. 11b), electron migration occurs and it brings the Fermi level to an equilibrium state due to the different Fermi levels of WO and PCN61. The transfer of charge will form a built-in electric field at contact interface of WO and PCN. Due to the migration of particles, the energy of PCN becomes higher, the energy band of PCN is bent upward,the energy of WO becomes lower, and the energy band of WO is bent downward. Under the action of band bending and Coulombic force, the useless electrons on CB of WO and the useless holes on VB of PCN recombine each other under light(Fig. 11c). Then leaving the useful holes on VB of WO and the useful electrons on CB of PCN. The electrons left on CB of PCN react with H+in H2O to produce H2, and the holes on the VB of WO are consumed by TEOA. The S-scheme provides a possibility for enhancing the redox capability in the photocatalytic system62-66.

4 Conclusions

In summary, the 1D/2D WO/PCN S-scheme heterojunction was designed and synthesized by hydrothermal preparation,which exhibited excellent performance and stability in photocatalytic H2evolution. The enhanced photocatalytic H2production performance can be ascribed to three factors: (1) the novel 1D/2D structure will offer massive active sites for its reaction with water; (2) WO/PCN heterojunction exhibit improved visible light harvesting by virtue of special structure of WO; (3) S-scheme mechanism endows WO/PCN system with strong redox capability. This work will provide a feasible solution for photocatalytic H2evolution.