物 理 化 学 学 报
Acta Phys. -Chim. Sin. 2022, 38 (7), 2108028 (1 of 9)
Received: August 19, 2021; Revid: August 30, 2021; Accepted: September 4, 2021; Published online: September 9, 2021. *Correspondingauthors.Emails:******************(K.D.);****************(J.Z.).Tel.: +86-561-3803256. †The authors contributed equally to this work.
This work was supported by the National Natural Science Foundation of China (51572103 and 519730
78), the Distinguished Young Scholar of Anhui Province, China (1808085J14) and the Major projects of Education Department of Anhui Province, China (KJ2020ZD005).海外军事基地
国家自然科学基金(51572103, 51973078), 安徽省杰出青年基金(1808085J14)和安徽省教育厅重大项目(KJ2020ZD005)
© Editorial office of Acta Physico-Chimica Sinica
[Article]
doi: 10.3866/PKU.WHXB202108028
www.whxb.炝锅面
Construction of 1D/2D W 18O 49/Porous g-C 3N 4 S-Scheme Heterojunction with Enhanced Photocatalytic H 2 Evolution
Yue Huang 1,†, Feifei Mei 1,†, Jinfeng Zhang 1,*, Kai Dai 1,*, Graham Dawson 2
1 Anhui Province Key Laboratory of Pollutant Sensitive Materials and Environmental Remediation, Huaibei Normal University,
夜光鞋
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 H 2 production is a key challenge in the field of energy conversion. There are numerous oxygen vacancies on the surface of non-stoichiometric W 18O 49 (WO), which result in suitable light absorption performance, but the hydrogen evolution effect is not ideal becau 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 miconductor. 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 unud electrons and holes while leaving ud electrons and holes, which can accelerate the paration 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 relead. Compared to WO and PCN, 20%WO/PCN composite has a higher H 2 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 H 2 production.
Key Words: S-scheme; Photocatalytic H 2 production; W 18O 49; Porous carbon nitride; Heterojunction
一维/二维W18O49/多孔g-C3N4梯形异质结构建及其光催化析氢性能研究
黄悦1,†，梅飞飞1,†，张金锋1,*，代凯1,*，Graham Dawson 2
1淮北师范大学，污染物敏感材料与环境修复安徽省重点实验室，安徽淮北235000
2西交利物浦大学化学系，江苏苏州215123
摘要：提高光催化分解水制氢的效率是能量转换领域的关键挑战。本研究首先合成了二维多孔氮化碳(PCN)，然后在二维PCN上原位生长了一维W18O49 (WO)，形成了一种新型的梯形(S型)异质结。该异质结可以加快界面电荷的分离和转移，赋予WO/PCN体系更好的氧化还原能力。此外，具有多孔结构的PCN提供了更多的催化活性位点。与WO和PCN 相比，20% WO/PCN复合材料具有更高的H2产率(1700 µmol∙g−1∙h−1)，是PCN (30 µmol∙g−1∙h−1)的56倍。本研究提供了一种新S型光催化剂用于光催化制氢领域。
关键词：S型；光催化制氢；W18O49；多孔氮化碳；异质结
中图分类号：O643
1 Introduction
Nowadays, our main energy source is fossil fuel, and its persistent combustion has caud a worldwide energy crisis and environment pollution, so looking for a clean energy source is an urgent matter for humanity 1–6. Solar energy is an inexhaustible energy source, so using photocatalytic technology to convert solar energy into clean energy is a feasible strategy 7,8, and developing H2production is one of the most investigated strategies to solve energy problems 9–11. Since Fujishima reported on TiO2, it has been gradually studied by scientific rearchers as a stable photo
catalyst 12–16. However, the wide band gap of TiO2(3.20 eV) caus 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 energy 17–19. 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 miconductor 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 sunlight 20–22. Nevertheless, WO cannot be ud alone for photocatalytic H2evolution becau its conduction band (CB) potential is more positive than H+/H2 redox potential. Xiong’s rearch team ud Mo doping to refine the defect state in WO for improving its photocatalytic nitrogen fixation activity 23. Cheng’s group made the CB of WO more negative and enhanced its CO2 reduction ability by doping Cu+ in WO 24. It can be en that the photogenerated charge paration and transport efficiency of WO can be effectively improved by doping. In addition, constructing heterojunction with band gap-matched miconductor can also overcome the limitations and get an effective strategy for the evolution of H2 driven by sunlight. Lu et al. loaded WO quantum dots on CdS nanorods to effectively parate the photogenerated carriers on the bulk and surface, thereby e
nhancing its photocatalytic H2 production performance and stability 25. Since Prof. Wang first reported on graphite carbon nitride (CN) in 2009, CN has been widely studied in photocatalytic systems 26. As a reduction miconductor, CN has a narrow band gap (~2.7 eV) and a negative CB potential 27,28. Using oxidation miconductors to construct heterojunctions with CN can improve the redox capability of the entire system 3,29. At the same time, it promotes the paration and transportation of photoexcited charges. For example, Bi2O3 QDs/CN 30 and BiOBr/CN 31 are prepared. However, how to construct and effectively u reduction photocatalysts and oxidation photocatalysts is still a challenge.
Step-scheme (S-scheme) photocatalysts were propod by Prof. Yu, it can solve the problem of low redox potential of photocatalytic system 32–34. S-scheme heterojunction generally contains two miconductors 35–37. One is an oxidation miconductor and the other is a reduction miconductor. The Fermi level of reduction miconductor is higher than that of oxidation miconductor. When two n-type miconductors 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 miconductor is positively charged, and the surface of the oxidation miconductor is negatively charged. The energy band of a miconductor with a high Fermi level is bent upward due to the formation of an electron-loss layer, and the energy band of a miconductor 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 miconductor with low Fermi level recombine with the holes on the VB of the miconductor 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 parated spatially, and effectively improves the paration of carriers and the transmission efficiency 38,39.
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 increas 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/P
CN S-scheme heterojunction effectively improves the paration and transport efficiency of charges, thereby the photocatalytic H2 production activity is enhanced. Our work will provide a feasible strategy for practical application of WO in the field of photocatalytic H2 production.
2 Experimental
刘方>陆间海2.1 Materials
Urea (AR 99%, CH4N2O), thiourea (AR 99%, CH4N2S), tungsten hexachloride (AR 99%, WCl6) and ethanol absolute (AR 99%) were purchad 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.能字成语
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 WCl6 is dissolved in 12 mL of ethanol absolute under stirring. After stirring for 30 min, WCl6 solution 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 ud to analyze the composition and crystal orientation of the samples. UV-Vis diffu 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−1 Na2SO4.
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 ud 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 × 10−6 eV∙atom−1. The maximum force is 0.01 eV∙Å−1(1 Å = 0.1 nm), the maximum stress and maximum displacement are 0.02 GPa and 5.0 × 10−4 Å, respectively.
2.6 Measurement of activity
Disper 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 aling, exclude air with N2 and irradiate with 300 W Xenon lamp (λ > 420 nm). Under light irradiation, 1 mL of mixed gas was sampl
ed 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 disperd in 20 mL ethanol solution for 30 min. WCl6 was disperd in 12 mL of ethanol and added dropwi to
Fig. 1 Schematic diagram of the formation process of WO/PCN
composite materials.
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. 3.2 Pha 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 en in WO/PCN, and as the ratio of WO increas in the composite, the peak intensity also increas. The prence 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 en 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
浮桥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 v
alue 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 clo contact between WO and PCN. 3.3 XPS and elemental analys
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 1s and N 1s elements, without other impurities. The N 1s high-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 1s high-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 1s spectrum (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 eV 47. The W 4f spectrum (Fig. 4e) has four characteristic peaks. Peaks at 37.4 and 35.3 eV are characteristic peaks of W 6+, and the two peaks at 36.9 and 33.8 eV are characteristic peaks of W 5+ 48.
3.4 Specific surface area analysis
To a certain extent, the BET surface area values (S BET ) of the catalyst has a certain influence on the photocatalytic activity. It has been deeply explored. Fig. 5a displays N 2 adsorption-
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. 2 XRD patterns of WO, WO/PCN and PCN.
直挂云帆济沧海的含义
10
15
20
25
30
35
4045
5055606570
(002)
(010)
JCPDS No. 87-1526
I n t e n s i t y (a . u .)
PCN
40%WO/PCN
WO
2 /(°)
10%WO/PCN
20%WO/PCN 30%WO/PCN JCPDS No. 71-2450
desorption 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 is S BET . The S BET values of WO, 20%WO/PCN and PCN are 93.6, 65.4 and 42.8 m 2∙g −1, respectively. After adding WO, S BET of 20%WO/PCN is much larger than that of PCN, which will i
mprove the photocatalytic performance. The S BET of the catalyst is an index to measure the performance of the catalyst. To a certain extent, the larger the S BET , the higher the reaction activity. However, the S BET is large due to the small pore diameter of the catalyst, which leads to an increa in internal diffusion resistance. The diffusion rate of the carrier decreas, 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 en at the edge of the absorption band. The band gaps of PCN and WO can be obtained by the following formula 49–51: (αhν)1/n = A (hν − E g ) (1) where h is Planck's constant, α is absorption coefficient, n is directly related to miconductor type. For direct-gap miconductor and indirect-gap miconductor, the value of n is 1/2 and 2, respectively. WO and PCN are direct-gap miconductors, where n is 1/2. It is calculated that the energy band gap (E g ) of PCN is 2.85 eV , and the E g of WO is 2.66 eV (Fig. 6b). The CB position (E CB ) and VB position (E VB ) of WO and PCN are bad on the following formula 52: E VB = X − E e + 0.5E g (
2) E CB = E VB − E g (3) Where: X is the electronegativity of the miconductor. E e is the energy of free electrons in hydrogen scale and its value is 4.5 eV . The electronegativity of WO is 6.49 eV , and the E CB of WO is 0.66 eV and the E VB is 3.32 eV by calculation. The electronegativity of PCN is 4.64 eV . The E CB of PCN is −1.29
535534533532531530529528
530.2 eV Binding energy (eV)
I n t e n s i t y (a . u .)
O 1s
531.6 eV
38363432
33.8 eV
36.9 eV
35.3 eV
I n t e n s i t y (a . u .)
Binding energy (eV)
W 4f
37.4 eV
290
288
286
284
282
284.9 eV
I n t e n s i t y (a . u .)
Binding energy (eV)
C 1s
288.5 eV 402400398396399.1 eV
I n t e n s i t y (a . u .)
Binding energy (eV)N 1s
401.2 eV
120010008006004002000I n t e n s i t y (a . u .)
Binding energy (eV)W 4f
C 1s
N 1s
O 1s
WO/PCN
e
b
a
c
d
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) N 2 adsorption-desorption isotherms curves and (b) S BET of WO, 20%WO/PCN and PCN.
S B E T (m 2⋅g -1)
V o l u m e a d s o r b e d (S T P )(c m 3⋅g -1)
0a
