High-efficient separation of photoinduced carriers on double Z-scheme heterojunction for superior photocatalytic CO2 reduction

Journal Pre-proofs High-efficient separation of photoinduced carriers on double Z-scheme heterojunction for superior photocatalytic CO2 reduction Linyu Zhu, Hong Li, Quanlong Xu, Dehua Xiong, Pengfei Xia PII: DOI: Reference:

S0021-9797(19)31555-3 https://doi.org/10.1016/j.jcis.2019.12.088 YJCIS 25828

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

5 September 2019 15 December 2019 19 December 2019

Please cite this article as: L. Zhu, H. Li, Q. Xu, D. Xiong, P. Xia, High-efficient separation of photoinduced carriers on double Z-scheme heterojunction for superior photocatalytic CO2 reduction, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.088

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High-efficient separation of photoinduced carriers on double Z-scheme heterojunction for superior photocatalytic CO2 reduction

Linyu Zhua, Hong Lia,*, Quanlong Xub, Dehua Xionga and Pengfei Xiac,d*

a State

Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, 122 Luoshi Road, Wuhan, 430070, P. R. China. bKey

laboratory of Carbon Materials of Zhejiang Province, College of Chemistry and

Materials Engineering, Wenzhou University, Wenzhou 325027, P.R. China cHefei

National Laboratory for Physical Sciences at the Microscale, CAS Center for E

xcellence in Nanoscience, iCHEM, University of Science and Technology of China, H efei , Anhui 230026, P.R. China. dNational

Synchrotron Radiation Laboratory, University of Science and Technology

of China, Hefei , Anhui 230026, P.R. China.

* Corresponding authors. E-mail addresses: [email protected] (H. Li); [email protected] (P. Xia).

Abstract: Developing heterojunction is one of the promising approaches to acquire desired photocatalysts with high-efficient photocatalytic activity. In this work, sheetlike ternary ZnO/ZnWO4/g-C3N4 composite was synthesized via stepwise calcination treatment. The double interface electric fields built in the ZnO/ZnWO4/g-C3N4 heterojunction can promote efficient separation of photogenerated charge carriers in space. Moreover, in contrast with the individual ZnO, g-C3N4, ZnWO4 and their binary composites, this double Z-scheme heterojunction achieves more light harvesting, larger pore volume, stronger photoreduction capacity and CO2 adsorption capacity. Therefore, the sheet-like ZnO/ZnWO4/g-C3N4 heterojunction exhibits efficient conversion of the CO2 molecules into solar fuels under the light irradiation. The production yield of photocatalytic CO2 reduction over the double Z-scheme heterojunction is 13.19 μmol h-1g-1 and the conversion rate of hydrocarbon fuel is highly up to 91.5%, which are much higher than that of other samples. This work offers a novel perspective to achieve high-efficiency heterojunction system for photoredox applications such as photocatalytic antibacterial, nitrogen fixation and degradation of pollutions. Keywords: photocatalysis, sheet-like heterojunction, double Z-scheme, CO2 reduction

1. Introduction

Photocatalysis has emerged as one of the promising means to alleviate the global energy shortage and greenhouse effect in modern industrial society [1,2]. Appropriate semiconductor photocatalysts plays a decisive role for a given photocatalytic reaction, especially for converting the greenhouse gas CO2 into solar fuels [3,4]. Owing to the intriguing properties, including non-toxicity, easy availability and excellent physicochemical stability, monoclinic ZnWO4 has been considered as a promising photocatalyst [5-7]. Up to now, the research works over ZnWO4 have been extensively focused on the photocatalytic generation of hydrogen and the mineralization of organic contaminants [6,8]. However, ZnWO4 suffers from large band gap (3.3 eV) and rapid recombination of

photogenerated

charge

carriers,

which

causes

low

photocatalytic

performance.9 To tackle these issues, many strategies have been carried out such as element or anion doping, deposition of noble metals, morphology control and construction of heterojunction [5, 10-12]. Particularly, constructing heterojunction is accepted to be a promising approach to facilitate the charge carrier separation and retard the recombination, as well as broaden the response range of solar spectrum. When the formed heterojunction is exposed to the light, the photogenerated electrons will migrate from one semiconductor to the other across the interfacial, while the photogenerated holes exhibit the inverse transfer route, thus achieving effective spatial separation [13]. Nowadays, many binary Zn-based heterojunction, such as ZnWO4/BiOI, ZnTiO3/TiO2 and CuO/ZnO etc, have been constructed, which

display improved photocatalytic performance [7,10,13]. Although constructing binary heterojunctions can improve the photocatalytic activity to a certain extent, these composite photocatalysts still encounter limited light response range, undesirable charge separation efficiency and low photocatalytic activities. Thanks to the further optimization of structures, ternary composites exhibit improved photocatalytic performance [9,14,15]. Ternary heterojunction not only has high light harvesting and utilization efficiency, but also possesses improved charge separation efficiency driven by built-in electric fields at the double interfaces. Graphitic carbon nitride (g-C3N4), a layered polymeric semiconductor photocatalyst, has emerged as a fascinating photocatalyst owing to its attractive characteristics such as nontoxicity, abundance, high chemical stability and unique electric property [16,17]. With the 2D layered structure and narrow bandgap energy (2.70 eV), g-C3N4 photocatalyst exhibits favorable charge migration and available visible-light absorption [18]. Moreover, previous reports indicate that g-C3N4 can be used to construct heterojunction with many other semiconductors to increase their photocatalytic performance [19, 20]. Herein, gC3N4 can be considered as an ideal candidate to optimize the binary ZnO/ZnWO4 heterojunction because of the suitable band offset between g-C3N4 and those two components, which would bring about multi-interfacial heterojunction for more effective transfer of photoinduced charge carriers [21-23].

In this work, sheet-like ZnO/ZnWO4/g-C3N4 heterojunction was fabricated via stepwise calcination strategy. Based on the experimental results, the Z-scheme mechanism works on the sheet-like ZnO/ZnWO4/g-C3N4 heterojunction with double interfaces, resulting in strong photoreduction and photooxidation ability under light irradiation. In this regards, the ZnO/ZnWO4/g-C3N4 composite exhibits high photocatalytic CO2 reduction activity. 2. Results and discussion The morphologies of the as-synthesized samples were investigated by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) as shown in Fig. 1. ZnO/ZnWO4/g-C3N4 composite displays 2D sheetlike morphology with textured surface (Fig. 1a), and the TEM image in Fig. 1b exhibits the different configurations of its components. Fig. 1c shows the highresolution TEM (HRTEM) image of ZnO/ZnWO4/g-C3N4 composite. The lattice space of 0.25 and 0.28 nm can be assigned to ZnWO4 (001) facet and ZnO (100) facet, respectively. Nevertheless, the lattice fringe of g-C3N4 with weak crystalline cannot be found [11, 24]. It is apparent that the intimate contacts between those three components are built, resulting in strong interaction, which benefits the interfacial charge transfer. The selected area electron diffraction (SAED) pattern of ZnO/ZnWO4/g-C3N4 in Fig. 1d shows the co-existence of the lattice planes of these three components. Meanwhile, the elemental mapping images of ZnO/ZnWO4/g-C3N4 composite exhibit the uniform distribution for all elements of C, N, Zn, W and O as presented in Fig. 1e-j, proving the

incorporation of those three components in this composite. These results further reveal the construction of ternary heterojunction.

Fig. 1. (a) SEM, (b) TEM, (c) HRTEM, (d) SARD and (e-j) elemental mapping images of ZnO/ZnWO4/g-C3N4.

The crystal structure of the as-prepared samples was detected by the X-ray diffraction (XRD), as shown in Fig. 2a. The diffraction peaks of pure ZnO and ZnWO4 are well matched with those of hexagonal wurtzite ZnO (JCPDS no.361451) and monoclinic ZnWO4 phase (JCPDS NO. 73-0554), respectively [11,

25]. For g-C3N4, the characteristic (002) facet located at 27.4° is assigned to the repeated stacking of graphitic layers [26]. Another weak peak located at 13.1° is due to the repeated stacking of tri-s-triazine rings. As for the binary composites such as ZnO/ZnWO4, ZnWO4/g-C3N4 and ZnO/g-C3N4, the diffraction peaks are all examined and well matched with their single component. As for the ternary ZnO/ZnWO4/g-C3N4 composite, all three phases, including ZnO, ZnWO4 and gC3N4, are well detected.

Fig. 2.

(a) XRD patterns, (b) FTIR and (c) UV-vis spectra of all the samples. (d)

Band gaps of ZnO, ZnWO4 and g-C3N4.

To examine the surface groups of the prepared samples, Fourier transform infrared spectroscopy (FTIR) was performed as displayed in Fig. 2b. As for the pure g-C3N4, the absorption bands located at 3100-3400 cm-1 reflect the appearance of amino groups [27]. The peaks appeared at the range of 1200-1700 cm-1 are originated from the characteristic stretching vibrations of C-N bond in the heterocyclic rings [28]. The peak at 810 cm-1 is attributed to the typical breathing mode of tri-s-triazine rings [29]. As for ZnO, the intense peak observed at 450 cm-1 corresponds to the stretching vibration of Zn-O bond [30]. On the spectrum of ZnWO4, the characteristic vibrations of W-O, Zn-O-W bending and Zn-O can be detected [31]. Especially, for the ternary ZnO/ZnWO4/g-C3N4 composite, the coexistence of characteristic absorption bands of ZnO, g-C3N4 and ZnWO4 can be observed. The results of XRD patterns and FTIR further demonstrate that ternary ZnO/ZnWO4/g-C3N4 composite is successfully synthesized through the stepwise calcination process. The optical properties of all samples were recorded as shown in Fig. 2c. Both pure ZnO and ZnWO4 only respond to ultraviolet absorption due to their large band gaps (see in Fig. 2d, 3.20 eV for ZnO, 3.33 eV for ZnWO4). In contrast, pure g-C3N4 shows intense visible-light absorption because of its relatively small band gap energy (see in Fig.2d, 2.88 eV). After introducing g-C3N4, both ZnO/gC3N4 and ZnWO4/g-C3N4 composites display enhanced efficiency of visible-light harvesting compared with pristine ZnO and ZnWO4. By contrast, the light absorption ability of ZnO/ZnWO4 composite is almost the same to their pure

components. The result indicates that the introduction of g-C3N4 can improve light absorption. As for the ternary ZnO/ZnWO4/g-C3N4 composite, it obviously shows enhanced visible-light harvesting ability in comparison with those binary composites, which is conductive to produce more photogenerated charge carriers in the photocatalytic system [32]. Mott-schottky plots were employed to investigate the band structures of ZnO, g-C3N4 and ZnWO4, as presented in Fig. S1. It is noted that the Mott-schottky plots of all asprepared samples show positive slope in the linear part, indicating n-type semiconductor [33]. Thus, the conduction band (CB) positions can be evaluated by the flat-band potentials, which can be obtained from the Mott-schottky plots as shown in Fig. S1a, b and c. The CB positions of ZnO, g-C3N4 and ZnWO4 are measured to be 0.61, -1.32 and -0.23 V vs NHE, respectively [34, 35]. Accordingly, the valance band (VB) positions of ZnO, g-C3N4 and ZnWO4 can be calculated to be 2.59, 1.56 and 3.10 V, respectively. Based on the abovementioned results, the band structures of ZnO, gC3N4 and ZnWO4 are summarized in Fig. S1d. Thermodynamically, both g-C3N4 and ZnO can produce superoxide (•O2−) radicals under light irradiation, while both ZnO and ZnWO4 can generate hydroxyl radicals (•OH) radicals [36].

Theoretical calculation is an effective strategy to further understand the band and electronic structures of as-prepared samples as presented in Fig. 3. Accordingly, ZnO belongs to direct-gap semiconductor, while g-C3N4 and ZnWO4 are indirect-gap

semiconductors. Furthermore, the calculated band gaps of ZnO, g-C3N4 and ZnWO4 are 3.08, 2.61 and 3.19 eV, respectively. Although the calculated band gaps are smaller than the measured results due to the limitation of theoretical calculation, it is still meaningful for the qualitative analysis. Additionally, the DOS of ZnO and ZnWO4 indicate that their conduction bands (CBs) are mainly originated from metal atoms such as the Zn or W atoms, whereas their valance bands (VBs) mainly stem from the metalfree O atoms. For g-C3N4, the VBs mainly consist of N 2p electronic orbits and the CBs are mainly composed of the C 2p and N 2p electronic orbits.

Fig. 3. The optimized structural models, calculated band structures and density of states (DOS) of (a) ZnO, (b) g-C3N4 and (c) ZnWO4. The specific surface area and pore structure of all the samples were studied via the Brunauer-Emmett-Teller method. In Fig. S2a, according to the Brunauer-DemingDeming-Teller classification, the measured isotherms of all as-prepared samples are assigned to IV type with H3 type hysteresis loop, indicating the existence of slit-like pores in these structures [37]. As shown in Fig. S2b, the pores of the as-prepared samples mainly belong to mesopore and macrospore, which can promote the transport of reactants and products during the photocatalytic reaction. Moreover, as shown in Table S2, although the specific surface area of the ternary composite only displays slight change relative to other samples, its pore volume is the largest, meaning that the adsorption and transfer of mass molecules involved in the ternary composite can be substantially enhanced [38]. X-ray photoelectron spectroscopy (XPS) measurement was conducted to probe the elemental chemical states and the charge transfer process over ZnO/ZnWO4/g-C3N4 composite. In Fig. S3, Zn and O peaks for ZnO, and C and N peaks for g-C3N4 are observed on their survey spectra, respectively. Meanwhile, Zn, W and O peaks appear on the survey spectrum of ZnWO4. For ZnO/ZnWO4/g-C3N4 composite, the Zn, W, O, C and N elements are all detected on its survey spectrum, revealing that it is composed of ZnO, g-C3N4 and ZnWO4 components. In Fig. 4a, the high-resolution W 4f XPS spectrum in pure ZnWO4 sample exhibits two peaks located at 35.1 and 37.3 eV, which are ascribed to W4f7/2 and W4f5/2, respectively [39]. Those two peaks on the spectrum

of ternary ZnO/ZnWO4/g-C3N4 composite shift to lower binding energies in comparison with the pure one, indicating that ZnWO4 acts as the electron acceptor. In Fig. 4b, the high-resolution Zn 2p XPS spectra in pure ZnO and ZnWO4 samples show two peaks at 1021.7 and 1044.8 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively [37, 40]. Similarly, those peaks on the spectra of ternary ZnO/ZnWO4/g-C3N4 composite also shift to the lower binding energies compared with the pure samples, suggesting that both ZnO and ZnWO4 components in this composite are the electron acceptor. Additionally, considering that the peaks of both high-resolution C 1s and N 1s XPS spectra in the ternary composite shift to higher binding energies compared with the pure g-C3N4, the g-C3N4 component in the ZnO/ZnWO4/g-C3N4 composite serves as the electron donor, as presented in Fig. 4c and 4d [41,42]. Based on the above results and analysis, there is electronic interaction and interfacial heterojunction among ZnO, g-C3N4 and ZnWO4 components [4,43].

Fig. 4. XPS of ZnO, g-C3N4, ZnWO4 and ZnO/ZnWO4/g-C3N4: (a) W4f, (b) Zn 2p, (c) C 1s and (d) N 1s high-resolution spectra.

To further understand the free electron transfer route, the work functions of ZnO (100) facet, g-C3N4 (100) facet and ZnWO4 (001) facet were calculated by the density functional theory (DFT) method as presented in Fig. 5. The work functions of ZnO (100) facet, g-C3N4 (100) facet and ZnWO4 (011) facet are 5.8, 4.5 and 6.14 eV, respectively. Thus, g-C3N4 has higher Fermi level than that of ZnO and ZnWO4. When they are closely connected, the electrons of g-C3N4 can transfer to the ZnO and ZnWO4, aligning their Fermi levels. In this case, g-C3N4 serves as the electron donor, while ZnO and ZnWO4 act as the electron acceptor. This is in good accordance with the XPS results.

Fig. 5. The optimized structural models and calculated work functions of (a) ZnO (100) facet, (b) g-C3N4 (100) facet and (c) ZnWO4 (001) facet. The transient photocurrent plots and electrochemical impedance spectroscopy (EIS) were measured to study the migration and separation behavior of photoinduced charge carriers. As shown in Fig. 6a, the transient photocurrent intensities of the prepared samples are sorted in the order of ZnO/ZnWO4/g-C3N4 > ZnO/g-C3N4 > ZnWO4/gC3N4 > ZnO/ZnWO4 > g-C3N4 > ZnO > ZnWO4, indicating that ZnO/ZnWO4/g-C3N4 has the highest separation efficiency and lowest recombination of charge carriers [41]. Fig. 6b presents the EIS plots of all the as-prepared samples with the analog circuit. Particularly, a small radius of impedance arc represents low resistance of charge transfer. Note that the ZnO/ZnWO4/g-C3N4 heterojunction presents smallest charge transfer resistance among the prepared samples, indicating more efficient charge

migration on the ternary heterojunction. These results reveal that the ternary ZnO/ZnWO4/g-C3N4 heterojunction constructed in this work can further effectively accelerate the charge transfer and separation compared with those binary heterocomposites.

Fig. 6. (a) Transient photocurrent curves and (b) EIS of all the as-prepared samples. In a typical photocatalytic CO2 reduction reaction, CO2 molecules can be converted into solar fuels through a series of complex processes such as adsorption and activation of CO2, the subsequent formation and release of solar fuels on the photocatalyst surface. Therefore, CO2 adsorption ability of the photocatalyst reasonably plays an important role in affecting its photocatalytic performance. Fig. 7 displays the CO2 adsorption plots of the as-prepared samples at room temperature. The pristine ZnWO4, ZnO and g-C3N4 samples exhibit relatively low adsorption capacity of CO2 molecules, which is detrimental to the CO2 reduction process. Differently, binary composites show increased

CO2

adsorption

capacities.

Furthermore,

the

sheet-like

ternary

ZnO/ZnWO4/g-C3N4 heterojunction has the highest CO2 adsorption capacity, suggesting the great merit in photocatalytic CO2 reduction reaction.

Fig. 7. The CO2 adsorption plots of the as-prepared samples at room temperature. Photocatalytic Activity. The as-prepared samples were evaluated by photocatalytic CO2 reduction as shown in Fig. 8. The production yields of CO2 conversion over the pristine ZnWO4, ZnO and g-C3N4 samples are 0.30, 0.24 and 3.39 μmol h-1 g-1, respectively. Considering the low adsorption capacity of CO2 molecules and high recombination rate of photoinduced charge carriers, it is reasonable that these individual samples display low photocatalytic CO2 reduction performance. In comparison with the individual counterparts, photocatalytic CO2 reduction activities over ZnO/ZnWO4, ZnWO4/g-C3N4 and ZnO/g-C3N4 are increased to 5.58, 9.59 and 8.96 μmol h-1 g-1, respectively. Furthermore, it is noteworthy that the production yield of photocatalytic CO2 reduction over the sheet-like ternary ZnO/ZnWO4/g-C3N4 heterojunction reaches 13.19 μmol h-1 g-1, and the corresponding conversion rate of hydrocarbon fuel is highly up to 91.5%. This result is better than many other Zn-based composites as presented in Table S3, indicating that the ternary ZnO/ZnWO4/g-C3N4

heterojunction has stronger photoreduction ability compared with other single and binary samples.

Fig. 8. Photocatalytic activity of CO2 reduction over all the as-prepared samples. The isotope labelling method is a useful technique to trace the carbon source during the photocatalytic CO2 reduction reaction as presented in Fig. 9. In the isotope labelling experiments, the

12CO

2

and

13CO

2

are used as the carbon source to

produce hydrocarbon fuels under light irradiation, respectively. Meanwhile, the produced methane is monitored by the gas chromatography and mass spectra (GC-MS) as described in Fig. 9a and b. It is noted that the retention time of methane stemmed from 12CO2 is obviously different from that of 13CO2. Furthermore, the m/z signal at 16 in the mass spectra can be assigned to the

12CH

4

using

12CO

2

as the carbon source,

whereas the m/z signal at 17 can be attributed to the 13CH4 using 13CO2 as the carbon source. The above examined results reveal that the carbon source of photoreduced

products is originated from the provided carbon dioxide rather than from other sources such as self-decomposition of photocatalyst and contamination of air.

Fig. 9. GC-MS results of the methane product using the

12CO

2

and

13CO

2

as carbon

source after the photocatalytic reaction of ternary ZnO/ZnWO4/g-C3N4 heterojunction. Electronic paramagnetic resonance (EPR) technique can be used to evaluate the photoreduction and photooxidation abilities of the photocatalysts by detecting the generated radicals as presented in Fig. 10. 5,5-dimethyl-L-pyrroline N-oxide (DMPO) serves as the trapping agent for capturing superoxide radicals (•O2−) in aqueous solution and hydroxyl radicals (•OH) in methanol, leading to the production of DMPO-•O2− and DMPO-•OH adducts, respectively [44, 45]. Fig. 10a shows the EPR spectra of DMPO•O2− adducts generated in the photocatalytic systems of blank experiment, ZnO, g-C3N4, ZnWO4 and ZnO/ZnWO4/g-C3N4 under the light irradiation. For the pure samples, the EPR signal is obviously strong in g-C3N4 photocatalytic system, whereas the EPR signal is relatively weak in ZnO photocatalytic system and negligible in ZnWO4 photocatalytic system. These results indicate that both g-C3N4 and ZnO can generate superoxide radicals (•O2−), but ZnWO4 sample is incapable of producing superoxide radicals (•O2−)

under light illumination. It is because CB positions of g-C3N4 (-1.32 V, NHE) and ZnO (-0.61 V, NHE) are more negative than the standard potential of O2/•O2− (-0.33 V, NHE), whereas the CB position (-0.23 V, NHE) of ZnWO4 is less negative than the standard potential of O2/•O2− (-0.33 V, NHE). According to the EPR results, pure gC3N4 can generate more •O2− radicals than ZnO, which is attributed to the stronger reduction ability of the photoelectrons in the CB of g-C3N4 than that of ZnO. More importantly, ternary ZnO/ZnWO4/g-C3N4 heterojunction displays the strongest EPR signal of DMPO-•O2− adducts among the as-prepared samples, revealing the abundant •O2− radicals are generated. Generally, if the reduction potential of photoelectron is satisfied with that of O2/•O2− the photoelectrons of a photocatalyst can react with the oxygen molecules to form •O2− radicals, which can be used to indirectly evaluate the photoelectron concentration of the photocatalyst. In this case, more •O2− radicals produced in the ternary ZnO/ZnWO4/g-C3N4 heterojunction means that ZnO/ZnWO4/gC3N4 could generate more photoelectrons under the same light irradiation. These photoelectrons can also be used to promote the photocatalytic activity of CO2 reduction. Correspondingly, the EPR signal of DMPO-•OH adducts is relatively strong in ZnWO4 photocatalytic system, whereas this EPR signal is relatively weak in ZnO photocatalytic system and negligible in g-C3N4 photocatalytic system as presented in Fig. 10b. These results suggest that both ZnWO4 and ZnO can generate hydroxyl radicals (•OH) but g-C3N4 sample cannot produce hydroxyl radicals (•OH) under illumination. It is reasonable because the VB positions of ZnWO4 (3.10 V, NHE) and ZnO (2.59 V, NHE) are more positive than the potential of H2O/•OH (2.24 V, NHE),

whereas the VB position (1.56 V, NHE) of g-C3N4 is less positive than the potential of H2O/•OH (2.24 V, NHE). It is noted that pure ZnWO4 can produce more •OH radicals than ZnO based on their EPR signal intensity, which can be assigned to the stronger oxidation ability of photoinduced holes in ZnWO4 VB position than that of ZnO. Furthermore, ZnO/ZnWO4/g-C3N4 heterojunction exhibits the strongest EPR signal among the prepared samples, indicating that this ternary composite generates the most •OH radicals. Hence, the above experimental results reveal that ternary ZnO/ZnWO4/gC3N4 heterojunction can produce more photoelectrons and holes than that of other samples, which can be attributed to the high-efficient separation of photogenerated carriers.

Fig. 10. EPR spectra of blank experiment, ZnO, g-C3N4, ZnWO4 and ZnO/ZnWO4/gC3N4 photocatalytic systems: (a) DMPO-•O2− and (b) DMPO-•OH. According to the abovementioned results and discussion, the photocatalytic CO2 reduction mechanism over ZnO/ZnWO4/g-C3N4 heterojunction can be proposed as presented in Fig. 11. Without illumination, when the intimate interfaces between gC3N4 and ZnO as well as between g-C3N4 and ZnWO4 are formed, the electrons could

migrate from g-C3N4 to ZnO and ZnWO4 because the Fermi level of g-C3N4 is obviously higher than those of ZnO and ZnWO4 [46,47]. Therefore, the interfaces near the g-C3N4 part are positively charged, whereas the interfaces near the ZnO and ZnWO4 parts are negatively charged as presented in Fig. 11a. The inner electric field at the abovementioned double interfaces is built from the g-C3N4 to ZnO, as well as from the g-C3N4 to ZnWO4, respectively. The built-in electric fields could hinder the continuous electron migration from g-C3N4 to ZnO and ZnWO4 until aligning their Fermi level. Under the light irradiation, the electrons in ZnO, ZnWO4 and g-C3N4 are excited to their CBs from the VBs to produce photoelectrons, leaving the holes in their VBs. Subsequently, the photoexcited electrons on the CBs of ZnO and ZnWO4 would easily transfer to the VB of g-C3N4 and then combine with its photoexcited holes through the facilitation of interface electric fields and the strong Coulomb attraction between the photoelectrons and holes [46]. As a result, the photoinduced charge carriers in this ternary heterojunction system can be effectively separated in space as shown in Fig. 8b, which can be regarded as a double Z-scheme heterojunction. Meanwhile, those photoinduced electrons accumulate in the CB of g-C3N4 and holes in VBs of ZnO and ZnWO4, resulting in strong redox ability. In this regards, these accumulated photoelectrons with strong reduction ability can participate in a series of photocatalytic reactions to promote the photoreduction of CO2 molecules [48].

Fig. 11. Diagram of charge transfer in the double ZnO/ZnWO4/g-C3N4 Z-scheme heterojunction (a) in darkness and (b) under irradiation. 4. Conclusions In summary, a novel sheet-like ZnO/ZnWO4/g-C3N4 Z-scheme heterojunction was synthesized through the stepwise calcination method. The XPS spectra, DFT calculations, electrochemical measurements and EPR tests are carried out to deeply understand the photocatalytic mechanism, confirming that the double electric fields are built in the ZnO/g-C3N4/ZnWO4 heterojunction. Thus, a double Z-scheme transfer process of photogenerated charge carriers is proposed. This double Z-scheme ZnO/gC3N4/ZnWO4 heterojunction not only facilitates the separation and transfer of photoinduced charge carriers, but also endows the strong photoreduction ability. As a result, this double Z-scheme heterojunction acquires a high-efficient conversion of CO2 molecules into various solar fuels such as CO (1.12 μmol h-1g-1), CH4 (6.24 μmol h-1g1),

CH3OH (3.85 μmol h-1g-1) and CH3CH2OH (1.98 μmol h-1g-1), and the conversion

rate of hydrocarbon fuel is highly up to 91.5%. This work provides a novel strategy to

obviously enhance the photocatalytic performance via constructing the double Zscheme heterojunction for photocatalytic CO2 reduction applications. Notes The authors declare no competing financial interest. Acknowledgements The authors of this study gratefully acknowledge the financial support of National Natural Science Foundation of China (51772224, 51372179) and National Postdoctoral Program for Innovative Talents (BX20190309). This study is also supported by the Fundamental Research Funds for the Central Universities (2019-YB-006). Appendix A. Supplementary data Supplementary data to this article can be found online at in the online version. References [1] J. Liu, Q. Zhang, J. Yang, H. Ma, M. Tade, S. Wang, J. Liu, Facile synthesis of carbon-doped mesoporous anatase TiO2 for the enhanced visible-light driven photocatalysis, Chem. Commun. 50(2014) 13971-13974. [2] X.Q. Qiao, Z.W. Zhang, Q.H. Li, D.F. Hou, Q.C. Zhang, J. Zhang, D.S. Li, P.Y. Feng, X.H. Bu, In situ synthesis of n-n Bi2MoO6 & Bi2S3 heterojunctions for highly efficient photocatalytic removal of Cr(vi), J. Mater. Chem. A 6 (2018) 2258022589. [3] S. Ye, C. Ding,

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Graphical abstract

Linyu Zhu: Conceptualization, Methodology, Investigation, Data Curation, Formal analysis, Writing-Original Draft,

Hong Li: Writing-Review Editing, Funding Acquisition

Quanlong Xu: Writing-Review Editing

Dehua Xiong: Resources

PengfeiXia: Conceptualization, Formal analysis, Writing-Review Editing, Funding Acquisition

Declaration of interests

 The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: