Î²-AgVO3 Nanocomposite as a Direct Z-Scheme Photocatalyst

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Energy, Environmental, and Catalysis Applications

InVO4/#-AgVO3 Nanocomposite as Direct Z-Scheme Photocatalyst toward Efficient and Selective Visible-light Driven CO2 Reduction Juan Yang, Jingyi Hao, Siyu Xu, Qi Wang, Jun Dai, Anchao Zhang, and Xinchang Pang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10758 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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ACS Applied Materials & Interfaces

InVO4/β-AgVO3 Nanocomposite as Direct Z‑Scheme Photocatalyst toward Efficient and Selective Visible-light Driven CO2 Reduction Juan Yang,*,† Jingyi Hao,† Siyu Xu,† Qi Wang,§ Jun Dai,† Anchao Zhang,† and Xinchang Pang*,‡

†Institute

of Applied Chemistry, College of Chemistry and Chemical Engineering, Henan

Polytechnic University, Jiaozuo 454003, P.R. China ‡School

of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P.R.

China §School

of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou

310018, P.R. China

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Photocatalytic CO2 reduction to solar fuel is a promising route to alleviate the ever-growing energy crisis and global warming. Herein, to enhance photoconversion efficiency of CO2 reduction, a series of direct Z-scheme composites consisting of β-AgVO3 nanoribbons and InVO4 nanoparticles (InVO4/β-AgVO3) are prepared via a facile hydrothermal method and subsequent in-situ growth process. The prepared InVO4/β-AgVO3 composites exhibit enhanced photocatalytic activity for reduction CO2 to CO under visible light illumination. CO evolution rate of 12.61 μmol·g-1·h-1 is achieved over the optimized 20% In-Ag without any cocatalyst or sacrificial agent, which is 11 times larger than that yielded by pure InVO4 (1.12 μmol·g-1·h-1). Moreover, the CO selectivity is more than 93% over H2 production from the side reaction of H2O reduction. Significantly, based on the results of electron spin resonance (ESR) and in-situ irradiated XPS tests, it is proposed that the synthesized InVO4/β-AgVO3 catalysts comply with direct Z-scheme transfer mechanism. Significantly improved photocatalytic activities for selective CO2 reduction could be primarily ascribed to effective separation of photoinduced electron-hole pairs and enhanced reducibility of photoelectrons at the conduction band of InVO4. This work provides a new insight for constructing highly efficient photocatalytic CO2 reduction systems toward solar fuel generation. Keywords: Direct Z-scheme, photocatalysis, CO2 reduction, InVO4, β-AgVO3

1. Introduction The rapid consumption of carbon-rich fossil fuels is accelerating global energy shortage and increasing dramatically atmospheric CO2 concentrations, which results in serious environmental issues including greenhouse effect and global warming.1,2 One of the most promising approaches to address these problems could be conversion CO2 into value-added fuel and chemicals through artificial photosynthesis using abundantly available solar energy.3-6 Although the photocatalysts can reduce the activation energy of CO2 reduction, solar energy conversion efficiency still requires much improvement. To this end, various strategies have been adopted to optimize the structure and composition of catalysts for improving photocatalytic performance of CO2 reduction, e.g., creating surface defects, exposed crystal facets control, loading metal co-catalysts, improving CO2 adsorption, and constructing heterojunctions etc.7-14 Among them, developing heterojunction 2


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structures by assembling two semiconductors with staggered band-structures has been proved to be an effective approach to improve photocatalytic performance due to the merits in separating photoinduced electron/hole pairs and coupling respective advantages of single component.12-14 In addition to the conventional type-II heterojunction photocatalysts, Z-scheme heterostructures have also been constructed to achieve high charge separation efficiency and strong redox capacity. Generally, traditional Z-scheme photocatalysts have employed an additional charge carrier mediator.15-19 For instance, Domen et al. have synthesized Z-scheme photocatalyst Pt-WO3: Pt-ZrO2/TaON by using IO3−/I− as redox mediator and achieved stoichiometric H2O splitting under visible light irradiation.17 Zou et al. have fabricated Z-scheme WO3/Au/In2S3 nanowire arrays showing excellent visible light catalytic activity toward CO2 conversion with Au nanoparticles as charge mediator.18 However, these redox mediators might induce undesirable backward reactions or inhibit the light absorption of catalytic active components.16,20 Hence, it is highly desirable to developing redox-mediator-free systems, namely direct Z-scheme photocatalytic systems for imitating natural photosynthesis and promoting CO2 reduction efficiency. For direct Z-scheme photocatalysts, intimate interfacial contact and electron interaction of two semiconductor components result in the formation of an internal electric field, ensuring the efficient charge transfer and separation without any shuttle mediator.21-25 More importantly, direct Z-scheme photocatalysts can simultaneously preserve photogenerated electrons and holes possessing strong reduction and oxidation ability, which contributes to achieving high photocatalytic reaction efficiency.26-31 Recently, metal vanadates (BiVO4, InVO4, AgVO3, and CuV2O6) have attracted increasing attention in photocatalytic field, owing to the advantages including suitable band structures and excellent chemical stability.32-36 Among of vanadate photocatalysts, Indium vanadate (InVO4) possessing narrow bandgap energy (~2.0 eV),37-40 has been extensively evaluated in water splitting and environment purification under visible light. Nonetheless, the catalytic performance of pristine InVO4 is not ideal, primarily owing to low separation efficiency of photoinduced electron/hole pairs. Photocatalytic performance of InVO4 can be improved by fabricating type-II heterojunction catalysts, such as g-C3N4/InVO4, BiVO4/InVO4, and In2S3/InVO4.37-39 In contrast, InVO4-based direct Z-scheme heterostructured composites seldom reported, especially for the 3



application to photocatalytic CO2 reduction. Therefore, it is essentially necessary to find suitable photoactive component and composite strategy to construct InVO4-based direct Z-scheme photocatalyst and evaluate its catalytic performance of selective CO2 reduction. Owing to the unique geometrical and electronic characteristics of one-dimensional nanostructures,41,42 β-AgVO3 nanoribbon is seen to be an appropriate choice to fabricate the direct Z-scheme heterojunction with InVO4.33,43 β-AgVO3 can be served as a V source, and thus in-situ synthesis of InVO4 nanocrystals can be implemented via cation exchange process.44,45 By using in-situ growth approach, InVO4 nanoparticles can be distributed and fixed uniformly upon the surface of β-AgVO3 nanoribbons, which dramatically inhibits the aggregation of InVO4 particles. The intimate interfacial contact and matched energy-band levels of InVO4 nanocrystals and β-AgVO3 nanoribbons provide good prerequisites of constructing direct Z-scheme photocatalysts. Herein, direct Z-scheme nanocomposites consisting of β-AgVO3 nanoribbons and InVO4 nanocrystals (InVO4/β-AgVO3) are fabricated through a facile hydrothermal and successive cation exchange process, utilizing β-AgVO3 nanoribbons as the V source and the support for in-situ growing InVO4 nanoparticles. To our knowledge, it is the first time to report the enhanced visible light catalytic activity for selective CO2 reduction into CO over InVO4/β-AgVO3 composites. The effect of InVO4 molar fraction on the photocatalytic performance is well scrutinized. The optimum InVO4/β-AgVO3 photocatalyst (20% In-Ag) shows 11-fold enhancement in CO evolution rate over pristine InVO4. The formation of direct Z-scheme heterojunction between InVO4 and β-AgVO3 is verified by in-situ irradiated XPS analysis and radical generation test. Direct Z-scheme charge transfer pathway of InVO4/β-AgVO3 nanocomposites facilitates the spatial separation of electron/hole pairs and preserves the reduction capacity of photoelectrons, leading to remarkably improved CO2 photoreduction activity.

2. Experimental section 2.1. Synthesis of InVO4/β-AgVO3 composites. All chemical reagents were of analytical grade from J&K Scientific Ltd and were used without extra treatment. β-AgVO3 nanoribbons were synthesized using a modified method based on our previous report.43 Typically, 1.0 mmol of NH4VO3 was added to 60 mL deionized water under 4


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magnetic agitation to obtain a homogeneous solution. Subsequently, 1.0 mmol of AgNO3 was put into the above-prepared NH4VO3 solution and stirred magnetically for another 20 min. The pH of this mixed solution was adjusted to 8.0 using 28 wt% NH3·H2O, and then was transferred to 100 mL Teflon-lined stainless autoclave and retained at 180 °C for 12 hrs. Afterwards, the resulting yellow sediments were collected by filtration and washed for three times with deionized water. Eventually, the product was dried at 60 °C in a vacuum oven and marked as pristine β-AgVO3. As illustrated in Figure 1, InVO4/β-AgVO3 nanocomposites were prepared through cation-exchange and in-situ growth approach. In detail, 100 mg of β-AgVO3 was firstly dispersed into 70 mL deionized water and ultrasonicated for 30 min. And then 40 mg of In(NO3)3·H2O was put into the above suspension and stirred magnetically for 30 min to obtain an uniform mixture, which was transferred into a 100 mL Teflon-lined stainless autoclave and kept at 140 °C for 8 hrs. After cooling down, the solid products were collected by centrifugation and dried at 60 °C for 10 hrs to obtain InVO4/β-AgVO3 composites. For convenience, the prepared InVO4/β-AgVO3 was labeled as x% In-Ag, where In, Ag, and x represent InVO4, β-AgVO3, and the molar ratios of InVO4 in InVO4/β-AgVO3 composites, respectively. The actual compositions of 5% In-Ag, 10% In-Ag, 20% In-Ag and 30% In-Ag samples were 0.89, 1.65, 2.58, and 3.74 mol %, respectively (Table S1), which were determined by inductively coupled plasma optical emission spectrometry (Agilent 725 ICP-OES). For comparison, pristine InVO4 sample was synthesized through the similar hydrothermal process, but without adding β-AgVO3 nanoribbons.

Figure 1. Schematic diagram for the synthesis process of InVO4/β-AgVO3 composite photocatalysts.

2.2. Characterization of Materials. The phase composition of as-synthesized composites were determined on X-ray diffractometer (XRD, Bruker AXS D8 Advances) using Cu K radiation (=0.15405 nm) and operating in a 2 5



range of 5-70°. The morphology and microstructure of the samples were examined by field-emission SEM (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Tecnai G2, FEI) with a JEOL JEM-2010 HRTEM instrument at 200 kV accelerating voltage. Energy dispersive X-ray spectrum was collected to analyze the elements distribution of the composites. The surface chemical state of the synthesized samples was determined via X-ray photoelectron spectrometer (XPS) characterization using a Thermo ESCALAB 250xi apparatus. In-situ XPS was performed on the same instrument equipped with a low-power 405 nm LED light (Beijing NBET Technology Co. Ltd.) as the illumination source. Nitrogen adsorption-desorption experiments of composite samples were measured on a Micromeritics ASAP 3020 gas sorption system at 77K. Ultraviolet-visible diffused reflectance spectra were recorded on a UV-vis spectrophotometer (UV-2550, Shimadzu). Photoluminescence spectra (PLs) were obtained on an Edinburgh FLS 980 spectrofluorometer with an excitation wavelength of 400 nm. Electron spin resonance (ESR) signals were recorded with a Bruker ER200-SRC spectrometer. 30 L of the sample suspension was put into quartz capillary tube, which was then inserted in the ESR cavity. Photoelectrochemical measurements were performed on a CHI-660C electrochemical workstation with a standard three-electrode cell and the working electrode was prepared according to our previous report.46 A Na2SO4 neutral solution (0.2 M) was used as the electrolyte. Periodic photocurrent responses were measured by using a 300 W Xe lamp (λ > 420 nm) as the illumination source at a bias of + 0.50 V. Electrochemical impedance spectrum was acquired using the above three-electrode system at 0.0 V. The Mott-Schottky plot was recorded with a scanning rate of 5 mV/s at a frequency of 0.5 kHz. 2.3. Photocatalytic Experiment. Photocatalytic activity measurements of CO2 reduction were carried out in a Teflon-lined stainless reactor with a quartz window at the top for light irradiation. Typically, 50 mg of the as-prepared powder samples were equably dispersed on a circular glass dish that was positioned 10 cm away from the light source. Before photoreaction, the reactor was vacuumed by a mechanical pump, and then ultrapure CO2 (99.99%) was filled into the reactor to attain ambient pressure. 100 L of deionized water as reducer was injected into the reaction system. Visible-light irradiation was obtained by using a 300W xenon lamp equipped with cutoff filter to remove radiation below 420 6


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nm. The photoreaction temperature was maintained at 25 ± 1 °C through a circulating cooling water system. Starting from the light illumination, about 1 mL of the mixed gas was extracted with a syringe injector at given time intervals and analyzed using a gas chromatograph (Shimadzu GC-2014) equipped with a thermal conductivity detector and flame ionization detector (FID). The yields of photocatalytic products were calculated using the working curves of standard gaseous samples. The origin of photocatalytic CO2 reduction products was determined using isotope-labeled

13CO

2

(99%) following the procedures similar to the abovementioned CO2

photoreduction test. The gaseous products were analyzed through gas chromatography-mass spectrometry (GC-MS) (Trace 1310 GC-ISQTM quadrupole MS, ThermoFisher, USA). The apparent quantum efficiency (AQE) was measured according to the previous reported method.47 Photocatalytic CO2 reduction experiment was performed by using a 400 nm band-pass filter. The intensity of irradiation was determined to be 2.4 mW·cm−2 and the illuminated area of photocatalyst was about 3.14 cm2. AQE (%) = Nelectron / Nphoton = [NCO × 2] / Nphoton × 100% = [NCO × 2] / [(I×A×t) / (Ephoton × NA)] × 100% where NCO represents the amounts of CO; I is the incident light intensity; A is the illumination area; Ephoton is the average single photon energy that is calculated with the function: Ephoton = hc/λ; NA is Avogadro’s constant.

3. Results and discussion Characterizations of InVO4/β-AgVO3 Composites. The crystal structures of as-synthesized InVO4/β-AgVO3 nanocomposites, along with pristine β-AgVO3 and InVO4, are determined by XRD. As displayed in Figure 2a, the diffraction peaks appeared at 20.3°, 22.8°, 25.7°, 29.9°, 33.5°, 34.5°, 40.3°, 44.1°, 49.3°, 54.8°, 56.5° and 62.1°, can be assigned to monoclinic phase β-AgVO3 (JCPDS No. 29-1154) for (400), (-202), (-501), (501), (-112), (-303), (303), (710), (204), (404), (-222) and (-123) planes.43,48 Pristine InVO4 sample (Figure 2f) shows the diffraction peaks at 18.5°, 20.8°, 23.0°, 24.9°, 27.1°, 31.0°, 33.0°, 35.2°, 41.6°, 47.0°, 51.0°, 56.3° and 60.9°, which can be indexed to (110), (020), (111), (021), (002), (200), (112), (130), (202), (222), (042), (150), and (242) planes of orthorhombic phase InVO4 7



(JCPDS No. 48-0898), respectively.49 As can be clearly observed from Figure 2b-2e, as In source increases (from 5% In-Ag to 30% In-Ag), the peak intensity corresponding to InVO4 gradually strengthens while that of β-AgVO3 weakens gradually, indicating the increased InVO4 molar fraction of as-synthesized nanocomposites. XRD results in Figure 2 indicate the successful

#

## ##

#

#

(112)

*

# InVO4

(-112)

(110)

(501)

incorporation of β-AgVO3 and InVO4.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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*#

* -AgVO3

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#

(f) (e) (d) (c) (b)

* * *

** * 20

30

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* * * 50

* 60

(a) 70

2 Theta (degree)

Figure 2. XRD patterns of (a) β-AgVO3 nanoribbons, (b) 5% In-Ag, (c) 10% In-Ag, (d) 20% In-Ag, (e) 30% In-Ag, and (f) pristine InVO4, respectively.

The morphologies of the synthesized InVO4/β-AgVO3 nanocomposites with different molar fractions of In source are firstly characterized by FE-SEM and the corresponding images are indicated in Figure 3. As can be seen from Figure S1, pristine β-AgVO3 sample shows nanoribbon morphology with an average diameter of about 300 nm and length of several micrometers. For the InVO4/β-AgVO3 composites, when the molar fraction of In(NO3)3·H2O is less than 20%, InVO4 nanoparticles with an average size of 35 nm are anchored upon the surface of β-AgVO3 nanoribbons through the cation-exchange and the followed in-situ growth process. And meanwhile, the one-dimensional ribbon-like morphology of β-AgVO3 remains unchanged in the obtained InVO4/β-AgVO3 heterostructures. Besides, it can be noticed from Figure 3d that the size distribution of InVO4 particles becomes uneven and large InVO4 particles with the size of about 500-600 nm appear when the molar fraction of In source increases up to 30%. It indicates the molar fraction of In(NO3)3·H2O play a crucial role in the size distribution of InVO4 particles. For comparison, the prepared pristine InVO4 sample displays microsphere-like morphology with a size 8


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of 2.0-3.5 m (Figure S2). Therefore, the in-situ growth approach can significantly inhibit the aggregation of InVO4, which contributes to forming the uniformly distributed InVO4 nanoparticles.

Figure 3. SEM images of (a) 5% In-Ag, (b) 10% In-Ag, (c) 20% In-Ag, (d) 30% In-Ag, (e)TEM and (f) HRTEM image of 20% In-Ag, (g-k) High-angle annular dark field (HAADF) STEM image and elemental mapping of 20% In-Ag sample.

Furthermore, the microstructure of 20% In-Ag composite is investigated by TEM and high-resolution TEM. As can be observed from Figure 3e, the InVO4 nanoparticles with the diameters ca. 20-30 nm are loaded on the surface of β-AgVO3 nanoribbons. Additionally, the lattice fringes are relatively clear in the HRTEM image (Figure 3f). The lattice spacing of ca. 0.298 nm can be indexed to (501) plane of β-AgVO3 (0.299 nm from JCPDS No. 29-1154),50 and another lattice spacing of ca. 0.269 nm corresponds to (112) plane of InVO4 (0.271 nm from JCPDS No. 48-0898).35,37 The HRTEM analysis confirms the generation of InVO4 nanoparticles 9



on β-AgVO3 nanoribbons, agreeing with the XRD results. The EDS in Figure S3 indicates that the coexistence of Ag, In, V, and O elements, resulting from the heterostructured β-AgVO3/InVO4 composite. The elemental mapping images in Figure 3g-3k further evidence that InVO4 nanoparticles are well anchored and evenly distributed upon the surface of β-AgVO3 nanoribbons. N2 adsorption-desorption experiments are performed to measure the specific surface area (SBET) and pore structure of InVO4/β-AgVO3 composites. As can be observed from Figure S4, all the samples exhibit IV-type isotherms with H3 type hysteresis loops, meaning the presence of slit-shaped mesopores,21,44 probably due to the interparticle stacking of β-AgVO3 nanoribbons or InVO4 nanoparticles. It can be seen from Table S2 that the SBET of InVO4/β-AgVO3 increases with increasing the molar ratios of InVO4 and 20% In-Ag shows the highest SBET of 10.67 m2·g-1, whereas the BET surface area decreases slightly with the further increase of InVO4 molar ratios. This can be attributed to the formation of larger InVO4 particles, as indicated in SEM images (Figure 3). Besides, a moderate drop in the average pore size can be noted, suggesting that there are more small-size pores in InVO4/β-AgVO3 samples than in β-AgVO3, especially the pores with diameter of 2-8 nm (inset of Figure S4), which might be beneficial for CO2 adsorption. XPS is further employed to investigate the surface composition and chemical states of InVO4/β-AgVO3 composites. The survey XPS spectrum (Figure 4a) indicates the presence of Ag, V, O and In elements for 20% In-Ag sample. The Ag 3d XPS spectrum (Figure 4b) shows two symmetrical peaks of Ag 3d5/2 around 367.4 eV and Ag 3d3/2 around 373.4 eV, corresponding to Ag+ in β-AgVO3.43 Note that the binding energy of Ag 3d for 20% In-Ag shifts by 0.4 eV towards lower value as compared to that for pristine β-AgVO3, indicating the presence of electron migration from InVO4 to β-AgVO3 in as-synthesized composites.51,52 For In 3d XPS spectrum (Figure 4c), two characteristic peaks centered at 452.1 eV and 444.6 eV can be assigned to In 3d3/2 and In 3d5/2 of In3+, matching well with the XPS results of InVO4.38,49 By contrast, the binding energy of In 3d for 20% In-Ag sample shows clear shift to higher values in comparison with pristine InVO4 (Figure 4c). It reaffirms the electrons migrate from InVO4 to β-AgVO3, which would create an internal electric field at the interface directing from InVO4 to β-AgVO3. The electron migration and the associated electric field would play great roles in the formation of heterostructure and hence affect photocatalytic activity of CO2 reduction. In-situ XPS analysis of 10


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the synthesized catalysts under visible-light illumination is presented in the following discussion section of photocatalysis mechanism.

Ag 3d C 1s

V 2p In 3d

Relative intensity (a.u.)

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20% In-Ag (Vis) 373.0

20% In-Ag 367.4

373.4 -AgVO3

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444.9

373.2

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380

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In 3d5/2

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445.2

20% In-Ag (Vis)

452.7

20% In-Ag 444.6

452.1

InVO4 440

Binding energy (eV)

445

450

455

460

Binding energy (eV)

Figure 4. (a) XPS survey spectrum of 20% In-Ag composite, High-resolution XPS spectra of (b) Ag 3d and (c) In 3d for the prepared samples.

The UV-vis DRS of InVO4/β-AgVO3 nanocomposite samples are displayed in Figure 5A. Pristine β-AgVO3 shows broad visible light absorption with a band edge of 602 nm, and InVO4 exhibit an absorption edge at ca. 545 nm. After coupling InVO4 nanoparticles, the absorbance intensities of InVO4/β-AgVO3 composites display the moderate enhancement in visible-light region. Moreover, the direct bandgap energies of β-AgVO3 and InVO4 can be calculated to be around 2.15 eV and 2.36 eV by extrapolating the linear region of the curves of (hv)2 versus hv to y=0 (Figure 5B), where  represents the absorption coefficient and hv is the photoenergy.53,54 The XPS valence-band spectrum can be used to study the energy gap from valence-band maximum to Fermi level (Ef) of a semiconductor.55,56 As presented in Figure S5 of Supporting Information, the VBM positions of β-AgVO3 and InVO4 can be estimated to be 2.05 and 2.24 eV below Ef. The 11



positive slope of Mott-Schottky curves presented in Figure 5C and 5D indicates that the synthesized β-AgVO3 and InVO4 belong to n-type semiconductor.55,56 The flatband potentials of β-AgVO3 and InVO4 are determined to be 0.41 and −0.54 V through extrapolating the linear region of Mott-Schottky plot to y=0. As a result, the VBM of β-AgVO3 and InVO4 are calculated to be 2.46 and 1.70 V since the Fermi level is close to flatband potential for n-type semiconductor.57 According to the measured bandgaps of β-AgVO3 and InVO4, their conduction-band edges can be obtained to be 0.31 and −0.66 V, respectively. Moreover, the difference in Fermi levels between β-AgVO3 and InVO4 can reasonably explain the electron migration, which is inferred by XPS analysis. When the InVO4 nanoparticles get into contact with β-AgVO3, the electrons will migration from InVO4 to β-AgVO3 until their Fermi levels are aligned since β-AgVO3 has lower Fermi level than InVO4. Then, β-AgVO3 and InVO4 become negatively and positively charged respectively, and hence an internal electric field is created directing from InVO4 to β-AgVO3 at the interface. 5

(A)

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-AgVO3

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(hv) (eV)

Absorbance (a.u.)

5% In-Ag 10% In-Ag 20% In-Ag 30% In-Ag pristine InVO4

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1.0

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Figure 5. (A) UV-vis DRS of pristine β-AgVO3 and InVO4/β-AgVO3 composites; (B) plots of (αhν)2 versus hν of β-AgVO3 and InVO4; (C) and (D) Mott-Schottky plots of pristine β-AgVO3 and InVO4. 12


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To investigate the electron-hole separation efficiency in InVO4/β-AgVO3 nanocomposites, a series of experiments, including transient photocurrent response, EIS and PL analysis, have been performed. Figure 6A displays the transient photocurrent responses of β-AgVO3, InVO4, and InVO4/β-AgVO3, which are recorded for several on-off cycles under visible-light illumination. Generally, instantaneous photocurrent response is positively correlated with the separation efficiency of photogenerated electrons and holes, and efficient charge separation can improve photocatalytic performance.46,58 As depicted in Figure 6A, the synthesized InVO4/β-AgVO3 composites exhibit higher photocurrent intensity than pristine β-AgVO3 or InVO4, indicating the improved electron-hole separation efficiency of these heterostructured samples. Among the three composite photocatalysts, 20% In-Ag sample shows the highest transient photocurrent, which further demonstrates that in-situ coupling a suitable amount of InVO4 nanocrystals is of crucial importance to achieve the effective separation of photogenerated charges. And meantime, the EIS curves of the synthesized samples are presented in Figure 6B. The arc radius of InVO4/β-AgVO3 composites is smaller than that of pure β-AgVO3 or InVO4, suggesting the charge migration resistance of nanocomposite samples is lower in comparison with single component. 20% In-Ag has the smallest arc radius, indicating a more efficient charge migration in this nanocomposite.41,58 Overall, these results indicate the separation and migration of charge carriers in InVO4/β-AgVO3 composites are more effective. The enhanced migration of photoinduced charge carriers over InVO4/β-AgVO3 was also evidenced by PLs analysis. As depicted in Figure S6, pristine β-AgVO3 shows a main emission peak centered at 480 nm and a shoulder peak around 580 nm.43 The relatively high intensity of β-AgVO3 emission peaks means the photogenerated electrons and holes over AgVO3 nanoribbons are easy to recombine. Pristine InVO4 displays an emission peak at ca. 512 nm, which could be assigned to the intrinsic luminescence of InVO4.59 InVO4 nanoparticles are anchored uniformly upon the surface of β-AgVO3 nanoribbons via in-situ synthesis strategy, which produces the intimate interfacial contact to improve the migration rate of photoinduced charges. Much weaker PLs intensity of 20% In-Ag sample further indicates the recombination of photogenerated electrons and holes can be more effectively suppressed, owing to the formation of 13



InVO4/β-AgVO3 heterostructures. 750

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Figure 6. (A) Transient photocurrent response of (a) pristine InVO4, (b) β-AgVO3, (c) 10% In-Ag, (d) 20% In-Ag, and (e) 30% In-Ag composites under visible light irradiation; (B) the corresponding EIS Nyquist plots of the synthesized InVO4, β-AgVO3, and InVO4/β-AgVO3 composites.

Photocatalytic Activity of InVO4/β-AgVO3. The CO2 adsorption capacity of a catalyst is a pivotal aspect determining the photocatalytic performance of CO2 reduction.4,11 Hence, the CO2 adsorption of the synthesized photocatalysts was determined at 298 K to investigate their CO2 adsorption abilities before photocatalytic reduction tests. As depicted in Figure S7, the InVO4/β-AgVO3 composites exhibit improved CO2 adsorption capacity compared with single component, probably owing to a larger BET surface area and comparatively smaller pore size (Table S2 and Figure S4). Besides, it can be noticed that the influence of InVO4 molar ratios on CO2 adsorption capacity is consistent with the SBET of InVO4/β-AgVO3 samples, suggesting the vital role of textural structure for CO2 adsorption capacity. Photocatalytic CO2 reduction performance of the prepared InVO4/β-AgVO3 composites was then evaluated under visible-light irradiation in a gas-solid reaction system. As displayed in Figure 7, CO was found to be the only carbonous product (CO2 + 2e− + 2H+ → CO + H2O)35 towards CO2 photoreduction without any cocatalyst or sacrificial reagent. Figure 7A compares the overall CO evolution rates of pristine InVO4, β-AgVO3 and InVO4/β-AgVO3 composites. In detail, no noticeable CO was detected over pure β-AgVO3, which is mainly ascribed to more positive CB edge than the reduction potential of CO2/CO (−0.53 V vs 14


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NHE)47,56. Pristine InVO4 presented a low CO production rate of 1.12 mol·g-1·h-1, probably owing to rapid recombination of photogenerated electron-hole pairs. For the constructed InVO4/β-AgVO3 nanocomposites, a significant enhancement of CO evolution was achieved, demonstrating that in-situ coupling of InVO4 nanocrystals upon the surface of β-AgVO3 nanoribbons is beneficial for improving the CO2 photoreduction performance. The uniformly distributed of InVO4 nanocrystals provides abundant active sites for CO2 reduction and the intimate interfacial interaction between InVO4 and β-AgVO3 facilitates the transfer and separation of photogenerated charges. The rate of CO evolution over InVO4/β-AgVO3 composites increases with increasing InVO4 content, achieving a maximum of 12.61 mol·g-1·h-1 over the optimal photocatalyst of 20% In-Ag, which is 11 times higher than its InVO4 counterpart. Further increase of InVO4 content leads to a reduction in CO evolution rate, which can be due to the fact that InVO4 nanoparticles are easily aggregated under high content. Besides, to study the influence of InVO4 particle size on the photocatalytic activity, we prepared InVO4 nanoparticles (InVO4-NP) with the average size of 40-50 nm,60 which is comparable with that of loaded InVO4 on the surface of β-AgVO3 (see Figure S8 and additional description in Supporting Information). As presented in Figure S9 of Supporting Information, the CO generation rate of InVO4-NP (3.05 mol·g-1·h-1) is nearly three times that of microspheric InVO4 sample, whereas it is still far below that of 20% In-Ag composite (12.61 mol·g-1·h-1). It suggests that smaller size of InVO4 particles facilitates the improvement of photocatalytic activity, but the enhanced separation of photoinduced charges over InVO4/β-AgVO3 composites plays more dominant role in significantly promoted catalytic activity of CO2 photoreduction. Generally, the enhanced CO2 adsorption may boost photocatalytic performance of CO2 reduction since the adsorption is considered to precede the conversion of CO2.4,11,55 As presented in Table S2, the maximum CO2 adsorption of 20% In-Ag is about 4 times that of pure InVO4, but the CO evolution rate of 20% In-Ag is 11 times higher than that of pristine InVO4 (Figure 7A). This means that higher CO2 adsorption capacity contributes to promoting photocatalytic performance for CO2 reduction, however, the remarkably improved photocatalytic activity over InVO4/-AgVO3 is more dependent on the efficient separation of photoinduced electron/hole pairs, as evidenced from transient photocurrent response, EIS and PL analysis (Figure 6 and S6). 15



Furthermore, considering the possible effects of BET surface area on catalytic activity, the CO evolution rates were also normalized with the SBET of the synthesized photocatalysts (shown in Table S2). It is found that the order of normalized rates is almost the same as the original order and 20% In-Ag sample still exhibits the best photocatalytic performance (Figure S10). Figure 7B indicates the kinetic curves of CO production for pristine InVO4 and InVO4/β-AgVO3 photocatalyst, indicating the yield of CO increases with irradiation time of visible light. Although the undesired H2 from water vapor reduction was also detected (Figure 7C), the CO selectivity is greater than 93% over H2 evolution. Especially, the CO selectivity of the optimal 20% In-Ag is almost 99%, suggesting that InVO4/β-AgVO3 composites possess the high selectivity for photoreduction of CO2 and effectively hinder the side reaction of H2O reduction.

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Figure 7. (A) Average CO evolution rate of pure InVO4 and InVO4/β-AgVO3 photocatalysts, (B) time-dependent CO evolution amount over pristine InVO4 and InVO4/β-AgVO3 samples, (C) H2 evolution and CO selectivity of pristine InVO4 and InVO4/β-AgVO3 samples for 7 h reaction, (D) O2 evolution of β-AgVO3, InVO4 and InVO4/β-AgVO3 photocatalysts under 7 h visible-light irradiation.

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The O2 evolution over InVO4/β-AgVO3 composites was also measured and the results are displayed in Figure 7D. All of InVO4/β-AgVO3 samples showed O2 evolution from H2O oxidation. To offer a better insight of the oxidation cycle and photocatalytic mechanism, the consumed numbers of electrons for CO2 reduction and holes for O2 evolution over 20% In-Ag catalyst were calculated and the ratio was around 1.12:1, indicating that the consumed electrons for CO2 reduction and holes in water oxidation are almost comparable. To verify the evolution of CO from CO2 reduction over InVO4/β-AgVO3 photocatalysts, the following two controlled experiments were conducted: (1) irradiation of catalysts under inert N2 condition and (2) the use of isotopically labeled 13CO2 as the reactants. Under inert N2 condition, no product was detected in the darkness or under visible-light illumination, indicating the carbon source is derived from input CO2. For the 13CO

2

isotope-labeling experiment, Figure 8A presents the mass spectrum of 20% In-Ag after 7 h

photocatalytic reaction, in which the signals of CO and O2 can be clearly observed. More importantly, the m/z of CO is 29 rather than 28, which further confirms the generated CO indeed originates from CO2 photoreduction. The isotopically labeled H218O experiment was also performed, and the signal of 18O2 (m/z = 36) appeared in the mass spectrum (Figure 8B), proving the evolved O2 gas came from the photocatalytic water oxidation. (B)

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Figure 8. (A) Mass spectrum of 13CO2 isotope-labeling experiments for 20% In-Ag sample, (B) Mass spectrum from the 18O-labeling H2O isotope experiments over 20% In-Ag photocatalyst.

The influence of introducing InVO4 nanoparticles on the photoelectrons generation was investigated by ESR spectroscopy using TEMPO as the spin-labeling agent. According to the previous report, blank TEMPO displays the characteristic triplet ESR signal, which would weaken 17



or even disappear when it is reduced by photogenerated electrons of the excited semiconductor.61,62 And thus, the generation of photoinduced energetic electrons can be detected indirectly through the changes in ESR signal intensities of TEMPO. Figure 9A displays the changes of TEMPO signals in different InVO4/β-AgVO3 suspensions. The ESR pattern of TEMPO aqueous solution contains the triplet peak with an intensity ratio of 1:1:1. In the absence of catalysts or visible light, the signal intensity of TEMPO remains almost unchanged. After visible light illumination for 10 min, the intensity of TEMPO signal decreases slightly over pristine InVO4 (ca. 13%), whereas the obvious declines can be noticed in the suspension of InVO4/β-AgVO3 composites. Moreover, the attenuation of TEMPO signal intensity increases firstly and then reduces with the molar fractions of InVO4 component. The greatest decline is obtained over 20% In-Ag sample, implying this composite exhibits highest capacity for producing the photoinduced electrons. It is noteworthy that the effect of introduced InVO4 amounts on the decline of TEMPO signal intensity is in accordance with the CO evolution, as illustrated in Figure 9B. This correlation further demonstrates that in-situ loading an appropriate content of InVO4 nanocrystals upon the β-AgVO3 surface is significant for efficiently generating photoinduced

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electrons and achieving optimal photocatalytic reduction performance.

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Different photocatalysts

Figure 9. (A) ESR spectra of InVO4/β-AgVO3 suspensions containing 0.02mm TEMPO under visible light irradiation, (B) the dependence of photoelectrons generation and CO evolution rate over different InVO4/β-AgVO3 catalysts.

The stability of photocatalysts is also crucial for the composite structure because of the possible leakage of one component from another. As displayed in Figure 10a, 20% In-Ag retains more than 18


Page 19 of 32

87% of the original efficiency for CO production, suggesting the stable heterostructure of InVO4/β-AgVO3 composites after five consecutive runs. Moreover, the fresh and used catalysts show no obvious difference in XRD patterns (Figure 10b), SEM image and XPS spectra (Figure S11 and S12), indicating the stable crystal structure and elemental composition of InVO4/β-AgVO3 during the photocatalytic reaction. The apparent quantum efficiency of CO evolution for 20% In-Ag catalyst is estimated to be 0.49% at 400 nm, which is higher than many reported state-of-the-art catalysts for CO2 photoreduction, as shown in Table S3 of Supporting Information. 100

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Figure 10. (a) Cycling experiments of photocatalytic CO2 reduction over 20% In-Ag catalyst, (b) XRD patterns of 20% In-Ag before and after the photocatalytic reaction.

Photocatalytic Mechanism. To elucidate the mechanism of improved CO2 photoreduction activity over InVO4/β-AgVO3 composites, the individual band structure and energy level of β-AgVO3 and InVO4 obtained from UV-vis DRS and MS plots are depicted in Figure 11. As can be clearly seen from Figure 11, the CB edge of InVO4 is just above the reduction potential of CO2/CO (−0.53 V vs NHE), whereas that of β-AgVO3 is below the reduction potential of CO2. This means that the photoelectrons located on the CB of β-AgVO3 cannot implement the CO evolution. Based on the energy level diagram presented in Figure 11, two charge transfer pathways are possible: traditional type-II transfer and direct Z-scheme migration. With regard to type-II heterojunction, the photoelectrons transfer from one higher CB to another relatively lower CB, and meanwhile the photoholes migrate from one lower VB to another higher VB. The reducibility of photoinduced electrons and oxidizability of photoholes are simultaneously weakened, although the separation of 19



photogenerated electron/hole pairs is improved. For direct Z-scheme mechanism, photoinduced electrons of one semiconductor with lower reduction potential would combine directly with the photoholes of another semiconductor with lower oxidation potential, leaving photoelectrons and photoholes with higher redox capacity to participate in surface catalytic reaction. If the migration behavior of photogenerated charge complies with type-II heterojunction mechanism (Figure S13 of Supporting Information), the photogenerated holes would migrate to the VB of InVO4 from that of β-AgVO3 under visible light illumination. Meanwhile, the excited electrons would migrate from the CB of InVO4 to that of β-AgVO3, where CO2 photoreduction is expected to occur. Nevertheless, as presented in Figure S13, the photoinduced electrons on the CB of β-AgVO3 are incapable of driving CO2 reduction to CO, since the CB edge of β-AgVO3 (0.31 V vs NHE) are more positive than the reduction potential of CO2. Therefore, owing to the matched band-structure, Z-scheme migration is more feasible in the present photocatalytic system. To validate direct Z-scheme mechanism, ESR tests were conducted to detect ·O2– and ·OH species generated in photocatalyst suspensions by using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping agent. As depicted in Figure 12A, the characteristic signals corresponding to DMPO/·O2– adduct could be clearly observed in methanol suspension of InVO4 and 20% In-Ag, and the signal intensity of DMPO/·O2– for 20% In-Ag composite is much higher than that of pristine InVO4.63,64 In contrast, no obvious signal of DMPO/·O2– is detected in the case of pristine β-AgVO3. The production of ·OH is also detected and the characteristic signals of DMPO/·OH in aqueous suspensions of β-AgVO3, InVO4, and 20% In-Ag are shown in Figure 12B. The ESR signals with relative intensity of 1:2:2:1 belonging to DMPO/·OH are detected for β-AgVO3 and 20% In-Ag samples, whereas no ESR signal was observed for pristine InVO4. Meanwhile, the ESR signal of DMPO/·OH in aqueous suspension of 20% In-Ag was also higher than that in pure β-AgVO3. Supposing the traditional type-II heterojunction is valid (Figure S13, Supporting Information), the photoelectrons and holes would accumulate at the CB position of β-AgVO3 and the VB position of InVO4, respectively. Owing to more positive CB edge of β-AgVO3 (0.31 V) than the reduction potential of O2/·O2– (−0.33 V vs NHE),65 the generation of ·O2– is forbidden thermodynamically. Similarly, the VB edge of InVO4 (1.70 V vs NHE) is more negative than oxidation potential of H2O/·OH or OH−/·OH (1.99 V or 2.34 V vs NHE), and 20


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thus no ·OH could be generated. Nonetheless, ·O2– and ·OH species are detected in the suspension of 20% In-Ag composite. These experimental results are contradicted with the abovementioned type-II migration mechanism.

Figure 11. Schematic illustration of energy band alignment of β-AgVO3 and InVO4, and direct Z-scheme migration of InVO4/β-AgVO3 composites.

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Figure 12. (A) ESR signals of DMPO/·O2– after 5 min of visible-light illumination in methanol dispersion of β-AgVO3, InVO4 and 20% In-Ag, (B) ESR signals of DMPO/·OH in aqueous suspension of β-AgVO3, InVO4 and 20% In-Ag samples.

Contrarily, according to direct Z-scheme mechanism, photoexcited electrons at the CB of β-AgVO3 readily migrate to the interface through the internal electric field and recombine with photoinduced holes at the VB of InVO4, which results in effective separation of photogenerated charge. Moreover, photogenerated electrons and holes would gather on more negative CB position of InVO4 and more positive VB position of β-AgVO3, respectively. And thus, the 21



InVO4/β-AgVO3 composites simultaneously possess strong reducibility of photoelectrons at the CB of InVO4 and strong oxidizability of photoholes at the VB of β-AgVO3, compared with the single component. Consequently, the increased intensity of ·O2– and ·OH signals can be observed for as-prepared 20% In-Ag composite. Based on the band alignment of InVO4/β-AgVO3 and the ESR results, a direct Z-scheme mechanism over the synthesized InVO4/β-AgVO3 composites can be proposed, as depicted in Figure 11. To further confirm the direct Z-scheme mechanism, in-situ XPS measurements under visible-light irradiation were performed. It can be observed from Figure 4b, the binding energy of Ag 3d in 20% In-Ag composite increases 0.2 eV under visible irradiation, compared to the value measured in the darkness. Positive shift of Ag 3d binding energy indicates a decrease in its electron density.51,52 Meanwhile, under light irradiation, the binding energy of In 3d (Figure 4c) in 20% In-Ag sample decreases 0.3 eV, compared to the value detected in the darkness. This negative shift suggests an increase in electron density on InVO4 component. The shifts of binding energy obtained from in-situ XPS measurements demonstrate the electrons migration from β-AgVO3 to InVO4 under light illumination,51,52 which is well consistent with direct Z-scheme mechanism. Based on the above results and discussion, direct Z-scheme migration mechanism is presented to elucidate the improved catalytic performance for CO2 photoreduction over InVO4/β-AgVO3 composites. According to direct Z-scheme mechanism (Figure 11), photogenerated electrons from the CB of β-AgVO3 transfer to the interface of composites and combine with photogenerated holes from the VB of InVO4, which dramatically inhibits the prevailing recombination of charge carriers in InVO4. Direct Z-scheme migration makes photoholes accumulate at the VB of β-AgVO3 and photoelectrons at the CB of InVO4. That is, high reduction potential of photoelectrons is greatly preserved, which facilitates CO2 reduction on InVO4 surface. Coupling of InVO4 with β-AgVO3 results in the formation of direct Z-scheme heterostructure, which facilitates the separation of photoinduced charge carriers in both β-AgVO3 and InVO4. And meanwhile, the reducibility of photogenerated electrons at the CB of InVO4 is highly retained, leading to 11-fold enhancement in photocatalytic activity for CO2 reduction.

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4. Conclusions In summary, direct Z-scheme InVO4/β-AgVO3 nanocomposites were constructed by a facile hydrothermal process and in-situ cation exchange strategy. β-AgVO3 nanoribbons serve as not only the V source, but also a promising support for in-situ growing InVO4 nanocrystals. The as-synthesized composites exhibited excellent photocatalytic activity toward selective reduction CO2 to CO without any sacrifice agent or cocatalyst. Coupling of β-AgVO3 with InVO4 nanocrystals can notably improve catalytic activities for CO2 photoreduction and the optimum 20% In-Ag composite exhibits a CO generation rate of 12.61 μmol·g−1·h−1, which is 11 times higher than pristine InVO4. Moreover, the CO selectivity of as-constructed composites is above 93%. The introduction of β-AgVO3 facilitates the uniform distribution of InVO4 nanocrystals and the formation of close interfacial contact. The greatly improved photocatalytic activity of CO2 reduction is ascribed to the formation of direct Z-scheme heterostructure InVO4/β-AgVO3, which can be certified by in situ XPS measurement and radical generation experiment. Direct Z-scheme heterostructure not only favors the separation of photoinduced electrons and holes, but also preserves the reducibility of photoelectrons in the CB of InVO4. This study offers a novel strategy for designing direct Z-scheme type visible-light catalysts to promote catalytic performance of artificial CO2 photoreduction.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx. SEM image of pristine β-AgVO3 and InVO4, molar percentages of Ag, In and V in InVO4/β-AgVO3 composites, EDS spectrum of 20% In-Ag sample, N2 adsorption-desorption and CO2 adsorption isotherms of the synthesized samples, XPS valence-band spectra of β-AgVO3 and InVO4, PLs spectra of -AgVO3, InVO4 and 20% In-Ag composite, XRD pattern and SEM image of InVO4 nanoparticles, SBET normalized CO evolution rate, SEM image and XPS spectra of 20% In-Ag before and after photocatalytic CO2 reduction, the performance comparison of 20% In-Ag with other reported photocatalysts, schematic diagram of type-II InVO4/β-AgVO3 heterojunction 23



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(PDF).

Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Juan Yang: 0000-0002-2468-4502 Anchao Zhang: 0000-0002-0704-6736 The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors of this work are deeply grateful for these financial supports by National Natural Science Foundation of China (21307027, 51676064, U1804128 and 21876154), the Scientific and Technological Project of Henan Province (172102310725) and the Foundation for Distinguished Young Scientists (J2016-4) of Henan Polytechnic University.

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Photocatalysts

for

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Î²-AgVO3 Nanocomposite as a Direct Z-Scheme Photocatalyst

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