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Fabrication of Heterogeneous-phase Solid-solution Promoting Band Structure and Charge Separation for Enhancing Photocatalytic CO2 Reduction: A Case of ZnxCa1-xIn2S4 Chao Zeng, Hongwei Huang, Tierui Zhang, Fan Dong, Yihe Zhang, and Yingmo Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08767 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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ACS Applied Materials & Interfaces

Fabrication of Heterogeneous-phase Solid-solution Promoting Band Structure and Charge Separation for Enhancing Photocatalytic CO2 Reduction: A Case of ZnxCa1-xIn2S4 Chao Zeng,† Hongwei Huang,*,† Tierui Zhang,‡ Fan Dong,§ Yihe Zhang,† Yingmo Hu*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid

Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡

Key Laboratory of Photochemical Conversion and Optoelectronic Materials,

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China §

Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College

of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing, 400067, China

E-mail: [email protected], [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Photocatalytic CO2 reduction into solar fuels illustrates huge charm for simultaneously settling energy and environmental issues. The photo-reduction ability of a semiconductor is closely correlated to its conduction band (CB) position. Homogeneous-phase

solid-solution

with

same

crystal

system

always

has

monotonously changed CB position, and the high CB level has to be sacrificed to achieve a benign photoabsorption. Herein, we report the fabrication of heterogeneous-phase solid-solution ZnXCa1-XIn2S4 between trigonal ZnIn2S4 and cubic CaIn2S4. The ZnXCa1-XIn2S4 solid solutions with orderly tuned photoresponsive range from 540 nm to 640 nm all present more negative CB level and highly enhanced charge separation efficiency. Profiting from these merits, all these ZnXCa1-XIn2S4 solid solutions exhibit remarkably strengthened photocatalytic CO2 reduction performance under visible light (λ > 420 nm) irradiation. Zn0.4Ca0.6In2S4 bearing the most negative CB position and highest charge separation efficiency casts the optimal photocatalytic CH4 and CO evolution rates, which reaches 16.7 and 6.8 times than that of ZnIn2S4, and 7.2 and 3.9 times higher than that of CaIn2S4, respectively. To verify the crucial role of heterogeneous-phase solid-solution in promoting band structure and photocatalytic

performance,

another

heterogeneous-phase

solid-solution

ZnXCd1-XIn2S4 has been synthesized. It also displays up-shifted CB level and promoted charge separation. This work may provide new perspective into development of efficient visible-light driven photocatalyst for CO2 reduction and other photo-reduction reaction. Keywords: ZnXCa1-XIn2S4; photocatalytic CO2 reduction; band structure; crystal 2


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system; charge separation 1. INTRODUCTION With the massive combustion of fossil fuel, the atmospheric carbon dioxide (CO2) concentration sharply increases, which results in the greenhouse effect and seriously destroys the ecological balance. At the same time, shortage of fossil resources also restrains industrial development. Solar photocatalytic conversion of CO2 to valuable chemicals is of great potential, which not only shrinks the content of atmospheric CO2 but also partly supplies industrially beneficial compounds (e.g. CO, CH3OH, CH4, HCOOH), thus attracting enormous research interests recently.1, 2 Metal-sulfide photocatalysts,3-5 for example, ZnS,6 CdS,7 ZnIn2S4,8 CdIn2S4,9 CaIn2S4,10 Cu2ZnSnS4

11

and ZnS-In2S3-CuS,12 have long been investigated as

superior semiconductor photocatalysts for water splitting into hydrogen (H2) under visible light illumination. To further improve the photocatalytic performance, construction of solid-solution with continuously adjustable content is regarded as an efficient approach through optimizing the light absorption and electronic structure. For example, Chen et al reported the synthesis of AgGa1-xInxS2 solid solutions, which exhibited greatly enhanced photocatalytic activity in photocatalytic H2 production.13 On the other hand, other types of materials, e.g. β-AgAl1-xGaxO2 solid-solution photocatalysts, reported by Ye et al, also exhibited much higher photocatalytic performance in iso-propanol (IPA) photodegradation under visible light irradiation.14 As is well known, the photo-reduction ability of a semiconductor is closely associated with its conduction band (CB) position, and a higher CB level could provide a larger reduction driving force.15 Nevertheless, the CB level of the above-mentioned solid solutions monotonically becomes more negative or positive with adjusting the x value, and thus the CB position of solid solution located between that of the two pristine 3



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materials. Namely, although the photocatalyst gains enhanced light absorption, the reduction capacity of the photogenerated electrons was weakened. So, a question comes into being that can we design a solid solution with more negative CB position than the two bare materials? Considering that similar crystal structure always gives rise to similar band structure, fabrication of a solid solution between the two pristine compounds with different crystal structures may meet the above requirement. The absorption edge of ZnIn2S4 and CaIn2S4 are reported as 480 ~ 570 nm and 580 ~ 650 nm, respectively.16-24 According to the previous articles, the crystal system of ZnIn2S4 can be hexagonal, cubic, trigonal, and CaIn2S4 is only cubic. Thereby, ZnXCa1-XIn2S4 solid solutions may be the appropriate model that we need. It is very interesting and we would investigate this issue. In this work, we prepare a series of flower-like heterogeneous-phase solid solutions ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) by a facile one-pot hydrothermal method. The crystal system of ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, and 0.4) is trigonal, while ZnXCa1-XIn2S4 (x = 0.2 and 0) is cubic. Noticeably, the CB level of the solid solutions orderly up-shifts and then decreases when x ranges from 1 to 0 in ZnXCa1-XIn2S4. All the solid solutions show enormously enhanced photocatalytic performance for CO2 reduction in comparison with ZnIn2S4 and CaIn2S4 with illumination of visible light (λ > 420 nm), and Zn0.4Ca0.6In2S4 that possesses the most negative CB position and highest charge separation efficiency shows the best photocatalytic CO2 reduction activity into CH4 and CO. Moreover, we also synthesize another heterogeneous-phase solid solutions ZnXCd1-XIn2S4, and discover the similar phenomena occurred on band structure

and

charge

separation,

which

confirms

heterogeneous-phase solid solution takes. 2. EXPERIMENTAL SECTION 4


the

positive

role

that

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2.1. Preparation of the photocatalyst. All the reagents, which were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), were of analytical grade and used without further purification. The flower-like solid solutions ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) were synthesized via a facile hydrothermal method. In a typical synthesis, ZnCl2, Ca(NO3)2▪4H2O, InCl2▪4H2O, and thioacetamide (TAA) were dissolved in 25 mL deionized water and 5 mL glycol. After being drastically stirred for 30 min at room temperature, the resulting heterogeneous solution was transferred into a 50ml Teflon-lined stainless steel autoclave and maintained at 120 °C for 12 h in an oven. After natural cooling, the products were collected by centrifugation, rinsed repeatedly with distilled water and ethanol, and then dried at 60 ℃ for 10 h. The as-obtained solid solutions were labeled as ZnIn2S4, Zn0.8Ca0.2In2S4, Zn0.6Ca0.4In2S4, Zn0.4Ca0.6In2S4, Zn0.2Ca0.8In2S4, and CaIn2S4 when the molar ratio of Zn/Ca was 1:0, 0.8:0.2, 0.6:0.4, 0.4:0.6, 0.2:0.8, and 0:1, which were in line with the value of x = 1, 0.8, 0.6, 0.4, 0.2, and 0 in ZnXCa1-XIn2S4, respectively. The schematic illustration of the preparation process for the flower-like ZnXCa1-XIn2S4 solid solutions was displayed in Scheme. 1. In addition, the ZnXCd1-XIn2S4 solid solutions were prepared by the same procedure by substituting Ca(NO3)2▪4H2O with Cd(NO3)2▪4H2O. 2.2. Characterization.

X-ray powder diffraction (XRD) was taken on a Bruker D8

focus with Cu Kα radiation (40 kV/40 mA) to verify the crystal structure of samples. X-ray photoelectron spectroscopy (XPS) was performed on An ESCALAB 250xi (ThermoFsher, England) electron spectrometer to investigate the surface chemical composition. Element content was mearsured on an inductively coupled plasma mass spectrometry (ICP-MS) (ICAP-MS--QC, Thermo, America). The morphologies and microstructure of the samples were recorded on a field emission scanning electron 5



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microscope (FE-SEM, Hitachi-S4800) and a transmission electron microscope (TEM, JEM2100), respectively. UV-vis diffuse reflectance spectra (DRS) were conducted on a Varian Cary 5000 UV-vis spectrophotometer to investigate optic property. The Brunauer–Emmett–Teller (BET) specific surface areas and pore diameter distribution of samples were analyzed by nitrogen adsorption-desorption (Micromeritics ASAP 2460, USA). All the measurements were measured at room temperature. 2.3. Photocatalytic activity. The photocatalytic activities of as-obtained samples were evaluated by the photocatalytic CO2 reduction experiment, which was carried out on a PLS-SXE300 Labsolar-IIIAG closed gas system (Perfectlight, China) with a reaction volume of 500 mL. 100 mg photocatalyst was evenly dispersed on a watch glass with an area of about 28 cm2. 1.7 g NaHCO3 was put into the reaction cell, and then the reaction was vacuum-treated. Prior to light irradiation, H2SO4 (4 M, 15 mL) was injected into the cell to achieve CO2 gas (1 atm) via the reaction between NaHCO3 and H2SO4. At given time intervals, 1 mL resulting gas was collected and then qualitatively analyzed by a GC9790II gas chromatograph (Zhejiang Fuli Analytical Instrument Co., Ltd., China) equipped with a GDX502 flame ionization detector and a TDX-01 thermal conductivity detector. The outlet gases were determined to be CO, and CH4. The production yield was calculated based on a calibration curve. Blank test was conducted with purging argon and without adding NaHCO3 in the reaction cell. To evaluate the stability, the photocatalytic performance Zn0.4Ca0.6In2S4 sample taken as a model was reevaluated in the light of the aforementioned procedure. All the above photocatalytic experiments were conducted triplicates. 2.4. Photoelectrochemical measurement. The photoelectrochemical measurements, including linear sweep voltammetry, photocurrent response and Mott-Schottky curve, were conducted on an electrochemical analyzer (CHI660E, Shanghai) equipped with a 6


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standard three-electrode system. Ag/AgCl (saturated KCl) and platinum (Pt) wire were used as reference electrode and the counter electrode, respectively. A 300W Xe arc lamp (with a UV light cut off filter (λ≥420 nm)) was taken as light source. The electrolyte solution was 0.35 M Na2SO3 and 0.25 M Na2S aqueous solution. To prepare the working electrodes, 10 mg of sample was dispersed in 800 µl ethanol. Subsequently, the obtained suspension liquid was drop-coated on a ITO glass with size of 20mm×40mm, and dried at 60 ℃ for 10 h to be dried in the air. 3. RESULTS AND DISCUSSION 3.1. Crystal structure evolution, microstructure and composition. The XRD patterns of ZnXCa1-XIn2S4 solid solutions are exhibited in Fig. 1a-c. As shown in Fig. 1a, the main peaks of ZnIn2S4 at 21.2°, 27.3°, 30.5°, 39.7°, 47.0°, 52.2°, 55.4°, and 75.9°, correspond well to the diffractions of the (006), (102), (104), (108), (110), (116), (022), and (212) planes of trigonal ZnIn2S4 (JCPDS﹟65-2023), respectively. Besides, we also carefully compared the XRD pattern of ZnIn2S4 with the standard peaks of three

types

of

ZnIn2S4

crystal-phase

(cubic

JCPDS#48-1778,

trigonal

JCPDS#65-2023, hexagonal JCPDS#72-0773) (Fig. S1), further corroborating the crystal system of ZnIn2S4 is trigonal. With regard to CaIn2S4, as shown in Fig. 1c, its main diffraction peaks at 22.9°, 27.1°, 28.3°, 33.0°, 41.0°, 43.4°, 47.6°, 56.1°, 59.3°, and 66.5° are ascribed to the (220), (311), (222), (400), (422), (511), (440), (533), (444), and (731) planes of cubic CaIn2S4 (JCPDS﹟31-0272), respectively. No other impurity peaks can be detected, evidencing the obtainment of pure-phase ZnIn2S4 and CaIn2S4. Notably, the crystal phase of ZnIn2S4 and CaIn2S4 is different, motivating our interest in the crystal structure evolution of their solid solutions. As shown in Fig. 1b, with decreasing the X value in ZnXCa1-XIn2S4 solid solutions, the peak at 27.3° ((102) plane of ZnIn2S4) gradually shifts to a smaller 2θ degree (27.1° for CaIn2S4). 7



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According to Bragg equation, a smaller 2θ means a larger lattice distance. As Ca2+ has an obviously bigger radius that of Zn2+, it demonstrates that the Ca2+ ions have been introduced into the unit cell of ZnIn2S4. In addition, a peak at 33.0° was observed for Zn0.2Ca0.8In2S4, and CaIn2S4, but no peak around 33.0° appears for ZnIn2S4, Zn0.8Ca0.2In2S4, Zn0.6Ca0.4In2S4, and Zn0.4Ca0.6In2S4. It can be deduced that the crystal system of Zn0.2Ca0.8In2S4 and CaIn2S4 is cubic, and ZnIn2S4, Zn0.8Ca0.2In2S4, Zn0.6Ca0.4In2S4, and Zn0.4Ca0.6In2S4 samples are trigonal. The unit cell of ZnIn2S4, Zn0.4Ca0.6In2S4, CaIn2S4 are portrayed in Fig. 1i-k. Obviously, trigonal ZnIn2S4 and Zn0.4Ca0.6In2S4 have layered crystal structures, which are composed of In-S chains and interleaved Zn-S or Zn/Ca-S slices. In contrast, the cubic CaIn2S4 shows a distinct three-dimensional framework. It is evident that ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) solid solutions with different crystallization phase are successfully obtained by the current one-pot hydrothermal method. The composition and chemical states of different elements for ZnIn2S4, Zn0.4Ca0.6In2S4, and CaIn2S4 samples are analyzed by X-ray photoelectron spectroscopy (XPS). As exhibited in Fig. 1d-h, the survey spectra (Fig. 1d) reveal the presence of Zn, In, S elements in ZnIn2S4, Ca, In, S elements in CaIn2S4, and Zn, Ca, In, S elements in the Zn0.4Ca0.6In2S4 sample, respectively. The C 1s peak locating at 283.7 eV results from the adventitious carbon of the XPS instrument. O 1s peak at 530.8 eV can be ascribed to O2 and H2O adsorbed on the surface of sample. The high resolution XPS spectra for Zn is depicted in Fig. 1e. For ZnIn2S4, two peaks at 1045.5 and 1022.5 eV are attributed to Zn2+ 2p1/2 and Zn2+ 2p3/2, and the two peaks at 1045.3 and 1022.4 eV are attributed to Zn2+ ions of Zn0.4Ca0.6In2S4. The binding energy of Ca2+ ions for Zn0.4Ca0.6In2S4 and CaIn2S4 is 347.6 and 347.5 eV, respectively (Fig. 1f). Fig. 1g displays the high-resolution XPS spectra for In 3d. The binding energies of 8


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In3+ ions are 452.6 and 444.9 eV for ZnIn2S4, 452.4 and 444.8 eV for Zn0.4Ca0.6In2S4, and 452.4 and 444.7 eV for CaIn2S4, which can be attributed to In3+ 3d3/2 and In3+ 3d5/2, respectively. The binding energies of S2- 2p1/2 and S2- 2p3/2 for ZnIn2S4, Zn0.4Ca0.6In2S4 and CaIn2S4 are 162.9 and 161.7 eV, 162.6 and 161.5 eV, and 162.6 and 164.4 eV, respectively (Fig. 1h). The electronegativity of Zn2+ and Ca2+ ions are 1.65 and 1.00, respectively. It is always believed that the increasement of the electron density of an atom would induce the shrinkage of the binding energies.21, 25 With the introduction of Ca2+, the binding energies of Zn, In, and S atoms all decline, supporting that the Ca2+ ions do enter the lattice cell of ZnIn2S4. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and EDX mapping are conducted to investigate the morphology and microstructure of as-synthesized samples (Fig. 2a-g). It can be found from Fig. 2a-c and Fig. S2 that the ZnIn2S4, Zn0.4Ca0.6In2S4, and CaIn2S4 are all composed of uniform flower-like microspheres that consist of a large quantity of nanosheets. The introduction of Ca2+ in the crystal lattice of ZnIn2S4 enlarges the diameter of nanosheets and the size of microspheres. TEM images (Fig. 2d-e) also evidence the inerratic flower-like morphology of Zn0.4Ca0.6In2S4. As revealed by the HRTEM image of Zn0.4Ca0.6In2S4 (Fig. 2f), the lattice fringes with interval of 0.330 nm can be indexed to the (110) plane of Zn0.4Ca0.6In2S4. EDX is conducted to verify the element composition of Zn0.4Ca0.6In2S4 (Fig. S3), which confirms the elemental constitution of Zn, Ca, In and S atoms. The signal of Pt and Al are assigned to the process of gilding and the carrier of aluminum foil. Moreover, the molar ratio of Zn2+ and Ca2+ in the ZnXCa1-XIn2S4 (x = 0.8, 0.6, 0.4, 0.2) samples is estimated to be almost consistent to the theoretical value from the results of ICP-MS (Fig. S4). The TEM-EDX elemental mapping (Fig. 2g) reveals that the Zn, Ca, In and S elements 9



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have a highly homogeneous distribution throughout the Zn0.4Ca0.6In2S4 microsphere. To explore the specific surface area and porous nature of ZnXCa1-XIn2S4 solid solutions, Brunauer–Emmett–Teller and Barrett–Joyner–Halenda (BET/BJH) methods are employed. All solid-solution samples display a type IV isotherm with a H3 hysteresis loop, suggesting the existence of mesoporous structure (Fig. 2h-i),26,

27

which should originate from the close piling of numerous nanosheets. The BET surface areas (SBET) of the ZnXCa1-XIn2S4 solid solutions (x = 1 - 0) were determined to be 56.3, 64.3, 54.8, 54.5, 84.2, and 81.8 m2/g, respectively (Table 1). The SBET of these solid solutions has not big difference, which means the specific surface area is not responsible for the enhanced photocatalytic activity. 3.2. Photocatalytic performance for CO2 reduction into CO and CH4 under visible light illumination (λ > 420 nm). The as-obtained ZnXCa1-XIn2S4 solid solutions are employed in CO2 reduction experiment under the illumination of visible light (λ > 420 nm), and the results are shown in Fig. 3a-d. As shown in Fig. 3a and c, the photocatalytic activity of all the ZnXCa1-XIn2S4 (x=0.8, 0.6, 0.4, and 0.2) solid solutions are better than ZnIn2S4 and CaIn2S4 in different degrees. The apparent reaction rate constants of CH4 or CO evolution in photoreducing CO2 are displayed in Fig. 3b and Fig. 3d, respectively. With decreasing the x value, the conversion rate constant of ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) solid solutions first ascends, achieving the maximum when x = 0.4, and then declines. For the reductant product CH4, the conversion rate constant of Zn0.4Ca0.6In2S4 is 0.877 umol/g/h, which reaches 16.7 and 7.2 times higher than that of ZnIn2S4 and CaIn2S4 samples. For the reductant product CO, the conversion rate constant of Zn0.4Ca0.6In2S4 is 0.224 umol/g/h, achieving 6.8 and 3.9 times higher than that of ZnIn2S4 and CaIn2S4 samples. The photocatalysis performance of ZnXCa1-XIn2S4 solid solutions is remarkably promoted 10


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compared with pristine ZnIn2S4 and CaIn2S4 samples. It is also worth noting that no CO and CH4 can be detected by purging argon and without adding NaHCO3 in the reaction cell. It indicates that CO and CH4 are truly generated from photocatalytic CO2 reduction of ZnXCa1-XIn2S4, other than any organic impurity. The stability of the as-synthesized photocatalysts, which is another important factor for practical application besides photocatalytic performance, was inspected by performing three-run cycling experiments of CO2 reduction. As shown in Fig. 3e, there is not even a slight shrinkage of photocatalytic activity after three successive cycles. In addition, the XRD patterns and SEM images of Zn0.4Ca0.6In2S4 sample before and after photoreaction also show no obvious difference (Fig. 3f and Fig. S5). These results confirm the superior stability of Zn0.4Ca0.6In2S4 photocatalyst, boding for its promising practical applications. 3.3. Continuously adjustable Light absorption. The optical absorption property, which takes a decisive role for photocatalytic performance of semiconductors, is analyzed by diffuse reflection spectroscopy (DRS). Fig. 4a-b depicts the DRS spectra and band gap for ZnXCa1-XIn2S4 solid solutions. With decreasing the x value from 1 to 0, the absorption edge of ZnXCa1-XIn2S4 solid solutions monotonically red-shifts from 546 nm to 638 nm, so the photoresponse of solid solutions in visible region is continuously strengthened compared to the pristine ZnIn2S4 (Fig. 4a). Particularly, the diffuse reflection spectra are all very steep, revealing that the visible-light absorption is ascribed to a band transition, instead of the transition from impurity levels.28 The Kubelka-Munk equation:29 αhv = A(αhv-Eg)n (1), is employed to calculate the band gap energy of the as-obtained solid solutions. Here h, α, v, A, and Eg represents the Planck’s constant, optical absorption coefficient, photon frequency, a constant, and photonic energy band gap, respectively. Both ZnIn2S4 and CaIn2S4 are 11



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direct-transition allowed semiconductors,23, 30 so the value of n is 2. The band gaps of ZnXCa1-XIn2S4 solid solutions (x = 1, 0.8, 0.6, 0.4, 0.2, 0) are estimated to be 2.46, 2.43, 2.35, 2.26, 2.18 and 2.04 eV, respectively (Fig. 4b, Fig. S6, and Table 1), which are consistent with the color change of sample suspensions from yellow to brownish red (Fig. S7). The DRS results disclose that the as-prepared samples are ZnXCa1-XIn2S4 solid solutions other than a simple physical mixture of ZnIn2S4 and CaIn2S4, and meanwhile the light absorption of solid solutions are conspicuously promoted compared with pristine ZnIn2S4. 3.4. Investigation on photocatalytic activity enhancement mechanism. The band structure would greatly impact the photocatalytic activity of semiconductor phototcatalyst.31 To explore the band structure of ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) solid solutions, Mott−Schottky (M-S) plots are conducted at frequency of 2000 and 3000 Hz. The flat-band potential values (Efb) can be determined by the following Mott-Schottky equation.32 1

C2

=

2  κT  E − E fb − B q εε 0N D 

(2)

  

Where ε are ε0 the dielectric constants of free space and the film electrode, and ND, C, E, T, κb, and q represents the donor density, space charge capacitance, applied potential, temperature, Boltzmann’s constant, and electronic charge, respectively. As shown in Fig. 4c-h, the intersection points (the flat potential (Efb)) in the Mott-Schottky plots for each photocatalyst are the same under different frequencies applied. The Efb of ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) solid solutions is determined to be -1.56, -1.58, -1.63, -1.70, -1.67 and -1.60 V versus Ag/AgCl, which is equal to -1.36, -1.38, -1.43, -1.50, -1.47 and -1.40 V versus the normal hydrogen electrode (NHE), respectively. The positive slop of Mott–Schottky curves manifests that ZnXCa1-XIn2S4 solid solutions are n-type semiconductors. It is generally believed 12


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for many n-type semiconductors that Efb is about 0.1 V more negative than their conduction band position (ECB).33 Thereby, the CB position for ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) can be approximately estimated as -1.46, -1.48, -1.53, -1.60, -1.57 and -1.50 V, and their valence band potential (EVB) are correspondingly determined to be 1.00, 0.95, 0.82, 0.66, 0.61 and 0.54 V, respectively. In addition, XPS valence band spectra are employed to further investigate the band edges of ZnIn2S4, Zn0.4Ca0.6In2S4, CaIn2S4 samples, the valence band potential of which is from positive to negative, as shown in Fig. S8. Although the values from XPS valence band spectra are slight different from the above calculated values, the trend is the same. The schematic band structures for the ZnXCa1-XIn2S4 solid solutions are portrayed in Fig. 4k. It is interesting that the CB position shows a trend of first up-shift and then down-shift, achieving the most negative potential of -1.60 V at Zn0.4Ca0.6In2S4. The CO2/CO and CO2/CH4 redox potentials are separately -0.53 and -0.24 eV,34 which is more positive than the CB level of ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) solid solutions, so the CO2 can be reduced by the electrons on the CB of ZnXCa1-XIn2S4. The most negative CB position of Zn0.4Ca0.6In2S4 should be responsible for its best photocatalytic activity in photocatalytic CO2 reduction among the series of ZnXCa1-XIn2S4 solid solutions. The photocatalytic performance of semiconductor phototcatalyst is also closely related to the recombination rate of photo-induced charge carriers. Linear sweep voltammetry, and transient photocurrent responses under visible light (λ > 420 nm) illumination for ZnXCa1-XIn2S4 solid solutions are exhibited in Fig. 4i-j. For transient photocurrent response, an enhanced current density clarifies an enhancement on separation efficiency of photogenerated holes and electrons in photocatalysts.35 As displayed in Fig. 4j, the photocurrent density of ZnXCa1-XIn2S4 solid solutions intensifies firstly and then declines with decreasing x value. The strongest 13



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photocurrent response was realized for Zn0.4Ca0.6In2S4 sample, which is about 1.5 and 2.0 times higher than that of ZnIn2S4 and CaIn2S4. Linear sweep voltammetry (Fig. 4i) also proves the higher photocurrent of Zn0.4Ca0.6In2S4 sample compared with ZnIn2S4 and CaIn2S4 at different voltages applied. The enhanced photocurrent density of solid solution, particularly Zn0.4Ca0.6In2S4, may be due to formation of some defects caused by introduction of Ca2+ ions into the lattice cell of ZnIn2S4, in view of the large difference between the ionic radius of Zn2+ and Ca2+. The as-formed defect could capture photogenerated electrons or holes, resulting in high separation of electrons and holes. Both the results from transient photocurrent response and linear sweep voltammetry elucidate the promoted separation efficiency of photogenerated charges for ZnXCa1-XIn2S4 (x = 0.8, x = 0.6, x = 0.4, x = 0.2) compared to ZnIn2S4 and CaIn2S4, which is also accounting for their enhanced photocatalytic activities. 3.5. Verification by another heterogeneous-phase solid solutions ZnXCd1-XIn2S4. Moreover, to confirm that the band structure promotion (e.g. up-shift of CB) can be induced by formation of heterogeneous-phase solid solutions, we also prepare another series of solid solutions ZnXCd1-XIn2S4 by a similar one-pot hydrothermal route. As exhibited in Fig. 5a, the crystal system of as-synthesized CdIn2S4 is cubic, which is different from the trigonal ZnIn2S4. There is a tiny peak at 25.2° in the CdIn2S4 XRD pattern, which does not exist in the standard data of cubic CdIn2S4 (ICSD #108215). This phenomenon has been reported in previous references,36, 37 so the as-prepared CdIn2S4 still can be determined as pure-phase sample. It can be deduced from the XRD patterns that the ZnXCd1-XIn2S4 solid solutions are successfully fabricated, other than a simple physical mixture of ZnIn2S4 and CdIn2S4 (Fig. 5b). For ZnXCd1-XIn2S4 solid solutions, the absorption edge continuously red-shifts from 540 nm to 640 nm with decreasing the x value, and their energy band gap (Eg) gradually narrowed, 14


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which are in accordance with the color change of sample suspensions from pale yellow to orange-yellow (Fig. 5c-d, and Table 2). Interestingly, Mott–Schottky curves (Fig. 5e-f) demonstrated that the CB position of Zn0.6Cd0.4In2S4 (-1.59 eV) is also more negative than that of ZnIn2S4 (-1.46 eV) and CdIn2S4 (-1.51 eV), as depicted in their schematic band structures (Fig. 5g). The band structure evolution is similar

to

that

occurred

for

ZnXCa1-XIn2S4 solid

solutions.

Additionally,

Zn0.6Cd0.4In2S4 also produces higher photocurrent density than that of ZnIn2S4 and CdIn2S4 (Fig. 5h). Therefore, it can be safely concluded that the band structure and charge separation can be highly promoted by fabrication of heterogeneous-phase solid solutions. 4. CONCLUSION In summary, heterogeneous-phase solid-solutions ZnXCa1-XIn2S4 (x = 1, 0.8, 0.6, 0.4, 0.2, 0) between trigonal ZnIn2S4 and cubic CaIn2S4 are prepared by hydrothermal approach for the first time. Compared to ZnIn2S4 and CaIn2S4, the ZnXCa1-XIn2S4 solid-solutions not only have more negative conduction band level affording powerful photo-reduction driving force, but also show much enhanced charge separation. The photocatalytic experiments reveal that all the ZnXCa1-XIn2S4 solid-solutions present enhanced photoactivity for CO2 reduction. The optimal performance is achieved for Zn0.4Ca0.6In2S4 with CH4 and CO production rates of roughly 4-17 fold enhancements in contrast to ZnIn2S4 and CaIn2S4. Additionally, another heterogeneous-phase solid-solutions ZnXCd1-XIn2S4 are also synthesized, which verifies its positive role in promoting band structure and charge separation. The current study shed new light on advancing the photocatalytic CO2 reduction performance by an alternative strategy.

■ASSOCIATED CONTENT 15



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Supporting Information XRD patterns of ZnIn2S4 and the standard peaks of three types of ZnIn2S4 crystal-phase. SEM and EDX images of Zn0.4Ca0.6In2S4 before and after photoreaction. ICP-MS results for the ZnXCa1-XIn2S4 solid solutions. Band gap and Digital photos of suspensions for the ZnXCa1-XIn2S4 solid solutions. XPS valence band spectra of ZnIn2S4, Zn0.4Ca0.6In2S4, CaIn2S4 samples. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant No. 51672258 and No. 51572246), the Fundamental Research Funds for the Central Universities (2652015296). REFERENCE (1) Chang, X. X.; Wang, T.; Gong, J. L. CO2 Photo-reduction: Insights into CO2 Activation and Reaction on Surfaces of Photocatalysts. Energy Environ. Sci. 2016, 9, 2177-2196. (2) Liu, H. M.; Li, M.; Dao, T. D.; Liu, Y. Y.; Zhou, W.; Liu, L. Q.; Meng, X. G.; Nagao, T.; Ye, J. H. Design of PdAu Alloy Plasmonic Nanoparticles for Improved Catalytic Performance in CO2 Reduction with Visible Light Irradiation. Nano Energy. 2016, 26, 398–404. (3) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253–278. (4) Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A. Z-Schematic Water Splitting into H2 and O2 using Metal Sulfide as a Hydrogen-Evolving Photocatalyst and Reduced Graphene Oxide as a Solid-State Electron Mediator. J. Am. Chem. Soc. 2015, 137, 604–607. 16


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21



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Scheme. 1 The schematic illustration of the preparation process for the flower-like ZnXCa1-XIn2S4 solid solutions.

22


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Fig. 1 XRD patterns of ZnXCa1-XIn2S4 solid solutions (a-c). Typical XPS survey spectra (d), high-resolution XPS spectra of Zn (e), Ca (f), In (g), and S (h) for ZnIn2S4, Zn0.4Ca0.6In2S4, and CaIn2S4. Schematic representation of the crystal structure for ZnIn2S4 (i), Zn0.4Ca0.6In2S4 (j), and CaIn2S4 (k).

23



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Table 1 Band energy levels, surface area and evolution rate of CO2 reduction productions of ZnXCa1-XIn2S4 solid solutions photocatalysts. X

Eg

ECB

EVB

specific surface

CH4 evolution

CO evolution

value

(eV)

(V)

(V)

area (m2/g)

rate (umol/g/h)

rate (umol/g/h)

1

2.46

-1.46

1.00

56.3

0.080

0.033

0.8

2.43

-1.48

0.95

64.3

0.174

0.082

0.6

2.35

-1.53

0.82

54.8

0.287

0.117

0.4

2.26

-1.60

0.66

54.5

0.877

0.224

0.2

2.18

-1.57

0.61

84.2

0.530

0.161

0

2.04

-1.50

0.54

81.8

0.122

0.057

24


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Fig. 2 SEM images of ZnIn2S4 (a), Zn0.4Ca0.6In2S4 (b), and CaIn2S4 (c). TEM (d-e), HRTEM (f), and EDX elemental mapping images (g) of Zn0.4Ca0.6In2S4 sample. Nitrogen adsorption/desorption isotherms (h) and pore diameter distribution (i) of ZnXCa1-XIn2S4 solid solutions.

25



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Fig. 3 Time dependence (a, c), and apparent rate constants (b, d) for CH4 or CO yields for photo-reduction of CO2 over ZnXCa1-XIn2S4 solid solutions under visible light irradiation (λ > 420 nm). Cycling runs for CO2 reduction (e), XRD pattern (f) and SEM images (inset of Fig. 3f) of Zn0.4Ca0.6In2S4 sample before and after photocatalytic reaction.

26


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Fig. 4 DRS spectra (a), band gap (b), Mott–Schottky curves (c-h), and schematic band structures (k) for the ZnXCa1-XIn2S4 solid solutions. Linear sweep voltammetry of ZnIn2S4 Zn0.4Ca0.6In2S4 and CaIn2S4 (i) and transient photocurrent responses of ZnXCa1-XIn2S4 solid solutions (j) under visible light (λ > 420 nm) illumination.

27



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Fig. 5 XRD patterns of CaIn2S4 (a) and ZnXCa1-XIn2S4 solid solutions (b). DRS spectra (band gap illustration in the inset) (c) and digital photos of suspensions (d) for ZnXCd1-XIn2S4 solid solutions. Mott–Schottky curves for Zn0.6Cd0.4In2S4 (e) and Cd0.4In2S4 (f). Schematic band structures (g) and transient photocurrent responses (h) for ZnIn2S4, Zn0.6Cd0.4In2S4 and CdIn2S4 under visible light (λ > 420 nm) illumination. 28


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Table 2 Characterization results of ZnXCd1-XIn2S4 solid solutions photocatalysts. X

Eg

ECB

EVB

value

(eV)

(V)

(V)

1

2.46

-1.46

1.00

0.6

2.34

-1.59

0.75

0

2.17

-1.51

0.66

29



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Table of Contents

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