Facial synthesis of two-dimensional In2S3/Ti3C2Tx heterostructures with boosted photoactivity for the hydrogenation of nitroaromatic compounds

Yisong Zhu a, Guanshun Xie a, Guohao Li a, Fei Song a, Changqiang Yu a, Zhenjun Wu *b, Xiuqiang Xie *a and Nan Zhang *a
aCollege of Materials Science and Engineering, Hunan Joint International Laboratory of Advanced Materials and Technology for Clean Energy, Hunan University, P. R. China. E-mail: xiuqiang_xie@hnu.edu.cn; nanzhang@hnu.edu.cn
bCollege of Chemistry and Chemical Engineering, Hunan University, P. R. China. E-mail: wooawt@hnu.edu.cn

Received 12th June 2021 , Accepted 17th July 2021

First published on 20th July 2021


Abstract

Two-dimensional (2D) heterostructures have gained increased interest in recent years due to the integrated functionalities of the individual building blocks and beyond. Herein, we have fabricated 2D In2S3/Ti3C2Tx heterostructures through a facile one-step low temperature refluxing method. While bare In2S3 tends to evolve as nanoparticles, it is found that Ti3C2Tx (MXene) acts as the platform for directing the growth of 2D In2S3 nanoflakes, thereby generating 2D In2S3/Ti3C2Tx heterostructures. For the probe reaction of nitroaromatic compound hydrogenation, In2S3/Ti3C2Tx composites exhibited boosted photocatalytic activities compared to bare In2S3, which can be ascribed to the favorable active site exposure in the 2D/2D heterostructures and the “electron sink” effect of the Ti3C2Tx nanosheets. This work distinctly indicates that Ti3C2Tx nanosheets are a promising platform for fabricating 2D/2D heterostructures for photoredox catalysis.


1. Introduction

As one of the effective ways to relieve the increasingly serious problems of environmental pollution and energy crisis, semiconductor-based photocatalysis has recently aroused tremendous research interest.1–5 However, there are still many scientific problems to be solved in the photocatalytic technology, such as the high electron–hole recombination rate and insufficient surface active-site exposure for photocatalysis, which greatly limit photocatalytic activity.6–8 In recent years, tremendous efforts have been applied to design effective semiconductor photocatalysts. Among numerous strategies for improving photocatalytic activity, coupling semiconductors with a cocatalyst to form 2D/2D heterostructures holds great superiority over 0D/2D and 1D/2D heterostructures.9 The 2D/2D heterostructures afford more conducive carrier transfer channels and greater contact area,10–12 which are beneficial to achieve improved photocatalytic activity. At present, many reported 2D-2D heterostructures exhibit improved photocatalytic activity, such as MXene/Bi2WO6,13 BiVO4/Ti3C2,11 Zn3In2S6/FCN,14 BiOBr/La2Ti2O7,15 SnS2/g-C3N4,16 and rGO/g-C3N4.17 However, there are some bottlenecks of the currently commonly used 2D cocatalysts, such as limited functional groups to establish intimate interfacial interaction with semiconductors, poor conductivity resulting in inefficient separation of charge carries, and lack of hydrophilic functionalities leading to instability in aqueous solution,18 which make the photocatalytic activity unsatisfactory. Therefore, it remains desirable to explore new 2D materials as cocatalysts to overcome these shortcomings in constructing 2D heterostructured photocatalysts.

Ti3C2Tx (Tx stands for the surface terminations, such as –OH, –O and –F), as a typical example in the family of 2D metal carbides/nitrides known as MXene, has attractive properties of metallic conductivity and anisotropic behavior of carrier mobility,19,20 and has shown great application prospects in energy-related applications.21–25 In addition, the surface functional groups of Ti3C2Tx not only endow it with good dispersion and great anti-aggregation ability in aqueous medium, but also provide anchoring sites for the growth of semiconductors on its surface.26 Furthermore, high work function and electrical conductivity contribute to the efficient transport of photogenerated electrons and inhibit their recombination with holes, thereby improving the photocatalytic activity of the composites.27–29 These merits of Ti3C2Tx make it a promising co-catalyst for constructing 2D heterostructured photocatalysts.

In this work, In2S3/Ti3C2Tx composites have been constructed through a facile one-step refluxing method. Intriguingly, it has been found that Ti3C2Tx plays an essential role in directing the 2D growth of In2S3 nanoflakes, resulting in the construction of 2D In2S3/Ti3C2Tx heterostructures. The underlying mechanism of Ti3C2Tx-directed growth of In2S3 nanoflakes has been further investigated. In addition, Ti3C2Tx also serves as an “electron sink”, which is responsible for the extraction of photogenerated electrons from semiconductor In2S3, thus improving the photocatalytic activity of In2S3/Ti3C2Tx composites. As compared to bare In2S3, the as-obtained In2S3/Ti3C2Tx composites exhibit improved photocatalytic activity towards the hydrogenation reduction of nitroaromatic compounds under visible light irradiation. Finally, a possible photocatalytic mechanism was proposed to explain the improved photocatalytic activity.

2. Experimental

2.1. Materials

All reagents and solvents were of analytical reagent grade and were used in accordance with information obtained from commercial suppliers. Hydrochloric acid (HCl), indium chloride tetrahydrate (InCl3·4H2O), thioacetamide (C2H5NS, TAA), ammonium formate (HCOONH4), potassium persulfate (K2S2O8) and 4-nitroaniline (C6H6N2O2) used in the experiments were all obtained from Sinopharm Chemical Reagent Co. The deionized water (DI water) used in the experiments was from local sources.

2.2. Synthesis of Ti3C2Tx nanosheets

Ti3C2Tx was prepared by selectively etching an Al layer with HF using Ti3AlC2 MAX powder as the raw material. The preparation process was as follows: 1 g of LiF was added to 10 mL HCl (9 mol L−1) solution and stirred evenly. The MAX powder was slowly added to the above solution. The mixture was kept at 35 °C for 24 h, and then was washed with deionized water until the pH of the supernatant was above 5. The obtained product was redispersed in deionized water. After ultrasonic treatment in a water bath for 1 h under N2 flow, the suspension was centrifuged for 1 h, and the dark green supernatant was collected to obtain delaminated Ti3C2Tx colloid solution. The concentration of the delaminated Ti3C2Tx was determined by filtering a known volume of the supernatant through a Celgard membrane and measuring the weight of the film after drying.

2.3. Synthesis of the In2S3/Ti3C2Tx composite and bare In2S3

The In2S3/Ti3C2Tx composite was synthesized by a facile one-step refluxing method. A certain volume of Ti3C2Tx suspension was dispersed into deionized water by ultrasonication and the total volume was kept as 100 mL. 250 mg of InCl3·4H2O was added into the above solution along with stirring for 30 min at room temperature. Then 120 mg of TAA was added into the above solution, and stirred for 30 min. The mixture was refluxed for 5 h at 95 °C under a N2 flow (60 mL min−1). After the reaction, the sample was cooled to room temperature, and washed three times with deionized water. The precipitate was freeze-dried for 24 h to obtain the In2S3/Ti3C2Tx-x% composite, where x (x = 1, 2, and 3) denotes the weight ratios of Ti3C2Tx. The synthetic process of bare In2S3 was consistent with that for the In2S3/Ti3C2Tx composite except for the addition of Ti3C2Tx.

2.4. Materials characterization

X-ray diffraction (XRD) measurements were carried out on a Rigaku SmartLab using Ni-filtered Cu Kα radiation at a scan rate of 0.1° s−1. Transmission electron microscopy (TEM) images and elemental mapping results were collected by using Tecnai G2 F20 S-TWIN at an accelerating voltage of 200 kV. Field-emission scanning electron microscopy (FESEM) was used to determine the morphology of the samples on a JSM-6700F spectrophotometer. UV-vis diffuse-reflectance (DRS) spectra were obtained on a UV-2600 UV-vis spectrophotometer equipped with a diffuse-reflectance accessory, in which BaSO4 was employed as a reference. X-ray photoelectron spectroscopy (XPS) measurements were performed using AXIS SUPRA. JW-BK200C equipment was used to determine the nitrogen adsorption–desorption isotherms and the Brunauer–Emmett–Teller (BET) surface areas. Raman spectra were obtained using an inVia-reflex Laser Raman spectrometer. The photo-luminescence (PL) spectra for the samples were analyzed on an Edinburgh Analytical Instrument F900 spectrophotometer with an excitation wavelength of 420 nm. Photoelectrochemical measurements were carried out on a CHI 660E electrochemical station (Shanghai Chenhua) with a three-electrode system consisting of a working electrode, Pt foil counter electrode, and Ag/AgCl reference electrode. The FTO glass was washed in ethanol ultrasonically for 30 minutes and dried at 60 °C. The boundary of FTO glass was protected with scotch tape. The sample slurry was obtained by dispersing 5 mg of samples in 0.5 mL of N,N-dimethylformamide (DMF) by ultrasonication, which was then coated on the pretreated FTO glass. After drying, the adhesive tape was removed and the uncoated part of the electrode was pasted with epoxy resin. The exposed area of the working electrode was 0.25 cm2. A 300 W Xe lamp system (PLS-SXE300D, Beijing Perfectlight) equipped with a UV light filter (to cut off light with λ < 420 nm) was used as the irradiation source. The working electrode was immersed in an aqueous solution of 0.1 M Na2SO4. After the visible light was irradiated for 60 s, the light was turned off to monitor the subsequent decay of photovoltage for 50 s, and the open circuit photovoltage decay (OCP) spectrum was obtained. The electrochemical impedance spectroscopy (EIS) and cyclic voltammogram (CV) experiments were conducted in 0.5 M KCl containing 0.01 M K3[Fe(CN)6]/K4[Fe(CN)6] under open-circuit potential conditions. The transient photocurrent response was conducted in 0.2 M Na2SO4 aqueous solution without voltage bias under visible light irradiation. The Mott–Schottky (M–S) experiments were obtained in 0.2 M Na2SO4 aqueous solution without additive. The contact angle of water droplet on the freestanding membranes of the exfoliated Ti3C2Tx nanosheets was measured using a DSA 100 (KRÜSS) instrument to characterize the surface properties.

2.5. Photocatalytic activity tests

The photocatalytic activity of the prepared samples was tested toward the hydrogenation reduction of nitroaromatic compounds at ambient temperature and N2 atmosphere using a 300 W Xe lamp with a UV light filter (to cut off light with λ < 420 nm). 40 mg of photocatalyst and 40 mg of ammonium formate as a hole trapping agent were mixed with 40 mL of 4-nitroaniline (10 ppm) in a reaction bottle with a circulating water system and the mixture was stirred in the dark for 1 h to ensure the establishment of the adsorption–desorption equilibrium between the sample and reactants. The whole experimental process was conducted under N2 bubbling at a flow rate of 60 mL min−1. During the process of the reaction, 2 mL of liquid was collected at a certain time interval and centrifuged to remove the catalyst completely at 8000 rpm. After that, the solution was analyzed using an ultraviolet-visible light (UV-vis) spectrophotometer (Shimadzu, UV-1780). Control experiments were carried out in a similar manner to the above photocatalytic reduction of nitroaromatic compounds except for the addition of active species scavengers.

3. Results and discussion

The synthesis procedure for the In2S3/Ti3C2Tx composite is illustrated in Fig. 1a. The exfoliated Ti3C2Tx nanosheets are mixed with InCl3, during which the In3+ cations are adsorbed on the surface of negatively charged Ti3C2Tx nanosheets through electrostatic attractions. Then TAA as the sulfur source is added into the above mixture to undergo a refluxing treatment. During this process, TAA is heated to release S2− ions to react with the adsorbed In3+ to form In2S3in situ grown on the surface of Ti3C2Tx nanosheets, thus giving rise to the formation of In2S3/Ti3C2Tx composites.
image file: d1qm00844g-f1.tif
Fig. 1 Illustration of the synthesis process for the In2S3/Ti3C2Tx composites (a). SEM image (b), TEM image (c), HRTEM image (d), and element mapping (e) of In2S3/Ti3C2Tx-1%. XRD patterns of bare In2S3 and In2S3/Ti3C2Tx composites (f).

The morphology of the as-prepared sample is firstly studied by scanning electron microscopy (SEM). Bare In2S3 consists of aggregated nanoparticles (Fig. S1, ESI). In contrast, well-defined In2S3 nanoflakes are observed for the In2S3/Ti3C2Tx-1% composite (Fig. 1b), suggesting that the introduction of Ti3C2Tx nanosheets plays a critical role in controlling the growth of In2S3 nanoflakes. The microscopic morphology and structure of the In2S3/Ti3C2Tx composite have been further analyzed by transmission electron microscopy (TEM). Fig. 1c shows a typical TEM image of the In2S3/Ti3C2Tx-1% composite. It can be clearly seen that many small In2S3 nanoflakes are attached to the Ti3C2Tx nanosheets, which is in accordance with the results of SEM. The high-resolution TEM (HRTEM) image of In2S3/Ti3C2Tx-1% in Fig. 1d reveals the distinguishable interfaces of In2S3 nanoparticles with Ti3C2Tx nanosheets. The lattice spacing of 0.285 nm is attributed to the (400) crystal plane of the In2S3 in In2S3/Ti3C2Tx composite. The elemental mapping analysis has been performed to further determine the element distribution. The results in Fig. 1e show that the In, S, Ti and C elements are identically distributed in the In2S3/Ti3C2Tx-1% composite. The mapping image of the In element is basically the same as that of the S element, which overlap with the Ti and C elements. These results indicate that In2S3 nanoflakes are uniformly grown on Ti3C2Tx nanosheets with intimate interfacial interactions, which facilitates the separation and transfer of photogenerated electrons. Contact angle measurements were conducted to explore the hydrophilicity of the Ti3C2Tx nanosheets. From Fig. S2 (ESI), a small contact angle of 39.4° is observed, indicating the preferable hydrophilicity of Ti3C2Tx, which is important for the role in directing the growth of 2D In2S3 nanoflakes. To explore the growth mechanism of In2S3 nanoflakes on the Ti3C2Tx nanosheets, the effects of Ti3C2Tx content and reaction time (t = 1, 4, and 5 h) were systematically studied. As shown in Fig. S3a–c (ESI), with the increase of refluxing time, In2S3 gradually nucleates and grows on the Ti3C2Tx surface. When the refluxing time is prolonged to 4 h, the surface of the Ti3C2Tx nanosheets gradually become rough, and small In2S3 nanoflakes are observed. After 5 h of refluxing, In2S3 grew vertically on the surface of the Ti3C2Tx nanosheets to form 2D In2S3/Ti3C2Tx heterostructures. In addition, when the Ti3C2Tx content in the In2S3/Ti3C2Tx composite increases to 5%, In2S3 nanoflakes can still be seen (Fig. S3d, ESI). However, when the content of Ti3C2Tx increases to 20% or 30%, In2S3 nanoparticles rather than nanoflakes are generated. These controlled experiments indicate that the content of Ti3C2Tx plays a significant role in the morphology regulation of In2S3. It is reasonable to propose that low content of Ti3C2Tx results in a high density of In2S3 nuclei, which favors the vertical growth of nanosheets based on the theory that the steric hindrance from adjacent seeds can hinder the in-plane direction growth.30

X-ray diffraction (XRD) measurements have been conducted to analyze the crystal structure of the as-prepared samples. As demonstrated in Fig. 1f, the characteristic diffraction peaks at 2θ = 14.2, 23.3, 27.4, 28.7, 33.2, 43.6, 47.7, 56.6, 59.4, 66.6 and 69.8° in the XRD spectrum of bare In2S3 are indexed to the (111), (220), (311), (222), (400), (511), (440), (622), (444), (731) and (800) crystal planes of the cubic In2S3 phase (β-In2S3) (JCPDS no. 65-0459), respectively. The In2S3/Ti3C2Tx composites exhibit similar XRD patterns to that of bare In2S3, demonstrating that the introduction of Ti3C2Tx does not change the crystal structure of In2S3. Therefore, the possible influence of the crystal structure on the photocatalytic performance of the samples can be eliminated. There are no obvious characteristic diffraction peaks of Ti3C2Tx in the XRD patterns, which should be ascribed to the low content of Ti3C2Tx. In addition, no XRD peaks of TiO2 can be observed, which clearly indicates that the stability of Ti3C2Tx is well sustained. The intensity of the diffraction peaks for In2S3 decreases with the increase of Ti3C2Tx content in the composites. This should be because the aggregation of In2S3 is effectively inhibited by the addition of Ti3C2Tx as 2D growing platforms. The composition of the samples has been further confirmed by Raman. As revealed in Fig. S4 (ESI), the Raman peaks located at 306 cm−1 can be associated with the typical vibration modes of β-In2S3.31 As for the In2S3/Ti3C2Tx-1% composite, it is obvious that besides the typical peak of In2S3 at 306 cm−1, there is an additional peak centered at 206 cm−1, which can be associated with the vibrations from Ti3C2Tx.32 Furthermore, no characteristic peaks of TiO2 can be seen, which further manifests that Ti3C2Tx is well sustained during the fabrication process.

The optical properties of bare In2S3 and In2S3/Ti3C2Tx composites have been studied by UV-vis diffuse reflectance spectra (DRS). As displayed in Fig. S5 (ESI), the introduction of Ti3C2Tx nanosheets leads to the enhanced light absorption of In2S3/Ti3C2Tx composites in visible light. The absorption gradually increases with the increase of Ti3C2Tx content, which could be ascribed to the background light absorption of Ti3C2Tx. This is consistent with the color of the samples, as shown in the insets of Fig. S5 (ESI). The above results show that the introduction of Ti3C2Tx has a similar effect on the morphology, crystal structure and optical properties of the In2S3/Ti3C2Tx composites.

The surface chemical states of the as-prepared samples have been analyzed by X-ray photoelectron spectroscopy (XPS). It can be seen from Fig. S6 (ESI) that In, S, Ti, and C elements are detected in In2S3/Ti3C2Tx-1%, which is consistent with the elemental mapping analysis. In addition, the peak of F element in the survey spectrum of In2S3/Ti3C2Tx-1% is almost extinct compared with bare Ti3C2Tx, indicating the F element has been exchanged during the refluxing process. In the high-resolution In 3d XPS spectrum of In2S3 (Fig. 2a), the binding energies of In 3d at 452.2 eV and 444.7 eV are ascribed to In 3d3/2 and In 3d5/2, respectively. Meanwhile, two peaks located at 161.2 eV and 162.3 eV are attributed to S 2p3/2 and S 2p1/2 (Fig. 2b).33 After the introduction of Ti3C2Tx, the binding energy of In 3d3/2 and In 3d5/2 shifted to 452.3 eV and 444.8 eV, while the binding energy of S 2p3/2 and S 2p1/2 shifted to 161.3 eV and 162.4 eV, respectively. This implies a decrease in surface electron density of In2S3 in the In2S3/Ti3C2Tx-1% composite, suggesting a Schottky effect between the semiconductor In2S3 and Ti3C2Tx, which is favorable for the transfer of photogenerated electrons. As shown in Fig. 2c, there are four peaks located at 281.5 eV, 284.6 eV, 286.5 eV and 288.1 eV, which can be assigned to the Ti–C bond, C–C bond, C–O bond, and C–F bond, respectively.34 As displayed in Fig. 2d, there are four doublets (Ti 2p3/2–Ti 2p1/2) in the Ti 2p XPS spectra, three of which correspond to C–Tiδ+–Tx (δ = 1, 2 and 3) while the fourth is attributed to Ti(VI)–O.35


image file: d1qm00844g-f2.tif
Fig. 2 XPS spectra of In2S3 and In2S3/Ti3C2Tx-1%. In 3d (a) and S 2p (b) of In2S3 and In2S3/Ti3C2Tx-1%. C 1s (c) and Ti 2p (d) of In2S3/Ti3C2Tx-1%.

The photocatalytic activities of the as-prepared samples have been comparatively evaluated by selective hydrogenation of 4-nitroaniline (4-NA) and other nitroaromatic compounds in water with the addition of ammonium formate as a hole scavenger under visible light irradiation. As shown in Fig. 3a, the conversion of 4-NA reaches 65% over bare In2S3 after 140 min. When Ti3C2Tx with a weight ratio of 1% is added, the conversion of 4-NA is improved as compared with bare In2S3. However, the addition of higher contents of Ti3C2Tx (i.e., 2 and 3 wt%) results in a decrease of photocatalytic activity and the In2S3/Ti3C2Tx-3% even exhibits a worse performance than bare In2S3. This indicates that an appropriate content of Ti3C2Tx combined with In2S3 can obtain the optimum photocatalytic activity. With the increase of Ti3C2Tx content in the In2S3/Ti3C2Tx composite, the light through the reaction suspension might be shielded by Ti3C2Tx with a dark color, which reduces the effective contact of In2S3 nanoflakes with incident photons, and thus leads to a decrease of photocatalytic activity. The transient photocurrent response of the as-prepared samples under intermittent visible light irradiation is shown in Fig. S7 (ESI). The In2S3/Ti3C2Tx-1% composite exhibits an improved transient photocurrent response compared to bare In2S3, indicating the increased transport and separation of photogenerated charge carriers. However, the addition of higher contents of Ti3C2Tx (i.e., 2 and 3 wt%) result in the decrease of transient photocurrent response and the In2S3/Ti3C2Tx-3% even exhibits a worse transient photocurrent response than bare In2S3. The transient photocurrent response results keep accordance with the above photocatalytic activity results, which further demonstrates an appropriate content of Ti3C2Tx combined with In2S3 can obtain the optimum photocatalytic activity. The dependence of photoactivity of In2S3/Ti3C2Tx on the weight ratio of Ti3C2Tx has also been found for the hydrogenation of other nitroaromatic compounds with different substituent groups (4-nitrotoluene, 1-chloro-4-nitrobenzene, 4-nitroanisole and 4-nitrophenol), as shown in Fig. 3b–e. In order to investigate the mechanism for the photocatalytic reduction of 4-NA, controlled experiment with the addition of K2S2O8 as a scavenger for photogenerated electrons has been conducted. As shown in Fig. 3f, photocatalytic activity of the In2S3/Ti3C2Tx-1% composite is strongly inhibited, and almost no conversion of 4-NA has been observed. The results give direct evidence to prove that the photocatalytic reduction of 4-NA is driven by photogenerated electrons. When N2 injection was cut off, the photocatalytic activities of In2S3/Ti3C2Tx-1% decreased significantly and the conversion of 4-NA was only 4.4%, illustrating that the inert atmosphere is indispensable for the conversion of 4-NA over the In2S3/Ti3C2Tx composite. In order to detect whether Ti3C2Tx in the In2S3/Ti3C2Tx composite has been oxidized during photocatalysis, XPS analysis of the In2S3/Ti3C2Tx composite after the photocatalytic activity test has been conducted. As shown in Fig. S8 (ESI), the contents of the Ti (IV)–O bond for In2S3/Ti3C2Tx before and after photocatalytic tests are 17.0% and 17.8%, respectively, which clearly unravels the stability of Ti3C2Tx.


image file: d1qm00844g-f3.tif
Fig. 3 Photocatalytic selective hydrogenation of 4-NA (a), 4-nitrotoluene (b), 1-chloro-4-nitrobenzene (c), 4-nitroanisole (d), and 4-nitrophenol (e) over In2S3 and In2S3/Ti3C2Tx composites under visible light irradiation. Control experiments for the photocatalytic selective hydrogenation of 4-NA over In2S3/Ti3C2Tx-1% (f).

To investigate the underlying reasons for the improved photocatalytic activity of In2S3/Ti3C2Tx-1% compared to bare In2S3, the surface areas of In2S3 and In2S3/Ti3C2Tx-1% have been revealed by the nitrogen (N2) adsorption–desorption measurements. However, as reflected in Fig. S9 (ESI), the specific surface area of In2S3/Ti3C2Tx-1% (49.6 m2 g−1) is smaller than bare In2S3, which can be attributed to the stacking of nanosheets.13 This result clearly demonstrates that the surface area is not the key factor determining the photoactivity in the current system. The separation of photogenerated electron–hole pairs is a significant step in photocatalysis,36 hence a set of electrochemical, photoelectrochemical and photoluminescence (PL) measurements have been performed over In2S3 and In2S3/Ti3C2Tx-1%. The cyclic voltammograms show clear anodic and cathodic peaks of K3[Fe(CN)6]/K4[Fe(CN)6] over bare In2S3 and In2S3/Ti3C2Tx-1% (Fig. 4a). A higher current density and smaller overpotential are observed in the In2S3/Ti3C2Tx-1% electrode, which corresponds to the improved electron transfer rate as compared to bare In2S3. Besides, as evidenced by the Nyquist plots displayed in Fig. 4b, the In2S3/Ti3C2Tx-1% electrode exhibits a smaller high-frequency semicircle as compared to bare In2S3. This result indicates that the lower charge transfer resistance in the In2S3/Ti3C2Tx-1% composite favors faster transport and more effective separation of photo-generated charges carriers.37 As shown in the photoluminescence (PL) spectra in Fig. 4c, bare In2S3 exhibits a broad peak at around 700 nm, while the PL intensity of the In2S3/Ti3C2Tx-1% composite decreases, suggesting that the introduction of Ti3C2Tx effectively inhibits the recombination of photogenerated electron–hole pairs. The photoelectron lifetime of two samples has been compared through an open circuit photovoltage (OCP) decay technique under visible light irradiation (Fig. S10, ESI). It is distinctly seen that from Fig. 4d, the calculated photoelectron lifetime as a function of Voc follows the order of In2S3/Ti3C2Tx-1% > In2S3, further demonstrating that the introduction of Ti3C2Tx nanosheets can effectively restrain the recombination of photo-induced electron–hole pairs and prolongs the lifetime of charge carriers. In combination with the electrochemical, photoelectrochemical and PL results, it can also be concluded that the 2D/2D In2S3/Ti3C2Tx heterostructures play a superior role in enhancing the separation and transfer of photogenerated electrons, thereby resulting in the improved photoactivity of the In2S3/Ti3C2Tx-1% composite as compared to bare In2S3.


image file: d1qm00844g-f4.tif
Fig. 4 Cyclic voltammograms (CV) (a), electrochemical impedance spectroscopy (EIS) results (b), photoluminescence (PL) spectra (c), and electron lifetime (d) of In2S3 and In2S3/Ti3C2Tx-1%.

As photogenerated electrons are the active species for the reduction of nitroaromatic compounds as aforementioned, the band positions of In2S3 have been calculated to investigate the transfer progress of photogenerated electrons upon visible light irradiation. According to the Kubelka–Munk function plots obtained by DRS spectral transformation (Fig. S5, ESI), the band gap (Eg) of In2S3 has been measured as 2.01 eV (Fig. S11a, ESI). Besides, the Mott–Schottky result shown in Fig. S11c (ESI) suggests that the flat-band potential of In2S3 is −0.62 V (vs. Ag/AgCl). As shown in Fig. S11b (ESI), the valence band of In2S3 is measured to be 1.30 eV, and then the conduction band is calculated as −0.71 eV according to EVB = ECB + Eg.

In light of the above results, a possible mechanism for the photocatalytic reduction of the nitroaromatic compounds has been proposed. As illustrated in Fig. 5, under visible light irradiation (λ ≥ 420 nm), the electrons are photoexcited from the valence band (VB) to the conduction band (CB) of In2S3 in the In2S3/Ti3C2Tx composites, during which holes in the VB are created. Because Ti3C2Tx has a more positive Fermi level (Ef) than the ECB of In2S3, photoexcited electrons migrate rapidly from the CB of In2S3 to Ti3C2Tx, which inhabits the recombination with holes, thus promoting the reduction reaction of nitroaromatic compounds. Simultaneously, holes in the VB are captured by ammonium formate, which restrains the oxidation of nitroaromatic compounds and their reduction products. Notably, an anaerobic atmosphere is critical to reduce the competition reaction of oxygen reduction to ensure the desirable hydrogenation of nitroaromatic compounds. The improved photocatalytic activity of In2S3/Ti3C2Tx-1% can be interpreted by the favorable active site exposure in the 2D/2D heterostructures and the “electron sink” effect of Ti3C2Tx nanosheets, which can boost the transport and separation of photogenerated charge carriers.


image file: d1qm00844g-f5.tif
Fig. 5 Illustration of the proposed reaction mechanism for the photocatalytic selective reduction of nitroaromatic compounds over In2S3/Ti3C2Tx composites.

4. Conclusions

In conclusion, In2S3/Ti3C2Tx composites have been fabricated through a low temperature refluxing method. Interestingly, the introduction of Ti3C2Tx nanosheets leads to the controlled growth of In2S3 nanoflakes, which results from the steric hindrance from adjacent seeds to hinder the in-plane direction growth. Ti3C2Tx nanosheets also serve as an efficient co-catalyst for further promoting the separation and transfer of photogenerated charge carriers from In2S3 under visible light irradiation. As a result, the 2D heterostructured In2S3/Ti3C2Tx composites exhibit improved photocatalytic activity compared to bare In2S3 toward the hydrogenation reduction of nitroaromatic compounds under visible light irradiation. This work provides a new vision for making better use of the rich surface properties and metal conductivity of Ti3C2Tx for the construction of 2D heterostructured photocatalysts.

Author contributions

Yisong Zhu: methodology, conceptualization, investigation, formal analysis, writing – original draft. Guanshun Xie: resources, investigation, visualization. Guohao Li: resources. Fei Song: resources. Changqiang Yu: resources. Zhenjun Wu: resources. Xiuqiang Xie: project administration, supervision, writing – review & editing. Nan Zhang: project administration, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to decalre.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52071137, 51977071, 51802040, 21802020) and the Natural Science Foundation of Hunan Province (2020JJ3004, 2020JJ4192). N. Z. and X. X. also acknowledge the financial support of the Fundamental Research Funds for the Central Universities.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00844g

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