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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23144−23151
Distorted 1T-ReS2 Nanosheets Anchored on Porous TiO2 Nanofibers for Highly Enhanced Photocatalytic Hydrogen Production Xinqian Wang,† Biao Chen,† Dedao Yan,† Xinyu Zhao,† Chenlu Wang,† Enzuo Liu,†,‡ Naiqin Zhao,†,‡,+ and Fang He*,†,+
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†
School of Materials Science and Engineering and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300350, P.R. China ‡ Collaborative Innovation Centre of Chemical Science and Engineering, Tianjin 300072, P.R. China + Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin University, Tianjin 300072, P.R. China S Supporting Information *
ABSTRACT: Recently, loading TiO2 with transition-metal disulfides (TMDs) to construct dual functional heterostructures has been widely researched as an effective strategy to improve the photocatalytic performance of a TiO2 photocatalyst. For the TMD cocatalysts, the 2H-MoS2 and 1TMoS2 have been widely studied and researched. However, they suffer from poor catalytic activity sites/low charge transfer ability and an unstable structure. In this regard, distorted 1T-phase TMDs with a stable structure are greatly fit for the cocatalyst due to their high charge transfer ability and rich catalytic sites on both the edge and basal plane. Therefore, it is highly desirable to develop distorted 1T-phase TMD/ TiO2 heterostructures with well-identified interfaces for highly enhanced photocatalytic performance. Herein, we first introduce distorted 1T-ReS2 anchored on porous TiO2 nanofibers as a promising photocatalyst for achieving an excellent photocatalytic hydrogen production. The excellent performance is attributed to the strong chemical interaction of the Ti−O−Re bond between TiO2 and ReS2, the excellent electron mobility of distorted 1T-ReS2, and the abundant catalytic activity sites on both the plane and edge of the ReS2 cocatalyst. KEYWORDS: ReS2, TiO2, Ti−O−Re bond, photocatalysis, hydrogen evolution reaction
1. INTRODUCTION Currently, the increasingly environmental pollution and energy crisis are greatly stimulating the development of the clean and renewable energy and technologies. Among them, hydrogen energy associated with photocatalysis technology has attracted a lot of attention, owing to the advantages of low cost, vast abundance, and eco-friendliness of solar energy.1 Since Fujishima and Honda first reported the generation of hydrogen by splitting water on the titanium dioxide (TiO2) electrode,2 TiO2 has received special interest as one kind of promising photocatalyst in the past few decades with the advantages of its strong redox ability, eco-friendliness, chemical stability, and low cost.3−6 However, the photocatalytic hydrogen evolution (PHE) activity of TiO2 was still restricted by its narrow spectral response range, high charge carrier recombination, and poor catalytic activity sites.7−9 To overcome these drawbacks, loading TiO2 with cocatalysts, especially noble metals, has been developed as an effective strategy.10,11 The noble metals have been demonstrated significantly enhanced efficiency of charge separation and the number of catalytic activity sites. However, the low reserves and high cost of noble metals restricted their practical application.12 Therefore, it is urgent to © 2019 American Chemical Society
develop new cocatalysts with low-cost, earth-abundant, and many catalytic activity sites for their widespread application.13,14 There has been increasing interest in transition-metal disulfides (TMDs) on account of their characteristic twodimensional structure, eminent optoelectronic and catalytic properties.15,16 As a typical layered TMD, molybdenum disulfide (MoS2) has been extensively investigated as one of alternative cocatalysts.17,18 For instance, Zhang et al. reported that the photocatalytic hydrogen production can be efficiently enhanced by preparing a few-layer MoS 2 -coated TiO 2 heterostructure by a hydrothermal method.19 It has been confirmed by Xu and co-workers that the MoS2 cocatalyst in the TiO2@MoS2 composite can effectively enhance the transfer of photogenerated carriers and the number catalytic activity sites, resulting in excellent photocatalytic hydrogen evolution activity.46 Recent advancements validate that the crystal structure of MoS2 plays a crucial role in the Received: March 1, 2019 Accepted: June 6, 2019 Published: June 6, 2019 23144
DOI: 10.1021/acsami.9b03772 ACS Appl. Mater. Interfaces 2019, 11, 23144−23151
Research Article
ACS Applied Materials & Interfaces photocatalytic hydrogen production performance.20 It is known to all that MoS2 has two phases, 2H and 1T.21 The 1T metallic phase displays greatly lower charge transfer resistance than the 2H-semiconducting phase.22 Moreover, the 1T phase exhibits catalytic activity sites on both the basal plane and edge, whereas the 2H phase has catalytic activity sites only on the edge.23 Therefore, the 1T-MoS2 shows higher catalytic activity than the most common phase, 2H-MoS2.24 However, the 1T phase is metastable and tends to convert to the stable 2H phase, which is harmful to the photocatalytic performance.22 Therefore, it remains challenging to design ideal TMD cocatalysts for boosting the hydrogen production of TiO2. Recently, distorted 1T-phase TMDs have been demonstrated to be fit for photocatalytic water splitting due to their high charge transfer ability and large catalytic sites on both the edge and basal plane.25,26 Moreover, the distorted 1T phase is more thermodynamically stable than the 1T phase,27 which is greatly capable of the photocatalytic application. Therefore, the distorted 1T-phase TMDs have been considered as promising cocatalysts. Recently, rhenium disulfide (ReS2), as an emerging member of TMDs, was discovered to have a distorted 1T structure and tunable direct bandgap.28−32 In addition, the weak interlayer coupling in ReS2 makes contributions to almost layer-independent optoelectronic characters and preferably exposed to unsaturated edge sites.33−35 Therefore, the distorted 1T-ReS2 is extremely fit for photocatalytic activities as an alternative cocatalyst. However, the promising TiO2/ ReS2 photocatalysts have not been reported at present. In this work, few-layer ReS2 nanosheets anchored on porous TiO2 nanofibers (TiO2@ReS2) were prepared for the first time by a hydrothermal method. The few-layer ReS2 nanosheets with a distorted 1T phase exhibit large catalytic activity sites, high electron mobility, and a matched band structure. Moreover, the chemical bond of Ti−O−Re between ReS2 and TiO2 can effectively accelerate the separation and transfer of the photogenerated electron−hole pairs. These unique structural advantages greatly facilitate the hydrogen evolution process. As a result, the optimized TiO2@ReS2 photocatalysts with 30 wt % ReS2 exhibit a surpassing PHE activity with a rate of 1404 μmol h−1 g−1, which is nearly 34 times higher than that of pure TiO2.
solid product was repeatedly washed with deionized water and ethanol and finally dried at 80 °C overnight to obtain TiO2@ReS2 nanocomposites (containing 30 wt % ReS2). The TiO2@ReS2 composites with 15 and 60 wt % ReS2 were also prepared with half and twice the mass of NH4ReO4 and CH4N2S materials, respectively. By contrast, we also prepared ReS2 nanosheets without the addition of TiO2 nanofibers. 2.3. Characterization. The crystal phases of TiO2, ReS2, and TiO2@ReS2 composites were determined by X-ray diffraction (XRD, Bruke D8 Advance). Field emission scanning electron microscopy (FE-SEM, HITACHI S4800) and transmission electron microscopy (TEM, JEOL JEM-2100f) were used to survey the core−shell structure of TiO2@ReS2. The mass ratio of ReS2 in the TiO2@ReS2 samples was analyzed by thermogravimetry (TG, PerkinElmer) with a temperature range of 25−800 °C in air. X-ray photoelectron spectroscopy (XPS, VersaProbe PHI-5000) was measured to study the elemental states of the samples. The N2 desorption and adsorption curves were analyzed by the Brunauer−Emmett−Teller (BET, Quantachrome U.S.) method. The density functional theory (DFT) method was used to analyze the pore size distribution. 2.4. Photoelectrochemical Measurements. A three-electrode electrochemical workstation (GAMRY 08030) was used to measure the photoelectrochemical performances of the samples (reference electrode: saturated calomel electrode (SCE); working electrode: FTO electrode deposited with the TiO2@ReS2 nanocomposites; counter electrode: platinum foil; 2 M Na2SO3 aqueous solution was used as the electrolyte at 25 °C). The light source was a 300 W xenon arc lamp (PL-X300, Happylab, China) at a wavelength range of 320− 780 nm, and the light intensity was 100 mW cm−2. Mott−Schottky plots were determined at a frequency of 5000 Hz in 0.5 M Na2SO4 aqueous solution. A voltage range of −0.5 to 1.2 V versus Ag/AgCl was converted to the reversible hydrogen electrode (RHE) scale by the Nernst equation 0 E RHE = 0.0592pH + EAg/AgCl + EAg/AgCl
in which ERHE is the converted potential versus RHE, E0Ag/AgCl is the standard potential of Ag/AgCl filled with 0.3 M KCl solution at 25 °C, and EAg/AgCl is the experimental potential versus Ag/AgCl. 2.5. Photocatalytic Hydrogen Production Tests. A slolution containing 80 mL of deionized water, 20 mL of ethanol, and 50 mg of as-prepared samples were added into a quartz reactor (200 mL). The reactor was assembled on a glass automatic online micro gas analysis system (Labsolar 6A) and then evacuated during the stirring process to remove bubbles from the solution. A 300 W xenon arc lamp (PLX300, Happylab, China) served as the light source. Gas chromatography (GC7900, Tianmei) was used to analyze hydrogen delivered by nitrogen. The apparent quantum efficiency (AQE) was tested under a light source with a 380 nm filter. The focused intensity was about 1.65 mW cm−2. The AQE was measured by the equation
2. EXPERIMENTAL SECTION 2.1. Synthesis of Porous TiO2 Nanofibers. Porous TiO2 nanofibers were synthesized through a simple electrospinning method together with calcination.27 Poly(vinyl pyrrolidone) (PVP) (450 mg) was dissolved in 6 mL of ethanol and stirred at room temperature for 2 h. Another solution containing 3 mL of ethanol, 3 mL of acetic, and 1.62 mL of titanium tetraisopropoxide (Ti(OiPr)4) was also prepared. The above two solutions were mixed and stirred until a transparent solution was obtained. An electrical potential of 10 kV and an electrode distance of 15 cm were performed for electrospinning at an ejected rate of 0.001 mm s−1. Finally, to form porous TiO2 nanofibers, the precursor was calcined at 500 °C for 2 h with a heating rate of 5 °C/min in air. 2.2. Preparation of TiO2@ReS2 Nanocomposites. Calcined TiO2 nanofibers (100 mg) were dispersed into 20 mL of deionized water by sonication. Another solution containing 43 mg of ammonium perrhenate (NH 4 ReO 4 ), 54.4 mg of thiourea (CH4N2S), 42 mg of hydroxylamine hydrochloride (HONH3Cl), and 10 mL of deionized water was also prepared by stirring. Then, the above two solutions were mixed to get a suspension. The suspension was transferred to a Teflon-lined stainless steel autoclave and heated at 220 °C for 24 h. When cooling down to room temperature, the
AQE =
NH2 × 2 NR × 100% = × 100% NI NI
in which NH2 is the number of formed H2 molecules, NR is the number of reacted electrons, and NI is the number of incident photons.
3. RESULTS AND DISCUSSION The prepared strategy of TiO2@ReS2 nanocomposites contains two steps, as illustrated in Figure 1. Porous TiO2 nanofibers were first prepared through electrospinning and calcination. As shown in the scanning electron microscopy (SEM, Figure S1, Supporting Information) images, the TiO2 nanofibers exhibit several micrometers in length and about 150 nm in diameter. Moreover, they display a rough surface with a great number of mesopores, which can provide sufficient sites for the nucleation and growth of ReS2 during the subsequent hydrothermal process.36 Figure 2a,b shows the typical SEM and transmission electron microscopy (TEM) images of 23145
DOI: 10.1021/acsami.9b03772 ACS Appl. Mater. Interfaces 2019, 11, 23144−23151
Research Article
ACS Applied Materials & Interfaces
assemble to form ReS2 spheres (Figure S5, Supporting Information). X-ray diffraction (XRD) patterns were performed to investigate the crystal structures. It is obvious that the characteristic peaks of anatase TiO2 (JCPDS 21-1272) and rutile TiO2 (JCPDS 21-1276) can be observed in porous TiO2 nanofibers (Figure S6a, Supporting Information), and such mixed phase TiO2 is expected to enhance charge separation, leading to significantly improve photoactivity. In addition, Figure S6b presents that the diffraction peaks located at 14.6°, 32.8°, 44.8°, and 57.1° assign to the (002), (−220), (006), and (3−13) planes of distorted 1T-ReS2 (JCPDS 89-0341), respectively. As shown in Figure 3a, both the diffraction peaks of porous TiO2 and pure ReS2 can be detected, suggesting that the TiO2@ReS2 (30%) nanocomposites consist of anatase/rutile TiO2 and distorted 1T-ReS2. Moreover, the actual weight content of ReS2 in TiO2@ReS2 (30%) was studied by using thermogravimetric analysis (TGA). As shown in Figure 3b, a low weight loss of 2.1% below 150 °C is ascribed to the volatilization of water. An evident weight loss of 20.4% in a range of 150−700 °C can be attributed to the oxidation of ReS2.35 Therefore, the real weight content of ReS2 (20.8%) is smaller than the designed value (30%) due to the inadequate hydrothermal reaction. The specific surface area and pore size distribution of the as-prepared TiO2 nanofibers and TiO2@ReS2 (30%) nanocomposites were analyzed by BET (Figure 3c,d). The larger specific surface area (45.5 m2 g−1) of TiO2 nanofibers is because of their extensive mesopores around 7.0 nm. The TiO2@ReS2 (30%) composites exhibit a reduced surface area and a negative pore size shift of about 2.2 nm, suggesting the ReS2 nanosheets were tightly anchored on the surface of porous TiO2 nanofibers. The X-ray photoelectron spectroscopy (XPS) analysis was further used to examine the chemical bonding and composition of the as-prepared samples. The survey spectra (Figure 4a) reveal that the TiO2@ReS2 (30%) composites are composed of Ti, O, Re, and S, which originate from porous TiO2 nanofibers and ReS2 spheres. Meanwhile, the C 1s peak at 284.8 eV is ascribed to the contamination of the XPS instrument.39,40 The Re and S fine peaks of Figure 4b,c show that the binding energies of Re 4f7/2, Re 4f5/2, S 2p3/2, and S 2p1/2 in pure ReS2 are located at 42.6, 45.0, 163.0, and 164.4 eV, respectively, agreeing well with the reported results in the previous literature.41 For TiO2@ReS2 (30%), all binding energies showcase a negative energy shift of 0.7 eV, implying the strong chemical coupling between ReS2 and TiO2. The Ti 2p fine peaks of TiO2 and TiO2@ReS2 (30%) are presented in Figure 4d. We can observe that the separations of binding energies of Ti 2p1/2 and Ti 2p3/2 peaks are all 5.8 eV in the two samples, further demonstrating the existence of Ti4+.42,43 Meanwhile, the two peaks of TiO2@ReS2 (30%) shift to a higher binding energy by 0.2 eV when compared with TiO2 nanofibers, further confirming the electronic interaction between ReS2 and TiO2.44,45 The O 1s fine peaks in TiO2 and TiO2@ReS2 (30%) were also performed to reveal the interaction between ReS2 and TiO2. As shown in Figure 4e, the O 1s spectrum of TiO2 exists three peaks, situated at 529.9, 532.1, and 533.6 eV, corresponding to the Ti−O−Ti bond, O vacancy, and hydroxyl groups, respectively. Notably, it is worth pointing out that an extra O 1s peak at 530.7 eV is observed in TiO2@ReS2 (30%), which can be attributed to the formation of Ti−O−Re bonds between ReS2 and TiO2 (illustrated in Figure 4e).46
Figure 1. Schematic illustration of the preparation of TiO2@ReS2 nanocomposites.
Figure 2. (a) SEM, (b) TEM, and (c, d) HRTEM images of TiO2@ ReS2 (30%). (e) Elemental mappings corresponding to the selected area by red box in (b). The insets in (c) are the HRTEM image and corresponding FFT pattern.
TiO2@ReS2 nanocomposites with 30 wt % ReS2. It can be clearly observed that the ReS2 nanosheets coat uniformly on the surface of porous TiO2 nanofibers in TiO2@ReS2 (30%). The high-resolution TEM (HRTEM) images in Figure 2c,d further reveal that the ReS2 nanosheets with few layers (
