Probing interfacial charge transfer dynamics in MoS2/TiO2 nanocomposites using scanning Kelvin probe for improved photocatalytic response
Introduction
The advent of nanomaterials with the emergence of low-dimensional physics has offered green, cheap and efficient solution for water and air pollution. TiO2 and MoS2 are considered novel materials owing to their high oxidizing power, superb chemical stability, low-cost and non-toxicity to be utilized as a next generation photocatalyst materials for efficient removal of organic pollutants [1], [2], [3]. In the nanocomposites (NC) form, TiO2 act as a catalyst and MoS2 basically serves as a co-catalyst/photosensitizer for high photocatalytic performance.
The basic principle involved in a photocatalysis reaction is utilization of photo-generated electron-hole (e-h) pairs for decomposition of the pollutants. The main challenges in this whole process require high photo-absorption capability, effective separation of photo-induced e-h pairs and efficient transfer of charge to the active sites (should be large) for achieving better performance [1], [2], [3]. Nanostructures are considered excellent candidates in providing these properties. Nano-sized MoS2 and TiO2 are highly beneficial in providing large active sites for photocatalytic reaction due to high surface area. However, TiO2 being a wide band gap (3.2 eV) material is unable to absorb visible light which limits its absorption ability [4]. In addition, TiO2 suffers from high rate of e-h recombination which is detrimental for the intended purpose [5,6]. Different strategies like doping with noble metals, transition metals and non metals have been adopted to overcome these drawbacks with limited success [2,[7], [8], [9]]. But, in recent years, coupling TiO2 with ultrathin semiconductors like MoS2 and graphene has produced excellent results due to many complementary advantages [6,[10], [11], [12], [13], [14]]. Bulk MoS2 is an indirect band gap (1.2 eV) semiconductor with Mo atoms sandwiched between S atoms and transitions to direct band gap with energy 1.9 eV in monolayer due to quantum confinements [11,12]. This rare property enables it to absorb incident visible light. In addition, the strong intra-layer covalent bonding with weak van der Waal forces between the layers makes it favorable for effective charge transport [10,14]. MoS2 in its layered nanosheet form can act as an excellent co-catalyst inducing accelerated suppression of e-h recombination rate and provides larger area for photocatalytic active sites.
Understanding the dynamics of charge transfer across the interface of MoS2/TiO2 is quite essential for improving the performance of photocatalyst materials. The charge transfer dynamics can be accurately probed by measuring the change in the WF of the constituent materials and their NC. The previous studies suggest the influence of WF in improvement of the Photocatalytic response of metal doped TiO2 nanostructures [15], [16], [17]. Another work by Tao et al. shows better photocalytic performance of MoS2/TiO2 hetero thin films due to better interface coupling [18]. However, the modified charge carrier dynamics through modulation of surface electronic properties in the nancomposite materials is not intensively studied.
In this work, the effect of modified surface electronic properties on the dynamics of interfacial charge transfer towards enhancement of photocatalytic activity (PCA) in MoS2/TiO2 NC synthesized by sol-gel method with varying content of MoS2 is reported. The modification in the surface electronic properties of MoS2/TiO2 NC is investigated by measuring their WF using scanning Kelvin probe microscopy in terms of contact potential difference (CPD). Surface morphology of the NC is observed by field effect scanning electron microscopy (FESEM). Size and crystallinity are investigated by X-ray diffraction (XRD) pattern and transmission electron microscopy (TEM). Further, the NCs are characterized by Raman spectroscopy to study the doping and strain effects in the MoS2/TiO2 hybrid nanostructures by studying their vibrational properties and the results support the SKPM results. UV-Vis spectrophotometer is used to find the extent of band gap modulation and measurement of PCA in terms of Rhodamine B (RhB) degradation. This study establishes SKPM as a unique tool for investigating the charge transfer dynamics for preparation of advanced photocatalytic composite materials
Section snippets
Materials
MoS2powder, N-methyl-2-pyrrolidone (NMP), titanium isopropoxide (TIP, C12H28O4Ti) and glacial acetic acid (C2H4O2) were purchased from Sigma-Aldrich. For thin film preparation, the single crystal silicon 〈100〉 wafer was used.
Synthesis of MoS2 nanosheets
To get the MoS2nanosheets, bulk MoS2powder (1 mg/ml) was chemically exfoliated in N-methyl-2-pyrrolidone (NMP) for 12 h using a bath sonicator. After ultrasonication process, the solution was centrifuged for 1 h at 5000 rpm and the supernatant was collected.
Synthesis of TiO2
To synthesize TiO
Morphological and structural characterization
Morphological characterizations of MoS2/TiO2 nanocomposites are presented in the Fig. 1. In the FESEM images (Fig. 1(a) and (b)), the TiO2 nanoparticles can be seen dispersed on the nanosheets of MoS2. Fig. 1(a) is the FESEM image of 0.5MT and Fig. 1(b) is of 1MT.
Further characterization was done using TEM to investigate the microstructures of NC and shown in Fig. 1(c), (d) and (e). TEM image of TiO2 nanoparticles are presented in Fig. 1(c). TiO2 nanoparticles incorporated in the MoS2
Discussion
The enhanced PCA of MT over TiO2 can be attributed to the fact that the valence band maximum and conduction band minimum of MoS2 lies above the these respective bands of TiO2 and thus it generates type II band alignment at the interface of MoS2 and TiO2 [41]. This band alignment creates band bending at the interface through flow of electrons from conduction band of MoS2 to the TiO2 conduction band until Fermi level on two sides becomes equal. The potential barrier is thus created at the
Conclusion
In summary, the interfacial charge transfer dynamics during modification of surface electronic properties of MoS2/TiO2 NC and its role in improving separation of e-h pairs have been studied by using SKPM and Raman spectroscopy. The analysis of observed enhancement in PCA demonstrates that SKPM can provide deep insight into the mechanism of photocatalysis. MoS2 being an efficient co-catalyst provides great platform to suppress the e-h pair recombination in the TiO2. MoS2 forms type II band
Funding sources
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
All the authors declare that they do not have any conflict of interest.
Acknowledgments
The authors are thankful to AIRF, JNU for FESEM, TEM and Raman spectroscopy and Dr. Shobhan Sen for UV-Visible spectroscopic measurements. SK and JS acknowledge UGC, India for research fellowship.
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