Understanding the relative efficacies and versatile roles of 2D conductive nanosheets in hybrid-type photocatalyst
Graphical abstract
Introduction
A great deal of research efforts has been devoted for the efficient harnessing of solar energy because of its remarkable advantage as one of the most promising sustainable energy sources [1,2]. As an efficient methodology to convert solar energy into user-friendly chemical or electrical energy [3,4], semiconductor-assisted photocatalysis boasts versatile applicabilities for the photosplitting of water to produce H2 and O2, artificial photosynthesis to convert CO2 into C1 compounds, and the photodegradation of harmful organic and inorganic species [[5], [6], [7], [8], [9], [10]]. Various research strategies such as chemical substitution, the control of crystal facet, and the introduction of crystal defects have been developed to explore high-performance photocatalysts [[11], [12], [13]]. Among them, the hybridization with conductive 2D nanosheet (NS) like reduced graphene oxide (rGO), transition metal dichalcogenide (TMD), and transition metal oxide (TMO) is one of the most effective ways to improve the photocatalytic activity of semiconductor [5,[14], [15], [16], [17], [18], [19]], because ultrathin thickness and wide 2D lateral dimension of exfoliated NSs make it possible to optimize the optical and electronic properties of hybridized semiconductor [[20], [21], [22]]. Since the photocatalytic efficiency of semiconductor is strongly dependent on the lifetime of photoexcited electrons and holes, light absorption region, reaction kinetics of photocatalysis, and transport of photoexcited charge carriers [[23], [24], [25], [26], [27]], the beneficial effect of hybridization with 2D NSs heavily relies on their roles as electron reservoirs, photosensitizers, cocatalysts, and charge transport pathways [[28], [29], [30], [31]]. Depending on the chemical composition, these inorganic NSs possess variable band structures, optical properties, and electrical conductivities, which are closely related to their capabilities for photosensitization, charge separation, and charge transport [32]. Also, tailorable surface natures of these 2D materials can provide valuable opportunity to finely control an interfacial electronic coupling between hybridized species and to study its influence on the photocatalyst performance of the resulting nanohybrid [5,33]. Among many exfoliated NSs ever-studied, the exfoliated 2D NSs of rGO, MoS2, and RuO2 are the most investigated representatives for carbon-based, TMD, and TMO NSs with high hydrophobicity, intermediate hydrophobicity/hydrophilicity, and high hydrophilicity, respectively [5,7,8,16,[28], [29], [30], [31]]. A comparative study about the hybridization effect of these NSs would be quite valuable not only in understanding mechanism responsible for the enhancement of photocatalyst performance upon hybridization but also in exploring novel high-performance NS-based hybrid photocatalyst. Despite many studies about the synthesis of conductive NS-based hybrid photocatalysts [14,34,35], we are unaware of any other systematic comparative investigation about the relative efficacies of various conductive NSs in optimizing the photocatalyst performance of semiconductor and their underlying mechanism.
In this work, a series of conductive NS (RuO2, MoS2, and rGO)-based nanohybrids are synthesized not only to explore efficient visible light-active photocatalysts but also to elucidate crucial factor governing the photocatalyst functionality of hybrid-type material. The versatile roles of TMO, TMD, and rGO NSs as electron reservoirs, photosensitizers, cocatalysts, and charge transport pathways are systematically investigated with a series of well-designed experiments. To understand the effect of the chemical bonding nature of NS on the functionality of nanohybrids, the interfacial electronic couplings in the present nanohybrids are also studied with diverse spectroscopies and density functional theory (DFT) calculations.
Section snippets
Sample preparation
The positively-charged CdS quantum dot (QD) was prepared by reacting cadmium acetate dehydrate (1.33 g, 5 mmol), 2-aminoethanethiol hydrochloride (1.42 g, 12.5 mmol), and thioacetamide (0.47 g, 6.25 mmol) in distilled water (250 mL) [36]. According to the previous report [37], the exfoliated RuO2 NS was obtained by the reaction of protonated Na0.2RuO2 material with excess tetrabutylammonium (TBA+) ions for >10 days. The exfoliation of MoS2 was achieved by the reaction of Li-intercalated MoS2
Characterization of the exfoliated RuO2, MoS2, and rGO NSs and their hybridized derivatives
As shown in Figs. S2A−C of Supporting information, the lateral sizes of these conductive NSs are determined to be ˜300 nm for RuO2 NS, ˜130 nm for MoS2 NS, and ˜730 nm for rGO NS based on the DLS analysis. According to N2 adsorption−desorption isotherm analysis, the restacked NSs possess surface areas of ˜12 m2 g−1 for RuO2 NS, ˜8 m2 g−1 for MoS2 NS, and ˜49 m2 g−1 for rGO NS (Fig. S2D of Supporting information). The difference in the surface area of the present NSs can be ascribed to their
Conclusions
In conclusion, systematic comparative experiments presented here enable to evaluate the relative efficiencies of the present representative NSs for versatile roles; (1) electron reservoir efficiency: rGO NS < MoS2 NS < RuO2 NS, (2) photosensitizer efficiency: rGO NS < MoS2 NS < RuO2 NS, (3) cocatalyst efficiency: MoS2 NS < rGO NS < RuO2 NS, and (4) electron pathway efficiency: MoS2 NS < rGO NS < RuO2 NS. Among the present materials, the RuO2 NS-based CdSR nanohybrid is one of the most promising
Declarations of interest
None.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2A1A17069463) and by the Korea government (MSIT) (No. NRF-2017R1A5A1015365). The experiments at PAL were supported in part by MOST and POSTECH.
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These authors contributed equally to this work.