Invited Review
2D inorganic nanosheet-based hybrid photocatalysts: Design, applications, and perspectives

https://doi.org/10.1016/j.jphotochemrev.2018.03.002Get rights and content

Highlights

  • A comprehensive account on research activity for 2D inorganic nanosheets is presented.

  • 2D inorganic nanosheets can be used as valuable building blocks for hybrid photocatalyst.

  • Diverse examples of 2D inorganic nanosheet-based hybrid photocatalysts are presented.

  • In-depth discussion about the crucial roles of 2D nanosheets in photocatalysis is provided.

  • Future perspectives in researches for 2D nanosheet-based hybrid photocatalysts are discussed.

Abstract

Highly anisotropic 2D nanosheets of inorganic solids with nanometer-level thickness attract a great deal of research activity because of their unique merits in exploring novel high performance photocatalysts applicable for environmental purification and production of renewable clean energy. The 2D inorganic nanosheets possess many valuable properties such as tailorable band structures and chemical compositions, large surface areas, well-defined defect-free surface structure, and tunable electrical conductivities. Due to these unique advantages of 2D inorganic nanosheets, these materials can be used as promising building blocks for hybrid-type photocatalysts with optimized band structures, expanded surface areas, improved charge separation behaviors, and enhanced reaction kinetics. Of prime importance is that unusually strong electronic coupling can occur between very thin 2D inorganic nanosheets and hybridized nanospecies, leading to the synergistic optimization of electronic and optical properties, and thus the remarkable enhancement of photocatalytic activity. Depending on the type of component nanosheets, diverse examples of inorganic nanosheet-based photocatalysts are presented along with the in-depth discussion about critical roles of inorganic nanosheet in these hybrid photocatalysts. Future perspectives in the researches for 2D inorganic nanosheet-based photocatalysts are discussed to offer useful directions for designing and synthesizing novel high performance photocatalysts applicable for renewable energy production and environmental purification.

Introduction

The steeply-increasing consumption of fossil fuels and the accompanying environmental pollution demand the development of renewable and clean energy sources to solve these serious challenges for humanity. Since the solar light is an ideal clean and immense energy resource to completely fulfill the current energy demands by humanity, research activity about the exploration of alternative energy technology has been focused on the harnessing and conversion of solar energy [[1], [2], [3]]. Since the first report of Honda–Fujishima effect about the photogeneration of H2/O2 through photoelectrochemical (PEC) water splitting on TiO2 electrode [4], photocatalysis has been regarded as one of the main options for providing renewable energy source via the harnessing of solar energy [[5], [6], [7]]. The generation of photoexcited electrons and holes caused by light absorption can catalyze various redox reactions of water, carbon dioxide, organic molecules, and others. The development of photocatalytically active materials attracts plenty of research efforts because of their effectiveness as a powerful and eco-friendly way of producing renewable energy as well as of purifying organic/inorganic pollutants [[8], [9], [10], [11], [12]]. Despite intense research activity devoted for the exploration of efficient photocatalyst materials, the photocatalytic efficiency of solar fuel production remains unsatisfactory for commercial applications of photocatalysis technology. To achieve high efficiency of solar energy harvesting by photocatalysts, there are several critical issues to be addressed such as an absorptivity of visible light, the life-time of photoexcited electrons and holes, charge transport kinetics to surface reaction sites, and the reaction kinetics of photocatalysis [[13], [14], [15]]. Many kinds of research strategies such as morphology/size control [[16], [17], [18]], crystal defect/facet/vacancy engineering [[19], [20], [21]], cation/anion substitution [22,23], and hybridization with other species [[24], [25], [26], [27]] have been pursued to optimize these factors. The significant effects of crystal shape and surface area on photocatalyst performance render highly porous nanostructured semiconductors promising candidates for efficient photocatalysts via the shortening of migration path and the enhancement of ion diffusion path [[28], [29], [30], [31]]. As another approach to improve the photocatalytic activity of semiconductor, intense research efforts have been devoted for optimizing the surface properties of photocatalyst with the control of crystal defects and crystal facets of semiconducting materials [[32], [33], [34], [35], [36]]. In one instance, the creation of the oxygen vacancies for BiVO4 via the electrochemical partial reduction of Bi3+ and V5+ ions can induce the remarkable enhancement of its photocatalytic activity for photocurrent generation [37]. Of prime interest is that TiO2 crystal exposed with thermodynamically unstable (001) facets exhibits excellent photocatalytic efficiency, which is attributable to the high surface energy of (001) facets [38]. Also, the substitution of heteroatom enables not only to modify the electronic structure of the parent semiconductor but also to introduce highly active heteroatomic sites, resulting in the broadening of light absorption range and the enhancement of photocatalysis kinetics, respectively [[39], [40], [41]]. Of noteworthy is that the significant narrowing of bandgap can be achieved via the substitution of anion like N, C, S, B, F, Br, I, and P, providing visible light-harvesting activity for wide bandgap semiconductor [[42], [43], [44], [45], [46], [47]].

Alternatively, the hybridization between nanostructured semiconductors can provide one of the most powerful ways to optimize the critical factors in photocatalysis and to explore high performance photocatalyst materials. Of prime importance is that the hybridization strategy is highly effective in depressing the recombination of photoexcited electron–hole pairs and in expanding the light absorption region via electronic coupling between hybridized species [[48], [49], [50]]. Taking into consideration a close relationship between the number of photoexcited charge carriers and photocatalytic activity, the efficient provision of photoexcited charge carriers caused by the depression of charge recombination result in the remarkable improvement of photocatalyst performance upon the hybridization. Judging from the high level of flexibility in the compositions of hybridized components, there are still wide rooms not only for the development of novel hybrid-type photocatalysts but also for the unexpected creation of novel functionalities derived from synergistic combination of existing properties.

Among diverse low-dimensional nanomaterials, highly anisotropic two-dimensional (2D) nanosheets with atom-level thickness possess unique advantages as efficient building blocks for hybridization because of their fascinating characteristics such as tunable chemical compositions, wide 2D surface area, and well-defined surface structures [51,52]. The 2D nanosheets can be defined as highly anisotropic 2D nanocrystalline structures with very thin thickness ranging from sub-nano meter to a few nanometers and much larger lateral dimension of several hundred nanometers to several micrometers. Generally, these 2D nanosheets consist of ∼1–10 stacked monolayers depending on their synthetic methods. The great diversity and tunability of the chemical compositions and crystal structures of 2D inorganic nanosheets offer valuable opportunity to optimize the photocatalytic activities of 2D nanosheet-based hybrid materials [[53], [54], [55], [56]]. In particular, unusually strong electronic coupling between hybridized components can be achieved with 2D inorganic nanosheets because all the component ions of these materials are exposed on their interfacial surfaces [57,58]. The exfoliated 2D inorganic nanosheets possess additional merit of well-defined defect-free surface, which is quite crucial in minimizing the charge recombination center and in enhancing the photocatalytic efficiency [55,56,58]. Also, the formation of hierarchically porous structure upon hybridization with 2D inorganic nanosheet is also effective in improving the photocatalyst performance of the resulting nanohybrid [57,58]. Despite such remarkable advantages of 2D inorganic nanosheets as building blocks for hybrid-type photocatalysts [59,60], we are aware of no other systematic review focused on the synthesis and photocatalyst application of diverse 2D inorganic nanosheets and their nanohybrids, together with the in-depth discussion about the crucial role of 2D inorganic nanosheets in the hybrid photocatalyst systems.

The main purpose of this review is to present a broad overview of 2D inorganic nanosheet-based hybrid materials in the field of photocatalysis. Although many reviews survey recent progress in the synthesis, characterization, and photocatalyst applications of 2D nanomaterials [9,[61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71]], there are only a few number of reviews focused on the beneficial role of hybridization in optimizing the photocatalytic activities of 2D nanosheets. In this review, we aim to offer a comprehensive account on emerging research activities devoted for the design, synthesis, and applications of 2D inorganic nanosheet-based hybrid photocatalysts. Depending on the type of component nanosheets, the 2D inorganic nanosheet-based photocatalysts are categorized as follows: 2D transition metal oxide (TMO) nanosheet-based nanohybrids, 2D transition metal dichalcogenide (TMD) nanosheet-based nanohybrids, 2D graphitic carbon nitride (g-C3N4)-based nanohybrids, and other class of 2D nanosheet-based nanohybrids. The remarkable advantages of 2D inorganic nanosheet in photocatalytic activities of nanohybrids are discussed for their roles as photocatalyst/sensitizer, cocatalyst, and charge carrier acceptor/diffusion pathway. In the end of this review, we present overall conclusion and future perspectives for research directions about the 2D inorganic nanosheet-based hybrid-type photocatalysts.

Section snippets

Basic principles of hybrid-type photocatalysts

All the heterogeneous photocatalysts should possess semiconductive band structure composed of a filled valence band (VB) and an empty conduction band (CB) with intrinsic bandgap. The light absorption range of semiconductor is totally dependent on its bandgap energy, since the photocatalysis reaction can occur with appropriate positions of CB and VB located in-between redox potentials of reactants and products [[72], [73], [74]]. Thus, the bandgap energy and band position are crucial factors for

Characteristics and advantages of 2D inorganic nanosheets

Since 2D inorganic nanosheets can be used as effective building blocks for novel functional hybrid materials including photocatalysts [[94], [95], [96]], a great deal of research efforts has been devoted for the synthesis, characterization, and applications of diverse 2D inorganic nanosheets [[97], [98], [99]]. As a fruit of research activities, many kinds of synthetic approaches have been developed for 2D inorganic nanosheets, which can be classified into two categories of top-down and

Nanosheet-based hybrid-type photocatalyst

The application of 2D inorganic nanosheet as a building block for hybridization can provide an effective way to improve the photocatalyst performance of semiconducting compound. As mentioned in the introduction section, there are several factors for optimizing the photocatalytic activity of semiconductor; (1) the maximization of light absorption region, (2) the increase of the lifetime of photoexcited electrons and holes, (3) the improvement of transport kinetics to surface reaction sites, and

Role of 2D inorganic nanosheet in hybrid-type photocatalysts

As surveyed in the Section 4, the hybridization with 2D inorganic nanosheet is highly useful in improving the photocatalytic activity of many semiconducting compounds, which provides efficient methodology to explore new class of photocatalysts. The remarkably enhanced photocatalyst performance of 2D inorganic nanosheet-based hybrids originates from several beneficial roles of 2D nanosheet as photocatalytically active component, light sensitizer, charge reservoir, cocatalyst, and charge

Overall conclusion and perspective

Semiconductor-based photocatalysis has received a great deal of increasing research interest as one of the most important solutions for deepening crisis of fossil energy depletion and environmental pollution. Currently, main focus in the research of photocatalysts moves from the sanitization of harmful organic compounds to the conversion of solar energy to user-friendly chemical energy. Special attention has been devoted for the emerging application of heterogeneous photocatalyst-assisted

Acknowledgements

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).

Yun Kyung Jo currently received her Ph.D. degree in inorganic chemistry from Ewha Womans University (Supervisor: prof. Seong-Ju Hwang). Her research focuses on the synthesis and characterization of porous nanoarchitectures including 2-dimensional inorganic nanosheets applicable for photocatalysts, gas adsorbents, and electrocatalysts.

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    Yun Kyung Jo currently received her Ph.D. degree in inorganic chemistry from Ewha Womans University (Supervisor: prof. Seong-Ju Hwang). Her research focuses on the synthesis and characterization of porous nanoarchitectures including 2-dimensional inorganic nanosheets applicable for photocatalysts, gas adsorbents, and electrocatalysts.

    Jang Mee Lee received a B.S. degree in chemistry (2011) and a M.S. degree in inorganic chemistry (2013) from Ewha Womans University (Korea). She is supposed to finish her Ph. D. study under prof. Seong-Ju Hwang at the same university by the Feb. of 2018. Her research focuses on the synthesis and characterization of 2-dimensional inorganic nanosheet-based nanohybrid for diverse applications such as photocatalysis, Li-ion battery, and electrocatalysis. Especially, she is working on the in-situ XAS analysis to demonstrate the catalytic mechanism based on the local structure alteration of nano crystalline material.

    Suji Son is currently a MS student in the Department of Chemistry and Nanoscience at Ewha Womans University (Supervisor: prof. Seong-Ju Hwang). Her research interests include the design and application of nanostructured materials including 2D nanosheets for solar energy conversion.

    Seong-Ju Hwang received a B.S. degree in chemistry (1992) and a M.S. degree in inorganic chemistry (1994) from Seoul National University (Korea), a Ph.D. degree in inorganic chemistry from Universit? Bordeaux I (France) in 2001, and worked as a postdoc in Michigan State University (Supervisor: M. G. Kanatzidis). Prof. Hwang joined Department of Applied Chemistry at Konkuk University in 2002 and moved to Department of Chemistry & Nanoscience at Ewha Womans University in 2005. In 2014, he was designated as Ewha Fellow. Currently he is a director of “Center for Hybrid Interfacial Chemical Structure (SRC program funded by KRF)". His research focuses on the synthesis and characterization of low-dimensional nanostructured transition metal compounds applicable for energy production, energy storage, and environmental purification”.

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    These authors contributed equally.

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