Single metal atoms confined in two dimensional (2D) materials have gained substantial attention as potential heterogeneous catalysts for various electrochemical applications. Single-atom catalysts ...(SACs) can be defined as a class of isolated metal atoms that are either atomically dispersed or coordinated with neighboring surface atoms of an appropriate support. Unlike nanoparticles or bulk materials, SACs offer unique characteristics which turn them as superior candidate for various catalytic applications. This review aims to summarize recent advances in various synthetic approaches and characterization techniques used to design different SACs. After this overview, we focus our discussion on single atom-2D support interactions, followed by recent progress in single atom incorporated 2D catalysts for water splitting applications, which includes both electrocatalytic and photocatalytic hydrogen production. Finally, we summarize the current challenges and the future outlook exists for the rational design of single atom based new catalyst with high catalytic activity, better stability and selectivity for various sustainable energy conversion applications.
•Overview on recent advances in single atom catalyst (SACs) synthetic approaches and characterizations.•A deep insights on interaction between SACs and two dimensional (2D) supports.•SACs/2D supports for water splitting applications.•Current challenges and future outlook on SACs/2D supports for energy conversion and storage applications.
Photoelectrochemical water splitting is one of the viable approaches to produce clean hydrogen energy from water. Herein, we report MoS2/Si-heterojunction (HJ) photocathode for PEC H2 production. The ...MoS2/Si-HJ photocathode exhibits exceptional PEC H2 production performance with a maximum photocurrent density of 36.33 mA/cm2, open circuit potential of 0.5 V vs. RHE and achieves improved long-term stability up to 10 h of reaction time. The photocurrent density achieved by MoS2/Si-HJ photocathode is significantly higher than most of the MoS2 coupled Si-based photocathodes reported elsewhere, indicating excellent PEC H2 production performance.
A titanium carbide (Ti3C2Tx) MXene is employed as an efficient solid support to host a nitrogen (N) and sulfur (S) coordinated ruthenium single atom (RuSA) catalyst, which displays superior activity ...toward the hydrogen evolution reaction (HER). X‐ray absorption fine structure spectroscopy and aberration corrected scanning transmission electron microscopy reveal the atomic dispersion of Ru on the Ti3C2Tx MXene support and the successful coordination of RuSA with the N and S species on the Ti3C2Tx MXene. The resultant RuSA‐N‐S‐Ti3C2Tx catalyst exhibits a low overpotential of 76 mV to achieve the current density of 10 mA cm−2. Furthermore, it is shown that integrating the RuSA‐N‐S‐Ti3C2Tx catalyst on n+np+‐Si photocathode enables photoelectrochemical hydrogen production with exceptionally high photocurrent density of 37.6 mA cm−2 that is higher than the reported precious Pt and other noble metals catalysts coupled to Si photocathodes. Density functional theory calculations suggest that RuSA coordinated with N and S sites on the Ti3C2Tx MXene support is the origin of this enhanced HER activity. This work would extend the possibility of using the MXene family as a solid support for the rational design of various single atom catalysts.
Ti3C2Tx MXene is demonstrated as a 2D solid support to host a ruthenium single atom (RuSA) catalyst for water splitting. The resultant RuSA‐N‐S‐Ti3C2Tx catalyst coupled with n+np+‐Si photocathode enables photoelectrochemical H2 production with exceptionally high photocurrent density of 37.6 mA cm−2 under AM 1.5G illumination.
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•Diffused sunlight is firstly used as an effective source for the degradation of organics.•More than 10 fold synergistic effect is achieved by sono-photocatalysis.•rGO enhances the ...degradation efficiency up to 54% as compared with CuO–TiO2 alone.•Plausible mechanism and intermediates formed are supported with experimental studies.
Diffused sunlight is found to be an effective light source for the efficient degradation and mineralization of organic pollutant (methyl orange as a probe) by sono-photocatalytic degradation using reduced graphene oxide (rGO) supported CuO–TiO2 photocatalyst. The prepared catalysts are characterized by XRD, XPS, UV–vis DRS, PL, photoelectrochemical, SEM-EDS and TEM. A 10 fold synergy is achieved for the first time by combining sonochemical and photocatalytic degradation under diffused sunlight. rGO loading augments the activity of bare CuO–TiO2 more than two fold. The ability of rGO in storing, transferring, and shuttling electrons at the heterojunction between TiO2 and CuO facilitates the separation of photogenerated electron–hole pairs, as evidenced by the photoluminescence results. The complete mineralization of MO and the by-products within a short span of time is confirmed by TOC analysis. Further, hydroxyl radical mediated degradation under diffused sunlight is confirmed by LC–MS. This system shows similar activity for the degradation of methylene blue and 4-chlorophenol indicating the versatility of the catalyst for the degradation of various pollutants. This investigation is likely to open new possibilities for the development of highly efficient diffused sunlight driven TiO2 based photocatalysts for the complete mineralization of organic contaminants.
Extending the absorption to the visible region by tuning the optical band-gap of semiconductors and preventing charge carrier recombination are important parameters to achieve a higher efficiency in ...the field of photocatalysis. The inclusion of reduced graphene oxide (rGO) support in photocatalysts is one of the key strategies to address the above-mentioned issues. In this study, rGO supported AgI-mesoTiO2 photocatalysts were synthesized using a sonochemical approach. The physical effects of ultrasound not only improved the crystallinity of AgI-mesoTiO2 but also increased the surface area and loading of the AgI-mesoTiO2 nanocomposite on rGO sheets. The low intense oxygen functionalities (C-O-C and COOH groups) peak observed in the high resolution C1s spectrum of a hybrid AgI-mesoTiO2-rGO photocatalyst clearly confirmed the successful reduction of graphene oxide (GO) to rGO. The interfacial charge transfer between the rGO and the p-n junction of heterostructured photocatalysts has decreased the band-gap of the photocatalyst from 2.80 to 2.65 eV. Importantly, the integration of rGO into AgI-mesoTiO2 composites serves as a carrier separation centre and provides further insight into the electron transfer pathways of heterostructured nanocomposites. The individual effects of photo-generated electrons and holes over rGO on the photocatalytic degradation efficiency of rhodamine (RhB) and methyl orange (MO) using AgI-mesoTiO2-rGO photocatalysts were also studied. Our experimental results revealed that photo-generated superoxide (O2(-)˙) radicals are the main reactive species for the degradation of MO, whereas photo-generated holes (h(+)) are responsible for the degradation of RhB. As a result, 60% enhancement in MO degradation was observed in the presence of rGO in comparison to that of the pure AgI-mesoTiO2 photocatalyst. This is due to the good electron acceptor and the ultrafast electron transfer properties of rGO that can effectively reduce the molecular oxygen to produce a large amount of reactive O2(-)˙ radicals. However, in the case of RhB degradation, h(+) is the main reactive species which showed a slightly increased photocatalytic activity (12%) in the presence of rGO support where the role of rGO is almost negligible. This study suggests the effective roles of rGO for the degradation of organics, i.e., the rate of photocatalytic degradation also depends on the nature of compound rather than rGO support.
Photocatalysis is an effective approach for the removal of heavy metal ions present in the aquatic bodies. In this report, TiO
2
nanoparticles were successfully functionalized with 2-naphthol (2-NAP) ...using simple and scalable condensation reaction. The prepared photocatalyst was demonstrated as superior visible light photocatalyst for the effective reduction of Cr(
vi
). The 2-NAP functionalized TiO
2
displayed a remarkable enhancement in the photocatalytic reduction of Cr(
vi
) under visible light irradiation (
λ
> 400 nm). The maximum Cr(
vi
) reduction of about 100% (7 fold higher activity than bare TiO
2
) was achieved within 3 h. The discernible enhancement in the photocatalytic reduction of TiO
2
-2-NAP can be ascribed to improved optical absorption in visible region, high crystallinity of TiO
2
and high surface area. In addition, the photogenerated electron transfer from 2-NAP to TiO
2
(ligand to metal transfer) can significantly improved the photocatalytic performance than bare TiO
2
counterparts. Therefore, the functionalization of metal oxides with organic ligands can open new directions to overcome the existing limitations in photocatalysis.
Visible light active 2-naphthol functionalized TiO
2
(TiO
2
-2-NAP) photocatalytic system is developed for the 100% reduction of Cr(
vi
) in aqueous medium.
Nitrogen-doped graphene quantum dots (N-GQDs) were decorated on a three-dimensional (3D) MoS2–reduced graphene oxide (rGO) framework via a facile hydrothermal method. The distribution of N-GQDs on ...the 3D MoS2–rGO framework was confirmed using X-ray photoelectron spectroscopy, energy dispersive X-ray elemental mapping, and high-resolution transmission electron microscopy techniques. The resultant 3D nanohybrid was successfully demonstrated as an efficient electrocatalyst toward the oxygen reduction reaction (ORR) under alkaline conditions. The chemical interaction between the electroactive N-GQDs and MoS2–rGO and the increased surface area and pore size of the N-GQDs/MoS2–rGO nanohybrid synergistically improved the ORR onset potential to +0.81 V vs reversible hydrogen electrode (RHE). Moreover, the N-GQDs/MoS2–rGO nanohybrid showed better ORR stability for up to 3000 cycles with negligible deviation in the half-wave potential (E 1/2). Most importantly, the N-GQDs/MoS2–rGO nanohybrid exhibited a superior methanol tolerance ability even under a high concentration of methanol (3.0 M) in alkaline medium. Hence, the development of a low-cost metal-free graphene quantum dot-based 3D nanohybrid with high methanol tolerance may open up a novel strategy to design selective cathode electrocatalysts for direct methanol fuel cell applications.
Ammonia (NH3) is attracted as a potential carbon free energy carrier and as important feedstock for most of the fertilizers, chemicals, pharmaceutical related products. NH3 is industrially produced ...by conventional Haber–Bosch process under harsh experimental conditions (high temperature and high pressure), and this process requires high-energy consumption and produces large amount of CO2 emissions into the atmosphere. Therefore, there is an urgent need to develop an alternative and sustainable route for NH3 production under ambient conditions. Recently, electrocatalytic N2 reduction to NH3 production has attracted as a potential approach, but achieving high NH3 yield and Faradaic efficiency, and avoiding competitive hydrogen-evolution reaction (HER) are still challenging. Nitrate/nitrite (NO3−/NO2−) is the widely reported contaminant for eutrophication and carcinogens, which can be utilized as a nitrogen resource for electrocatalytic NO3−/NO2− reduction to NH3 (NRA) via eight/six-electron transfer process. Unfortunately, electrocatalytic NRA using metal nanomaterials are rarely investigated. In this review, we discuss the electrocatalytic NRA performance containing reactivity, selectivity, Faradaic efficiency and cycling stability of metal nanocatalysts, bio-inspired metalloenzymes and bioelectrochemical system. After this overview, we investigate the key factors, rate-determining step and the reaction mechanism that controlling the NRA performance. Finally, we summarize the challenges and future pathways guiding the design of effective nanomaterials and reaction systems to promote the industrial application of electrocatalytic NRA.
This paper comprehensively reviews electrocatalytic NRA performance of the metal nanomaterials, bio-inspired metalloenzymes and bioelectrochemical systems. An insight into the rate-determining step and reaction mechanism of NRA process is summarized. The challenges and perspectives are put forward for the future design and application of advanced electrocatalytic NRA systems under low-temperature conditions. Display omitted
•This review summarizes the recent advances in electrocatalytic NRA.•The strategies for NH3 yield and Faradic efficiency improvement are discussed.•Key factors and fundamental mechanisms of NRA are described.•Challenges and future pathways of NRA are summarized.
Photoelectrochemical water splitting is one of the viable approaches to produce clean hydrogen energy from water. Herein, we report MoS
/Si-heterojunction (HJ) photocathode for PEC H
production. The ...MoS
/Si-HJ photocathode exhibits exceptional PEC H
production performance with a maximum photocurrent density of 36.33 mA/cm
, open circuit potential of 0.5 V vs. RHE and achieves improved long-term stability up to 10 h of reaction time. The photocurrent density achieved by MoS
/Si-HJ photocathode is significantly higher than most of the MoS
coupled Si-based photocathodes reported elsewhere, indicating excellent PEC H
production performance.
Solar water splitting provides a promising path for sustainable hydrogen production and solar energy storage. In recent times, metal–organic frameworks (MOFs) have received considerable attention as ...promising materials for diverse solar energy conversion applications. However, their photocatalytic performance is poor and rarely explored due to rapid electron–hole recombination. Herein, we have developed a material MOF@rGO that exhibits highly enhanced visible-light photocatalytic activity. A real-time investigation reveals that a strong π–π interaction between MOF and rGO is responsible for efficient separation of electron–hole pairs, and thereby enhances the photocatalytic hydrogen production activity. Surprisingly, MOF@rGO showed ∼9.1-fold enhanced photocatalytic hydrogen production activity compared to that of pristine MOF. In addition, it is worth mentioning here that remarkable apparent quantum efficiency (0.66%) is achieved by π–π interaction mediated charge carrier separation.
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