Confined catalysis under the cover of two-dimensional (2D) materials has emerged as a promising approach for achieving highly effective catalysts in various essential reactions. Here, a porous cover structure is designed to boost the interfacial charge and mass transfer kinetics of 2D-covered catalysts. The improvement in catalytic performance is confirmed by the photoelectrochemical oxidation evolution reaction (OER) on a photoanode based on an n-Si substrate modified with a NiOx thin-film model electrocatalyst covered with a porous graphene (pGr) monolayer. Experimental results demonstrate that the pGr cover enhances the OER kinetics by balancing the charge and mass transfer at the photoanode and electrolyte interface compared to the intrinsic graphene cover and cover-free control samples. Theoretical investigations further corroborate that the pore edges of the pGr cover boost the intrinsic catalytic activity of active sites on NiOx by reducing the reaction overpotential. Furthermore, the optimized pores, which can be easily controlled by plasma bombardment, allow oxygen molecules produced in the OER to pass through without peeling off the pGr cover, thus ensuring the structural stability of the catalyst. This study highlights the significant role of the porous cover structure in 2D-covered catalysts and provides new insight into the design of high-performance catalysts.

Single-layer graphene (SLG) has drawn considerable interest in photoelectrochemical (PEC) cells due to its atomically flat pinhole-free structure and remarkable in-plane carrier mobility. It is challenging, however, to obtain efficient SLG-modified photoelectrodes for PEC water splitting mainly due to the inefficient charge transfer interface. Here, a transition metal oxide/SLG/transition metal sandwich structure modified n-Si-based model photoanode is constructed to regulate the interfacial charge transfer behavior for enhanced PEC water oxidation performance. In this sandwich configuration, SLG tailors the morphology, structure, and work function properties of surface metal electrocatalysts to obtain both higher thermodynamic photovoltage and faster kinetical charge transfer at the semiconductor/electrolyte interface. In addition, SLG promotes the surface catalytic reaction as an effective charge trap and storage layer. This study provides a new structural design to engineer the SLG interfacial properties for high-performance energy conversion devices.

Charge recombination is a critical problem limiting the efficiency of catalysts for solar water splitting. Constructing both single- and polycrystalline structures have been proposed to tackle this issue, however, comparison of the two is mainly focused on the crystallinity and a comprehensive analysis of the underlying reasons is lacking. Herein, we show that the enhancement in water-oxidation activity of the single crystalline photoanode is dominated by the lower surface charge recombination as compared to the polycrystalline one, taking hematite nanorod arrays prepared by gas phase cation exchange to exclude the influence of shape as the model catalyst. In contrast, the unexpected lower bulk charge separation efficiency of single crystal than that of polycrystal indicates that increasing the crystallinity is actually not the major factor for improving bulk charge transport efficiency. Our study sheds light on the structure-property relationship of monocrystal versus polycrystal in the hematite photoelectrochemical cell, beneficial to design of high-efficient catalysts for solar energy conversion.

Abstract As a powerful complement to positive photoconductance (PPC), negative photoconductance (NPC) holds great potential for photodetector. However, the slow response of NPC relative to PPC devices limits their integration. Here, we propose a facile covalent strategy for an ultrafast NPC hybrid 2D photodetector. Our transistor-based graphene/porphyrin model device with a rise time of 0.2 ms and decay time of 0.3 ms has the fastest response time in the so far reported NPC hybrid photodetectors, which is attributed to efficient photogenerated charge transport and transfer. Both the photosensitive porphyrin with an electron-rich and large rigid structure and the built-in graphene frame with high carrier mobility are prone to the photogenerated charge transport. Especially, the intramolecular donor-acceptor system formed by graphene and porphyrin through covalent bonding promotes photoinduced charge transfer. This covalent strategy can be applied to other nanosystems for high-performance NPC hybrid photodetector.

Conversion and storage of solar energy into fuels and chemicals by artificial photosynthesis has been considered as one of the promising methods to address the global energy crisis. However, it is still far from the practical applications on a large scale. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. This article provides a comprehensive review on the utilization of nanoarray structures in artificial photosynthesis of renewable fuels and high value‐added chemicals. First, basic principles of solar energy conversion and superiorities of using nanoarray structures in this field are described. Recent research progress on nanoarray structures in both abiotic and abiotic–biotic hybrid systems is then outlined, highlighting contributions to light absorption, charge transport and transfer, and catalytic reactions (including kinetics and selectivity). Finally, conclusions and outlooks on future research directions of nanoarray structures for artificial photosynthesis are presented. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. The recent research progress in the development of various nanoarray structures in both abiotic and abiotic–biotic hybrid artificial photosynthetic systems is summarized, highlighting their contributions to light absorption, charge transport and transfer, and catalytic reactions.

A comprehensive understanding of the role of the electrocatalyst in photoelectrochemical (PEC) water splitting is central to improving its performance. Herein, taking the Si-based photoanodes (n+p-Si/SiOx/Fe/FeOx/MOOH, M = Fe, Co, Ni) as a model system, we investigate the effect of the transition-metal electrocatalysts on the oxygen evolution reaction (OER). Among the photoanodes with the three different electrocatalysts, the best OER activity, with a low-onset potential of ∼1.01 V RHE, a high photocurrent density of 24.10 mA cm−2 at 1.23 V RHE, and a remarkable saturation photocurrent density of 38.82 mA cm−2, was obtained with the NiOOH overlayer under AM 1.5G simulated sunlight (100 mW cm−2) in 1 M KOH electrolyte. The optimal interfacial engineering for electrocatalysts plays a key role for achieving high performance because it promotes interfacial charge transport, provides a larger number of surface active sites, and results in higher OER activity, compared to other electrocatalysts. This study provides insights into how electrocatalysts function in water-splitting devices to guide future studies of solar energy conversion.

Two-dimensional (2D) van der Waals heterojunctions have many unique properties, and energy band modulation is central to applying these properties to electronic devices. Taking the 2D graphene/MoS2heterojunction as a model system, we demonstrate that the band structure can be finely tuned by changing the graphene structure of the 2D heterojunction via ultraviolet/ozone (UV/O3). With increasing UV/O3exposure time, graphene in the heterojunction has more defect structures. The varied defect levels in graphene modulate the interfacial charge transfer, accordingly the band structure of the heterojunction. And the corresponding performance change of the graphene/MoS2field effect transistor indicates the shift of the Schottky barrier height after UV/O3treatment. The result further proves the effective band structure modulation of the graphene/MoS2heterojunction by UV/O3. This work will be beneficial to both fundamental research and practical applications of 2D van der Waals heterojunction in electronic devices.

As a fourth fundamental two‐terminal circuit element, memristors have received great research interest as resistive random access memory (RRAM). Herein, a new memristor structure based on TiOx/HfOx or AlOx/HfOx multilayers with gradually varied thickness as the switching material is fabricated. The devices show forming‐free, self‐compliance, reliable multilevel resistive switching, and low switching voltage properties, and are promising for applications in future advanced integrated circuits. A novel memristor based on multilayer TiOx/HfOx or AlOx/HfOx with gradually varied thickness simultaneously realizes the properties of forming‐free, multilevel resistive switching, low switching voltage, and self‐compliance.

We synthesized single-crystal (Ga1–xZnx)(N1–xOx) nanowires with fully tunable compositions (0 < x < 1) using a customized chemical vapor deposition strategy. Despite the uniform distributions of component elements at the nanometer scale, X-ray absorption fine structure analysis in combination with ab initio multiple-scattering calculation verified the existence of a strong clustering tendency, i.e., the energetic preference of the valence-matched Ga-N and Zn-O pairs, in the synthesized nanowires. The strong clustering tendency plays a dominant role in determining the electronic band structures of the nanowires, causing a continuous band-gap reduction with increasing ZnO content, which is interpreted via a type II band alignment among the intracrystalline heterojunctions formed between the incorporated clusters and the host material. This, ultimately, makes the sample with the highest ZnO content show the highest water-splitting activity. Atomic arrangement engineering will provide an additional tool for band-gap engineering of semiconductor alloys, greatly benefiting the development of new functional materials for energy conversion applications.

For photocatalysts with a strong exciton effect, overall understanding and regulating exciton dynamics, which is central to optimizing performance, have been ignored. Herein, taking a carbon nitride derivative as the model system, we demonstrate the alteration of multiple band structures for regulating the overall exciton dynamics and accordingly the photocatalytic hydrogen evolution performance by introducing cyano defects. Both experimental and theoretical data show that with increasing defect concentration, the defect-mediated trap state (TS) increases the singlet exciton generation, facilitates the exciton transfer from the first singlet excited state to TS rather than to the triplet excited state, promotes exciton dissociation, and suppresses the singlet exciton recombination until reaching to the highest activity. However, excess defects will act as recombination centers, leading to severe exciton recombination and decreased activity. This work provides a new insight into an in-depth understanding of comprehensive regulation of exciton dynamics for high-efficiency solar energy conversion.

The sputtering deposition commonly adopted in the photoelectrochemical water splitting device usually causes structure damage and accordingly performance degradation. Taking the Si-based photoanode as a model system, we propose a facile strategy to enable sputtering deposition of the film without damaging the underlying layer by introducing a metal sputtering buffer layer, which ensures high-quality interface structure and excellent water oxidation performance. In our work, the Ni sputtering buffer layer avoids the damage on the Al2O3 passivation layer during the NiFe oxide electrocatalyst sputtering process. Consequently, a Si/Al2O3/Ni metal–insulator–semiconductor junction with a precise configuration is formed, which increases the Si barrier height by reducing the interface state density and suppressing Fermi-level pinning, thus enhancing the photovoltage in thermodynamics and accelerating the oxygen evolution reaction (OER) in kinetics. In addition, the dense NiFe oxide with high transmittance and antireflection properties functions as both an efficient OER electrocatalyst and a robust protective layer and contributes to efficient use of the incident light. The photoanode affords a high water oxidation activity with an onset potential of ∼0.92 V vs reversible hydrogen electrode (VRHE) and a high photocurrent density of ∼31 mA cm–2 at 1.23 VRHE after further incorporating the cocatalyst NiFe-layered double hydroxide and has no obvious decay after 330 h of continuous operation. This strategy is beneficial for developing energy conversion devices on a large scale.

Photocatalytic CO2 reduction is promising for reducing the greenhouse effect and producing renewable energy, but still shows low activity and selectivity due to the ineffective utilization of photogenerated charge carriers and insufficient active sites for CO2 adsorption and activation. Taking CdS nanocrystals as a model semiconductor, we demonstrate that graphdiyne, a new type of two-dimensional carbon allotrope uniquely formed from sp- and sp2-hybridized carbon, enhances CO2 photoreduction over CdS with higher activity, selectivity, and stability in the gas phase without any sacrificial agent compared to graphene. Both experimental and theoretical results prove that the chemical bonding between graphdiyne and CdS and sufficient CO2 adsorption sites due to the strong interfacial interaction-induced sulfur vacancies in CdS and more electron-deficient acetylenic linkages in graphdiyne lead to more efficient electron transfer and storage for the subsequent CO2 reduction reaction. The excellent properties of graphdiyne make it promising for applications in solar energy conversion.

Simultaneously improving the efficiency and stability on a large scale is significant for the development of photoelectrochemical (PEC) water splitting systems. Here, we demonstrated a novel design of GaP/GaPN core/shell nanowire (NW) decorated p-Si photocathode for improved PEC hydrogen production performance compared to that of bare p-Si photocathode. The formation of the p-n junction between p-Si and GaP NW promotes charge separation, and the lower conduction band position of GaPN relative to that of GaP further facilitates the transfer of photogenerated electrons to the electrode surface. In addition, the NW morphology both shortens the carrier collection distance and increases the specific surface area, which result in superior reaction kinetics. Moreover, introduction of N in GaP is beneficial for enhancing the light absorption as well as stability. Our efficient and facile strategy can be applied to other solar energy conversion systems as well.

For photoelectrochemical (PEC) devices, light irradiation direction changes photogenerated charge distribution and thus will affect the charge collection efficiency of the photoelectrodes. Herein, we studied the influence of irradiation direction on charge collection properties of α‐Fe2O3 photoanodes with different geometries, and demonstrated the controlled charge collection by modifying surface geometry. An irradiation direction‐dependent surface charge recombination was identified. In comparison with backside irradiation, frontside irradiation was found to be favorable to obtain high photocurrent because of the effective hole collection, except for the thick planar film due to its high surface recombination. This problem can be solved by patterning the planar film surface with nanonet geometry to promote surface charge transfer. As a result, the nanonet photoanode exhibited a higher photocurrent than the planar counterpart under frontside irradiation. Our finding provides useful guidance on designing high efficiency PEC devices. Irradiation direction affects charge collection: Frontside irradiation is generally beneficial for photohole collection and high photocurrent, but the high surface charge recombination in thick planar films makes it inferior compared to backside irradiation. Patterning planar film surfaces with nanonet structures can suppress charge recombination by accelerating photohole injection at surface.

Electrocatalysts (ECs) are indispensable for high-efficiency photoelectrochemical (PEC) water splitting, but the underlying mechanism for performance modulation is still not clear. Taking the α-Fe2O3 semiconductor (SC) decorated with the cobalt oxide ECs as the model photoanode system, we demonstrate the opposite changes of PEC water oxidation activities by tuning the charge-storage capacity of ECs. Holes stored in the EC can increase the hole density on the photoanode surface, which can benefit the multihole surface water oxidation reaction, as well as aggravate the SC–EC interfacial charge recombination due to the Coulomb attraction. Both experimental and theoretical data prove that the EC with low hole-storage capacity brings limited interfacial charge recombination, enabling faster hole injection to improve the water oxidation activity. In contrast, the EC with high hole-storage capacity causes severe interfacial charge recombination, hindering the hole injection, thus decreasing the water oxidation activity. As a result, the PEC activity of photoanodes changes nonmonotonically with increasing surface hole density. This study can provide insightful guidance to interface design for solar energy-conversion systems.

Integration of electrocatalysts (ECs) with photoabsorbers is indispensable for high-performance solar water splitting. However, how the interaction between different ECs affects the performance is less explored. In the model system of the Si-based photoanode paired with different transition-metal-based EC dual layers, the oxygen evolution reaction with high efficiency and stability is obtained at low cost via synergetic effects of dual ECs. The spontaneous mutual doping between the EC dual layers greatly increases the conductivity of the electrode, thus facilitating the interfacial charge transfer, and the EC interlayer with high hole-accumulation ability can dramatically improve the hole collection capacity of the EC overlayer. In addition, the dynamic cycle of dissolution, diffusion, and deposition of a tiny amount of metal species between the EC dual layers favors the electrode stability. This work provides insightful guidance to interface design of high-performance devices for solar energy conversion.

Severe charge recombination in solar water-splitting devices significantly limits their performance. To address this issue, we design a frustum of a cone nanograting configuration by taking the hematite and Au-based thin-film photoanode as a model system, which greatly improves the photoelectrochemical water oxidation activity, affording an approximately 10-fold increase in the photocurrent density at 1.23 V versus the reversible hydrogen electrode compared to the planar counterpart. The surface plasmon polariton-induced electric field in hematite plays a dominant role in efficiency enhancement by facilitating charge separation, thus dramatically increasing the incident photon-to-current efficiency (IPCE) by more than 2 orders of magnitude in the near band gap of hematite. And the relatively weak electric field caused by light scattering in the nanograting structure is responsible for the approximate maximum 20-fold increase in IPCE within a broadband wavelength range. Our scalable strategy can be generalized to other solar energy conversion systems.

Dual-band-gap systems are promising for solar water splitting due to their excellent light-harvesting capability and high charge-separation efficiency. However, a fundamental understanding of interfacial charge-transfer behavior in the dual-band-gap configuration is still incomplete. Taking CdS/reduced graphene oxide (CdS/RGO) nanoheterojunctions as a model solar water splitting system, we attempt here to highlight the interaction-dependent interfacial charge-transfer behavior based on both experimental observations and theoretical calculations. Experimental evidence points to charge transfer at the CdS-RGO interface playing a dominant role in the photocatalytic hydrogen production activity. By tuning the degree of reduction of RGO, the interfacial interaction, and, thereby, the charge transfer can be controlled at the CdS-RGO interface. This observation is supported by theoretical analysis, where we find that the interfacial charge transfer is a balance between the effective single-electron- and hole-transfer probability and the surface free electron and hole concentration, both of which are related to the surface potential and tailored by interfacial interaction. This mechanism is applicable to all systems for solar water splitting, providing a useful guidance for the design and study of heterointerfaces for high-efficiency energy conversion.

Because of inefficient charge utilization caused by localized π-electron conjugation and large exciton binding energy, the photoelectrochemical water-splitting efficiency of organic polymers is seriously limited. Taking the graphitic carbon nitride (g-CN) polymer as an example, we report a novel photoanode based on a vertically aligned g-CN porous nanorod (PNR) array prepared in situ, using a thermal polycondensation approach, with anodic aluminum oxide as the template. The g-CN PNR array exhibits an excellent photocurrent density of 120.5 μA cm-2 at 1.23 VRHE under one sun illumination, the highest reported incident photon-to-current efficiency of ∼15% at 360 nm, and an outstanding oxygen evolution reaction stability in 0.1 M Na2SO4 aqueous solution, which constitutes a benchmark performance among the reported g-CN-based polymer photoanodes without any sacrificial reagents. When compared with its planar counterpart, the enhanced performance of the PNR array results principally from its unique structure that enables a high degree of aromatic ring π-electron conjugation for higher mobility of charge carriers, provides a direct pathway for the electron transport to the substrate, produces a large portion of hole-accepting defect sites and space charge region to promote exciton dissociation, and also withstands more strain at the interface to ensure intimate contact with the substrate. This work opens a new avenue to develop nanostructured organic semiconductors for large-scale application of solar energy conversion devices.

Designing high-quality interfaces is crucial for high-performance photoelectrochemical (PEC) water-splitting devices. Here, we demonstrate a facile integration between polycrystalline n+p-Si and NiFe-layered double hydroxide (LDH) nanosheet array by a partially activated Ni (Ni/NiOx) bridging layer for the excellent PEC water oxidation. In this model system, the thermally deposited Ni interlayer protects Si against corrosion and makes good contact with Si, and NiOx has a high capacity of hole accumulation and strong bonding with the electrodeposited NiFe-LDH due to the similarity in material composition and structure, facilitating transfer of accumulated holes to the catalyst. In addition, the back illumination configuration makes NiFe-LDH sufficiently thick for more catalytically active sites without compromising Si light absorption. This earth-abundant multicomponent photoanode affords the PEC performance with an onset potential of ∼0.78 V versus reversible hydrogen electrode (RHE), a photocurrent density of ∼37 mA cm-2 at 1.23 V versus RHE, and retains good stability in 1.0 M KOH, the highest water oxidation activity so far reported for the crystalline Si-based photoanodes. This bridging layer strategy is efficient and simple to smooth charge transfer and make robust contact at the semiconductor/electrocatalyst interface in the solar water-splitting systems.

The semiconductor/electrolyte interface plays a crucial role in photoelectrochemical (PEC) water-splitting devices as it determines both thermodynamic and kinetic properties of the photoelectrode. Interfacial engineering is central for the device performance improvement. Taking the cheap and stable hematite (α-Fe2O3) wormlike nanostructure photoanode as a model system, we design a facile sacrificial interlayer approach to suppress the crystal overgrowth and realize Ti doping into the crystal lattice of α-Fe2O3 in one annealing step as well as to avoid the consequent anodic shift of the photocurrent onset potential, ultimately achieving five times increase in its water oxidation photocurrent compared to the bare hematite by promoting the transport of charge carriers, including both separation of photogenerated charge carriers within the bulk semiconductor and transfer of holes across the semiconductor-electrolyte interface. Our research indicates that understanding the semiconductor/electrolyte interfacial engineering mechanism is pivotal for reconciling various strategies in a beneficial way, and this simple and cost-effective method can be generalized into other systems aiming for efficient and scalable solar energy conversion.

CdS modified with reduced graphene oxide (RGO) has been widely demonstrated to be effective in the field of solar-energy conversion. However, the inherent mechanism of this superior property is still not thoroughly understood. Thus the photoelectrochemical method was employed to systemically investigate the synergetic effect between CdS and RGO. The result shows that the photoelectrochemical properties of RGO/CdS samples are sensitive to the relative ratio of RGO to CdS, and the photoelectrode with 1.0 wt % ratio of RGO possesses the best photoelectrochemical performance. Further investigation demonstrates that the synergetic effect between CdS and RGO directly influences the charge-transport property and band-structure of the composite, which is also supported by the X-ray photoelectron spectroscopy data and first-principle simulation, respectively.

Hydrogen production from solar water splitting has been considered as an ultimate solution to the energy and environmental issues. Over the past few years, graphene has made great contribution to improving the light-driven hydrogen generation performance. This article provides a comprehensive overview of the recent research progress on graphene-based materials for hydrogen evolution from light-driven water splitting. It begins with a brief introduction of the current status and basic principles of hydrogen generation from solar water splitting, and tailoring properties of graphene for application in this area. Then, the roles of graphene in hydrogen generation reaction, including an electron acceptor and transporter, a cocatalyst, a photocatalyst, and a photosensitizer, are elaborated respectively. After that, the comparison between graphene and other carbon materials in solar water splitting is made. Last, this review is concluded with remarks on some challenges and perspectives in this emerging field.