Nanometal materials play very important roles in solar-to-chemical energy conversion due to their unique catalytic and optical characteristics. They have found wide applications from semiconductor photocatalysis to rapidly growing surface plasmon-mediated heterogeneous catalysis. The recent research achievements of nanometals are reviewed here, with regard to applications in semiconductor photocatalysis, plasmonic photocatalysis, and plasmonic photo-thermocatalysis. As the first important topic discussed here, the latest progress in the design of nanometal cocatalysts and their applications in semiconductor photocatalysis are introduced. Then, plasmonic photocatalysis and plasmonic photo-thermocatalysis are discussed. A better understanding of electron-driven and temperature-driven catalytic behaviors over plasmonic nanometals is helpful to bridge the present gap between the communities of photocatalysis and conventional catalysis controlled by temperature. The objective here is to provide instructive information on how to take the advantages of the unique functions of nanometals in different types of catalytic processes to improve the efficiency of solar-energy utilization for more practical artificial photosynthesis.

The production of hydrogen fuels by using sunlight is an attractive and sustainable solution to the global energy and environmental problems. Platinum (Pt) is known as the most efficient co-catalyst in hydrogen evolution reaction (HER). However, due to its high-cost and limited-reserves, it is highly demanded to explore alternative non-precious metal co-catalysts with low-cost and high efficiency. Transition metal disulfides (TMDs) including molybdenum disulfide and tungsten disulfide have been regarded as promising candidates to replace Pt for HER in recent years. Their unique structural and electronic properties allow them to have many opportunities to be designed as highly efficient co-catalysts over various photo harvesting semiconductors. Recent progress in TMDs as photo-cocatalysts in solar hydrogen production field is summarized, focusing on the effect of structural matchability with photoharvesters, band edges tunability, and phase transformation on the improvement of hydrogen production activities. Moreover, recent research efforts toward the TMDs as more energy-efficient and economical co-catalysts for HER are highlighted. Finally, this review concludes by critically summarizing both findings and current perspectives, and highlighting crucial issues that should be addressed in future research activities.

Exploiting noble-metal-free cocatalysts is of huge interest for photocatalytic water splitting using solar energy. As an efficient cocatalyst in photocatalysis, MoS₂ is shown promise as a low-cost alternative to Pt for hydrogen evolution. Here we report a systematical study on controlled synthesis of MoS₂ with layer number ranging from ≈1 to 112 and their activities for photocatalytic H₂ evolution over commercial CdS. A drastic increase in photocatalytic H₂ evolution is observed with decreasing MoS₂ layer number. Particularly for the single-layer (SL) MoS₂, the SL-MoS₂/CdS sample reaches a high H₂ generation rate of ≈2.01 × 10⁻³m h⁻¹ in Na₂S–Na₂SO₃ solutions and ≈2.59 × 10⁻³m h⁻¹ in lactic acid solutions, corresponding to an apparent quantum efficiency of 30.2% and 38.4% at 420 nm, respectively. In addition to the more exposed edges and unsaturated active S atoms, valence band–XPS and Mott–Schottky plots analysis indicate that the SL MoS₂ has the more negative conduction band energy level than the H⁺/H₂ potential, facilitating the hydrogen reduction.

Hematite photoanodes are decorated with nanostructured FeOOH by photoelectrodeposition. An obvious cathodic shift in the photocurrent onset potential is observed, while four-times enhancement of photocurrent density enhancement is acheived with FeOOH present. This can be ascribed to the high reaction area for the structure and high electrocatalytic activity of nanostructured FeOOH, which increases the amount of photogenerated holes involved in the water oxidation reaction and accelerates the kinetics of water oxidation. Furthermore, the obtained Fe₂O₃/FeOOH photoanode achieves considerable O₂ evolution rate (10.1 μmol h⁻¹ cm⁻²) under AM 1.5 G illumination and is maintained for as long as 70 h. The Fe₂O₃/FeOOH films show visible light response, high photocurrent density, and long-term stability, and they are well qualified photoanode materials and a promising candidate for photoelectrochemical water splitting.

UiO-66, a zirconium based metal–organic framework, is incorporated with nanosized carbon nitride nanosheets via a facile electrostatic self-assembly method. This hybrid structure exhibits a large surface area and strong CO₂ capture ability due to the introduction of UiO-66. We demonstrate that electrons from the photoexcited carbon nitride nanosheet can transfer to UiO-66, which can substantially suppress electron–hole pair recombination in the carbon nitride nanosheet, as well as supply long-lived electrons for the reduction of CO₂ molecules that are adsorbed in UiO-66. As a result, the UiO-66/carbon nitride nanosheet heterogeneous photocatalyst exhibits a much higher photocatalytic activity for the CO₂ conversion than that of bare carbon nitride nanosheets. We believe this self-assembly method can be extended to other carbon nitride nanosheet loaded materials.

The photocatalytic reduction of Cr(VI) is investigated over iron(III)-based metal–organic frameworks (MOFs) structured as MIL-88B. It is found that MIL-88B (Fe) MOFs, containing Fe₃-μ₃-oxo clusters, can be used as photocatalyst for the reduction of Cr(VI) under visible light irradiation, which is due to the direct excitation of Fe₃-μ₃-oxo clusters. The amine-functionalized MIL-88B (Fe) MOFs (denoted as NH₂–MIL-88B (Fe)) shows much higher efficiency for the photocatalytic Cr(VI) reduction under visible-light irradiation compared with MIL-88B (Fe). It is revealed that in addition to the direct excitation of Fe₃-μ₃-oxo clusters, the amine functionality in NH₂–MIL-88B (Fe) can also be excited and then transferred an electron to Fe₃-μ₃-oxo clusters, which is responsible for the enhanced photocatalytic activity for Cr(VI) reduction. The enhanced photocatalytic activity for Cr(VI) reduction is also achieved for other two amine-functionalized iron(III)-based MOFs (NH₂–MIL-53 (Fe) and NH₂–MIL-101 (Fe)).

Efficient photocatalytic conversion of CO₂ into CO and hydrocarbons by hydrous hydrazine (N₂H₄⋅H₂O) is achieved on SrTiO₃/TiO₂ coaxial nanotube arrays loaded with Au–Cu bimetallic alloy nanoparticles. The synergetic catalytic effect by the Au–Cu alloy nanoparticles and the fast electron-transfer in SrTiO₃/TiO₂ coaxial nanoarchitecture are the main reasons for the efficiency, while N₂H₄⋅H₂O as the H source and electron donor provides a reducing atmosphere to protect the surface Cu atoms from oxidation, therefore maintaining the alloying effect which is the basis for the high photocatalytic activity and stability. This approach opens a feasible route to enhance the photocatalytic efficiency, which also benefits the development of photocatalysts and co-catalysts.

A novel CO₂ photoreduction method, CO₂ conversion through methane reforming into syngas (DRM) was adopted as an efficient approach to not only reduce the environmental concentration of the greenhouse gas CO₂ but also realize the net energy storage from solar energy to chemical energy. For the first time it is reported that gold, which was generally regarded to be inactive in improving the performance of a catalyst in DRM under thermal conditions, enhanced the catalytic performance of Rh/SBA-15 in DRM under visible-light irradiation (1.7 times, CO₂ conversion increased from 2100 to 3600 μmol g⁻¹ s⁻¹). UV/Vis spectra and electromagnetic field simulation results revealed that the highly energetic electrons excited by local surface plasmon resonances of Au facilitated the polarization and activation of CO₂ and CH₄ with thermal assistance. This work provides a new route for CO₂ photoreduction and offers a distinctive method to photocatalytically activate nonpolar molecules.

Efficient photocatalytic conversion of CO₂ into CO and hydrocarbons by hydrous hydrazine (N₂H₄⋅H₂O) is achieved on SrTiO₃/TiO₂ coaxial nanotube arrays loaded with Au–Cu bimetallic alloy nanoparticles. The synergetic catalytic effect by the Au–Cu alloy nanoparticles and the fast electron-transfer in SrTiO₃/TiO₂ coaxial nanoarchitecture are the main reasons for the efficiency, while N₂H₄⋅H₂O as the H source and electron donor provides a reducing atmosphere to protect the surface Cu atoms from oxidation, therefore maintaining the alloying effect which is the basis for the high photocatalytic activity and stability. This approach opens a feasible route to enhance the photocatalytic efficiency, which also benefits the development of photocatalysts and co-catalysts.

A novel CO₂ photoreduction method, CO₂ conversion through methane reforming into syngas (DRM) was adopted as an efficient approach to not only reduce the environmental concentration of the greenhouse gas CO₂ but also realize the net energy storage from solar energy to chemical energy. For the first time it is reported that gold, which was generally regarded to be inactive in improving the performance of a catalyst in DRM under thermal conditions, enhanced the catalytic performance of Rh/SBA-15 in DRM under visible-light irradiation (1.7 times, CO₂ conversion increased from 2100 to 3600 μmol g⁻¹ s⁻¹). UV/Vis spectra and electromagnetic field simulation results revealed that the highly energetic electrons excited by local surface plasmon resonances of Au facilitated the polarization and activation of CO₂ and CH₄ with thermal assistance. This work provides a new route for CO₂ photoreduction and offers a distinctive method to photocatalytically activate nonpolar molecules.

Asymmetric Janus nanostructures containing a gold nanocage (NC) and a carbon–titania hybrid nanocrystal (AuNC/(C–TiO₂)) are prepared using a novel and facile microemulsion-based approach that involves the assistance of ethanol. The localized surface plasmon resonance of the Au NC with a hollow interior and porous walls induce broadband visible-light harvesting in the Janus AuNC/(C–TiO₂). An acetone evolution rate of 6.3 μmol h⁻¹ g⁻¹ is obtained when the Janus nanostructure is used for the photocatalytic aerobic oxidation of iso-propanol under visible light (λ = 480–910 nm); the rate is 3.2 times the value of that obtained with C–TiO₂, and in photo-electrochemical investigations an approximately fivefold enhancement is obtained. Moreover, when compared with the core–shell structure (AuNC@(C–TiO₂) and a gold–carbon–titania system where Au sphere nanoparticles act as light-harvesting antenna, Janus AuNC/(C–TiO₂) exhibit superior plasmonic enhancement. Electromagnetic field simulation and electron paramagnetic resonance results suggest that the plasmon–photon coupling effect is dramatically amplified at the interface between the Au NC and C–TiO₂, leading to enhanced generation of energetic hot electrons for photocatalysis.

The photothermal conversion of CO₂ provides a straightforward and effective method for the highly efficient production of solar fuels with high solar-light utilization efficiency. This is due to several crucial features of the Group VIII nanocatalysts, including effective energy utilization over the whole range of the solar spectrum, excellent photothermal performance, and unique activation abilities. Photothermal CO₂ reaction rates (mol h⁻¹ g⁻¹) that are several orders of magnitude larger than those obtained with photocatalytic methods (μmol h⁻¹ g⁻¹) were thus achieved. It is proposed that the overall water-based CO₂ conversion process can be achieved by combining light-driven H₂ production from water and photothermal CO₂ conversion with H₂. More generally, this work suggests that traditional catalysts that are characterized by intense photoabsorption will find new applications in photo-induced green-chemistry processes.

Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

A series of upconversion luminescent erbium-doped SrTiO₃ (ABO₃-type) photocatalysts with different initial molar ratios of Sr/Ti have been prepared by a facile polymerized complex method. Er³⁺ ions, which were gradually transferred from the A to the B site with increasing Sr/Ti, enabled the absorption of visible light and the generation of high-energy excited states populated by upconversion processes. The local internal fields arising from the dipole moments of the distorted BO₆ octahedra promoted energy transfer from the high-energy excited states of Er³⁺ with B-site occupancy to the host SrTiO₃ and thus enhanced the band-to-band transition of the host SrTiO₃. Consequently, the erbium-doped SrTiO₃ species with B-site occupancy showed higher photocatalytic activity than those with A-site occupancy for visible-light-driven H₂ or O₂ evolution in the presence of the corresponding sacrificial reagents. The results generally suggest that the introduction of upconversion luminescent agents into host semiconductors is a promising approach to simultaneously harnessing low-energy photons and maintaining redox ability for photocatalytic H₂ and O₂ evolution and that the site occupancy of doped elements in ABO₃-type perovskite oxides greatly determines the photocatalytic activity.

A facile solvothermal epitaxial growth combined with a mild oxidation route has been developed for the fabrication of a magnetically recyclable Fe₃O₄/WO₃ core–shell visible-light photocatalyst. In this core–shell structured photocatalyst, visible-light-active WO₃ nanoplates (the shells) with high surface area are used as a medium to harvest absorbed photons and convert them to photogenerated charges, while conductive Fe₃O₄ microspheres (the cores) are used as charge collectors to transport the photogenerated charges. This is a new role for magnetite. The Fe₃O₄/WO₃ core–shell structured photocatalysts possess large surface-exposure area, high visible-light-absorption efficiency, stable recyclability, and efficient charge-separation properties, the combination of which has rarely been reported in other visible-light-active photocatalysts. Photoelectrochemical investigations verify that the core–shell structured Fe₃O₄/WO₃ has a more effective photoconversion capability than pure WO₃ or Fe₃O₄. At the same time, the visible-light photocatalytic ability of the Fe₃O₄/WO₃ photocatalyst has significantly enhanced activity in the photodegradation of organic-dye materials. The results presented herein provide new insights into core–shell materials as high-performance visible-light photocatalysts and their potential use in environmental protection.

Cubic perovskite structure photocatalysts of Na_0.5La_0.5TiO₃ and (Na_0.5La_0.5TiO₃)_1.00(LaCrO₃)_0.08 solid solution that consisted of well-defined single-crystal nanocubes were successfully prepared by means of facile and surfactant-free hydrothermal reactions for the first time. The results from different instrumental characterizations and theoretical calculations consistently confirmed the formation of nanocubic single-crystal solid solution of (Na_0.5La_0.5TiO₃)_1.00(LaCrO₃)_0.08, and clearly revealed the modification of its physicochemical properties compared with those of Na_0.5La_0.5TiO₃. In particular, the effective narrowing of the bandgap (from 3.19 to 2.25 eV) by Cr³⁺ in the solid solution made it possible to utilize visible light. The solid-solution configuration maintained the charge balance to preserve the valence of Cr³⁺ rather than Cr⁶⁺, and accommodated Cr³⁺ with high content to form new energy bands instead of localized impurity levels. The hydrothermal preparation strategy ensured the formation of single crystals with high purity, few defects, and regulated morphology; it also guaranteed the valences of Ti⁴⁺ and Cr³⁺ in the solid solution. Consequently, the recombination of photogenerated carriers could be effectively suppressed to benefit photocatalytic H₂ evolution. (Na_0.5La_0.5TiO₃)_1.00(LaCrO₃)_0.08 nanocubic single-crystal solid solution showed stable photocatalytic activity, and thus was proved to be a promising candidate for visible-light-driven photocatalytic H₂ evolution.

We present a general method to fabricate niobate nanoparticles (NPs) by using hexaniobate Lindqvist ions as niobium source. First, soft-chemical synthesis was adopted to prepare amorphous compound NPs, which subsequently served as parent bodies, affording not only nanoscale size but also optimal compositions and elements for the final annealed nanocrystalline niobates. By this method, a series of nanocrystalline niobates, including MNbO₄ (M = In, Ga, Fe), MNb₂O₆ (M = Ba, Sr, Ni, Cd, Pb), M_xFe_1–xNbO₄ (M = In, Ga), M_xNi_1–xNb₂O₆ (M = Ba, Sr), and (AgNb)_1–x(SrTi)_xO₃, were successfully prepared. Experimental results presented herein show that the compositions and components of niobate NPs can be adjusted as desired, markedly influencing the crystal phase, energy band structure, and photocatalytic performance of the niobate NPs.

A hierarchical macro-/mesoporous Ce_0.49Zr_0.37Bi_0.14O_1.93 solid-solution network has been synthesized on a large scale by means of a simple and general polymerization–carbonization–oxidation synthetic route. The as-prepared product has been characterized by SEM, XRD, TEM, BET surface area measurement, UV/Vis diffuse-reflectance spectroscopy, energy-dispersive X-ray spectroscopy (EDS), and photoelectrochemistry measurements. The photocatalytic activity of the product has been demonstrated through the photocatalytic degradation of methyl orange. Structural characterization has indicated that the hierarchical macro-/mesoporous solid-solution network not only contains numerous macropores, but also possesses an interior mesoporous structure. The mesopore size and BET surface area of the network have been measured as 2–25 nm and 140.5 m² g⁻¹, respectively. The hierarchical macro-/mesoporous solid-solution network with open and accessible pores was found to be well-preserved after calcination at 800 °C, indicating especially high thermal stability. Due to its high specific surface area, the synergistic effect of the coupling of macropores and mesopores, and its high crystallinity, the Ce_0.49Zr_0.37Bi_0.14O_1.93 solid-solution material shows a strong structure-induced enhancement of visible-light harvest and exhibits significantly improved visible-light photocatalytic activity in the photodegradation of methyl orange compared with those of its other forms, such as mesoporous hollow spheres and bulk particles.

In this work, hierarchical WO₃ hollow shells: dendrites, spheres and dumbbells have been successfully synthesized by simply calcining the acid-treated PbWO₄ and SrWO₄ as precursors at 500 °C for 2 h. These hollow structures were characterized using X-ray diffraction, scanning and transmission electron microscopies, Brunauer–Emmet–Teller surface area and diffuse reflectance spectroscopy, respectively. Possible formation mechanisms for hollow WO₃ shells consisted of tiny nanoplatelets were investigated in detail. Comparing with commercial WO₃ particles, all the obtained hollow shells with larger BET surface areas showed enhanced photocatalytic activities for the degradation of organic contaminants under visible light irradiation.