Noble Metal Cocatalysts Research Articles

Dynamics of Electron Transfer in CdS Photocatalysts Decorated with Various Noble Metals.

To address charge recombination in photocatalysis, the prevalent approach involves the use of noble metal cocatalysts. However, the precise factors influencing this performance variability based on cocatalyst selection have remained elusive. In this study, CdS hollow spheres loaded with distinct noble metal nanoparticles (Pt, Au, and Ru) are investigated by femtosecond transient absorption (fs-TA) spectroscopy. A more pronounced internal electric field leads to the creation of a larger Schottky barrier, with the order Pt-CdS > Au-CdS > Ru-CdS. Owing to these varying Schottky barrier heights, the interface electron transfer rate (Ke) and efficiency (ηe) of metal-CdS in acetonitrile (ACN) exhibit the following trend: Ru-CdS > Au-CdS > Pt-CdS. However, the trends of Ke and ηe for metal-CdS in water are different (Ru-CdS > Pt-CdS > Au-CdS) due to the influence of water, leading to the consumption of photogenerated electrons and affecting the metal/CdS interface state. Although Ru-CdS displays the highest Ke and ηe, its overall photocatalytic performance, particularly in H2 production, lags behind that of Pt-CdS due to the electron backflow from Ru to CdS. This work offers a fresh perspective on the origin of performance differences and provides valuable insights for cocatalyst design and construction.

Enhancing Z-scheme photocatalytic CO2 methanation at extended visible light (>600 nm): Insight into charge transport and surface catalytic reaction mechanisms

This article reports Z-scheme photocatalytic CO2 methanation at the extended visible light (>500 nm) in absence of sacrificial electron donor/acceptor and noble metal cocatalysts. This Z-scheme photocatalyst consists of reduced graphene oxide (rGO) sandwiched between two different redox active photocatalysts (Ca-doped Bi2O3 and Cu2O). The rGO acts as a mediator by relaying electrons from Ca-doped Bi2O3 and holes from Cu2O that enables Z-scheme spatial confinement of electrons in Cu2O while holes in Ca-doped Bi2O3. This charge carrier delocalization enforces the enhanced oxidation strength of holes in Ca-doped Bi2O3 and reduction strength of electrons in Cu2O. As a result, it has exhibited methane (CH4) production rate under wider range of visible light (up to 700 nm) absorption. This Z-scheme photocatalyst therefore advents a new ‘600 nm-class photocatalyst’ for CO2 methanation. With the help of 13C isotopic measurements, first principles calculations and photoelectrochemical measurements, we have revealed the atomistic insight into the charge transport and surface catalytic reactions mechanisms to understand the rate limiting factors towards unassisted PCR.

Metastable Ni(I)-TiO2-x Photocatalysts: Self-Amplifying H2 Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent.

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni0, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni+ surface states act as electron relay for electron transfer to form H2 from water, in the absence of sacrificial species or noble metal cocatalysts.

P-N Heterojunction Embedded CuS/TiO2 Bifunctional Photocatalyst for Synchronous Hydrogen Production and Benzylamine Conversion.

The coupling of photocatalytic hydrogen production and selective oxidation of benzylamine is a topic of significant research interest. However, enhancing the bifunctional photocatalytic activity in this context is still a major challenge. The construction of Z-scheme heterojunctions is an effective strategy to enhance the activity of bifunctional photocatalysts. Herein, a p-n type direct Z-scheme heterojunction CuS/TiO2 is constructed using metal-organic framework (MOF)-derived TiO2 as a substrate. The carrier density is measured by Mott-Schottky under photoexcitation, which confirms that the Z-scheme electron transfer mode of CuS/TiO2 is driven by the diffusion effect caused by the carrier concentration difference. Benefiting from efficient charge separation and transfer, photogenerated electrons, and holes are directedly transferred to active oxidation and reduction sites. CuS/TiO2 also exhibits excellent bifunctional photocatalytic activity without noble metal cocatalysts. Among them, the H2 evolution activity of the CuS/TiO2 is found to be 17.1 and 29.5 times higher than that of TiO2 and CuS, respectively. Additionally, the yields of N-Benzylidenebenzylamine (NBB) are 14.3 and 47.4 times higher than those of TiO2 and CuS, respectively.

Enhanced photocatalytic reaction activity of Cu2WS4/Ag2S composite photocatalyst

Coupling noble metal cocatalysts are considered to be one of the most effective approaches to improve photocatalytic performance, but their expensive price limits their practical application. Herein, a series of Cu2WS4/Ag2S nanocomposite photocatalysts were successfully synthesized by a simple ion exchange method using Ag2S as the cocatalyst. Compared with pure Cu2WS4 and Ag2S, the optimized 25 wt% Cu2WS4/Ag2S (CA25) nanocomposites exhibited excellent photocatalytic degradation capability of RhB (96 %,20 min). More importantly, the as-prepared CA25 sample showed a significant photocatalytic hydrogen yield (19.8 μmol/h), which is 9.4 times that of pure Cu2WS4. Systematic studies revealed that the introduction of Ag2S can not only broaden the light absorbance capacity of the composite photocatalyst, but also effectively improve the separation efficiency of the photogenerated carriers. In addition, our results showed that photogenerated electrons simultaneously reduce Ag2S into metallic Ag in the photocatalytic reaction process, that is, metallic Ag can act as the bridge to capture electrons and accelerate the transmission of photogenerated carriers, thus enabling further enhancement of photocatalytic performance. This work provides a new insight for the design of highly efficient composite photocatalysts, and highlights the important role of photocarrier separation in the photocatalytic process.

Promoted Photocatalytic Hydrogen Evolution by Tuning the Electronic State of Copper Sites in Metal‐Organic Supramolecular Assemblies

AbstractThe electronic state in terms of charge and spin of metal sites is fundamental to govern the catalytic activity of a photocatalyst. Herein, we show that modulation of the electronic states of Cu sites, without changing the coordination environments, of two metal‐organic supramolecular assemblies based on π⋅⋅⋅π stacking can significantly improve photocatalytic activity. The use of these heterogeneous photocatalysts, without using noble metal cocatalysts, resulted in an increase of the hydrogen production rate from 522 to 3620 μmol h−1 g−1. A systematical analysis revealed that the charge density and spin density of the metal centers are efficiently modulated via the modulation of the coordination fields around active copper (II) centers by the variation of the non‐coordination groups of terminal ligands, leading to the significant enhancement of photocatalytic activity. This work provides an insight into the electronic state of active metal centers for designing high‐performance photocatalysts.

Enhanced photocatalytic hydrogen production utilizing few-layered 1T-WS2/g-C3N4 heterostructures prepared with one-step calcination route

Two-dimensional sulfide is considered as one of the best alternative for noble metal cocatalyst, whereas few-layered 1T WS2 structure is very helpful to promote the photocatalytical activity for hydrogen generation. Herein, few-layered 1T-WS2/g-C3N4 composites have been prepared via a facile one-step calcination route and used as a noble metal-free catalyst for H2 evolution. The results showed that the optimized 1T-WS2/g-C3N4 has a H2 evolution rate of 10.08 mmol∙g−1 h−1, which is 180 and 2.44 times larger than that of pure pristine g-C3N4 and g-C3N4 (Pt), respectively. The enhanced photocatalytic activity can be explained as the optimized 1T-WS2/g-C3N4 sample has an increased light absorption (from visible to infrared region), better electrical conductivity and high density of active sites. The studies will be helpful for constructing high efficient heterostructured catalysts by loading 1T phase metal dichalcogenides 2D of material onto a two-dimensional material surface.

CoO/g-C3N4 p-n heterojunction catalyst in-situ loading CoP for enhanced photocatalytic H2 evolution

A prospective and pollution-free approach for producing hydrogen is photocatalytic water-splitting. Designing and developing a catalyst that can separate the photogenerated carriers and offer enough H+ reduction active sites are of paramount importance in boosting the efficiency of photocatalytic H2 evolution. Herein, the cocatalyst CoP were loaded on CoO/g-C3N4 p-n heterojunction (CoP/CoO/CN, referring g-C3N4 to as CN) by an in-situ phosphating method, and its H2 evolution activity was measured by simulating sunlight irradiation without adding any noble metal cocatalyst. The CoP/CoO/CN photocatalyst exhibited a better H2 evolution activity (4036.97 μmol·g−1·h−1) than Pt/CN. The mechanism study demonstrated that the excellent activity is resulted from the formed internal electric field in p-n heterojunction, and the enough active reduction sites from cocatalyst CoP. This work provides a rational design for in-situ loading cocatalysts on heterojunction and is helpful for developing efficient photocatalysts using the abundant low-cost elements.

Highly selective photocatalytic CO 2 reduction to ethylene in pure water by Nb 2O 5 nanoparticles with enriched surface –OH groups under simulated solar illumination

Photocatalytic CO₂ reduction to valuable chemical compounds could be a promising approach for carbon neutral practice. In this work, a simple and robust thermal decomposition process was developed with ammonium carbonate as both the precipitation agent and sacrificial template to produce fine Nb₂O₅ nanoparticles with a rich existence of surface -OH groups. It was found by Density Functional Theory (DFT) calculations and experiments that the rich existence of surface -OH groups enhanced the adsorption of both reactants (CO₂ and H₂O molecules) for the photocatalytic CO₂ reduction on these fine Nb₂O₅ nanoparticles, and a highly selective conversion of CO₂ to a high value chemical compound of ethylene (~ 68 mmol·g^-1·h^-1 with ~ 100% product selectivity) was achieved under simulated solar illumination without the usage of any sacrificial agents or noble metal cocatalysts. This synthesis process may also be readily applied as a surface engineering method to enrich the existence of surface -OH groups on various metal oxide-based photocatalysts for a broad range of technical applications.

Hollow Zn0.01Co0.99 Se2/ZnIn2S4 Z-scheme heterogeneous photocatalyst with Strong internal electric field and Excellent surface electron transfer capability for Efficient Hydrogen Production

Reasonable design and optimization of photocatalyst structure is an important strategy to realize sustainable hydrogen production. The rational design and interfacial tuning of Z-scheme heterojunctions remain challenging. Herein, a series of hollow cage-like Zn0.01Co0.99Se2/ZnIn2S4 Z-scheme heterojunction photocatalysts templated by cobalt-based bimetallic organic frameworks is successfully obtained via a hydrothermal approach. As expected, the optimized catalysts exhibit excellent hydrogen production performance without noble metal co-catalysts, which is mainly attributable to the synergistic effect of abundant active sites, strong light trapping and effective charge separation. Spectroscopic characterization and density functional theory (DFT) calculations indicate that the doping of Zn ions leading to more photogenerated carriers and faster charge transfer rates inside the heterojunction is another key factor in improving the catalytic performance. This work provides a feasible strategy for the optimization and design of Z-scheme heterojunctions through structural and interfacial engineering.

Variation of Ni and Mg dopants in spinel ZnIn2(O,S)4 microflower to generate [formula omitted] and [formula omitted] substitutional defects for a high-rate visible-light-induced hydrogen generation

Carbon-free based fuels are a promising solution to overcome the world energy crisis and environmental pollution. Therefore, hydrogen fuel is a critical and sustainable energy source to replace the limited carbon-based fossil fuels, which can be generated using a simple photocatalysis method. Herein, we demonstrated that Ni- and Mg-doped ZnIn2(O,S)4 (NMZI) photocatalysts prepared with a hydrothermal process performed an unprecedented HER rate without a noble-metal cocatalyst under visible-light illumination. The amounts of Ni and Mg dopants in NMZI were varied and the as-prepared NMZI were characterized with SEM, TEM, XRD, Raman, FTIR, XPS, EPR, DRS, PL, EIS, TPC, CV, and tested for photocatalytic HER. It was found that the Ni and Mg co-dopants induced substitutional surface defects of ZnIn−, NiIn−, and MgIn−. As a result, the formation of oxygen vacancy (VO2+) compensated the induced defect charges, which are crucial in the HER mechanism. EDS, ICP, and XPS analyses suggest a non-stoichiometric Zn(Zn,Ni,Mg,In)2(O,S)3.5 with Zn, Ni, and Mg occupying In sites in the structure. The NMZI catalyst could achieve a maximum HER rate of 9.4 mmol/g․h under visible-light illumination. It is believed that the HER performance improvement is due to the defect formation on NMZI catalyst that subsequently enhances the optical, electrochemical, and photocatalytic properties. The photocatalytic HER mechanism has been elucidated and discussed thoroughly in this work.

Waste ilmenite sludge-derived low-cost mesoporous Fe-doped TiO2: A versatile photocatalyst for enhanced visible light photocatalysis without a cocatalyst

Ilmenite (FeTiO3), a waste of the titania production industry, is proposed as a potential candidate for visible-light-driven photocatalysis due to its narrow band gap (Eg = 2.01–2.90 eV). However, to date, its photocatalytic performance has been very poor, even significantly lower than TiO2 (Eg = 3.2 eV) due to its high iron content. Herein we report waste ilmenite sludge-derived mesoporous Fe-doped titania (m-FDT) photocatalyst with significantly enhanced activity for a variety of photocatalytic processes, including photocatalytic CO2 reduction, water splitting, and dye-degradation. The m-FDT is developed via a simple hydrothermal process and has desirable properties such as a low band gap (1.72 eV), high surface area (137 m2/g), and lower charge recombination. This can be attributed to the formation of various defects (Fe2+, Fe3+, Ti3+, and O vacancies) due to the doping of Fe in the TiO2 lattice, as confirmed by various experiments and theoretical studies. The champion photocatalyst with an optimal Fe content (1.8 wt%) exhibits high methanol production (86.1 mmol/gcat), which is 100-fold higher than commercial TiO2 without noble metal cocatalyst and sacrificial agent. The designed photocatalyst shows excellent water-splitting activity (55.03 μmol/gcat) and dye degradation ability. This work presents a high-performance, low-cost, versatile photocatalyst for visible light photocatalysis and opens new avenues in semiconductor photocatalysis.

Photocatalytic hydrogen production from water and wastepaper on Pt/TiO2 composites

Ultra-small Pt nanoparticles loaded on TiO2 through in-situ photo-deposition for H2 production via photo reforming of wastepaper was prepared in this study. The effect of noble metal cocatalyst, Pt loading, content of paper (e.g., napkin), and concentration of NaOH was investigated. Consequently, a highest H2 production rate of 614.7 µmol/g/h was achieved in the optimum condition. It was found that both the solid paper and the dissolved part in the NaOH solution could scavenge the photo-generated holes and contribute to H2 production. Our research provides a potential route for the reuse of wastepaper to produce clean fuel.

TiN anchored mesoporous single-crystalline LaTiO2N with ohmic contact to expedite photocarrier separation for efficient photocatalytic water splitting

LaTiO2N is a promising photocatalyst for solar water splitting but is generally subject to a low activity due to inadequate separation of photocarriers (i.e. e- and h+). Loading noble metal cocatalysts such as Pt often has limited improvements because of their Schottky-type interfaces with LaTiO2N, resulting in a deficient electron collection at the surface catalytic sites. In this work, TiN nanoparticles have been firmly anchored at the surface of mesoporous single-crystalline LaTiO2N. As opposed to Pt, TiN has an ohmic contact with LaTiO2N due to a comparable work function and the structurally matched heterointerfaces. Such a tight and ohmic connection enables TiN to readily collect electrons from LaTiO2N, which in turn substantially improves the photocarrier separation and photocatalytic activity. These findings justify that TiN is an ideal relay for electron transfer between n-type semiconductors and noble metal cocatalysts which would otherwise be very difficult when the Schottky-type interfaces are formed.

Facile synthesis of a Z-scheme CeO2/C3N4 heterojunction with enhanced charge transfer for CO2 photoreduction

Using solar energy to convert CO2 into value-added fuel is crucial for the goal of global carbon neutrality. Effective separation of photogenerated charges is important for improving photocatalytic CO2 reduction efficiency. Herein, we report a facile in situ exfoliation and conversion strategy to synthesize a novel CeO2/C3N4 heterostructure by uniformly distributing CeO2 nanoparticles onto ultrathin porous C3N4 nanosheets. The ultrathin porous structure of C3N4 not only increases the specific surface area to provide more active sites but also effectively shortens the migration distance of photogenerated electron holes to avoid their recombination. In addition, the well-dispersed CeO2 on C3N4 shows an intimate interface contact, which allows more charges to be transferred through the increased interface surface area. The as-synthesized CeO2/C3N4 heterojunction with well-matched band gaps and a Z-scheme structure prolongs the lifetime of photo-induced charge carriers and maximizes the redox ability of the photocatalyst. Without a noble metal cocatalyst or a sacrificial agent, the CO2 photoreduction performance of the CeO2/C3N4 heterojunction is approximately 5-fold enhanced compared with that of bulk C3N4. This study provides a facile strategy for the design and practical application of direct Z-scheme pho-tocatalysts for sustainable energy conversion.

Transition-Metal Single Atom Anchored on MoS2 for Enhancing Photocatalytic Hydrogen Production of g-C3N4 Photocatalysts.

Single-atom catalyst technology with near-100% atomic utilization and a well-defined coordination structure has provided new ideas for designing high-performance photocatalysts, which is also beneficial for reducing the usage of noble metal cocatalysts. Herein, a series of single-atomic MoS2-based cocatalysts where monoatomic Ru, Co, or Ni modify MoS2 (SA-MoS2) for enhancing the photocatalytic hydrogen production performance of g-C3N4 nanosheets (NSs) are rationally designed and synthesized. The 2D SA-MoS2/g-C3N4 photocatalysts with Ru, Co, or Ni single atoms show similar enhanced photocatalytic activity, and the optimized Ru1-MoS2/g-C3N4 photocatalyst has the highest hydrogen production rate of 11115 μmol/h/g, which is about 37 and 5 times higher than that of pure g-C3N4 and MoS2/g-C3N4 photocatalysts, respectively. Experimental and density functional theory calculation results reveal that the enhanced photocatalytic performance is mainly attributed to the synergistic effect and intimate interface between SA-MoS2 with well-defined coordination single-atomic structures and g-C3N4 NSs, which is conducive to the rapid interfacial charge transport, and the unique single-atomic structure of SA-MoS2 with modified electronic structure and appropriate hydrogen adsorption performance offers abundant reactive sites for enhancing the photocatalytic hydrogen production performance. This work provides new insight into improving the cocatalytic hydrogen production performance of MoS2 by a single-atomic strategy.

Synergism of sulfur vacancy and Schottky junction in Ni/ZnIn2S4 nanosheet assembly for efficient charge separation and photocatalytic hydrogen evolution

Surface and interface engineering play vital roles on improving the photocatalytic hydrogen evolution. Herein, Ni/ZnIn2S4 nanosheet assembly with sulfur vacancy and Schottky junction (Ni/DZIS) is fabricated through facile ethanediol-assisted solvothermal method. Density functional theory (DFT) calculation reveals that the Ni is absorbed at the hollow site of three S atoms next to sulfur vacancy on the ZnIn2S4 (006) face due to the formation of sulfur vacancy. The resultant Ni/DZIS Schottky junction exhibits high photocatalytic H2 evolution rate of 703 μmol g−1h−1 without any noble metal cocatalysts, which is 2.6 times higher than that of pristine ZnIn2S4 nanosheets. The improved photocatalytic performance is mainly attributed to the synergism of sulfur vacancy and Schottky junction accelerating the photogenerated charge transfer and thus inhibiting the charge recombination. Moreover, the photocatalytic performance of Ni/DZIS nearly keeps constant even after six recycles, indicating the high stability and potential applications in field of energy. This work provides a convenient strategy for constructing efficient defective heterojunction assembly photocatalysts via surface and interface engineering.

An electron-hole rich dual-site nickel catalyst for efficient photocatalytic overall water splitting

Photocatalysis offers an attractive strategy to upgrade H2O to renewable fuel H2. However, current photocatalytic hydrogen production technology often relies on additional sacrificial agents and noble metal cocatalysts, and there are limited photocatalysts possessing overall water splitting performance on their own. Here, we successfully construct an efficient catalytic system to realize overall water splitting, where hole-rich nickel phosphides (Ni2P) with polymeric carbon-oxygen semiconductor (PCOS) is the site for oxygen generation and electron-rich Ni2P with nickel sulfide (NiS) serves as the other site for producing H2. The electron-hole rich Ni2P based photocatalyst exhibits fast kinetics and a low thermodynamic energy barrier for overall water splitting with stoichiometric 2:1 hydrogen to oxygen ratio (150.7 μmol h−1 H2 and 70.2 μmol h−1 O2 produced per 100 mg photocatalyst) in a neutral solution. Density functional theory calculations show that the co-loading in Ni2P and its hybridization with PCOS or NiS can effectively regulate the electronic structures of the surface active sites, alter the reaction pathway, reduce the reaction energy barrier, boost the overall water splitting activity. In comparison with reported literatures, such photocatalyst represents the excellent performance among all reported transition-metal oxides and/or transition-metal sulfides and is even superior to noble metal catalyst.

Long-range electron synergy over Pt1-Co1/CN bimetallic single-atom catalyst in enhancing charge separation for photocatalytic hydrogen production

The development of novel single-atom catalysts with optimal electron configuration and economical noble-metal cocatalyst for efficient photocatalytic hydrogen production is of great importance, but still challenging. Herein, we fabricate Pt and Co single-atom sites successively on polymeric carbon nitride (CN). In this Pt1-Co1/CN bimetallic single-atom catalyst, the noble-metal active sites are maximized, and the single-atomic Co1N4 sites are tuned to Co1N3 sites by photogenerated electrons arising from the introduced single-atomic Pt1N4 sites. Mechanism studies and density functional theory (DFT) calculations reveal that the 3d orbitals of Co1N3 single sites are filled with unpaired d-electrons, which lead to the improved visible-light response, carrier separation and charge migration for CN photocatalysts. Thereafter, the protons adsorption and activation are promoted. Taking this advantage of long-range electron synergy in bimetallic single atomic sites, the photocatalytic hydrogen evolution activity over Pt1-Co1/CN achieves 915.8 mmol·g-1Pt·h−1, which is 19.8 times higher than Co1/CN and 3.5 times higher to Pt1/CN. While this electron-synergistic effect is not so efficient for Pt nanoclusters. These results demonstrate the synergistic effect at electron-level and provide electron-level guidance for the design of efficient photocatalysts.

Implanting nitrogen-doped graphene quantum dots on porous ultrathin carbon nitride for efficient metal-free photocatalytic hydrogen evolution

Mimicking natural photosynthesis to make solar power into hydrogen energy is widely perceived as a promising and sustainable way to alleviate the energy crisis and environmental pollution. However, adding metallic cocatalysts and even noble metal cocatalysts into carbon nitride-based photocatalysts still seriously restrict the practical development of photocatalytic water splitting. To address these issues, we carefully design and construct a metal-free photocatalytic system through grafting 0D nitrogen-doped graphene quantum dots (N-GQDs) on 2D porous ultrathin carbon nitride (PUCN). The incorporated N-GQDs can act as electron collectors to promote the charge separation/transfer on PUCN, and also can extend the photoabsorption region of PUCN. Thus, the metal-free N-GQDs/PUCN photocatalyst exhibits a drastically enhanced hydrogen production rate of 1248 μmol g−1 h−1, which fetches up to about 208 times as high as that of PUCN (6 μmol g−1 h−1). Notably, the photocatalytic performance of N-GQDs/PUCN is close to that of Pt/PUCN with the same cocatalyst amount (1706 μmol g−1 h−1), and metal-free N-GQDs/PUCN photocatalyst outperforms most of the reported carbon nitride-based photocatalysts with metallic cocatalysts for photocatalytic water splitting. This work offers new insight to construct metal-free carbon nitride-based photocatalysts, which will be beneficial for the practical development of photocatalytic water splitting.