Oxygen vacancies mediated in-situ growth of noble-metal (Ag,Au, Pt) nanoparticles on 3D TiO2 hierarchical spheres for efficient photocatalytic hydrogen evolution from water splitting

Defect engineering is effective to extend the light absorption range of TiO₂. However, the oxygen vacancy defects in TiO₂ may serve as recombination centers, hampering the separation and transfer of photo-generated charges. Here, we present a strategy of in-situ depositing noble-metal (M = Ag, Au or Pt) nanoparticles (NPs) on defective 3D TiO₂ hierarchical spheres (THS) with large surface area through the redox reaction between metal ions in solution and the electrons trapped at oxygen vacancies in THS. The oxygen vacancies at the THS surface are consumed, resulting in direct contact between TiO₂ and noble-metal NPs, while the other oxygen vacancies in the bulk are retained to promote visible light absorption. The noble-metal NPs with well-controlled size and distribution throughout the porous hierarchical structure not only facilitate the generation of electron-hole pairs in THS due to the effect of surface plasmon-induced resonance energy transfer (SPRET) from noble-metal NPs to TiO₂, but also expediate the electron transfer from TiO₂ to noble-metal NPs due to the Schottky junction at the TiO₂/M interface. Therefore, THS-M shows improved photocatalytic performance in water splitting compared to THS. The optimum performance is achieved on THS-Pt (13.16 mmol h⁻¹g⁻¹) under full-spectrum (UV–Vis) irradiation but on THS-Au (1.49 mmol h⁻¹g⁻¹) under visible-light irradiation. The underlying mechanisms are proposed from the surface plasmon resonance of noble-metal NPs as well as the Schottky junction at the TiO₂/M interface.     

Photoelectrocatalytic hydrogen production of heterogeneous photoelectrodes with different system configurations of CdSe nanoparticles,Au nanocrystals and TiO2 nanotube arrays

The different configurations of CdSe nanoparticles, Au nanocrystals and TiO₂ nanotube arrays play an important role in the photoelectrochemical behavior and photoelectrocatalytic hydrogen production of this heterogeneous photoelectrode system. It is discovered that the photoelectrocatalytic hydrogen production of the TiO₂–CdSe–Au photoelectrode (1.724 mmol g⁻¹ h⁻¹) is about 4 times that of the TiO₂–Au–CdSe photoelectrode (0.430 mmol g⁻¹ h⁻¹) under visible light irradiation. From the comprehensive investigation of their photoelectrochemical behaviors, it is illustrated that the interfacial electrical field has distinct effects on the separation and transportation of photogenerated carriers in these heterostructure photoelectrodes. The directions of the interfacial electrical fields formed at TiO₂–Au and Au–CdSe interfaces are opposite in the TiO₂–Au–CdSe photoelectrode, which hinders the separation of photogenerated electron-hole pairs and subsequent transportation of photogenerated carriers. On the contrary, the directions of the interfacial electrical fields formed at TiO₂–CdSe and CdSe–Au interfaces are identical in the TiO₂–CdSe–Au photoelectrode, which promotes the separation of photogenerated excitons and subsequently enhances their transportation for enlarged photocurrent density. The results of photoelectrocatalytic hydrogen production also confirm our assumption.     

Biomass-derived hierarchical porous CdS/M/TiO2 (M = Au,Ag, pt,pd) ternary heterojunctions for photocatalytic hydrogen evolution

We demonstrate a general method for the synthesis of biomass-derived hierarchical porous CdS/M/TiO₂ (M = Au, Ag, Pt, Pd) ternary heterojunctions for efficient photocatalytic hydrogen evolution. A typical biomass—wood are used as the raw sources while five species of wood (Fir, Ash, White Pine, Lauan and Shiraki) are chosen as templates for the synthesis of hierarchical porous TiO₂. The as-obtained products inherited the hierarchical porous features with pores ranging from micrometers to nanometers, with improved photocatalytic hydrogen evolution activity than non-templated counterparts. Noble metals M (M = Pt, Au, Ag, Pd) and CdS are loaded via a two-step photodeposition method to form core (metal)/shell (CdS) structures. The photocatalytic modules—CdS(shell)/metal (core)/TiO₂ heterostructures, have demonstrated to increase visible light harvesting significantly and to increase the photocatalytic hydrogen evolution activity. The H₂ evolution rates of CdS/Pd/TiO₂ ternary heterostructures are about 6.7 times of CdS/TiO₂ binary heterojunctions and 4 times higher than Pd/CdS/TiO₂ due to the vertical electron transfer process. The design of such system is beneficial for enhanced activity from morphology control and composition adjustment, which would provide some new pathways for the design of promising photocatalytic systems for enhanced performance.     

TiO2 hollow spheres with separated Au and RuO2 co-catalysts for efficient photocatalytic water splitting

Hollow mesoporous TiO₂ photocatalysts with dual co-catalysts, located at specific positions, were prepared using Polystyrene (PS) as sacrificial templates. Au nanoparticles (NPs) were in situ loaded on the surface of PS spheres and the resulting nanocomposites were coated with TiO₂ shell using sol-gel reaction. The outer surface of core-shell spheres was impregnated with Ru and the subsequent calcination produced hollow anatase spheres with Au and RuO₂ dual co-catalysts. The hollow mesoporous spheres of Au@TiO₂@RuO₂ were proved by various techniques such as TEM, EDX, and SEM images. Photocatalysts were applied for hydrogen generation from water splitting and that with dual co-catalysts showed efficient catalytic activity under simulated solar light. The catalytic activity of photocatalysts with both oxidation and reduction co-catalysts (Au@TiO₂@RuO₂) showed hydrogen evolution (3165 μmol g⁻¹) almost two times more than that Au@TiO₂ and TiO₂@RuO₂ with single co-catalysts. And the hydrogen evolved is more than three times as compared to TiO₂ (935 μmol g⁻¹) without any co-catalyst. Hollow mesoporous morphology with different co-catalysts on inner and outer surfaces is believed to enhance photocatalytic activity which is due to better separation of photo-generated charges.     

Remarkable charge separation and photocatalytic efficiency enhancement through TiO2(B)/anatase hetrophase junctions of TiO2 nanobelts

Light harvesting and charge separation are both significant to the photocatalysis, but it is challenging to synchronously realize both in a single-component material. The surface coarsened TiO₂ nanobelts with TiO₂(B)/anatase hetrophase junctions and large BET surface area are prepared via a hydrothermal/annealing method. The presence of surface coarsened nanobelt structure enhances the light absorption through reflection/refraction of light. The TiO₂(B)/anatase hetrophase junctions can efficiently promote the separation of photoinduced electrons and holes pairs and therefore decrease the charge recombination. The large BET surface area provides abundant active sites for the absorption and diffusion of reactants. As a consequence, the obtained TiO₂ nanobelts exhibit an enhanced photocatalytic H₂ evolution activity at the optimal annealing temperature (450 °C) with Pt as co-catalysts (0.786 mmol h⁻¹g⁻¹), exceeding that of pure anatase TiO₂ nanobelts (TiO₂ nanobelts-600 °C, 0.265 mmol h⁻¹g⁻¹). Interestingly, TiO₂ nanobelts-450 °C still show a high hydrogen evolution rate of 0.601 mmol h⁻¹g⁻¹ in the absence of co-catalysts.     

Cactus shaped FeOOH/Au/BiVO4 photoanodes for efficient photoelectrochemical water splitting

In this work, a FeOOH/Au/BiVO₄ photoanode was fabricated through dual modification with Au nanoparticles (NPs, ∼5 nm) and FeOOH nanoneedles (NNs) on nanoporous BiVO₄ surface. Both the Au NPs and FeOOH NNs were distributed uniformly onto the BiVO₄ by using the electrodeposition and chemisorption routes, respectively. After parameter optimization, the FeOOH/Au/BiVO₄ photoanode displayed a photocurrent density of 4.64 mA cm⁻² at 1.23 V_RHE, which was 3.74 times higher than the pristine BiVO₄ one. Besides, the FeOOH/Au/BiVO₄ photoanodes also displayed a maximum H₂ yield amount of 23.9 μmol cm⁻² h⁻¹. The significantly enhanced performance could be attributed to the following reasons: Introduction of Au NPs enhanced the visible light absorption through localized surface plasmon resonance (LSPR) effect; the photo-excited “hot electrons” of Au NPs were more likely to flow into the conduction band (CB) of BiVO₄ via a direct electron transfer mechanism, leading to an improvement of the charge separation/transfer efficiency; surface modification of FeOOH extracted photogenerated holes, leading to an acceleration of the surface catalytic kinetics. In a word, the present study suggests that co-sensitization of Au NPs and FeOOH might be an effective method for improving the PEC water oxidation kinetics of BiVO₄ photoanodes.     

Accelerated transfer and separation of charge carriers during the photocatalytic production of hydrogen over Au/ZrO2–TiO2 structures by interfacial energy states

Energy states and surface plasmon resonance (SPR) play an important role in photocatalytic processes and power generation energy, for they improve the separation, transport, and mobility of charge carriers. The creation of Au/semiconductor heterostructures with different amounts of Au forms energy states that can modulate surface plasmon excitation, interfacial charge transport and photocatalytic activity to generate hydrogen. However, the Au loading effect on the interfacial charge transport and photocatalysis of plasmonic Au/semiconductors is unclear. For this reason, in this study, Au/ZrO₂–TiO₂ materials with different Au loadings were synthesized and evaluated in the photocatalytic production of hydrogen. The results confirmed boosted photoactivity with increased gold loading up to 5 wt.%, obtaining four times more hydrogen production than with the base material. The (photo) electrochemical measurements revealed that the Au inclusion provoked the adjustment of Fermi level values associated with the variation of surface energy states at the Au/ZrO₂–TiO₂ interface, which can be related to the modulation of SPR. This phenomenon can be explained by two simultaneous effects: i) the creation of energy states at the Au/ZrO₂–TiO₂ interface that modify the Fermi level to more negative potentials with respect to the base material, in order to have photogenerated electrons with higher reducing power to catalyze the hydrogen production; and ii) the Au metallic nanoparticles with SPR act as electronic reservoirs that extend the life time of photogenerated electron-hole pairs, thus enhancing the separation of charge carriers and the mobility of photogenerated electrons.     

Three-dimensional ZnO@TiO2 core-shell nanostructures decorated with plasmonic Au nanoparticles for promoting photoelectrochemical water splitting

A novel three-dimensional (3D) core-shell nanostructure decorated with plasmonic Au nanoparticles (NPs) was prepared for photoelectrochemical water splitting. In the new nanostructure, ZnO nanorods (NRs) are perpendicular to ZnO nanosheets (NSs), and the ZnO NSs-NRs are coated with a thin TiO₂ shell formed by liquid phase deposition. The plasmonic Au NPs were formed in situ on the surface of ZnO NSs-NRs@TiO₂ by thermal reduction. A thin TiO₂ shell and uniformly distributed Au NPs were successfully obtained. The photoconversion efficiency and photocurrent density of the 3D ZnO NSs-NRs@TiO₂-Au nanostructure respectively reached 0.48% and 1.73 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode, 4.80 and 4.33 times higher than those of ZnO NSs, respectively. The thin TiO₂ shell effectively promoted charge separation, while the surface plasmon resonance effects of the Au NPs improved the photocurrent density. The findings suggest that the 3D ZnO NSs-NRs@TiO₂-Au nanostructure is a promising photoanode for photoelectrochemical water splitting.     

Simultaneously efficient light absorption and charge transport of CdS/TiO2 nanotube array toward improved photoelectrochemical performance

CdS has been widely used to modify TiO₂-based photoanodes for photoelectrochemical (PEC) water splitting. Due to the poor interface contact between chalcogenides and oxides, however, such CdS modified TiO₂ materials usually exhibit inefficient separation and transport of charges, leading to an unsatisfactory efficiency during the PEC water splitting process. Addressing this issue, we herein report a CdS/TiO₂ nanotube array (CdS/TNA) photoanode that was fabricated through a successive ion layer absorption and reaction (SILAR) method with an additional subsequent annealing. This post-annealing process is essential to enhance the interface contact between the CdS and the TNAs, resulting in an accelerated transfer of photogenerated electrons from the CdS to the TNAs. In addition, the post-annealing also improves the light absorption capability of the CdS/TNA photoanode. The simultaneous enhancement of charge transport and light absorption provided by the post-annealing is essential for improving the PEC performance of the CdS/TNA photoanode. The CdS/TNA photoanode obtained by this strategy exhibits a much enhanced PEC performance in water splitting, and its photocurrent density and solar-to-hydrogen conversion efficiency could reach 4.56 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode and 5.61%, respectively. This simple but effective route can provide a general strategy for obtaining high-performance oxide-based photoelectrodes.     

Improvement of photocatalytic hydrogen generation from CdSe/CdS/TiO2 nanotube-array coaxial heterogeneous structure

The fabrication and characterization of CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure that has potential applications in photocatalytic water splitting and toxic pollutants degradation are investigated. CdSe(top)/CdS(under) double-layer is conformally deposited onto TiO₂ nanotubes by successive ionic layer adsorption and reaction (SILAR) and electrochemical atomic layer deposition (ECALD), respectively, for the CdS under layer and the CdSe top layer. Such double sensitized TiO₂ nanotubular photoelectrode exhibits significant enhancements in photoconversion efficiency, visible light response, and efficient hydrogen generation. The detailed synthesis process and the surface morphology, phase structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure are discussed. The photoconversion efficiency of 9.47% and hydrogen generation rate of 10.24 ml h⁻¹ cm⁻² were observed. Both values are a 7-fold enhancement compared with that of the pure TiO₂ nanotube. The as-prepared photoelectrode presents potential application for industrialized photocatalytic hydrogen generation in the future.     

TiO2–CdS supported CuNi nanoparticles as a highly efficient catalyst for hydrolysis of ammonia borane under visible-light irradiation

TiO₂–CdS nanotubes (NTs) were used for the first time as a support to load metal nanoparticles (NPs) for the hydrolysis of ammonia borane (AB) which is a new strategy. The TiO₂–CdS NTs support was first synthesized using a hydrothermal method, and then the CuNi NPs were loaded using a liquid-phase reduction method. The synthesized samples were characterized by XRD, SEM-EDS, TEM, XPS, ICP, UV–Vis, and PL analyses. The characterization results show that the CuNi NPs existed in the form of an alloy with a size of ~1.2 nm and uniformly dispersed on the support. Compared with their single metal counterparts, the bimetallic CuNi-supported catalysts showed a higher catalytic activity in the hydrolysis of AB under visible-light irradiation: Cu_0·45Ni_0·55/TiO₂–CdS catalyst had the fastest hydrogen evolution rate with a high conversion frequency (TOF) of 25.9 mol_H2·mol_cat⁻¹ min⁻¹ at 25 °C and low activation energy of 32.8 kJ mol⁻¹. Cu_0.45Ni_0.55/TiO₂–CdS catalyst showed good recycle performance, maintaining 99.3% and 85.6% of the original hydrogen evolution rate even after five and ten recycles, respectively. Strong absorption of visible light, improved electron–hole separation efficiency, and metal synergy between Cu and Ni elements played a crucial role in improving the catalytic hydrolysis performance of AB. The catalyst prepared in this study provides a new strategy for the application of photocatalysts.     

CuS-modified ZnO rod/reduced graphene oxide/CdS heterostructure for efficient visible-light photocatalytic hydrogen generation

Solar energy utilization is a promising strategy for the photocatalytic generation of H₂ from water. Herein, a CuS-modified ZnO rod/reduced graphene oxide (rGO)/CdS heterostructure was fabricated via Cu-induced electrochemical growth with Zn powder at room temperature. The resulting powder revealed good interfacial bonding and promoted photoexcited carrier transport. The CuS nanoparticles played a pivotal role in enhancing visible-light responses and demonstrated excellent catalytic performance. A high visible-light photocatalytic H₂ generation rate of 1073 μmol h⁻¹ g⁻¹ was obtained from the CuS–ZnO/rGO/CdS heterostructure containing 0.23% CuS and 1.62% CdS. Increased photoexcited electron lifetimes, improved carrier transport rates, and decreased fluorescence intensities confirmed the synergistic effects of each of the components of the heterostructure. This study provides an innovative strategy for constructing multi-component heterostructures to achieve efficient visible-light H₂ evolution.     

Fabrication of TiO2 nanoflowers with bronze (TiO2(B))/anatase heterophase junctions for efficient photocatalytic hydrogen production

Light harvesting and charge separation are both significant in the photocatalysis, but it is challenging to synchronously realize both in a single-component material. The novel porous TiO₂ nanoflowers (NFs) photocatalysts with stable bronze (TiO₂(B))/anatase heterophase junctions and large pore sizes are prepared via a hydrothermal/annealing method. The presence of porous nanoflower structure enhances the light absorption through reflection/refraction of light. The stable TiO₂(B)/anatase heterophase junctions can efficiently promote the separation of photoinduced electrons and holes pairs and therefore suppress the charge recombination. The large pore sizes provide multi-level channels for the absorption and diffusion of reactants. With the increase of annealing temperatures from 350 to 550 °C, the H₂ evolution activity is promoted. However, overhigh annealing temperature (650 °C) cause the broken of nanoflower structure and TiO₂(B)/anatase heterophase junctions, thus inducing even decrease of H₂ evolution activity. As a consequence, the obtained TiO₂ NFs exhibit an enhanced photocatalytic H₂ evolution activity at the optimal annealing temperature (550 °C) with Pt as co-catalysts (5.013 mmol h⁻¹g⁻¹), exceeding that of TiO₂ NFs without annealing (0 mmol h⁻¹g⁻¹) and pure anatase TiO₂ NFs (TiO₂ NFs-650 °C, 4.722 mmol h⁻¹g⁻¹), respectively. Interestingly, TiO₂ NFs-550 °C still show a high hydrogen evolution rate of 4.317 mmol h⁻¹g⁻¹ in the absence of co-catalysts.     

Hydrogen generation by photocatalytic reforming of glucose with heterostructured CdS/MoS2 composites under visible light irradiation

A binary heterostructured CdS/MoS₂ flowerlike composite photocatalysts was synthesized via a simple one-pot hydrothermal method. This photocatalyst demonstrated higher photocatalytic hydrogen production activity than pure MoS₂. The heterojunction formed between MoS₂ and CdS seems to promote interfacial charge transfer (IFCT), suppress the recombination of photogenerated electron–hole pairs, and enhance the hydrogen generation. Based on the good match between the conduction band (CB) edge of CdS and that of MoS₂, electrons in the CB of CdS can be transferred to MoS₂ easily through the heterojunction between them, which prevents the accumulation of electrons in the CB of CdS, inhibiting photocorrosion itself and greatly enhancing stability of catalyst. Hydrogen evolution reaction (HER) using Na₂S/Na₂SO₃ or glucose as sacrificial agents in aqueous solution was investigated. The ratio between CdS and MoS₂ plays an important role in the photocatalytic hydrogen generation. When the ratio between CdS and MoS₂ reaches 40 wt%, the photocatalyst showed a superior H₂ evolution rate of 55.0 mmol g⁻¹ h⁻¹ with glucose as sacrificial agent under visible light, which is 1.2 times higher than using Na₂S/Na₂SO₃ as sacrificial agent. Our experimental results demonstrate that MoS₂-based binary heterostructured composites are promising for photocorrosion inhibition and highly efficient H₂ generation.     

Surface defect and rational design of TiO2−x nanobelts/ g-C3N4 nanosheets/ CdS quantum dots hierarchical structure for enhanced visible-light-driven photocatalysis

TiO_2-x/g-C₃N₄/CdS ternary heterojunctions are fabricated through thermal polymerization-chemical bath deposition combined with in-situ solid-state chemical reduction approach. The prepared materials are characterized by X-ray diffraction, Fourier transform infrared spectra, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption, and X-ray photoelectron spectroscopy. The results show that the ternary heterojunctions are formed successfully and CdS quantum dots (QDs) and TiO₂ are anchored on surface of g-C₃N₄ nanosheets simultaneously. The visible-light-driven photocatalytic degradation ratio of Bisphenol A and hydrogen production rate are up to 95% and ∼254.8 μmol h⁻¹, respectively, which are several times higher than that of pristine TiO₂. The excellent visible-light-driven photocatalytic activity can be ascribed to the synergistic effect of TiO_2−x, g-C₃N₄ and CdS QDs which extend the photoresponse to visible light region and favor the spatial separation of photogenerated charge carriers.     

A nanoreactor based on SrTiO3 coupled TiO2 nanotubes confined Au nanoparticles for photocatalytic hydrogen evolution

A TiO₂ nanotube-based nanoreactor was designed and fabricated by facile two steps synthesis: firstly, hydrothermal synthesized SrTiO₃ was deposited on TiO₂ nanotubes (TiO₂NTs). Secondly, the Au nanoparticles (NPs) were encapsulated inside the TiO₂NTs followed by vacuum-assisted impregnation. The as-synthesized composites were characterized using Transmission electron microscopy (TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Photoluminescence spectra (PL) and Ultraviolet–visible absorption spectroscopy (UV–vis). The photocatalytic performance was evaluated by the hydrogen evolution reaction. The results revealed that the SrTiO₃ modified TiO₂NTs confined Au NPs (STO-TiO₂NTs@Au) achieved an enhanced hydrogen evolution rate at 7200 μmol h⁻¹ g⁻¹, which was 2.2 times higher than that of bald TiO₂NTs@Au at 3300 μmol h⁻¹ g⁻¹. The improved photocatalytic activity could be attributed to the synergistic effect of the electron-donating of SrTiO₃ and TiO₂NTs confinement. The as-designed nanoreactor structure provides an example of efficient carriers' separation photocatalyst.     

Refined Z-scheme charge transfer in facet-selective BiVO4/Au/CdS heterostructure for solar overall water splitting

Artificial Z-scheme systems that mimic natural photosynthesis are well applicable to photocatalytic overall water splitting for hydrogen (H₂) production free of electricity. However, it commonly confronts low efficiency with huge challenge of steering charge transfer between H₂ evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP). Here we report an all-solid-state Z-scheme system with facet-selective construction that favors charge spatial separation toward HEP and OEP for high efficient solar overall water splitting. Based on the spontaneous separation of photogenerated electrons and holes on the different crystal facets of BiVO₄ decahedra, we successively implemented the selective depositions of Au and CdS nanoparticles (NPs) onto the electron-rich {010} facets, to fortify the Z-scheme charge transfer between BiVO₄ and CdS across Au mediators upon two-step photoexcitation. In-situ photoelectron dynamics ascertains Z-scheme model of resultant BiVO₄/Au/CdS, which enables an impressive overall water splitting with stoichiometric H₂ and O₂ evolution rates of 281 and 138 μmol g^?1 h^?1, respectively, under 1 sun irradiation (100 mW cm^?2, AM 1.5G) without using any sacrificial agents and external bias. This work not only presents a refined Z-scheme overall water splitting system, but also gains insights into photo-induced charge transfer dynamics.     

Efficient photocatalytic hydrogen evolution on amorphous MoSx-modified CdS/KTaO3 heterojunctions under visible light irradiation

Constructing heterostructures with efficient charge separation is a promising route to improve photocatalytic hydrogen production. In this paper, MoS_x/CdS/KTaO₃ ternary heterojunction photocatalysts were successfully prepared by a two-step method (hydrothermal method and photo deposition method), which improved the photocatalytic hydrogen evolution activity. The results show that the rate of hydrogen evolution for the optimized photocatalyst is 2.697 mmol g^?1·h^?1under visible light, which is 17 times and 2.6 times of the original CdS (0.159 mmol g^?1 h^?1) and the optimal CdS/KTaO₃(1.033 mmol g^?1 h^?1), respectively, and the ternary photocatalyst also shows good stability. The improvement on photocatalytic hydrogen evolution performance can be attributed to the formation of heterojunction between the prepared composite materials, which effectively promotes the separation and migration of photo-generated carriers. Amorphous MoS_x acts as an electron trap to capture photogenerated electrons, providing active sites for proton reduction. This provides beneficial enlightenment for hydrogen production by efficiently utilizing sunlight to decompose water.     

Rational regulation of vacancy species to manage migration paths of carriers in MoS2/TiO2 heterojunctions for efficient photocatalytic H2 generation

Photocatalytic water splitting to produce hydrogen (H₂), as one means to solve environmental pollution and energy shortage, is limited by the serious recombination of photogenerated electrons and holes, resulting in low solar energy conversion efficiency. Thus, steering the behaviors of charge carriers by rationally designing their transport pathway is essential, which can effectively suppress the recombination of electrons and holes. Herein, we designed a MoS₂/TiO₂ heterojunction with different vacancy species to manage the migration paths of photogenerated charge carriers. As demonstrated by experimental characterizations and density functional theory (DFT) calculations, oxygen and sulfur vacancies can induce defect energy levels in heterostructures, which can capture photogenerated holes and electrons, respectively, resulting in substantially promoted charge separation efficiency and longer lifetime of electrons. As expected, the optimized photocatalyst shows a stable H₂ production rate of 1.41 mmol g^?1 h^?1, which is significantly better than that of the bare MoS₂/TiO₂ heterojunction. This finding informs the significance in rational design of the nanostructures for promoting the photocatalytic performance.     

Effect of Pt loading on the hydrogen production of CNT/Pt composites functionalized with carboxylic groups

This work reports the morphological and photocatalytic hydrogen generation properties of CNT/Pt composites with and without functionalization by carboxylic/oxygen groups. The composites with and without functionalization were named f-CNT/Pt and CNT/Pt, respectively. Several f-CNT/Pt and CNT/Pt composites with different content of Pt NPs (from 0 to 30 wt%) were synthesized and analyzed by scanning electron microscopy (SEM). Those images revealed that the composites without functionalization presented higher agglomerations of Pt nanoparticles (NPs). Furthermore, the average sizes of the Pt NPs in the named f-CNT/Pt composites (2.3–2.9 nm) were lower than these in the CNT/Pt composites (2.5–3.1 nm). The hydrogen generation rates were also calculated from the decomposition of pure water under UV irradiation (365 nm) and found maximum values of 45.4 and 193.9 μmol·h⁻¹ g⁻¹ for the CNT/Pt and f-CNT/Pt composites (they contained 20 wt% of Pt NPs), respectively. Additional experiments for hydrogen generation were achieved using sodium sulfite as sacrificial agent; in this case, a maximum value of 13850 μmol·h⁻¹ g⁻¹ was obtained for the f-CNT/Pt composite. The f-CNT/Pt composites produced more hydrogen than the CNT/Pt composites because they presented higher content of defects; this was confirmed by the Raman spectra. We also showed that the Pt NPs acted as electron trap centers, which delayed the recombination of the photogenerated electrons and holes, this in turn, enhanced the hydrogen generation rates of the composites (the hydrogen generation was maximized by varying the content of Pt NPs deposited on the CNTs). The CNT/Pt composites presented here were simpler and easier to synthesize than the previous published ternary systems based on TiO₂, CNTs and Pt NPs.