A Simple Fabrication of Sb2S3/TiO2 Photo-Anode with Long Wavelength Visible Light Absorption for Efficient Photoelectrochemical Water Oxidation

An Sb₂S₃-sensitized TiO₂ (Sb₂S₃/TiO₂) photo-anode (PA) exhibiting a high photo-electrochemical (PEC) performance in water oxidation has been successfully prepared by a simple chemical bath deposition (CBD) technique. Herein, the Raman spectra and XPS spectrum of Sb₂S₃/TiO₂ confirmed the formation of Sb₂S₃ on the TiO₂ coatings. The Sb₂S₃/TiO₂ photo-anode significantly shifted the absorption edge from 395 nm (3.10 eV) to 650 nm (1.90 eV). Furthermore, the Sb₂S₃/TiO₂ photo-anode generated a photo-anodic current under visible light irradiation below 650 nm due to the photo-electrochemical action compared with the TiO₂ photo-anode at 390 nm. The incident photon-to-current conversion efficiency (IPCE = 7.7%) at 400 nm and -0.3 V vs. Ag/AgCl was 37 times higher than that (0.21%) of the TiO₂ photo-anodes due to the low recombination rate and acceleration of the carriers of Sb₂S₃/TiO₂. Moreover, the photo-anodic current and photostability of the Sb₂S₃/TiO₂ photo-anodes improved via adding the Co²⁺ ions to the electrolyte solution during photo-electrocatalysis.

Photocatalyst-Mediator Interface Modification by Surface-Metal Cations of a Dye-Sensitized H2 Evolution Photocatalyst

To develop highly active H₂ evolving dye-sensitized photocatalysts (DSPs) applicable for Z-scheme water splitting, we synthesized a series of Ru(II)-dye-double-layered DSPs, X'-RuCP⁶-Zr-RuP⁶@Pt-TiO₂ (X'-DSP) with different surface-bound metal cations (X' = Fe²⁺, Y³⁺, Zr⁴⁺, Hf⁴⁺, and Bi³⁺). In 0.5 M KI aqueous solution, the photocatalytic H₂ evolution activity under blue light irradiation (λ = 460 ± 15 nm) increased in the following order: nonmetal-modified DSP, H⁺-DSP (turn over number for 6 h irradiation = 35.2) < Fe²⁺-DSP (54.9) ≈ Bi³⁺-DSP (55.2) < Hf⁴⁺-DSP (65.5) ≈ Zr⁴⁺-DSP (68.3) ≈ Y³⁺-DSP (71.5), suggesting that the redox-inactive and highly charged metal cations tend to improve the electron donation from the iodide electron mediator. On the other hand, DSPs having heavy metal cations, Hf⁴⁺-DSP (18.4) and Bi³⁺-DSP (16.6), exhibited better activity under green light irradiation (λ = 530 ± 15 nm) than Zr⁴⁺-DSP (15.7) and H⁺-DSP (7.80), implying the contribution of a heavy atom effect of the surface-bound metal cation to partially allow the spin-forbidden metal-to-ligand charge-transfer excitation.

Highly stable halide perovskite with Na incorporation for efficient photocatalytic degradation of organic dyes in water solution

To overcome water instability and low photocatalytic activity of lead-free halide perovskite for the degradation of organic dyes, we report a novel photocatalyst of lead-free halide perovskite with Na incorporation and employ it for the photocatalytic degradation of organic dyes in water solution under visible light irradiation. The main purpose of this work is to confirm the feasibility of lead-free halide perovskite with Na incorporation for improving the photocatalytic efficiency and recyclability in water solution and further to explore the mechanism behind the enhancement of photocatalytic performance after Na incorporation. The results show that Cs₂Ag_0.60Na_0.40InCl₆ can increase the dye degradation rate by at least 50% than the lead-free halide perovskite (Cs₂AgInCl₆) and the photocatalyst of Ag substituted by Na (Cs₂NaInCl₆). The degradation efficiency of rhodamine 6G catalyzed by Cs₂Ag_0.60Na_0.40InCl₆ reaches 94.94% over 60 min, which is 72% higher than that catalyzed by Cs₂NaInCl₆ and 27% higher than that catalyzed by Cs₂AgInCl₆. What's more, the degradation efficiency of methyl orange catalyzed by Cs₂Ag_0.60Na_0.40InCl₆ is 90.39% within 150 min, which is 66% higher than that catalyzed by Cs₂NaInCl₆ and 54% higher than that catalyzed by Cs₂AgInCl₆. Moreover, the photocatalyst of Cs₂Ag_0.60Na_0.40InCl₆ exhibits a desirable recyclability by water exposure, retaining the degradation efficiency over 90% after five cycles. The strengthened photocatalytic performance in the presence of Cs₂Ag_0.60Na_0.40InCl₆ is ascribed to an increase of radiative recombination rate and an improvement of average lifetime to 204 ns since an appropriate Na incorporation at the atomic ratio of Na/Ag=4:6 breaks the original crystal lattice and meanwhile increases the electron and hole overlap. The work proves a great potential of halide perovskite with Na incorporation for the highly efficient photocatalytic degradation of organic dyes in water solution.

Two Excited State Collaboration of Heteroleptic Ir(III)-Coumarin Complexes for H2 Evolution Dye-Sensitized Photocatalysts

Interfacial electron injection from a photoexcited surface-immobilized dye to a semiconductor substrate is a key reaction for dye-sensitized photocatalysts. We previously reported that the molecular orientation of heteroleptic Ir(III) photosensitizer on the TiO2 nanoparticle surface was important for efficient interfacial electron injection. In this work, to overcome the weak light absorption ability of heteroleptic Ir(III) photosensitizer and to improve the photoinduced charge-separation efficiency at the dye–semiconductor interface, we synthesized two heteroleptic Ir(III) complexes with different coumarin dyes, [Ir(C6)2(H4CPbpy)]Cl and [Ir(C30)2(H4CPbpy)]Cl [Ir-CX; X = 6 or 30; HC6 = 3-(2-enzothiazolyl)-7-(diethylamino)coumarin, HC30 = 3-(2-N-methylbenzimidazolyl)-7-N,N-diethylaminocoumarin, H4CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine], as the cyclometalated ligands and immobilized them on the surface of Pt-cocatalyst-loaded TiO2 nanoparticles. Ultraviolet-visible absorption and emission spectroscopy revealed that the singlet ligand-centered (1LC) absorption and triplet 3LC emission bands of Ir-C30 occurred at shorter wavelengths than those of Ir-C6, while time-dependent density-functional-theory data suggested that the ligand-to-ligand charge transfer (LLCT) excited states of the two complexes were comparable. The photocatalytic H2 evolution activity of the Ir-C6-sensitized Pt-TiO2 nanoparticles (Ir-C6@Pt-TiO2) under visible light irradiation (λ > 420 nm) was higher than that of Ir-C30@Pt-TiO2. In contrast, their activities were comparable under irradiation with monochromatic light (λ = 450 ± 10 nm), which is absorbed comparably by both Ir-CX complexes. These results suggest that the internal conversion from the higher-lying LC state to the LLCT state effectively occurs in both Ir-CX complexes to trigger electron injection to TiO2.

CdS decorated MnWO4 nanorod nanoheterostructures: a new 0D-1D hybrid system for enhanced photocatalytic hydrogen production under natural sunlight

Constructing a heterostructure is an effective strategy to reduce the electron-hole recombination rate, which enhances photocatalytic activity. Here, we report a facile hydrothermal method to grow CdS nanoparticles on MnWO₄ nanorods and their photocatalytic hydrogen generation under solar light. A structural study shows the decoration of hexagonal CdS nanoparticles on monoclinic MnWO₄. Morphological studies based on FE-TEM analysis confirm the sensitization of CdS nanoparticles (10 nm) on MnWO₄ nanorods of diameter-35 nm with mean length ∼100 nm. The lower PL intensity of MnWO₄ was observed with an increasing amount of CdS nanoparticles, which shows inhibition of the charge carrier recombination rate. A CdS@MnWO₄ narrow band gap semiconductor was employed for photocatalytic hydrogen generation from water under solar light and the highest amount of hydrogen, i.e. 3218 μmol h^-1 g^-1, is obtained which is 21 times higher than that with pristine MnWO₄. The enhanced photocatalytic activity is ascribed to the formation of a CdS@MnWO₄ nanoheterostructure resulting in efficient spatial separation of photogenerated electron-hole pairs due to vacancy defects. More significantly, direct Z-scheme electron transfer from MnWO₄ to CdS is responsible for the enhanced hydrogen evolution. This work signifies that a CdS decorated MnWO₄ nanoheterostructure has the potential to improve the solar to direct fuel conversion efficiency.

Composite photocatalysts based on Cd1−xZnxS and TiO2 for hydrogen production under visible light: effect of platinum co-catalyst location

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd_1−xZn_xS/TiO₂ were compared. In the first case, platinum was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25; in the second one, Cd_1−xZn_xS (x = 0.2–0.3) was deposited on the surface of Pt/TiO₂ P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/Pt (1 wt%)/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/TiO₂ ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ in the amount of 26 mmol g⁻¹ h⁻¹ (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.

The determination of the preferred location of platinum particles in TiO₂–Cd_1−xZn_xS systems was carried out for the first time.

Recent Advancements and Future Prospects in Ultrathin 2D Semiconductor-Based Photocatalysts for Water Splitting

Ultrathin two-dimensional (2D) semiconductor-mediated photocatalysts have shown their compelling potential and have arguably received tremendous attention in photocatalysis because of their superior thickness-dependent physical, chemical, mechanical and optical properties. Although numerous comprehensions about 2D semiconductor photocatalysts have been amassed up to now, low cost efficiency, degradation, kinetics of charge transfer along with recycling are still the big challenges to realize a wide application of 2D semiconductor-based photocatalysis. At present, most photocatalysts still need rare or expensive noble metals to improve the photocatalytic activity, which inhibits their commercial-scale application extremely. Thus, developing less costly, earth-abundant semiconductor-based photocatalysts with efficient conversion of sunlight energy remains the primary challenge. In this review, it begins with a brief description of the general mechanism of overall photocatalytic water splitting. Then a concise overview of different types of 2D semiconductor-mediated photocatalysts is given to figure out the advantages and disadvantages for mentioned semiconductor-based photocatalysis, including the structural property and stability, synthesize method, electrochemical property and optical properties for H2/O2 production half reaction along with overall water splitting. Finally, we conclude this review with a perspective, marked on some remaining challenges and new directions of 2D semiconductor-mediated photocatalysts.

Metal Chalcogenides Based Heterojunctions and Novel Nanostructures for Photocatalytic Hydrogen Evolution

The photo-conversion efficiency is a key issue in the development of new photocatalysts for solar light driven water splitting applications. In recent years, different engineering strategies have been proposed to improve the photogeneration and the lifetime of charge carriers in nanostructured photocatalysts. In particular, the rational design of heterojunctions composites to obtain peculiar physico-chemical properties has achieved more efficient charge carriers formation and separation in comparison to their individual component materials. In this review, the recent progress of sulfide-based heterojunctions and novel nanostructures such as core-shell structure, periodical structure, and hollow cylinders is summarized. Some new perspectives of opportunities and challenges in fabricating high-performance photocatalysts are also discussed.

Hydrogen and Propane Production From Butyric Acid Photoreforming Over Pt-TiO2

Photocatalysis is a promising technology from economic, energetic, and ecological points of view because it takes advantage of solar light. Hence, it is one of the investigated green routes to produce hydrogen from renewable energy resources. Butyric acid (BA) is largely present in wastewater and as an intermediate product in anaerobic digestion and therefore it is an inexpensive resource, which can be converted to valuable chemicals. In this work, photoreforming of butyric acid (BAPR) under UV light in aqueous suspensions of platinum-modified titanium dioxide-based catalysts is reported for the first time. Titania nanotubes (TNT) synthesized and calcined at different temperatures (300, 400, 500°C) and commercial TiO₂ (P25), decorated with platinum nanoparticles, have been tested and characterized through different techniques including X-ray powder diffraction, UV-vis diffuse reflectance and photoluminescence spectroscopy, transmission electron microscopy, BET and porosimetry analysis. The main identified products of the BAPR were H₂, propane, CO₂ and several organic acids (e.g., pentanoic and 3-methylhexanoic acid). It has been found that the morphology and crystallinity of the photocatalysts affected dramatically their optical properties and, consequently, the reaction rate and the product distribution. Specifically, the highest conversion of BA (X_BA) and selectivity toward H₂ (S_H2) was recorded with P25-Pt (X_BA = 26.9%, S_H2 = 47.2% after 8 h of irradiation). TNT-400-Pt showed the highest selectivity toward propane (S_C3H8 = 16.1%) with X_BA = 23.4% and S_H2 = 36.2%. The activity results in conjunction with the characterization of the catalysts highlighted that the main factor affecting the activity in terms of X_BA and generation of H₂ was the crystallinity, and in particular the presence of rutile phase in TiO₂, whereas S_C3H8 appears to increase when the electron-holes recombination is lower.

Recent Progress on Metal Sulfide Composite Nanomaterials for Photocatalytic Hydrogen Production

Metal sulfide-based photocatalysts have gained much attention due to their outstanding photocatalytic properties. This review paper discusses recent developments on metal sulfide-based nanomaterials for H2 production, acting as either photocatalysts or cocatalysts, especially in the last decade. Recent progress on key experimental parameters, in-situ characterization methods, and the performance of the metal sulfide photocatalysts are systematically discussed, including the forms of heterogeneous composite photocatalysts, immobilized photocatalysts, and magnetically separable photocatalysts. Some methods have been studied to solve the problem of rapid recombination of photoinduced carriers. The electronic density of photocatalysts can be investigated by in-situ C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra to study the mechanism of the photocatalytic process. The effects of crystal properties, nanostructure, cocatalyst, sacrificial agent, electrically conductive materials, doping, calcination, crystal size, and pH on the performance of composite photocatalysts are presented. Moreover, the facet effect and light trapping (or light harvesting) effect, which can improve the photocatalytic activity, are also discussed.

Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes

Cuprous oxide (Cu₂O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu₂O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu₂O surface as well as the interface between Cu₂O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu₂O and provide guidance for future material development.

While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. Here, the authors use femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses.

Novel metal doped carbon quantum dots/CdS composites for efficient photocatalytic hydrogen evolution

Carbon quantum dots (CDs) are rising stars for photocatalytic applications due to their low toxicity and excellent electron transfer characteristics. Doping with heteroatoms is expected to adjust the band levels and electron transfer properties of CDs, and understanding the effect of doping on CDs can aid the rational preparation of highly efficient CD-based photocatalysts. Herein, we prepared a series of metal atom (Zn, Co, Bi, Cd, or Ti) doped CDs by pyrolysis and explored the photocatalytic application of these metal doped CDs for the first time. The metal doped CDs were combined with CdS nanowires as a co-catalyst for photocatalytic hydrogen production. The Bi, Cd and Ti doped CDs/CdS composites show much better hydrogen production performance than the undoped CDs/CdS composite. Among these composites, Bi-CDs/CdS presents the optimal interfacial charge separation and the best hydrogen production performance. The hydrogen evolution rate of Bi-CDs/CdS is 4.2 times and almost one time higher than that of pure CdS and undoped CDs/CdS, respectively. Bi doping can make the CDs metallic and promote the charge transfer of CDs. Such a great enhancement originates from the outstanding electron transfer properties of Bi-doped CDs, as well as the effective charge separation between Bi-doped CDs and CdS. Bi doping was demonstrated to be an effective strategy for optimizing the photocatalytic activity of CD based composites.

In situ photodeposition of amorphous NixP on CdS nanorods for efficient visible-light photocatalytic H2 generation

Noble metal-free amorphous Ni_xP modified CdS with superior photocatalytic H₂ evolution activity has been successfully fabricated.

Blue Ti3+ self-doped TiO2 nanosheets with rich {001} facets for photocatalytic performance

The as-prepared BT_1.5TNs exhibits superior photocatalytic performance for RhB degradation due to Ti³⁺ doping.

Effective Photocatalytic Hydrogen Evolution by Cascadal Carrier Transfer in the Reverse Direction

Visible-light-responsive photocatalysts used in the highly efficient hydrogen production exhibit several disadvantages such as photocorrosion and fast recombination. Because of the potential important applications of such catalysts, it is crucial that a simple, effective solution is developed. In this respect, in this study, we combined SiC (β modification) and TiO₂ with CdS to overcome the challenges of photocorrosion and fast recombination of CdS. Notably, we found that when irradiated with visible light, CdS was excited, and the excited electrons moved to the conduction band of TiO₂, thereby increasing the efficiency of charge separation. In addition, by moving the holes generated on CdS to the valence band of SiC, in the opposite direction of TiO₂, photocorrosion and fast recombination were prevented. As a result, in the sulfide solution, the CdS/SiC composite catalyst exhibited 4.3 times higher hydrogen generation ability than pure CdS. Moreover, this effect was enhanced with the addition of TiO₂, giving 10.8 times higher hydrogen generation ability for the CdS/SiC/TiO₂ catalyst. Notably, the most efficient catalyst, which was obtained by depositing Pt as a cocatalyst, exhibited 1.09 mmol g^-1 h^-1 hydrogen generation ability and an apparent quantum yield of 24.8%. Because water reduction proceeded on the TiO₂ surface and oxidative sulfide decomposition proceeded on the SiC surface, the exposure of CdS to the solution was unnecessary, and X-ray photoelectron spectroscopy confirmed that photocorrosion was successfully suppressed. Thus, we believe that the effective composite photocatalyst construction method presented herein can also be applied to other visible-light-responsive powder photocatalysts having the same disadvantages as CdS, thereby improving the efficiency of such catalysts.

CO2-based hydrogen storage – hydrogen liberation from methanol/water mixtures and from anhydrous methanol

New strategies for the reforming of methanol under mild conditions on the basis of heterogeneous and molecular catalysts have raised the hopes and expectations on this fuel. This contribution will focus on the progress achieved in the production of hydrogen from aqueous and anhydrous methanol with molecular and heterogeneous catalysts. The report entails thermal approaches, as well as light-triggered dehydrogenation reactions. A comparison of the efficiency and mechanistic aspects will be made and principles of catalytic pathways operating in biological systems will be also addressed.

Recent Progress on Titanium Dioxide Nanomaterials for Photocatalytic Applications

Environmental and energy problems have drawn much attention owing to rapid population growth and accelerated economic development. For instance, photocatalysis, "a green technology", plays an important role in solar-energy conversion owing to its potential to solve energy and environmental problems. Recently, many efforts have been devoted to improving visible-light photocatalytic activity by using titanium dioxide as a photocatalyst as a result of its wide range of applications in the energy and environment fields. However, fast charge recombination and an absorption edge in the UV range limit the photocatalytic efficiency of TiO₂ under visible-light irradiation. Many investigations have been undertaken to overcome the limitations of TiO₂ and, therefore, to enhance its photocatalytic activity under visible light. The present literature review focuses on different strategies used to promote the separation efficiency of electron-hole pairs and to shift the absorption edge of TiO₂ to the visible region. Current synthesis techniques used to elaborate several nanostructures of TiO₂ -based materials, recent progress in enhancing visible photocatalytic activity, and different photocatalysis applications will be discussed. On the basis of the studies reported in the literature, we believe that this review will help in the development of new strategies to improve the visible-light photocatalytic performance of TiO₂ -based materials further.

Improving Photocatalytic Water Treatment through Nanocrystal Engineering: Mesoporous Nanosheet-Assembled 3D BiOCl Hierarchical Nanostructures That Induce Unprecedented Large Vacancies

Vacancy control can significantly enhance the performance of photocatalytic semiconductors for water purification. However, little is known about the mechanisms and approaches that could generate stable large vacancies. Here, we report a new mechanism to induce vacancy formation on nanocrystals for enhanced photocatalytic activity: the introduction of mesopores. We synthesized two nanosheet-assembled hierarchical 3D BiOCl mesoporous nanostructures with similar morphology and exposed facets but different nanosheet thickness. Positron annihilation analysis detected unprecedentedly large V_Bi^‴ V_O^•• V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates (as well as V_Bi^‴ V_O^•• V_Bi^‴) on BiOCl assembled from 3-6 nm nanosheets but only V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates on BiOCl assembled from thicker (10-20 nm) nanosheets. Comparison of vacancy properties with 2D ultrathin 2.7 nm nanosheets (with V_Bi^‴ V_O^•• V_Bi^‴ and V_Bi^‴) indicates that nanosheet thinness alone cannot explain the formation of such large atom vacancies. On the basis of density functional theory computations of formation energy of isolated Bi vacancy, we show that mesopores facilitate the formation of large vacancies to counterbalance thermodynamic instability caused by incompletely coordinated Bi and O atoms along the mesopore perimeters. We corroborate that the extraordinarily large V_Bi^‴ V_O^•• V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates facilitate photoexcitation of electrons and prevent the recombination of electron-hole pairs, which significantly enhances photocatalytic activity. This is demonstrated by the rapid mineralization of bisphenol A (10^-5 M) with low photocatalyst loading (1 g L^-1), as well as enhanced bacterial disinfection. Improved electron-hole separation is also corroborated by enhanced photocatalytic reduction of nitrate.

Low temperature synthesis of BiFeO3 nanoparticles with enhanced magnetization and promising photocatalytic performance in dye degradation and hydrogen evolution

In this investigation, we have synthesized BiFeO₃ nanoparticles by varying hydrothermal reaction temperatures from 200 °C to 120 °C to assess their visible-light driven photocatalytic activity along with their applicability for hydrogen production via water splitting. The rhombohedral perovskite structure of BiFeO₃ is formed for hydrothermal reaction temperature up to 160 °C. However, for a further decrement of hydrothermal reaction temperature a mixed sillenite phase is observed. The XRD Rietveld analysis, XPS analysis and FESEM imaging ensure the formation of single-phase and well crystalline nanoparticles at 160 °C reaction temperature with 20 nm of average size. The nanoparticles fabricated at this particular reaction temperature also exhibit improved magnetization, reduced leakage current density and excellent ferroelectric behavior. These nanoparticles demonstrate considerably high absorbance in the visible range with a low band gap (2.1 eV). The experimentally observed band gap is in excellent agreement with the calculated band gap using first-principles calculations. The favorable photocatalytic performance of these nanoparticles has been able to generate more than two times of solar hydrogen compared to that produced by bulk BiFeO₃ as well as commercially available Degussa P25 titania. Notably, the experimentally observed band gap is almost equal for both bulk material and nanoparticles prepared at different reaction temperatures. Therefore, in solar energy applications, the superiority of BiFeO₃ nanoparticles prepared at 160 °C reaction temperature may be attributed not only to their band gap but also to other factors, such as reduced particle size, excellent morphology, good crystallinity, large surface to volume ratio, ferroelectricity and so on.

Multiferroic BiFeO₃ nanoparticles were synthesized using low temperature hydrothermal technique to assess their visible-light driven photocatalytic activity along with their applicability for the production of hydrogen via water splitting.

Pt nanoparticles decorated rose-like Bi2O2CO3 configurations for efficient photocatalytic removal of water organic pollutants

Pt nanoparticles decorated with rose-like Bi₂O₂CO₃ configurations were synthesized via a simple photoreduction method at room temperature.

Sensitization of Pt/TiO2 Using Plasmonic Au Nanoparticles for Hydrogen Evolution under Visible-Light Irradiation

Au nanoparticles with different sizes (10, 20, 30, and 50 nm) were synthesized using a seed-assisted approach and anchored onto Pt/TiO₂ employing 3-mercaptopropionic acid as the organic linker. The sizes of the Au nanoparticles were controlled within a narrow range so that the size-dependent surface plasmonic resonance effect on sensitizing Pt/TiO₂ can be thoroughly studied. We found that 20 nm Au nanoparticles (Au₂₀) gave the best performance in sensitizing Pt/TiO₂ to generate H₂ under visible-light illumination. Photoelectrochemical measurements indicated that Au₂₀-Pt/TiO₂ exhibited the most efficient "hot" electrons separation among the studied catalysts, correlating well with the photocatalytic activity. The superior performance of Au-supported Pt/TiO₂ (Au₂₀-Pt/TiO₂) compared with Au anchored to TiO₂ (Au₂₀/TiO₂) revealed the important role of Pt as a cocatalyst for proton reduction. To elucidate how the visible-light excited hot electrons in Au nanoparticles involved in the proton-reduction reaction process, Au₂₀/TiO₂ was irradiated by visible light (λ > 420 nm) with the presence of Pt precursor (H₂PtCl₆) in a methanol aqueous solution under deaerated condition. Energy-dispersive X-ray spectroscopy mapping analysis on the recovered sample showed that Pt ions could be reduced on the surfaces of both Au nanoparticles and TiO₂ support. This observation indicated that the generated hot electrons on Au nanoparticles were injected into the TiO₂ conduction band, which were then subsequently transferred to Pt nanoparticles where proton reduction proceeded. Besides, the excited hot electrons could also participate in the proton reduction on Au nanoparticles surface.

Impact of Photosensitizing Multilayered Structure on Ruthenium(II)-Dye-Sensitized TiO2-Nanoparticle Photocatalysts

To improve the efficiency of photoinduced charge separation on the surface of dye-sensitized TiO₂ nanoparticles, we synthesized the Ru(II)-photosensitizer-immobilized, Pt-cocatalyst-loaded TiO₂ nanoparticles RuCP ² @Pt-TiO₂, RuCP ² -Zr-RuP ⁶ @Pt-TiO₂, and RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂ (RuCP ² = [Ru(bpy)₂(mpbpy)]^2-, RuP ⁴ = [Ru(bpy)(pbpy)₂]^6-, RuP ⁶ = [Ru(pbpy)₃]^10-, H₄mpbpy = 2,2'-bipyridine-4,4'-bis(methanephosphonic acid), and H₄pbpy = 2,2'-bipyridine-4,4'-bis(phosphonic acid)) using phosphonate linkers with bridging Zr⁴⁺ ions. X-ray fluorescence and ultraviolet-visible absorption spectra revealed that a layered molecular structure composed of Ru(II) photosensitizers and Zr⁴⁺ ions (i.e., RuCP ² -Zr-RuP ⁶ and RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ ) was successfully formed on the surface of Pt-TiO₂ nanoparticles, which increased the surface coverage from 0.113 nmol/cm² for singly layered RuCP ² @Pt-TiO₂ to 0.330 nmol/cm² for triply layered RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂. The photocatalytic H₂ evolution activity of the doubly layered RuCP ² -Zr-RuP ⁶ @Pt-TiO₂ was three times higher than that of the singly layered RuCP ² @Pt-TiO₂, whereas the activity of triply layered RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂ was less than half of that for RuCP ² @Pt-TiO₂. The photosensitizing efficiencies of these Ru(II)-photosensitizer-immobilized nanoparticles for the O₂ evolution reaction catalyzed by the Co(II)-containing Prussian blue analogue [Co^II(H₂O)₂]_1.31[{Co^III(CN)₆}_0.63{Pt^II(CN)₄}_0.37] decreased as the number of Ru(II)-photosensitizing layers increased. Thus, crucial aspects of the energy- and electron-transfer mechanism for the photocatalytic H₂ and O₂ evolution reactions involve not only the Ru(II)-complex-TiO₂ interface but also the multilayered structure of the Ru(II)-photosensitizers on the Pt-TiO₂ surface.

Controlling Configurational Isomerism in Three-Component Colloidal Hybrid Nanoparticles

Colloidal hybrid nanoparticles are solution-dispersible constructs that join together multiple distinct nanoscale materials through direct solid-solid interfaces. Given their multifunctionality and synergistic properties that emerge from interfacial coupling, hybrid nanoparticles are of interest for applications in biomedical imaging, solar energy conversion, heterogeneous catalysis, nanophotonics, and beyond. High-order hybrid nanoparticles, which incorporate three or more nanocrystal domains, offer greater tunability and functional diversity relative to one or two-component nanoparticles. The multiple heterojunctions within these structures can facilitate complex electromagnetic coupling as well as cooperative surface processes. Additionally, these materials can be used as model systems for studying fundamental structure-property relationships at the nanoscale that arise from particle coupling and interfacial exchanges. Limiting these advances is the inability to synthesize hybrid nanoparticles with precise morphologies and geometries. High-order hybrid nanoparticles can adopt more than one configuration, and each unique arrangement will have different heterointerfaces and, accordingly, different functions. Seeded-growth methods are among the most effective methods for producing high-quality hybrid nanoparticles. Engineering complex heterostructures using these stepwise reactions is in some ways conceptually analogous to the total synthesis of large organic molecules. However, unlike in molecular synthesis, the rules and guidelines that underpin the formation of hybrid nanoparticles are less understood. For example, when a third domain is added to a two-component heterodimer nanoparticle seed, several distinct types of hybrid nanoparticle products are possible, but only one is typically observed due to preferred growth at specific locations. The three-component heterotrimer products that preferentially form are not necessarily those that have the domain configurations and heterojunctions required to facilitate a targeted application. Different arrangements of the three nanoparticles that comprise a heterotrimer lead to distinct configurational isomers. Accordingly, understanding and controlling configurational isomerism in nanoparticle heterotrimers is foundational for engineering high-order hybrid nanostructures with targeted heterointerfaces, properties, and functionalities. This Account highlights recent insights into the pathways by which three-component nanoparticle heterotrimers form and how their configurations can be controlled and modified. In-depth microscopic investigations into the formation of heterotrimers by growing a third nanoparticle domain on a two-component heterodimer seed have revealed that in some cases indiscriminate nucleation first occurs on all exposed surfaces followed by surface-mediated migration and coalescence to the preferred interface. This insight helps to rationalize observed site-specific, chemoselective growth phenomena. Additionally, new approaches for directing growth in heterotrimer synthesis, such as protection-deprotection schemes inspired by organic chemistry, are becoming effective tools for constructing hybrid nanoparticles having nonpreferred domain configurations. Alternatives to traditional seeded-growth approaches are also emerging, including insertion reactions driven by saturation-precipitation processes and orthogonal transformations of preformed hybrid constructs using ion exchange. These and other recent advances are providing a powerful suite of synthetic tools that are anticipated to enable function-driven design of high-order hybrid nanoparticles having targeted properties and applications.

CdS/Zr:Fe2 O3 Nanorod Arrays with Al2 O3 Passivation Layer for Photoelectrochemical Solar Hydrogen Generation

CdS-sensitized 1 D Zr:Fe₂ O₃ nanorod arrays were synthesized on fluorine-doped tin oxide substrates by a two-step hydrothermal method. The photoelectrochemical results demonstrate that the current density (4.2 mA cm^-2 at 0 V vs. Ag/AgCl) recorded under illumination for the CdS/1 D Zr:Fe₂ O₃ photoanodes is 2.8 time higher than the bare 1 D Zr:Fe₂ O₃ . The extended absorbance spectrum, the reduced recombination, and the effective transport of photogenerated holes in CdS to the electrolyte facilitate enhancement in the photoelectrochemical performance. From X-ray photoelectron spectroscopy and TEM observations of the bare and aluminum oxide-treated CdS/1 D Zr:Fe₂ O₃ photoanodes, we could confirm that the 1 D Zr:Fe₂ O₃ nanorods were covered by the CdS layer and Al₂ O₃ layer present on surface of CdS. Furthermore, the photocurrent and stability of the CdS/1 D Zr:Fe₂ O₃ nanorods was significantly enhanced by Al₂ O₃ compared to bare CdS/1 D Zr:Fe₂ O₃ heterojunction owing to its ability to act as an effective holetransport- as well as photocorrosion-protecting layer. These remarkable enhancements in light-energy harvesting, improvement in charge transport, and stability directly suggest the usefulness of photoanodes for solar hydrogen generation.

Imaging the photoinduced charge injection in CdS/TiO2 nanoparticles by the sequential fluorescence mapping method

The combination of a sensitizer and TiO₂ nanoparticles forming a photocatalytic material is a central issue in many fields of applied photochemistry. The charge injection of emissive sensitizers into the conduction band of the semiconductor TiO₂ may form a photoactive region that becomes dark, or it has a very low emission signal due to the generation of sensitizer radicals. However, by sequential coupling of a selected photoredox dye, such as resazurin, the dark region may become fluorescent at the interfaces where the charge injection has taken place due to the concomitant formation of fluorescent resorufin by cascade electron transfer. Using this strategy and a total internal reflection fluorescence microscopy (TIRFM) image, the charge injection in TiO₂/CdS and SiO₂/TiO₂/CdS nanoparticles is investigated The method allows the charge injection efficiency of the excited CdS into TiO₂ to be evaluated qualitatively, explaining the differences observed for these photocatalytic materials in H₂ generation.

Assembling Polyoxo-Titanium Clusters and CdS Nanoparticles to a Porous Matrix for Efficient and Tunable H2 -Evolution Activities with Visible Light

Polyoxo-titanium clusters (PTCs) are applied to construct highly efficient ternary PTC/CdS/MIL-101 visible-light-range H₂ -evolution photocatalysts. The application of PTCs not only increases the photocatalytic activities but also provides a molecular tool to tune them.

Hybrid metal–Cu2S nanostructures as efficient co-catalysts for photocatalytic hydrogen generation

Unique colloidal synthesis of monodispersed dumbbell shaped metal–Cu₂S (M–Cu₂S where M = Pt, FePt) HNs and their utilization in hydrogen production.

A hybrid of CdS/HCa2Nb3O10 ultrathin nanosheets for promoting photocatalytic hydrogen evolution

A hybrid of CdS/HCa₂Nb₃O₁₀ ultrathin nanosheets with a tough heterointerface was successfully fabricated. Efficient interfacial charge transfer from CdS to HCa₂Nb₃O₁₀ nanosheets was achieved to realize the enhanced photocatalytic H₂ evolution activity.

Photocatalytic H2 production on trititanate nanotubes coupled with CdS and platinum nanoparticles under visible light: revisiting H2 production and material durability

The photocatalytic production of molecular hydrogen (H₂) on ternary composites of Pt, CdS, and sodium trititanate nanotubes (Na_xH_2−xTi₃O₇, TNTs) is examined in an aqueous 2-propanol (IPA) solution (typically 5 vol%) at a circum-neutral pH under visible light (λ > 420 nm). The H₂ production rates are dependent on the Pt-loading level, and the optimum production rate in the Pt/CdS/TNTs is approximately six times higher than that in Pt/CdS/TiO₂. A D₂O solution containing 5 vol% IPA leads only to the production of D₂ molecules, whereas increasing the IPA amount to 30 vol% leads to the production of DH molecules. This indicates that the Pt/CdS/TNTs composites enable H₂ production via true water splitting under our typical experimental conditions. X-ray photoelectron spectroscopy (XPS) analyses of the as-synthesized Pt/CdS/TNTs and those used for 6 and 12 h show that metallic Pt on the CdS/TNTs is less susceptible to oxidation than Pt on CdS/TiO₂. In addition, photocorrosion of CdS (i.e., sulfate formation) is significantly inhibited during the photocatalytic H₂ production reactions in the Pt/CdS/TNTs because of the efficient charge transfer via the TNTs framework. The Pt/CdS/TNTs samples are thermally more stable than Pt/CdS/TiO₂ and CdS/TNTs, effectively inhibiting the formation of CdO during the thermal synthesis. Detailed surface characterizations of the as-synthesized ternary composites are performed using X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and XPS.

Nanostructured CdS sensitized CdWO4 nanorods for hydrogen generation from hydrogen sulfide and dye degradation under sunlight

In this report, CdS nanoparticles have been grown on the surface of CdWO₄ nanorods via an in-situ approach and their high photocatalytic ability toward dye degradation and H₂ evolution from H₂S splitting under visible light has been demonstrated. The structural and optical properties as well as morphologies with varying amount of CdS to form CdS@CdWO₄ have been investigated. Elemental mapping and high resolution transmission electron microscopy (HRTEM) analysis proved the sensitization of CdWO₄ nanorods by CdS nanoparticles. A decrease in the PL emission of CdWO₄ was observed with increasing amount of CdS nanoparticles loading possibly due to the formation of trap states. Considering the band gap in visible region, the photocatalytic study has been performed for H₂ production from H₂S and dye degradation under natural sunlight. The steady evolution of H₂ was observed from an aqueous H₂S solution even without noble metal. Moreover, the rate of photocatalytic H₂ evolution over CdS modified CdWO₄ is ca. 5.6 times higher than that of sole CdWO₄ under visible light. CdS modified CdWO₄ showed a good ability toward the photo-degradation of methylene Blue. The rate of dye degradation over CdS modified CdWO₄ is ca. 7.4 times higher than that of pristine CdWO₄ under natural sunlight. With increase in amount of CdS nanoparticle loading on CdWO₄ nanorods the hydrogen generation was observed to be decreased where as dye degradation rate is increased. Such nano-heterostructures may have potential in other photocatalytic reactions.