Recent progress in metal-doped TiO2, non-metal doped/codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis

Titanium dioxide remains a benchmark photocatalyst with high stability, low cost, and less toxicity, but it is active only under UV light; thus, in practical applications using visible light, its catalytic reactions are stalled. To enhance its catalytic activity under visible light, non-metal/codoped TiO₂ structures are being studied. These structures improve the photocatalytic activity of TiO₂ in visible light by reducing its energy bandgap. This might be useful in wastewater treatment for the photocatalytic degradation of organic contaminants under visible and UV light irradiation. In this intensive review, we describe recent developments in TiO₂ nanostructured materials for visible-light driven photocatalysis, such as (i) mechanistic studies on photo-induced charge separation to understand the photocatalytic activity and (ii) synthesis of non-metal doped/codoped TiO₂ and TiO₂ nanostructured hybrid photocatalysts. Furthermore, the effects of various parameters on their photocatalytic efficiency, photodegradation of various organic contaminants present in wastewater, and photocatalytic disinfection are delineated.     

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.     

Photocatalytic decomposition of formic acid and methyl formate on TiO2 doped with N and promoted with Au. Production of H2

The photo-induced vapor-phase decompositions of formic acid and methyl formate were investigated on pure, N-doped and Au-promoted TiO₂. Infrared (IR) spectroscopic studies revealed that illumination initiated the decomposition of adsorbed formate formed in the dissociation of formic acid and located mainly on TiO₂. The photocatalytic decompositions of formic acid and methyl formate vapor on pure TiO₂ occurred to only a limited extent. The deposition of Au on pure or doped TiO₂ markedly enhanced the extent of photocatalytic decomposition of formic acid. The main process was dehydrogenation to give H₂ and CO₂. The formation of CO occurred to only a very small extent. Addition of O₂ or H₂O to the formic acid decreased the CO level from ∼0.8% to ∼0.088%. Similar features were experienced in the photocatalytic decomposition of methyl formate, which dissociated in part to give surface formate. Experiments over Au deposited on N-doped TiO₂ revealed that the photo-induced decomposition of both compounds occurs even in visible light.     

Pt – g-C3N4 – (Au/TiO2): Electronically integrated nanocomposite for solar hydrogen generation

A potential nanocomposite photocatalyst was designed by integrating Pt nanoclusters (co-catalyst and electron sink) with graphitic carbon nitride (g-C₃N₄ (gcn)) (charge diffusion) and 0.5 wt % Au containing Au-TiO₂ (AuT) (plasmonic on semiconductor) for solar water splitting (SWS). Variety of Pt-gcn-AuTiO₂ compositions has been evaluated for SWS under one sun conditions. Complexity of the photocatalyst was increased systematically from Au-TiO₂, gcn-TiO₂ to Pt-gcn-Au-TiO₂ to explore the influence of different combinations. Electronic integration of charge separation/diffusion component (gcn) with light absorbing sensitizer components (Au and gcn), and co-catalyst (Pt) seems to be the critical factor to improve hydrogen yield (HY) or overall efficiency. Although addition of gcn increase the HY of composites, there is no SWS activity observed on bare TiO₂ or gcn. Au or Pt on gcn enhances the charge separation effectively and interface between Au and/or Pt with gcn works as the Schottky barrier. A monodispersion of Au over TiO₂ and Pt nanoclusters over gcn/AuTiO₂ composite lead to the maximum solar hydrogen yield (1.52 mmol/h g) with an apparent quantum yield (AQY) of 7.5%. Photoelectron and photoluminescence spectral studies confirm the electron transfer from Au to gcn, and Au and/or gcn to titania. A thorough physico-chemical investigation of various composites underscores the electronic integration aspects of the nanocomposite towards storage of electrons in the Pt co-catalyst and hence an effective charge separation and an increase in AQY.     

Promoted photoinduced charge separation and directional electron transfer over dispersible xanthene dyes sensitized graphene sheets for efficient solar H2 evolution

Directional electron transfer and effective charge separation facilitated by graphene sheets have provided an inspiring approach to enhance the efficiencies of photoelectric conversion and photocatalysis. Herein, we demonstrated the feasibility of constructing a high-performance of the dye-sensitized H₂ evolution system using dispersible graphene sheets as both efficient electron transfer carrier and catalyst scaffold. Among the xanthene dyes sensitized H₂ evolution catalysts in this study, photocatalyst of Rose Bengal (RB) sensitized graphene decorated with Pt is the most active one and exhibits the highest apparent quantum efficiency (AQE) of 18.5% at wavelength of 550 nm and rather long-term stability for H₂ evolution. Dispersible graphene sheets can not only capture electrons from the excited dye and then transfer them to the decorated catalysts efficiently for improving charge separation with a small energy loss, but also afford large interfaces for highly dispersing catalyst nanoparticles with more active sites, thereby significantly enhancing the H₂ evolution efficiency than graphite oxide (GO) and multiwall carbon nanotubes (MWCNTs). This work proposes a potential strategy to develop efficient photocatalytic systems for solar-energy-conversion and provides a new insight into mechanistic study of photoinduced electron transfer by effective synergetic combination of dispersible graphene sheets with an efficient dye and a H₂ evolution catalyst.     

Additional electron transfer channels of thermostable 0D Cs(Pb: Pt)Br3 perovskite quantum dots /2D accordion-like Ni-MOF nanojunction for photocatalytic H2 evolution

Overcoming the low charge transfer efficiency and poor photothermal stability of halide perovskite quantum dots (QDs) is the booster to achieve photocatalytic applications. In this paper, the Pt²⁺-doped CsPbBr₃ QDs/two-dimensional accordion-like Ni-MOF (CPPB QDs/Ni-MOF) composite was firstly synthesized by fixing the CPPB QDs into the pores of Ni-MOF. Electron separation and transfer efficiency were analyzed by PL spectra and electrochemical data. The photocatalyst exhibited outstanding photocatalytic performance in hydrogen (H₂) evolution. The optimal H₂ evolution efficiency of the composite reached 153.6 μmol h^?1, which was about 9 times than that of pure Ni-MOF and remained 134.8 μmol h^?1 after the cycle test. The splendid efficiency could be benefited from the advantages of 2D layered structure of Ni-MOF and the high charge separation and transmission efficiency of CPPB QDs. Finally, the mechanism of electron migration and additional electron transfer channels between composite interfaces was further demonstrated by density functional theory (DFT) calculations. The present work opens up a novel perspective for photocatalytic applications of doped halide perovskite QDs/Ni-MOF nanocomposites.     

Mesoporous black TiO2 array employing sputtered Au cocatalyst exhibiting efficient charge separation and high H2 evolution activity

The separation and transfer of photogenerated carriers are the key issue in the design of high performance TiO₂ photocatalysts. In order to overcome the kinetic limitations and achieve rapid charge transfer, TiO₂-related multi-component catalysts have been extensively studied. Among all the TiO₂ supports, the impressive black TiO₂ (BT) with broad visible light absorption spectrum and oxygen vacancies are preferable, but still suffers from low quantum efficiency. Meanwhile, poor control of cocatalyst placement by conventional loading method can also severely impede photocatalytic efficiency. Herein a fast and simple metal magnetron sputter approach was used to place highly-uniformed Au nanoparticles cocatalyst on the top of the mesoporous TiO₂–BT nanotube array fabricated by in situ electrochemical anodization approach on a Ti film. This confined plasmonic photocatalyst with highly uniformly distributed Au cocatalysts exhibited greatly enhanced charge-separation and charge-transfer behavior, and a remarkable 10 times enhancement of the photocatalytic H₂ evolution reactivity over conventional TiO₂ nanotube. The TiO₂-BT-Au electron transfer cascade structure is proposed in which black TiO₂ acts as a buffer layer for TiO₂ conduction band electrons, allowing efficient photogenerated electrons to be transferred to Au nanoparticles and then into the TiO₂ pores that suitable for H₂ generation. Since the nanotube walls themselves are curved upwards, the short diffusion length allows electrons to be easily transferred to the cocatalyst, where recombination of photogenerated electron pairs is limited. The metal magnetron sputter technique for noble metal cocatalyst immobilization and the unique TiO₂–BT–Au electron-transfer system are promising and can be extended to the design of other supported catalysts.     

Facile fabrication of CdSe/CuInS2 microflowers with efficient photocatalytic hydrogen production activity

Photocatalytic water splitting to produce hydrogen has attracted extensive attention and exhibited broad development prospects. In this work, CuInS₂ microflowers were fabricated through the solvothermal method, and decorated with CdSe quantum dots on the surface. As-prepared CdSe/CuInS₂ microflowers exhibited high photocatalytic hydrogen production activity (10610.37 μmol g^?1 h^?1) and high AQE of 48.97% at 420 nm. The enhanced photocatalytic hydrogen production activity owing to the construction of p-n heterostructure improved light absorption ability, increased electrons transfer efficiency and reduced recombination of photo-induced electrons and holes. Moreover, high stability and cyclic utilization of CdSe/CuInS₂ microflowers were beneficial to photocatalytic hydrogen production application.     

Enhanced photocatalytic performance and stability of multi-layer core-shell nanocomposite C@Pt/MoS2@CdS with full-spectrum response

The nanocomposite material C@Pt/MoS₂@CdS was prepared by a simple microwave-assisted hydrothermal method combined with photoreduction method. The crystal structure, microstructure, and surface physical chemistry properties of the material were analyzed by X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance absorption spectroscopy (UV–vis/DRS), X-ray photoelectron energy spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), nitrogen adsorption–desorption measurement, photoluminescence spectroscopy (PL), and electrochemical tests. As a result, this material has full-spectrum light absorption property and the composited CdS presents a good hexagonal phase. Moreover, the composite material presents a nanorod-like multi-layer core-shell structure, wherein the rod-like MoS₂@CdS surface is covered with Pt and C. The formation of the multi-layer core-shell structure increases the specific surface area of as-composite material and strengthens its light absorption performance. The electrochemical impedance and transient photocurrent test results show that C@Pt/MoS₂@CdS has the highest charge separation efficiency and enhanced photocurrent density compared with other systems. Photogenerated charge carriers have higher separation efficiency, and photogenerated electrons and holes exhibit longer life. During the photocatalysis experiments, the nanocomposite C@Pt/MoS₂@CdS shows enhanced photodegradation activity under multi-modal photocatalytic experiments and excellent stability under visible light irradiation. In addition, C@Pt/MoS₂@CdS has a strong photocatalytic water splitting ability. Under the same experimental conditions, its hydrogen production is 60 times that of commercially available P25. Through capture experiments, the reactive species in the photocatalytic reaction process were determined, and the possible photocatalytic reaction mechanism of this multi-layer core-shell C@Pt/MoS₂@CdS nanocomposite was inferred.     

Synthesis of NiS2/polyvinylpyrrolidone/(CuIn)0·2Zn1·6S2 type II heterojunction photocatalysts for high-efficiency photocatalytic hydrogen production under visible light

A robust NiS₂/polyvinylpyrrolidone/(CuIn)_0·2Zn_1·6S₂ (NiS₂/PVP/CIZS) photocatalyst was successfully synthesized through a sequential hydrothermal treatment. Firstly, the addition of PVP reduces the size of CIZS nanoparticles, resulting in appearing surface effect, and hence an improvement of chemical activity. Moreover, the small size of PVP/CIZS possesses a shorten transfer distance of photo-induced carriers. The photocatalytic H₂ evolution rate of 0.8 g PVP/CIZS elevates to 3112.7 μmol/g/h under visible light. After coupling with NiS₂, the light absorption range and separation efficiency of photo-induced carriers for NiS₂/PVP/CIZS composites have been elevated and the optimal photocatalytic hydrogen evolution rate of 15% NiS₂/PVP/CIZS reaches up to 5369.4 μmol/g/h. There forms a type Ⅱ heterostructure on NiS₂/PVP/CIZS, and the heterostructure facilitates to suppress the recombination and elevate the separation of photo generated electrons and holes. Therefore, the synergistic effect of size control and constructing a type Ⅱ heterostructure with NiS₂ on PVP/CIZS floriform photocatalyst helps to enhance photocatalytic performance of the composites. This work opens up a new way to prepare highly efficient photocatalysts under visible light.     

Photocatalytic hydrogen production with aid of simultaneous metal deposition using titanium dioxide from aqueous glucose solution

The photocatalytic hydrogen production with aid of simultaneous metal deposition using TiO₂ was investigated in biomass glucose solution. Because the hydrogen production was very trace with pure TiO₂, the simultaneous metal deposition was applied into the glucose solution. The photocatalytic H₂ production activity with TiO₂ was significantly enhanced by simultaneous metal deposition for Au and Pd. The experimental factors such as glucose concentration, metal ion concentration and reaction temperature were investigated. The photocatalytic hydrogen production increased with increasing the concentration of glucose, and it followed Langmuir–Hinshelwood mechanism. Under the optimal conditions, the photocatalytic hydrogen generations from aqueous glucose solution with in-situ Au and Pd deposited TiO₂ were about 203 and 362 times larger compared with those observed with pure TiO₂. The enhanced photocatalytic activity could be explained in terms of reduced electron hole recombination via electron transfer from conductance band of TiO₂ to metal.     

New method for improving the bulk charge separation of hematite with enhanced water splitting

Due to its poor bulk charge separation efficiency, the photoelectrochemical (PEC) performance of pristine hematite prepared directly from an electrodeposited Fe film is limited. Au-modification of hematite via a simple immersion method improves the PEC performance two-fold to 0.31 mA cm⁻². The Au nanoparticles deposited from HAuCl₄ act as plasmonic photosensitizers and electron collectors to improve the light absorption and bulk charge separation efficiency of the photoanode. In addition, the increase in the (110) plane and specific surface area induced by HAuCl₄ enhances the bulk charge separation efficiency. After further modification with Ti, the photocurrent response of the resulting Ti/Au/α-Fe₂O₃ photoanode improves to 0.51 mA cm⁻²; this increase is attributed to its increased light absorption, bulk charge separation efficiency (η_bulk), and surface charge injection efficiency (η_surface). In this work, the effect of Au and Ti on the crystalline structure, morphology and PEC performance of the novel electrodeposited hematite photoanode are investigated by systematical characterization.     

Green synthesis of fibrous hierarchical meso-macroporous N doped TiO2 nanophotocatalyst with enhanced photocatalytic H2 production

A new approach to prepare hierarchical and fibrous meso-macroporous N-doped TiO₂ is attempted at room temperature without using templates by the addition of titanium isopropoxide droplets to the ammonia solution. The catalysts are thoroughly characterized by physico-chemical and spectroscopic method to explore the structural, electronic and optical properties. The photocatalytic activities of the catalyst were evaluated with hydrogen generation. NTP catalyst calcined at 400 °C (NTP-400) exhibited 602.7 μmol/3 h H₂ generation from 10 vol.% methanol under visible light. The excellent photocatalytic activity for NTP-400 is attributed to the porous networks existing in our system with uniform N dispersion throughout the catalyst. The hierarchical and fibrous structures allow easy channelization of electron as in the case of nanotubes for effective surface charge transfer. Along with macroporosity, nitrogen incorporation and mesoporosity play some important roles for enhanced photoactivities.     

Dye-cosensitized graphene/Pt photocatalyst for high efficient visible light hydrogen evolution

In this work, a highly efficient and stable photocatalytic H₂ evolution catalyst was constructed on Pt deposited graphene sheets cosensitized by Eosin Y (EY) and Rose Bengal (RB). Under two-beam monochromic light irradiation (520 and 550 nm), a high quantum yield (QY) up to 37.3% has been achieved owing to maximum utilization of incident visible light. As a result of the excellent electron transport properties of graphene, it can greatly facilitate the forward electron transfer from photoexcited dye molecules to Pt catalyst and suppress back electron transfer, which significantly enhances photocatalytic efficiency for H₂ evolution. This efficient cosensitization strategy is also effective in enhancing H₂ evolution efficiencies of dye sensitized TiO₂ and multiwall carbon nanotubes (MWCNTs).     

High-efficiency charge separation of Z-scheme 2D/2D C3N4/C3N5 nonmetal VdW heterojunction photocatalyst with enhanced hydrogen evolution activity and stability

The interfacial charge transfer control is a key and arduous issue for propelling the migration/separation of photogenerated carriers for heterojunction photocatalysts. Here, a new 2D/2D C₃N₄/C₃N₅ nonmetal van der Waals (VdW) heterojunction is fabricated by the simple self-assembly technique in acidic medium, whose charge separation efficiency is promoted dramatically, thus being endowed with the high-efficiency photocatalytic hydrogen evolution (PHE) performance. The PHE rate reaches up to 3.33 mmol h^?1 g^?1 under the visible light and the apparent quantum efficiency (AQE) of 20.6% is achieved at 420 nm on the optimal 2D/2D C₃N₄/C₃N₅-5% sample. Furthermore, the 2D/2D C₃N₄/C₃N₅ nonmetal VdW heterojunction also exhibits the desired stability because there was no significant decrease after PHE reaction of 10 cycles with total 40 h. Such outstanding PHE activity and stability originate from the impelled separation of photoinduced charge carriers and the powerful interfacial interaction through forming Z-Scheme charge transfer path and π-π coupling effect between C₃N₄ and C₃N₅ nanosheets. This work takes a significant guiding and demonstration for designing and exploiting other novel nonmetallic polymer-based VdW heterojunctions in the photocatalytic application field.     

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.     

Synergistic effect of gold NPs modified graphitic carbon nitride nanotubes (g-CNT) with the role of hot electrons and hole scavengers for boosting solar hydrogen production

The efficient and selective photocatalysts for the evolution of hydrogen are highly demanding, however, many semiconductors are complex in nature and have lower photocatalytic efficiency. This is because of their small exposed surface area, poor light penetration, and unregulated charge recombination rate. Herein, well-designed graphitic carbon nitride with hierarchical nanotextures loaded with plasmonic gold (Au) nanoparticles has been investigated for stimulating photocatalytic H₂ evolution. Hole scavengers, diffusion effects, duration, and mass transfer were used to evaluate the photoactivity activity in a slurry type continuous flow photoreactor system. Due to better charge carrier separation and enhanced light permeability, H₂ evolution was boosted by two times when bulk g-C₃N₄ structure was alternated with graphitic carbon nitride nanotubes (g-CNT). The maximum H₂ generation rate was 455 μmol g⁻¹ h⁻¹ with 0.3% Au/g-CNT nanotexture, which was 17.8 and 8.9 times greater than utilizing g-C₃N₄ and g-CNT samples, respectively. The key factors influencing this improvement in photoactivity were the unique interlayer opening, higher light penetration, more light utilization due to plasmonic effect, enhanced surface reactive active sites, and decreased charge carrier recombination. The hot electrons due to plasmonic gold was another important feature to promote H₂ evolution rate under solar energy. It is possible to employ these freshly developed nanotextures, which have a gold plasmonic effect, for solar energy conversion and other energy-related applications.     

NiSe as an effective co-catalyst coupled with TiO2 for enhanced photocatalytic hydrogen evolution

Construction of semiconductor heterojunctions can effectively accelerate the separation of photo-induced charge carriers and thereby enhance photocatalytic activity. Here, NiSe was used as an effective co-catalyst to construct an active NiSe/TiO₂ heterojunction for improving the photocatalytic H₂ production of TiO₂. The resultant 10%NiSe/TiO₂ heterojunction exhibited 11 times higher photocatalytic H₂-production activity than that of bare TiO₂. The NiSe/TiO₂ heterojunction and the photo-reduction of partial Ni²⁺ to Ni⁰ notably accelerated the separation and transfer of photo-excited electron-hole pairs, and thus resulted in obvious improvement of H₂-evolution activity. This work holds promise for the application of NiSe in photocatalysis as a high-efficiency photocatalytic cocatalyst.     

The structure and photocatalytic studies of N-doped TiO2 films prepared by radio frequency reactive magnetron sputtering

N-doped TiO₂ films were prepared by a radio frequency reactive magnetron sputtering (RF-MS) deposition method from an undoped TiO₂ target in a mixture of Ar/N₂ atmosphere on heated quartz glass substrates. The structures and properties of the N-doped were studied by XRD, Raman, XPS, TEM, ultraviolet (UV)-vis and PL spectroscopy. By analyzing the structures and photocatalytic activities of undoped and N-doped TiO₂ films under ultraviolet and visible light irradiation, the probable photocatalytic mechanism of N-doped TiO₂ films was investigated. Because many oxygen defects are caused in films by nitrogen doping, it is presumed that nitrogen doping and oxygen defect induced the formation of new states closed to the valence band and conduction band, respectively. The cooperation of nitrogen and oxygen defects leads to a significant narrowing of the band gap and greatly improves the absorption in the visible light region. It is found that the degradation efficiencies of N-doped TiO₂ films greatly decreased under ultraviolet irradiation, but slowly improved under visible light irradiation, compared with the undoped TiO₂ film. It is suggested that the N-doped TiO₂ films are formed for the nitrogen to occupy oxygen defect sites directly. The doped nitrogen ions and oxygen defects act as recombination centers that reduce the lifetime of photo-induced electrons and holes, thereby resulting in the decrease of photocatalytic activity under ultraviolet light illumination.     

Optimal energy offsets for organic solar cells containing a donor/acceptor pair

Optimal energy levels and offsets of an organic donor/acceptor binary-type solar cell have been analyzed using the classic Marcus electron transfer theory to identify the most efficient photo-induced charge separation. This study reveals that, an exciton quenching parameter (Y_eq) yields one optimal donor/acceptor energy offset where the photo-induced exciton is converted to charges most efficiently, and a recombination quenching parameter (Y_rq) yields a second optimal donor/acceptor energy offset where the ratio of charge separation rate constant over charge recombination rate constant becomes maximum. Another energy offset is also identified where charge recombination becomes most severe. This information would be useful for evaluating and fine tuning frontier energy levels of a donor/acceptor pair for optimum solar cell applications.