Enhanced photocatalytic water splitting activity of carbon-modified TiO2 composite materials synthesized by a green synthetic approach

We report a green and facile approach for the preparation of carbon-modified (C-modified) TiO₂ composite materials by hydrothermal synthesis followed by pyrolytic treatment. The resultant materials were characterized by powder X-ray diffraction (XRD), nitrogen physisorption studies, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). The photocatalytic performances of these materials were evaluated by calculating the amount of hydrogen evolved from the decomposition of water under solar simulated irradiation conditions. An improvement was achieved from no H₂ evolution at all with the bare TiO₂, to an evolution of 0.21 mL g⁻¹ h⁻¹ from a composite material modified with an optimum carbon loading of 3.62%. These results suggested that the interaction of carbon with predominantly rutile form of TiO₂ can promote shallow trapping of photogenerated electrons in the oxygen vacancies. This phenomenon consequently enhances the photocatalytic activity by minimizing charge carrier recombination, a characteristic demonstrated by fluorescence quenching of the TiO₂ emission.     

Highly stabilized Ag2O-loaded nano TiO2 for hydrogen production from glycerol: Water mixtures under solar light irradiation

Nano TiO₂ prepared by a hydrothermal method and silver-loaded nano TiO₂ prepared by impregnation were studied for the photocatalytic production of hydrogen from glycerol:water mixtures. The structural characteristics were revealed using XRD, EDAX, DRS, TEM, XPS, BET surface area and Raman techniques. The photocatalytic hydrogen production has been investigated under solar light irradiation. Effects of nano TiO₂ calcination temperature, silver loading, photocatalyst content, light source and Ag oxidation state on hydrogen production have been systematically studied. Maximum hydrogen production of 200 μmol h^?1 g^?1 is observed on 4wt% silver-loaded nano TiO₂ catalyst in pure water and the maximum hydrogen production of 7030 μmol h^?1 g^?1 is observed on 3wt% silver-loaded nano TiO₂ catalyst in glycerol: water mixtures. Silver-loaded nano TiO₂ reduced and photodeposited catalysts show similar hydrogen production activities in glycerol: water mixtures under solar irradiation. The optimum catalyst modified with conducting carbon materials (graphene oxide, graphene, carbon nanotubes) by a solid-state dispersion method were also studied for hydrogen production under solar light irradiation. Compared with pure nano TiO₂, a 3wt% silver-loaded nano TiO₂/graphene composite exhibited an approximately 17-fold enhancement of hydrogen production leading to hydrogen production rates of 12,100 μmol h^?1 g^?1. Based on the characterization results and hydrogen production activity on these catalysts, a structure–activity correlation has been proposed wherein the interacting Ag₂OAg phases on the surface of nano TiO₂ play an important role in maintaining a high hydrogen production activity under solar irradiation.     

A hydrothermal route for constructing reduced graphene oxide/TiO2 nanocomposites: Enhanced photocatalytic activity for hydrogen evolution

A series of reduced graphene oxide/TiO₂ (RGO/TiO₂) nanowire microsphere composites were synthesized with a facile one-step hydrothermal method using TiCl₃ and graphene oxide (GO) as the starting materials, during which the formation of TiO₂ and the reduction of GO occur simultaneously. The obtained nanocomposites were characterized with X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, respectively. UV–vis absorption spectra showed that the absorption edges of TiO₂ were extended into visible light region with the addition of RGO. The photocatalytic activities of the samples with and without Pt as cocatalysts were evaluated by hydrogen evolution from water photo-splitting under UV–vis light illumination. Enhanced photocatalytic properties were observed for the as-prepared RGO/TiO₂ nanocomposites. The amount of hydrogen evolution from the optimized photocatalyst reached to 43.8 μmol h⁻¹, which was about 1.6 times as high as that of bare TiO₂. The results shown here indicate a convenient and applicable approach to further exploitation of high activity materials for photocatalytic water splitting applications.     

Photocatalytic hydrogen evolution on graphene quantum dots anchored TiO2 nanotubes-array

The inversely structured TiO₂ nanotubes-array (TiO₂-NA) and CdS-modified TiO₂ nanotubes-array (CdS/TiO₂-NA) with graphene quantum dots (GQDs) anchored inside were prepared through a facile impregnation method. The catalysts were characterized by multiple techniques of SEM, TEM, XRD, Raman spectroscopy, XPS, TG and diffuse reflectance UV/Vis absorption spectroscopy. The results of TEM, XPS and Raman spectroscopy indicate that the GQDs were really formed and successfully anchored into the TiO₂-NA and CdS/TiO₂-NA. The activity evaluation results show that the hydrogen evolution rate during photocatalytic water splitting was greatly improved after loading GQDs into TiO₂-NA and CdS/TiO₂-NA. By breaking graphene into GQDs, the light-filtering effect of graphene was remarkably inhibited as compared with that of conventional large graphene sheets. Moreover, the overall morphology of TiO₂ nanotube array could be well maintained after anchoring GQDs inside, which is favorable to mass transfer. The catalyst design strategy proposed in present work can be extended to other photocatalytic systems.     

Copper modified-TiO2 catalysts for hydrogen generation through photoreforming of organics. A short review

An intense scientific activity was recorded during the last several years in the field of preparation, characterization and use of copper-based TiO₂ photocatalysts for hydrogen generation through photocatalytic reforming of organics. Different copper species were used dissolved in aqueous solution or incorporated on the TiO₂ surface as single co-catalyst or in the presence of a second catalyst (e.g., graphene, carbon fibers) to (1) effectively separate the electron–hole pairs, thus reducing the occurrence of the recombination reaction, and (2) extend the light absorption to the visible range of the solar spectrum. Many organic species (e.g., methanol, glycerol, formic acid) were proposed as sacrificial agents for hydrogen generation, although the prevailing idea is that of using organic compounds currently found in industrial wastewaters. The pH value was recognized as a fundamental variable in photocatalytic H₂ generation via copper modified-TiO₂ catalysts. A positive effect to promote hydrogen generation was associated to an increase in pH until moderate alkaline values. On the other hand, a release in the solution of cupric ions and a consequent decrease in photocatalytic activity were observed when decreasing pH. A relevant lack of information was recorded about the efficiencies of hydrogen generation which were reported only in few papers. Therefore, this critical literature review has been performed with the aim of providing a complete background to select the most efficient approaches and eventually promote new competitive systems for hydrogen generation via photoreforming for industrial applications.     

A green and facile synthesis of TiO2/graphene nanocomposites and their photocatalytic activity for hydrogen evolution

This work reports a green and facile approach to synthesize chemically bonded TiO₂/graphene sheets (GS) nanocomposites using a one-step hydrothermal method. The as-prepared composites were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and ultraviolet visible (UV-Vis) diffuse reflectance spectra. The photocatalytic activity was evaluated by hydrogen evolution from water splitting under UV-Vis light illumination. An enhancement of photocatalytic hydrogen evolution was observed over the TiO₂/GS composite photocatalysts, as 1.6 times larger for TiO₂/2.0 wt%GS than that of Degussa P25. This fabrication process features the reduction of graphene oxide and formation of TiO₂ simultaneously leading to the well dispersion of generated TiO₂ nanoparticles on the surface of GS.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

New insights in the performance and reuse of rGO/TiO2 composites for the photocatalytic hydrogen production

The viability of the photocatalytic hydrogen production is closely related to the performance and long term stability of the photocatalyst. In this work rGO/TiO₂ composites have been synthetized with graphene oxide (GO) ratios from 1% to 10% and experimentally assessed towards hydrogen generation from methanol solutions. The performance of the composite with 2% of rGO (2 GT) has been compared to bare TiO₂ working with 20% volume methanol solution. The hydrogen production initial rate showed similar values with both photocatalysts decreasing after about 24 h. Further analysis of the photocatalytic process at longer times showed the negative influence of hydrogen accumulation in the reaction system. Thus, an experimental procedure with argon purge was developed and the behavior of TiO₂ and 2 GT photocatalysts was compared. It is concluded that TiO₂ keeps its activity after 8 operation cycles while 2 GT performance reduces progressively. This can be attributed to the further reduction of GO and the increase of defects in its structure.     

Enhancement of photocatalytic hydrogen production by liquid phase plasma irradiation on metal-loaded TiO2/carbon nanofiber photocatalysts

Enhanced hydrogen production by photocatalytic decomposition was assessed using liquid phase plasma over metal-loaded photocatalysts. Effects of irradiation of the liquid phase plasma were evaluated in the photocatalytic hydrogen production of hydrogen. Carbon nanofiber was introduced as photocatalytic support for the Ni-loaded TiO₂ photocatalyst. The influence of addition of organic reagents into water on hydrogen evolution was also evaluated. The photocatalytic decomposition by irradiation of the liquid phase plasma without photocatalyst produced some hydrogen evolution. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. The carbon nanofiber acted as a useful photocatalytic support for the fixation of TiO₂. Hydrogen evolution was enhanced by the Ni loading on the TiO₂ nanocrystallites supported on the carbon nanofiber support. Hydrogen evolution was increased significantly by the addition of organic reagents, which acted as a type of sacrificial reagent promoting photocatalysis.     

Synthesis of reduced graphene oxide–TiO2 nanoparticle composite systems and its application in hydrogen production

The utilization of solar energy for the conversion of water to hydrogen and oxygen has been considered to be an efficient strategy to solve crisis of energy and environment. Here, we report the synthesis of reduced graphene oxide–TiO₂ nanoparticle composite system through the photocatalytic reduction of graphite oxide using TiO₂ nanoparticles. Photoelectrochemical characterizations and hydrogen evolution measurements of these nanocomposites reveal that the presence of graphene enhances the photocurrent density and hydrogen generation rate. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm⁻² and 127.5 μmole cm⁻²h⁻¹ in 0.5 M Na₂SO₄ electrolyte solution under 1.5AM solar irradiance of white light with illumination intensity of 100 mW cm⁻². In graphene–TiO₂ nanocomposite, photogenerated electrons in TiO₂ are scavenged by graphene sheets and percolate to counter electrode to reduce H⁺ to molecular hydrogen thus increasing the performance of water-splitting reaction.     

Life cycle assessment of earth-abundant photocatalysts for enhanced photocatalytic hydrogen production

Hydrogen (H₂) can play a critical role in global greenhouse gas (GHG) mitigation. Photocatalytic water splitting using solar radiation is a promising H₂ technology. Titanium dioxide (TiO₂) and carbon nitride (g–C₃N₄)–based photocatalysts are the most widely used photocatalytic materials because of their activity and abundance. Several attempts have been made to improve the photocatalytic performance of these materials in terms of their activity level, life span, response to visible radiation, and stability. However, the environmental impacts of these modifications are often not included in existing studies. This research, therefore, develops a cradle-to-grave life cycle assessment (LCA) framework to evaluate and compare the GHG footprints of four alternative pathways: TiO₂ nanorods and fluorine-doped carbon nitride quantum dots embedded with TiO₂ (CNF: TNR/TiO₂), g-C₃N₄, and g-C₃N₄/BiOI composite. Unlike most studies that focus only on certain stages such as laboratory-scale photocatalytic fabrication, this study includes utility-scale cell production, assembly, operation, and end of life to give a more accurate and precise environmental performance estimation. The results show that g-C₃N₄/BiOI has the lowest GHG footprint (0.38 kg CO₂ eq per kg of H₂) and CNF: TNR/TiO₂ has the lowest energy payback time (0.4 years). In every pathway, energy use in material extraction processes makes up the largest GHG contribution, between 83% and 89%. Sensitivity and uncertainty analyses were conducted under the impact of various input parameters on the life cycle GHG emissions of hydrogen production. Photocatalytic water splitting is highly feasible for adaptation as a mainstream hydrogen production pathway in the future.     

Green synthesis of well dispersed TiO2/Pt nanoparticles photocatalysts and enhanced photocatalytic activity towards hydrogen production

Deposition of Pt NPs with preferred dispersion and morphologies on TiO₂ have been the focus of studies in photocatalytic and photoelectrochemical hydrogen production. Green synthesis of TiO₂/Pt NPs nanocomposites with narrow size distribution of Pt NPs still remain a challenge. Herein, we report that sucrose is highly efficient for the preparation of well-dispersed TiO₂/Pt NPs photocatalysts. Moreover, the sucrose could act as an electron donor, showing higher hydrogen production activity under simulated sunlight than pure water. The as-synthesized photocatalysts have been characterized by techniques of transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), and diffuse reflectance spectroscopy (DRS). Compared with TiO₂/Pt NPs photocatalysts prepared through conventional photodeposition, the photocatalysts as prepared showed higher photocatalytic efficiency. Moreover, the catalyst could be reused easily without apparent degradation of their original photocatalytic activities. This approach presents a promising and low-cost strategy to improve the photocatalytic performance of TiO₂ from biomass.     

Enhanced photocatalytic hydrogen evolution from reduced graphene oxide-defect rich TiO2-x nanocomposites

Solar-driven photocatalytic hydrogen generation by splitting water molecules requires an efficient visible light active photocatalyst. This work reports an improved hydrogen evolution activity of visible light active TiO_2-x photocatalyst by introducing reduced graphene oxide via an eco-friendly and cost-effective hydrothermal method. This process facilitates graphene oxide reduction and incorporates intrinsic defects in TiO₂ lattice at a one-pot reaction process. The characteristic studies reveal that RGO/TiO_2-x nanocomposites were sufficiently durable and efficient for photocatalytic hydrogen generation under the visible light spectrum. The altered band gap of TiO_2-x rationally promotes the visible light absorption, and the RGO sheets present in the composites suppresses the electron-hole recombination, which accelerates the charge transfer. Hence, the noble metal-free RGO/TiO_2-x photocatalyst exhibited hydrogen production with a rate of 13.6 mmol h^?1g^?1_cat. under solar illumination. The appreciable photocatalytic hydrogen generation activity of 947.2 μmol h^?1g^?1_cat with 117 μAcm^?2 photocurrent density was observed under visible light (>450 nm).     

Investigation of the appropriate content of graphene in AgTiO2-graphene ternary nanocomposites applied as photocatalysts

AgTiO₂-graphene ternary nanocomposites with varying graphene contents were fabricated by photocatalytic reduction. SEM and TEM imaging of the nanocomposites showed that TiO₂ nanoparticles decorated with Ag nanoparticles covered graphene nanosheets. A higher content of graphene was shown to be favorable for dye photodegradation. The results of electrochemical analysis revealed that a higher graphene content contributed to increased conductivity and reduced interfacial impedance, which led to more efficient electron transport and thus higher photocatalytic activity. The highest efficiency in dye photodegradation and hydrogen production from water splitting was achieved when the ratio of TiO₂ to graphene in the nanocomposite was 5: 1. The corresponding mass-normalized hydrogen evolution rate and quantum efficiency were 129.5 μmol g^?1 h^?1 and 4.8%, respectively. A mechanism for photocatalysis was proposed and discussed. This study demonstrates that the AgTiO₂-graphene ternary nanocomposite could be a promising photocatalyst.     

The design of a hierarchical photocatalyst inspired by natural forest and its usage on hydrogen generation

A novel photocatalyst was designed from the inspiration of natural forest's high efficient on light harvesting and energy conversion. This novel “forest-like” photocatalyst was successfully synthesized by a facile continuously-conducted three steps methods: electrospinning TiO₂ nanofiber acts as the trunks, hydrothermal growth ZnO nanorods on the surface of TiO₂ nanofiber acts as the branches, while photodeposition of Cu nanoparticles on the surface of TiO₂ nanofiber and ZnO nanorods act as the leaves. This novel photocatalyst demonstrated higher photocatalytic hydrogen generation rate than most of semiconductor catalysts and many newly developed catalysts such as Pt/TiO₂ catalyst and artificial leaves Pt/N–TiO₂ catalyst in a water/methanol sacrificial reagent system under the light irradiation as a result of its enhanced light absorption ability, enlarged specific surface area promoting mass transfer and providing more reaction sites and its potential on anti-recombination of electrons and holes. Meanwhile, it is interesting to note that the photocatalytic hydrogen generation activity has a liner relationship with the hierarchy of materials, which means higher hierarchy materials display higher photocatalytic hydrogen generation activity. It is reasonable to believe that this natural mimic photocatalyst without noble metals will benefit the energy generation and novel materials development.     

Carbon nitride quantum dots (CNQDs)/TiO2 nanoparticle heterojunction photocatalysts for enhanced ultraviolet-visible-light-driven bisphenol a degradation and H2 production

The extension of the absorption band of solar energy is an efficient strategy to dramatically enhance the application value of TiO₂. Based on this, we have prepared carbon nitride quantum dots (CNQDs)/TiO₂ nanoparticle heterojunctions by mixing TiO₂ and the as-prepared CNQDs by the simple mechanical stirring method. The synthesized CNQDs-x/TiO₂ composites were systematically characterized in term of their physicochemical properties, the performance of photocatalytic degradation of Bisphenol A, and their photocatalytic hydrogen evolution performance under stimulated sunlight. The CNQDs/TiO₂ nanoparticle exhibited a lattice spacing of 0.352 nm, assigning to the (101) crystal plane of anatase phase TiO_2. Intriguingly, the modification of TiO₂ nanoparticle with CNQDs can indeed get a narrower optical band gap of 3.02 eV, with a wider absorption range extending to visible light region and could enhance their overall photocatalytic performance over the commercially TiO₂ nanoparticles. In Addition, it was demonstrated that the ratios of CNQDs to TiO₂ exhibited obvious influence on the photocatalytic performance of the obtained composite catalysts.By contrast to the pure TiO₂, all the CNQDs-x/TiO₂ composites displayed higher photocatalytic activities, and the CNQDs-2/TiO₂ possessed the highest photocatalytic degradation capacity towards bisphenol A with a reaction rate constant 0.30 (0.17 for pure TiO₂). Meanwhile, the H₂ production rate of CNQDs-2/TiO₂ sample is about 30 μmol g⁻¹ h⁻¹ higher than that of the pure TiO₂ nanoparticles. Moreover, the photocurrent intensity of CNQDs-2/TiO₂ was about 25 times higher compared to that of pure TiO₂ nanoparticles. Therefore, our research results can provide valuable guidance for exploring high-performance photocatalytic materials.     

Co-application of liquid phase plasma process for hydrogen production and degradation of acetaldehyde over NiTiO2 supported on porous materials

Photocatalytic decomposition of acetaldehyde-contained wastewater was assessed for the degradation of pollutants and the production of hydrogen. Liquid phase plasma was applied in the photoreaction as a light source. The evolution of hydrogen and acetaldehyde degradation were characterized by the photocatalytic decomposition system. Ni-loaded TiO₂ photocatalysts and various porous materials were introduced to the photocatalytic reaction. The photochemical decomposition by irradiation of the liquid phase plasma without photocatalysts produced some hydrogen evolution with the degradation of acetaldehyde, which was attributed to the decomposition of the reactant by active species generated by the irradiation of liquid phase plasma. The Ni loading on TiO₂ brought out an enhancement of acetaldehyde degradation and hydrogen evolution. In the photocatalysis of acetaldehyde-contained wastewater using the liquid phase plasma, hydrogen evolution was accelerated due to the additional hydrogen production by the photocatalytic decomposition of acetaldehyde. The porous materials could be used as an effective photocatalytic support. MCM-41 mesoporous material has acted as a highly efficient photocatalytic support to the TiO₂ photocatalyst.     

Vertically-heterostructured TiO2-Ag-rGO ternary nanocomposite constructed with {001} facetted TiO2 nanosheets for enhanced Pt-free hydrogen production

TiO₂ nanosheets with high ratio of {001} facets were coupled with reduced graphene oxide (rGO) nanosheets through the link of silver (Ag) nanoparticles, forming a novel ternary nanocomposite photocatalyst with a vertical heterostructure, TiO₂-Ag-rGO. The vertical anchoring of TiO₂-Ag nanosheets between rGO sheets was confirmed by transmission electron microscopy (TEM), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Due to excellent separation of electron-hole pairs in the TiO₂ nanosheets, enhanced electron transfer to rGO via Ag nanoparticles, the TiO₂-Ag-rGO nanocomposite exhibited an outstanding performance in photocatalytic hydrogen production, with a hydrogen production rate of 593.56 μmol g^?1 h^?1. This study provides new insights to the development of Pt-free photocatalysts for hydrogen production.     

Enhanced photocatalytic activities of three-dimensional graphene-based aerogel embedding TiO2 nanoparticles and loading MoS2 nanosheets as Co-catalyst

A novel graphene-based three-dimensional (3D) aerogel embedded with two types of functional nanomaterials had been prepared by a facile one-pot hydrothermal process. During the hydrothermal reaction, graphene, TiO₂ nanoparticles and MoS₂ nanosheets were self-assembled into the 3D interconnected networks aerogel, where the uniformly dispersed TiO₂ nanoparticles were densely anchored onto the graphene nanosheets and decorated with the ultrathin MoS₂ nanosheets. The UV–vis DRS and PL spectra measurement shows that the MoS₂/P25/graphene aerogel exhibits enhanced light absorption and efficient charge separation properties. As a new photocatalyst, the photocatalytic activity was evaluated by photoelectrochemical test and photodegradation methyl orange (MO) under UV irradiation, an improvement of photocurrent was observed, as 6 times higher for MoS₂/P25/graphene aerogel (37.45 mA/cm²) than pure P25 at +0.6 V, and the fastest photodegradation of MoS₂/P25/graphene aerogel was found within 15 min. The improved photocatalytic activity is attributed to the porous structure, good electrical conductivity and the maximization of accessible sites of the unique 3D graphene aerogel, the increasing active adsorption sites and photocatalytic reaction centers for the introduction of MoS₂ nanosheets, and the positive synergetic effect between the three components in this hybrid. This work demonstrates that the as-prepared MoS₂/P25/graphene aerogel may have a great potential application in photoelectrochemical hydrogen production and pollution removal.     

Controllable O2 oxidization graphene in TiO2/graphene composite and its effect on photocatalytic hydrogen evolution

TiO₂/reduced graphene oxide composite (T-rGO) was synthesized and its performance was evaluated with photocatalytic hydrogen evolution. It was found that the hydrogen evolution rate of T-rGO increased significantly after injecting small amount of air into the vacuum pumped and UV irradiated sealed reaction cell. The IR, XPS, Raman and ESR spectra analysis indicated that the O₂^•−, which generated from the reaction of photoinduced electrons and the injected O₂ can moderately and controllably increase the oxygen groups on graphene planar of T-rGO at ambient condition. The amount of oxygen groups on graphene planar of T-rGO will affect the p-doping concentration of graphene, thus affect the p–n junction and the performance of T-rGO for photocatalytic hydrogen evolution.