Photocatalytic hydrogen production by water splitting over Au/Al-SrTiO3

Visible light water splitting activity of Au-Al/SrTiO₃ was tested in this work. Al/SrTiO₃ was synthesized via solid state reaction while Au loading was done with homogenous deposition precipitation method. The effects of Au loading and Al doping were investigated in 10, 20 and 30% aqueous solutions of methanol, ethanol, and isopropyl alcohol. The methanol was performed better over 0.25%Au-1.0%Al/SrTiO₃ at 20% alcohol concentration while the isopropyl alcohol resulted in better performance over the same catalyst at 30% concentration; the latter was also the best result obtained in this work with the hydrogen evolution rate of 347 μmol/h.gcat. Ethanol showed lower performance than other two alcohols. It was found from UV–vis analysis that Al doping increased the band energy of SrTiO₃. XRD and XPS analyses clearly showed that the dominant structure was SrTiO₃ in all samples. Au was found to be generally loaded as 30–40 nm particles by SEM.     

A new insight into Au/TiO2-catalyzed hydrogen production from water-methanol mixture using lamps containing simultaneous ultraviolet and visible radiation

Different amounts of Au NPs were deposited on a modified-TiO₂ using the deposition-precipitation method with urea and used for hydrogen production via water splitting at room temperature and atmospheric pressure. Methanol and simultaneous UV and visible radiation were used as sacrificial reagent and excitation sources, respectively. Both modified-support and photocatalysts were characterized by XRD, HRTEM or STEM-HAADF, FE-SEM-EDS, N₂ physisorption and UV–vis DRS. The emission spectra of the excitation sources were also obtained by spectrofluorometry. XRD, HRTEM and UV–vis DRS results showed that TiO₂ anatase was the predominant crystalline phase, with a relative high specific surface area. STEM-HAADF and FE-SEM-EDS techniques revealed that the average Au NPs size was increased with Au loading from 3.2 to 14.9 nm and that the estimated Au contents were close to the expected theoretical values. On the other hand, the photo-generated hydrogen was significantly increased with Au NPs incorporation and it could be associated to a slightly decrease of the energy band gap and the intrinsic localized surface plasmon resonance that can suppress the high rate of electron-hole pair recombination. The photocatalytic performance also depended on multiples experimental factors, such as: stirring speed, amount and size of Au NPs, as well as the radiation source. The highest hydrogen production rate (2336 μmol-H₂/g_cat⋅h) was obtained using the Au/TiO₂ photocatalyst with 0.5 wt% Au, a stirring speed of 800 rpm and purple lamp (13 W) simultaneously emitting UV (52%) and visible (48%) radiation.     

Noble metal-free cobalt oxide (CoOx) nanoparticles loaded on titanium dioxide/cadmium sulfide composite for enhanced photocatalytic hydrogen production from water

In this present paper, cobalt oxide (CoO_x) is studied as an effective cocatalyst in a photocatalytic hydrogen production system. CoO_x-loaded titanium dioxide/cadmium sulfide (TiO₂/CdS) semiconductor composites were prepared by a simple solvothermal method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production was studied using the as-synthesized photocatalysts in aqueous solution containing sodium sulfide (Na₂S)/sodium sulfite (Na₂SO₃) as hole scavengers under visible light irradiation (λ > 400 nm). The optimal cobalt content in CoO_x-loaded TiO₂/CdS composite is determined to be 2.1 wt% and the corresponding rate of hydrogen evolution is 660 μmol g⁻¹ h⁻¹, which is about 7 times higher than TiO₂/CdS and CdS photocatalysts under the same condition. Visible light-driven photocurrents of the semiconductor composites were further measured on a photoelectrochemical electrode, revealing that the photocorrosion of CdS can be prevented due to the presence of TiO₂–CoO_x.     

Hydrogen production performance of active Ce/N co-doped SrTiO3 for photocatalytic water splitting

An efficient visible light responsive photocatalyst Ce/N co-doped SrTiO₃ was prepared via a hydrothermal method for hydrogen production. The phase structure, morphology, contents and valence states of the dopant elements, specific surface area, optical properties, and photocatalytic activity of the samples were characterized. The transient photocurrent response and electrochemical impedance spectra under visible light illumination indicated that Ce/N co-doped SrTiO₃ possessed a more intense photo-current response and lower surface resistance than N–SrTiO₃ and Ce–SrTiO₃. The water splitting rate of Ce/N-co-doped SrTiO₃ is 4.28 mmol/g/h, which is 84.49 times higher than that of pure SrTiO₃. The enhanced photocatalytic performance is due to the narrowing of the band gap of SrTiO₃ by Ce ion and N ion impurities.     

Visible-light photo-assisted synthesis of GO-TiO2 composites for the photocatalytic hydrogen production

The main aim of this study is to propose a fast and simple synthesis method for GO-TiO₂ nanocomposites, which present self-tuning optoelectronic properties, advantageous features, for their application towards the photocatalytic hydrogen production. Graphene oxide (GO) was prepared through a modification of the Tour method from powder graphite by applying a microwave pretreatment. GO-TiO₂ nanocomposites were synthesized by visible-light assisted anchoring, while the study of morphology, crystalline structure, size, surface area and optical characterization of the material was carried out by SEM, XRD, TEM, BET and UV-Vis spectroscopy, respectively. Photocatalytic activity evaluation of the composite material (GO-TiO₂) towards hydrogen production through the water splitting reaction from water under visible light was followed by gas chromatography, reaching a production of 6500 μmolH₂/g, thus showing an enhancement effect compared to the conventional H₂ production using TiO₂ only.     

A review on integrated thermochemical hydrogen production from water

Hydrogen production from water splitting is considered one of the most environmentally friendly processes for replacing fossil fuels. Among the various technologies to produce hydrogen from water splitting, thermochemical cycles using chemical reagents have the advantage of scale up compared to other specific facilities or geological conditions required. According to thermochemical processes using chemical redox reactions, 2-, 3-, 4-step thermochemical water splitting cycles can generate hydrogen more efficiently due to reducing temperatures. Increasing the number of cycles or steps of thermochemical hydrogen production could reduce the required maximum temperature of the facility. In addition, recently developed hybrid thermochemical processes combined with electricity or solar energy have been studied on a large scale because of the reduced cost of hydrogen production. Additionally, hybrid thermochemical water splitting combined with renewable energy can result in not only reducing the cost, but also increasing hydrogen production efficiency in terms of energy. As for a green energy, hydrogen production from water splitting using sustainable and renewable energy is significant to protect biological environment and human health. Additionally, hybrid thermochemical water splitting is conducive to large scale hydrogen production. This paper reviews the multi-step and highly developed hybrid thermochemical technologies to produce hydrogen from water splitting based on recently published literature to understand current research achievements.     

Pulsed current water splitting electrochemical cycle for hydrogen production

A pulsed current 3 D MnO₂ electrode water splitting electrochemical cycle is being proposed for hydrogen production. In 3D MnO₂ electrochemical cycle, the reactions take place at the solid/liquid and solid/gas two phase boundaries. Also, this electrochemical cycle should be able to generate hydrogen and oxygen gas separately at different periods of time. Here, we applied an interrupted pulsed current to reduce the overpotential caused by diffusion layers in conventional direct current electrolysis. The pulsed current, which disturbs the formation of the ion diffusion layer in the vicinity of the electrodes, is observed to be effective above 50 Hz. The best electrolysis performance was recorded at a current density of 0.2 A cm^?2, and the observed cell voltage was 1.69 V at 25 °C for a pulse frequency of 500 Hz, which is less than the corresponding conventional alkaline electrolysis.     

Shining light on panchromatic ruthenium sensitizers towards dye-sensitized photocatalytic hydrogen evolution

Five new photocatalysts have been synthesized in order to extend the photo response upto visible range, by adsorbing MC113-MC117 ruthenium complexes on TiO₂-Pt composites. Highlight harvesting properties of these ruthenium complexes instigated us to evaluate for photocatalytic activity. The absorption curves of the synthesized photocatalysts extended up to 750 nm. Morphological studies of photocatalysts have been carried out using SEM and powder X-ray crystallography. Among all photocatalysts, MC113PC showed high photocatalytic activity i.e. 9474 TONs. IPCE and fluorescence quenching studies of the catalysts revealed the light harvesting nature and electron injection efficiency. The photocatalytic activity of MC photocatalysts were systematically screened at different pH and employing different sacrificial electron donors (SED) in order to obtain optimal photocatalytic performance.     

Synthesis of carbon-doped SnO2 nanostructures for visible-light-driven photocatalytic hydrogen production from water splitting

Environmental issues: global warming, organic pollution, CO₂ emission, energy shortage, and fossil fuel depletion have become severe threats to the future development of humans. In this context, hydrogen production from water using solar light by photocatalytic/photoelectrochemical technologies, which results in zero CO₂ emission, has received considerable attention due to the abundance of solar radiation and water. Herein, a single-step thermal decomposition procedure to produce carbon-doped SnO₂ nanostructures (C–SnO₂) for photocatalytic applications is proposed. The visible-light-driven photocatalytic performance of the as-prepared materials is evaluated by photocatalytic hydrogen generation experiments. The bandgaps of the photocatalysts are determined by ultraviolet–visible diffused reflectance spectroscopy. The crystallinity, morphological features (size and shape), and chemical composition and elemental oxidation states of the samples are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The proposed simple thermal decomposition method has significant potential for producing nanostructures for metal-free photocatalysis.     

NaFeTiO4: A novel visible light active photocatalyst for water splitting and environmental remediation

Single phase, crystalline NaFeTiO₄ with tunnel structure is prepared by a solid state method and explored as a novel photocatalyst for the first time. Structural, optical and morphological properties of NaFeTiO₄ are investigated by various characterization techniques such as X-ray diffraction (XRD), scanning & transmission electron microscopy (SEM & TEM), Energy dispersive X-ray spectroscopy (EDS), N₂ adsorption-desorption study (BET), UV-vis, X-ray photoelectron, X-ray absorption (UV-vis DRS, XPS and XANES) and photoluminescence (PL) spectroscopy. The interfacial charge transfer ability of the prepared n-type NaFeTiO₄ was also investigated by transient photocurrent response and electrochemical impedence spectroscopy which proved to be an efficient tool for better understanding of electronic properties of NaFeTiO₄. The photocatalytic efficiency of NaFeTiO₄ is evaluated for decomposition of methylene blue (MB) and Rhodamine B (RhB) dyes as well as for H₂ evolution through water splitting reaction under visible light. NaFeTiO₄ exhibits efficient charge separation properties, excellent photocatalytic activities and reusability.     

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H₂ evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H₂ production have been discussed. In the mainstream, recent developments in TiO₂ and g-C₃N₄ based photo-catalysts and their potential for H₂ production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H₂ production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H₂ production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.     

Design of cocatalyst loading position for photocatalytic water splitting into hydrogen in electrolyte solutions

Photocatalytic water splitting into hydrogen is a very attractive and desirable technology to realize sustainable and renewable green energy conversion. Up to now, many research results have confirmed that cocatalyst such as Pt is essential for a high efficiency photocatalytic H₂ evolution system. In a traditional view, the cocatalyst should be closely combined with the photocatalyst to achieve a high H₂ or O₂ photo-productive rate. In this work, an unusual point has been put forward that the suitable loading position of cocatalyst Pt for film-type TiO₂ catalyst is on the bare Fluorine-doped tin oxide (FTO) substrate instead of on the surface of TiO₂ in the electrolyte solutions. Especially, in acidic electrolyte, the hydrogen production rate of this new designed catalyst with Pt loaded on FTO (TiO₂-Pt/FTO) reaches 2.4 times that of the common catalyst with Pt loaded on TiO₂ (Pt/TiO₂-FTO). According to the experiment results, it is supposed that this loading way of cocatalyst on the substrate can construct a self-bias photocatalytic electrochemical cell system, drive electrolyte ions' movement directionally, and obtain high photocatalytic H₂ production efficiency. The universality of this innovation has been verified by CdS and CdS@TiO₂ film-type catalysts. This study provides a new guide in exploring high-efficiency film-type photocatalytic system for water splitting into hydrogen in the electrolyte solution.     

A perspective on the fabrication of heterogeneous photocatalysts for enhanced hydrogen production

This perspective provides an insight to the possibility of adopting hydrogen as a key energy-carrier and fuel source, through Photocatalytic water splitting in the near future. The need of green and clean energy is increasing to overcome the growing demand of sustainable energy throughout globe, owing to CO₂ emission using fossil fuels. To generate highly efficient and cost-competitive hydrogen, the semiconductor based heterojunction nanomaterials have gained tremendous consideration as a promising way. Currently, the efficiency for hydrogen generation through UV–Vis active photocatalysts is relatively low. The key issues are found to be poor separation of photogenerated electron/hole, less surface area, and low absorption region of electromagnetic spectrum. Such issues arise due to inappropriate band edge potentials and large bandgap of present catalyst. A lot of schemes has been devoted to design and fabricate efficient photocatalysts for improved photocatalytic performance in recent years. However, it seems still a challenge and imperative to greatly comprehend the fundamental aspects, photocatalysis and transfer mechanisms for complete deployment of electron/hole pairs. Further, to produce hydrogen to a larger extent through photocatalytic water splitting, the photocatalyst has been modified through co-catalysts/dopants using numerous techniques including the Z-scheme, hybridization, crystallinity, morphology, tuning of band edge positions, reduction of the band gap, surface structure etc., such that these heterogeneous photocatalysts may have ability to absorb enough light in the UV-VIS-IR region. This type of heterogeneous photocatalysts has the ability to improve the rate of efficiency for hydrogen evolution through absorption of sufficient light of solar spectrum and enhance the separation of charge-carriers by inhibiting recombination of electron/hole pairs. We surmise that taking into account the aforesaid factors should support in scheming an efficient photocatalysts for hydrogen production through water splitting, eventually prompting technological developments in this field.     

Review on hydrogen production photocatalytically using carbon quantum dots: Future fuel

They are sometimes identified as zero-dimensional (0D) nanoparticles. These particles have gained much attention in water splitting into hydrogen and oxygen through photocatalytic conversion. CQDs act as semiconductor few nm sizes, due to very small size; their optical and electronic properties differ from larger particles. CQDs particle has high stability, mild toxicity besides conductivity. These particles are environmentally friendly due to low toxicity and also have excellent luminescence. Therefore they can be utilized as a potential source for the splitting of water photocatalytically. The parting of water into H₂ and O₂ will enable us to produce or collect hydrogen to be used as a future fuel. The review summarizes the efforts made by various researchers in the field of utilizing carbon quantum dabs for water splitting which may be further followed by future researchers for commercial-scale hydrogen production. Thus, the study concludes the methods for the production of CQDs and their utilization under sunlight by catalytically hydrogen gas production from water.     

A novel three-step GeO2/GeO thermochemical water splitting cycle for solar hydrogen production

This investigation reports the thermodynamic exploration of a novel three-step GeO₂/GeO water splitting (WS) cycle. The thermodynamic computations were performed by using the data obtained from HSC Chemistry thermodynamic software. Numerous process parameters allied with the GeO₂/GeO WS cycle were estimated by drifting the thermal reduction ($T_H$) and water splitting temperature ($T_L$). The entire analysis was divided into two section: a) equilibrium analysis and b) efficiency analysis. The equilibrium analysis was useful to determine the $T_H$ and $T_L$ required for the initiation of the thermal reduction (TR) of GeO₂ and re-oxidation of GeO via WS reaction. Furthermore, the influence of $P_{O_2}$ on the $T_H$ required for the comprehensive dissociation of GeO₂ into GeO and O₂ was also studied. The efficiency analysis was conducted by drifting the $T_H$ and $T_L$ in the range of 2080 to 1280 K and 500–1000 K, respectively. Obtained results indicate that the minimum ${\dot{Q}}_{solar-cycle}=624.3\text{kW}$ and maximum ${\eta }_{solar-to-fuel}=45.7\text{\%}$ in case of the GeO₂/GeO WS cycle can be attained when the TR of GeO₂ was carried out at 1280 K and the WS reaction was performed at 1000 K. This ${\eta }_{solar-to-fuel}=45.7\text{\%}$ was observed to be higher than the SnO₂/SnO WS cycle (39.3%) and lower than the ZnO/Zn WS cycle (49.3%). The ${\dot{Q}}_{solar-cycle}$ can be further decreased to 463.9 kW and the ${\eta }_{solar-to-fuel}$ can be upsurged up to 61.5% by applying 50% heat recuperation.     

Chloride ion: A promising hole scavenger for photocatalytic hydrogen generation

The investigation pertains to elucidation of promising role of in-situ chloride ions generated during the photoreduction of HAuCl₄ as an internal sacrificial donor for photocatalytic hydrogen generation. The hydrogen evolution rate (HER) observed was 4.16 mmol h^?1 using in-situ route of photocatalyst formation which is significantly higher than the conventional route of formation of recovered photocatalyst. This unreported and unprecedented enhancement is explained on the basis of role of chloride ions released from the gold precursor. Experimental data inferring the effect of chloride ions on photocatalytic hydrogen generation using AuTiO₂ are also briefly explained. The role of anionic sacrificial donors suggests several potential possibilities for their applications in photocatalysis considering their presence in wastewater as well as their low cost and abundant availability. The work also introduces one-step photodeposition and hydrogen generation process against traditional recovered photocatalyst, wherein the catalysts were prepared first by normal route of photodeposition, recovered and then employed for hydrogen generation.     

Ti3C2 MXene coupled with CdS nanoflowers as 2D/3D heterostructures for enhanced photocatalytic hydrogen production activity

As a novel co-catalyst, Ti₃C₂ MXene has an excellent prospect in the field of photocatalysis. Herein, the 2D/3D Ti₃C₂ MXene@CdS nanoflower (Ti₃C₂@CdS) composite was successfully synthesized by a hydrothermal method. The combination of 2D Ti₃C₂ MXene and 3D CdS nanoflowers can promote carrier transfer and separation, which can improve the performance of CdS. Compared to pure CdS nanoflowers, Ti₃C₂@CdS composite presents lower photoluminescence intensity, longer fluorescence lifetime, higher photocurrent density and smaller electrochemical impedance. The Ti₃C₂@CdS composite with 15 wt% Ti₃C₂ adding amount presents high photocatalytic hydrogen evolution activity (88.162 μmol g^?1 h^?1), 91.57 times of pure CdS. The improved photocatalytic activity of Ti₃C₂@CdS composite is ascribed to the addition of lamellar Ti₃C₂ MXene, which improves the electrical conductivity of the photocatalytic system and effectively accelerates the excited electrons transfer from CdS to Ti₃C₂ MXene.     

Chemical strategies in molybdenum based chalcogenides nanostructures for photocatalysis

Since the last two decades, plenty of environmental issues have risen up due to the damage which humans have caused to the planet for the sake of development. The continual ignorance of global climate change and the stalemate approach of major oil producing industries led to the catastrophic melting of glaciers in Arctic and Antarctica and very recently the highest mountain peak of Sweden have become 24 m shorter which is the evident outcome of climatic disturbance. The chaotic unbalancing in the reservoirs of natural resources is leading us to the several crisis which has a potential to affect the livelihood. Among the various techniques used for the development of sustainable energy, photocatalysis is regarded as one of the simplest technique which can yield enormous amount of energy by the utilization of solar energy for meeting the world's demand of an energy requirement and which can be exploited in the degradation of toxic pollutants i.e. organic as well as inorganic pollutants for environment remediation. Transition metal chalcogenides (TMCs) have a potential to get adsorb easily and be utilized for solving the energy-related problems. Large number of photocatalysts has been fabricated, among them Molybdenum (Mo) chalcogenides nanostructures, which also belong to the class of TMCs exhibit exceptional properties such as non-toxicity, low cost and structural flexibility which give them edge over the other materials. Furthermore, the tunable band gap of Mo chalcogenides nanostructures makes them the promising candidates for efficient hydrogen evolution via photocatalytic water splitting in the visible light illumination. This review deals with the photocatalytic applications of Mo based chalcogenides nanostructures in efficient hydrogen production via water splitting and degradation of dyes. It also discusses the recent developments in fabricating Molybdenum chalcogenides nanostructures, their role in the photocatalytic water splitting and discusses the efforts which have been made to improve their photocatalytic activity for extending their applications to the scalable point.     

TiO2 nanoparticles modified with 2D MoSe2 for enhanced photocatalytic activity on hydrogen evolution

As an emerging two-dimensional (2D) nanomaterial, 2D MoSe₂ nanosheets has the advantages of wide light response and rapid charge migration ability. In this work, 2D MoSe₂/TiO₂ nanocomposites were successfully synthesized through a simple hydrothermal method. The microstructure and photocatalytic activity of the nanocomposites were systematically investigated and determined. The corresponding Raman peaks and crystal planes of MoSe₂ were analysed by Raman spectroscopy and transmission electron microscopy respectively, demonstrating the successful combination of the MoSe₂ nanosheets and TiO₂ nanoparticles. UV-vis diffused reflectance spectra demonstrated that the introduction of MoSe₂ did increase the light absorption ability of the nanocomposites. A lower recombination of electrons and holes was demonstrated for the MoSe₂/TiO₂ heterojunction from photoluminescence results. The photocatalytic hydrogen evolution test showed that the hydrogen production rate was 4.9 μmol h⁻¹ for the sample with 0.1 wt.% MoSe₂, 2 times higher than that of bare TiO₂. This work provides a novel strategy for improving the photocatalytic properties of semiconductor photocatalyst.