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.     

Ionothermal synthesis of TiO2 nanoparticles for enhanced photocatalytic H2 generation

Photocatalytic hydrogen generation is one of the most promising solutions to convert light energy into green chemical energy. In the present work, methoxy ethyl methyl imidazolium methyl sulphonate ionic liquid is used for the synthesis of i-TiO₂ nanoparticles via ionothermal method at 120 °C. The obtained products were characterized by various spectroscopic techniques like XRD, FTIR, Raman, UV–visible, DRS, TEM and TG-DSC analysis. XRD pattern confirmed the anatase phase with minor rutile phase having average crystallite size of 5 nm. From the FTIR spectrum, the band appeared at ～547 cm^?1 confirmed the Ti–O–Ti stretching and also few bands of ionic liquid. UV–vis spectrum clearly reveals the blue shift due to size effect of TiO₂. The spherical surface structure and particle size (15–30 nm) have been studied in detail using TEM images. Finally, the practical applicability of the as synthesized i-TiO₂ nanoparticles is shown by using it as a photocatalyst towards the generation of H₂ through water splitting reaction and it is found to be 462 μmol h^?1g^?1.     

Enhanced photocatalytic activity in water splitting for hydrogen generation by using TiO2 coated carbon fiber with high reusability

In this study, TiO₂ coated carbon fiber (TiO₂@CF) was synthesized and used for the improvement of hydrogen (H₂) evolution. Obtained results from scanning electron microscopy (SEM), X-ray diffraction (XRD), gas adsorption analysis (BET), UV–vis diffuse (UV–vis), and X-ray photoelectron spectroscopy (XPS) confirmed that the surface area and light absorption of the material was significantly improved. The synthesized TiO₂@CF photocatalyst exhibited improved photocatalytic performance toward hydrogen generation. The enhancement of photocatalytic H₂ evolution capacity by TiO₂@CF was ascribed to its narrowed bandgap energy (2.76eV) and minimized recombination of photogenerated electron-hole pairs The hydrogen production rate by the TiO₂@CF reached 3.238 mmolg^?1h^?1, which was 4.8 times higher than unmodified TiO₂ (0.674 mmolg^?1h^?1). The synthesized TiO₂@CF was relatively stable with no distinct reduction in photocatalytic activity after five recycling runs. The photoluminescence and photocurrent were employed to support the photocatalytic H₂ production mechanism proposed mechanism.Based on these results, TiO₂@CF with unique properties, easy handle, and high reusability could be suggested as an efficient strategy to develop a high-performance photocatalyst for H₂ production.     

Highly-efficient photocatalytic activity of TiO2-AC nanocomposites for hydrogen production from sulphide wastewater

Photocatalytic water splitting is one of the prospective green energy technologies, particularly, for hydrogen production from wastewater under natural sunlight irradiation. Herein, we report experimental data on the synthesis and characteristics of the biomass activated carbon (b-AC)-anchored anatase titanium oxide (a-TiO₂) nanocomposites, which were ultrasonically prepared by using the sol-gel-grown a-TiO₂ spherical nanoparticles and the KOH-activated citron-derived b-AC nanoflakes. The a-TiO₂/b-AC nanocomposites displayed an aggregated morphology with the interconnected spherical nanoparticles, and they showed a high surface area (495 m²/g). When using a-TiO₂/b-AC as a photocatalyst for hydrogen production from sulphide wastewater (0.15 M) under solar irradiation (740 W/m²), the superb hydrogen production efficiency was achieved up to 400 mL/h. The results suggest the a-TiO₂/b-AC nanocomposites to hold an ample potential for photocatalytic hydrogen production from the wastewater.     

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.     

Hierarchical CuO/ZnO “corn-like” architecture for photocatalytic hydrogen generation

Novel high efficient photocatalyst is the key for photocatalytic hydrogen generation from water splitting. In this study, a novel hierarchical CuO/ZnO “corn-like” architecture was designed and synthesized via a combination of hydrothermal and photodeposition method. The as-prepared nanostructured materials was shown to effectively generate hydrogen in the mixture of methanol and water (v/v = 1:10). This is because the hierarchical CuO/ZnO “corn-like” architecture: 1) greatly enhances the light utilization rate due to its special architecture, 2) enlarges the specific surface area, providing more reaction sites and promoting mass transfer, 3) promotes the photogenerated electrons transfer from ZnO to CuO, achieving the anti-recombination effect of electrons and holes, and 4) avoids the photocorrosion of ZnO to improve the stability of ZnO as a catalyst during water splitting. Moreover, the novel hierarchical CuO/ZnO “corn-like” architecture is easily recovered for reuse.     

Heterogeneous photocatalytic hydrogen generation in a solar pilot plant

Few studies have been published about large scale heterogeneous photocatalysis hydrogen generation with simultaneous removal of organic pollutants. The purpose of the present work was to study the simultaneous photocatalytic hydrogen production and organic pollutant removal under direct solar irradiation at pilot-plant scale. The experiments were performed in a Compound Parabolic Collector (CPC) at the Plataforma Solar de Almería (PSA). The efficiencies of two different photocatalytic systems, one based on a nitrogen doped and platinized TiO₂, and the other using a platinized CdS–ZnS composite were evaluated. Formic acid and glycerol were used as sacrificial electron donors. Also, experiments using real municipal wastewaters were carried out showing simultaneous hydrogen generation and partial water pollutant removal. The largest amounts of hydrogen were obtained with aqueous solutions of formic acid, although the experiments with real wastewater gave moderate amounts of hydrogen, pointing towards the possible future use of such waters for photocatalytic hydrogen generation.     

Hydrogenated F-doped TiO2 for photocatalytic hydrogen evolution and pollutant degradation

In this work, both hydrogenated and F-doping strategies are adopted to synergistically enhance the photocatalytic properties of TiO₂ under solar light irradiations. The hydrogenated F-doping TiO₂ is successfully synthesized by a facial two-step method and characterized with physicochemical and spectroscopy methods. It is found that the F-doping strategy can effectively enhance the UV-light absorption property of TiO₂, and the hydrogen treatments can further increase the light absorption performance of TiO₂ in the visible light region. As a result, the hydrogenated F-doping TiO₂ exhibits better photocatalytic properties than pure, hydrogenated or F-doped TiO₂, in either hydrogen production or organic pollutant degradation. The present work provides a suitable and synergistic strategy to enhance the photocatalytic activity of TiO₂.     

A novel AlBiOCl composite for hydrogen generation from water

In this paper, a novel AlBiOCl material has been prepared by milling Al powder and BiOCl firstly. Experimental results show that BiOCl-doped can prevent an inert alumina film forming on the surface of Al particles and induce the rapid hydrogen generation as well as high conversion rate. SEM, XRD, EDS, TEM, XPS and calorimetric techniques are used for the mechanism analysis of the samples. The results demonstrate the fresh surface of Al, AlCl₃, Bi and Bi₂O₃ are produced in situ under ball milling Al and BiOCl, which play an important role in hydrolysis reaction of Al. The hydrogen yield of Al-15 wt% BiOCl rises to 1058.1 mL g⁻¹ in about 5 min, corresponding to the high conversion yield of 91.6% at room temperature. After doping additives (such as LiH, Bi or AlCl₃), hydrogen generation performances of AlBiOCl-additive are further improved. For example, the conversion yield and maximum hydrogen generation rate (MHGR) of AlBiOClLiH can increase to 94.9% and 3178.5 mL g⁻¹ min⁻¹, respectively. Therefore, the proposed materials in this paper are expected to serve as a hydrogen generation material for the fuel cells.     

Magnetic CoOx@C-Reduced graphene oxide composite with catalytic activity towards hydrogen generation

Cobalt-containing magnetic core-shell structures are alternative catalysts with better catalytic performance for hydrogen evolution. In this study, a facile route is adopted to fabricate a CoO_x@carbon-reduced graphene oxide composite containing Co nanoparticle cores, carbon shells, and reduced graphene oxide. During hydrogen generation through the catalytic hydrolysis of NaBH₄ and NH₃BH₃, the surface of CoO_x cores provides active catalytic sites, and the carbon shells protect the CoO_x cores from aggregating into gigantic cobalt oxide granules. The magnetism of CoO_x anchored onto reduced graphene oxide sheets achieves effectively a momentum transfer assisted by a motional external magnetic field. In a batch reactor, the composite exhibits a higher catalytic activity in a self-stirring mode than that in a magneton-stirring mode. This simple and efficient synthesis strategy is highly promising for the next development of both facile hydrogen generation and core-shell composite functional materials.     

Significant improvement of TiO2 photocatalytic hydrogen generation by photothermic synergistic action and underlying mechanism

This article reported an extremely easy method of optical radiation-assisted thermal excitation to dramatically increase photocatalytic hydrogen generation ability of water splitting with P25 as a model compound. This method compensated for the time waste, high cost and operational complexity of traditional catalytic material modification methods, and largely improved the photocatalytic hydrogen production ability of photocatalytic materials. The hydrogen generation rates at room temperature is 1090 μmol/g/h. At 50 °C, the rate increase to 10670 μmol/g/h. The quantum rates at room temperature and 50 °C are 6.5 and 63.3, respectively. It is clear that appropriate low-temperature heating could largely accelerate the hydrogen generation rate of P25.This work presents the detailed mechanism how this method largely enhances photocatalytic hydrogen generation of P25 as well as the laws. The new method offers some evidences and reference for research on how the photothermic synergistic action facilitates the photocatalytic hydrogen generation of catalytic materials.     

Ascorbic acid-assisted hydrothermal route to create mesopores in polymeric carbon nitride for increased photocatalytic hydrogen generation

The facile synthesis of mesoporous polymeric carbon nitride (PCN) can enlarge the surface area and provide mass transport channels, which thus potentially boosts its photocatalytic H₂ generation. However, the formation of mesopores in PCN are mainly relying on the hard and soft templates with tedious operations, and it remains a grand challenge in preparing mesoporous PCN without the assistance of templates. Herein we report on an ascorbic acid-assisted hydrothermal route to effectively create the mesoporous in bulk PCN. The mesopores thus formed a typical IV isotherm with an H3 hysteresis loop, and have a pore size distribution of ∼3.8 nm. Moreover, the crystallinity of mesoporous PCN was improved with ascorbic acid-assisted hydrothermal treatment. Meanwhile, ascorbic acid can be converted to carbon materials under hydrothermal conditions. As a result, an increased photocatalytic H₂ generation was realized under visible light exposure. The highest H₂ generation rate is up to 26.8 μmol h⁻¹ for Pt/CN-A10%, near 10 times higher than that of Pt/PCN (2.7 μmol h⁻¹). This work highlights the effectiveness of organic acid-assisted hydrothermal treatment for synthesizing mesoporous PCN for increased photocatalytic performance.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Highly active and stable CeO2 supported nickel-complex catalyst in hydrogen generation

Cerium oxide supported 5-Amino-2,4-dichlorophenol-3,5-ditertbutylsalisylaldimine-Nickel complex for the first time was used to produce H₂ from hydrolysis of sodium borohydride. Cerium oxide supported Nickel complex catalyzed hydrolysis system was studied depend on temperature, concentration of sodium hydroxide, amount of Cerium oxide supported Ni complex catalyst, concentration of Ni complex and concentration of sodium borohydride. Cerium oxide supported Ni(II) complex display highly effective catalytic activity in sodium borohydride hydrolysis reaction. The obtained Cerium oxide supported Ni(II) complex catalyst was characterized by using Fourier Transform Infrared Spectroscopy, Scanning Electron Microscope, Transmission Electron Microscope, Brunauer-Emmett-Teller Surface Area Analysis, X-Ray Diffraction Analysis techniques. The catalyst stability was tested, even the fifth recycle the catalytic activity was maintained at 100%. Additionally the proposed Cerium oxide supported-Ni (II) complex catalyzed sodium borohydride hydrolysis mechanism was determined carefully. The experimental results showed that Cerium oxide supported Ni (II) complex catalyst accelerate sodium borohydride hydrolysis with 43,392 and 19,630 mL H₂ g_cat^?1 min^?1 hydrogen production rates at 50 °C and 30 °C respectively and 20,587 kJ mol^?1 activation energy.     

In situ preparation of N doped orthorhombic Nb2O5 nanoplates /rGO composites for photocatalytic hydrogen generation under sunlight

The synthesis of nitrogen doped orthorhombic niobium oxide nanoplates/reduced graphene oxide composites (NNb₂O₅/rGO) and their photocatalytic activity towards hydrogen generation from water and H₂S under natural sunlight has been demonstrated, uniquely. Nanostructured NNb₂O₅/rGO is synthesized by in situ wet chemical method using urea as a source of nitrogen and optimized by varying percentage of graphene oxide (GO). X?ray diffraction (XRD) study reveals that NNb₂O₅ have orthorhombic crystal structure with crystalline size, 35 nm. Further, X?ray photoelectron spectroscopy (XPS) confirm the presence of nitrogen and rGO in NNb₂O₅/rGO nanocomposite. Morphological features of (NNb₂O₅/rGO) were examined by FE?SEM and FE?TEM showed Nb₂O₅ nanoplates of diameter 25–40 nm anchored on 2D rGO. Diffuse reflectance spectra depicts the extended absorbance in the visible region with band gap of 2.2 eV. Considering the band gap in the visible region, the photocatalytic hydrogen generation from water and H₂S has been performed. The 1 wt % rGO hybridized NNb₂O₅ (S2) exhibited superior photocatalytic hydrogen generation (537 μmol/h) from water and (1385 μmol/h) from H₂S under sunlight. The improved photocatalytic activity is attributed due to an extended absorbance in the visible region, modified electronic structure upon doping and formation of well defined NNb₂O₅/rGO interface, provides large surface area, accelerates the supression of electron and hole pairs recombination rate. In our opinion, this works may provides facile route for energy efficient and economic approach for fabrication of NNb₂O₅/rGO nanocomposites as a visible light active photocatalyst.     

Boosting charge transfers in cadmium sulfide nanorods with a few layered Ni-doped MoS2 nanosheets for enhanced photocatalytic hydrogen evolution

The molybdenum sulfide (MoS₂) is a promising low-cost photocatalyst aimed at the hydrogen production reactions, however, obtaining a detailed understanding of its catalytic site has proved to be a challenging task. Several studies indicated that the active sites for catalytic reaction are mainly associated with the edge sites of 2D-layered MoS₂, and their basal plane (in-plane) displays poor activity toward catalytic reactions. Herein, we established the simple approaches to enhance the activity of MoS₂ by conversion of in-plane active sites into active surface edge sites by transition metal (Ni) doping followed by exfoliation. These activated MoS₂ was utilized for enormous upgrading of CdS photocatalytic activity for hydrogen production and is roughly 249 mmol h^?1 g^?1, which is 70 times higher than pure CdS, showed ～140 h stable H₂ production. The amended conductivity, improved surface area and huge active sites are extremely advantageous properties expanded by metal doping to MoS₂ and exfoliation. Additionally, another reason for the enhanced activity of Ni–MoS₂/CdS system was due to promotion of catalytic kinetics by Ni and Mo sits, they are admirable activity of water dissociation and higher ability of hydrogen adsorption correspondingly. These modifications made of superior photogenerated charge carriers’ separation and migration for effective utilization. As far as we know, this system demonstrates the utmost effective performance among inclusive reported MoS₂ based CdS composites. Remarkably, these outcomes will have abundant potential for the progress of immensely actual photocatalytic systems.     

Enhancing the hydrogen generation of TiO2 nanoparticles by decorating its surface with BiI3 and PbI2 quantum dots

This work reports the performance of TiO₂/BiI₃ and TiO₂/PbI₂ nanocomposites for hydrogen generation. BiI₃ and PbI₂ quantum dots (QDs) were grown on TiO₂ (P25 Degussa) using a fast injection method. According to the analysis by X-ray diffraction, the nanocomposites have a mixture of anatase, rutile and cubic phases from TiO₂, BiI₃ and PbI₂. The images obtained from transmission electron microscopy revealed that the TiO₂ support have sizes in the range of 70–220 nm while the QDs of BiI₃ and PbI₂ (co-catalysts) grown on TiO₂ have sizes in the range of 12–17 nm. The presence of these iodides on TiO₂ created oxygen vacancies defects (confirmed by photoluminescence measurements) that extended the light absorption of TiO₂ from the UV to the VIS range. According to the results from the photocatalytic experiments for hydrogen generation (achieved using pure water and UV-VIS light), the hydrogen generation rates produced by the TiO₂/BiI₃ and TiO₂/PbI₂ nanocomposites were 437–580 times, 81–108 times and 21–30 times, higher than these for pure TiO₂, PbI₂ and BiI₃, respectively. The maximum hydrogen generation rates of the TiO₂/BiI₃ and TiO₂/PbI₂ nanocomposites were 290.7 and 219.2 μmol h^?1 g^?1, respectively. In addition, the TiO₂/BiI₃ and TiO₂/PbI₂ nanocomposites contained defects that acted as electron trapping centers, which in turn, delayed the electron-hole recombination and this favored the photocatalytic generation of H₂. Moreover, the heterojunction formed between the TiO₂ and the iodides allowed the transfer of electrons from the conduction band of TiO₂ toward the conduction band of the iodides, creating a “sink” for the electrons which delayed the electron hole recombination. The results presented here demonstrated that the deposition of iodide co-catalyst on TiO₂ is a feasible option to enhance the hydrogen generation.     

Hybridization of g-C3N4 quantum dots with 1D branched TiO2 fiber for efficient visible light-driven photocatalytic hydrogen generation

The hybrid 1D branched TiO₂ loaded with g-C₃N₄ QDs was successfully fabricated that plays a significant role in photocatalysis. The 1D branched TiO₂ prepared by electrospinning followed by alkali-hydrothermal process, and g-C₃N₄ QDs were grafted over it by a chemical vapor deposition method. The composite display enhancement in photocatalytic hydrogen evolution is about 10.57 mmol. g⁻¹.h⁻¹ in comparison to the g-C₃N₄ sample that only produces 0.32 mmol. g⁻¹.h⁻¹ while the HBTiO₂ sample evolved a negligible amount of hydrogen under visible light. The composite sample shows quantum efficiency for HER at 420 nm light is 18.6% that is much higher than the other two samples. The specific surface area of the composite sample is 92.39 m²g^-1 that is about 13 times more than bulk g-C₃N_4. The bandgap of HBTiO₂/g-C₃N₄ QDs, g-C₃N₄, and HBTiO₂ samples calculated as 2.71 eV, 2.67eV, and 3.24eV, respectively. The TRPL spectra imply that the duration of the lifetime of composite becomes longer which effectually overwhelm the electron-hole recombination. The 1D branched TiO₂ fiber reduces the charge recombination by fast transfer of electron while g-C₃N₄ QDs facilitate the visible light absorption by improving the optical properties. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.     

Carbon vacancies improved photocatalytic hydrogen generation of g-C3N4 photocatalyst via magnesium vapor etching

Vacancies engineering was widely reported as the promising strategy for the improvement of the photocatalytic performance of semiconductor photocatalysts. In current work, carbon vacancies are constructed successfully in graphitic carbon nitride (g-C₃N₄) photocatalyst via magnesium vapor etching. Experimental results show that the formed carbon vacancies in g-C₃N₄ photocatalyst can significantly improve the photocatalytic H₂ generation performance. XRD, FTIR, SEM/TEM, XPS and PL characterization data are employed to evidence the construction of carbon vacancies, which are revealed to be the reason for the enhancement of photocatalytic H₂ evolution. This work develops an alternative route to construct carbon vacancies in g-C₃N₄ materials and gives an insight into the influence of vacancies on the photocatalytic performance of photocatalysts.