A nano heterostructure with step-accelerated system toward optimized photocatalytic hydrogen evolution

Adequate light absorption and high carrier separation/transfer efficiency are central to elevate the development of highly efficient photocatalytic hydrogen evolution. Herein, a unique nano heterostructure is constructed by translocating 0D CdSe@(Zn, Cd)Se@ZnS quantum dots (CSS QDs) into the 3D hollow spherical graphite carbon nitride (SCN). The ultrafast TA spectroscopy and electrochemical measurements were measured to reveal the enhanced surface dependent electron transfer efficiency. Besides, the density functional theory (DFT) calculations further explained the mechanism of electrons transfer between interfaces. As expected, benefiting from the structural advantages of SCN and the channel-driven effectiveness produced by a step-accelerated system which is composed of (Zn, Cd)Se and ZnS double-shell layers of CSS QDs, the optimal hydrogen evolution rate of the prepared material in the photocatalytic hydrogen evolution reaction reached 132.5 μmol h^?1, which was 7.6 times higher than that of the pure SCN under visible light irradiation. This work provides a novel avenue into the construction of nano heterostructure for solar hydrogen evolution.     

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.     

Novel CoOx/CdS/TiO2 with multi-shelled porous heterostructure for enhanced charge separation efficiency of photoelectrochemical water splitting

Hydrogen production by solar energy is an efficient and clean approach to fulfill the future energy demand. Herein, a novel multi-shelled porous heterostructure CoO_x/CdS/TiO₂ photoanode was fabricated by the hydrothermal and chemical method. There were more active sites, suitable surface defects and heterojunction structures in the homogeneous-porous-multi-shelled CoO_x/CdS/TiO₂ photoanode. It showed a photocurrent density of 2.89 mA/cm² at 1.23V vs. RHE, which is 2.22 fold of the original TiO₂ photoanode. The heterostructure fabrication of the CdS/TiO₂ could broaden the visible light absorption and enhance the charge separation efficiency. The multi-shelled homogeneous porous structure of the CoO_x/CdS/TiO₂ further enhanced the charge separation efficiency and accelerated the interfacial oxygen evolution kinetics. The mechanism for the enhanced photoelectrochemical water splitting of favorable CoO_x/CdS/TiO₂ photoanode is proposed.     

TiO2 hollow spheres with separated Au and RuO2 co-catalysts for efficient photocatalytic water splitting

Hollow mesoporous TiO₂ photocatalysts with dual co-catalysts, located at specific positions, were prepared using Polystyrene (PS) as sacrificial templates. Au nanoparticles (NPs) were in situ loaded on the surface of PS spheres and the resulting nanocomposites were coated with TiO₂ shell using sol-gel reaction. The outer surface of core-shell spheres was impregnated with Ru and the subsequent calcination produced hollow anatase spheres with Au and RuO₂ dual co-catalysts. The hollow mesoporous spheres of Au@TiO₂@RuO₂ were proved by various techniques such as TEM, EDX, and SEM images. Photocatalysts were applied for hydrogen generation from water splitting and that with dual co-catalysts showed efficient catalytic activity under simulated solar light. The catalytic activity of photocatalysts with both oxidation and reduction co-catalysts (Au@TiO₂@RuO₂) showed hydrogen evolution (3165 μmol g⁻¹) almost two times more than that Au@TiO₂ and TiO₂@RuO₂ with single co-catalysts. And the hydrogen evolved is more than three times as compared to TiO₂ (935 μmol g⁻¹) without any co-catalyst. Hollow mesoporous morphology with different co-catalysts on inner and outer surfaces is believed to enhance photocatalytic activity which is due to better separation of photo-generated charges.     

MoS2 supported on hydrogenated TiO2 heterostructure film as photocathode for photoelectrochemical hydrogen production

The substitution of noble metal platinum catalyst is one of the important research contents for sustainable development and is also the key to the practical application of photoelectrochemical (PEC) hydrogen production. In this work, we loaded the 1T-2H mixed phase MoS₂ on the hydrogenated anatase/rutile heterophase TiO₂ (A-H-RTNA) by hydrothermal method to prepare a new MoS₂/A-H-RTNA electrode material. The prepared material exhibited higher carrier density, lower PL intensity and higher conductivity than Pt/A-H-RTNA because 1T-MoS₂ has more active sites and lower charge transfer resistance than Pt. With the bias voltage of −0.4 V, the optimized 16MoS₂/A-H-RTNA as photocathode shows the largest PEC hydrogen production rate of 1840 mmol m⁻² h⁻¹, which is 2.9 and 2.2 times higher than those of A-H-RTNA (625 mmol m⁻² h⁻¹) and Pt/A-H-RTNA (848 mmol m⁻² h⁻¹), respectively. We innovatively used the prepared 16MoS₂/A-H-RTNA film as counter electrode instead of Pt electrode to construct a PEC system without any noble-metal. The result demonstrates that the noble-metal-free MoS₂ loaded on TiO₂ electrode as counter electrode has 75% PEC activity of noble metal Pt electrode. This study develops a PEC method for hydrogen evolution, which no longer depends on precious metal platinum as cathode.     

Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4

Efficient catalysts with high selectivity in products are highly desirable for photocatalytic CO₂ reduction. In this work, hydroxyapatite (HAP) decorated TiO₂ (HAP/TiO₂) were successfully fabricated via in-situ deposition of Ca(OH)₂ on rutile TiO₂ followed by a facile hydrothermal reaction. Comparing with TiO₂, HAP/TiO₂ exhibited significant enhancement (ca. 40 times) toward photocatalytic CO₂ reduction in the presence of H₂O with a >95% selectivity of CH₄. The characterizations revealed HAP possessed Lewis basic sites (O²⁻ in -PO₄^3- groups) and Lewis acidic sites (Ca²⁺ or OH⁻ vacancies), where Lewis basic sites could enhance the adsorption/activation of CO₂ and Lewis acidic sites facilitated the adsorption/dissociation of H₂O respectively, thus promoting the photocatalytic reduction and oxidation half-reactions of CO₂ and H₂O over Pt/TiO₂. The formation of much more stable intermediates over HAP/TiO₂ would be responsible for the high selectivity of CH₄. Moreover, photoelectrochemical and electrochemical characterizations revealed HAP could also promote the charge separation of TiO₂ and the charge transfer between TiO₂ and adsorbed species. The findings demonstrate HAP has a great potential as efficient assistant for photocatalytic CO₂ reduction with H₂O and will stimulate us to design novel semiconductor-based materials with tuned Lewis acidic and Lewis basic sites to achieve highly efficient photocatalysts.     

Application of SiO2 and TiO2 nano particles to enhance the efficiency of polymer-surfactant floods

Polymer-surfactant flooding is one of the most novel chemical enhanced oil recovery methods. Application of Nano particles in enhanced oil recovery has attracted much interest as well. This work concerns the application of Nano particles to increase the efficiency of polymer-surfactant flooding in heavy oil five-spot systems. In this investigation, micromodel setup was used to monitor the role of Nano particles on wettability alteration during polymer-surfactant floods. Two common Nano particles, SiO₂ and TiO₂ as well as HPAM and SDS as commercial chemicals in enhanced oil recovery, were used to create five solutions containing Nano particles at different levels of concentration. Then, contact angle tests and flooding tests were performed by taking microscopic/macroscopic pictures. According to the results, since SiO₂ Nano particle decrease the contact angle more severely, it results in a higher oil recovery. Although this decrease is more when SiO₂ is dispersed in water, due to its better absorption on a surface, the wettability alteration is more obvious during polymer-surfactant flooding because of the presence of a thinner oily film that intensifies mass transformation from fluid to the surface. In addition, an increase in the concentration of Nano particles leads to an increase in the efficiency of oil recovery and wettability alteration. Furthermore, according to the microscopic pictures, pulling and emulsification mechanisms are more effective than the wettability alteration mechanism.     

Preparation of TiO2/MoSe2 heterostructure composites by a solvothermal method and their photocatalytic hydrogen production performance

TiO₂/MoSe₂ composite photocatalysts were prepared by a solvothermal method using different MoSe₂ loadings on the surface of TiO₂ to improve the catalytic activity. Upon increasing the MoSe₂ loading, the fluorescence intensity of the catalyst gradually decreased, indicating that the MoSe₂ loading increased the utilization of photogenerated carriers in TiO₂ and improved the catalytic activity. The formic acid produced by the oxidation of lactic acid and methanol dissociated into oxygen-containing anions that were electrostatically adsorbed on the catalyst surfaces. During photocatalysis, the photogenerated electrons on TiO₂ were transferred to MoSe₂, which separated the photogenerated electrons and holes and increased the quantum efficiency. Therefore, the hydrogen production rate of the composite catalyst was higher than that of pure TiO₂, with a quantum efficiency of about 36.8%.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Well-defined FeP/CdS heterostructure construction with the assistance of amine for the efficient H2 evolution under visible light irradiation

The photocatalyst is a crucial factor in determining solar-to-H₂ efficiency for solar-driven water splitting. Here, the FeP/CdS well-defined heterostructure was elaborately designed and successfully constructed in-situ to achieve efficient water splitting by using a simple and green solvothermal approach. In the synthetic process, the ethylenediamine plays an important role in the construction of intimate contact interface between FeP and CdS. This good quality FeP/CdS heterostructure can efficiently promote charge separation and transportation, and therefore the charge recombination of CdS was significantly suppressed. As a result, the as-synthesized FeP/CdS heterostructure showed excellent photocatalytic performance under visible-light irradiation with an optimal hydrogen evolution rate of 37.92 mmol g⁻¹ h⁻¹ and an apparent quantum yield of 31.50% at 420 nm far exceeding that of pristine CdS by more than 122 folds. This rate, to the best of our knowledge, outperforms other similar catalytic systems.     

Facile construction of well-defined hierarchical NiFe2O4/NiFe layered double hydroxides with a built-in electric field for accelerating water splitting at the high current density

Interfacial charge redistribution induced by a strong built-in electric field can expertly optimize the adsorption energy of hydrogen and hydroxide for improving the catalytic activity. Herein, we develop a well-defined hierarchical NiFe₂O₄/NiFe layered double hydroxides (NFO/NiFe LDH) catalysts, exhibiting superior performance due to the strong interfacial electric field interaction between NiFe₂O₄ nanoparticle layers and NiFe LDH nanosheets. In 1 M KOH, NFO/NiFe LDH needs 251 mV and 130 to drive 50 and 10 mA cm^?2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Moreover, only 1.517 V cell voltage is needed to reach 10 mA cm^?2 towards overall water splitting. Notably, under simulated industrial electrolysis conditions, NFO/NiFe LDH only needs 289 mV to drive 1000 mA cm^?2. This work puts a deep insight into the role of the built-in electric field in transition metal-based catalysts for accelerating water splitting and scalable industrial electrolysis applications.     

Photo-chemical property evolution of superior thin g-C3N4 nanosheets with their crystallinity and Pt deposition

The crystallinity and composition of superior thin g-C₃N₄ nanosheets were adjusted at different temperature settings to study property evolution via a two-step thermal polymerization procedure. g-C₃N₄ sample prepared at 600 °C shows high N/C ratio and amorphous structure while a crystalline g-C₃N₄ sample with C/N ratio of nearly 0.75 was obtained at 750 °C. The band gap varies for each type of g-C₃N₄ sample and photodegradation kinetics were examined to be related to the crystallinity of the g-C₃N₄ sample. The RhB photocatalytic degradation plots for g-C₃N₄ nanosheets samples prepared at more than 600 °C are fitted using the pseudo-first-order model while the reaction for bulk g-C₃N₄ sample prepared at 550 °C follows zero-order kinetics. This phenomenon is ascribed to the varying surface states of the g-C₃N₄ samples. g-C₃N₄ nanosheets prepared at 700 °C showed the best photocatalytic performance, in which the sample with both amorphous and crystalline structural features is assumed to be amorphous/crystalline homojunctions. Moreover, Pt deposition confirms that g-C₃N₄ nanosheets prepared at 700 °C reveal the highest photocatalytic H₂ evolution rate of 4892 μmol/hg which is about 21 times high compared with amorphous g-C₃N₄ nanosheets prepared at 600 °C.     

Fabrication of one-dimensional MnxCd1-xS@D-MoSeyS2-y heterostructure with enhanced visible-light photocatalytic hydrogen evolution

Developing a visible light photocatalysts for hydrogen (H₂) production to replacing fossil fuels is a huge challenge. Herein, one-dimensional Mn_xCd_1-xS@D-MoSe_yS_2-y heterostructure was prepared for improving the hydrogen generation. The molar ratio of Mn to Cd in Mn_xCd_1-xS solid solution was adjusted, which effectively enhances the photocatalytic H₂ evolution efficiency. MoSe_yS_2-y with defects (D-MoSe_yS_2-y) was synthesized by solvothermal method during the photocatalytic process as cocatalyst, which wrapping on the Mn_xCd_1-xS solid solution. Mn_xCd_1-xS@D-MoSe_yS_2-y shows improved photocatalytic H₂ evolution activity, which was endowed by suitable energy band structure, exposure of active sites, high carrier concentration and fast electrons transfer. The highest photocatalytic H₂ evolution rate of sample among prepared Mn_xCd_1-xS@D-MoSe_yS_2-y is 12.46 mmol/g/h, which is equivalent to 37 times that of CdS. This present study offers a insight to the preparation of CdS-based photocatalysts.     

Tunable band gap of NV co-doped Ca:TiO2B (CaTi5O11) for visible-light photocatalysis

Visible-light photocatalysis in layered perovskite CaTi₅O₁₁ is measured by band-gap tuning with co-doped anionic nitrogen (N) and cationic vanadium (V). The screening of hybrid density functional (HSE06) calculation reveals that strong Coulomb interaction between the dopants and other atoms makes NV co-doping energetically favorable and effective for narrowing the band gap. More importantly, co-doping can eliminate the impurity states to reduce the electron-hole combination and improve the efficiencies of photocatalysis, since the mono-doped N or V ion produces impurity near the Fermi level. The impurity state captures the photoexcitation-generated carriers and accelerates the recombination process of the electron-hole pairs, thus suppressing their photocatalytic performance. The alignment of the band edge position with respect to the water oxidation/reduction potential indicates that NV co-doped CaTi₅O₁₁ can act as a potential photocatalytic water catalyst. Co-doping is expected to be an effective way of improving the visible-light photocatalytic activity in layered perovskite due to the elimination of recombining the electron-hole pairs.     

Highly efficient photocatalytic conversion of gas phase CO2 by TiO2 nanotube array sensitized with CdS/ZnS quantum dots under visible light

With the massive consumption of fossil fuels, energy crisis and effectively reducing CO₂ to curb global warming have become urgent and severe problems in the world. Photocatalytic conversion of CO₂ technology which can convert CO₂ into combustible compounds by using solar energy can solve both of the problems mentioned above. However, the photocatalytic conversion of CO₂ exhibits too low efficiency, especially under visible light. So, in order to improve the photocatalytic efficiency, the composite photocatalysts of TiO₂ nanotube array (TNTA) sensitized by CdS/ZnS quantum dots (QDs) were successfully prepared by anodization method and successive ionic layer adsorption and reaction (SILAR) method in this work. And the composite photocatalysts exhibited a high performance for photocatalytic conversion of gas-phase CO₂ to methanol under visible light. X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), and X-ray photoelectric spectroscopy (XPS) were employed to characterize the ingredients and morphologies of the synthesized photocatalysts. And, UV–vis diffuse reflectance spectra (UV–Vis DRS) revealed that CdS/ZnS QDs enhanced the photo-absorption of composite photocatalyst in the visible light region. The main product methanol yield of CdS/ZnS-TNTA under visible light was 2.73 times that of bare TNTA when TNTA was treated by 10 SILAR cycles. Meanwhile, the product yield first increased before decreasing with the increase of the CO₂ flow rate. And the greatest product yield reached up to 255.49 nmol/(cm²-cat·h) with the increase of light intensity. The reaction mechanism was discussed in this paper. This high performance for photocatalytic reduction of CO₂ was primarily attributed to the CdS/ZnS QDs sensitization, which widens the response wavelength range of the catalyst to include visible light and partly inhibits the recombination of electron-hole pairs.     

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.     

Decolouration of textile dyes in wastewaters by photocatalysis with TiO2

The photocatalytic removal of colour of a synthetic textile effluent, using TiO₂ suspensions under solar radiation, has been studied at pilot plant scale. A synthetic dye solution was prepared by a mix of six commercial textile dyes. A photochemical reactor of parallel CPC reflectors with UV-transparent tubular receivers was used. The study of photodegradation was carried out using the Taguchi’s parameter design method. Following this methodology, the reaction was conducted under different flow conditions, pH and H₂O₂ concentrations. The results show that all dyes used in the experiences can be degraded successfully by photo-oxidation. The process shows a significant enhancement when it is carried out at high flows, alkaline media and high H₂O₂ concentration. Colour removal from the effluent was reached at 55 min operating time.     

Highly dispersed metal nanoparticles on TiO2 acted as nano redox reactor and its synergistic catalysis on the hydrogen storage properties of magnesium hydride

In this paper, we have synthesized highly dispersed Co metal nanoparticles with the particle size about 5–10 nm on TiO₂ (25–50 nm) for the first time through an extremely facile solvothermal method. It is supposed that the synthesized Co/TiO₂ composite can combine the catalytic advantage of both Co and TiO₂, exhibiting the superior catalytic effect on the hydrogen de/absorption properties of MgH₂. The experimental data confirmed the above supposition and demonstrated that Co/TiO₂ additive highly enhances the hydrogen de/absorption kinetics of MgH₂ as compared to separate Co or TiO₂ additive. Specifically, the MgH₂Co/TiO₂ composite begins to desorb hydrogen at about 190 °C with a low apparent activation energy of 77 kJ/mol. Besides, the MgH₂Co/TiO₂ composite has a desorption peak temperature of 235.2 °C, which is 53.2, 94.2 and 132.2 °C lower than that of MgH₂TiO₂ (288.4 °C), MgH₂Co (329.4 °C) and ball-milled MgH₂ (367.4 °C). Moreover, MgH₂Co/TiO₂ composite also exhibits low temperature rehydrogenation properties, which can absorb 6.07, 5.56 and 4.24 wt% H₂ within 10 min at the temperature of 165, 130 and 100 °C, respectively. It is supposed that such excellent hydrogen desorption properties and low desorption energy barrier of MgH₂Co/TiO₂ composite are mainly ascribed to the novel synergistic catalytic effects of Co and TiO₂. Herein, we propose a novel catalytic mechanism and think that Co/TiO₂ acts as “nano redox reactor”, which can facilitate the dissociation and recombination process of hydrogen, thus reducing the reaction energy barrier and enhancing the de/rehydrogenation of MgH₂.     

Design and preparation of CdS/H-3D-TiO2/Pt-wire photocatalysis system with enhanced visible-light driven H2 evolution

In recent years, tremendous efforts have been devoted to develop new photocatalyst with wide spectrum response for H₂ generation from water or aqueous solution. In this work, CdS nanoparticles (NPs) have been immobilized on hydrogenated three-dimensional (3D) branched TiO₂ nanorod arrays, resulting in a highly efficient photocatalyst, i.e, CdS/H-3D-TiO₂. In addition, electrochemical reduction of H⁺ ion is identified as a limiting step in the photocatalytic generation of H₂ at this catalyst, while here a Pt wired photocatalysis system (CdS/H-3D-TiO₂/Pt-wire) is designed to overcome this barrier. Without the application of potential bias, visible light photocatalytic hydrogen production rate of CdS/H-3D-TiO₂/Pt-wire is 18.42 μmol cm^?2 h^?1, which is 11.2 times that of CdS/H-3D-TiO₂ without Pt (1.64 μmol cm^?2 h^?1). The Pt wire acts as an electron super highway between the FTO substrate and H⁺ ions to evacuate the generated electrons to H⁺ ions and catalyze the reduction reaction and consequently generate H₂ gas. This work successfully offers a novel direction for dramatic improvement in H₂ generation efficiency in photocatalysis field.     

Facile fabrication of mesoporous In2O3/LaNaTaO3 nanocomposites for photocatalytic H2 evolution

The incorporation of In₂O₃ nanoparticles on mesoporous La_0.02Na_0.98TaO₃ photocatalysts is very interesting for promoting the H₂ production under UV illumination in the presence of [10%] glycerol as a hole scavenger. It is demonstrated that an outstanding mesoporous In₂O₃/La_0.02Na_0.98TaO₃ photocatalyst can be constructed by incorporating In₂O₃ nanoparticles (0-2 wt%) and mesoporous La_0.02Na_0.98TaO₃ nanocomposites for highly promoting photocatalytic H₂ evolution. The maximum yield of H₂ ~ 2350 μmol g⁻¹ was obtained over mesoporous 1%In₂O₃/La_0.02Na_0.98TaO₃ nanocomposite. The mesoporous 1%In₂O₃/La_0.02Na_0.98TaO₃ nanocomposite exhibited further enhancement H₂ production, in which the rate of H₂ evolution can be as high as 235 μmol g⁻¹ h⁻¹, 435 times higher than those of mesoporous La_0.02Na_0.98TaO₃. The results showed that the 1%In₂O₃/La_0.02Na_0.98TaO₃ photocatalyst possesses high stability and durability for H₂ evolution by implying almost no photoactivity reduce after five cycles for 45 h continuous illumination. The measurement of photoluminescence spectroscopy, transient photocurrent spectra and UV- diffuse reflectance spectra for all synthesized samples exhibited that the promoted H₂ production is mainly explained by its effective electron-hole separation and broaden photoresponse region due to its compositions and structures of the obtained heterostructures.