One-pot synthesis of TiO2/Sb2S3/RGO complex multicomponent heterostructures for highly enhanced photoelectrochemical water splitting

The development of new sources of renewable energy fuels like hydrogen remains challenging considering the nowadays society energy needs on a day-to-day basis. In this context, hybrid nanostructures conformed by TiO₂ nanoparticles sensitized with bundles of rod-like Sb₂S₃ (stibnite) on the surface of reduced graphene oxide (TiO₂/Sb₂S₃/RGO), can pave the way in this direction, as offering heterostructures that can be employed as the active phase in photo-anodes for photoelectrochemical water oxidation. For that, these TiO₂/Sb₂S₃/RGO heterostructures are able to extend the light absorption to the visible range, enhance the charge separation and transportation, and improve the conductivity of the catalyst. Furthermore, the method of synthesis, though simple, implies a one-pot strategy by which the TiO₂ nanoparticles and the Sb₂S₃ rod-like particles are independently produced at the surface of RGO sheets, warranties the proper improved function of the hybrids and offers the engineering of future chalcogenide-based catalysts with promising water splitting photoelectrochemical properties.     

Facile synthesis of cobalt modified 2D titanium carbide with enhanced hydrogen evolution performance in alkaline media

2D transition metal carbides, nitrides and carbonitrides, namely the MXenes, attract more and more attentions due to their unique properties. Here, we report a simple one-step molten salt etching method to prepare Co modified MXene hybrid (Ti₃C₂T_x:Co) by the reaction of Ti₃AlC₂ with Lewis acid CoCl₂ at 750 °C. Most of Co atoms aggregates in the interlayered space of Ti₃C₂T_x. Benefitting from the improved electron charge transfer efficiency and increased active sites, the sulfuric acid treated Ti₃C₂T_x:Co-12h hybrid exhibits excellent electrocatalytical activity for hydrogen evolution reaction in alkaline media, delivering a current density of 10 mA cm⁻² at an overpotential of 103.6 mV, which is lower than most noble metal free MXene based electrocatalysts. The results illustrate that the proposed method is very facile and useful to incorporate mid-to-late transition metals into the MXene phase to prepare MXene based HER electrocatalysts.     

Boosting the visible-light photoelectrochemical performance of C3N4 by coupling with TiO2 and carbon nanotubes: An organic/inorganic hybrid photocatalyst nanocomposite for photoelectrochemical water spitting

Developing photoanode with proficient sunlight harvesting, stability as well as enhancing the electron injection across the interface remains a major challenge in the photoelectrochemical water splitting strategy to generate hydrogen. Herein, we design and fabricate an organic/inorganic TiO₂/C₃N₄/CNT photoanode by a hydrothermal technique which exhibits much enhanced photoelectrochemical properties. The TiO₂/C₃N₄/CNT photoanode exhibits a photocurrent density of 2.94 mA/cm², which is ~6.4 time higher than pristine graphitic carbon nitride (C₃N₄) at an applied bias potential of 0.6 V vs. Ag/AgCl. The excellent photoelectrochemical performance benefits from the impactful migration of photo-induced electrons at the TiO₂/C₃N₄ interface from C₃N₄ to TiO₂ and their intimate interface contact with CNT. Kelvin probe force microscopy result shows a smaller interface barrier height (~10 meV) between TiO₂ and C₃N₄, suggesting that electrons transport is favored through TiO₂/C₃N₄ interfaces in a ternary photoanode. The TiO₂/C₃N₄/CNT photoanode exhibited an onset potential of 0.25 V vs. Ag/AgCl which is much lower compared to pristine C₃N_4. The electrochemical impedance spectroscopy results also confirmed the enhanced electron injection across the interface in a ternary photoanode. These results demonstrate a promising approach to develop a highly proficient and visible light active photoanode with excellent stability for renewable energy applications.     

Enhanced hydrogen production over incorporated Cu and Ni into titania photocatalyst in glycerol-based photoelectrochemical cell: Effect of total metal loading and calcination temperature

The solar driven hydrogen production was successfully investigated in a glycerol-based photoelectrochemical cell (PEC) over nanostructured TiO₂ supported bimetallic Cu and Ni by adjusting total metal loading (5, 10, and 15 mol%) and calcination temperature (400, 450, 500, and 600 °C). The effects of the mentioned parameters on physicochemical and photoelectrochemical properties of prepared Cu–Ni/TiO₂ photoanodes were explored by using different characterization techniques. The hydrogen evolution was experimentally found to be affected total metal loading and calcination temperature. The calcined photocatalyst with the total metal loading of 5 mol% at 450 °C was identified as the most efficient photocatalyst by producing maximum accumulative hydrogen of 694.84 μmol. A high performance of this photocatalyst is mainly attributed to its proper particle size and great ratio of Ti³⁺:Ti⁴⁺ and Cu⁺:Cu²⁺ in TiO₂ matrix. These better physicochemical properties enhanced charge carrier separation, which retarded the charge recombination and enhanced the transportation of photo-induced electrons at the photoelectrode/electrolyte interface. The intermediates from photooxidation of glycerol were verified using high performance liquid chromatography, indicating a partial oxidation of glycerol with selective pathway in KOH (1 M) solution. This work demonstrates that optimization Cu–Ni/TiO₂ photoanode has the practical potential in PEC cell to generate hydrogen from solar and biomass energy.     

Construction of Co-decorated 3D nitrogen doped-carbon nanotube/Ti3C2Tx-MXene as efficient hydrogen evolution electrocatalyst

Rational design of transition metal catalysts with robust and durable electrocatalytic activity for hydrogen evolution reactions (HER) is extremely important for renewable energy conversion and storage, as well as water splitting. Heteroatom doping has emerged as a feasible strategy for enhancing electrocatalytic activity. Here, cobalt nanoparticles (Co-NPs) were coated with nitrogen-doped carbon nanotubes (NCNTs) prepared via an in situ growth on accordion-like Ti₃C₂T_x-MXene (Co-NCNT/Ti₃C₂T_x). Such an intriguing structure showed great features: abundant anchoring sites for NCNT in situ growth, intimate integration of Co-NPs and NCNTs, high-speed electron transfer between 1D NCNTs and 2D Ti₃C₂T_x-MXenes, and a large number of effective catalytic active sites. This Co-NCNT/Ti₃C₂T_x hybrid catalyst was demonstrated to possess excellent HER performance with low overpotential (η₁₀, 190 mV), small Tafel slope (78.4 mV dec⁻¹), large electrochemically active surface area, and good long-term stability, thus outperforming many reported electrocatalysts. The present strategy provided a facile route for the design of transition metal HER catalysts with NCNT and MXene.     

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.     

Improved n-Si/oxide junctions for environmentally safe solar energy conversion

The considerable interest in the practical use of solar energy has increased the importance of photovoltaic and photoelectrochemical systems. Metal oxide films like tin dioxide (SnO₂), titanium oxide (TiO₂) or indium tin oxide (ITO) are known to form stable photovoltaic junction with semiconductors of practical relevance like silicon (Si). Thin films of SnO₂ and TiO₂ were prepared easily and conveniently on the surface of silicon wafers by the spray pyrolysis technique. The prepared heterojunctions, i.e. the Si/oxide junction represent the main part of stable and efficient solar energy converter. In these systems, the solid/solid junction (n-Si/oxide) is separated from the site of the environmental interaction by the stable oxide film (SnO₂ or TiO₂) that protect the conventional semiconductor from photocorrosion.Besides its use in the fabrication of photovoltaic cells, the n-Si/oxide was used in the preparation of photoelectrochemical cells. The characteristics of the oxide film were subjected to a series of improvements either in the surface conductivity or the band gap energy, i.e. the position of the Fermi level of the semiconducting oxide by incorporating foreign atoms in the oxide film matrix during its preparation. The incorporation of Ru in the thin oxide film leads to the improvement of the solar conversion efficiency by improving the fill factor of the photovoltaic or photoelectrochemical cells. Such improvement enables the use of the prepared cells as clean energy converters. The fabricated solar cells have an average open-circuit potential of 0.44–0.62 V and a short circuit current of 28–30 mA/cm². A conversion efficiency up to 14% was achieved.     

MXene boosted metal-organic framework-derived Fe–N–C as an efficient electrocatalyst for oxygen reduction reactions

Iron-nitrogen-carbon (Fe–N–C) electrocatalysts offer great promise to replace their noble metal-based counterparts for oxygen reduction reactions (ORR). However, the practical applications of this type of catalyst are hindered by insufficient accessible active sies, low electrical conductivity, and poor durability. Here, we report a Ti₃C₂ MXene supported metal-organic framework (MOF)-derived Fe–N–C (Fe-N_x/N/Ti₃C₂) catalyst to simultaneously address the issues. Owing to the negatively charged characteristics, NH₂-MIL-53(Fe) is firmly anchored on Ti₃C₂ MXene, which not only serves as a conductive substrate to alleviate the collapse and agglomeration of MOFs during the pyrolysis, but also modulates the electronic properties of active FeN_x sites to improve the electrocatalytic activity and stability. As a result, the as-prepared Fe-N_x/N/Ti₃C₂ catalyst exhibits superb ORR activity and long-term stability in both alkaline and acidic electrolytes.     

Boosting the dehydrogenation efficiency of dodecahydro-N-ethylcarbazole by assembling Pt nanoparticles on the single-layer Ti3C2Tx MXene

As the candidates for large-scale hydrogen storage, liquid organic hydrogen carriers (LOHCs) exhibit evident advantages in hydrogen storage density and convenience of storage and transportation. Among them, NECZ (N-ethylcarbazole)/12H-NECZ (dodecahydro-N-ethylcarbazole) is considered as a typical system with the lower hydrogenation/dehydrogenation temperature. However, the low dehydrogenation efficiency restrict its commercial applications. In this work, the single-layer Ti₃C₂T_x MXene was employed as the support to load the Pt nanoparticles for the 12H-NECZ dehydrogenation reaction. The effect of transition metals, loading amounts and morphologies of catalysts were analyzed. It was found that the 3 wt% Pt/S–Ti₃C₂T_x catalyst exhibited the best catalytic performance with 100% conversion, 91.55% selectivity of NECZ and 5.62 wt% hydrogen release amount at 453 K, 101.325 kPa for 7 h. The product distributions and kinetics analysis suggested that the elementary reaction from 4H-NECZ to NECZ was the rate-limiting step. The selectivity of NECZ is sensitive to the dehydrogenation temperature. Combined with the XRD, SEM, HRTEM, XPS, BET and FT-IR results, it could be indicated that the special two-dimension structure of S–Ti₃C₂T_x and electronic effect between Pt and S–Ti₃C₂T_x enhanced the dehydrogenation efficiency of 12H-NECZ. The measurements of cyclic dehydrogenation indicated that the Pt/S–Ti₃C₂T_x catalyst exhibited good stability after 42 h. This work brought a new strategy for the design of efficient catalysts using two-dimensional materials in the applications of the liquid organic storage hydrogen technology.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Multifunctional Co3O4/Ti3C2Tx MXene nanocomposites for integrated all solid-state asymmetric supercapacitors and energy-saving electrochemical systems of H2 production by urea and alcohols electrolysis

Co₃O₄/Ti₃C₂T_x MXene nanocomposites have been fabricated by vacuum filtration and hydrothermal-annealing methods, and their electrochemical performance were investigated for energy storage and conversion, systematically. As electrode materials, Co₃O₄/Ti₃C₂T_x MXene nanocomposites in 6 M KOH solution demonstrated the specific capacitance of 240.1 F g^?1 at 0.1 A g^?1 and the long-term cycle stability. The solid-state asymmetric supercapacitors exhibited an operating potential window of 1.4 V, a specific capacitance of 97.9 F g^?1at 0.25 A g^?1, an energy density of 95.9 Wh kg^?1 at a power density of 630.4 W kg^?1, and excellent long-term durability. Furthermore, the connected solid-state asymmetric supercapacitors inseries and parallels presented the promising practical applications. Besides, Co₃O₄/Ti₃C₂T_x nanocomposites displayed outstanding catalytic behaviors for energy-saving H₂ generation by urea and alcohols electrolysis. The electrolyzer in KOH + CH₃CH₂OH electrolyte required only 1.33 V potential to deliver the current density of 0.5 A g^?1. Especially, the elctrochemical system of H₂ production by The electrolyzer and the powered solid-state asymmetric supercapacitors based on Co₃O₄/Ti₃C₂T_x nanocomposites was constructed, demonstrating outstanding properties of H₂ production. Therefore, this study not only shows enormous potential of Co₃O₄/Ti₃C₂T_x nanocomposites as a portable power supply but also indicates its great opportunities in energy-saving H₂ production in practical applications.     

Hierarchical MoS2 nanosheets integrated Ti3C2 MXenes for electrocatalytic hydrogen evolution

In this study, conductive Ti₃C₂ MXenes were used as a promoter to accelerate charger transfer of MoS₂, realizing highly efficient HER electrocatalysis. A facile hydrothermal strategy is demonstrated to be effective for in situ growth of MoS₂ nanosheets vertically standing on planar Ti₃C₂ nanosheets to form hierarchical heterostructures. Beneficial from the opened layer structures and strong interfacial coupling effect, the resulting MoS₂/Ti₃C₂ heterostructures achieve a giant enhancement in HER activity compared with pristine MoS₂ nanosheets. More specifically, the catalytic current density induced by MoS₂/Ti₃C₂ heterostructures at an overpotential of ∼400 mV is nearly 6.2 times as high as that of the pristine MoS₂ nanosheets. This work uncovers that the Ti₃C₂ nanosheets are ideal candidates for construction of highly active electrocatalysts for water splitting.     

In-situ growth of N–TiO2 on delaminated N–Ti3C2 with highly strengthened photocatalytic activity

Herein, a collection of N–TiO₂/delaminated N–Ti₃C₂ (NTTx) composites is designed and synthesized by one-step calcination of NH₄Cl–Ti₃C₂ precursors. The thermal decomposition of NH₄Cl not only serves as the gas template to make the delamination of Ti₃C₂, but also acts as N source to for doping. As expected, the N–TiO₂ nanoparticles uniformly anchor on the surface and interlayer of Ti₃C₂ nanosheets with intimate contact. Both the photocatalytic degradation rate of Rh–B and photocatalytic nitrogen fixation experiments of NTT2 composites show higher performance than that of pure P25 and NTT0 under visible light irradiation. The strengthened photocatalytic activity is due to the decrease of band gap by N doping in TiO₂ and excellent electrical conductivity of N–Ti₃C₂, which leads to enhanced light response and photogenerated electron-hole separation, respectively. This study develops a new strategy to design efficient photocatalysts for degradation of contaminants and fixation of nitrogen.     

High-efficiency photoelectrochemical water splitting with heterojunction photoanode of In2O3-x nanorods/black Ti–Si–O nanotubes

Constructing heterojunction was an efficient way to promote photoelectrochemical (PEC) water splitting performance of TiO₂-based nano-photoanode. In this work, we demonstrated the feasible preparation of oxygen vacancies-induced In₂O₃ (In₂O_3-x) nanorods/black Si-doped TiO₂ (Ti–Si–O) nanotubes heterojunction photoanode for enhanced PEC water splitting. Black Ti–Si–O nanotubes were fabricated through Zn reduction of the as-annealed Ti–Si–O nanotubes, followed by In₂O_3-x nanorods coupling by a facile electrodepositing and Ar heat treatment. Solar to hydrogen conversion efficiency of the heterojunction photoanode reached as high as 1.96%, which was almost 10 times that of undoped TiO₂. The improved PEC properties were mainly attributed to co-doping effects of Si and Ti³⁺/oxygen vacancy as well as In₂O_3-x decoration, which resulted in enhanced optical absorption and facilitated separation-transport process of photogenerated charge carriers. Charge transfer process in the composite system and hydrogen production mechanism were proposed. This work will facilitate designing TiO₂-based nano-photoanodes for promoting water splitting by integrating with elements doping, oxygen vacancies self-doping and semiconductors coupling.     

Application of PEO based gel network polymer electrolytes in dye-sensitized photoelectrochemical cells

Poly(ethyleneoxide) (PEO) based gel network polymer electrolytes prepared from crosslinking reaction were applied in fabricating quasi-solid-state dye-sensitized TiO₂ photoelectrochemical cells. Incident photon-to-current conversion efficiencies up to 48% and 40%, overall energy conversion efficiencies up to 3.6% and 2.9% have been achieved respectively for the resulting cells containing PEO₂₀₀₀ and PEO₁₅₀₀ segments.     

Monolayered Ti3C2O/(001) TiO2 photocatalyst with Schottky junction and active facets for hydrogen production

Monolayered Ti₃C₂O hybrid (001) TiO₂ (Ti₃C₂O/(001) TiO₂) photocatalyst was synthesized via a facile hydrothermal subsequent annealing method. The Schottky junction was constructed in situ by regulating surface functional groups, exposing highly active facets, and controlling structural dimension. Among the composite, monolayered Ti₃C₂O nanosheets acted as the electron reservoir to effectively separate the photogenerated electrons/holes. The exposure of the (001) facet largely improved the catalytic activity and surface energy of the TiO₂ material. Through the well-planned triple modifications, the highest activity of photocatalytic hydrogen evolution is 13.63 times than the contrast. In this work, excellent photocatalytic material with Schottky junction was prepared by structural modification and in-situ oxidation of Ti₃C₂T_x (Mxene), which revealed considerable reference values for expanding the application scope and modification methods of Mxene material.     

Influence of van der waals heterostructures of 2D materials on catalytic performance of ZnO and its applications in energy: A review

Recently, photocatalysis has received huge attention in order to overcome energy crisis worldwide. Many semiconductors, potential schemes and hierarchies have come to light during past few decades to fabricate efficient catalysts however, among all these methods heterostructures have taken the world by surprise. With the advancement in post-graphene 2D materials, van der Waals heterostructures have come to light exploring enhancement in photocatalysis. During a very short period a number of ZnO-based van der Waal heterostructures have taken the limelight in the field of photocatalysis. First principles calculations and DFT approach towards the heterostructures of GeC, GaN, WSe₂, WS₂ and other layered 2D materials unleased a series of properties and facts for the provision of enhanced catalysis. Reduction in bandgap of ZnO has also been observed which widens the pathways towards visible light irradiation. However, energy applications of zinc oxide are also fascinating feature as it can serve as a photoanode to replace TiO₂. Whereas the famous hydrogen production, batteries and solar cells have also been fabricated by the use of this semiconductor.     

N-doped and sulfur vacancy-rich TiO2@SnS2 nanoporous arrays for the plasmonic photocatalytic H2 evolution

An elegant templating method has been developed for the rational design and synthesis of hierarchical SnS₂ nanoclusters composed of ultrathin nanosheets and embedded inside TiO₂ nanoporous arrays. Herein, benefiting from their unique structural merits and metal-like plasmonic activity, the TiO₂@SnS₂ heterostructures exhibit enhanced photocatalytic H₂ evolution properties in terms of good cycling performance. S vacancies and N-doping are proved to be vitally important to the electronic structures and bandgap of SnS₂, thus influence the plasmonic property and separation of photo-carriers. The optimized TiO₂@6nmSnS₂/N nanoporous arrays give an ultra-high H₂ yield rate of 285 μmol h⁻¹cm⁻² under a low catalyst loading mass, that comparable to most noble metal catalysts. Remarkable cycling performance with a capability retention of 90% is achieved after 30 h under solar light illumination. As an innovative exploration, this study demonstrates that the photocatalytic activities of nonmetal, earth-abundant SnS₂ can be enhanced with plasmonic effect, which may serve as an excellent catalytic agent for solar energy conversion to chemical fuel.     

Titanium dioxide for solar-hydrogen I. Functional properties

The present work considers the concept of photoelectrochemical generation of hydrogen through water splitting using solar energy (solar-hydrogen). The focus is on functional material properties that are essential for the performance of photoelectrochemical cell for solar-hydrogen. The performance of the cell is discussed in terms of the energy conversion efficiency (ECE). It is argued that TiO₂ and TiO₂-based materials are the most promising candidates for photoelectrodes for solar-hydrogen. The modification of TiO₂ in order to achieve desired performance parameters is discussed in terms of the electronic structure, concentration of charge carriers and segregation-induced surface properties, which are critical to the ECE. Challenges to the development of a bi-photoelectrode cell, equipped with both n-type and p-type TiO₂, forming photoanode and photocathode, respectively, are discussed. The research strategies and pressing issues related to the optimization of key functional properties necessary for the commercialization of solar-hydrogen are outlined. It is shown that defect chemistry is the most appropriate framework for tailoring the functional properties of TiO₂-based oxide systems in order to obtain high-performance photoelectrodes. The present work provides an overview of the research progress on solar-hydrogen.     

Ti3C2Tx nanosheets with uniformly anchored Ru nanoparticles for efficient acidic and basic hydrogen evolution reaction

Developing an efficient and stable electrocatalyst for hydrogen evolution reaction (HER) remains critically signi?cance for renewable hydrogen production. Herein, a facile electrochemical reduction method was proposed to fabricate Ru nanoparticles (NPs) evenly anchored on Ti₃C₂T_x nanosheets (Ti₃C₂T_x-NS) electrocatalyst (Ru@Ti₃C₂T_x-NS). Interestingly, owing to the interaction between Ru NPs and Ti₃C₂T_x-NS, the resultant Ru@Ti₃C₂T_x-NS electrocatalyst performed a Pt-like electrocatalytic property for HER under the acidic solution with an ultra-low overpotential of 46.75 mV to reach ?10 mA/cm², a small Tafel slope of 30.6 mV/dec, and long-term stability. Simultaneously, the Ru@Ti₃C₂T_x-NS also displayed splendid HER electrocatalytic performance in the basic condition. Furthermore, Ru@Ti₃C₂T_x-NS showed a lower value of Gibbs free energy for HER (?0.21 eV) than either pure Ru or Ti₃C₂T_x-NS from the theoretical calculation results. It is expected that such a promising approach would be extended to design and fabricate other noble metal NPs anchored MXene nanosheets for HER application.