Photocatalytic Hydrogen Evolution via Water Splitting: A Short Review

Elucidating Structural Stability, Bandgap, and Photocatalytic Hydrogen Evolution of (H2O/DMF)@HKUST-1 Host-Guest Systems

The H2O@HKUST-1 and DMF@HKUST-1 systems were experimental and computationally assessed, employing XRD/TGA/FT-IR/DFT-calculations, evidencing that H2O or DMF coordinated to Cu, modulating HKUST-1 photocatalytic properties. DMF@HKUST-1 has narrower bandgap promoting higher-crystallinity and light-harvesting. H2O@HKUST-1 showed smaller particle sizing and sharp morphology. Theoretical models, (H2O)1@HKUST-1 and (DMF)1@HKUST-1, containing one coordinated molecule, elucidated bandgap modulation associated with infiltration. H2O@HKUST-1/DMF@HKUST-1 presented bandgaps [eV] of 3.6/3.4, by Tauc plots, and 3.55/3.26, by theoretical calculations, narrowing bandgap, compared with non-solvated HKUST-1(HKUST-1NS). Both composites raised the valence band (VB) and lowered the conduction band (CB), but DMF@HKUST-1 most raised VB. Topological analysis revealed that guests i) with higher electronic density, raised VB, and ii) induced π-backbonding, lowering CB. DMF@HKUST-1 presented a higher photocatalytic hydrogen evolution (μmol), 26.45, in the first 30 min of the reaction, nevertheless, H2O@HKUST-1 presented a competitive activity, of 17.32. In large periods, H2O@HKUST-1/DMF@HKUST-1 showed practically the same hydrogen evolution, 45.50/49.03.

Efficient photodegradation of polystyrene microplastics integrated with hydrogen evolution: Uncovering degradation pathways

Photocatalytic microplastics (MPs) conversion into valuable products is a promising approach to alleviate MPs pollution in aquatic environments. Herein, we developed an amorphous alloy/photocatalyst composite (FeB/TiO₂) that can successfully convert polystyrene (PS) MPs to clean H₂ fuel and valuable organic compounds (92.3% particle size reduction of PS-MPs and 103.5 μmol H₂ production in 12 h). FeB effectively enhanced the light-absorption and carrier separation of TiO₂, thereby promoting more reactive oxygen species generation (especially ‧OH) and combination of photoelectrons with protons. The main products (e.g., benzaldehyde, benzoic acid, etc.) were identified. Additionally, the dominant PS-MPs photoconversion pathway was elucidated based on density functional theory calculations, by which the significant role of ‧OH was demonstrated in combination with radical quenching data. This study provides a prospective approach to mitigate MPs pollution in aquatic environments and reveals the synergistic mechanism governing the photocatalytic conversion of MPs and generation of H₂ fuel.

Photocatalytic H2 Production by Visible Light on Cd0.5Zn0.5S Photocatalysts Modified with Ni(OH)2 by Impregnation Method

Nowadays, the study of environmentally friendly ways of producing hydrogen as a green energy source is an increasingly important challenge. One of these potential processes is the heterogeneous photocatalytic splitting of water or other hydrogen sources such as H₂S or its alkaline solution. The most common catalysts used for H₂ production from Na₂S solution are the CdS-ZnS type catalysts, whose efficiency can be further enhanced by Ni-modification. In this work, the surface of Cd_0.5Zn_0.5S composite was modified with Ni(II) compound for photocatalytic H₂ generation. Besides two conventional methods, impregnation was also applied, which is a simple but unconventional modification technique for the CdS-type catalysts. Among the catalysts modified with 1% Ni(II), the impregnation method resulted in the highest activity, for which a quantum efficiency of 15.8% was achieved by using a 415 nm LED and Na₂S-Na₂SO₃ sacrificial solution. This corresponded to an outstanding rate of 170 mmol H₂/h/g under the given experimental conditions. The catalysts were characterized by DRS, XRD, TEM, STEM-EDS, and XPS analyses, which confirmed that Ni(II) is mainly present as Ni(OH)₂ on the surface of the CdS-ZnS composite. The observations from the illumination experiments indicated that Ni(OH)₂ was oxidized during the reaction, and that it therefore played a hole-trapping role.

Simultaneous Photocatalytic Sugar Conversion and Hydrogen Production Using Pd Nanoparticles Decorated on Iron-Doped Hydroxyapatite

Pd nanoparticles (PdNPs) were successfully deposited on the surface of Fe(III)-modified hydroxyapatite (HAp), which was subsequently used as a photocatalyst for simultaneous photocatalytic H2 evolution and xylose conversion. The structural phase and morphology of the pristine HAp, FeHAp, and Pd@FeHAp were examined using XRD, SEM, and TEM instruments. At 20 °C, Pd@FeHAp provided a greater xylose conversion than pristine HAp and FeHAp, about 2.15 times and 1.41 times, respectively. In addition, lactic acid and formic acid production was increased by using Pd@FeHAp. The optimal condition was further investigated using Pd@FeHAp, which demonstrated around 70% xylose conversion within 60 min at 30 °C. Moreover, only Pd@FeHAp produced H2 under light irradiation. To clarify the impact of Fe(III) doping in FeHAp and heterojunction between PdNPs and FeHAp in the composite relative to pure Hap, the optical and physicochemical properties of Pd@FeHAp samples were analyzed, which revealed the extraordinary ability of the material to separate and transport photogenerated electron-hole pairs, as demonstrated by a substantial reduction in photoluminescence intensity when compared to Hp and FeHAp. In addition, a decrease in electron trap density in the Pd@FeHAp composite using reversed double-beam photoacoustic spectroscopy was attributed to the higher photocatalytic activity rate. Furthermore, the development of new electronic levels by the addition of Fe(III) to the structure of HAp in FeHAp may improve the ability to absorb light by lessening the energy band gap. The photocatalytic performance of the Pd@FeHAp composite was improved by lowering charge recombination and narrowing the energy band gap. As a result, a newly developed Pd@FeHAp composite might be employed as a photocatalyst to generate both alternative H2 energy and high-value chemicals.

Supported nanostructured photocatalysts: the role of support-photocatalyst interactions

Heterogeneous photocatalysis employing semiconductor oxide photocatalysts is a sustainable and promising method for environmental remediation and clean energy generation. In this context, nanostructured photocatalysts, with at least one dimension in the 1‒100 nm size regime, have attracted ever-growing attention due to their unique and often enhanced size-dependent physicochemical properties. While their reduced size ensures enhanced photocatalytic performance, the same makes it difficult and time/energy-demanding to remove/recover such nanostructured photocatalysts from aqueous media. This fundamental limitation has paved the way towards developing supported nanophotocatalysts where the active photocatalytic nanostructures are coated on the surface of polymeric or inorganic support materials, often in a core@shell conformation. This arrangement solves the problem of photocatalysts' recovery for effective reuse or recycling and leads to improved and desired target properties due to specific photocatalyst-support interactions. While the enhanced physicochemical properties of supported photocatalysts have been widely studied in many target applications, the role of support-photocatalysts interactions in improving these properties remains unexplored. This review article provides an updated viewpoint on the photocatalyst-support interactions and the resulting unique physiochemical properties important for diverse photochemical applications and the design of practical devices. While exploring the properties of supported nanostructured metal oxide/sulfides photocatalysts such as TiO₂ and MoS₂, we also briefly discuss the common strategies employed to coat the active nanomaterials on the surface of different supports (organic/polymeric, inorganic, active, inert, and magnetic).

Enhancing the activity and stability of Cu2O nanorods via coupling with a NaNbO3/SnS2 heterostructure for photoelectrochemical water-splitting

We synthesized a stable copper-based heterostructure catalyst, NaNbO3/SnS2/Cu2O for photoelectrochemical water-splitting applications with improved activity, stability, and inhibited photocorrosion in Cu2O.

Cobalt Incorporated Graphitic Carbon Nitride as a Bifunctional Catalyst for Electrochemical Water-Splitting Reactions in Acidic Media

Non-noble metal-based bifunctional electrocatalysts may be a promising new resource for electrocatalytic water-splitting devices. In this work, transition metal (cobalt)-incorporated graphitic carbon nitride was synthesized and fabricated in electrodes for use as bifunctional catalysts. The optimum catalytic activity of this bifunctional material for the hydrogen evolution reaction (HER), which benefitted at a cobalt content of 10.6 wt%, was promoted by the highest surface area and conductivity. The activity achieved a minimum overpotential of ~85 mV at 10 mA/cm² and a Tafel slope of 44.2 mV/dec in an acidic electrolyte. These values of the HER were close to those of a benchmark catalyst (platinum on carbon paper electrode). Moreover, the kinetics evaluation at the optimum catalyst ensured the catalyst flows (Volmer-Heyrovsky mechanism), indicating that the adsorption step is rate-determining for the HER. The activity for the oxygen evolution reaction (OER) indicated an overpotential of ~530 mV at 10 mAcm^-2 and a Tafel slope of 193.3 mV/dec, which were slightly less or nearly the same as those of the benchmark catalyst. Stability tests using long-term potential cycles confirmed the high durability of the catalyst for both HER and OER. Moreover, the optimal bifunctional catalyst achieved a current density of 10 mAcm^-2 at a cell voltage of 1.84 V, which was slightly less than that of the benchmark catalyst (1.98 V). Thus, this research reveals that the present bifunctional, non-noble metallic electrocatalyst is adequate for use as a water-splitting technology in acidic media.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications

Photocatalytic water splitting, CO₂ reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H₂ generation potential, and an excellent ability to capture and activate CO₂ molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H₂ evolution, CO₂ reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

Effects of Preparation Conditions on the Efficiency of Visible-Light-Driven Hydrogen Generation Based on Ni(II)-Modified Cd0.25Zn0.75S Photocatalysts

Hydrogen as an environmentally friendly fuel can be produced by photocatalytic procedures from aqueous systems, utilizing H₂S, an industrial side-product, by conversion and storage of renewable solar energy. Although composites of CdS and ZnS prepared by co-precipitation are very efficient in heterogeneous photocatalytic H₂ generation, the optimal conditions for their synthesis and the effects of the various influencing factors are still not fully clarified. In this work, we investigated how the efficiency of Cd₀.₂₅Zn₀.₇₅S composites modified with Ni(II) was affected by the doping method, Ni-content, hydrothermal treatment, and presence of a complexing agent (ammonia) used in the preparation. The composition, optical, and structural properties of the photocatalysts prepared were determined by ICP, DRS, XRD, TEM, and STEM-EDS. Although hydrothermal treatment proved preferable for Ni-free composites, Ni-modification was more efficient for untreated composites precipitated from ammonia-containing media. The best efficiency (14.9% quantum yield at 380 nm irradiation, 109.8 mmol/g/h hydrogen evolution rate) achieved by surface modification with 0.1–0.3% Ni(II) was 15% and 20% better than those for hydrothermally treated catalyst and similarly prepared Pt-modified one, respectively. Structural characterization of the composites clearly confirmed that the Ni²⁺ ions were not embedded into the CdS-ZnS crystal lattice but were enriched on the surface of particles of the original catalyst in the form of NiO or Ni(OH)₂. This co-catalyst increased the efficiency by electron-trapping, but its too high amount caused an opposite effect by diminishing the excitable surface of the CdS-ZnS particles.

Effects of Preparation Conditions on the Efficiency of Visible-Light-Driven Hydrogen Generation Based on Cd0.25Zn0.75S Photocatalysts

Photocatalytic H2 production utilizing H2S, an industrial side-product, is regarded as an environmentally friendly process to produce clean energy through direct solar energy conversion. For this purpose, sulfide-based materials, such as photocatalysts, have been widely used due to their good solar response and high photocatalytic activity. In this work, a ZnS–CdS composite was studied, and special attention was dedicated to the influence of the preparation parameters on its H2 production activity. The ZnS–CdS composite, with an enhanced photoactivity for H2 production, was synthesized both from ammine complexes and, in a conventional way, directly from acetates at various pH values. Deviating from the traditional method, the photoactivity of ZnS–CdS prepared from ammine complexes was not affected by the pH. Besides, the hydrothermal treatment and the ammonia content strongly influenced the rate of H2 production in this system. DRS, TEM, SEM, XRD, and quantum yield measurements prove the dependence of the photoactivity of these catalysts on the structural and morphological properties determined by the preparation conditions. The promising photocatalytic efficiency achieved with the application of these ZnS–CdS catalysts, prepared without any metal deposition, encourages further investigations to enhance the rate of hydrogen generation by optimization of the reaction conditions for practical utilization.

Hydrogen production via water splitting over graphitic carbon nitride (g-C3N4 )-based photocatalysis

Photocatalytic splitting of water into hydrogen and oxygen using semiconductor photocatalysts and light irradiation has been attracted much attention and considered to be an alternative for nonrenewable fossil fuel to solve environmental problems and energy crisis and also an as promising approach to produce clean, renewable hydrogen fuel. Owing to their various advantages such as low cost and environmental friendly, chemical, and thermal stability, appropriate band structure, graphitic carbon nitride (g-C3N4 ) photocatalysts have gained multitudinous attention because of their great potential in solar fuels production and environmental remediation. However, due to its fast charge carrier’s recombination, low surface, and limited absorption of the visible light restrict their activity toward hydrogen evolution and numerous modification techniques were applied to solve these problems such as structural modification, metal/nonmetal doping, and noble metal loading, and coupling semiconductors. In this chapter, we summarize recent progress in the synthesis and characterization of the g-C3N4-based photocatalyst. Several modification methods used to enhance the photocatalytic hydrogen production of g-C3N4-based photocatalyst were also highlighted. This chapter ends with the future research and challenges of hydrogen production over g-C3N4-based photocatalyst.

Plasmon-enhanced hydrogen evolution reaction on a Ag-branched-nanowire/Pt nanoparticle/AgCl nanocomposite

A plasmon-enhanced photocatalytic system was designed with Ag-Pt-AgCl nanocomposites. Branched nanowires of Ag (AgBNWs) were first synthesized on indium-doped tin oxide-coated glass by electrodeposition. Then, the AgBNWs were dipped into an aqueous solution of Na2[PtCl6] at different concentrations from 1 to 5 mM to deposit Pt nanoparticles (PtNPs) on the AgBNWs via galvanic displacement. During the PtNP deposition, eluted Ag+ ions reacted with Cl- ions to precipitate AgCl on the AgBNWs. The obtained AgBNW/PtNP/AgCl nanocomposites exhibited plasmonic absorption at approximately 465 nm. The nanocomposites were then examined as photoelectrodes for hydrogen evolution. The hybridization of the PtNPs on the AgBNWs significantly decreased the overpotential for water splitting in the dark, and the large number of PtNPs resulted in a higher efficiency compared to a conventional catalyst. Under blue-light irradiation (479 nm, 100 mW cm-2), the overpotential decreased by -110 mV, and the current density increased by 27.8 mA cm-2. Under red-light irradiation (631 nm, 100 mW cm-2), the shift in onset potential was small, which could be attributed to the mismatching of the plasmonic absorption band with the excitation wavelength. The nanocomposite without AgCl (AgBNW/PtNP) was less effective at lowering the overpotential but more effective at improving the onset potential than AgBNW/PtNP/AgCl. These electrochemical behaviors were explained by the synergistic effect of the plasmon-induced photocurrent and charge transfer between Ag, Pt, and AgCl. The nanocomposite retained its photocatalytic activity after 400 cycles; therefore, the AgBNW/PtNP/AgCl nanocomposite could be useful for hydrogen evolution devices.

Biopolymer-derived (nano)catalysts for hydrogen evolution via hydrolysis of hydrides and electrochemical and photocatalytic techniques: A review

Over the course of a few decades, the concern of environmental damages of fossil fuels, an increase in CO₂ emission and a decrease of hydrogen have been growing more and more. Accordingly, hydrogen production is a crucial issue nowadays. Different polymers are applied to attain the purpose. Among all polymers, biodegradables polymers are the best choices to develop the main aim. Polysaccharides and proteins are biodegradable polymers with unique places and advantages with regards to their ecofriendly properties. There are different techniques to apply and achieve the foremost purpose. It is worthwhile to mention that green and facile methods are always attracting attention in different aspects and fields. The three non-polluting and economical techniques, that is, electrochemical hydrogen evolution reaction (HER), photocatalytic technique, and hydrolysis of hydrides, are reviewed in this paper. This review helps researchers, who are environment supporters, to evaluate and choose the most ecological biopolymers and processes in their work.

Polyampholytic Graft Copolymers as Matrix for TiO2 /Eosin Y/[Mo3 S13 ]2- Hybrid Materials and Light-Driven Catalysis

An effective strategy to enhance the performance of inorganic semiconductors is moving towards organic‐inorganic hybrid materials. Here, we report the design of core–shell hybrid materials based on a TiO₂ core functionalized with a polyampholytic (poly(dehydroalanine)‐graft‐(n‐propyl phosphonic acid acrylamide) shell (PDha‐g‐PAA@TiO₂). The PDha‐g‐PAA shell facilitates the efficient immobilization of the photosensitizer Eosin Y (EY) and enables electronic interactions between EY and the TiO₂ core. This resulted in high visible‐light‐driven H₂ generation. The enhanced light‐driven catalytic activity is attributed to the unique core–shell design with the graft copolymer acting as bridge and facilitating electron and proton transfer, thereby also preventing the degradation of EY. Further catalytic enhancement of PDha‐g‐PAA@TiO₂ was possible by introducing [Mo₃S₁₃]²⁻ cluster anions as hydrogen‐evolution cocatalyst. This novel design approach is an example for a multi‐component system in which reactivity can in future be independently tuned by selection of the desired molecular or polymeric species.

Photocatalytic Hydrogen Evolution

Solar energy conversion is one of the sustainable technologies that tackles the global warming and energy crisis [...]

Inartificial Two-Dimensional Ge4Se9 Janus Structures with Appropriate Direct Band Gaps and Intrinsic Polarization Boosted Charge Separation for Photocatalytic Water Splitting

Two-dimensional (2D) Janus structures, which are totally different from prevailing 2D structures, are more interesting for photocatalytic water splitting. Here we proposed some inartificial 2D Ge₄Se₉ Janus structures. Excellent photocatalytic properties are revealed: (a) Ge₄Se₉ structures exhibit layer-independent direct gap character with appropriate band gaps of 2.53, 2.22, 2.11, and 2.03 eV for monolayered, bilayered, triple-layered, and four-layered structures, respectively. (b) Band edge positions of these 2D structures are suitable for the driving of the evolution reaction of water splitting. (c) More importantly, owning to intrinsic electric polarization, the charge densities of the valence band maximum (VBM) and the conduction band minimum (CBM) of triple-layered and four-layered Ge₄Se₉ structures can be notably separated. (d) In addition, we also observed that these 2D structures can possess rather pronounced optical absorption in the visible light region. This work discloses some inartificial 2D Janus structures whose fascinating properties render them as promising photocatalysts for water splitting.

Graphitic Carbon Nitride Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review

The generation of photocatalytic hydrogen via water splitting under light irradiation is attracting much attention as an alternative to solve such problems as global warming and to increase interest in clean energy. However, due to the low efficiency and selectivity of photocatalytic hydrogen production under solar energy, a major challenge persists to improve the performance of photocatalytic hydrogen production through water splitting. In recent years, graphitic carbon nitride (g-C3N4), a non-metal photocatalyst, has emerged as an attractive material for photocatalytic hydrogen production. However, the fast recombination of photoexcited electron–hole pairs limits the rate of hydrogen evolution and various methods such as modification, heterojunctions with semiconductors, and metal and non-metal doping have been applied to solve this problem. In this review, we cover the rational design of g-C3N4-based photocatalysts achieved using methods such as modification, metal and non-metal doping, and heterojunctions, and we summarize recent achievements in their application as hydrogen production photocatalysts. In addition, future research and prospects of hydrogen-producing photocatalysts are also reviewed.

Photoelectrochemical Water Splitting using Adapted Silicon Based Multi-Junction Solar Cell Structures: Development of Solar Cells and Catalysts, Upscaling of Combined Photovoltaic-Electrochemical Devices and Performance Stability

Thin film silicon based multi-junction solar cells were developed for application in combined photovoltaic electrochemical systems for hydrogen production from water splitting. Going from single, tandem, triple up to quadruple junctions, we cover a range of open circuit voltages from 0.5 V to 2.8 V at photovoltaic cell (PV) efficiencies above 13%. The solar cells were combined with electrochemical (EC) cells in integrated devices from 0.5 cm2 to 64 cm2. Various combinations of catalyst pairs for the oxygen and hydrogen evolution reaction side (OER and HER) were investigated with respect to electrochemical activity, stability, cost and – important for the integrated device – optical quality of the metal catalyst on the HER side as back reflector of the attached solar cell. The combined PV-EC systems were further investigated under varied operation temperatures and illumination conditions for estimation of outdoor performance and annual fuel production yield. For 0.5 cm2 size combined systems a maximum solar-to-hydrogen efficiency ηSTH = 9.5% was achieved under standard test conditions. For device upscaling to 64 cm2 various concepts of contact interconnects for reduced current and fill factor loss when using large size solar cells were investigated. To replace high performance noble metal based catalyst pairs (Pt/RuO2 or Pt/IrOx), more abundant and cheaper NiMo (HER) and NiFeOx (OER) compounds were prepared via electrodeposition. With the NiMo/NiFeOx catalyst pair we obtained ηSTH = 5.1% for a 64 cm2 size solar cell which was even better than the performance of the Pt/IrO2 system (ηSTH = 4.8%). In simulated day-night cycle operation the NiMo/NiFeOx catalyst pair showed excellent stability over several days. The experimental studies were successfully accompanied by simulation of the entire PV-EC device using a series connection model which allowed studies and pre-estimations of device performance by varying individual components such as catalysts, electrolytes, or solar cells. Based on these results we discuss the prospects and challenges of integrated PV-EC devices on large area for hydrogen and solar fuel production in general.

Modified Nimo Nanoparticles for Efficient Catalytic Hydrogen Generation from Hydrous Hydrazine

Precious metal-free NiMoM (M = Pr2O3, Cu2O) catalysts have been synthesized through a simple coreduction method, without any surfactant or support material, and characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The resultant Pr2O3- or Cu2O-modified NiMo catalysts exhibit different structures, which is due to a difference in the synergistic effects of NiMo and the modifying elements. NiMoPr2O3 has an amorphous structure, with low crystallinity and uniform particle dispersion, while NiMo@Cu2O adopts the core–shell structure, where the core and shell are synergistic with each other to promote electron transfer efficiency. The support material-free nanocatalysts Ni9Mo1(Pr2O3)0.375 and Ni4Mo@Cu2O are both highly efficient compared with bimetallic NiMo catalysts, in terms of hydrogen generation from hydrous hydrazine (N2H4·H2O) at 343 K, with total turnover frequencies (TOFs) of 62 h−1 and 71.4 h−1, respectively. Their corresponding activation energies (Ea) were determined to be 43.24 kJ mol−1 and 46.47 kJ mol−1, respectively. This is the first report on the use of Pr-modified NiMo and core–shell NiMo@Cu2O catalysts, and these results may be used to promote the effective application of noble metal-free nanocatalysts for hydrogen production from hydrous hydrazine.