On the use of DFT+U to describe the electronic structure of TiO2 nanoparticles: (TiO2)35 as a case study

One of the main drawbacks in the density functional theory (DFT) formalism is the underestimation of the energy gaps in semiconducting materials. The combination of DFT with an explicit treatment of the electronic correlation with a Hubbard-like model, known as the DFT+U method, has been extensively applied to open up the energy gap in materials. Here, we introduce a systematic study where the selection of the U parameter is analyzed considering two different basis sets: plane-waves and numerical atomic orbitals (NAOs), together with different implementations for including U, to investigate the structural and electronic properties of a well-defined bipyramidal (TiO₂)₃₅ nanoparticle. This study reveals, as expected, that a certain U value can reproduce the experimental value for the energy gap. However, there is a high dependence on the choice of basis set and on the U parameter employed. The present study shows that the linear combination of the NAO basis functions, as implemented in Fritz Haber Institute ab initio molecular simulation (FHI-aims), requires, requires a lower U value than the simplified rotationally invariant approach, as implemented in the Vienna ab initio simulation package (VASP). Therefore, the transfer of U values between codes is unfeasible and not recommended, demanding initial benchmark studies for the property of interest as a reference to determine the appropriate value of U.

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon-neutral solar fuels in a low-cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co-catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe₂ S₂ (CO)₆ ] or [Fe₃ Te₂ (CO)₉ ], using an interface-directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 μmol of H₂ per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time-resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

The development of new functional materials builds on an understanding of the intricate relationship between material structure and properties, and structural characterization is a crucial part of materials chemistry. However, elucidating the atomic structure of nanomaterials remains a challenge using conventional diffraction techniques due to the lack of long-range atomic order. Over the past decade, Pair Distribution Function (PDF) analysis of X-ray or neutron total scattering data has become a mature and well-established method capable of giving insight into the atomic structure in nanomaterials. Here, we review the use of PDF analysis and modelling in characterization of a range of different nanomaterials that exhibit unique atomic structure compared to the corresponding bulk materials. A brief introduction to PDF analysis and modelling is given, followed by examples of how essential structural information can be extracted from PDFs using both model-free and advanced modelling methods. We put an emphasis on how the intuitive nature of the PDF can be used for understanding important structural motifs, and on the diversity of applications of PDF analysis to nanostructure problems.

Porous Hybrid Materials Based on Mesotetrakis(Hydroxyphenyl) Porphyrins and TiO2 for Efficient Visible-Light-Driven Hydrogen Production

A series of highly robust nano-micro hybrid materials based on meso-tetra(4-hydroxyphenyl) porphyrins (M = H, Pd, Zn) and titanium dioxide (denoted as THPP-TiO2, THPP-Pd-TiO2, and THPP-Zn-TiO2) have been prepared by a facile sol-gel method for the first time. When Pt nanoparticles are incorporated in these hybrids, Pt/THPP-Pd-TiO2 achieves good H2 production activity (2025.4 μmol g−1 h−1 and 12.03 μmol m−2 h−1), higher than that of Pt/THPP-Zn-TiO2 (1239.8 μmol g−1 h−1 and 7.46 μmol m−2 h−1) and Pt/THPP-TiO2 (576.8 μmol g−1 h−1 and 4.02 μmol m−2 h−1), owing to the different central metal ions in porphyrins. The best activity of Pt/THPP-Pd-TiO2 would be attributed to the two-center catalysis from coordination Pd metal ions and Pt nanoparticles, while the higher activity of Pt/THPP-Zn-TiO2 than Pt/THPP-TiO2 could be ascribed to the more effective light harvesting and electron transfer between THPP-Zn and TiO2. In addition, the hybridized Pt/THPP-Pd-TiO2 catalyst exhibits unattenuated hydrogen production stability even after recycling the experiment 10 times (cumulative turnover number of 5111 after 50 h), far superior to that of the surface-sensitized Pt/THPP-Pd/TiO2 catalyst with analogous components, due to the more stable Ti-O bonds between four phenols in porphyrins and TiO2 for the hybrid system. The present study provides a promising approach for constructing stable organic–inorganic hybrid systems with unique hierarchical structures for efficient light absorption and electron transfer.

Functional carbon nitride materials for water oxidation: from heteroatom doping to interface engineering

Polymeric carbon nitrides (PCNs) are promising photocatalysts and electrocatalysts for water oxidation, as they are environmentally benign materials with an adjustable structure and facilely synthesized from inexpensive and abundant starting materials. In this minireview, we examine the state-of-the-art strategies for tailoring PCNs for efficient photocatalytic, electrocatalytic, and photoelectrochemical water oxidation, including heteroatom doping and interface engineering from band structure alignment (e.g., by coupling inorganic or organic semiconductors) to hybridization with nanoscale cocatalysts (e.g., nanosheets, nanoarrays, nanoparticles, and quantum dots) and sub-nanoscale cocatalysts (e.g., metallic molecular clusters and single-atom catalysts). Through establishing the structure-activity correlations, we aim to present a clear roadmap for providing insights into the design strategies, structure modification, and the improved catalytic performances of PCN-based materials in different catalytic water oxidation processes. For future guidance, we also propose some outlooks on the perspective and challenges of PCNs towards a better application in catalytic water oxidation.

Recent Progress and Development in Inorganic Halide Perovskite Quantum Dots for Photoelectrochemical Applications

Inorganic halide perovskite quantum dots (IHPQDs) have recently emerged as a new class of optoelectronic nanomaterials that can outperform the existing hybrid organometallic halide perovskite (OHP), II-VI and III-V groups semiconductor nanocrystals, mainly due to their relatively high stability, excellent photophysical properties, and promising applications in wide-ranging and diverse fields. In particular, IHPQDs have attracted much recent attention in the field of photoelectrochemistry, with the potential to harness their superb optical and charge transport properties as well as spectacular characteristics of quantum confinement effect for opening up new opportunities in next-generation photoelectrochemical (PEC) systems. Over the past few years, numerous efforts have been made to design and prepare IHPQD-based materials for a wide range of applications in photoelectrochemistry, ranging from photocatalytic degradation, photocatalytic CO₂ reduction and PEC sensing, to photovoltaic devices. In this review, the recent advances in the development of IHPQD-based materials are summarized from the standpoint of photoelectrochemistry. The prospects and further developments of IHPQDs in this exciting field are also discussed.

Photocatalysts for H2 Generation from Starburst Triphenylamine/Carbazole Donor-Based Metal-Free Dyes and Porous Anatase TiO2 Cube

A series of novel triphenylamine/carbazole-based D-D-π-π-A dyes DH1-4 and a mesoporous anatase cubic "microcage" TiO₂ material (denoted as MC-TiO₂ ) were synthesized and combined to obtain dye-sensitized photocatalysts (denoted as DHn/Pt/MC-TiO₂ , n=1-4). These catalysts showed better performances in visible-light-driven H₂ evolution from water than DHn/Pt/P25-TiO₂ catalysts based on commercial P25-TiO₂ bulk semiconductor under similar conditions. Compared with P25-TiO₂ particles, the porous MC-TiO₂ had a large Brunauer-Emmett-Teller surface area, porosity, and exposed {0 0 1} crystal plane, which greatly contributed to the photocatalytic activity. The optimized DH2/Pt/MC-TiO₂ photocatalyst exhibited an attractive H₂ production rate (16.28 mmol g^-1  h^-1 based on catalyst mass), and the optimized DH4/Pt/MC-TiO₂ photocatalyst showed good stability [turnover number (TON) of 16 699 in 105 h based on dye number], which represents one of the best performances among all reported visible-light-driven heterogeneous catalytic systems. Compared with the other dyes in this series, the high H₂ production rate of DH2 on Pt/MC-TiO₂ can be attributed to its size-matching effect and thus high dye loading amount, whereas the high TON and durability of DH4/Pt/MC-TiO₂ are probably related to the rapid regeneration kinetics of DH4.

Precise Surface State Control of Carbon Quantum Dots to Enhance Charge Extraction for Solar Cells

Dye-sensitized solar cells are regarded as promising candidates to resolve the energy and environmental issues in recent years, arising from their solution-processable fabrication technology and high power conversion efficiency. However, there are still several problems regarding how to accelerate the development of this type of photovoltaics, including the limited light-harvesting ability and high-production cost of molecular dye. In the current work, we have systematically studied the role of nitrogen-doped carbon quantum dots (N-CQDs) as co-sensitizers in traditional dye sensitized solar cells. A series of N-CQDs have been prepared by employing chitosan as a precursor via one-pot hydrothermal technology for various times, demonstrating a maximized efficiency as high as 0.089% for an only N-CQDs-based device. Moreover, the co-sensitized solar cell based on N719 dye (C₅₈H₈₆N₈O₈RuS₂) and optimized N-CQDs shows significantly enhanced performance, yielding a solar-to-electric conversion efficiency of up to 9.15% under one standard sun (AM 1.5G) irradiation, which is much higher than the 8.5%-efficiency of the controlled device without N-CQDs. The matched characteristics of energy level, excellent up-convention, and FRET (Förster resonance energy transfer) abilities of N-CQDs are responsible for their improved power conversion efficiency.

Mesocrystalline Ta3N5 superstructures with long-lived charges for improved visible light photocatalytic hydrogen production

Highly ordered mesocrystalline semiconductors often indicate tremendous prospects in the clean energy production and environmental photocatalysis mainly because of their unique superstructure for efficient charge transport pathways and long-lived charges. Here, superstructure Ta₃N₅ mesocrystals with the high-energy surface {2 0 0} planes exposed were the first time to be successfully fabricated by topological transformation of Ta₂O₅ mesocrystals. The prepared Ta₃N₅ mesocrystals showed enhanced visible-light photocatalytic hydrogen production activity of 98.67 μmol g^-1 for 180 min irradiation, which was approximately 5.28 times that of comm-Ta₃N₅ prepared with commercial Ta₂O₅ as the starting material, mainly due to the formation of long-distance electron conduction pathways and long-lived charges. The detailed electronic band structures of the prepared Ta₃N₅ mesocrystals were also investigated by electrochemical method. Finally, possible visible-light photocatalytic mechanisms of Ta₃N₅ mesocrystals for enhanced hydrogen production was also proposed in detail. Current work also indicates that tantalum-based mesocrystals show great potential to enhance the charge separation for efficient photocatalytic water splitting.

Recent Advances in Fiber Supercapacitors: Materials, Device Configurations, and Applications

Fiber supercapacitors (SCs), with their small size and weight, excellent flexibility and deformability, and high capacitance and power density, are recognized as one of the most robust power supplies available for wearable electronics. They can be woven into breathable textiles or integrated into different functional materials to fit curved surfaces for use in day-to-day life. A comprehensive review on recent important development and progress in fiber SCs is provided, with respect to the active electrode materials, device configurations, functions, integrations. Active electrode materials based on different electrochemical mechanisms and intended to improve performance including carbon-based materials, metal oxides, and hybrid composites, are first summarized. The three main types of fiber SCs, namely parallel, twist, and coaxial structures, are then discussed, followed by the exploration of some functions including stretchability and self-healing. Miniaturized integration of fiber SCs to obtain flexible energy fibers and integrated sensing systems is also discussed. Finally, a short conclusion is made, combining with comments on the current challenges and potential solutions in this field.

Fabrication of a Hierarchical Ni(OH)2 @Ni3 S2 /Ni Foam Electrode from a Prussian Blue Analogue-Based Composite with Enhanced Electrochemical Capacitive and Electrocatalytic Properties

Developing high-efficiency, cost-effective, and durable electrodes is significant for electrochemical capacitors and electrocatalysis. Herein, a 3D bifunctional electrode consisting of nickel hydroxide nanosheets@nickel sulfide nanocubes arrays on Ni foam (Ni(OH)₂ @Ni₃ S₂ /NF) obtained from a Prussian blue analogue-based precursor is reported. The 3D higher-order porous structure and synergistic effect of different compositions endow the electrode with large specific surface area, facile ion/electron transport path, and improved conductivity. As a result, the Ni(OH)₂ @Ni₃ S₂ /NF electrode exhibits a high specific capacity of 211 mA h g^-1 at a current density of 1 A g^-1 and 73 % capacity retention after 5000 cycles at 5 A g^-1 . Moreover, the Ni(OH)₂ @Ni₃ S₂ /NF electrode has superior electrocatalytic activity for the hydrogen evolution reaction with low overpotentials of 140 and 210 mV at current densities of 10 and 100 mA cm^-2 , respectively. The synthetic strategy for the unique higher-order porous structure can be extended to fabricate other composite materials for energy storage and conversion.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

A dinuclear iron complex as an efficient electrocatalyst for homogeneous water oxidation reaction

A novel dinuclear iron complex of a Schiff base ligand has been exploited as a homogeneous water splitting electrocatalyst having possible real life application in renewable energy.

Phase-Selective Disordered Anatase/Ordered Rutile Interface System for Visible-Light-Driven, Metal-Free CO2 Reduction

Visible-light-driven photocatalytic CO₂ reduction using TiO₂ that can absorb light of all wavelengths has been sought for over half a century. Herein, we report a phase-selective disordered anatase/ordered rutile interface system for visible-light-driven, metal-free CO₂ reduction using a narrow band structure, whose conduction band position matches well with the reduction potential of CO₂ to CH₄ and CO. A mixed disordered anatase/ordered rutile (A_d/R_o) TiO₂ was prepared from anatase and rutile phase-mixed P25 TiO₂ at room temperature and under an ambient atmosphere in sodium alkyl amine solutions. The A_d/R_o TiO₂ showed a narrow band structure due to multi-internal energy band gaps of Ti³⁺ defect sites in the disordered anatase phase, leading to high visible light absorption and simultaneously providing fast charge separation through the crystalline rutile phase, which was faster than that of pristine P25 TiO₂. The band gap of A_d/R_o TiO₂ is 2.62 eV with a conduction band of -0.27 eV, which matches well with the reduction potential of -0.24 V_NHE of CO₂/CH₄, leading to effective electron transfer to CO₂. As a result, the A_d/R_o TiO₂ provided the highest CH₄ production (3.983 μmol/(g h)), which is higher than that of even metal (W, Ru, Ag, and Pt)-doped P25, for CO₂ reduction under visible light.

Data-Driven Systematic Search of Promising Photocatalysts for Water Splitting under Visible Light

Searching for highly efficient, environmentally friendly, and noble metal free photocatalysts is an important topic in the photocatalytic field. The combination of data-driven high-throughput screening and density functional theory (DFT) might be one of the most effective ways. To this end, we have carried out a systematic search of the Materials Project Database. A high-throughput screening method is employed, and six criteria are selected as the indicators. The screening reduces the list of candidates from 83989 to 22 structures that show promise as photocatalysts. The electronic and optical properties are then determined by DFT calculations. Our results indicate that ZnSe, Ga₂Se₃, and Na₂Zn₂O₃ show suitable direct band gaps, efficient optical absorption, and appropriate band edge positions, which are suitable for photocatalytic water splitting in the visible light region. We believe that this work not only proposes novel photocatalysts but also provides an efficient way of searching for advanced photocatalysts for water splitting.

Core-Shell Fe3O4@NCS-Mn Derived from Chitosan-Schiff Based Mn Complex with Enhanced Catalytic Activity for Oxygen Reduction Reaction

A core-shell type of Fe3O4/NCS-Mn composite was prepared by pyrolyzing a precursor fabricated by coating a chitosan-Schiff base Mn complex on Fe3O4 cores. For comparison purposes, the Fe3O4@NCS sample in the absence of Mn and the Fe3O4@NC sample derived from just chitosan coating Fe3O4 were also prepared. Among the three catalysts, Fe3O4@NCS-Mn demonstrates the best electrocatalytic activity compared to commercial Pt/C (20%) for oxygen reduction reaction (ORR). The average of the transferred electron number (n) approached 3.6 in the range of −0.3 to −0.8 V (vs. Ag/AgCl). Moreover, the catalyst exhibited high stability and durability against methanol and may potentially be a promising ORR catalyst for fuel cells.

Design, Performance, and Application of Thermoelectric Nanogenerators

Thermal energy harvesting from the ambient environment through thermoelectric nanogenerators (TEGs) is an ideal way to realize self-powered operation of electronics, and even relieve the energy crisis and environmental degradation. As one of the most significant energy-related technologies, TEGs have exhibited excellent thermoelectric performance and played an increasingly important role in harvesting and converting heat into electric energy, gradually becoming one of the hot research fields. Here, the development of TEGs including materials optimization, structural designs, and potential applications, even the opportunities, challenges, and the future development direction, is analyzed and summarized. Materials optimization and structural designs of flexibility for potential applications in wearable electronics are systematically discussed. With the development of flexible and wearable electronic equipment, flexible TEGs show increasingly great application prospects in artificial intelligence, self-powered sensing systems, and other fields in the future.

Electronic Structure and Lithium Diffusion in LiAl2(OH)6Cl Studied by First Principle Calculations

First-principles calculations based on the density functional theory (DFT) were carried out to study the atomic structure and electronic structure of LiAl2(OH)6Cl, the only material in the layered double hydroxide family in which delithiation was found to occur. Ab initio molecular dynamics (AIMD) simulations were used to explore the evolution of the structure of LiAl2(OH)6Cl during a thermally induced delithiation process. The simulations show that this process occurs due to the drastic dynamics of Li+ at temperatures higher than ~450 K, in which the [Al2(OH)6] host layers remain stable up to 1100 K. The calculated large value of the Li+ diffusion coefficient D, ~ 3.13 × 10 - 5 c m 2 / s , at 500 K and the high stability of the [Al2(OH)6] framework suggest a potential technical application of the partially-delithiated Li1-xAl2(OH)6Cl1-x (0 < x < 1) as a superionic conductor at high temperatures.

Graphene Supported Tungsten Carbide as Catalyst for Electrochemical Reduction of CO2

Electrochemical reduction of CO2 to useful chemical and fuels in an energy efficient way is currently an expensive and inefficient process. Recently, low-cost transition metal-carbides (TMCs) have been proven to exhibit similar electronic structure similarities to Platinum-Group-Metal (PGM) catalysts and hence, can be good substitutes for some important reduction reactions. In this work, we test graphene-supported WC (Tungsten Carbide) nanoclusters as an electrocatalyst for the CO2 reduction reaction. Specifically, we perform density functional theory (DFT) studies to understand various possible reaction mechanisms and determine the lowest thermodynamic energy landscape of CO2 reduction to various products, such as CO, HCOOH, CH3OH, and CH4. This in-depth study of reaction energetics could lead to improvements and development of more efficient electrocatalysts for CO2 reduction.

A thermo-photo hybrid process for steam reforming of methane: highly efficient visible light photocatalysis

Steam reforming of methane (SRM) is one of the most important industrial processes, which produces 95% of hydrogen used in the USA. However, SRM is an endothermic reaction, which requires a high energy input and a high reaction temperature (>800 °C) for the current process. Furthermore, its products must be subjected to a water-gas shift (WGS) process. A photocatalytic process is expected to solve the energy issue and to eliminate the necessity of WGS for SRM. However, the hydrogen yield from the current photocatalytic steam reforming of methane (PSRM) is very low (μmol h^-1 g^-1 level), which is far below industrial interest. This work demonstrates that a Pt/blackTiO₂ catalyst dispersed on a light-diffuse-reflection-surface is excellent for efficient visible-light PSRM. Under visible light illumination on the catalyst by filtering UV light from AM 1.5G sunlight, CH₄ and H₂O were directly converted into H₂ and CO₂ without WGS, leading to a high H₂ yield of 185 mmol h^-1 g^-1 with a quantum efficiency of 60% at 500 °C. The yield is 3 orders of magnitude larger than the reported values, which can be attributed to the synergistic effect between potential and kinetic energies. This opens up a new opportunity for hydrogen production from water and natural gas using solar energy.

Understanding the interplay between size, morphology and energy gap in photoactive TiO2 nanoparticles

Anatase TiO2 nanoparticles (NPs) have the potential to photocatalyse water splitting using UV light, to thus provide hydrogen fuel in a clean and sustainable manner. Such NPs have optical gaps covering a small range of relatively high energy solar photons, giving rise to low photo-efficiencies. Although anatase NPs with 10-20 nm diameters thermodynamically prefer crystalline faceted morphologies, application of physico-chemical procedures can produce more rounded NPs with amorphous shells. Such engineered metastable core-shell NPs (so-called black TiO2 NPs) have reduced band gaps due to shell-induced band edge broadening, resulting in higher photoactivities. For <5 nm diameters, TiO2 NPs typically exhibit spherical-like NP morphologies, which also display enhanced photoactivity. For smaller NPs it is difficult to experimentally determine their thermodynamic stability and internal atomic structure, to help rationalise their higher photoactivities. Employing accurate electronic structure calculations, we establish the relative stability of spherical and faceted stoichiometric TiO2 NPs with 1-3.4 nm diameters. Mirroring experimental preparation, simulated thermal annealing is found to significantly stabilise relaxed spherical cut anatase NPs. We find that the smallest spherical NPs become amorphized by annealing, but, for diameters >2 nm, annealing yields NPs with anatase-cores and amorphous-shells. Like larger black TiO2 core-shell NPs, we confirm that our core-shell NPs are metastable relative to faceted anatase NPs and have significantly smaller optical gaps than faceted NPs. Our calculated gaps are in excellent agreement with experimental data, strongly supporting the validity of our NP models. Energy gap narrowing in these core-shell NPs is found to be due to broadening of valence band states induced by the amorphous shell, analogous to the mechanism proposed for black TiO2 NPs. Our stoichiometric NPs also show that this band narrowing effect does not require the disordered shells to be non-stoichiometric or for incorporation of other atom types. Instead, we find that this effect mainly arises from 4-coordinated Ti atoms in the amorphous shell. Our careful and systematic computational investigation, using NP models of unprecedented realism, thus provides direct confirmation that the enhanced photoactivity in small spherical TiO2 NP observed in experiment is due to the formation of metastable core-shell NPs with 4-coordinated Ti centres.

Synergistic Effect of Co-Ni Hybrid Phosphide Nanocages for Ultrahigh Capacity Fast Energy Storage

Rational design of metal compounds in terms of the structure/morphology and chemical composition is essential to achieve desirable electrochemical performances for fast energy storage because of the synergistic effect between different elements and the structure effect. Here, an approach is presented to facilely fabricate mixed-metal compounds including hydroxides, phosphides, sulfides, oxides, and selenides with well-defined hollow nanocage structure using metal-organic framework nanocrystals as sacrificial precursors. Among the as-synthesized samples, the porous nanocage structure, synergistic effect of mixed metals, and unique phosphide composition endow nickel cobalt bimetallic phosphide (NiCo-P) nanocages with outstanding performance as a battery-type Faradaic electrode material for fast energy storage, with ultrahigh specific capacity of 894 C g-1 at 1 A g-1 and excellent rate capability, surpassing most of the reported metal compounds. Control experiments and theoretical calculations based on density functional theory reveal that the synergistic effect between Ni and Co in NiCo-P can greatly increase the OH- adsorption energy, while the hollow porous structure facilitates the fast mass/electron transport. The presented work not only provides a promising electrode material for fast energy storage, but also opens a new route toward structural and compositional design of electrode materials for energy storage and conversion.

Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts

Photocatalytic water splitting is a sustainable technology for the production of clean fuel in terms of hydrogen (H2). In the present study, hydrogen (H2) production efficiency of three promising photocatalysts (titania (TiO2-P25), graphitic carbon nitride (g-C3N4), and cadmium sulfide (CdS)) was evaluated in detail using various sacrificial agents. The effect of most commonly used sacrificial agents in the recent years, such as methanol, ethanol, isopropanol, ethylene glycol, glycerol, lactic acid, glucose, sodium sulfide, sodium sulfite, sodium sulfide/sodium sulfite mixture, and triethanolamine, were evaluated on TiO2-P25, g-C3N4, and CdS. H2 production experiments were carried out under simulated solar light irradiation in an immersion type photo-reactor. All the experiments were performed without any noble metal co-catalyst. Moreover, photolysis experiments were executed to study the H2 generation in the absence of a catalyst. The results were discussed specifically in terms of chemical reactions, pH of the reaction medium, hydroxyl groups, alpha hydrogen, and carbon chain length of sacrificial agents. The results revealed that glucose and glycerol are the most suitable sacrificial agents for an oxide photocatalyst. Triethanolamine is the ideal sacrificial agent for carbon and sulfide photocatalyst. A remarkable amount of H2 was produced from the photolysis of sodium sulfide and sodium sulfide/sodium sulfite mixture without any photocatalyst. The findings of this study would be highly beneficial for the selection of sacrificial agents for a particular photocatalyst.

Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting

Overall water splitting based on particulate photocatalysts is an easily constructed and cost-effective technology for the conversion of abundant solar energy into clean and renewable hydrogen energy on a large scale.

Mesoporous hollow black TiO2 with controlled lattice disorder degrees for highly efficient visible-light-driven photocatalysis

Black TiO₂ has received tremendous attention because of its lattice disorder-induced reduction in the TiO₂ bandgap, which yields excellent light absorption and photocatalytic ability. In this report, a highly efficient visible-light-driven black TiO₂ photocatalyst was synthesized with a mesoporous hollow shell structure. It provided a higher specific surface area, more reaction sites and enhanced visible light absorption capability, which significantly promoted the photocatalytic reaction. Subsequently, the mesoporous hollow black TiO₂ with different lattice disorder-engineering degrees were designed. The structure disorder in the black TiO₂ obviously increased with reduction temperature, leading to improved visible light absorption. However, their visible-light-driven photocatalytic efficiency increased first and then decreased. The highest value can be observed for the sample reduced at 350 °C, which was 2-, 1.4- and 5-fold that of the samples reduced at 320 °C, 380 °C and 400 °C, respectively. This contradiction can be ascribed to the varied functions of the surface defects with different concentrations in the black TiO₂ during the catalytic process. In particular, the defects at low concentrations boost photocatalysis but reverse photocatalysis at high concentrations when they act as charge recombination centers. This study provides significant insight for the fabrication of high-efficiency visible-light-driven catalytic black TiO₂ and the understanding of its catalysis mechanism.

Our work provides significant insights into the design of hollow black TiO₂ spheres and the mechanism accounting for their high-efficient visible-light-driven catalysis.

Benzophenone assisted UV-activated synthesis of unique Pd-nanodendrite embedded reduced graphene oxide nanocomposite: a catalyst for C–C coupling reaction and fuel cell

In this work we report the use of benzophenone (BP) for the synthesis of a palladium (Pd) embedded on reduced graphene oxide (rGO) nanocomposite (Pd/rGO) using a simple aqueous solution and UV irradiation. The simple and facile evolution of thermodynamically unstable branched Pd(0) nanodendrites was achieved by BP photoactivation, circumventing the growth of more stable nanomorphologies. The synthesis of Pd(0)-embedded rGO nanosheets (PRGO-nd) was made possible by the simultaneous reduction of both the GO scaffold and PdCl₂ by introducing BP into the photoactivation reaction. The nanocomposites obtained in the absence of BP were common triangular and twinned Pd(0) structures which were also implanted on the rGO scaffold (PRGO-nt). The disparity in morphologies presumably occurs due to the difference in the kinetics of the reduction of Pd²⁺ to Pd⁰ in the presence and absence of the BP photoinitiator. It was observed that the PRGO-nd was composed of dense arrays of multiple Pd branches around nucleation site which exhibited (111) facet, whereas PRGO-nt showed a mixture of (100) and (111) facets. On comparing the catalytic efficiencies of the as-synthesized nanocatalysts, we observed a superiority in efficiency of the thermodynamically unstable PRGO-nd nanocomposite. This is due to the evolved active facets of the dendritic Pd(0) morphology with its higher surface area, as testified by Brunauer–Emmett–Teller (BET) analysis. Since both PRGO-nd and PRGO-nt contain particles of similar size, the dents and grooves in the structure are the cause of the increase in the effective surface area in the case of nanodendrites. The unique dendritic morphology of the PRGO-nd nanostructures makes them a promising material for superior catalysis, due to their high surface area, and the high density of surface atoms at their edges, corners, and stepped regions. We investigated the efficiency of the as-prepared PRGO-nd catalyst in the Suzuki–Miyaura coupling reaction and showed its proficiency in a 2 h reaction at 60 °C using 2 mol% catalyst containing 0.06 mol% active Pd. Moreover, the electrochemical efficiency for the catalytic hydrogen evolution reaction (HER) was demonstrated, in which PRGO-nd provided a decreased overpotential of 68 mV for a current density of 10 mA cm⁻², a small Tafel slope of 57 mV dec⁻¹ and commendable stability during chronoamperometric testing for 5 h.

Benzophenone photoinitiator aided synthesis of Pd-nanodendrite embedded rGO nanocatalyst possessing superior potential in C–C coupling reaction and fuel cell application.

The Fine Characterization and Potential Photocatalytic Effect of Semiconducting Metal Minerals in Danxia Landforms

The Danxia landform is representative of the Cretaceous continental red sediment. The careful identification and potential environmental effects of minerals in Danxia red beds have yet to be clearly reported. In this work, reddish sandstone samples were collected from Lang Mountain Danxia landform in Xinning, Hunan province, China, and their mineral phases, element distribution, microstructure, and the spatial relationship of different minerals were investigated using polarizing optical microscope, environmental scanning electron microscopy, energy-dispersive X-ray analysis, electron probe microanalysis, micro-Raman spectra, micro- X-ray diffraction, X-ray fluorescence spectroscopy, and high-resolution transmission electron microscopy. The results revealed that iron oxide (mainly hematite) and titanium oxide (mainly anatase) were the dominant minerals in Danxia red layers. Microcrystalline hematite was suggested as being the coloring mineral. Anatase, reported here for the first time in Danxia red beds, constituted the content of titanium in the red layer (0.17–0.57%) and was present in a significantly higher amount than the adjacent limestone formation (0.13%). Over 95% of Fe/Ti oxides served as a cementation agent along the framework of coarse-grain minerals (quartz and feldspar). The hematite and anatase were visible-light-responsive semiconductors, with a band gap of 2.01 eV and 3.05 eV, respectively. Photoelectrochemical experiments were performed on synthetic hematite, anatase, and their coupled material. The inactive hematite displayed an enhanced 23-fold photocurrent at 0.8 V (vs. Ag/AgCl) when coupled with anatase. Furthermore, in a photodegradation experiment using methyl orange dye under simulated sunlight, the coupled material showed decolorizing efficiency 2.4 times that of hematite. The anatase, therefore, prominently improved the photocatalytic activities of hematite. It is proposed that these semiconducting minerals in red beds produce oxygen reactive species and have significant environmental effects, which is of great importance.

Multifunctional Photostable Nanocomplex of ZnO Quantum Dots and Avobenzone via the Promotion of Enolate Tautomer

Ideal multifunctional ultraviolet radiation (UVR) absorbents with excellent photostability, high molar absorptivity, broadband UVR screening, and desired skin sensorial properties remain a significant challenge for the sunscreen industry. The potential of the nanocomplex (NCx) formed by microwave synthesis of ZnO quantum dots (QDs) in the presence of Avobenzone (Av) for achieving these goals is reported. The NCx exhibits unique synergy between ZnO QD and Av components, which enhances the photostability and molar absorptivity, extends UVA filtering range, and provides a visible emission that matches the typical human in vivo skin emission color. Density functional theory (DFT) and time-dependent DFT calculations of ZnO-Av hybrid structures and comparison of their spectroscopic features with experiments suggest that ZnO QDs catalyze the formation of highly photostable surface enolate species via aldol condensation reaction. The combination of experiments and computations used in this study can advance the science and technology of photoprotection.