n-p Heterojunction of TiO2-NiO core-shell structure for efficient hydrogen generation and lignin photoreforming

Solar-Driven Biomass Reforming for Hydrogen Generation: Principles, Advances, and Challenges

Hydrogen (H2) has emerged as a clean and versatile energy carrier to power a carbon-neutral economy for the post-fossil era. Hydrogen generation from low-cost and renewable biomass by virtually inexhaustible solar energy presents an innovative strategy to process organic solid waste, combat the energy crisis, and achieve carbon neutrality. Herein, the progress and breakthroughs in solar-powered H2 production from biomass are reviewed. The basic principles of solar-driven H2 generation from biomass are first introduced for a better understanding of the reaction mechanism. Next, the merits and shortcomings of various semiconductors and cocatalysts are summarized, and the strategies for addressing the related issues are also elaborated. Then, various bio-based feedstocks for solar-driven H2 production are reviewed with an emphasis on the effect of photocatalysts and catalytic systems on performance. Of note, the concurrent generation of value-added chemicals from biomass reforming is emphasized as well. Meanwhile, the emerging photo-thermal coupling strategy that shows a grand prospect for maximally utilizing the entire solar energy spectrum is also discussed. Further, the direct utilization of hydrogen from biomass as a green reductant for producing value-added chemicals via organic reactions is also highlighted. Finally, the challenges and perspectives of photoreforming biomass toward hydrogen are envisioned.

Wireless Gas Sensor Based on the Mesoporous ZnO-SnO2 Heterostructure Enables Ultrasensitive and Rapid Detection of 3-Methylbutyraldehyde

Achieving ultrasensitive and rapid detection of 3-methylbutyraldehyde is crucial for monitoring chemical intermediate leakage in pharmaceutical and chemical industries as well as diagnosing ventilator-associated pneumonia by monitoring exhaled gas. However, developing a sensitive and rapid method for detecting 3-methylbutyraldehyde poses challenges. Herein, a wireless chemiresistive gas sensor based on a mesoporous ZnO-SnO2 heterostructure is fabricated to enable the ultrasensitive and rapid detection of 3-methylbutyraldehyde for the first time. The mesoporous ZnO-SnO2 heterostructure exhibits a uniform spherical shape (∼79 nm in diameter), a high specific surface area (54.8 m2 g-1), a small crystal size (∼4 nm), and a large pore size (6.7 nm). The gas sensor demonstrates high response (18.98@20 ppm), short response/recovery times (13/13 s), and a low detection limit (0.48 ppm) toward 3-methylbutyraldehyde. Furthermore, a real-time monitoring system is developed utilizing microelectromechanical systems gas sensors. The modification of amorphous ZnO on the mesoporous SnO2 pore wall can effectively increase the chemisorbed oxygen content and the thickness of the electron depletion layer at the gas-solid interface, which facilitates the interface redox reaction and enhances the sensing performance. This work presents an initial example of semiconductor metal oxide gas sensors for efficient detection of 3-methylbutyraldehyde that holds great potential for ensuring safety during chemical production and disease diagnosis.

Catalyzing Artificial Photosynthesis with TiO2 Heterostructures and Hybrids: Emerging Trends in a Classical yet Contemporary Photocatalyst

Titanium dioxide (TiO2) stands out as a versatile transition-metal oxide with applications ranging from energy conversion/storage and environmental remediation to sensors and optoelectronics. While extensively researched for these emerging applications, TiO2 has also achieved commercial success in various fields including paints, inks, pharmaceuticals, food additives, and advanced medicine. Thanks to the tunability of their structural, morphological, optical, and electronic characteristics, TiO2 nanomaterials are among the most researched engineering materials. Besides these inherent advantages, the low cost, low toxicity, and biocompatibility of TiO2 nanomaterials position them as a sustainable choice of functional materials for energy conversion. Although TiO2 is a classical photocatalyst well-known for its structural stability and high surface activity, TiO2-based photocatalysis is still an active area of research particularly in the context of catalyzing artificial photosynthesis. This review provides a comprehensive overview of the latest developments and emerging trends in TiO2 heterostructures and hybrids for artificial photosynthesis. It begins by discussing the common synthesis methods for TiO2 nanomaterials, including hydrothermal synthesis and sol-gel synthesis. It then delves into TiO2 nanomaterials and their photocatalytic mechanisms, highlighting the key advancements that have been made in recent years. The strategies to enhance the photocatalytic efficiency of TiO2, including surface modification, doping modulation, heterojunction construction, and synergy of composite materials, with a specific emphasis on their applications in artificial photosynthesis, are discussed. TiO2-based heterostructures and hybrids present exciting opportunities for catalyzing solar fuel production, organic degradation, and CO2 reduction via artificial photosynthesis. This review offers an overview of the latest trends and advancements, while also highlighting the ongoing challenges and prospects for future developments in this classical yet rapidly evolving field.

Interfacial effect between Ni2P/CdS for simultaneously heightening photocatalytic hydrogen production and lignocellulosic biomass photorefining

Photorefining of biomass is increasingly recognized as a pivotal technology for the simultaneous production of hydrogen and value-added chemicals. The intrinsic recalcitrance of lignocellulosic biomass puts high demands on the rational design of bifunctional photocatalyst. Herein, Ni2P/CdS with a strong interfacial effect in this work was designed to overcome lignocellulosic biomass photorefining. The strong interfacial effect between Ni2P and CdS not only improved the light absorbance, but also optimized the spatial redistribution of photogenerated electrons and holes. Therefore, Ni2P/CdS exhibited an unprecedented H2 evolution activity (ca. 199.7 mmol·h-1·g-1) in the presence of lactic acid as the traditional sacrificial agent. Considerable H2 generation was also achieved in the presence of lignin (ca. 322.8 μmol·h-1·g-1), cellulose (ca. 534.3 μmol·h-1·g-1) and hemicellulose (ca. 382.2 μmol·h-1·g-1) as the electron donor respectively. Theoretical calculation results indicated that establishing the interfacial effect between Ni2P and CdS optimized their work functions. This optimization fosters improved the redistribution between electrons and holes, as a result, photocatalytic hydrogen production from biomass solution was greatly enhanced. Significantly, Ni2P/CdS showed dual functionalities to produce H2 and value-added compounds from raw biomass directly. This present work demonstrates the potential of raw biomass photorefining through astutely designing photocatalysts.

Preparation of Pt/WO3@ZnO hollow spheres for low-temperature and high-efficiency detection of triethylamine

In this work, hollow spherical Pt-loaded WO3/ZnO heterostructured composites were prepared by a chemical liquid phase synthesis method. The morphology, crystal structure and components of the composites were characterized by SEM, TEM, XRD, XPS, etc. The sensing performance for various gases was also tested. Compared with the pristine WO3 (S = 44@225 °C, 50 ppm) gas sensor, the gas sensor that is functionalized with 1 wt% Pt and 0.5 mmol ZnO (1Pt/WZ-2) has a high response of 842-50 ppm at a relatively low temperature of 100 °C for TEA, with a quick response/recovery time of 34/120 s, a lower detection limit of 50 ppb, and good selectivity and moisture resistance. This study provides a highly efficient synthesis method of composite materials for TEA gas detection and the sensitivity mechanism is also discussed in detail.

Radical-Mediated Photocatalysis for Lignocellulosic Biomass Conversion into Value-Added Chemicals and Hydrogen: Facts, Opportunities and Challenges

Photocatalytic biomass conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. In short, this process can cleave Cβ -O/Cα -Cβ bonds in lignin to aromatic platform chemicals, and further conversion of the polysaccharides to other platform chemicals and H2 . From the chemistry point of view, the optimization of the unique cooperative interplay of radical oxidation species (which are activated via molecular oxygen species, ROSs) and substrate-derived radical intermediates by appropriate control of their type and/or yield is key to the selective production of desired products. Technically, several challenges have been raised that face successful real-world applications. This review aims to discuss the recently reported mechanistic pathways toward selective biomass conversion through the optimization of ROSs behavior and materials/system design. On top of that, through a SWOT analysis, we critically discussed this technology from both chemistry and technological viewpoints to help the scientists and engineers bridge the gap between lab-scale and large-scale production.

Recent advances and progress in biotemplate catalysts for electrochemical energy storage and conversion

Complex structures and morphologies in nature endow materials with unexpected properties and extraordinary functions. Biotemplating is an emerging strategy for replicating nature structures to obtain materials with unique morphologies and improved properties. Recently, efforts have been made to use bio-inspired species as a template for producing morphology-controllable catalysts. Fundamental information, along with recent advances in biotemplate metal-based catalysts are presented in this review through discussions of various structures and biotemplates employed for catalyst preparation. This review also outlines the recent progress on preparation routes of biotemplate catalysts and discusses how the properties and structures of these templates play a crucial role in the final performance of metal-based catalysts. Additionally, the application of bio-based metal and metal oxide catalysts is highlighted for various key energy and environmental technologies, including photocatalysis, fuel cells, and lithium batteries. Biotemplate metal-based catalysts display high efficiency in several energy and environmental systems. Note that this review provides guidance for further research in this direction.

Photoreforming for Lignin Upgrading: A Critical Review

Photoreforming of lignocellulosic biomass to simultaneously produce gas fuels and value-added chemicals has gradually emerged as a promising strategy to alleviate the fossil fuels crisis. Compared to cellulose and hemicellulose, the exploitation and utilization of lignin via photoreforming are still at the early and more exciting stages. This Review systematically summarizes the latest progress on the photoreforming of lignin-derived model components and "real" lignin, aiming to provide insights for lignin photocatalytic valorization from fundamental to industrial applications. Considering the complexity of lignin physicochemical properties, related analytic methods are also introduced to characterize lignin photocatalytic conversion and product distribution. We finally put forward the challenges and perspective of lignin photoreforming, hoping to provide some guidance to valorize biomass into value-added chemicals and fuels via a mild photoreforming process in the future.

p-n heterojunction of nickel oxide on titanium dioxide nanosheets for hydrogen and value-added chemicals coproduction from glycerol photoreforming

Selective photocatalysis to simultaneously produce sustainable hydrogen and value-added chemicals from biomass or biomass derivates is attracting extensive investigations. However, the lack of bifunctional photocatalyst greatly limits the possibility to realize the "one stone kills two birds" scenario. Herein, anatase titanium dioxide (TiO₂) nanosheets are rationally designed as the n-type semiconductor, combining with nickel oxide (NiO) nanoparticles, the p-type semiconductor, resulting in the formation of a p-n heterojunction structure. The shorten charge transfer path and the spontaneous formation of p-n heterojunction endow the photocatalyst with efficient spatial separation of photogenerated electrons and holes. As a result, TiO₂ accumulates electrons for efficient hydrogen generation while NiO collects holes to selectively oxidize glycerol into value-added chemicals. The results showed that by loading 5% nickel into the heterojunction caused a remarkable rise in the generation of hydrogen (H₂). The combination of NiO-TiO₂ created 4000 µmolh^-1g^-1 of H₂, which is 50% greater than the H₂ production from pure nanosheet TiO₂ and 63 times more than the H₂ production from commercial nanopowder TiO₂. Then, by changing loading amount of Ni, it is found that when 7.5 % of Ni is loaded the highest amount of hydrogen production achieved, 8000 µmolh^-1g^-1. By employing best sample (S3), 20 % of glycerol converted to value added products, glyceraldehyde and dihydroxyacetone. The feasibility study revealed that glyceraldehyde generates the largest portion of yearly earnings at 89%, while dihydroxyacetone and H₂ account for 11% and 0.03% of the annual revenue, respectively. This work provides a good example to simultaneously produce green hydrogen and valuable chemicals with the rational design of dually functional photocatalyst.

Floating Carbon Nitride Composites for Practical Solar Reforming of Pre-Treated Wastes to Hydrogen Gas

Solar reforming (SR) is a promising green-energy technology that can use sunlight to mitigate biomass and plastic waste while producing hydrogen gas at ambient pressure and temperature. However, practical challenges, including photocatalyst lifetime, recyclability, and low production rates in turbid waste suspensions, limit SR's industrial potential. By immobilizing SR catalyst materials (carbon nitride/platinum; CN_x |Pt and carbon nitride/nickel phosphide; CN_x |Ni₂ P) on hollow glass microspheres (HGM), which act as floating supports enabling practical composite recycling, such limitations can be overcome. Substrates derived from plastic and biomass, including poly(ethylene terephthalate) (PET) and cellulose, are reformed by floating SR composites, which are reused for up to ten consecutive cycles under realistic, vertical simulated solar irradiation (AM1.5G), reaching activities of 1333 ± 240 µmol_H2 m^-2 h^-1 on pre-treated PET. Floating SR composites are also advantageous in realistic waste where turbidity prevents light absorption by non-floating catalyst powders, achieving 338.1 ± 1.1 µmol_H2 m^-2 h^-1 using floating CN_x versus non-detectable H₂ production with non-floating CN_x and a pre-treated PET bottle as substrate. Low Pt loadings (0.033 ± 0.0013% m/m) demonstrate consistent performance and recyclability, allowing efficient use of precious metals for SR hydrogen production from waste substrates at large areal scale (217 cm² ), taking an important step toward practical SR implementation.

Fabrication of Z-scheme Cd0.85Zn0.15S/Co9S8dual-functional photocatalyst for effective hydrogen evolution and organic pollutant degradation

A Z-scheme Cd_0.85Zn_0.15S/Co₉S₈(CZS-CS) photocatalyst was reasonably fabricated by a simple solvothermal method for the effective visible-light-driven H₂evolution and organic pollutants degradation. The precise construction of the CZS-CS composites provided an efficient heterogeneous contact interface and abundant reaction sites for the proposed photocatalytic reaction. The homogeneous Co₉S₈nanocrystals were uniformly wrapped on the surface of Cd_0.85Zn_0.15S nanorods, forming an intimate-contact interface, markedly contributed to the light collection and effectively inhibited the charge-carrier recombination. The optimized CZS-CS-15 composites exhibited a special H₂production rate reaching 19.15 mmol·h^-1·g^-1, roughly 1915 and 4.5 times of pure Co₉S₈and Cd_0.85Zn_0.15S samples and 85% of tetracycline (TC) molecule within 15 min was degraded. Furthermore, trapping experiments confirmed that h⁺was the main active species for TC photodegradation. Moreover, the obtained photocatalysts manifested stability without apparent activity declines during the proposed reactions. Finally, the Z-scheme photocatalytic mechanism was verified to illustrate the characteristics of efficient charge transfer and high redox ability. This study provided a rational and learnable strategy for designing dual-functional Z-scheme heterojunction photocatalysts.

Heterojunction Design between WSe2 Nanosheets and TiO2 for Efficient Photocatalytic Hydrogen Generation

Design and fabrication of efficient and stable photocatalysts are critically required for practical applications of solar water splitting. Herein, a series of WSe2/TiO2 nanocomposites were constructed through a facile mechanical grinding method, and all of the nanocomposites exhibited boosted photocatalytic hydrogen evolution. It was discovered that the enhanced photocatalytic performance was attributed to the efficient electron transfer from TiO2 to WSe2 and the abundant active sites provided by WSe2 nanosheets. Moreover, the intimate heterojunction between WSe2 nanosheets and TiO2 favors the interfacial charge separation. As a result, a highest hydrogen evolution rate of 2.28 mmol/g·h, 114 times higher than pristine TiO2, was obtained when the weight ratio of WSe2/(WSe2 + TiO2) was adjusted to be 20%. The designed WSe2/TiO2 heterojunctions can be regarded as a promising photocatalysts for high-throughput hydrogen production.

Design of TiO2/Ag3BiO3 n-n heterojunction for enhanced degradation of tetracycline hydrochloride under visible-light irradiation

Pharmaceutical residual contaminants in aquatic ecosystems have caused severe risks to human health. Affordable, eco-friendly and effective photocatalysts to deal with these pollutants has become a hot topic in the scientific community. In this research, Ag₃BiO₃ nanoparticles were embedded on TiO₂ to form n-n heterojunction through a facile hydrothermal method. According to scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR), brunauer emmett teller (BET), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), photoluminescence (PL), X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) tests, the successful construction of TiO₂/Ag₃BiO₃ heterojunction is proved. TiO₂/Ag₃BiO₃ heterojunctions were employed as photocatalysts to remove tetracycline hydrochloride (TCH) under visible light irradiation in aqueous solution. Optimum TCH photodegradation efficiency was observed for TiO₂/Ag₃BiO₃ (10%), 15.4 times superior to that of TiO₂. The enhanced TCH photodegradation efficiency of TiO₂/Ag₃BiO₃ results from improved light absorption capacity and the reduction of recombination of photogenerated charge carriers via generation of n-n heterojunctions. The mechanism of increasing the photodegradation efficiency of TCH was determined by employing reactive species quenching experiments. TiO₂/Ag₃BiO₃ (10%) also exhibited an acceptable stability.

Recent advances in self-powered and flexible UVC photodetectors

Ultraviolet-C (UVC) radiation is employed in various applications, including irreplaceable applications in military and civil fields, such as missile guidance, flame detection, partial discharge detection, disinfection, and wireless communication. Although most modern electronics are based on Si, UVC detection technology remains a unique exception because the short wavelength of UV radiation makes efficient detection with Si difficult. In this review, recent challenges in obtaining ideal UVC photodetectors with various materials and various forms are introduced. An ideal photodetector must satisfy the following requirements: high sensitivity, fast response speed, high on/off photocurrent ratio, good regional selectivity, outstanding reproducibility, and superior thermal and photo stabilities. UVC detection is still in its infancy compared to the detection of UVA as well as other photon spectra, and recent research has focused on different key components, including the configuration, material, and substrate, to acquire battery-free, super-sensitive, ultra-stable, ultra-small, and portable UVC photodetectors. We introduce and discuss the strategies for fabricating self-powered UVC photodetectors on flexible substrates in terms of the structure, material, and direction of incoming radiation. We also explain the physical mechanisms of self-powered devices with various architectures. Finally, we present a brief outlook that discusses the challenges and future strategies for deep-UVC photodetectors.

Waste Biomass Selective and Sustainable Photooxidation to High-Added-Value Products: A Review

Researchers worldwide seek to develop convenient, green, and ecological production processes to synthesize chemical products with high added value. In this sense, lignocellulosic biomass photocatalysis is an excellent process for obtaining various outcomes for the industry. One issue of biomass transformation via heterogeneous catalysis into valuable chemicals is the selection of an adequate catalyst that ensures high conversion and selectivity at low costs. Titanium oxide (TiO2), is widely used for several applications, including photocatalytic biomass degradation, depolymerization, and transformation. Graphite carbon nitride (g-C3N4) is a metal-free polymeric semiconductor with high oxidation and temperature resistance and there is a recent interest in developing this catalyst. Both catalysts are amenable to industrial production, relatively easy to dope, and suited for solar light absorption. Recent investigations also show the advantages of using heterojunctions, for biomass derivates production, due to their better solar spectrum absorption properties and, thus, higher efficiency, conversion, and selectivity over a broader spectrum. This work summarizes recent studies that maximize selectivity and conversion of biomass using photocatalysts based on TiO2 and g-C3N4 as supports, as well as the advantages of using metals, heterojunctions, and macromolecules in converting cellulose and lignin. The results presented show that heterogeneous photocatalysis is an interesting technology for obtaining several chemicals of industrial use, especially when using TiO2 and g-C3N4 doped with metals, heterojunctions, and macromolecules because these modified catalysts permit higher conversion and selectivity, milder reaction conditions, and reduced cost due to solar light utilization. In order to apply these technologies, it is essential to adopt government policies that promote the use of photocatalysts in the industry, in addition to encouraging active collaboration between photooxidation research groups and companies that process lignocellulosic biomass.

Enhanced solar light photocatalytic and antimicrobial activity of green noble metal/TiO2 nanorods

Au/TiO2 and Pt/TiO2 nanocomposites have been processed using a green method. Au and Pt colloidal nanoparticles have been primarily synthesized by mixing their corresponding metal ions with an aqueous solution of corn husk extract, followed by anchoring on the as-synthesized TiO2 nanorods. The structural and morphological properties of green-prepared nanomaterials were systematically investigated by various techniques. The UV–VIS absorption measurements confirmed the formation of colloidal Au and Pt with λmax at 550 and 345 nm, respectively. TEM results show anchoring of spherical Au particles (40 nm) on TiO2 nanorods while smaller Pt particles have been observed on Pt/TiO2 composite. It has been shown that the highest visible light harvesting capability was earned for Au/TiO2 composite due to the surface plasmon resonance (SPR) of Au nanoparticles. The photocatalytic and antimicrobial activity of the nanomaterials was investigated for disinfection of Escherichia coli under solar light irradiation. The green synthesized nanocomposites showed enhanced solar light photocatalytic and antimicrobial activity. The best photocatalytic and antimicrobial activity was obtained for Au/TiO2. This evidences the enhanced SPR of Au nanoparticles and hence an enhancement in the solar light accessible by TiO2 so that a higher amount of reactive oxygen species can be generated, which enhances photocatalytic and antibacterial activity.

Biomass Photoreforming for Hydrogen Production over Hierarchical 3DOM TiO2-Au-CdS

Photocatalytic hydrogen production is a promising route to the provision of sustainable and green energy. However, the excess addition of traditional electron donors as the sacrificial agents to consume photogenerated holes greatly reduces the feasibility of this approach for commercialization. Herein, considering the abundant hydroxyl groups in cellulose, the major component of biomass, we adopted glucose (a component unit of cellulose), cellobiose (a structure unit of cellulose) and dissolving pulp (a pretreated cellulose) as electron donors for photocatalytic hydrogen production over a TiO2-Au-CdS material. The well-designed ternary TiO2-Au-CdS possesses a hierarchical three-dimensional ordered macroporous (3DOM) structure, which not only benefits light harvesting but can also facilitate mass diffusion to boost the reaction kinetics. As expected, the fabricated photocatalyst exhibits considerable hydrogen production from glucose (645.1 μmol·h−1·g−1), while the hydrogen production rates gradually decrease with the increased complexity in structure from cellobiose (273.9 μmol·h−1·g−1) to dissolving pulp (79.7 μmol·h−1·g−1). Other gaseous components such as CO and CH4 are also produced, indicating the partial conversion of biomass during the photoreforming process. This work demonstrates the feasibility of sustainable hydrogen production from biomass by photoreforming with a rational photocatalyst design.

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems

Photocatalytic reforming of biomass has emerged as an area of significant interest within the last decade. The number of papers published in the literature has been steadily increasing with keywords such as 'hydrogen' and 'visible' becoming prominent research topics. There are likely two primary drivers behind this, the first of which is that biomass represents a more sustainable photocatalytic feedstock for reforming to value-added products and energy. The second is the transition towards achieving net zero emission targets, which has increased focus on the development of technologies that could play a role in future energy systems. Therefore, this review provides a perspective on not only the current state of the research but also a future outlook on the potential roadmap for photocatalytic reforming of biomass. Producing energy via photocatalytic biomass reforming is very desirable due to the ambient operating conditions and potential to utilise renewable energy (e.g., solar) with a wide variety of biomass resources. As both interest and development within this field continues to grow, however, there are challenges being identified that are paramount to further advancement. In reviewing both the literature and trajectory of the field, research priorities can be identified and utilised to facilitate fundamental research alongside whole systems evaluation. Moreover, this would underpin the enhancement of photocatalytic technology with a view towards improving the technology readiness level and promoting engagement between academia and industry.

Fabrication of TiO2/NiO p-n Nanocomposite for Enhancement Dye Photodegradation under Solar Radiation

A p-n nanocomposite based on TiO₂ nanotubes (NTs) and NiO nanoparticles (NPs) was designed and optimized in this study to improve the photocatalytic performance of methylene blue (MB). The hydrothermal technique has been used to produce TiO₂/NiO nanocomposites with different NiO NPs weight ratios; 1TiO₂:1NiO, 1TiO₂:2NiO, and 1TiO₂:3NiO. The crystal phase, chemical composition, optical properties, and morphology of TiO₂/NiO were explored by various techniques. TiO₂ NTs have a monoclinic structure, while NiO NPs have a cubic structure, according to the structural study. The bandgap of TiO₂ NTs was reduced from 3.54 eV to 2.69 eV after controlling the NiO NPs weight ratio. The TiO₂/2NiO nanocomposite showed the best photodegradation efficiency. Within 45 min of solar light irradiation, the efficiency of MB dye degradation using TiO₂/2NiO hits 99.5% versus 73% using pure TiO₂ NTs. Furthermore, the catalytic photodegradation efficiency did not deteriorate significantly even after five reusability cycles, intimating the high stability of the TiO₂/2NiO nanocomposite. This suggests that the loading of NiO NPs into TiO₂ NTs lowers the recombination of photo-produced electron/hole pairs and enlarged solar spectral response range, which results in improved photocatalytic activity. The mechanism of charge transfer in the TiO₂/NiO and kinetic models were discussed for the photodegradation of MB.

NiO-TiO2 p-n Heterojunction for Solar Hydrogen Generation

Photocatalytic water splitting for hydrogen production has been widely recognized as a promising strategy for relieving the pressure from energy crisis and environmental pollution. However, current efficiency for photocatalytic hydrogen generation has been limited due to a low separation of photogenerated electrons and holes. p-n heterojunction with a built-in electric field emerges as an efficient strategy for photocatalyst design to boost hydrogen evolution activities due to a spontaneous charge separation. In this work, we investigated the effect of different preparation methods on photocatalytic hydrogen production over NiO-TiO2 composites. The results demonstrated that a uniform distribution of NiO on a surface of TiO2 with an intimate interfacial interaction was formed by a sol-gel method, while direct calcination tended to form aggregation of NiO, thus leading to an uneven p-n heterojunction structure within a photocatalyst. NiO-TiO2 composites fabricated by different methods showed enhanced hydrogen production (23.5 ± 1.2, 20.4 ± 1.0 and 8.8 ± 0.7 mmolh−1g−1 for S1-20%, S2-20% and S3-10%, respectively) as compared with pristine TiO2 (6.6 ± 0.7 mmolh−1g−1) and NiO (2.1 ± 0.2 mmolh−1g−1). The current work demonstrates a good example to improve photocatalytic hydrogen production by finely designing p-n heterojunction photocatalysts.

Ultrasensitive ppb-level trimethylamine gas sensor based on p-n heterojunction of Co3O4/WO3

Trace poisonous and harmful gases in the air have been harming and affecting people's health for a long time. At present, effective and accurate detection of ppb-level harmful gas is still a bottleneck to be overcome. Herein, we report a ppb-level triethylamine (TEA) gas sensor based on p-n heterojunction of Co₃O₄/WO₃, which is prepared with ZIF-67 as the precursor and provides Co₃O₄deposited tungsten oxide flower-like structure. Due to the introduction of Co₃O₄and the 3D flower-like structure of WO₃, the Co₃O₄/WO₃-2 gas sensor shows excellent gas sensing performance (1101 for 10 ppm at 240 °C), superb selectivity, good long-term stability and linear response for TEA concentration. Moreover, the experimental results indicate that the Co₃O₄/WO₃-2 gas sensor also possesses a good response to 50 ppb TEA, in fact, the theoretical limit of detection is 0.6 ppb. Co₃O₄not only improves the efficiency of electron separation/transport, but also accelerates the oxidation rate of TEA. This method of synthesizing p-n heterojunction with ZIF as the precursor provides a new idea and method for the preparation of low detection limit gas sensors.

Nickel clusters accelerating hierarchical zinc indium sulfide nanoflowers for unprecedented visible-light hydrogen production

As a typical two-dimensional (2D) metal chalcogenides and visible-light responsive semiconductor, zinc indium sulfide (ZnIn₂S₄) has attracted much attention in photocatalysis. However, the high recombination rate of photogenerated electrons and holes seriously limits its performance for hydrogen production. In this work, we report in-situ photodeposition of Ni clusters in hierarchical ZnIn₂S₄ nanoflowers (Ni/ZnIn₂S₄) to achieve unprecedented photocatalytic hydrogen production. The Ni clusters not only provide plenty of active sites for reactions as evidenced by in-situ photoluminescence measurement, but also effectively accelerate the separation and migration of the photogenerated electrons and holes in ZnIn₂S₄. Consequently, the Ni/ZnIn₂S₄ composites exhibit good stability and reusability with highly enhanced visible-light hydrogen production. In particular, the best Ni/ZnIn₂S₄ photocatalyst exhibits an unprecedented hydrogen production rate of 22.2 mmol·h^-1·g^-1, 10.6 times that of the pure ZnIn₂S₄ (2.1 mmol·h^-1·g^-1). And its apparent quantum yield (AQY) is as high as 56.14% under 450 nm monochromatic light. Our work here suggests that depositing non-precious Ni clusters in ZnIn₂S₄ is quite promising for the potential practical photocatalysis in solar energy conversion.

Visible Light-Driven Reforming of Lignocellulose into H2 by Intrinsic Monolayer Carbon Nitride

The photoreforming of lignocellulose is a novel method to produce clean and sustainable H₂ energy. However, the catalytic systems usually show low activity under ultraviolet light; thus, this reaction is very limited at present. Visible light-responsive metal-free two-dimensional graphite-phased carbon nitride (g-C₃N₄) is a good candidate for photocatalytic hydrogen production, but its activity is hindered by a bulky architecture. Although reported layered g-C₃N₄ modified with active functional groups prepared by the chemical exfoliation enhances the photocatalytic activity, it lost the intrinsic structure and thus is not conducive to understand the structure-activity relationship. Herein, we report an intrinsic monolayer g-C₃N₄ (∼0.32 nm thickness) prepared by nitrogen-protected ball milling in water, which shows good performance of photoreforming lignocellulose to H₂ driven by visible light. The exciton binding energy of g-C₃N₄ was estimated from the temperature-dependent photoluminescence spectra, which is a key factor for subsequent charge separation and energy transfer. It is found that monolayer g-C₃N₄ with smaller exciton binding energy increases the free exciton concentrations and promotes the separation efficiency of charge carriers, thereby effectively improving its performance of photocatalytic reforming of lignocellulose, even the virgin lignocellulose and waste lignocellulose. This result could lead to more active catalysts to photoreform the raw biomass, making it possible to provide clean energy directly from locally unused biomass.

Size effect of bifunctional gold in hierarchical titanium oxide-gold-cadmium sulfide with slow photon effect for unprecedented visible-light hydrogen production

Gold nanoparticles (Au NPs) with surface plasmonic resonance (SPR) effect and excellent internal electron transfer ability have widely been combined with semiconductors for photocatalysis. However, the in-depth effects of Au NPs in multicomponent photocatalysts have not been completely understood. Herein, ternary titanium oxide-gold-cadmium sulfide (TiO₂-Au-CdS, TAC) photocatalysts, based on hierarchical TiO₂ inverse opal photonic crystal structure with different Au NPs sizes have been designed to reveal the SPR effect and internal electron transfer of Au NPs in the presence of slow photon effect. It appears that the SPR effect and internal electron transfer ability of Au NPs, depending on their sizes, play a synergistic effect on the photocatalytic enhancement. The ternary TAC-10 photocatalyst with ~ 10 nm Au NPs demonstrates an unprecedented hydrogen evolution rate of 47.6 mmolh^-1g^-1 under visible-light, demonstrating ~ 48% enhancement comparing to the sample without slow photon effect. In particular, a 9.83% apparent quantum yield under 450 nm monochromatic light is achieved for TAC-10. A model is proposed and finite-difference time-domain (FDTD) simulations reveal the size influence of Au NPs in ternary TAC photocatalysts. This work suggests that the rational design of bifunctional Au NPs coupling with slow photon effect could largely promote hydrogen production from visible-light driven water splitting.

ZIF-8 derived ZnO/TiO2 heterostructure with rich oxygen vacancies for promoting photoelectrochemical water splitting

Due to the serious recombination of electron-hole and weak photoresponse ability, achieving highly efficient photoelectrochemical (PEC) water splitting activity for TiO₂ photoelectrode has become a key issue. In this paper, we reported a new method for preparing ZnO/TiO₂ photoelectrodes by electrostatic adsorption from zeolitic imidazolate framework-8 (ZIF-8) as the precursor. ZIF-8 was combined with TiO₂ nanorods (NRs) through electrostatic interaction and then calcined to obtain ZnO/TiO₂ heterojunction photoelectrodes with abundant oxygen vacancies (O_vac). The introduced ZnO with O_vac provides a large number of active sites which enhanced the electrical conductivity and charges separation of ZnO/TiO₂ photoelectrode. The optimal photocurrent density of ZnO/TiO₂ photoelectrodes at 1.23 V versus (vs.) reversible hydrogen electrode (RHE) under illumination (100 mW/cm²) has reached 1.76 mA/cm², almost 2.75 times that of the pure TiO₂. Meanwhile, the incident photon-to-electron conversion efficiency (IPCE) of the best photoelectrode has increased to 58.2% at 390 nm, the charge injection (η_injection) and separation (η_separation) efficiency have reached to 93.53% and 51.62% (1.23 V vs. RHE), respectively.

Coproduction of hydrogen and lactic acid from glucose photocatalysis on band-engineered Zn1-xCdxS homojunction

Photocatalytic transformation of biomass into value-added chemicals coupled with co-production of hydrogen provides an explicit route to trap sunlight into the chemical bonds. Here, we demonstrate a rational design of Zn_1-xCd_xS solid solution homojunction photocatalyst with a pseudo-periodic cubic zinc blende (ZB) and hexagonal wurtzite (WZ) structure for efficient glucose conversion to simultaneously produce hydrogen and lactic acid. The optimized Zn_0.6Cd_0.4S catalyst consists of a twinning superlattice, has a tuned bandgap, and displays excellent efficiency with respect to hydrogen generation (690 ± 27.6 μmol·h⁻¹·g_cat.⁻¹), glucose conversion (~90%), and lactic acid selectivity (~87%) without any co-catalyst under visible light irradiation. The periodic WZ/ZB phase in twinning superlattice facilitates better charge separation, while superoxide radical (⋅O₂^-) and photogenerated holes drive the glucose transformation and water oxidation reactions, respectively. This work demonstrates that rational photocatalyst design could realize an efficient and concomitant production of hydrogen and value-added chemicals from glucose photocatalysis.

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Zn_1-xCd_xS ZB-WZ homojunction was designed to improve charge separation efficiency

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Bandgap engineering improved the hydrogen production from glucose photoreforming

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Optimized Zn_0.6Cd_0.4S ZB-WZ exhibited high lactic acid yield and selectivity

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Rational photocatalyst design realizes biomass valorization and H₂ coproduction

Zinc sulfide quantum dots/zinc oxide nanospheres/bismuth-enriched bismuth oxyiodides as Z-scheme/type-II tandem heterojunctions for an efficient charge separation and boost solar-driven photocatalytic performance

A novel zinc sulfide quantum dot (ZnS QD)/zinc oxide (ZnO) nanosphere/bismuth-enriched bismuth oxyiodide (Bi₄O₅I₂) tandem heterojunction photocatalyst is fabricated through two-step solvothermal, calcination and one-step hydrothermal strategies. The successfully constructed core-shell nanostructure can increase the interface area and the active sites of the composite photocatalysts. The formation of a Z-scheme/Type-II tandem heterojunction favors the transfer and spatial separation of charge carriers, in which Bi₄O₅I₂ plays a bridging role to connect ZnO and ZnS. Simultaneously, the participation of Bi₄O₅I₂ significantly shortens the band gap of the composite photocatalyst. This dual functional ZnO@Bi₄O₅I₂/ZnS composite photocatalyst has a high photocatalytic hydrogen evolution rate of 578.4 µmol g^-1h^-1 and an excellent photocatalytic degradation efficiency for bisphenol A (BPA) and 2,4,5-trichlorophenol (TCP). In addition, cycling tests show that ZnO@Bi₄O₅I₂/ZnS has a high stability, which is favorable for practical applications. This novel ZnO@Bi₄O₅I₂/ZnS Z-scheme/Type-II tandem heterojunction photocatalyst will provide new ideas for the multichannel charge transfer of other highly efficient heterojunction photocatalysts.

Composite photocatalysts based on Cd1−xZnxS and TiO2 for hydrogen production under visible light: effect of platinum co-catalyst location

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd_1−xZn_xS/TiO₂ were compared. In the first case, platinum was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25; in the second one, Cd_1−xZn_xS (x = 0.2–0.3) was deposited on the surface of Pt/TiO₂ P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/Pt (1 wt%)/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/TiO₂ ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ in the amount of 26 mmol g⁻¹ h⁻¹ (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.

The determination of the preferred location of platinum particles in TiO₂–Cd_1−xZn_xS systems was carried out for the first time.