Enhanced Photoelectrochemical Water-Splitting through Application of Layer by Layer (LBL) and Hydrothermal (HT) Methods in TiO2 and α-Fe2O3 Hierarchical Structures

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

Two methods, viz., the hydrothermal (HT) and co-precipitation (CP) methods, were used to prepare CeO2–NiO composite oxides; with them as the supports, Au/CeO2–NiO catalysts were prepared by the colloidal deposition method and used in the preferential oxidation (PROX) of CO in H2-rich stream. Various characterization measures such as N2 sorption, XRD, TEM, H2-TPR, Raman spectroscopy and XPS were used to clarify the influence of preparation method on the structure of CeO2–NiO support and the performance of Au/CeO2–NiO catalyst. The XPS and TEM results reveal that the CeO2–NiO(HT) support prepared by hydrothermal method displays a uniform rod-like shape and exposes preferentially the (110) and (100) planes of CeO2, whereas the CeO2–NiO(CP) support prepared by co-precipitation method is composed of nanorods and irregular nanoparticles dominated by (111) facets of CeO2. After deposition of gold, both the Au/CeO2–NiO(HT) and Au/CeO2–NiO(CP) catalysts are alike in the state and size distribution of deposited Au nanoparticles. The H2-TPR results indicate that the presence of Au strongly promotes the reduction of CeO2 in the Au/CeO2–NiO catalyst. Raman spectra illustrate that the incorporation of Ni ions into CeO2 remarkably increases the amount of oxygen vacancies in the CeO2–NiO supports, especially in CeO2–NiO(HT) prepared by hydrothermal method, which is beneficial to the dispersion and stabilization of gold species. The structure of CeO2–NiO support and catalytic activity of Au/CeO2–NiO in CO PROX is strongly related to the preparation method; Au/CeO2–NiO(HT) exhibits much higher activity than Au/CeO2–NiO(CP). The larger fraction of (110) and (100) CeO2 facets in CeO2–NiO(HT) can promote the dispersion of gold species, formation of oxygen vacancies and migration of oxygen species, which are effective to enhance the redox capacity and activity of the obtained Au/CeO2–NiO(HT) catalyst for CO PROX in H2-rich stream. Au supported on CeO2–NiO nanorods prepared by hydrothermal method exhibits much higher catalytic activity for CO PROX in H2-rich stream.

A review of recent modification strategies of TiO2-based photoanodes for efficient photoelectrochemical water splitting performance

Forward osmosis (FO) membrane has becomes a promising membrane technology due to its lower energy consumption and high salt rejection performance. However, in prolong application, FO membrane is susceptible to internal concentration polymerization (ICP) that reduce water flux through the membrane. Therefore, layer by layer (LbL) method was proposed to improve the FO performance. This study focused on the effect of fabrication method to cast thin film composite (TFC) membrane using SL method and LbL method. The membranes were prepared using interfacial polymerization technique and tested through salt rejection experiment. Theoretically, LbL method should provide high salt rejection value as it is good at controlling the structures and thickness compare to SL method. However, from the testing, the results showed that the salt rejection value of TFC membrane using SL technique is higher (97.17%) compared to LbL technique (73.33%). Thus, it is suggested, co-casting method should be used towards the TFC membrane for future studies due to its advantageous over SL and LbL method.

Photoelectrochemical (PEC) water splitting based on semiconductor photoelectrodes provides a promising platform for reducing environmental pollution and solving the energy crisis by developing clean, sustainable and environmentally friendly hydrogen energy. In this context, metal oxides with their advantages including low cost, good chemical stability and environmental friendliness, have attracted extensive attention among the investigated candidates. However, the large bandgap, poor charge transfer ability and high charge recombination rate limit the PEC performance of metal oxides as photoelectrodes. To solve this limitation, many approaches toward enhanced PEC water splitting performance, which focus on surface and interface engineering, have been presented. In this topical review, we concentrate on the heterostructure design of some typical metal oxides with narrow bandgaps (e.g. Fe2O3, WO3, BiVO4 and Cu2O) as photoelectrodes. An overview of the surface- and interface-engineered heterostructures, including semiconductor heterojunctions, surface protection, surface passivation and cocatalyst decoration, will be given to introduce the recent advances in metal oxide heterostructures for PEC water splitting. This article aims to provide fundamental references and principles for designing metal oxide heterostructures with high activity and stability as photoelectrodes for PEC solar hydrogen generation.

Thin film-capped metallic nanoparticles (TFCMNPs) hold big promise for rapid, low-cost, and portable tracing of gas analytes. We show that sensing properties can be controlled by the configuration of the TFCMNPs. To this end, two methods were developed: layer by layer (LbL) and drop-by-drop, i.e., drop casting (DC). The TFCMNP prepared via LbL method was homogeneous and gradually increased in thickness, absorbance, and conductivity relative to TFCMNP prepared via DC method. However, our results indicate that the sensing of TFCMNP devices prepared via DC is significantly higher than that of equivalent LbL devices. These discrepancies can be explained as follows: LbL forms a high dense layer of TFCMNPs without vacancies, and a well-controlled deposition of NPs. The distance between the adjacent NPs is controlled by the capped ligands and the linker molecules making a rigid TFCMNP. Thus, exposing LbL devices to analyte induces a marginal change in the NP–NP distance. However, in DC devices, the analyte induces major change in the NP distances and permittivity due to their lack of connection, making the sensing much more pronounced. The DC and LbL methods used thiol and amine ligands-capped metallic nanoparticles to demonstrate the applicability of the methods to all types of ligands. Our results are of practical importance for integrating TFCMNPs in chemiresistive sensing platforms and for (bio) and chemical sensing applications.

The layer-by-layer (LbL) polyelectrolyte self-assembly encapsulation method has attracted much interest because of its versatility to use various polymers for capsule formation, ability to encapsulate different templates, and capability to control capsule permeability. Traditionally, the LbL method was performed in water as solvent and limited to poorly or non-water-soluble templates. Using the matrix-assisted LbL method, complex mixtures of water-soluble proteins or DNA could be encapsulated within agarose microbeads templates but leakage of biomolecules into the water phase during the LbL process results in low encapsulation efficiency. Recently, the reverse-phase LbL (RP-LbL) method was introduced to perform LbL and encapsulation of water-soluble templates in organic solvents, thus preventing the templates from dissolving and allowing high encapsulation efficiency. However, encapsulation of complex mixtures of biomolecules or other substances with quantitative encapsulation efficiency remained impossible. Here we present a new approach for encapsulation of biomolecules or complex mixtures thereof with almost 100% encapsulation efficiency. The ability of our method to achieve high encapsulation efficiency arises from the combination of two strategies. (1) Using microparticles as surface stabilizer to create stable biomolecule-loaded hydrogel microbeads, termed matrix-assisted colloidosome (MAC), that are able to disperse in oil and organic solvents. (2) Using the RP-LbL method to fabricate polymeric capsule "membranes", thereby preventing diffusion of the highly water-soluble biomolecules. Using an oil phase during emulsification and an organic solvent phase during encapsulation could completely prevent leakage of water-soluble biomolecules and almost 100% encapsulation efficiency is achieved. Microcapsules fabricated with our method retained nearly 100% of encapsulated proteins during a 7 day incubation period in water. The method was demonstrated on model proteins and may be extended to other biomolecules or mixtures. Our method is a valuable addition to the family of encapsulation techniques and can significantly contribute to the fields of bioreactors and bioanalytical microcapsules.

It is a promising strategy that the photoelectrode with high charge transfer rate and separation efficiency is designed in the photoelectrochemical (PEC) water splitting. Herein, the nanoflakes/nanoflakes (2D/2D) MoS2/ZnIn2S4 heterojunction is synthesized by two‐step hydrothermal methods. The carrier transport path in heterojunction is optimized through the Schottky contact calculated by density functional theory (DFT) and large specific surface area between 2D nanoflakes MoS2 and ZnIn2S4. Besides, the bimetallic oxyhydroxide NiFeOOH can be used as co‐catalyst to improve the separation of carriers. The results exhibit that MoS2/ZnIn2S4/NiFeOOH photoelectrode with dramatically enhanced photocurrent density of 0.74 mA/cm2 at 1.23 V versus the reversible hydrogen electrode (VRHE), which is 2.6 and 5.7 times higher than the bare MoS2 and ZnIn2S4, respectively. And it has the largest charge separation efficiency (ηbulk) (22.8 %) and charge transfer efficiency (ηsurface) (64.6 %) in the as‐prepared samples. This work provides a strategy for design of the photoelectrode with excellent charge transfer efficiency and separation efficiency in PEC water splitting.

NiFe layered double-hydroxide nanoparticles for efficiently enhancing performance of BiVO4 photoanode in photoelectrochemical water splitting

Enhanced photoelectrochemical water splitting performance of α-Fe2O3 nanostructures modified with Sb2S3 and cobalt phosphate

Developing an efficient photocathode system using abundant earth materials is crucial for the effective process of Photoelectrochemical (PEC) water splitting. Achieving successful charge transfer between heterojunctions is a key consideration in creating an innovative photocathode that considers cost-effectiveness, material abundance, and PEC performance. In our study, we synthesized a p-type narrow band gap photocathode using copper oxide (CuO) with an energy gap (Eg) of 1.5 eV through a hydrothermal method. We then enhanced its performance by decorating it with antimony sulfide (Sb2S3) nanospheres (NSs) using a simple chemical bath deposition (CBD) technique, resulting in the formation of a CuO/Sb2S3 NSs heterojunction.The fabricated heterojunction demonstrated superior PEC performance compared to the bare CuO. Specifically, we observed a significant increase in photocurrent density for the CuO/Sb2S3 NSs photocathode (J = -1 mA.cm-2) compared to CuO alone (J = -0.3 mA.cm-2) at a standard potential of 0 V vs RHE in a 0.5 M Na2SO4 solution with a pH of 6.85. This improvement can be attributed to the enhanced generation and separation of charge carriers. Furthermore, the CuO/Sb2S3 heterojunction exhibited remarkable photostability over a period of 2.5 h, with no degradation in photocurrent density. The Sb2S3 acted as a sensitizer, reducing the recombination rate of electron-hole pairs (e-/h+) within the CuO/Sb2S3 NSs, thereby enhancing the overall performance. The CuO/Sb2S3 NSs photocathode exhibited an ABPE% of 0.1%, while the ABPE% for CuO alone was 0.03%. This indicates a significant improvement in the performance of the CuO/Sb2S3 NSs photocathode. Onset potential (Eonset) was also observed to shift in a positive direction in CuO/Sb2S3 NSs as compared to CuO only. Eonset for the films followed a trend having CuO/Sb2S3 NSs (0.85 VRHE) > CuO (0.78 VRHE). Notably, this improvement is attributed to increased light absorption and a reduced recombination rate compared to the pristine CuO photocathode. Moreover, electrochemical impedance spectroscopy (EIS) studies provided evidence of enhanced electron transfer rates occurring at the interface between the heterojunction and the electrode/electrolyte interface. UV-Visible and photoluminescence (PL) emission spectra results indicated that the CuO/Sb2S3 NSs heterojunction exhibited an extended absorption spectrum and a reduced rate of recombination, further supporting the enhanced performance. Additionally, electrochemical impedance spectroscopy studies revealed lower charge transfer resistance in the CuO/Sb2S3 NSs heterojunction compared to CuO alone.These findings open up new avenues for the development of novel photocathodic materials and heterojunctions utilizing copper-based binary oxides/chalcogenides for efficient solar harvesting applications.References(1) Kumar, M.; Singh, A.; Meena, B.; Sahu, P. K.; Subrahmanyam, C. Decoration of Spherical Sb2S3 over CuO Nanoflakes for Efficient Photoelectrochemical Hydrogen Generation. Results Eng. 2023, 101513. https://doi.org/10.1016/j.rineng.2023.101513.(2) Kumar, M.; Meena, B.; Subramanyam, P.; Suryakala, D.; Subrahmanyam, C. Emerging Copper-Based Semiconducting Materials for Photocathodic Applications in Solar Driven Water Splitting. Catalysts 2022, 12 (10), 1198. https://doi.org/10.3390/catal12101198.(3) Kumar, M.; Meena, B.; Subramanyam, P.; Suryakala, D.; Subrahmanyam, C. Recent Trends in Photoelectrochemical Water Splitting: The Role of Cocatalysts. NPG Asia Mater. 2022, 14 (1), 88. https://doi.org/10.1038/s41427-022-00436-x. Figure 1

Titanium dioxide is one of the most investigated materials for photoelectrochemical (PEC) water splitting. However, TiO2 has a wide band gap of approximately 3.2 eV, which limits its absorption energy to UV only, and the photoexcitation products (i.e., electron and hole) easily recombine. Developing a composite with other semiconductor materials is one of the efforts for improving the performance of TiO2 photoelectrode. In this study, a composite of TiO2/Co3O4 thin films was developed, and its performance in PEC water splitting was investigated. Co3O4 was synthesized using a hydrothermal method, and TiO2 P25 was used. TiO2 and Co3O4 were mixed by ball milling before coating on a clean FTO and annealing at 550 °C. Two weight variations of TiO2:Co3O4 were used (95:5 and 90%:10%). XRD and FESEM analysis were used to investigate crystallinity phase and surface morphology. The optical band gap of the thin films was determined by UV-Vis spectroscopy, where the film with 90:10 weight ratio of TiO2:Co3O4 obtained the smallest band gap of 2.9 eV. Co3O4 improved the current density and TiO2 performance by approximately three times.

Photoelectrochemical (PEC) water splitting is the potent technology to solve the global issues of energy crisis and environmental pollution. It converts solar energy to chemical energy stored in the form of hydrogen. In practical application, no single material can meet all the requirements for PEC water splitting. Two-dimensional (2D) nanostructures have been developed as potential materials for application in photoelectrochemical (PEC) water splitting due to their unique structure and fascinating properties. However, the light harvesting ability of bulk or nanosized materials are always limited due to the low light transmittance and high reflection at the grain boundaries. Also, the photo generated charge carriers inside the semiconductor with atomistic thickness will reach the surface faster relative to bulk materials due to the shortened transport distance. SnS2 is a two-dimensional (2D) transition metal dichalcogenide with a layered cadmium iodide (CdI2) structure. It has a narrow band gap of 2.18-2.44 eV. It is inexpensive, nontoxic and chemically stable in acidic and neutral solutions. A metal free semiconductor, known as graphitic carbon nitride (g-C3N4) also possesses a layered 2D structure and has been reported to have potential applications in photocatalysis. It exhibits various interesting properties, including visible-light absorption with a band gap of 2.7 eV. However, g-C3N4 suffers from low electrical conductivity which can be enhanced by the use of reduced graphene oxide (RGO). RGO has been extensively used by combining with various semiconductors to develop efficient photocatalysts. In the present work, ternary composites consisting of 2D nanomaterials of SnS2, reduced graphene oxide (RGO), and mesoporous graphitic carbon nitride (mpg-C3N4) were synthesized by using hydrothermal method. The presence of SnS2, mpg-C3N4 and RGO was confirmed by XRD, UV-Vis spectroscopy, and TEM. The single SnS2 nanosheet acquires a hexagonal structure with a lateral size of 20-50 nm. Generally, these SnS2 nanosheets tend to agglomerate which is prevented by RGO nanosheets. Due to the intimate face-to-face interactions offered by the 2D structured constituents, the obtained composite showed excellent performance as photoanode. The ternary composite showed photocurrent density of 1.4 mA/cm 2 at 1.2 V versus RHE, which is over 28 times higher than that of the pure SnS2 material and almost 5 times more than C3N4-SnS2 composite. The superior photocatalytic activity of the mpg-C3N4/SnS2/RGO composite is attributed to enhanced separation of the photogenerated electron-hole pairs, as well as increased visible-light absorption. [1] J. Di, J. Xia, H. Li, Z. Liu, Freestanding atomically-thin two-dimensional materials beyond graphene meeting photocatalysis: Opportunities and challenges, Nano Energy. 35 (2017) 79-91. [2] Y. Sun, H. Cheng, S. Gao, Z. Sun, Q. Liu, Q. Leu, F. Lei, T. Yao, J. He, S. Wei, Y. Xie, Freestanding tin disulfide single-layers realizing efficient visible-light water splitting, Angew. Chemie - Int. Ed. 51 (2012) 8727-8731. [3] Z. Zhang, J. Huang, M. Zhang, Q. Yuan, B. Dong, Ultrathin hexagonal SnS2 nanosheets coupled with g-C3N4 nanosheets as 2D/2D heterojunction photocatalysts toward high photocatalytic activity, Appl. Catal. B Environ. 163 (2015) 298-305. [4] M.S.A. Sher Shah, A.R. Park, A. Rauf, S.H. Hong, Y. Choi, J. Park, J. Kim, W.-J. Kim, P.J. Yoo, Highly interdigitated and porous architected ternary composite of SnS2 , g-C3N4 , and reduced graphene oxide (rGO) as high performance lithium ion battery anodes, RSC Adv. 7 (2017) 3125-3135. Figure 1

Enhancing photocatalytic water splitting: Comparative study of TiO2 decorated nanocrystals (Pt and Cu) using different synthesis methods

Nickel incorporated graphitic carbon nitride supported copper sulfide for efficient noble-metal-free photo-electrochemical water splitting