Spatial decoupling of light absorption and reaction sites in n-Si photocathodes for solar water splitting

Template-Guided Silicon Micromotor Assembly for Enhanced Cell Manipulation

Light-driven micro/nanorobots (LMNRs) are tiny, untethered machines with great potential in fields like precision medicine, nano manufacturing, and various other domains. However, their practicality hinges on developing light-manipulation strategies that combine versatile functionalities, flexible design options, and precise controllability. Our study introduces an innovative approach to construct micro/nanorobots (MNRs) by utilizing micro/nanomotors as fundamental building blocks. Inspired by silicon Metal-Insulator-Semiconductor (MIS) solar cell principles, we design a new type of optomagnetic hybrid micromotors (OHMs). These OHMs have been skillfully optimized with integrated magnetic constituent, resulting in efficient light propulsion, precise magnetic navigation, and the potential for controlled assembly. One of the key features of the OHMs is their ability to exhibit diverse motion modes influenced by fracture surfaces and interactions with the environment, streamlining cargo conveyance along "micro expressway"-the predesigned microchannels. Further enhancing their versatility, a template-guided assembly strategy facilitates the assembly of these micromotors into functional microrobots, encompassing various configurations such as "V-shaped", "N-shaped", and 3D structured microrobots. The heightened capabilities of these microrobots, underscore the innovative potential inherent in hybrid micromotor design and assembly, which provides a foundational platform for the realization of multi-component microrobots. Our work moves a step toward forthcoming microrobotic entities boasting advanced functionalities.

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

A silicon photoanode protected with TiO2/stainless steel bilayer stack for solar seawater splitting

Photoelectrochemical seawater splitting is a promising route for direct utilization of solar energy and abundant seawater resources for H2 production. However, the complex salinity composition in seawater results in intractable challenges for photoelectrodes. This paper describes the fabrication of a bilayer stack consisting of stainless steel and TiO2 as a cocatalyst and protective layer for Si photoanode. The chromium-incorporated NiFe (oxy)hydroxide converted from stainless steel film serves as a protective cocatalyst for efficient oxygen evolution and retarding the adsorption of corrosive ions from seawater, while the TiO2 is capable of avoiding the plasma damage of the surface layer of Si photoanode during the sputtering of stainless steel catalysts. By implementing this approach, the TiO2 layer effectively shields the vulnerable semiconductor photoelectrode from the harsh plasma sputtering conditions in stainless steel coating, preventing surface damages. Finally, the Si photoanode with the bilayer stack inhibits the adsorption of chloride and realizes 167 h stability in chloride-containing alkaline electrolytes. Furthermore, this photoanode also demonstrates stable performance under alkaline natural seawater for over 50 h with an applied bias photon-to-current efficiency of 2.62%.

Magnetically Manipulated Optoelectronic Hybrid Microrobots for Optically Targeted Non-Genetic Neuromodulation

Optically controlled neuromodulation is a promising approach for basic research of neural circuits and the clinical treatment of neurological diseases. However, developing a non-invasive and well-controllable system to deliver accurate and effective neural stimulation is challenging. Micro/nanorobots have shown great potential in various biomedical applications because of their precise controllability. Here, a magnetically-manipulated optoelectronic hybrid microrobot (MOHR) is presented for optically targeted non-genetic neuromodulation. By integrating the magnetic component into the metal-insulator-semiconductor junction design, the MOHR has excellent magnetic controllability and optoelectronic properties. The MOHR displays a variety of magnetic manipulation modes that enables precise and efficient navigation in different biofluids. Furthermore, the MOHR could achieve precision neuromodulation at the single-cell level because of its accurate targeting ability. This neuromodulation is achieved by the MOHR's photoelectric response to visible light irradiation, which enhances the excitability of the targeted cells. Finally, it is shown that the well-controllable MOHRs effectively restore neuronal activity in neurons damaged by β-amyloid, a pathogenic agent of Alzheimer's disease. By coupling precise controllability with efficient optoelectronic properties, the hybrid microrobot system is a promising strategy for targeted on-demand optical neuromodulation.

Highly selective photoelectrochemical CO2 reduction by crystal phase-modulated nanocrystals without parasitic absorption

Photoelectrochemical (PEC) carbon dioxide (CO2) reduction (CO2R) holds the potential to reduce the costs of solar fuel production by integrating CO2 utilization and light harvesting within one integrated device. However, the CO2R selectivity on the photocathode is limited by the lack of catalytic active sites and competition with the hydrogen evolution reaction. On the other hand, serious parasitic light absorption occurs on the front-side-illuminated photocathode due to the poor light transmittance of CO2R cocatalyst films, resulting in extremely low photocurrent density at the CO2R equilibrium potential. This paper describes the design and fabrication of a photocathode consisting of crystal phase-modulated Ag nanocrystal cocatalysts integrated on illumination-reaction decoupled heterojunction silicon (Si) substrate for the selective and efficient conversion of CO2. Ag nanocrystals containing unconventional hexagonal close-packed phases accelerate the charge transfer process in CO2R reaction, exhibiting excellent catalytic performance. Heterojunction Si substrate decouples light absorption from the CO2R catalyst layer, preventing the parasitic light absorption. The obtained photocathode exhibits a carbon monoxide (CO) Faradaic efficiency (FE) higher than 90% in a wide potential range, with the maximum FE reaching up to 97.4% at -0.2 V vs. reversible hydrogen electrode. At the CO2/CO equilibrium potential, a CO partial photocurrent density of -2.7 mA cm-2 with a CO FE of 96.5% is achieved in 0.1 M KHCO3 electrolyte on this photocathode, surpassing the expensive benchmark Au-based PEC CO2R system.

Electrochemical carbon-carbon coupling with enhanced activity and racemate stereoselectivity by microenvironment regulation

Enzymes are characteristic of catalytic efficiency and specificity by maneuvering multiple components in concert at a confined nanoscale space. However, achieving such a configuration in artificial catalysts remains challenging. Herein, we report a microenvironment regulation strategy by modifying carbon paper with hexadecyltrimethylammonium cations, delivering electrochemical carbon-carbon coupling of benzaldehyde with enhanced activity and racemate stereoselectivity. The modified electrode-electrolyte interface creates an optimal microenvironment for electrocatalysis-it engenders dipolar interaction with the reaction intermediate, giving a 2.2-fold higher reaction rate (from 0.13 to 0.28 mmol h-1 cm-2); Moreover, it repels interfacial water and modulates the conformational specificity of reaction intermediate by facilitating intermolecular hydrogen bonding, affording 2.5-fold higher diastereomeric ratio of racemate to mesomer (from 0.73 to 1.82). We expect that the microenvironment regulation strategy will lead to the advanced design of electrode-electrolyte interface for enhanced activity and (stereo)selectivity that mimics enzymes.

Tandem cells for unbiased photoelectrochemical water splitting

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

Stability of Photocathodes: A Review on Principles, Design, and Strategies

Photoelectrochemical devices based on semiconductor photoelectrode can directly convert and store solar energy into chemical fuels. Although the efficient photoelectrodes with commercially valuable solar-to-fuel energy conversion efficiency have been reported over past decades, one of the most enormous challenges is the stability of the photoelectrode due to corrosion during operation. Thus, it is of paramount importance for developing a stable photoelectrode to deploy solar-fuel production. This Review commences with a fundamental understanding of thermodynamics for photoelectrochemical reactions and the fundamentals of photocathodes. Then, the commercial application of photoelectrochemical technology is prospected. We specifically focus on recent strategies for designing photocathodes with long-term stability, including energy band alignment, hole transport/storage/blocking layer, spatial decoupling, grafting molecular catalysts, protective/passivation layer, surface element reconstruction, and solvent effects. Based on the insights gained from these effective strategies, we propose an outlook of key aspects that address the challenges for development of stable photoelectrodes in future work.

An integrated n-Si/BiVO4 photoelectrode with an interfacial bi-layer for unassisted solar water splitting

Integrated n-Si/BiVO4 is one of the most promising candidates for unbiased photoelectrochemical water splitting. However, a direct connection between n-Si and BiVO4 will not attain overall water splitting due to the small band offset as well as the interfacial defects at the n-Si/BiVO4 interface that severely impede carrier separation and transport, limiting the photovoltage generation. This paper describes the design and fabrication of an integrated n-Si/BiVO4 device with enhanced photovoltage extracted from the interfacial bi-layer for unassisted water splitting. An Al2O3/indium tin oxide (ITO) interfacial bi-layer was inserted at the n-Si/BiVO4 interface, which promotes the interfacial carrier transport by enlarging the band offset while healing interfacial defects. When coupled to a separate cathode for hydrogen evolution, spontaneous water splitting could be realized with this n-Si/Al2O3/ITO/BiVO4 tandem anode, with an average solar-to-hydrogen (STH) efficiency of 0.62% for over 1000 hours.

Scaling-Up of Thin-Film Photoelectrodes for Solar Water Splitting Based on Atomic Layer Deposition

Atomic layer deposition (ALD) is an established method to prepare protective layers for Si-based photoelectrodes for photoelectrochemical (PEC) water splitting. Although ALD has been widely used in microelectronics and photovoltaics, it remains a great challenge to design simple and effective ALD systems to deposit large and uniform protective films for Si-based photoelectrodes with industrial sizes. This paper describes the design and realization of a simple ALD chamber configuration for photoelectrodes with large sizes, in which the influence of a gas redistributor over the gas flow and heat transfer during film growth was revealed by computational fluid dynamics simulations and experimental investigations. A simple circular baffle-type redistributor was proposed to establish a uniform gas flow field throughout the ALD reactor, resulting in a uniform temperature profile. With this simple baffle redistributor, the large-area Al₂O₃ monitor film (46 nm thickness) reached a good nonuniformity (Nu %) of 0.88% over a large area of 256 cm². This design enables the fabrication of large-scale photocathodes from standard industrial-grade 166 mm Si(100) wafers (276 cm²) by depositing 50 nm TiO₂ protective films with Nu % less than 5%. The obtained photocathode achieves a saturation current of 6.45 A with a hydrogen production rate of 43.2 mL/min under outdoor illumination. This work elucidates how flow pattern and heat transfer may influence the deposition of protective layers over large photoelectrodes, providing guidance for future industrial applications of PEC water splitting.

Photovoltage memory effect in a portable Faradaic junction solar rechargeable device

Two-electrode solar rechargeable device is one of the promising technologies to address the problem of solar energy storage in large scale. However, the mechanism of dark output voltage remains unclear and the low volumetric energy density also limits its practical applications. Herein, we report that a Si/CoO_x/KBi_(aq)/MnO_x Faradaic junction device exhibits a photovoltage memory effect, that is, the dark output voltage can precisely record the value of the photovoltage in the device. To investigate the mechanism of the effect, we develop an open circuit potential method to real-time monitor the photo charge and dark discharge processes in the Faradaic junction device. This effect leads to minimized interface energy loss in the Faradaic junction device, which achieves much higher performances than the devices without the effect. Moreover, we realize a portable device with a record value of the dark volumetric energy density (∼1.89 mJ cm⁻³) among all reported two-electrode solar rechargeable devices. These results offer guidance to improve the performance of a solar rechargeable device and design other photoelectric devices for new applications.

A Faradaic junction solar rechargeable device is one of the promising technologies to address the problem of solar energy storage but the working mechanism remains unclear. Here, the authors report a photovoltage memory effect in a portable Si/CoO_x/KBi_(aq)/MnO_x Faradaic junction device.