Stabilizing Silicon Photocathodes by Solution-Deposited Ni–Fe Layered Double Hydroxide for Efficient Hydrogen Evolution in Alkaline Media

An important pathway toward cost-effective photoelectrochemical (PEC) solar water-splitting devices is to stabilize and catalyze silicon (Si) photocathodes for hydrogen evolution reaction (HER), especially in alkaline solutions. To date, the most stable Si photocathode in alkaline media is protected by the atomic layer deposited (ALD) dense TiO₂ layer and catalyzed by noble metal-based catalysts on top. However, the ALD process is difficult to scale up, and the noble metals are expensive. Herein, we report the first demonstration of using a scalable hydrothermal method to deposit earth-abundant NiFe layered double hydroxide (LDH) to simultaneously protect and catalyze Si photocathodes in alkaline solutions. The NiFe LDH-protected/catalyzed p-type Si photocathode shows a current density of 7 mA/cm² at 0 V vs RHE, an onset potential of ∼0.3 V vs RHE that is comparable to that of the reported p–n⁺ Si photocathodes, and durability of 24 h at 10 mA/cm² in 1 M KOH electrolyte

Conformal Electroless Nickel Plating on Silicon Wafers, Convex and Concave Pyramids, and Ultralong Nanowires

Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro- and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory and mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate, and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides a more conformal Ni coating on complex and high-aspect-ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications

One-Step Hydrothermal Deposition of Ni:FeOOH onto Photoanodes for Enhanced Water Oxidation

The realization of efficient photoelectrochemical (PEC) water splitting requires effective integration of earth-abundant active oxygen evolution catalysts (OECs) with diverse photoanodes. Although many good OECs have been investigated on conductive substrates under dark conditions, further studies are needed to evaluate their performance when integrated with photoanodes under illumination. Such studies will be facilitated by developing effective coating methods of OECs onto diverse photoanodes. Here, we report a one-step hydrothermal process that conformally coats various photoanodes with ultrathin Ni-doped FeOOH (Ni:FeOOH) OECs. The coated Ni:FeOOH, due to its unique open tunnel structure, tunable Ni doping concentration, and high coating/interface quality, lowers the onset potential of all of the photoanodes investigated, including WO₃/BiVO₄, WO₃, α-Fe₂O₃, TiO₂ nanowires, BiVO₄ films, and Si wafers. We believe that this simple and yet effective hydrothermal method is a useful addition to the existing deposition techniques for coupling OECs with photoanodes and will greatly facilitate the scale-up of efficient PEC devices

High-Performance Ultrathin BiVO₄ Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting

Photoelectrochemical (PEC) water splitting devices rely on light-absorbers to absorb sunlight, and the photogenerated electrons and holes further react with water to generate hydrogen and oxygen. Fabricating light-absorbers on textured substrates offers alternative routes for optimizing their PEC performance. Textured substrates would greatly enhance both light absorption and surface reactions of photoanodes and thus reduce the total amount of light-absorbers needed. Herein, we report the fabrication of ultrathin BiVO₄ photoanode film on textured polydimethylsiloxane (PDMS) substrates by using a modified water-assisted transfer printing method. Significantly, a pristine BiVO₄ photoanode of only 80 nm thick shows a photocurrent density of 1.37 mA/cm² at 1.23 V_RHE on patterned PDMS substrates, which is further increased to ∼2.0 mA/cm² at 1.23 V_RHE when FeOOH oxygen evolution catalyst is added. We believe that our transfer printing method can be broadly applied to integrate photoelectrodes and other thin-film optoelectronic devices (e.g., solar cells and electronics) onto diverse textured substrates to enhance their performance