Enabling Silicon for Solar-Fuel Production

Copper Tantalate by a Sodium-Driven Flux-Mediated Synthesis for Photoelectrochemical CO2 Reduction

Copper-tantalate, Cu2Ta4O11 (CTO), shows significant promise as an efficient photocathode for multi-carbon compounds (C2+) production through photoelectrochemical (PEC) CO2 reduction, owing to its suitable energy bands and catalytic surface. However, synthesizing CTO poses a significant challenge due to its metastable nature and thermal instability. In this study, this challenge is addressed by employing a flux-mediated synthesis technique using a sodium-based flux to create sodium-doped CTO (Na-CTO) thin films, providing enhanced nucleation and stabilization for the CTO phase. To evaluate the PEC performance and catalytic properties of the films, copper(II) oxide (CuO) at the Na-CTO surface is selectively etched. The etched Na-CTO shows a lower dark current, with decreased contribution from photocorrosion, unlike the non-etched Na-CTO which has remaining CuO on the surface. Furthermore, Na-CTO exhibits 7.3-fold ethylene selectivity over hydrogen, thus highlighting its promising potential as a photocathode for C2+ production through PEC CO2 reduction.

Interface engineering for photoelectrochemical oxygen evolution reaction

Photoelectrochemical (PEC) water splitting provides a promising approach for solving sustainable energy challenges and achieving carbon neutrality goals. The oxygen evolution reaction (OER), a key bottleneck in the PEC water-splitting system occurring at the photoanode/electrolyte interface, plays a fundamental role in sustainable solar fuel production. Proper surface or interface engineering strategies have been proven to be necessary to achieve efficient and stable PEC water oxidation. This review summarizes the recent advances in interface engineering, including junction formation, surface doping, surface passivation or protection, surface sensitization, and OER cocatalyst modification, while highlighting the remarkable research achievements in the field of PEC water splitting. The benefits of each interface engineering strategy and how it enhances the device performance are critically analyzed and compared. Finally, the outlook for the development of interface engineering for efficient PEC water splitting is briefly discussed. This review illustrates the importance of employing rational interface engineering in realizing efficient and stable PEC water splitting devices.

Photoinduced Electrochemiluminescence Immunoassays

Optimization of electrochemiluminescence (ECL) immunoassays is highly beneficial for enhancing clinical diagnostics. A major challenge is the improvement of the operation conditions required for the bead-based immunoassays using the typical [Ru(bpy)3]2+/tri-n-propylamine (TPrA) system. In this study, we report a heterogeneous immunoassay based on near-infrared photoinduced ECL, which facilitates the imaging and quantitative analysis of [Ru(bpy)3]2+-modified immunobeads at low anodic potential. The photovoltage generated by the photoanode under near-infrared light promotes oxidation processes at the electrode/electrolyte interface, thus considerably lowering the onset potential for both TPrA oxidation and ECL emission. The anti-Stokes shift between the excitation light (invisible to the human eyes) and the visible emitted light results in a clear and stable signal from the immunobeads. In addition, it offers the possibility of site-selective photoexcitation of the ECL process. This approach not only meets the performance of traditional ECL immunoassays in accuracy but also offers the additional benefits of lower potential requirements and enhanced stability, providing a new perspective for the optimization of commercial immunoassays.

Photoelectrochemical Lithium Extraction from Waste Batteries

The amount of global hybrid-electric and all electric vehicle has increased dramatically in just five years and reached an all-time high of over 10 million units in 2022. A good deal of waste lithium (Li)-containing batteries from dead vehicles are invaluable unconventional resources with high usage of Li. However, the recycle of Li by green approaches is extremely inefficient and rare from waste batteries, giving rise to severe environmental pollutions and huge squandering of resources. Thus, in this mini review, we briefly summarized a green and promising route-photoelectrochemical (PEC) technology for extracting the Li from the waste lithium-containing batteries. This review first focuses on the critical factors of PEC performance, including light harvesting, charge-carrier dynamics, and surface chemical reactions. Subsequently, the conventional and PEC technologies applying in the area of Li recovery processes are analyzed and discussed in depth, and the potential challenges and future perspective for rational and healthy development of PEC Li extraction are provided positively.

Light-Induced Quantum Reconfiguration of Oxyhydroxides for Photoanodes with 4.24% Efficiency and Stability Beyond 250 Hours

Photoelectrochemical (PEC) water splitting is attracting significant research interest in addressing sustainable development goals in renewable energy. Current state-of-the-art, however, cannot provide photoanodes with simultaneously high efficiency and long-lasting lifetime. Here, large-scale NiFe oxyhydroxides-alloy hybridized co-catalyst layer that exhibits an applied bias photon-to-current efficiency (ABPE) of 4.24% in buried homojunction-free photoanodes and stability over 250 h is reported. These performances represent an increase over the present highest-performing technology by 408% in stability and the most stable competitor by over 330% in efficiency. These results originate from a previously unexplored mechanism of light-induced atomic reconfiguration, which rapidly self-generates a catalytic-protective amorphous/crystalline heterostructure at low biases. This mechanism provides active sites for reaction and insulates the photoanode from performance degradation. Photon-generated NiFe oxyhydroxides are more than 200% higher than the quantity that pure electrocatalysis would otherwise induce, overcoming the threshold for an efficient water oxidation reaction in the device. While of immediate interest in the industry of water splitting, the light-induced NiFe oxyhydroxides-alloy co-catalyst developed in this work provides a general strategy to enhance further the performances and stability of PEC devices for a vast panorama of chemical reactions, ranging from biomass valorization to organic waste degradation, and CO2-to-fuel conversion.

Light Conversion by Electrochemiluminescence at Semiconductor Surfaces

ConspectusElectrochemiluminescence (ECL) is the electrochemical generation of light. It involves an interfacial charge transfer that produces the excited state of a luminophore at the electrode surface. ECL is a powerful readout method that is widely employed for immunoassays and clinical diagnostics and is progressively evolving into a microscopy technique. On the other hand, photoelectrochemistry at illuminated semiconductors is a field of research that deals with the transfer of photogenerated charge carriers at the solid-liquid interface. This concept offers several advantages such as a considerable lowering of the onset potential required for triggering an electrochemical reaction as well as light addressable chemistry, via the spatial confinement of redox reactions at locally illuminated semiconductor electrodes. The combination of ECL with photoelectrochemistry at illuminated semiconductors is termed photoinduced ECL (PECL). It deals with the triggering of an ECL reaction through the transfer of photogenerated minority charge carriers at the illuminated solid/liquid interface. PECL results in the conversion of incident photons (λexc), that are absorbed by the semiconductor photoelectrode to emitted photons (λPECL), produced by the ECL reaction. Although demonstrated in the 1970s by Bard et al. in ultradry organic solvents, PECL remained unexplored until the last five years. Nowadays, as a result of the considerable progress achieved in semiconductor photoelectrodes and ECL systems, a large variety of PECL systems can be designed by combining photoelectrode materials with ECL luminophores, making it a versatile tool for light conversion in aqueous media.In this Account, we introduce the fundamentals of ECL and photoelectrochemistry at illuminated semiconductors and review the recent developments in PECL. We discuss the two main PECL light conversion schemes: downconversion (where λexc < λPECL) and upconversion (where λexc > λPECL). Besides, PECL can be used to simplify considerably the common electrochemical setups employed for ECL. Indeed, by engineering the photoelectrode material and carefully considering the reactivity involved for ECL and its counter-reaction, PECL enables the ultimate concept of all-optical ECL (AO-ECL), i.e., ECL generation at an illuminated monolithic device immersed into the electrolyte solution. As discussed in this Account, AO-ECL is an important breakthrough that allows the simplest ECL experimental configuration ever reported, eliminating constraints such as an electrical power supply, wires, electrodes, connections, and specific electrochemical knowledge. As shown at the end of this Account, due to the robustness of recently manufactured PECL systems, several applications can already be envisioned for microscopy, elucidation of solar conversion mechanisms, near-infrared imaging, and bioanalysis.

Photoelectrocatalytic Hydrogen Generation: Current Advances in Materials and Operando Characterization

Photoelectrochemical (PEC) hydrogen generation is a promising technology for green hydrogen production yet faces difficulties in achieving stability and efficiency. The scientific community is pushing toward the development of new electrode materials and a better understanding of the underlying reactions and degradation mechanisms. Advances in photocatalytic materials are being pursued through the development of heterojunctions, tailored crystal nanostructures, doping, and modification of solid-solid and solid-electrolyte interfaces. Operando and in situ techniques are utilized to deconvolute the charge transfer mechanisms and degradation pathways. In this review, both materials development and Operando characterization are covered for advancing PEC technologies. The recent advances made in the PEC materials are first reviewed including the applied improvement strategies for transition metal oxides, nitrites, chalcogenides, Si, and group III-V semiconductor materials. The efficiency, stability, scalability, and electrical conductivity of the aforementioned materials along with the improvement strategies are compared. Next, the Operando characterization methods and cite selected studies applied for PEC electrodes are described. Operando studies are very successful in elucidating the reaction mechanisms, degradation pathways, and charge transfer phenomena in PEC electrodes. Finally, the standing challenges and the potential opportunities are discussed by providing recommendations for designing more efficient and electrochemically stable PEC electrodes.

Enhanced Capability of Hydrogen Evolution Photocathode by Laminated Interface Engineering of Co/MoS2 QDs/pyramid-black Si

We present a novel and stable laminated structure to enhance the performance and stability of silicon (Si) photocathode devices for photoelectrochemical (PEC) water splitting. First, by utilizing Cu nanoparticle catalysts to work on a n+p-black Si substrate via the metal-assisted chemical etching, we can achieve the black silicon with a porous pyramid structure. The low depth holes on the surface of the pyramid caused by Cu etching not only help enhance the light capture capability with quite low surface reflectivity (<5%) but also efficiently protect the p-n junction from damage. To improve the charge migration efficiency and mitigate parasitic light absorption from cocatalysts at the same time, we drop casted quantum dots (QDs) MoS2 with the size of nanometer scale as the first layer of catalyst. Hence, we then can safely electrodeposit cocatalyst Co nanoparticles to further enhance interface transfer efficiency. The synergistic effects of cocatalysts and optimized light absorption from the morphology and QDs contributed to the overall enhancement of PEC performance, offering a promising pathway for an efficient, low cost, and stable (over 100 h) hydrogen production photocathode.

Silicon photocathode functionalized with osmium complex catalyst for selective catalytic conversion of CO2 to methane

Solar-driven CO2 reduction to yield high-value chemicals presents an appealing avenue for combating climate change, yet achieving selective production of specific products remains a significant challenge. We showcase two osmium complexes, przpOs, and trzpOs, as CO2 reduction catalysts for selective CO2-to-methane conversion. Kinetically, the przpOs and trzpOs exhibit high CO2 reduction catalytic rate constants of 0.544 and 6.41 s-1, respectively. Under AM1.5 G irradiation, the optimal Si/TiO2/trzpOs have CH4 as the main product and >90% Faradaic efficiency, reaching -14.11 mA cm-2 photocurrent density at 0.0 VRHE. Density functional theory calculations reveal that the N atoms on the bipyrazole and triazole ligands effectively stabilize the CO2-adduct intermediates, which tend to be further hydrogenated to produce CH4, leading to their ultrahigh CO2-to-CH4 selectivity. These results are comparable to cutting-edge Si-based photocathodes for CO2 reduction, revealing a vast research potential in employing molecular catalysts for the photoelectrochemical conversion of CO2 to methane.

Tailoring Second Coordination Sphere for Tunable Solid-Liquid Interfacial Charge Transfer toward Enhanced Photoelectrochemical H2 Production

The recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid-liquid interface via molecular catalyst design. Specifically, the surface of a p-Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda-SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda-SO3H)2, which contains an electron-withdrawing -SO3H group in the second coordination sphere, elevates the flat-band potential of the polished p-Si photocathode and nanoporous p-Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band-edge engineering by molecular catalysts could be an important design consideration for semiconductor-catalyst hybrids toward PEC H2 production.

Synthesis of Well-Ordered Functionalized Silicon Microwires Using Displacement Talbot Lithography for Photocatalysis

Metal-assisted chemical etching (MACE) is a cheap and scalable method that is commonly used to obtain silicon nano- or microwires but lacks spatial control. Herein, we present a synthesis method for producing vertical and highly periodic silicon microwires, using displacement Talbot lithography before wet etching with MACE. The functionalized periodic silicon microwires show 65% higher PEC performance and 2.3 mA/cm2 higher net photocurrent at 0 V compared to functionalized, randomly distributed microwires obtained by conventional MACE at the same potentials.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Near-IR Photoinduced Electrochemiluminescence Imaging with Structured Silicon Photoanodes

Infrared (IR) imaging devices that convert IR irradiation (invisible to the human eye) to a visible signal are based on solid-state components. Here, we introduce an alternative concept based on light-addressable electrochemistry (i.e., electrochemistry spatially confined under the action of a light stimulus) that involves the use of a liquid electrolyte. In this method, the projection of a near-IR image (λexc = 850 or 840 nm) onto a photoactive Si-based photoanode, immersed into a liquid phase, triggers locally the photoinduced electrochemiluminescence (PECL) of the efficient [Ru(bpy)3]2+-TPrA system. This leads to the local conversion of near-IR light to visible (λPECL = 632 nm) light. We demonstrate that compared to planar Si photoanodes, the use of a micropillar Si array leads to a large enhancement of local light generation and considerably improves the resolution of the PECL image by preventing photogenerated minority carriers from diffusing laterally. These results are important for the design of original light conversion devices and can lead to important applications in photothermal imaging and analytical chemistry.

Polymeric viologen-based electron transfer mediator for improving the photoelectrochemical water splitting on Sb2Se3 photocathode

The photogenerated charge carrier separation and transportation of inside photocathodes can greatly influence the performance of photoelectrochemical (PEC) H2 production devices. Coupling TiO2 with p-type semiconductors to construct heterojunction structures is one of the most widely used strategies to facilitate charge separation and transportation. However, the band position of TiO2 could not perfectly match with all p-type semiconductors. Here, taking antimony selenide (Sb2Se3) as an example, a rational strategy was developed by introducing a viologen electron transfer mediator (ETM) containing polymeric film (poly-1,1'-dially-[4,4'-bipyridine]-1,1'-diium, denoted as PV2+) at the interface between Sb2Se3 and TiO2 to regulate the energy band alignment, which could inhibit the recombination of photogenerated charge carriers of interfaces. With Pt as a catalyst, the constructed Sb2Se3/PV2+/TiO2/Pt photocathode showed a superior PEC hydrogen generation activity with a photocurrent density of -18.6 mA cm-2 vs. a reversible hydrogen electrode (RHE) and a half-cell solar-to-hydrogen efficiency (HC-STH) of 1.54% at 0.17 V vs. RHE, which was much better than that of the related Sb2Se3/TiO2/Pt photocathode without PV2+ (-9.8 mA cm-2, 0.51% at 0.10 V vs. RHE).

Infrared photoinduced electrochemiluminescence microscopy of single cells

Electrochemiluminescence (ECL) is evolving rapidly from a purely analytical technique into a powerful microscopy. Herein, we report the imaging of single cells by photoinduced ECL (PECL; λem = 620 nm) stimulated by an incident near-infrared light (λexc = 1050 nm). The cells were grown on a metal-insulator-semiconductor (MIS) n-Si/SiOx/Ir photoanode that exhibited stable and bright PECL emission. The large anti-Stokes shift allowed for the recording of well-resolved images of cells with high sensitivity. PECL microscopy is demonstrated at a remarkably low onset potential of 0.8 V; this contrasts with classic ECL, which is blind at this potential. Two imaging modes are reported: (i) photoinduced positive ECL (PECL+), showing the cell membranes labeled with the [Ru(bpy)3]2+ complex; and (ii) photoinduced shadow label-free ECL (PECL-) of cell morphology, with the luminophore in the solution. Finally, by adding a new dimension with the near-infrared light stimulus, PECL microscopy should find promising applications to image and study single photoactive nanoparticles and biological entities.

All-Optical Electrochemiluminescence at Metal-Insulator-Semiconductor Diodes

Pt/InGa/n-Si/SiOx/Pt devices were prepared by using standard chemical and sputtering processes. These systems are diodes comprising a frontside photoactive metal-insulator-semiconductor (MIS) n-Si/SiOx/Pt junction and a backside Pt/InGa/n-Si Ohmic contact. Pt/InGa/n-Si/SiOx/Pt was first characterized by dark-solid-state electrical and impedance measurements. Then, each side of the device was investigated by electrochemical means in the dark and under near-IR illumination at 850 nm in the luminol-H2O2 electrochemiluminescence (ECL) electrolyte. The results suggested the possibility of triggering an all-optical ECL (AO-ECL) at Pt/InGa/n-Si/SiOx/Pt. This was confirmed by studying AO-ECL at the monolithic, all-integrated Pt/InGa/n-Si/SiOx/Pt device, immersed in the ECL electrolyte. The conversion process can occur with good stability and the intensity of the visible emission (440 nm) depends on tunable parameters such as the illumination power density, O2 concentration, or the concentration of added H2O2. These results are important for the next developments of AO-ECL in sensing and microscopy.

An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoOx , a NiOx /NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoOx /NiOx /NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoOx /NiOx /NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm-2 at 1.23 V vs. RHE for OER, as compared to n-Si/CoOx , even significantly surpassing the counterpart n-Si/CoOx /NiOx /FeOOH and n-Si/CoOx /NiOx /NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoOx /NiOx /NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.

Engineering the Inhomogeneity of Metal-Insulator-Semiconductor Junctions for Photoelectrochemical Methanol Oxidation

Si-based inhomogeneous metal-insulator-semiconductor (MIS) junctions with a discontinuous metal nanostructure on the Si/insulator layer are expected to be efficient photoelectrodes for solar energy conversion. However, the formation of a metal nanostructure with an optimized arrangement on semiconductors for efficient charge carrier collection is still a big challenge. Herein, we report a method for the in situ formation of an n-Si inhomogeneous MIS junction with well-dispersed metal nanocontacts through a self-assembly process during photoelectrochemical (PEC) methanol oxidation. The photovoltage shows a strong dependence on the inhomogeneity of the n-Si MIS junction, which can be precisely tuned by the applied electrode potential and operation time. The appropriate inhomogeneity of the Schottky junction as well as the high barrier regions induced by the metal oxide/(oxy)hydroxide layer synergistically produces a large photovoltage of 500 mV for the n-Si inhomogeneous MIS junction. Finally, the n-Si-based photoanode is coupled with a CO2-to-formate reaction to realize the production of formate at both electrodes, resulting in a high faradic efficiency (FE) of 86 and 93% for anode and cathode reactions at an operational current of 30 mA/cm2, respectively. These findings provide important insights into the design of highly efficient inhomogeneous MIS junctions through an in situ self-assembly route for solar energy conversion and storage.

Composite p-Si/Al2O3/Ni Photoelectrode for Hydrogen Evolution Reaction

A photoelectrode for hydrogen evolution reaction (HER) is proposed, which is based on p-type silicon (p-Si) passivated with an ultrathin (10 nm) alumina (Al₂O₃) layer and modified with microformations of a nickel catalyst. The Al₂O₃ layer was formed using atomic layer deposition (ALD), while the nickel was deposited photoelectrochemically. The alumina film improved the electronic properties of the substrate and, at the same time, protected the surface from corrosion and enabled the deposition of nickel microformations. The Ni catalyst increased the HER rate up to one order of magnitude, which was comparable with the rate measured on a hydrogen-terminated electrode. Properties of the alumina film on silicon were comprehensively studied. Grazing incidence X-ray diffraction (GI-XRD) identified the amorphous structure of the ALD oxide layer. Optical profilometry and spectroscopic ellipsometry (SE) showed stability of the film in an acid electrolyte. Resistivity measurements showed that annealing of the film increases its electric resistance by four times.

Quasi-band structure of quantum-confined nanocrystals

We discuss the electronic properties of quantum-confined nanocrystals. In particular, we show how, starting from the discrete molecular states of small nanocrystals, an approximate band structure (quasi-band structure) emerges with increasing particle size. Finite temperature is found to broaden the discrete states in energy space forming even for nanocrystals in the quantum-confinement regime quasi-continuous bands in k-space. This bands can be, to a certain extend, interpreted along the lines of standard band structure theory, while taking also finite size and surface effects into account. We discuss this on various prototypical nanocrystal systems.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)₅Cl to form complexes of the type Re(bpy)(CO)₃Cl, which are related to known catalysts for CO₂ reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO₂ on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO₂ reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Microstructure-regulated inverted pyramidal Si photocathodes for efficient hydrogen generation

Black silicon electrodes with inverted pyramid arrays (SiIPs) are promising for efficient photoelectrochemical water splitting due to their excellent photoelectric properties and quasi-hydrophilicity. In this work, an elaborate study on microstructure regulation of SiIP photocathodes is reported. We find that on SiIPs where sidewalls have been processed with copper-assisted chemical etching (Cu-ACE), there are vast numbers of micro-pits distributed (deep holes and shallow grooves) that exactly determine electrode performance, which is a result of homogeneous Cu²⁺ oxidation of Si. Furthermore, SiIP microstructural features can be effectively adjusted via controlling the etchant composition and introducing alkali post-treatment. Taking the trade-off between light trapping ability and charge separation capacity into consideration, we optimized the hydrogen evolution reaction (HER) activity of a SiIP photocathode, and its onset potential was decreased to -0.35 V vs. RHE. On this basis, we constructed reliable heterojunctions to further improve the sluggish HER kinetics. The optimized SiIPs/TiO₂/MoS_x cathode exhibits a considerable photocurrent density of 9.45 mA cm^-2 at zero HER overpotential for 18 h in acidic media. Notably, our work presents a detailed physical insight into micro-pit formation and elimination in Cu-ACE, and describes the dependency of SiIP-based electrode performance on the microstructure morphology, paving a new way for its potential application in unbiased overall water splitting.

Electrodeposition of Molybdenum Disulfide (MoS2) Nanoparticles on Monocrystalline Silicon

Molybdenum disulfide (MoS2) has attracted great attention for its unique chemical and physical properties. The applications of this transition metal dichalcogenide (TMDC) range from supercapacitors to dye-sensitized solar cells, Li-ion batteries and catalysis. This work opens new routes toward the use of electrodeposition as an easy, scalable and cost-effective technique to perform the coupling of Si with molybdenum disulfide. MoS2 deposits were obtained on n-Si (100) electrodes by electrochemical deposition protocols working at room temperature and pressure, as opposed to the traditional vacuum-based techniques. The samples were characterized by X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Rutherford Back Scattering (RBS).

Evaluating the Optical Response of Heavily Decorated Black Silicon Based on a Realistic 3D Modeling Methodology

Combining black silicon (BS), a nanostructured silicon containing highly roughened surface morphology with plasmonic materials, is becoming an attractive approach for greatly enhancing light-matter interactions with promising applications of sensing and light harvesting. However, precisely describing the optical response of a heavily decorated BS structure is still challenging due to the increasing complexity in surface morphology and plasmon hybridization. Here, we propose and fully characterize BS-based multistacked nanostructures with randomly distributed nanoparticles on the highly roughened nonflat surface. We demonstrate a realistic 3D modeling methodology based on parametrized scanning electron microscopy images that provides high-precision morphology details, successfully linking the theoretical analysis with experimental optical response of the complex nanostructures. Far-field calculations very nicely reproduce experimental reflectance spectra, revealing the dependency of light trapping on the thickness of the conformal reflector and the atop nanoparticle size. Near-field analysis clearly identifies three types of stochastic "hotspots". Their contribution to the overall field enhancement is shown to be very much sensitive to the nanoscale surface morphology. The simulated near-field property is then used to examine the measured surface-enhanced Raman scattering (SERS) response on the multistacked structures. The present modeling approach combined with spectroscopic characterizations is expected to offer a powerful tool for the precise description of the optical response of other large-scale highly disordered realistic 3D systems.

Anti-Stokes photoinduced electrochemiluminescence at a photocathode

Anti-Stokes photoinduced electrochemiluminescence (PECL) converts infrared photons to visible photons and is usually triggered at a narrow band gap-protected photoanode. Here, we report the first example of PECL with the model [Ru(bpy)₃]²⁺/benzoyl peroxide system at a bare p-type Si photocathode. The reported PECL system, which allows a notable decrease of the cathodic potential required for ECL generation, should open new opportunities for imaging and light-addressable devices.

Metal-Insulator-Semiconductor Anodes for Ultrastable and Site-Selective Upconversion Photoinduced Electrochemiluminescence

Photoinduced electrochemiluminescence (PECL) allows the electrochemically assisted conversion of low-energy photons into high-energy photons at an electrode surface. This concept is expected to have important implications, however, it is dramatically limited by the stability of the surface, impeding future developments. Here, a series of metal-insulator-semiconductor (MIS) junctions, using photoactive n-type Si (n-Si) as a light absorber covered by a few-nanometer-thick protective SiO_x /metal (SiO_x /M, with M=Ru, Pt, and Ir) overlayers are investigated for upconversion PECL of the model co-reactant system involving the simultaneous oxidation of tris(bipyridine)ruthenium(II) and tri-n-propylamine. We show that n-Si/SiO_x /Pt and n-Si/SiO_x /Ir exhibit high photovoltages and record stabilities in operation (35 h for n-Si/SiO_x /Ir) for the generation of intense PECL with an anti-Stokes shift of 218 nm. We also demonstrate that these surfaces can be employed for spatially localized PECL. These unprecedented performances are extremely promising for future applications of PECL.

Chemi-Inspired Silicon Allotropes-Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets, Single-Walled Nanotubes, and Nanoparticles

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si9]4- clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si-Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si9 clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si9]4- based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.

Surface Engineering of Nanoporous Silicon Photocathodes for Enhanced Photoelectrochemical Hydrogen Production

Silicon (Si) is a promising semiconductor material in photoelectrochemical (PEC) H2 evolution due to its advantages of Earth-abundant element, non-toxicity, broad absorption of the solar spectrum, high saturated-current and industrial...

Improved hydrogen evolution with SnS2 quantum dot-incorporated black Si photocathode

Black silicon (bSi), possessing appealing light-trapping properties and large specific surface area, ranks high among many other photocathode materials. However, the insufficient dynamics towards HER seriously bother black Si. Herein, a novel photoelectrode with ultrasmall size tin sulfide quantum dot (SnS₂ QD) incorporated black silicon was employed. Nanosized SnS₂ possesses numerous active sites for electrochemical reactions. Impressively, benefiting from SnS₂ QDs, the downward band bending of the Si Fermi level at the interface of electrolyte becomes higher, which remarkably suppresses the recombination of photo-generated carriers, thereby facilitating the participation of photo-generated electrons in PEC-HER. As a result, the thus-designed SnS₂/bSi reveals an exceptional PEC-HER activity with a positive onset potential of 0.235 V vs. reversible hydrogen electrode (RHE), a high photocurrent of 1.23 mA cm^-2 at 0 V vs. RHE, and long-term stability. Besides, the saturated photocurrent of ∼41 mA cm^-2 is achieved at about -0.51 V vs. RHE.

Solution-processed silicon quantum dot photocathode for hydrogen evolution

The photoelectrochemical response of a photocathode made from a colloidal solution of boron (B) and phosphorus (P) codoped silicon (Si) quantum dots (QDs) 2-11 nm in diameters is studied. Since codoped Si QDs are dispersible in alcohol and water due to the hydrophilic surface, a photoelectrode with a smooth surface is produced by drop-coating the QD solution on an indium tin oxide substrate. The codoping provides high oxidation resistance to Si QDs and makes the electrode operate as a photocathode. The photoelectrochemical response of a Si QD photoelectrode depends strongly on the size of QDs; there is a transition from anodic to cathodic photocurrent around 4 nm in diameter. Below the size, anodic photocurrent due to self-oxidation of Si QDs is observed, while above the size, cathodic photocurrent due to electron transfer across the interface is observed. The cathodic photocurrent increases with increasing the size, and in some samples, it is observed for more than 3000 s under intermittent light irradiation.

Immobilising molecular Ru complexes on a protective ultrathin oxide layer of p-Si electrodes towards photoelectrochemical CO2 reduction

Photoelectrochemical CO2 reduction is a promising approach for renewable fuel generation and to reduce greenhouse gas emissions. Owing to their synthetic tunability, molecular catalysts for the CO2 reduction reaction can give rise to high product selectivity. In this context, a RuII complex [Ru(HO-tpy)(6-mbpy)(NCCH3)]2+ (HO-tpy = 4'-hydroxy-2,2':6',2''-terpyridine; 6-mbpy = 6-methyl-2,2'-bipyridine) was immobilised on a thin SiOx layer of a p-Si electrode that was decorated with a bromide-terminated molecular layer. Following the characterisation of the assembled photocathodes by X-ray photoelectron spectroscopy and ellipsometry, PEC experiments demonstrate electron transfer from the p-Si to the Ru complex through the native oxide layer under illumination and a cathodic bias. A state-of-the-art photovoltage of 570 mV was determined by comparison with an analogous n-type Si assembly. While the photovoltage of the modified photocathode is promising for future photoelectrochemical CO2 reduction and the p-Si/SiOx junction seems to be unchanged during the PEC experiments, a fast desorption of the molecular Ru complex was observed. An in-depth investigation of the cathode degradation by comparison with reference materials highlights the role of the hydroxyl functionality of the Ru complex to ensure its grafting on the substrate. In contrast, no essential role for the bromide function on the Si substrate designed to engage with the hydroxyl group of the Ru complex in an SN2-type reaction could be established.

Molecular Triad Containing a TEMPO Catalyst Grafted on Mesoporous Indium Tin Oxide as a Photoelectrocatalytic Anode for Visible Light-Driven Alcohol Oxidation

Photoelectrochemical cells based on semiconductors are among the most studied methods of artificial photosynthesis. This study concerns the immobilization, on a mesoporous conducting indium tin oxide electrode (nano-ITO), of a molecular triad (NDADI-P-Ru-TEMPO) composed of a ruthenium tris-bipyridine complex (Ru) as photosensitizer, connected at one end to 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) as alcohol oxidation catalyst and at the other end to the electron acceptor naphthalenedicarboxyanhydride dicarboximide (NDADI). Light irradiation of NDADI-P-Ru-TEMPO grafted to nano-ITO in a pH 10 carbonate buffer effects selective oxidation of para-methoxybenzyl alcohol (MeO-BA) to para-methoxybenzaldehyde with a TON of approximately 150 after 1 h of photolysis at a bias of 0.4 V vs. SCE. The faradaic efficiency is found to be of 80±5 %. The photophysical study indicates that photoinduced electron transfer from the Ru complex to NDADI is a slow process and must compete with direct electron injection into ITO to have a better performing system. This work sheds light on some of the important ways to design more efficient molecular systems for the preparation of photoelectrocatalytic cells based on catalyst-dye-acceptor arrays immobilized on conducting electrodes.

Charge and discharge profiles of repurposed LiFePO4 batteries based on the UL 1974 standard

Owing to the popularization of electric vehicles worldwide and the development of renewable energy supply, Li-ion batteries are widely used from small-scale personal mobile products to large-scale energy storage systems. Recently, the number of retired power batteries has largely increased, causing environmental protection threats and waste of resources. Since most of the retired power batteries still possess about 80% of their initial capacity, their second use becomes a possible route to solve the emergent problem. Safety and performance are important when using these second-use repurposed batteries. Underwriters Laboratories (UL), a global safety certification company, published the standard for evaluating the safety and performance of repurposed batteries, i.e., UL 1974. In this work, the test procedures are designed according to UL 1974, and the charge and discharge profile datasets of the LiFePO₄ repurposed batteries are provided. Researchers and engineers can use the characteristic curves to evaluate the quality of the repurposed batteries. Furthermore, the profile datasets can be applied in the model-based engineering of repurposed batteries, e.g., fitting the variables of an empirical model or validating the results of a theoretical model.

Measurement(s)	Charge and discharge profiles
Technology Type(s)	Two-tier DC load method

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14495604

Effect of morphology on the photoelectrochemical performance of nanostructured Cu2O photocathodes

Cu₂O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (J_ph), onset potential (E_onset), band edges, carrier density (N_A), and interfacial charge transfer resistance (R_ct) are affected by the morphology and method used to deposit Cu₂O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values ofN_AandR_ct. However, these differences are not observed to translate to other photocatalytic properties of Cu₂O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (E_fb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively.E_onsetof 0.48 ± 0.04 eV obtained for these systems is close to theE_fbindicating negligible water reduction overpotential. Electrochemically deposited planar Cu₂O provides the highest photocurrent density of 5.0 mA cm^-2at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu₂O photocathodes.

Hole-Storage Enhanced a-Si Photocathodes for Efficient Hydrogen Production

Ferrihydrite (Fh) has been demonstrated as an effective interfacial layer for photoanodes to achieve outstanding photoelectrochemical (PEC) performance for water oxidation reaction owing to its unique hole-storage function. However, it is unknown whether such a hole-storage layer can be used to construct highly efficient photocathodes for hydrogen evolution reaction (HER). In this work, we report Fh interfacial engineering of amorphous silicon photocathode (with nickel as HER cocatalyst) achieving a photocurrent density of 15.6 mA cm^-2 at 0 V vs. the reversible hydrogen electrode and a half-cell energy conversion efficiency of 4.08 % in alkaline solution, outperforming most of reported a-Si based photocathodes including multi-junction configurations integrated with noble metal cocatalysts in acid solution. Besides, the photocurrent density is maintained above 14 mA cm^-2 for 175 min with 100 % Faradaic efficiency for HER in alkaline solution. Our results demonstrate a feasible approach to construct efficient photocathodes via the application of a hole-storage layer.

Recent advances of monoelemental 2D materials for photocatalytic applications

As a sustainable environmental governance strategy and energy conversion method, photocatalysis has considered to have great potential in this field due to its excellent optical properties and has become one of the most attractive technologies today. Among 2D materials, the emerging two-dimensional (2D) monoelemental materials mainly distributed in the -IIIA, -IVA, -VA and -VIA groups and show excellent performance in solar energy conversion due to their graphene-like 2D atomic structure and unique properties, thereby drawing increasing attention. This review briefly summarizes the preparation processes and fundamental properties of 2D single-element nanomaterials, as well as various modification strategies and adjustment mechanisms to enhance their photocatalytic properties. In particular, this article comprehensively discusses the related practical applications of 2D single-element materials in the field of photocatalysis, including photocatalytic degradation for contaminants removal, photocatalytic pathogen inactivation, photocatalytic fouling control and photocatalytic energy conversion. This review will provide some new opportunities for the rational design of other excellent photocatalysts based on 2D monoelemental materials, as well as present tremendous novel ideas for 2D monoelemental materials in other environmental conservation and energy-related applications, such as supercapacitors, electrocatalysis, solar cells, and so on.

Custom plating of nanoscale semiconductor/catalyst junctions for photoelectrochemical water splitting

Photoelectrochemical water splitting under harsh chemical conditions can be promoted by dispersed transition metal nanoparticles electrodeposited on n-Si surfaces, without the need for classical protection layers. Although this method is simple, it only allows for poor control of metal morphology and geometry on the photoanode surface. Herein, we introduce templated nanoscale electrodeposition on photoactive n-Si for the customization of nanoscale inhomogeneous Schottky junctions and demonstrate their use as stable photoanodes. The photoelectrochemical properties of the so-manufactured photoanodes exhibit a strong dependence on the photoanodes' geometrical features, and the obtained experimental trends are rationalized using simulation.

Impact of structure, doping and defect-engineering in 2D materials on CO2 capture and conversion

2D material based strategies for adsorption and conversion of CO2 to value-added products.

Integration of Oxygen-Vacancy-Rich NiFe-Layered Double Hydroxide onto Silicon as Photoanode for Enhanced Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting has the potential to efficiently convert intermittent solar energy into storable hydrogen fuel. However, poor charge separation and transfer ability as well as sluggish surface oxygen evolution reaction (OER) kinetics of the photoelectrode severely hinder the advance in PEC performance. Herein, a facile electrodeposition method was used to integrate Mo-doped NiFe-layered double hydroxide onto a NiOx /Ni-protected Si photoanode for enhanced PEC water oxidation. Mo doping contributed to an increased amount of oxygen vacancies, whereas a dynamic surface self-reconstruction was induced by Mo leaching under PEC OER conditions. This led to enhanced PEC performance with an onset potential of 0.87 V vs. reversible hydrogen electrode (RHE), a photocurrent density of 39.3 mA cm-2 at 1.23 V vs. RHE, a fill factor of 0.38, and a solar-to-oxygen conversion efficiency of 5.3 %, along with a stability of 130 h continuous PEC reaction. The performance was superior to that of the undoped NiFe-LDH/NiOx /Ni/Si (4.3 %), which was attributed to the elevated interface charge separation, fast charge transfer, and accelerated OER kinetics.

Spontaneous solar water splitting with decoupling of light absorption and electrocatalysis using silicon back-buried junction

Converting sunlight into a storable form of energy by spontaneous water splitting is of great interest but the difficulty in simultaneous management of optical, electrical, and catalytic properties has limited the efficiency of photoelectrochemical (PEC) devices. Herein, we implemented a decoupling scheme of light harvesting and electrocatalysis by employing a back-buried junction (BBJ) PEC cell design, which enables >95% front side light-harvesting, whereas the electrochemical reaction in conjunction with carrier separation/transport/collection occurs on the back side of the PEC cell. The resultant silicon BBJ-PEC half-cell produces a current density of 40.51 mA cm⁻² for hydrogen evolution by minimizing optical, electrical, and catalytic losses (as low as 6.11, 1.76, and 1.67 mA cm⁻², respectively). Monolithic fabrication also enables three BBJ-PEC cells to be connected in series as a single module, enabling unassisted solar water-splitting with a solar-to-hydrogen conversion efficiency of 15.62% and a hydrogen generation rate of 240 μg cm⁻² h⁻¹.

The simultaneous management of optical, electrical, and catalytic properties is challenging for photoelectrochemical devices. Here, authors design Si back-buried junction photoelectrodes that can be series connected for unassisted water-splitting with a high solar-to-hydrogen efficiency of 15.62%.