Efficient Photoelectrochemical Water Splitting over Anodized <i>p</i>‑Type NiO Porous Films

NiO photocathodes were fabricated by alkaline etching-anodizing nickel foil in an organic-based electrolyte. The resulting films have a highly macroporous surface structure due to rapid dissolution of the oxide layer as it is formed during the anodization process. We are able to control the films’ surface structures by varying the anodization duration and voltage. With an onset potential of +0.53 V versus the reversible hydrogen electrode (RHE), the photocurrent efficiency of the NiO electrodes showed dependencies on their surface roughness factor, which determines the extent of semiconductor-electrolyte interface and the associated quality of the NiO surface sites. A maximum incident photon-to-current conversion efficiency (IPCE_max) of 22% was obtained from NiO film with a roughness factor of 8.4. Adding an Al₂O₃ blocking layer minimizes surface charge recombination on the NiO and hence increased the IPCE_max to 28%. The NiO/Al₂O₃ films were extremely stable during photoelectrochemical water splitting tests lasting up to 20 h, continuously producing hydrogen and oxygen in the stoichiometric 2:1 ratio. The NiO/Al₂O₃ and NiO films fabricated using the alkaline anodization process produced 12 and 6 times as much hydrogen, respectively, as those fabricated using commercial NiO nanoparticles

Shuttling Photoelectrochemical Electron Transport in Tricomponent CdS/rGO/TiO₂ Nanocomposites

Composite photoelectrodes consisting of CdS sensitizer, reduced graphene oxide (rGO) transporter, and TiO₂ acceptor were synthesized in a solvothermal synthesis. Under solvothermal conditions, the dimethyl sulfoxide (DMSO) solvent medium decomposed to form free sulfides, which facilitated the formation of CdS and, at the same time, which also reduced graphene oxide sheets by forming disulfide moieties. Compared to pure CdS and TiO₂, coupling of these materials either as bi- or tricomponent composites (including rGO) allowed efficient interfacial charge separation and prolonged electron lifetimes. In particular, in the CdS/rGO/TiO₂ tricomposite case, the rGO plays vital roles in alleviating trapped electrons at the heterojunction and serves as a platform for shuttling electrons between CdS and TiO₂. Taking into account all of the structure-related charge-transport characteristics, including interfacial contacts, the highest quantum efficiency (incident photon-to-current efficiency, IPCE, at 460 nm = 12%) was achieved for the CdS/rGO/TiO₂ tricomposite, and this was 6-fold that of CdS/TiO₂

Investigation of the Exchange Kinetics and Surface Recovery of Cd_<i>x</i>Hg_1–<i>x</i>Te Quantum Dots during Cation Exchange Using a Microfluidic Flow Reactor

Detailed analyses of coupled photoluminescence, emission lifetime, and absorption measurements have been made on the products of cation exchange reactions between CdTe nanocrystals and Hg²⁺ salt/ligand solutions in a microfluidic flow reactor and capillary measurement cell to probe the reaction kinetics over the seconds to hours time scale and to establish the influence of the reaction conditions on the spatial distribution of the mixed cations within the resulting Cd_<i>x</i>Hg_1–<i>x</i>Te colloidal quantum dots. The establishment of the evolution of the radiative and nonradiative rates allowed the recovery of the emission quantum yield in Cd_<i>x</i>Hg_1–<i>x</i>Te quantum dots to be quantified to almost 50% and the necessary time scales to be determined for each set of reaction conditions. The reaction kinetics showed clear indication of a fast surface exchange process followed by a slower internal rearrangement of the cation distribution

Semiconductor Nanocrystals as Luminescent Down-Shifting Layers To Enhance the Efficiency of Thin-Film CdTe/CdS and Crystalline Si Solar Cells

A simple optical model is presented to describe the influence of a planar luminescent down-shifting layer (LDSL) on the external quantum efficiencies of photovoltaic solar cells. By employing various visible light-emitting LDSLs based on CdTe quantum dots or CdSe/CdS core–shell quantum dots and tetrapods, we show enhancement in the quantum efficiencies of thin-film CdTe/CdS solar cells predominantly in the ultraviolet regime, the extent of which depends on the photoluminescence quantum yield (PLQY) of the quantum dots. Similarly, a broad enhancement in the quantum efficiencies of crystalline Si solar cells, from ultraviolet to visible regime, can be expected for an infrared emitting LDSL based on PbS quantum dots. A PLQY of 80% or higher is generally required to achieve a maximum possible short-circuit current increase of 16 and 50% for the CdTe/CdS and crystalline Si solar cells, respectively. As also demonstrated in this work, the model can be conveniently extended to incorporate LDSLs based on organic dyes or upconverting materials