Soon Hyung Kang

TiO2 nanotubes with a ZnO thin energy barrier for improved current efficiency of CdSe quantum-dot-sensitized solar cells

This paper reports the formation of a thin ZnO energy barrier between a CdSe quantum dot (Q dots) sensitizer and TiO2 nanotubes (TONTs) for improved current efficiency of Q dot-sensitized solar cells. The formation of a ZnO barrier between TONTs and the Q dot sensitizer increased the short-circuit current under illumination and also reduced the dark current in a dark environment. The power conversion efficiency of Q dot-sensitized TONT solar cells increased by 25.9% in the presence of the ZnO thin layer due to improved charge-collecting efficiency and reduced recombination.

Hole transport in sensitized CdS–NiO nanoparticle photocathodes

A general chemical approach was used to synthesise NiO-CdS core-shell nanoparticle films as photocathodes for p-type semiconductor-sensitized solar cells. Compared to dye-sensitized NiO photocathodes, the CdS-sensitized NiO cathodes exhibited two orders of magnitude faster hole transport (attributable to the passivation of surface traps by the CdS) and almost 100% charge-collection efficiencies.

A facile and green synthesis of colloidal Cu2ZnSnS4 nanocrystals and their application in highly efficient solar water splitting

The development of solution-processable routes as well as compounds consisting of earth abundant elements is highly desirable to reduce the fabrication cost. Recently, kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have attracted great attention for photoelectrochemical (PEC) water splitting owing to their suitable low-cost, earth-abundancy and suitable band gap energy. However, the environmentally benign synthesis of high-quality CZTS NCs without toxic solvents remains elusive. Here, a green chemistry approach employing vegetable oil as a non-toxic solvent for the synthesis of monodisperse and size-tunable CZTS NCs is introduced for the first time. Additionally, the relationship between the abnormal size behavior of the CZTS NCs and the degree of decomposition in the vegetable oil using electrospray ionization mass spectrometry (ESI-MS) measurements is elucidated for the first time. As a conceptual strategy, a ternary abundant compound based heterojunction nanostructure for efficient solar water splitting by introducing CZTS NCs onto 5 nm Zn(O,S) passivated layer/hydrothermally grown TiO2 nanorod arrays (TNRs) is designed and developed. Remarkably, this ternary CZTS NCs/Zn(O,S)/TNR photoelectrode shows a photocurrent density as high as 15.05 mA cm−2 at 1.23 V (vs. the NHE), which is the highest ever for previously reported CZTS NC-based photoelectrodes. The reasons for the enhanced PEC performance are discussed in detail based on different PEC characterizations. More importantly, this work reflects the sophistication of eco-friendly solution phase synthesized CZTS NCs without using any toxic chemicals as an earth abundant sensitizer and constitute a new paradigm towards the enhanced PEC performance with quantum dot based hetero-nanostructures.

Enhancing photoelectrochemical water splitting performance of TiO 2 nanotube arrays by controlling morphological properties

Anodic TiO2 nanotube (TONT) with various morphological features was developed by simple electrochemical anodization in the variation of applied voltage where the morphological properties such as pore diameter, wall thickness, and inter-tube spacing can be adjusted to survey the influence on the photoelectrochemical (PEC) activity. Thus, in the applied voltage from 20 V to 60 V at different reaction times, the identical thickness of 3.0 ± 0.2 µm was maintained. From the field-emission scanning electron microscopy, the geometric factors were compared and the porosity of TONT films was calculated. The results demonstrated that the porosity of TONT film grown at 20 V (denoted as 20 V) is the highest and that the porosity steadily reduced with increasing applied voltage. On the basis of these geometric factors, the PEC performance of TONT film (20 V) shows the highest photocurrent of 1.64 mA/cm2 at 0 V vs. Ag/AgCl. This was attributed to the lowest series resistance due to the high porosity for the hole or ions pathway and to the high crystallite size for the beneficial charge transport. On the other hand, TONT films (above 40 V) exhibit a similar photocurrent with high series resistance for opposite reasons.

Facile, Room Temperature, Electroless Deposited (Fe1− x ,Mn x )OOH Nanosheets as Advanced Catalysts: The Role of Mn Incorporation

Herein, bimetallic iron (Fe)-manganese (Mn) oxyhydroxide ((Fe1-x, Mnx )OOH, FeMnOOH) nanosheets on fluorine-doped tin oxide conducting substrates and on semiconductor photoanodes are synthesized by a facile, room temperature, electroless deposition method as catalysts for both electrochemical and photo-electrochemical (PEC) water splitting, respectively. Surprisingly, Mn-doped FeOOH can significantly modulate the nanosheet morphology to increase the active surface area, boost more active sites, and augment the intrinsic activity by tuning the electronic structure of FeOOH. Due to the 2D nanosheet architecture, the optimized FeMnOOH exhibits superior electrochemical activity and outstanding durability for the oxygen evolution reaction with a low overpotential of 246 mV at 10 mA cm-2 and 414 mV at 100 mA cm-2 , and long-term stability for 40 h without decay, which is comparable to the best electrocatalysts for water oxidation reported in the literature. By integrating with semiconductor photoanodes (such as α-Fe2 O3 nanorod (NR) arrays), bimetallic FeMnOOH catalysts achieve solar-driven water splitting with a significantly enhanced PEC performance (3.36 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE)) with outstanding long-term stability (≈8 h) compared to that of the bare Fe2 O3 NR (0.92 mA cm-2 at 1.23 V vs RHE).

Nanotube- and Nanorod-Based Dye-Sensitized Solar Cells

Considerable efforts have been devoted to the design and synthesis of low-dimensional, nanostructured materials due to their morphology-dependent performances. In particular, one-dimensional (1-D) TiO2 nanostructures, including nanorods (NRs), nanowires (NWs), and nanotubes (NTs), have attracted considerable interest due to their unique characteristics. In dye-sensitized solar cell (DSSC) operation, 1-D nanostructure-based photoanodes can contribute to rapid electron transport, ensuring efficient charge collection by the conducting substrate in competition with recombination. Relying on the ordering of 1-D TiO2 nanomaterial, the conversion efficiency of DSSCs was affected because electron collection is determined by trapping/detrapping events at the site of the electron traps, such as defects, surface states, grain boundaries, and self-trapping. This point has promoted research on self-ordered, 1-D photoanodes stretched on a substrate with enhanced electron transport properties due to their desirable features: highly decreased intercrystalline contacts and a structure with a specified directionality. In this literature review, the preparation of various 1-D nanomaterials from disordered to ordered states and their electron dynamics in the application of DSSCs are reviewed.

Unassisted visible solar water splitting with efficient photoelectrodes sensitized by quantum dots synthesized via an environmentally friendly eutectic solvent-mediated approach

Deep eutectic solvents (DESs) based on choline chloride/ethylene glycol have been explored as synthetic media for recently introduced Cu–Sb–S based colloidal quantum dots (CQDs) decorated on NiO/fluorine-doped tin oxide (FTO) and TiO2/FTO photoelectrodes for unassisted solar water splitting for hydrogen generation. The feasibility of the use of an environmentally benign solvent-based synthetic process is demonstrated herein by preparing the earth-abundant Cu–Sb–S-based CQDs and utilizing them in a solar energy harvesting material for photoelectrochemical (PEC) water splitting while avoiding the use of sacrificial agents. The band alignment between CQDs and NiO or TiO2 clearly suggests that the CQD-modified NiO and TiO2 electrodes act as a potential photocathode (NiO/Cu3SbS4/ZnS) and photoanode (TiO2/CuSbS2/ZnS) with faradaic efficiencies of up to 74 and 86%, respectively, which allows us to construct an efficient PEC cell to split water at an overall solar-to-hydrogen (STH) efficiency of ∼0.28%. The tandem photoelectrode configuration in an unassisted mode of solar-driven water splitting based on a wire-linked system shows ∼0.97 mA cm−2 of current density, and can split water under zero-bias conditions. Enhancement of the PEC device by accelerating electron and hole transport and broadening the diffusion length using photosensitizer materials while avoiding typical recombination with a thin passivation layer was achieved. The charge transport mechanism through combining experimental results in half and overall water splitting reactions is proposed. The success of such efficient multi-layered heterojunction photoelectrodes is essential for the future development of green energy harvesting devices.

The effect of a metallic Ni core on charge dynamics in CdS-sensitized p-type NiO nanowire mesh photocathodes

We report on the synthesis and photoelectrochemical characterization of photocathodes based on CdS-sensitized Ni–NiO core-shell nanowire mesh inverse opals (IOs). Compared to the NiO–CdS nanowire mesh IO electrode, the hole diffusion coefficient of the CdS-sensitized Ni–NiO core-shell nanowire mesh IO photocathode was one order of magnitude larger, indicating that the Ni core facilitated hole transfer in nanostructured p-type electrodes. As a result, the charge-collection efficiency of the Ni–NiO core-shell nanowire mesh IO electrode was shown to be essentially 100%.