Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion.

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO2 reduction, and N2 reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Exploiting defects in TiO2 inverse opal for enhanced photoelectrochemical water splitting.

In this work, we report on defects generation in TiO2 inverse opal (IO) nanostructures by electrochemical reduction in order to increase photocatalytic activity and improve photoelectrochemical (PEC) water splitting performance. Macroporous structures, such as inverse opals, have attracted a lot of attention for energy-related applications because of their large surface area, interconnected pores, and ability to enhance light-matter interaction. Photocurrent density of electrochemically reduced TiO2-IO increased by almost 4 times, compared to pristine TiO2-IO photoelectrodes. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses confirm the presence of oxygen vacancies in electrochemically reduced TiO2-IO photoelectrodes. Oxygen vacancies extend the absorption of TiO2 from the UV to visible region. The incident photon-to-current efficiency (IPCE) increased by almost 3 times in the absorption (UV) region of TiO2 and slightly in the visible region. Impedance studies show improved electrical conductivity, longer photogenerated electron lifetime, and a negative shift of the flatband potential, which are attributed to oxygen vacancies acting as electron donors. The Fermi level shifts to be closer to the conduction band edge of TiO2-IO.

Photoelectrochemical studies of InGaN/GaN MQW photoanodes.

The research interest in photoelectrochemical (PEC) water splitting is ever growing due to its potential to contribute towards clean and portable energy. However, the lack of low energy band gap materials with high photocorrosion resistance is the primary setback inhibiting this technology from commercialisation. The ternary alloy InGaN shows promise to meet the photoelectrode material requirements due to its high chemical stability and band gap tunability. The band gap of InGaN can be modulated from the UV to IR regions by adjusting the In concentration so as to absorb the maximum portion of the solar spectrum. This paper reports on the influence of In concentration on the PEC properties of planar and nanopillar (NP) InGaN/GaN multi-quantum well (MQW) photoanodes, where NPs were fabricated using a top-down approach. Results show that changing the In concentration, while having a minor effect on the PEC performance of planar MQWs, has an enormous impact on the PEC performance of NP MQWs, with large variations in the photocurrent density observed. Planar photoanodes containing MQWs generate marginally lower photocurrents compared to photoanodes without MQWs when illuminated with sunlight. NP MQWs with 30% In generated the highest photocurrent density of 1.6 mA cm-2, 4 times greater than that of its planar counterpart and 1.8 times greater than that of the NP photoanode with no MQWs. The InGaN/GaN MQWs also slightly influenced the onset potential of both the planar and NP photoanodes. Micro-photoluminescence, diffuse reflectance spectroscopy and IPCE measurements are used to explain these results.

Improved photoelectrochemical performance of GaN nanopillar photoanodes.

In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.

Applications of atomic layer deposition in solar cells.

Atomic layer deposition (ALD) provides a unique tool for the growth of thin films with excellent conformity and thickness control down to atomic levels. The application of ALD in energy research has received increasing attention in recent years. In this review, the versatility of ALD in solar cells will be discussed. This is specifically focused on the fabrication of nanostructured photoelectrodes, surface passivation, surface sensitization, and band-structure engineering of solar cell materials. Challenges and future directions of ALD in the applications of solar cells are also discussed.

Quantum-dot-sensitized TiO2 inverse opals for photoelectrochemical hydrogen generation.

A new nanoarchitecture photoelectrode design comprising CdS quantum-dot-sensitized, optically and electrically active TiO(2) inverse opals is developed for photoelectrochemical water splitting. The photoelectrochemical performance shows high photocurrent density (4.84 mA cm(-2) at 0 V vs. Ag/AgCl) under simulated solar-light illumination.

Monocrystalline InP Thin Films with Tunable Surface Morphology and Energy Band gap.

InP is currently being used in various (opto)electronic and energy device applications. However, the high cost of InP substrates and associated epitaxial growth techniques has been huge impediments for its widespread use. Here, large-area monocrystalline InP thin films are demonstrated via a convenient cracking method, and the InP thin films show material properties identical to their bulk counterparts. Furthermore, the same substrate can be reused for the production of additional InP thin films. This cracking technique is also shown to be a versatile tool to form an ultrasmooth surface or a microscale periodic triangular grating structure on the surface, depending on the orientation of the donor substrate used. Strain-induced band gap energy shift is also observed in localized regions of the thin film with a grating structure. The simplicity of this technique, which does not require any sophisticated equipment and complex fabrication process, is promising to reduce the cost of InP thin-film devices.

Engineering III-V Semiconductor Nanowires for Device Applications.

III-V semiconductor nanowires offer potential new device applications because of the unique properties associated with their 1D geometry and the ability to create quantum wells and other heterostructures with a radial and an axial geometry. Here, an overview of challenges in the bottom-up approaches for nanowire synthesis using catalyst and catalyst-free methods and the growth of axial and radial heterostructures is given. The work on nanowire devices such as lasers, light emitting nanowires, and solar cells and an overview of the top-down approaches for water splitting technologies is reviewed. The authors conclude with an analysis of the research field and the future research directions.

Ultrathin Ta2O5 electron-selective contacts for high efficiency InP solar cells.

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta2O5) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta2O5 by investigating band energetics at the InP-Ta2O5 interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the Ta2O5 inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta2O5 layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, Voc, of 822 mV, a short circuit current density, Jsc, of 30.1 mA cm-2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The Voc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta2O5 material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc.

15% Efficiency Ultrathin Silicon Solar Cells with Fluorine-Doped Titanium Oxide and Chemically Tailored Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) as Asymmetric Heterocontact.

In order to achieve a high performance-to-cost ratio to photovoltaic devices, the development of crystalline silicon (c-Si) solar cells with thinner substrates and simpler fabrication routes is an important step. Thin-film heterojunction solar cells (HSCs) with dopant-free and carrier-selective configurations look like ideal candidates in this respect. Here, we investigated the application of n-type silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HSCs on periodic nanopyramid textured, ultrathin c-Si (∼25 µm) substrates. A fluorine-doped titanium oxide film was used as an electron-selective passivating layer showing excellent interfacial passivation (surface recombination velocity ∼10 cm/s) and contact property (contact resistivity ∼20 mΩ/cm2). A high efficiency of 15.10% was finally realized by optimizing the interfacial recombination and series resistance at both the front and rear sides, showing a promising strategy to fabricate high-performance ultrathin c-Si HSCs with a simple and low-temperature procedure.

InGaAsP as a Promising Narrow Band Gap Semiconductor for Photoelectrochemical Water Splitting.

While photoelectrochemical (PEC) water splitting is a very promising route toward zero-carbon energy, conversion efficiency remains limited. Semiconductors with narrower band gaps can absorb a much greater portion of the solar spectrum, thereby increasing efficiency. However, narrow band gap (∼1 eV) III-V semiconductor photoelectrodes have not yet been thoroughly investigated. In this study, the narrow band gap quaternary III-V alloy InGaAsP is demonstrated for the first time to have great potential for PEC water splitting, with the long-term goal of developing high-efficiency tandem PEC devices. TiO2-coated InGaAsP photocathodes generate a photocurrent density of over 30 mA/cm2 with an onset potential of 0.45 V versus reversible hydrogen electrode, yielding an applied bias efficiency of over 7%. This is an excellent performance, given that nearly all power losses can be attributed to reflection losses. X-ray photoelectron spectroscopy and photoluminescence spectroscopy show that InGaAsP and TiO2 form a type-II band alignment, greatly enhancing carrier separation and reducing recombination losses. Beyond water splitting, the tunable band gap of InGaAsP could be of further interest in other areas of photocatalysis, including CO2 reduction.

Perovskite Photovoltaic Integrated CdS/TiO2 Photoanode for Unbiased Photoelectrochemical Hydrogen Generation.

Photoelectrolysis of water using solar energy into storable and environment-friendly chemical fuel in the form of hydrogen provides a potential solution to address the environmental concerns and fulfill future energy requirements in a sustainable manner. Achieving efficient and spontaneous hydrogen evolution in water using solar light as the only energy input is a highly desirable but a difficult target. In this work, we report perovskite solar cell integrated CdS-based photoanode for unbiased photoelectrochemical hydrogen evolution. An integrated tandem device consisting of mesoporous CdS/TiO2 photoanode paired with a triple-cation perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) solar cell is developed via a facile fabrication route. The proposed photovoltaic integrated photoanode presents an efficient tandem configuration with high optical transparency to long-wavelength photons and strong photoelectrochemical conversions from short-wavelength photons. On the basis of this integrated tandem device, an unbiased photocurrent density of 7.8 mA/cm2 is demonstrated under AM1.5G illumination.

Photon upconversion in hetero-nanostructured photoanodes for enhanced near-infrared light harvesting.

A hetero-nanostructured photoanode with enhanced near-infrared light harvesting is developed for photo-electrochemical cells. By spatially coating upconversion nanoparticles and quantum dot photosensitizers onto TiO2 inverse opal, this architecture allows direct irradiation of upconversion nanoparticles to emit visible light that excites quantum dots for charge separation. Electrons are injected into TiO2 with minimal carrier losses due to continuous electron conducting interface.

Atomic layer deposition for nanofabrication and interface engineering.

Atomic layer deposition (ALD) provides a tool for conformal coating on high aspect-ratio nanostructures with excellent uniformity. It has become a technique for both template-directed nanofabrications and engineering of surface properties. This Feature Article highlights the application of ALD in selected fields including photonics, SERS and energy materials. Specifically, the topics include fabrication of plasmonic nanostructures for the SERS applications, fabrication of 3-D nanoarchitectured photoanodes for solar energy conversions (dye-sensitized solar cells and photoelectrochemical cells), and coating of electrodes to enhance the cyclic stability and thus device life span of batteries. Dielectric coating for tailoring optical properties of semiconductor nanostructures is also discussed as exemplified by ZnO nanowires. Future direction of ALD in these applications is discussed at the end.

Homogeneous photosensitization of complex TiO2 nanostructures for efficient solar energy conversion.

TiO2 nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO2 in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO2 such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO2 substrates. PEC tests made to CdSe-sensitized TiO2 inverse opal photoanodes result in a drastically improved photocurrent level, up to ~15.7 mA/cm² at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction.