Delafossite CuGaO2-Based Chemiresistive Sensor for Sensitive and Selective Detection of Dimethyl Disulfide

Dimethyl disulfide (DMDS) is a common odor pollutant with an extremely low olfactory threshold. Highly sensitive and selective detection of DMDS in ambient humid air background, by metal oxide semiconductor (MOS) sensors, is highly desirable to address the increased public concern for health risk. However, it has still been a critical challenge up to now. Herein, p-type delafossite CuGaO2 has been proposed as a promising DMDS sensing material owing to its striking hydrophobicity (revealed by water contact angle measurement) and excellent partial catalytic oxidation properties (indicated by mass spectroscopy). The present CuGaO2 sensor shows a selective DMDS response, with satisfied humidity resistance performance and long-term stability at a relatively low operation temperature of 140 °C. An ultrahigh response of 100 to 10 ppm DMDS and a low limit of detection of 3.3 ppb could be achieved via a pulsed temperature modulation strategy. A smart sensing system based on a CuGaO2 sensor has been developed, which could precisely monitor DMDS vapor in ambient humid air, even with the presence of multiple interfering gases, demonstrating the practical application capability of MOS sensors for environmental odor monitoring.

Nanocrystals of CuCoO2 derived from MOFs and their catalytic performance for the oxygen evolution reaction

In this work, two different solvothermal synthesis routes were employed to prepare MOF-derived CuCoO₂ (CCO) nanocrystals for electrocatalytic oxygen evolution reaction (OER) application. The effects of the reductants (ethylene glycol, methanol, ethanol, and isopropanol), NaOH addition, the reactants, and the reaction temperature on the structure and morphology of the reaction product were investigated. In the first route, Cu-BTC derived CCO (CCO1) nanocrystals with a size of ∼214 nm and a specific surface area of 4.93 m² g^-1 were prepared by using Cu-BTC and Co(NO₃)₂·6H₂O as the Cu and Co source, respectively. In the second route, ZIF-67 derived CCO (CCO2) nanocrystals with a size of ∼146 nm and a specific surface area of 11.69 m² g^-1 were prepared by using ZIF-67 and Cu(NO₃)₂·3H₂O as the Co and Cu source, respectively. Moreover, the OER performances of Ni foam supported CCO1 (Ni@CCO1) and CCO2 (Ni@CCO2) electrodes were evaluated in 1.0 M KOH solution. Ni@CCO2 demonstrates a better OER catalytic performance with a lower overpotential of 394.5 mV at 10 mA cm^-2, a smaller Tafel slope of 82.6 mV dec^-1, and long-term durability, which are superior to those of some previously reported delafossite oxide or perovskite oxide catalysts. This work reveals the preparation method and application potential of CCO electrocatalysts by using Cu-BTC/ZIF-67 as the precursor, providing a new approach for the preparation of delafossite oxide CCO and the enhancement of their OER performances.

Bismuth Vacancy-Induced Efficient CO2 Photoreduction in BiOCl Directly from Natural Air: A Progressive Step toward Photosynthesis in Nature

Photocatalytic CO₂ conversion into carbonaceous fuels through artificial photosynthesis is beneficial to global warming mitigation and renewable resource generation. However, a high cost is always required by special CO₂-capturing devices for efficient artificial photosynthesis. For achieving highly efficient photocatalytic CO₂ reduction (PCR) directly from natural air, we report rose-like BiOCl that is rich in Bi vacancies (V_Bi) assembled by nanosheets with almost fully exposed active {001} facets. These rose-like BiOCl with V_Bi assemblies provide considerable adsorption and catalytic sites, which hoists the CO₂ capture and reduction capabilities, and thus expedites the PCR to a superior value of 21.99 μmol·g^-1·h^-1 CO generation under a 300 W Xe lamp within 5 h from natural air. The novel design and construction of a photocatalyst in this work could break through the conventional PCR system requiring compression and purification for CO₂, dramatically reduce expenses, and open up new possibilities for the practical application of artificial photosynthesis.

Hybrid 3D Nanostructure-Based Hole Transport Layer for Highly Efficient Inverted Perovskite Solar Cells

In this study, we demonstrate a new hybrid three-dimensional (3D) nanostructure system as an efficient hole transport layer (HTL) by a facile design of a low-temperature solution process. It is realized by integrating high-conductive chromium-doped CuGaO₂ nanoplates synthesized with choline chloride (denoted as Cr/CuGaO₂-CC) into ultrasmall NiO_x nanoparticles. First, we propose to incorporate a Cr-doped strategy under hydrothermal synthesis conditions together with controllable intermediates and surfactants' assistance to synthesize fine-sized Cr/CuGaO₂-CC nanoplates. Subsequently, these two-dimensional (2D) nanoplates serve as the expressway for improving hole transportation/extraction properties. Meanwhile, the ultrasmall-sized NiO_x nanoparticles are employed to modify the surface for achieving unique surface properties. The HTL formed from the designed hybrid 3D-nanostructured system exhibits the advantages of smooth and full-covered surface, remarkable charge collection efficiency, energy level alignment between the electrode and perovskite layer, and the promotion of perovskite crystal growth. Consequently, nearly 20% of power conversion efficiency with negligible hysteresis is achieved in inverted perovskite solar cells (PSCs). This work not only demonstrates the potential applications of a 3D-nanostructured Cr/CuGaO₂-CC/NiO_x hybrid HTL in PSCs but also provides a fundamental insight into the design of hybrid material systems by manipulating electric behavior and morphology structure for achieving high-performance photovoltaic devices.

Investigation of the structural, optical and electrical properties of Ca2+ doped CuCoO2 nanosheets

In this work, we present the hydrothermal synthesis of delafossite oxide Ca-doped CuCoO2 (CCCaO) nanosheets at a low temperature of 100 °C. The crystal phase, morphology and chemical composition of these CuCoO2 (CCO) based samples were comprehensively characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The size of CCCaO nanosheets decreased with increasing Ca dopant concentration, and the optimized CCCaO nanosheets (∼490 nm in lateral size and ∼15 nm in thickness) were much smaller than CCO nanocrystals (∼540 nm in lateral size and 85 nm in thickness). The specific surface area of these CCO based samples increased with increasing Ca content, and the optimized CCCaO nanosheets present a high BET surface area of 28 m2 g-1. XPS and Raman spectroscopy analyses indicate Ca2+ dopant substitution on the Cu+ site in CCCaO nanosheets. Moreover, the effects of Ca2+ doping on the optical and electrical properties of these CCO based samples were further studied. The optical properties measured at room temperature show high absorbability (up to 90%) in the ultraviolet-visible-near infrared (UV-VIS-NIR) region, and the indirect band gap shows a significant blue-shift with increasing Ca2+ concentration. The CCO nanocrystals possess a higher electrical conductivity than the CCCaO nanosheets, and present good conductivities of around 12.81, 4.47 and 0.69 s m-1 for the CCO and CCCaO samples at room temperature. The facile fabrication process, tunable crystallite sizes, and excellent optical absorption and electrical properties of these CCO based nanomaterials are encouraging for the development of future applications in photoelectric devices.

Transparent p-Type Semiconductors: Copper-Based Oxides and Oxychalcogenides

While p-type transparent conducting materials (TCMs) are crucial for many optoelectronic applications, their performance is still not satisfactory. This has impeded the development of many devices such as photovoltaics, sensors, and transparent electronics. Among the various p-type TCMs proposed so far, Cu-based oxides and oxychalcogenides have demonstrated promising results in terms of their optical and electrical properties. Hence, they are the focus of this current review. Their basic material properties, including their crystal structures, conduction mechanisms, and electronic structures will be covered, as well as their device applications. Also, the development of performance enhancement strategies including doping/co-doping, annealing, and other innovative ways to improve conductivity will be discussed in detail.

An ab initio study of Cu-based delafossites as an alternative to nickel oxide in photocathodes: effects of Mg-doping and surface electronic features

CuMO2 delafossites (M = Al, Ga, and Cr) are p-type semiconductor oxides that have been recently proposed as the electrode in p-type dye-sensitized solar cells (p-DSSC) which is an alternative to the standard, low-performing nickel oxide. To assess this potential application of delafossites, we report here a DFT-based investigation of the structural and electronic properties of CuAlO2, CuGaO2 and CuCrO2. In particular, we address the role of Mg doping to obtain the p-type semiconducting character: the substitution of an M3+ cation with Mg2+ is easier with Ga than with Al and Cr, and, in all cases, the hole introduced by Mg2+ leads to the formation of Cu2+ species. Moreover, we address surface electronic features in order to characterize the most exposed delafossite surface termination and, more importantly, to predict the valence band maximum energy value, which determines the p-DSSC open circuit potential. From analysis of all our results, CuGaO2 emerges as the most promising system that can boost the development of new photocathodes for p-DSSCs.

Dielectric, optical and enhanced photocatalytic properties of CuCrO2 nanoparticles

Delafossite CuCrO₂ nanoparticles with band gap energy of 3.09 eV and surface area of 235 m² g⁻¹ were prepared by citrate precursor route showed enhanced catalytic degradation of methylene blue in H₂O under the sunlight irradiation.

Photoluminescence and photocatalytic properties of rhombohedral CuGaO2 nanoplates

Rhombohedral phase CuGaO₂ nanoplates with a diameter of about 10 μm were synthesized via low temperature hydrothermal method. Room temperature and low temperature photoluminescence of the obtained CuGaO₂ nanoplates were characterized. CuGaO₂ nanoplates exhibited blue emission at room temperature and free exciton emission were appeared at low temperature. The blue emission is originated from defects such as Cu vacancies, which is the possible origin of p-type conductivity. The appearance of free exciton emission can demonstrate the direct bandgap transition behavior of CuGaO₂ nanoplates. The as-prepared p-type CuGaO₂ nanoplates were further decorated by n-type ZnO nanoparticles via calcination method to fabricate p-n junction nanocomposites. The nanocomposites exhibited enhanced photocatalytic activity which can be ascribed to the effective separation of photogenerated carriers by the internal electrostatic field in the p-n junction region, and the enhanced light absorption properties resulted from sub-bandgap absorption effect of p-n junction. This work has offered a new insight into the design of p-n junction devices using p-type CuGaO₂ nanoplates.

Low temperature hydrothermal synthesis mechanism and thermal stability of p-type CuMnO2 nanocrystals

We first report an oxidation–reduction reaction mechanism for the hydrothermal synthesis of CuMnO₂ nanocrystals at the low temperature of 80 °C.

Copper borate as a photocathode in p-type dye-sensitized solar cells

Copper borate as a photocathode in p-type dye-sensitized solar cells.

Wurtzite-derived ternary I-III-O2 semiconductors

Ternary zincblende-derived I-III-VI2 chalcogenide and II-IV-V2 pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I-III-O2 oxide semiconductors with a wurtzite-derived β-NaFeO2 structure are limited. Wurtzite-derived β-LiGaO2 and β-AgGaO2 form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. β-CuGaO2, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I-III-O2 semiconductors is still limited. In this paper we review previous studies on β-LiGaO2, β-AgGaO2 and β-CuGaO2 to determine guiding principles for the development of wurtzite-derived I-III-O2 semiconductors.

Solution Synthesized p-Type Copper Gallium Oxide Nanoplates as Hole Transport Layer for Organic Photovoltaic Devices

p-Type metal-oxide hole transport layer (HTL) suppresses recombination at the anode and hence improves the organic photovoltaic (OPV) device performance. While NiOx has been shown to exhibit good HTL performance, very thin films (<10 nm) are needed due to its poor conductivity and high absorption. To overcome these limitations, we utilize CuGaO2, a p-type transparent conducting oxide, as HTL for OPV devices. Pure delafossite phase CuGaO2 nanoplates are synthesized via microwave-assisted hydrothermal reaction in a significantly shorter reaction time compared to via conventional heating. A thick CuGaO2 HTL (∼280 nm) in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) devices achieves 3.2% power conversion efficiency, on par with devices made with standard HTL materials. Such a thick CuGaO2 HTL is more compatible with large-area and high-volume printing process.

Cu(I)-based delafossite compounds as photocathodes in p-type dye-sensitized solar cells

The research of p-type dye-sensitized solar cells (p-DSSCs) has attracted growing attention because of the potential for integration with conventional n-type DSSCs (n-DSSCs) into the more efficient tandem-DSSCs. However, to date the performance of p-DSSCs is lagging behind that of n-DSSCs. One main reason is the lack of optimal photocathode materials. This article reviews the most recent progress in utilizing Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr), as photocathodes in p-DSSCs. As alternative materials to the commonly used NiO, the CuMO2 compounds have their intrinsic advantages such as lower valence band edge, larger optical bandgap and higher conductivity. By providing an insight into these materials and their applications in p-DSSCs, this perspective aims to stimulate more exciting research in the development of p-DSSCs as well as of tandem-DSSCs.

Mechanisms in the solution growth of free-standing two-dimensional inorganic nanomaterials

Free-standing two-dimensional (2D) nanomaterials have attracted extensive and growing research interest owing to their exotic physical and mechanical properties, which have enabled their applications in electronics, optoelectronics, electrochemical and biomedical devices. Current synthesis strategies rely largely on top-down approaches such as etching and exfoliation. Among bottom-up approaches in literature, there lacks a systematic understanding of the mechanisms of 2D crystal growth, unlike one-dimensional nanomaterials whose growth mechanisms have been well documented. To date, the growth design of free-standing 2D nanomaterials has remained a case-by-case practice. This review focuses on the bottom-up solution synthesis of free-standing 2D nanomaterials and summarizes the general mechanisms and empirical methodologies that can lead to 2D crystal growth. A brief outlook on the development of synthesis and application of solution-grown 2D nanomaterials is also presented.

Three-dimensional mesoscale heterostructures of ZnO nanowire arrays epitaxially grown on CuGaO2 nanoplates as individual diodes

We report a three-dimensional (3D) mesoscale heterostructure composed of one-dimensional (1D) nanowire (NW) arrays epitaxially grown on two-dimensional (2D) nanoplates. Specifically, three facile syntheses are developed to assemble vertical ZnO NWs on CuGaO2 (CGO) nanoplates in mild aqueous solution conditions. The key to the successful 3D mesoscale integration is the preferential nucleation and heteroepitaxial growth of ZnO NWs on the CGO nanoplates. Using transmission electron microscopy, heteroepitaxy was found between the basal planes of CGO nanoplates and ZnO NWs, which are their respective (001) crystallographic planes, by the observation of a hexagonal Moiré fringes pattern resulting from the slight mismatch between the c planes of ZnO and CGO. Careful analysis shows that this pattern can be described by a hexagonal supercell with a lattice parameter of almost exactly 11 and 12 times the a lattice constants for ZnO and CGO, respectively. The electrical properties of the individual CGO-ZnO mesoscale heterostructures were measured using a current-sensing atomic force microscopy setup to confirm the rectifying p-n diode behavior expected from the band alignment of p-type CGO and n-type ZnO wide band gap semiconductors. These 3D mesoscale heterostructures represent a new motif in nanoassembly for the integration of nanomaterials into functional devices with potential applications in electronics, photonics, and energy.

Enhanced performance of p-type dye-sensitized solar cells based on ultrasmall Mg-doped CuCrO2 nanocrystals

Herein, we present ultrasmall delafossite-type Mg-doped CuCrO2 nanocrystals prepared by using hydrothermal synthesis and their first application as photocathodes in efficient p-type dye-sensitized solar cells. The short-circuit current density (Jsc ) is notably increased by approximately 27% owing to the decreased crystallite size and the enhanced optical transmittance associated with Mg doping of the CuCrO2 nanocrystalline sample. An open-circuit voltage (Voc ) of 201 mV, Jsc of 1.51 mA cm(-2) , fill factor of 0.449, and overall photoconversion efficiency of 0.132% have been achieved with the CuCr0.9 Mg 0.1 O2 dye photocathode sensitized with the P1 dye under optimized conditions. This efficiency is nearly three times higher than that of the NiO-based reference device, which is attributed to the largely improved Voc and Jsc . The augmentation of Voc and Jsc can be attributed to the lower valance band position and the faster hole diffusion coefficient of CuCr0.9 Mg 0.1 O2 compared to those of the NiO reference, respectively, which leads to a higher hole collection efficiency.

p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO2 Nanoplates with Saturation Photovoltages Exceeding 460 mV

Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (Voc). Here we report the first application of delafossite CuGaO2 nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO2 nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of Voc on the illumination intensity to estimate the maximum obtainable Voc from the CuGaO2-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co(3+/2+)(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a Voc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs.