Semiconductor hierarchically structured flower-like clusters for dye-sensitized solar cells with nearly 100% charge collection efficiency

Solution-Processed Anatase Titania Nanowires: From Hyperbranched Design to Optoelectronic Applications

The utilization of solar energy and the development of its related optoelectronic devices have become more important than ever. Solar cells or photoelectrochemical (PEC) cells that require the design of light harvesting assemblies for efficiently converting solar light into electricity or solar fuels are of particular interest. Semiconductor TiO₂, serving as the photoelectrode for photovoltaic devices (e.g., dye- or quantum dot-sensitized solar cells (DSSCs/QDSSCs) or perovskite solar cells (PSCs)) and PEC cells, has aroused intense research interest owing to its inherent characteristics of wide band gap and promising optical and electrical properties. TiO₂ nanowires (TNWs) have been widely used in optoelectronic devices due to their unique 1D geometry and salient optical and electrical properties. However, the insufficient surface area resulting from the relatively large diameter of NWs and considerable free space between adjacent NWs restricts their optoelectronic performance. Hence, it is desirable to explore every feasible aspect of TNWs in terms of structural design and optical management, aiming to further improve the performance of optoelectronic devices. In this Account, we present a brief survey of strategies for designing branched or hyperbranched TNW-based photoelectrodes and their applications in solar cells and PEC cells. The general strategies (e.g., alkaline/acid hydrothermal method, lift-off transfer, and self-assembly approach) are discussed to address the challenges associated with fabricating TNWs on transparent conducting oxide (TCO) substrates. A series of strategies to fabricate judiciously designed 3D branched array architectures, including length tuning and sequential surface branched or hyperbranched modification, are proposed. The versatile implantation of the TNWs onto other backbones (nanosheets, nanotubes, hollow spheres, or multilayered electrodes) and substrates (fiber-shaped metal wire or mesh, flexible metal foil, or plastic sheet) is demonstrated to construct a new class of the TNW-embedded composite electrode materials with desired morphological characteristics and optoelectronic properties, for example, favorable energy level alignment for cascade charge transfer and rational homogeneous/heterogeneous interfacial engineering. The functionalities of TNW-based electrodes include enlarged surface area and superior light scattering for maximized light harvesting, as well as facilitated charge transport and suppressed charge recombination for enhanced charge collection, which are promising in optoelectronic fields such as solar cells, photocatalysis, and PEC cells. Beyond TNWs, one can also integrate other types of semiconductor (e.g., Fe₂O₃ or WO₃) NWs into rationally designed structures for preparing novel photocatalytic materials with panchromatic absorption, efficient charge transfer, and excellent catalytic properties. Finally, an insightful perspective for rational design of advanced NW-based materials is provided.

Optical radiation stability of ZnO hollow particles

Zinc oxide has multifunctional physical properties depending on its microstructure and morphology. Herein, we reported the in situ investigations of the radiation stability of ZnO particles with hollow, ball, star and flower shapes under electron and proton irradiation. 100 keV protons with a fluence of 5 × 10¹⁵ cm^-2 and 50 keV electrons with fluence ranging from 0.5 to 7 × 10¹⁶ cm^-2 are employed to investigate the radiation stability of nanostructured ZnO particles. In situ reflectance, X-ray photoelectron spectra and photoluminescence were characterized in the irradiation environment to avoid the effects of the atmospheric environment on radiation induced defects. The experimental results reveal that, compared to the other shapes, the hollow structure with the best radiation stability due to the hollow structure facilitates the decrease of the accumulation of radiation defects. This study clearly demonstrates the promise of ZnO hollow particles as a plasmonic nanostructure for achieving high radiation stability, and they could be easily employed to serve as the radiation stability pigment for coatings.

Paper-Based Origami Photoelectrochemical Sensing Platform with TiO2/Bi4NbO8Cl/Co-Pi Cascade Structure Enabling of Bidirectional Modulation of Charge Carrier Separation

A bidirectional modulation of photoinduced charge carrier separation strategy based on TiO₂/Bi₄NbO₈Cl/Co-Pi was proposed in microfluidic paper based photoelectrochemical analytical device (μ-POAD). Perovskite Bi₄NbO₈Cl with high charge carrier mobility was employed as visible light absorber, sandwiching between electron transporting material (ETM) and hole transporting material (HTM). Paper based TiO₂ nanosheet arrays (PTNAs) serve as the ETM to provide a direct pathway for electron transport and Co-Pi works as the HTM to extract holes. Driven by a built-in electric field, the generated electrons of Bi₄NbO₈Cl are extracted by PTNAs, while holes are drawn toward Co-Pi, achieving efficient carrier separation. Remarkably, it is the first time that the HTM was introduced into μ-POAD to efficiently output holes and enhance the sensitivity. With the aid of ETM and HTM, 2.59 and 14.6 times higher photocurrent density was obtained compared with PTNAs/Bi₄NbO₈Cl and Bi₄NbO₈Cl photoelectrode, respectively. Benefiting from this dramatic photocurrent signal, ultrasensitive detection of β human chorionic gonadotrophin is realized with the linear range of 0.01-3000 IU L^-1 and detection limitation of 0.005 IU L^-1. This work demonstrates the importance of efficient carrier separation to the sensitivity of μ-POAD and paves the way for developing a high-performance analytical device.

Complex ZnO-TiO2 Core-Shell Flower-Like Architectures with Enhanced Photocatalytic Performance and Superhydrophilicity without UV Irradiation

ZnO-TiO₂ core-shell photocatalysts of a complex flower-like architecture were synthesized, using a well-controlled sol-gel coating reaction of presynthesized ZnO flower-like structures. The samples were characterized by X-ray diffraction, field emission scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance UV-vis and attenuated total reflectance-Fourier transform infrared spectroscopy, nitrogen adsorption-desorption measurements, and photoluminescence measurements. Well-defined, core-shell flowers with a wurtzite ZnO core and anatase TiO₂ shells, with variable shell thickness, were acquired by appropriately adjusting the ZnO/Ti precursor mass feed ratio in the reaction. Moreover, hollow TiO₂ flowers were obtained, and they retained their morphology following the etching of the ZnO core in an acidic solution. The photocatalytic performance of the core-shell and hollow semiconductors was evaluated via the decoloration of a methylene blue dye solution under UV-vis irradiation. The core-shell flowers exhibited a higher decoloration rate, when compared with bare ZnO flowers, TiO₂ particles, and hollow TiO₂ flowers, and the photoactivity was dependent on the TiO₂ shell thickness. This was attributed to the efficient separation of the photogenerated holes and electrons at the ZnO-TiO₂ interface. Moreover, the most photoactive core-shell catalyst exhibited excellent reusability and stability for at least three photocatalytic cycles and excellent superhydrophilicity without UV irradiation, which is due to the increased roughness of the flower-like structures.

A Simple Route to Reduced Graphene Oxide-Draped Nanocomposites with Markedly Enhanced Visible-Light Photocatalytic Performance

Nanocomposites (denoted RGO/ZnONRA) comprising reduced graphene oxide (RGO) draped over the surface of zinc oxide nanorod array (ZnONRA) were produced via a simple low-temperature route, dispensing with the need for hydrothermal growth, electrochemical deposition or other complex treatments. The amount of deposited RGO can be readily tuned by controlling the concentration of graphene oxide (GO). Interestingly, the addition of Sn(2+) not only enables the reduction of GO, but also functions as a bridge that connects the resulting RGO and ZnONRA. Remarkably, the incorporation of RGO improves the visible-light absorption and reduces the bandgap of ZnO, thereby leading to the markedly improved visible-light photocatalytic performance. Moreover, RGO/ZnONRA nanocomposites exhibit a superior stability as a result of the surface protection of ZnONRA by RGO. The mechanism on the improved photocatalytic performance based on the cophotosensitizations under the visible-light irradiation has been proposed. This simple yet effective route to the RGO-decorated semiconductor nanocomposites renders the better visible-light utilization, which may offer great potential for use in photocatalytic degradation of organic pollutants, solar cells, and optoelectronic materials and devices.

A novel aptasensor for thrombin detection based on alkaline phosphatase decorated ZnO/Pt nanoflowers as signal amplifiers

To remedy the problems caused by the introduction of an additional electron mediator and realize signal amplification, a new strategy has been presented to construct an electrochemical aptasensor for thrombin detection based on the cascade electrocatalysis of alkaline phosphatase (ALP) and Pt nanoparticle (PtNP)-functionalized ZnO nanoflowers.

Mesoporous titania-vertical nanorod films with interfacial engineering for high performance dye-sensitized solar cells

Working electrode (WE) fabrication offers significant challenges in terms of achieving high-efficiency dye-sensitized solar cells (DSCs). We have combined the beneficial effects of vertical nanorods grown on conducting glass substrate for charge transport and mesoporous particles for dye loading and have achieved a high photoconversion efficiency of (η) > 11% with an internal quantum efficiency of ∼93% in electrode films of thickness ∼7 ± 0.5 μm. Controlling the interface between the vertical nanorods and the mesoporous film is a crucial step in attaining high η. We identify three parameters, viz., large surface area of nanoparticles, increased light scattering of the nanorod-nanoparticle layer, and superior charge transport of nanorods, that simultaneously contribute to the improved photovoltaic performance of the WE developed.

Periodic layered inverse micelle multilayers with tunable photonic band gap: fabrication and application in dye-sensitized solar cells

Periodic organic-inorganic multilayer films are constructed by stepwise alternate build-up of UV-stabilized poly(styrene-block-vinylpyridine) block copolymer inverse micelles and poly(styrene-block-ethylene oxide) block copolymer layers containing inorganic moieties at the polar core blocks. The layered block copolymer inverse micelle films show strong reflective color and well-defined photonic stop bands in the entire wavelength region from visible to near IR, which can be fine-tuned by controlling the inner architectures, i.e., the periodic size of the layered structure. The layered block copolymer films are integrated into the back-side of counter electrodes as a light reflection layer and thereby an enhancement ratio of ∼11% in the cell efficiency is achieved, which can be attributed to the increased light harvesting by the sensitized dye molecules. Tailoring the inner structure of the photonic band gap multilayers, the wavelength of reflected light can be adjusted to the wavelength of dye absorption, leading to a noticeable enhancement in photocurrent and power conversion efficiency.