Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Review on nanoscale Bi-based photocatalysts

Photocatalysis represents a unique class of chemical transformations. It utilizes the energy delivered by light and drives reactions that are difficult, sometimes even impossible, to carry out in dark. When used for thermodynamically uphill reactions such as photosynthesis, photocatalysis promises a sustainable solution to large scale solar energy storage. Despite the longstanding interest in this process and research efforts, existing photocatalysis demonstrations are limited to academic laboratory settings. Chief among the reasons for the slow progress is the lack of suitable photocatalyst materials for large scale applications. For the purpose of effective light absorption, charge separation, and charge transfer, a large number of photocatalytic materials, including conventional semiconductors and emerging photoelectronic materials such as nanoscale plasmonic metal particles, quantum dots, and 2D materials, have been studied. This Review is written to summarize these recent efforts from a broad materia...

Photocatalysis: From Fundamental Principles to Materials and Applications

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e--h+) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO2 reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface

We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.

Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications

BiVO4 is a considerably promising semiconductor for photoelectrochemical water splitting due to its stability, low cost and moderate band gap. In this research, g-C3N4 was proposed in Z-scheme configuration which boosted the performance of BiVO4 up to four times. The experimental observations were counterchecked with Density Functional Theory (DFT) simulations. A TiO2/BiVO4 heterojunction was developed and its performance was compared with that of g-C3N4/BiVO4. The photocurrent for g-C3N4/BiVO4 was 0.42 mAcm−2 at 1.23 V vs. RHE which was the highest among g-C3N4 based Z-scheme heterojunction devices. Lower charge transfer resistance, higher light absorption and more oxygen vacancy sites were observed for the g-C3N4 based heterojunction. The simulated results attested that g-C3N4 and BiVO4 formed a van der Waals type heterojunction, where an internal electric ﬁeld facilitated the separation of electron/hole pair at g-C3N4/BiVO4 interface which further restrained the carrier recombination. Both the valence and conduction band edge positions of g-C3N4 and BiVO4 changed with the Fermi energy level. The resulted heterojunction had small effective masses of electrons (0.01 me) and holes (0.10 me) with ideal band edge positions where both CBM and VBM were well above and below the redox potential of water.

Enhanced photoelectrochemical performance of Z-scheme g-C3N4/BiVO4 photocatalyst

Solar light is widely recognized as one of the most valuable renewable energy sources for the future. However, the development of solar-energy technologies is severely hindered by poor energy-conversion efficiencies due to low optical-absorption coefficients and low quantum-conversion yield of current-generation materials. Huge efforts have been devoted to investigating new strategies to improve the utilization of solar energy. Different chemical and physical strategies have been used to extend the spectral range or increase the conversion efficiency of materials, leading to very promising results. However, these methods have now begun to reach their limits. What is therefore the next big concept that could efficiently be used to enhance light harvesting? Despite its discovery many years ago, with the potential for becoming a powerful tool for enhanced light harvesting, the slow-photon effect, a manifestation of light-propagation control due to photonic structures, has largely been overlooked. This review presents theoretical as well as experimental progress on this effect, revealing that the photoreactivity of materials can be dramatically enhanced by exploiting slow photons. It is predicted that successful implementation of this strategy may open a very promising avenue for a broad spectrum of light-energy-conversion technologies.

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Slow Photons for Photocatalysis and Photovoltaics.

Novel 3DOM BiVO4/TiO2 nanocomposites with intimate contact were for the first time synthesized by a hydrothermal method in order to elucidate their visible-light-driven photocatalytic performances. BiVO4 nanoparticles and 3DOM TiO2 inverse opal were fabricated respectively. These materials were characterized by XRD, XPS, SEM, TEM, N2 adsorption–desorption and UV-vis diffuse (UV-vis) and photoluminescence spectroscopies. As references for comparison, a physical mixture of BiVO4 nanoparticles and 3DOM TiO2 inverse opal powder (0.08 : 1), and a BiVO4/P25 TiO2 (0.08 : 1) nanocomposite made also by the hydrothermal method were prepared. The photocatalytic performance of all the prepared materials was evaluated by the degradation of rhodamine B (RhB) as a model pollutant molecule under visible light irradiation. The highly ordered 3D macroporous inverse opal structure can provide more active surface areas and increased mass transfer because of its highly accessible 3D porosity. The results show that 3DOM BiVO4/TiO2 nanocomposites possess a highly prolonged lifetime and increased separation of visible light generated charges and extraordinarily high photocatalytic activity. Owing to the intimate contact between BiVO4 and large surface area 3DOM TiO2, the photogenerated high energy charges can be easily transferred from BiVO4 to the 3DOM TiO2 support. BiVO4 nanoparticles in the 3DOM TiO2 inverse opal structure act thus as a sensitizer to absorb visible light and to transfer efficiently high energy electrons to TiO2 to ensure long lifetime of the photogenerated charges and keep them well separated, owing to the direct bandgap of BiVO4 of 2.4 eV, favourably positioned band edges, very low recombination rate of electron–hole pairs and stability when coupled with photocatalysts, explaining the extraordinarily high photocatalytic performance of 3DOM BiVO4/TiO2 nanocomposites. It is found that larger the amount of BiVO4 in the nanocomposite, longer the duration of photogenerated charge separation and higher the photocatalytic activity. This work can shed light on the development of novel visible light responsive nanomaterials for efficient solar energy utilisation by the intimate combination of an inorganic light sensitizing nanoparticle with an inverse opal structure with high diffusion efficiency and high accessible surface area.

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Novel 3DOM BiVO4/TiO2 nanocomposites for highly enhanced photocatalytic activity

Three dimensionally ordered macroporous inverse opal TiO2 nanocomposites decorated by ZnO quantum dots (ZnO QDs@3DOM TiO2) with an intimate contact were successfully synthesized using the sol-gel technique and characterized in terms of structure, porosity, chemical composition and optical properties. The photocatalytic activity of ZnO QDs@3DOM TiO2 nanocomposites with different ZnO QDs amounts was evaluated in the aqueous phase of dye pollutant molecules and compared with the state-of-the-art 3DOM TiO2 and P25 photocatalysts. The symbiotic effect of ZnO QDs and 3DOM photonic structure on the light absorption and further on the photocatalytic activity of the nanocomposites was observed. The sample with the highest ZnO QDs amount exhibits extraordinarily high photocatalytic activity, which is attributed to firstly, the formation of an intimate junction between the two semiconductors, hence favoring the separation of photo-introduced electron–hole pairs in ZnO-TiO2 photocatalyst, and, secondly, to the quantum size effect (QSE). The QSE results in an increase in the width of the forbidden electronic band, which increases the energy of electrons (holes) in the conduction (valence) and particularly leads to the displacement of the conduction band potentials of ZnO to more negative energy values compared to TiO2. Thanks to the heterojuction formed between ZnO QDs and 3DOM TiO2, the energy difference between conduction bands of both semiconductors acts as a driving force for rapid electron/hole transfer between the coupled materials. Due to the extremely short diffusion time, the lifetime of photogenerated charge carriers is extended and the effectiveness of reduction and oxidation process is increased with faster reaction kinetics. Increasing the amount of ZnO QDs can boost the photocatalytic activity. On the other hand, 3DOM photonic structure of TiO2 with its open meso-macroporosity can facilite the diffusion of dye molecules and light propagation. This first successful example of symbiose of a series of physical effects can open a new window for solar energy conversion by the synergitic association of QSEs, photonic effect and other effects such as plasmonic effects, in one solid material to develop highly efficient solar light havesting system to enhance solar energy conversion effeciency for photocatalysis and photovoltaics.

ZnO quantum dots decorated 3DOM TiO2 nanocomposites: Symbiose of quantum size effects and photonic structure for highly enhanced photocatalytic degradation of organic pollutants

Benoit Van der Schueren

Papers

Slow Photons for Photocatalysis and Photovoltaics.

Novel 3DOM BiVO4/TiO2 nanocomposites for highly enhanced photocatalytic activity

ZnO quantum dots decorated 3DOM TiO2 nanocomposites: Symbiose of quantum size effects and photonic structure for highly enhanced photocatalytic degradation of organic pollutants

A comprehensive study of the reaction parameters involved in the synthesis of silica thin films with well-ordered uni-directional mesopores.