Nanosized tubular clay minerals as inorganic nanoreactors for energy and environmental applications: A review to fill current knowledge gaps

Modern society pays further and further attention to environmental protection and the promotion of sustainable energy solutions. Heterogeneous photocatalysis is widely recognized as one of the most economically viable and ecologically sound technologies to combat environmental pollution and the global energy crisis. One challenge is finding a suitable photocatalytic material for an efficient process. Inorganic nanotubes have garnered attention as potential candidates due to their optoelectronic properties, which differ from their bulk equivalents. Among them, clay nanotubes (halloysite, imogolite, and chrysotile) are attracting renewed interest for photocatalysis applications thanks to their low production costs, their unique physical and chemical properties, and the possibility to functionalize or dope their structure to enhance charge-carriers separation into their structure. In this review, we provide new insights into the potential of these inorganic nanotubes in photocatalysis. We first discuss the structural and morphological features of clay nanotubes. Applications of photocatalysts based on clay nanotubes across a range of photocatalytic reactions, including the decomposition of organic pollutants, elimination of NOx, production of hydrogen, and disinfection of bacteria, are discussed. Finally, we highlight the obstacles and outline potential avenues for advancing the current photocatalytic system based on clay nanotubes. Our aim is that this review can offer researchers new opportunities to advance further research in the field of clay nanotubes-based photocatalysis with other vital applications in the future.

Engineering Directional Charge Carrier Transport Using Ferroelectric Polarization for Enhanced Photoelectrochemical Water Oxidation

Introducing ferroelectric polarization has shown great potential to facilitate interfacial charge separation in photoelectrochemical (PEC) water splitting. However, unambiguous evidence of the actual influence of spontaneous ferroelectric polarization, as compared to heterojunction formation, on electron extraction and PEC water splitting is still lacking to date. Herein, core-shell BaTiO3/TiO2 nanostructures are designed as photoanodes based on paraelectric cubic and ferroelectric tetragonal phases BaTiO3 (BTO) perovskite. The cubic and tetragonal crystalline phases are stabilized using selected elaboration methods. Compared to the paraelectric cubic (c-BTO), the ferroelectric tetragonal (t-BTO) leads to a favorable ferroelectric polarization, enhancing directional charge separation and as a consequence to increased photocurrent up to a factor of 1.95. More interestingly, the charge separation efficiency can be tuned by applying positive or negative polarization, with the highest charge separation obtained for the positive one. When loading Ni(OH)2 as a cocatalyst on the t-BTO@TiO2 photoanode, the Ni(OH)2 /TiO2 /t-BTO exhibits a high performance and superior stability toward PEC water oxidation with a photocurrent almost 6.7 times that of the reference SiO2 @TiO2 . The proposed facilitation may open an avenue to engineer charge separation and transport for high-performance PEC water oxidation.

Ba3(PO4)2 Photocatalyst for Efficient Photocatalytic Application

Barium phosphate (Ba3(PO4)2) is a class of material that has attracted significant attention thanks to its chemical stability and versatility. However, the use of Ba3(PO4)2 as a photocatalyst is scarcely reported, and its use as a photocatalyst has yet to be reported. Herein, Ba3(PO4)2 nanoflakes synthesis is optimized using sol-gel and hydrothermal methods. The as-prepared Ba3(PO4)2 powders are investigated using physicochemical characterizations, including XRD, SEM, EDX, FTIR, DRS, J-t, LSV, Mott-Schottky, and EIS. In addition, DFT calculations are performed to investigate the band structure. The oxidation capability of the photocatalysts is investigated depending on the synthesis method using rhodamine B (RhB) as a pollutant model. Both Ba3(PO4)2 samples prepared by the sol-gel and hydrothermal methods display high RhB photodegradation of 79% and 68%, respectively. The Ba3(PO4)2 obtained using the sol-gel process exhibits much higher stability under light excitation after four regeneration cycles. The photocatalytic oxidation mechanism is proposed based on the active species trapping experiments where O2 •‒ is the most reactive species. The finding shows the promising potential of Ba3(PO4)2 photocatalysts and opens the door for further investigation and application in various photocatalytic applications.

Highly Dispersed Ni-Pt Bimetallic Cocatalyst: The Synergetic Effect Yields Pt-Like Activity in Photocatalytic Hydrogen Evolution

Achieving high photocatalytic activity with the lowest possible platinum (Pt) consumption is crucial for reducing the cost of Pt-based cocatalysts and enabling large-scale applications. Bimetallic Ni-Pt cocatalysts exhibit excellent photocatalytic performance and are considered one of the most promising photocatalysts capable of replacing pure Pt for hydrogen evolution reaction (HER). However, the synergistic photocatalytic mechanism between bimetallic Ni-Pt cocatalysts needs to be further investigated. Herein, we deposit highly dispersed Ni-Pt bimetallic cocatalysts on the surface of TiO2 by radiolytic reduction. We study the dynamics of photogenerated charge carriers of the Ni-Pt-comodified TiO2 and propose their underlying electron transfer mechanisms, in which Pt acts as an electron trap, whereas Ni serves as an electron supplier. The synergistic effect is Ni/Pt ratio-dependent and can confer bimetallic Ni-Pt to pure Pt-like photocatalytic activity in HER. The Ni2-Pt1-comodified TiO2 is optimized to be the most cost-effective photocatalyst with robust stability, which exhibits about 40-fold higher performance than bare TiO2.

Selective Cocatalyst Decoration of Narrow-Bandgap Broken-Gap Heterojunction for Directional Charge Transfer and High Photocatalytic Properties

Narrow-bandgap semiconductors are promising photocatalysts facing the challenges of low photoredox potentials and high carrier recombination. Here, a broken-gap heterojunction Bi/Bi2 S3 /Bi/MnO2 /MnOx , composed of narrow-bandgap semiconductors, is selectively decorated by Bi, MnOx nanodots (NDs) to achieve robust photoredox ability. The Bi NDs insertion at the Bi2 S3 /MnO2 interface induces a vertical carrier migration to realize sufficient photoredox potentials to produce O2 •- and • OH active species. The surface decoration of Bi2 S3 /Bi/MnO2 by Bi and MnOx cocatalysts drives electrons and holes in opposite directions for optimal photogenerated charge separation. The selective cocatalysts decoration realizes synergistic surface and bulk phase carrier separation. Density functional theory (DFT) calculation suggests that Bi and MnOx NDs act as active sites enhancing the absorption and reactants activation. The decorated broken-gap heterojunction demonstrates excellent performance for full-light driving organic pollution degradation with great commercial application potential.

A Deprotection-free Method for High-yield Synthesis of Graphdiyne Powder with In Situ Formed CuO Nanoparticles

With a direct band gap, superior charge carrier mobility, and uniformly distributed pores, graphdiyne (GDY) has stimulated tremendous interest from the scientific community. However, its broad application is greatly limited by the complicated multistep synthesis process including complex deprotection of hexakis-[(trimethylsilyl)ethynyl]benzene (HEB-TMS) and peeling of GDY from the substrates. Here, we describe a deprotection-free strategy to prepare GDY powder by directly using HEB-TMS as the monomer. When CuCl was used as the catalysts in DMF solvent, the yield of GDY powder reached ≈100 %. More interestingly, uniformly dispersed CuO nanoparticles with an average diameter of ≈2.9 nm were in situ formed on GDY after the reaction. The prepared CuO/GDY was demonstrated an excellent co-catalyst for photocatalytic hydrogen evolution, comparable to the state-of-art Pt co-catalyst. The deprotection-free approach will widen the use of GDY and facilitate its scaling up to industrial level.

Z-scheme WO3/PANI heterojunctions with enhanced photocatalytic activity under visible light: A depth experimental and DFT studies

A WO₃@PANI heterojunction photocatalyst with a various mass ratio of polyaniline to WO₃ was obtained via the in situ oxidative deposition polymerization of aniline monomer in the presence of WO₃ powder. The characterization of WO₃@PANI composites was carried via X-ray diffraction (XRD), scanning electron microscopy (SEM-EDS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). The photocatalytic efficiency of WO₃@PANI photocatalysts was assessed by following the decomposition of the Rhodamine B (RhB) dye under visible light irradiation (λ >420 nm). The results evidenced the high efficiency of the WO₃@PANI (0.5 wt %) nanocomposite in the photocatalytic degradation of RhB (90% within 120 min) under visible light irradiation 3.6 times compared to pure WO₃. The synergistic effect between PANI and WO₃ is the reason for the increased photogenerated carrier separation. The superior photocatalytic performance of the WO₃@PANI catalyst was ascribed to the increased visible light in the visible range and the efficient charge carrier separation. Furthermore, the Density Functional Theory study (DFT) of WO₃@PANI was performed at the molecular level, to find its internal nature for the tuning of photocatalytic efficiency. The DFT results indicated that the chemical bonds connected the solid-solid contact interfaces between WO₃ and PANI. Finally, a plausible photocatalytic mechanism of WO₃@PANI (0.5 wt %) performance under visible light illumination is suggested to guide additional photocatalytic activity development.

Facile Vacuum Annealing-Induced Modification of TiO2 with an Enhanced Photocatalytic Performance

In this work, the photocatalytic performance enhancement of hydrothermally prepared TiO₂ was achieved by facile vacuum annealing treatment. Calcination of TiO₂ powder in air (CA-TiO₂) maintained its white color, while gray powder was obtained when the annealing was performed under vacuum (CV-TiO₂). Fourier transform infrared, total organic carbon, X-ray photoelectron spectroscopy, and electron paramagnetic resonance analyses proved that vacuum annealing transformed ethanol adsorbed on the surface of TiO₂ into carbon-related species accompanied by the formation of surface oxygen vacancies (Vo). The residual carbon-related species on the surface of CV-TiO₂ favored its adsorption of organic dyes. Compared with TiO₂ and CA-TiO₂, CV-TiO₂ exhibited an improved charge carrier separation with surface Vo as trapping sites for electrons. Vacuum annealing-induced improvement of crystallinity, enhancement of adsorption capacity, and formation of surface Vo contributed to the excellent photocatalytic activity of CV-TiO₂, which was superior to that of commercial TiO₂ (P25, Degussa). Obviously, vacuum annealing-triggered decomposition of ethanol played an important role in the modification of TiO₂. In the presence of ethanol, vacuum annealing was also suitable for the introduction of Vo into P25. Therefore, the current work offers an easy approach for the modification of TiO₂ to enhance its photocatalytic performance by facile vacuum annealing in the presence of ethanol.

A soft-chemistry assisted strong metal-support interaction on a designed plasmonic core-shell photocatalyst for enhanced photocatalytic hydrogen production

Engineering photocatalysts based on gold nanoparticles (AuNPs) has attracted great attention for the solar energy conversion due to their multiple and unique properties. However, boosting the photocatalytic performance of plasmonic materials for H2 generation has some limitations. In this study, we propose a soft-chemistry method for the preparation of a strong metal-support interaction (SMSI) to enhance the photocatalytic production of H2. The TiO2 thin overlayer covering finely dispersed AuNPs (forming an SMSI) boosts the photocatalytic generation of hydrogen, compared to AuNPs deposited at the surface of TiO2 (labelled as a classical system). The pathway of the charge carriers' dynamics regarding the system configuration is found to be different. The photogenerated electrons are collected by AuNPs in a classical system and act as an active site, while, unconventionally, they are injected back in the titania surface for an SMSI photocatalyst making the system highly efficient. Additionally, the adsorption energy of methanol, theoretically estimated using the density functional theory (DFT) methodology, is lower for the soft-chemistry SMSI photocatalyst accelerating the kinetics of photocatalytic hydrogen production. The SMSI obtained by soft-chemistry is an original concept for highly efficient photocatalytic materials, where the photon-to-energy conversion remains a major challenge.

Plasmonic core-shell nanostructure as an optical photoactive nanolens for enhanced light harvesting and hydrogen production

Hydrogen production using plasmonic photocatalyst has attracted increasing attention since it improves light harvesting and photoefficiency. Herein, we have designed a plasmonic photocatalyst in a core-shell nanostructure that enabled an improvement of light harvesting and photocatalytic production of hydrogen using a very low amount of gold nanoparticles. The core-shell nanostructure was found to mimic the focusing of light observed for the lens-like epidermal cells. Thus, the core-shell nanostructure acts as a convex nanolens to reinforce the electromagnetic field at the nanostructure surface. The electric field was also found to be enhanced, which improves the energy absorbed for gold particles located in the core-shell nanostructure. Thus, by adjusting the diameter of the core-shell nanostructure, an optimal intensity for the localized surface plasmon resonance of gold was obtained. Tuning the size of the core-shell nanostructure enabled to improve the absorption at the reactive surface, thus increasing the photocatalytic hydrogen production efficiency by 5-fold.

Effect of SiN x diffusion barrier thickness on the structural properties and photocatalytic activity of TiO2 films obtained by sol-gel dip coating and reactive magnetron sputtering

We investigate the effect of the thickness of the silicon nitride (SiN x ) diffusion barrier on the structural and photocatalytic efficiency of TiO2 films obtained with different processes. We show that the structural and photocatalytic efficiency of TiO2 films produced using soft chemistry (sol-gel) and physical methods (reactive sputtering) are affected differentially by the intercalating SiN x diffusion barrier. Increasing the thickness of the SiN x diffusion barrier induced a gradual decrease of the crystallite size of TiO2 films obtained by the sol-gel process. However, TiO2 obtained using the reactive sputtering method showed no dependence on the thickness of the SiN x barrier diffusion. The SiN x barrier diffusion showed a beneficial effect on the photocatalytic efficiency of TiO2 films regardless of the synthesis method used. The proposed mechanism leading to the improvement in the photocatalytic efficiency of the TiO2 films obtained by each process was discussed.

Study of mesoporous CdS-quantum-dot-sensitized TiO2 films by using X-ray photoelectron spectroscopy and AFM

CdS quantum dots were grown on mesoporous TiO2 films by successive ionic layer adsorption and reaction processes in order to obtain CdS particles of various sizes. AFM analysis shows that the growth of the CdS particles is a two-step process. The first step is the formation of new crystallites at each deposition cycle. In the next step the pre-deposited crystallites grow to form larger aggregates. Special attention is paid to the estimation of the CdS particle size by X-ray photoelectron spectroscopy (XPS). Among the classical methods of characterization the XPS model is described in detail. In order to make an attempt to validate the XPS model, the results are compared to those obtained from AFM analysis and to the evolution of the band gap energy of the CdS nanoparticles as obtained by UV-vis spectroscopy. The results showed that XPS technique is a powerful tool in the estimation of the CdS particle size. In conjunction with these results, a very good correlation has been found between the number of deposition cycles and the particle size.