Construction of Highly Ordered ZnO–TiO₂ Nanotube Arrays (ZnO/TNTs) Heterostructure for Photocatalytic Application

In recent years, strenuous efforts have been devoted to exploring ZnO functionalized TiO₂ nanotube arrays (ZnO/TNTs) nanocomposites; however, there is still a paucity of reports on the construction of well-defined ZnO/TNTs heterostructure via efficient and easily accessible approach. In this work, drawing on a two-step anodization combined pyrolysis strategy, we attained a highly ordered ZnO/TNTs hybrid nanostructure. Combined with a collection of characterizations including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), diffusion reflectance spectrum (DRS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), we found that, in this coupling, in situ formed ZnO phases were uniformly grafted to TNTs framework giving rise to hybrid nanostructure, which is ascribed to cooperative interfacial interaction between polar TiO₂ layer and ZnO precursor. The underlying interaction leading to judicious combination of TNTs and ZnO was unveiled by Fourier transformed infrared spectrum (FTIR) and XPS. Alternatively, it has been shown that ZnO nanocrystals distributed on the TNTs could serve as favorable hole channels and receptors for efficient separation of photoexcited charge carriers, which results in significantly enhanced photocatalytic performances of ZnO/TNTs heterostructure in comparison with pure TNTs, ZnO film, and P25 particulate film. Furthermore, it is found that the hybrid photocatalyst demonstrated excellent photostability. It is hoped that our work could present a straightforward paradigm for preparation of hierarchical semiconductor/1-D semiconductor heterostructures

Photosensitization Efficiency Modulation of Atomically Precise Silver Nanoclusters for Photoelectrocatalysis

Atomically precise metal nanoclusters (NCs) have emerged as feasible alternatives to traditional photosensitizers in solar energy conversion due to the unique atomic stacking mode, quantum size effect, and abundant active sites. Despite the sporadic advancement in fabricating metal NC-based photosystems, most of which are predominantly centered on Au NCs, unleashing atomically precise silver nanoclusters as light-harvesting antennas has still been in the infant stage, with the charge transfer mechanism remaining elusive. Herein, we comprehensively demonstrate the photosensitization effect of Ag NCs in the photoelectrochemical (PEC) water-splitting reaction and strictly evaluate the correlation of photosensitization efficiency with atomic architecture. To these ends, tailor-made negatively charged l-glutathione (GSH)-capped Ag NCs [Agx, Ag9(GSH)6, Ag16(GSH)9, Ag31(GSH)19] as building blocks are controllably deposited on the metal oxide (MOs = TiO2, WO3, Fe2O3) substrate by a facile self-assembly strategy. Benefiting from the highly efficient photosensitization effect of atomically precise Ag NCs, these self-assembled MOs/Ag NC heterostructured photoanodes with an elegant charge transfer interface demonstrate significantly enhanced photoelectrochemical water oxidation performances under visible-light irradiation on account of efficient charge transport from Ag NCs to the MO substrate, substantially prolonging the charge lifetime of Ag NCs. Our work would significantly inspire ongoing interest in unlocking the generic photosensitization capability of atomically precise metal NCs for solar energy conversion

Crafting Insulating Polymer Mediated and Atomically Precise Metal Nanoclusters Photosensitized Photosystems Towards Solar Water Oxidization

Atomically precise metal nanoclusters (NCs) have been deemed as a new generation of metal nanomaterials because of their characteristic atomic stacking fashion, quantum confinement effect, and multitude of active sites. The discrete molecular-like energy band structure of metal NCs endows them with photosensitization capability for light harvesting and conversion. However, applications of metal NCs in photoelectrocatalysis are limited by the ultrafast charge recombination and unfavorable stability, impeding the construction of metal NC-based photosystems. In this work, we elaborately crafted multilayered metal oxide (MO)/(metal NCs/insulating polymer)n photoanodes by a facile layer-by-layer (LbL) assembly technique. In these well-defined heterostructured photoanodes, glutathione (GSH)-wrapped metal NCs (Agx@GSH, Ag9@GSH6, Ag16@GSH9, and Ag31@GSH19) and an insulating poly(allylamine hydrochloride) (PAH) layer are alternately deposited on the MO substrate in a highly ordered integration mode. We found that photoelectrons of metal NCs can be tunneled into the MO substrate via the intermediate ultrathin insulating polymer layer by stimulating the tandem charge transfer route, thus facilitating charge separation and boosting photoelectrochemical water oxidation performances. Our work would open a new frontier for judiciously regulating directional charge transport over atomically precise metal NCs for solar-to-hydrogen conversion

Nonconjugated Polymers Enabled Solar Water Oxidation

Wholly distinct from conjugated polymers which are featured by generic charge transfer capability stemming from a conjugated molecular structure, solid nonconjugated polymers mediated charge transport has long been deemed as theoretically impossible because of the deficiency of π electrons along the molecular skeleton, thereby retarding their widespread applications in solar energy conversion. Herein, we first conceptually unveil that intact encapsulation of metal oxides (e.g., TiO2, WO3, Fe2O3, and ZnO) with an ultrathin nonconjugated polyelectrolyte of branched polyethylenimine (BPEI) can unexpectedly accelerate the unidirectional charge transfer to the active sites and foster the defect generation, which contributes to the boosted charge separation and prolonged charge lifetime, ultimately resulting in considerably improved photoelectrochemical (PEC) water oxidation activities. The interfacial charge transport origins endowed by BPEI adornment are elucidated, which include acting as a hole-withdrawing mediator, promoting vacancy generation, and stimulating the directional charge flow route. We additionally ascertain that such charge transport characteristics of BPEI are universal. This work would unlock the charge transfer capability of nonconjugated polymers for solar water oxidation. The nonconjugated insulating polymer was utilized as a charge transport mediator for boosting charge migration and separation over metal oxides toward solar water oxidation

Bridging the Gap: Electron Relay and Plasmonic Sensitization of Metal Nanocrystals for Metal Clusters

In recent years, enormous attention has been paid to the construction of metal cluster-semiconductor nanocomposites because of the fascinating and unique properties of metal clusters; however, investigations on photoelectrochemical (PEC) and photocatalytic properties of metal cluster-semiconductor systems are still rare. Moreover, to date, intrinsic correlation between metal clusters and bulk metal nanocrystals has yet to be elucidated. In this work, a facile layer-by-layer (LbL) self-assembly strategy has been developed to judiciously and intimately integrate gold nanocrystals (Au) within the interface between gold clusters (Au_<i>x</i>) and hierarchically ordered TiO₂ nanotube arrays framework, by which imperative roles of Au nanocrystals as electron relay mediator and plasmonic sensitizer for Au_<i>x</i> clusters were revealed. In addition, it was found that synergistic interaction between Au nanocrystals and Au_<i>x</i> clusters contributed to promising visible-light-driven photocatalytical and PEC performances. It is anticipated that our work could provide a general way for rationally constructing metal and metal clusters codecorated semiconductor heterostructures and, more significantly, bridge the gap between metal clusters and metal nanocrystals for a diverse range of applications

Layer-by-Layer Self-Assembly of CdS Quantum Dots/Graphene Nanosheets Hybrid Films for Photoelectrochemical and Photocatalytic Applications

In recent years, increasing interest has been devoted to synthesizing graphene–semiconductor nanocomposites as efficient photocatalysts for extensive applications. Unfortunately, it is still challenging to make uniform graphene–semiconductor composite films with controllable film thickness and architecture, which are of paramount importance to meet the application requirements. In this work, stable aqueous dispersion of polymer-modified graphene nanosheets (GNs) was prepared via in situ reduction of exfoliated graphite oxide in the presence of cationic poly­(allylamine hydrochloride) (PAH). The resultant water-soluble PAH-modified GNs (GNs-PAH) in conjunction with tailor-made negatively charged CdS quantum dots (QDs) were utilized as nanobuilding blocks for sequential layer-by-layer (LbL) self-assembly of well-defined GNs–CdS QDs hybrid films, in which CdS QDs overspread evenly on the two-dimensional (2D) GNs. It was found that the alternating GNs–CdS QDs multilayered films showed significantly enhanced photoelectrochemical and photocatalytic activities under visible light irradiation as compared to pure CdS QDs and GNs films. The enhancement was attributed to the judicious integration of CdS QDs with GNs in an alternating manner, which maximizes the 2D structural advantage of GNs in GNs–CdS QDs composite films. In addition, photocatalytic and photoelectrochemical mechanisms of the GNs–CdS QDs multilayered films were also discussed. It is anticipated that our work may open new directions for the fabrication of uniform semiconductor/GNs hybrid films for a wide range of applications

Boosting Charge-Transfer Efficiency by Simultaneously Tuning Double Effects of Metal Nanocrystal in Z‑Scheme Photocatalytic Redox System

Integrating individual functional materials into elegant nanoarchitectures holds great promise for creating high-efficiency photosynthesis systems with unique structure-directing merits. Herein, an all-solid-state metal-based Z-scheme photocatalytic system consisting of well-defined one-dimensional WO₃@Au@CdS core–shell heterostructure has been progressively and rationally designed by a green and facile two-step wet-chemistry approach. Significantly, it was uncovered that Au ingredient sandwiched in between the interfacial domain of WO₃ and CdS layer plays simultaneous dual roles in boosting the visible-light-driven photoactivities of core–shell ternary heterostructure, that is, as interfacial charge-transfer mediator to expedite vectorial Z-scheme electron transfer between CdS and WO₃ and plasmonic photosensitizer to trigger the generation of plasmon-induced hot electrons, thereby substantially augmenting the photoelectron density in a photoredox catalytic system. Such cooperative concurrent dual roles of Au nanocrystal in Z-scheme photocatalytic system results in the versatile and considerably enhanced photoredox performances of plasmonic WO₃@Au@CdS core–shell heterostructure toward anaerobic reduction of aromatic nitro compounds to corresponding amines and mineralization of organic pollutants under visible light irradiation at ambient conditions. Moreover, predominant active species during the photoredox catalysis were accurately determined, on the basis of which the photocatalytic mechanism was reasonably deduced and clearly elucidated. This work would provide a quintessential paradigm to uncover the essential roles of metal nanocrystals along with their cooperative synergy in Z-scheme photocatalytic system for substantial solar energy conversion

Self-Transformation of Atomically Precise Alloy Nanoclusters to Plasmonic Alloy Nanocrystals: Evaluating Photosensitization in Solar Water Oxidation

Atomically precise alloy nanoclusters (NCs) inherit the advantages of homometal NC counterparts such as atomic stacking fashion, quantum confinement effect, and enriched catalytic active sites and simultaneously possess the advantageous physicochemical properties such as significantly enhanced photostability, ideal photosensitization efficiency, and favorable energy band structure. Nevertheless, elucidation of the roles of alloy NCs and alloy nanocrystals (NYs) in boosting solar water oxidation has so far not yet been reported owing to the deficiency of applicable alloy NC photosystems. Herein, utilizing the generic thermal-induced self-transformation of alloy NCs to alloy NYs, we comprehensively explore the photosensitization properties of glutathione (GSH)-capped alloy NCs (AgxAu1–x@GSH and CuxAu1–x@GSH) and the corresponding alloy NY (AgAu and CuAu) counterparts in solar water oxidation reaction. The results imply that photoelectrons of alloy NCs surpass the hot electrons over plasmonic alloy NYs in stimulating the PEC water oxidation reaction. The photoelectrons of alloy NCs demonstrate lower interfacial charge-transfer resistance, longer carrier lifetime, and a more enhanced photosensitization effect with respect to the plasmonic alloy NYs, contributing to the significantly boosted photoelectrochemical water oxidation activities. Moreover, we found that our result is universal

Light-Induced In Situ Transformation of Metal Clusters to Metal Nanocrystals for Photocatalysis

In situ transformation of glutathione-capped gold (Au_<i>x</i>) clusters to gold (Au) nanocrystals under simulated solar light irradiation was achieved and utilized as a facile synthetic approach to rationally fabricate Au_<i>x</i>/Au/TiO₂ ternary and Au/TiO₂ binary heterostructures. Synergistic interaction of Au_<i>x</i> clusters and Au nanocrystals contributes to enhanced visible-light-driven photocatalysis

Simultaneous Photocatalytic Tetracycline Oxidation and Cr(VI) Reduction by Z‑Scheme Multiple Layer TiO₂/SnIn₄S₈

Wastewater pollutants are a major threat to natural resources, with antibiotics and heavy metals being common water contaminants. By harnessing clean, renewable solar energy, photocatalysis facilitates the synergistic removal of heavy metals and antibiotics. In this paper, MXene was both a template and raw material, and MXene-derived oxide (TiO2) and SnIn4S8 Z-scheme composite materials were synthesized and characterized. The synergistic mode of photocatalytic reduction and oxidation leads to the enhanced utilization of e–/h+ pairs. The TiO2/SnIn4S8 exhibited a higher photocatalytic capacity for the simultaneous removal of tetracycline (TC) (20 mg·L–1) and Cr(VI) (15 mg·L–1). The main active substances of TC degradation and Cr(VI) reduction were identified via free radical scavengers and electron paramagnetic resonance (EPR). Additionally, the potential photocatalytic degradation route of TC was thoroughly elucidated through liquid chromatography–mass spectrometry (LC-MS)