Cellulose-Derived Hierarchical g-C3N4/TiO2-Nanotube Heterostructured Composites with Enhanced Visible-Light Photocatalytic Performance

Research progress, trends, and updates on pollutants removal by Bi2WO6-based photocatalysts under visible light irradiation

In recent years, extensive research has been conducted on bismuth tungstate (Bi2WO6) in the field of photocatalysis owing to its unique crystal structure and favorable bandgap. This study offers a comprehensive review of the research on Bi2WO6-based photocatalysts from 2007 to 2022 using bibliometric analysis. The analysis utilized the Web of Science Core Collection Database and encompassed a dataset of 2064 publications. The bibliometric analysis and science mapping were carried out using the bibliometix R-package and CiteSpace software. This analysis examined and discussed the network of relationships among countries, journals, organizations, authors, and keywords pertaining to the topic and subtopics under investigation. The findings demonstrate that China has played a significant role in this research area and has formed close collaborations with other countries. The identification of highly-cited emerging terms suggests that enhancing the photocatalytic performance of Bi2WO6-based nanomaterials is a primary research focus. Moreover, there has been increasing interest in exploring the synergistic effects of photocatalysis and adsorption as a means to improve catalytic efficiency. This study provides valuable insights for researchers seeking a deeper understanding of Bi2WO6-based photocatalysts.

Sunlight active cellulose/g-C3N4/TiO2nano-photocatalyst for simultaneous degradation of methylene blue dye and atenolol drug in real wastewater

The paper critically addresses two contemporary environmental challenges, the water crisis and the unrestricted discharge of organic pollutants in waterways together. An eco-friendly method was used to fabricate a cellulose/g-C3N4/TiO2photocatalytic composite that displayed a remarkable degradation of methylene blue dye and atenolol drug under natural sunlight. Introducing graphitic carbon nitride (g-C3N4) onto pristine TiO2improved hybrid material's photonic efficacy and enhanced interfacial charge separation. Furthermore, immobilizing TiO2/g-C3N4on a semi-interpenetrating cellulose matrix promoted photocatalyst recovery and its reuse, ensuring practical affordability. Under optimized conditions, the nano-photocatalyst exhibited ∼95% degradation of both contaminants within two hours while retaining ∼55% activity after ten cycles demonstrating a promising photostability. The nano-photocatalyst caused 66% and 57% reduction in COD and TOC values in industrial wastewater containing these pollutants. The photocatalysis was fitted to various models to elucidate the degradation kinetics, while LC-MS results suggested the mineralization pathway of dye majorly via ring opening demethylation. >98% disinfection was achieved againstE. coli(104-105CFU·ml-1) contaminated water. This study thus paves multifaceted strategies to treat wastewater contaminants at environmental levels employing nano-photocatalysis.

Microtubular cellulose-derived kapok fibre as a solid electron donor for boosting photocatalytic H2O2 production over C-doped g-C3N4 hybrid complexation

Cellulose continues to play an important and emerging role in photocatalysis, and its favourable properties, such as electron-rich hydroxyl groups, could enhance the performance of photocatalytic reactions. For the first time, this study exploited the kapok fibre with microtubular structure (t-KF) as a solid electron donor to enhance the photocatalytic activity of C-doped g-C₃N₄ (CCN) via ligand-to-metal-charge-transfer (LMCT) to improve hydrogen peroxide (H₂O₂) production performance. As confirmed by various characterisation techniques, the hybrid complex consisting of CCN grafted on t-KF was successfully developed in the presence of succinic acid (SA) as a cross-linker via a simple hydrothermal approach. The complexation formation between CCN and t-KF results in the CCN-SA/t-KF sample displaying a higher photocatalytic activity than pristine g-C₃N₄ to produce H₂O₂ under visible light irradiation. The enhanced physicochemical and optoelectronic properties of CCN-SA/t-KF imply that the LMCT mechanism is crucial in improving photocatalytic activity. This study promotes utilising the unique t-KF material's properties to develop a low-cost and high-performance cellulose-based LMCT photocatalyst.

Preparation of Oxygen-Doping Nongraphitic Carbon Nitride via Efficiency Exfoliation for the Application of Photocatalytic Degradation

E-OLCN photocatalyst was synthesized by oxygen doping of low molecular weight carbon nitride (LCN) with ethanol solvent stripping. The enhanced light absorption, fast electron transport rate, and photogenerated carrier separation efficiency of E-OLCN leads to the excellent photocatalytic degradation performance compared with the original materials. The synergistic effect of oxygen doping and ethanol solvent stripping plays a significant role for the modulation of electronic and structural properties of the prepared catalysts. Methyl orange (MO) and rhodamine B (RhB) are chosen as typical pollutants for the application of photocatalytic degradation. The E-OLCN sample exhibits outstanding photocatalytic degradation performance, where the rate constant k (1 × 10-2 min-1) of E-OLCN (1.68) is 2.9 times than that of O-LCN (0.58) and 8.8 times than that of pristine LCN (0.19) for MO. Moreover, modulated E-OLCN shows good stability after cycling experiments and the activity still achieved 90%. The detailed mechanism for MO degradation was proposed with the technical support of liquid chromatography-mass spectrometry (LC-MS) and electron spin resonance (EPR). The superoxide radical (·O2-) is the main active species and the MO molecule could be decomposition completely.

Hierarchical Bi2WO6/TiO2-nanotube composites derived from natural cellulose for visible-light photocatalytic treatment of pollutants

A series of Bi2WO6/TiO2-nanotube (Bi2WO6/TiO2-NT) heterostructured composites were prepared by utilizing natural cellulose (e.g., laboratory filter paper) as the structural template. The obtained nanoarchitectonics, namely Bi2WO6/TiO2-NT nanocomposites, displayed three-dimensionally interwoven structures which replicated the initial cellulose template. The composite Bi2WO6/TiO2-NT nanotubes were formed by TiO2 nanotubes that uniformly anchored with Bi2WO6 nanoparticles of various densities on the surface. The composites exhibited improved photocatalytic activities toward the reduction of Cr(VI) and degradation of rhodamine B under visible light (λ > 420 nm), which were attributed to the uniform anchoring of Bi2WO6 nanoparticles on TiO2 nanotubes, as well as strong mutual effects and well-proportioned formation of heterostructures in between the Bi2WO6 and TiO2 phases. These improvements arose from the cellulose-derived unique structures, leading to an enhanced absorption of visible light together with an accelerated separation and transfer of the photogenerated electron-hole pairs of the nanocomposites, which resulted in increased effective amounts of photogenerated carriers for the photocatalytic reactions. It was demonstrated that the photoinduced electrons dominated the photocatalytic reduction of Cr(VI), while hydroxyl radicals and reactive holes contributed to the photocatalytic degradation of rhodamine B.

Continuous g-C3N4 layer-coated porous TiO2 fibers with enhanced photocatalytic activity toward H2 evolution and dye degradation

TiO₂/g-C₃N₄ composite photocatalysts with various merits, including low-cost, non-toxic, and environment friendliness, have potential application for producing clean energy and removing organic pollutants to deal with the global energy shortage and environmental contamination. Coating a continuous g-C₃N₄ layer on TiO₂ fibers to form a core/shell structure that could improve the separation and transit efficiency of photo-induced carriers in photocatalytic reactions is still a challenge. In this work, porous TiO₂ (P-TiO₂)@g-C₃N₄ fibers were prepared by a hard template-assisted electrospinning method together with the g-C₃N₄ precursor in an immersing and calcination process. The continuous g-C₃N₄ layer was fully packed around the P-TiO₂ fibers tightly to form a TiO₂@g-C₃N₄ core/shell composite with a strong TiO₂/g-C₃N₄ heterojunction, which greatly enhanced the separation efficiency of photo-induced electrons and holes. Moreover, the great length–diameter ratio configuration of the fiber catalyst was favorable for the recycling of the catalyst. The P-TiO₂@g-C₃N₄ core/shell composite exhibited a significantly enhanced photocatalytic performance both in H₂ generation and dye degradation reactions under visible light irradiation, owing to the specific P-TiO₂@g-C₃N₄ core/shell structure and the high-quality TiO₂/g-C₃N₄ heterojunction in the photocatalyst. This work offers a promising strategy to produce photocatalysts with high efficiency in visible light through a rational structure design.

TiO₂@g-C₃N₄ core/shell fibers with a continuous g-C₃N₄ layer packing around exhibit high photocatalytic efficiency toward H₂ production and RhB degradation due to the intimate core/shell structure with a high-quality TiO₂/g-C₃N₄ heterojunction.

Synergistic Generation of Radicals by Formic Acid/H2O2/g-C3N4 Nanosheets for Ultra-efficient Oxidative Photodegradation of Rhodamine B

Water pollution is a global challenge endangering people's health. In this work, an ultra-efficient photodegradation system of Rhodamine B (RhB) has been established using a graphitic carbon nitride nanosheet (CNNS) as the semiconductor photocatalyst, from which energy is harvested on both the conduction band and valence band by formic acid and hydrogen peroxide, respectively. The optimized FA/H₂O₂/CNNS system increases the apparent photodegradation rate of RhB by 25 folds, from 0.0198 to 0.4975 min^-1. Through a comprehensive investigation with reactive oxygen species scavengers, electron paramagnetic resonance, high-performance liquid chromatography-mass spectrometry, etc., an oxidative mechanism for RhB photodegradation has been proposed, which combines enhanced charge carrier migration and synergistic generation of multiple radicals. Comparable performance improvements have also been observed for similar systems with different semiconductors, suggesting that such a catalytic system could afford a general approach to enhance semiconductor-catalyzed photodegradation.

A critical review on N-modified TiO2 limits to treat chemical and biological contaminants in water. Evidence that enhanced visible light absorption does not lead to higher degradation rates under whole solar light

Intensive research has been focused on the synthesis of N-modified TiO₂ materials having visible light absorption in order to get higher solar photocatalytic degradation rates of pollutants in water. However, an exhaustive revision of the topic underlines several controversial issues related to N-modified TiO₂ materials; these issues concern (a) the methodology used for preparation, (b) the assessment of the structural characteristics, (c) the mechanistic action modes and (d) the raisons argued to explain the limited performances of the prepared materials for organic and biological targets photodegradation in water. Taking advantage of last year's progress in analytical chemistry and in material characterization methods, the authors show, for example, that some works in the literature controversially attribute the term nitrogen doping without enough analytical evidence. Additionally, some papers describe N-modified TiO₂ photocatalysts as being able to generate holes with enough oxidative potential to form hydroxyl radicals under visible light. This last assertion often derives from a no pertinent use of illumination sources, light filters, or targets or a limited understanding of the thermodynamic aspects of the studied systems. None of N-containing materials prepared by herein presented methods leads, under solar light, to a significant enhancement in pollutants degradation and microorganism's inactivation kinetics.

Synergistic Promotion Effect of ZnCoS Solid Solution and Co1-xS on Photocatalytic Hydrogen Production of the CdS Composite

Photocatalytic reactions over effective photocatalysts are attractive to explore clean hydrogen energy from water with the utilization of solar energy. Ternary Co_1-xS@ZnCoS/CdS (ZCS/CdS) composites are constructed as photocatalysts through the hydrothermal formation of Co_1-xS and ZnCoS nanoparticles on CdS nanorods. Superior to the binary Co_1-xS/CdS composite, ZCS/CdS shows the improved photocatalytic activity with a hydrogen production rate of 58.4 mmol·g^-1·h^-1, which is 31.4 and 2.1 times higher than those of CdS and Co_1-xS/CdS, respectively. Different from binary Co_1-xS/CdS, the participation of a small amount of zinc favors the formation of ZnCoS solid solution in ZCS/CdS. A synergistic promotion effect of ZnCoS and Co_1-xS is confirmed due to tight heterojunctions among Co_1-xS, ZnCoS, and CdS in ZCS/CdS. The unique heterostructure of ZCS/CdS benefits its enhanced absorption ability of visible light, accelerating the separation of photoinduced electron-hole pairs and the electron transfer. ZCS/CdS exhibits the strong reduction ability and superior photocatalytic stability due to the role of double Z-scheme electron transfer pathways in the ternary composite. This work provides a suitable way to tune noble metal-free composite photocatalysts for efficient H₂ production.

Three-Dimensional Cross-Linked Nb2O5 Polymorphs Derived from Cellulose Substances: Insights into the Mechanisms of Lithium Storage

Niobium pentoxide (Nb₂O₅)-based materials have been regarded as promising anodic materials for lithium-ion batteries due to their abundant crystalline phases and stable and safe lithium storage performances. However, there is a lack of systematic studies of the relationship among the crystal structures, electrochemical characteristics, and lithium storage mechanisms for the various Nb₂O₅ polymorphs. Herein, pure pseudohexagonal Nb₂O₅ (TT-Nb₂O₅), orthorhombic Nb₂O₅ (T-Nb₂O₅), tetragonal Nb₂O₅ (M-Nb₂O₅), and monoclinic Nb₂O₅ (H-Nb₂O₅) with three-dimensional interconnected structures are reported, which were synthesized via a hydrothermal method using the commercial filter paper as the structural template followed by specific annealing processes. Impressively, the TT- and T-Nb₂O₅ species both possess bronze-like phases with "room and pillar" structures, while M- and H-Nb₂O₅ ones are both in the Wadsley-Roth phases with crystallographic shear structures. Among the pristine Nb₂O₅ materials, H-Nb₂O₅ exhibits the highest initial specific capacity (270 mA h g^-1), while T-Nb₂O₅ performs with the lowest (197 mA h g^-1) at 0.02 A g^-1, meaning that crystallographic shear structures provide more lithium storage sites. TT-Nb₂O₅ realizes the best rate capability (207 mA h g^-1 at 0.02 A g^-1 and 103 mA h g^-1 at 4.0 A g^-1), indicating that the "room and pillar" crystal structures favor fast lithium storage. Electrochemical analyses reveal that the TT- and T-Nb₂O₅ phases with "room and pillar" crystal structures utilize a pseudocapacitive intercalation mechanism, while the M- and H-Nb₂O₅ phases with the Wadsley-Roth shear structures follow a typical battery-type intercalation mechanism. A fresh insight into the further understanding of the intercalation pseudocapacitance on the basis of the unit cells of the electrode materials and a meaningful guidance for crystalline structural design/construction of the electrode materials for the next-generation LIBs are thus provided.

Regulation of the Volume Flow Rate of Aqueous Methyl Blue Solution and the Wettability of CuO/ZnO Nanorods to Improve the Photodegradation Performance of Related Microfluidic Reactors

Six CuO/ZnO nanorod (CuO/ZnONR)-based microfluidic reactors were constructed for different UV irradiation durations, with which an aqueous methylene blue (MB) solution was photodegraded at varied volume flow rate Q. Via numerical and experimental routes, the effects of the Q on the kinetic adsorption rate constant K_a and the initial rate constant K_A of the CuO/ZnONR-based microfluidic reactors were discussed. Moreover, a reverse contacting angle (CA) trend of CuO/ZnONRs to the reaction constant K curve of corresponding CuO/ZnONR-based microfluidic reactor suggested that the CA of CuO/ZnONRs was another key influencing factor that affected greatly the photodegradation performance of the microfluidic reactors. The Q of the aqueous MB solution and the UV irradiation duration for the photodeposition of CuO/ZnONRs were optimized to be 125 μL/min and 1.0 h, the K of the CuO/ZnONR-based microfluidic reactors reached 4.84 min^-1, and the related ΔK_A/K was less than 6%. Similarly, these methods and results can be employed not only to enhance the mass transport and adsorption of specific species within other nanostructured matrix material-coated microchannels but also to enlarge the actual contacting surface areas between these microchannels and the related solution, which further improve the performance of other nanostructured catalyst-based microfluidic reactors, rGO microfluidic voltage generation, and a GOx/AuNW enzymatic glucose microfluidic sensor.

Conjugated Cationic Pp-x Formed on g-C3N4 for Photocatalyzed Water Splitting

Polycationic Pp-x@g-C₃N₄ composite was synthesized through an in situ polymerization process of N-alkylpyridinium acetylenic alcohol bromide (p-x) above the surface of g-C₃N₄. The structure of p-0 and the Pp-x@g-C₃N₄ properties were checked by modern technologies. Photocatalytic tests of Pp-x@g-C₃N₄ in water splitting unveiled much better Pp-x@g-C₃N₄ hydrogen evolution activities by comparison with both g-C₃N₄ and Pp-0. The hydrogen production by Pp-0@g-C₃N₄ was 1654.5 μmol h^-1 g^-1, which is ∼26- and 22-fold greater in relation to what g-C₃N₄ and Pp-0 produced (62.7 and 75.0 μmol h^-1 g^-1, respectively), suggesting strong bilateral and synergistic interactions of g-C₃N₄ with Pp-0. Although the lengthening methylene chain in the polymers weakened the hydrogen generation ability of Pp-x@g-C₃N₄, the conjugated double bonds, solubilization, and dispersion of Pp-x polycationic surfactants made Pp-x@g-C₃N₄ superior to g-C₃N₄ in water splitting. Due to the readily available raw materials, a simple way of preparation (starting chemicals to p-0 to Pp-0@g-C₃N₄), high photocatalysis efficiency, light irritation stability, recyclable ability, and low toxicity, Pp-0@g-C₃N₄ is a good candidate for water splitting.

An Interface Optimization Strategy for g-C3N4-Based S-Scheme Heterojunction Photocatalysts

Graphitic carbon nitride (CN) has attracted much attention in photocatalytic fields due to its unique electronic band structure. However, the rapid recombination of photogenerated carriers severely inhibits its catalytic activity. The heterojunction structure has been widely confirmed to significantly improve the photocatalytic activity of CN through the formed interface structure. However, researchers often give attention to the band matching and conductivity of the cocatalyst, while the importance of the interface as a migration channel for photogenerated carriers is often overlooked. In this work, we adopt the strategy of morphology engineering to regulate the morphology of the CN photoactive component so as to achieve the interface optimization of the traditional heterojunction structure. The photocatalytic degradation experiment of rhodamine B shows that compared with the traditional CeO₂@CN heterojunction structure, the photocatalytic activity of the interface-optimized CeO₂/CN is increased by more than 20%. The following points could be used to explain the improvement of photocatalytic activity: (I) the formed S-scheme heterojunction structure, which inhibits the recombination of useful electrons and holes but expedites the recombination of relatively useless electrons and holes, (II) the increased interface area, which provides more carrier migration channels, and (III) the reduced interface contact resistance, which facilitates the separation and migration of photogenerated carriers. Furthermore, the interface optimization of the traditional Al₂O₃@CN and Fe₂O₃@CN heterojunction structures also achieved consistent results. This shows that the strategy in this work is a universal method for interface optimization, which provides potential alternative for further improving the catalytic activity of other heterojunction composites.

Bifunctional Nitrogen-Doped Carbon Dots in g-C3N4/WOx Heterojunction for Enhanced Photocatalytic Water-Splitting Performance

A novel metal-free all-solid-state z-scheme g-C₃N₄/NCDs/WO_x photocatalyst was fabricated using nitrogen-doped carbon dots (NCDs) as the electron mediator. As-prepared sandwich-structured composites displayed enhanced visible and NIR light photocatalytic activity. Under visible light irradiation, the hydrogen evolution rate reached 3.27 mmol g^-1 h^-1, which increased to roughly seven times higher than that of pure g-C₃N₄ and roughly twice that of g-C₃N₄/NCDs or g-C₃N₄/WO_x binary heterojunctions. The apparent quantum efficiency is 7.58% at 420 nm. The localized surface plasmon resonance effect of WO_x and the up-converted photoluminescence property of NCDs enhanced the utilization efficiency of NIR light together. In addition, the matched energy band structures of WO_x and g-C₃N₄ as well as the effective electron conductor (NCDs) between them accelerate electron transfer at the interface. The all-solid-state z-scheme g-C₃N₄/NCDs/WO_x photocatlyst was confirmed by a series of characterizations and experiment results. This report offered new insights into constructing an efficient all-solid-state z-scheme photocatalyst to be applied during the photocatalytic water-splitting reaction in the visible and NIR light regions.

A Bio-Inspired Nanotubular Na2MoO4/TiO2 Composite as a High-Performance Anodic Material for Lithium-Ion Batteries

A train of bio-inspired nanotubular Na₂MoO₄/TiO₂ composites were synthesized by using a natural cellulose substance (e.g., commercial ordinary filter paper) as the structural template. The TiO₂ gel films were coated on the cellulose nanofiber surfaces via a sol-gel method firstly, followed with the deposition of the poly(diallyldimethylammonium chloride)/Na₂MoO₄ (PDDA/Na₂MoO₄) bi-layers several times, through the layer-by-layer self-assembly route, yielding the (PDDA/Na₂MoO₄)_n/TiO₂-gel/cellulose composite, which was calcined in air to give various Na₂MoO₄/TiO₂ nanocomposites containing different Na₂MoO₄ contents (15.4, 24.1, and 41.4%). The resultant nanocomposites all inherited the three-dimensionally porous network structure of the premier cellulose substance, which were formed by hierarchical TiO₂ nanotubes anchored with the Na₂MoO₄ layers. When employed as anodic materials for lithium-ion batteries, those Na₂MoO₄/TiO₂ nanocomposites exhibited promoted electrochemical performances in comparison with the Na₂MoO₄ powder and pure TiO₂ nanotubes, which was resulted from the high capacity of the Na₂MoO₄ component and the buffering effects of the TiO₂ nanotubes. Among all the nanotubular Na₂MoO₄/TiO₂ composites, the one with a Na₂MoO₄ content of 41.4% showed the best electrochemical properties, such as the cycling stability with a capacity of 180.22 mAh g⁻¹ after 200 charge/discharge cycles (current density: 100 mA g⁻¹) and the optimal rate capability.