Visible-light Water Research Articles

Preliminary Studi of Dye-Sensitized Solar Cell Photoelectrochemical for CO2 Conversion to Methanol Using CuO-modified Dark TiO2 Nanotubes Array as Cathode

The increased atmospheric carbon dioxide (CO2) levels can lead to climate change and adversely affect human health. Therefore, it is necessary to develop a method to capture CO2 and convert it into a more valuable substance, such as methanol. In this study, we established a tandem system involving dye-sensitized solar cells and photoelectrochemical (DSSC-PEC), which included the PEC zone using CuO/dark TiO2 nanotube array (CuO/dTNA) as the dark cathode where CO2 reduction takes place, and Co-Pi/blue TiO2 nanotube array (Co-Pi/bTNA) as the counter photoanode. For the DSSC zone, N719/TNA was used as the photoanode, I-/I3- electrolyte, and Pt/FTO as the cathode. The tandem system was constructed by connecting the PEC cathode to the DSSC photoanode and the PEC photoanode to the DSSC cathode using silver wire. Under solely visible light induction and water containing sodium bicarbonate electrolyte saturated with CO2, the proposed devise produced methanol at 1.292 μmol/hour. Keyword: Carbon dioxide, copper oxide, dark TiO2 nanotube, DSSC-PEC

Two-dimensional pnictogen carbides and their Janus structures: Potential on visible-light photocatalytic water splitting for hydrogen production

This study explored eight novel and highly stable monolayer configurations: C3X2 (X = N, P, As, Sb, and Bi) and C3XY (X = P and As, YAs and Sb) by analyzing their electronic, optical, and photocatalytic properties based on first-principles density functional theory. The findings indicated that the C3X2 monolayers, except for C3Bi2, exhibit indirect band gap semiconductor properties, and their band gaps are 1.497, 1.862, 2.272, 1.448, and 1.561 eV, respectively. Notably, only C3P2 and C3As2 fulfilled the prerequisites for photocatalytic water splitting, boast high electron mobility (up to ∼103 cm2/Vs), strong absorption in the visible light range, and high solar-to-hydrogen (STH) efficiency (30.16% for C3P2 and 23.05% for C3As2). Moreover, the Janus C3PAs, C3PSb, and C3AsSb monolayers exhibited indirect band gaps ranging from 1.616 to 2.070 eV and satisfied the band structure criteria for photocatalytic water splitting. These three Janus monolayers outperformed C3X2 in terms of optical absorption, carrier mobility, and STH efficiency owing to their inherent electric fields. The STH efficiency of C3PSb and C3AsSb reach 31.26% and 29.17%, respectively, with electron mobility reaching up to 4642.01 and 11902.64 cm2/Vs, respectively, indicating their exceptional potential for photocatalytic water splitting.

The Link between Surface Visible Light Spectral Features and Water–Salt Transfer in Saline Soils—Investigation Based on Soil Column Laboratory Experiments

Monitoring soil salinity with remote sensing is difficult, but knowing the link between saline soil surface spectra, soil water, and salt transport processes might help in modeling for soil salinity monitoring. In this study, we used an indoor soil column experiment, an unmanned aerial vehicle multispectral sensor camera, and a soil moisture sensor to study the water and salt transport process in the soil column under different water addition conditions and investigate the relationship between the soil water and salt transport process and the spectral reflectance of the image on the soil surface. The observation results of the soil column show that the soil water and salt transportation process conforms to the basic transportation law of “salt moves together with water, and when water evaporates, salt is retained in the soil weight”. The salt accumulation phenomenon increases the image spectral reflectance of the surface layer of the soil column, while soil temperature has no effect on the reflectance. As the water percolates down, water and salt accumulate at the bottom of the soil column. The salinity index decreases instantly after the addition of brine and then tends to increase slowly. The experimental results indicate that this work can capture the relationship between the water and salt transport process and remote sensing spectra, which can provide theoretical basis and reference for soil water salinity monitoring.

CuxS films as photoelectrodes for visible-light water splitting

Copper sulfides are appealing semiconductors for optoelectronics applications requiring visible-light activity, such as solar water splitting, due to their relatively low band gaps and earth-abundance. This study investigates the effect of deposition conditions, including substrate temperature and background gas pressure, on the characteristics and photoelectrochemical performance of copper sulfide (CuxS) films grown by pulsed laser deposition (PLD) on Si (100) substrates. The results reveal that varying the deposition parameters influences the crystalline phases, morphology, and optoelectronic properties of the films. For deposition at room temperature, the films exhibited covellite CuS phase, while elevated deposition temperatures (250 °C and 500 °C) led to the formation of Cu1.8S and Cu2S phases, resulting in narrowing of the band gap, with the increased temperature also enhancing the film crystallinity and surface roughness. As a result of the changes in film morphology and crystal structure, photocurrent density magnitudes increased with higher deposition temperatures, reaching 0.747 mA/cm2 at −0.8 V for films deposited at 500 °C under vacuum, at least an order of magnitude higher than reported in comparable previous studies of the photoelectrochemical performance of CuxS. Background gas pressure only slightly affected the photocurrent density, with changes in photoactivity also correlating with changes in film roughness and band gap. The results demonstrate the good visible-light activity and behavior of CuxS as a stand-alone photoelectrode material, with tunability based on the fabrication conditions, suggesting their potential use in photoelectrochemical and other solar energy conversion applications.

A universal molecular oxygen-mediated photocatalysis strategy to boost visible-light induced hydrogen evolution through partial water splitting

A universal oxygen-mediated, stepwise strategy is proposed for efficiently inducing visible-light photocatalytic partial water decomposition into hydrogen over various semiconductor photocatalysts with conduction band bottoms below the single-electron oxygen reduction potential. In this scenario, molecular O2 can be transformed into reactive oxygen species, serving as both an oxidant and a homogeneous catalyst for producing hydrogen from alkaline aqueous solution containing various organic substrates. Further enhancement the performance is achieved by doping with phosphorous and oxygen, which constructs a local internal electric field and introduces sulfur vacancies, thereby facilitating the transport of photogenerated charge carriers, particularly on a representative CdS photocatalyst. The optimal hydrogen evolution performance reaches 2321.4 and 8521.4 μmol·gcatatlyst−1·h−1 in methanol and formaldehyde solution systems, respectively, with an apparent quantum efficiency exceeding 59.4 % under 450 nm visible light irradiation. Mechanistic studies demonstrate that the oxygen-mediated, sequential single-electron transfer process can occur with virtually zero activation energy.

Analysing the efficacy of TiO2/g-C3N4 nanohybrid electrospun membranes for visible-light photocatalytic water purification

The significance and reliability of membrane-based water purification in wastewater recycling are emphasized in this study. Hydrophilic cellulose acetate and hydrophobic polycaprolactam membranes incorporated with TiO2/g-C3N4 nanohybrids were fabricated by electrospinning. Synthesis of TiO2/g-C3N4 nanohybrids employed a sol–gel-assisted hydrothermal method, while g-C3N4 through fractional thermal polymerization of acetic acid-modified melamine. The optimized photocatalytic hybrid exhibited methylene blue degradation by a 1.5-fold increase in photoactivity compared to pristine TiO2 and a 3-fold increase compared to g-C3N4. Upon incorporation into electrospun membranes, the hybrids demonstrated effective degradation of Methylene blue dye by photocatalysis, and the membranes exhibited excellent stability over multiple cycles.

Two-Dimensional Janus XY (X/Y=P, As, Sb, Bi, X≠Y) material with excellent piezoelectricity and carrier mobility for Visible-Light water splitting

Janus materials, with unique physical and chemical properties, show promise in turning solar energy into clean hydrogen through photocatalytic water splitting. Herein, utilizing first-principles calculations, we have confirmed the stabilities of newly discovered two-dimensional XY materials (X/Y=P, As, Sb, Bi, X≠Y), unveiling their promising capabilities in photocatalytic water splitting. These Janus structures exhibit broad bandgaps, paired with remarkable electron carrier mobilities. Their out-of-plane piezoelectric coefficients (d31) varying from 0.10 pm/V to 0.31 pm/V, surpass those of many typical two-dimensional materials, highlighting their considerable potential in energy conversion applications. These materials stand out in water splitting due to their favorable band edges and electrostatic potential gradients, achieving solar-to-hydrogen (STH) efficiencies above 20 % and visible light absorption efficiencies over 10 %. Remarkably, Janus PBi showcases STH and visible light absorption efficiencies up to 29.01 % and 19.33 %, respectively. By using biaxial strain, Janus PAs and PSb can reach a peak STH efficiency of over 30 % and a top visible light absorption efficiency above 20 %. Janus AsSb (PBi) exhibits exceptionally high photocatalytic activity for the oxygen (hydrogen) evolution reaction. These attributes suggest that these Janus XY materials are promising contenders as photocatalytic catalysts for water-splitting applications.

Dynamic structural twist in metal-organic frameworks enhances solar overall water splitting.

Photocatalytic overall water splitting holds great promise for solar-to-hydrogen conversion. Maintaining charge separation is a major challenge but is key to unlocking this potential. Here we discovered a metal-organic framework (MOF) that shows suppressed charge recombination. This MOF features electronically insulated Zn2+ nodes and two chemically equivalent, yet crystallographically independent, linkers. These linkers behave as an electron donor-acceptor pair with non-overlapping band edges. Upon photoexcitation, the MOF undergoes a dynamic excited-state structural twist, inducing orbital rearrangements that forbid radiative relaxation and thereby promote a long-lived charge-separated state. As a result, the MOF achieves visible-light photocatalytic overall water splitting, in the presence of co-catalysts, with an apparent quantum efficiency of 3.09 ± 0.32% at 365 nm and shows little activity loss in 100 h of consecutive runs. Furthermore, the dynamic excited-state structural twist is also successfully extended to other photocatalysts. This strategy for suppressing charge recombination will be applicable to diverse photochemical processes beyond overall water splitting.

Systematic evaluations and integration of Assam indigenous Joha rice starch in intelligent packaging films for monitoring food freshness using beetroot extract

It is becoming increasingly important to have starch sources with different physicochemical properties to meet the needs of new applications in food, packaging, bioplastic, and pharmaceutical industries. The first part of this study dealt with the isolation of starch from culturally, geographically, nutritionally esteemed, and high-yielding Assam Joha rice. Fine and uniform particle size (6.3 ± 0.09 μm), high amylose content (28 ± 1.03 %), swelling behavior, viscoelastic rheological behavior, moderate gelatinization temperature (66 ± 1.7 °C), thermostable nature, type A crystallographic pattern with high (45 ± 3.3 %) crystallinity, and suitable microbial quality make the Joha rice derived starch physico-chemically and functionally suitable for potential applications in diverse domains. The latter part of the study focuses on one of the applications of derived starch as a suitable matrix for intelligent packaging films with the incorporation of betanin-enriched beetroot extract (BRE) as a bio-based pH sensor. The addition of 1.0 % w/v BRE to the starch film (starch-BRE III) significantly increased its functionality by reducing UV–visible light transmittance and water vapor permeability, along with enhancing flexibility and hydrophobicity due to intermolecular bonding between BRE and the starch film matrix. Moreover, starch-BRE films with different BRE concentrations were successfully used to monitor the real-time freshness of white meat (chicken and fish) and Indian cottage cheese samples. Overall, the results indicated that starch-BRE III has great potential as an intelligent packaging material for monitoring food freshness.

Full prediction of band potentials in semiconductor materials

A machine learning (ML) framework to predict the physical band potentials for a range of semiconductor materials, from metal sulfide, oxide, and nitride, to oxysulfide and oxynitride, is hereby described. A valence band maximum (VBM) model was established via the transfer learning of a large dataset of 2D materials (1382 samples, but with incorrect VBM potentials) onto a much smaller dataset of physically measured VBM for bulk 3D materials (87 samples) on a crystal graph convolutional neural network. This resulted in predictions with experimental accuracy (RMSE = 0.27 eV), which is a 3-fold improvement compared with ML trained on the physical dataset without transfer learning (RMSE = 0.75 eV). When combined with the bandgap prediction model (RMSE = 0.29 eV), a full prediction of conduction and valence band potentials can be made, which to the best of our knowledge, is the first for any ML framework. The variation of band potentials across low-index surfaces was predicted correctly and verified with reported physical potentials. In fact, the framework is able to capture variation in band potentials associated with minor atomic position alterations. Based on this, a general trend between surface atomic displacement and VBM shift was observed across various semiconductor materials. The model is not yet able to cope with major rearrangement of atomic sequence on surface layers, i.e., surface reconstructions, since it was not trained with such data but can be easily done so with specifically designed dataset. As an example application, the ML framework was used for the screening of potential photocatalytic materials for visible light water splitting. A total of 824 materials was successfully identified, including those experimentally-verified in the literature.

Graphitic Carbon Nitride Enters the Scene: A Promising Versatile Tool for Biomedical Applications.

Graphitic carbon nitride (g-C3N4), since the pioneering work on visible-light photocatalytic water splitting in 2009, has emerged as a highly promising advanced material for environmental and energetic applications, including photocatalytic degradation of pollutants, photocatalytic hydrogen generation, and carbon dioxide reduction. Due to its distinctive two-dimensional structure, excellent chemical stability, and distinctive optical and electrical properties, g-C3N4 has garnered a considerable amount of interest in the field of biomedicine in recent years. This review focuses on the fundamental properties of g-C3N4, highlighting the synthesis and modification strategies associated with the interfacial structures of g-C3N4-based materials, including heterojunction, band gap engineering, doping, and nanocomposite hybridization. Furthermore, the biomedical applications of these materials in various domains, including biosensors, antimicrobial applications, and photocatalytic degradation of medical pollutants, are also described with the objective of spotlighting the unique advantages of g-C3N4. A summary of the challenges faced and future prospects for the advancement of g-C3N4-based materials is presented, and it is hoped that this review will inspire readers to seek further new applications for this material in biomedical and other fields.

Economical, one-pot, and green synthesis of plant-based carbon quantum dots for efficient visible-light photocatalytic dye degradation and water purification

BackgroundGreen, in-situ, one-pot, facile, and economical synthesis of efficient photocatalysts to destroy emerging aqueous contaminants is essential. MethodBiocompatible carbon quantum dots (CQDs) photocatalysts were successfully synthesized from a plant source called garden thyme as a low-cost precursor by one-step calcination method. CQDs were synthesized in both pure form and co-doped with fluorine and boron indicating size of <10 nm and favorable quantum yield of 40%. Four key factors including synthesis temperature, synthesis time, fluorine and boron weight ratios, and CQDs size were investigated on removal rates of organic pollutant Rhodamine B (RhB). Significant findings10%F-10%B co-doped CQDs with synthesized at 400 ºC for 5 h with a size of <10 nm, showed the highest photocatalytic RhB degradation performances of 86.1% and 89% within 40 and 60 min, respectively. This improved performance was originated from synergetic influences of quantum size, co-doping, promising quantum yield, and great visible light harvesting, that satisfactorily separated and transported light-created electron-hole pair. Trapping experimental data approved that hydroxyl radicals were dominant reactive species that oxidized RhB on F,B co-doped CQDs catalysts upon visible light radiation. Overall, CQDs could be easily synthesized as a green and super-cheap photocatalyst and would be effective for wastewater treatments.

Boosting visible-light photoelectrochemical water splitting response of WO3–MoS2 nanocomposites by transition metal doping: Experimental and first-principles studies

The current study employs both theoretical and experimental methods to investigate the effect of Ti and Co doping on the WO3 nanocomposite with MoS2 photoanode's photoelectrochemical (PEC) water-splitting reaction. A simple hydrothermal and sol-gel method was adopted for the preparation of samples. The Ti-doped WO3–MoS2 (1.96 at %) sample showed the maximum optical absorption with a photocurrent density of 1.15 mA/cm2 at 1.6 V vs Ag/AgCl. The improvement in photocurrent density may be ascribed to the upward shifting of the conduction band edge towards the H+/H2 redox potential as well as the presence of a large number of active sites in MoS2. A high value of flat band potential of −0.3 V vs Ag/AgCl was achieved in Ti-doped WO3–MoS2, which was confirmed by Mott-Schottky measurements with the corresponding lowest charge transfer resistance at the semiconducting electrode/electrolyte interface. Furthermore, density functional theory calculations have also indicated that on doping WO3 with Ti and Co, there is a slight upward shift in the conduction band minimum, without any significant change in the valence band maximum, with respect to the Fermi level. This study provides new insight into the impact of transition metal doping and nanocomposite of chalcogenide materials for WO3-based PEC electrodes.

Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting.

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.

Metal-free carbazole-thiophene photosensitizers designed for a dye-sensitized H2-evolving photocatalyst in Z-scheme water splitting.

Dye-sensitized photocatalysts with molecular dyes and widegap semiconductors have attracted attention because of their design flexibility, for example, tunable light absorption for visible-light water splitting. Although organic dyes are promising candidates as metal-free photosensitizers in dye-sensitized photocatalysts, their efficiency in H2 production has far been unsatisfactory compared to that of metal-complex photosensitizers, such as Ru(II) tris-diimine-type complexes. Here, we demonstrate the substantial improvement of carbazole-thiophene-based dyes used for dye-sensitized photocatalysts through systematic molecular design of the number of thiophene rings, substituents in the thiophene moiety, and the anchoring group. The optimized carbazole-thiophene dye-sensitized layered niobate exhibited a quantum efficiency of 0.3% at 460nm for H2 evolution using a redox-reversible I- electron donor, which is six-times higher than that of the best coumarin-based metal-free dye reported to date. The dye-sensitized photocatalyst also facilitated overall water splitting when combined with a WO3-based O2-evolving photocatalyst and an I3-/I- redox shuttle mediator. The present metal-free dye provided a high dye-based turnover frequency for water splitting, comparable to that of the state-of-the-art Ru(II) tris-diimine-type photosensitizer, by simple adsorption onto a layered niobate. Thus, this study highlights the potential of metal-free organic dyes with appropriate molecular designs for the development of efficient water splitting.

A novel GO hoisted SnO2-BiOBr bifunctional catalyst for the remediation of organic dyes under illumination by visible light and electrocatalytic water splitting.

It is imperative to develop affordable multi-functional catalysts based on transition metals for various applications, such as dye degradation or the production of green energy. For the first time, we propose a simple chemical bath method to create a SnO2-BiOBr-rGO heterojunction with remarkable photocatalytic and electrocatalytic activities. After introducing graphene oxide (GO) into the SnO2-BiOBr nanocomposite, the charge separation, electron mobility, surface area, and electrochemical properties were significantly improved. The X-ray diffraction results show the successful integration of GO into the SnO2-BiOBr nanocomposite. Systematic material characterization by scanning and transmission electron microscopy showed that the photocatalysts are composed of uniformly distributed SnO2 nanoparticles (∼11 nm) on the regular nanosheets of BiOBr (∼94 nm) and rGO. The SnO2-BiOBr-rGO photocatalyst has outstanding photocatalytic activity when it comes to reducing a variety of organic dyes like rhodamine B (RhB) and methylene blue (MB). Within 90 minutes of visible light illumination, degradation of a maximum of 99% for MB and 99.8% for RhB was noted. The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance was also tested for the ternary nanocomposite, and significantly lower overpotential values of 0.34 and -0.11 V (vs. RHE) at 10 mA cm-2 were observed for the OER and HER, respectively. Furthermore, the Tafel slope values are 34 and 39 mV dec-1 for the OER and HER, respectively. The catalytic degradation of dyes with visible light and efficient OER and HER performance offer this work a broad spectrum of potential applications.

A direct Z-scheme BS/PtO2 van der Waals heterojunction for enhanced visible-light photocatalytic water splitting: a first-principles study.

A direct Z-scheme heterostructure holds a unique advantage in solar-driven overall water splitting, while the rational design of efficient photocatalysts for water splitting in such heterostructures remains a challenge. Based on first-principles calculations, this study proposes a novel direct Z-scheme two-dimensional (2D) van der Waals (vdW) heterostructure photocatalyst, denoted as BS/PtO2. Its band edges match the oxidation-reduction potentials of water, satisfying the conditions for the oxidation and reduction of water. Under acidic conditions (pH = 0), the results of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) indicate that BS/PtO2 can drive the OER without the need for an external bias, while the HER requires catalytic assistance. Interestingly, compared to single-layer materials, this heterostructure exhibits a significant enhancement in visible light absorption, implying a more efficient solar energy conversion capability. Therefore, the BS/PtO2 heterostructure holds the potential to become a promising direct Z-scheme photocatalyst with efficient visible light activity.

Wide-direct-band-gap monolayer carbon nitride CN2: a potential metal-free photocatalyst for overall water splitting.

Two dimensional metal-free semiconductors with high work function have attracted extensive research interest in the field of photocatalytic water splitting. Herein, we have proposed a kind of highly stable monolayer carbon nitride CN2 with an anisotropic structure based on first principles density functional theory. The calculations of electronic structure properties, performed using the HSE06 functional, indicate that monolayer CN2 has a wide direct band gap of 2.836 eV and a high work function of 6.54 eV. And the suitable band edge alignment, high electron mobility (∼103 cm2 V-1 s-1) and visible-light optical absorption suggest that monolayer CN2 has potential on visible-light photocatalytic water splitting at pH ranging from 0 to 14. Moreover, we have observed that uniaxial strain can effectively control the electronic structure properties and optical absorption of monolayer CN2, which can further improve its solar to hydrogen efficiency from 9.6% to 16.02% under 5% uniaxial tension strain along the Y direction. Our calculations have not only proposed a new type of potential metal-free photocatalyst for water splitting but also provided a functional part with high work function for type-I and scheme-Z heterojunction applied in photocatalytic water splitting.

Bifunctional conjugated polymer photocatalysts for visible light water oxidation and CO2 reduction: function- and site-selective hybridisation of Ru(ii) complex catalysts

Function-/site-selective hybridisation of two specific Ru(ii) complex catalysts on donor–acceptor conjugated polymers enables bifunctional visible-light water oxidation and CO2 reduction.

Band engineering of layered oxyhalide photocatalysts for visible-light water splitting.

The band structure offers fundamental information on electronic properties of solid state materials, and hence it is crucial for solid state chemists to understand and predict the relationship between the band structure and electronic structure to design chemical and physical properties. Here, we review layered oxyhalide photocatalysts for water splitting with a particular emphasis on band structure control. The unique feature of these materials including Sillén and Sillén-Aurivillius oxyhalides lies in their band structure including a remarkably high oxygen band, allowing them to exhibit both visible light responsiveness and photocatalytic stability unlike conventional mixed anion compounds, which show good light absorption, but frequently encounter stability issues. For band structure control, simple strategies effective in mixed-anion compounds, such as anion substitution forming high energy p orbitals in accordance with its electronegativity, is not effective for oxyhalides with high oxygen bands. We overview key concepts for band structure control of oxyhalide photocatalysts such as lone-pair interactions and electrostatic interactions. The control of the band structure of inorganic solid materials is a crucial challenge across a wide range of materials chemistry fields, and the insights obtained by the development of oxyhalide photocatalysts are expected to provide knowledge for diverse materials chemistry.