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16 pages, 3259 KiB

Open AccessFeature PaperArticle

Influence of Vacancy Defects on the Interfacial Structural and Optoelectronic Properties of ZnO/ZnS Heterostructures for Photocatalysis

by Sajjad Hussain, Lingju Guo and Tao He

Catalysts 2023, 13(8), 1199; https://doi.org/10.3390/catal13081199 - 10 Aug 2023

Viewed by 818

Abstract

Hybrid density functional theory has been employed to study the influence of interfacial oxygen (O), sulfur (S) and zinc (Zn) vacancies on the optoelectronic properties of ZnO/ZnS heterostructure. The results show that the O, S, and Zn vacancies can decrease cell volume of [...] Read more.

Hybrid density functional theory has been employed to study the influence of interfacial oxygen (O), sulfur (S) and zinc (Zn) vacancies on the optoelectronic properties of ZnO/ZnS heterostructure. The results show that the O, S, and Zn vacancies can decrease cell volume of the ZnO/ZnS heterostructure, leading to slight deformation from the perfect heterostructure. The quasi-band gap of ZnO/ZnS heterostructure is remarkably reduced compared to the ZnO surface. Hence, the visible light response is enhanced in ZnO/ZnS heterostructure, which can be further improved by creating an interfacial S or O vacancy. Moreover, the removal of S or O atoms can generate lone electrons in the system, which can enhance n-type conductivity of the heterostructure. The O and S vacancies improve the contribution of the atomic orbitals of Zn_ZnO (Zn atom in ZnO), Zn_ZnS (Zn atom in ZnS), S and O to the valence band maximum (VB) of the heterostructure; while the Zn-vacancy remarkably improves the contribution of S states to the conduction band minimum (CB). The resultant type-II band alignment and large difference between the migration speed of electrons and holes can efficiently separate the photogenerated electron-hole pairs. The CB edge positions are more negative than the redox potentials of CO₂/CO and H₂O/H₂, and the VB edge positions are more positive than the redox potential of O₂/H₂O. Hence, all the systems under investigation can be potentially used as efficient photocatalysts for various applications like CO₂ reduction and water splitting. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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13 pages, 2420 KiB

Open AccessArticle

A Novel ZnO/Co₃O₄ Nanoparticle for Enhanced Photocatalytic Hydrogen Evolution under Visible Light Irradiation

by Tsung-Mo Tien and Edward L. Chen

Catalysts 2023, 13(5), 852; https://doi.org/10.3390/catal13050852 - 08 May 2023

Cited by 6 | Viewed by 1348

Abstract

In recent years, ZnO/Co₃O₄ nanoparticles (NPs) have been reflected as typical of the most promising photocatalysts utilized in the field of photocatalysis for potentially solving energy shortages and environmental remediation. In this work, a novel ZnO/Co₃O₄ NP [...] Read more.

In recent years, ZnO/Co₃O₄ nanoparticles (NPs) have been reflected as typical of the most promising photocatalysts utilized in the field of photocatalysis for potentially solving energy shortages and environmental remediation. In this work, a novel ZnO/Co₃O₄ NP photocatalyst was fabricated and utilized for photocatalytic hydrogen evolution with visible light activity. ZnO/Co₃O₄ NPs display an improved photocatalytic hydrogen production rate of 3963 μmol/g through a five-hour test under visible light activity. This is much better than their single components. Hence, bare ZnO NPs loaded with 20 wt% Co₃O₄ NPs present optimum efficiency of hydrogen evolution (793.2 μmol/g/h) with 10 vol% triethanolamine (TEOA), which is 11.8 times that of pristine ZnO NPs. An achievable mechanism for improved photocatalysis is endowed in terms of the composite that promotes the operative separation rate of charge carriers that are produced by visible light irradiation. This study yields a potential process for the future, proposing economical, high-function nanocomposites for hydrogen evolution with visible light activity. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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21 pages, 4808 KiB

Open AccessArticle

Adsorption and Photo-Degradation of Organophosphates on Sulfate-Terminated Anatase TiO₂ Nanoparticles

by Fredric G. Svensson and Lars Österlund

Catalysts 2023, 13(3), 526; https://doi.org/10.3390/catal13030526 - 04 Mar 2023

Viewed by 1438

Abstract

The adsorption and photocatalytic degradation of trimethyl phosphate (TMP) and triethyl phosphate (TEP), two environmentally relevant model pollutants, have been studied on commercial anatase TiO₂ and sulfate-terminated anatase TiO₂ nanoparticles by means of operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy [...] Read more.

The adsorption and photocatalytic degradation of trimethyl phosphate (TMP) and triethyl phosphate (TEP), two environmentally relevant model pollutants, have been studied on commercial anatase TiO₂ and sulfate-terminated anatase TiO₂ nanoparticles by means of operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and 2D correlation spectroscopy (2D COS). It is concluded that both TMP and TEP adsorb dissociatively on anatase TiO₂, while on the sulfate-terminated anatase TiO₂, TMP and TEP adsorb associatively. Upon UV illumination, TMP and TEP are completely oxidized on sulfate-terminated anatase TiO₂, as evidenced by the evolution of the IR bands characteristic for water and carbon dioxide. In contrast, on anatase TiO₂, UV illumination leads to the formation of stable surface-coordinated carboxylate products, which impedes complete oxidation. 2D COS analysis suggests that parallel reaction pathways occur during oxidation under UV illumination, viz. methoxide/ethoxide (ads) → carboxylates (ads) and methoxide/ethoxide (ads) → aldehydes (ads) → carboxylates (ads). A parallel reaction occurs on sulfated TiO₂ that yields CO₂ and H₂O by direct radical reactions with the methoxide groups with little, or no, formation of surface-coordinated intermediates. Sulfated TiO₂ favor the formation of aldehyde intermediates, with reaction rates 10 times and 30 times faster for TMP and TEM, respectively, compared with commercial anatase TiO₂. About 37% (33%) and 32% (24%) of TMP (TEP) were degraded on sulfated-terminated TiO₂ and pure TiO₂, respectively, after the first 9 min of UV illumination. We show that the sulfate-functionalization of TiO₂ has two main functions. First, it prevents the formation of strongly bonded bridging carboxylates, thereby alleviating deactivation. Second, it promotes full oxidation of the organic side-chains into carbon dioxide and water. Improved electron-hole separation by the electrophilic S(VI) in combination with the blocking of bridging reaction intermediates is proposed to contribute to the improved activity. The presented results give insights into how acidic surface modifications change adsorbate surface chemistries, which can be used to increase the sustained activity of low-temperature photocatalysts. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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16 pages, 2541 KiB

Open AccessArticle

A Novel Application of Photocatalysis: A UV-LED Photocatalytic Device for Controlling Diurnal Evaporative Fuel Vapor Emissions from Automobiles

by Catherine B. Almquist, Julia Kocher, Kylie Saxton, Lauryn Simonson, Alex Danciutiu, Phat John Nguyen and Jack Bain

Catalysts 2023, 13(1), 85; https://doi.org/10.3390/catal13010085 - 31 Dec 2022

Cited by 3 | Viewed by 1730

Abstract

A novel application of photocatalysis was investigated to reduce diurnal evaporative fuel vapor emissions from automobiles. A light-weight annulus photocatalytic device was designed, fabricated, and characterized for its performance for the oxidation of diurnal evaporative fuel vapor emissions. The prototype photocatalytic device was [...] Read more.

A novel application of photocatalysis was investigated to reduce diurnal evaporative fuel vapor emissions from automobiles. A light-weight annulus photocatalytic device was designed, fabricated, and characterized for its performance for the oxidation of diurnal evaporative fuel vapor emissions. The prototype photocatalytic device was made with PVC pipe and ultraviolet (λ = 365 nm) light emitting diodes (UV LEDs) as light sources. Commercially available Evonik P25 TiO₂ was used as the photocatalyst. The study results demonstrate that the UV LED photocatalytic device is capable of reducing diurnal evaporative fuel vapor emissions from automobiles by 60 wt%. However, the presence of high concentrations of light alkanes and aromatic fuel vapors in the diurnal emissions may limit the longevity of the device due to photocatalyst deactivation. Further development of the idea to enhance the longevity of its performance is recommended. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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16 pages, 3456 KiB

Open AccessArticle

Correlation between Photocatalytic Properties of ZnO and Generation of Hydrogen Peroxide—Impact of Composite ZnO/TiO₂ Rutile and Anatase

by Nouha Mediouni, Frederic Dappozze, Lhoussain Khrouz, Stephane Parola, Abdesslem Ben Haj Amara, Hafsia Ben Rhaiem, Nicole Jaffrezic-Renault, Philippe Namour and Chantal Guillard

Catalysts 2022, 12(11), 1445; https://doi.org/10.3390/catal12111445 - 15 Nov 2022

Cited by 4 | Viewed by 1371

Abstract

The generation of hydrogen peroxide on commercial and synthesized ZnO from different precursors was studied using two model molecules, formic acid (FA) and phenol (Ph), as well as phenolic intermediates, hydroquinone (HQ), benzoquinone (BQ), and catechol (CAT). The samples were characterized using X-ray [...] Read more.

The generation of hydrogen peroxide on commercial and synthesized ZnO from different precursors was studied using two model molecules, formic acid (FA) and phenol (Ph), as well as phenolic intermediates, hydroquinone (HQ), benzoquinone (BQ), and catechol (CAT). The samples were characterized using X-ray Diffraction (XRD), Transmission Electronic Microscopy (TEM), RAMAN, and Electron Paramagnetic Resonance (EPR) before evaluating their photocatalytic properties. We found that the improved efficiency is accompanied by a high level of H₂O₂ production, fewer oxygen vacancies, and that the number of moles of H₂O₂ formed per number of carbon atoms removed is similar to the degradation of FA and Ph with a factor of 1. Moreover, a comparative study on the formation of H₂O₂ was carried out in the presence of TiO₂ rutile and TiO₂ anatase, with commercial ZnO. Our results exhibit the impact of the presence of TiO₂ on the decomposition of hydrogen peroxide and the formation of phenolic intermediates, which are much lower than those of ZnO only, which is in agreement with the formation of hydroxyl radicals °OH and superoxide O₂°⁻ degrading significantly hydroquinone (HQ), benzoquinone (BQ), and cathecol (CAT). Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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13 pages, 5800 KiB

Open AccessArticle

Effect of Biogenic Silica Behavior in the Incorporation of Mesoporous Anatase TiO₂ for Excellent Photocatalytic Mineralization of Sodium Diclofenac

by Christian Brice Dantio Nguela, Ngomo Horace Manga, Clément Marchal, Aimé Victoire Abega, Ndi Julius Nsami and Didier Robert

Catalysts 2022, 12(9), 1001; https://doi.org/10.3390/catal12091001 - 05 Sep 2022

Cited by 3 | Viewed by 1393

Abstract

TiO₂/SiO₂ composites were synthesized via a simple sol gel method by surface reduction of Ti⁴⁺ ions to Ti³⁺ using titanium isopropoxide as a TiO₂ precursor and rice husks (RHA) as a SiO₂ source. The silica content [...] Read more.

TiO₂/SiO₂ composites were synthesized via a simple sol gel method by surface reduction of Ti⁴⁺ ions to Ti³⁺ using titanium isopropoxide as a TiO₂ precursor and rice husks (RHA) as a SiO₂ source. The silica content and calcination temperature of the materials were evaluated. Thermal, crystallographic and physicochemical aspects suggest that biogenic silica (SiO₂) can improve the thermal stability of the anatase phase of TiO₂, when the SiO₂ content reaches 20%. The N₂ adsorption-desorption isotherms showed that the SiO₂-modified samples have uniform pore diameters and a large specific surface area. The XPS analysis showed the surface reduction of Ti⁴⁺ ions to Ti³⁺ within the TiO₂ network via oxygen vacancies after SiO₂ introduction, which is beneficial for the photocatalytic reaction. Photocatalytic degradation of sodium diclofenac (SDFC) shows that TiO₂/SiO₂ composites have better activity compared to commercial P25. Mesoporous TiO₂ composite modified with 20 wt% SiO₂ showed better photocatalytic mineralization than P25 (83.7% after 2 h instead of 57.3% for P25). The excellent photocatalytic mineralization of the photocatalysts can be attributed to the high anatase crystallinity exhibited by XRD analysis, high specific surface area, surface hydroxyl groups, and the creation of oxygen vacancy, as well as the presence of Ti³⁺ ions. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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Review

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26 pages, 94706 KiB

Open AccessReview

The Advanced Progress of MoS₂ and WS₂ for Multi-Catalytic Hydrogen Evolution Reaction Systems

by Haoxuan Yu, Mengyang Zhang, Yuntao Cai, Yanling Zhuang and Longlu Wang

Catalysts 2023, 13(8), 1148; https://doi.org/10.3390/catal13081148 - 25 Jul 2023

Cited by 3 | Viewed by 1263

Abstract

Two-dimensional transition-metal dichalcogenides (TMDs) are considered as the next generation of hydrogen evolution electrocatalysts due to their adjustable band gap, near-zero Gibbs free energy, and lower cost compared to noble metal catalysts. However, the electrochemical catalytic hydrogen evolution performance of TMDs with two-dimensional [...] Read more.

Two-dimensional transition-metal dichalcogenides (TMDs) are considered as the next generation of hydrogen evolution electrocatalysts due to their adjustable band gap, near-zero Gibbs free energy, and lower cost compared to noble metal catalysts. However, the electrochemical catalytic hydrogen evolution performance of TMDs with two-dimensional properties is limited by innate sparse catalytic active sites, poor electrical conductivity, and weak electrical contact with the substrate. It remains challenging for the intrinsic activity of TMDs for electrocatalytic and photocatalytic hydrogen evolution reactions (HERs) to compete with the noble metal platinum. In recent years, significant development of transition metal chalcogenides, especially MoS₂ and WS₂, as catalysts for electrocatalytic and photocatalytic HERs has proceeded drastically. It is indispensable to summarize the research progress in this area. This review summarizes recent research results of electrocatalysts and photocatalysts for hydrogen evolution reactions based on two-dimensional materials, mainly including MoS₂, WS₂, and their compounds. The challenges and future development directions of two-dimensional hydrogen evolution reaction electrocatalysts and photocatalysts are summarized and prospected as well. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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15 pages, 3917 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Water Photo-Oxidation over TiO₂—History and Reaction Mechanism

by Yoshio Nosaka

Catalysts 2022, 12(12), 1557; https://doi.org/10.3390/catal12121557 - 01 Dec 2022

Cited by 7 | Viewed by 2050

Abstract

Photocatalytic water oxidation over titanium dioxide (TiO₂) was overviewed by surveying briefly the history of water photo-oxidation, followed by profiling the research for the molecular mechanism of oxygen evolution reaction (OER) at the TiO₂ surface. As the experimental approach to [...] Read more.

Photocatalytic water oxidation over titanium dioxide (TiO₂) was overviewed by surveying briefly the history of water photo-oxidation, followed by profiling the research for the molecular mechanism of oxygen evolution reaction (OER) at the TiO₂ surface. As the experimental approach to investigate the reaction mechanism, ESR, NMR, and STM were described as well as FTIR spectroscopy. Detection of reactive oxygen species, which are the intermediate species in the OER, was also involved in discussing the mechanism. As the theoretical approach to the reaction mechanism, some research with density functional theory (DFT) for anatase (101) surface was illustrated. Since the OER activity of rutile TiO₂ is higher than that of anatase, and the rutile (011) surface has been assigned to the oxidation facet, we performed a DFT calculation for a (011) surface model molecule. The results were successfully discussed with the reported mechanism. The first oxidation step occurs at the bridging OH site, which faces a Ti_5C site. The water molecule which coordinates both sites is oxidized, and the resultant radical coordinates the Ti_5C site to form a trapped hole Ti-O•. In the second step, a coordinated water molecule is oxidized at the Ti-O• site to form a Ti-OOH structure. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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28 pages, 4895 KiB

Open AccessReview

Towards the Sustainable Production of Ultra-Low-Sulfur Fuels through Photocatalytic Oxidation

by Artem S. Belousov and Iqrash Shafiq

Catalysts 2022, 12(9), 1036; https://doi.org/10.3390/catal12091036 - 12 Sep 2022

Cited by 5 | Viewed by 2528

Abstract

Nowadays, the sulfur-containing compounds are removed from motor fuels through the traditional hydrodesulfurization technology, which takes place under harsh reaction conditions (temperature of 350–450 °C and pressure of 30–60 atm) in the presence of catalysts based on alumina with impregnated cobalt and molybdenum. [...] Read more.

Nowadays, the sulfur-containing compounds are removed from motor fuels through the traditional hydrodesulfurization technology, which takes place under harsh reaction conditions (temperature of 350–450 °C and pressure of 30–60 atm) in the presence of catalysts based on alumina with impregnated cobalt and molybdenum. According to the principles of green chemistry, energy requirements should be recognized for their environmental and economic impacts and should be minimized, i.e., the chemical processes should be carried out at ambient temperature and atmospheric pressure. This approach could be implemented using photocatalysts that are sensitive to visible light. The creation of highly active photocatalytic systems for the deep purification of fuels from sulfur compounds becomes an important task of modern catalysis science. The present critical review reports recent progress over the last 5 years in heterogeneous photocatalytic desulfurization under visible light irradiation. Specific attention is paid to the methods for boosting the photocatalytic activity of materials, with a focus on the creation of heterojunctions as the most promising approach. This review also discusses the influence of operating parameters (nature of oxidant, molar ratio of oxidant/sulfur-containing compounds, photocatalyst loading, etc.) on the reaction efficiency. Some perspectives and future research directions on photocatalytic desulfurization are also provided. Full article

(This article belongs to the Special Issue 50 Years of Research in Photocatalysis: Scientific Advances, Discoveries and New Perspectives)

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