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Photocatalytic Water Splitting—the Untamed Dream

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Photochemistry".

Deadline for manuscript submissions: closed (10 May 2016) | Viewed by 64537

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Prof. Dr. Nick Serpone

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F. EurASc, Visiting Professor, PhotoGreen Laboratory, Dipartimento di Chimica, Università di Pavia, ViaTaramelli 12, 27100 Pavia, Italy
Interests: fundamentals and systematization of heterogeneous photocatalysis; photochemistry and photophysics of semiconductor metal oxides; applied heterogeneous photocatalysis for environmental remediation; photochemistry of physical (TiO₂, ZnO) and chemical (e.g. oxybenzone, octlymethoxycinnamate, etc.) UV filters; microwave-assisted organic reactions and nanoparticle syntheses

Special Issue Information

Dear Colleagues,

Water splitting to produce dihydrogen and dioxygen by the assistance of some catalyst activated by sunlight has been a dream of many researchers ever since Jules Verne first introduced the idea in his book, “The Mysterious Island”, nearly 150 years ago: “Yes, but water decomposed into its primitive elements, and decomposed doubtless, by electricity, which will then have become a powerful and manageable force, for all great discoveries, by some inexplicable laws, appear to agree and become complete at the same time. Yes, my friends, I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable. Someday the coal rooms of steamers and the tenders of locomotives will, instead of coal, be stored with these two condensed gases, which will burn in the furnaces with enormous calorific power. There is, therefore, nothing to fear. As long as the earth is inhabited it will supply the wants of its inhabitants, and there will be no want of either light or heat as long as the productions of the vegetable, mineral or animal kingdoms do not fail us. I believe, then, that when the deposits of coal are exhausted we shall heat and warm ourselves with water. Water will be the coal of the future." The notion of using sunlight to carry out many photochemical reactions for the benefit of mankind was later taken up by the Italian chemist, Giacomo Ciamician, some 100 years ago at the University of Bologna. The last 40 years has witnessed a frenzy of research activities, ever since the seminal paper by Fujishima and Honda (Nature, 1972), that showed that water could be split photoelectrochemically into its two elements in the presence of titanium dioxide. We believe it is appropriate at this time to take stock of the efforts and results of the many that have shared in this untamed dream in the search for the “holy grail” photocatalysts or photocatalytic composites that might realize Jules Verne’s dream that water will indeed be the fuel of the future—and this in one Special Issue “Photocatalytic Water Splitting—the Untamed Dream” in Molecules.

Prof. Dr. Nick Serpone
Guest Editor

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Keywords

Water splitting
Solar fuels
Hydrogen
Photocatalysis
Photocatalysts
Photocatalyst nanocomposites
Semiconductors
Nanomaterials
Metal oxides
Nanotechnology

Published Papers (6 papers)

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Research

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4529 KiB

Open AccessArticle

Photoelectrochemical Behavior of Electrophoretically Deposited Hematite Thin Films Modified with Ti(IV)

by Nicola Dalle Carbonare, Rita Boaretto, Stefano Caramori, Roberto Argazzi, Maurizio Dal Colle, Luca Pasquini, Renzo Bertoncello, Marcello Marelli, Claudio Evangelisti and Carlo Alberto Bignozzi

Molecules 2016, 21(7), 942; https://doi.org/10.3390/molecules21070942 - 20 Jul 2016

Cited by 6 | Viewed by 5696

Abstract

Doping hematite with different elements is a common strategy to improve the electrocatalytic activity towards the water oxidation reaction, although the exact effect of these external agents is not yet clearly understood. Using a feasible electrophoretic procedure, we prepared modified hematite films by introducing in the deposition solution Ti(IV) butoxide. Photoelectrochemical performances of all the modified electrodes were superior to the unmodified one, with a 4-fold increase in the photocurrent at 0.65 V vs. SCE in 0.1 M NaOH (pH 13.3) for the 5% Ti-modified electrode, which was the best performing electrode. Subsequent functionalization with an iron-based catalyst led, at the same potential, to a photocurrent of ca. 1.5 mA·cm⁻², one of the highest achieved with materials based on solution processing in the absence of precious elements. AFM, XPS, TEM and XANES analyses revealed the formation of different Ti(IV) oxide phases on the hematite surface, that can reduce surface state recombination and enhance hole injection through local surface field effects, as confirmed by electrochemical impedance analysis. Full article

(This article belongs to the Special Issue Photocatalytic Water Splitting—the Untamed Dream)

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3710 KiB

Open AccessArticle

Photocatalytic Activities of Copper Doped Cadmium Sulfide Microspheres Prepared by a Facile Ultrasonic Spray-Pyrolysis Method

by Jinzhan Su, Tao Zhang, Yufeng Li, Yubin Chen and Maochang Liu

Molecules 2016, 21(6), 735; https://doi.org/10.3390/molecules21060735 - 15 Jun 2016

Cited by 36 | Viewed by 8098

Abstract

Ultrasonic spray pyrolysis is a superior method for preparing and synthesizing spherical particles of metal oxide or sulfide semiconductors. Cadmium sulfide (CdS) photocatalysts with different sizes and doped-CdS with different dopants and doping levels have been synthesized to study their properties of photocatalytic hydrogen production from water. The CdS photocatalysts were characterized with scanning electron microscopy (SEM), X-ray fluorescence-spectrometry (XRF), UV-Vis absorption spectra and X-ray diffraction (XRD) to study their morphological and optical properties. The sizes of the prepared CdS particles were found to be proportional to the concentration of the metal nitrates in the solution. The CdS photocatalyst with smaller size showed a better photocatalytic activity. In addition, Cu doped CdS were also deposited and their photocatalytic activities were also investigated. Decreased bandgaps of CdS synthesized with this method were found and could be due to high density surface defects originated from Cd vacancies. Incorporating the Cu elements increased the bandgap by taking the position of Cd vacancies and reducing the surface defect states. The optimal Cu-doped level was found to be 0.5 mol % toward hydrogen evolution from aqueous media in the presence of sacrificial electron donors (Na₂S and Na₂SO₃) at a pH of 13.2. This study demonstrated that ultrasonic spray pyrolysis is a feasible approach for large-scale photocatalyst synthesis and corresponding doping modification. Full article

(This article belongs to the Special Issue Photocatalytic Water Splitting—the Untamed Dream)

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7787 KiB

Open AccessArticle

From Nanorods to Nanowires of CdS Synthesized by a Solvothermal Method: Influence of the Morphology on the Photoactivity for Hydrogen Evolution from Water

by Fernando Vaquero, José Luis G. Fierro and Rufino M. Navarro Yerga

Molecules 2016, 21(4), 401; https://doi.org/10.3390/molecules21040401 - 24 Mar 2016

Cited by 20 | Viewed by 6396

Abstract

The effect of temperature and water/thiourea ratio on the growth, crystallinity and morphological characteristics of CdS nanostructures synthetized by a solvothermal method using ethylenediamine as solvent were studied. The temperature and water/thiourea ratio used in the synthesis determine the surface area, shape, length and degree of crystallinity of the CdS nanostructures obtained. Nanowires of high crystallinity and length were obtained when the solvothermal synthesis was performed at 190 °C, while nanorods with lower length and crystallinity were obtained as the solvothermal temperature decreased to 120 °C. The change in the water/thiourea ratio affects the crystallinity and length of the CdS nanostructures to a lesser extent than temperature. Nevertheless an increase in the water/thiourea ratio used during the solvothermal synthesis resulted in CdS nanorods with higher crystallinity, lower aspect ratio and lower specific surface area. Textural, structural and surface properties of the prepared CdS nanostructures were determined and related to the activity results in the production of hydrogen from aqueous solutions containing SO₃²⁻ + S²⁻ under visible light. Full article

(This article belongs to the Special Issue Photocatalytic Water Splitting—the Untamed Dream)

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Review

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1774 KiB

Open AccessReview

Water Will Be the Coal of the Future—The Untamed Dream of Jules Verne for a Solar Fuel

by Vladimir K. Ryabchuk, Vyacheslav N. Kuznetsov, Alexei V. Emeline, Yurii M. Artem’ev, Galina V. Kataeva, Satoshi Horikoshi and Nick Serpone

Molecules 2016, 21(12), 1638; https://doi.org/10.3390/molecules21121638 - 29 Nov 2016

Cited by 17 | Viewed by 6172

Abstract

This article evokes the futuristic visions of two giants, one a writer, Jules Verne, who foresaw water as the coal of the future, and the other a scientist, Giacomo Ciamician, who foresaw the utilization of solar energy as an energy source with which to drive photochemical and photocatalytic reactions for the betterment of mankind. Specifically, we examine briefly the early work of the 1960s and 1970s on the photosplitting of free water and water adsorbed on solid supports, based mostly on metal oxides, from which both hydrogen and oxygen evolve in the expected stoichiometric ratio of 2 to 1. The two oil crises of the 1970s (1973 and 1979) spurred the interest of researchers from various disciplines (photochemistry, photo-catalysis and photoelectrochemistry) in search of a Holy Grail photocatalyst, process, or strategy to achieve efficient water splitting so as to provide an energy source alternative to fossil fuels. Some approaches to the photosplitting of water adsorbed on solid insulators (high bandgap materials; E_bg ≥ 5 eV) and semiconductor photocatalysts (metal oxides) are described from which we deduce that metal oxides with bandgap energies around 5 eV (e.g., ZrO₂) are more promising materials to achieve significant water splitting on the basis of quantum yields than narrower bandgap photocatalysts (e.g., TiO₂; E_bg ≈ 3.0–3.2 eV), which tend to be relatively inactive by comparison. Although proof of concept of the photosplitting of water has been demonstrated repeatedly in the last four decades, much remains to be done to find the Holy Grail photocatalyst and/or strategy to achieve significant yields of hydrogen. Full article

(This article belongs to the Special Issue Photocatalytic Water Splitting—the Untamed Dream)

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2222 KiB

Open AccessReview

Photocatalytic Water Splitting—The Untamed Dream: A Review of Recent Advances

by Tahereh Jafari, Ehsan Moharreri, Alireza Shirazi Amin, Ran Miao, Wenqiao Song and Steven L. Suib

Molecules 2016, 21(7), 900; https://doi.org/10.3390/molecules21070900 - 09 Jul 2016

Cited by 445 | Viewed by 29499

Abstract

Photocatalytic water splitting using sunlight is a promising technology capable of providing high energy yield without pollutant byproducts. Herein, we review various aspects of this technology including chemical reactions, physiochemical conditions and photocatalyst types such as metal oxides, sulfides, nitrides, nanocomposites, and doped materials followed by recent advances in computational modeling of photoactive materials. As the best-known catalyst for photocatalytic hydrogen and oxygen evolution, TiO₂ is discussed in a separate section, along with its challenges such as the wide band gap, large overpotential for hydrogen evolution, and rapid recombination of produced electron-hole pairs. Various approaches are addressed to overcome these shortcomings, such as doping with different elements, heterojunction catalysts, noble metal deposition, and surface modification. Development of a photocatalytic corrosion resistant, visible light absorbing, defect-tuned material with small particle size is the key to complete the sunlight to hydrogen cycle efficiently. Computational studies have opened new avenues to understand and predict the electronic density of states and band structure of advanced materials and could pave the way for the rational design of efficient photocatalysts for water splitting. Future directions are focused on developing innovative junction architectures, novel synthesis methods and optimizing the existing active materials to enhance charge transfer, visible light absorption, reducing the gas evolution overpotential and maintaining chemical and physical stability. Full article

(This article belongs to the Special Issue Photocatalytic Water Splitting—the Untamed Dream)

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7165 KiB

Open AccessReview

Doping-Promoted Solar Water Oxidation on Hematite Photoanodes

by Yuchao Zhang, Hongwei Ji, Wanhong Ma, Chuncheng Chen, Wenjing Song and Jincai Zhao

Molecules 2016, 21(7), 868; https://doi.org/10.3390/molecules21070868 - 01 Jul 2016

Cited by 24 | Viewed by 7926

Abstract

As one of the most promising materials for solar water oxidation, hematite has attracted intense research interest for four decades. Despite their desirable optical band gap, stability and other attractive features, there are great challenges for the implementation of hematite-based photoelectrochemical cells. In particular, the extremely low electron mobility leads to severe energy loss by electron hole recombination. Elemental doping, i.e., replacing lattice iron with foreign atoms, has been shown to be a practical solution. Here we review the significant progresses in metal and non-metal element doping-promoted hematite solar water oxidation, focusing on the role of dopants in adjusting carrier density, charge collection efficiency and surface water oxidation kinetics. The advantages and salient features of the different doping categories are compared and discussed. Full article

(This article belongs to the Special Issue Photocatalytic Water Splitting—the Untamed Dream)

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