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2017, Vol. 26, No. 2　Online: 2017-03-15


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Nanocarbon photocatalytic materials

This perspective paper introduces the concept that nanocarbons and related materials such as carbon dots are an interesting intrinsic photocatalytic semiconducting material. The semiconducting properties of the nanocarbons, and the possibility to have the band gap within the visible-light region through defect band engineering, introduction of light heteroatoms and control/manipulation of the curvature or surface functionalization are discussed. These materials show semiconductor domains, with tunable characteristics, embedded in a conductive carbon matrix, with the presence of functional groups enhancing charge separation by trapping electrons. These nanocarbons open a range of new possibilities for photocatalysis both for energetic and environmental applications.

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Preface

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Special Issue on New Concepts and Advances in Photocatalytic Materials for Sustainable Energy 2017, 26(2): 0-0.  摘要 ( 1637 )   PDF

PERSPECTIVES

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Photoactive materials based on semiconducting nanocarbons-A challenge opening new possibilities for photocatalysis Siglinda Perathoner, Claudio Ampelli, Shiming Chen, Rosalba Passalacqua, Dangsheng Su, Gabriele Centi 2017, 26(2): 207-218.  DOI: 10.1016/j.jechem.2017.01.005 摘要 ( 3392 )   This perspective paper introduces the concept that nanocarbons and related materials such as carbon dots are an interesting intrinsic photocatalytic semiconducting material, and not only a modifier of the existing (semiconducting) materials to prepare hybrid materials. The semiconducting properties of the nanocarbons, and the possibility to have the band gap within the visible-light region through defect band engineering, introduction of light heteroatoms and control/manipulation of the curvature or surface functionalization are discussed. These materials are conceptually different from the “classical” semiconducting photocatalysts, because semiconductor domains with tuneable characteristics are embedded in a conductive carbon matrix, with the presence of various functional groups (as C=O groups) enhancing charge separation by trapping electrons. These nanocarbons open a range of new possibilities for photocatalysis both for energetic and environmental applications. The use of nanocarbons as quantum dots and photoluminescent materials was also analysed.

REVIEWS

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Semiconductor, molecular and hybrid systems for photoelectrochemical solar fuel production Rosalba Passalacqua, Siglinda Perathoner, Gabriele Centi 2017, 26(2): 219-240.  DOI: 10.1016/j.jechem.2017.03.004 摘要 ( 3218 )   The paper shortly reviews the basic direct approaches applied in searching for viable solutions to solar fuel production. These are generally distinguished in molecular and semiconductor (non-molecular) systems, however, hybrid strategies, proposed recently, have also been included. The most promising efforts are considered, highlighting key aspects and emerging critical issues. Special attention is paid to aspects such as electrode architecture, device design, and main differences in the scientific vision and challenges to directly produce solar fuels. This overview could be useful to orientate the readers in the wide panorama of research activities concerning water splitting, natural and artificial photosynthesis, and solar fuel production through the identification of common aspects, specialties and potentialities of the many initiatives and approaches that are developing worldwide in this field with the final aim to meet world energy demand.

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Coupling metal oxide nanoparticle catalysts for water oxidation to molecular light absorbers Heinz Frei 2017, 26(2): 241-249.  DOI: 10.1016/j.jechem.2017.03.001 摘要 ( 2753 )   Water oxidation, as a mandatory reaction of solar fuels conversion systems, requires the use of light absorbers with electronic properties that are well matched with those of the multi-electron catalyst in order to achieve high efficiency. Molecular light absorbers offer flexibility in fine tuning of orbital energetics, and metal oxide nanoparticles have emerged as robust oxygen evolving catalysts. Hence, these material choices offer a promising approach for the development of photocatalytic systems for water oxidation. However, efficient charge transfer coupling of molecular light absorbers and metal oxide nanoparticle catalysts has proven a challenge. Recent new approaches toward the efficient coupling of these components based on synthetic design improvements combined with direct spectroscopic observation and kinetic evaluation of charge transfer processes are discussed.

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TiO2-based materials for photocatalytic hydrogen production G. L. Chiarello, M. V. Dozzi, E. Selli 2017, 26(2): 250-258.  DOI: 10.1016/j.jechem.2017.02.005 摘要 ( 5041 )   Hydrogen, the cleanest and most promising energy vector, can be produced by solar into chemical energy conversion, either by the photocatalytic direct splitting of water into H2 and O2, or, more efficiently, in the presence of sacrificial reagents, e.g., in the so-called photoreforming of organics. Efficient photocatalytic materials should not only be able to exploit solar radiation to produce electron-hole pairs, but also ensure enough charge separation to allow electron transfer reactions, leading to solar energy driven thermodynamically up-hill processes. Recent achievements of our research group in the development and testing of innovative TiO2-based photocatalytic materials are presented here, together with an overview on the mechanistic aspects of water photosplitting and photoreforming of organics. Photocatalytic materials were either (i) obtained by surface modification of commercial photocatalysts, or produced (ii) in powder form by different techniques, including traditional sol gel synthesis, aiming at engineering their electronic structure, and flame spray pyrolysis starting from organic solutions of the precursors, or (iii) in integrated form, to produce photoelectrodes within devices, by radio frequency magnetron sputtering or by electrochemical growth of nanotube architectures, or photocatalytic membranes, by supersonic cluster beam deposition.

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Photophysics and electrochemistry relevant to photocatalytic water splitting involved at solid-electrolyte interfaces Tatsuya Shinagawa, Zhen Cao, Luigi Cavallo, Kazuhiro Takanabe 2017, 26(2): 259-269.  DOI: 10.1016/j.jechem.2016.07.007 摘要 ( 3178 )   Direct photon to chemical energy conversion using semiconductor-electrocatalyst-electrolyte interfaces has been extensively investigated for more than a half century. Many studies have focused on screening materials for efficient photocatalysis. Photocatalytic efficiency has been improved during this period but is not sufficient for industrial commercialization. Detailed elucidation on the photocatalytic water splitting process leads to consecutive six reaction steps with the fundamental parameters involved: The photocatalysis is initiated involving photophysics derived from various semiconductor properties (1: photon absorption, 2: exciton separation). The generated charge carriers need to be transferred to surfaces effectively utilizing the interfaces (3: carrier diffusion, 4: carrier transport). Consequently, electrocatalysis finishes the process by producing products on the surface (5: catalytic efficiency, 6: mass transfer of reactants and products). Successful photocatalytic water splitting requires the enhancement of efficiency at each stage. Most critically, a fundamental understanding of the interfacial phenomena is highly desired for establishing “photocatalysis by design” concepts, where the kinetic bottleneck within a process is identified by further improving the specific properties of photocatalytic materials as opposed to blind material screening. Theoretical modeling using the identified quantitative parameters can effectively predict the theoretically attainable photon-conversion yields. This article provides an overview of the state-of-the-art theoretical understanding of interfacial problems mainly developed in our laboratory. Photocatalytic water splitting (especially hydrogen evolution on metal surfaces) was selected as a topic, and the photophysical and electrochemical processes that occur at semiconductor-metal, semiconductor- electrolyte and metal-electrolyte interfaces are discussed.

ARTICLES

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Rare earth oxides in zirconium dioxide: How to turn a wide band gap metal oxide into a visible light active photocatalyst Chiara Gionco, Maria C. Paganini, Elio Giamello, Olga Sacco, Vincenzo Vaiano, Diana Sannino 2017, 26(2): 270-276.  DOI: 10.1016/j.jechem.2016.07.006 摘要 ( 2643 )   In the present study, we investigated the effect of cerium and erbium doping of the zirconium dioxide matrix. We synthesized doped samples using hydrothermal process. The amounts of dopant used were 0.5%, 1% and 5% molar (rare earth oxide over zirconium dioxide) respectively. The samples have been studied via X-ray Diffraction measurements for the structural characterization. UV visible diffuse reflectance was used for the optical analysis, Brunauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally the samples have been analysed via electron paramagnetic resonance (EPR) for the electronic characterization. Then we tested the new synthetized materials to determine their photocatalytic activity in the reaction of degradation of methylene blue performed under irradiation by diodes (LEDs) emitting exclusively visible light.

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Evaluation of the plasmonic effect of Au and Ag on Ti-based photocatalysts in the reduction of CO2 to CH4 Martin Dilla, Anna Pougin, Jennifer Strunk 2017, 26(2): 277-283.  DOI: 10.1016/j.jechem.2016.09.009 摘要 ( 2931 )   Crystalline TiO2 (P25) and isolated titanate species in a ZSM-5 structure (TS-1) were modified with Au and Ag, respectively, and tested in the gas-phase photocatalytic CO2 reduction under high purity conditions. The noble metal modification was performed by photodeposition. Light absorbance properties of the catalysts are examined with UV-Vis spectroscopy before and after the activity test. In the gas-phase photocatalytic CO2 reduction, it was observed that the catalysts with Ag nanostructures are more active than those with Au nanostructures. It is thus found that the energetic difference between the band gap energy of the semiconductor and the position of the plasmon is influencing the photocatalytic activity. Potentially, plasmon excitation due to visible light absorption results in plasmon resonance energy, which affects the excitation of the semiconductor positively. Therefore, an overlap between band gap energy of the semiconductor and metal plasmon is needed.

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Analysis of the factors controlling performances of Au-modified TiO2 nanotube array based photoanode in photo-electrocatalytic (PECa) cells Claudio Ampelli, Francesco Tavella, Chiara Genovese, Siglinda Perathoner, Marco Favaro, Gabriele Centi 2017, 26(2): 284-294.  DOI: 10.1016/j.jechem.2016.11.004 摘要 ( 3109 )   The efficiency of photo-electrocatalytic (PECa) devices for the production of solar fuels depends on several limiting factors such as light harvesting, charge recombination and mass transport diffusion. We analyse here how they influence the performances in PECa cells having a photo-anode based on Au-modified TiO2 nanotube (TNT) arrays, with the aim of developing design criteria to optimize the photo-anode and the PECa cell configuration for water photo-electrolysis (splitting) and ethanol photo-reforming processes. The TNT samples were prepared by controlled anodic oxidation of Ti foils and then decorated with gold nanoparticles using different techniques to enhance the visible light response through heterojunction and plasmonic effects. The activity tests were made in a gas-phase reactor, as well as in a PECa cell without applied bias. Results were analysed in terms of photo-generated current, H2 production rate and photoconversion efficiency. Particularly, a solar-to-hydrogen efficiency of 0.83% and a Faradaic efficiency of 91% were obtained without adding sacrificial reagents.

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Structure-activity relationship in Ti phosphate-derived photocatalysts for H2 evolution Diego Mateo, Francisco Gonell, Josep Albero, Avelino Corma, Hermenegildo García 2017, 26(2): 295-301.  DOI: 10.1016/j.jechem.2016.09.016 摘要 ( 4549 )   Photocatalytic H2 production has emerged as one of the most clean and promising renewable energy sources. In spite of the efforts to obtain efficient photocatalysts able to produce H2 from Sun light and water, there is still the need to prepare cheaper and environmental friendlier photocatalysts. Phosphatebased materials could be good candidates to fulfill these requirements. In this manuscript we have prepared a set of mixed Ti3+/Ti4+ valence, open-framework titanium phosphates (of-TiPO4) and mixed titanium oxide/phosphate derivatives (cr-TiP), correlating their structure and composition with the photocatalytic activity for H2 production. We determined that mixed titanium oxide/phosphate crystalline phases produced enhanced H2 evolution under Sun simulated light irradiation than mixed Ti3+/Ti4+ valence, open-framework titanium phosphates and titanium oxide phases.

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Applied bias photon-to-current conversion efficiency of ZnO enhanced by hybridization with reduced graphene oxide Sharifah Bee Abdul Hamid, Swe Jyan Teh, Chin Wei Lai, Siglinda Perathoner, Gabriele Centi 2017, 26(2): 302-308.  DOI: 10.1016/j.jechem.2016.11.006 摘要 ( 3379 )   The role of reduced graphene oxide (rGO) in the enhancement of photo-conversion efficiency of ZnO films for photoelectrochemical (PEC) water-splitting applications was analyzed. ZnO and rGO-hybridized ZnO (rGO/ZnO) films were prepared via a two-step electrochemical deposition method followed by annealing at 300 ℃ under argon gas flow. The physical, optical and electrochemical properties of the films were characterized to identify the effect of rGO-hybridization on the applied bias photon-to-current efficiency (ABPE) of ZnO. Scanning electron microscopy and X-ray diffraction indicated the formation of verticallyaligned, wurtzite-phase ZnO nanorods. Diffuse-reflectance UV-visible spectroscopy indicated that rGOhybridization was able to increase the light absorption range of the rGO/ZnO film. UPS analysis showed that hybridization with rGO increased the band gap of ZnO (3.56 eV) to 3.63 eV for rGO/ZnO sample, which may be attributed to the Burstein-Moss effect. Photoluminescence (PL) spectra disclosed that rGOhybridization suppressed electron-hole recombination due to crystal defects. Linear sweep voltammetry of the prepared thin films showed photocurrent density of 1.0 and 1.8mA/cm2 for ZnO and rGO/ZnO at +0.7 V, which corresponded to an ABPE of 0.55% and 0.95%, respectively. Thus, this report highlighted the multi-faceted role of rGO-hybridization in the enhancement of ZnO photo-conversion efficiency.

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Stability of Ag@SiO2 core-shell particles in conditions of photocatalytic overall water-splitting Sun-Young Park, Kai Han, Devin B. O'Neill, Guido Mul 2017, 26(2): 309-314.  DOI: 10.1016/j.jechem.2016.12.010 摘要 ( 4793 )   Core-shell nanoparticles containing plasmonic metals (Ag or Au) have been frequently reported to enhance performance of photo-electrochemical (PEC) devices. However, the stability of these particles in water-splitting conditions is usually not addressed. In this study we demonstrate that Ag@SiO2 core- shell particles are instable in the acidic conditions in which WO3-based PEC cells typically operate, Ag in the core being prone to oxidation, even if the SiO2 shell has a thickness in the order of 10 nm. This is evident from in situ voltammetry studies of several anode composites. Similar to the results of the PEC experiments, the Ag@SiO2 core-shell particles are instable in slurry-based, Pt/ZnO induced photocatalytic water-splitting. This was evidenced by in situ photodeposition of Ag nanoparticles on the Pt-loaded ZnO catalyst, observed in TEM micrographs obtained after reaction. We explain the instability of Ag@SiO2 by OH-radical induced oxidation of Ag, yielding dissolved Ag+. Our results imply that a decrease in shell permeability for OH-radicals is necessary to obtain stable, Ag-based plasmonic entities in photo-electrochemical and photocatalytic water splitting.

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Facile preparation of Nd2Zr2O7-ZrO2 nanocomposites as an effective photocatalyst via a new route Sahar Zinatloo-Ajabshir, Zahra Zinatloo-Ajabshir, Masoud Salavati-Niasari, Samira Bagheri, Sharifah Bee Abd Hamid 2017, 26(2): 315-323.  DOI: 10.1016/j.jechem.2016.11.005 摘要 ( 3120 )   Nd2Zr2O7-ZrO2 nanocomposites were prepared via a facile process with propylene glycol as novel connecting agent and benzene tricarboxylic acid as a new complexing agent. The as-obtained Nd2Zr2O7-ZrO2 nanocomposites were characterized by transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy, energy dispersive X-ray microanalysis (EDX), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). According to the morphological studies of the as-synthesized nanocomposites, it was found that the shape and particle size of Nd2Zr2O7-ZrO2 nanocomposites depended on the space-filling template type, dosage of space-filling template and tricarboxylic acid as complexing agent. Nd2Zr2O7-ZrO2 nanocomposites with different shapes and grain sizes have been synthesized. The photocatalytic behavior of as-produced Nd2Zr2O7-ZrO2 nanocomposites was also investigated through photodegradation of methylene blue dye and 2-naphthol as water pollutants.