Nanohybrid-sensitized photoelectrochemical cells for solar-to-hydrogen conversion
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Nanohybrid-sensitized photoelectrochemical cells for solar-to-hydrogen conversion Hiroaki Tada, Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Address all correspondence to Hiroaki Tada at [email protected] (Received 9 April 2018; accepted 12 July 2018)
Abstract This article reviews the semiconductor and metal-based nanohybrid-sensitized photoelectrochemical (PEC) cells for hydrogen generation from water. The nanoscale hybridization of sensitizers in the photoanode can enhance light harvesting, interfacial charge transfer, charge separation, and induce a catalytic effect in dependence on the kind of the components and interfacial junction state. Subsequent to the introduction, second and third sections present the basic structure and design of the nanohybrid-sensitized PEC cell. Fourth section deals with the effect of the interfacial bond between quantum dots and TiO2 on the electron injection process. Fifth section mainly describes the formation of heteroepitaxial junction between the components of nanohybrids. In the sixth section, the state-of-the-art nanohybrid-sensitized PEC cells are treated with a particular emphasis placed on the interface state.
Introduction The development of the efficient and green processes for producing hydrogen (H2) is vital for realizing sustainable society. At present, most H2 is produced by the steam reforming of methane at high temperature with the emission of carbon dioxide. If we can produce H2 from water at low temperature using the sunlight as an energy source, an ideal “solar hydrogen cycle” is completed by combining with the fuel cell (Scheme 1). For this purpose, water splitting with a standard Gibbs energy of reaction (ΔrG0) of +237.13 kJ/mol (Eq. 1) by visible-light semiconductor photocatalysts[1–3] and photoelectrochemical (PEC) cells[4] has intensively been studied. H2 O H2 + 1/2O2
(1)
The semiconductors developed for water splitting need a band gap above ∼2 eV, which is much larger than the thermodynamic value of 1.23 eV because of the large overpotential particularly for water oxidation.[5] The increase in the band gap lowers the light harvesting efficiency (LHE), while it increases the stability. So far, no efficient and stable water splitting by a single photocatalytic material has been achieved. An alternative way is using metal chalcogenide quantum dot (MX QD, X = S, Se, Te)-sensitized PEC (SPEC) cells.[6,7] The metal chalcogenide QDs possess unique features as the photosensitizer: (i) tunability of the band gap or the band edge levels by the size quantization[8,9] and the concentrations of S2− and OH− ions in the electrolyte solution[10] to enhance the redox abilities of photogenerated charge carriers, (ii) great extinction coefficient >104/M/cm or large LHE,[11,12] and (iii) large dipole moment
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enhancing charge separation.[13] On the other hand, Au nanoparticle (NP)-loaded metal oxides (Au/MOs) represented by Au/ TiO2 have emerged as a
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