Abstract
The crystal-plane dependence of the photoelectrochemical (PEC) water-splitting property of rutile-structured Nb-doped TiO2 (TiO2:Nb) single-crystal substrates was investigated. Among the crystal planes, the (001) plane was a very promising surface for attaining good photocurrent. Under 1 sun illumination at 1.5 V vs. a reversible hydrogen electrode, the TiO2:Nb(001) single-crystal substrate showed the highest photocurrent (0.47 mA/cm2) among the investigated substrates. The doped Nb ions were segregated inward from the top surface, and the TiO2 ultrathin layer was formed at the surface of the crystal, resulting in the formation of a heterointerface between the TiO2 and the TiO2:Nb. The enhancement of the PEC properties of the TiO2:Nb(001) single-crystal substrate originated from favorable atomic configurations for water molecule absorption and facilitation of transport of photoexcited electron–hole pairs in the depletion layer formed around the heterointerface of TiO2 thin layers on the base crystal.
Highlights
Photoelectrochemical (PEC) reactions have been widely studied, especially for solar hydrogen production and water purification [1,2,3,4,5]
The enhancement of the PEC properties of the TiO2 :Nb(001) single-crystal substrate originated from favorable atomic configurations for water molecule absorption and facilitation of transport of photoexcited electron–hole pairs in the depletion layer formed around the heterointerface of TiO2 thin layers on the base crystal
TiO2 :Nb has been reported as a conductive material that exhibits photocatalytic and PEC reaction properties [18,19,20,21]. For such TiO2 :Nb crystals, we first revealed the formation of a heterointerface between the TiO2 ultrathin layer and the Nb-doped base single crystal via segregation of doped Nb ions inward from the top surface; we investigated the potential significance of the (001) surface plane of the TiO2 :Nb crystals on their PEC properties
Summary
Photoelectrochemical (PEC) reactions have been widely studied, especially for solar hydrogen production and water purification [1,2,3,4,5]. To improve the performance of PEC cells, the development of photoelectrodes with high activity under an excitation light is critical. Photoelectrodes for PEC reactions are usually composed of semiconductor layers and conducting bottom electrodes. Several key factors favor the improvement of photoelectrodes: (1) A large optical absorption and active surface area [6,7], (2) the effective adsorption of reactants onto the photoelectrode surface [8,9], (3) good excited charge separation [10], and (4) efficient excited charge transfer to the bottom electrodes [11,12,13] Among these key factors, we focused on the properties of the crystal surface to attain new insights into the surface adsorption of reactants and photocarrier transport properties. In the case of crystal-plane surface dependence, numerous reports on crystal-facet-growth nanomaterials have appeared in the literature, and the role of each crystal plane in PEC reactions has been discussed [14,15,16,17]
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