(Invited) Core-Shell Double Doping for the Remarkable Photocatalytic Water Splitting on Ga2O3
Authors: Yamakata Akira , Vequizo Junie Jhon M. , Kato Kosaku , Sakata Yoshihisa 
Published: July 2021

The production of H2 gas from water using solar energy with photocatalysts is a promising method to realize a sustainable society. As H2 is clean and free from CO2 emissions, photocatalysts are expected to address many problems such as the energy crisis, environmental pollution, and global warming. However, the efficiency of photocatalysis is still not sufficient for industrial use; hence, further activity enhancement is required. The efficiency of photocatalysis is determined by the competition between electron-hole pair recombination and the rate of charge transfer to the reactant molecules. Therefore, in order to improve the activity, the recombination should be suppressed and the rate of charge transfer to the reactant molecules should be enhanced. It has long been assumed that the fabrication of fine crystals with low defects and impurities is essential for improving photocatalytic activity, since defects and impurities have previously been believed to accelerate the recombination process. However, the discovery of activity enhancement of NaTaO3 photocatalysts by La3+ doping, has led to the development of doping as a new methodology to increase the photocatalytic activity. This method has been applied to SrTiO3, Ga2O3, and TiO2, etc. However, the role of these dopants, except for band gap narrowing for visible absorption, is still a controversial issue. Several mechanisms have been proposed, such as the change in surface morphology to enhance the charge separation, the formation of a band-gap gradient, an increase in the n-type or p-type conductivity, and a reduction of oxygen vacancies (VO), etc., but their actual role still remains unclear.
Recently, a study by Sakata et al.,[1] succeeded in upgrading the world record of the highest quantum efficiency (QE) of water-splitting photocatalysts; a remarkably high QE of ~71% was achieved under 254 nm light illumination by doping Zn and Ca in Ga2O3. To our knowledge, this remarkably high QE surpasses the highest value of La-doped NaTaO3 with a QE of 56% at 270 nm, reported in 2000.[2] Previous studies have already reported that surface doping of Ga2O3 with divalent cations such as Zn or Ca increases its activity. However, the combination of surface Zn-doping and bulk Ca-doping further enhanced the activity. Double doping is often applied to prevent VO formation and/or to fix the valence number of doped cations by allowing for charge balance within the material. However, in the present case, Zn2+ and Ca2+ have the same valence number; hence, it is expected that some co-operative effects are present between Zn and Ca. This double-doping technique could be applied to many other photocatalysts, but the underlying mechanism of the enhancement of QE by Zn and Ca double-doping still remains uncertain.
In this work, we have unraveled the mechanism of the enhancement of the activity of Ga2O3 induced by Zn and Ca double-doping. Visible-to-mid-IR transient absorption spectroscopy (TAS) was adapted to elucidate the effects of dopants on the behavior of photogenerated charge carriers as this technique is quite effective in differentiating the behaviors of free electrons and electrons trapped at the defects on photocatalysts, such as TiO2, Fe2O3, NaTaO3, SrTiO3, LaTiO2N, and Ga2O3. Free electrons in conduction band (CB) and trapped electrons at the defects contribute to transient absorption (TA) signals at different wavenumbers;[3] hence, the behavior of these electrons can be separately investigated. Furthermore, the depth of the electron traps can be estimated from the absorption energy of the trapped electrons. Based on the TAS results, we found that the lifetime of photoelectrons increases due to surface Zn-doping and bulk Ca-doping, but the combination of Zn and Ca doping further increased the electron lifetime. It is also elucidated that the lifetime-extension of photocarriers is responsible for the formation of shallow mid-gap states by Zn and Ca doping; electron-trapping at these traps reduces the probability of interacting with holes, and thus, the lifetime of electrons increases further. Notably, most electrons are trapped at these mid-gap states, but they have a higher reactivity with water than those in the undoped sample. The mechanism of drastic activity enhancement by Zn and Ca double doping is discussed based on the TAS results, first-principles density functional theory calculations, and scanning transmission electron microscopy with EDS analysis.
[1] Y. Sakata, T. Hayashi, R. Yasunaga, N. Yanaga, H. Imamura, Chem. Commun., 51 (2015) 12935.
[2] A. Kudo, H. Kato, Chem. Phys. Lett., 331 (2000) 373.
[3] A. Yamakata, M. Kawaguchi, N. Nishimura, T. Minegishi, J. Kubota, K. Domen, J. Phys. Chem. C, 118 (2014) 23897.