Hydrogen has been in the spotlight because it is one of the most promising energy carriers for the future. It combines with oxygen to generate enormous energy and water without any greenhouse gases or other pollutants. In addition, unlike the electricity produced from the most common photovoltaic cells, hydrogen can store solar energy in the form of high-density chemical energy. However, in order for hydrogen to play a key role as the next-generation energy carrier, it is required to produce hydrogen in a cost-effective, large-scale, and sustainable manner. Photocatalytic water splitting is an ideal route to cost-effective, large-scale, and sustainable hydrogen production, but it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band-gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, I have conducted the research for upgrading photocatalytic properties of scheelite-monoclinic BiVO4.
Scheelite-monoclinic (m-) BiVO4 is a well-documented photocatalyst having earth-abundant element composition and suitable band-gap energy (Eg ~ 2.4 eV) for absorbing visible light. Also, it is chemically stable in aqueous solution under light irradiation. However, pristine m-BiVO4 usually shows a low photocatalytic activity owing to poor charge-transport characteristics and the weak surface adsorption properties. In order to overcome these weaknesses, I have doped phosphorous into the vanadium sites in the host lattice of m-BiVO4, for the first time, by replacing some of the VO4 oxoanions in BiVO4 with PO4 oxoanions. This oxoanion doping into the photocatalyst represents a previously unidentified concept, and thus it can be applied to other various VO4-based semiconductors to improve their electronic, catalytic, and photochemical properties.
The PO4 oxoanion doping did not bring about significant changes in the optical absorption behavior and crystal structure of m-BiVO4. When an appropriate amount of PO4 oxoanion was doped, however, the activity of photocatalytic water oxidation increased very significantly by a factor of about 30. The physical origin of the enhanced photocatalytic properties of PO4 oxoanion-doped BiVO4 was elucidated by using electrochemical impedence spectroscopy (EIS) and density functional theory (DFT) calculations. EIS measurements revealed that PO4 oxoanion doping lowered the charge transfer resistance of m-BiVO4 remarkably. DFT calculations demonstrated that an internal electric field was built inside PO4 oxoanion doped m-BiVO4, due to both the lattice strain imposed by the different V?O (1.737 Å) and P?O (1.563 Å) bond lengths and the charge redistribution around the PO4 oxoanion dopant. These results from EIS measurements and DFT calculations confirm that PO4 oxoanion-doping effect is very advantageous for the separation of electron-hole pairs and the charge-transfer characteristics of m-BiVO4.
Thanks to the PO4 oxoanion-doping study, it is possible to make m-BiVO4 function as an excellent photocatalyst for water oxidation (2H2O(l) + 4h+ => O2(g) + 4H+) under visible light. However, because the bottom of its conduction band is located at a more positive potential than that of proton reduction (0 VRHE at pH 7, RHE: reversible hydrogen electrode), it is still incapable of evolving H2 (2H+ + 2e- => H2(g)). This is why overall water splitting (2H2O(l) + 4h+ + 4e- => O2(g) + 2H2(g)) under visible light irradiation over BiVO4-based photocatalysts has never been fully achieved.
To meet this challenge, I have developed ‘greenish’ BiVO4 (GBVOx, x = atom ratio of In and Mo), Bi1-xInxV1-xMoxO4, by simultaneously substituting In3+ for Bi3+ and Mo6+ for V5+ in the host lattice of m-BiVO4. This new GBVOx photocatalyst has a slightly larger band-gap energy (Eg ~ 2.5 eV) than usual ‘yellow’ m-BiVO4 (Eg ~ 2.4 eV), as supported by the unique color change to green, and higher (more negative) conduction band than H+/H2 potential (0 VRHE at pH 7). Consequently, as Figure 1 illustrates, GBVOx is able to split water into H2 and O2 under visible-light irradiation without using any sacrificial reagents (e.g. CH3OH or AgNO3). This outcome is the first example of a pure water-splitting photocatalyst responding to visble light without any noble-metal cocatalyst.
In phase I of Figure 2, GBVO0.10 (the most active GBVOx, Bi0.9In0.1V0.9Mo0.1O4) achieved stoichiometric H2/O2 evolution even without modification by any co-catalyst. This result indicates that GBVO0.10 can split pure water into H2 and O2 by visible light-driven overall water splitting (OWS) without any sacrificial reagents or additives. Next, the reactor was purged with N2, and OWS reaction was tested again after photo-depositing 3 wt% RuO2 on GBVO0.10 as a cocatalyst. The photocatalytic activity was significantly improved by adding a RuO2 co-catalyst that collects electrons/holes and provides active sites for catalytic water reduction and/or oxidation. Phase II-IV presents repeated runs of the same catalysts to verify the photochemical stability of GBVO0.10. In phase V, heat treatment (623 K in air for 1 hour) further intensified the photocatalytic activity of RuO2-cocatalyzed GBVO0.10. This treatment was aimed at converting the photo-deposited RuO2•xH2O on GBVO0.10 into the more stable and active form of cocatalyst (i.e. RuO2).
The physical origin of the augmented photocatalytic behaviors of greenish BiVO4 was illuminated through DFT calculations as well as a variety of physical and electrochemical characterizations. Briefly, the In3+/Mo6+-dopant formation is more promoted within tetragonal BiVO4 rather than monoclinic BiVO4. This physicochemical tendency triggers the partial phase transition from pure monoclinic BiVO4 to a mixture of monoclinic and tetragonal BiVO4, which sequentially leads to unit-cell volume growth, compressive lattice-strain increase, conduction-band edge uplift, and band-gap widening. This In3+/Mo6+ doping-induced domino effect from phase transition to band edge engineering enables a previously unidentified mechanism to create a noble metal-free photocatalyst, accomplishing overall water splitting under visible-light irradiation. Therefore, the findings from this research possess great potential to realize a platform technology toward cost-effective, large-scale, and sustainable hydrogen production from water.
Figure 1. Overall water splitting reaction mechanism by GBVO0.10
Figure 2. Overall water splitting is realized by GBVO0.10 under the visible-light (λ ≥ 420 nm) irradiation. There are five reaction phases: Phase I, unmodified GBVO0.10; Phase II-IV, 3 wt% RuO2/GBVO0.10; and Phase V, heat treated 3 wt% RuO2/GBVO0.10. Solid and dashed lines indicate evolved H2 and O2, respectively. Base line represents a control experiment by pristine BiVO4.
1. Won Jun Jo et al, Phosphate Doping into Monoclinic BiVO4 for Enhanced Photoelectrochemical Water Oxidation Activity, Angewandte Chemie International Edition, 2012, 51, 3147-3151. Citations: 141
2. Won Jun Jo et al, Phase Transition-Induced Band Edge Engineering of BiVO4 to Split Pure Water under Visible Light, Proceedings of the National Academy of Sciences, 2015, 112(45), 13774-13778.
|Category||Short file description||File description||File Size|
|Präsentation||Overall water splitting reaction mechanism by GBVO0.10||This is the table of content (ToC) figure of the abstract.||861 KB||Download|
|Präsentation||H2 and O2 Evolution Data||This is the key experiment data of the submitted abstract.||187 KB||Download|
|Manuskript||Figure 1||Overall water splitting reaction mechanism by GBVO0.10||861 KB||Download|
|Manuskript||Figure 2||Overall water splitting realized by GBVO0.10 under the visible-light (? ? 420 nm) irradiation||187 KB||Download|