Role of Hydrogen in Defining the n-Type Character of BiVO₄ Photoanodes

Abstract

The roles of hydrogen impurity and oxygen vacancy defects on defining the conductivity, and hence photoelectrochemical (PEC) performance characteristics, of monoclinic scheelite bismuth vanadate (BiVO4) are investigated using a combination of experiment and theory. We find that elemental hydrogen is present as an impurity in as-synthesized BiVO4 and that increasing its concentration by annealing in H2 at temperatures up to 290 °C leads to near-complete elimination of majority carrier transport limitations, a beneficial shift in the photoanodic current onset potential, and improved fill factor. Magnetic resonance measurements reveal that hydrogen can be incorporated in at least two different chemical environments, which are assigned to interstitial and substitutional sites. Incorporation of hydrogen leads to a shift of the Fermi level toward the conduction band edge, indicating that n-type character is correlated with increased hydrogen content. This finding is in agreement with theory and reveals that hydrogen acts as a donor in BiVO4. Sub-bandgap photoluminescence is observed from as-synthesized material and is consistent with deep electronic states associated with oxygen vacancies. Hydrogen treatment leads to reduced emission from these states. These findings support the conclusion that hydrogen, rather than oxygen vacancies, is dominant in determining the n-type conductivity of BiVO4. These findings have important implications for controlling the electronic properties and functional characteristics of this promising photoanode material.

Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment

“…The observed reduction of trap states density upon hydrogen treatment is in agreement with a recent report by Cooper et al, [29] where hydrogen-treated BiVO4 was found to exhibit a lower sub-bandgap photoluminescence intensity as compared to untreated BiVO4. The reduction in the number of trap states is also consistent with the increase in carrier lifetimes ( Figure 3 and Table 1), which in turn results in the cathodic shift of the onset potential in the photocurrent-voltage curve ( Figure 2).…”

“…Thus, a hydrogen treatment appears to be a generally applicable strategy for improving the performance of BiVO4 photoanodes, as has also been reported by others. [24,29,38] Nevertheless, the main mechanism that causes the improvement of the PEC performance remains unclear.…”

“…This improvement is consistent with the increased photocurrent plateau and improved charge separation we found for these hydrogentreated BiVO4 films. Since hydrogen has been suggested to act as a donor-type dopant in BiVO4, [24,29] we compare the TRMC response with that for a tungsten-doped BiVO4 film. Along with Mo, W is the most common effective donor dopant in BiVO4.…”

“…[24][25][26][27][28] The improved performance was initially assumed to originate from the increased electrical conductivity due to oxygen vacancy formation and/or hydrogen insertion (i.e., an increase in carrier density through donor doping). [23][24][25][26][27][28][29] However, to the best of our knowledge, no report has yet provided direct evidence for the influence of hydrogen treatment on the carrier transport properties of BiVO4.…”

Assembly and Photocarrier Dynamics of Heterostructured Nanocomposite Photoanodes from Multicomponent Colloidal Nanocrystals

“…[47,48] For the case of the pure BiVO 4 film, a broad peak near 700 nm is observed, which has been assigned to radiative recombination at bulk oxygen vacancy sites. [36] PL from the TiO 2 /BiVO 4 nanocomposite film is characterized by broad spectral emission.…”