The central objective of the present study is to improve the solar energy absorptio n capacity by making the core-shell heterostructure using two semiconductor materials having different optical properties. Shell of a wide band gap (TiO2) materials and
nanoparticle core of smaller band gap (BiVO4) semiconductor have been used and the
properties of core shell nanoparticle has been compared with those of BiVO4 and TiO2
nanoparticles of similar sizes. BiVO4 nanoparticles samples showed lower photocurrent
density due to poor carrier transport properties because of lower hole diffusion length201. The present strategy of using core-shell heterostructure resulted in
photogenerated carriers very close to BiVO4-TiO2 interface. Surface catalytic
properties of the semiconductor surface in contact with liquid electrolyte is also very important. TiO2 surface is known to have superior surface photocatalytic properties in
comparison to other semiconductors, as well as high stability in liquid electrolyte202.
Therefore, the use of TiO2 shell in core-shell nanoparticle is expected to result in
stability and better surface catalytic properties. Further, for improving the charge transfer in a heterojunction, a favorable band alignment is an important pre-requisite. The favorable potential difference between BiVO4 and TiO2 for efficient charge transfer
has also been reported in thin film heterojunction203. The improved properties at BiVO 4
and TiO2 interface explained in terms of favorable Fermi level alignment in the
BiVO4/TiO2 interface with conduction band of BiVO4 higher than that of TiO2 resulting
in a type II band alignment. This will favor photoelectrons created in BiVO4 being
efficiently transferred to TiO2 preventing recombination and efficient PEC reaction at
TiO2/ electrolyte interface. Based on our photo-electrochemical results, it can be
confirmed that the separation of photogenerated charge carriers of BiVO4 nanoparticles
101 depositing a thin TiO2 shell. Accordingly, there is an increase in photocatalytic activity
for the degradation of methylene blue dye under visible-light irradiation. In previous studies, it has been reported that the increased separation of photogenerated charges in BiVO4-TiO2 heterostructure, when TiO2 is in contact with electrolyte as a top layer, are
mainly attributed to the uncommon transfers of photo-excited highenergy electrons from bottom BiVO4 layer to top TiO2 layer. When BiVO4-TiO2 core-shell is illuminated
by one sun radiation to excite the charge carriers, the TiO2 shell layer absorbs the UV
radiation shorter than 375 nm. The remaining light with radiation larger than 375nm and shorter wavelengths than 530 nm reaches to excite the BiVO4 core and generate a
large concentration of high-energy electrons (e) in BiVO4 above the conduction band
(CB) with positive holes (h+) left in the valence band (VB). Generally, these high-
energy electrons are active for water reduction, because they possess a higher energy level than that standard hydrogen electrode (SHE) of water reduction, while the holes would be captured by water to oxidize. However, it should be noted that the high-energy electrons migrated easily to the bottom of the CB in a remarkably short time, as they relax and then immediately recombine with the holes at the VB, leading to weakened charge separation with a lowered photoactivity. Interestingly, when the BiVO4 is
coupled with TiO2 in core–shell configuration, the visible-light-excited high-energy
electrons of BiVO4 would thermodynamically transfer to the CB of TiO2, which
prolongs the lifetimes of the high-energy electrons. In addition, it is well known that the CB energy level of TiO2 is higher than that of water reduction potential, meaning
that the transferred high-energy electrons can be used directly for hydrogen generation. One of the most important finding in the result of the present study is the change in the nature of PEC response from anodic behavior of TiO2 and BiVO4 nanoparticle samples
102 PEC response. The change in the work function due to nanoparticle size, core-shell nanoparticle configuration and interaction of nanoparticle with water may be responsible for the change in this electronic behavior. The above discussion supports this conjecture. the enhancement mechanism of charge separation in BiVO4-TiO2 type-
2 heterojunction204.
Work function of a semiconductor surface is one of the most important parameters determining band alignment at the interface important for metal-semiconductor contacts, photovoltaic junction and photo-electro-chemical devices. In the context of the present study, effect of interaction of semiconductor surface with water adsorbed is important. Studies for measuring the surface properties of semiconductors and changes on water adsorption are normally performed at liquid nitrogen or lower temperature in vacuum conditions involving adsorption of few layers of water. It is clear that this can give results different from when a semiconductor is in direct contact with liquid electrolyte as in case of photo-electro-chemical cell. Despite its importance, work function of widely used TiO2 semiconductor has only been investigated in limited
studies205. In an interesting study, the effect of surface treatment (oxidation and
annealing) has been carried out206. The results show overall variation of work function to be 1.74, 2.14 and 1.39 eV for Anatase (001), Anatase (101) and polycrystalline Anatase, respectively. Work function is found to be in the order: oxidized> stoichiometric> annealed>sputtered. As discussed above, work function values can get modified by surface conditions, stoichiometry, doping, surface charge layers or the surface dipole 207. As work function is difference in the Fermi energy and vacuum level energy, it may also change with any change in these parameters. In the context of present study involving application of TiO2 and BiVO4 nanoparticles for water splitting,
103 Distribution of vacancies may also influence interface of TiO2 and water206. There is a
general agreement that water interaction with TiO2 will affect work function of a
semiconductor surface, but it is not clear which way and how much is the effect on the change in work function values as adsorption affect the work function values of surface207. In a study on effect of work function on water adsorption, work function has been measured and shown that it can vary in the range of 4.70-6.44, 4.62-6.76 and 4.51- 5.62 eV for (001), (101) and polycrystalline surfaces, respectively206.
One of the most important result of the present study is the change in the nature of PEC response from anodic response for TiO2 and BiVO4 nanoparticle samples to cathodic
response for BiVO4/TiO2 core- shell nanoparticles along with enhanced photo-electro-
chemical response. The change in the work function due to nanoparticle size, core-shell nanoparticle configuration and interaction of nanoparticle with water may be responsible for the change in this electronic behavior. The above discussion supports this conjecture.
5.3 Conclusions
In summary, pristine BiVO4, TiO2, and BiVO4-TiO2 core-shell heterostructure
nanoparticle have been fabricated by chemical method. TEM analysis confirm the formation of core-shell nanoparticles.The present study shows that the fabricated core– shell heterostructure can not only provide a high optical absorption in visible region, but also cause the formation of a staggered BiVO4-TiO2 core -shell nanoparticle
heterojunction to promote the charge separation and transfer, leading to a significantly enhanced photo-electrochemical water splitting efficiency and high rate of photocatalytic degradation of organic pollution as compared to pristine BiVO4 and TiO2
nanoparticles.The change in PEC response from anodic (for TiO2 and BiVO4
104 sensitive dependence of work function values to surface conditions, nanoparticle effect, core-shell configuration and interaction of core-shell nanoparticle with water.
105