How TiO₂facets determine arsenic adsorption and photooxidation: spectroscopic and DFT studies

Abstract

Anatase TiO 2 nanomaterials have been widely used in arsenic (As) remediation, although reports on their adsorption and photocatalytic capacity have been controversial. The motivation for our study is to explore the As adsorption and photooxidation processes on different TiO 2 facets at the molecular level. Our results from multiple complementary characterization techniques suggest that anatase {001} facets have stronger Lewis acid sites than those on {101} facets, resulting in a higher As adsorption affinity. Density functional theory (DFT) calculations confirmed that the As surface complex is more energetically favorable on {001} than on the {101} facets. In addition, the strong interaction of the {001} facets with molecular O 2 facilitates the transfer of photo-excited electrons to the adsorbed O 2 to generate a superoxide radical (O 2˙− ), which is the primary AsIJIII) oxidant as evidenced by our radical-trapping experiments. Meanwhile, the oxygen vacancies on the {001} facets could expedite the interfacial electron transfer and electron-hole separation, which promote the generation of O 2˙− and, ultimately, the catalytic efficiency. The insights gained from this study provide a firm basis for the proposition that As adsorption and photoactivity can be mediated by tailoring the exposed TiO 2 facets, which is of essence in the design and application of TiO 2 -based environmental technologies.

Recent progress of arsenic adsorption on TiO 2 in the presence of coexisting ions: A review

“…Meanwhile, TiO 2 material should have a large particle size and strong hardness to be operated easily in field filtration. Recent studies have shown that the adsorption performance of TiO 2 largely depends on its exposed crystal facets (Liu et al, 2014;Yan et al, 2016). The study of As adsorption on different anatase TiO 2 surfaces suggests that {001} facets have stronger Lewis acid sites than {101} facets, resulting in a higher As adsorption affinity, with As surface complexes being more energetically favorable on {001} than on {101} facets .…”

TiO2 crystal facet-dependent antimony adsorption and photocatalytic oxidation

“…For example, water molecules [7,8], dye molecules [6,9], and Au clusters [10] could preferentially bind to {001} facet rather than {101} facet. Our previous study also showed that arsenic adsorption was more favourable on the high-energy {001} facet than on the thermodynamically stable {101} facet [11]. However, to the best of our knowledge, no specific linear relationship has been derived between adsorption capacity and facets exposed on TiO 2 .…”

Mechanistic study of simultaneous arsenic and fluoride removal using granular TiO2-La adsorbent

“…The overlap of PDOS in Fig. 3c and d indicates that the five-coordinated Ti atoms on the TiO 2 facets covalently bonded with O atoms from La oxides, as evidenced by the electron sharing between the O-2p and Ti-3d orbitals [25]. The newly formed La-O bond was mainly attributed to the electron sharing between the O-2p and La-5d orbitals.…”

“…2b. The two peaks at about 458.3 eV and 464.0 eV can be assigned to the Ti 2p 3/2 and Ti 2p 1/2 spin orbital splitting photoelectrons in the Ti 4+ chemical state [25,29], respectively. Fig.…”

“…The k-point over the Brillouin zone was set as a 2 Â 2 Â 1 grid. The geometry optimization was calculated with the BFGS method until meeting the convergence criteria, 1.0 Â 10 À6 and 1.0 Â 10 À5 eV/ atom, respectively, for SCF and energy tolerances [25]. All atoms were relaxed during calculations.…”