Structure of a photocatalyst-water interface

The interaction of water with TiO₂ is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago [1] there have been numerous studies of the rutile single crystal TiO₂(110) interface with water [2]. This has provided an atomic level understanding of the water/TiO₂ interaction. However, nearly all the previous studies of water/TiO₂ interfaces involve water in the vapour phase.

In this work, we have explored the interfacial structure between liquid water and a rutile TiO₂(110) surface pre-characterised at the atomic level. Scanning-tunnelling microscopy and surface X-ray diffraction were used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O₂ and H₂O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO₂ photocatalysis.

A novel approach was employed to provide the first quantitative structure of a well-defined metal oxide-water interface formed by either temporarily immersing (dipping) a rutile TiO₂(110) surface into water or by depositing a water droplet on the surface, respectively. Both ex situ and in situ measurements of the liquid water-TiO₂ interface were carried out in an aerobic environment using SXRD at beamline ID32 (a former XSW and surface diffraction beamline). Pre-characterisation at the atomic level carried out at both the UCL and the ESRF was essential to the success of the SXRD experiment. Figure 1a displays STM images of the as-prepared TiO₂(110) surface. In the high-resolution image of the inset, bright (Ti) rows are seen that run in the [001] direction. Bright spots are also present and these are a mixture of bridging oxygen vacancies (O_b-vacs) and bridging hydroxyls (OH_b), the latter being formed by water dissociation at defect sites. After venting to N₂ and immersion in 10 ml water (18.2 MΩ.cm, total organic content < 2 ppm) for 5 mins, the sample (H₂O_dip sample) was reintroduced to UHV. STM images of the H₂O_dip sample are shown in Figure 1b. The high-resolution image shown in the inset evidences a (2×1) overlayer. Antiphase domains of this overlayer can form by an offset of one unit cell along the [001] direction.

Figure 1. STM images (0.2 nA, Vs = 1.2 V) of the TiO2(110) surface before (a) and after immersion in liquid water (b). In the large-area images, the scale bars are 100 Å and in the insets 20 Å.  Before immersion, the TiO2(110) surface is characterised by bright rows that arise from Ti5c atoms and bright spots that correspond to Ob-vac (0.11 ML) and OHb (0.10 L). After immersion in liquid water to form the H2Odip sample, the surface is characterised by bright spots arranged with a (2×1) periodicity. The blue and green shapes mark areas that are particularly well ordered. Note that these two areas are antiphase domains; the bright spots are offset by one TiO2 surface unit cell in the [001] direction.

To probe the chemical nature of the (2×1) overlayer, we employed X-ray and UV photoelectron spectroscopy (XPS, UPS). XPS spectra taken before and after immersion of TiO₂(110) in water display a peak at ~532 eV binding energy in the O 1s spectrum that originates from OH, which becomes more intense after immersion. Similarly, in UPS spectra taken after immersion in water, peaks appear at 8.0 and 10.2 eV below the Fermi level (EF) that are characteristic of chemisorbed OH. UPS also detects a band-gap state (BGS) associated with O_b-vac and OH_b that lies ~0.8 eV below EF. This BGS is quenched after immersion in water, which can be explained by healing of the O_b-vacs/OH_b by exposure to O₂ during the N₂ venting procedure.

Figure 2a shows selected SXRD results in the form of crystal truncation rods (CTRs) recorded from the H₂O_dip and H₂Odrop surfaces together with those from the as-prepared UHV surface. Best fits to the data are also shown in Figure 2a. A model for the clean surface is shown in Figure 2b and the best-fit model for the H₂O_dip and H₂O_drop surfaces are shown in Figure 2, c and d, respectively. Due to its low X-ray scattering contribution, hydrogen is not included in the analysis and is not shown in the model. Instead, water molecules or hydroxyls are represented only by their oxygen atoms. The bond distance between O(1#) and Ti(2) (see Figure 2c) is 1.95 ± 0.03 Å, which is in good agreement with the literature value for the Ti–OH_t (terminal OH) bond of 1.85 ± 0.08 Å. Hence, we assign O(1#) to OH_t, which is also consistent with our photoelectron spectroscopy data. The OH_t molecule has occupancy of 0.45±0.1, consistent with the STM results. For the H₂O_drop surface, the best-fit model is essentially the same as that of the H₂O_dip model (Figure 2c), except for the presence of a hydration layer above the (2×1) OHt contact layer.

Figure 2. Selected CTRs from the SXRD measurements alongside proposed models. (a) The structure factors of the different TiO2(110) surfaces are plotted for the (0,1,l), (1,2,l) and (2,1,l) CTRs. Black, red, and blue error bars represent the data from the as-prepared surface recorded before the H2Odrop experiment, H2Odip, and H2Odrop samples, respectively, with solid lines being the calculated data. Profiles are offset for clarity. Dashed vertical lines correspond to Bragg peaks. (b-d) Ball-stick models for clean, H2Odip, and H2Odrop samples, respectively.

Principal publication and authors

Structure of a model TiO₂ photocatalytic interface, H. Hussain (a,b), G. Tocci (a), T. Woolcot (a), X. Torrelles (c), C.L. Pang (a), D.S. Humphrey (a), C.M. Yim (a), D.C. Grinter (a), G. Cabailh (d,e), O. Bikondoa (f), R. Lindsay (g), J. Zegenhagen (b), A. Michaelides (a), G. Thornton (a), Nature Materials (2016); doi: 10.1038/nmat4793.
(a) London Centre for Nanotechnology and Department of Chemistry, University College London (UK)
(b) ESRF
(c) Institut de Ciència de Materials de Barcelona (CSIC), Bellaterra (Spain)
(d) Sorbonne Universités, Paris (France)
(e) CNRS, Institut des NanoSciences de Paris (France)
(f) Department of Physics, University of Warwick, Coventry (UK)
(g) Corrosion and Protection Centre, School of Materials, The University of Manchester (UK)

References
[1] A. Fujishima & K. Honda, Nature 238, 37 (1972).
[2] C.L. Pang et al., Chem. Rev. 113, 3887 (2013).