Understanding the anatase–rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting

The key to phase junctions for efficient charge separation is to consider both the phase alignment and interface structure.


Material synthesis
TiO 2 films with tunable phase structure were fabricated using direct-current reactive magnetron sputtering technique. This method includes two main steps: deposition of precursor film and rapid thermal annealing (RTA) treatment. TiO 2 precursor films were first deposited on commercial FTO glass (Nippon Sheet Glass, Japan, sheet resistance ca. 14 ohm per square, FTO coating thickness ca. 350nm, glass thickness ca. 2.2 mm) by the DC reactive physical vapor deposition (PVD) method using a commercial sputtering system equipped with a turbo molecular pump. A titanium metal disk (99.995% purity, 3 mm thickness, 60 mm diameter) was used as the target. After evacuation to 10 -3 Pa, the argon and oxygen gases were introduced into the chamber respectively through the mass flow controller. The oxygen outlet is over the target about 40mm. The total sputtering pressure was kept at 2.0 Pa with the oxygen partial pressure in the range of 0-15% (PO 2 % = PO 2 / (PO 2 +PAr) × 100%). A constant current mode was used and the sputtering current was kept at 0.6 A. The distance between the target and the sample holder was 60 mm. The titanium deposition rate was about 40 nm/min. At 12% O 2 partial pressure, the deposition rate of amorphous TiO 2 was about 8 nm/min. A corundum boat was used as the sample holder during the annealing procedure.
The resulting films were annealed under air atmosphere in a muffle furnace preheated to a certain temperature.
The TiO 2 -AR film with gradually introduced phase junction was fabricated by adjusting the O 2 partial pressures gradually from 12% to 0% during the deposition process. The deposition time for the TiO 2 -AR electrode with 80 nm thickness was ca.130 s. After rapid thermal annealing (RTA) treatment, the internal amorphous TiO 2 zone of the film deposited at oxygen-rich conditions was crystallized into anatase phase TiO 2 , and the external zone deposited at oxygen-deficient conditions was oxidized into rutile phase TiO 2 .
The TiO 2 -dAR with abrupt phase junction was fabricated by deposition of the internal amorphous TiO 2 layer at oxygen-rich conditions (12% O 2 ) first, followed by deposition of the external titanium layer at oxygen-free conditions (0% O 2 ). After RTA treatment, the internal layer was crystallized into anatase phase TiO 2 and the external titanium layer was oxidized into rutile phase TiO 2 .
For comparison, the TiO 2 -RA electrode was also fabricated. The titanium layer was firstly deposited on the FTO substrate, calcined at 1073K for 4min, followed by the amorphous TiO 2 layer deposition at 12% O 2 and calcinations treatment at 1073 k for 4 min.

Characterization
Raman: The anatase and rutile phase was verified by visible Raman and UV Raman spectroscopy recorded on a Renishaw inVia Raman microscope. Single-frequency laser (532 nm, DPSS 532 Model 200) was used as the exciting source for the visible Raman with an output of 2 -10 mW and a He-Cd laser (325 nm) was used as the exciting source for the UV Raman. Need to mention that, if the UV resonance Raman spectra were recorded with the Renishaw inVia Raman microscope with the Rayleigh scattering filter, the Rayleigh scattering lower than 200 cm -1 could be filtered from the spectrum. In order to verify this point, we also recorded on UV Raman spectrum with a Jobin-Yvon T64000 triple-stage spectrograph which can detect Raman peaks down to 100 cm -1 . The spectrum of FTO-AR shows only three characteristic bands of rutile phase TiO 2 at 247, 443 and 610 cm -1 as shown followed. This confirms the absence of UV Raman scattering features due to either anatase or rutile phase TiO 2 at wavenumber lower than 200 cm -1 for our TiO 2 -AR electrode. In the article we use the visible Raman spectra and UV Raman spectra recorded on the Renishaw inVia Raman microscope for easy comparison.
The morphology of TiO 2 films were characterized using a field emission scanning           Figure S8.