The potential for applying UV light pre-treatment to enhance the oxygen activation capacity of Au/TiO2 under ambient conditions was examined. Catalytic formic acid oxidation in an aqueous environment was employed as the test reaction. Pre-illuminating Au/TiO2 with UV light can amplify the catalytic formic acid oxidation rate by up to four times with the degree of enhancement governed by system parameters such as Au loading, pre-illumination time, and initial formic acid loading. X-ray photoelectron spectroscopy, photoluminescence spectroscopy and electrochemical assessment of the Au/TiO2 indicated light pre-illumination invokes photoexcited electron transfer from the TiO2 support to the Au deposits. The Au deposits then utilise the additional electrons to catalyse molecular oxygen activation and promote the oxidation reaction. Scanning tunneling spectroscopy analysis and first principle calculations indicated the Au deposits introduced new electronic states above the TiO2 valence band. The new electronic states were most intense at the Au–TiO2 interface suggesting the Au deposit:TiO2 perimeter may be the key region for oxygen activation. The current study has demonstrated that pre-illuminating Au/TiO2 with light can be used to augment reactions here oxygen activation is a critical component, such as for the oxidation of organic pollutants and for the oxygen reduction reaction in fuel cells or energy storage systems.

1. e-
e-
e-
e-
Charge traps
e- Electron
Oxygen
Carbon
Hydrogen
Au
Au perimeter atom
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G
Key Contact
Particles and Catalysis Research Group,
School of Chemical Engineering,
The University of New South Wales,
Sydney, NSW 2052, Australia.
E-mail: r.amal@unsw.edu.au
jason.scott@unsw.edu.au

2. Introduction
Oxidation
https://www.thinglink.com/scene/620029595583774722
Glucose + O2 CO2 + H2O
Respiration
CO2
Volatile organic compound
Carbon monoxide
+ O2
Pollutant removal
http://butane.chem.uiuc.edu/pshapley
/GenChem2/C10/index.html
Energy generation
Fuel + O2 CO2 + H2O
+ Energy
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

3. Fig. 1 (a) HRTEM image of Au/TiO2 prepared by deposition–precipitation. Red arrows indicate a selection
of the Au deposits; (b) size distribution of the Au deposits on TiO2 as evaluated from HRTEM. Au deposit
count: 971.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

4. Fig. 2 The effect of: (a) Au loading on the formic acid oxidation rate (R50) for Au/TiO2 (dark = no pre-
illumination, pre-30 = 30 min UV light pre-illumination, light = photocatalytic); (b) pre-illumination time on the
formic acid oxidation rate for 1.0 at% Au/TiO2. The dotted line indicates the R50 of Au/TiO2 under the ‘light’
condition (i.e. UV-illuminated); (c) initial formic acid loading on its oxidation rate for 1.0 at% Au/TiO2 either
following light pre-illumination or under the light condition. The dotted line highlights the formic acid oxidation
rate under the ‘dark’ condition; (d) instantaneous rate of formic acid oxidation by Au/TiO2 as a function of time
for various formic acid loadings. The dotted line represents the ideal oxidation rate profile for a 1000 μmol
formic acid loading whereby no decay in activity (i.e. loss of active species) occurs. Catalyst loading = 1.0 g L−1,
initial formic acid loading = 100 μmol, light pre-illumination period = 30 min unless otherwise indicated.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

5. Fig. 3 Photoluminescence spectra of neat TiO2 and 1.0 at% Au/TiO2 in water. Excitation wavelength: 320 nm.
The presence of Au deposits (Au/TiO2 – red profile) on the TiO2 (black profile) decrease the electron–hole
recombination rate as shown by the reduction in PL intensity at 412 nm. Light pre-illumination of the Au/TiO2
for 30 min (purple profile) and 60 min (pink profile) further reduces the recombination rate.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

6. Fig. 4 XPS spectra of: Au4f (i–iii), O1s (iv–vi), and Ti2p (vii–ix) for 1.0 at% Au/TiO2 as-prepared, at 0 min
light pre-illumination (i.e. immersed in water), and following 30 min light pre-illumination. Binding energies
are normalised to adventitious carbon C1s = 285.0 eV.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

7. Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G
Fig. 5 Impact of light pre-illumination period on oxygen reduction reaction electrocatalytic activity (vs. Ag/AgCl)
for 1.0 at% Au/TiO2: effect on (a) current density at −1.0 V bias; (b) onset potential.

8. Fig. 6 STM images showing: (a) Au nanoparticle (pink cross) on rutile (110) single crystal; (b) Au-free
region (blue cross) on rutile (110) single crystal; and (c) STS of the corresponding spots marked on the
images. STS reveal the presence of new valence states above the valence band of TiO2 at the edge of the
Au nanoparticle near the Au/TiO2 interface and a shift in the conduction band towards lower energy.
(Vsample = 1.5 V, I = 100 pA, lock-in modulation V = 20 mV at 937 Hz).
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

9. Fig. 7 Total and projected electron densities of states in the energy region of the band gap for
Au/TiO2: (a) anatase (101) surface; (b) rutile (110) surface. The vertical dotted line shows the energy
of the highest occupied state (for the ground state). The yellow band in (a) highlights the new
valence states from the Au–TiO2 interaction.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

10. Scheme 1 Proposed electron pathways in the process of molecular oxygen activation on
Au/TiO2 during UV light pre-illumination. Photoexcited electrons are: (i) trapped by sites on/in
the TiO2; (ii) transferred into and then through the Au deposit where they are trapped by
adsorbed molecular O2.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G

11. Fig. 8 The effect of storage time in the dark and under ambient conditions on the: (a) formic acid oxidation
rate of Au/TiO2 following 30 min pre-illumination. Catalyst loading = 1.0 g L−1 , initial formic acid loading =
1000 μmol; (b) Au deposit size distribution on TiO2 when freshly prepared (top, particle count = 971) and
following 5 months of storage (bottom, particle count = 327). The particle size range below 2 nm is
highlighted in green.
Catalysis Science & Technology
Volume 6, Number 23, pages 8188-8199, 7 December 2016, DOI: 10.1039/C6CY01717G