Contributed paper
OPTO-ELECTRONICS REVIEW 12(1), 53–56 (2004)
Nanocermet TiO2:Au thin film electrodes for wet electrochemical
solar cells
M. RADECKA*1, A. GORZKOWSKA-SOBAŒ1, K. ZAKRZEWSKA2, and P. SOBAŒ
1Faculty
of Material Science and Ceramics, AGH-University of Science and Technology
30-059 Cracow, 30 Mickiewicza Ave., Poland
2Faculty of Electronics, Automatics, Computer Science and Electronics,
AGH-University of Science and Technology, 30-059 Cracow, 30 Mickiewicza Ave., Poland
TiO2:Au nanocermets and TiO2 thin films covered with gold precipitates were obtained from RF reactive sputtering of mosaic
Au:Ti or metallic Au targets. The influence of gold content, deposition temperature and post-deposition annealing on
photoanodic behaviour and the structure of the obtained materials was examined by optical, structural and electrochemical
measurements.
Keywords: nanocermets, titanium dioxide, water photolysis.
1. Introduction
2. Experimental
Wet electrochemical solar cells convert solar energy into
chemical (hydrogen fuel) or electrical energy (photocurrent). For these applications, a titanium dioxide is the material of particular interest. TiO2, when immersed in a water
solution under illumination, acts as a photoanode in photolysis of water [1]. Possible reactions include formation of
gaseous hydrogen which can be used in solid fuel cells to
provide electrical energy.
Titanium dioxide photoanodes are inert and electrochemically stable. However, in terrestrial conditions, the efficiency of photolysis is poor because a fundamental absorption edge of titania is situated over UV region at about
350–375 nm. Therefore to increase the efficiency of water
photolysis it is necessary to increase the photoresponse over
the visible range of light. Several attempts on the TiO2 modifications were made, e.g., by doping it with transition metal
alloys or sensibilizing the surface with organic dyes [2–4].
Also, it was shown that one can shift the TiO2 absorption edge by introducing noble metal particles inside the
TiO2 matrix. The obtained nanocermets may have various
applications in electrochemistry or in non-linear optics
[5–7].
In this work, we introduced gold nanoparticles into the
bulk of TiO2 thin films (co-sputtering from Ti-Au mosaic
target) or covered the TiO2 film surface with a discontinuous gold film (sequential sputtering from Ti and Au targets) to examine the influence of such modifications on the
TiO2 anodic photoresponse.
2.1. Thin films preparation
*e-mail:
[email protected]
Opto-Electron. Rev., 12, no. 1, 2004
The samples were obtained from RF reactive sputtering
process in Ar + O2 or Ar controlled flow. The samples with
gold nanoparticles in the bulk (A series, see Table 1) were
prepared by co-sputtering in Ar + O2 gas mixture from a
mosaic Ti-Au target. The samples covered with gold (B series, see Table 2) were prepared by sputtering a titanium
target in Ar + O2 flow and sequential sputtering of metallic
Au or the mosaic Ti-Au targets in Ar flow. They were sputtered on a corning glass and a titanium foil.
2.2. Characterisation of the samples
In order to examine influence of the sputtering conditions,
as well as post-deposition thermal treatment, on photoanodic behaviour some of the substrates were heated to
350°C during the deposition. Furthermore, after deposition
process almost all samples from A series were annealed in
the deposition chamber in vacuum at about 5×10–5 mbar.
Samples 3 and 5 were annealed in Ar atmosphere at
4×10–2 mbar pressure. The annealing temperature was
400°C and the time was 1 hour. Sample 1 was measured
as-sputtered. Optical measurements were carried on a Lambda
19 Perkin Elmer spectrophotometer over 200–2200 nm wavelength range. The structural information was obtained from
the X-ray diffraction measurements on Seifert 7 diffractometer at a grazing interference angle (GID). The sample
thickness was determined mechanically by a Tallystep
profilometer.
M. Radecka
53
Nanocermet TiO2:Au thin film electrodes for wet electrochemical solar cells
Table 1.
A series
1
Substrate temperature (°C)
2
3
4
5
350
Post-deposition annealing (400°C, 1 h)
Thickness (nm)
6
7
8
9
10
RT
11
12
350
None
Vac
Ar
Vac
Ar
Vac
Vac
Vac
50
50
50
50
70
90
140
170
75
175
115
60
0.85
< 0.2
3.18
1.93
< 0.2
Au amount (at %)
< 0.2
Vacuum
Table 2.
B series
1
2
3
4
5
Gold deposition time (s)
0
1
8
60
300
Target
Metallic Au
Mosaic
(Ti+Au)
2.3. Measurements
Electrochemical measurements were carried in a standard
cell consisting of a photoanode, working electrode, and reference electrode (SCE) [Fig. 1(a)], with TRIAX 180 Jobin
Yvon monochromator and 450W Xe lamp as a light source
[Fig. 1(b)]. The electrodes were immersed in a buffer solution with pH = 8. Basic information on the samples is listed
in Tables 1 and 2.
Fig. 2. X-ray diffraction patterns for TiO2:Au nanocermets.
Fig. 1. Schematic view of the measurement system: light is
provided via optical fibre from monochromator to PEC (a);
photoelectrochemical cell with TiO2 photoanode and reactions
occurring at the electrodes during photolysis (b).
3. Results
The X-ray diffraction pattern for the chosen A series are
shown in Fig. 2. Generally, the samples are amorphous and
the substrate temperature seems to play the most important
54
role for crystalline structure of the samples. The samples
sputtered onto the heated substrates are slightly better crystallised than the rest, with anatase as a dominant phase.
A metallic gold phase is revealed only for the sample with
the highest gold content (peaks pointed with arrows in Fig.
2). However, in our previous works it was indicated from
TEM observations of the TiO2:Au nanocermets microstructure that for samples obtained in this manner gold
nanoparticles are well crystallized inside the TiO2 matrix
[5]. The effects of post-deposition annealing treatment did
not reveal in the structural measurements. This may be
caused by low annealing temperature. However, the higher
annealing temperatures lead to crystallization of a rutile
phase [3].
Absorptance spectra, calculated from the measured
transmittance T(l) and R(l) for the samples are shown in
Fig. 3(a) and 3(b). For nanocermets with the low gold con-
Opto-Electron. Rev, 12, no. 1, 2004
© 2004 COSiW SEP, Warsaw
Contributed paper
Fig. 3. Optical absorptance spectra for chosen samples: nanocermets (a) and TiO2 thin films covered with discontinuous gold film (b).
Fig. 4. Photocurrent densities for TiO2:Au nanocermets.
Fig. 6. IPEC calculated for chosen samples from both series vs.
light wavelength.
Fig. 5. Photocurrent densities for TiO2 thin films covered with
discontinuous gold film.
Opto-Electron. Rev., 12, no. 1, 2004
tent, the absorption edge is sharp with an additional absorption peak clearly visible at l about 500 nm. This peak is
clearly visible for nanocermets obtained at RT but broadens for these sputtered onto the heated substrates [Fig.
3(a)]. For the samples of series B there is also the additional absorption peak which broadens and shifts towards
longer wavelengths with increasing gold content on the
surface [Fig. 3(b)]. Furthermore, for the sample B3 a shoulder in the absorption edge occurs which is due to the additional states in the TiO2 band gap. The spectral characteristic of the absorption is strongly modified by Au in the region of the fundamental absorption edge.
The effect of Au nanoparticles on TiO2 photoresponse
is presented in Fig. 4. For nanocermets, a local maximum
in the photocurrent appears at about 540 nm. The samples
sputtered on the heated substrates give significantly better
M. Radecka
55
Nanocermet TiO2:Au thin film electrodes for wet electrochemical solar cells
photoresponse which affects the incident photon-to-current
conversion efficiency (IPCE) presented in Fig. 6. The IPCE
was calculated from a formula given by Gerischer [8].
The photoresponse of the samples sputtered on the
non-heated substrates worsens with the growing thickness
of these samples. Also, photocurrent values decrease with
increasing gold content in nanocermets. On the contrary,
the photocurrent values for samples covered with gold increases with increasing gold content (Fig. 5). The photoresponse of these samples is higher than for pure TiO2 over
visible range of light as well as for UV range of light.
4. Conclusions
Nanocermets TiO2:Au sputtered onto heated substrates are
better crystallised and show the higher photoefficiency. The
presence of gold increases the absorptance over the visible
light range, probably due to surface plasmon resonance phenomenon [5,7]. However, the higher concentrations of gold in
the bulk worsen the photoresponse of the samples. The atmosphere during post deposition annealing treatment does not
seem to affect the absorptance spectra and photoelectrochemical behaviour of the samples with Au in the bulk.
The SPR is also observed for the samples with gold precipitates on the surface. A small amount of gold sputtered onto the
TiO2 surface works as a photosensibilizer, increasing the photoresponse of the samples at l = 600 nm and at l < 400 nm.
Acknowledgement
The project is supported by the Polish State Commitee for
Scientific Research (KBN) under the grant No. 4T08A
02524.
56
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Opto-Electron. Rev, 12, no. 1, 2004
© 2004 COSiW SEP, Warsaw