Supplementary Information
Composition dependence of the photochemical reduction of Ag+
by as-grown Pb(ZrxTi1-x)O3 films on indium tin oxide electrode
Man Zhang, Chunxiang Jiang, Wen Dong, Fengang Zheng, Liang Fang, Xiaodong Su
and Mingrong Shen
Jiangsu Key Laboratory of Thin Films and Department of Physics, Soochow University, Suzhou, 215006, People’s
Transimittance / %
100
80
(a)
60
PbTiO3
40
PZT10
PZT20
PZT30
PZT52
20
0
200
400
600
Wavelength / nm
Absorption (a. u.)
Republic of China
200
800
(b)
400
PbTiO3
PZT30
PZT52
600
Wavelength / nm
800
Figure S1. The UV-vis transmittance (a) and absorption (b) of the PZT films with
different Zr concentrations.
Robertson reported that the band gap of PbTiO3 (3.4 eV) is less than that of
ZrTiO3 (3.7 eV).1 As Zr is added to PbTiO3, the band gap may increase and less
light may be absorbed. To check this point, we measured the UV-vis
transmittance and absorption of the PZT films with different Zr concentrations
[see Fig. S1 (a)&(b) in supplementary material S1]. For PbTiO3, PZT10 and
PZT20, no significant difference for both the transmittance and absorption
spectra was observed, and small blue shifts for PZT30 and PZT52 can be seen.
1
According to the process described in our recent work,2 we determine that the
optical band gaps for PbTiO3, PZT10, PZT20, PZT30 and PZT52 are 3.47, 3.48,
3.48, 3.55 and 3.58 eV, respectively. Such results illustrate that the band gap does
not change obviously when Zr concentration is within 20%. Thus the absorption
can not lead to the result that the PEC photocurrent in Fig. 4(a) decreases
sharply from PbTiO3 to PZT20. However, for PZT with Zr concentration larger
than 20%, the change in absorption may be another reason for the change of the
PEC properties.
PZT are interesting since their extraordinary dielectric and piezoelectric
behaviors around morphotropic phase boundary (MPB, about Zr:Ti=52:48)
where the properties are enhanced.3 For Ba1-xSrxTiO3 ceramics4,5 and Gd doped
BiFeO3 nanoparticles6, the reported results indicated that the high dielectric
constant at the phase boundary can increase the width of the space charge
region and charge carrier separation in the near surface region, and therefore
enhance the photochemical processes. Our previous study showed that the
dielectric constant also reaches a maximum at MPB for PZT films.7 However, in
this study, we did not observe the enhancement of the PEC processes near the
MPB composition, indicating that such effect may be weak enough for PZT films
and/or be overweighed by other effects.
2
Figure S2. SEM images of the surface of PbTiO3 exposed in AgNO3 solution with
different illumination time: (a) 5 min, (b) 10 min, (c) 15 min, and (d) 20 min.
Fig. S2 presents the Ag nanoparticles photochemically grown on the
as-grown PbTiO3 film surfaces at different stages. It is interesting to note from
Fig. S2(a) that no any Ag nanoparticles appear on the film surface after an initial
illumination of 5 min. When the illumination time is larger than 10 min, both the
Ag particle size and density increase with the increase of illumination time, as
shown in Fig. S2(b) to 2(d). The photocathodic behavior shown in Fig. 4(a)
illustrates there is a net internal electric field in the ITO/PZT electrode with
direction pointing from ITO to PZT surface, and then the Stern layer may form on
PZT surface due to the surface potential that forms when the internal charge
compensation process is only partially screening the internal field.8 With the
aqueous AgNO3 solution the layer would consist of desolvated NO3 ions and
polarized H2O molecules, which acts as a barrier to the Ag+ ions in the outer
Helmholtz and diffusion region, as proposed by Dunn et al.8 In order for the Ag+
3
ion to reach the PZT surface and nucleate there must be a disturbance in the
Stern layer. Under illumination, the photoexcited electrons migrate to reduction
sites on the PZT surface and accumulate, causing a localized variation in
electrical potential V to a more negative one. Within the first 5 min illumination,
the accumulated electrons is not large enough and thus the V is not negative
enough to repel the NO3 anions and attracting the Ag+ ions. Therefore, no Ag
nanoparticles can be observed. Above 5 min illumination, the accumulated
electrons is larger enough to attract the Ag+ ions which subsequently accept the
electron from the PZT surface, resulting in the growth of Ag nanoparticles, as
illustrated in Fig. 4(b) to 4(d). However, Dunn et al.9 reported that the nucleation
density does not change significantly from the initial nucleation density as the
illumination time increases. This point is not consistent with our finding that
both the Ag particle size and density increase with the increase of illumination
time. In this study, the factor dominating the Ag growth behavior is the ITO/PZT
interface barrier instead of polarized domain surface. The former may produce
an internal electric field more uniformly distributed in the PZT film, making the
photogenerated electrons arriving at the film surface and accumulating there
more evenly and resulting in the uniform distribution of the Ag nanoparticles.
Therefore, new Ag nanoparticles can nucleate on PZT surface as the illumination
time increases.
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1J.
Robertson, J. Vac. Sci. Technol. B 18, 1785 (2000).
4
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3I.
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4A.
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8P.
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9S.
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5