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Fig. 2. (Left) Absorption spectra (DRS) of Ag@TiO2 taken directly after silver photodeposition (for suspended photocatalyst in methanol (50 vol%), before sample washing
and drying); (right) photocatalytic decomposition of acetic acid (5 vol%) under visible light irradiation on Ag@TiO2 (ST-01) with corresponding DRS spectra (after preparation
sample was washed, dried, suspended in acetic acid and irradiated).
alcohol dehydrogenation [22,23]. Recently, the possibility of alco-
hol dehydrogenation by complexes of noble metals has been
reported [24,25].
metal NPs resulted in their enrichment on the surfaces [29,30].
A much larger amount of silver might indicate the presence of
extremely fine (nano-sized) silver NPs in the case of Ag@TiO2
(ST-41). XPS analysis of Ag 3d showed the presence of three com-
During irradiation, the colour of titania suspension changed
from white to brown or violet, due to silver NPs formation, as shown
in Table S2 (Supplementary Information). The maxima LSPR val-
ues for small anatase titanias were observed at 430 nm, while for
large anatase/rutile titanias at 440 nm, indicating that larger sil-
ver NPs were formed on larger titanias. The influence of support on
metal NPs formation was also observed previously, e.g., larger silver
NPs were formed on larger polystyrene globules [26], and a linear
increase of mean gold NPs size with increase of particle size of sup-
port was observed during gold photodeposition on titania [14]. It is
thought, that similar to gold deposition on titania, silver is prefer-
ably deposited on surface defects [27], and thus larger amount of
surface defects on titania with large surface area (small NPs, [28])
caused formation of a large amount of small silver NPs.
0
+
2+
ponents attributed to Ag (368.1 eV), Ag (Ag O, 367.3 eV) and Ag
2
(AgO, 366.1 eV) [29]. Surface silver was mainly in an oxidized form
+
(82–89%, Ag ) as shown in Supplementary Information (Fig. S2 and
Table S3).
Microscopic observation confirmed the existence of silver NPs
of various sizes and shapes, as well as rod-like NPs, as shown in
Fig. 3. In the case of large rutile (ST-G1, crystalline size: 250 nm)
much broader distribution of silver size/shapes was detected, i.e.,
from nano-sized to 100 nm (Fig. 3, top), than for large anatase (ST-
41, crystalline size: 208 nm). For large anatase the NPs of silver
were more uniform appearing mainly in the sizes from 10 to 40 nm,
similarly to results obtained from XRD analysis. On the other hand,
small silver NPs were formed on fine titania. It is thought that
similar to gold-modified titania metal NPs were mainly formed
on crystalline defects [27] and that fine particles of titania pos-
sessing a large amount of such defects [31] caused formation
of fine metallic NPs. While, large metallic NPs were formed on
the surface of well-crystallized rutile titania with large NPs. It
must be mentioned that in the case of silver deposition a large
amount of nano-sized particles (1–3 nm) was easily detected
(Fig. 3, top).
In contrast with Au@TiO photocatalysts, the colour of Ag@TiO2
2
samples was unstable and changed after drying (Supplementary
Information, Table S2), probably due to partial oxidation of silver.
XPS data confirmed that surface of silver NP was mainly positively
charged (>80%), as shown in Table S3 and Fig. S1 (Supplementary
Information). Fortunately, during subsequent irradiation the plas-
monic properties returned to original ones, e.g., during acetic acid
oxidation under visible light irradiation, as shown in the right part
of Fig. 2 and Table S2. It must be pointed that colour change did not
influence the properties, and linear evolution of CO2 during acetic
acid decomposition was observed from the beginning of irradiation
3.3. Photocatalytic activities under UV and/or visible light
irradiation
(
without induction period, Fig. 2, right).
The activities of Ag@TiO2 photocatalysts during acetic acid and
3
.2. Photocatalyst characterization
2-propanol oxidation under UV and visible light irradiation are
shown in Figs. 4 and 5, respectively, in comparison with previously
reported data for gold-modified titanias [14].
The sizes of crystallites and crystalline composition were deter-
mined by XRD analysis. The exemplary XRD patterns are shown in
Fig. S2. Three photocatalysts, i.e., ST-41, ST-01 and TIO10, consisted
mainly anatase phase, while ST-G1 and Ald R contained rutile
phase. The crystalline sizes of titania are summarized in Table S2.
XRD analysis proved the deposition of silver crystallites on titania,
and sizes of silver NPs were similar for large anatase (ST-41) and
rutile (ST-G1) samples, i.e., 40 and 38 nm, respectively.
Atomic composition and chemical characters of elements incor-
porated in the surface layer of modified titania were investigated
by XPS analysis. The presence of silver was confirmed in all tested
samples, and its amount exceeded that which was used for pho-
todeposition, reaching 13 and 4 wt% for silver-modified titania
ST-41 and ST-G1, respectively. It is known that deposition of
Under UV–vis irradiation all modified photocatalysts exhibited
much higher photocatalytic activities than those of correspond-
ing bare titanias. Interestingly, the highest photoactivity for acetic
acid oxidation showed Ag@TiO2 (ST-41) sample of large anatase
particles which showed the lowest photoactivity for methanol
dehydrogenation among all modified samples (Fig. 1, right). The
highest enhancement of photocatalytic activity (ca. 3.5, 4 and 7
times) was observed for Ag-modified titania with large crystallites
(517, 208 and 250 nm, TiO : Ald R, ST-41 and ST-G1, respectively).
2
It is proposed that storage of photogenerated electrons in noble
(large titania NPs).
Please cite this article in press as: E. Kowalska, et al., Silver-modified titania with enhanced photocatalytic and antimicrobial properties
Kowalska
