photocatalytic h2 production from ethanol–water mixtures over pt/tio2 and au/tio2 photocatalysts:...
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ORIGINAL PAPER
Photocatalytic H2 Production from Ethanol–Water Mixtures OverPt/TiO2 and Au/TiO2 Photocatalysts: A Comparative Study
Vedran Jovic • Zakiya H. N. Al-Azri •
Wan-Ting Chen • Dongxiao Sun-Waterhouse •
H. Idriss • Geoffrey I. N. Waterhouse
Published online: 15 June 2013
� Springer Science+Business Media New York 2013
Abstract Pt/TiO2 (Pt loadings 0–4 wt%) and Au/TiO2 (Au
loadings 0–4 wt%) photocatalysts were synthesized, char-
acterized and tested for H2 production from ethanol–water
mixtures (80 vol% ethanol, 20 vol% H2O) under UV exci-
tation. Average metal nanoparticle sizes determined by TEM
were 1–3 nm for Pt in the Pt/TiO2 photocatalysts and 5–7 nm
for Au in the Au/TiO2 photocatalysts. Au/TiO2 showed an
intense localized surface plasmon resonance feature at
*570 nm, typical for metallic Au nanoparticles of diameter
*5 nm supported on TiO2. X-ray photoelectron spectros-
copy and X-ray diffraction analyses established that Pt and
Au were present in metallic form on the TiO2 support. X-ray
fluorescence revealed close accord between nominal and
actual Pt and Au loadings. The Au/TiO2 and Pt/TiO2 phot-
ocatalysts both displayed very high activities for H2 pro-
duction under UV irradiation, with the Au/TiO2 samples
affording slightly superior rates of H2 production at most
metal loadings. The 2 wt% Au/TiO2 and 1 wt% Pt/TiO2
photocatalysts showed the highest H2 production
rates (32–34 mmol g-1 h-1). Photoluminescence studies
confirmed that Pt and Au nanoparticles positively enhance
the photocatalytic properties of P25 TiO2 for H2 production
by acting as electron acceptors and thereby suppressing
electron–hole pair recombination in TiO2.
Keywords Photocatalysis � Hydrogen production �Titania � Platinum � Gold � Ethanol
1 Introduction
The development of a sustainable hydrogen economy, in
which H2 will replace fossil fuels for electricity generation,
hinges on the discovery of clean, low cost and sustainable
technologies for H2 manufacture, distribution and storage.
Current hydrogen production is based primarily around
steam methane reforming, which is an energy intensive
process with a significant carbon footprint [1, 2]. Alternative
H2 manufacturing technologies, which preferably use
renewable H2 sources such as water or biofuels and harness
solar, wind, geothermal or hydroelectric energy to facilitate
H2 manufacture, must be found. Light from the Sun is by far
the most abundant source of energy on Earth, though pres-
ently\0.05 % of the total power used by humans annually
(15,000 GW) is generated using the Sun’s solar energy
(excluding solar heating and biomass combustion) [3]. Solar
H2 production is expected to be the cornerstone of future H2
economies, yet direct conversion of solar energy to H2
remains economically and technically challenging.
Semiconductor photocatalysis is one of the most prom-
ising technologies for harvesting sunlight and generating
energy carriers such as H2 from renewable sources (water
and biofuels) [1, 4–7]. Excitation of a semiconductor
photocatalyst, such as TiO2, with photons of appropriate
energy produces electron–hole (e-–h?) pairs which may
Electronic supplementary material The online version of thisarticle (doi:10.1007/s11244-013-0080-8) contains supplementarymaterial, which is available to authorized users.
V. Jovic � Z. H. N. Al-Azri � W.-T. Chen �D. Sun-Waterhouse � G. I. N. Waterhouse (&)
School of Chemical Sciences, The University of Auckland,
Private Bag 92019, Auckland, New Zealand
e-mail: [email protected]
H. Idriss (&)
SABIC Research Centres, Riyadh, Saudi Arabia
e-mail: [email protected]
H. Idriss
KAUST, Thuwal, Saudi Arabia
123
Top Catal (2013) 56:1139–1151
DOI 10.1007/s11244-013-0080-8
either recombine or react with adsorbed species. To realize
efficient H2 production from water or biofuels, the bottom
of the conduction band of the semiconductor should be
more negative than the redox potential of the H2O/H2
couple (0.00 V vs. NHE) and the top of the valence band of
the semiconductor should be more positive than redox
potential of the O2/H2O couple (1.23 V vs. NHE) [8, 9].
TiO2 satisfies both these criteria, and is typically modified
with electron accepting co-catalysts (typically noble metals
such as Pd, Pt or Au; graphene or CNTs or semiconductors
such as CuO) to provide active sites for H2 generation and
to suppress electron–hole pair recombination in TiO2 [4, 8–
14]. Over the last decade, a large number of papers have
been published relating to photocatalytic H2 production
over TiO2-based systems, yet many crucial aspects of
reaction mechanisms remain poorly understood. Presently
it is very difficult to compare the work of different groups,
and to gauge the relative merits of different co-catalysts for
promoting H2 production, due to differences in catalyst
formulation (e.g. co-catalyst loading and semiconductor
support) and experimental conditions used to evaluate
photocatalyst performance. Comparative studies of differ-
ent photocatalysts under standardized testing conditions
would help understanding the role of each component on
the reaction.
Noble metals with suitable work functions, such as plat-
inum (U = 5.39 eV) or gold (U = 5.31 eV), when highly
dispersed as nanoparticles over TiO2 surfaces, have strong
ability to activate TiO2 photocatalysts towards H2 produc-
tion [10, 15, 16]. The apparent Fermi level for supported Pt or
Au nanoparticles on TiO2 sits ideally between the bottom of
the conduction band of TiO2 (-500 mV vs. NHE) and above
the H2O/H2 couple (0.00 V vs. NHE). Au/TiO2 is -270 mV
relative to NHE for Au particles of 5 nm diameter [17, 18].
Accordingly, electrons photoexcited in TiO2 will migrate to
the semiconductor surface and on to the supported Pt or Au
nanoparticles, thereby reducing e-–h? pair recombination
and enhancing H2 production (metallic Pt or Au co-catalysts
are the active centers for H2 production). The activity of Pt/
TiO2 or Au/TiO2 photocatalysts generally depends on three
key parameters. (a) the polymorphic nature of the TiO2
support (anatase, rutile, brookite or combinations thereof),
(b) the nature and loading of the noble metal particles, and
(c) the surface structure of the noble metal-TiO2 interface.
Murdoch et al. recently demonstrated that photocatalysts
comprising gold nanoparticles (diameter 3–12 nm) depos-
ited on nanocrystalline anatase TiO2 supports, are extremely
active for H2 production from ethanol–water mixtures under
UV irradiation, and about 100 times more active than Au
nanoparticles deposited on nanocrystalline rutile TiO2 sup-
ports [15]. Liquid slurry photoreaction studies produced
hydrogen at a rate of 2 L/kgcat. min on a 2 wt% Au/anatase
TiO2 catalyst under UV irradiation of comparable intensity
to that provided by the Sun [11]. Putting this into perspective,
a 1 kW PEM fuel cell needs 15 L of H2 per min, hence Au/
TiO2 photocatalysts are promising first generation photo-
catalysts for hydrogen production from renewable biofuels.
Waterhouse et al. observed that photocatalytic activity of
Au/TiO2 photocatalysts for H2 production from bioethanol
could be further increased when Degussa P25 TiO2 (85 %
anatase and 15 % rutile) was the support phase instead of
nanocrystalline anatase, on account of synergistic electron
transfer between the rutile and anatase phases following
photoexcitation [19–24]. Available data suggests that the
activity of Au/TiO2 photocatalysts for H2 production is
dependent on the Au loading, but largely independent of
the Au nanoparticle size (for Au particles in the size range
3–12 nm). This contrasts dark reactions over Au–TiO2 cat-
alysts (e.g. low temperature CO oxidation or hydrogenation
reactions), where strong relationships between Au particle
size and catalytic performance have been established [25].
Studies by other groups have also demonstrated the high
catalytic activity of Pt/TiO2 materials for H2 production [10,
26–28]. However, confusion exists about the relative activ-
ities of Au and Pt co-catalysts in activating TiO2 for H2
production (especially from biofuels), motivating the present
investigation.
This study aimed to systematically evaluate and com-
pare the photocatalytic activity of Pt/TiO2 (0–4 wt% Pt)
and Au/TiO2 (0–4 wt% Au) photocatalysts for H2 pro-
duction from ethanol–water mixtures under UV irradiation.
The photocatalysts were prepared using the archetypal
semiconductor photocatalyst, Degussa P25 TiO2, to facil-
itate comparison with the prior work of other groups. The
synthesis of the photocatalysts, catalyst pretreatments
before testing, and the photocatalytic tests were standard-
ized as much as possible to allow the relative activities of
the Pt and Au co-catalysts in promoting H2 production to
be ascertained. These results could then be used in the
rational design of new and improved photocatalysts for H2
production from biofuels, such as ethanol.
2 Experimental Section
2.1 Materials
Hexachloroplatinic acid hexahydrate (H2PtCl6�6H2O,
98 %), Tetrachloroauric acid trihydrate (HAuCl4�3H2O,
C99 %), urea (C99.5 %) and absolute ethanol (Puriss,
C99.5 %) were all obtained from Sigma-Aldrich and used
without further purification. Degussa P25 TiO2 (85 wt%
anatase, 15 wt% rutile, C99.5 %) was obtained from a
local supplier. All solutions were prepared using milli-Q
water (18.2 MX cm resistivity).
1140 Top Catal (2013) 56:1139–1151
123
2.2 Au/TiO2 Photocatalyst Preparation
Gold nanoparticles (1–4 wt%) were deposited on Degussa
P25 TiO2 using the deposition–precipitation with urea
method described by Zanella et al., with some modifica-
tions [29]. Briefly, HAuCl4�3H2O (1.654 g) was dissolved
in milli-Q water (1 L) to give a gold stock solution
([Au3?] = 4.2 9 10-3 M). Gold stock solution (25, 50, 75
or 100 mL for 1, 2, 3 or 4 wt% Au/TiO2 photocatalysts,
respectively), milli Q water (175, 150, 125 or 100 mL,
respectively) and urea (5.04 g) were added to a glass Schott
bottle and vigorously stirred. Degussa P25 TiO2 (2 g) was
then added, and the resulting suspension thermostated at
80 �C in a dark room for 8 h. After 8 h, the yellow Au(III)
impregnated TiO2 powders were collected by vacuum fil-
tration, washed repeatedly with milli-Q, and then air dried
at 70 �C overnight. The samples were then calcined at
300 �C for 2 h to thermally reduce surface Au(III) species
on TiO2 to metallic gold. The final Au/TiO2 powders were
all purple in colour.
2.3 Pt/TiO2 Photocatalyst Preparation
Platinum nanoparticles (1–4 wt%) were deposited on De-
gussa P25 TiO2 using the deposition–precipitation with
urea method. Briefly, H2PtCl6�6H2O (2.175 g) was dis-
solved in milli-Q water (1 L) to give a platinum stock
solution ([Pt4?] = 4.2 9 10-3 M). All other procedures
were identical to those described above for preparation of
the Au/TiO2 photocatalysts, except that here the platinum
stock solution was used instead of the gold stock solution.
After calcination at 300 �C for 2 h, the Pt/TiO2 photocat-
alysts were additionally treated under a H2/N2 flow
(10 vol% H2, 100 mL min-1) at 350 �C for 2 h to reduce
adsorbed Pt(IV) or Pt(II) species to metallic form. The
reduced samples were all grey in colour [30, 31].
2.4 Characterization
TEM images were taken on a JEOL 2012F TEM operating
at 200 kV, equipped with an Oxford Instruments ISIS
energy-dispersive X-ray spectroscopy system for qualita-
tive analysis of elements present in each sample. Powders
samples were dispersed in isopropanol and then several
drops of the resulting dispersion placed on holey carbon
coated copper TEM grids for analysis.
Powder XRD patterns were collected using Philips PW-
1130 diffractometer, equipped with a Cu anode X-ray tube
and a curved-graphite filter monochromator. Data was
taken from 2h = 2–100� (0.02�, 2� min-1) using Cu Ka X-
rays (k = 1.5418 A). The rutile:anatase ratio in the sam-
ples was determined according to the method described by
Fu et al. [32].
%Rutile =1
ðA/R)� 0:884þ1½ � � 100
where A is the peak area for the anatase (101) reflection at
2h = 25.3�, and R is the peak area for the rutile (110)
reflection at 2h = 27.4�.
N2 physisorption isotherms were determined at liquid
nitrogen temperature (-195 �C) using a Micromeritics
Tristar 3000 instrument. Specific surfaces areas were cal-
culated from the N2 adsorption data according to the
Brunauer–Emmett–Teller (BET) method using P/Po values
in the range 0.05–0.2. Cumulative pore volumes and pore
diameters were calculated from the adsorption isotherms
by the Barrett–Joyner–Halenda (BJH) method. Samples
were degassed at 100 �C under vacuum for 1 h prior to the
N2 physisorption measurements.
UV–Vis absorbance spectra were collected over the
wavelength range 250–900 nm on a Shimadzu UV-2101PC
scanning spectrophotometer fitted with a Shimadzu ISR-
260 integrating sphere attachment. BaSO4 powder was
used as a reference.
XPS data was collected using a Kratos Axis UltraDLD
equipped with a hemispherical electron energy analyzer and
an analysis chamber of base pressure *1910-9 Torr.
Spectra were excited using monochromatic Al Ka X-rays
(1486.69 eV), with the X-ray source operating at 100 W.
Sample were gently pressed into thin pellets of *0.1 mm
thickness for the analyses. A charge neutralisation system
was used to alleviate sample charge build up, resulting in a
shift of *3 eV to lower binding energy. Survey scans were
collected at a pass energy of 80 eV over the binding energy
range 1200–0 eV, whilst core level scans were collected with
a pass energy of 20 eV. The spectra were calibrated against
the C 1s signal at 285 eV from adventitious hydrocarbons.
Photoluminescence measurements were performed in air
at 25 �C using a Perkin-Elmer LS-55 Luminescence
Spectrometer. Spectra were excited at 310 nm and photo-
luminescence spectra were recorded over a range of
330–600 nm using a standard photomultiplier. A 290 nm
cutoff filter was used.
2.5 Photocatalytic Testing
Photocatalytic hydrogen production tests on the Pt/TiO2 and
Au/TiO2 photocatalysts were carried out in a tubular Pyrex
reactor (100 mL volume). Photocatalyst (0.0065 mg) was
placed in the reactor and evacuated under a nitrogen flow for
30 min to remove oxygen. Ethanol (15 mL) and milli-Q
water (3.75 mL) were then injected into the reactor through a
rubber septum and the mixture stirred continuously in the
dark for 1 h. The reactor was then exposed to UV light,
supplied from a Spectraline model SB-1000P/F lamp
Top Catal (2013) 56:1139–1151 1141
123
(200 W, 365 nm) at a distance of 5 cm from the reactor. The
photon flux at the sample was *6.5 mW cm-2 (the UV flux
from the Sun is *5 mW cm-2). Hydrogen evolution was
monitored by taking gas head space samples (1 mL) at
20 min intervals and injecting these into a Shimadzu GC
2014 equipped with a TCD detector and Carboxen-1010 plot
capillary column (L 9 I.D. 30 m 9 0.53 mm, average
thickness 30 lm). H2 produced through photoreaction was
quantified against an external calibration curve. Photocata-
lytic tests for each sample were carried out in triplicate.
3 Results and Discussion
3.1 Photocatalyst Characterization
Figure 1 shows TEM images of the 1–4 wt% Pt/TiO2 phot-
ocatalysts after H2 treatment at 350 �C for 2 h. For all
samples, supported Pt nanoparticles of size \3 nm can be
seen in the images. No attempt was made to determine the
exact Pt nanoparticle size distributions, as the size limit for
detection of metal nanoparticles by TEM is *1 nm. Pt
loadings determined by XRF for the Pt/TiO2 photocatalysts
(Table 1) were in excellent agreement with the nominal
loadings of 1–4 wt%. Powder XRD patterns for the Pt/TiO2
samples (Online Resource Figure 1) contained only peaks
characteristic for anatase and rutile in the P25 TiO2 support.
For all samples, the anatase:rutile weight ratio was calcu-
lated to be *5:1. The fact that no peaks characteristic for Pt
were observed by XRD, even at high Pt loadings (4 wt%), is
attributed to the very small size of the supported Pt clusters.
Extreme line broadening due to quantum size effects is
expected for Pt nanoparticles of size 1–3 nm. Also, Pt
nanoparticles of diameter 1.5 nm contain only 39 atoms and
hence would not be expected to have a well-developed f.c.c.
lattice that could be probed by XRD. XPS data below pro-
vides strong evidence for the presence of metallic Pt in these
samples. UV–Vis absorption spectra for the Pt/TiO2 samples
(Online Resource Figure 2) showed strong absorption below
400 nm due to the TiO2 support, as well as intense absorption
across the entire visible spectrum due to Pt interband tran-
sitions from Pt 5d ? 6sp and within the 6sp band [33].
TEM images for the 1–4 wt% Au/TiO2 photocatalysts are
shown in Figure 2. Au nanoparticles of size range 1–12 nm
Fig. 1 TEM images of a 1 wt%
Pt/TiO2, b 2 wt% Pt/TiO2,
c 3 wt% Pt/TiO2, and d 4 wt%
Pt/TiO2 photocatalysts. The Pt
nanoparticles are of size 1–3 nm
and highly dispersed over the
TiO2 support
1142 Top Catal (2013) 56:1139–1151
123
are seen in the images, located preferentially at the interface
between two TiO2 crystallites. The Au nanoparticles dis-
played pseudo-spherical morphologies with metal-support
contact angles[90�, evidence for a relatively weak metal-
support interaction. On increasing the Au loading, the
average Au particle size did not increase significantly.
Figure 3a–d shows Au particle size distributions for the
1–4 wt% Au/TiO2 samples. The mean Au size (d/nm),
standard deviation and number of particles counted are
shown in the figure. It should be noted that Au/TiO2 catalyst
pretreatment (either calcination at 300 �C or H2 exposure at
350 �C) did not affect the size distribution of Au nanopar-
ticles. XRD patterns for the 1–4 wt% Au/TiO2 photocata-
lysts (Online Resource Figures 3 and 4) were dominated by
reflections characteristic of nanocrystalline anatase and
rutile in the P25 TiO2 support. The weight fraction of anatase
and rutile in the samples was 5:1 for all samples. The Au/
TiO2 samples showed additional broad and weak XRD fea-
tures that intensified with nominal Au loading, which were
readily assigned to metallic gold nanoparticles with f.c.c.
structure on the TiO2 support. Online Resource Figure 4
shows an expanded view of the XRD patterns between
2h = 35–55�, where the development of Au(111) and
Au(200) reflections with increasing gold loading can be seen.
Identification of metallic gold by XRD for the Au/TiO2
photocatalysts agrees qualitatively with TEM and UV–Vis
absorbance data above for the same samples (Figs. 1, 3e), as
well as XPS data presented below. Figure 3e shows UV–Vis
absorbance spectra for the Au/TiO2 photocatalysts. The
samples have a broad absorption *570 nm due to the Au
nanoparticle LSPR, as well as absorption across the whole
visible spectrum because of Au interband transitions from
Au 5d ? 6sp and within the 6sp band [33–36]. The intensity
of the Au LSPR signal increased linearly with Au loading
(Online Resource Figure 5). The position and intensity of the
Au LSPR signal depends on Au particle size, shape and
dielectric constant of the support [33, 34]. Kimura et al.
reported that the LSPR for Au nanoparticles of size 5 nm on
anatase TiO2 is *570 nm [33], in good accordance with the
results of the present study (anatase is the dominant TiO2
polymorph in P25 TiO2). The asymmetry on the long
wavelength side of the Au/Anatase LSPR feature may be due
to Au/rutile, the LSPR for which is predicted to be *40 nm
higher than that of Au/Anatase LSPR at the same Au nano-
particle size [33]. Plots of (ahm)1/2 versus E, calculated from
the spectra of Fig. 3e, are shown in Fig. 3f. The extrapolated
band gap energy for P25 TiO2 is 3.15 eV, in good agreement
with literature reports [37, 38]. As the Au loadings were
increased, the titania absorption edge showed a distinctive
red shift. Extrapolated bandgap energies were 3.01, 2.95,
2.85 and 2.80 eV for the 1, 2, 3 and 4 wt% Au/TiO2 samples,
respectively. Comparable shifts with increasing gold loading
have been observed by other groups [36, 38, 39].Ta
ble
1S
um
mar
yo
fth
ep
hy
sica
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alan
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ho
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tic
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per
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0–
4w
t%P
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Sam
ple
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)
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Ato
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PS
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Tsu
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(cm
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BJH
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eter
(nm
)
H2
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(mm
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H2
pro
du
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(mm
ol
m-
2h
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Ti
OC
M=
Pt
or
Au
(M/T
i)
P2
5T
iO2
–0
.02
5.2
52
.12
2.7
–4
9.6
0.1
45
14
.61
.26
0.0
25
1w
t%P
t/T
iO2
\2
1.0
24
.75
0.9
23
.90
.5(0
.02
0)
48
.10
.36
02
7.8
31
.70
.65
9
2w
t%P
t/T
iO2
\2
1.8
23
.55
0.7
25
.30
.5(0
.02
1)
48
.40
.35
02
7.6
28
.70
.59
4
3w
t%P
t/T
iO2
\2
2.9
25
.05
0.6
23
.80
.6(0
.02
4)
47
.00
.36
62
8.0
27
.50
.58
4
4w
t%P
t/T
iO2
\2
4.1
25
.35
0.8
23
.20
.7(0
.02
8)
48
.20
.36
02
8.5
27
.50
.57
0
Pt
foil
––
3.4
26
.96
9.7
––
––
–
1w
t%A
u/T
iO2
4.8
0.9
23
.05
1.8
25
.00
.2(0
.00
9)
48
.10
.39
02
9.2
32
.50
.67
5
2w
t%A
u/T
iO2
6.4
2.0
23
.64
9.5
26
.50
.4(0
.01
7)
47
.10
.32
02
5.8
33
.40
.70
9
3w
t%A
u/T
iO2
6.6
2.8
25
.05
1.2
23
.20
.6(0
.02
4)
46
.50
.35
02
8.6
31
.50
.67
6
4w
t%A
u/T
iO2
4.8
4.1
25
.15
0.9
23
.20
.8(0
.03
2)
46
.50
.31
12
6.6
24
.70
.53
2
Au
foil
––
––
–1
00
.0
Top Catal (2013) 56:1139–1151 1143
123
XPS was used to probe the oxidation states of platinum
and gold at the surface of the Pt/TiO2 and Au/TiO2 phot-
ocatalysts, respectively. Figure 4a shows Pt 4f XPS spectra
for the Pt/TiO2 photocatalysts. The samples show peaks at
71.3 and 74.7 eV, respectively, in a 4:3 area ratio, which
indicate the presence of metallic Pt (Pt 4f7/2 and Pt 4f5/2,
respectively). As expected, the intensity of these peaks
increased with an increase in the nominal Pt loading of the
samples. Pt 4f peaks for metallic Pt are inherently asym-
metric, so we cannot completely exclude a minor contri-
bution from Pt(OH)2 or PtO (Pt 4f7/2 = 74.0 eV) or PtO2
(Pt 4f7/2 = 74.8 eV) species. However, on the basis of the
XPS data we can conclude that metallic Pt is the dominant
Pt species on the surface of the Pt/TiO2 photocatalysts. Pt
loadings determined by quantitative XPS analysis were
higher than those determined by XRF (Table 1), which is
not unexpected as XPS is a surface-analytical technique.
Figure 4b shows Au 4f XPS spectra for the Au/TiO2
photocatalysts, which contain features at 83.6 and 87.3 eV
in a 4:3 peak area ratio. These features intensify linearly on
increasing the nominal Au loading, and are assigned to the
Au 4f7/2 and Au 4f5/2 peaks of supported metallic gold
nanoparticles. The binding energies of these features are
*0.4 eV lower than those determined for a metallic gold
foil (Au 4f7/2 = 84.0 eV, and Au 4f7/2 = 87.7 eV), con-
sistent with the finding of other groups [11, 15, 37]; this is
attributed to the shift in the Au Fermi level towards the
conduction band of TiO2 thus affecting Au 4f as well as Ti
2p lines [40]. The XPS data eliminates the possibility that
amorphous Au2O3 (Au 4f7/2 = 86.9 eV) or other Au3?-
containing species, like Au(OH)3, coexist with the Au
nanoparticles on the TiO2 surface. Au loadings determined
by quantitative XPS analysis were higher than those
determined by XRF (Table 1).
N2 physisorption data for Degussa P25 TiO2 and the Au/
TiO2 and Pt/TiO2 photocatalysts is summarized in Table 1.
The BET specific surface areas for all the metal-containing
photocatalysts were similar (46–48 m2 g-1) and slightly
lower than that of the P25 TiO2 support (49.6 m2 g-1). The
BJH cumulative pore volume and BJH average pore
diameters of all the metal-containing photocatalysts were
also similar (0.31–0.39 cm3 g-1 and 25.8–29.2 nm,
respectively) and largely independent of metal loading.
Compared to data collected for the Degussa P25 TiO2
support, the BJH cumulative pore volume and BJH average
pore diameter of samples increased after noble metal
deposition, which is simply explained by Au and Pt
nanoparticles blocking micropores in the TiO2 support.
3.2 Photoluminescence Measurements
Photocatalytic H2 production relies on using excited elec-
trons to reduce hydrogen ions and at the same time holes
are trapped by a sacrificial agent (as well as water mole-
cules) resulting in oxidation. In competition with this
e-–h? recombination occurs. In the bulk and at the surface
a fraction of these e- and h? is lost via three routes:
radiative recombination, Auger recombination and
Fig. 2 TEM images of a 1 wt%
Au/TiO2, b 2 wt% Au/TiO2,
c 3 wt% Au/TiO2; and d 4 wt%
Au/TiO2 photocatalysts. The Au
nanoparticles appear as dark
spots in the TEM images. For
all samples, the Au nanoparticle
size is *3–8 nm. Selected area
diffraction patterns confirm the
presence of nanocrystalline
anatase, rutile and Au in the
samples
1144 Top Catal (2013) 56:1139–1151
123
Schockely Read Hall recombination (defects). The energy
released during the e-–h? recombination in the form of
light gives rise to photoluminescence [41–43]. Therefore
the higher the luminescence signal the lower is the
expected catalytic activity. There are methods of estimat-
ing the e-–h? life time from the attenuation of the PL
Au Particle Size (nm)
0 2 4 6 8 10 12
Pe
rce
ntag
e (
%)
0
5
10
15
20
25
d = 4.8 nm s = 1.9 nm cts. = 129
Au Particle Size (nm)
0 2 4 6 8 10 12
Per
cent
age
(%)
0
5
10
15
20
25
d = 6.4 nm s = 2.1 nm cts. = 119
Au Particle Size (nm)
0 2 4 6 8 10 12
Pe
rce
ntag
e (
%)
0
5
10
15
20
25
d = 6.6 nmσ = 2.1 nm
cts. = 91
Au Particle Size (nm)
0 2 4 6 8 10 12
Pe
rce
ntag
e (
%)
0
5
10
15
20
25
(a) (b)
(c) (d)
1 wt.% Au 2 wt.% Au
3 wt.% Au 4 wt.% Au d = 4.8 nm
σ = 1.9 nm cts. = 129
Wavelength (nm)
300 400 500 600 700 800 900
Ab
sorb
ance
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6P25 TiO2
1 wt.% Au2 wt.% Au3 wt.% Au4 wt.% Au
(e)
Energy (eV)
1.5 2.0 2.5 3.0 3.5 4.0
( αh ν
)1/2 /
(eV
1/2cm
-1/2)
0.0
0.5
1.0
1.5
2.0
2.5
3.0P25 TiO2
1 wt.% Au2 wt.% Au3 wt.% Au4 wt.% Au
(f)
Fig. 3 Au nanoparticle size distributions for a 1 wt% Au/TiO2,
b 2 wt% Au/TiO2, c 3 wt% Au/TiO2; and d 4 wt% Au/TiO2
photocatalysts, e UV–Vis absorbance spectra for the Au–TiO2
photocatalysts, where Au nanoparticles gives rise to an intense LSPR
feature at *570 nm, f corresponding (ahv)1/2 versus E plots. The
electronic absorption edge shifts to shorter wavelengths with
increasing Au loading
Top Catal (2013) 56:1139–1151 1145
123
luminescence signal but they require considerable cali-
bration that is beyond the objective of the work [43]. Au
and Pt nanoparticles are postulated to enhance the photo-
catalytic activity of TiO2 for H2 production by acting as
‘‘electron sinks’’ and suppressing electron–hole pair
recombination in TiO2 [44]. To further study this, photo-
luminescence measurements were carried out on P25 TiO2
and the Pt/TiO2 and Au/TiO2 photocatalysts (Fig. 5). P25
TiO2 gave a very intense photoluminescence signal, indi-
cating that electron–hole pair recombination occurred
rapidly following photoexcitation in the absence of a noble
metal co-catalyst. An intense feature is seen at *3.1 eV,
which corresponds to direct (X1b ? X1a, 3.45 eV and
X1b ? X2b, 3.59 eV) and phonon-assisted indirect transi-
tions (X1b ? C3, 3.19 eV; C1b ? X2b 3.05 eV; and
C1b ? X1a 2.91 eV) in anatase TiO2 (possible contribu-
tions by rutile are masked by the large excess of anatase in
P25 TiO2) [45, 46]. Weaker features at longer wavelengths
(lower energy) are due to shallow traps, (VO) ? C3,
involving oxygen defects [45]. Figure 6 shows a partial
energy level diagram for anatase TiO2 indicating these
allowed direct and indirect transitions (X denotes the edge,
and C the center, of the Brillouin Zone) [45–47]. Deposi-
tion of Pt or Au nanoparticles on the surface of the TiO2
support strongly attenuated the photoluminescence signal
of P25 TiO2, with the signal becoming progressively
weaker with increasing metal loading (Fig. 5). These
results confirm that Au or Pt nanoparticles suppress elec-
tron–hole pair recombination in TiO2 following photoex-
citation, by acting as ‘‘electron sinks’’ and in doing so
increasing the number of charge carriers (h? or e-) avail-
able for photoreactions on TiO2 surfaces. Online Resource
Figure 6 shows a plot of the normalized PL intensity versus
metal loading for the Pt/TiO2 and Au/TiO2 photocatalysts.
At a metal loading of 1 wt%, the abilities of the Pt and Au
nanoparticles to suppress electron hole-pair recombination
are similar, but Au becomes more effective than Pt at high
metal loadings. Interestingly, the photoluminescence signal
maximum for the Pt/TiO2 and Au/TiO2 photocatalysts were
slightly blue-shifted relative to that of P25 TiO2. A similar
photoluminescence blue-shift was reported by Nakajima
et al. for Pt-loaded TiO2 powders [48, 49]. Nakajima et al.
argued that following initial electron transfer from TiO2 to
Pt, further transfer of photoinduced electrons to Pt was
inhibited, and the additional photoinduced electrons could
then move in a restricted region in the TiO2, leading to a
blue shift in the PL. An alternative explanation we offer is
that supported metal nanoparticles selectively suppress
phonon-assisted indirect transitions (X1b ? C3, 3.19 eV;
C1b ? X2b 3.05 eV; and C1b ? X1a 2.91 eV) in anatase
relative to the other transitions, which would blue-shift the
photoluminescence maximum accordingly. The energies of
the direct (X1b ? X1a, 3.45 eV and X1b ? X2b, 3.59 eV)
and shallow trap transitions (VO) ? C3 were largely
unaffected by the presence of Pt or Au nanoparticles.
3.3 Photocatalytic Hydrogen Production Tests
The photocatalytic activity of Degussa P25 TiO2, and the
Pt/TiO2 and Au/TiO2 photocatalysts, for H2 production
from ethanol–water mixtures (80 vol% ethanol, 20 vol%
Binding energy (eV)68707274767880
Inte
nsity
(ar
bitr
ary
units
)Pt 4f7/2
Pt 4f5/2
3 wt.% Pt
2 wt.% Pt
1 wt.% Pt
0 wt.% Pt
Pt foil
4 wt.% Pt
(a)
Binding Energy (eV)8284868890
Inte
nsity
(ar
bitr
ary
units
)
0 wt.% Au
1 wt.% Au
2 wt.% Au
3 wt.% Au
4 wt.% Au
Au foil
(b) Au 4f7/2
Au 4f5/2
Fig. 4 Core level a Pt 4f XPS spectra for the Pt/TiO2 photocatalysts
and b Au 4f XPS spectra for the Au/TiO2 photocatalysts. The Pt 4f7/2
and Au 4f7/2 binding energies are consistent with the metallic form of
Pt and Au, respectively. The peak areas of the Pt 4f and Au 4f signals
increase almost linearly with nominal metal loading
1146 Top Catal (2013) 56:1139–1151
123
H2O) were evaluated under UV light of intensity compa-
rable to that present in Sunlight. The mechanism of this
reaction has been the subject of numerous studies and will
not be discussed in detail here [11, 50]. However, the key
overall steps are as follows. TiO2 absorbs photons with
energy greater than the electronic band gap of TiO2 (Eg
*3.1–3.2 eV), resulting in the formation of electron–hole
pairs (e-–h?). Photoexcited holes (h?) in the valence band
of TiO2 oxidise ethanol (CH3CH2OH ? 2 h? ? CH3-
CHO ? 2H?) and H2O (H2O ? 2 h? ? 0.5O2 ? 2H?),
whilst photoexcited electrons in the conduction band of
TiO2 reduce H? to H2 (2H? ? 2e- ? H2) or H2O to H2
(2H2O ? 4e- ? H2 ? 2OH-). Figure 7a shows a plot of
H2 production (mmol g-1) versus time (min) for Degussa
P25 TiO2 and the 1–4 wt% Pt/TiO2 photocatalysts. Fig-
ure 7b shows corresponding plots for the Au/TiO2 photo-
catalysts. The Degussa P25 TiO2 support, in the absence of
a co-catalyst, exhibited minimal H2 production activity
(1.26 mmol g-1 h-1). The H2 production activity of De-
gussa P25 TiO2 increased dramatically after the deposition
of Pt or Au co-catalysts. For both sets of photocatalysts,
linear H2 production rates were observed with UV expo-
sure (we have extended these tests over longer periods up
to 24 h and the same linearity was observed). Photocata-
lytic H2 production rates for all samples are plotted in
Fig. 8. For the Pt/TiO2 photocatalysts, the 1 wt% Pt sample
gave the highest H2 production rate (31.7 mmol g-1 h-1).
At higher loadings, the activity decreased progressively.
For the Au/TiO2 photocatalysts, a 2 wt% Au loading
appears optimal, affording a H2 production rate of
33.4 mmol g-1 h-1. All of the samples possessed similar
specific surface areas and pore size distributions (Table 1),
so H2 production rates normalized against photocatalyst
surface area followed the same trend. The optimum metal
loading can be rationalised in terms of (1) suppression of
electron–hole pair recombination at the TiO2 surface due to
electron trapping by metal nanoparticles; (2) loss of TiO2
surface for ethanol and water adsorption with increasing
metal loading. It is noteworthy to indicate that a similar
loading of Au on Anatase has been seen to give the highest
hydrogen production activity from water with 0.1 vol% of
methanol [51].
The decrease in the photoluminescence spectra in the
case of Pt/TiO2 and Au/TiO2 cannot be strictly correlated
with the photocatalytic activity. The quantum yield of a
photocatalytic reaction is a function of two main param-
eters: the rate constant of charge transfer and the rate
constant of electron–hole recombination [8, 52]. The
decrease in photoluminescence on addition of Pt or Au
cocatalysts is clear and may partly be linked to increasing
the life time of charge carriers. In simple terms the
presence of impurities and/or defects results in changing
the wave vector directions of the excited electrons upon
relaxation within the conduction band thus increases their
life time. While the rutile band gap is direct (allowing for
fast electron–hole recombination) the anatase band gap is
indirect (although still under debate) and the charge car-
rier life time in anatase is orders of magnitude higher than
in rutile [52, 53]. As stated in the introduction, the pres-
ence of both phases has been seen to considerably
increase the photocatalytic activity of TiO2. Various
mechanisms have been proposed to explain the synergistic
effect of both phases on the photoreaction, and these have
been recently summarized [53, 54]. The TEM images of
Fig. 2 showed that most Au particles are in between the
rutile and anatase particles and this might allow for the
Energy (eV)2.02.22.42.62.83.03.23.43.63.8
Abs
orba
nce
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
PL
Inte
nsity
0
200
400
600
800
1000
1200
1400
1600
P25 TiO2 (abs.)
P25 TiO2
1 wt.% Pt2 wt.% Pt3 wt.% Pt4 wt.% Pt
2.34
eV
2.56
eV
2.70
eV
2.80
eV
2.91
eV
3.05
eV
3.19
eV
3.45
eV
3.59
eV(a)
Energy (eV)
2.02.22.42.62.83.03.23.43.63.8
Abs
orba
nce
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4P
L In
tens
ity
0
200
400
600
800
1000
1200
1400
1600
P25 TiO2 (abs.)
P25 TiO2
1 wt.% Au2 wt.% Au3 wt.% Au4 wt.% Au
2.34
eV
2.56
eV
2.70
eV
2.80
eV
2.91
eV
3.05
eV
3.19
eV
3.45
eV
3.59
eV(b)
Fig. 5 Photoluminescence spectra for a 0–4 wt% Pt/TiO2 photocat-
alysts and b 0–4 wt% Au/TiO2 photocatalysts. The photolumines-
cence signal of P25 TiO2 is progressively suppressed with increasing
metal loading. Vertical dotted lines show the characteristic transitions
of anatase TiO2
Top Catal (2013) 56:1139–1151 1147
123
charge transfer to occur more efficiently. Therefore the
presence of Au particles (and by analogy Pt particles)
might have a role in increasing the life time of excited
electrons. Alternatively, one could simply attribute the
decrease of the photoreaction rate with increasing Au
loading to the decrease of available surface sites (three
phases together: anatase, rutile and gold particles) because
statistically increasing the % of one phase (Au in this
case) may, after a given threshold, decrease the number of
reaction sites (three-phase requirement). We will make an
argument that this is not the case. From Table 1, it can be
seen that the at.% Au detected by XPS increased linearly
with Au loading (i.e. it doubles with doubling the amount
of gold). Knowing the mean particle diameter of the Au
particles (from TEM) and number of surface Ti sites per
unit area in TiO2 the fraction of surface Ti atoms (and O
atoms) that have been covered by Au particles can be
estimated. The geometric surface area of Au particle of
5 nm size is 7.85 9 10-17 m2 (and its cross sectional area
is about 2 9 10-17 m2); a 5 nm particle of Au contains
about 3000 atoms of Au. Because the escape depth of the
XPS Au4f is relatively high (about 2.5 nm) it is reason-
able to assume that a large fraction of all Au atoms are
seen by XPS (assuming perfectly spherical particles).
Therefore the at.% Au values determined by XPS can be
taken as a true count of the number of the number of Au
atoms. The TiO2 unit cell is about 20 9 10-20 m2 in area,
therefore the number of TiO2 unit cells that is covered by
Au particles can be estimated from total number of Au
particles and their cross section area: 6.6 9 1012 particles
of Au/m2 9 100 unit cells of TiO2 = 6.6 9 1014 unit
cells covered). In 1 m2 there is about 0.23 9 1019 TiO2
unit cells. Therefore the argument of geometrical cover-
age reducing the photoreaction rate at high metal loadings
cannot be made. The reason is therefore purely electronic.
It was recognized early on that the photoreaction for
hydrogen production from alcohol and water is very
sensitive to the % of noble metals on the semiconductor,
with the optimal metal loading depending on the metal
and the support [54, 55]. One of the arguments invoked is
that the distance between the metal particles affects the
electron–hole recombination rate [55, 56]. In the absence
of more fundamental studies on this, it is not possible to
give a definitive explanation without speculation. How-
ever, changes in the electric field can cause changes in the
electron–hole recombination rate [57] and it is not
unreasonable to consider local changes in the electric field
due to changes in the distance separating the noble metal
particles [56]. Table 2 presents H2 production rates nor-
malized against the noble metal/Ti XPS ratio as well as
by the number of noble metal atoms. It is clear that rates
cannot be normalized the same way thermal catalytic
reactions are normalized and the present numbers are
merely given to show the trends.
Transition E (eV) λ(nm)
X2b ↔ X1b 3.59 345.4
X1a ↔ X1b 3.45 359.4
Γ3
Γ3
Γ3
↔ X1b 3.19 388.7
X2b↔ Γ1b
X1a ↔ Γ1b
3.05 406.6
2.91 426.1
↔ (VO)4 2.80 442.9
↔ (VO)3 2.70 459.3
↔ (VO)2 2.56 484.4
↔ (VO)1 2.34 529.93
1b
1b
1a2b
(VO)
E
(VO)(VO)3
(VO)
X
Γ
Γ3
Γ3
Fig. 6 Partial energy diagram
for anatase TiO2. Possible
transitions are highlighted
1148 Top Catal (2013) 56:1139–1151
123
The contribution of the plasmon effect to the photore-
action is also worth consideration. While the Au LSPR
increases linearly with increasing the number of Au parti-
cles (i.e. the Au loading), the photoreaction rate does not.
One simple explanation would be that the plasmonic effect
of Au into the reaction is small compared to that of the
semiconductor. Another explanation is that under the
present study (UV excitation only) only a fraction of the
excitation source will have the exact frequency needed to
excite Au particles efficiently. However the same argument
as above can be invoked which is related to the consider-
able enhancement of the electric field due to Au (or Pt)
particles [57]. Therefore a balance has to be found between
the increase in the rate due to the plasmonic effect and the
decrease in the rate due to the increasing local electric field.
The slightly lower activity of the Pt based catalysts
could be attributed to the non-rectifying Ohmic contact
formed at the interface between Pt and TiO2 particles
aiding the rapid dispersion of electrons to the surrounding
medium [17]. The rectifying nature of the Schottky barrier
in the Au/TiO2, manifests in a stronger ability of Au
nanoparticles to prevent charge recombination (evidenced
in the photoluminescence measurements of Fig. 5). Alter-
natively, metal nanoparticle size may play an important
role, with Au nanoparticles of average size 4–7 nm (Fig. 2)
offering superior electron scavenging properties compared
to the 1–3 nm Pt nanoparticles (Fig. 1). The energy sepa-
ration between the Fermi level of supported Pt or Au
nanoparticles and the bottom of the conduction band of
TiO2 becomes larger with increasing metal nanoparticle
size in the range 1–10 nm, facilitating electron capture and
H2 production as the metal nanoparticle size increases.
Conversely, for Pt and Au nanoparticles \2 nm, a loss of
metallic character could decrease electron transfer effi-
ciency and photocatalytic activity.
(a)
Time (min)
0 100 200 300 400
H2 P
rodu
ced
(mm
ol g
-1)
0
50
100
150
200
250P25 TiO2
1 wt.% Pt2 wt.% Pt3 wt.% Pt4 wt.% PtRegression
Time (min)
0 100 200 300 400
H2 P
rodu
ced
(mm
ol g
-1)
0
50
100
150
200
250P25 TiO2
1 wt.% Au2 wt.% Au3 wt.% Au4 wt.% AuRegression
(b)
Fig. 7 H2 evolution vs. UV exposure time plots for a Pt/TiO2
photocatalysts and b Au/TiO2 photocatalysts in ethanol–water
mixtures (80 vol% ethanol, 20 vol% H2O). Platinum and gold
nanoparticles dramatically enhance the activity of TiO2 for H2
production. No deactivation of the samples was noticeable over
extended periods
0 w
t.% M
etal
1 w
t.% M
etal
2 w
t.% M
etal
3 w
t.% M
etal
4 w
t.% M
etal
Rat
e of
H2 P
rodu
ctio
n (m
mol
g-1
h-1)
0
10
20
30
40P25 TiO2
Pt/TiO2
Au/TiO2
Fig. 8 Summarized H2 production rates over Pt/TiO2 and Au/TiO2
photocatalysts. The Au/TiO2 samples showed higher rates of H2
production up to 4 wt% loading
Top Catal (2013) 56:1139–1151 1149
123
4 Conclusions
Pt/TiO2 and Au/TiO2 photocatalysts, based on a Degussa
P25 TiO2 support (anatase 85 wt%, rutile 15 wt%), exhibit
very high photocatalytic activities for H2 production from
ethanol-water mixtures under UV excitation. Metallic Pt or
Au nanoparticles on the surface of TiO2 suppress electron–
hole pair recombination in TiO2 by acting as electron
acceptors, and serve as active sites for H2 production.
Under the applied testing conditions, the photocatalytic H2
production activities of Au/TiO2 photocatalysts were
marginally superior to those of the corresponding Pt/TiO2
photocatalysts at metal loadings in the range 1–3 wt%. The
high photocatalytic activities and stabilities reported here
for the Pt/TiO2 photocatalysts (31.7 mmol g-1 h-1 for
the 1 wt% Pt sample) and Au/TiO2 photocatalysts (33.4
mmol g-1 h-1 for the 2 wt% Au/TiO2 sample) suggest that
these are promising first generation materials for solar
hydrogen production from biofuels.
Acknowledgments We gratefully acknowledge funding support
from the University of Auckland, the Australian Institute for Nuclear
Science and Technology (AINSE) and the MacDiarmid Institute for
Advanced Materials and Nanotechnology.
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Table 2 Normalised H2 production rates for the Pt/TiO2 and Au/TiO2 photocatalysts
Catalyst x wt%
M/TiO2
Rate mmol
m-2 h-1XPS (M/Ti)
M = Pt or Au
Rate (in mmol m-2 h-1)/
(M/Ti) M = Pt or Au
Rate 1020
molecules m-2 h-1Rate (103 molecules
Matom-1 h-1)a M = Pt or Au
1 wt% Pt 0.659 0.020 33.0 3.97 7.94
2 wt% Pt 0.594 0.021 28.3 3.58 7.16
3 wt% Pt 0.584 0.024 24.3 3.52 5.87
4 wt% Pt 0.570 0.028 20.4 3.43 4.90
1 wt% Au 0.675 0.009 75.0 4.06 20.30
2 wt% Au 0.709 0.017 41.7 4.27 10.68
3 wt% Au 0.676 0.024 28.2 4.07 6.78
4 wt% Au 0.532 0.032 16.6 3.20 4.00
a Rate is in 1020 molecules/m2 h divided by the number of metals in 1 m2 assuming 1019 atoms per m2. For example for 1 wt% Pt the rate of
3.97 9 1020 molecules of H2 m-2 h-1 is divided by 0.5/100 9 1019 = 7940 molecules Matom-1 h-1. 0.5/100 is taken from Table 1
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