Photocatalytic H2 generation on macro-mesoporous oxide-supported Pt nanoparticles

Article abstract of DOI:10.1039/c5ra25358f

A series of photocatalysts were prepared with crystalline macro-mesoporous oxides and Pt nanoparticles (Pt-TiO₂, Pt-Ta₂O₅, Pt-Nb₂O₅, Pt-ZrO₂, and Pt-Al₂O₃). Transmission electron microscopy, X-ray diffraction, and N₂ adsorption-desorption isotherms reveal that the oxides have prominent macro- and mesoporosity in the crystalline walls. The high surface area and crystalline walls of the oxides play significant roles in photocatalytic H₂ production. Pt-TiO₂ catalysts show enhanced photocatalytic water splitting efficiency for H₂ generation (solar energy conversion efficiency of 1.06%) under a 150 W and 1.5 AM solar simulator. Pt-Ta₂O₅ and Pt-Nb₂O₅ also generate noticeable photocatalytic activities of 0.21% and 0.16%, respectively. The enhanced photocatalytic activity is attributed to correct band alignment of the porous oxides with absorption in the UV-visible range, and ordered macro- and mesoporosity of the crystalline oxides for efficient charge transfer.

Full text of DOI:10.1039/c5ra25358f

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Photocatalytic H generation on macro-
2
mesoporous oxide-supported Pt nanoparticles†
Cite this: RSC Adv., 2016, 6, 18198
ab
ab
c
ab
Song Yi Moon,‡ Brundabana Naik,‡ Kwangjin An, Sun Mi Kim
ab
and Jeong Young Park*
A series of photocatalysts were prepared with crystalline macro-mesoporous oxides and Pt nanoparticles
(
Pt–TiO
2
2 5 2 5 2 2 3
, Pt–Ta O , Pt–Nb O , Pt–ZrO , and Pt–Al O ). Transmission electron microscopy, X-ray
diffraction, and N
2
adsorption–desorption isotherms reveal that the oxides have prominent macro- and
mesoporosity in the crystalline walls. The high surface area and crystalline walls of the oxides play
significant roles in photocatalytic H production. Pt–TiO catalysts show enhanced photocatalytic water
splitting efficiency for H generation (solar energy conversion efficiency of 1.06%) under a 150 W and
.5 AM solar simulator. Pt–Ta and Pt–Nb also generate noticeable photocatalytic activities of
.21% and 0.16%, respectively. The enhanced photocatalytic activity is attributed to correct band
2
2
2
Received 29th November 2015
Accepted 29th January 2016
1
O
2 5
2 5
O
0
DOI: 10.1039/c5ra25358f
alignment of the porous oxides with absorption in the UV-visible range, and ordered macro- and
www.rsc.org/advances
mesoporosity of the crystalline oxides for efficient charge transfer.
metal oxides with a crystalline wall possess not only high
thermal and mechanical stability, but also specic electronic
1
Introduction
Photocatalytic water splitting for the generation of H
under solar light illumination has been studied as a clean and and photocatalytic efficiencies.
renewable energy technology that overcomes the limitations of mesoporous oxides have been prepared using a so template,
fuel orbitals and lattice defects that inuence catalytic performance
2
18,19
Many different kinds of
1
–7
fossil fuels.
This thermodynamically uphill, two-electron such as block copolymers that have both hydrophilic and
ꢀ1
20,21
reaction requires 237 kJ mol to split a water molecule into hydrophobic groups.
As an alternative route, the nano-
8
H
2
and O
2
. Despite the revolutionary Fujishima–Honda effect casting method is employed, in which a hard template e.g.
discovered in 1972, photocatalytic/photo-electrochemical H
2
a polymer bead or silica sphere is used as a sacricial
9
22
production is still challenging to achieve. Construction of template. However there are some limitations to directly
a stable and practical solar power system with economic comparing the catalytic effect of different mesoporous oxides
viability is highly demanding. Several factors have been proven because each oxide has a different pore size, channel structure,
to affect the solar-to-H2 energy conversion efficiencies, and surface area. Furthermore, some mesoporous oxides do not
including morphology, surface area, crystallinity, and band have large enough pore sizes and volumes to load metal nano-
structure of the catalyst and charge transfer at oxide–metal particles and create supported catalysts. Recently, mesoporous
1
0,11
interfaces.
Transition metal oxides with ordered pore oxides with macropores were prepared using dual templates.
12–15
structures have numerous catalytic applications.
geneous catalysis, it is known that the interaction between aluminas with bimodal pore structures. In their report, Plur-
metal nanoparticles and oxide supports plays a crucial role in onic P123 triblock copolymer was used as the structuring agent
In hetero- Dacquin et al. reported highly ordered macro-mesoporous
23
16,17
determining the catalytic activity.
Especially, mesoporous to form the mesopores, while monodispersed polystyrene beads
created macropores in the crystalline walls of the
mesostructures.
a
Graduate School of EEWS, Korea Advanced Institute of Science and Technology
KAIST), Daejeon, 305-701, Republic of Korea. E-mail: jeongypark@kaist.ac.kr
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science,
In this study, we prepared macro-mesoporous oxides that
have crystalline and well-dened macro- and meso-frameworks.
Using P123 as the so template and polystyrene beads with an
average diameter of 500 nm as the hard template, we generated
(
b
Daejeon, 305-701, Republic of Korea
c
School of Energy and Chemical Engineering, Ulsan National Institute of Science and
crystalline macro-mesoporous oxides of TiO
2 2 5 2 5
, Ta O , Nb O ,
Technology(UNIST), Ulsan 689-798, Republic of Korea
ZrO , and Al that contain 300 nm macropores in meso-
2
2 3
O
†
Electronic supplementary information (ESI) available: Experimental data
including FE-SEM (Fig. S1), HR-TEM (Fig. S2), small-angle XRD patterns porous frameworks. By supporting colloidal Pt nanoparticles in
(
Fig. S3) and XPS (Fig. S4). Photoluminescence (Fig. S5). See DOI: the oxides, we prepared supported photocatalysts i.e. Pt–TiO
,
2
1
0.1039/c5ra25358f
2 5 2 5 2 2 3
Pt–Ta O , Pt–Nb O , Pt–ZrO , and Pt–Al O . With a set of oxide-
‡
These authors contributed equally to this work.
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supported Pt-nanoparticle photocatalysts, photocatalytic water the precursor were then added to ethylene glycol in a 50 mL three-
ꢁ
splitting experiments were conducted for the generation of H2. necked ask. The mixture was then heated to 200 C and main-
We observed enhanced photocatalytic activity following the tained at this temperature for 10 min under Ar gas. The solution
25
introduction of Pt and macro-mesoporous structures with was precipitated with excess acetone and re-dispersed in ethanol.
crystalline walls. Following which, we investigated the correla- To prepare the oxide-supported Pt-nanoparticle photocatalysts,
tion between the type of macro-mesoporous oxide and the the desired amount of macro-mesoporous oxide was added to
photocatalytic water splitting efficiency.
poly(vinylpyrrolidone) (PVP)-capped Pt nanoparticles dispersed in
26
ethanol and sonicated for 2 h. The catalysts were collected by
centrifugation and dried. To remove the organic capping mole-
2
Experimental
ꢁ
cules, the catalysts were calcined at 360 C for 6 h in air.
2.1 Synthesis of macro-mesoporous oxide
2
.3 Characterization of photocatalysts
Macro-mesoporous oxides were prepared using the dual tem-
plating method, in which Pluronic P123 surfactant and poly-
styrene beads were used as so and hard templates,
respectively. Potassium persulfate (0.03 g dissolved in water)
was added as an initiator for the polymerization. The poly-
styrene beads were synthesized through emulsier-free emul-
High-resolution transmission electron microscopy (HR-TEM)
images of the Pt nanocatalyst were obtained using a JEOL
JEM-ARM200F (Cs-corrected scanning transmission electron
microscope). The morphology was characterized using scan-
ning electron microscopy (SEM, Magellan 400). The oxidation
states of the catalysts were investigated using XPS. The XPS
spectra were taken using a Thermo VG Scientic Sigma Probe
system equipped with an Al-Ka X-ray source (1486.3 eV) with an
24
sion polymerization. Briey, 14 g styrene monomer and 0.7 g
divinyl benzene as a cross linker were washed 4 times with
a 0.1 M NaOH solution to remove inhibitors, and then washed
with DI water 4 times. The solution was mixed with 140 mL DI
energy resolution of 0.47 eV FWHM under ultra-high vacuum at
ꢁ
ꢀ10
water at 70 C and purged by owing Ar for 1 h. For initiation,
10
Torr. The amount of Pt nanoparticles loaded on the oxide
0.03 g potassium persulfate dissolved in 2 mL water was added
powder was measured by inductively coupled plasma atomic
emission spectroscopy (ICP-AES). XRD patterns were measured
on a Bruker D8 GADDS diffractometer using Co Ka radiation
to the solution. Upon addition of the initiator, polymerization
ꢁ
took place; the reaction was then maintained at 70 C for 12 h.
White solids containing the polystyrene beads were obtained by
centrifugation aer washing with methanol and water. To
synthesize the meso-macroporous oxides, metal chloride or
alkoxide precursors were dissolved in an acidic solution in the
presence of P123 and mixed thoroughly under vigorous stirring
for the sol–gel reaction. The polystyrene beads were added to
the precursor solution before the evaporation-induced self-
assembly step. For the macro-mesoporous alumina, 4.0 g
P123 ((EO) (PO) (EO) triblock copolymer (EO ¼ ethylene
˚
(
1.79 A). Nitrogen physisorption data was obtained on a Micro-
meritics ASAP 2020.
2
.4 Photocatalytic hydrogen evolution measurement
The photocatalytic H production reactions were performed in
2
a Pyrex top, irradiation-type vessel connected to a closed gas
circulation system at room temperature. A 0.1 g sample was
dispersed in 100 mL of an aqueous solution i.e. 10 mL methanol
2
0
70
20
(
10 vol%) and 90 mL ultrapure water. The reactant solution was
oxide, PO ¼ propylene oxide, M
0 mL ethanol for 4 h. For the precursor solution, 8.16 g
aluminum isopropoxide was dissolved in 20 mL ethanol and
.4 mL 68–70 wt% nitric acid, and then this precursor solution
w
¼ ꢂ5800)) was dissolved in
evacuated several times to ensure complete air removal, fol-
lowed by the introduction of back-lled argon (ca. 5 kPa) into
the system. The reaction was initiated by irradiation with
a 150 W solar simulator (1.5 AM, 1 sun). The reaction was
4
6
was added drop wise into the P123 solution. For the macro-
mesoporous Nb O and Ta O , 30 mmol niobium chloride or
ꢀ2
carried out at a power of 50 mW cm with an irradiation area of
2
5
2 5
2
1
9.6 cm . The evolved gases were analyzed by online gas chro-
matography (DS Science with a TCD detector and MS-5A
column, and argon carrier gas). The photocatalytic H evolu-
tion activity was estimated from the initial gas evolution rate.
tantalum chloride, respectively, was dissolved in 30 mL ethanol
with 3 g P123. For the macro-mesoporous TiO2 and ZrO2,
2
20 mmol titanium isopropoxide or zirconium isopropoxide,
respectively, in 70 wt% 1-propanol was added to a P123 solution
that also contained ethanol and HCl. Each precursor solution
mixed with P123 was stirred for 5 h; 4 g dried polystyrene beads 3 Results and discussion
(
powder) was then added to the solution. The mixture solution
3
.1 Characterization of oxide-supported Pt-nanoparticle
ꢁ
was placed on a hot plate at 60 C for 48 h for solvent evapora-
tion. The collected solid was calcined in a muffle furnace at
7
nation, we obtained crystalline macro-mesoporous Al
Nb , Ta , and ZrO with an average pore size of 300 nm.
photocatalysts
ꢁ ꢁ
00 C for 6 h (for Al O , 900 C for 10 h) in air. Through calci-
2 3
Macro-mesoporous oxides are prepared using the dual tem-
plating method in which Pluronic P123 surfactant and poly-
styrene beads are used as so and hard templates, respectively.
When metal chloride or alkoxide precursors are cured by the
sol–gel reaction in the presence tri-block copolymer P123 and
polystyrene beads, porous oxides having both meso- and mac-
2 3 2
O , TiO ,
O
2 5
O
2 5
2
2
.2 Deposition of Pt on macro-mesoporous oxide
To synthesize the 2.7 nm Pt nanoparticles, we mixed H
xH O, and Pt(acac)
2 6
PtCl $
ropores are obtained aer calcination. At high temperatures i.e.
ꢁ
2
2
as the precursor; poly(vinylpyrrolidone) and above 700 C, the inorganic frameworks are crystallized and the
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organic polymers are burned out, generating dual-pore struc-
tures (Fig. 1a). When PVP-capped Pt nanoparticles with an
average diameter of 2.7 nm are deposited into the pores of the
oxides, oxide-supported Pt-nanoparticle photocatalysts are
produced with high loading (ca. 1 wt% Pt in the catalyst).
Fig. 1a illustrates the detailed synthesis procedure for the
preparation of the macro-mesoporous oxides and their sup-
ported Pt nanoparticle catalysts. Representative transmission
electron microscopy (TEM) images in Fig. 1b and d reveal that
the as-prepared Nb O and Ta O have distinct macropores with
2
5
2 5
an average diameter of 300 nm traced by the removal of the
polystyrene beads at high temperature. In addition, the mac-
roporosity of the oxide-supported Pt-nanoparticle photo-
catalysts is observed using FE-SEM (Fig. S1 in ESI†). The FE-SEM Fig. 2 (a) Nitrogen adsorption–desorption isotherms and (b) pore size
images in Fig. S1(a) and (b)† show an ordered macroporous distribution curves deduced from the adsorption branches of TiO
2
,
2 5 2 5 2 2 3
Ta O , Nb O , ZrO , and Al O .
structure and well-interconnected macropores. It is clear that
the Pt nanoparticles are evenly incorporated in the macropores
without agglomeration, as shown in Fig. S1(c) and (d).† The
TEM images shown in Fig. 1c and e of Pt–Nb O and Pt–Ta O ,
ꢁ
00 C, indicating high thermal stability. Ta O , Nb O , and
2 5 2 5
7
2
5
2 5
Al
2
2
3
O show higher surface areas of 96.7, 102.9, and 123.6
respectively, contain mesoporosity that can shorten the diffu-
sion length to a few nanometers, thus facilitating charge
transfer. From these morphological studies, we can conrm
that the oxide-supported Pt nanocatalysts have dual-pore
structural morphologies consisting of macro- and meso-
porosity and that the Pt nanoparticles are incorporated into the
inner pores. The presence of ordered macroporosity as well as
mesoporosity could synergistically boost light harvesting and
ꢀ1
m g , respectively, whereas TiO
2
and ZrO
2
show much lower
surface areas. Through TEM images (Fig. S2†), we conrmed
that the low surface area in the case of Pt–TiO is associated
with mesopores created by agglomerated TiO nanoparticles.
The characteristic features of the crystalline walls can be iden-
tied from X-ray diffraction (XRD) studies (Fig. 3). Anatase TiO2
2
2
with dominant (101) peaks, orthorhombic L- or b-Ta O ,
pseudo-hexagonal Nb
Al are identied by the Joint Committee on Powder
Diffraction Standards i.e. JCPDS. Well-dened peaks corre-
sponding to the crystal structure of each oxide are clearly
evident. Small-angle X-ray diffraction (XRD) patterns of the
2
5
27
2 5 2
O , tetragonal ZrO , and gamma-phase
charge transfer. The mesoporous nature of the oxide photo-
catalysts is conrmed from N sorption studies (Fig. 2). In the
adsorption–desorption isotherms in Fig. 2a, each oxide i.e.
TiO Nb ZrO and Al —excluding Ta —display
2 3
O
2
2
,
2
O
5
,
2
,
2
O
3
2 5
O
a typical type-IV hysteresis loop, which is a characteristic of
mesoporous materials with uniform pore structure. The pore
size distributions calculated from the nitrogen sorption
isotherm are shown in the Fig. 2b. The mesopores created by
P123 ranged in size from 2.8 to 11.8 nm, which are all in the
mesoporous range. The physicochemical characterization data
i.e. surface area, pore size, and pore volume are summarized in
Table 1. The porosity of the oxide-supported Pt-nanoparticle
photocatalysts can be retained at temperatures as high as
calcined Pt-oxide photocatalysts (Pt–TiO , Pt–Ta O , Pt–Nb O )
2
2
5
2 5
are shown in Fig. S3b in ESI.† Strong peaks in the region of 2q ¼
ꢁ
0.44–0.632 correspond to mesostructures, which imply that the
photocatalysts keep their porous structure aer calcination at
high temperature. Mesoporosity with crystalline walls results in
faster diffusion and more efficient transfer of charge carriers
from the bulk to the surface by suppressing the recombination
of holes and electrons. To access the optical response of the
oxide-supported Pt nanoparticle catalysts, we studied the
UV-Vis absorbance spectra (Fig. 4). The UV-Vis absorbance
19
spectra showed Pt–TiO absorbed in the visible light (>400 nm)
2
region while Pt–Nb
UV range. The onset of absorbance for Pt–Al
2
O
5
and Pt–Ta
2
O
5
only absorbed light in the
and Pt–ZrO
2
2
O
3
started near the IR region; alumina and zirconia are generally
considered to be insulators because of direct wide band gaps.
The band gap energy of macro-mesoporous oxides obtained
from the UV-Vis absorbance is similar to their theoretical band
gap values. To conrm the efficiency of charge carrier transfer
by Pt doping, photoluminescence (PL) measurements were
carried out. PL spectra of Pt–TiO
exhibit quenching, in comparison with the oxide support
without Pt nanoparticles (Fig. S4 in ESI†). However, Pt–Al
and Pt–ZrO did not show any quenching behaviour even aer
Pt nanoparticles were introduced to the oxide support. In
particular, the PL intensity of Pt–TiO2 shows the most
2 2 5 2 5
, Pt–Ta O , and Pt–Nb O
Fig. 1 (a) Illustration of the preparation of macro-mesoporous oxides
using P123 and polystyrene beads and the corresponding oxide-sup-
ported Pt nanoparticle catalysts. (b–e) Representative TEM images of
2 3
O
2
(
2 5 2 5
b and c) Pt–Nb O and (d and e) Pt–Ta O catalysts showing the
macro-mesoporous dual structures.
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Table 1 Physico-chemical characterization data of BET surface area, pore size, pore volume, theoretical band gaps, and solar energy conversion
efficiency of the oxide-supported Pt nanoparticle catalysts
2
ꢀ1
a
ꢀ1
b
c
d
Photocatalyst
S
BET (m g
)
V
pore (cc g
)
D
pore (nm)
Pt (wt%)
Conv. (%)
Pt–TiO
2 5
Pt–Ta O
2
12.9
96.7
102.9
19.2
0.045
0.068
0.098
0.047
0.206
11.8
2.8
3.8
9.7
6.6
1.20
1.11
1.04
1.16
1.09
1.06
0.21
0.17
—
Pt–Nb
2
O
5
Pt–ZrO
2
Pt–Al
2
O
3
123.3
—
a
b
c
d
Pore volume. Pore size. Pt loading/ICP. Solar energy conversion efficiency.
3
.2 Photocatalytic water splitting for H generation
2
Ordered macro-mesoporous oxides have been employed for
higher photocatalytic activity in the past few decades because of
better mass and radiation transport through the ordered porous
28,29
network.
N-doped TiO
in solar light. To investigate the effect of macro-mesoporous
oxides, photocatalytic H production under simulated solar
Recently, Naik et al. reported hierarchical porous
2
photocatalysts for enhanced photogeneration of
5
H
2
2
light was carried out using 10 volume% methanol solutions as
sacricial agents over the oxide-supported Pt-nanoparticle
photocatalysts. We did not observe any H production in the
2
absence of either light or catalyst. Among the oxide-supported
Pt-nanoparticle photocatalysts, Pt–TiO2 showed the highest
ꢀ
1
ꢀ1
photocatalytic H
2
generation activity (157.5 mmol g
h
), as
) and Pt–Nb
) also exhibited a considerable amount of
generation. However, Pt–Al and Pt–ZrO
signicantly diminished PL intensity, compared with the bare catalysts did not evolve any photocatalytic H because they can
oxide; a lower PL intensity indicates a reduced charge recom- only be excited under UV-light irradiation due to their large
bination process. Thus, photoinduced electrons from the TiO
bandgaps. The measured solar energy conversion efficiencies
can be injected into the Pt nanoparticles by formation of are 1.06% for Pt–TiO , 0.21% for Pt–Ta , and 0.17% for
a Schottky barrier at the Pt and TiO
interface that demonstrates Pt–Nb (Table 1). Fig. 5b shows the time-dependent study of
efficient charge transfer. In addition, we carried out X-ray
evolution for Pt–TiO i.e. the best-performing catalyst; the
photoelectron spectroscopy to study the inuence of the catalyst evolved 28.4 mmol of H in the initial two hours of
Fig. 3 X-ray diffraction (XRD) patterns of TiO
and Al
2 2 5 2 5 2
, Ta O , Nb O , ZrO ,
ꢀ1
ꢀ1
shown in Fig. 5a. Pt–Ta
2
O
5
(31.3 mmol g
h
2 5
O
O .
2 3
ꢀ1
ꢀ1
(24.9 mmol g
h
photocatalytic H
2
2
O
3
2
2
2
O
2 5
2
2 5
O
2
H
2
2
2
oxidation state of Pt on photocatalytic activity. Fig. S5 in ESI† photocatalytic reaction. Aer two hours of reaction, the reactor
shows the XPS peaks of Pt in the oxide-supported Pt- was evacuated, backlled with Ar, and then the photocatalytic
nanoparticle photocatalysts; in all of the photocatalysts, Pt is experiment was repeated. The activity was found to be almost
mainly present in its metallic state. The presence of a high the same in four successive runs, without any decrease aer 8 h.
percentage of metallic Pt nanoparticles acts as a co-catalyst for Additionally, the Pt–Ta
O and Pt–Nb O catalysts revealed no
2 5 2 5
th
efficient charge transfer.
decrease in activity even aer the 4 cycle i.e. aer 8 h. This
Fig. 5 (a) Photocatalytic H
oxides. (b) Time-dependent study of photocatalytic H
Fig. 4 UV-Vis absorbance of the oxide-supported Pt nanoparticle Pt-mesoporous TiO , Pt-mesoporous Ta and Pt-mesoporous
catalysts i.e. Pt–TiO , Pt–Ta , Pt–Nb , Pt–ZrO , and Pt–Al Nb
2
generation by Pt-macro-mesoporous
2
generation by
2
2 5
O
2
O
2 5
O
2 5
2
O
2 3
.
2 5
O .
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Fig. 6 (a) Band structures of oxide photocatalysts relative to redox potential. (b) Mechanism for photocatalytic H
2
generation by light absorption
2
and charge transfer through Pt–TiO photocatalysts.
demonstrates the photo-stability of these catalysts and that they the dual pore structure of the catalyst with crystalline domains
can be used for a long time. Photocatalytic water-splitting plays the most important role for the photocatalytic process.
results for the Pt–TiO photocatalyst i.e. that has the highest Ordered macroporosity serves as the light harvesting medium
2
3
solar energy conversion efficiency of 1.06% are compared with and provides multiple reections owing to its capacity for easy
those in the recent report by Zhang et al. where hierarchical transport of the reactant mass and radiation. The presence of
ꢁ
ordered macro-mesoporous anatase calcined at 400 C exhibi- mesoporosity helps to shorten the diffusion length of charge
30
ted 0.23% quantum efficiency. Previously, our ordered hier- carriers from the bulk to the surface for efficient charge trans-
31
archical porous N-doped TiO
2
photocatalyst with Pt fer. Apart from charge separation, the mesoporosity intro-
nanoparticles had a quantum efficiency of 1.38%. For compar- duces a higher surface area and hence more active sites for
ison, the N-doped Pt–TiO2 photocatalyst with macroporosity photocatalysis. The deposited Pt nanoparticles create a Schottky
exhibits lower efficiency (0.58%), indicating that the macro- barrier with porous semiconductor oxides acting as co-catalysts
3
2–35
mesoporous structure has an advantage in improving photo- and promoting charge transfer.
It is believed that the red
evolution. The exceptionally high shi in the longer wavelength range is related to the presence of
photocatalytic water splitting activity of Pt–TiO could also be new electronic states produced by the deposition of Pt nano-
15
catalysis activity for H
2
2
35
attributed to the red shi of the absorption and the presence of particles. The synergistic effects of visible light absorption,
the macro-mesoporous structure with crystalline walls. Obvi- fast diffusion of charge carriers, and effective transfer are vital
ously, the UV-Vis diffuse absorption spectra of Pt–TiO2 dis- parameters for enhancing visible-light photocatalytic activity.
played a signicant shi in absorption to the visible light
region. Moreover, the suitable band position and band gap of
TiO
2
is valuable to initiate hydrogen production. In photo-
4
, the position of
Conclusions
catalytic water splitting for the production of H
2
the conduction band should be above the hydrogen reduction
potential and the valence band should be below the oxygen
oxidation potential; all three photoactive oxides full these
requirements (Fig. 6a). As the solar simulator intensity is much
In conclusion, we investigated the role of ordered macro-
mesoporous oxides with crystalline walls for photocatalytic
water splitting to generate H
Pt-nanoparticle photocatalysts, Pt–TiO
energy conversion efficiency, followed by Pt–Ta O (0.21%) and
2
. Among the oxide-supported
2
exhibits 1.06% solar
higher in the visible range, the Pt–TiO catalyst therefore shows
2
2
5
much higher activity than Pt–Ta O and Pt–Nb O . Fig. 6b
2
5
2
5
Pt–Nb O (0.16%), while there is no generation of H on Pt–
2
5
2
2
illustrates the mechanism for photocatalytic H generation by
Al
2
O
3
and Pt–ZrO
2
. The high photocatalytic activity of the Pt–
light absorption and charge transfer in the macro-mesoporous
oxide-supported Pt-nanoparticle photocatalysts. The presence
of ordered meso- and macro-porosities with a higher effective
surface area facilitates more active sites for the reaction,
resulting in higher activity. Apart from surface area, the crys-
talline walls of the porous oxides play a signicant role in the
high photocatalytic activity. The crystallized anatase phase
facilitated rapid transfer of photoelectrons from the bulk to the
surface, which could effectively inhibit recombination of the
photo-generated electrons and holes to improve the photo-
catalytic activity. The higher photocatalytic activity may also be
attributed to a combination of the following synergetic effects:
TiO photocatalyst is attributed to proper position of the band
2
structure. Ordered macro-mesoporosity of the oxide is respon-
sible for enhanced mass and electromagnetic radiation trans-
port, and multiple internal reections boosting light harvest.
The crystalline walls of the porous oxides facilitate charge
separation with faster diffusion in the crystalline channel. The
presence of Pt at the surface results in the formation of
a Schottky barrier at the metal–oxide interface, which facilitates
interfacial electron transfer and subsequently encourages
charge carrier separation. As a result, charge carrier transfer is
remarkably improved, which in turn results in enhanced pho-
2
tocatalytic H generation.
18202 | RSC Adv., 2016, 6, 18198–18203
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Acknowledgements
The work was supported by IBS-R004-G4 and partly by the 2015 20 G. J. D. A. Soler-Illia, E. L. Crepaldi, D. Grosso and
Research Fund (1.150109.01) of UNIST.
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