The photocatalytic activity of the Pt deposited TiO2catalysts synthesized at different temperatures was evaluated by means of hydrogen evolution reaction under both UVevis and visible li
Trang 1Pt deposited TiO 2 catalyst fabricated by thermal decomposition of
titanium complex for solar hydrogen production
Quang Duc Truonga,b, Thanh Son Leb, Yong-Chien Linga,*
a Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
b Department of Chemistry, Vietnam National University, Hanoi 10000, Viet Nam
a r t i c l e i n f o
Article history:
Received 10 June 2014
Received in revised form
9 September 2014
Accepted 26 September 2014
Available online
Keywords:
Hydrogen evolution
Semiconductors
Chemical synthesis
Photocatalyst
a b s t r a c t
C, N codoped TiO2catalyst has been synthesized by thermal decomposition of a novel water-soluble titanium complex The structure, morphology, and optical properties of the synthesized TiO2catalyst were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UVevis diffuse reflectance spectroscopy The photocatalytic activity of the Pt deposited TiO2catalysts synthesized at different temperatures was evaluated by means of hydrogen evolution reaction under both UVevis and visible light irradiation The investigation results reveal that the photocatalytic H2 evolution rate strongly depended on the crystalline grain size as well as specific surface area of the synthesized catalyst Our studies successfully demonstrate a simple method for the synthesis of visible-light responsive Pt deposited TiO2catalyst for solar hydrogen production
© 2014 Elsevier Masson SAS All rights reserved
1 Introduction
A great concern has been paid at global level due to the shortage
of natural energy resources such as fossil fuels, coal, and natural
gas The development of alternative and renewable energy source,
therefore, has gained increasing attention Since Honda and
Fujishima introduced the water splitting using a TiO2electrode in
early 1970s, the conversion of solar energy into chemical energy
has been intensively studied[1] Hydrogen is among one of most
promising alternative fuels, the transformation of solar energy into
hydrogen storage offers intriguing opportunity of achieving solar
fuels[2] For instance, considerable efforts have been devoted to
explore the photocatalytic water splitting into hydrogen A variety
of photocatalysts have been studied for hydrogen evolution such as
titanates [3], tantalates[4], niobates[5], oxynitrides [6], and
Ti-based materials[7e12] Among various catalysts, titanium oxide
(TiO2) is one of the most promising candidates for photocatalytic
reaction, such as organic degradation or water splitting, owing to
its powerful oxidation properties, availability, efficiency, and
long-term stability[7e12] The current research has been focused on the
improvement of photocatalytic efficacy for water splitting by
enhancement of photo-induced holeeelectron pair separation and
extension the photo-responsive region to the visible light region To
improve the photoactivity of TiO2under the visible light irradiation, various methods have been proposed to shift its photo-responsive region from UV to visible light region Among them, doping TiO2 with nonmetals such as N and C may lead to narrower band gap and alter the optical property of TiO2materials, resulting in enhance-ment of photocatalytic H2production activity under visible light irradiation[13,14]
Regarding the synthesis of N-doped TiO2, several methods have been proposed including ion implantation, solegel, microemulsion, hydrothermal, laser ablation, and sputtering methods[15e20] In these processes, N-doped TiO2has been successfully synthesized either by the reduction using gaseous NH3, oxidation of TiN, decomposition of TiO2 and urea mixture, sputtering of the TiO2 target in N2 atmosphere, or by hydrolysis of a titanium-dioxide precursor with NH3solution Therefore, the delicate control over the reaction conditions at high temperature, high pressure and tedious synthetic procedure are required which make them unat-tractive for chemical industry Therefore, it is of great importance to develop a facile, eco-friendly, and scalable approach for the syn-thesis of nonmetal-doped titania
It should be noted that the codoping of TiO2with two or more elements, such as NeF, CeN, and SeN offers higher visible-light responses as compared to the TiO2 doped with single element [7,11,21e23] For example, NeF codoped TiO2showed higher pho-tocatalytic decompositions of organic compounds compared to N-doped and F-N-doped TiO2 The enhanced photoactivities were attributed to the synergistic effect of increased visible-light
* Corresponding author.
E-mail address: ycling@mx.nthu.edu.tw (Y.-C Ling).
Contents lists available atScienceDirect Solid State Sciences
j o u r n a l h o me p a g e : w w w e l s e v i e r c o m/ l o ca t e / s s s c i e
http://dx.doi.org/10.1016/j.solidstatesciences.2014.09.009
1293-2558/© 2014 Elsevier Masson SAS All rights reserved.
Solid State Sciences 38 (2014) 18e24
Trang 2absorption, presence of surface oxygen vacancy, and formation of
Ti3þions (promoted by F-doping)[21] The CeN codoped TiO2also
exhibited higher photocatalytic activity under visible light
compared to C-doped and N-doped TiO2 [22] The enhanced
adsorption to the carbonate species formed on the surface of the
CeN codoped TiO2 and visible-light photoactivity induced by
N-doping into the TiO2 lattice resulted in the such improvement
Herein, we demonstrate that thermal decomposition of the
com-plex at relatively low temperature resulted in the formation of C, N
codoped titania More importantly, the synthesized particles
exhibited the photocatalytic production of H2under both UV and
visible light irradiation The investigation results reveal that the
photocatalytic H2evolution rate strongly depends on the crystalline
grain size, specific surface area, as well as the doping nature of the
synthesized catalyst
2 Experimental section
2.1 Synthesis of TiO2
TiO2was prepared by a hydrothermal method using titanium
complex with oxalic acid as a chelating agent[24e26] Typically,
metallic titanium powder (5 mmol, 99.4%, Alfa Aesar, America) was
completely dissolved in a cold mixture of aqueous ammonia
solu-tion (10 cm3, 28%, J.T Baker, Germany) and hydrogen peroxide
so-lution (40 cm3, 30%, J.T Baker, Germany) After removing the excess
reagents by aging at 353 K, a yellowish gel-like specimen of
peroxo-titanic acid was obtained This gel-like specimen was dissolved in
15 mL distilled water containing 2% H2O2 to produce a
peroxo-titanic acid solution with pH 5.0 In the typical synthesis, oxalic
acid (7.5 mmol, 99.5%, Merk, Germany) was added to the solution of
peroxo-titanic acid The solution color changed from yellow to red,
suggesting the formation of a titanium oxalate complex (Fig S1)
The solution of the complex was heated again at 353 K until a
gel-like specimen formed The complex gel was then allowed to
decompose at 623 K, 723 K, 823 K, 923 K for 2 h to produce titania
catalyst, hereafter namely A623, A723, A823, A923, respectively
The resultant powders were used for further characterization and
evaluation of photocatalytic activity
2.2 Characterization of the synthesized particles The crystalline phase of the samples was characterized using powder X-ray diffraction (XRD; Rigaku D/MAX-IIB, 40 kV and
30 mA) with Cu Karadiation (l¼ 1.5406 Å) Data were collected in a
2qeqscanning mode with a scan speed of 4min1and a step size
of 0.02 The morphology of the nanoparticles was examined using field-emission scanning electron microscopy (FE-SEM; Hitachi S-4700) at an accelerating voltage of 15 kV TEM (JEOL 2010) and high-resolution TEM (TOPCOM EM-002B, 200 kV) were conducted using specimens dispersed in ethanol and then dropped onto Cu microgrid coated with a holey carbonfilm, followed by the evap-oration of the ethanol N2 adsorption and desorption isotherms were measured at 77 K (Micromeritics ASAP 2010) to evaluate the BrunauereEmmetteTeller (BET) specific surface area All samples were also characterized by diffuse reflectance spectroscopy The experiment was carried out using diffuse reflectance scanning spectrophotometer instrument, Shimadzu, UV 2450 The surface composition and binding energy of the samples were determined
by X-ray photoelectron spectroscopy (Perkin Elmer PHI 5600) The shift of the binding energy owing to relative surface charging was corrected using the C 1s level at 284.6 eV as an internal standard and Arþsputtering was employed to clean the surface of samples
2.3 Photocatalytic production of hydrogen The experimental setup for the photocatalytic production of hydrogen were conducted using water/methanol mixtures under solar irradiation Briefly, photocatalytic hydrogen evolution was carried out in a closed gas circulation system using Pyrex reactor Each sample contained 50 mL of aqueous methanol solution of 10/2 water/methanol (v/v) Typically, 0.1 g of TiO2particles were sus-pended in water by sonication for a minute, then H2PtCl6solution was directly injected to that solution for adjustment to 1% Pt loading on the catalyst powder Generally, it is accepted that a limited amount of H2PtCl6 will be completely deposited on the catalyst upon the light irradiation Finally, methanol was used tofill
to 50 mL and the reactor was sealed with rubber septum Prior to photocatalytic H2evolution, the solution was degassed by a circu-lation N2 gas and evacuation for 30 min 500-W Xe lamp (light intensity, 2.5 mW cm2) was employed as light source delivering from the top of the cell through a Pyrex window with a distance of
10 cm to the solution This light is considered as artificial solar light and the Pyrex window will cut off light with the wavelength
<290 nm[9] Thus, the wavelength of irradiated light was290 nm and the reaction temperature was kept at 25C using a cooling water bath A filter was used to cut off entire UV light with the wavelength <420 nm The amount of hydrogen evolved was measured using an online gas chromatograph (Shimadzu GC-14C) equipped with an MS-5A separation column, thermal
20 25 30 35 40 45 50 55 60 65 70 75 80
2θ/ degree, CuKα
(d) (c) (b) (a)
A
A A
A A
Fig 1 XRD patterns of the particles synthesized by thermal treatment of titanium
oxalate complex at different temperatures: (a) 623 K; (b) 723 K; (c) 823 K and (d)
923 K A: anatase.
Table 1 Chemical and physical properties and H 2 evolution rate on the TiO 2 catalysts Sample Atom (%) a Crys b (nm) Band gap (eV) BET (m 2 g1) H 2 (mmol h1)/
QE (%)
a Estimated with relative sensitivity factor excluded adventitious carbon.
b Crystallite size estimated according to the Scherrer equation: D (hkl) ¼ (K$l)/(b cosq) where K is the shape factor,lthe wavelength of the Cu Karadiation,bthe full width at half-maximum (fwhm) of the (hkl) peak, andqthe diffraction angle.
Trang 3conductivity detector (TCD) High-purity nitrogen (99.999%) was
used as the carrier gas
3 Results and discussion
3.1 Structure of TiO2catalyst
Thermal treatment of the titanium oxalate complex at
623e923 K yielded powders of different colors XRD patterns of the
particles obtained by the thermal treatment of the titanium oxalate
complex at different temperature are shown inFig 1 XRD patterns
indicate that all the samples consist of crystalline titanium oxide
without impurity phase Anatase is the only crystalline phase,
ob-tained by the treatment of the complex at 623e923 K The
crys-tallinity of the synthesized particles increases with the increase of
the temperature Based on the XRD patterns and the Scherrer
equation, the crystallite size of the anatase TiO2 could also be
calculated as given inTable 1 The crystallite size of anatase
syn-thesized at low temperature is readily reduced in with respect to
that at high temperature which is in range of 15e28 nm The
morphology of the synthesized catalyst was also investigated by
SEM, TEM As shown inFig 2(andFig S2), the samples consist of
nano-sized particles which are aggregated in large particles The
very small Pt particles were homogeneously decorated on the surface of TiO2 particles as shown on TEM image The HR-TEM image (Fig 2(c)) exhibits lattice distances of 0.228 nm and 0.19 nm, which correspond to the (111) spacing of the Pt particles and the (200) or (020) spacing of the anatase TiO2
3.2 UVevis DRS and band gap energy studies The UVevis diffuse reflectance spectra of the synthesized TiO2 catalysts are shown inFig 3 The catalyst obtained at low tem-perature calcination (A623, A723, curve a, b) show strong absorp-tion of visible light region in the range of 400e600 nm; whereas the catalysts synthesized at higher temperature display a weak absorption in this region The absorption intensity of the catalyst A723 dominates those of other samples The absorption intensity in the visible-light region increases in the order A923z A823 < A723 < A623 which is consistent with appearance
of the color of the sample as shown in the inset inFig 3(A) To further study the doping effect on the optical property of TiO2 catalysts, we estimated the band gap energy by plotting the modified KubelkaeMunk function [F(R)E]1/2 calculated from the optical absorption spectrum versus the energy of irradiation light (Fig 3(B)) The respective band gap energy is listed in Table 1, Fig 2 TEM images of the Pt deposited TiO 2 particles synthesized at (a) 623 K and (b) 823 K HR-TEM image of Pt/TiO 2 synthesized at 823 K (c).
Q.D Truong et al / Solid State Sciences 38 (2014) 18e24 20
Trang 4indicating that the band gap energy decrease from 3.20 eV to
3.08 eV of A723 and 3.15 eV of others (Fig S3), matching well with
the absorption shift to visible region observed in the DRS spectra
(Fig 3(A))
The remarkable absorption of the synthesized TiO2catalysts in
visible light region was presumably attributed to the carbon and
nitrogen doping In previous report, we demonstrated that carbon
and nitrogen may be doped in titania nano-powders synthesized
from the titanium complex by hydrothermal method The strong
binding of small organic molecules with Ti4þusually facilitates the
carbon and/or nitrogen doping of synthesized particles during the
crystallization process[24,25] The possible formula of titanium
complex is (NH4)2[Ti2(O2) (C2O4)2]$2H2O [25] Therefore, it is
reasonable that TiO2catalyst synthesized by the thermal
decom-position of the titanium complex might also be doped by carbon
and/or nitrogen which present in the complex molecule To confirm
this assumption, the XPS measurement was carried out to
investi-gate the surface compositions and chemical state of the
synthe-sized particles The survey spectra (Fig 4) show additional peak of C
1s and N 1s at binding energy around 285 eV and 400 eV,
respec-tively, indicating the presence of surface carbon and nitrogen
The rescaled plots of N 1s spectra regions are shown inFig 5(A)
It can be seen that there are two different peaks at 398.4 eV and
401.6 eV The observed peak at 398.4 eV (detected in A623, A723,
A923) is attributed to the presence of nitrogen in the NeTieO
oxynitride[27,28] The observed peak at 401.6 eV (detected in A623, A823, A923) is due to the presence of oxidized state of N as NO or
NO2in the N-doped TiO2sample[29,30] The rescaled plots of C 1s spectra regions are shown in Fig 5(B) The peak detected in all samples at 284.8 eV is ascribed to the adventitious carbon derived from the XPS instrument or absorbed species The peak, around 286.4 eV (detected in all samples), can be attributed to CeO, sug-gesting the incorporation of C into the lattice of TiO2by replacing titanium atoms in the form of TieOeC structure[31] Furthermore, the peak at 282.4 eV appeared in A623, A723 and shoulders in A823, A923, attributing to the binding energy of CeTi bond[32,33] The doping amount of C and N was calculated based on XPS spectra and elemental analysis results (Table 1) Sample A823 shows the highest content of C (6.0 at.%); whereas sample A723 possesses the highest amount of N (1.9 at.%)
3.3 Photocatalytic hydrogen evolution The photocatalytic activity of the synthesized TiO2was evalu-ated in terms of photocatalytic H2evolution in methanol solution under UVevis (l> 290 nm) and visible light (l> 420 nm) irradi-ation Generally, Pt was used as co-catalyst which plays a role as electron trap to prevent the electronehole combination Conse-quently, photo-excited electrons and holes were driven to react with adsorbed agent, leading to the photocatalytic reaction
Fig 3 (A) UVevis DRS spectra of the synthesized particles at (a) 623 K, (b) 723 K (c)
823 K, and (d) 923 K The inset shows the photograph image of the synthesized
samples (B) Plot of the KubelkaeMunk function versus the energy of light absorbed of
the synthesized particles at (a) 623 K, (b) 723 K (c) 823 K, and (d) 923 K.
Fig 4 XPS spectra of the synthesized particles at (a) 623 K, (b) 723 K (c) 823 K, and (d)
Trang 5Methanol was used as electron donor or sacrificial reagent to
consume the photogenerated holes on the TiO2surface (Fig 6) In
particular, the photogenerated holes can initiate the oxidation of
water to produce
OH radicals or transferred directly to adsorbed methanol molecules[34,35]to form hydroxymethyl radicals, which
could subsequently injects an additional electron into the TiO2
conduction band On the other side, the photogenerated electrons
will be trapped at the Pt islands followed by the reduction of a
proton from water to produce adsorbed H
radicals[36] Recently, it has been determined that D2 generates from deuterated water
using mass spectrometry[37] Finally, methanol is photooxidized to
carbon dioxide via the formation of the stable intermediates
formaldehyde and formic acid and hydrogen is evolved on the Pt
islands (Fig 6)
Table 1 lists the photocatalytic performance in terms of the
hydrogen evolution rate, as well as the specific BET surface area of
the synthesized TiO2catalysts Upon UVevis light (l> 290 nm)
irradiation, the synthesized catalyst afforded H2evolution rate of
365e810mmol h1 The H2yield afforded by A923 TiO2is signi
fi-cantly lower than that by others Above all, the A623 exhibited
remarkable photoactivity with 810mmol h1of H2evolution rate
that is 2.2 times higher than that by A923 catalyst (Fig 7(a)) The
solar hydrogen production over Pt/TiO2 has been investigated in
terms of advantageous surface structures For instance, Yu et al
have reported photocatalytic hydrogen production using Pt/TiO2
nanosheets with a production rate of 333.5mmol h1[10,11] Teng
et al applied Pt@CuO/TiO2nanosheets for photogeneration of H2
with a total amount of 1222mL for 3 h[12] It was found that the
TiO2nanosheets exhibited much higher photocatalytic activity than
Degussa P25 TiO2and pure TiO2nanoparticles[10,11]and a stable
pen heterojunction form at the interface between CuO and TiO2
nanosheets significantly refrains the recombination rate of
elec-trons and holes[12]
Upon visible light (l> 420 nm) irradiation, a similar trend was
observed In particular, A623 catalyst showed the highest evolution
rate among all samples The H2 generation rate by A623 is
0.60mmol h1, which is higher than those by others (Fig 7(b)) In
the regard of photocatalytic hydrogen evolution, the current
research has been focused on the effect of crystal facet and crys-talline phase on the evolution rate [7e12] The photocatalytic production of hydrogen under visible light irradiation has not been considered appreciably[13,14] Till now, there is only few reports
on photocatalytic production of H2on Pt/TiO2under visible light irradiation[13,14] Compared with their results, the photoproduc-tion rate obtained in this work (H2of 13.4ml h1/0.1 g catalyst) is two times higher than that obtained using N-doped TiO2[13,14] The apparent quantum yield (QE) is estimated using H2yield noting that two electrons are required to reduce 2Hþto H2 The equation is as follows:
FHydrogenð Þ ¼ 100 2 mole of H% ½ 2yield.
mole of photons absorbed by catalyst½ (1) Mole of photon¼ ½I S=½NA E (2)
270 275 280 285 290 295 300
Binding energy / eV
380 385 390 395 400 405 410
Binding energy / eV
284.8
286.4
282.4 398.4
) A (
(d)
(c)
(b)
(a)
H+
H2
TiO2
Pt
e
-h+
Fig 6 Schematic representing the oxidative and reductive reaction on titania surface for the photocatalytic hydrogen production from aqueous methanol solution Q.D Truong et al / Solid State Sciences 38 (2014) 18e24
22
Trang 6where I is light intensity (2.5 mW cm2and 0.12 mW cm2for UV
and visible light, respectively); S is the irradiated area of the reactor
(p 12 12 cm2); E is the photon energy, (6.85 1019J at 290 nm
and 4.73 1019 J at 420 nm); N
A is the Avogadro number (6.022 1023mol1) The quantum efficiency (QE) of the
synthe-sized Pt/TiO2 catalyst was estimated and listed in Table 1 The
CH3OH QE obtained under UV/visible light irradiation ranges from
0.68 to 1.52 %, which is lower than the H2QE obtained with Pt/TiO2
Degussa reported by Lasa et al.[38]
The A623 catalyst exhibited significantly higher photocatalytic
activity than those of other samples which may be attributed to the
small crystallite size of this sample Ohtani et al have reported that
the photocatalytic hydrogen evolution activity of a TiO2strongly
depends on the crystallite size For instance, sample with smaller
crystallite size exhibited higher photocatalytic H2generation[39] The small crystallite size offers an enhancement for the photo-catalytic reaction by the significant shortening of the migration length of photo-excited carriers to surface active sites Our result is also in good agreement with such conclusion For instance, A623 has smallest grain size which is beneficial for its higher photo-catalytic activity Furthermore, A623 showed a great improvement
of photocatalytic production of H2which may also be attributed to its high specific surface area The increase in photocatalytic activity can be explained by an increase of the specific surface areas (101.5 m2/g,Table 1) because the increase in the number of the catalytic active sites on the particles surfaces may allow the ef fi-cient transport of photogenerated electronseholes to the absorbed reactant molecules
4 Conclusions The well-defined titania catalyst has been synthesized by a thermal decomposition of a titanium complex The titania phases of anatase can be successfully synthesized at temperature as low as
623 K Among the synthesized samples, the photocatalytic pro-duction of H2activity increases with the decrease in the crystallite size Particles synthesized at lower temperature exhibited a higher photocatalytic activity for H2 evolution under both UVevis and visible light irradiation Our studies demonstrate a facile approach for synthesis of Pt deposited titania for photocatalytic water splitting
Acknowledgments Financial support by the Ministry of Science and Technology (NSC-95-2113-M-007-044-MY3 and NSC-101-2113-M-007-006-MY3) of Taiwan are gratefully acknowledged Authors thank Dr Jen-Yu Liu for his help on XPS measurement This work was partly supported by Grant-in-Aid for Basic Research from the National Foundation for Science and Technology Development (NAFOSTED, Grant no 104.06-136.09) of Vietnam
Appendix A Supplementary data Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.solidstatesciences.2014.09.009
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