DSpace at VNU: Pt deposited TiO2 catalyst fabricated by thermal decomposition of titanium complex for solar hydrogen production

Solid State Sciences 38 (2014) 18e24 Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie Pt deposited TiO2 catalyst fabricated by thermal decomposition of titanium complex for solar hydrogen production Quang Duc Truong a, b, Thanh Son Le b, Yong-Chien Ling a, * a b Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Chemistry, Vietnam National University, Hanoi 10000, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received 10 June 2014 Received in revised form September 2014 Accepted 26 September 2014 Available online C, N codoped TiO2 catalyst has been synthesized by thermal decomposition of a novel water-soluble titanium complex The structure, morphology, and optical properties of the synthesized TiO2 catalyst 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 TiO2 catalysts 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 visiblelight responsive Pt deposited TiO2 catalyst for solar hydrogen production © 2014 Elsevier Masson SAS All rights reserved Keywords: Hydrogen evolution Semiconductors Chemical synthesis Photocatalyst 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 TiO2 electrode 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 Tibased 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 longterm 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 * Corresponding author E-mail address: ycling@mx.nthu.edu.tw (Y.-C Ling) http://dx.doi.org/10.1016/j.solidstatesciences.2014.09.009 1293-2558/© 2014 Elsevier Masson SAS All rights reserved improve the photoactivity of TiO2 under 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 TiO2 materials, resulting in enhancement of photocatalytic H2 production 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 TiO2 has 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 NH3 solution Therefore, the delicate control over the reaction conditions at high temperature, high pressure and tedious synthetic procedure are required which make them unattractive for chemical industry Therefore, it is of great importance to develop a facile, eco-friendly, and scalable approach for the synthesis of nonmetal-doped titania It should be noted that the codoping of TiO2 with 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 TiO2 showed higher photocatalytic decompositions of organic compounds compared to Ndoped and F-doped TiO2 The enhanced photoactivities were attributed to the synergistic effect of increased visible-light Q.D Truong et al / Solid State Sciences 38 (2014) 18e24 absorption, presence of surface oxygen vacancy, and formation of Ti3ỵ ions (promoted by F-doping) [21] The CeN codoped TiO2 also 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 Ndoping into the TiO2 lattice resulted in the such improvement Herein, we demonstrate that thermal decomposition of the complex at relatively low temperature resulted in the formation of C, N codoped titania More importantly, the synthesized particles exhibited the photocatalytic production of H2 under both UV and visible light irradiation The investigation results reveal that the photocatalytic H2 evolution rate strongly depends on the crystalline grain size, specific surface area, as well as the doping nature of the synthesized catalyst Experimental section 2.1 Synthesis of TiO2 TiO2 was 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 solution (10 cm3, 28%, J.T Baker, Germany) and hydrogen peroxide solution (40 cm3, 30%, J.T Baker, Germany) After removing the excess reagents by aging at 353 K, a yellowish gel-like specimen of peroxotitanic acid was obtained This gel-like specimen was dissolved in 15 mL distilled water containing 2% H2O2 to produce a peroxotitanic 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 gellike specimen formed The complex gel was then allowed to decompose at 623 K, 723 K, 823 K, 923 K for 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 Intensity (arb units) A A A A A (d) (c) (b) 19 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 Ka radiation (l ¼ 1.5406 Å) Data were collected in a 2qeq scanning mode with a scan speed of 4 minÀ1 and a step size of 0.02 The morphology of the nanoparticles was examined using field-emission scanning electron microscopy (FE-SEM; Hitachi S4700) 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 carbon film, followed by the evaporation 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 Arỵ sputtering 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 TiO2 particles were suspended in water by sonication for a minute, then H2PtCl6 solution 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 to fill to 50 mL and the reactor was sealed with rubber septum Prior to photocatalytic H2 evolution, the solution was degassed by a circulation N2 gas and evacuation for 30 500-W Xe lamp (light intensity, 2.5 mW cmÀ2) 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 420 nm) irradiation Generally, Pt was used as co-catalyst which plays a role as electron trap to prevent the electronehole combination Consequently, photo-excited electrons and holes were driven to react with adsorbed agent, leading to the photocatalytic reaction Fig (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 indicating 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 TiO2 catalysts 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 Ti4ỵ usually 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 TiO2 catalyst synthesized by the thermal decomposition 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 investigate the surface compositions and chemical state of the synthesized particles The survey spectra (Fig 4) show additional peak of C 1s and N 1s at binding energy around 285 eV and 400 eV, respectively, indicating the presence of surface carbon and nitrogen The rescaled plots of N 1s spectra regions are shown in Fig 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 Fig XPS spectra of the synthesized particles at (a) 623 K, (b) 723 K (c) 823 K, and (d) 923 K 22 Q.D Truong et al / Solid State Sciences 38 (2014) 18e24 Methanol was used as electron donor or sacrificial reagent to consume the photogenerated holes on the TiO2 surface (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 lists the photocatalytic performance in terms of the hydrogen evolution rate, as well as the specific BET surface area of the synthesized TiO2 catalysts Upon UVevis light (l > 290 nm) irradiation, the synthesized catalyst afforded H2 evolution rate of 365e810 mmol hÀ1 The H2 yield afforded by A923 TiO2 is significantly lower than that by others Above all, the A623 exhibited remarkable photoactivity with 810 mmol hÀ1 of H2 evolution 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 333.5 mmol hÀ1 [10,11] Teng et al applied Pt@CuO/TiO2 nanosheets for photogeneration of H2 with a total amount of 1222 mL for h [12] It was found that the TiO2 nanosheets exhibited much higher photocatalytic activity than Degussa P25 TiO2 and pure TiO2 nanoparticles [10,11] and a stable pen heterojunction form at the interface between CuO and TiO2 nanosheets significantly refrains the recombination rate of electrons 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 0.60 mmol hÀ1, which is higher than those by others (Fig 7(b)) In the regard of photocatalytic hydrogen evolution, the current H+ e- H2 Pt HCHO + H+ TiO2 h+ CH3OH •CH OH + H+ Fig Schematic representing the oxidative and reductive reaction on titania surface for the photocatalytic hydrogen production from aqueous methanol solution research has been focused on the effect of crystal facet and crystalline 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 H2 on Pt/TiO2 under visible light irradiation [13,14] Compared with their results, the photoproduction rate obtained in this work (H2 of 13.4 ml hÀ1/0.1 g catalyst) is two times higher than that obtained using N-doped TiO2 [13,14] The apparent quantum yield (QE) is estimated using H2 yield noting that two electrons are required to reduce 2Hỵ to H2 The equation is as follows: FHydrogen %ị ẳ 100 ẵ2 mole of H2 yieldŠ  ½mole of photons absorbed by catalyst Mole of photon ẳ ẵI S=ẵNA E (A) (B) 401.6 284.8 (c) 398.4 282.4 (b) 286.4 (a) 405 400 395 390 Binding energy / eV 385 380 300 295 290 285 280 275 (1) (2) (d) 410 270 Binding energy / eV Fig (A) N 1s and (B) C 1s XPS spectra of the synthesized particles at (a) 623 K, (b) 723 K (c) 823 K, and (d) 923 K Q.D Truong et al / Solid State Sciences 38 (2014) 18e24 crystallite size exhibited higher photocatalytic H2 generation [39] The small crystallite size offers an enhancement for the photocatalytic 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 photocatalytic activity Furthermore, A623 showed a great improvement of photocatalytic production of H2 which 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 efficient transport of photogenerated electronseholes to the absorbed reactant molecules (a) 1800 1600 A623 1400 Hydrogen / μmol 1200 A823 1000 A723 800 A923 600 Conclusions 400 200 0 0.5 1.5 2.5 Irradiation time / h (b) Hydrogen / μmol 23 Acknowledgments A623 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 production of H2 activity 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 Financial support by the Ministry of Science and Technology (NSC-95-2113-M-007-044-MY3 and NSC-101-2113-M-007-006MY3) 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 A723 A823 A923 Appendix A Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2014.09.009 0 Irradiation time / h Fig The time course of H2 evolution on the synthesized particles under (a) UVevis light irradiation and (b) visible light irradiation where I is light intensity (2.5 mW cmÀ2 and 0.12 mW cmÀ2 for UV and visible light, respectively); S is the irradiated area of the reactor (p  12  12 cm2); E is the photon energy, (6.85  10À19 J at 290 nm and 4.73  10À19 J at 420 nm); NA is the Avogadro number (6.022  1023 molÀ1) The quantum efficiency (QE) of the synthesized Pt/TiO2 catalyst was estimated and listed in Table The CH3OH QE obtained under UV/visible light irradiation ranges from 0.68 to 1.52 %, which is lower than the H2 QE 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 TiO2 strongly depends on the crystallite size For instance, sample with smaller References [1] A Fujishima, K Honda, 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