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Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

iO2 promoted by two different non-noble metal cocatalysts fornhanced photocatalytic H2 evolution

ing-Dong Lina,b,c,∗, Shi Yana, Qin-Dong Huanga, Mei-Ting Fana, You-Zhu Yuana,b,c,d,imothy Thatt-Yang Tane,∗∗, Dai-Wei Liaoa,b,c,d

Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, ChinaNational Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, Xiamen University, Xiamen 361005, ChinaInstitute of Physical Chemistry, Xiamen University, Xiamen 361005, ChinaState Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, ChinaSolar Fuel Lab, School of Chemical and Biomedical Engineering, Nanyang Technological University, 637457, Singapore

r t i c l e i n f o

rticle history:eceived 20 January 2014eceived in revised form 30 April 2014ccepted 1 May 2014vailable online xxx

eywords:hotocatalytic hydrogen evolution

a b s t r a c t

TiO2 photocatalysts modified by cobalt and nickel cocatalysts were prepared via polymerized complexmethod (PCM) and evaluated by photocatalytic hydrogen evolution. Hydrogen generation in 6 h for theTiO2 promoted by cobalt and nickel (0.1%Co + 0.2%Ni/TiO2) is about two times (2456 �mol H2) com-pared to that of TiO2 promoted only by cobalt (1180 �mol H2 for 0.1%Co/TiO2) or nickel (1127 �molH2 for 0.2%Ni/TiO2), and mechanically mixed TiO2 promoted by cobalt and TiO2 promoted by nickel(0.1%Co/TiO2:0.2%Ni/TiO2 = 1:1 (m/m), 1282 �mol H2). The high photocatalytic H2 evolution activity overTiO2 promoted by cobalt and nickel is ascribed to enhanced photo response due to the presence of cobalt

on-nobel metalocatalystynergistic effectobaltickel

and nickel impurity level, and effective separation of photogenerated electrons and holes due to the syn-ergistic effect of cobalt and nickel, which serve as active sites for H2 evolution reaction (HER) and oxidationreaction (OR) respectively. This study demonstrates a viable strategy to design more active photocata-lysts for photocatalytic H2 evolution by substituting noble metals with more abundant elements usingas HER and OR cocatalysts, respectively.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Photocatalytic H2 evolution from water by solar energy overemiconductor offers a promising way for clean and renewable pro-uction of hydrogen [1]. The TiO2-based materials have receivedonsiderable attention in recent years due to their extensive appli-ation in photocatalysis for hydrogen evolution [2–5]. Though TiO2s the most widely investigated photocatalyst for hydrogen evo-ution, it still provides low photocatalytic activity. Development

Please cite this article in press as: J.-D. Lin, et al., TiO2 promoted by two dH2 evolution, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsus

f new photocatalysts with excellent photocatalytic activity forydrogen evolution is still an important topic [2].

∗ Corresponding author at: Department of Chemistry, Xiamen University, Xiamen61005, China. Tel.: +86 592 218 3045; fax: +86 592 218 3043.∗∗ Corresponding author at: School of Chemical and Biomedical Engineering,anyang Technological University, 62 Nanyang Drive, 637457, Singapore.el.: +65 6316 8829; fax: +65 6794 7553.

E-mail addresses: jdlin@xmu.edu.cn, jdlin@189.cn (J.-D. Lin),ytan@ntu.edu.sg (T.T.-Y. Tan).

ttp://dx.doi.org/10.1016/j.apsusc.2014.05.008169-4332/© 2014 Elsevier B.V. All rights reserved.

Photocatalytic hydrogen evolution process mainly includesthree steps: generation electron–hole pairs, separation and migra-tion of photogenerated electrons and holes, redox reactions byphotogenerated electrons or holes on catalyst surface [1,6]. Suit-able energy structure of photocatalysts is required for generationelectron–hole pairs at first. The photo response, for example, canbe effectively improved by doping for TiO2 [5,7–9]. However,even if the photogenerated electrons and holes possess thermo-dynamically sufficient potentials for water splitting, they sufferfrom recombination if the surface active sites for redox reactionsare absent on the catalyst surface [6]. Only when the three stepsmentioned above are optimized, photocatalytic activities can beobtained effectively [6].

For a given kind of catalyst, an effective strategy to reduce chargerecombination is to enable quick transfer of the photogeneratedelectrons or holes to cocatalysts. Cocatalysts, such as Pt and NiO,are usually loaded to introduce active sites for HER, which play

ifferent non-noble metal cocatalysts for enhanced photocatalyticc.2014.05.008

an important role to capture the photogenerated electrons, hencelower the recombination of photogenerated electron–hole pairs[10–13]. Similarly, RuO2 and CoOx are usually loaded as activesites for OR, which play a role to capture the photogenerated holes

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resolution TEM (HRTEM) images (see Fig. S1), cobalt and nickel withtypical layered structure are deposited on TiO2 for 0.1%Co/TiO2 and0.2%Ni/TiO2, respectively. The fringes with lattice spacing of 0.245and 0.208 nm can be indexed to (1 1 1) plane of cubic CoO (JCPDF

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14,15]. If only one type of cocatalyst is loaded, the overall photore-ction could be limited by the other half-reaction. The addition of

sacrifice agent, such as methanol, Fe3+ or Ag+, can overcome thisroblem to some extent [1]. Another effective way to overcomehis limitation is to load both reduction and oxidation reactionsocatalysts onto the same photocatalyst [6,16–18]. The well knownocatalysts for the HER and OR are Pt and RuOx, respectively [11,14].owever, Pt and Ru are expensive and scarce noble metals. Non-oble metal alternatives such as nickel and cobalt are the promisingandidates for HER and OR, respectively [13,15]. The current workeports a first study of TiO2 promoted by cobalt and nickel coca-alysts for photocatalytic H2 evolution, which is prepared by PCM.he synergistic effect of cobalt and nickel is discussed.

. Experimental

.1. Sample preparation

Titanium tetrabutoxide (TBOT) was supplied by Jintan Chemi-al Reagent Factory and used as received. Ethanol, cobalt nitrate,ickel nitrate, citric acid (CA) and ethylene glycol (EG) were sup-lied by Sinopharm Chemical Reagent Co. Ltd. and used as received.iO2 promoted by cobalt and nickel cocatalysts for photocat-lytic H2 evolution were synthesized by PCM. The fabricationas described as follows in detail. Ethanol, TBOT, cobalt nitrate,ickel nitrate, CA and EG were mixed with the molar proportionf 50:1:0.001:0.002:10:40 at 333 K with stirring for 1 h. The mix-ure was condensed at 413 K for 5 h to polymerize EG and CA, andemove solvent. A black spongy powder, which was obtained byalcination at 623 K for 3 h, was ground and then sintered at 923 Kor 2 h. TiO2 promoted by cobalt or nickel cocatalysts alone wererepared using a similar procedure for comparison. The obtainedowder was named according to the molar ratio of Co to Ti andi to Ti. For example, the molar ratio of Co to Ti was 0.001 andi to Ti was 0.002, the product was named 0.1%Co + 0.2%Ni/TiO2.he 0.1%Co/TiO2 and 0.2%Ni/TiO2 with ratio of 1/1 (m/m) was alsorepared by mechanical mixing for comparison.

.2. Photocatalytic test

For photocatalytic H2 evolution, the reaction was carried out at08 K using inner irradiation-type quartz reaction cell with a 250 Wigh pressure mercury lamp (GY-250, Beijing Tianmai Henghui)19]. Typically, 1.0 g of catalyst was dispersed in the mixture solu-ion of methanol (10.0 mL) and water (≥18.0 M� cm, 500.0 mL) by

agnetic stirring. The amount of H2 evolution was analyzed bysing on-line gas chromatography (GC-950, Shanghai Haixin) with

thermal conductivity detector (TCD) and molecular sieve 5 A col-mn using Ar as the carrier gas.

.3. Photocatalyst characterizations

The XRD patterns were recorded using Panalytical X‘pert Prouper X-ray diffractometer with Cu K� radiation (� = 0.15418 nm).he accelerating voltage of 40 kV and emission current of 30 mAere used. The 2� angular regions between 10◦ and 90◦ were

ecorded at a scan rate of 0.0167◦ for 10 s. Raman spectra werebtained using a Renishaw inVia Raman microscope with exci-ation wavelength at 532 nm. Transmission electron microscopyTEM) specimens were prepared by dispersing powder in ethanolnd picked up with holey carbon supporting films on copper

Please cite this article in press as: J.-D. Lin, et al., TiO2 promoted by two dH2 evolution, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsus

rids. TEM was carried out on JEOL JEM-2100 with 200 kV accel-rating voltage. The N2 absorption–desorption isotherms wereeasured using an automatic micropore and chemisorption ana-

yzer (Micrometric ASAP 2020). Specific surface area was calculated

PRESScience xxx (2014) xxx–xxx

using Brunauer–Emmett–Teller (BET) equation. Pore size distri-butions were obtained by Barrett–Joyner–Halenda (BJH) equationusing desorption branch. Ultraviolet-visible diffuse reflection spec-troscopy (UV-vis/DRS) spectra were recorded using a Varian Cary5000 UV-vis spectrophotometer. BaSO4 was used as reference. Thephotoluminescence spectra (PL) for catalysts were investigated on aHitachi F-7000 spectrophotometer, using an excitation wavelengthof 309 nm, 5 nm excitation slits and 5 nm emission slits.

3. Results and discussion

Fig. 1 shows the XRD patterns of TiO2, 0.1%Co/TiO2, 0.2%Ni/TiO2and 0.1%Co + 0.2%Ni/TiO2. As shown in Fig. 1, the samples treatedby thermal annealing at 923 K result in high degrees of crystallinitywith anatase as the principal phase. No additional XRD peaks cor-responding to cobalt and nickel cocatalysts are revealed. This maybe attributed to the well-dispersed cobalt and nickel, which is notlarge enough for XRD detection. The XRD patterns also do not revealany other characteristic peaks of a mixed oxide phase being formeddue to an interaction between nickel (or cobalt) and TiO2. Similarresults were also reported by Chang and Ramírez-Meneses et al.[20]. Metal-ion dopants such as Co2+ and Ni2+ ions are most likelylocated in interstitial positions of the lattice rather than directlyin Ti4+ sites because of the relatively large size difference betweendopant ions (Co2+, 0.885 A; Ni2+, 0.830 A) and Ti4+ (0.745 A) [21]. Itis also noticed that small diffraction peak at 2� = 27.5◦ which camefrom rutile phase appears when TiO2 was loaded with cobalt or(and) nickel. The addition of ions with valence less than 4+ canincrease oxygen vacancies of TiO2. It is generally accepted thatanatase-rutile (A–R) phase transformation can be accelerated byincreasing oxygen vacancies, since the A–R phase transformationinvolves a contraction or shrinking of the oxygen structure [22].

In order to visualize the junction structure of cobalt, nickeland TiO2, TEM characterizations were carried out. From the low-resolution TEM images (see Fig. 2 and Fig. S1), uniform TO2particles with ∼30–50 nm size are observed for TiO2, 0.1%Co/TiO2,0.2%Ni/TiO2, 0.1%Co + 0.2%Ni/TiO2 and the mechanical mixture of0.1%Co/TiO2 and 0.2%Ni/TiO2 photocatalysts which were preparedby PCM. TEM results indicate that loading with 0.2%Ni or (and)0.1%Co has minimal effect on the dispersities of TiO2. From the high-

ifferent non-noble metal cocatalysts for enhanced photocatalyticc.2014.05.008

Fig. 1. XRD patterns of (a) TiO2, (b) 0.1%Co/TiO2, (c) 0.2%Ni/TiO2 and (d)0.1%Co + 0.2%Ni/TiO2.

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2 and

N00piw(cMffacgmdocataTHtp

0.1%Co/TiO2, 0.2%Ni/TiO2 and 0.1%Co + 0.2%Ni/TiO2. All theisotherms show a typical type IV mesopore sorption behavior with∼17 nm diameter of pores. An obvious hysteresis loop appearsin the range of P/Po at 0.6–1.0, which corresponds to capillary

Fig. 2. TEM images of photocatalysts. (a) and (b) 0.1%Co + 0.2%Ni/TiO

o. 00-009-0402) and (2 0 0) plane of cubic phase NiO (JCPDF No.1-071-1179), respectively. Uniform fringes with an interval of.352 nm corresponding to the (1 0 1) lattice spacing of anatasehase (JCPDF No. 00-021-1272) are observed. The HRTEM results

ndicate NiO and CoO are the main phases of nickel and cobalt,hich are firmly contacted with TiO2. During the PCM process, Co2+

or Ni2+) will react with CA to form soluble M(Co2+ or Ni2+)-CAomplexes before polyesterification. When heated in EG, soluble(Co2+ or Ni2+)-CA complexe can undergo polyesterification to

orm an uniform organic polymeric glass, which is favorable fororming a more homogeneous structure than other methods suchs solid-state reaction, precipitation or impregnation [23]. Afteralcination, the complex multicomponent oxide with good homo-eneity through mixing of each component (Ni, Co and Ti) at theolecular level would be favorable in improving the doping or

eposition of cobalt or nickel in TiO2 and forming firm contact NiOr (and) CoO with TiO2 [24], as evident from Figs. 1 and 2. The firmontact NiO or (and) CoO with TiO2 is the key to effectively capturend transfer photogenerated electrons or holes from TiO2 to coca-alysts, leading to low recombination of photogenerated electronsnd holes. Significantly, nickel and cobalt deposited on the same

Please cite this article in press as: J.-D. Lin, et al., TiO2 promoted by two dH2 evolution, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsus

iO2 particle were observed for 0.1%Co + 0.2%Ni/TiO2 (see Fig. 2b).owever, nickel and cobalt were deposited on different TiO2 par-

icles for the mechanical mixture of 0.1%Co/TiO2 and 0.2%Ni/TiO2hotocatalyst (see Fig. 2d).

(c) and (d) the mechanical mixture of 0.1%Co/TiO2 and 0.2%Ni/TiO2.

Fig. 3 shows N2 adsorption–desorption isotherms of TiO2,

ifferent non-noble metal cocatalysts for enhanced photocatalyticc.2014.05.008

Fig. 3. Nitrogen adsorption–desorption isotherms of photocatalysts.

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Table 1Specific surface area of different catalysts.

Samples TiO2 0.1%Co/TiO2 0.2%Ni/TiO2 0.1%Co + 0.2%Ni/TiO2

cifd0ls

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SBET (m2/g)a 52.6 41.5 53.0 47.3

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ondensation within the inter-particle mesopores. Nitrogensothermal adsorption data are used to calculate the specific sur-ace area, which are provided in Table 1. There are no significantifferences of specific surface area among TiO2, 0.1%Co/TiO2,.2%Ni/TiO2 and 0.1%Co + 0.2%Ni/TiO2. These results indicate that

oading with 0.2%Ni or (and) 0.1%Co has minimal effect on thepecific surface area and pore structure of TiO2.

Fig. 4 displays the photocatalytic activities for hydrogen evolu-ion over TiO2, 0.1%Co/TiO2, 0.2%Ni/TiO2, 0.1%Co + 0.2%Ni/TiO2 andhe mixture of 0.1%Co/TiO2 and 0.2%Ni/TiO2. Pure TiO2 shows aery low activity of 429 �mol H2 generation in the first 6 h. How-ver, the amounts of H2 generation in the first 6 h increase to180 and 1127 �mol H2 with promoting by 0.1%Co and 0.2%Ni,espectively. This result indicates nickel or cobalt cocatalyst oniO2 can enhance the photocatalytic activity for H2 evolutionffectively. It is interesting that TiO2 promoted by cobalt andickel cocatalysts simultaneously affords much higher H2 evolu-ion activity (2456 �mol H2) than those of 0.1%Co/TiO2 (1180 �mol2), 0.2%Ni/TiO2 (1127 �mol H2) and the mechanical mixture of.1%Co/TiO2 and 0.2%Ni/TiO2 (1282 �mol H2) in the first 6 h. Thisesult implies that the cobalt and nickel cocatalysts play remark-ble synergistic role in H2 evolution, which results in higher H2volution activity over 0.1%Co + 0.2%Ni/TiO2.

To better understand the roles of nickel and cobalt on TiO2,V–vis/DRS and PL characterizations were carried out. Fig. 5 shows

he UV–vis/DRS spectra of TiO2, 0.1%Co/TiO2, 0.2%Ni/TiO2 and.1%Co + 0.2%Ni/TiO2. A strong absorption below 400 nm for TiO2

s attributed to the absorption edge of anatase. The band gap (Eg)s determined based on diffuse reflectance measurements usingulbeka–Munk function and Tauc theory [25]. The following rela-

ional expression is used.

F(R) · h�]1/n = A(h� − Eg) (1)

Please cite this article in press as: J.-D. Lin, et al., TiO2 promoted by two dH2 evolution, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsus

here, h: Planck’s constant, �: frequency of vibration,(R) = (1 − R)2/2R, where R is the percentage reflectance, Eg:and gap, A: proportional constant, the value of the exponent n of

ig. 4. Time-profiled hydrogen evolution. (a) TiO2, (b) 0.1%Co/TiO2, (c) 0.2%Ni/TiO2,d) 0.1%Co + 0.2%Ni/TiO2 and (e) the mechanical mixture of 0.1%Co/TiO2 and.2%Ni/TiO2.

Fig. 5. UV–vis/DRS spectra of (a) TiO2, (b) 0.1%Co/TiO2, (c) 0.2%Ni/TiO2 and (d)0.1%Co + 0.2%Ni/TiO2. (Insets show Tauc plots calculated from the data).

anatase is 2. The band gap energies are calculated by plotting the[F(R)·h�]1/2 versus h� (eV). The line tangent to the plotted curveinflection point is extrapolated to [F(R)·h�]1/2 = 0 to get the bandgap energy. The band gap of TiO2 obtained based on UV–vis/DRSusing Kulbeka–Munk function and Tauc theory is 3.2 eV, whichis consistent with the existing reports [26]. The absorption edgeshifts to a longer wavelength as cobalt or (and) nickel loading.Fig. 4 shows that the absorption band edge shifted to 400, 408 and415 nm (3.10, 3.04 and 2.99 eV) for 0.1%Co/TiO2, 0.2%Ni/TiO2 and0.1%Co + 0.2%Ni/TiO2, respectively. The shift of absorption edgecan be ascribed to the extra cobalt or (and) nickel impurity energylevel within the energy band gap which causes the decreasingof band gap energy [27]. Thus TiO2 can be excited by a longerwavelength light. Compared with nickel loading, TiO2 promotedby cobalt not only possesses good response in visible light butalso has an obvious absorption in the visible light region about620 nm (2.0 eV). The absorption around 2.0 eV is characteristic fortetrahedrally coordinated Co2+ [28,29]. It also should be notedthat 0.1%Co + 0.2%Ni/TiO2 has the best light absorption perfor-mance. The UV–vis/DRS results indicate that nickel or (and) cobaltcocatalyts play the role of enhancing the photo response of TiO2.

ifferent non-noble metal cocatalysts for enhanced photocatalyticc.2014.05.008

Photoluminescence is the result of electron–hole pair recom-bination after semiconductor being irradiated, which can be usedfor investigating the rate of photogenerated electron–hole pairsrecombination. Fig. 6 shows the PL spectra of TiO2, 0.1%Co/TiO2,

Fig. 6. PL spectra of (a) TiO2, (b) 0.1%Co/TiO2, (c) 0.2%Ni/TiO2 and (d)0.1%Co + 0.2%Ni/TiO2.

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Fig. 7. Schematic illustration of the proposed mechanism for photocatalytic hydro-g

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.2%Ni/TiO2 and 0.1%Co + 0.2%Ni/TiO2. TiO2 shows an abrupt PLmission band similar to the results of previous study [30]. Ashown in Fig. 6, loading with nickel or cobalt cocatalyst decreaseshe PL intensity, indicating that the recombination of photogenera-ed electrons and holes in TiO2 can be inhibited by nickel or cobaltoading. It is also found that 0.1%Co + 0.2%Ni/TiO2 shows the lowestL intensity. This result indicates that the recombination of photo-enerated electrons and holes can be suppressed more effectivelyy co-loading nickel and cobalt than by loading either nickel orobalt alone.

Based on the above results, a mechanism of enhancementhotocatalytic H2 evolution activity over TiO2 using cobalt andickel as cocatalysts can be proposed and is shown in Fig. 7. N2dsorption–desorption results indicate loading with nickel or (and)obalt has minimal effect on the specific surface area and poretructure of photocatalyts. Therefore, the higher photo activitiesf TiO2 promoted with nickel or (and) cobalt are attributed to themproved light absorption and lower recombination rate of pho-ogenerated electrons and holes. Nickel is a well known effectiveER catalyst, which works as photogenerated electrons collector,

nhibiting the recombination of electrons and holes efficiently [13].herefore, the PL intensity of 0.2Ni/TiO2 is lower than that of TiO2nd results in its higher photocatalytic activity of H2 evolution. Sim-larly, cobalt is a well known effective OR catalyst, which workss the photogenerated holes trap, inhibiting the recombination oflectrons and holes [15]. Therefore, the PL intensity of 0.1Co/TiO2s lower than that of TiO2 and results in its higher photocatalyticctivity of H2 evolution. When TiO2 is promoted by nickel alone,he rate of OR would remain unchange, whereas the rate of HERver Ni/TiO2 would be greater than that of TiO2. Similarly, wheniO2 is promoted by cobalt alone, the rate of HER would remainnchange, whereas the rate of OR over Co/TiO2 would be greaterhan that of TiO2. In the case of the mechanical mixture of Co/TiO2nd Ni/TiO2, nickel and cobalt are deposited on different TiO2 par-icles, hence the H2 evolution activity over the mechanical mixturef Co/TiO2 and Ni/TiO2 is similar to the average H2 evolution activ-ties of Ni/TiO2 and Co/TiO2. However, when TiO2 is promoted byoth cobalt and nickel, nickel and cobalt may deposited on the sameiO2 particle, HER and OR will be enhanced more simultaneously,esulting in lower recombination rate of photogenerated electronsnd holes in TiO2. Therefore, the recombination of photogenerated

Please cite this article in press as: J.-D. Lin, et al., TiO2 promoted by two dH2 evolution, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsus

lectrons and holes can be inhibited more effectively by co-loadingickel and cobalt than by loading either nickel or cobalt alone. Theynergistic effect of two different cocatalysts was also observed inhotocatalysts containing Pt and RuO2 as cocatalysts [6,18].

[

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4. Conclusions

TiO2 promoted by cobalt and nickel cocatalysts was synthesizedby PCM. TiO2 promoted by homogenously distributed cobalt andnickel cocatalysts shows much higher photocatalytic activity for H2evolution than those of 0.1%Co/TiO2, 0.2%Ni/TiO2 and the mixtureof 0.1%Co/TiO2 and 0.2%Ni/TiO2. A synergistic effect in photocat-alytic H2 evolution was found for TiO2 promoted by cobalt andnickel cocatalysts simultaneously. This work shows that non nobleelements such as cobalt and nickel can be used as substitutes fornoble metals for enhanced photocatalytic H2 evolution. This studyalso provides a promising strategy to develop lower cost photocata-lysts for photocatalytic H2 evolution by substituting noble metalswith more abundant elements using as HER and OR cocatalysts,respectively.

Acknowledgements

This work was supported by the National Key Basic ResearchProgram of China (973) (2011CBA00508) and the National Foundfor Fostering Talents of Basic Science of China (NFFTBS) (No.J1310024).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2014.05.008.

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[Co-SBA-15 mesoporous materials by the pH-adjusting approach, Microporous

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