Stabilizing Metallic Cu0on the Surface of m-TiO2for Photocatalytic H2Production

LIU Juan-JuanQIAO Pei-ShengGUO Jun-FangZOU Shi-HuiXIAO Li-PingFAN Jie

(Department of Chemistry,Zhejiang University,Hangzhou,Zhejiang 310027,China)

Stabilizing Metallic Cu0on the Surface of m-TiO2for Photocatalytic H2Production

LIU Juan-JuanQIAO Pei-ShengGUO Jun-FangZOU Shi-HuiXIAO Li-Ping＊FAN Jie＊

(Department of Chemistry,Zhejiang University,Hangzhou,Zhejiang 310027,China)

Dodecanethiol was introduced as a protective agent to stabilize the in situ generated Cu0species on the surface of mesoporous TiO2(m-TiO2).The as-produced samples were characterized by XRD,XPS,HRTEM and HADDF-STEM.It is noteworthy that only Cu0species were detected in these samples.The system thus served as an excellent model to investigate Cu0-incorporated m-TiO2.Photocatalytic measurements suggested that the Cu0species could greatly enhance the photocatalytic H2-evolution activity of m-TiO2in formaldehyde solution. Moreover,we found that the activity depends on the concentration of Cu0.The maximum H2evolution rate of 725 μmol·h-1·g-1is obtained on 1.0%Cu/m-TiO2,with average Cu0particle size of(4.2±0.9)nm.It is interesting to find that in our case,the molar ratio of produced H2to CO2is 2∶1,which indicates the involvement of H2O as hydrogen source.

Cu0species;mesoporous TiO2;photocatalytic H2evolution;formaldehyde

Hydrogen(H2)has been considered as a promising fuel candidate of next generation in industries due to its high energy capacity(142 MJ·kg-1),environmental friendliness and recycling possibility[1-6].Currently,H2is mainly produced by steam reforming of fossil, which is accompanied with the emission of harmful gases(NOxor SO2)and particulate matters[7-9].From a clean-energyperspective,fabricating emission-free pathway to produce hydrogen is important for applications of“hydrogen economy”,which drive people′s attention to water splitting,especially semiconductorbased photocatalytic hydrogen production[10-12].

Over the past 40 years,a large number of semiconductors have been developed as photocatalysts to split water into H2and O2[13-20].Among them,TiO2is the most investigated due to its low cost,high chemical stability,excellent photostability and environmentalfriendly characters[21-26].However,the H2evolution efficiency of photocatalytic water splitting over bare TiO2remains quite limited because of the fast recombination of photogenerated electron/hole pairs as well as the rapid backward reaction between hydrogen and oxygen[27-29].To overcome these shortcomings,extensive efforts have been devoted to develop modification techniques of TiO2,including noble metal loading, heteroelement doping,sacrificial reagents addition, dye sensitization and so on[30-35].From a cost efficiency perspective,it is of great interest to fabricate transition metal modified TiO2with organic wastes(formaldehyde, glycerol etc.)as sacrificial reagents to produce H2[36-38].

It was shown that Cu-incorporated TiO2are efficient in photocatalytic H2production[39-41].Conventionally,the majority studies are focused on CuO or Cu2O modified TiO2.For example,Bandara et al. fabricatedahighlystableCuOdepositedTiO2photocatalyst and found that CuO could promote the charge separation and act as a water reduction site[39]. Yu and coworkers investigated the possibility of using CuO and Cu(OH)2cluster as effective co-catalyst to enhance the photocatalytic H2-production activity of TiO2[5,42].A quantum size effect of CuO cluster was observed to alter the energy levels of conduction and valence band edges in the CuO-TiO2semiconductor systems while the formation of Cu clusters was believed to facilitate the electron transfer from the conductive band(CB)of TiO2to Cu(OH)2and the reduction of H+[11].On the other hand,Wu et al.fabricated different CuOxspecies over TiO2and discovered that Cu+species could promote photocurrent generation while Cu2+species inhibits the activity[43].Considering all these aforementioned studies,the actual functions of different Cu species,especially Cu0are still unclear since Cu0is easily oxidized in air[44].This in return motivates us to develop techniques to stabilize the metastable state(Cu0and Cu+)of Cu species.

In this study,we introduced dodecanethiol(DDT) as a protective agent to stabilize the in situ generated Cu0species on the surface of m-TiO2.DDT was chosen because it could form self-assembled monolayers around Cu species but does not change Cu species chemical state[45].Catalytic measurements showed that Cu0-incoporated m-TiO2exhibited much better H2-evolution performance than bare m-TiO2.The molar ratio of produced H2and CO2was determined to be 2∶1,indicating the involvement of H2O as hydrogen source.Besides,we found that H2production activities were strongly dependent on the concentration of Cu0. The maximum H2-evolution rate of 725 μmol·h-1·g-1was obtained on 1.0%Cu/m-TiO2,with average particle size of 4.2±0.9 nm.

1 Experimental

1.1 Synthesis of m-TiO2

All the chemical reagents used in this study were of analytical grade and were used without further purification.m-TiO2was synthesized via a sol-gel process according to reported literatures with some modifications[46].In a typical synthesis,10 mmol of Ti(OBu)4,40 mmol HOAc,12 mmol HCl,and 1.6 g of F127(EO96PO70EO96,Mr=12 000)were dissolved in 30 mL of ethanol.The mixture was stirred vigorously for 1 h to obtain a clear solution and then the solution was transferred into a petri dish(i.d.=125 mm).The ethanol was evaporated at 40℃with a relative humidity of 30%～80%.After the solvent was evaporated,it was transferred into a 65℃oven and aged for 24 h.The as-synthesized mesostructured hybrids were calcined at 350℃in air for 6 h(ramp rate 2℃·min-1) to obtain m-TiO2.

1.2 CxT catalysts preparation and photocatalytic test of H2production

CxT catalysts preparation and photocatalytic H2evolution experiments were carried out at 25℃under light irradiation by a 300 W high-pressure Hg lamp. In a typical reaction system,40 mg of m-TiO2and a certain amount of CuCl2aqueous solution(0.05 mol· L-1)were mixed into 10 mL of formaldehyde solution (2%),and then the oxygen was completely eliminatedby Ar.The reaction tube was sealed in absence of air. The amount of produced H2and CO2was monitored by GC-TCD.After photocatalysis,200 μL of DDT was injected into the reaction mixture.The solid was collected after 10 min vigorous stirring by centrifugation,washed twice with water and ethanol.Then,the solid was dried in a vacuum oven at 30℃overnight. The samples were labeled as CxT,where x is the molar ratio of Cu to Ti(x=0.1,0.5,1.0,5.0,10.0).

1.3 Characterization

The small-angle X-ray scattering(SAXS)patterns were collected on a Nanostar U SAXS system(Germany) using Cu Kα radiation at 40 kV and 35 mA to determine structural quality and symmetry.Nitrogen adsorption isotherms were measured at-196℃on a Micromeritics ASAP 2020 adsorption analyzer.Wide-angle X-ray diffraction(XRD)patterns were recorded on a Rigaku UltimateⅣoperated at 40 mA and 40 kV with Cu Kα radiation(λ=0.154 178 nm)at a scan rate of 5°·min-1.XPS measurements were performed on a VG Scientific ESCALAB MarkⅡspectrometer equipped with two ultra-high vacuum(UHV)chambers.All binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon.High resolution transmission electron microscopy(HRTEM) images,HADDF-STEM and EDS measurements were recorded on a TECNAI G2 F20 operated at 200 kV.

2 Results and discussion

2.1 Characterization of materials

Fig.1a shows the SAXS pattern of the as-produced TiO2.The well-resolved diffraction peaks can be indexed to the(100)reflections of a two-dimensional hexagonal phase with an interplanar distance of 10.0 nm,indicating an ordered mesoporous structure of TiO2[47].The conclusion is further confirmed by N2sorption isotherms of TiO2which show a type-Ⅳcurve with a clear capillary condensation step(Fig.1b)[48]. The pore size of the produced TiO2is ca.4.2 nm and the surface area of TiO2is as high as 220.2 m2·g-1.In addition,the typical mesoporous structure can be seen from TEM images(Fig.1a inset)as well.

Fig.1(a)SAXS(inset:TEM)data and(b)N2adsorptiondesorption isotherms of m-TiO2

Fig.2XRD patterns of m-TiO2and CxT(x=0.1,0.5,1.0, 5.0 and 10.0)

XRDmeasurementswerecarriedoutto determine the phase structure and crystalline size of the collected samples.As can be seen from Fig.2,no characteristic diffraction peaks of Cu species were detected when the CuCl2loading content is lower than 1.0%,implying the small particle size and good dispersion of Cu species.In contrast,once the Cu content is higher than 5.0%,two sharp peaks at 43.4° and 55.6°were observed,corresponding to(111)and(200)of Cu0(JCPDS 65-9743),respectively[49-50].The crystalline sizes of Cu0particles in C5.0T and C10.0T were calculated to be ca.27.4 nm and 39.7 nm by Scherrer formula,respectively,indicating that the sizes of Cu0particles are closely related to the Cu content.On the other hand,all samples exhibited similar XRD peaks for anatase without evident shifts, implying that there was no significant change in crystalline structure of m-TiO2.And the deposited Cu0mainly attached on the surface of m-TiO2rather than incorporated into the lattice of m-TiO2.

The chemical states of Cu species in CxT were further verified by XPS measurements.As shown in Fig.3,two symmetrical peaks at ca.932.4 and 952.3 eV were observed for all samples,which could be attributed to thedominantCu02p3/2andCu02p1/2, respectively[51-52].No other peaks belonging to Cu species (Cu+or Cu2+)were detected,indicating a completely conversion from Cu2+to Cu0.Notably,in the absence of DDT,all Cu species(Cu0,Cu+and Cu2+)were detected,confirming the instability of Cu0in air. These results,on the other hand,verify the feasibility of utilizing DDT to stabilize Cu0.Besides,Ti2p peaks were in good agreement with those of Ti4+reported in literatures[53].The system can thus serve as a model system to investigate the effect of Cu0as co-catalyst of m-TiO2.

The microstructures of CxT were further investigated by STEM,HRTEM and EDX analysis.EDX analyses(Fig.4b and 4e)display that the typical brighter spots in cycles in Fig.4a and Fig.4d are Cu species,while the background circles mainly consist of TiO2support.The lattice fringe of typical Cu nanoparticle(Fig.4c)displays inter-planar spacing of 0.209 nm,which matches well with the(111)plane of Cu0species[54].STEM images shown in Fig.4a demonstrate that the Cu0nanoparticles in C1.0T catalyst are well dispersed on the m-TiO2framework.The sizes of the Cu0particles are rather small with a narrow size distribution((4.2±0.9)nm).Compared with C1.0T sample,the size of Cu0particle in C10.0T catalyst is much bigger with an average of(38.4±5.2)nm(Fig.4d and 4f).These results coincide with the XRD results, again confirming that the Cu0particle sizes are determined by the Cu content.

The conclusion is also confirmed by the UV-Vis spectra.As can be seen in Fig.5,there is no remarkable difference between m-TiO2and CxT in the UV-absorption region(i.e.300～400 nm),suggesting they have similar band structures.Nevertheless,CxT samples display broad peaks in the range of 500～800 nm with the intensities increasing along with the Cu0loading amount.These results,in line with the HRTEM data and literature results[55-57],indicate that the Cu0particle size is increased due to the increased loading amount.

2.2 Photocatalytic H2evolution from HCHO/H2O

Fig.3XPS spectra of CxT(x=0.1,0.5,1.0,5.0,10.0)

Fig.4HAADF-STEM images,EDX analysis,HR-TEM images of(a)～(c)C1.0T and(d)～(f)C10.0T

Fig.5UV-Vis spectra of the CxT catalysts

Photocatalytic activities of various samples were evaluated under UV-irradiation using formaldehyde as a sacrificial agent.We chose this reaction because it is an efficient and low-cost procedure which combines the abatement of organic wastewater pollutants with energy generation.Control experiments indicated that no appreciable hydrogen production was detected in the absence of either irradiation or photocatalyst, suggestingthat hydrogenwas producedby photocatalytic reactions on catalyst.Fig.6 shows a comparison of the photocatalytic performance on various samples.As can be seen in this figure,all of the CxT(x=0.1,0.5,1.0, 5.0,10.0)catalysts exhibit superior activities than pure m-TiO2,suggesting that Cu0could significantly enhance the photocatalytic H2-production activity of m-TiO2.Interestingly,the activities are also related to the Cu content.When the Cu content in CxT is lower than 1.0%(C0.1T,C0.5T and C1.0T),the H2-evolution rate increases along with the Cu content.It is important to highlight that with a small amount of CuCl2addition (C0.1T),the H2production is significantly improved from ca.16 μmol to 400 μmol.The highest photocatalytic H2evolution rate,725 μmol·g-1·h-1,is obtained on C1.0T,which is ca.10 times higher than that of bare m-TiO2.To further increase the CuCl2loading content to 5.0%and 10.0%(C5.0T and C10.0T),a decrease in the H2evolution rate is observed.Especially, there is a drastic decrease in the H2production on C10.0Tsample.It is interesting to find that the variation of H2-evolution rate to Cu content is similar to that of the Cu0particle size.The drastic decrease in the H2production on C10.0T is likely due to the significantly increased Cu0size,which is unfavorable for charge transfer.

Fig.6Amount of H2and CO2produced by CxT within 6 h under UV-irradiation

On the other hand,we find that for all samples except C10.0T,the molar ratio of the produced H2and CO2is ca.2∶1.Given that there are only two H atoms within a HCHO molecule,the H2/CO2molar ratio of 2∶1 indicates that H2O is involved in the reaction.The overall reaction equation is then:

HCHO+H2O→2H2+CO2

The H2/CO2molar ratio of C10.0T is less than 2:1 because certain amount of CO2is produced during the reduction of Cu2+to Cu0.As shown in Fig.7,at the beginning of reaction,more CO2is produced than H2on C10.0T.

Fig.7Time course of H2/CO2molar ratio for CxT

To better understand the catalytic mechanisms, as well as to further verify the function of Cu0as cocatalyst for m-TiO2,the anion effect with equal molar ratio of Cu species was investigated.As can be seen in Fig.8,there is only a tiny difference between the varied Cu salts for H2evolution rates,with the order of CuCl2＞Cu(Ac)2＞CuSO4＞Cu(NO3)2,suggesting that the Cu0species rather than Cl-are the main active species to enhance the activity of m-TiO2.Combining the results in Fig.6,we can safely conclude that Cu0species play a decisive role in photocatalytic H2evolution.

Fig.8H2evolution rates of m-TiO2with various Cu salts (molar ratio of Cu to Ti is fixed at 1.0%)

On the other hand,the promoting effect of Cu0is not limited to m-TiO2.As shown in Fig.9,the addition of 1.0%CuCl2salt in ZnO,WO3and P25 systems can also significantly improve the H2-evolution rate.The sequences of H2-evolution rates is P25＞ZnO＞WO3, largely in agreement with the capability of semiconductors in producing electron/hole pairs under UV irradiation.In addition,we also found that the structural variation can regulate the catalytic performance.It shows the activity on m-TiO2is much higher than that on P25(TiO2particles).This difference mainly results from the higher surface areas(50 m2·g-1vs 220 m2·g-1for P25 vs m-TiO2,respectively)and easier mass diffusion/adsorption on ordered mesoporous TiO2.

Fig.9H2evolution rates(within 6 h)of WO3,ZnO,P25 with and without 1.0%CuCl2

Based on above evidences,the mechanism of photocatalytic H2evolution in formaldehyde solution over Cu0/m-TiO2is illustrated in Scheme 1.Under UV irradiation,Cu2+species is quickly reduced to Cu0species,which is the main factor responsible for photocatalytic H2evolution improvement.The promotion effect is similar with noble metal supported on TiO2, such as Au or Pt[58-59].In current system,the electron/ hole recombination is decreased due to the presence of Cu0nanoparticles,which could capture the electrons and reduce HCHO and H2O to H2.On the other hand, the organic sacrificial molecules(formaldehyde)can combine with the holes,and then be oxidized into CO2.The reduced electron/hole recombination improves the utilization efficiency of excited electrons and thus results in an enhanced H2-evolution performance.

Scheme 1Schematic illustration for the charge transfer on CxT in photocatalytic H2evolution

Moreover,there exists a particle size effect of metallic Cu0in photocatalytic H2evolution in formaldehyde solution.For C1.0Tsamples,sub-5 nm Cu0nanoparticles are uniformly distributed on m-TiO2,which facilitates the electron transfer from m-TiO2to Cu0and subsequently HCHO and H2O can be reduced to H2. By comparison,C10.0T sample with high CuCl2content forms large Cu nanoparticles,which is unfavorable for chargetransfer.Thisissimilarwiththesizedependent photocatalytic properties of Au nanoparticles deposited on TiO2[59].The photocatalysis activity can be decreased with the growing bigger particles.

3 Conclusions

In summary,we successfully stabilized the in situ generated Cu0species by DDT.Characterizations on the samples suggested the only presence of Cu0species,which make this system a perfect model to investigate the function of Cu0as co-catalyst for m-TiO2.Photocatalytic measurements showed that Cu0could greatly improve the H2-evolution rate of m-TiO2. The molar ratio of H2/CO2on Cu0/m-TiO2is 2:1, indicating the involvement of H2O as hydrogen source. Besides,we found that the H2-evolution rate also depends on the particle size of Cu0.The sub-5 nm Cu0nanoparticles favor the charge transfer andthus improve the H2evolution.Our study clarifies the function of Cu0as co-catalyst for m-TiO2,which may provide valuable insights into a detailed understanding of the whole Cu-incorporated TiO2systems in photocatalysis.

Acknowledgements:We are grateful to Department of Chemistry and Department of Materials in Zhejiang University for XPS and TEM measurements.

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表面稳定单质铜的介孔二氧化钛的光催化产氢性能

刘娟娟乔培胜过军芳邹世辉肖丽萍＊范杰＊
(浙江大学化学系，杭州310027)

采用十二烷基硫醇作为保护剂有效地稳定住了光催化过程中介孔二氧化钛(m-TiO2)表面原位生成的Cu0物种。通过X射线衍射，X射线光电子能谱，高分辨透射电镜，高角环形暗场扫描透射电镜等手段对催化剂的组成结构进行了表征，发现催化剂中仅有Cu0物种存在。在紫外光照射下，以甲醛水溶液为牺牲试剂测试了Cu0物种对介孔二氧化钛产氢性能的影响，发现适量的Cu0纳米颗粒能够极大地提高介孔二氧化钛的产氢性能。当Cu0的物质的量分数为1.0%时，Cu0/m-TiO2表现出最高的产氢速率，为725 μmol·h-1·g-1。该样品中Cu0纳米颗粒的尺寸为(4.2±0.9)nm。此外，通过气相色谱检测到产生的H2和CO2的物质的量之比为2∶1，表明部分氢气来自于水分解。

单质铜；介孔二氧化钛；光催化产氢；甲醛

O643.3；O614.41+1；O614.121

A

1001-4861(2016)06-1063-08

2016-03-03。收修改稿日期：2016-03-26。

10.11862/CJIC.2016.143

国家自然科学基金(No.21271153，21373181，21222307，U1402233)，国家自然科学基金重大研究计划(No.91545113)资助项目。

＊通信联系人。E-mail：lpxiao@zju.edu.cn，jfan@zju.edu.cn；会员登记号：S06N4298S1406。