with wide-range visible-light harvesting based on localized

Photocatalysis DOI:10.1002/anie.201300239 Gold-Nanorod-Photosensitized Titanium Dioxide with Wide-Range Visible-Light Harvesting Based on Localized Surface Plasmon Resonance**

Lequan Liu,Shuxin Ouyang,and Jinhua Ye*

In the quest to solve environmental remediation and solar

energy conversion issues,photocatalysis using sunlight have

been attracting tremendous attention.[1]As a green chemistry

technology,photocatalysis is an ambient temperature process

that can completely decompose organic pollutants even at low

levels.[2]Since the discovery of its photocatalytic activity

under UV light,[3]TiO2has become the most extensively

studied semiconductor in applications such as environmental

cleaning and hydrogen energy.[4]Nevertheless,TiO2possesses

a wide band gap which limits its photo-absorption to only the

UV region,accounting for about4%of the total sunlight.

From the perspective of both chemistry and practical

applications,it is undoubtedly important to develop photo-

catalytic materials that harvest a wide range of visible

photons.Many strategies,including metal-ion[5]and nonmetal

doping,[6]have been proposed to extend the absorption of

TiO2to visible spectrum.However,to date,the doped

materials typically suffer from thermal instability,photo-

corrosion,and fast eà/h+recombination rates.

The recent and rapid development of localized surface

plasmon resonance(LSPR)photosensitization has offered

a new opportunity to overcome the limited efficiency of

photocatalysts.[7]That is,semiconductors loaded with coinage

metals,such as Au,Ag and Cu,exhibited visible-light activity

based on LSPR,which has been observed in several photo-

chemical processes.[8]These enhancement effects might be

caused by the charge transfer from photoexcited metal to the

semiconductor and/or LSPR-induced electromagnetic fields

in the vicinity of the plasmonic nanostructure.[7b]From this

perspective,in any case,not only improved absorption of

photons by LSPR of metal nanoparticles but also a closed

interface between metal and semiconductor is important to

enhance visible-light activity in plasmonic composite photo-

catalysts.The presence of an obstacle on the interface,such as

surfactant,will hinder the direct charge transfer or dramat-

ically decrease the intensity of LSPR-induced electromag-

netic field close to semiconductor,as this field decays

exponentially with distance.[9]Calcination is typically the

most simple way to remove surfactants,but it could not be

easily applied to metal nanoparticles with specific morphol-

ogy,such as rods,wires,or cubes,which cannot survive in high

temperatures.Some efforts were made to coat those nano-

structures with semiconductors.[10]Nevertheless,active sites

are also masked in this process while surfactant still adheres

onto metal nanostructures.[11]On the other hand,among

plasmonic composite photocatalysts,tremendous efforts were

focused on harvesting visible light around characteristic

adsorption of coinage metal nanoparticles(Ag:ca.410nm,

Au:ca.520nm),[7b]while little effort was put into harvesting

longer-wavelength visible light which accounts for a larger

proportion of solar energy.[12]We report herein a wide-range

visible light harvesting of TiO2by introducing Au nanorods

(NRs)as antennas,while surfactant removal is achieved by an

HClO4oxidative method without any noticeable change in

Au NR morphology.It was found that not only transversal

plasma which is similar to sphere Au particles,but also

longitudinal plasma of Au NRs(with adsorption centered

from630nm to810nm)could induce photocatalytic oxida-

tion of2-propanol(IPA).

Five Au NR samples with different aspect ratios(defined

as length pided by width of the nanorod)were synthesized

by seed-mediated synthetic route.[13]The plasmon resonance

absorption splits into two modes corresponding to the

oscillation of the free electrons along and perpendicular to

the long axis of the rods.The transverse mode shows

a resonance at about520nm,while the resonance of the

longitudinal mode is red-shifted and strongly depends on

aspect ratio of the nanorod(Supporting Information,Fig-

ure S1).[14]This is caused by charge accumulation difference

along the rod axis(longitudinal plasma)and along a perpen-

dicular direction(transversal plasma),and the charge accu-

mulation will be maximum for the latter.As the restoring

force is proportional to this charge accumulation,smaller

forces and consequently smaller resonance frequencies are

required for exciting longitudinal plasmon resonance.As can

be seen from the TEM images(Supporting Information,

Figure S2),Au NRs in five samples possess similar longitudi-

nal length(ca.40nm),while different aspect ratios(ca.1.8,

2.3,2.7,

3.3,

4.0)were obtained with the decrease of transverse

length.A lattice space measured as0.204nm,corresponding

to Au(200)lattice plane,was easily discerned in HRTEM

[*]Dr.L.Q.Liu,Dr.S.X.Ouyang,Prof.J.H.Ye

International Center for Materials Nanoarchitectonics(WPI-MANA)

and Environmental Remediation Materials Unit

National Institute for Materials Science(NIMS)

1-1Namiki,Tsukuba,Ibaraki305-0044(Japan)

E-mail:jinhua.ye@nims.go.jp

Prof.J.H.Ye

TU-NIMS Joint Research Center

School of Material Science and Engineering

Tianjin University(P.R.China)

[**]This work received financial support from the World Premier

International Research Center Initiative(WPI Initiative)on Materials

Nanoarchitectonics,and MEXT(Japan).

Supporting information for this article is available on the WWW

under 5f9d96c652d380eb63946dc9/10.1002/anie.201300239.

6689 Angew.Chem.Int.Ed.2013,52,6689–6693 2013Wiley-VCH Verlag GmbH&Co.KGaA,Weinheim

image,indicating good crystallinity of Au NRs.Au NR-loaded TiO 2photocatalysts were prepared by impregnating colloidal gold nanorods with TiO 2(5m m in diameter).Calcination is not suitable in this case to remove bilayer surfactant adhere onto Au NRs as the morphology of colloidal gold nanorod will be destroyed by heating before the release of the inner layer surfactant.[15]Here,an oxidative method with HClO 4is developed to remove this bilayer surfactant (see catalyst preparation in the Supporting Information).An Au NR-loaded TiO 2photocatalyst with longitudinal plasmon reso-nance adsorption peak centered at 660nm was prepared through the route mentioned above,denoted as Au NR/TiO 2-660,with the purpose of studying surfactant removal and morphology of Au NRs on TiO 2.The surfactant removal over Au NR/TiO 2-660is verified by differential thermal analysis (DTA)spectra (Figure 1).Over as-prepared Au NR/TiO 2,

there is an obvious exothermic peak centered at 2208C,corresponding to surfactant combustion with oxygen.This exothermic peak disappeared after the catalyst was treated with HClO 4,indicating at least most surfactant has been successfully removed.A slow endothermic peak tailed to higher temperature over Au NR/TiO 2-660might be caused by desorption of adsorbents,which is similar to that of TiO 2.The morphology maintenance of Au NRs during HClO 4treatment was studied by HAADF-STEM,HRTEM and diffusion reflectance spectrophotometer.As can be seen from Fig-ure 2a and b,well dispersed gold nanorods (bright rods)can be discerned from the background.This assignment is confirmed by STEM-elemental mapping (Supporting Infor-mation,Figure S3).In HRTEM image,Figure 2c,Au NRs with an aspect ratio of 2.3as well as TiO 2crystals can be easily discerned.As compared with TEM and STEM images before HClO 4treatment (Supporting Information,Figure S4),there is no significant change in morphology of Au NRs.This is in accordance with adsorption position maintenance in normal-ized diffusion reflectance spectra except for slight broadening (Supporting Information,Figure S5).More importantly,as shown in Figure 2d,a close contact is formed at the interface of Au NRs and TiO 2,which would facilitate the electron or

energy transfer in this composite photocatalyst.These studies indicate that HClO 4oxidative method is an efficient method to remove surfactant over Au NRs without noticeably destroying its morphology.Five TiO 2-supported Au NR catalysts were prepared by loading Au NRs with different aspect ratios,and the surfactant was removed by the HClO 4oxidative method.By adjusting the aspect ratio of Au NRs,longitudinal plasmon resonance adsorptions are tuned from 630nm to 810nm (Figure 2e).The corresponding catalysts are denoted as Au NR/TiO 2-630,Au NR/TiO 2-660,Au NR/TiO 2-710,Au NR/TiO 2-760,and Au NR/TiO 2-810.It should be pointed out that there is an overlap between transversal and longitudinal plasmon resonance adsorption in Au NR/TiO 2-630,which is mainly caused by the relative low aspect ratio (ca. 1.8)and slight transformation of nanorod to sphere during preparation procedure.[16]From XRD patterns (Sup-porting Information,Figure S6),Au (200),(220),and (311)diffraction planes can also be recognized along with charac-teristic anatase and rutile TiO 2diffraction peaks,which agrees well with TEM results.Photocatalytic oxidation of IPA

served

Figure 1.DTA curves of a)TiO 2and b,c)Au NR/TiO 2-660before (b)and after (c)HClO 4oxidative

treatment.

Figure 2.a,b)HAADF-STEM images and c,d)HRTEM images of the Au NR/TiO 2-660photocatalyst;e)UV/Vis absorption spectra of Au NR/TiO 2photocatalysts.

.Angewandte Communications

5f9d96c652d380eb63946dc9

2013Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim

Angew.Chem.Int.Ed.2013,52,6689–6693

as a model reaction to evaluate the wavelength dependence

photocatalytic activity over those photocatalysts.To identify

the activity derived from transversal or longitudinal plasma of

Au NRs,the reactions were carried out under different

broadband light irradiation.That is,broadband light I:visible

light cover transversal plasmon resonance of Au NRs only (ca.

400

broadband light II:visible light cover both transversal and

longitudinal plasmon resonance of Au NRs (ca.400

or 910nm;Supporting Information,Figure S7b).Details on

catalyst preparation,characterization,and photocatalytic

evaluation can be found in the Supporting Information.

Photocatalytic activity evaluation results under different

reaction conditions are given in Table 1and the Supporting

Information,Table S1.Taking Au NR/TiO 2-660and Au NR/

TiO 2-710for example,12.0m mol acetone and 0.61m mol CO 2

were obtained over Au NR/TiO 2-660under broadband light I

irradiation for 10h (Figure 3a).In contrast,only 0.12m mol

acetone and 0.08m mol CO 2were obtained under dark

reaction carried out at 308C,which primarily indicated

a visible-light photocatalytic activity was achieved by intro-

ducing Au onto TiO 2,as reported earlier.[17]Furthermore,this

activity is enhanced by 2.8times in terms of acetone

production under broadband lightI irradiation as compared

with that of Au NR/TiO 2without HClO 4oxidative treatment

(Table 1,Supporting Information,Figure S8),suggesting the

removal of surfactant facilitate charge carriers or energy

transfer in plasmonic composite photocatalysts.Dramatic

activity increase was observed after increasing wavelength of

light to cover not only transversal but also longitudinal

plasmon resonance of Au NRs (broadband lightII);that is,

75.0m mol acetone and 19.7m mol CO 2were obtained.The enhancement factor (EF),calculated as the ratio of the IPA photooxidation rate R II (under broadband light II irradiation)

to R I (under broadband light I irradiation),is as high as 6.6.This activity enhancement can be (at least roughly)ascribed to driving effect of longitudinal plasma of Au NRs.To further

confirm the activity inspired from longitudinal plasma of

Au NRs,plasmon-induced photooxidation was carried out under monochromic light irradiation at l =660nm (Figure 3a inset).1.1m mol acetone and 0.15m mol CO 2were obtained,which are about 9and 2times,respectively,as many as that obtained under dark reaction.On the other hand,the IPA photooxidation rate over Au NR/TiO 2-660under UV irradi-ation is about 1.8times as high as that of TiO 2(Supporting Information,Figure S9),which might be mainly caused by co-catalyst effect of the surface Au NRs.As for Au NR/TiO 2-710,7.3m mol acetone and 0.86m mol CO 2were obtained by irradiation under broadband lightI for 10h (Figure 3b).The photocatalytic activity induced by

longitudinal plasma of Au NRs

could also be clarified by compar-ing with the photocatalytic activ-

ity induced not only transversal

but also longitudinal plasma of

Au NRs;that is,52.3m mol ace-

tone and 4.7m mol CO

2under broadband light II irradiation for 10h with an EF of 7.1.Moreover,this activity is also higher than that of Au nanoparticles loaded

TiO 2(13.4m mol acetone and

1.7m mol CO 2;Supporting Infor-mation,Figure S10),further sug-gesting longitudinal plasma of Table 1:Photocatalytic oxidation of 2-propanol (IPA)over Au NR-TiO 2catalysts.Catalyst Au Broadband I (400

1.79.50.4656.510.2 1.40.07Au NR-TiO 2-660 1.51

2.00.6175.019.7 1.10.15Au NR-TiO 2-710 2.27.30.8652.3 4.7 1.00.11Au NR-TiO 2-760 2.618.30.4562.526.5 1.20.12Au NR-TiO 2-810 2.125.00.5575.810.2 1.60.23Au NR-TiO 2-660[d] 1.5 4.20.2216.40.53––[a]Achieved by combining L42+CM500S filters.[b]Achieved by combining L42+R810or R900filters.[c]Photoreaction for 10h,and the amounts of acetone and CO 2have been subtracted with the corresponding values under dark reaction.[d]Au NR-TiO 2-660before HClO 4treatment.Units for acetone and CO 2[m

mol].Figure 3.Curves of acetone (filled symbols)and CO 2evolution (open symbols)in potocatalytic oxidation of IPA over Au NR/TiO2-660(a)and over Au NR/TiO 2-710(b)under broadband light I (*,*)and II

(&,&)irradiation as a function of reaction time.Inset:reaction carried

out under monochromic light irradiation for 10h (gray:a)660,b)710nm);shaded:dark reaction).

6691Angew.Chem.Int.Ed.2013,52,6689–6693 2013Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim 5f9d96c652d380eb63946dc9

Au NRs are involved in photocatalytic oxidation of IPA. Though with higher gold loading,the activity over Au NR/ TiO2-710is a little inferior to that of Au NR/TiO2-660, indicating the photocalytic activity of Au NR/TiO2is not directly related to Au content.The ability to drive photo-catalytic IPA oxidation by longitudinal plasma of Au NRs is also confirmed by reaction carried out under monochromic light irradiation at l=710nm(Figure3b inset).Additionally, the catalyst after being irradiated under broadband light II for 10h was characterized by UV/Vis diffusion reflectance spectrophotometer(Supporting Information,Figure S11). There is no significant change in the position of both transversal and longitudinal plasmon resonance adsorption, while the slight intensity increase of transversal plasmon resonance adsorption might be caused by transformation of trace gold nanorods to spheres through surface diffusion.[16b] Over the other three catalysts,Au NR/TiO2-630,Au NR/ TiO2-760,and Au NR/TiO2-810,photocatalytic activities induced by longitudinal plasma of Au NRs are also identified by carrying out the reaction under irradiation of broadband light I and II,respectively(Supporting Information,Figur-es S12–14).Dramatic activity increases were achieved when irradiation covers both transversal and longitudinal plasmon resonance of Au NRs with EFs in the range of3.1–6.2.It is worthy to point out that the enhancement effects decrease for Au NR/TiO2-760and Au NR/TiO2-810catalysts,which should be caused by the obvious light intensity decrease of Xe lamp in this near infrared region though broadband II were also extended in these two cases.[18]This result,in turn,indicates that this photocatalytic process,even under near-infrared light irradiation,is derived by longitudinal plasma of Au NRs. This assumption is further evidenced by reaction carried out under monochromc light irradiation.These results indicate that gold nanorods are the species responsible for light absorption and trigger the photochemical reactions.A comparison study and wavelength dependence activity give clear evidence that the IPA photooxidation can be driven by longitudinal plasma of Au NRs,which largely extends the light harvesting from about520nm of spherical Au particles to about810nm.Furthermore,Au NR/TiO2was subjected to photodegradation of aldehyde(Supporting Information,Fig-ure S15).Aldehyde can also be photooxidized to CO2with long-wavelength visible-light irradiation,suggesting these Au NRs photosensitized catalysts seems to have a good universality.

Two main mechanisms have been proposed to explain the enhancement in photocatalytic activity in those composite systems compared to pure TiO2.One is electron injection from Au to TiO2and the other is energy transfer from excited Au LSPR to TiO2.[19]High intensity of LSPR is manifested as amplification effect for electromagnetic field in the vicinity of Au NRs as observed in Raman scattering[20](Supporting Information,Figure S16).Meanwhile,it is suggested that the intensity of plasmon resonance of Au NRs is enhanced compared with that of sphere Au nanoparticles,[21]which agrees with the large activity enhancement induced by longitudinal plasma of Au NRs as compared with that of transversal plasma only.At this stage,charge carriers in TiO2 with sufficient redox potentials excited by photons from

Au NR LSPR decaying are proposed to account for the visible-light activities achieved over Au NR/TiO2in photo-catalytic IPA oxidation,while the heating effect can be excluded in this case as it is characteristic of small particles.[22] Despite this analysis,the underlying physical mechanism is still obscure.Therefore,further study on this issue is required, which is import in developing effective plasmonic composite phototatalysts.

In summary,broadband visible and even near-infrared light harvesting over TiO2has been successfully achieved by introducing gold nanorods as antennas,while surfactant adhered onto the Au NR surface was successfully removed by an HClO4oxidative method without noticeably changing its morphology.Photocatalytic activities induced by not only transversal but also longitudinal plasma of gold nanorods were confirmed,and the broadband visible-light activities are speculated to be caused by LSPR of Au NRs.Moreover,the adjustable light adsorption resulting from the tunability of gold nanorod aspect ratio would benefit to designing photo-catalyst with specific light harvesting.The strategy of introducing gold nanorods and surfactant removing by HClO4oxidative method reported here might open an avenue to develop broadband visible-light sensitive photo-catalysts.

Received:January11,2013

Revised:April8,2013

Published online:May10,2013

.Keywords:gold·photocatalysis·surface plasmon resonance·titania·visible light

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