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Applied Surface Science 356 (2015) 1289–1299

Contents lists available at ScienceDirect

Applied Surface Science

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

old-nanoparticle-modified TiO2 nanowires for plasmon-enhancedhotocatalytic CO2 reduction with H2 under visible light irradiation

uhammad Tahir1, Beenish Tahir, Nor Aishah Saidina Amin ∗

hemical Reaction Engineering Group (CREG)/Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM,kudai, Johor Baharu, Johor, Malaysia

r t i c l e i n f o

rticle history:eceived 18 June 2015eceived in revised form 14 August 2015ccepted 26 August 2015vailable online 31 August 2015

eywords:hotocatalysis

a b s t r a c t

Gold-nanoparticles (Au-NPs) incorporated TiO2-nanowires (TiO2-NWs) of controlled sizes prepared viahydrothermal and chemical reduction method have been investigated for CO2 photoreduction with H2

under visible light irradiations. The nanocatalysts have been characterized by XRD, FESEM, TEM, N2

adsorption–desorption, XPS, UV–vis and PL spectroscopy. Highly crystalline TiO2 nanowires of meso-porous structure were obtained in Au-deposited TiO2 NWs. Au-NPs, uniformly distributed over TiO2-NWsas Au-metal state, hindered charges recombination rate and increased TiO2 activity under visible lightthrough plasmon excitation. With the deposition of Au-NPs, the efficiency of CO2 reduction to CO was

urface plasmon resonanceiO2 nanowiresold nanoparticlesO2 photoreduction2 reductant

greatly enhanced under lower energy visible light irradiations. The maximum CO and CH3OH yield rateover 0.5 wt.% Au-NPs/TiO2 NWs reached to 1237 and 12.65 �mole g catal.−1 h−1, respectively. High quan-tum yield was also observed over Au-NPs/TiO2 NWs using H2 as reductant. The significantly improvedphotoactivity was evidently due to efficient electron–hole separation and surface plasmon responseof Au-NPs. The reaction-pathway is proposed to provide insights over the mechanism of Au-plasmon-enhanced CO2 conversion.

. Introduction

Energy and environmental issues have been prevalent at thelobal stage lately. Concerted efforts are being mobilized to abateO2 concentration in the atmosphere [1,2]. The solar photocatalyticonversion of CO2 into industrially beneficial compounds offers onerospective path [3]. Among the various semiconductor materials,iO2 is the most widely studied photocatalysts for CO2 reductionpplications due to its powerful oxidation potential, low cost, non-oxic and higher chemical and thermal stability [4]. However, TiO2s only functional under ultraviolent irradiations because of the

ide band gap (3.20 eV), while fast electron–hole pairs recom-ination limited TiO2 photoactivity [5]. TiO2 nanomaterials such

s nanotubes, nanowires, and nanorods are considered as supe-ior candidates for achieving higher performances in photocatalysispplications. The nanoparticles offer direct pathways for photo-nduced electron transfer, effectively collecting photons and/or

∗ Corresponding author.E-mail addresses: bttahir@yahoo.com (M. Tahir), noraishah@cheme.utm.my

N.A.S. Amin).1 Permanent address: Department of Chemical Engineering, COMSATS Institute

f Information Technology Lahore, Punjab, Pakistan.

ttp://dx.doi.org/10.1016/j.apsusc.2015.08.231169-4332/© 2015 Elsevier B.V. All rights reserved.

© 2015 Elsevier B.V. All rights reserved.

electrons, facile charge transport along the longitudinal dimen-sions and have low electron–hole recombination rate [6,7]. TiO2nanowires and nanorods can be served not only as photocatalysts,but also as good substrates for the enhancement of photocatalyticactivity. However, these materials are only active under UV-lightirradiation.

The photoactivity of TiO2 nanomaterials under solar spectrumcan be increased by loading and dispersing metals on varioussupports [8]. Loading TiO2 with nobel metals (Au, Pt and Ag)has been widely demonstrated as efficient surface modifiers toenhance TiO2 photoactivity [9–11]. The gold nanoparticles (Au-NPs) are extensively investigated to improve the photocatalysisprocess due to the localized surface plasmon resonance (LSPR) bygold nanoparticles. The light energy can be coupled into Au-NPsowing to SPR absorption, accordingly enhanced in photocatalyticactivity of metal-semiconductor [12]. There are limited reportsto demonstrate LSPR driven CO2 reduction with H2 as reductantusing metal-nanoparticles-decorated titanium nanowires undervisible light irradiations. Liu et al. [13] reported photocatalytic CO2

reduction with H2O to CH3OH over plasmonic Ag/TiO2 nanorodsunder visible light irradiations. The higher TiO2 NRs photoactivityfor CO2 reduction was observed due to Ag-NPs SPR effect. Recently,we investigated Au-In/TiO2 nanoparticles for photocatalyticCO2 reduction with H2 under UV light irradiation in a monolith

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290 M. Tahir et al. / Applied Surfa

hotoreactor. The significantly enhanced TiO2 photoactivity wasbserved due to the presence of Au-metal [14].

Herein, we envisage gold (Au) nanoparticles deposition on TiO2anowires (NWs) could absorb visible light and improve photore-uction of CO2 with H2 to selective fuels. The TiO2 NWs hybrids,ith homogenously deposited Au-nanoparticles on the surface,ere fabricated by a hydrothermal and chemical reduction process.

he activity test was conducted for photocatalytic CO2 reductionith H2 under visible light irradiation. The influence of Au-loading

n the photocatalytic efficiency was investigated. The samples wereharacterized by XRD, FESEM, TEM, N2 adsorption–desorption, XPS,V–vis and PL spectroscopy. The reactor products were analyzednd compared to elucidate the role of Au-NPs. The reaction pathwayas proposed to provide the reaction mechanism and determine

he SPR effect of Au-NPs for CO2 reduction.

. Experimental

.1. Preparations of TiO2 nanowires

The TiO2 nanowires were prepared according to the followingteps: 1 g anatase TiO2 powder prepared by sol–gel method [15]as suspended in 8 M NaOH solution (50 mL) and then transferred

nto Teflon-lined autoclave (volume 75 mL) and heated at 110 ◦Cor 1 h under continuous stirring at 250 rpm. After being cooled tooom temperature, the samples were washed several times witheionized water until the pH reached below 9. The products werehen dispersed in 0.1 M HCl and stirred for 1 h and then washedgain with deionized water until there were no residual ions. Aftereing dried at 80 ◦C for 12 h, the products were calcined at 500 ◦Cor 5 h. The different sizes of TiO2 NWs were prepared using theame procedure, but at different times and identified as TiO2 NWs-

h, TiO2 NWs-2 h, TiO2 NWs-3 h and TiO2 NWs-4 h, respectively.he scheme for the formation of TiO2 nanowires at different timess depicted in Fig. 1. When the TiO2 powder was immersed for ahorter time (1 h) in NaOH solution, longer TiO2 NWs were pro-uced. Consequently, then the contact time between TiO2 powdernd NaOH solution was increased to 2, 3 and 4 h, shorter lengthiO2 NWs were produced.

.2. Preparations of Au-NPs/TiO2 NWs

The deposition of gold (Au) on TiO2 NWs was performed withhemical reduction method. Typically, 0.50 g of TiO2 NWs was dis-ersed in 50 mL ethylene glycol. The mixture was transferred to

two-necked flask and heated under magnetic stirring to reachemperature of 160 ◦C. A certain volume of 0.025 M AuCl2 solu-ion was injected rapidly and continued to be stirred for another

0 min before the mixture was filtered and kept at 80 ◦C for 12 h.fter drying, the samples were calcined at 500 ◦C for 5 h before thenal Au-loading TiO2 NWs were obtained. The different volumes ofuCl2 solution used corresponded to Au loading of 0.1%, 0.3%, 0.5%nd 0.7 wt.%, respectively.

8M N

TiO2 po wder

Ti(C3H7)4+ C3H7O CH3COO H

Calcinati on 500oC, 5h Tita nium

precurs or

Fig. 1. Scheme for the synthesis of TiO2 na

ence 356 (2015) 1289–1299

2.3. Structure characterization

The crystalline structure and phase transformation of sampleswere identified by X-ray powder diffraction (XRD) performed onBruker D 8 advance diffractometer (Cu-K� radiation, � = 1.54 A,operated at 40 kV and 40 mA). The morphology of the products wasinvestigated using field-emission scanning electron microscopy (FESEM) with Carl Zeiss Supra 35 VP FE-SEM instrument. The struc-tures of the samples were analyzed using transmission electronmicroscope (TEM, JEOL), operating at an accelerating voltage of200 kV. Textural characterization of the samples was carried outwith a Micromeritics ASAP 2020. BET (Brunauer–Emmett–Teller)surface area and pore diameters were determined by nitrogenadsorption examined at −196 ◦C. All the samples were degasifiedat 250 ◦C for 4 h under vacuum. The XPS measurement was per-formed using Omicron DAR 400 analyzer. The photocatalyst wasfixed to the sample holder using a carbon tape. The pass energy usedwas 20 eV while the instrument was operated at 15 kV. The sur-vey spectra were recorded in the range of 0–1400 eV. The bindingenergies were calibrated against the C 1s signal (284.60 eV) as theinternal standard. Ultraviolet–visible (UV–vis) diffuse reflectanceabsorbance spectra of the samples were determined using Agilent,Cary 100 UV-Vis spectrophotometer equipped with an integratedsphere. Initially, blank runs were conducted to correct the base line.The absorbance spectra were analyzed at ambient temperature inthe wavelength range of 200–800 nm. The band gap energies ofthe photocatalysts were determined from the extrapolation of Taucplot to the abscissa of photon energy. The charges recombinationrate was measured using photoluminescence (PL, PerkinElmer LS55 Luminescence Spectrometer).

2.4. Photocatalytic testing

The photocatalytic reduction of CO2 with H2 was carried out ina batch rectangular photocatalytic reactor with 3 cm × 4 cm × 9 cmdimensions, and a total volume of 108 cm3 as shown in Fig. 2 [16].The visible light source was a HID 35 W Xenon car head lamp. Theaverage light intensity passing through the glass window of thick-ness 10 mm was 10 mW cm−2 measured using an online opticalprocess monitor ILT OPM-1D and sensor XRD340A. Typically, 10 mgof powdered photocatalyst was distributed uniformly inside thereactor. The reactor was purged using helium (He) flow and checkedfor leakage at 2 bar pressure. Compressed CO2 and H2 were regu-lated by mass flow controllers (MFC). Next, the reactor was purgedfor some time using mixtures of CO2, H2 and helium prior to main-tain gauge pressure of 0.25 bars in all the experiments. The pressureinside the reactor was kept above the atmospheric pressure for thepurpose of gaseous products analysis. The products were analyzed

using an online gas chromatograph (GC-Agilent Technologies 6890N, USA) equipped with FID and TCD detectors (GC/FID/TCD). TheFID detector was connected with HP PLOT Q column while the TCDdetector was connected to DC-200, ucw-982, PoraPak Q and MS13X columns.

aOH, 110oC 2 h

1 h

4 h

Nanowires

Nanowi res

Nanowires

nowires with different dimensions.

M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299 1291

lytic C

3

3

ea2aNcrdtNo(

Fig. 2. Schematic of experimental setup for photocata

. Results and discussion

.1. Characterization of nanocatalysts

The XRD patterns of TiO2, TiO2 NWs and Au-NPs/TiO2 NWs arexhibited in Fig. 3. The prepared TiO2 nanoparticles were in purenatase phase with crystalline structure and 1 0 1 peak located at-theta of 25.62◦. The TiO2 NWs are also composed of anatasend crystalline phase. However, a slight shift in 1 0 1 peak of TiO2Ws (2-theta 25.34◦) could be seen compared to TiO2 nanoparti-les. Similarly, all the patterns of Au-NPs deposited over TiO2 NWsevealed anatase TiO2 phase and no rutile phase was detected. Theepositing Au-NP, the diffraction peaks of TiO2 NWs, were unal-

ered (2-theta 25.34◦). The shift in TiO2 NWs and Au-NPs/TiO2Ws peaks toward lower diffraction angles indicated fully devel-ped single crystal of anatase TiO2 (2-theta 25.356◦, JCPDS-ICSD89-4921)).

20 30 40

Au (2

Inte

nsity

(a.u

)

2-theta (

TiO2

TiO2 NW 0.1% A u/T iO 0.5% Au/TiO

TiO2(101)

Fig. 3. XRD patterns of TiO2, TiO2 NWs and

O2 reduction with H2 under visible light irradiations.

The cell parameters and cell volumes of TiO2, TiO2 NWs andAu-NPs/TiO2 NWs were calculated as listed in Table 1. Both cellparameters and cell volumes were compared with those reported inJCPDS-ICSD (89-4921) standards for anatase TiO2 i.e. a = b = 3.777 A,c = 9.501 A, and V = 135.54 A3. The TiO2 and single crystal TiO2 NWswere predominantly tetragonal structure. The cell parameters andcell volume of TiO2 NWs were much closer to anatase TiO2 stan-dards, confirming fully developed tetragonal crystal shape of TiO2NWs. The increment of TiO2 NWs crystalline size is attributed tolarger crystal structure of TiO2 NWs compared to TiO2. A furtherincrease in the crystal size with Au was probably due to incorpora-tion of Au into the TiO2 matrix.

The morphology of TiO2 NWs of different sizes and Au-NPs

deposited TiO2 NWs is depicted in Fig. 4. Fig. 4(a) and (b)revealed prepared TiO2 NWs consisted of smooth and homoge-nous nanowires with length up to several micrometers. Similarly,smooth, shorter length and homogenous nanowires could be seen

50 60 70

Au (220)00)

degree)

2 NW

2 NW

various Au-NPs/TiO2 NWs samples.

1292 M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299

Table 1Cell parameters and crystallite sizes of pure TiO2, TiO2 NWs and Au-NPs/TiO2 NWs.

Sample type Cell parameter (A) Cell volume (A3) Crystallite sizea (nm) EDX analysis (Au-content) (wt.%)

a = b c

TiO2 3.7773 9.482 134.94 19 –TiO2 NWs 3.786 9.504 136.29 34 –0.1 wt.% Au/TiO2 NWs 3.786 9.516 136.46 45 0.085

ilmiFa

NAfItrpca

FN

0.5 wt.% Au/TiO2 NWs 3.786 9.494 135.85

a Crystallite sizes calculated using Scherrer Equation.

n Fig. 4(c). It can be evident from Fig. 4(d) and (e) that Au-NPs areoaded on the TiO2 NWs and dispersed uniformly with spherical

orphology on the surface of TiO2 NWs. The size of Au-NPs var-ed from 30 to 50 nm, leading to a broad visible light absorption.ig. 4(f) highlights the EDX analysis of 0.5 wt.% Au-NPs/TiO2 NWsnd confirmed the presence of Au-metal in TiO2 NWs (Table 1).

Fig. 5 illustrates TEM and HRTEM images of a typical 0.5 wt.% Au-Ps/TiO2 NWs sample to examine the morphology TiO2 NWs andu-deposition on TiO2 NWs. TEM images in Fig. 5(a) indicate uni-

orm TiO2 NWs of mesoporous structure with diameter 14–25 nm.t can be clearly observed that Au-nanoparticles were deposited on

he surface of the TiO2 NWs. The dark spots of TEM image in Fig. 5(b)eveals that Au-nanoparticles were well dispersed in the meso-orous TiO2 NWs matrix. The HR-TEM was used to investigate therystal structure of the Au-NPs/TiO2 NWs as presented in Fig. 5(c)nd (d). The spacing between two adjacent lattice fringes of 0.35

ig. 4. SEM images of TiO2 NWs and Au-NPs/TiO2 NWs: (a) TiO2 NWs-1 h, (b) TiO2 NWs-Ws.

47 0.473

and 0.23 nm, corresponding to anatase TiO2 and Au-metal, respec-tively. The electron dispersion is illustrated in inset of Fig. 5(d),confirming anatase TiO2.

The N2 adsorption–desorption isotherms of TiO2, TiO2 NWs andAu-NPs deposited TiO2 NWs are presented in Fig. 6(a). The plot ofpure TiO2 reveals isotherms similar to type IV curve with the hys-teresis loops, conforming to mesoporous materials. The isothermsof TiO2 NWs also belong to type IV curve which presents meso-porous material. BJH pore size distribution of Au-NPs/TiO2 NWs isdepicted in inset Fig. 6(b), which confirmed mesoporous pore sizedistribution of Au-NPs/TiO2 NWs.

The BET surface area, pore volume and pore diameter of allthe samples are tabulated in Table 2. BET specific surface area(SBET) of pure TiO2 was 42 m2 g−1 increased to 72 and 71 m2 g−1 forTiO2 NWs-1 h and TiO2 NWs-2 h, respectively. The BET surface areadecreased to 59 and 47 m2 g−1 for 0.1 and 0.5 wt.% Au-NPs/TiO2

2 h, (c) TiO2 NWs-4 h, (d, e) Au-NPs/TiO2 NWs and (f) EDX patterns of Au-NPs/TiO2

M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299 1293

Fig. 5. TEM and HRTEM images of Au-NPs/TiO2 NWs at different magnification: (a, b) TEM image of Au-NPs/TiO2 NWs at different magnification and (c, d) HRTEM image ofAu-NPs/TiO2 NWs with d-spacing and electron dispersion patterns.

Fig. 6. (a) N2 adsorption–desorption isotherms of TiO2, TiO2 NWs and Au-NPs/TiO2 NWs and (b) BJH pore size distribution of Au/TiO2 NWs.

Table 2Summary of physiochemical characteristics of TiO2 and Au-NPs/TiO2 NWs samples.

Sample type BET surface area (m2 g−1) BJH adsorption pore volume (cm3/g) BJH pore diameter (nm)

TiO2 42 0.15 10TiO2 NWs-1 h 72 0.35 25TiO2 NWs-2 h 75 0.64 260.1 wt.% Au/TiO2 NWs 59 0.49 280.5 wt.% Au/TiO2 NWs 47 0.43 34

1294 M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299

455 460 465 83 84 85 86 87 88 89

528 53 0 53 2 53 4 53 6280 28 2 28 4 28 6 28 8 29 0

465.15

C 1sO 1 s

Au 4f

(d)(c)

(b)In

tens

ity (

a.u)

Inte

n sity

(a.

u)

Inte

n sity

(a.

u)In

ten s

ity (

a.u)

Bind ing Ene rgy (eV)Bind ing Ene rgy (eV)

(a)

Ti 2p459.46

87.11

84.02

533.12

530.75284.60

288.58

NWs:

NadpdNT

NTTstOtgbCcsto

Tal

Bind ing Ene rgy (eV)

Fig. 7. Plot of XPS spectra of Au-TiO2

Ws samples, respectively. The decrease in surface area can beccounted for partial surface coverage of TiO2 NWs surface by theeposited Au-NPs. Similar trends were observed in BJH adsorptionore volumes. However, in the case of pore diameter, the poreiameter of TiO2 increased from 10 nm to 25 and 26 nm for TiO2Ws-1 h and TiO2 NWs-2 h, respectively, obviously due to largeriO2 NWs diameter at different synthesis time.

The chemical states of the component element of Au-NPs/TiO2Ws were analyzed by XPS as exhibited in Fig. 7. The spectrum ofi2p is shown in Fig. 7(a) with the presence of two peaks namelyi2p 3/2 and Ti2p 1/2 observed at 459.46 and 465.15 eV, corre-ponded to Ti4+ or TiO2. The spectrum of Au4f in Fig. 7(b) confirmedhe presence of Au in metal state. Fig. 7(c) shows the spectrum of1s with BE values located at 530.55 and 533.12 eV, attributed to

he lattice oxygen O2 and possibly due to H2O or the free hydroxylroup (O H) on the surface, respectively. The C 1s peaks, with ainding energy located at ∼284.60 and 588.58 eV, correspond to

C, and C O, respectively as shown in Fig. 7(d). The presence ofarbon may be recognized as the carbon from carbon tap used forample analysis. The XPS data together with TEM images, suggestedhat Au-nanoparticles were deposited on the TiO2 NWs as similarly

bserved previously [9].

The UV–vis diffuse reflectance absorbance spectra of the TiO2,iO2 NWs and Au-NPs/TiO2 NWs are exhibited in Fig. 8(a). The lightbsorption of TiO2 NWs somewhat shifted toward shorter wave-ength compare to TiO2 because of its pure anatase phase structure

Bind ing Ene rgy (eV)

(a) Ti2p, (b) Au4f, (c) O1s and (d) C1s.

as evidenced by XRD. However, visible light absorption of TiO2nanowires remarkably increased with the deposition of Au-NPs.Au-NPs/TiO2 NWs registers a broad light absorption in the visibleregion arising from the SPR-absorption of Au-NPs, indicating thatthe metallic Au exists in the sample, as already confirmed in XPSanalysis. The visible light absorption of Au-NPs culminate to a broadresponse ranging from 450 to 700 nm, which is possibly due to widesize distribution of Au-NPs. It is obvious that Au-NPs are responsi-ble for the visible light absorption in the Au-NPs/TiO2 NWs samples.The band gap energy of the samples were calculated according toplot of (˛hv)2 vs. the energy of absorbed light as depicted in Fig. 8(b).The band gap energy of TiO2 increased from 3.12 eV to 3.17 eV inTiO2 NWs due to the pure anatase phase and larger crystallite sizeof TiO2 NWs compared to TiO2 nanoparticles. The band gap energyof Au-NPs/TiO2 NWs obtained was 3.15 eV, much closer to TiO2NWs. This reveals introducing Au-metal has no significant influ-ence on the band gap energy of TiO2 NWs. However, in contrastto pure TiO2 NWs, Au-NPs-decorated TiO2 NWs have significantlyenhanced light absorption in the visible region with a broadbandpeak located at around 580 nm arising from the SPR of Au-NPs inthe TiO2 matrix.

Fig. 9 reveals the photoluminescence (PL) spectra of pure TiO2NWs and Au-NPs/TiO2 NWs. The fluorescence peaks of Au-NPs/TiO2NWs were the same as pure TiO2 NWs and exhibit a wide and strongPL signal in the range of 450–550 nm with the excited wavelengthof 350 nm. The spectral peak located at 458 nm relates to anatase

M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299 1295

O2 NW

TttTiooe

3

iCactw

Fl

Fig. 8. (a) UV–vis absorbance spectra of TiO2, TiO2 NWs and Au-NPs/Ti

iO2, while two peaks positioned at 483 and 527 nm attributedo the transition of electrons from the oxygen vacancies with tworapped electrons and one trapped electron in TiO2 valence band.he lower peak intensities of Au-NPs/TiO2 NWs revealed the signif-cant decrease in the recombination rate of photogenerated chargesn the Au-NPs/TiO2 NWs surface. The positively charged plasmasf Au-NPs attracted electrons in the CB of TiO2, thus enhancedlectron–hole separation process.

.2. Photocatalytic CO2 reduction with H2

The photocatalytic activity of the catalyst samples was testedn a gas–solid system. The control experiments for photocatalytic

O2 reduction with H2 using different catalysts were conductedt 100 ◦C for 2 h under visible light irradiation. Carbon containingompounds were not detected in the reaction system without reac-ants or light irradiations. However, significant amount of productsas observed during CO2 reduction with H2 over photocatalyst

450 460 470 480 490 500 510 520 530 5400

100

200

300

400

500

600

700

800

900

Excitatio n= 350 nm

PL

inte

nsity

Wavelength (nm)

TiO2 NWs Au/TiO2 NWs

ig. 9. PL spectra of TiO2 NWs and Au-NPs/TiO2 NWs measured at excitation wave-ength of 350 nm.

s samples; (b) band gap energy calculation of corresponding samples.

under light irradiations, indicating the CO2 reduction requiresphotocatalysis activation. These observations proved that the CO,CH3OH and hydrocarbons originated from CO2 and not from anycontamination.

The effects of Au-NPs on the performance of TiO2 NWs forphotocatalytic CO2 reduction with H2 to CO under visible lightirradiations is illustrated in Fig. 10(a). Pure TiO2 has very low pho-toactivity but the production of CO gradually increased using TiO2NWs. This was perhaps due to efficient charge transfer, higher sur-face area and improved adsorption–desorption process due to themesoporous structure of TiO2 NWs. Loading Au-NPs into TiO2 NWshas significantly improved CO2 photoreduction and yield of COincreased with the contents of Au-NPs to a maximum of about0.5 wt.%, suggesting Au-NPs have great effect on the performance ofmesoporous TiO2 NWs. The enhanced Au-NPs/TiO2 NWs photoac-tivity was noticeably due to hindered charges recombination byAu-metal, enhanced visible light absorption and surface plasmonicresponse of Au-NPs [10].

The performance of Au-NP loaded TiO2 NWs for the produc-tion of CH4 and CH3OH during CO2 reduction with H2 is presentedin Fig. 10(b). Loading Au-NPs into TiO2 NWs has significantlyimproved CO2 photoreduction to CH4. The maximum CH4 pro-duction could be seen at optimum Au-loading of 0.5 wt.% beforegradually decreased. In addition, CH3OH production was detectedin Au-loaded TiO2 NWs samples. The Au-NPs/TiO2 NWs exhib-ited higher photoactivity for CO2 reduction, and yield of CH3OHincreased gradually by increasing the contents of Au-NPs, suggest-ing Au-NPs have great effect on the performance of TiO2 NWs.Conceivably by contacting Au-NPs with TiO2, the plasmon-excitedelectron can be injected into TiO2 and being trapped by Au-metalresulting in prolonged recombination time [17]. The energy of thetrapped electrons is enhanced via SPR effect of Au-NPs. In this way,an instant charge separation can be achieved on the gold nanopar-ticles that are able to accept electrons from the external donors forthe reduction of CO2 to CO, CH4 and CH3OH. The higher TiO2 NWsmesoporosity also enhanced the mobility of charges in Au-NPs and

TiO2 NWs heterojunction, resulting in improved productivity.

The effect of TiO2 NWs size on the reduction of CO2 to COis depicted in Fig. 11(a). Photocatalytic activity of TiO2 catalystwas enhanced using Au-NPs/TiO2 and Au/TiO2 NWs of differ-ent sizes prepared at different reaction times. Using Au-NPs/TiO2

1296 M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299

0

50

100

150

200

250

300

350

0.5% Au/TNW 0.7% Au/TNWTiO20.2% Au/TNW

Yie

ld o

f CO

(um

ole)

Au-lo adin g (wt. %)

TNW

TiO2 TNW 0.2% Au-TNW 0.5% Au-TNW 0.7% Au-TNW

0

2

4

6

8

10

12

Yie

ld o

f CH

4 an

d C

H3O

H (

umol

e)

u-lo

CH4

MeOH

(a)

(b)

F tio 1.0

NAiop2sepoCdT

A

ig. 10. Effect of Au-loading onto TiO2 NWs for CO2 reduction with H2 at CO2/H2 ra

Ws-1 h, significant amount of CO was produced compared tou/TiO2 nanoparticles. This was due to the larger surface area with

ncreased mesoporosity in TiO2 NWs, resulting in higher mobilityf charges, more light absorption and efficient mass transfer. Theroduction of CO was further enhanced over Au-NPs/TiO2 NWs-

h, but reduced in Au-NPs/TiO2 NWs-3 h and Au-NPs/TiO NWs-4 hamples. Therefore, an optimum size of TiO2 NWs is suitable for thefficient CO2 reduction to CO. On the other hand, much higher CH4

roduction was achieved using Au/TiO2 nanoparticles, but the yieldf CH4 gradually reduced over TiO2 NWs as illustrated in Fig. 11(b).onversely, Au-NPs/TiO2 NWs were more suitable for CH3OH pro-uction and its yield rate was enhanced using larger size TiO2 NWs.hese observations confirmed that Au-NPs/TiO2 NWs gave higher

adin g (wt. %)

and irradiation time 2 h; (a) CO production and (b) CH4 and CH3OH production.

performance for CO, CH4 and CH3OH production during photocat-alytic CO2 reduction with H2. The results infer that yield rates ofdifferent products are dependent on the pore structure and mor-phology of the materials.

The summary of yield rates of different products over variousTiO2 based catalysts during photocatalytic CO2 reduction with H2under visible light irradiations is summarized in Table 3. The CO2reduction products observed were CO, CH3OH, CH4, C2H4, C2H6,

C3H6 and C3H8 over different photocatalysts. The yield rate of COover Au-NPs/TiO2 NWs was 1237 �mole g catal.−1 h−1 which was29 times more than TiO2 NWs and 137 times higher than pureTiO2. It is encouraging to observe that 12.65 �mole g catal.−1 h−1 ofCH3OH was produced over Au-NPs/TiO2 NWs. However, no CH3OH

M. Tahir et al. / Applied Surface Science 356 (2015) 1289–1299 1297

0.5% Au-TiO2 0.5% Au-TNW-1h 0.5% Au-TNW-2h 0.5% Au-TNW-3h 0.5% Au-TNW-4h0

50

100

150

200

250

300

350

400

Yie

ld o

f CO

(um

ole)

Effect of time for TNWs preparation

0.5% Au-TiO2 0.5% Au-TNW-1h 0.5% Au -TNW-2h 0.5% Au -TNW-3h 0.5% Au -TNW-4h

0

2

4

6

8

10

12

14

16

18

20

Yie

ld o

f CH

4 and

CH

3OH

(um

ole)

of T

CH4

CH3OH

(a)

(b)

F ratio o

yANArmN

TS

Efefct

ig. 11. Effect of TiO2 NWs sizes on photocatalytic CO2 reduction with H2 at CO2/H2

ield was detected in pure TiO2 and TiO2 NWs. These revealedu-NPs were more favorable for the production of CH3OH in TiO2

Ws. The yield rate of CH4 production was higher in the order ofu-NPs/TiO2 NWs > TiO2-NWs > TiO2. The much higher productionate over Au-NPs/TiO2 NWs was obviously due to higher electronsobility with hindered recombination rate and SPR effect of Au-Ps loaded into mesoporous TiO2 NWs matrix.

able 3ummary of yield rates and quantum yields over TiO2, TiO2 NW and Au-NPs/TiO2 NWs-ca

Catalysts Production rate (�mole g catal.−1 h−1)a

CO CH4 C2H4 C2H6

TiO2 9 3 0.40 –

TiO2 NWs 43 9 0.57 1.51

0.5 wt.% Au-NPs/TiO2 NWs 1237 13 1.36 8.25

a Yield rates calculated at 100 ◦C, CO2/H2 ratio 1.0, irradiation time 4 h and pressure gab Quantum yield calculated based on mole/s of product by mole/s of photon entering in

NWs Sizes

f 1.0 and irradiation time 2 h: (a) CO production and (b) CH4 and CH3OH production.

The performance of Au-NPs/TiO2 NWs for CO2 reductionwith H2 is compared with the reported values in the literature.

Kong et al. [9] described photocatalytic CO2 reduction with H2Ovapors to CH4 over Ag-nanoparticles-decorated TiO2 nanorods.Deposition with Ag nanoparticles was observed to enhance thephotocatalytic up to five times with respect to undecorated TiO2nanorods with peak production of 2.64 �mole g catal−1 h−1. Au/Pt

talysts.

Quantum yield (%)b

C3H6 C3H8 CH3OH CO CH4 CH3OH

– – – 0.003 0.005 –0.50 0.75 – 0.010 0.008 –7.50 6.00 12.65 0.283 0.012 0.010

uge 0.25 bars.to the reactor.

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anoparticles modified TiO2 nanofibers were investigated by Zhangt al. [10] for plasmon-enhanced CO2 photoreduction with H2Oapors. Over Au/TiO2, yield rate of H2, CH4 and CO observedere 0.39, 0.31 and 0.20 �mole h−1, respectively. The production

ate increased due to hindered charges recombination and SPRffect in Au/Pt-nanoparticles. Similarly, CH4 production rate of.91 ppm g catal.−1 h−1 was achieved during CO2 reduction with2O over Cu-NPs-modified TiO2 nanorods. The enhanced TiO2anorods photoactivity was due to plasmonic effect of Cu-NPs [18].

n another study, CH3OH production of 1.8 �mole cm−2 h−1 waseported during CO2 reduction over Cu-modified TiO2 nano-flower19]. By comparing these results with the current study, it is evi-ent that CO2 reduction to CO, CH3OH and CH4 was significantlynhanced using mesoporous Au-NPs/TiO2 NWs with H2 as reduc-ant.

The performance of Au-NPs/TiO2 NWs for CO2 photoreductionith H2 was further evaluated by calculating their quantum effi-

iencies. If one electron–hole pair to be generated by one photonctivation, then the quantum efficiency of CO2 photoreduction cane defined as the ratio of production rate (mole/s) with photonicux (mole/s) as described in Eq. (1) [14]:

uantum Yield(QY, %)

= n × moles of production rate (mol/s)moles of photon flux (mol/s)

× 100 (1)

here n is the number of electrons needed for a specific prod-cts. Two, six and eight electrons are needed to transform CO2

nto CO, CH3OH and CH4, respectively. The moles of photon coulde calculated from the visible-light input energy of maximum

ight intensity 10 mW cm−2 at wavelength 450 nm. The quantumfficiency of CO2 reduction over various types of catalysts andhotoreactor systems is summarized in Table 3. The quantumfficiencies of CO, CH4 and CH3OH production over 0.5% Au-

Ps/TiO2 NWs were 0.283%, 0.012% and 0.010%, respectively. Byomparing the performance of Au-NPs/TiO2 NWs, the efficiency ofu-NPs/TiO2 NWs was 28.3 times higher than TiO2 NWs and 83 foldigher than TiO2 nanoparticles. The quantum efficiency for CH3OHroduction was 0.01%, which was larger than the many reported

ig. 12. Schematic illustration of the proposed mechanism for photocatalytic CO2 reductrradiation.

ence 356 (2015) 1289–1299

values in the literature. The significantly enhanced performance ofAu-NPs/TiO2 NWs is attributed to the absorption of visible lightowing to plasmon-excitation and SPR effect. Therefore, the visiblelight CO2 reduction is conceivably due to SPR excitation of goldnanoparticles. Similar observations are reported in literature forwater splitting to produce H2 over Au-doped TiO2 nanoparticles[12].

3.3. Photocatalytic mechanism over the Au-NPs/TiO2 NWs

Based on the above discussion, Au-NPs play crucial rule in photo-catalytic CO2 reduction with H2. The photoactivity of TiO2 and TiO2NWs under visible light irradiation may be due to the extendedabsorption of light in the visible region caused by the excitation ofvalence electrons into defects of impurity states located below theTiO2 conductance band. However, significantly enhanced photoac-tivity in Au-NPs/TiO2 NWs revealed plasmon-excitation of Au-NPsenhanced visible light absorption through strong localized elec-tric field and improves charge separation in TiO2 NWs [12]. Theplasmon-excited electrons with sufficient energy can transfer fromAu-NPs to the contacting TiO2 to reduce CO2 to CO and other prod-ucts. The possible reaction mechanims is illustrated in Eqs. (2)–(9)to provide insights about the reaction process [13]:

TNWs + hvAu/TNWs−→ h+

vb + e−cb (2)

e−cb → e−

Au (3)

e−Au + hv(visible) → e−

SPR (4)

H2 + h+vb → H+ + H+ (5)

CO2 + 2H+ + 2e−SPR → CO + H2O (6)

CO2 + 6H+ + 6e−SPR → CH3OH + H2O (7)

CO2 + 8H+ + 8e−SPR → CH4 + 2H2O (8)

nCO2 + nH+ + ne−SPR → C2H4 + C2H6 + C3H6 + C3H8 + nH2O (9)

First, under visible light irradiations, the SPR effect of Au-NPson the TiO2 surface promoted electron to conductance band ofTiO2 semiconductor for the e−/h+ excitation as explained in Eq. (2).

ion with H2 to CO, CH4 and CH3OH over Au-NPs/TiO2 nanowires under visible light

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ext, photogenerated TiO2 CB electrons (e−cb) are trapped by Au-

Ps (Eq. (3)), and enhanced charges separation. The energy of therapped electrons is enhanced via SPR effect of Au-NPs (Eq. (4)) forO2 species to react effectively [19]. The active valance band holesh+

vb) in TiO2 NWs and/or Au can oxidize H2 to produce H+ (Eq. (5)).hen, H+ and active electrons are able to reduce CO2 to CO, CH3OHnd hydrocarbons as explained in Eqs. (6)–(9). Therefore, photo-atalytic activity enhancement of Au-NPs-decorated TiO2 NWs cane explained based on the charge trapping ability and transfer ofu-NPs and visible light harvesting ability of Au-NPs/TiO2 NWs cat-lysts. The schematic of the reaction mechanism is presented inig. 12. The higher photoactivity of mesoporous TiO2 NWs for CO2eduction to CO was likely due to more defects or impurity states iniO2 NWs causing slight absorption in visible region. However, theeak visible light absorption of TiO2 NWs can be promoted by theu SPR localized electric field and improves the charge separationrocess on the adjacent TiO2 surface. Thus, Au-NPs-decorated TiO2Ws exhibits much higher photocatalytic activity for CO produc-

ion than pure TiO2 NWs and TiO2 samples.

. Conclusions

In this work, Au-NPs/TiO2 NWs were successfully prepared by aacile hydrothermal and a chemical reduction route. The characteri-ation results revealed higher crystalinity in mesoporous TiO2 NWsf anatase phase with uniform distribution of Au-NPs. The BET sur-ace area of TiO2 was greatly increased in TiO2 NWs but reduced inu-loaded TiO2 NWs. Gold, deposited over TiO2 NWs as Au-metal,indered charges recombination rate and exhibited remarkable vis-

ble light activities for photocatalytic CO2 reduction with H2 toO, CH3OH and hydrocarbons. Au-NPs deposited over TiO2 NWsavored CH3OH and its yield increased with Au-concentrations.he highest photocatalytic activity of the Au-NPs/TiO2 NWs cat-lyst for CO2 reduction could be obtained at 0.5 wt.% Au-loading.he improved photocatalytic activity of Au-loaded TiO2 NWs isscribed to the charger transfer property and surface plasmon-xcitation of Au-NPs. The Au-modified plasmonic photocatalystsay bring new insight for photocatalytic CO2 reduction into useful

ompounds.

cknowledgements

The authors would like to extend their deepest appreciationo the Ministry of Education (MOE), Malaysia for NanoMITE LRGSLong Term Research Grant Scheme, Vot 4L839) and Universitieknologi Malaysia for the RUG (Research University Grant, Vot2G14), and FRGS (Fundamental Research Grant Scheme).

[

ence 356 (2015) 1289–1299 1299

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