Appl Phys A (2011) 103: 251–256DOI 10.1007/s00339-010-6025-1

Composition dependence of AgSbO3/NaNbO3 compositeon surface photovoltaic and visible-light photocatalytic properties

Guoqiang Li · Wanling Wang · Na Yang · W.F. Zhang

Received: 20 May 2010 / Accepted: 28 July 2010 / Published online: 9 September 2010© Springer-Verlag 2010

Abstract The visible-light-active xAgSbO3/NaNbO3

(x = 0.5, 1, 2, 4, 6) composite photocatalysts were preparedby a conventional solid-state reaction method. Composi-tion dependences on the structure, optical, surface photo-electronic and photocatalytic properties were investigated.The absorption edge of the composites could be red-shiftedincreasing the amount of AgSbO3 in comparison withthat of NaNbO3. The surface photovoltage response showsthe selectivity for the amount of AgSbO3 in the samples.The photocatalytic activities for Rhodamine B degrada-tion exhibit a parabola-like behavior with the amount ofAgSbO3. The highest photocatalytic activity is observed onthe AgSbO3/NaNbO3 composite due to the better disper-siveness, electron transfer and surface photoelectric proper-ties.

1 Introduction

In quest of environmental purification, more and more at-tention has been paid to developing visible-light-activecomposite photocatalysts because of their unique prop-erties, which are unattainable by the pristine materials.In recent years, besides the TiO2-based composites suchas TiO2/CdS [1], TiO2/AgGaS2 [2], and TiO2/Fe2O3

[3], some new visible-light-active composite photocatalystshave been reported, such as Fe2O3/SnO2 [4], BiOCl/Bi2O3

[5], Co3O4/BiVO4 [6], (AgIn)xZn2(1−x)S2, (CuIn)x

G. Li · W. Wang · N. Yang · W.F. Zhang (�)Key Laboratory of Photovoltaic Materials of Henan Province,School of Physics and Electronics, Henan University, Kaifeng475004, Chinae-mail: wfzhang@henu.edu.cn

Zn2(1−x)S2, and ZnS/CuInS2/AgInS2 [7]. These previ-ous studies on composite photocatalysts reveal that mak-ing composite photocatalysts is an effective method to de-velop a photocatalyst with high photocatalytic activity undervisible-light irradiation.

AgSbO3 has been reported to be a promising visible-light sensitive photocatalyst with the narrow bandgap [8].From its energy band structure, the conduction band bottommainly consists of the hybridized Ag 5s and Sb 5s orbitals,and the valence band top is composed of the hybridizedAg 4d and O 2p. The hybridization of obitals broughtabout a continuous dispersion in a relative wide energyrange, leading to the high photocatalytic activity [8–10]. Re-cently, Jyoti Singh et al. reported ilmenite AgSbO3 showshigher photocatalytic activity for the organic compoundsdegradation in the visible-light region [11]. At the sametime, sodium niobate with the wide bandgap of 3.4 eVis one of interesting and attractive materials due to itsmany properties such as ferroelectric, piezoelectric, dielec-tric, ionic conductive, photorefractive and photocatalyticproperties [12–14]. Recently, some NaNbO3-based photo-catalysts were reported, such as NaNbO3 nanowires andcubes [15], Na0.3Ag0.7NbO3 modified by a platinum com-plex [16], (AgNbO3)1−x (NaNbO3)x solid solutions [17],NaNbO3 thin film [18], N-doped NaNbO3 [19]. These previ-ous studies motivate us to investigate the photophysical andphotocatalytic properties of AgSbO3/NaNbO3 compositeswith the different molar ratios of AgSbO3 with pyrochlorestructure to NaNbO3 with orthorhombic structure.

In this work, AgSbO3/NaNbO3 composites with the dif-ferent molar ratios of AgSbO3 to NaNbO3 were preparedby a solid-state reaction method. Composition dependenceon the structure, optical, surface photoelectronic and pho-tocatalytic properties were investigated. The surface pho-tovoltage response shows the selectivity for the amount of

252 G. Li et al.

AgSbO3 in the samples. The photocatalytic activities exhibita parabola-like behavior with the amount of AgSbO3.

2 Experimental section

2.1 Samples preparation

Pristine AgSbO3, NaNbO3 and the xAgSbO3/NaNbO3

(molar ratio: x = 0.5, 1, 2, 4, 6) composites were preparedby a conventional solid-state reaction method. First, appro-priate amounts of Ag2O (99.7%), Sb2O3 (99%), Na2CO3

(99.8%) and Nb2O5 (99%) were mixed well in mortar, andthen the mixtures were preheated at 1023 K for 4 h, thenannealed at 1173 K for 8 h in a furnace. It should be men-tioned that excess Ag2O and Na2CO3 (1%) were added inthe mixtures to compensate for the vaporization of Ag andNa elements during the annealing process. The mixture wasprepared via mixing AgSbO3 and NaNbO3 at the ratio of 1in the mortar.

2.2 Characterization

The structure was estimated with an X-ray diffractometer(DX-2500, Fangyuan, Dandong) (XRD) with Cu Kα radia-tion with λ = 0.154145 nm, scanning range from 10o to 80o

with a step of 0.08o s−1, tube voltage with 35 kV and cur-rent with 30 mA. The diffuse reflectance spectra were deter-mined with a UV–vis spectrophotometer (Varian Cary 5000)with BaSO4 as the reference standard, and the diffuse re-flectance spectra were transformed into the absorption spec-tra by the Kubelka–Munk method. Raman spectra were ob-tained on Laser-light Raman spectrograph (LRS-III). Thesurface photovoltaic property was measured with a home-built apparatus, which was made up of a Xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd. China), a light chop-per (SR540), a lock-in amplifier (SR830-DSP), a double-grating monochromator (Zolix SP500) and sample cell witha sandwich structure consisting of two pieces of ITO quartzglass electrodes. Electric field-induced surface photovoltagespectroscopy (EFISPS) is recorded by external bias appliedto the two sides of the sample on the basis of linking theelectric field-effect principle with the SPS method. The mor-phology was observed by using a scanning electron micro-scope (SEM, JSM 5600LV, JEOL Ltd.).

Photocatalytic activity was estimated via the photocat-alytic degradation of Rhodamine B (RhB) under visible-light irradiation from a 300-W Xe lamp with a cut-off filter(L-42). Typical experimental process is that 0.2 g powdersample was suspended in a 100 ml RhB solution with theinitial concentration of 2.5 mg L−1 in a glass cell, then darkreaction for 30 min to get the adsorption equilibrium be-fore light illumination. The concentration of RhB was deter-mined by measuring the absorption intensity at a wavelength

of about 554 nm. In this paper, the photocatalytic propertywas characterized with the rate constant of RhB degradation,which was calculated according to the first-order plot withinfirst one hour under visible-light irradiation.

3 Results and discussion

3.1 Crystal structure

Figure 1a exhibits the XRD patterns of AgSbO3, NaNbO3

and xAgSbO3/NaNbO3 (x = 0.5, 1, 2, 4, 6) composites.The intensities have been normalized to the strongest peakof AgSbO3 to investigate the change tendency of NaNbO3

in the composites. The AgSbO3 and NaNbO3 in the com-posite are indexed in PDF#74-0125 (pyrochlore, Fd−3m)and PDF#33-1270 (orthorhombic, Pbma), respectively. Ob-viously, AgSbO3 and NaNbO3 in the composites (x ≥ 1)separately retain their peaks and no peak shifts were found,

Fig. 1 (a) XRD patterns of the prepared samples. (b) Variation in theratio of integrated intensity of AgSbO3 to that of NaNbO3 with themolar ratio of AgSbO3 to NaNbO3

Composition dependence of AgSbO3/NaNbO3 composite on surface photovoltaic and visible-light 253

Fig. 2 Raman spectra of the prepared samples

indicating that the products are composites, but not solidsolutions. The ratio of integrated intensities of AgSbO3 tothose of NaNbO3 (IAgSbO3

/INaNbO3 ) could reflect the mo-lar ratio of AgSbO3 to NaNbO3 (MAgSbO3

/MNaNbO3 ) in thecomposites. The variation in IAgSbO3

/INaNbO3 as a func-tion of MAgSbO3

/MNaNbO3 is shown in Fig. 1b. Obviously,the IAgSbO3

/INaNbO3 increases linearly with the increasingMAgSbO3

/MNaNbO3 . In addition, some new peaks belong-ing to NaSbO3 in the composite at x = 0.5 were observed,which was possibly caused by the remains of the formedNaSbO3 during the solid-state reaction.

3.2 Raman scattering spectra

Figure 2 shows the Raman spectra of AgSbO3, NaNbO3,and the composites from 180 to 670 cm−1. When x is notless than 1, the profiles of Raman spectra of the compositesare similar to that of pristine AgSbO3; when x is less than 1,it is similar to that of pristine NaNbO3. The Raman spec-trum reflects the main compound of the sample in the micro-region or surface. The profile likeness implies that AgSbO3

possibly is covered on the surface of the xAgSbO3/NaNbO3

(x ≥ 1) composites.

3.3 Optical absorption properties

Figure 3 displays the optical absorption spectra of NaNbO3,AgSbO3 and the composites. From the picture, the absorp-tion edge of NaNbO3 is in the UV region, however, that ofAgSbO3 in visible-light region, and the bandgap of NaNbO3

and AgSbO3 are estimated to be about 3.4 and 2.3 eV, whichare in good agreement with the previous reports [17, 20].All the composites could absorb the visible light. And theabsorption band edges of the composites (x ≥ 1) exhibit ared-shift relative to that of NaNbO3 with increasing contentof AgSbO3 in the composites.

Fig. 3 UV–vis absorption spectra of the prepared samples

Fig. 4 SPS spectra of the prepared samples. The inset is the SPS spec-tra of NaNbO3 without and with the external bias

3.4 Surface photovoltage properties

The surface photovoltage (SPV) is the difference in surfacepotential before and after illumination for semiconductormaterials, which could be used to study the surface photo-electric properties in depth [21]. The inset of Fig. 4 showsthe SPS spectra of NaNbO3 without and with the externalbias. It is found that the SPV of NaNbO3 is too weak to bedetected under illumination without external bias. The ex-ternal electric field can promote the separation of the pho-toinduced charge carriers and hence affect the semiconduc-tor surface photovoltaic effect by reconstructing the distrib-ution of charge carriers [22]. Obviously, the SPS spectra ofNaNbO3 at +1.5 and +2 V are similar and appear a peak at∼360 nm, which is in accordance with the absorption bandedge of NaNbO3 in Fig. 3. The increase in photovoltagewith the rise in external bias indicates that NaNbO3 is an

254 G. Li et al.

n-type semiconductor as reported previously [23]. The re-sponse peak at ∼360 nm is possibly caused by the electrontransition from the valence band top (O 2p) to the conduc-tion band bottom (Nb 4d) under illumination, which is sim-ilar to the response peak of Nb2O5 [24].

Figure 4 displays the SPS spectra of the compositeswithout external bias in the wavelength range from 300 to550 nm. The profiles of the SPS spectra are sensitive to thecontent of AgSbO3 in the composites. When x is not lessthan 1, the SPS spectra of the composites show the simi-lar profile; while x is less than 1, the profile of SPS spec-trum is similar to that of NaNbO3. The SPS spectra reflectthe surface photoelectric properties. The selectivity of SPSresponse implies that the rich compound on the surface ofthe composites is silver antimonite for x ≥ 1 and NaNbO3

for x < 1, respectively. For the composites (x ≥ 1), the SPSspectra appear with two response peaks, which are at ∼375and ∼400 nm, respectively. The first SPS peak (at ∼375 nm)could be ascribed to the transition from O 2p to Nb 4d [25].For the second peak (at ∼400 nm), the intensity increaseswith raising the content of AgSbO3 in the composite, whichsuggests that the peak at ∼400 nm is possibly connectedwith AgSbO3 in the visible region. The band structure ofAgSbO3 indicates that the bottom of the conduction bandand the top of the valence band mainly consist of hybridiza-tion orbitals of Ag 5s and Sb 5s, Ag 4d and O 2p, respec-tively [8]. This peak, therefore, is considered to be origi-nating from electron transition from Ag 4d to Sb 5s underillumination.

The SPV intensity of pristine AgSbO3 is almost the sameas that of NaNbO3 without external bias. However, the SPVof the AgSbO3/NaNbO3 composites is enhanced signifi-cantly in comparison with these of pristine end materials ofAgSbO3 and NaNbO3, which is similar to the results ob-served in α-Fe2O3 modified by the Zn2SnO4 system [26].

3.5 Photocatalytic activities

Photocatalytic activities of the samples were evaluated fromthe degradation of RhB in a liquid medium under visible-light irradiation (λ ≥ 420 nm). In this system, when the ini-tial concentration of RhB is dilute, the reaction obeys thefirst-order kinetics mechanism [27, 28]. The reaction rate(r) can be expressed as: r = kKC, where K refers to ad-sorption equilibrium constant, k is the true rate constant, andC is the instantaneous concentration of the reactant. The truerate constants of RhB photodegradation for all the samplesare shown in Fig. 5.

Obviously, for the two end materials of NaNbO3 andAgSbO3, NaNbO3 shows a low photocatalytic activity. Al-though NaNbO3 could not absorb the visible light, the RhBdegradation could be possibly caused by the RhB-sensitized

Fig. 5 Rate constant of RhB photodegradation over the prepared sam-ples under visible-light irradiation. The inset is the variation of RhBabsorption spectra with the irradiation time over the composite (x = 1)

degradation similar to that over TiO2 under visible-light ir-radiation [29, 30]. The composites show significantly en-hanced activities compared to the end materials. With rais-ing the amount of AgSbO3, the rate constant value first in-creases, then decreases. And the composite (x = 1) exhibitsthe highest activity, which is four- and eight-fold higherthan that of AgSbO3 and NaNbO3, respectively. The insetof Fig. 5 shows the absorption spectra of RhB solution atthe range from 506 to 554 nm over the composite (x = 1).The absorbance of RhB decreases with the irradiation time,which indicates that the concentration of RhB is decreaseddue to the degradation.

The photocatalytic activity of composites could be af-fected by many factors, such as dispersiveness of the com-pounds, surface area, the electron transfer between the com-pounds, the surface photoelectric properties and so on. Fig-ure 6 displays the SEM images of the samples. The disper-siveness of the composite (x = 1) seems better than thoseof the others, which implies that the ratio of AgSbO3 toNaNbO3 could tune the crystallization procedure. And theBET surface area for the samples (x = 0.5, 1, 2, 4, 6) are 2.3,2.5, 1.9, 2.5, 2.8 m2 g−1, respectively. However, the differ-ences in the dispersiveness and BET surface area are sosmall that it could not be considered as the dominant fac-tors for the enhanced photocatalytic activity. The fact of thelower photocatalytic activity of the mixture in comparisonwith that of the composite, as shown in Fig. 5, implies thatthe electron transfer between AgSbO3 and NaNbO3 is veryimportant. The reason why the mixture shows the lowest ac-tivity is possibly related to the light scattering and absorp-tion, and photogenerated electron transfer. The better con-tact between NaNbO3 and AgSbO3 in the composite is fa-vorable for the photogenerated electron transfer. Althoughthe exact band position of NaNbO3 and AgSbO3 is not clear,we speculate that the photogenerated electron in AgSbO3

could be transferred to NaNbO3 because of the sensitiv-

Composition dependence of AgSbO3/NaNbO3 composite on surface photovoltaic and visible-light 255

Fig. 6 SEM images of the samples. (a) AgSbO3, (b) x = 0.5, (c) x = 1, (d) x = 2, (e) x = 4, (f) x = 6, (g) NaNbO3

ity of visible-light photocatalytic activity to the content ofAgSbO3. Moreover, the surface photoelectric properties ofphotocatalyst plays an important role in the photocatalyticreaction because it occurs on the surface of photocatalyst.The SPS signal of the composite (x = 1) is the smallest asshown in Fig. 4, which indicates that its difference of theband bending before and after illumination is smallest, lead-ing to the large potential drop between the bulk and surfacein the composite (x = 1), which is favorable for the electrontransfer to the surface or the reagent.

4 Conclusions

The xAgSbO3/NaNbO3 composites are prepared by a solid-state reaction method. The surface photovoltage and photo-catalytic activities are sensitive to the molar percentage ofAgSbO3 in the samples. The highest photocatalytic activ-ity is observed on the AgSbO3/NaNbO3 composite due tothe better dispersiveness, electron transfer and surface pho-toelectric properties. These results indicate that making thecomposite is an effective method of modifying the photo-physical and photochemical properties of photocatalyst.

Acknowledgements This work was supported by the Project ofCultivating Innovative Talents for Colleges & Universities of Henan

Province (2002006), the Natural Science Foundation of Departmentof Education of Henan Province (2009B48003), and the Key Tech-nologies R & D Program of Henan Province (092102210005). Partlyit is supported by Province and Ministry co-building Henan Univer-sity Funding (SBGJ090512) and Educational Commission of HenanProvince of China (2010B140002).

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