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Solid State Sciences 46 (2015) 37e42

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Solid State Sciences

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Hydrothermal synthesis, nonlinear optical property andphotocatalytic activity of a non-centrosymmetric AgIO3 photocatalystunder UV and visible light irradiation

Hongwei Huang a, *, Ying He a, Yuxi Guo a, Ran He b, Zheshuai Lin b, Yihe Zhang a, *

a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of MaterialsScience and Technology, China University of Geosciences, Beijing 100083, Chinab Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics andChemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

Article history:Received 16 December 2014Received in revised form19 May 2015Accepted 23 May 2015Available online 27 May 2015

Keywords:AgIO3

Photocatalytic activityNon-centrosymmetric structureElectronic structureElectric field

* Corresponding authors.E-mail addresses: hhw@cugb.edu.cn (H. Huang), z

http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.01293-2558/© 2015 Elsevier Masson SAS. All rights res

a b s t r a c t

AgIO3 as a novel photocatalyst was prepared via a facile hydrothermal route. The microstructure, elec-tronic structure, optical and nonlinear optical (NLO) properties of AgIO3 were investigated by a series ofexperimental and theoretical methods, including X-ray powder diffraction (XRD), scanning electronmicroscope (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), BrunauereEmmetteTeller (BET), UVevis diffuse reflectance spectra (DRS), second harmonic generation (SHG)measurements and the first principle calculation. The results revealed that AgIO3 exhibits a strong SHGresponse and excellent photocatalytic performance with high stability under both UV and visible lightirradiation. The advantages of this material, such as large polarizability resulted from the NCS structure,polar IO3

� anion and layered structure should be responsible for the high photocatalytic activity of AgIO3.The present work may shed new light on the design of multifunctional materials.

© 2015 Elsevier Masson SAS. All rights reserved.

1. Introduction

Photocatalysis has aroused extensive interest for their prom-ising applications in environmental remediation and energy gen-eration [1e3]. Though traditional UV light photocatalysts showpotentials, numerous efforts were made to develop visible-light-driven (VLD) photocatalysts [4e9]. As separation efficiency ofphotogenerated charge carriers is crucial to photocatalytic perfor-mance, it is highly desirable that a strong driving force for thecharge separation is present throughout the entire photocatalyst[10]. Lately, it has been reported that nonlinear optical (NLO) ma-terials, displaying a second harmonic generation (SHG), can serveas efficient photocatalysts, like K3B6O10Br [11]. The non-centrosymmetric (NCS) structure can give rise to an intrinsiclarge polarization effect, which promotes the efficient separation ofphotogenerated electronehole pairs, thus resulting in a high pho-tocatalytic activity.

Recently, metal iodates have been extensively studied due to

yh@cugb.edu.cn (Y. Zhang).

08erved.

the IO3 group with a lone pair of electrons, favoring the forma-tion of asymmetry structure and polarity. These iodate photo-catalysts, including Y(IO3)3 [12], Ln(IO3)3 (Ln ¼ Ce, Nd, Eu, Gd, Er,Yb) [13], BiIO4 [14] and Bi(IO3)3 [15], all exhibit excellent pho-tocatalytic activity for dye photodegradation under UV light.Their high performance should be mainly attributed to the in-ternal polar field resulted from the IO3 pyramids. It suggests thatiodates can serve as a good source to develop newphotocatalysts.

Besides, semiconductors with specific d10/d10s2 metal ion Agþ

show great potential, as the Ag cation manages to elevate thevalence band by means of the hybridization of their 4d orbital withthe O 2p orbital, resulting in a higher valence band, and narrowingthe band gap of the semiconductor. Ag3PO4 [16], Ag2CO3 [17],AgGaO2 [18], AgSbO3 [19] and Ag6Si2O72 [20] all possess excellentVLD photooxidation abilities. However, the above silver-containingphotocatalytsts suffer from the drawback of photocorrosion, whichcan seriously deactivate photocatalysts. Thus, it is of great interestand desirable to develop new Ag-based photocatalysts which areresistant to photochemical or chemical corrosion.

Herein, we successfully developed a visible-light-active photo-catalyst AgIO3 by a facile hydrothermal method. By investigating

Fig. 1. Crystal structure of AgIO3 (a) Unit cell. (b) [AgO7] plane. (c) Distortion direction of IO3.

Fig. 2. XRD pattern of AgIO3.

H. Huang et al. / Solid State Sciences 46 (2015) 37e4238

the photocatalytic activity, AgIO3 was found to be an efficient andstable photocatalyst superior to TiO2 for RhB degradation underboth UV light and visible light irradiation. The advantages of thismaterial, such as large polarizability resulted from the NCS struc-ture, polarized IO3

� anion and layered structure should be respon-sible for the high photocatalytic activity of AgIO3.

2. Experimental

2.1. Preparation of the photocatalyst

All chemicals were in analytic grade and used as received. AgIO3

was prepared by a hydrothermal method. Typically, 1 mmol AgNO3and stoichiometric I2O5 were dissolved in 30 mL deionized waterunder stirring. After that, the suspension was transferred into aTeflon-lined stainless steel autoclave. The autoclave was sealed andheated at 180 �C for 24 h. The product was collected by filtration,washed repeatedly with ethanol and distilled water, and then driedat 80 �C for 10 h.

2.2. Characterization

X-ray powder diffraction (XRD) patterns of samples weremeasured on a D8 Advance X-ray diffractometer (Bruker AXS,Germany) with Cu Ka radiation. A Cary 5000 UVevisibleeNIRspectrophotometer was employed to record the UVevis diffusereflectance spectra (DRS). The morphology and microstructure ofthe products were investigated by a transmission electron micro-scopy (TEM), high resolution TEM (HRTEM) and S-4800 scanningelectron microscope (SEM). X-ray photoelectron spectroscopy(XPS) was performed at 150 W (XPS: Thermo ESCALAB 250, USA)with Al Ka X-ray radiation (ht ¼ 1486.6 eV). Specific surface area ofwas characterized by nitrogen adsorption BET method with aMicromeritics 3020 instrument.

2.3. Photocatalytic activity

The photocatalytic performance of AgIO3 was evaluated bydecomposition of Rhodamine B (RhB) under UV (300 W high-pressure lamp) and visible light (l > 400 nm, 500 W Xe lamp). Ina typical procedure, 50 mg of photocatalyst was dispersed into100 mL of RhB (1 � 10�5 mol/L) solution. Before photoreaction, thesuspension was vigorously stirred in dark for 1 h to reach anadsorptionedesorption equilibrium. Afterward, about 3 mL of themixture was taken at given time intervals, and separated throughcentrifugation. The concentration of upper centrifuged liquid wasanalyzed by using a U-3010 UVevis spectrophotometer.

2.4. Second harmonic generation (SHG)

SHG measurements were performed on AgIO3 with particle sizeof 50e75 mm by means of the Kurtz-Perry method [21]. A Q-switched 1064 nm Nd:YAG laser was used as the light sourceproducing a pulsed infrared beam to irradiate the samples. TheKH2PO4 (KDP) microcrystalline powders with the same size rangeserved as the standard.

2.5. Density functional calculations

Planewave pseudopotential methodwas employed to obtain theband structure, as well as total and partial densities of states (DOS)

Fig. 3. (a, b) SEM images, (c) TEM image, (d) SAED pattern, (e) HRTEM image and (f) Theoretical angle between the (220) and (041) planes of AgIO3.

Fig. 4. (a) Band structure and total density of states. (b) DRS and band gap (inset) of AgIO3.

of AgIO3 [22]. The calculation was performed by local densityapproximation (LDA) with adopting a relatively high kinetic energycutoff of 500 eV and a density of (2 � 2 � 2) Monkhorste-Pack k-point mesh [23].

3. Results and discussion

3.1. Crystal structure and optical property

AgIO3 crystallizes in the NCS orthorhombic space group Pbc2. Asshown in Fig. 1aec, it possesses a layered structure with an infinitesandwich-like [AgIO7]∞ layer composed of (AgO7)∞ plane and theIO3 anions. These sandwich-like layers are further stacked togetherby the nonbonding interaction to form three-dimensional crystalstructure of AgIO3. XRD patterns of the as-prepared AgIO3 (Fig. 2)showed that all the diffraction peaks were in good agreement withthe data from the Inorganic Crystal Structure Database (ICSD) [24],indicating the pure phase of AgIO3. The sharp peaks suggest thehigh crystallinity of obtained product.

The typical SEM images of the as-prepared AgIO3 sample wereshown in Fig. 3a and b. It can be observed that most of these AgIO3products have a spindly shaped morphology, and their particle sizewas about 1e6 mm. The crystal feature was identical with theorthorhombic Pbc2 space group. Fig. 3b shows the enlarged SEMimage of a single crystal of AgIO3. The AgIO3 single crystal possessesvery sharp corners, neat cutting edges and smooth surfaces.Transmission electronmicroscopy (TEM) image (Fig. 3c) verifies thespindly shaped structure. The selected area electronic diffraction(SAED) pattern (Fig. 3d) confirmed the single crystal nature andhigh crystallinity of AgIO3 [25]. The HRTEM image (Fig. 3e)demonstrated two sets of lattice fringes with spacing of 0.324 and0.318 nm, which correspond well to the {220} and {041} facets ofAgIO3. Furthermore, the angle indicated in the SAED pattern is 68�,which is in accordance with the theoretical value between the(220) and (041) planes (Fig. 3f).

The electronic structure of AgIO3 is illustrated in Fig. 4a. Asindicated, the highest occupied states locate at X point and thelowest unoccupied states are between G and Z points, demon-strating that AgIO3 is an indirect band gap semiconductor with a

Fig. 5. Photocatalytic degradation curves of RhB under (a) UV light and (b) visible light (l > 400 nm). Temporal absorption spectra of RhB under (c) UV light and (d) visible light.

Fig. 6. (a) Cycling runs in the photocatalytic degradation of RhB in the presence of AgIO3 under UV light irradiation. (b) XPS spectra of AgIO3 samples before and after photocatalyticprocess.

H. Huang et al. / Solid State Sciences 46 (2015) 37e4240

theoretical band gap of 2.63 eV. The DRS of the AgIO3 sample wasdisplayed in Fig. 4b. It showed an absorption edge around 420 nm.The experimental band gap was calculated to be 2.99 eV slightlylarger than the theoretical value. It is because that DFT calculationusually underestimates the band gap [26,27].

3.2. Photocatalytic activity and mechanism of AgIO3

The photocatalytic activity of AgIO3 was evaluated by decom-position of RhB under UV and visible light irradiation. RhB is quitestable and its self-photolysis is negligible either under UV light orvisible light irradiation (Fig. 5a and b). Fig. 5a shows the change ofRhB concentration over AgIO3 and P25 under UV light irradiation.

AgIO3 exhibits a higher photocatalytic activity than P25 under UVlight, though the BET surface area of AgIO3 (0.844 m2/g) is muchsmaller than that of P25 (48.6 m2/g). The degradation curves of RhBover AgIO3 and NeTiO2 under visible light were depicted in Fig. 5b.After 6 h irradiation, the photodecomposition of RhB by AgIO3 wasmore than 70% while only about 30% of RhB was removed byNeTiO2. From Fig. 5c and d, it can be observed that the intensity ofRhB absorption spectra obviously decreased with increasing theirradiation time. Besides, the blue shift of the maximum absorptionband at 554 nm demonstrates the occurrence of N-demethylationand de-ethylation during the degradation reaction. To compare thedegradation rate quantitatively, the pseudo-first-order kineticcurves were also plotted. The apparent rate constant k calculated

Fig. 7. Total and partial density of states of AgIO3.

Fig. 8. Oscilloscope traces of the SHG signals for the powders (50e75 mm) of KDP andAgIO3.

H. Huang et al. / Solid State Sciences 46 (2015) 37e42 41

from the experimental data was 0.222 and 0.118 min�1 for AgIO3and P25 under UV light, respectively, and 0.536 and 0.199 min�1 forAgIO3 and NeTiO2 under visible light, respectively. In other words,the photocatalytic activity of AgIO3 is 1.9 times that of P25 under UV

Fig. 9. Schematic diagram of separation and transfer of ph

light, and 2.7 times that of NeTiO2 under visible light (Figs. S1 andS2). The stability of AgIO3 for photodecomposition of RhB wasstudied. AgIO3 were reclaimed and re-examined for three extracycles. As shown in Fig. 6a, AgIO3 did not exhibit any significant lossof activity. XPS analysis is performed to confirm the above results.As shown in Fig. 6b, the peaks of I 3d, I 3p, Ag 3d and O 1s all can bedetected, and the C peak is from the adventitious hydrocarbon ofthe XPS instrument. The two sets of XPS spectra of AgIO3 before andafter photocatalytic process show good consistency and no impu-rity peaks can be detected, demonstrating the high stability ofAgIO3. The XRD patterns of AgIO3 samples before and after pho-tocatalytic reaction were also compared (Fig. S3). There is nochange in the XRD pattern of AgIO3 after photodegradation process.These experimental results indicated that AgIO3 was stable and notphotocorroded during the photocatalytic oxidation process of thecontainments.

The density of states (DOS) of AgIO3 was shown in Fig. 7. Thebottom of the conduction bands (CB) mainly consists of I 5p andO 2p orbitals, whereas the top of the valence bands (VB) isoccupied by Ag 4d and O 2p orbitals. The separate occupancy inCB and VB by the orbitals from different groups is beneficial forthe separation of photoinduced electrons and holes. NLO mea-surements (Fig. 8) revealed that AgIO3 exhibits a high SHGresponse, which is approximately 6.5 times that of KH2PO4 (KDP),indicating that it could also be used as a promising NLO materialfor the laser harmonic generation [28]. The large NLO intensity ofAgIO3 verified the large intrinsic polarization effect in its crystalstructure. The presence of the dipole moment can work as anaccelerator for efficient photoexcited carrier separation in thelocal structure [29]. Under visible light irradiation, the photo-generated electronehole pairs appeared over AgIO3. Based on theDOS results, the photogenerated electrons will travel toward theI5þ ions, while the holes migrate to the Agþ ions. Due to theelectrostatic fields derived from the layered configuration andNCS structure of AgIO3, which could provide the large space topolarize the related atoms and orbitals, the photogeneratedelectron and hole would travel in opposite directions along the b-axis direction (the polarization vector direction) as shown inFig. 9. Then, the reactive radicals, like superoxide (�O2

�), hydroxylradicals (�OH) and holes (hþ) are generated and play crucial rolesin the photooxidation process. Moreover, the polar IO3 pyramidspossess a large dipole moment of 63.32 D, which could produce apyroelectric polarization along the c-axis direction [30]. Thispolar field is believed to further promote the separation andtransfer of electrons and holes in the AgIO3, enhancing its

otogenerated eeh pairs under the internal polar field.

H. Huang et al. / Solid State Sciences 46 (2015) 37e4242

photocatalytic activity.

4. Conclusions

In summary, we have hydrothermally synthesized AgIO3 andshown that it possesses excellent photocatalytic activity with highstability under both UV and visible light irradiation. Nonlinearoptical (NLO) measurements revealed that AgIO3 exhibits a highsecond harmonic generation (SHG) response, which is approxi-mately 6.5 times that of KH2PO4 (KDP) standard. The large NLOeffects of AgIO3 verified the large intrinsic polarization effect in itscrystal structure. The combined effects of internal polar electricfield and static electric field derived from its NCS structure, polarIO3 groups and heterolayered structure can greatly facilitate theseparation of photogenerated electronehole pairs, thus endowingAgIO3 with an efficient photocatalytic reactivity. These uniquefeatures of AgIO3 also suggest that it is potentially applicable forNLO materials, ferroelectric materials, solar cells, and optoelec-tronic devices.

Acknowledgments

This work was supported by the National Natural ScienceFoundations of China (Grant No. 51302251), the FundamentalResearch Funds for the Central Universities (2652013052).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.008.

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