Hydrothermal synthesis, nonlinear optical property and photocatalytic activity of a non-centrosymmetric AgIO3 photocatalyst under UV and visible light irradiation

Accepted Manuscript Hydrothermal synthesis, nonlinear optical property and photocatalytic activity of a non-centrosymmetric AgIO3 photocatalyst under UV and visible light irradiation Hongwei Huang, Ying He, Yuxi Guo, Ran He, Zheshuai Lin, Yihe Zhang PII:

S1293-2558(15)00118-1

DOI:

10.1016/j.solidstatesciences.2015.05.008

Reference:

SSSCIE 5134

To appear in:

Solid State Sciences

Received Date: 16 December 2014 Revised Date:

19 May 2015

Accepted Date: 23 May 2015

Please cite this article as: H. Huang Y. He, Y. Guo, R. He, Z. Lin, Y. Zhang Hydrothermal synthesis, nonlinear optical property and photocatalytic activity of a non-centrosymmetric AgIO3 photocatalyst under UV and visible light irradiation, Solid State Sciences (2015), doi: 10.1016/ j.solidstatesciences.2015.05.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

Hydrothermal synthesis, nonlinear optical property and

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photocatalytic activity of a non-centrosymmetric AgIO3 photocatalyst under UV and visible light irradiation

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid

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Hongwei Huanga,∗, Ying Hea, Yuxi Guoa, Ran Heb, Zheshuai Linb, Yihe Zhanga,∗

Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China b

Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology

of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese

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Academy of Sciences, Beijing 100190, China

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Hydrothermal synthesis, nonlinear optical property and photocatalytic activity of a non-centrosymmetric AgIO3

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photocatalyst under UV and visible light irradiation

a

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Hongwei Huanga,∗, Ying Hea, Yuxi Guoa, Ran Heb, Zheshuai Linb, Yihe Zhanga,∗

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid

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Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China b

Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser

Technology of Chinese Academy of Sciences, Technical Institute of Physics and

∗

Corresponding author

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Chemistry, Chinese Academy of Sciences, Beijing 100190, China

[email protected] (H.W. Huang); [email protected] (Y.H.

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Zhang). Tel.: +86-10-82332247.

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ACCEPTED MANUSCRIPT ABSTRACT AgIO3 as a novel photocatalyst was prepared via a facile hydrothermal route. The

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microstructure, electronic structure, optical and nonlinear optical (NLO) properties of AgIO3 were investigated by a series of experimental and theoretical methods, including X-ray powder diffraction (XRD), scanning electron microscope (SEM),

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transmission electron microscopy (TEM), high resolution TEM (HRTEM), Brunauer-

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Emmett-Teller (BET), UV-vis diffuse reflectance spectra (DRS), second harmonic generation (SHG) measurements and the first principle calculation. The results revealed that AgIO3 exhibits a strong SHG response and excellent photocatalytic performance with high stability under both UV and visible light irradiation. The

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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

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photocatalytic activity of AgIO3. The present work may shed new light on the design

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of multifunctinal materials.

Keywords:

AgIO3;

Photocatalytic

activity;

Electronic structure; Electric field

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Non-centrosymmetric

structure;

ACCEPTED MANUSCRIPT 1. Introduction Photocatalysis has aroused extensive interest for their promising applications in

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environmental remediation and energy generation [1-3]. Though traditional UV light photocatalysts show potentials, numerous efforts were made to develop visible-lightdriven (VLD) photocatalysts [4-9]. As separation efficiency of photogenerated charge

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carriers is crucial to photocatalytic performance, it is highly desirable that a strong

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driving force for the charge separation is present throughout the entire photocatalyst [10]. Lately, it has been reported that nonlinear optical (NLO) materials, displaying a second harmonic generation (SHG), can serve as efficient photocatalysts, like K3B6O10Br [11]. The non-centrosymmetric (NCS) structure can give rise to an

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intrinsic large polarization effect, which promotes the efficient separation of photogenerated electron−hole pairs, thus resulting in a high photocatalytic activity.

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Recently, metal iodates have been extensively studied due to the IO3 group with a

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lone pair of electrons, favoring the formation of asymmetry structure and polarity. These iodate photocatalysts, including Y(IO3)3 [12], Ln(IO3)3 (Ln = Ce, Nd, Eu, Gd, Er, Yb) [13], BiIO4 [14] and Bi(IO3)3 [15], all exhibit excellent photocatalytic activity for dye photodegradation under UV light. Their high performance should be mainly attributed to the internal polar field resulted from the IO3 pyramids. It suggests that iodates can serve as a good source to develop new photocatalysts.

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ACCEPTED MANUSCRIPT Besides, semiconductors with specific d10/d10s2 metal ion Ag+ show great potential, as the Ag cation manages to elevate the valence band by means of the hybridization of

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their 4d orbital with the O 2p orbital, resulting in a higher valence band, and narrowing the band gap of the semiconductor. Ag3PO4 [16], Ag2CO3 [17], AgGaO2 [18], AgSbO3 [19] and Ag6Si2O72 [20] all possess excellent VLD photooxidation

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abilities. However, the above silver-containing photocatalytsts suffer from the

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drawback of photocorrosion, which can seriously deactivate photocatalysts. Thus, it is of great interest and desirable to develop new Ag-based photocatalysts which are resistant to photochemical or chemical corrosion.

Herein, we successfully developed a visible-light-active photocatalyst AgIO3 by a

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facile hydrothermal method. By investigating the photocatalytic activity, AgIO3 was found to be an efficient and stable photocatalyst superior to TiO2 for RhB degradation

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under both UV light and visible light irradiation. The advantages of this material, such

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as large polarizability resulted from the NCS structure, polarized IO3- anion and layered structure should be responsible 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, 1mmol AgNO3 and stoichiometric I2O5 were

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ACCEPTED MANUSCRIPT dissolved in 30 mL deionized water under stirring. After that, the suspension was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and for 24 h. The product was collected by filtration, washed repeatedly

with ethanol and distilled water, and then dried at 80 2.2. Characterization

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heated at 180

for 10 h.

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X-ray powder diffraction (XRD) patterns of samples were measured on a D8

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Advance X-ray diffractometer (Bruker AXS, Germany) with Cu Kα radiation. A Cary 5000 UV−visible−NIR spectrophotometer was employed to record the UV-vis diffuse reflectance spectra (DRS). The morphology and microstructure of the products were investigated by a transmission electron microscopy (TEM), high resolution TEM

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(HRTEM) and S-4800 scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS) was performed at 150 W (XPS: Thermo ESCALAB 250, USA)

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with Al Ka X-ray radiation (ht = 1486.6 eV). Specific surface area of was

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characterized by nitrogen adsorption BET method with a Micromeritics 3020 instrument.

2.3. Photocatalytic activity The photocatalytic performance of AgIO3 was evaluated by decomposition of Rhodamine B (RhB) under UV (300W high-pressure lamp) and visible light (λ > 400 nm, 500 W Xe lamp). In a typical procedure, 50 mg of photocatalyst was dispersed

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ACCEPTED MANUSCRIPT into 100 mL of RhB (1× 10-5 mol/L) solution. Before photoreaction, the suspension was vigorously stirred in dark for 1 h to reach an adsorption-desorption equilibrium.

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Afterwards, about 3 ml of the mixture was taken at given time intervals, and separated through centrifugation. The concentration of upper centrifuged liquid was analyzed by using a U-3010 UV-vis spectrophotometer.

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2.4. Second harmonic generation (SHG)

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SHG measurements were performed on AgIO3 with particle size of 50-75 µm by means of the Kurtz-Perry method [21]. A Q-switched 1064 nm Nd:YAG laser was used as the light source producing a pulsed infrared beam to irradiate the samples.

standard.

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The KH2PO4 (KDP) microcrystalline powders with the same size range served as the

2.5. Density functional calculations

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Planewave pseudopotential method was employed to obtain the band structure, as

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well as total and partial densities of states (DOS) of AgIO3 [22]. The calculation was performed by local density approximation (LDA) with adopting a relatively high kinetic energy cutoff 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

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ACCEPTED MANUSCRIPT AgIO3 crystallizes in the NCS orthorhombic space group Pbc2. As shown in Fig. 1a-c, it possesses a layered structure with an infinite sandwich-like [AgIO7]∞

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layer composed of (AgO7)∞ plane and the IO3 anions. These sandwich-like layers are further stacked together by the nonbonding interaction to form threedimensional crystal structure of AgIO3. XRD patterns of the as-prepared AgIO3

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(Fig. 2) showed that all the diffraction peaks were in good agreement with the data

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from the Inorganic Crystal Structure Database (ICSD) [24], indicating the pure phase of AgIO3. The sharp peaks suggest the high crystallinity of obtained product.

The typical SEM images of the as-prepared AgIO3 sample were shown in Fig. 3a

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and b. It can be observed that most of these AgIO3 products have a spindly shaped morphology, and their particle size was about 1

6µm. The crystal feature was

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identical with the orthorhombic Pbc2 space group. Fig. 3b shows the enlarged SEM

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image of a single crystal of AgIO3. The AgIO3 single crystal possesses very sharp corners, neat cutting edges and smooth surfaces. Transmission electron microscopy (TEM) image (Fig. 3c) verifies the spindly shaped structure. The selected area electronic diffraction (SAED) pattern (Fig. 3d) confirmed the single crystal nature and high crystallinity of AgIO3. The HRTEM image (Fig. 3e) demonstrated two sets of lattice fringes with spacing of 0.324 and 0.318 nm, which correspond well to the {220}

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ACCEPTED MANUSCRIPT and {041} facets of AgIO3. Furthermore, the angle indicated in the SAED pattern is 68°, which is in accordance with the theoretical value between the (220) and (041)

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planes (Fig. 3f). The electronic structure of AgIO3 is illustrated in Fig. 4a. As indicated, the highest occupied states locate at X point and the lowest unoccupied states are between G and

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Z points, demonstrating that AgIO3 is an indirect band gap semiconductor with a

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

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3.2. Photocatalytic activity and Mechanism of AgIO3 The photocatalytic activity of AgIO3 was evaluated by decomposition of RhB

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under UV and visible light irradiation. RhB is quite stable and its self-photolysis is

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negligible either under UV light or visible light irradiation (Fig. 5a and 5b). Fig. 5a shows the change of RhB concentration over AgIO3 and P25 under UV light irradiation. AgIO3 exhibits a higher photocatalytic activity than P25 under UV light, though the BET surface area of AgIO3 (0.844 m2/g) is much smaller than that of P25 (48.6 m2/g). The degradation curves of RhB over AgIO3 and N-TiO2 under visible light were depicted in Fig. 5b. After 6 h irradiation, the photodecomposition of RhB

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obviously decreased with increasing the irradiation time. Besides, the blue shift of the maximum absorption band at 554 nm demonstrates the occurrence of Ndemethylation and de-ethylation during the degradation reaction. To compare the

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degradation rate quantitatively, the pseudo-first-order kinetic curves were also plotted.

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The apparent rate constant k calculated from the experimental data was 0.222 and 0.118 min−1 for AgIO3 and P25 under UV light, respectively, and 0.536 and 0.199 min−1 for AgIO3 and N-TiO2 under visible light, respectively. In other words, the photocatalytic activity of AgIO3 is 1.9 times that of P25 under UV light, and 2.7 times

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that of N-TiO2 under visible light (Fig. S1 and S2). The stability of AgIO3 for photodecomposition of RhB was studied. AgIO3 were reclaimed and re-examined for

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three extra cycles. As shown in Fig. 6a, AgIO3 did not exhibit any significant loss of

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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 be detected, and the C peak is from the adventitious hydrocarbon of the XPS instrument. The two sets of XPS spectra of AgIO3 before and after photocatalytic process show good consistency and no impurity peaks can be detected, demonstrating the high stability of AgIO3. The XRD patterns of AgIO3 samples before and after photocatalytic reaction were also compared (Fig.

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ACCEPTED MANUSCRIPT S3). There is no change in the XRD pattern of AgIO3 after photodegradation process. These experimental results indicated that AgIO3 was stable and not photocorroded

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during the photocatalytic oxidation process of the containments. The density of states (DOS) of AgIO3 was shown in Fig. 7. The bottom of the conduction bands (CB) mainly consists of I 5p and O 2p orbitals, whereas the top of

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the valence bands (VB) is occupied by Ag 4d and O 2p orbitals. The separate

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occupancy in CB and VB by the orbitals from different groups is beneficial for the separation of photoinduced electrons and holes. NLO measurements (Fig. 8) revealed that AgIO3 exhibits a high SHG response, which is approximately 6.5 times that of KH2PO4 (KDP), indicating that it could also be used as a promising NLO material for

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the laser harmonic generation [28]. The large NLO intensity of AgIO3 verified the large intrinsic polarization effect in its crystal structure. The presence of the dipole

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moment can work as an accelerator for efficient photoexcited carrier separation in the

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local structure [29]. Under visible light irradiation, the photogenerated electron-hole pairs appeared over AgIO3. Based on the DOS results, the photogenerated electrons will travel toward the I5+ ions, while the holes migrate to the Ag+ ions. Due to the electrostatic fields derived from the layered configuration and NCS structure of AgIO3, which could provide the large space to polarize the related atoms and orbitals, the photogenerated electron and hole would travel in opposite directions along the b-axis

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ACCEPTED MANUSCRIPT direction (the polarization vector direction) as shown in Fig. 9. Then, the reactive radicals, like superoxide (·O2-), hydroxyl radicals (·OH) and holes (h+) are generated

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and play crucial roles in the photooxidation process. Moreover, the polar IO3 pyramids possess a large dipole moment of 63.32 D, which could produce a pyroelectric polarization along the c-axis direction [30]. This polar field is believed to

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further promote the separation and transfer of electrons and holes in the AgIO3,

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enhancing its photocatalytic activity. 4. Conclusions

In summary, we have hydrothermally synthesized AgIO3 and shown that it possesses excellent photocatalytic activity with high stability under both UV and

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visible light irradiation. Nonlinear optical (NLO) measurements revealed that AgIO3 exhibits a high second harmonic generation (SHG) response, which is approximately

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6.5 times that of KH2PO4 (KDP) standard. The large NLO effects of AgIO3 verified

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the large intrinsic polarization effect in its crystal structure. The combined effects of internal polar electric field and static electric field derived from its NCS structure, polar IO3 groups and heterolayered structure can greatly facilitate the separation of photogenerated electron-hole pairs, thus endowing AgIO3 with an efficient photocatalytic reactivity. These unique features of AgIO3 also suggest that it is

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optoelectronic devices.

Acknowledgements

This work was supported by the National Natural Science Foundations of China

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(Grant No. 51302251), the Fundamental Research Funds for the Central Universities

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(2652013052).

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

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direction of IO3.

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Fig. 2 XRD pattern of AgIO3.

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Fig. 3 (a,b) SEM images, (c) TEM image, (d) SAED pattern, (e) HRTEM image and

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(f) Theoretical angle between the (220) and (041) planes of AgIO3.

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

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

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Fig. 6 (a) Cycling runs in the photocatalytic degradation of RhB in the presence of

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AgIO3 under UV light irradiation. (b) XPS spectra of AgIO3 samples before and after

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photocatalytic process.

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

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AgIO3.

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Fig. 8 Oscilloscope traces of the SHG signals for the powders (50-75 µm) of KDP and

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Fig. 9 Schematic diagram of separation and transfer of photogenerated e-h pairs under

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the internal polar field.

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Highlights

AgIO3 as a novel photocatalyst was synthesized by a facile hydrothermal method. It exhibits excellent photocatalytic activity both under UV and visible light.

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The high photocatalytic activity is attributed to the non-centrosymmetric structure.

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AgIO3 also shows high stability resistant to photocorrosion.

ACCEPTED MANUSCRIPT Supporting Information for

Hydrothermal

synthesis,

photocatalytic

activity

nonlinear of

a

optical

property

non-centrosymmetric

and

AgIO3

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photocatalyst under UV and visible light irradiation

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes,

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a

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

National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China. b

Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of

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Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of

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Sciences, Beijing 100190, China

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Fig. S1 Kinetic curves for the photocatalytic degradation of RhB under UV light.

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Fig. S2 Kinetic curves for the photocatalytic degradation of RhB under visible light.

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Fig. S3 XRD patterns of AgIO3 samples before and after photocatalytic process.

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