Photoinduced In Situ Growth of Ag Nanoparticles on AgNbO3

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Photoinduced in-Situ Growth of Ag Nanoparticles on AgNbO Yang Lu, Qianyun Shen, Qiaonan Yu, Feng Zhang, Guoqiang Li, and Weifeng Zhang

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10961 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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The Journal of Physical Chemistry

Photoinduced In-situ Growth of Ag Nanoparticles on AgNbO3 †

†

Yang Lu1, , Qianyun Shen1, , Qiaonan Yu1,2, Feng Zhang1, Guoqiang Li*,1 and Weifeng Zhang*,1 1. Henan Key Laboratory of Photovoltaic Materials, Henan University, Kaifeng 475004, P.R. China. 2. College of Electronics and Electrical Engineering, Nanyang Institute of Technology, Nanyang 473200, P.R. China. †Equally contribution. * Corresponding author: Tel.: +86-378-3881-940. Fax: +86-378-3880-659. E-mail: [email protected]; [email protected]

Abstract: Semiconductor modified with metal nanoparticles shows many interesting properties. Ag nanoparticles with size of 10 nm were in situ grown on AgNbO3 using the light as the energy source. After Ag nanoparticles decoration, the sample changes the color from yellow to brown. The SEM images show that the Ag nanoparticles dispersed on the surface randomly. The apparent localized plasma resonance (LPR) absorption was observed from UV-vis spectrum, and the wavelength and intensity of LPR could be affected by the experimental conditions, such as light intensity, surrounding atmosphere, and the ratio of Ag to Nb. Furthermore, we found the color change is a quasi-reversible photochromism effect.

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1. Introduction Semiconductor modified with metal nanoparticles shows many interesting properties, such as surface-enhanced Raman scattering,1 enhanced photocatalytic activity2 and multicolor photochromism.3-5 Most of the phenomena are relative to the localized plasma resonance absorption (LPR) generated by metal nanoparticles. Metal nanoparticles were mainly focused on noble metals, for example, Ag, Au and Pt. Many semiconductors were studied from the traditional semiconductors to oxide semiconductors.6-9 Metal nanoparticles decorated on the surface of semiconductors were prepared mainly through two methods distinguishing from the sources of metal nanoparticles. One is from colloid; the other is from the reduction of metal salt.2, 5, 10-11 The former seems like a mixture, which contains a weak contact. The better contact between the metal nanoparticle and semiconductor is very important to obtain the better catalytic performance. For example, the Pt loaded TiO2 prepared by photodeposition exhibited a better activity of hydrogen evolution than that by impregnation method. 12 The second one usually introduces some additional species, such as NO3-, Cl- etc. We expected to develop an in-situ method of metal nanoparticles decorating on the surface of semiconductors. In situ growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO3 stoichiometry, during the crystalization.13 Xu et al reported a simple one-step reduction process that can in situ grow plasmonic silver nanoparticles on the surface of AgTaO3. Briefly, the AgTaO3 was reduced in the ethylene glycol heated up to 160oC in a glass beaker using a hot plate.14 All above two in-situ growth method are forced by the thermal effects. Light could be used as the energy source in photochemistry synthesis of Au nanocystal.15-16 In this study, the light was used as the energy source to in situ grow Ag nanoparticles on AgNbO3, which is a visible light reactive pseudo-perovskite photocatalyst possessing the properties of splitting water to oxygen and decomposing organic pollutants.17-18 The Ag nanoparticles could be created in a solid-gas phase system which is similar to photocatalytic decomposition of isopropanol (IPA).19-20 After certain time light irradiation, the sample changes the color from yellow to brown. The SEM images show that the Ag nanoparticles dispersed on the surface randomly. The obvious LPR was observed from UV-vis spectrum, and the wavelength and


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intensity of LPR could affected by the experimental conditions, such as light intensity, surrounding atmosphere, and the ratio of Ag to Nb. Furthermore, we found the color change is a quasi-reversible photochromism effect. 2. Experimental Section 2.1. Sample preparation Powder samples of AgNbO3 were synthesized by a conventional solid-state reaction method using Ag2O and Nb2O5 as starting materials as reported before.19 It is noted that the intermediate grinding will affect the position of peak in the UV-vis spectra of the sample after light irradiation. We also controlled the ratio of Ag to Nb to obtain the sample with non-stoichiometric ratio. 2.2. Characterization The final sample was confirmed by an X-ray diffractometer (DX-2700 diffractometer, Fangyuan) with Cu Kα radiation (λ = 0.154145 nm). All of samples are indexed as AgNbO3 without impurity. The diffuse reflectance spectrum was recorded with a UV-Vis spectrophotometer (UV2550) with BaSO4 as the reference standard, and transformed into the absorption spectra by the Kubelka-Munk method. The morphology was observed using a scanning electron microscope (SEM, JSM7001F, JEOL Ltd). In the case of X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250), an Al anode with a monochromator was used to significantly reduce the background signal. The binding energy was referenced to the C 1s peak taken at 284.8 eV. 2.3. In-situ growth of Ag nanoparticles on AgNbO3 First, the samples were pretreated at 350oC for 1h in an oven and then cooled to room temperature before use, removing the organic compounds adsorbed on the surface of the photocatalyst. A certain amount of AgNbO3 was evenly spread over a vessel which could be used to measure the UV-vis spectrum and placed on the bottom of reactor. We can get the UV-vis spectrum of initial sample. The reactor was sealed and filled with the saturated gaseous IPA via injecting the liquid IPA (5ml). Then, the reactor was stored in the dark for 30min. The sample did not change the color. Finally, the reactor was irradiated with the light emitted by Xe lamp with and without optical fibers. The light intensity could be controlled by the current. The thermal effect of light was measured by thermometer. The adsorption property of AgNbO3 was recorded with a UV-Vis spectrophotometer (UV2550). We can stop the reaction when the special measurement is



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needed. The recover sample was obtained from the sample under 30min irradiation and kept in dark. In some control experiments, the reaction will be carried out in N2 and O2, even without IPA. 3. Results and Discussion 3.1 Color change

Figure 1. Color of the samples at different states, (a) origin, (b) after irradiation and (c) isolated from air and kept in dark for 48h. The fresh AgNbO3 appears light yellow as shown in Fig 1a. After light irradiation in presence of IPA in air, we got the sample with the color of brown as displayed in Fig 1b. When the post-irradiated sample was sealed and stored in dark for 48h, no apparent color change was obtained, as illustrated in Fig 1c. However, when the sample was exposed in air, the color will return to the original whatever the sample is previously sealed or not. The color change indicates that something happens when it is irradiated and exposed in air. We will address the origin and impact factors of above phenomenon. 3.2 Origin of color change under irradiation The XRD patterns are the same before and after illumination, we could not get any useful information yet, although Xu et al found metallic Ag (111) diffraction peak in the Ag/AgTaO3 system.14 We thought the amount of Ag is under the limitation of XRD. The surface change was investigated by field emission scanning electron microscopy (FESEM), as shown in Figure 2. Obviously, only some steps lay on the surface of fresh AgNbO3, and almost no particles existed on the surface. After 10 min illumination, some particles with the size of 10 nm appeared on the surface (see figure 2b). They are dispersed on the surface randomly, implying that there are no any preferential geometry locations, such as, edges and corns. When using the heat as the energy


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source, the edges and corns are likely to appear nanoparticles.13 With the longer time of illumination, the nanoparticles become more and more (see figure 2c). The similar phenomenon was found when AgTaO3 was heated at 160oC in ethylene glycol for a long time.14 For quantifying the amount of nanoparticles, we calculated the density of particles from the SEM images, which is defined as the number of particles divided into the area of the image. It increases rapidly in the initial and seems reach a platform after 10 min illumination, as shown in figure 2d.

Figure 2. SEM images obtained from the samples at different states, (a) origin, (b) after 10 min irradiation, (c) after 30 min irradiation, (d) plot of density of nanoparticles vs. irradiation time.

Ag 3d5/2

(c)

O 1s

(c)

Ag 3d3/2

84%

Nb 3p1/2

6%

10%

6%

8%

Nb 3p3/2

Intensity(a.u.)

Intensity (a.u.)

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(b)

(a)

(b) 86%

(a) 87% 13%

385

380

375

370

365

Binding Energy (eV)

360

534

532

530

528

Binding Energy(eV)

Figure 3. Ag 3d and O1s XPS line obtained from the sample at different states, (a) origin, (b) after 30 min irradiation and sealed, (c) after 30 min irradiation and then exposed to air. We investigated the chemical state of the sample before and after irradiation. The Ag 3d and O 1s XPS line were shown in figure 3. The Nb 3p XPS lines overlap with the Ag 3d lines. We carefully deconvoluted the spectra with the same full width at half maximum (FWHM) and with



the standard area ratio. We did not find the difference of Ag 3d line in the samples before and after irradiation. Three peaks at 529.6, 531.2 and 532.5eV were found in the O 1s spectra of the sample after irradiation. The former two peaks were found in the original sample. Kruczek et al reported that the O 1s line in single crystal and ceramic AgNbO3 have two peaks with 1.9 eV chemical shift, and

they were considered to be related to the lattice oxygen and adsorbed oxygen.21 We

carefully checked the handbook of XPS spectroscopy. We thought: the first peak should be related to lattice oxygen in AgNbO3; the second peak should be originated from the adsorbed hydroxide ion; the third peak is possibly resulted from the adsorbed H2O.

0.3

0.3

0.1

0.0

Abs. (a.u.)

0.2

Time 40 min 30 min 10 min 0

0.2 501 0.1 498

0.0 500

Wavelength (nm)

504

Abs. (a.u.)

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600

700

Wavelength (nm)

800

0 10 20 30 40

Time (min)

Figure 4. UV-vis spectra of AgNbO3 irradiated for different time (left) and the intensity and wavelength of peak (right). Conditions: Xe with the optical fiber; 5mL IPA in air. Some silver nanoparticles would be formed when silver salt is reduced.14 Metal nanoparticles will generate localized plasma resonance absorption (LPR), which is easily obtained by UV-visible absorption spectroscopy. The UV-vis spectra of sample varied with the irradiation time is shown in figure 4. The fresh AgNbO3 could absorb the light with a wavelength less than 473 nm, which is consistent with literatures.22 After 10 min illumination, a new absorption peak appeared at 503 nm and the baseline was lifted up. We thought the new peak originates from the localized plasma resonance of Ag nanoparticles on the surface. The size and shape of the particles, and surrounding materials significantly impact the absorption of electromagnetic radiation by small particles. For example, assuming silver particles as spherical ones, too large or too small particle size, different supporting materials will significantly broaden or shift the localized plasma resonance absorption peak.7, 23 Considering the SEM image and the absorption spectra, the silver


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nanoparticles would be exsoluted out of crystal structure of AgNbO3 during photodecompositing the IPA. The lifted baseline comes from the enhanced quadrupole or higher multipole resonance in some, well-grown nanoparticales.4 The similar phenomenon was reported on the Ag nanoparticles on the TiO2 single crystals. 4 The peak intensity and baseline increased with the irradiation time and reached a platform after long time irradiation, which is consistent with the trend of the density of particles. In other words, the peak intensity could reflect the amount of Ag nanoparticles on the surface. Moreover, the LPR wavelength shifted from 504 nm to 498 nm with prolonging the irradiation time. The LPR wavelength of Ag nanoparticles was usually red-shifted with increasing the particle size.24 The decrease in the refractive index of surrounding materials will cause the blue-shift of LPR wavelength.24-25 IPA could be degraded to acetone in the presence of AgNbO3.19 the refractive index of IPA and acetone is 1.38 and 1.36. After long time irradiation, the fraction of acetone will increase, the refractive index in gas phase will be decrease. Consequently, we speculated that the blue-shift of LPR is caused by the decrease in the refractive index of surrounding atmosphere. 3.3 Impact factors

0.50 (c)

Absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

0.25 (d)

(a)

0.00

500

600

700

800

Wavelength (nm)

Figure 5. UV-vis spectra of AgNbO3 under different irradiation conditions (a) fresh AgNbO3, (b and c) Xe lamp without optical fiber at current of 10A and 20A, (d) Xe lamp with optical fiber. Here we corrected the baseline to show the peak and absolute Abs value clearly. Hereafter, we will investigate the effects of experimental conditions on the exsolution of Ag nanoparticles. First, we carried out the experiment under different irradiation conditions. In order



to output the same irradiation spectrum, we used Xe lamp as the light source. We controlled irradiation conditions using the current and optical fiber to get the different light intensity and thermal effect on the surface of sample. The light intensity is higher when the electric current is larger, and the optical fiber will reduce the light intensity. The temperature on the surface under illumination is 71oC at the electric current of 20 A, 43 oC at the electric current of 10 A, and 28 oC with optical fiber. When the sample was irradiated for the same time, the absorption spectra were shown in figure 5. The peak intensity will be reduced when decreasing the light intensity and temperature, implying that the amount of Ag nanoparticles is sensitive to the light intensity and/or temperature. The peak will be shifted to higher wavelength when we used the optical fiber in comparison with that without the optical fiber. (a)

(b)

0.3

0.3

Absorbance (a.u.)

0.2 With IPA

0.1

Peak position (nm)

N2

Without IPA

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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air 0.2

505 500 495 490

0.1

N2

Air

O2

O2

AgNbO3

0.0

0.0

500

600

700

Wavelength (nm)

800

500

600

700

800

Wavelength (nm)

Figure 6. UV-vis spectra of AgNbO3 under different atmosphere. (a) with and without IPA in air, (b) with IPA in N2, air and O2. Here we corrected the baseline to show the peak and absolute Abs value clearly. Secondly, we also carried out the experiment under different atmospheres. We investigated the effect of IPA, as shown in figure 6a. The intensity is almost same, implying that the amount of Ag exsoluted was not affected by the IPA. The profile of spectra without IPA is similar to that without optical fiber. The boiling point of IPA is 82.45oC, which is close to the temperature under Xe lamp without optical fiber. So few amount of IPA could adsorb on the surface, resulting in the similar profile of spectra. The experiments were carried out in the N2, air and O2. The intensity of LPR decreased with increasing the O2 content in surrounding atmosphere. When the O2 existed in the system, the Ag


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nanoparticles are easy to form Ag2O, resulting in the reduced intensity of LPR generated by Ag nanoparticles. The red-shift of LPR peak was observed with increasing the O2 content. When Ag nanoparticle was covered with a thin layer of Ag2O, it is mean that the surrounding materials changed from air to Ag2O. The refractive indexes of surrounding materials increased, leading to

0.3

Peak position (nm)

the red shift of LPR peak.

(a) (c)

510

500

490 0.98

(b)

∆ Abs

0.2

1.00

1.02

Ag/Nb

0.1

500

600

700

Wavelength (nm)

Figure 7. UV-vis spectra of samples with different ratios of Ag to Nb, (a) Ag:Nb=0.98, (b) Ag:Nb=1, (c) Ag:Nb=1.02. The effect of defects was investigated by controlling the ratio of Ag to Nb during preparation. The intensity is the smallest, when the ratio equals 1, indicating that the amount of Ag nanoparticles is sensitive to the stoichiometry. The peak will shift to longer wavelength with increasing the ratio of Ag to Nb. Kityk et al

26

reported that the dielectric susceptibility will be

increased when the ratio of the Li content and the total amount of cations (Li+Nb) in LiNbO3 is far away from the stoichiometric composition. Therefore, the observed red shift is considered as the results of the increase in dielectric susceptibility due to the rise in the amount of intrinsic cationic defects. The detailed structure of location appearing Ag nanoparticles is not clear yet. 0.5

1st cycle

2nd cycle

3rd cycle

4th cycle

∆ Abs at 470 nm

0.4

Abs.(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.2 0.1 0.0

420

560

700 420

560

700

420

560

Wavelength(nm)

700 420

560

700

0

1

2

3

4

5

Figure 8. Recycle results under Xe lamp at current of 20A without optical fiber.



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The color change is qusi-reversible, as displayed in figure 8. The difference in Abs at 470nm changed a lot in first cycle. However, the changes in Abs in the last three cycles are almost the same, indicating that the sample reached a steady state. 4. Conclusion Ag nanoparticle was in situ grown on the surface of AgNbO3 using light as the energy source. The intensity and wavelength of LPR is sensitive to light intensity, temperature, surrounding atmosphere and the stoichiometry. The new system could be used as a new alternative multicolor photochromism except for Ag-TiO2.

Acknowledgements This work was supported by the Program for Science & Technology Innovation Talents in Universities of Henan Province, China (17HASTIT014), the Young Core Instructor Foundation from the Education Commission of Henan Province (2015GGJS-021), the National Natural Science Foundation of China (51402088), and the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology (Nanjing University).

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Photoinduced In Situ Growth of Ag Nanoparticles on AgNbO3

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