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AgCl modified self-doped TiO2 hollow sphere with enhanced visible light photocatalytic activity
Journal of Alloys and Compounds 657 (2016) 44e52
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Ag/AgCl modified self-doped TiO2 hollow sphere with enhanced visible light photocatalytic activity Haoyong Yin a, Xulong Wang a, Ling Wang b, QLin Nie a, *, Yang Zhang c, Qiuli Yuan c, Weiwei Wu a a b c
Institute of Environmental Materials and Applications, Hangzhou Dianzi University, Hangzhou, 310018, PR China Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China College of Science, Hangzhou Dianzi University, Hangzhou 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 14 September 2015 Accepted 6 October 2015 Available online xxx
The Ag/AgCl modified self-doped TiO2 hollow spheres were successfully prepared firstly by a solegel method using carbon sphere as template and subsequently deposition-precipitation and photoreduction process. Characteristics and properties of the products were investigated using X-ray diffraction (XRD), UVevis diffuse reflectance spectra (UVevis DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the self-doped TiO2 hollow spheres, with diameters about 800e950 nm and shell thickness of 20e30 nm, are constituted with thousands of self-doped TiO2 nanoparticles with diameters of 20e30 nm. Ag/AgCl modified self-doped TiO2 hollow spheres showed broad absorption in the region of 400e700 nm arising from surface plasmon resonance of Ag nanoparticles. The as prepared samples showed improved solar light and visible light induced photocatalytic activities by decomposing Rhodamine B. The improved visible light photocatalytic activities may be due to the oxygen vacancies and Ti3þ localized states of the self-doped TiO2 hollow spheres and the surface plasmonic resonance effect. © 2015 Elsevier B.V. All rights reserved.
Keywords: Self-doped TiO2 Hollow sphere Surface plasmon resonance Photocatalyst
1. Introduction Searching for efficient semiconductor photocatalysts utilizing visible-light solar energy for environmental purification and energy conversion remains a great challenge [1e4]. Many efforts have been focused on enhancing visible light absorption by narrowing the band gap of TiO2, including element doping and oxygen deficiency [2,5,6]. Unfortunately, the doped impurity species in TiO2 often lead to severe thermal instability and a relatively fast carrier recombination rate since the foreign introduced atoms can serve as carrier recombination centers, which undoubtedly impair the photocatalytic activities of TiO2 [2,7]. Recently, Mao and co-workers [1] have demonstrated a novel approach to enhance the solar harvesting by introducing disorders in the surface layers of high crystalline TiO2 nanoparticles through hydrogenation. This reduced TiO2 with Ti3þ self-doping has attracted enormous attention for its enhanced visible light
* Corresponding author. E-mail address: [email protected] (Q. Nie). http://dx.doi.org/10.1016/j.jallcom.2015.10.055 0925-8388/© 2015 Elsevier B.V. All rights reserved.
absorption owing to the interband level of the Ti3þ (oxygen vacancy) and increased electrical conductivity arising from its high donor density, which lead to significantly enhanced visible light induced photocatalytic activity compared with pure samples [1,8,9]. Not surprisingly, the work triggered a large amount of follow-up studies where the self-doped TiO2 was prepared by various reductive treatments methods, mostly using hydrogenation methods, chemical vapor deposition, and high energy particle (laser, electron or Arþ) bombardment [1,8,10e12]. Although these methods have their own merits, they are severely limited by high cost and complicated procedures and not suitable for practical applications. Moreover, the surface Ti3þ and oxygen defects on the TiO2 are usually not stable enough in air, since the surface Ti3þ is easily oxidized into Ti4þ by the dissolved oxygen in water. Very recently, the low-cost NaBH4 was used as the reductant to prepare the TiO2 catalysts with self-doped Ti3þ by a solvothermal method to resolve these issues [13]. Nevertheless, the study of self-doped TiO2 is still in its infancy and there are still many problems to be resolved in this field, such as the morphology control of the self-doped TiO2, the improvement of the photocatalytic performance of self-doped TiO2, the relationship between the Ti3þ doping and photocatalytic
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activity of TiO2 and the investigation of its photocatalytic activity both under solar light and visible light. In oxide materials chemistry, solegel process is mostly known for its versatility, scalability and economic application. Besides, it is easy to control the materials morphology in a simple and safe way [14,15]. Recently, plasmonic photocatalysts comprising semiconductors and metal nanoparticles (NPs) have exhibited excellent photocatalytic activities under visible light due to the surface plasmon resonance (SPR) effect of the metal NPs. Silver halides, when combined with silver NPs, and particularly TiO2, exhibit good stability, allowing their application as photocatalysts [16e18]. To the best of our knowledge, there are no reports on the preparation of Ag/AgCl modified self-doped TiO2 hollow spheres. In the present research we demonstrated the synthesis of Ag/AgCl modified selfdoped TiO2 hollow spheres by a simple method with self-doped TiO2 hollow spheres prepared by a solegel method using carbon spheres as templates and Ag/AgCl modified self-doped TiO2 hollow spheres synthesized by a deposition-precipitation method and photo-reduction process. The as-prepared photocatalysts display much improved visible and solar light induced photocatalytic activity. Since the process of self-doped TiO2 hollow spheres formation undergoes the calcination of the sample in air the as prepared photocatalysts possess more stable Ti3þ both in the bulk and surface. The combined surface plasmon resonance effect and the band gap engineering of TiO2 enable its greatly enhanced visible light induced photocatalytic activities. 2. Experimental 2.1. Preparation of the carbon sphere templates In a typical procedure, 5 g of glucose was firstly dissolved in 40 mL of water. Then the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. Finally, hydrothermal treatments were carried out at 180 C for 8 h and then cooled down to room temperature naturally. The obtained products were then centrifuged, washed with absolute alcohol and deionized water for several times, then dried at 80 C for 10 h. 2.2. Preparation of the self-doped TiO2 hollow spheres
45
method. Typically, the synthesized RTiO2HS powders were dispersed in 50 mL deionized water and sonicated for 10 min. Then 10 mL of 0.1 mol/L AgNO3 solution was added into the RTiO2HS suspension and sonicated for another 10 min. After stirring magnetically for 20 min, 20 mL of 0.1 mol/L HCl aqueous solution was added, sonicated for 10 min and stirred magnetically for 20 min. Subsequently, the resulting suspension was irradiated by a 250 W high pressure mercury lamp for 30 min to reduce partial Agþ ions in the AgCl particles to Ag0 species by photochemical decomposition of AgCl. The whole reaction process was kept at room temperature. Finally, the dark yellow Ag/AgCl modified selfdoped TiO2 hollow spheres (Ag-RTiO2HS) powders were collected, washed for several times until the pH of the suspension was about 7, centrifuged and dried at 80 C for several hours. 2.4. Photocatalysts characterization The morphologies and microstructures of the obtained products were analyzed by a Hitachi S-4700 field emission scanning electron microscope (SEM, scanning voltages 15 kV) and JEOL 200CX highresolution transmission electron microscopy (HRTEM, accelerating voltage 200 kV). The phase structures of the samples were measured by X-ray diffraction (XRD) studies using an X-ray diffractometer (Thermo ARL SCINTAG X'TRA) with CuKa radiation (l ¼ 0.154056 nm) under 40 kV with the 2q ranging from 20 to 70 . UVevis diffuse reflectance spectra (DRS) of the products was performed on an UVevis spectrophotometer (UV-2550, SHIMADZU, Japan) using BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5400 spectrometer and the shift of binding energy due to relative surface charge was corrected using the C1s level at 284.6 eV as an internal standard. The Electrochemical impedance spectroscopy (EIS) analysis were measured on an electrochemical workstation (Zennium, Zahner, Germany), using a conventional three-electrode cell. FTO glass coated with the as-prepared samples (0.1 mg) served as the working electrode. The counter and the reference electrodes were a platinum plate and a saturated AgeAgCl electrode, respectively. A 0.1 M NaOH aqueous solution was used as electrolyte. The working electrode (the active area 0.5 cm 0.5 cm) was prepared using an amended doctor blade method. Firstly 0.1 g of sample was ground with 1 mL terpineol. The slurry was then coated onto FTO glass by the doctor blade method which was cleaned in distilled water and ethanol by ultrasonication. These electrodes were preliminarily dried at 70 C and calcined at 200 C for 1 h under Ar atmosphere.
As prepared carbon spheres (0.5 g) were firstly dispersed in 30 mL of ethanol solution (25 mL CH3CH2OH þ 5 mL CH3COOH) ultrasonically for 10 min. Then 10 mL of tetrabutyltitanate solution (5 mL Ti(OBu)4 þ 5 mL CH3CH2OH) was added dropwise to the stirred solution to form a sol. Subsequently 0.2 g of NaBH4 was added into the sol with continuously stirring until it formed a gel. The gel was then dried at 80 C overnight and the obtained powder was grinded and calcinated at 500 C in air for 3 h. Finally, the product was washed in 1 M HCl solution by stirring for 8 h and then washed by distilled water for several times and dried at 80 C in vacuum for 6 h to get the self-doped TiO2 hollow spheres (RTiO2HS). For comparison, the reduced TiO2 nanoparticles (RTiO2NP) were prepared without carbon sphere addition and the pure TiO2 hollow spheres (PTiO2HS) and pure TiO2 nanoparticles (PTiO2NP) were also prepared under the same condition without the NaBH4 addition. The color of the obtained PTiO2NP and RTiO2HS was white and pale yellow respectively which may suggest the possible visible light absorption ability for the partial reduction of RTiO2HS.
The photocatalytic activity of the catalysts was evaluated by degradation of RhodamineB (RhB) under solar and visible light irradiation. A 300 W Xe lamp was used as the simulated solar light. The 420 nm cut off filter was used to get the visible light source. In each experiment, 0.1 g of the as-prepared photocatalyst was added into 200 mL of RhB solution (5 mg/L). Before irradiation, the suspensions were placed in dark and stirred for 30 min to ensure the establishment of adsorptionedesorption equilibrium between the catalyst and RhB. Subsequently, at every interval, about 5 mL of suspension was sampled and centrifuged to remove the photocatalyst particles. The concentration of filtrates was analyzed by measuring the maximum absorbance at 553 nm for RhB using a UV-2550 UVevis spectrophotometer.
2.3. Preparation of Ag/AgCl modified self-doped TiO2 hollow spheres
3. Results and discussion
Ag/AgCl nanoparticles were deposited on the as-prepared RTiO2HS samples by a precipitationephotoreduction reaction
The SEM morphologies of the prepared photocatalysts and carbon spheres are shown in Fig. 1. It can be seen that the as
2.5. Photocatalytic studies
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Fig. 1. SEM images of Carbon spheres (A), RTiO2HS (B, C, and D) and Ag-RTiO2HS (E and F).
prepared carbon sphere templates are smooth and regular spherical shape with diameters of about 300e450 nm. The surfaces of the sample become rough after coating with the self-doped TiO2 and removal of the carbon sphere templates (Fig. 1B). The microspheres also grow larger after coated with TiO2 with diameter increased to about 800e950 nm. The cracked RTiO2HS indicated that the as prepared microspheres have hollow structures when the templates were annealed and removed in the form of CO2 (Fig. 1C). The escaped templates left the hole on the RTiO2HS surface with similar diameter of carbon spheres. The DSC/TG curves (Fig. S1) show distinctly much higher weight loss of RTiO2HS (28% for PTiO2NP versus 35% for RTiO2HS) which can also suggest that the carbon sphere templates can be removed and the hollow structured microspheres was formed at the temperature higher than 400 C. Detailed hollow structures of the RTiO2HS was shown in Fig. 1D which revealed that RTiO2HS are composed by single layer of selfdoped TiO2 with diameter of TiO2 nanoparticles less than 30 nm and accordingly the shell thickness of the RTiO2HS is about 20e30 nm. The SEM images of the Ag/AgCl modified TiO2 hollow spheres (Ag-RTiO2HS) are shown in Fig. 1E and F. It can be observed that the spherical hollow structures of RTiO2HS have not been damaged by the deposition of Ag/AgCl nanoparticles. However, the surface of RTiO2HS becomes much rougher and the nanoparticles of TiO2 grow a little bigger which may suggest that the Ag/AgCl
nanoparticles were deposited on the RTiO2HS surface. The specific surface areas of PTiO2NP, RTiO2NP, PTiO2HS, RTiO2HS and AgRTiO2HS were measured to be 34.7, 35.3, 88.4, 89.2 and 89.8 m2/g respectively which may also indicates the hollow structure formation of the template removed samples. Substructures of RTiO2HS and Ag-RTiO2HS can be further observed by TEM and HRTEM (Fig. 2). TEM image (Fig. 2A) distinctly shows that the surface of RTiO2HS was constituted by thousands of self-doped TiO2 nanoparticles (RTiO2NP) with diameter about 20e30 nm which is consistent with the shell thickness of the RTiO2HS (Fig. 1D). High-resolution TEM (Fig. 2B) detected a distinct crystal spacing of 0.35 nm, which corresponds to the (101) interplanar spacing of anatase TiO2. The insert in Fig. 2B shows a selected electron diffraction diagram, and bright, clear anatase concentric diffraction rings can be observed. Fig. 2C and E shows the HRTEM and enlarged HRTEM images of Ag-RTiO2HS. It can be seen that the distinct (101) interplanar spacing of anatase TiO2 still can be clearly observed on the surface of Ag-RTiO2HS. However, there also exhibit different fringes with lattice spacing of 0.235 nm and 0.196 nm, which correspond to the (111) plane of metal Ag and the (220) plane of AgCl, respectively. The chemical composition of the prepared sample was determined by EDS (Fig. 2D). The EDS spectrum of the Ag-RTiO2HS sample reveals that Ti, Ag, O, and Cl are the major elements. Pt peaks in the spectrum are attributed to
H. Yin et al. / Journal of Alloys and Compounds 657 (2016) 44e52
Fig. 2. TEM (A) and HRTEM (B) images of RTiO2HS (Inset is the ED of the RTiO2HS), EDS (C) and HRTEM (D and E) of Ag-RTiO2HS.
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the Pt coating used in SEM analysis. The HRTEM and EDS of AgRTiO2HS indicate that the Ag/AgCl was successfully deposited on the surface of RTiO2HS. Fig. 3 shows the XRD patterns of RTiO2NP, RTiO2HS and AgRTiO2HS. As shown in Fig. 3, all the diffraction peaks of RTiO2NP and RTiO2HS could be indexed to the anatase of TiO2 (JCPDS Card No. 21-1272), suggesting that the reduction of TiO2 by NaBH4 does not change its crystal structures. In addition to RTiO2NP and RTiO2HS diffraction peaks, there are some strong peaks observed in the XRD patterns of Ag-RTiO2HS hybrids at about 2q ¼ 27.91, 32.21, 46.21, 57.51 and 67.4 which can be indexed to (111), (200), (220), (222), (400) planes of the cubic phase AgCl (JCPDS No.85-1355). It is similar to the results reported by Liu et al. [17]. However, no distinct diffraction peaks of metallic Ag could be found, probably due to its low content, high dispersity or small crystallite. The chemical composition of the Ag-RTiO2HS sample was further characterized by XPS spectra (Fig. 4). The survey spectrum indicates that the sample consists mainly of the elements Ti, Ag, O, Cl and C. No other impurity peaks were detected, evidencing the high purity of the resulting sample. High resolution XPS spectrum of Ti2p and O1s for PTiO2NP and RTiO2HS are shown in Fig. 5A and B. The characteristic peaks of Ti2p XPS has a slight shift to lower binding energy after reduction of the TiO2 by NaBH4, which can be ascribed to the exist of Ti3þ in the prepared samples [1]. The result suggests that oxygen vacancies (Ti3þ) were created in RTiO2HS during NaBH4 reduction. The peak of O 1s XPS spectra (Fig. 5B) also exhibits a slight shift to lower energy (from 529.8 to 529.2 eV) after the chemical reduction, which may be the result of electron transfer from the conduction band to oxygen vacancy state [8]. The results also showed that even after calcination at high temperature in air there are still existing stable Ti3þ in the RTiO2HS. Electron paramagnetic resonance (EPR) spectra (Fig. S2A) shows a very strong response at g-value of 2.002 than that of blank test and PTiO2NP which may be assigned to the presence of an electron trapped on an oxygen vacancy [19]. The signal of Ti3þ species at g < 2 was not distinct on the curves, which may be due to the testing environment (at room temperature). As is known the Ti3þ species can only be detected by EPR measurement at 77 K [19]. However, a superparamagnetic behavior due to localized Ti3þ 3d1 states can be observed (Fig. S2B) which is consistent with the previous study [20]. Therefore, a strong EPR signal attributed to the existence of oxygen vacancies and super paramagnetic behavior may also suggest the existence of Ti3þ in RTiO2HS. The high-resolution spectrum of Ag 3d species is shown in Fig. 6A. It consists of two individual peaks at approximately 373.08 eV and 367.35 eV,
150000
O1s
C1s 100000
Count(a.u)
48
Ag3p
O(KLL)
Ti2p1/2
50000 Cl2p
Ag3d
0 0
200
400 600 800 Binding Energy(eV)
1000
Fig. 4. A wide survey scan XPS spectra of as-synthesized Ag-RTiO2HS.
which are ascribed to the peaks of Ag 3d3/2 and Ag3d5/2, respectively. The peaks ascribe to Ag 3d5/2 exhibit the negative shift in the Ag-RTiO2HS sample, which was different from the pure metallic Ag (The standard binding energy of Ag 3d5/2 for bulk Ag is about 368.2 eV) [21,22]. It might be due to the interaction between Ag and AgCl. Compared the peak strength of Ag and Cl XPS (Fig. 6B), it can be inferred that the surface atomic ratio of silver to chlorine was much higher than the stoichiometric ratio in AgCl (1:1), indicating the existence of excessive Ag on the surface of Ag-RTiO2HS. Considering the fact that there was metallic silver in the as-prepared Ag-RTiO2HS sample from the HRTEM and XPS results, this implied that the excessive amount of silver might have been produced from the photo-reduction of AgCl on the surface. So, based on the result of XRD, TEM and XPS, the coexistence of Ag0 and AgCl species in the samples could be confirmed. Fig. 7 shows the UVevis diffuse reflectance spectra of PTiO2NP, RTiO2HS and Ag-RTiO2HS. It can be seen that the self-doped samples of RTiO2HS exhibit a slight enhancement in the UV light region, and strong absorbance in the visible light region than the undoped sample of PTiO2NP due to the presence of Ti3þ (oxygen vacancies), leading to mid-gap energy levels corresponding to the excitation from the valence band to the impurity band [8]. This result indicates that the reduction treatment extended the visible light absorption of the RTiO2HS. For the Ag-RTiO2HS, the band gap absorption edge shifts slightly to a higher wavelength, and moreover, an additional broad absorption is observed in the region of 400e700 nm, which could be due to surface plasmon resonance of the Ag nanoparticles produced by the photoreduction of AgCl. The estimated value of the band gap (Eg) of the samples could be calculated by the following formula [13].
Aðhv EgÞn=2 ¼ ahv
Fig. 3. XRD of Ag-RTiO2HS (A), RTiO2HS (B) and RTiO2NP (C).
Ti2p3/2
(1)
where A, a, v, and Eg are the constant, absorption coefficient, light frequency, and band gap, respectively. According to the reference, the value of n for PTiO2NP, RTiO2HS and Ag- RTiO2HS is 4. The Eg of PTiO2NP, RTiO2HS and Ag-RTiO2HS are calculated to be 3.15 eV, 2.98 eV and 2.93 eV (Fig. 7B), respectively, which implied that AgRTiO2HS may have best visible light induced photocatalytic activities. The effect of Ag/AgCl loading on the photocatalytic activity of Ag-RTiO2HS catalysts has been studied by degradation of RhB under simulated solar and visible light irradiation. Fig. 8 shows the photocatalytic degradation of RhB (A) and comparison of kapp (B) with different TiO2 under simulated solar light. It can be seen that all the self-doped TiO2 show higher photocatalytic activities than
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Fig. 5. Fine scan XPS spectra of Ti2p (B) and O1s (A) of RTiO2HS and PTiO2NP.
1400
3000
B Counts(a.u)
Counts(a.u)
A 1200
2500
1000
2000
1500
800 360
370
380
190
Binding Energy(eV)
200
210
Binding Energy(eV)
Fig. 6. Fine scan XPS spectra of Ag3d (B) and Cl2p (A) of Ag-RTiO2HS.
that of undoped samples and the hollow structured TiO2 have better photocatalytic performance than the particle TiO2 which may due to fact that the larger surface area of hollow structured TiO2 can introduce more absorption sites and active centers. Moreover, Ag-RTiO2HS has the best photocatalytic performance. It is well known that photocatalytic oxidation of organic pollutants follows first-order kinetics. The linear relationship between ln(C0/ C) and t confirms that the photocatalytic degradation process of RhB followed the apparent pseudo-first-order model expressed in Eq. (2) [16]:
lnðC0 =CÞ ¼ kappt
(2)
Where kapp is the apparent pseudo-first-order rate constant (min1), C is RhB concentration in aqueous solution at time t (mg/ L), C0 is initial RhB concentration (mg/L). Thus, from the comparison of kapp (Fig. 8B) it can be seen that the Ag-RTiO2HS catalysts have the highest photocatalytic activities under solar light irradiation. Enhanced visible light photocatalytic activity was also investigated on Ag-RTiO2HS (Fig. 9). The similar first-order kinetics
Fig. 7. UVevis diffuse reflectance spectra (A) and Band gap calculation by Tauc's method (B).
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Fig. 8. Photocatalytic degradation of rhodamine B (A) and comparison of kapp (B) with different TiO2 under simulated solar light.
reaction rate constant (kapp) was calculated and illustrated on Fig. 9B. As can be seen, the effect of photolysis and absorption of catalyst on the decomposition of RhB was negligible during the test period. It can also be observed that both the self-doped samples of RTiO2NP and RTiO2HS showed higher photocatalytic activities than the undoped samples of PTiO2NP and PTiO2HS with the hollow structured photocatalyst displaying better photocatalytic activities than the particles. It should be noted that the kapp of Ag-RTiO2HS is 7.8 times higher than that of PTiO2NP under visible light irradiation which is much higher than the ratio of 4.3 for the two samples under solar light. This result may suggest that higher visible light photocatalytic performance can be obtained on Ag-RTiO2HS catalysts. The better photocatalytic performance of the hollow structured photocatalysts compared with the nanoparticles catalysts may be due to the increase of the specific surface area and the enhanced photocatalytic activity of the Ag-RTiO2HS may be owing to the synthetic effect of the coupling of Ag/AgCl and reduced TiO2 hollow structures. The temporal evolution of the absorption spectral changes during the photocatalytic degradation of RhB by the Ag-RTiO2HS catalystsis was shown in Fig. 10. It is found that the absorption peak of the solution decreases quickly with the increase of irradiation time. The color of the suspension changed to colorless after irradiation (insert in Fig. 10), which indicates that the RhB molecular was indeed decomposed in the reaction process. The stability of a practical photocatalyst is as same important as its photocatalytic activity. The plasmonic photocatalyst Ag-RTiO2HS was here investigated through recycling experiments. As shown in
Fig. 10. UVevis spectra changes in the degradation of RhB on the photocatalysts.
Fig. 11, after four cycles of photodegradation of RhB, the sample did not show any significant loss of photocatalytic activity, which indicates that the catalyst can keep stable during the photocatalytic reaction. EIS is a high effective method for probing the photoelectrochemical characterization of surface-modified electrodes. In this study, EIS was carried out to investigate the changes of electron transfer resistance that aroused from the surface modification step as shown in Fig. 12. The RTiO2HS electrode exhibits poor
Fig. 9. Photocatalytic degradation of rhodamine B (A) and comparison of kapp (B) with different TiO2 under visible light.
H. Yin et al. / Journal of Alloys and Compounds 657 (2016) 44e52
C/C0
1.0
0.5
0.0 0
40
80
Time(min) Fig. 11. Cycling degradation of RhB under visible light irradiation over the Ag-RTiO2HS.
conductivity with an EIS of about 532 U. After deposited with Ag/ AgCl, the EIS of Ag-RTiO2HS electrode shows a small semicircle (137 U), only about a quarter of that of RTiO2HS, which suggests the successful deposition of Ag/AgCl on the RTiO2HS surface. Importantly, a smaller impedance arc radius in Nyquist plots indicates better electron/hole pair separation efficiency [23]. In this experiment, Ag-RTiO2HS had better electron/hole pair separation efficiency, suggesting that the Ag/AgCl is favorable to the separation of photogenerated electron/hole pair which can introduce higher photocatalytic activities. The mechanism for the remarkably enhanced photocatalytic performance of the Ag-RTiO2HS photocatalysts can be understood from the following aspects. Firstly, the reduction of RTiO2HS by NaBH4 introduces abundant oxygen vacancies into the bulk of RTiO2HS, which induces a narrowed band gap and a slight VB tailing. In addition, self-doped Ti3þ species introduce localized states below the CB minimum of RTiO2HS. Both the VB tailing and Ti3þ localized states of RTiO2HS are responsible for their significant visible light absorption. Then, the existence of Ag/AgCl nanocrystals on the surfaces of RTiO2HS forms a uniquely hierarchical nanostructure, which provides a high surface area and a large number of interfaces between the Ag/AgCl and RTiO2HS. The high surface areas and profuse interfaces are accessible to the outer environment, and provide numerous active sites for the photodegradation of dye molecules. Thirdly, the metal Ag clusters formed in situ on the semiconductors AgCl and RTiO2HS remarkably enhance the absorption in the visible light region due to the surface plasmonic
Fig. 12. Nyquist plots of electrochemical impedance of the RTiO2HS and Ag-RTiO2HS.
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resonance effect, which photogenerates transient holes that can oxidize the dye molecules [24,25]. Last but not least, the positively synergistic effects of the coupling of Ag/AgCl and RTiO2HS improve the effective separation of the photo-generated electronehole pairs. The charge separation and transfer in the Ag-RTiO2HS photocatalysts under visible-light irradiation was illustrated in Fig. 13. As can be seen in the illustration, under visible light irradiation, photogenerated electronehole pairs can be formed in Ag NPs due to surface plasmon resonance. The photoexcited electrons at the silver NPs are injected into the TiO2 conduction band. Accordingly, the electron could be transferred to molecular oxygen, which is adsorbed on the surface of the catalyst to generate superoxide radical (O 2 ). Meanwhile, the holes transfer to the surface of the AgCl particles and cause the oxidation of Cl and OH ions to Cl0 and OH atoms, which are reactive radical species and can oxidize organic pollutant. Similar processes were also proposed in AgeAgCleTiO2 [26] and Ag/AgCl/TiO2 [27] systems and other photocatalysts [28,29]. However, in the Ag-RTiO2HS system, except the general electron and charge transfer the RTiO2HS could also absorb the visible light. Therefore, under the visible light irradiation, the photogenerated electrons of RTiO2HS can transfer from VB to CB of RTiO2HS to recombine with the plasmon-induced electrons produced by plasmonic absorption of Ag nanoparticles, while the VB holes remain on RTiO2HS to oxidize organic substances. So, the positively synergistic effects of the coupling of Ag/AgCl and RTiO2HS may greatly improve the effective separation of the photogenerated electronehole pairs and hence enhance the photocatalytic activities of the catalysts. 4. Conclusions The Ag/AgCl modified self-doped TiO2 hollow spheres were prepared firstly by a solegel method and subsequently depositionprecipitation and photo-reduction process. The as prepared hollow
Fig. 13. Schematic illustration of the charge separation and transfer in the Ag-RTiO2HS photocatalysts under visible-light irradiation.
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structured photocatalysts, with diameter about 800e950 nm and shell thickness of 20e30 nm, show highly efficient solar light and visible light induced photocatalytic activities. The improved visible light induced photocatalytic activities may be due to the oxygen vacancies and Ti3þ localized states of RTiO2HS and the surface plasmonic resonance effect of Ag nanoparticles. The present work suggests that the Ag/AgCl modified self-doped TiO2 hollow spheres may be a promising photocatalyst for degrading organic pollutants and environmental remediation. Acknowledgment This work is financially supported by the National Nature Science Foundation of China (No. 41271249), Educational Commission of Zhejiang Province (No.Y201329269) and Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Zhejiang Sci-Tech University), Ministry of Education (No. 2013008). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.10.055. References [1] X. Chen, L. Liu, Y.Y. Peter, S.S. Mao, Science 331 (2011) 746e750. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269e271. [3] Y.H. Hu, Angew. Chem. Int. Ed. 51 (2012) 12410e12412. [4] W. Zhou, W. Li, J.-Q. Wang, Y. Qu, Y. Yang, Y. Xie, K. Zhang, L. Wang, H. Fu, D. Zhao, J. Am. Chem. Soc. 136 (2014) 9280e9283. [5] X. Lü, X. Mou, J. Wu, D. Zhang, L. Zhang, F. Huang, F. Xu, S. Huang, Adv. Funct. Mater 20 (2010) 509e515.
[6] H. Tong, N. Umezawa, J. Ye, Chem. Commun. 47 (2011) 4219e4221. [7] O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates, J. Phys. Chem. B 108 (2004) 6004e6008. [8] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3026e3033. [9] F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt, P. Feng, J. Am. Chem. Soc. 132 (2010) 11856e11857. [10] Z. Wang, C. Yang, T. Lin, H. Yin, P. Chen, D. Wan, F. Xu, F. Huang, J. Lin, X. Xie, M. Jiang, Adv. Funct. Mater 23 (2013) 5444e5450. [11] F. Teng, M. Li, C. Gao, G. Zhang, P. Zhang, Y. Wang, L. Chen, E. Xie, Appl. Catal. B Environ. 148e149 (2014) 339e343. [12] Z. Wang, C. Yang, T. Lin, H. Yin, P. Chen, D. Wan, F. Xu, F. Huang, J. Lin, X. Xie, M. Jiang, Energy Environ. Sci. 6 (2013) 3007e3014. [13] M. Xing, W. Fang, M. Nasir, Y. Ma, J. Zhang, M. Anpo, J. Catal. 297 (2013) 236e243. [14] D. Bu, H. Zhuang, Catal. Commun. 29 (2012) 24e28. [15] X. Lin, F. Rong, X. Ji, D. Fu, J. Sol Gel Sci. Technol. 59 (2011) 283e289. [16] J.G. McEvoy, W. Cui, Z. Zhang, Appl. Catal. B Environ 144 (2014) 702e712. [17] W. Liu, D. Chen, S.H. Yoo, S.O. Cho, Nanotechnology 24 (2013). [18] Y. Xu, H. Xu, H. Li, J. Xia, C. Liu, L. Liu, J. Alloys Compd. 509 (2011) 3286e3292. [19] C. Marchiori, G. Liberto, G. Soliveri, L. Loconte, L. Presti, D. Meroni, M. Ceotto, C. Oliva, S. Cappelli, G. Cappelletti, C. Aieta, S. Ardizzone, J. Phys. Chem. C 118 (2014) 24152e24164. [20] Q. Zhao, P. Wu, B. Li, Z. Lu, E. Jiang, J. Appl. Phys. 104 (2008), 073911-073911073915. [21] Y. Xu, H. Xu, H. Li, J. Yan, J. Xia, S. Yin, Q. Zhang, Colloid Surf. A Physicochem. Eng. Asp. 416 (2013) 80e85. [22] W. Lu, S. Gao, J. Wang, J. Phys. Chem. C 112 (2008) 16792e16800. [23] C. Wang, J. Wu, P. Wang, Y. Ao, J. Hou, J. Qian, Sens. Actuator B Chem. 181 (2013) 1e8. [24] J.-F. Guo, B. Ma, A. Yin, K. Fan, W.-L. Dai, J. Hazard. Mater. 211e212 (2012) 77e82. [25] D. Chen, T. Li, Q. Chen, J. Gao, B. Fan, J. Li, X. Li, R. Zhang, J. Sun, L. Gao, Nanoscale 4 (2012) 5431e5439. [26] Y.Q. Yang, G.K. Zhang, W. Xu, J. Colloid Interface Sci. 376 (2012) 217e223. [27] J. Zhou, Y. Cheng, J. Yu, J. Photochem. Photobiol. A Chem. 223 (2011) 82e87. [28] Z. Zhou, M. Long, W. Cai, J. Cai, J. Mol. Catal. A Chem. 353e354 (2012) 22e28. [29] Y. Liang, S. Lin, J. Hu, L. Liu, J. McEvoy, W. Cui, J. Mol. Catal. A Chem. 383e384 (2014) 231e238.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Ag/AgCl modified self-doped TiO2 hollow sphere with enhanced visible light photocatalytic activity Haoyong Yin a, Xulong Wang a, Ling Wang b, QLin Nie a, *, Yang Zhang c, Qiuli Yuan c, Weiwei Wu a a b c
Institute of Environmental Materials and Applications, Hangzhou Dianzi University, Hangzhou, 310018, PR China Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China College of Science, Hangzhou Dianzi University, Hangzhou 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 14 September 2015 Accepted 6 October 2015 Available online xxx
The Ag/AgCl modified self-doped TiO2 hollow spheres were successfully prepared firstly by a solegel method using carbon sphere as template and subsequently deposition-precipitation and photoreduction process. Characteristics and properties of the products were investigated using X-ray diffraction (XRD), UVevis diffuse reflectance spectra (UVevis DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the self-doped TiO2 hollow spheres, with diameters about 800e950 nm and shell thickness of 20e30 nm, are constituted with thousands of self-doped TiO2 nanoparticles with diameters of 20e30 nm. Ag/AgCl modified self-doped TiO2 hollow spheres showed broad absorption in the region of 400e700 nm arising from surface plasmon resonance of Ag nanoparticles. The as prepared samples showed improved solar light and visible light induced photocatalytic activities by decomposing Rhodamine B. The improved visible light photocatalytic activities may be due to the oxygen vacancies and Ti3þ localized states of the self-doped TiO2 hollow spheres and the surface plasmonic resonance effect. © 2015 Elsevier B.V. All rights reserved.
Keywords: Self-doped TiO2 Hollow sphere Surface plasmon resonance Photocatalyst
1. Introduction Searching for efficient semiconductor photocatalysts utilizing visible-light solar energy for environmental purification and energy conversion remains a great challenge [1e4]. Many efforts have been focused on enhancing visible light absorption by narrowing the band gap of TiO2, including element doping and oxygen deficiency [2,5,6]. Unfortunately, the doped impurity species in TiO2 often lead to severe thermal instability and a relatively fast carrier recombination rate since the foreign introduced atoms can serve as carrier recombination centers, which undoubtedly impair the photocatalytic activities of TiO2 [2,7]. Recently, Mao and co-workers [1] have demonstrated a novel approach to enhance the solar harvesting by introducing disorders in the surface layers of high crystalline TiO2 nanoparticles through hydrogenation. This reduced TiO2 with Ti3þ self-doping has attracted enormous attention for its enhanced visible light
* Corresponding author. E-mail address: [email protected] (Q. Nie). http://dx.doi.org/10.1016/j.jallcom.2015.10.055 0925-8388/© 2015 Elsevier B.V. All rights reserved.
absorption owing to the interband level of the Ti3þ (oxygen vacancy) and increased electrical conductivity arising from its high donor density, which lead to significantly enhanced visible light induced photocatalytic activity compared with pure samples [1,8,9]. Not surprisingly, the work triggered a large amount of follow-up studies where the self-doped TiO2 was prepared by various reductive treatments methods, mostly using hydrogenation methods, chemical vapor deposition, and high energy particle (laser, electron or Arþ) bombardment [1,8,10e12]. Although these methods have their own merits, they are severely limited by high cost and complicated procedures and not suitable for practical applications. Moreover, the surface Ti3þ and oxygen defects on the TiO2 are usually not stable enough in air, since the surface Ti3þ is easily oxidized into Ti4þ by the dissolved oxygen in water. Very recently, the low-cost NaBH4 was used as the reductant to prepare the TiO2 catalysts with self-doped Ti3þ by a solvothermal method to resolve these issues [13]. Nevertheless, the study of self-doped TiO2 is still in its infancy and there are still many problems to be resolved in this field, such as the morphology control of the self-doped TiO2, the improvement of the photocatalytic performance of self-doped TiO2, the relationship between the Ti3þ doping and photocatalytic
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activity of TiO2 and the investigation of its photocatalytic activity both under solar light and visible light. In oxide materials chemistry, solegel process is mostly known for its versatility, scalability and economic application. Besides, it is easy to control the materials morphology in a simple and safe way [14,15]. Recently, plasmonic photocatalysts comprising semiconductors and metal nanoparticles (NPs) have exhibited excellent photocatalytic activities under visible light due to the surface plasmon resonance (SPR) effect of the metal NPs. Silver halides, when combined with silver NPs, and particularly TiO2, exhibit good stability, allowing their application as photocatalysts [16e18]. To the best of our knowledge, there are no reports on the preparation of Ag/AgCl modified self-doped TiO2 hollow spheres. In the present research we demonstrated the synthesis of Ag/AgCl modified selfdoped TiO2 hollow spheres by a simple method with self-doped TiO2 hollow spheres prepared by a solegel method using carbon spheres as templates and Ag/AgCl modified self-doped TiO2 hollow spheres synthesized by a deposition-precipitation method and photo-reduction process. The as-prepared photocatalysts display much improved visible and solar light induced photocatalytic activity. Since the process of self-doped TiO2 hollow spheres formation undergoes the calcination of the sample in air the as prepared photocatalysts possess more stable Ti3þ both in the bulk and surface. The combined surface plasmon resonance effect and the band gap engineering of TiO2 enable its greatly enhanced visible light induced photocatalytic activities. 2. Experimental 2.1. Preparation of the carbon sphere templates In a typical procedure, 5 g of glucose was firstly dissolved in 40 mL of water. Then the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. Finally, hydrothermal treatments were carried out at 180 C for 8 h and then cooled down to room temperature naturally. The obtained products were then centrifuged, washed with absolute alcohol and deionized water for several times, then dried at 80 C for 10 h. 2.2. Preparation of the self-doped TiO2 hollow spheres
45
method. Typically, the synthesized RTiO2HS powders were dispersed in 50 mL deionized water and sonicated for 10 min. Then 10 mL of 0.1 mol/L AgNO3 solution was added into the RTiO2HS suspension and sonicated for another 10 min. After stirring magnetically for 20 min, 20 mL of 0.1 mol/L HCl aqueous solution was added, sonicated for 10 min and stirred magnetically for 20 min. Subsequently, the resulting suspension was irradiated by a 250 W high pressure mercury lamp for 30 min to reduce partial Agþ ions in the AgCl particles to Ag0 species by photochemical decomposition of AgCl. The whole reaction process was kept at room temperature. Finally, the dark yellow Ag/AgCl modified selfdoped TiO2 hollow spheres (Ag-RTiO2HS) powders were collected, washed for several times until the pH of the suspension was about 7, centrifuged and dried at 80 C for several hours. 2.4. Photocatalysts characterization The morphologies and microstructures of the obtained products were analyzed by a Hitachi S-4700 field emission scanning electron microscope (SEM, scanning voltages 15 kV) and JEOL 200CX highresolution transmission electron microscopy (HRTEM, accelerating voltage 200 kV). The phase structures of the samples were measured by X-ray diffraction (XRD) studies using an X-ray diffractometer (Thermo ARL SCINTAG X'TRA) with CuKa radiation (l ¼ 0.154056 nm) under 40 kV with the 2q ranging from 20 to 70 . UVevis diffuse reflectance spectra (DRS) of the products was performed on an UVevis spectrophotometer (UV-2550, SHIMADZU, Japan) using BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5400 spectrometer and the shift of binding energy due to relative surface charge was corrected using the C1s level at 284.6 eV as an internal standard. The Electrochemical impedance spectroscopy (EIS) analysis were measured on an electrochemical workstation (Zennium, Zahner, Germany), using a conventional three-electrode cell. FTO glass coated with the as-prepared samples (0.1 mg) served as the working electrode. The counter and the reference electrodes were a platinum plate and a saturated AgeAgCl electrode, respectively. A 0.1 M NaOH aqueous solution was used as electrolyte. The working electrode (the active area 0.5 cm 0.5 cm) was prepared using an amended doctor blade method. Firstly 0.1 g of sample was ground with 1 mL terpineol. The slurry was then coated onto FTO glass by the doctor blade method which was cleaned in distilled water and ethanol by ultrasonication. These electrodes were preliminarily dried at 70 C and calcined at 200 C for 1 h under Ar atmosphere.
As prepared carbon spheres (0.5 g) were firstly dispersed in 30 mL of ethanol solution (25 mL CH3CH2OH þ 5 mL CH3COOH) ultrasonically for 10 min. Then 10 mL of tetrabutyltitanate solution (5 mL Ti(OBu)4 þ 5 mL CH3CH2OH) was added dropwise to the stirred solution to form a sol. Subsequently 0.2 g of NaBH4 was added into the sol with continuously stirring until it formed a gel. The gel was then dried at 80 C overnight and the obtained powder was grinded and calcinated at 500 C in air for 3 h. Finally, the product was washed in 1 M HCl solution by stirring for 8 h and then washed by distilled water for several times and dried at 80 C in vacuum for 6 h to get the self-doped TiO2 hollow spheres (RTiO2HS). For comparison, the reduced TiO2 nanoparticles (RTiO2NP) were prepared without carbon sphere addition and the pure TiO2 hollow spheres (PTiO2HS) and pure TiO2 nanoparticles (PTiO2NP) were also prepared under the same condition without the NaBH4 addition. The color of the obtained PTiO2NP and RTiO2HS was white and pale yellow respectively which may suggest the possible visible light absorption ability for the partial reduction of RTiO2HS.
The photocatalytic activity of the catalysts was evaluated by degradation of RhodamineB (RhB) under solar and visible light irradiation. A 300 W Xe lamp was used as the simulated solar light. The 420 nm cut off filter was used to get the visible light source. In each experiment, 0.1 g of the as-prepared photocatalyst was added into 200 mL of RhB solution (5 mg/L). Before irradiation, the suspensions were placed in dark and stirred for 30 min to ensure the establishment of adsorptionedesorption equilibrium between the catalyst and RhB. Subsequently, at every interval, about 5 mL of suspension was sampled and centrifuged to remove the photocatalyst particles. The concentration of filtrates was analyzed by measuring the maximum absorbance at 553 nm for RhB using a UV-2550 UVevis spectrophotometer.
2.3. Preparation of Ag/AgCl modified self-doped TiO2 hollow spheres
3. Results and discussion
Ag/AgCl nanoparticles were deposited on the as-prepared RTiO2HS samples by a precipitationephotoreduction reaction
The SEM morphologies of the prepared photocatalysts and carbon spheres are shown in Fig. 1. It can be seen that the as
2.5. Photocatalytic studies
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Fig. 1. SEM images of Carbon spheres (A), RTiO2HS (B, C, and D) and Ag-RTiO2HS (E and F).
prepared carbon sphere templates are smooth and regular spherical shape with diameters of about 300e450 nm. The surfaces of the sample become rough after coating with the self-doped TiO2 and removal of the carbon sphere templates (Fig. 1B). The microspheres also grow larger after coated with TiO2 with diameter increased to about 800e950 nm. The cracked RTiO2HS indicated that the as prepared microspheres have hollow structures when the templates were annealed and removed in the form of CO2 (Fig. 1C). The escaped templates left the hole on the RTiO2HS surface with similar diameter of carbon spheres. The DSC/TG curves (Fig. S1) show distinctly much higher weight loss of RTiO2HS (28% for PTiO2NP versus 35% for RTiO2HS) which can also suggest that the carbon sphere templates can be removed and the hollow structured microspheres was formed at the temperature higher than 400 C. Detailed hollow structures of the RTiO2HS was shown in Fig. 1D which revealed that RTiO2HS are composed by single layer of selfdoped TiO2 with diameter of TiO2 nanoparticles less than 30 nm and accordingly the shell thickness of the RTiO2HS is about 20e30 nm. The SEM images of the Ag/AgCl modified TiO2 hollow spheres (Ag-RTiO2HS) are shown in Fig. 1E and F. It can be observed that the spherical hollow structures of RTiO2HS have not been damaged by the deposition of Ag/AgCl nanoparticles. However, the surface of RTiO2HS becomes much rougher and the nanoparticles of TiO2 grow a little bigger which may suggest that the Ag/AgCl
nanoparticles were deposited on the RTiO2HS surface. The specific surface areas of PTiO2NP, RTiO2NP, PTiO2HS, RTiO2HS and AgRTiO2HS were measured to be 34.7, 35.3, 88.4, 89.2 and 89.8 m2/g respectively which may also indicates the hollow structure formation of the template removed samples. Substructures of RTiO2HS and Ag-RTiO2HS can be further observed by TEM and HRTEM (Fig. 2). TEM image (Fig. 2A) distinctly shows that the surface of RTiO2HS was constituted by thousands of self-doped TiO2 nanoparticles (RTiO2NP) with diameter about 20e30 nm which is consistent with the shell thickness of the RTiO2HS (Fig. 1D). High-resolution TEM (Fig. 2B) detected a distinct crystal spacing of 0.35 nm, which corresponds to the (101) interplanar spacing of anatase TiO2. The insert in Fig. 2B shows a selected electron diffraction diagram, and bright, clear anatase concentric diffraction rings can be observed. Fig. 2C and E shows the HRTEM and enlarged HRTEM images of Ag-RTiO2HS. It can be seen that the distinct (101) interplanar spacing of anatase TiO2 still can be clearly observed on the surface of Ag-RTiO2HS. However, there also exhibit different fringes with lattice spacing of 0.235 nm and 0.196 nm, which correspond to the (111) plane of metal Ag and the (220) plane of AgCl, respectively. The chemical composition of the prepared sample was determined by EDS (Fig. 2D). The EDS spectrum of the Ag-RTiO2HS sample reveals that Ti, Ag, O, and Cl are the major elements. Pt peaks in the spectrum are attributed to
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Fig. 2. TEM (A) and HRTEM (B) images of RTiO2HS (Inset is the ED of the RTiO2HS), EDS (C) and HRTEM (D and E) of Ag-RTiO2HS.
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the Pt coating used in SEM analysis. The HRTEM and EDS of AgRTiO2HS indicate that the Ag/AgCl was successfully deposited on the surface of RTiO2HS. Fig. 3 shows the XRD patterns of RTiO2NP, RTiO2HS and AgRTiO2HS. As shown in Fig. 3, all the diffraction peaks of RTiO2NP and RTiO2HS could be indexed to the anatase of TiO2 (JCPDS Card No. 21-1272), suggesting that the reduction of TiO2 by NaBH4 does not change its crystal structures. In addition to RTiO2NP and RTiO2HS diffraction peaks, there are some strong peaks observed in the XRD patterns of Ag-RTiO2HS hybrids at about 2q ¼ 27.91, 32.21, 46.21, 57.51 and 67.4 which can be indexed to (111), (200), (220), (222), (400) planes of the cubic phase AgCl (JCPDS No.85-1355). It is similar to the results reported by Liu et al. [17]. However, no distinct diffraction peaks of metallic Ag could be found, probably due to its low content, high dispersity or small crystallite. The chemical composition of the Ag-RTiO2HS sample was further characterized by XPS spectra (Fig. 4). The survey spectrum indicates that the sample consists mainly of the elements Ti, Ag, O, Cl and C. No other impurity peaks were detected, evidencing the high purity of the resulting sample. High resolution XPS spectrum of Ti2p and O1s for PTiO2NP and RTiO2HS are shown in Fig. 5A and B. The characteristic peaks of Ti2p XPS has a slight shift to lower binding energy after reduction of the TiO2 by NaBH4, which can be ascribed to the exist of Ti3þ in the prepared samples [1]. The result suggests that oxygen vacancies (Ti3þ) were created in RTiO2HS during NaBH4 reduction. The peak of O 1s XPS spectra (Fig. 5B) also exhibits a slight shift to lower energy (from 529.8 to 529.2 eV) after the chemical reduction, which may be the result of electron transfer from the conduction band to oxygen vacancy state [8]. The results also showed that even after calcination at high temperature in air there are still existing stable Ti3þ in the RTiO2HS. Electron paramagnetic resonance (EPR) spectra (Fig. S2A) shows a very strong response at g-value of 2.002 than that of blank test and PTiO2NP which may be assigned to the presence of an electron trapped on an oxygen vacancy [19]. The signal of Ti3þ species at g < 2 was not distinct on the curves, which may be due to the testing environment (at room temperature). As is known the Ti3þ species can only be detected by EPR measurement at 77 K [19]. However, a superparamagnetic behavior due to localized Ti3þ 3d1 states can be observed (Fig. S2B) which is consistent with the previous study [20]. Therefore, a strong EPR signal attributed to the existence of oxygen vacancies and super paramagnetic behavior may also suggest the existence of Ti3þ in RTiO2HS. The high-resolution spectrum of Ag 3d species is shown in Fig. 6A. It consists of two individual peaks at approximately 373.08 eV and 367.35 eV,
150000
O1s
C1s 100000
Count(a.u)
48
Ag3p
O(KLL)
Ti2p1/2
50000 Cl2p
Ag3d
0 0
200
400 600 800 Binding Energy(eV)
1000
Fig. 4. A wide survey scan XPS spectra of as-synthesized Ag-RTiO2HS.
which are ascribed to the peaks of Ag 3d3/2 and Ag3d5/2, respectively. The peaks ascribe to Ag 3d5/2 exhibit the negative shift in the Ag-RTiO2HS sample, which was different from the pure metallic Ag (The standard binding energy of Ag 3d5/2 for bulk Ag is about 368.2 eV) [21,22]. It might be due to the interaction between Ag and AgCl. Compared the peak strength of Ag and Cl XPS (Fig. 6B), it can be inferred that the surface atomic ratio of silver to chlorine was much higher than the stoichiometric ratio in AgCl (1:1), indicating the existence of excessive Ag on the surface of Ag-RTiO2HS. Considering the fact that there was metallic silver in the as-prepared Ag-RTiO2HS sample from the HRTEM and XPS results, this implied that the excessive amount of silver might have been produced from the photo-reduction of AgCl on the surface. So, based on the result of XRD, TEM and XPS, the coexistence of Ag0 and AgCl species in the samples could be confirmed. Fig. 7 shows the UVevis diffuse reflectance spectra of PTiO2NP, RTiO2HS and Ag-RTiO2HS. It can be seen that the self-doped samples of RTiO2HS exhibit a slight enhancement in the UV light region, and strong absorbance in the visible light region than the undoped sample of PTiO2NP due to the presence of Ti3þ (oxygen vacancies), leading to mid-gap energy levels corresponding to the excitation from the valence band to the impurity band [8]. This result indicates that the reduction treatment extended the visible light absorption of the RTiO2HS. For the Ag-RTiO2HS, the band gap absorption edge shifts slightly to a higher wavelength, and moreover, an additional broad absorption is observed in the region of 400e700 nm, which could be due to surface plasmon resonance of the Ag nanoparticles produced by the photoreduction of AgCl. The estimated value of the band gap (Eg) of the samples could be calculated by the following formula [13].
Aðhv EgÞn=2 ¼ ahv
Fig. 3. XRD of Ag-RTiO2HS (A), RTiO2HS (B) and RTiO2NP (C).
Ti2p3/2
(1)
where A, a, v, and Eg are the constant, absorption coefficient, light frequency, and band gap, respectively. According to the reference, the value of n for PTiO2NP, RTiO2HS and Ag- RTiO2HS is 4. The Eg of PTiO2NP, RTiO2HS and Ag-RTiO2HS are calculated to be 3.15 eV, 2.98 eV and 2.93 eV (Fig. 7B), respectively, which implied that AgRTiO2HS may have best visible light induced photocatalytic activities. The effect of Ag/AgCl loading on the photocatalytic activity of Ag-RTiO2HS catalysts has been studied by degradation of RhB under simulated solar and visible light irradiation. Fig. 8 shows the photocatalytic degradation of RhB (A) and comparison of kapp (B) with different TiO2 under simulated solar light. It can be seen that all the self-doped TiO2 show higher photocatalytic activities than
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49
Fig. 5. Fine scan XPS spectra of Ti2p (B) and O1s (A) of RTiO2HS and PTiO2NP.
1400
3000
B Counts(a.u)
Counts(a.u)
A 1200
2500
1000
2000
1500
800 360
370
380
190
Binding Energy(eV)
200
210
Binding Energy(eV)
Fig. 6. Fine scan XPS spectra of Ag3d (B) and Cl2p (A) of Ag-RTiO2HS.
that of undoped samples and the hollow structured TiO2 have better photocatalytic performance than the particle TiO2 which may due to fact that the larger surface area of hollow structured TiO2 can introduce more absorption sites and active centers. Moreover, Ag-RTiO2HS has the best photocatalytic performance. It is well known that photocatalytic oxidation of organic pollutants follows first-order kinetics. The linear relationship between ln(C0/ C) and t confirms that the photocatalytic degradation process of RhB followed the apparent pseudo-first-order model expressed in Eq. (2) [16]:
lnðC0 =CÞ ¼ kappt
(2)
Where kapp is the apparent pseudo-first-order rate constant (min1), C is RhB concentration in aqueous solution at time t (mg/ L), C0 is initial RhB concentration (mg/L). Thus, from the comparison of kapp (Fig. 8B) it can be seen that the Ag-RTiO2HS catalysts have the highest photocatalytic activities under solar light irradiation. Enhanced visible light photocatalytic activity was also investigated on Ag-RTiO2HS (Fig. 9). The similar first-order kinetics
Fig. 7. UVevis diffuse reflectance spectra (A) and Band gap calculation by Tauc's method (B).
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Fig. 8. Photocatalytic degradation of rhodamine B (A) and comparison of kapp (B) with different TiO2 under simulated solar light.
reaction rate constant (kapp) was calculated and illustrated on Fig. 9B. As can be seen, the effect of photolysis and absorption of catalyst on the decomposition of RhB was negligible during the test period. It can also be observed that both the self-doped samples of RTiO2NP and RTiO2HS showed higher photocatalytic activities than the undoped samples of PTiO2NP and PTiO2HS with the hollow structured photocatalyst displaying better photocatalytic activities than the particles. It should be noted that the kapp of Ag-RTiO2HS is 7.8 times higher than that of PTiO2NP under visible light irradiation which is much higher than the ratio of 4.3 for the two samples under solar light. This result may suggest that higher visible light photocatalytic performance can be obtained on Ag-RTiO2HS catalysts. The better photocatalytic performance of the hollow structured photocatalysts compared with the nanoparticles catalysts may be due to the increase of the specific surface area and the enhanced photocatalytic activity of the Ag-RTiO2HS may be owing to the synthetic effect of the coupling of Ag/AgCl and reduced TiO2 hollow structures. The temporal evolution of the absorption spectral changes during the photocatalytic degradation of RhB by the Ag-RTiO2HS catalystsis was shown in Fig. 10. It is found that the absorption peak of the solution decreases quickly with the increase of irradiation time. The color of the suspension changed to colorless after irradiation (insert in Fig. 10), which indicates that the RhB molecular was indeed decomposed in the reaction process. The stability of a practical photocatalyst is as same important as its photocatalytic activity. The plasmonic photocatalyst Ag-RTiO2HS was here investigated through recycling experiments. As shown in
Fig. 10. UVevis spectra changes in the degradation of RhB on the photocatalysts.
Fig. 11, after four cycles of photodegradation of RhB, the sample did not show any significant loss of photocatalytic activity, which indicates that the catalyst can keep stable during the photocatalytic reaction. EIS is a high effective method for probing the photoelectrochemical characterization of surface-modified electrodes. In this study, EIS was carried out to investigate the changes of electron transfer resistance that aroused from the surface modification step as shown in Fig. 12. The RTiO2HS electrode exhibits poor
Fig. 9. Photocatalytic degradation of rhodamine B (A) and comparison of kapp (B) with different TiO2 under visible light.
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C/C0
1.0
0.5
0.0 0
40
80
Time(min) Fig. 11. Cycling degradation of RhB under visible light irradiation over the Ag-RTiO2HS.
conductivity with an EIS of about 532 U. After deposited with Ag/ AgCl, the EIS of Ag-RTiO2HS electrode shows a small semicircle (137 U), only about a quarter of that of RTiO2HS, which suggests the successful deposition of Ag/AgCl on the RTiO2HS surface. Importantly, a smaller impedance arc radius in Nyquist plots indicates better electron/hole pair separation efficiency [23]. In this experiment, Ag-RTiO2HS had better electron/hole pair separation efficiency, suggesting that the Ag/AgCl is favorable to the separation of photogenerated electron/hole pair which can introduce higher photocatalytic activities. The mechanism for the remarkably enhanced photocatalytic performance of the Ag-RTiO2HS photocatalysts can be understood from the following aspects. Firstly, the reduction of RTiO2HS by NaBH4 introduces abundant oxygen vacancies into the bulk of RTiO2HS, which induces a narrowed band gap and a slight VB tailing. In addition, self-doped Ti3þ species introduce localized states below the CB minimum of RTiO2HS. Both the VB tailing and Ti3þ localized states of RTiO2HS are responsible for their significant visible light absorption. Then, the existence of Ag/AgCl nanocrystals on the surfaces of RTiO2HS forms a uniquely hierarchical nanostructure, which provides a high surface area and a large number of interfaces between the Ag/AgCl and RTiO2HS. The high surface areas and profuse interfaces are accessible to the outer environment, and provide numerous active sites for the photodegradation of dye molecules. Thirdly, the metal Ag clusters formed in situ on the semiconductors AgCl and RTiO2HS remarkably enhance the absorption in the visible light region due to the surface plasmonic
Fig. 12. Nyquist plots of electrochemical impedance of the RTiO2HS and Ag-RTiO2HS.
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resonance effect, which photogenerates transient holes that can oxidize the dye molecules [24,25]. Last but not least, the positively synergistic effects of the coupling of Ag/AgCl and RTiO2HS improve the effective separation of the photo-generated electronehole pairs. The charge separation and transfer in the Ag-RTiO2HS photocatalysts under visible-light irradiation was illustrated in Fig. 13. As can be seen in the illustration, under visible light irradiation, photogenerated electronehole pairs can be formed in Ag NPs due to surface plasmon resonance. The photoexcited electrons at the silver NPs are injected into the TiO2 conduction band. Accordingly, the electron could be transferred to molecular oxygen, which is adsorbed on the surface of the catalyst to generate superoxide radical (O 2 ). Meanwhile, the holes transfer to the surface of the AgCl particles and cause the oxidation of Cl and OH ions to Cl0 and OH atoms, which are reactive radical species and can oxidize organic pollutant. Similar processes were also proposed in AgeAgCleTiO2 [26] and Ag/AgCl/TiO2 [27] systems and other photocatalysts [28,29]. However, in the Ag-RTiO2HS system, except the general electron and charge transfer the RTiO2HS could also absorb the visible light. Therefore, under the visible light irradiation, the photogenerated electrons of RTiO2HS can transfer from VB to CB of RTiO2HS to recombine with the plasmon-induced electrons produced by plasmonic absorption of Ag nanoparticles, while the VB holes remain on RTiO2HS to oxidize organic substances. So, the positively synergistic effects of the coupling of Ag/AgCl and RTiO2HS may greatly improve the effective separation of the photogenerated electronehole pairs and hence enhance the photocatalytic activities of the catalysts. 4. Conclusions The Ag/AgCl modified self-doped TiO2 hollow spheres were prepared firstly by a solegel method and subsequently depositionprecipitation and photo-reduction process. The as prepared hollow
Fig. 13. Schematic illustration of the charge separation and transfer in the Ag-RTiO2HS photocatalysts under visible-light irradiation.
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H. Yin et al. / Journal of Alloys and Compounds 657 (2016) 44e52
structured photocatalysts, with diameter about 800e950 nm and shell thickness of 20e30 nm, show highly efficient solar light and visible light induced photocatalytic activities. The improved visible light induced photocatalytic activities may be due to the oxygen vacancies and Ti3þ localized states of RTiO2HS and the surface plasmonic resonance effect of Ag nanoparticles. The present work suggests that the Ag/AgCl modified self-doped TiO2 hollow spheres may be a promising photocatalyst for degrading organic pollutants and environmental remediation. Acknowledgment This work is financially supported by the National Nature Science Foundation of China (No. 41271249), Educational Commission of Zhejiang Province (No.Y201329269) and Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Zhejiang Sci-Tech University), Ministry of Education (No. 2013008). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.10.055. References [1] X. Chen, L. Liu, Y.Y. Peter, S.S. Mao, Science 331 (2011) 746e750. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269e271. [3] Y.H. Hu, Angew. Chem. Int. Ed. 51 (2012) 12410e12412. [4] W. Zhou, W. Li, J.-Q. Wang, Y. Qu, Y. Yang, Y. Xie, K. Zhang, L. Wang, H. Fu, D. Zhao, J. Am. Chem. Soc. 136 (2014) 9280e9283. [5] X. Lü, X. Mou, J. Wu, D. Zhang, L. Zhang, F. Huang, F. Xu, S. Huang, Adv. Funct. Mater 20 (2010) 509e515.
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