CHEN Yi-Ln XU Yu-Xin LIN Di-Feng LUO Yong-Jin XUE Hun CHEN Qing-Hu

a (Fujian Key Laboratory of Pollution Control & Resource Reuse, College of Environment Science and Engineering, Fujian Normal University, Fuzhou 350007, China)

b (School of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou 350118, China)

c (Fujian Polytechnic Normal University, Fuqing 350300, China)

ABSTRACT Ag2O/TiO2 heterostructure has been constructed by loading corner-truncated cubic Ag2O on the TiO2 hollow nanofibers via an electrospinning-precipitation method. Compared to individual Ag2O and TiO2, Ag2O/TiO2 heterostructure exhibits obviously enhanced photocatalytic activity for the photodegradation of methyl orange (MO) under visible light irradiation. The composite with molar ratio of Ag2O to TiO2 at 4:10 exhibits the best photocatalytic performance with MO degraded 93% in 6 min. The superior activity is mainly attributed to the surface plasmon resonance (SPR) effect of metallic Ag in-situ produced during the photocatalytic process, which can favor electron transfer to the conduction band of TiO2. This leads to the efficient separation of photogenerated carriers, thus a superior photodegradation activity. Moreover, the energy band alignments of Ag2O/TiO2 heterostructure are calculated, which provides strong support for the proposed mechanism.

Keywords: visible light photocatalysis, corner-truncated cubic Ag2O, TiO2 hollow nanofibers, surface plasmon resonance, heterostructure;

1 INTRODUCTION

Nowadays, the major concern of the environ- mentalist is the presence of recalcitrant dyes in effluents because they cause serious environmental and health threats[1,2]. There are more than 100,000 commercial dyes with an annual production of over 7 × 105tonnes/year[3]. The total dye consumption in the textile industry worldwide is more than 10,000 tonnes/year and approximately 100 tonnes/year of dyes entered into water streams[4]. Methyl orange (MO) is one of the well-known acidic/anionic dyes, which has been widely used in textile, printing and paper industries and research laboratories[5]. Thus, it is imperative to develop alternative processes to remove MO from wastewater.

Semiconductor photocatalysis offers great poten- tial opportunities to remove MO from waters due to its high efficiency, no secondary pollution and low cost. TiO2is one of the most extensively used photocatalysts for photodegradation of toxic organic pollutants[6]. However, its wide band gap and rapid recombination of photogenerated electron and hole pairs limit its application[7]. In recent years, fabricating p-n heterojunction photocatalysts is one of the most ingenious approaches to improve the visible light responsive activity and facilitate the separation of photogenerated carriers.

Ag2O has the strong absorption in the visible light region. However, individual Ag2O still exhibits poor photocatalytic activity for the low quantum yield, poor light stability and short life of charge carriers[8-10]. Thereby, Ag2O/TiO2hetero- structure has been proposed by researchers. As Zhou et al. reported[11,12]that cubic Ag2O (c-Ag2O) decorated TiO2has better photocatalytic perfor- mance than individual Ag2O and TiO2under ultraviolet (UV) irradiation, but the degradation curve for Ag2O/TiO2was observed to be very close to that for Ag2O under visible-light irradiation. It should be because the conduction band (CB) minimum of Ag2O is more positive than that of TiO2, and the photogenerated electron from Ag2O cannot transfer to the CB of TiO2. However, amorphous TiO2/hexagonal Ag2O (h-Ag2O) hetero- structures prepared by Xu et al[13]exhibited excellent photocatalytic activity under UV and visible light irradiation, much better than TiO2and Ag2O, which is based on the 2.45 ± 0.05 eV band gap ofh-Ag2O. It is controversial with the Yu et al.’s study[14], in which the band gap ofh-Ag2O is found to be lower than 1.55 eV. Therefore, how two different phases of Ag2O play roles in Ag2O/TiO2for enhanced visible light photocatalytic perfor- mance is confusing. It is worth investigating the role of Ag2O structure and content, as well as the energy band engineering in Ag2O/TiO2. Meanwhile, our recent work demonstrates that TiO2hollow nanofibers can contribute to higher surface area for enhancing adsorption capability and improves the stability of Ag/AgCl[15].

Hence, in this paper, we synthesized TiO2hollow nanofibers via an electrospinning method, then corner-truncated cubic Ag2O was loaded on the surface of TiO2hollow nanofibers by a simple precipitation method. The photodegradation experi- ments indicate thatc-Ag2O in Ag2O/TiO2plays important roles in enhancing the visible-light photodegradation activity. The enhanced photo- catalytic activity over Ag2O/TiO2is due to the formation of metallic Ag during the photocatalytic process which exhibits the SPR effect. Combining the characterization analysis and energy band calculation results, a possible photocatalytic mechanism was proposed.

2 EXPERIMENTAL

2. 1 Synthesis

Ag2O/TiO2was prepared by an electrospinning- precipitation method. The preparation of TiO2hollow nanofiber was following our previous report[15]. Then, 0.16 g above TiO2was dispersed in 30 mL deionized water, and then an appropriate amount of AgNO3was added into the solution. After that, 0.2 mol NaOH was slowly added to the mixture suspension followed by continuous stirring for 5 min. Then the resulting precipitate was collected, washed and dried. By controlling the amount of AgNO3to be 0.4, 0.8, 1.2, 1.6 and 2.0 mmol, the obtained Ag2O/TiO2was labeled as AT-1 to AT-5, respectively. A similar process was adopted to synthesize Ag2O without the addition of TiO2.

2. 2 Characterization

The phase of samples was determined on an X-ray diffraction analyzer (XRD, D8 Advance, Bruker) using graphite-monochromatized CuKαradiation, with the accelerating voltage and the applied current to be 40 kV and 40 mA, respectively. Samples morphology was observed by a field emission scanning electron microscope (FESEM, JEOL, JSM-7500F). Transmission electron micro- scopy (TEM) images were taken with a JEM-2100F transmission electron microscope with an accelera- ting voltage of 200 kV. The BET specific surface areas (SBET) were analyzed by N2adsorption- desorption at 77 K on a BELSORP-mini surface area analyzer (BELSORP Co., Japan). X-ray photoelectron spectroscopy (XPS) was measured using a Thermo ESCALAB 250Xi XPS, and the C 1speak at 284.8 eV of the adventitious carbon was referenced to rectify the binding energies. The UV-visible diffuse reflectance absorption features were investigated on a UV-visible spectrophoto- meter (Cary 500 Scan Spectrophotometers, Varian, USA).

2. 3 Photocatalytic activity test

Each sample (100 mg) was suspended in 100 mL methyl orange (MO) aqueous solution with a concentration of 10 mg·L-1in a Pyrex glass vessel. A 300 W Xe lamp (PLS-SXE300C, Perfectligt, China) with a UV cutoff filter (providing visible light with ≥ 420 nm) was used as a light source. Prior to irradiation, the suspension was magneti- cally stirred for 1 h to reach adsorption/desorption equilibrium. At a given time interval, 3 mL of suspension was withdrawn regularly and centri- fuged, and the supernatant was analyzed to deter- mine the residual concentration of MO with the colorimetric method at 464 nm by a UV-vis spectro- photometer (Shimadzu UV-1750).

3 RESULTS AND DISCUSSION

The morphology of the samples is investigated by SEM and TEM. The representative SEM image of PAN/Ti(OiPr)4composite nanofibers is shown in Fig. 1a. It is clearly that composite nanofibers are continuous and smooth. After calcination (Fig. 1b), the composite nanofibers breakage and bend, and the hollow porous structure is confirmed by TEM images (the inset of Fig. 1b). From Fig. 1c, corner-truncated cubic Ag2O already exhibit slightly etched square faces. Regarding Ag2O/TiO2(Fig. 1d), corner-truncated cubic Ag2O is distributed on the surface of TiO2. Compared to individual Ag2O, the morphology of Ag2O in Ag2O/TiO2has change, the face-etching is not observed.

Fig. 1. SEM images of (a) PAN/Ti(OiPr)4 composite nanofibers; (b) TiO2 hollow nanofibers; (c) Individual Ag2O and (d) AT-4

N2adsorption-desorption analysis is used to examine the BET specific surface area of the obtained samples, and the results are shown in Fig. 2. The adsorption-desorption isotherm of TiO2can be classified as a type IV isotherm with a H1-type hysteresis loop, which is characteristic of meso- pores, giving a high SBETof 55 m2/g. Compared with individual Ag2O, the N2adsorption-desorption isotherm of AT-4 shifts upward, indicating the increase of BET specific surface area. The SBETof Ag2O and AT-4 are 3 and 20 m2/g, respectively. It demonstrates that the presence of TiO2hollow structure and mesopores in AT-4 results in a larger surface area, which will provide the enhanced capability for pollutant adsorption.

The phase analysis of the samples is performed using XRD and is reported in Fig. 3. In individual Ag2O, the diffraction peaks at 2θof 33.0°, 38.3° and 55.2° are attributed to the respective (111), (200) and (220) planes ofc-Ag2O (JCPDS No.01-1041). An additional peak for (003) planes ofh-Ag2O is also detected at 34.2° (JCPDS No.42-0874). For TiO2, most of the diffraction peaks can be clearly indexed as the anatase TiO2(JCPS No. 84-1286), and the additional peaks attribute to (110) planes of rutile TiO2(JCPDS no. 65-0192). In the case of Ag2O/TiO2, the peaks of Ag2O and TiO2are observed without any other impurities. For AT-1, only the peaks ofh-Ag2O and TiO2are observed. When increasing the content of Ag2O in Ag2O/TiO2, the peak intensity ofc-Ag2O appears and gradually increases. Therefore, the change of morphology for Ag2O in AT-4 (Fig. 1d) may be due to the co-existence ofc-Ag2O andh-Ag2O in AT-4.

Fig. 2. N2 adsorption/desorption isotherms for samples

Fig. 3. XRD patterns of samples: (a) TiO2; (b) AT-1; (c) AT-2; (d) AT-3; (e) AT-4; (f) AT-5; (g) Ag2O

The elemental compositions and chemical states of AT-4 are measured by XPS. Fig. 4a demonstrates the high-resolution XPS spectra for Ag 3d3/2 and Ag 3d5/2 photoelectrons at 374.37 and 368.37 eV, respectively. These binding energies are consistent with previous report for Ag2O[16]. As for Ti 2pelement (Fig. 4b), two peaks at 458.2 and 464.6 eV for AT-4 are assigned to Ti 2p1/2 and Ti 2p3/2, respectively, which are corresponding to Ti4+in individual TiO2[17]. The O 1sspectrum region shows two peaks with approximately binding energies of 531.17 and 530.17 eV (Fig. 4c), which belong to surface-adsorbed hydroxyl groups and the lattice O. The result clearly indicates the presence of Ag2O and TiO2. The HRTEM image of AT-4 (Fig. 4d) shows that the interplanar spacing of 0.272 and 0.236 nm corresponds to the (111) and (200) planes ofc-Ag2O, and that of 0.262 nm matches with the lattice spacing ofh-Ag2O (003). While the distinct lattice fringes of 0.351 and 0.325 nm are related to the crystallographic planes of anatase TiO2(101) and rutile TiO2(110), respectively, the observation of HRTEM image matches with the XRD and XPS analysis. Moreover, the continuity of lattice fringes between the interface of Ag2O and TiO2can favor the charge transfers between them.

Fig. 4. High-resolution XPS spectra of AT-4 (a) Ag, (b) Ti, and (c) O; (d) TEM images of AT-4

Photocatalytic performance is evaluated for degrading aqueous solution of MO under visible light irradiation. As presented in Fig. 5a, the removal of MO for individual Ag2O is 42.6% within 3 min, while there is almost no degradation for individual TiO2under the same condition. The content of Ag2O in Ag2O/TiO2plays a significant impact on their photocatalytic performances. The removal e ciencies of MO are about 16.7%, 47.5%, 60.7%, 78.9% and 76.2% for AT-1, AT-2, AT-3, AT-4 and AT-5 within 3 min, respectively. Note that the photocatalytic activity enhanced when the content of AgNO3increased from 0.4 to 1.6 mmol. When the loading amount of AgNO3increases to 2.0 mmol, the photocatalytic activity is slightly decreased. AT-4 exhibits the highest degradation e ciency towards MO degradation. Combined with XRD analysis, the photocatalytic performance of AT-1 is obviously lower than individual Ag2O, indicating theh-Ag2O plays a negative role in Ag2O/TiO2. However, more contents ofc-Ag2O decrease the photocatalytic activity, since AT-4 has better photodegradation performance than AT-5. It may be attributed to the fact that the higher content of Ag2O covers the adsorption sites for MO[18].

In addition, the pseudo-first-order model[19]is applied to quantitatively compare the reaction kinetics of decomposition rate of MO. The various rate constant (k) is depicted in the form of columns of different colors (Fig. 5b). Undoubtedly, AT-4 displays the highest rate constant (0.28 min-1) 1.5 times higher than that of individual Ag2O. Moreover, the observed photocatalytic performance of AT-4 is far better compared to most of the reported Ag2O/TiO2which has been summarized in Table 1.

Fig. 5. (a) Degradation curves and (b) the corresponding reaction rate constants for the photodegradation of MO

Table 1. Performances of Photocatalytic Activity in the Reported Literature

In order to verify the enhanced visible-light photocatalytic activity and improve the charge separation of Ag2O/TiO2, the UV-vis diffuse reflec- tance spectra of Ag2O, TiO2and AT-4 are measured (Fig. 6). The UV-vis diffuse reflectance spectra show that TiO2has a clear edge around 396 nm, and the band gap value is estimated to be 3.13 eV, corresponding to the absorbance in the ultraviolet region. Afterp-Ag2O/n-TiO2heterojunction being formed, the optical absorption increases in the visible-light region, and the band gap is determined to be around 2.01 eV thanks to the band gap of the obtained Ag2O (1.18 eV). Thus, Ag2O/TiO2has great effects on its optical property and enhances the utilized efficiency of visible light for TiO2.

Fig. 6. UV-vis diffuse reflectance spectra for the samples

Theoretical band structure of Ag2O/TiO2hetero- junction is constructed to evaluate the e ects of structure on their properties. First, the band gaps of TiO2and Ag2O are characterized by UV-vis diffuse reflectance spectra, and the band positions of Ag2O and TiO2can be predicted by the following Eqs. 1 and 2[23], respectively. whereEgis the band gap energy of semiconductor,EVBis the valence band (VB) edge potentials,ECBis the conduction band (CB) edge potentials,Xis the electronegativity of semiconductor (XTi= 3.45 eV,XAg= 4.44 eV,XO= 7.54 eV) andEeis the energy of free electrons on hydrogen scale (～4.5 eV). The values of calculatedX,EVB,ECB, andEgare listed in Table 2.

Table 2. Values of Calculated X, EVB, ECB, and Eg for Ag2O and TiO2

Based on the data listed in Table 2, the theoretical band structure of Ag2O/TiO2heterojunctions can be proposed. From the valence and conduction band positions for Ag2O and TiO2, it can be concluded that photogenerated electrons in the CB of Ag2O cannot drift to that of TiO2under visible light illumination. Thus, it should have a similar visible light photocatalytic performance to Ag2O. However, in this work, Ag2O/TiO2exhibits obviously enhan- ced visible light photocatalytic activity. Hence, XRD patterns, XPS patterns and UV-vis DRS spectra of AT-4 before and after photocatalytic reaction are performed to reveal the cause of excellent photocatalytic activity.

The comparison of XRD patterns of AT-4 before and after photocatalytic reaction is shown in Fig. 7a. After photocatalytic reaction, the XRD patterns exhibit an obvious change, and the diffraction peaks at 38.1° and 44.3° appear. The two diffraction peaks respectively match with the (111) and (200) crystalline planes of metallic Ag, which are marked with “#” (38.1° also belong to (200) planes of Ag2O, marked with “·”). To further provide the presence of Ag0, XPS spectra of Ag 3din AT-4 after photocatalysis are performed (Fig. 7b). Compared with Fig. 4a, the Ag species add two bands at 367.17 and 373.17 eV after photocatalysis, which ascribes to the metallic Ag0. From Fig. 7c, it can be found a stronger broad absorption band in the range of 400～700 nm in used AT-4, which attributes to the SPR effect of metallic Ag. These observations conclude that the Ag2O undergoesin-situphotoreduction during photocatalysis reaction and forms metallic Ag on the TiO2surface. The SPR absorption of Ag under visible light may have a great influence on the photocatalysis.

Fig. 7. (a) XRD patterns and (b) XPS spectra of Ag 3d in used AT-4; (c) UV-vis diffuse reflectance spectra for AT-4 before and after photocatalysis

In the photodegradation process, the active species including holes (h+), hydroxyl radicals (·OH) and superoxide radicals (O2-·) can be formed. In order to better understand the mechanism of photo- degradation of MO over AT-4 under visible-light irradiation, the trapping experiments of active species involved in the photocatalytic reaction are investigated (Fig. 8). In this study, benzoquinone (BQ)[24], ammonium oxalate (AO)[25]and dimethyl sulfoxide (DMSO)[24]acting as the scavengers for O2-·, h+and ·OH radicals are introduced into the photocatalytic process, respectively. From Fig. 8, photocatalytic activities of AT-4 decreased obviously with the addition of BQ (The conversion of MO was 30.4%) and AO (The conversion of MO was 13.1%), and reduced slightly with adding DMSO (The conversion of MO was 84.6%), indicating O2-· and h+radicals are the main oxidative species.

Fig. 8. Effects of scavengers on the degradation of MO inAT-4

Based on the above results, a possible photoca- talytic mechanism for Ag2O/TiO2under visible-light irradiation is proposed in Fig. 9. Under irradiation with visible light, only Ag2O can be excited to generate electrons and holes. However, the photo- generated electrons on Ag2O cannot transfer to TiO2because of the more positive conduction band potential of Ag2O than that of TiO2. Fortunately, Ag2O can be photo-reduced to form metallic Ag under visible light irradiation. The produced metallic Ag contributes to the activity by trans- porting the SPR excited electrons into CB of TiO2, and then electrons react with O2to produce O2-·. Meanwhile, holes in the VB of TiO2can transfer to Ag2O surface due to the heterostructure between TiO2and Ag2O. The· and holes are directly involved in the oxidized decomposition of MO. In this way, the recombination of electrons and holes pairs is effectively inhibited due to the SPR effect induced by metallic Ag and Ag2O/TiO2hetero- structure, resulting in a higher reaction rate.

Fig. 9. Photocatalytic mechanism scheme for Ag2O/TiO2 under visible light illumination

The good repeatability and stability of photoca- talysts are beneficial to reduce water treatment costs and avoid secondary pollution. To confirm the stability of AT-4, the circulating runs in the photocatalytic degradation of MO is checked (Fig. 10). The photocatalytic performance of AT-4 is main- tained at a high level, indicating good photo- stability of the as-prepared samples.

Fig. 10. Cycling runs in the photocatalytic degradation of MO over AT-4

4 CONCLUSION

In this study, corner-truncated cubic Ag2O decorated TiO2hollow nanofibers was successfully synthesized through electrospinning combined with precipitation method. The optimal catalyst is AT-4, with which MO can be degraded 93% within 6 min under visible light irradiation. In the case of AT-1, onlyh-Ag2O was observed, showing the negative effect in photodegradation compared to individual Ag2O. Regarding AT-5, however, excessivec-Ag2O will cover the adsorption sites for MO, leading to a decreased photocatalytic activity related to AT-4. UV-Vis DRS confirms the SPR effect of metallic Ag in Ag2O/TiO2, benefiting the transfer of excited electrons to CB of TiO2. The existence of metallic Ag is evidenced by XRD and XPS analysis of used AT-4. The energy band calculation indicates that the holes can transfer from VB of TiO2to that of Ag2O due to the successful construction of Ag2O/TiO2heterostructure. As a result, the separation effi- ciency of photogenerated carriers is promoted. Scavenger tests demonstrate that· and h+radicals play the dominant role in the photo- degradation of MO. Moreover, AT-4 shows good stability during the photocatalytic reaction, which can be a promising candidate for the photodegrada- tion of dye organic pollution.