English

• 1. INTRODUCTION

  Nowadays, the major concern of the environmentalist 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 × 10⁵ tonnes/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 potential opportunities to remove MO from waters due to its high efficiency, no secondary pollution and low cost. TiO₂ is 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/TiO₂ heterostructure has been proposed by researchers. As Zhou et al. reported^{[11, 12]} that cubic Ag₂O (c-Ag₂O) decorated TiO₂ has better photocatalytic performance than individual Ag₂O and TiO₂ under ultraviolet (UV) irradiation, but the degradation curve for Ag₂O/TiO₂ was 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) heterostructures prepared by Xu et al^[13] exhibited excellent photocatalytic activity under UV and visible light irradiation, much better than TiO₂ and Ag₂O, which is based on the ∼2.45 ± 0.05 eV band gap of h-Ag₂O. It is controversial with the Yu et al.'s study^[14], in which the band gap of h-Ag₂O is found to be lower than 1.55 eV. Therefore, how two different phases of Ag₂O play roles in Ag₂O/TiO₂ for enhanced visible light photocatalytic performance 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 TiO₂ hollow 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 TiO₂ hollow nanofibers via an electrospinning method, then corner-truncated cubic Ag₂O was loaded on the surface of TiO₂ hollow nanofibers by a simple precipitation method. The photodegradation experiments indicate that c-Ag₂O in Ag₂O/TiO₂ plays important roles in enhancing the visible-light photodegradation activity. The enhanced photocatalytic activity over Ag₂O/TiO₂ is 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/TiO₂ was prepared by an electrospinning-precipitation method. The preparation of TiO₂ hollow nanofiber was following our previous report^[15]. Then, 0.16 g above TiO₂ was dispersed in 30 mL deionized water, and then an appropriate amount of AgNO₃ was 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 AgNO₃ to be 0.4, 0.8, 1.2, 1.6 and 2.0 mmol, the obtained Ag₂O/TiO₂ was 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 CuKα radiation, 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 microscopy (TEM) images were taken with a JEM-2100F transmission electron microscope with an accelerating voltage of 200 kV. The BET specific surface areas (S_BET) were analyzed by N₂ adsorption-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 1s peak 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 spectrophotometer (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^-1 in 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 magnetically stirred for 1 h to reach adsorption/desorption equilibrium. At a given time interval, 3 mL of suspension was withdrawn regularly and centrifuged, and the supernatant was analyzed to determine the residual concentration of MO with the colorimetric method at 464 nm by a UV-vis spectrophotometer (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)₄ composite 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/TiO₂ has change, the face-etching is not observed.

  Figure 1

  

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

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  N₂ adsorption-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 TiO₂ can be classified as a type Ⅳ isotherm with a H1-type hysteresis loop, which is characteristic of mesopores, giving a high S_BET of 55 m²/g. Compared with individual Ag₂O, the N₂ adsorption-desorption isotherm of AT-4 shifts upward, indicating the increase of BET specific surface area. The S_BET of Ag₂O and AT-4 are 3 and 20 m²/g, respectively. It demonstrates that the presence of TiO₂ hollow structure and mesopores in AT-4 results in a larger surface area, which will provide the enhanced capability for pollutant adsorption.

  Figure 2

  

  Figure 2.  N₂ adsorption/desorption isotherms for samples

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  The phase analysis of the samples is performed using XRD and is reported in Fig. 3. In individual Ag₂O, the diffraction peaks at 2θ of 33.0°, 38.3° and 55.2° are attributed to the respective (111), (200) and (220) planes of c-Ag₂O (JCPDS No.01-1041). An additional peak for (003) planes of h-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 TiO₂ are observed without any other impurities. For AT-1, only the peaks of h-Ag₂O and TiO₂ are observed. When increasing the content of Ag₂O in Ag₂O/TiO₂, the peak intensity of c-Ag₂O appears and gradually increases. Therefore, the change of morphology for Ag₂O in AT-4 (Fig. 1d) may be due to the co-existence of c-Ag₂O and h-Ag₂O in AT-4.

  Figure 3

  

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

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  The elemental compositions and chemical states of AT-4 are measured by XPS. Fig. 4a demonstrates the high-resolution XPS spectra for Ag 3d 3/2 and Ag 3d 5/2 photoelectrons at 374.37 and 368.37 eV, respectively. These binding energies are consistent with previous report for Ag₂O^[16]. As for Ti 2p element (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 1s spectrum 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 of c-Ag₂O, and that of 0.262 nm matches with the lattice spacing of h-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 TiO₂ can favor the charge transfers between them.

  Figure 4

  

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

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  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 TiO₂ under the same condition. The content of Ag₂O in Ag₂O/TiO₂ plays a significant impact on their photocatalytic performances. The removal efficiencies 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 AgNO₃ increased from 0.4 to 1.6 mmol. When the loading amount of AgNO₃ increases to 2.0 mmol, the photocatalytic activity is slightly decreased. AT-4 exhibits the highest degradation efficiency towards MO degradation. Combined with XRD analysis, the photocatalytic performance of AT-1 is obviously lower than individual Ag₂O, indicating the h-Ag₂O plays a negative role in Ag₂O/TiO₂. However, more contents of c-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].

  Figure 5

  

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

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  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/TiO₂ which has been summarized in Table 1.

  Table 1

  

  Table 1.  Performances of Photocatalytic Activity in the Reported Literature

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  Photocatalyst	Conc. and volume of MO	Light source	Degradation time (min)	Degradation (%)	Reference
  Ag2O/TiO2 hollow NFs (100 mg)	3*10-5 M, 100 mL	Visible	6	93	Our work
  3D Ag2O/TiO2 heterojunction (40 mg)	10−5 M, 40 mL	UV	15	93	[20]
  Ag2O/TiO2 nanobelts (20 mg)	20 mg/L, 20 mL	Visible	24	80	[8]
  Ag2O/TiO2 heterostructure (20 mg)	20 mg/L, 20 mL	UV	25	100	[9]
  Ag2O/TiO2 sphere (20 mg)	14 mg/L, 50 mL	Simulated sunlight	60	98	[21]
  Ag2O/TiO2 nanobelts (10 mg)	10 mg/L, 20 mL	UV	20	100	[18]
  TiO2(B)/Ag2O (20 mg)	10 mg/L, 100 mL	Visible	30	88	[22]

  In order to verify the enhanced visible-light photocatalytic activity and improve the charge separation of Ag₂O/TiO₂, the UV-vis diffuse reflectance spectra of Ag₂O, TiO₂ and AT-4 are measured (Fig. 6). The UV-vis diffuse reflectance spectra show that TiO₂ has 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. After p-Ag₂O/n-TiO₂ heterojunction 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/TiO₂ has great effects on its optical property and enhances the utilized efficiency of visible light for TiO₂.

  Figure 6

  

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

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  Theoretical band structure of Ag₂O/TiO₂ heterojunction is constructed to evaluate the effects of structure on their properties. First, the band gaps of TiO₂ and Ag₂O are characterized by UV-vis diffuse reflectance spectra, and the band positions of Ag₂O and TiO₂ can be predicted by the following Eqs. 1 and 2^[23], respectively.

  $ E_{VB} = X − E_{e }+ 0.5E_{g } $

  (1)

  $ E_{CB} = E_{VB} − E_{g } $

  (2)

  where E_g is the band gap energy of semiconductor, E_VB is the valence band (VB) edge potentials, E_CB is the conduction band (CB) edge potentials, X is the electronegativity of semiconductor (X_Ti = 3.45 eV, X_Ag = 4.44 eV, X_O = 7.54 eV) and Ee is the energy of free electrons on hydrogen scale (~4.5 eV). The values of calculated X, E_VB, E_CB, and E_g are listed in Table 2.

  Table 2

  

  Table 2.  Values of Calculated X, E_VB, E_CB, and E_g for Ag₂O and TiO₂

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  Semiconductor	X(eV)	EVB(eV)	ECB(eV)	Eg(eV)
  Ag2O	5.30	1.39	0.21	1.18
  TiO2	5.81	2.87	-0.26	3.13

  Based on the data listed in Table 2, the theoretical band structure of Ag₂O/TiO₂ heterojunctions 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 TiO₂ under visible light illumination. Thus, it should have a similar visible light photocatalytic performance to Ag₂O. However, in this work, Ag₂O/TiO₂ exhibits obviously enhanced 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 3d in 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 undergoes in-situ photoreduction during photocatalysis reaction and forms metallic Ag on the TiO₂ surface. The SPR absorption of Ag under visible light may have a great influence on the photocatalysis.

  Figure 7

  

  Figure 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

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  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 photodegradation 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.

  Figure 8

  

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

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  Based on the above results, a possible photocatalytic mechanism for Ag₂O/TiO₂ under 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 photogenerated electrons on Ag₂O cannot transfer to TiO₂ because 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 transporting the SPR excited electrons into CB of TiO₂, and then electrons react with O₂ to produce O₂⁻•. Meanwhile, holes in the VB of TiO₂ can transfer to Ag₂O surface due to the heterostructure between TiO₂ and Ag₂O. The O₂⁻• 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/TiO₂ heterostructure, resulting in a higher reaction rate.

  Figure 9

  

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

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  The good repeatability and stability of photocatalysts 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 maintained at a high level, indicating good photostability of the as-prepared samples.

  Figure 10

  

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

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  4. CONCLUSION

  In this study, corner-truncated cubic Ag₂O decorated TiO₂ hollow 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, only h-Ag₂O was observed, showing the negative effect in photodegradation compared to individual Ag₂O. Regarding AT-5, however, excessive c-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 TiO₂ to that of Ag₂O due to the successful construction of Ag₂O/TiO₂ heterostructure. As a result, the separation efficiency of photogenerated carriers is promoted. Scavenger tests demonstrate that O₂⁻• and h⁺ radicals play the dominant role in the photodegradation of MO. Moreover, AT-4 shows good stability during the photocatalytic reaction, which can be a promising candidate for the photodegradation of dye organic pollution.

  
  
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• 

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

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  Figure 2  N₂ adsorption/desorption isotherms for samples

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

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

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

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  Figure 6  UV-vis diffuse reflectance spectra for the samples

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  Figure 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

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  Figure 8  Effects of scavengers on the degradation of MO inAT-4

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

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

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  Table 1.  Performances of Photocatalytic Activity in the Reported Literature

  Photocatalyst	Conc. and volume of MO	Light source	Degradation time (min)	Degradation (%)	Reference
  Ag2O/TiO2 hollow NFs (100 mg)	3*10-5 M, 100 mL	Visible	6	93	Our work
  3D Ag2O/TiO2 heterojunction (40 mg)	10−5 M, 40 mL	UV	15	93	[20]
  Ag2O/TiO2 nanobelts (20 mg)	20 mg/L, 20 mL	Visible	24	80	[8]
  Ag2O/TiO2 heterostructure (20 mg)	20 mg/L, 20 mL	UV	25	100	[9]
  Ag2O/TiO2 sphere (20 mg)	14 mg/L, 50 mL	Simulated sunlight	60	98	[21]
  Ag2O/TiO2 nanobelts (10 mg)	10 mg/L, 20 mL	UV	20	100	[18]
  TiO2(B)/Ag2O (20 mg)	10 mg/L, 100 mL	Visible	30	88	[22]

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  Table 2.  Values of Calculated X, E_VB, E_CB, and E_g for Ag₂O and TiO₂

  Semiconductor	X(eV)	EVB(eV)	ECB(eV)	Eg(eV)
  Ag2O	5.30	1.39	0.21	1.18
  TiO2	5.81	2.87	-0.26	3.13

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