Facile Synthesis of Mesoporous Ag2O-ZnO Heterojunctions for Efficient Promotion of Visible Light Photodegradation of Tetracycline.

Fabrication of 3D mesoporous Ag2O-ZnO heterojunctions at varying Ag2O contents has been achieved through poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic F-108) as the structure-directing agent for the first time. The mesoporous Ag2O-ZnO nanocomposites exhibited a mesoporous structure, which revealed a large pore volume and high surface area. The photocatalytic efficiency over mesoporous Ag2O-ZnO nanocomposites for tetracycline (TC) compared with that over commercial P-25 and pristine ZnO NPs through the visible light exposure was studied. Mesoporous 1.5% Ag2O-ZnO nanocomposites indicated the highest degradation efficiency of 100% of TC during 120 min of the visible light exposure compared with 5% and 10% for pristine ZnO NPs and commercial P-25, respectively. The TC degradation rate took place much rapidly over 1.5% Ag2O-ZnO nanocomposites (0.798 μmol L-1 min-1) as compared to either commercial P-25 (0.097 μmol L-1 min-1) or ZnO NPs (0.035 μmol L-1 min-1). The mesoporous 1.5% Ag2O-ZnO nanocomposite revealed the highest degradation rate among all synthesized samples, and it was 23 and 8 orders of magnitudes greater than those of pristine ZnO NPs and P-25, respectively. The photoluminescence and transient photocurrent intensity behaviors have been discussed to explore photocatalysis mechanisms. It is anticipated that the present work will contribute some suggestions for understanding other heterojunctions with outstanding behaviors.

Introduction

Much exploration regarding the semiconductor photocatalysts mainly focused to promote energy and environmental applications during the past few decades.[1−4] Semiconductor materials have received a considerable transact of attention owing to their extensive environmental treatment application, for instance, removal of heavy metals and detoxification of organic pollutants (dyes, pharmaceuticals, and endocrines).[5−9] For example, ZnO, an n-type semiconductor with an excitation binding energy of ∼60 meV and a direct band gap energy of ∼3.37 eV, has drawn wide research observation for diverse potential applications such as chemical sensors, photocatalysis, nanolasers, solar cells, and piezoelectric nanogenerators.[10−13] ZnO nanomaterials have been synthesized with the assistance of various structure-directing agents in solution conditions.[10−12] To address the demand for various ZnO nanostructure applications, they have been synthesized, including 2D nanobelts and nanosheets, 1D nanorods, nanowires, and 3D hollow spheres.[14−17] The 3D hollow structures or hierarchical meso-/micropores with a large pore are much desirable for photocatalysis, lithium-ion battery, catalysts, and chemical sensors because of their easy mass transmission in materials and large surface area.[18,19] Also, the 3D hierarchical structures exhibited promotion characteristics for photocatalysts[23] and gas sensor applications.[20−22] However, until now, there remains a challenge to promote a time-saving and facile approach in the absence of toxic reagents to construct either 2D porous subunits or 3D hierarchical architectures for sophisticated applications.[24−26] Thus, the fabrication of ZnO NPs with high nanocrystallinity could successfully reduce the defects in the ZnO surface and hence promote the photocorrosion features.

Noble metals have received tremendous attention because of their appealing catalytic behaviors at nanoscale sizes.[27−29] Noble metals such as Ag, Pd, Pt, and Au are deposited onto the surface of the semiconductor as promoters or sensitizers to enhance the photocatalysis applications.[30−32] On the other hand, as a class of Ag2O (Eg = 1.2 eV) p-type semiconductors, a wide visible light region could be harvested. Ag2O NPs have been extensively impregnated with outspread band gap semiconductor materials as an effective photocatalyst to provide their visible light response, for instance, Ag2O quantum dots on the ZnIn2S4 nanosheet surface, Ag2O–Bi2WO6, Ag2O–TiO2, ZnO-based Ag2S–Ag2O, Ag2O–ZnO,[33−38] and so on. Thus, incorporation of ZnO with Ag2O NPs to extend the absorption of the solar spectrum is achieved in an optimistically superior photocatalyst. Particularly, well-matched between Ag2O and ZnO band structures can create an effective separation of photoinduced electron–hole pairs. Therefore, we suggest a mesoporous Ag2O–ZnO heterostructure system to improve the absorption at longer wavelengths with a narrow band gap photocatalyst.[33] Taking into consideration the environmental and energy applications and fundamental research studies on the ZnO NP nanostructure, the optimum morphologies are found to be porous structures and highly crystalline with large surface areas, owing to such metal oxides which can show large interfacial surface areas and high electroconductivity.

Tetracycline (TC) is one of the antibiotics employed for promoting animal husbandry growth and treating some diseases. The antibiotic sources in the environment comprise medical wastewater, aquaculture wastewater, animal feed discharge, and domestic sewage. The residual of antibiotic may enter the body through vegetables, meat, and drinking water, which may go back to the human during ecological cycles.[39] Subsequently, the claim for eliminating antibiotics should be addressed.[39] To date, diverse processes comprising photocatalytic degradation, adsorption, ion exchange, and oxidation have been employed to degrade antibiotics from aqueous solution.[40−42]

In this contribution, an easy fabrication of mesoporous 3D Ag2O–ZnO nanocomposites has been achieved at varying Ag2O contents using Pluronic F-108 as the structure-directing agent. The 3D mesoporous ZnO NPs have been utilized as a support network to deposit Ag2O NPs to achieve functional materials for TC photodegradation under visible light. It is observed that there are no reports on the superior system of 3D mesoporous Ag2O–ZnO nanocomposites, which integrates the utilities of 3D mesoporous ZnO and Ag2O NPs. Mesoporous 1.5% Ag2O–ZnO nanocomposites indicated the highest photodegradation efficiency of 100% of TC during 120 min of the visible light exposure compared with 5% and 10% for pristine ZnO NPs and commercial P-25, respectively. Our findings illustrate that the mesoporous Ag2O–ZnO nanocomposites revealed crucially improved photocatalytic efficiency, which can provide a new avenue to enhance advanced photocatalyst materials.

Experimental Section

Materials

Ag(NO3)3, CH3COOH, Zn(NO3)3·6H2O, C2H5OH, HCl, nonionic surfactant Pluronic F-108, and PEG-PPG-PEG, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) with Mn ∼ 14,600 were obtained from Sigma-Aldrich.

Fabrication of Mesoporous Ag2O–ZnO Nanocomposites

The mesoporous ZnO network was fabricated using Pluronic F-108 as a structure-directing agent, followed by integrated Ag2O NPs at varying contents (0.5–2 wt %). The molar ratios of precursors ZnO, F-108, C2H5OH, HCl, and CH3COOH were estimated to be 1:0.02:50:2.25:3.75. In particular, 0.2 g of F-108 surfactant was added into 30 mL of C2H5OH with magnetic agitation within 60 min. Afterward, 20.3 g of Zn(NO3)3·6H2O, 2.3 mL of CH3COOH, and 0.74 mL of HCl were also added through movingagitation. The obtained sol was maintained in a humidity chamber (40–80% relative humidity) at 40 °C to achieve gel formation and subsequently dried at 65 °C for 12 h and annealed at 450 °C for 4 h. Mesoporous Ag2O–ZnO nanocomposites were fabricated. Typically, 1 g of mesoporous ZnO NPs was added into 100 mL of C2H5OH through sonication for 5 min. Subsequently, a certain amount of Ag(NO3)3 was gradually added to mesoporous ZnO NP solution through agitation for 60 min to reach mesoporous 0.5, 1, 1.5, and 2 wt % Ag2O–ZnO nanocomposites. Then, C2H5OH in the suspension solution was evaporated at 110 °C overnight. The obtained powder was calcined at 400 °C for 3 h.

Characterizations

XRD patterns were recorded through a Bruker AXS D4 Endeavour X diffractometer. TEM images were determined using a JEOL JEM-2100F electron microscope (Japan) operating at 200 kV. N2 adsorption–desorption isotherms were recorded at 77 K employing the Quantachrome Autosorb equipment after outgassing at 200 °C overnight. A spectrofluorophotometer was used to record photoluminescence (PL) by applying a xenon lamp (150 W) as an excitation source at λ ∼ 365 nm (RF-5301 PC, 400 W, 50/60 Hz) at room temperature. X-ray photoelectron spectroscopy (XPS) data were analyzed using a Thermo Scientific K-ALPHA spectrometer. Fourier transform infrared spectrometry (FT-IR) spectra were measured at 400–4000 cm–1 via a PerkinElmer after mixing with KBr. Zahner Zennium electrochemical workstation was used for determining transient photocurrent measurements. Diffuse reflectance spectra were recorded using a Varian Cary 100 Scan UV–vis system at λ ∼ 200–800 nm.

Photocatalytic Test

Mesoporous Ag2O–ZnO nanocomposites were evaluated through visible light exposure with a wavelength of more than 420 nm for the degradation of TC [20 mg/L]. The xenon lamp (300 W) was used with a 10 cm distance over the photoreactor (250 mL) including the H2O cooling circulation system. The mesoporous Ag2O–ZnO was sonicated in 200 mL of aqueous solution TC [20 mg/L] with air pumping to be an oxygen source. The suspension was agitated in the dark for 30 min to acquire the adsorption equilibrium of TC over the mesoporous Ag2O–ZnO. The photocatalytic performances for mesoporous Ag2O–ZnO were conducted for 2 h during visible light exposure. The photocatalytic efficiency was determined by withdrawing 3 mL of suspension, and it was separated by a 0.22 μm nylon filter at an interval time for TC analysis employing a spectrophotometer with an absorbance peak at 361 nm. % Photocatalytic performance = (Co – C)/Co × 100%, where C and Co are the TC concentration at certain times and zero time, respectively.

Results and Discussion

The XRD patterns for mesoporous ZnO NPs and 0.5, 1, 1.5, and 2% Ag2O–ZnO nanocomposites are demonstrated in Figure . The results demonstrated that the peaks of pure mesoporous ZnO NPs appeared at 31.66°, 34.29°, 36.23°, 47.48°, 56.60°, 62.77°, and 67.85° matching with the planes of (100), (002), (101), (102), (110), (103), and (200) for the ZnO hexagonal structure (JCPDS files 89–1397), respectively. XRD analysis for all synthesized Ag2O–ZnO nanocomposites at varying Ag2O NP samples indicated the existence of the ZnO hexagonal structure as well. The peak intensity of mesoporous ZnO NPs significantly confirm the construction of polycrystalline nanostructures. Interestingly, the characteristics of the peak showed no impurities and Ag2O NPs rather than the ZnO phase, which emphasize that the synthesized samples are the pure ZnO phase. It might be that Ag2O NPs are highly dispersed with very small particle sizes. It is observed that after Ag2O NPs’ impregnation, all ZnO mean peaks were slightly moved to lower 2θ values, indicating that Ag+ replaced the Zn atom in the lattice matrix. Based on the first-principles theoretical computations, replacement of Ag+ on the Zn atom surface is energetically appropriate.[43] Thus, the Zn2+ionic radius (0.74 Å) is lesser than that of Ag+ (1.15 Å), which leads to lattice distortion.[44] The average particle sizes of the mesoporous ZnO NPs were estimated from the main diffraction peak (101) plane applying Scherrer’s formula:[45]D = kλ/(β cos θ), where D = average particle sizes, θ = diffraction angle, k = 0.89, β = fwhm, and λ = 1.54 Å; D for all synthesized samples were calculated to be in the range of ∼5–10 nm.

Figure 1

X-ray diffraction for mesoporous ZnO NPs (a), Ag2O–ZnO nanocomposites at varying Ag2O contents 0.5% (b), 1% (c), 1.5% (d), and 2% (e). Shifted for the sake of clarity.

The FT-IR spectra of pristine ZnO NPs and Ag2O–ZnO nanocomposites are displayed in Figure A. The broad peak at 3220–3550 cm–1 and the peak assigned at 1634.90 cm–1 are ascribed to the O–H stretching vibration and bending modes of the adsorbed H2O and in the hydrated oxide surface.[46,47] The peak was located at 776 cm–1 corresponding to the Zn–O vibration. After Ag2O NP impregnation, the characteristic peak of ZnO was shifted to a lower wavenumber at 734.9 cm–1 with the boost of Ag2O NPs, which underlines the coexistence of Ag2O and ZnO NPs and demonstrates the fabrication of Ag2O–ZnO nanocomposites. Interestingly, the Zn–O stretching peak intensity was diminished in the mesoporous Ag2O–ZnO nanocomposites, which may be associated with the replacement of Ag+ into the lattice matrix of mesoporous Ag2O–ZnO nanocomposites.[48] To evaluate the behavior of pore structure and their pore volume and size in the mesoporous ZnO NPs and Ag2O–ZnO nanocomposites, N2 adsorption–desorption isotherms were determined, as presented in Figure B. The IV isotherm type with a hysteresis loop of H3 type was observed for mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O NP contents.[49] The isotherm of type IV for the synthesized samples corresponded to the capillary condensation occurring in its mesopore structure. There is a hysteresis of the pristine mesoporous ZnO NPs that first appears at low relative p/po ∼ 0.41, followed by a linear boost up to p/po ∼ 0.8, while low relative p/po of the 1.5% Ag2O–ZnO nanocomposite is the range of 0.6–0.92. The increment in slope at relative p/po correlates with the capillary condensation of uniform mesopore systems, while the additional increment at higher relative p/po suggested considerable interparticle porosity. The average pore diameter of pristine ZnO NPs was around 6.4 nm which reduced up to 5.37 nm after impregnating Ag2O NPs, respectively. The pore volume and the surface area of mesoporous ZnO NPs are about 0.31 cm3/g and 140 m2/g, which decreased to 0.24 cm3/g and 117 m2/g, respectively, after impregnating 2% Ag2O NPs. The high surface area of mesoporous ZnO NPs was attributed to the small particle size of pore structures. Furthermore, the hydrogen bonding between F108 and Zn2+ and Ag+ NPs produced very small particles, which led to a high surface area of the synthesized mesoporous materials.

Figure 2

(A) FTIR spectra of the for mesoporous ZnO NPs (a), Ag2O–ZnO nanocomposites at varying Ag2O contents 0.5% (b), 1% (c), 1.5% (d), and 2% (e); (B) N2 sorption isotherms and pore size distributions (inset) the mesoporous ZnO NPs and 1.5% Ag2O–ZnO nanocomposites.

TEM images were observed for the pristine mesoporous ZnO NPs and mesoporous 1.5 and 2% Ag2O–ZnO nanocomposites (Figure ). Figure a shows that the ZnO NPs were highly uniformly distributed in terms of size and shape, and the average particle sizes were determined to be ∼5 nm. Figure b,c shows the TEM images of 1.5 and 2% Ag2O–ZnO nanocomposites. The images showed that ZnO NPs with a diameter of about 5 nm were not agglomerated after Ag2O impregnated ZnO NPs with a quite uniform shape and size, which comprised lots of pores. The HR-TEM image of 1.5% Ag2O–ZnO nanocomposites showed that distances between two close planes were 0.25 nm matching with the (101) plane ZnO, indicating the construction of ZnO polycrystalline crystals; however, the Ag2O NPs interplanar spacing was not observed because the formed Ag2O NPs were very small and the amount was tiny. The corresponding selected area electron diffraction (SAED) image exhibited a polycrystalline ZnO hexagonal crystal formation, as shown in Figure d, inset. To explore further characteristics of the mesoporous Ag2O–ZnO nanocomposite, XPS analysis was measured, as illustrated in Figure . The binding energy of C 1s was calibrated at 284.6 eV. Ag 3d spectra were located in their two peak forms, indicating the existence of 3d3/2 and 3d5/2,[50−52] which has characteristics at around 374.13 and 368.13 eV partially lesser than the reported ones, corresponding to Ag–O, as obviously observed in Figure a.[53] It was observed that two peaks were assigned at 1044.68 and 1021.58 eV for Zn 2p1/2 and Zn 2p3/2, respectively, corresponding to the existence of Zn2+. Figure c shows one mean peak at O 1s assigned at 530 eV corresponding to the formation of Zn–O–Zn.[36] The BE of the Ag peak moves to the lower region, due to which electrons can readily transmit from the conduction band (CB) of Ag to the CB of ZnO until mesoporous Ag2O–ZnO systems reach an equilibrium of the Fermi level; thus, a new generation of Fermi level was constructed in the Ag2O and ZnO heterojunction.[54] XPS of the 1.5% Ag2O–ZnO showed that the weight percentages are consistent with that of Ag2O–ZnO. Also, the atomic percentages of Ag, Zn, and O are determined to be 1.12, 53.77, and 45.11%, respectively.

Figure 3

TEM images of mesoporous ZnO NPs (a), 1.5% Ag2O–ZnO (b), and 2% Ag2O–ZnO nanocomposites (c); HRTEM image of 1.5% Ag2O–ZnO nanocomposite (d). The corresponding SAED image 3d, inset.

Figure 4

XPS analysis of 1.5% Ag2O–ZnO nanocomposite emerging from the emissions of the Ag, Zn, and O elements; Ag 3d (a), Zn 2b (b), and O 1s (c).

To explore the enhanced effect of light absorption and band gap energy on the photocatalytic performance of pristine mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents, UV–vis spectra of the synthesized nanocomposites were evaluated, as depicted in Figure . The absorption edge of mesoporous ZnO NPs was assigned about 376 nm (Figure a), while the mesoporous 0.5, 1, 1.5, and 2% Ag2O–ZnO nanocomposites were located at 400, 427, 450, and 456 nm, respectively; thus, a red shift was observed compared with that of pristine ZnO NPs. Meanwhile, the visible light harvest of Ag2O–ZnO nanocomposites was outstandingly higher than that of pristine ZnO NPs, which substantially occurs from impregnated Ag2O NPs (1.2 eV) and the fabrication of Ag2O–ZnO nanocomposites. The following formula was applied to determine the band gap energy[55]

Figure 5

(a) Diffuse reflectance spectra of mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents (0.5–2 wt %); (b) plot of transferred Kubelka–Munk vs energy of the light absorbed for mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents (0.5–2 wt %).

Therefore, the Eg of the Ag2O–ZnO nanocomposites was calculated from the plot of (αhν)1/2 versus hν, as shown in Figure b. The band gap value was calculated to be 3.07 eV of mesoporous ZnO NPs. The band gap energy of the mesoporous 0.5, 1, 1.5, and 2% Ag2O–ZnO nanocomposites was calculated as 2.86, 2.74, 2.66, and 2.65 eV, respectively. We observed a decrease of band gap energy of mesoporous ZnO NPs by introducing Ag2O NPs. Therefore, mesoporous Ag2O–ZnO nanocomposites provide more opportunities for the photogenerated electron–hole separation and promote visible light absorption.

Photocatalytic Performance

The photocatalytic efficiency and kinetics for the degradation of TC over mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying contents of Ag2O NPs compared with that over commercial P-25 during 120 min of the visible light exposure are illustrated in Figure . The photolysis of TC was determined to be less than 2% through visible light exposure for 30 min. However, the maximum adsorption capacity in the dark was achieved in the range of 5–8% over the synthesized samples within 120 min. The photodegradation of TC over mesoporous ZnO and Ag2O–ZnO samples was compared with that over P-25 under visible light illumination (Figure ). Compared to the pristine mesoporous ZnO NPs, the photocatalytic efficiency of mesoporous Ag2O–ZnO nanocomposites at varying contents of Ag2O NPs was enhanced owing to the impregnation of Ag2O NPs, which can harvest moderately visible light. The TC photodegradation efficiency over ZnO NPs can reach 5% through visible light exposure for 120 min, while the photocatalytic efficiency of the commercial P-25 was estimated to be 10%. With the increase of Ag2O NPs on mesoporous Ag2O–ZnO nanocomposites, the photocatalytic efficiency of Ag2O–ZnO nanocomposites was dramatically promoted. When the Ag2O NP content was boosted from 0.5 to 1.5%, the photocatalytic performance was increased from 30 to 100%, respectively, exhibiting the highest photocatalytic performance of 100% of TC during 120 min of the visible light exposure, as depicted in Figure a. At this point, we could observe that TC was completely degraded. Moreover, the photocatalytic performance over 1.5 and 2% Ag2O–ZnO nanocomposites for the photodegradation of TC was determined to be the highest because oftheir narrow band gap energy, small particle size, large surface area, and mesoporous structure. However, the photocatalytic efficiency of pristine ZnO NPs was lesser than that of Ag2O–ZnO nanocomposites, which elucidates that the outstanding photocatalytic efficiency of Ag2O–ZnO nanocomposites is attributed to the synergistic impact between ZnO and Ag2O NPs. The TC degradation rates over mesoporous Ag2O–ZnO nanocomposites compared to either commercial P-25 and pristine ZnO NPs were calculated (Table ). The TC degradation rate takes place much rapidly over the 1.5% Ag2O–ZnO nanocomposite (0.798 μmol L–1 min–1) as compared to either commercial P-25 (0.097 μmol L–1 min–1) and ZnO NPs (0.035 μmol L–1 min–1). Importantly, the degradation rate was boosted linearly with the increment of Ag2O NPs (0.5 to 1.5%). It is insignificant to impregnate Ag2O contents more than 1.5%. The mesoporous 1.5% Ag2O–ZnO nanocomposite revealed the highest degradation rate and it is 23 and 8 orders of magnitudes greater than those of pristine ZnO NPs and P-25, respectively.

Figure 6

(a) Time courses of the photodegradation of TC over that of mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents (0.5–2 wt %) compared with P-25 under visible light; (b) linear relationship between illumination time and ln(Co/C), where Co and C are the photodegradation of TC over that of mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents (0.5–2%) compared with P-25 under visible light (photocatalyst dose = 1 g/L, volume of aqueous solution = 200 mL, and TC concentration = 20 mg/L).

Table 1

Physical Properties of Mesoporous ZnO and Ag2O–ZnO Nanocomposites at Varying Ag2O Contents and Their Photodegradation of TC under Visible Lighta

photocatalysts	SBET/m2 g–1	bandgap (eV)	rate constant k, min–1	r (μmol g–1 min–1)
meso-ZnO	140	3.07	0.0010	0.035
0.5% Ag2O–ZnO	133	2.86	0.0068	0.199
1% Ag2O–ZnO	128	2.74	0.0172	0.485
1.5% Ag2O–ZnO	123	2.66	0.0414	0.798
2% Ag2O–ZnO	117	2.65	0.0423	0.802
P-25	50	3.20	0.0026	0.097

SBET surface area, and r photodegradation rate of TC.

The kinetic reaction over Ag2O–ZnO nanocomposites for TC degradation has been categorized by the pseudo-first-order as shown in the following formula, ln(Co/C) = kt,[30] where k and t are the rate constant and illumination time, C and Co are the TC concentration of residual after illumination time t, and t = 0, respectively. The linear correlation coefficient square (R2) values were estimated to be 0.98 ± 0.1. Figure b exhibits the correlation between ln(Co/C) and illumination time. The calculated k value for mesoporous Ag2O–ZnO nanocomposites compared with that for either commercial P-25 and pristine ZnO NPs is tabulated in Table . The k value of the 1.5% Ag2O–ZnO nanocomposite (0.0414 min–1) was 41 and 16 times greater than that of pristine ZnO NPs and P-25, respectively, showing the enhancement impact of Ag2O NPs and the superior photocatalytic efficiency of mesoporous Ag2O–ZnO nanocomposites. With the increase of Ag2O NP content from 0 to 1.5, the k values and photocatalytic efficiency were promoted. In general, the findings further revealed that Ag2O NPs were the key factors and exhibited a significant effect on the photocatalytic efficiency of mesoporous Ag2O–ZnO nanocomposites.

The influence of the loading amount of the mesoporous 1.5% Ag2O–ZnO nanocomposite for TC degradation was performed at a varying loading amount from 0.6 to 3 g/L (Figure a). The findings revealed that the photocatalytic performance was enhanced from 45 to 100%, with the boost of the loading amount from 0.6 g/L up to 1.8 g/L, respectively, whereas with the increment of the loading amount at 3 g/L, the photocatalytic performance was reduced to 80% because of the reduction in light scattering and penetration. Notwithstanding, it took almost 120 min over 1.8 g/L of the 1.5% Ag2O–ZnO nanocomposite to fully degrade TC, while less than 30 min was required when 2.4 g/L of the 1.5% Ag2O–ZnO nanocomposite was employed. At the loading amount of mesoporous 1.5% Ag2O–ZnO which was optimum, the high loading amount of Ag2O NPs might be present as the center for recombination of charge carriers and thus suppress the photocatalytic performance of the Ag2O–ZnO photocatalyst.[56,57] The stability of mesoporous 1.5% Ag2O–ZnO was a considerable factor in its potential photocatalysis application. To explore its stability of photocatalytic performance, five repeated TC degradation over mesoporous 1.5% Ag2O–ZnO was conducted as depicted in Figure b. As elucidated in Figure b, the photocatalytic performance was reduced only by 3% after five cycles. The 1.5% Ag2O–ZnO nanocomposite is stable during the photocatalytic reaction. To explore the stability, XRD patterns of the mesoporous 1.5% Ag2O–ZnO photocatalyst before and after illumination were evaluated. The phase structure of the mesoporous 1.5% Ag2O–ZnO photocatalyst did not alter before and after illumination within 10 h (Figure S1). Therefore, the 1.5% Ag2O–ZnO nanocomposite exhibited excellent stability of the photocatalytic performance for the potential applications in pollutants. This shows that the synthesized mesoporous Ag2O–ZnO nanocomposites exhibited advantages of small nanoparticle size around 5 nm and the eminent nanocrystalline, and the Ag2O NP-impregnated ZnO NPs led to the reduction of surface defect proportion in the mesoporous Ag2O–ZnO nanocomposites and significantly prohibited the photocorrosion, resulting in the enhancement of photostability in the mesoporous Ag2O–ZnO photocatalyst.

Figure 7

(a) Effect of loading amount of 1.5% Ag2O–ZnO nanocomposite on the photodegradation of TC; (b) time courses for recyclability of the photodegradation of TC for 5 times over mesoporous 1.5% Ag2O–ZnO nanocomposite.

Figure a exhibits PL spectra of pristine ZnO NPs and mesoporous Ag2O–ZnO nanocomposites at various Ag2O NPs at room temperature to examine and confirm the electronic structure and photocatalytic performance. In Figure a, mesoporous ZnO NPs exhibited a strong UV peak at 378 nm as a result of the photoinduced charge carrier recombination. Therefore, the presence of sharp and intense emission in the PL spectrum of the mesoporous ZnO NPs suggested that a low content of surface defects in ZnO nanocrystals was found. It was revealed that with the increase of Ag2O NPs, the PL intensity of the Ag2O–ZnO nanocomposites was decreased and it was red-shifted to longer wavelengths up to 427, 466, and 501 nm for 0.5, 1, and 1.5% Ag2O–ZnO nanocomposites, respectively. The decrease of PL intensity of Ag2O–ZnO nanocomposites indicated that the prohibition of photoinduced charge carrier recombination due to Ag2O NPs would be present as traps of the photogenerated electrons and prohibit the electron–hole recombination. Mesoporous 1.5% Ag2O–ZnO nanocomposites exhibited the weakest PL intensity, indicating the minimum of electron–hole recombination.[38] The transfer efficiency and surface charge separation of the mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at various Ag2O NPs were conducted by the transient photocurrent intensity (Figure b). The higher photocurrent intensity of Ag2O–ZnO nanocomposites revealed a better separation capability of photogenerated carriers. It is revealed that the order of their photocurrent intensities is 2% Ag2O–ZnO ≥ 1.5% Ag2O–ZnO > 1% Ag2O–ZnO > 0.5% Ag2O–ZnO > ZnO, indicating that 1.5 and 2% Ag2O–ZnO nanocomposites exhibited the highest separation efficiency compared with pristine ZnO NPs (Figure b). Thus, it is thought that the Ag2O loading can enhance the electron diffusion with high mobility and expedite charge pair separation.[58,59] The effect of scavengers was observed to determine the main active species for TC degradation through visible light. Benzoquinone (BQ), isopropanol (IPA), and EDTA were employed to study the roles of •O2–, •OH, and h+ radicals in the photocatalytic TC degradation, respectively, as depicted in Figure . The results indicated that IPA and EDTA were slightly suppressed TC photodegradation, which reduced the photocatalytic performance, indicating that •OH and h+ radicals had taken part in TC degradation. In contrast, the highest suppression effect was verified in the presence of BQ, which quenches the •O2– radical, indicating that it was the active species in the mesoporous 1.5% Ag2O–ZnO photocatalyst for TC degradation.

Figure 8

(a) PL spectra of mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents (0.5–2%); (b) transmission efficiency of photoexcited electrons in mesoporous ZnO NPs and Ag2O–ZnO nanocomposites at varying Ag2O contents (0.5–2%).

Figure 9

Effect of scavengers on the photodegradation of TC over mesoporous 1.5% Ag2O–ZnO nanocomposite under visible light.

Photocatalytic Enhancement Mechanism

The band gap energies of Ag2O and ZnO NPs are 3.07 and 1.2 eV, respectively. The matching energy band in both Ag2O and ZnO according to the synthesized Ag2O–ZnO heterostructures is presented in Scheme . The mesoporous Ag2O–ZnO heterostructures were constructed, the flat band potential of mesoporous Ag2O–ZnO heterostructures shows a more positive transportation in comparison with that of pristine ZnO NPs, which demonstrates that the CB of mesoporous Ag2O–ZnO heterostructures is more positive than that of ZnO. Upon illumination, Ag2O and ZnO NPs are readily induced to create electrons on the CB and leave holes on the valence band; therefore, the photoexcited electrons on Ag2O NPs can move to the CB of ZnO. When the Ag2O NPs only closes with ZnO with heterojunction structure, the photoinduced electrons transportation is obviously taken place in the TC photodegradation according to the different band and contact structure. The photoinduced electrons possessed intense activation energy and react with the adsorbed O2 on the mesoporous Ag2O–ZnO surface to produce superoxide radicals •O2–. Then, H+ reacted with •O2– partially to yield a strong oxidizing agent H2O2, which is additionally induced by electrons to produce •OH. All the oxidizing species produced during the photocatalysis mechanism effectively degraded TC into friendly environmental molecules such as CO2, H2O, and so forth.

Scheme 1

Proposed and Reaction Mechanisms for Photodegradation of TC to Explore the Promotion Photocatalytic Efficiency of Mesoporous Ag2O–ZnO Nanocomposites; Absorption of Visible Light by the Photocatalyst Enhances an Electron from the CB of Ag2O NPs to the CB of ZnO; Then, H+ Reacted with •O2– Partially to Yield a Strong Oxidizing Agent H2O2, Which Is Induced by Electrons to Produce •OH; All the Oxidizing Species Produced during the Photocatalysis Mechanism Were Effectively Degraded TC into Friendly Environmental Molecules Such As CO2, H2O, and So Forth

Conclusions

Mesoporous ZnO NPs have been successfully constructed through the sol–gel method employing Pluronic F-108 as a structure-directing agent with a large pore volume and high surface area. The uniform Ag2O NPs are facilely distributed onto the lattice and surface of ZnO networks. The pore volume and the surface area of mesoporous ZnO NPs are about 0.31 cm3/g and 140 m2/g, which decreased to 0.24 cm3/g and 117 m2/g, respectively, after impregnating 2% Ag2O NPs. The mesoporous Ag2O–ZnO nanocomposites have exhibited significant photocatalytic efficiency for TC degradation through visible-light exposure compared with pristine ZnO and commercial P-25. Mesoporous 1.5% Ag2O–ZnO nanocomposites indicated the highest photodegradation efficiency of 100% of TC during 120 min of the visible light exposure compared with 5% and 10% for pristine ZnO NPs and commercial P-25, respectively. The mesoporous 1.5% Ag2O–ZnO nanocomposite revealed the highest degradation rate among all synthesized samples, and it is 23 and 8 magnitudes greater than those of pristine ZnO NPs and P-25, respectively. The photostability and significantly photocatalytic performance were explained by the superior mesoporous Ag2O–ZnO structures with high crystallinity. Ag2O NPs were the key factors and exhibited an outstanding effect on the photocatalytic performance of mesoporous Ag2O–ZnO nanocomposites.