Fabrication of Ag/ZnO hollow nanospheres and cubic TiO2/ZnO heterojunction photocatalysts for RhB degradation


 ZnO nanomaterials with the stereochemical structure were becoming a research focus in the scope of photocatalytic materials, but the ZnO was sensitive to UV light rather than the solar light source, which considerably prohibited its extended application. ZnO nanomaterials coupled with other nanomaterials could generate the alternative composite heterojunction nanomaterials to promote the photocatalytic activity. Herein, we reported two facile and feasible synthesis methods to fabricate TiO2/ZnO cube nanocomposites and Ag/ZnO hollow spheres by hydrothermal reaction and chemical deposition, respectively. In this regard, these composited nanomaterials have been successfully fabricated with high purities, good morphology, and crystal structure. Noticeably, in contrast with TiO2/ZnO and Ag/ZnO bulk nanocomposites, the Ag/ZnO hollow spheres could offer the higher activity for RhB degradation under the visible light. Moreover, the photocatalytic performance of Ag/ZnO for RhB degradation could be improved synergistically, and the effect of RhB degradation was highest when the Ag mass ratio was modulated at 10% in the sample. Furthermore, it remained a high photocatalytic efficiency even after four cycles. This protocol provided an approvable approach to fabricate efficient photocatalysts with persistent photostability in the wastewater treatment process.


Introduction
Photocatalysis was typically considered as the prominent technologies for the application in the scope of environmental pollutants elimination, hydrolysis, CO 2 reduction, metal corrosion, and protection [1][2][3][4]. Their practical value and application potential have triggered continuous interests in developing effective and stable photocatalysts for pollutants reduction [5]. For decades, semiconducting metal oxides have widely developed for their favorable properties of suitable redox potentials, high photosensitivity, and low expenditure [6]. Especially for ZnO, it often exhibited the desirable photocatalytic activity due to its suitable band-gap energy and superior physicochemical properties [7][8][9][10]. However, the low separating efficiency of photogenerated charges and poor quantum utilization rate disapproved its further application to a certain extent [11][12][13].
Various strategies have been attempted to address the aforementioned challenges, including doping, constructing composite inhibition, and designing novel nanostructures [14][15][16]. Among them, constructing composite heterojunction between ZnO and TiO 2 has been demonstrated to be a facile and effective strategy for increasing the photocatalytic performance [5,17]. Besides, the closeness of energy band and forbidden bandwidth between ZnO and TiO 2 could make them well engage to form a composite material, which could become a promising candidate for RhB degradation [19][20][21]. Nevertheless, the dilemma of insufficient controllable preparation method restricted its further photocatalytic application due to the physical incompatibility during the preparation process [18][19][20][21]. Therefore, the architecture development of heterojunction nanomaterials as photocatalysis was highly desirable to improve photocatalytic efficiency.
Benefiting from the higher surface areas, controllable pore sizes, good dispersion, and high sensitivity, the hollow ZnO could also be utilized to favor the decomposition of pollutants in the solvent during the photocatalytic process [6,[22][23][24][25][26]. Besides the introduction of TiO 2 into ZnO, the composite metals into the ZnO including Ag, Pd, Pt, and Au have been extensively reported so far [27]. Comparatively, Ag has many advantages, such as low expenditure, less toxic, and easy to industrialize, which could be used to improve the activity of photocatalyst via hybridation [28][29][30]. Herein, we disclosed new strategies for photocatalytic RhB degradation by virtue of TiO 2 /ZnO and Ag/ZnO nanocomposites, which were synthesized through two effective methods of hydrothermal reaction and chemical deposition, respectively. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-vis optical absorption confirmed these nanocomposites with good morphology, crystal structure, and high purities. Photocatalytic degradation of RhB was studied in the irradiation with visible light. What's more, the photocatalytic efficiency and photostability for Ag/ZnO with different mass ratios were also investigated.

Materials
The reagents such as CaCl 2 , PEG-1000, Ti(OC 4 H 9 ) 4 , CaTiO 3 , KOH, and PVP were of analytical grade, obtained from Aladdin Company and used without further purification.

Chemical synthesis 2.2.1 Synthesis of TiO 2
A mixture of 0.11 g CaCl 2 , 0.5 g PEG-1000, and 0.33 mL Ti (OC 4 H 9 ) 4 was dissolved in 35 mL absolute ethanol and stirred at 25°C for 20 min. A total of 0.24 g NaOH was added under stirring. Then, the mixture was transferred to a 50 mL autoclave and held at 180°C for 15 h in an electric oven. After natural cooling to room temperature, the samples were washed with ethanol, deionized water, and then centrifuged and recovered to obtain the CaTiO 3 sample. Subsequently, 0.075 g CaTiO 3 and 0.34 g of Na 2 EDTA were dissolved in 40 mL of ionized water and ethylene glycol for ultrasonic treatment. Then, the solution was poured into a 50 mL autoclave and meantime raised the temperature to 180°C in a closed environment for 12 h. When cooled down, the sample was washed thoroughly with deionized water and ethanol and then dried the material by controlling the oven temperature at 80°C for 12 h, which was subsequently annealed at 400°C for 2 h. Eventually, the TiO 2 sample was successfully prepared.

Synthesis of ZnO seed crystal
A total of 0.115 g zinc acetate was added into 125 mL absolute ethyl alcohol, and then, the solution was stirred evenly at 60°C until the zinc acetate was completely dissolved. A total of 0.055 g KOH was dissolved into 65 mL of absolute ethyl alcohol, and the, it was stirred evenly until the KOH was completely dissolved, then the solution of KOH was transferred into the solution of zinc acetate at 60°C with stirring, and the sol-gel solution was finally obtained by stirring for 2 h at 60°C, and the seed solution of ZnO was used for the next step.

Synthesis of TiO 2 /ZnO
The fabricated TiO 2 was weighed and added into 5 g/L PVP aqueous solution and stirred for 4 h in a shaker. After the TiO 2 was fully coated by PVP, the solution was centrifuged and dried. The dried intermediate sample was dispersed in 20 mL of ZnO seed solution and placed in a shaker at room temperature for 4 h. Then, the intermediate sample was obtained by centrifugal washing and dried. This sample was added into 20 mL of a 10 mM Zn (NO 3 ) 2 ·6H 2 O growth medium and HMT growth medium, incubated at 85°C for 8 h, and then washed with deionized water and anhydrous ethanol. Finally, TiO 2 /ZnO photocatalyst was obtained by drying at 80°C for 4 h.

Synthesis of C ball
A mixture of 4 g anhydrous glucose and 150 mL deionized water was stirred, placed in a polytetrafluoroethylene reaction kettle, and stirred at 180°C for 10 h. The sample was centrifuged and washed by using deionize water and ethanol and then dried in an oven at 60°C. Finally, the C ball was successfully made for standby.

Synthesis of hollow ZnO
A mixture of 0.1 g C ball, 0.298 g Zn(NO 3 ) 2 .6H 2 O, and 7 g hexamethylenetetramine (HMT) was added into 100 mL ethanol. After ultrasonic treatment for 30 min, the mixture was put into the water bath and stirred for 2 h at 60°C, centrifuged three times with ethanol, and dried. The dried material was put into a muffle furnace, which had a temperature program of raising to 550°C at a speed of 5°C/min and maintained for 2 h to get hollow ZnO. Then, it was kept in the air for 2 h to remove the organic template and promote the ZnO film crystallization, leaving only a closed ZnO hollow spheres array.

Synthesis of hollow Ag/ZnO
A total of 0.1 g hollow ZnO was dispersed into 50 mL of deionized water containing 0.0075-0.030 g AgNO 3 , respectively, followed by the addition of 10 mL methanol under continuous stirring. Then, the solution was put into a xenon lamp (an instrument for photocatalytic degradation of RhB) and irradiated it for 30 min with revolution and rotation, followed by centrifugally washing for three times using water and ethanol. After the samples were dried, the hollow ZnO with 5-20% Ag deposited was obtained. The hollow Ag/ZnO nanocomposites were fabricated through the aforementioned three processes, as illustrated in Figure 1.

Synthesis of ZnO bulk
A mixture of 0.298 g Zn(NO 3 ) 2 .6H 2 O and 7 g HMT was put into 100 mL ethanol. After ultrasonic treatment for 30 min, this solution was added into the water bath, stirred for 2 h at 60°C, centrifuged for three times with ethanol and dried, then placed in a muffle furnace to raise 550°C at a speed of 5°C/min, and maintained for 2 h to synthesize ZnO bulk.

Characterization
The surface and interior morphology were checked by using a field-emission scanning electron microscope (FESEM) (TESCAN VEGA3) and a transmission electron microscope (TEM) (JEM-2100). The crystallinity and phase composition of all products were characterized by X-ray diffraction (Bruke-D8 ADVANCE) with a Cu-Kα source of radiation (λ = 1.5418 Å). The average crystallite size of the Ag nanoparticles was determined using the Scherrer formula (1): where K was the value of the Scherrer constant, λ was the wavelength of the X-ray, and c is the full width of the half peak (111) with radius (1/2). The optical properties were characterized by a UV-vis diffuse reflectance spectrophotometer (Varian Cary 500) with the samples supported on ZnO. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics Tristar 3000 instrument. The thermal spectrometer using a laser with an excitation wavelength of 532 nm was used to record Raman spectra.

Photocatalytic activity
The photocatalytic degradation of organic pollutant rhodamine B was studied. The hollow Ag/ZnO with different mass ratios of Ag was prepared, and the hollow Ag/ZnO spheres with the best morphology and mass ratio were characterized and analyzed. The photocatalytic experiments were carried out in the JH-GHXII catalytic reactor. The photocatalytic degradation of organic pollutants RhB was used to measure the photocatalytic performance of the material, and the TiO 2 /ZnO hollow spheres were also characterized and analyzed. Specifically, 30 mg TiO 2 /ZnO was dispersed into 5 mg/L RhB aqueous solution. Before irradiation, the dark reaction was carried out for 1 h to make the solution reach adsorption-desorption equilibrium. Then, a 500 W xenon lamp was turned on, a filter with a wavelength of 420 nm was put on, and the material was kept stirring for photocatalytic reaction. A total of 0.3 mL solution was taken out from the reactor every 1 minute. After the sample was centrifuged, the separated liquid-phase products were taken for the absorbance experiment. The degradation rate of the photocatalytic material was calculated via the following equation: where R was the degradation rate; A 0 and A represented the absorbance of rhodamine B solution before and after degradation, respectively. After the photocatalytic degradation experiments were finished, the catalyst in the RhB aqueous solution was centrifuged by absolute ethyl alcohol and deionized water, dried, and then dried in an oven at 60°C to test the repeatability and stability. Finally, this material was found still served as the photocatalyst in the later period.

Results and discussion
The SEM photograph of TiO 2 surface morphology is shown in Figure 2(a) and (b). It can be clearly found that the asfabricated TiO 2 was cubic, and some broken samples were seen in the nanocubic TiO 2 particles, some samples had cracks on the surface, and the overall size of each cube was 1-3 μm 3 . Figure 2(c) and (d) show a SEM image of the TiO 2 /ZnO nanocomposite. It was demonstrated that the surface of the TiO 2 /ZnO sample was rough, and ZnO nanoparticles were densely grown and uniformly distributed on the surface of the cube TiO 2 . Interestingly, although the TiO 2 /ZnO nanocubes were closely stacked together, their shape still presented a more regular cubic structure. Furthermore, SEM images of the morphology of Ag/ZnO photocatalyst are shown in Figure 2(e). It can be detected that the ZnO delivered the evident hollow spheres shape and the size of the sphere was 1-2 μm. In addition, numerous Ag nanoparticles were highly dispersed on the surface of ZnO hollow spheres, and the sizes of the spheres ranged from 5 to 10 nm. The representative TEM and highresolution TEM (HR-TEM) images of Ag/ZnO are displayed in Figure 2(f). The low-magnification TEM image clearly demonstrates that the Ag/ZnO was identified as the hollow spheres, where its surface consisted of a layer of ZnO particles. Furthermore, the HRTEM image disclosed that the Ag was attached on the hollow sphere surface, as evidenced by the lattice fringes of Ag (111) at 0.212 nm and ZnO (100) at 0.281 nm [5,[31][32][33].
The powder XRD patterns of TiO 2 and TiO 2 /ZnO nanoheterojunction photocatalysts were displayed in Figure 3.  . 04-783). Moreover, after the deposition of Ag, the position of ZnO diffraction peaks did not change, suggesting that Ag did not enter the crystal lattice of ZnO, but adhered to the ZnO surface. Besides, the peak intensity was decreased due to the encapsulation of Ag on the outer surface of ZnO. Since the coated Ag particles were nanosized, the peaks of the composite particles become broadened. Accordingly, the Ag/ZnO composite was well formed.
There was a close relationship between the bandgap and the photocatalytic properties of semiconductor photocatalysts. When the photoresponse range of the materials in the visible light region shifted toward the long wavelength direction, the photocatalytic properties would certainly be better. Figure 5 shows the UV-vis diffuse reflectance spectra of ZnO bulk, ZnO hollow spheres, 10% Ag/ZnO bulk, and 10% Ag/ZnO hollow spheres. The characteristic absorption peak of pure ZnO bulk appeared at 380 nm, which was consistent with its intrinsic absorption band at 390 nm. The composite nanomaterial of Ag/ZnO hollow spheres presented the adsorption peak shift from 380 to 428 nm after the Ag particles deposited on the ZnO surface, which was attributed to the strong interaction between them [35,36]. Compared with Ag/ZnO hollow spheres, the absorption peak of Ag/ZnO bulk had blue-shift to a certain degree due to the weaker interaction existing between them. These results demonstrated that Ag/ZnO hollow spheres could exhibit favorable photocatalytic properties because of its adsorption peak at the long wavelength range.
The nitrogen adsorption-desorption isotherms of ZnO bulk, ZnO hollow spheres, 10% Ag/ZnO bulk, and 10% Ag/ZnO hollow spheres are shown in Figure 6(a), which were used to determine the specific surface area by the BET method. All the samples exhibited a typical IV-type isotherm with the H 3 hysteresis loop, confirming the existence of both mesopores and macropores in these samples. The pore size distribution curves of these samples illustrated in Figure 6(b) presented the pores size distribution from 5 to 130 nm. Correspondingly, the BET surface of ZnO bulk, ZnO hollow spheres, 10% Ag/ZnO bulk, and 10% Ag/ZnO hollow spheres had been calculated to be 8.7, 13.7, 11.2, and 4.5 m 2 /g, respectively. This result disclosed  that 10% Ag/ZnO hollow spheres possessed the larger surface area among the aforementioned samples due to their abundant pores surviving on the hollow structure surface, which was beneficial to the increase of the photocatalytic activity.
The degradation efficiency of pollutant RhB on ZnO, TiO 2 , TiO 2 /ZnO catalyst under visible light irradiation is shown in Figure 7(a). In the blank experiment, the concentration of RhB had no obvious fluctuation in four curves, illustrating that the self-decomposition of RhB pollutants under visible light could be ignored. However, when ZnO catalyst was added, the RhB was degraded slowly and the relevant degradation rate was only about 5% after 120 min. Meantime, the RhB degradation rate of TiO 2 was about 17%, and the result was also not desirable. Moreover, the RhB degradation rate of TiO 2 /ZnO had no significant change, which was even lower than that of pure TiO 2 in the same period after repeating results of many experiments. Hence, we supposed that ZnO formed a film on the surface of TiO 2 , which made TiO 2 unable to make full use of visible light for photocatalytic activities and thus reduced photocatalytic performance. Therefore, excluding the error of data, it could be judged that the photocatalytic effect of TiO 2 /ZnO cubic nano-heterojunction composite material was not as expected. Significantly, it should be noted that the RhB degradation rates of Ag/ZnO photocatalysts are shown in Figure 7(b) and (c). The photocatalytic efficiency of Ag/ZnO hollow spheres photocatalysts is higher than that of Ag/ZnO bulk photocatalysts, which were also better than both ZnO and blank experiment. Because when Ag was loaded, the electrons of ZnO were excited under visible light and then jumped to its conduction band, and simultaneously, the Ag could temporarily store photogenerated electrons and improve its photocatalytic performance. More specifically, the overall trend of the curves was that the degradation rate of pollutant RhB changed significantly in the first 5 min, and then, the degradation rate began to slow down, mainly because at the beginning of the experiment, the amounts of pollutants were large, so the opportunity and area of contact with catalyst were large, which led to faster degradation in the early stage. When the reaction went to the later stage, the concentration of RhB pollutants was low, and the number of molecules that could react was reduced, so the degradation efficiency would become worse. Moreover, the photocatalytic performance of Ag/ZnO hollow spheres was the best when the loading mass fraction of Ag was 10%. As the mass fraction of Ag increased from 0 to 10%, the photocatalytic effect increased all the time. What's more, as the doping amount of Ag increased from 10% to 20%, the photocatalytic activities decreased gradually. Accordingly, the loading amount of Ag over the ZnO determined the photocatalytic activity of RhB degradation. This was because that the doping of Ag over the ZnO was conductive to promote its separation of photocarriers, which could prolong its lifetime and transmission time, thus enhancing its photocatalytic activities of RhB degradation. Moreover, the stability of the photocatalyst was the basic requirement for its practical application in the future. For the 10% Ag/ZnO hollow sphere photocatalyst, we tested repeatedly and found that its RhB degradation rate was basically not reduced even after five cycles, which remained at a high level of 90%, displaying its good photostability (Figure 7d).

Conclusion
In summary, we have demonstrated the synthesis of TiO 2 / ZnO and hollow Ag/ZnO for RhB degradation through two facial methods of low-temperature hydrothermal and chemical deposition, respectively. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-Vis optical absorption confirmed that these composited nano-materials have good morphology, crystal structure, and high purities. The modified photocatalysts for RhB degradation were researched under visible light, which revealed that the TiO 2 /ZnO has almost the same photocatalytic activities for RhB degradation. However, the Ag/ZnO hollow spheres could synergistically enhance the photocatalytic performance. An optimal concentration of 10% Ag loaded on the hollow-sphere heterojunction material shows the highest photocatalytic efficiency. Noticeably, its photocatalytic activity still remains excellent even after four cycles. More importantly, this methodology represented an effective solution to obtain good photostability of Ag/ZnO hollow spheres, enriching the arsenal of ZnO heterojunction nanomaterials, which also showed great potential for its application in wastewater treatment.  Author contributions: X.C.: conceptualization; C.S. and Z.Y.: methodology; N.W.: validation; X.C. and X.G.: formal analysis; X.C., N.W., and Z.Y.: investigation; N.W. and L.C.: resources; Y.L.: data curation; X.C. and X.G.: writingoriginal draft preparation; N.W., and X.G.: writing-review and editing; N.W. and L.C.: supervision; C.S.: project administration; X.C., N.W., Y.L., and L.C.: funding acquisition. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

Conflict of interest:
The authors declare no conflict of interest.