ZnO Film Photocatalysts

We have synthesized high-quality, nanoscale ultrathin ZnO films at relatively low temperature using a facile and effective hydrothermal approach. ZnO films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, photoluminescence spectra (PL), and UV-vis absorption spectroscopy. The products demonstrated 95% photodegradation efficiency with Congo red (CR) after 40min irradiation. The photocatalytic degradation experiments of methyl orange (MO) and eosin red also were carried out.The results indicate that the as-obtained ZnOfilmsmight be promising candidates as the excellent photocatalysts for elimination of waste water.


Introduction
Zinc oxide (ZnO), an important II-VI semiconductor with a bandgap energy of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, has been extensively studied because of its potential applications in solar cells [1], sensors [2,3], photocatalysis [4], and so forth.Among them, the important application of ZnO as a photocatalyst in environmental protection cannot be ignored [5][6][7][8][9][10].In the past decades, zero-dimensional (0D) and one-dimensional (1D) ZnO nanostructures have been extensively studied with the aims of developing novel applications [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25].However, twodimensional (2D) nanostructures have not been extensively explored [26,27].Since the photocatalytic reaction occurs at surface of the materials, the nanosized semiconductor will increase the decomposition rate because of the increased surface area.Therefore, the synthesis of novel ZnO nanostructure that is stable against aggregation and possesses a higher surface-to-volume ratio is still an important task for its environmental remediation applications.In the fabrication of 2D ZnO nanostructures, previous methods required either multiple operation steps [28,29] or using of the templates or the toxic reactants [30,31].Therefore, developing a simple and efficient green method to synthesize ZnO films will be highly required.
Herein, we used a facile hydrothermal approach to obtain ultrathin ZnO films without using any surfactants or templates.Such ZnO film structures exhibit a significantly improved photocatalytic activity in the photodegradation of MO than that of other structured ZnO.This work provides a way to improve the photocatalytic performance by designing a desirable nanoarchitecture.

Experimental Details
All reagents were of analytical grade and were used without further purification.In a typical procedure, 20 mL of Zn (NO 3 ) 2 solution was added to 20 mL urea aqueous solution.After a continuous stirring for 30 min, the mixed solution was transferred into a 100 mL stainless steel autoclave, which was sealed subsequently and kept at 150 ∘ C for 3 h.The white precipitation was centrifugated and washed several times with deionized water, followed by drying in air at 60 ∘ C for 8 h.
The morphology and microstructures of the asobtained products were characterized by scanning electron microscope (SEM; Hitachi S-4800), XRD (D/max2600, Rigaku), and Raman spectroscope (HR800).Photoluminescence spectra (PL) of the samples were characterized by the micro-Raman spectrometer (HR800) under the excitation  wavelength of 325 nm.The efficiency of the photocatalytic degradation was analyzed by monitoring dye decolorization at the maximum absorption wavelength, using a UV-vis spectrometer (Shimadzu UV-2550).
The photocatalytic experiment of the as-synthesized ZnO samples for decomposing MO was conducted as follows: 0.1 g ZnO films were suspended in 200 mL MO aqueous solution (20 mg L −1 ).The solution was continuously stirred for 1 h in the dark to ensure the establishment of an adsorptiondesorption equilibrium between ZnO film and MO.Then the solution was exposed to UV irradiation from a 500 W Hg lamp at room temperature.The samples were collected at regular interval to measure MO degradation by UV-vis spectra.The products were then separated from the solution by centrifuging, washed with ethanol to fully remove the residual organic species then with water, and reused for the next run.Finally, the experiments of the photocatalytic degradation of CR aqueous solution and eosin red aqueous solution also were conducted under the same conditions.

Results and Discussion
The general morphology of ZnO products was investigated by SEM.Figures 1(a)-1(c) show the SEM images of the as-synthesized ZnO products at different magnifications, finding that the as-obtained product consists of a layer of film with an average thickness of 30 nm.XRD pattern for ZnO film is shown in Figure 1(d).All of the diffraction peaks can be well indexed to hexagonal wurtzite ZnO (JCPDS number 36-1451) with lattice constants of  =  = 3.25 Å and  = 5.2 Å.
No diffraction peaks from any other impurities are identified, indicating high purity of the product.To further investigate the structures of the ZnO films, PL spectra of the product were conducted.Figures 1(e) and 1(f) are SEM images of ZnO nanocones and ZnO commercial powder, respectively.Figure 2(a) showed a strong ultraviolet emission peak and a weak green light emission.It is known that the UV peak arises from the near band-edge exciton recombination, and the green emission comes from the various defect states.Figure 2(b) presents Raman spectrum of the as-obtained product at room temperature.Two peaks are observed at 437 and 563 cm −1 , respectively.ZnO with wurtzite structure belongs to the C 6v space group with the two formula units per primitive cell and all the atoms occupy the C 3v symmetry.Near the center of the Brillouin zone, the group theory predicts the existence of the different optical modes.Raman active modes for wurtzite ZnO are Γ =  1 + 2 2 +  1 , where the  1 ,  1 , and 2 2 modes are Raman active and split into longitudinal (LO) and transverse (TO) optical modes [32,33].The peak at 437 cm −1 in Figure 2(b) is assigned to  2 optical phonon which corresponds to the band characteristic of ZnO wurtzite hexagonal phase [34].Peaks located at 563 cm −1 correspond to the LO phonon of  1 and longitudinal  1 , respectively.
In order to investigate the photocatalytic efficiency of ZnO structures with different morphologies, we examined the decomposition of MO in water under irradiation of a 500 W Hg lamp as the light source.For comparison, decomposition of ZnO nanocones and that of commercial powder were also conducted under the same experimental condition.Figure 3(a) shows the adsorption spectra of MO solution in the presence of ZnO films under Hg lamp light.The absorption peak corresponding to MO at 465 nm diminished gradually and the photocatalytic degradation rate of MO is 96% after 90 min.The adsorption spectra of MO solution in the presence of ZnO nanocones are shown in Figure 3(b), revealing its photocatalytic degradation rate of 73%.For commercial powder, the degradation rate is 84% (seen in Figure 3(c)).Figure 3(d) shows the curves of the degradation rate of MO solution for blank experiment (black curve), ZnO films (pink curve), ZnO nanocones (red curve), and commercial powder (blue curve).Experimental results show that the degradation rate of MO in the presence of ZnO films is the fastest.The superior photocatalytic activities of ZnO films may arise from their unique structures and surface reaction sites.Specifically, ZnO films possess several outstanding features, such as the large surface volume ratio, the effective electron-hole separation of the Schottky barriers, and thin thickness.It might be that higher surface area increases the number of active sites and promotes separation efficiency of the electron-hole pairs, resulting in the improvement of photocatalytic activity.And the separation and mobility of the electron-hole pairs were intensely suppressed in wide band gap.Hence, ZnO films can absorb and transport more dye molecules on their surface.
Finally, the photocatalytic activities of the as-synthesized ZnO films for the degradation of different organic pollutants (MO, eosin red, and CR) were carried out.Figure 4(a) shows the adsorption spectra of MO solution in the presence of ZnO films under ultraviolet light at different intervals of time.Figure 4(b) shows the adsorption spectra of eosin red solution.The main absorption peak is centered at 517 nm before and after irradiation.When the illumination time was extended to 60 min, the absorption peak diminished gradually and the photodegradation ratio of eosin red was up to 98%. Figure 4(c) shows the adsorption spectra of CR with the absorption peak of 495 nm.Nearly 95% of CR dye molecules were decomposed in 40 min.In order to illustrate for which dyes ZnO film are highly selective, we take the same 40 min to compare the degradation efficiency of different dyes according to Figures 4(a)-4(c).The changes of the organic pollutants concentration under visible irradiation can be calculated as follows: where  0 is the initial concentration of the organic pollutants when the ultraviolet light is turned on, while the real time concentration of organic pollutants under the ultraviolet light irradiation is expressed by .Photocatalytic efficiency derived from the changes of the organic dyes concentration can be represented by the relative ratio / 0 .The order of degradation rate was MO (58%) < eosin red (88%) < CR (95%), as shown in Figure 4(d).It show that ZnO films possess the highest degradation efficiency to CR solution than to the others.

Conclusions
In summary, ultrathin ZnO films have been successfully synthesized by a simple hydrothermal approach without any surfactants or templates.The as-obtained films possess the average thickness of 30 nm.The photocatalytic experiments revealed that ZnO films possess the highest photocatalytic activity for the degradation of CR dye under ultraviolet light irradiation.And the degradation rate is 95% in 40 min.It is expected that such ZnO films could have potential application in eliminating organic pollutant in wastewater.

Figure 1 :
Figure 1: (a-c) SEM images of the as-synthesized ZnO films at different magnification.(d) XRD pattern of as-synthesized ZnO products.(e) SEM images of ZnO nanocones.(f) SEM images of ZnO commercial powder.

Figure 2 :
Figure 2: (a) PL spectra of the synthesized product.(b) Raman spectrum of the synthesized product.

Figure 3 :
Figure 3: Adsorption spectra of MO solution in the presence of different ZnO nanostructures.(a) ZnO films.(b) ZnO nanocones.(c) ZnO commercial powder.(d) Degradation rate curve of ZnO films, ZnO nanocones, and ZnO commercial powder.

Figure 4 :
Figure 4: Variations of adsorption spectra of the organics dye solution in the presence of ZnO films irradiated by a Hg lamp for different time; (a) MO, (b) eosin red, and (c) CR (d); photocatalysis degradation rate of MO, eosin red, and CR.