Filter Paper Inspired Zinc Oxide Nanomaterials with High Photocatalytic Activity for Degradation of Methylene Orange

Nanometer-sized zinc oxide (ZnO) has been synthesized through sol-gel method with natural cellulose substance (commercial filter paper) as template. The structure of zinc oxide nanomaterial was characterized by nitrogen adsorption-desorption and XRD. The morphology was observed by SEM and TEM. The results show that the hexagonal wurtzite phase is actually the only crystal phase in the sample and the product faithfully inherits the hierarchical morphology and the complex network structure of the original filter paper, which is composed of many randomly intersecting zinc oxide microfibers and nanosheets with lots of close stacked particles adsorbed on the surface. Moreover, these zinc oxide nanomaterials possess abundant mesoporous structure with an average pore diameter of ca. 21 nm and a wide pore size distribution (3–30 nm). Due to the strong absorption ability in the UV range, the zinc oxide nanomaterial prepared by this method displays significantly high photocatalytic activity for degrading methyl orange. In a short period of 20 minutes, the zinc oxide nanomaterial has degraded about 50% of the original MO, and the MO dye is fully degraded after UV irradiation for 80 minutes.


Introduction
In recent years, the environmental pollution has been increasing.The photocatalytic processes in polluted air and water through photocatalytic oxidation of semiconductor catalysts have been studied extensively in recent years [1][2][3].Anatase titania has been widely used as a catalyst both in liquid and in gaseous systems showing good performances concerning photoreactivity and photostability [4][5][6].Zinc oxide is a direct wide band gap (3.37 eV) oxide semiconductor photocatalyst, which is close to band gap energy of TiO 2 (3.2 eV), and the mechanism of photocatalysis has been proven to be similar to that of TiO 2 [7].Nanometer-sized zinc oxide as an important inorganic function nanomaterial was widely used in rubber [8], coat [9], catalyst [10], and electronic engineering fields [11] due to its unique properties such as high photosensitivity, transparency in the visible, high infrared reflectivity, acoustic characteristics, high electrochemical stability, and excellently electronic properties.The dispersion and surface area of zinc oxide, which depend on the synthesis method, are important factors for determining its photocatalytic activity [12][13][14].Zinc oxide nanomaterials can be prepared by various methods, such as alkali precipitation [15], pulsed laser deposition [16], magnetron sputtering [17], chemical vapor deposition [18], and sol-gel processing [19][20][21].However, among all of these techniques, the sol-gel process is one of the versatile methods to prepare various morphologies nanosized zinc oxide without complicated instruments.The most important advantages are the simplicity of equipment, the ability to accurately control stoichiometry, high homogeneity, and relatively low process temperature.Therefore, sol-gel technique could be a suitable method for the preparation of various morphologies nanomaterials.
Synthesis by using natural materials as template is a facile strategy to yield functional materials with complex and hierarchical natural structures, which are generally difficult to prepare even through the most advanced synthetic methods.Among these biotemplates, natural cellulosic substances such as the common filter paper possess a macroscopic-tonanoscopic random morphological hierarchy.Using these common and low-cost natural cellulose substances as the structural templates, many nanomaterials with complex functional nanostructures can be formed.With the natural cellulosic materials as structural templates, unique and intricate morphologies originated from nature substances become the choice for the design and preparation of zinc oxide nanomaterials, which extensively expands the structural varieties of ZnO materials.
In this paper, we synthesized nanometer-sized zinc oxide through sol-gel method with natural cellulose substance (commercial filter paper) and polyethylene glycol (PEG) as dual templates, Zn(CH 3 COO) 2 ⋅2H 2 O as precursor, ethanol as solvent, and NH(C 2 H 2 OH) 2 as chelating agent.The resulting zinc oxide nanomaterials faithfully inherit the hierarchical morphologies and the complex network structures of the initial cellulose substance and exhibit relatively high photocatalytic activity.O] was dissolved in 50 mL of absolute ethanol and stirred with a magnetic stirrer (70 ∘ C) to be mixed thoroughly.When the solution changed into an emulsion, an amount of diethanolamine [NH(C 2 H 2 OH) 2 ; DEA] as chelating agent was added to the emulsion (the molar ratio of DEA/zinc acetate was 1 : 1).When the emulsion became clear after a period of stirring, 0.7 g polyethylene glycol (PEG) was added and continually stirred for 2 h until a transparent sol was obtained.After the sol solution was cooled to the room temperature, 150 mg precutted squared filter paper fractions of ca. 5 mm × 5 mm were immersed in the sol solution for 3 h and then the filter paper fractions were filtered off, washed with absolute ethanol, and finally vacuum-dried at room temperature overnight.The as-prepared filter paper was firstly calcined in a tubular furnace at 100 ∘ C for 1 h and then heated up to 500 ∘ C and kept for 3 h.The whole process was in air atmosphere with a ramping rate of 2 ∘ C/min.

Characterizations.
The morphologies of samples were observed by a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G20/JEM 2010, operated at 200 kV).To prepare the specimens for SEM and TEM observation, a small piece of the sample sheet specimen was suspended in ethanol by ultrasonication for several seconds.The suspension was dropped onto silicon wafer followed by sputtering with gold or platinum to increase conductivity for SEM observation or onto copper mesh for TEM observation, and the specimens were dried in air.Crystal phase of the obtained materials was determined by powder X-ray diffraction (XRD) recorded on a Dandong TD3500 Advanced Diffractometer with Cu-K radiation ( = 0.15418 nm) and scan rate 0.05 ∘ 2/ and within range of 2 from 20 ∘ to 90 ∘ at room temperature.The nitrogen adsorption and desorption isotherms were measured at 77 K on a TriStar II 3020 surface area and pore analyzer.The linear part of the Brunauer-Emmett-Teller (BET) equation was used for the specific surface area determination.The pore size distributions were obtained from the adsorption branch of the nitrogen physisorption isotherms using the BJH method.Ultraviolet-visible diffuse reflectance (UV-vis DRS) absorption spectra were tested using a U-4100 spectrometer (Hitachi, Japan) with BaSO 4 powder as the reference standard.

2.4.
Photodegradation Experiment.Methyl orange (C 14 H 14 N 3 NaO 3 S), a widely used dye, was employed as a representative dye pollutant to evaluate the photocatalytic activity of zinc oxide nanomaterials under UV light.Firstly, 100 mL aqueous solution of zinc oxide catalyst (0.2 g) was treated with ultrasonication for 30 min to promote dispersion uniformity and 400 mL dye aqueous solution of MO (30 mg/L) was prepared for subsequent photocatalytic activity test.Then, the two prepared solutions above were mixed in a quartz tube.Before irradiation, the above mixed solution was stirred for 30 min in the dark to ensure the establishment of adsorption-desorption equilibrium.Under ambient conditions and stirring, the quartz tube was exposed to the UV irradiation produced by a 500 W Hg arc lamp equipped with a band-pass light filter (365 ± 15 nm).At selected time intervals, about 3 mL of the suspension was withdrawn for analysis on a Varian UV-vis spectrophotometer (Cary-50, Varian Co.).The percentage of residual dye is reported as / 0 .Here,  is the concentration of dye solution at each irradiated time interval, and  0 is the initial concentration of dye solution before photodegradation experiment.
The widening XRD peak enables estimation of the crystallite size via the Scherrer equation as below: where  (radian) is the full-width at half-maximum for the peak at 2 = 36.3∘ ;  is a constant (0.89);  is the X-ray wavelength (1.5418 Å for Cu K);  is the particle diameter; and  is the angle of the diffraction peak (degree).This analysis yields an estimate of ca.25.3 nm for the average crystallite size based on the (101) peak.
The morphology of zinc oxide product was investigated by SEM and TEM, and typical images are shown in Figure 2. Figure 2(a) is the overview FE-SEM image of the sample, which shows that the zinc oxide sample faithfully inherits the hierarchical morphology and the complex network structure of the original filter paper template, and the resulting zinc oxide is composed of many randomly intersecting zinc oxide microfibers and nanosheets with lots of particles adsorbed on the surface.With further magnification, we can observe that the solid zinc oxide microtubes and nanosheets are composed of lots of closely packing zinc oxide particles (Figure 2(b)).Figure 2(c) is a typical TEM image of zinc oxide nanosheet, in which the solid nanosheet structure of sample is clearly identified and the zinc oxide particles with diameters of ca.28 nm are measured.The HR-TEM image (Figure 2(d)) shows that the measured lattice spacing of sample is 0.281 nm, which corresponds to the lattice distance of hexagonal wurtzite (100) phase and confirms the formation of hexagonal wurtzite nanoparticles.Several multicrystal diffraction rings to be indexed to the anatase phase can be observed obviously in the SAED pattern of sample (Figure 2(d) insert), in particular, (from small to large rings) indexed as 100 ( = 0.281 nm), 002 ( = 0.26 nm), 101 ( = 0.247 nm), 102 ( = 0.191 nm), 110 ( = 0.162 nm), 103 ( = 0.147 nm), and 200 ( = 0.141 nm).This result is in good agreement with XRD analysis.
The nitrogen adsorption and desorption curve of the synthesized zinc oxide nanomaterials is shown in Figure 3.The hysteresis loops of H4 type proved the existence of mesopores, which was observed in the case of aggregates of particles giving rise to pores.The hysteresis loop at a relative pressure (/ 0 ) between 0.5 and 0.95 is attributed to the stacking of zinc oxide particles.The capillary condensation step at relative pressure about 0.8 (/ 0 ) is a contribution of the typical mesopores in the sample.The sharp capillary condensation step occurs in the relative pressure range of 0.8-1.0(/ 0 ) due to the existence of mesopores in the materials.The pore distribution plot of the synthesized materials was determined by the BJH (Barrett-Joyner-Halenda) method from the desorption branch of isotherm as shown in the inset of Figure 3, which indicates that the synthesized zinc oxide has macropores with an average pore diameter of ∼21 nm and a wide pore size distribution.The BET specific surface area of the synthesized zinc oxide is estimated to be about 14.4293 m 2 /g and the total pore volume is about 0.119175 cm 3 /g.
The UV-vis adsorption spectrum of the prepared zinc oxide nanomaterials was measured by UV-vis optical absorbance pattern (Figure 4(a)), showing that the broad absorption region mainly concentrated in less than 400 nm, which indicates that this sample is mainly to absorb ultraviolet light.Semiconductors are classified as either direct or indirect according to the lowest allowed electronic transition [22].As we all know the zinc oxide crystal is a direct semiconductor and its absorption index (ℎ]) should follow the (ℎ] −   ) 3/2 dependence.Normally nanomaterials exhibit an absorption spectrum such as that of the direct band gap semiconductor, and its absorption coefficient (ℎ]) follows the (ℎ] −   ) 1/2 law.So, the relationship between the absorption coefficient () and incident  photon energy (ℎ]) can be written as [23] where   is the absorption constant for direct transitions [24].Therefore, the band gap energy can be estimated from a plot of (ℎ]) 2 versus the photon energy (ℎ]).As the absorbance () is proportional to the absorption coefficient (),  can substitute for .Plots of (ℎ]) 2 versus the photon energy (ℎ]) are shown in Figure 4(b).The intercept of the tangent to the plot gives a good approximation of the direct band gap energies of the samples.Extrapolating the linear part of the curve for the zinc oxide sample gives a direct band gap of 3.18 eV, which is narrower than commercial zinc oxide semiconductor (3.37 eV).The reduction of band gap value for the synthesized ZnO nanomaterials here should arise from the structural and electronic defects such as interstitials and vacancies created during synthesis process.In the sample, the band gap was obtained from the electronic transition between the filled valence states to the energy level of the generated defects instead of the transition between the valence band and the conduction band as usual and resulted in a lower band gap value than the bulk ZnO with the perfect structure [25].
The results above indicate that our zinc oxide samples possess stronger capability of absorption than commercial zinc oxide semiconductor in the ultraviolet light range.

Photocatalytic Degradation of MO.
The photocatalytic activity of our porous zinc oxide sample was further evaluated by monitoring the degradation of methyl orange (MO) under UV irradiation (Figure 5).Before UV irradiation, the mixture containing the catalyst and MO was stirred in the dark for 30 min to ensure that MO was adsorbed to saturation on the surface of catalysts.With the time increasing, gradually weakening orange liquid can be observed in Figure 5(a), implying the decrease of MO concentration in mixture.ZnO-based photocatalysts is much lower than that of the currently developed filter paper inspired ZnO nanomaterials, suggesting their superior photocatalytic activity.The kinetics of photocatalytic degradation of MO can be depicted by pseudo-first-ordered kinetics equation as below [28]: where  is the pseudo-first-order rate constant (min −1 ),  0 is initial concentration, and  is the concentration of MO at time  (min).Figure 5(c) shows the plot of ln(/ 0 ) versus time, in which linear plots were observed with  2 values equal to 0.96168.The result indicates that photodegradation of MO obeys pseudo-first-order kinetics and the calculated pseudofirst-order rate is 0.04734 min −1 .Figure 5 shows that the filter paper inspired zinc oxide nanomaterials exhibit relatively high photocatalytic activity.Though these zinc oxide samples possess a low surface area (14.4293 m 2 /g) and weak adsorptive capacity in the dark adsorption of photocatalytic test (Figure 5(b)), they still exhibit relatively efficient photocatalytic rates during the photochemical process, which contribute from its stronger absorption ability in the UV range.

Conclusion
In this paper, ZnO porous thin films were prepared on natural cellulose substance (commercial filter paper) by sol-gel method with polyethylene glycol (PEG) as organic template, Zn(CH 3 COO) 2 ⋅2H 2 O as precursor, ethanol as solvent, and NH(C 2 H 2 OH) 2 as chelating agent.The obtained material was characterized by XRD, SEM, TEM, SAED, N 2 adsorption/desorption, and UV-vis.From the results of the XRD and SAED, we can deduce that hexagonal wurtzite phase is actually the only crystal phase in the sample.The SEM results show that the product faithfully inherits the hierarchical morphologies of the initial cellulose substances and consists of many randomly intersecting zinc oxide microfibers and nanosheets with lots of particles adsorbed on the surface.With further magnification, we can observe that the solid zinc oxide microtubes and nanosheets are composed of lots of closely packing zinc oxide particles.Analysis of TEM and N 2 adsorption/desorption isotherm plots show that these zinc oxide nanomaterials possess the mesoporous structure with an average pore diameter of ca.21 nm and a wide pore size distribution (3-30 nm).The photocatalytic activity for methyl orange degradation was tested under UV irradiation.The result shows the zinc oxide nanomaterial prepared by this method displays significantly high photocatalytic activity due to its stronger absorption ability in the UV range.Under the UV irradiation in a short period of 20 minutes, the zinc oxide nanomaterials have degraded about 50% of the original MO, and all of the dye is almost fully degraded after UV irradiation for 80 minutes.The current facile but effective method, which uses the common commercial filter paper as structural template, will reduce the production cost of nanomaterials, expand the structural varieties of ZnO materials, and provide a straightforward approach to fabricate other metal oxides with distinct and complicated structures and morphologies.

Figure 1 :
Figure 1: X-ray diffraction pattern of the prepared filter paper inspired zinc oxide nanomaterial.

Figure 5 :
Figure 5: (a) Photographs of MO under UV irradiation at different times, (b) liquid-phase photocatalytic degradation of MO under the irradiation of UV light, and (c) kinetics of photocatalytic degradation of MO under UV for filter paper inspired zinc oxide nanomaterials.