Template-Free Synthesis of Star-Like ZrO 2 Nanostructures and Their Application in Photocatalysis

School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and Equipments, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Xuzhou City Key Laboratory of High Efficient Energy Storage Technology and Equipments, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China


Introduction
Zirconia (ZrO 2 ) is one of the most important ceramic materials with three different phases: monoclinic stable below 1175 °C, tetragonal stable at 1175-2370 °C, and cubic stable at 2370-2680 °C, respectively [1,2].Nanosized zirconia has specific optical and electrical properties which suits it for prospective applications in transparent optical devices, electrochemical capacitor electrodes, fuel cells, catalyst, and advanced ceramics [3][4][5][6][7][8].Numerous synthetic strategies have been developed to obtain zirconia nanostructures including solution combustion synthesis [9], microwave-hydrothermal [10], sol-gel [11], spray pyrolysis [12], chemical vapour synthesis [13], and precipitation approach [14].Among them, the hydrothermal method has attracted much attention because of its simple operation, mild experimental conditions, and high product purity.Catalytic properties of inorganic nanomaterials not only are related to their phase structure and chemical composition but also depend on their morphology [15].e morphology of ZrO 2 has a significant effect on its properties [16][17][18][19][20][21][22][23] because it can control a variety of physical and chemical properties at the same time.For instance, with special morphology, flower-like zirconia nanomaterials [24] showed an excellent photocatalytic activity on the degradation of rhodamine B. In addition, the spinous ZrO 2 core-shell morphology [25] exhibited a superior hydrogen storage performance, reaching a hydrogen uptake of 1.521 wt.% at 298 K under 5 MPa.
Wastewater pollution has become a serious problem in many countries [26].e removal of dyes from wastewater through heterogeneous photocatalysis has drawn an increasing attention over the last few decades.For degradation of organic pollutants, many studies on the heterogeneous photocatalysis were performed with oxide semiconductors such as TiO 2 [27], ZnO [28], Fe 2 O 3 [29], ZrO 2 [30], and CuO [31] being applied.
ese nanomaterials showed an excellent photocatalytic activity on the degradation of organic dyes.In recent years, ZrO 2 -based materials have gained a considerable scientific and technological attention in heterogeneous catalysis.ey have been used in the photodegradation of dye compound due to their high photocatalytic activity in the ultraviolet range, high thermal stability, chemical stability, low cost, nontoxicity, and environmentally friendly nature [32].
In recent years, many different approaches [17,[33][34][35]] have been utilized to prepare zirconia nanomaterials with different morphologies using suitable templates and surfactants.For example, ZrO 2 nanowires and nanobelts were prepared by an alumina template and pyrolysis of Zr(OH) 4 : RE particles, respectively.ZrO 2 mesopore microfibers have been prepared with a pluronic P-123 template [17].A ZrO 2 hollow fiber membrane was successfully synthesized employing a polypropylene hollow fiber as the template, and the prepared zirconia hollow fiber was demonstrated to be a highly selective adsorbent for the phosphonic acidcontaining compounds with high sensitivity [35].Flake-like ZrO 2 nanocrystallites were prepared using cetyltrimethyl ammonium bromide (CTAB) as the surfactant, and the use of surfactant led to the formation of stabilized tetragonal ZrO 2 nanoparticles (15 nm) [21].
ese studies demonstrated successful synthesis of ZrO 2 nanostructures with special morphology; however, most of these approaches required either suitable templates or surfactants to prepare ZrO 2 nanostructures and also included the removal of the template in the process.erefore, searching for a simple but effective method to get a special morphology of ZrO 2 in the absence of template remains a challenge.
In this paper, a facile route was employed to synthesize the ZrO 2 nanostructures with Zr(NO 3 )  − is easy to remove from the system, which prevents the contamination of the products.CH 3 COO − anions tend to adsorb on the surface of ZrO 2 and play an important role in the formation of ZrO 2 particles with special morphology.In this work, the degradation of rhodamine B (RhB) with nanosized ZrO 2 was studied in aqueous solution.e novel nanostructure of ZrO 2 may lead to superior performance in the photodegradation of RhB.

Materials.
e chemicals used were of analytical grade and purchased from Aladdin Chemistry Co. Ltd. e chemicals were used as received without further purification.
2.2.Synthesis.ZrO 2 nanostructures were synthesized using the hydrothermal method.Figure 1 shows the synthesis route of nanosized ZrO 2 .Typically, 0.123 g (0.0015 mol) of CH 3 COONa was added into the solution of Zr(NO 3 ) 4 •5H 2 O under magnetic stirring.en, the solution was transferred into a 25 mL beaker in a Teflon-lined stainless steel autoclave and heat treated at 180 °C for 6 h.After reaction, the autoclave was left to cool down to room temperature.e products were centrifuged and collected.en, the products were washed for several times with deionized water and ethanol.In the end, the ZrO 2 products were obtained by drying at 90 °C for 8 h.

Characterization.
e morphology was investigated using the high-resolution transmission electron microscope (HR-TEM, Tecnai G2 F20) working with an accelerating voltage of 200 kV.
e phase constitution of the products were analyzed by an X-ray diffractometer (Rigaku D/M4X 2500, Rigaku Co., Japan) with Cu Kα radiation (λ � 0.15418 Å). e infrared (FTIR) spectra were measured by a Nicolet is 35 using the KBr pellet technique in the range of 4000-400 cm −1 .ermogravimetric (TG) analysis was recorded on a Netzsch STA 449 F3 at a heating rate of 10 °C•min −1 in a flowing air.e UV-visible spectrum of ZrO 2 was measured using the Cary 300 UV-visible spectrophotometer.e X-ray photoelectron spectrum (XPS) was recorded on an ESCALAB 250Xi spectrometer with an energy analyzer working in the pass energy mode at 20.0 eV, and the Al Kα line was used as the excitation source.e binding energy reference was taken at 284.8 eV for the C1s peak arising from surface hydrocarbons.Brunauer-Emmett-Teller (BET) surface area was obtained with N 2 adsorption by using a Micromeritics ASAP 2020 nitrogen adsorption apparatus via determination of nitrogen adsorption isotherm at 77 K.

Photocatalytic Activity Test.
e photocatalytic experiments were carried out by following the RhB degradation under UV irradiation in a separate chamber.Prior to irradiation, the suspensions were magnetically stirred in a complete darkness for 20 min to attain adsorption equilibrium.During the RhB photodecomposition, the samples were withdrawn at regular intervals and centrifuged to separate solid particles for analysis.
e concentration of RhB was determined by a UV-Vis spectroscopy at its maximum absorption wavelength (about 554 nm).

Characterizations of ZrO 2 Nanostructures.
e XRD results of the products, prepared at 180 °C with different reaction times, are shown in Figure 2. It can be seen that the 2 Advances in Materials Science and Engineering product heat treated for 120 min was crystalline, but the peaks were not sharp.e onset of monoclinic ZrO 2 was observed in the product heat treated for 200 min.e crystallinity of the products increased with the reaction time.When the hydrothermal time was prolonged to 360 min, the products changed into monoclinic ZrO 2 completely (JCPDS number 37-1484), and no other phase was observed.
e mean crystallite size of the products was calculated by the following Scherrer equation: where τ is the mean size, k is the shape factor, β is the full width at half the maximum (FWHM) intensity, θ is the Braggs angle, and λ is the wavelength of X-ray source applied in XRD. e calculated average crystallite size of ZrO 2 at 120 min reaction time was 6.97 nm.When the reaction time was 360 min, the crystallite size of ZrO 2 was found to be approximately 9.02 nm (Table 1).ese results suggest that the crystallite size of ZrO 2 increases with increasing reaction time.e small crystallite size of ZrO 2 may be due to the groups adsorbed on the surface as well as on the grain boundary.ese adsorbed groups may prevent the continuous growth of zirconium oxide nanocrystals, which can be achieved by reducing their surface energy and surface activity.
e morphologies of ZrO 2 were investigated by FE-SEM and TEM.From Figures 3(a) and 3(b), it can be seen that ZrO 2 exhibited a beautiful star-like shape, and stars were in the range of 30-80 nm. e HR-TEM image in Figure 3(c) shows that ZrO 2 stars were composed of short nanorods of ca.15-20 nm in length and 3-5 nm in diameter.e corresponding selected area electron diffraction (SAED) pattern of a single structure is a ring pattern (Figure 3(d)), suggesting that ZrO 2 has a short-range crystalline structure on the nanoscale.e HR-TEM image shown in Figure 3(f ) is the magnification at the area denoted by the black arrow in Figure 3(e).e lattice fringe with an interplanar spacing of 0.28 nm is consistent with the value of the (111) lattice planes of ZrO 2 (JCPDS number 37-1484).
Figure 4(a) presents the FTIR spectrum of the star-like ZrO 2 .
e absorption bands at 751 cm −1 , 679 cm −1 , and 504 cm −1 are assigned to the vibration of Zr-O [36].e absorption bands between 1000 and 1400 cm −1 are due to NO 3 − anions.However, nitrate groups were no longer coordinated in a chemical bond fashion, and nitrate anions only remained on the ZrO 2 nanostructure surface.e absorption at 1531 cm −1 is due to the symmetric vibration absorption of COO − [15].e bands at 3234 cm −1 and 1638 cm −1 are attributed to the surface hydroxyl groups or adsorbed water strongly bound to the ZrO 2 surfaces [37].It is reported that surface hydroxyl groups play an important role in heterogeneous photocatalysis and act by capturing light-induced holes thereby, producing reactive hydroxyl radicals with high oxidation capacity [38,39].
is result suggests that the surface of ZrO 2 was probably covered by acetate groups, hydroxyl, and adsorbed water.
XPS measurements were performed on the product (Figure 4(b)), and the signals of Zr, O, C, and N were detected in the survey XPS of ZrO 2 .e signals at 181.6 and 183.9 eV correspond to Zr3d5/2 and Zr3d3/2 of ZrO 2 (Figure 4(c)), which are found to be those related to the presence of zirconium in the composite, that is, Zr 4+ of ZrO 2 as reported in [40].
e O1s peaks at 529.4, 531.1, and 532.4 eV can be ascribed to the lattice oxygen, the adsorbed oxygen (−OH, H 2 O), and acetate groups, respectively [41] (Figure 4(d)).e high-resolution C1s XPS of ZrO 2 shows a strong peak at 284.2 eV, which can be attributed to C-C, and another relatively weak C1s peak is also observed, which can be ascribed to -COOH adsorbed on the surface of ZrO 2 in the form of acetate [42] (Figure 4(e)).In addition, Figure 4(f ) shows a weak N1s peak at 406.6 eV, which may result from a small amount of residual nitrate as confirmed by checking the binding energy table [42].According to literature results [43], XPS peaks of N in N-doped ZrO 2 are at 396.8 eV(Zr-N) and 400.0eV (N-O), respectively.erefore, there is no nitrogen doping in the as-prepared ZrO 2 (406.6 eV) (Figure 4(f )). e XPS results indicate that the sample possesses a surface-adsorbed water, hydroxyl, and acetate groups, which is consistent with the FTIR spectra.
e TG curve of the ZrO 2 product is shown in Figure 5. e TG curve shows three-stage weigh loss events at 25-165 °C, 165-500 °C, and 500-1200 °C, respectively.ere is ca.1.94% weight loss upon heating from room temperature to 165 °C, which is due to the removal of adsorbed water on the surface of the product.e weight loss from 165 °C to 500 °C may be ascribed to the decomposition of the CH 3 COO − and NO 3 − anions, as the acetate and nitrate anions generally combust at about 350 °C [44] and 192 °C [45], respectively.e weight loss between 500 °C and 1200 °C is a result of the elimination of hydroxyls adsorbed  Advances in Materials Science and Engineering on the surface.From the analysis of the TG curve and IR spectrum, the final product is covered by adsorbed water, CH 3 COO − , NO 3 − , and hydroxyl, respectively.However, it was difficult to analyze the TG curve quantitatively due to the overlap of the weight loss.

Photocatalytic Activities of ZrO 2 .
e degradation of RhB under UV irradiation was carried out to evaluate the photocatalytic activity of as-prepared ZrO 2 nanostructures.For comparison, a blank experiment without catalyst was also conducted under identical conditions.Figure 6(a) indicates that the RhB concentration decreases with increasing irradiation time.When the solution was irradiated for 30 min without catalyst, a small amount of RhB was degraded (<20%).is result is similar to the results reported by other authors [24].When the as-prepared star-like ZrO 2 was added into the solution, the degradation of RhB increased  4 Advances in Materials Science and Engineering up to nearly 100% in 30 min, which shows an improved photocatalytic activity compared to the previous reports [24] (RhB degradation 100% in 40 min with flower-like ZrO 2 ).All products with different molar ratios (NO 3 ) 4 •5H 2 O to CH 3 COONa showed a superior photocatalytic performance (Figure 6(b)).e excellent photocatalytic activity of star-like ZrO 2 nanostructures can be attributed to several factors.Firstly, the star-like nanostructures may provide more adsorption sites and stronger surface adsorption ability to the RhB molecules, so the photocatalytic reaction can take place efficiently.Secondly, the improved surface functions may also contribute to the high photocatalytic activity of catalysts.To confirm this suggestion, the as-prepared star-like ZrO 2 was calcined (C-ZrO 2 ) at 600 °C for 2 h and used as the catalyst at the same condition.It was found that only 60% of the RhB degraded after irradiation for 30 min with the calcined ZrO 2 as shown in Figure 6(a).Compared with the as-prepared ZrO 2 , the ascalcined ZrO 2 showed a decreased photocatalytic activity toward the degradation of RhB.Furthermore, the FTIR was used to characterize the surface-adsorbed groups of ZrO 2 (Figure 7) .It can be seen that the intensity of OH (3246 cm −1 and 1632 cm −1 ) and NO 3 − absorption bands (1384 cm −1 ) of calcined ZrO 2 at 600 °C was significantly reduced compared to that of as-prepared ZrO 2 .Furthermore, the vibration absorption bands of acetate groups completely disappeared after    Advances in Materials Science and Engineering calcination.is indicates that the surface-adsorbed groups (hydroxyl or acetate groups) of the star-like ZrO 2 were destroyed during calcination and the surface chemistry of ZrO 2 changed.e acidity comes from hydroxyl groups since they capture light-induced holes [46], the generation of strong species (e.g., _OH), capable of oxidizing adsorbed organic substrates [47].us, the surfaceadsorbed hydroxyl groups or water plays an important role in the high photocatalytic activity of star-like ZrO 2 .Finally, to further probe the reason of superior catalytic property of ZrO 2 , the UV-Vis adsorption spectra of ZrO 2 and ZrO 2 calcined at 600 °C for 2 h were measured (Figure 8(a)).It can be seen from Figure 8(a) that all star-like ZrO 2 showed a strong absorption in UV-Vis region with the maximum intensity at 300 nm, and an absorption band was also found at 348 nm. e absorption of the calcined ZrO 2 was, on the contrary, much weaker, and a considerable shift towards lower wavelength was found.For the obtained UV-Vis spectra, a bandgap was calculated using the Kubelka-Munk theory and Tauc method.e following equation was used to calculate the bandgap: where A is the absorbance, K is the proportionality constant, and E g is the bandgap energy.e plot of (Ahv) 2 versus hv based on the direct transition is shown in Figure 8(b).e measured bandgap for the as-prepared ZrO 2 was found to be 3.56 eV, while the calcined ZrO 2 exhibited a wide bandgap of 5.22 eV.In our experiments, the bandgap of synthesized star-like ZrO 2 with various molar ratios of Zr(NO 3 ) 4 •5H 2 O to CH 3 COONa was in the range of 3.50-3.85eV, according to the results of UV-Vis spectra (Figures 9(a) and 9(b)).e narrow bandgap of the synthesized star-like ZrO 2 nanostructures could also relate to its surface functions and contribute to the superior photocatalytic activity.e N 2 adsorption-desorption isotherm plots for the asprepared ZrO 2 and the as-calcined ZrO 2 at 600 °C are shown in Figure 10.Both samples exhibited type IV isotherms, indicating a typical mesoporous structure.
e BET specific surface area of the as-prepared ZrO 2 was approximately 70.9 m 2 /g (Figure 10(a)), which is higher than that of ZrO 2 calcined at 600 °C.
e ZrO 2 calcined at 600 °C showed a smaller BET specific surface area of 26.5 m 2 /g (Figure 10(b)), which indicates that the BET specific surface area of ZrO 2 decreased with calcining at 600 °C. is may be another reason for the decreased photocatalytic activity towards the degradation of RhB by calcined ZrO 2 .
As reported in the literature, when the solid material is irradiated with ultraviolet light, some holes are left in the  Advances in Materials Science and Engineering valence band along with the process of electron transition from the valence band to the conduction band [48].e photogenerated electrons and holes are the origin of the photocatalytic reaction.
e results of the valence band (VB) XPS showed that the valence band energy of prepared ZrO 2 was 2.08 eV (Figure 11(a)).
e optical bandgap energy of prepared ZrO 2 was 3.56 eV.According to the relationship E VB � E CB + E g , the conduction band (CB) of prepared ZrO 2 would occur at −1.48 eV.Because the CB edge potential of ZrO 2 is more negative than E • O 2 /O 2 (−0.046V) [49], the electrons in ZrO 2 can capture O 2 and reduce it to _O 2 − , which could effectively suppress the electron-hole recombination rates [50].Meanwhile, the holes are captured by OH groups or H 2 O on the surface of ZrO 2 to produce hydroxyl radicals.Finally, the radicals formed, such as superoxide and hydroxyl, react with the RhB and degrade it completely.e abovementioned reactions take place on the surface of ZrO 2 with a high efficiency.e radicals produced react powerfully in the RhB solution and cause their degradation.e possible mechanism may be described as follows: Sudrajat and Babel [43] studied the mechanism of nitrogen-doped ZrO 2 -catalyzed degradation of rhodamine 6G.ey found that•OH was the most dominant reactive species.e photogenerated h + also seems to play an important role in the dye degradation through direct attack of R6G molecules on the catalyst surface.• O 2 − is more easily produced through reduction of O 2 by the electron in the CB of N-ZrO 2 due to high CB potential of N-ZrO 2 .Other references present a similar reaction mechanism.
Combining the above presented results with the literature reports, the possible photodegradation mechanism can be inferred (Figure 11(b)).In all the cases, the role of photogenerated electrons is negligible.
is is an indication of an effective electron transfer from the catalyst surface to the adsorbed molecules to produce reactive species [43].

Conclusions
To sum up, the synthesis of star-like ZrO 2 nanostructures has been successfully carried out by using the hydrothermal method without any template and surfactant.
e crystallite size of ZrO 2 increased with increasing reaction time, and the crystallite size was approximately 9.02 nm when the reaction time was 360 min.
e FTIR and XPS spectra showed the surface of ZrO 2 was covered by acetate groups, hydroxyl, and adsorbed water.
e bandgap of the assynthesized star-like ZrO 2 with various molar ratios of Zr(NO 3 ) 4 •5H 2 O to CH 3 COONa was in the range of 3.50-3.85eV, according to the results of UV-Vis spectroscopy.e as-synthesized nano-ZrO 2 showed excellent photocatalytic activities in RhB degradation under UV irradiation which may be attributed to the surface functions, special morphology, and narrow bandgap of the starlike ZrO 2 nanostructures.e possible photodegradation mechanism was proposed, and potential applications of the synthesized star-like ZrO 2 have been considered.Reactive species•OH and_O 2 − may play an important role in the RhB degradation.Overall, the star-like ZrO 2 nanostructures have been proven to be effective catalysts for the degradation of RhB under UV-Vis radiation and could be suitable candidates in environmental photocatalysis.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.

Figure 1 :
Figure 1: Flowchart of the preparation route of star-like ZrO 2 .

Figure 2 :
Figure 2: XRD patterns of the products prepared at different reaction times.

Figure 3 :
Figure 3: SEM image (a), TEM image (b, c), SAED patterns (d), TEM image (e) of the synthesized ZrO 2 , and (f ) HR-TEM image of the area denoted by the black arrow in (e).

Figure 4 :
Figure 4: FTIR (a) and XPS (b-f ) results showing the survey spectra of ZrO 2 products.

Figure 5 :
Figure 5: TG curve of ZrO 2 products obtained by reacting at 180 °C

Figure 6 :Figure 7 :Figure 8 :Figure 9 :Figure 10 :
Figure 6: RhB concentration as a function of UV irradiation time over the product and calcined ZrO 2 (a) and the products with different molar ratios (b).

Figure 11 :
Figure 11: (a) Valence band (VB) XPS of the prepared ZrO 2 and (b) possible photocatalytic mechanism scheme with as-prepared ZrO 2 nanostructures. 4 •5H 2 O and CH 3 COONa as starting materials without any template or surfactant.Starting materials Zr(NO 3 ) 4 •5H 2 O and CH 3 COONa are cheap and affordable.Furthermore, Zr(NO 3 ) 4 •5H 2 O is water soluble and suitable for hydrothermal synthesis.Also NO 3

Table 1 :
Estimated crystallite sizes of ZrO 2 synthesized at different reaction times.