Introduction
Owing to industrialization and urbanization, organic pollutants in water become a serious environmental killer along with the advancement of technology and industry.[Reference Cheng, Zhi, Zhang, Zhou, Peng, Xie, Zhang and Pang1–Reference He, Mudar, Xiong, Shu and Li4] Thus, it is important to develop simple and effective methods to address these issues. A promising method for destroying a wide spectrum of organic compounds is photo-degradation route, which has been studied extensively for destructive oxidation of various organic pollutants. This has proved to be very effective, since many organic compounds can be degraded successfully to safer end-products such as CO2, H2O, and mineral acids.[Reference Galindo, Jacques and Kalt5–Reference Wang, Jiang, Ding, Wang, Sun, Wang and Cheng7] In particular, titanium dioxide (TiO2) has been the focus in this field due to its unique physical and chemical properties, including high efficiency, good stability, nontoxicity, etc.[Reference Chen and Mao8–Reference Roy, Berger and Schmuki10] However, the easy recombination of the electron-hole pairs and the absorption of light only at ultraviolet (UV) wavelengths have hindered its further applications.[Reference Qu, Xie, Cao and Du11] Therefore, it is still a challenge to find a highly reactive photocatalyst to meet the present demands for environmental and fuel application.
In order to improve its photocatalytic activity, researchers have made many efforts to modify TiO2 with different methods, including doping with noble metal deposition,[Reference Liu, Zhang, Zhang, Zhou, Meng and Ruan12] nonmetals deposition,[Reference Huo, Hu, Jiang, Hou and Li13] and establishing junctions between TiO2 and other semiconductors.[Reference Li, Renata and Witold14–Reference Jun, Sheng, Su, Xu, Wang, Zheng and Wu18] Zirconium dioxide (ZrO2) is an inexpensive, stable, non-toxic, wide bandgap oxide semiconductor (E g ≈ 5.0 eV), and has appropriate flat-band potentials for photocatalytic purposes.[Reference Vishwanathan, Roh and Kim19–Reference Karunakaran and Senthilvelan22] In previous studies, TiO2 coupled with zirconia has been investigated extensively in degradation of organic compounds.[Reference Pany and Parida23] However, due to the large band gap of ZrO2, there is almost no charge transfer between TiO2 and ZrO2. It impedes the efficient separation of charge carriers and inhibits the corresponding photocatalytic performance.[Reference Neppolian, Wang and Yamashita24] Other metal ions such as iron, nickel, vanadium, chromium, platinum, ruthenium, copper, manganese, cerium, and tin have been investigated as potential dopant of TiO2 to promote the photocatalytic activity of TiO2. Metal ion doping is able to enhance interfacial charge–transfer reactions.[Reference Tripathi, Mathpal, Kumar, Singh, Soler and Agarwal25] It has also been proven that the transition metal dopants also inhibit the photogenerated charge recombination as the Meta sites are considered to act as trapping site by accepting the photogenerated electrons from the TiO2 valence band (VB).[Reference Sofianou, Tassi, Psycharis, Boukos, Thanos, Vaimakis, Yu and Trapalis26]
In this paper, the Mn-doped ZrO2/TiO2 nanostructured composites were synthesized via a simple hydrothermal method. The photocatalytic activities of the samples were determined by degradation of Rhodamine B (RhB). Compared with the pure TiO2 nanoparticles and non-doped sample, the 0.5% Mn-doped ZrO2/TiO2 composite has showed better photocatalytic activity due to the narrowed band gap and shifted up the conduction band (CB) edge.
Experimental
Preparation of materials
The Mn-doped ZrO2/TiO2 catalysts were synthesized by the hydrothermal reaction. 3 mL of zirconium n-propoxide (70 wt%) and 3 mL of tetrabutyl titanate (98 wt%) were added into deionized water under magnetic stirring. Next, the appropriate amounts of Mn(NO3)2 were added. Then, the resultant solution was transferred into a Teflon-lined stainless steel autoclave, followed by a hydrothermal reaction at 160 °C for 24 h. After cooling to room temperature, the precipitates were filtered and washed with deionized water and ethanol six times. The products were dried in a vacuum oven at 60 °C for 12 h to get the fine photocatalysts. The Mn concentrations in the samples were 0%, 0.5%, 1%, and 1.5%, respectively.
Characterization
The structures and crystal phase of the as-prepared samples were analyzed with a SIEMENS Diffraktometer D5000 x-ray diffractometer with Cu Kα radiation source at 35 kV, with a scan rate of 4°/s in the 2θ range of 10°–80°. x-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. JSM-2100 transmission electron microscopy (TEM) was used to characterize the morphology of the as-prepared samples. The Brunauer–Emmet–Teller (BET) specific surface area was measured using a specific surface and pore size analysis instrument (3H-2000PS1/2 static volume method, China). Electron paramagnetic resonance (EPR) spectra recorded at room temperature was run on an X-band CW-EPR Bruker EMX spectrometer equipped with a cylindrical cavity operating at 100 kHz field modulation frequency. TEM map (Tecnai G2 F30 S-Twin), UV-visible (JASCO V-570, Japan).
Measurement of photocatalytic activity
The photocatalytic activities of the samples were evaluated by the photo-degradation of RhB in aqueous solution under UV light irradiation with a 300 W lamp to provide UV light (λ ≤ 400 nm). Before irradiation, 20 mg amount of the as-prepared catalysts were added into 50 mL of RhB solution under magnetic stirring for 1 h to ensure the adsorption-desorption equilibrium. Then the solution was stirred and exposed to the UV light irradiation. An amount of aliquots were taken away and separated as-prepared catalysts with centrifugation at the designated time. The upper layer was appropriately diluted and the RhB concentration (C) was measured by JASCO V-570 UV-vis-NIR spectrophotometer (Japan).
The degradation rate could be expressed as the following formula.
$$\hbox{Decolonization rate of RhB}\!:R = ({C_0} - {C_t})/{C_0}*100\% $$Results and discussion
TEM is employed to determine the morphology of the products. Typical morphology of the 0.5% Mn-doped ZrO2/TiO2 nanocomposite is shown in Fig. 1(a). The size of the products is less than 10 nm and can be founded for the 0.5% Mn-doped ZrO2/TiO2 nanocomposite sample. The ZrO2/TiO2 nanocomposite without Mn doping shows the similar morphology and size (see Fig. S1 in support information). The selected area electron diffraction (SAED) image of 0.5% Mn-doped ZrO2/TiO2 nanocomposite exhibits the combination of bright ring and single crystalline spot, as shown in Fig. 1(b). It indicates the poly-crystalline nature of the as-prepared product. The crystalline nature of the 0.5% Mn-doped ZrO2/TiO2 nanocomposite is further investigated by high resolution TEM (HR-TEM). The HRTEM image presented in Fig. 1(c) shows the lattice spacing at 0.341 nm which is close to the (101) (0.35 nm) plane of the anatase phase of TiO2. The lattice spacings at 0.323 and 0.295 nm are contributed to the (111) (0.3164 nm) monoclinic and [111] (0.2964 nm) tetragonal crystalline structures of ZrO2, as shown in Figs. 1(d) and 1(e). The crystalline phases of the as-prepared powders are further identified by x-ray diffraction (XRD) analysis. Figure 1(f) shows the XRD patterns of the products with different Mn doping content. It can be found that all as-prepared samples exhibit a peak located around 26° which indicates the presence of anatase crystal structure of TiO2. The distinctive peak at ~28° and 30° is the typical signal of tetragonal and tetragonal ziirconia structure.[Reference Kolen'ko, Kovnir, Gavrilov, Garshev, Frantti, Lebedev, Churagulov, Van Tendeloo and Yoshimura27, Reference Khataee, Sheydaei and Hassani28] No detectable Mn dopant-related peaks can be observed in the samples except a little asymmetry of the peaks due to the low content and high dispersity of dopants.
TEM image (a), SAED pattern (b), HR-TEM images of 0.5% Mn-doped ZrO2/TiO2 (c–e), and XRD patterns of the doped ZrO2/TiO2 samples with different doping ration of Mn (f).
Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves are conducted to investigate the porous structure of the as-prepared samples. The as-prepared samples exhibit the Type IV isotherms characteristic of mesoporosity (see Fig. S2 in the support information). The BET specific surface areas, mean pore diameters, and total pore volumes of the as-prepared samples are summarized in Table S1. The 0.5% Mn-doped ZrO2/TiO2 exhibits the highest specific surface area (160.51 m2/g) and the lowest mean pore diameter (6.60 nm). The absorption edge and band-gap energies of the as-prepared samples are determined by UV-vis diffuse reflectance spectra analysis (see Fig. S2 in the support information). It can be founded that the 0.5% Mn-doped ZrO2/TiO2 nanocomposite exhibits the strongest absorption under the UV light region. The band gap energy of the 0.5% Mn-doped ZrO2/TiO2 nanocomposite is estimated to be 3.31 eV by fitting to a plot of Kubelka–Munk function versus energy (hv) which is much lower than that of other samples. The stronger absorption and the smaller band gap are contributed to the enhancement of photocatalytic activity of the as-prepared samples.[Reference Jiang, Tang, Li, Wei, Wang, Chena, Wan and Shen29, Reference Vadivel, Keerthi, Vanitha, Muthukrishnaraj and Balasubramanian30]
Fig. 2(a) shows a bright field TEM image taken under scanning TEM (STEM) mode. The morphology is corresponding with the TEM. The energy dispersive x-ray spectroscopy (EDS) analysis (shown in Fig. 2(b)) suggests the presence of Mn, O, Ti, and Zr elements in the product. It can be known that the Mn is well doped in 0.5% Mn-doped ZrO2/TiO2 nanocomposites. Mn, O, Ti, and Zr K edge mapping are shown in Figs. 2(c)–2(f), respectively. It can be seen that the green color (assigned to Mn), red color (assigned to O), yellow color (assigned to Ti), and blue color (assign to Zr) are uniformly distributed in the sample of ZrO2/TiO2 which indicates the formation of uniform nanocomposite.
STEM mode image (a), EDS (b), and Elemental mapping (c−f) of 0.5% Mn-doped ZrO2/TiO2 nanocomposite.
The chemical composition of the obtained composites is confirmed by XPS measurement. The XPS spectrum of the 0.5% Mn-doped ZrO2/TiO2 nanocomposite exhibits prominent peaks of Ti, Zr, O, and relatively feeble peaks of Mn, as shown in Fig. 3(a). The high-resolution XPS spectrum of Zr 3d on the composite photocatalyst is shown in Fig. 3(b), which can be fitted as two peaks with binding energies at 184.5 and 182.7 eV. It implies two different chemical environments of Zr existed in the products that are assigned to the Ti-Zr, and Zr-O, respectively. The high-resolution XPS spectrum of Ti 2p can be fitted as two peaks with binding energies at 464.8 and 458.5 eV which are assigned to Ti-Zr and Ti-O, respectively (Fig. 3(c)). The O 1s peak localized at 531.38 eV can be ascribed to Ti-O, while the peak at 529.8 eV is assigned to Ti-O and Zr-O (Fig. 3(d)). And the presence of Mn dopant is confirmed by Mn 2p core level, as shown in Fig. 3(e). As for Mn 2p peak with the binding energy at 642.4 eV originates from the Mn4+.[Reference Sofianou, Tassi, Psycharis, Boukos, Thanos, Vaimakis, Yu and Trapalis26] The VB of ZrO2/TiO2 nanocomposite and 0.5% Mn-doped ZrO2/TiO2 nanocomposite that measured by XPS valence spectra (Fig. 3(f)) are 2.84 and 2.58 eV, respectively. The composition of the 0.5% Mn-doped ZrO2/TiO2 nanocomposite from XPS analysis is showed in Table S2. The amount of Mn element is 0.41% in the resultant sample which closes to the composition of feed ratio. However, the high-resolution XPS spectra of Zr 3d, Ti 2p, and O 1s of non-doped ZrO2/TiO2 product exhibits the similar chemical environments compared with the Mn doped ZrO2/TiO2 product which indicates the doped Mn has no change in the structure of ZrO2/TiO2 composite (see Fig. S3 in the support information). The composition of the non-doped ZrO2/TiO2 nanocomposite from XPS analysis is showed in Table S3.
XPS spectrum of 0.5% Mn-doped ZrO2/TiO2 nanocomposite (a), the high-resolution XPS spectra of Zr 3d (b), Ti 2p (c), O 1s (d) Mn 2p (e), and VB of samples (f).
The photocatalytic activity of the as-prepared products is evaluated by degradation of RhB under the UV light irradiation. As shown in Fig. 4(a), the adsorption-desorption equilibrium for photocatalysts can be reached after stirring for 1 h in the dark. The 0.5% Mn-doped ZrO2/TiO2 photocatalyst exhibits the highest activity, and more than 95% of RhB could be degraded within 50 min. However, only about 77.6% of RhB has been degraded over non-doped ZrO2/TiO2 composite with the same irradiation time and about 58.2% RhB are removed by ZrO2 under same conditions. No higher activities have been observed for the photocatalysts with Mn doping at higher concentration (1% and 1.5%). It implies that the introduction of Mn doping play an important role for the improvement of the photocatalytic activities and excess of the Mn doping is of no help to improve their activities. The relatively higher photocatalytic activity of Mn-doped ZrO2/TiO2 nanocomposite photocatalysts may result from the doped of foreign species. Owing to its relatively smaller ionic radius, Mn4+ ions may enter into the lattice of ZrO2/TiO2 composities. It is beneficial to improve of photo-adsorption property, forming interband-gap localized levels to reduce band gap and enlarge specific surface area and the pore volume.[Reference Ksapabutr, Gulari and Wongkasemjit31, Reference Guo and Chen32] In order to test the re-use performances of composite photocatalyst, the 0.5% Mn-doped ZrO2/TiO2 photocatalyst is used to take the recycle photocatalytic decoloration of RhB solution, as shown in Fig. 4(b). The products still maintained their higher photocatalytic activates after 5 time cycles which indicates the good durability of the nanocomposite photocatalysts.
Photocatalytic degradation of RhB over different photocatalytic materials (a) and cycling runs for the photodegradation of RhB over 0.5% Mn-doped ZrO2/TiO2 nanocomposite (b).
For investigation of a plausible reaction mechanism for the excellent photocatalytic activity of 0.5% Mn-doped ZrO2/TiO2 nanocomposite and detecting the active species during photocatalytic reactivity, hydroxyl radicals (•OH), superoxide radical (•O2−), and holes (h+) are investigated by adding 1.0 mM isopropyl alcohol (IPA) (a quencher of •OH), p-benzoquinone (BQ) (a quencher of •O2−), and triethanolamine (TEOA) (a quencher of h+), respectively.[Reference Kolen'ko, Kovnir, Gavrilov, Garshev, Frantti, Lebedev, Churagulov, Van Tendeloo and Yoshimura27] The method is similar to the former photocatalytic activity test. Figure 5(a) displays the trapping experiment of active species during the photocatalytic reaction. It can be seen that the photocatalytic degradation of RhB obviously decreased with the addition of TEOA, BQ, and IPA. Therefore, it can be concluded that the h+, •O2−, and •OH are the main active species of the obtained products in aqueous solution under UV light irradiation. Many types of point defects present in solids, in particular, oxides are paramagnetic. EPR spectroscopy is one of the most suitable techniques to investigate defective materials. It can be seen in the Fig. 5(b), the intensity of 0.5% Mn-doped ZrO2/TiO2 nanocomposite is stronger than that of non-doped product. Under UV light irradiation, the intensity of EPR signal of doped product increased obviously which indicates the production of more active radicals assisted by the UV irradiation. More active radicals can improve the photocatalysis performances.[Reference Mario, Elio and Michel33, Reference Dyrek and Che34]
Trapping experiment of active species during the photocatalytic reaction (a) and EPR spectra of the ZrO2/TiO2 doped with different mass ratio of Mn (b).
A possible photocatalytic mechanism of the Mn-doped ZrO2/TiO2 photocatalysts is depicted in Scheme 1. Under the UV light irradiation, the photogenerated h+, •O2−, and •OH are in their VB and CB, respectively. The Mn doping not only narrows the band gap but also shifts up the CB edge. The doping with Mn introduced a VB of impurities which can narrow the band gap of ZrO2/TiO2.[Reference Wang, Valentin and Pacchioni35] The doping of Mn ions improved the quantity of the photogenerated electron-hole under visible light irradiation which formed new states closed to the CB and VB. The doping of Mn ions also inhibited the recombination of the photogenerated electron-hole pairs. The Mn ions with suitable concentration could trap the photogenerated electrons. As can be observed, 0.5% Mn-doped ZrO2/TiO2 exhibited the maximum photocatalytic activity for RhB degradation.[Reference Sofianou, Tassi, Psycharis, Boukos, Thanos, Vaimakis, Yu and Trapalis26] In fact, although RhB is active to light, the lowest unoccupied molecular orbital (LUMO) potential of RhB (−0.75 eV)[Reference Heinz and Frank36] is more positive than the CB minimum (CBM) of ZrO2 (−1 eV). The CBM of RhB is more negative than TiO2 (−0.4 eV), the excited electrons of RhB can have a further react with O2 on the surface of TiO2. The photogenerated h+ in CB also has a strong oxidation potential and serve as active sites responsible for RhB photodegradation.
Schematic illustration of the ZrO2/TiO2 photocatalytic mechanism of RhB degradation over Mn-doped ZrO2/TiO2 photocatalyst.
Conclusion
In summary, the Mn-doped ZrO2/TiO2 nanocomposites with UV light photocatalytic properties have been prepared by hydrothermal reaction. The doping with Mn can narrow the band gap of ZrO2/TiO2, which is beneficial to improve photo-adsorption property. At the same time, it also proves that the h+, •OH, and •O2− are the main active species of Mn-doped ZrO2/TiO2 photocatalysts in aqueous solution. The photocatalytic activity of the as-prepared products was evaluated by degradation of RhB under the UV light irradiation. The optimal concentration of dopant that improves the photocatalytic activity of the nanoplates was determined. These observations provide a new possibility in the investigation of photocatalytic performance of nanocomposites and are expected to promote its practical applications in the field of environmental science.
Supplementary materials
For supplementary material for this article, please visit http://dx.doi.org/10.1557/mrc.2015.59
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (51373155, 51133006) and “521 Talents Training Plan” in Zhejiang Sci-Tech University (ZSTU). H. C. thanks the Cultivation Fund Program for Excellent Dissertation in Zhejiang Sci-Tech University (2014XSKY25).
