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Facile synthesis of metal-doped titania nanospheres with tunable size exhibiting highly efficient photoactivity for degradation
Materials Chemistry and Physics xxx (2016) 1e9
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Facile synthesis of metal-doped titania nanospheres with tunable size exhibiting highly efficient photoactivity for degradation Xinzheng Yue a, Xu Jin b, Runwei Wang a, Ling Ni a, Shang Jiang a, Shilun Qiu a, Zongtao Zhang a, * a
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China Research Institute of Petroleum Exploration and Development, China University of Petroleum, Beijing 100191, PR China
b
h i g h l i g h t s The The The The
metal-doped TiO2 nanospheres were synthesized via a solegel technique. size of nanospheres can be tuned by varying the amounts of doped metal ions. size of nanospheres can be tuned by varying the metal element species. Ni/TiO2 shows much more remarkably photocatalytic activity than P25.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2015 Received in revised form 21 December 2015 Accepted 3 January 2016 Available online xxx
A simple, room-temperature synthesis approach is reported for obtaining metal-doped titania nanospheres to achieve the highly efficient visible-light-driven photocatalyst. A possible explanation of the photodegradation mechanism and the relationship between behavior of photogenerated charges and photocatalytic reactions were discussed. Morever, we find that the size of nanospheres can be easily tuned not only by changing the concentration of metal ions, but also by varying the metal element species. The probable formation mechanism has been discussed in detail which will initiate new preparation method of photocatalysts for synthesizing TiO2 nanospheres meaningfully. © 2016 Published by Elsevier B.V.
Keywords: Semiconductors Chemical synthesis Solegel growth Optical materials
1. Introduction Semiconductor-based photocatalytic materials have emerged through decades of efforts, and act as one of the most magnetic techniques to convert natural sunlight into chemical energy, which mainly used in hydrogen fuel, photo-decomposition or photooxidization of hazardous substances, etc [1e3]. TiO2, a widely used photocatalyst, has been extensively investigated in recent decades. Nevertheless, the relatively large wide band gap (3.2 eV) and high recombination of photogenerated carriers hinder its practical application to some extent [4,5]. The energy band configuration of a semiconductor is an effective approach to the exploration and development of visible-light-sensitive
* Corresponding author. E-mail address: [email protected] (Z. Zhang).
photocatalysts with advanced performance [6,7]. Therefore, doping with a second metal (e.g., Fe, Co, Ni) or nonmetal (e.g., N, S, C) have been proved the very effective ways to improve the catalytic property of TiO2 [8e11]. Recently, Ni-doped TiO2 has drawn a great deal of attention in photocatalysts for degradation and elimination of environmental pollutants under UV and visible light irradiation [12,13]. Wee et al. demonstrated for the first time that CNT (carbon nanotubes)@Ni/ TiO2 nanocomposites were active in the photoreduction of CO2 into methane (CH4) under visible light irradiation; the high catalytic performance of CNT@Ni/TiO2 was attributed to the photogenerated electrons of scavengers: CNTs and Ni metals, which effectively delayed the recombination rate of holes and electrons and hence increased the life span of the electronehole pairs [13]. Joseph et al. presented visible active metal decorated titania catalysts for the photocatalytic degradation of Amidoblack-10B; the obtained
http://dx.doi.org/10.1016/j.matchemphys.2016.01.001 0254-0584/© 2016 Published by Elsevier B.V.
Please cite this article in press as: X. Yue, et al., Facile synthesis of metal-doped titania nanospheres with tunable size exhibiting highly efficient photoactivity for degradation, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.001
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increased photocatalytic efficiencies in the visible region were attributed to the suppressed recombination of photoinduced electronehole pairs and decreased bandgap with the formation of an acceptor level below the conduction band (CB) after the impregnation of metal ions [12]. However, the synthesis of highly monodisperse spherical Ni-doped TiO2 and the role and local environment of the Ni doping site in TiO2 have rarely been elucidated in detail in these doped systems reported so far. Monodisperse nickel-doped titania nanospheres with controllable sizes were synthesized via a simple solegel technique. As compared with P25 (commercial titania) and bare TiO2, Ni/TiO2 nanospheres extended the light absorption range to long wavelength, and exhibited significantly enhanced photoactivity in visible light region. We put the superior visible photoresponse down to a strong electronic interaction between the Ni and TiO2. Surface photovoltage (SPV) was used to investigate photophysical mechanism of the visible light photocatalytic activity and the results revealed that there was an electronic interaction between the Ni and TiO2, which played a significant role in improving the efficiency of photocatalysis.
2. Results and discussion 2.1. Synthesis of monodisperse TiO2 nanospheres The metal-doped titania nanospheres with tunable size are obtained by a room-temperature synthesis approach and the formation process is schematically illustrated in Fig. 1. The growth mechanisms of the titania spheres prepared by introducing of metal ions were expounded in following sections. The morphology of the as-prepared Ni-doped TiO2 samples is characterized by SEM. As can be seen in Fig. 2, before introducing Ni ions, the obtained pure TiO2 particles are irregular and agglomerate. However, it is interesting to note that ordered and monodisperse nanospheres were obtained by only introducing nickel nitrate hexahydrate into the reaction system without changing any other conditions. We conclude preliminary that Ni2þ serves as a stabilizing agent for the synthesis of monodisperse titania spheres, and the detailed explanations were given in hereinafter sections.
Compared to the diameter of pure titanium dioxide nanospheres of approximately 75 nm, the diameter of doped samples present the trend of increases which are 200, 250, 270, and 300 nm, respectively, corresponding to 0.5%, 1%, 1.5%, and 2% Ni/TiO2 samples. The data of EDX indicated that the nanocrystals were composed mostly of Ti, O and Ni elements (Fig. 2f). Detailed structural information on the Ni/TiO2 sample was investigated using TEM and HRTEM. The lattice fringes with an interplanar spacing of 0.35 nm correspond to the (101) crystal plane of TiO2 nanoparticles, and 0.354 nm for that of Ni/TiO2 samples; suggesting that Ni ions have influenced the environments of TiO2 lattice. The SAED patterns taken from an individual nanoparticle were shown in Fig. 3b6 and inset in Fig. 3a2. All diffraction rings on this pattern can be attributed to anatase tetragonal TiO2, suggesting the polycrystalline nature.
3. Microstructure of Ni/TiO2 photocatalyst The crystal structure and crystallinity of the samples were investigated using XRD. From Fig. 4a, it can be clearly seen that all the indexed peaks in the obtained spectra are well matched with that of anatase structure of TiO2 with lattice constants of a ¼ b ¼ 3.785 Å and c ¼ 9.514 Å (JCPDS 65-5714). No diffraction peaks corresponding to a nickel oxide phase were observed. In addition, an interesting situation displays in the inset of Fig. 4a, that is, the main peak of TiO2 at 2q ¼ 25.5 shifts slightly to lower angles with the increasing of Ni doping content, for the difference in size of the ionic radii of Ni2þ (0.074 nm) and Ti4þ (0.068 nm) ions [14], which indicating that there is an interaction between the Ni and Ti. Fig. 4b shows the XRD patterns of the 1% Ni/TiO2 calcined at different temperatures. No peaks of an impurity phase appeared even under the calcination temperature of 650 C, proving that the sample has unexceptionable thermal stability against phase transformation. However, some significant peaks of rutile phase appeared when the calcination temperature was reached to 750 C. This may be due to the rearrangement of the TiO2 octahedral 6 under high temperature [15,16]. The compositions and chemical states of Ni/TiO2 were further tested by XPS for comparison. As observed in Fig. 5a, the XPS survey
Fig. 1. Formation process of the TiO2 nanospheres.
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Fig. 2. SEM images of the as-prepared products with different Ni/Ti molar ratio calcinated at 450 C: (a1, a2) 0%, (b1, b2) 0.5%, (c1, c2) 1%, (d1, d2) 1.5%, (e1, e2) 2%. (f) EDX spectrum of 1% Ni/TiO2 sample.
Fig. 3. a1 and b1 are SEM images of TiO2 and 1% Ni/TiO2 samples, respectively; a2 and b2eb4 are TEM images of TiO2 and 1% Ni/TiO2 samples, respectively; a3 and b5 are HRTEM images of TiO2 and 1% Ni/TiO2 samples, respectively; b6 and inset in a2 are SAED patterns of 1% Ni/TiO2 and TiO2 samples, respectively.
spectra contains only Ti, O and C, but unconspicuous for element Ni. The peaks of C 1s are from to the adhesive hydrocarbon from the XPS instrument itself. Fig. 5b presents the XPS spectra of Ti 2p, two strong peaks of Ti 2p1/2 and Ti 2p3/2 centered at 464.74 and 459.04 eV for pure TiO2, respectively. This confirms that Ti is present as Ti4þ [17]. However, it is amazing to discovery that compared with the typical peak position of Ti 2p, all doped samples shift toward lower binding energies, which indicate that the chemical environment of Ti4þ has been altered [18,19]. Also, based on the publications have been reported, it can be ascribed to the formation defects such as oxygen vacancies [20]. The broad and asymmetric O
1s profiles are shown in Fig. 5c and can be fitted to three symmetrical peaks locating at 529.3, 530.2 and 531.8 eV, respectively, which present three different kinds of O species in the samples. As we all know, the O 1s peak is broad and complicated due to the nonequivalence of surface oxygen ions. The strong component at 530.2 eV can be attributed to TieO in TiO2; the peak at 531.8 eV may mainly be associated with the OH group absorbed on the surface; and finally, the component with lower binding energy at 529.3 eV are usually attributed to lattice oxygen (TieOeTi) [19,21,22]. In addition, the O 1s peak for pure TiO2 is located at 530.19 eV, while the O 1s peaks shift to lower energies for all the Ni doped TiO2
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Fig. 4. XRD patterns of (a) the as-prepared samples calcined at 450 C; (b) 1% Ni/TiO2 calcined at different temperatures.
Fig. 5. XPS spectra of Ni/TiO2 samples with different Ni/Ti molar ratio: (a), (b), (c) and (d) show high-resolution survey, Ti 2p, O 1s spectra, Ni 2p, respectively.
products. In summary, from both the shifting of the O 1s and Ti 2p peaks to lower energies, we can obtain that a strong interaction may be existed between Ni and TiO2. From Fig. 5d, it can be seen clearly that, after the increased Ni-dopant, the characteristic banding energy peaks of Ni 2p appear, and its intensities become stronger and stronger from 2% to 10% Ni/TiO2. It demonstrates the successful incorporation of the Ni ions into the TiO2 lattice [23]. This result is also confirmed by the elemental mapping pictures, which proved that Ni is uniformly doped on the surface of TiO2 (Fig. S1).
optical response of Ni/TiO2 samples has a red shift toward the longer wavelength region (l ¼ 400e520 nm), with no significant changes on the degree of absorption as the amount of Ni content increases. Some previous studies suggested that the red shift was likely attributed to the formation of a new dopant energy level within the band gap in TiO2 by the doping of Ni ions, thus, a visiblelight response was obtained [1,25]. From the absorption edge of the samples in Fig. 6b, we calculate the bands gaps are 3.34, 3.28, 3.29, 3.29 and 3.30 eV for TiO2, 0.5%, 1%, 1.5% and 2% Ni/TiO2, respectively.
3.2. Enhancement of visible-light photocatalytic performances 3.1. Extension of visible-light optical properties To investigate the optical properties and electronic band structure of Ni/TiO2, UVevis DRS characterization is performed, as is shown in Fig. 6. There is a strongly absorption at 325 nm, which related to the electronic excitation from O 2p valence band to Ti 3d conduction band [24]. Meanwhile, compared with pure TiO2, the
The photocatalytic activity of as-obtained specimens for RhB aqueous solution degradation under visible-light irradiation (l > 400 nm) are explored. The results were shown in Fig. 7a. The blank experiment in the absence of any catalyst shows that the degradation of RhB is negligible. Also it can be seen that all Ni/TiO2 samples have photocatalytic activities that are much higher than
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Fig. 6. (a) UVevis DRS of pure TiO2 and Ni/TiO2 nanospheres; (b) plots of the (ahn)1/2 vs photon energy (hn) for pure TiO2 and Ni/TiO2 nanospheres.
Fig. 7. (a) Photodegradation of RhB over Ni/TiO2 samples; (b) plot of ln(C0/C) versus irradiation time; (c) recycling photocatalytic test of 1% Ni/TiO2 for photodegradation of RhB dye three times under visible light irradiation (l > 400 nm); (d) the influence of calcination temperature on photocatalytic preference of 1% Ni/TiO2. Inset: calculated pseudo-first order constants, kapp (min1).
those of bare TiO2. The degradation efficiency of RhB are about 32, 60, 91, 74 and 86% for the TiO2, 0.5%, 1%, 1.5% and 2% Ni/TiO2 nanospheres after 45 min, respectively. This may be the results of the increased absorption in the visible light range after introducing Ni. However, pure TiO2 also has a little catalytic activity, which is insensitive to visible-light. We deduced that it is the photosensitization effect of RhB [26,27]. RhB molecules can absorb visible light, the excited electrons are injected into the conduction band of TiO2 and finally, the RhB will decompose due to the loss of electrons. The 1% Ni/TiO2 nanospheres show the highest photocatalytic activity, with about 91% of RhB degraded after 45 min. With further increase of molar ratio, photocatalytic degradation decreases. But it still maintains better photocatalytic activities compared to single TiO2, as well as P25. This indicates both that the doped Ni ions have remarkably impacted the photocatalytic activity of TiO2 and that there is an optimal dopant concentration of Ni ions in TiO2. For a better comparison of the photocatalytic efficiency of the TiO2 and Ni/TiO2 nanospheres, the photocatalytic degradation
kinetic was investigated. Fig. 7b shows the plot of ln(C0/C) versus irradiation time. The result indicates that the RhB photodegradation on each Ni/TiO2 nanospheres follows the pseudo-firstorder kinetics model, ln(C0/C) ¼ kappt, where C is the concentration of the RhB at time t, C0 is the initial concentration of the RhB solution, kapp is the apparent first-order rate constant (min1). The rate constant, k, of RhB photodegradation were derived from the ln(C/C0) ~ t plots and listed in Fig. 7b. The kapp value for the 1% Ni/ TiO2 nanosphere is estimated to be 0.05048 min1, which is 6.3 times larger than that of pure TiO2. In order to investigate the stability of the photocatalysts, experiments of visible-light-driven photodegradation of RhB were repeated for three times on the recycled Ni/TiO2 catalysts, and the results are shown in Fig. 7c. It can be seen that after three cycles of the photocatalytic reaction, there was no obvious reduction of activity for the Ni/TiO2 nanospheres, indicating that the photocatalysts were relatively effective and stable. And this result is also confirmed by the XRD patterns in Fig. S2.
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Fig. 7d presents the influence of calcination temperature on the photocatalytic activity of 1% Ni/TiO2 nanospheres. It is interesting to find that the photocatalytic activity reduced with the increased calcination temperature from 450 C to 650 C. However, the sample calcined at 750 C also possesses the excellent photocatalytic activity, same as calcined at 450 C. According to the results of XRD, we deduce that the phase structure significantly affects the photocatalytic activity of our catalysts. When TiO2 is the mixed-phase, there may be a built-in electric field formed between the anatase and rutile, which played an important role in promoting the separation of photogenerated electrons and holes pairs and improved the photocatalytic activity [28e30]. 3.3. Photovoltaic properties The good understanding of photogenerated charge separation and transition properties in the TiO2 system may provide useful information for understanding the better performance in the photocatalytic process. SPV is a technique for semiconductor characterization that relies on analyzing illumination-induced changes in the surface voltage [31,32]. Thus, SPV was employed to carefully investigate the photogenerated charge separation and transport properties of Ni/TiO2 photocatalysts. Fig. 8 presents the (a) SPV and (b) the corresponding phase spectra of the as-prepared samples. As presented in Fig. 8a, a pronounced SPV response was observed for all the Ni/TiO2 products, which can be attributed to the electron transitions from the valence band (VB) to the conduction band (CB) of TiO2, that is the intrinsic band-to-band transition (300e400 nm) [33]. Interestingly, in contrast to the spectrum of pure TiO2, the sub-band-gap transition of the Ni/TiO2 samples all significantly extended into the visible light region (400e480 nm) (Fig. 8a, inset). Referring to the SPV data and the reason that photocatalytic activity of Ni/TiO2 samples in the presence of visible light was enhanced, the exceptional transfer pathway of photogenerated charge carriers was discussed below. Compared with that of bulk TiO2 (~3.34 eV), the response for all doped samples in the range 400e480 nm belongs to the dopant energy levels of Ni in the band gap of TiO2. According to previous report, when doped with the transition metal of Ni, the 3d orbitals in the band gap split into two states, eg state in the higher energy region and t2g state in the lower energy region [7], seen in Fig. 9. Because there is
Fig. 9. Schematic illustration of the photogenerated charge transfer events in the Ni/ TiO2 system under visible light irradiation.
an interaction between the Ni species and the bulk TiO2, the photogenerated electrons can easily transfer from the valence band of TiO2 to the dopant energy level under the visible light irradiation, then we can ensure that Ni acts as the favorable effect in separation and transport of photogenerated electronehole pairs and inhibit their recombination. The corresponding phase spectra (Fig. 8b) show the statistic kinetic characteristics for each SPV with a distinguishable response. From the phase spectra (Fig. 8b) we could obtain that the phase values of all doped Ni/TiO2 present the retardation with respect to 180 , which means that the photogenerated electrons generally transport to the top electrode. This trend of phase retardation indicates that much more photogenerated charges accumulate on the sample surface and lead to recombination decrease of the photogenerated carriers [33]. Based on the above discussion, the improved photocatalytic activity of Ni/TiO2 samples can be ascribed to the electronic interaction between doped Ni and TiO2. The doping Ni sites act as trapping sites to capture the electrons, and effectively inhibit the recombination of electronehole pairs. As shown in Fig. 8a, the intensity of the photovoltage response reached a maximum when the Ni content was 1%, the maximum photocatalytic efficiency was, for the maximum photogenerated charge carriers. That is to say, the incorporation of Ni accelerates the charge separation and transport in TiO2 and thereby restrains the recombination and improves the visible light photocatalytic activity of TiO2. Nevertheless, Fig. 8a shows that when the Ni content exceeds 1%, the response intensity began to decrease. That is, the excessed doping Ni becoming the center of recombination of photogenerated charges, resulting in the observed decrease in photocatalytic activity.
3.4. Growth mechanisms of the titania spheres prepared by introducing of metal ions
Fig. 8. SPV spectra (a) and corresponding phase spectra (b) for pure TiO2 and Ni/TiO2 samples with different molar ratios; inset: the SPV in the region of 400e500 nm; schematic setup of SPV measurement.
To confirm the role of Ni ions for the synthesis of monodisperse titania spheres, which were proposed in the article, we conduct the confirmatory experiments by introducing another seven metal ions into the reaction system, respectively, as shown in Fig. 10. According to the SEM results above, we can find that all doped TiO2 samples are monodisperse titania spheres. We take Ni ions for example and detailed explanations are as follows: after introducing of nickel nitrate, the Ni2þ will adsorb onto the surface of the nucleus instantly and to form a positively charged layer; this charged layer can prevent aggregation under the action of electrostatic repulsion, which will finally induce the formation of the monodisperse titania spheres. The size of nanospheres could be tailored easily from approximately 70e420 nm by varying the metal ions added. That is, when Ba2þ, Sr2þ, Ni2þ, Mn2þ, Ca2þ, Bi3þ, Zn2þ and Pb2þ were introduced into the reaction system, ordered nanospheres with
Please cite this article in press as: X. Yue, et al., Facile synthesis of metal-doped titania nanospheres with tunable size exhibiting highly efficient photoactivity for degradation, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.001
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Fig. 10. SEM images of titania spheres synthesized by doping varying of metal ions (molar ratio: M/Ti ¼ 2%): (a) Ba, (b) Sr, (c) Ni, (d) Mn, (e) Ca, (f) Bi, (g) Zn, (h) Pb; (i) pure TiO2. Scale bar: 400 nm.
uniform diameters of 400, 420, 300, 300, 240, 110, 100 and 70 nm, respectively, were obtained. Fig. 11 shows the comparison photograph of titania solution whether or not introducing metal ions. It should be stressed that the stability of sol solution without introducing metal ions was poor, with some white precipitates gathered at the bottom of the bottle after aging 24 h. However, the sol solution with metal ion was extremely stable and almost no precipitate was observed, which provides evidence that metal ions serve as the stabilizing agent in the process of synthesis nanospheres. In summary, several conclusions are summarized as follows: (i) as for the diverse particle sizes of TiO2 nanospheres with different metal ions doping, it is explained that the different metal ions, the difference in electronic structure, resulting in the electrostatic interactions; (ii) the nanospheres exert excellent photocatalytic activities under visible light illumination, for an example of Ni/TiO2 photocatalysts above; (iii) this mechanism will initiate new preparation method of photocatalysts for synthesizing TiO2
nanospheres meaningfully; (iv) based on TEM, the doped TiO2 nanospheres is mesoporous structure, and this template-free method open a new approach for synthesizing mesoporous materials.
4. Conclusions A facile route is presented for the synthesis of metal-doped titania nanospheres with controllable sizes, which can be easily controlled between approximately 70 and 420 nm by varying the metal ions added. The probable formation mechanism has been discussed in detail and it was found that metal ions serve as the stabilizing agent in the process of synthesis nanospheres. Furthermore, the highly photocatalytic activity of Ni-doped TiO2 was explored for visible-light-driven RhB dye degradation. The surface photovoltage investigations showed that there is an electronic interaction between the Ni and TiO2, which plays a significant effect in improving the efficiency of photocatalysis.
Fig. 11. Photograph of titania solution with introducing varying of metal ions after aging for one day: (a) Ba, (b) Sr, (c) Ni, (d) Mn, (e) Ca, (f) Bi, (g) Zn, (h) Pb; (i) pure TiO2.
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5. Experimental section 5.1. Sample preparation All the chemicals were of analytical grade and were used as received without further purification. The experiment was conducted using a typical solegel method procedure according to our previous study [34]. A computed amount of Ni(NO3)2$6H2O was dispersed in 25 mL of ethylene glycol under vigorously stirred at room temperature, and then 1 mL of tetrabutyl titanate was added to above solution. After that, the system was sealed with Parafilm and stirred for 24 h. The resulting solution was then poured into 100 mL of acetone containing 1 mL of ultrapure water and stirred vigorously for 10 min. After aging for 24 h, the obtained precipitate was centrifugation and washed with acetone and ethanol several times to remove possible impurities, and subsequently dried for 8 h at 80 C vacuum circumstance. Finally, the products were calcined at 450 C for 2 h with a heating rate of 1 C min1. Undoped TiO2 were also synthesized using an identical procedure for comparison. The as-prepared samples were denoted as x% Ni/TiO2, where x% refers to the Ni/Ti molar ratio. The Ba-doped (Sr, Mn, Ca, Bi, Zn, Pb) nanospheres and undoped TiO2 were also synthesized using an identical procedure for comparison. 5.2. Characterization The resulting samples were characterized by several techniques. The crystalline phase was determined by powder X-ray diffraction (XRD) with a Rigaku D/Max-2550 diffractometer using Cu Ka1 radiation (l ¼ 1.54056 Å) at 50 kV and 200 mA in the 2q range 20e70 at a scanning rate of 10 min1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo VG Scientific ESCALAB 250 spectrometer using monochromatized Al Ka excitation. UVevis diffuse reflectance spectra (UVevis DRS) were measured on dispersions using a UVeviseNIR spectrophotometer (Shimadzu UV-3600) to detect absorption over the range 300e800 nm. SEM images were obtained on field emission scanning electron microscope (JSM-6700F, Japan). The transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images were performed on a TECNAIG2 TEM microscope (FEI Company). Energy-dispersive X-ray spectroscopy (EDX) was obtained on a microscope (IE350, Japan). The element mappings were applied on a HITACHI SU-8020 transmission electron microscopy. Surface photovoltage was measured with a lock-in based SPV measurement system, which was composed of a source of monochromatic light, a lock-in amplifier (SR830-D SP) with a light chopper (SR540), a sample cell, and a computer. A low chopping frequency of 24 Hz was used. A 500 W xenon lamp (CHFXQ 500 W, Global xenon lamp power) and a double-prism monochromator (Hilger and Watts, D 300) provided monochromatic light. The photovoltaic cell was a structure of indium tin oxide ITO-sample-ITO. 5.3. Photocatalytic tests The photodegradation efficiency of RhB was assessed in a cylindrical Pyrex flask (50 mL) at room temperature. A 500 W Xenon lamp (CHFXQ 500 W, Global Xenon Lamp Power) was employed as the visible light source and a cut-off filter was used to remove the UV light (l < 400 nm). Initially, 20 mL of aqueous dye solution (10 mg/L) and 20 mg of Ni/TiO2 photocatalyst were put into the reactor and sonicated for 5 min, then continuously stirred for 1 h in the dark to ensure the establishment of adsorptionedesorption equilibrium of RhB molecules on the nanospheres surface before illumination. Analytical samples were taken from the suspension at
intervals of 15 min under visible light, and then immediately centrifuged at 10,000 rpm for 5 min. The photodegradation efficiency was detected by measuring the absorption at 554 nm of the RhB, using a UVevis spectrometer (Maya 2000 Pro) at room temperature. Acknowledgments For financial support, we are grateful to the National Natural Science Foundation of China (20841003 and 20741001) and the Porous Material: Green Synthesis and Application (3A812E571461). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.01.001. References [1] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Nano-photocatalytic materials: possibilities and challenges, Adv. Mater. 24 (2012) 229e251. [2] Q. Zhang, E. Uchaker, S.L. Candelaria, G. Cao, Nanomaterials for energy conversion and storage, Chem. Soc. Rev. 42 (2013) 3127e3171. [3] P.D. Tran, L.H. Wong, J. Barber, J.S.C. Loo, Recent advances in hybrid photocatalysts for solar fuel production, Energy Environ. Sci. 5 (2012) 5902e5918. [4] H. Tada, T. Kiyonaga, S.-i. Naya, Rational design and applications of highly efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium(IV) dioxide, Chem. Soc. Rev. 38 (2009) 1849e1858. [5] M. Anpo, M. Takeuchi, The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation, J. Catal. 216 (2003) 505e516. [6] X. Li, N. Kikugawa, J. Ye, Nitrogen-doped lamellar niobic acid with visible light-responsive photocatalytic activity, Adv. Mater. 20 (2008) 3816e3819. [7] J.H. Ye, Z.G. Zou, Visible light sensitive photocatalysts In1-xMxTaO4 (M¼3d transition-metal) and their activity controlling factors, J. Phys. Chem. Solids 66 (2005) 266e273. [8] K. Obata, K. Kishishita, A. Okemoto, K. Taniya, Y. Ichihashi, S. Nishiyama, Photocatalytic decomposition of NH3 over TiO2 catalysts doped with Fe, Appl. Catal. B 160 (2014) 200e203. [9] L. Liu, Z. Ji, W. Zou, X. Gu, Y. Deng, F. Gao, C. Tang, L. Dong, In situ loading transition metal oxide clusters on TiO2 nanosheets as co-catalysts for exceptional high photoactivity, ACS Catal. 3 (2013) 2052e2061. [10] S. Tada, R. Kikuchi, A. Takagaki, T. Sugawara, S.T. Oyarria, K. Urasaki, S. Satokawa, Study of Ru-Ni/TiO2 catalysts for selective CO methanation, Appl. Catal. B 140 (2013) 258e264. [11] C. Yang, Z. Wang, T. Lin, H. Yin, X. Lu, D. Wan, T. Xu, C. Zheng, J. Lin, F. Huang, X. Xie, M. Jiangl, Core-shell nanostructured “Black” rutile titania as excellent catalyst for hydrogen production enhanced by sulfur doping, J. Am. Chem. Soc. 135 (2013) 17831e17838. [12] J.A.I. Joice, T. Sivakumar, R. Ramakrishnan, G. Ramya, K.P.S. Prasad, D.A. Selvan, Visible active metal decorated titania catalysts for the photocatalytic degradation of Amidoblack-10B, Chem. Eng. J. 210 (2012) 385e397. [13] W.-J. Ong, M.M. Gui, S.-P. Chai, A.R. Mohamed, Direct growth of carbon nanotubes on Ni/TiO2 as next generation catalysts for photoreduction of CO2 to methane by water under visible light irradiation, Rsc Adv. 3 (2013) 4505e4509. [14] M. Raileanu, M. Crisan, A. Ianculescu, D. Crisan, N. Dragan, P. Osiceanu, S. Somacescu, N. Stanica, L. Todan, I. Nitoi, The influence of Ni dopant on the structure and photocatalytic properties of sol-gel TiO2 nanopowders, Water Air Soil Pollut. 224 (2013) 1773e1782. [15] Y. Matsumoto, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 294 (2001), 1003e3. [16] S.C. Padmanabhan, S.C. Pillai, J. Colreavy, S. Balakrishnan, D.E. McCormack, T.S. Perova, Y. Gun'ko, S.J. Hinder, J.M. Kelly, A simple sol-gel processing for the development of high-temperature stable photoactive anatase titania, Chem. Mater. 19 (2007) 4474e4481. [17] W. Zhu, G. Wang, X. Hong, X. Shen, One-step fabrication of Ni/TiO2 Core/Shell nanorod arrays in anodic aluminum oxide membranes, J. Phys. Chem. C 113 (2009) 5450e5454. [18] H. Li, D. Wang, H. Fan, T. Jiang, X. Li, T. Xie, Synthesis of ordered multivalent Mn-TiO2 nanospheres with tunable size: a high performance visible-light photocatalyst, Nano Res. 4 (2011) 460e469. [19] W.J. Hong, M. Kang, The super-hydrophilicities of Bi-TiO2,V-TiO2, and Bi-VTiO2 nano-sized particles and their benzene photodecompositions with H2O addition, Mater. Lett. 60 (2006) 1296e1305. [20] S. Sharma, S. Chaudhary, S.C. Kashyap, S.K. Sharma, Room temperature ferromagnetism in Mn doped TiO2 thin films: electronic structure and Raman investigations, J. Appl. Phys. 109 (2011) 083905e083911. [21] N. Bahadur, R. Pasricha, Govind, S. Chand, R.K. Kotnala, Effect of ni doping on
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Facile synthesis of metal-doped titania nanospheres with tunable size exhibiting highly efficient photoactivity for degradation Xinzheng Yue a, Xu Jin b, Runwei Wang a, Ling Ni a, Shang Jiang a, Shilun Qiu a, Zongtao Zhang a, * a
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China Research Institute of Petroleum Exploration and Development, China University of Petroleum, Beijing 100191, PR China
b
h i g h l i g h t s The The The The
metal-doped TiO2 nanospheres were synthesized via a solegel technique. size of nanospheres can be tuned by varying the amounts of doped metal ions. size of nanospheres can be tuned by varying the metal element species. Ni/TiO2 shows much more remarkably photocatalytic activity than P25.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2015 Received in revised form 21 December 2015 Accepted 3 January 2016 Available online xxx
A simple, room-temperature synthesis approach is reported for obtaining metal-doped titania nanospheres to achieve the highly efficient visible-light-driven photocatalyst. A possible explanation of the photodegradation mechanism and the relationship between behavior of photogenerated charges and photocatalytic reactions were discussed. Morever, we find that the size of nanospheres can be easily tuned not only by changing the concentration of metal ions, but also by varying the metal element species. The probable formation mechanism has been discussed in detail which will initiate new preparation method of photocatalysts for synthesizing TiO2 nanospheres meaningfully. © 2016 Published by Elsevier B.V.
Keywords: Semiconductors Chemical synthesis Solegel growth Optical materials
1. Introduction Semiconductor-based photocatalytic materials have emerged through decades of efforts, and act as one of the most magnetic techniques to convert natural sunlight into chemical energy, which mainly used in hydrogen fuel, photo-decomposition or photooxidization of hazardous substances, etc [1e3]. TiO2, a widely used photocatalyst, has been extensively investigated in recent decades. Nevertheless, the relatively large wide band gap (3.2 eV) and high recombination of photogenerated carriers hinder its practical application to some extent [4,5]. The energy band configuration of a semiconductor is an effective approach to the exploration and development of visible-light-sensitive
* Corresponding author. E-mail address: [email protected] (Z. Zhang).
photocatalysts with advanced performance [6,7]. Therefore, doping with a second metal (e.g., Fe, Co, Ni) or nonmetal (e.g., N, S, C) have been proved the very effective ways to improve the catalytic property of TiO2 [8e11]. Recently, Ni-doped TiO2 has drawn a great deal of attention in photocatalysts for degradation and elimination of environmental pollutants under UV and visible light irradiation [12,13]. Wee et al. demonstrated for the first time that CNT (carbon nanotubes)@Ni/ TiO2 nanocomposites were active in the photoreduction of CO2 into methane (CH4) under visible light irradiation; the high catalytic performance of CNT@Ni/TiO2 was attributed to the photogenerated electrons of scavengers: CNTs and Ni metals, which effectively delayed the recombination rate of holes and electrons and hence increased the life span of the electronehole pairs [13]. Joseph et al. presented visible active metal decorated titania catalysts for the photocatalytic degradation of Amidoblack-10B; the obtained
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increased photocatalytic efficiencies in the visible region were attributed to the suppressed recombination of photoinduced electronehole pairs and decreased bandgap with the formation of an acceptor level below the conduction band (CB) after the impregnation of metal ions [12]. However, the synthesis of highly monodisperse spherical Ni-doped TiO2 and the role and local environment of the Ni doping site in TiO2 have rarely been elucidated in detail in these doped systems reported so far. Monodisperse nickel-doped titania nanospheres with controllable sizes were synthesized via a simple solegel technique. As compared with P25 (commercial titania) and bare TiO2, Ni/TiO2 nanospheres extended the light absorption range to long wavelength, and exhibited significantly enhanced photoactivity in visible light region. We put the superior visible photoresponse down to a strong electronic interaction between the Ni and TiO2. Surface photovoltage (SPV) was used to investigate photophysical mechanism of the visible light photocatalytic activity and the results revealed that there was an electronic interaction between the Ni and TiO2, which played a significant role in improving the efficiency of photocatalysis.
2. Results and discussion 2.1. Synthesis of monodisperse TiO2 nanospheres The metal-doped titania nanospheres with tunable size are obtained by a room-temperature synthesis approach and the formation process is schematically illustrated in Fig. 1. The growth mechanisms of the titania spheres prepared by introducing of metal ions were expounded in following sections. The morphology of the as-prepared Ni-doped TiO2 samples is characterized by SEM. As can be seen in Fig. 2, before introducing Ni ions, the obtained pure TiO2 particles are irregular and agglomerate. However, it is interesting to note that ordered and monodisperse nanospheres were obtained by only introducing nickel nitrate hexahydrate into the reaction system without changing any other conditions. We conclude preliminary that Ni2þ serves as a stabilizing agent for the synthesis of monodisperse titania spheres, and the detailed explanations were given in hereinafter sections.
Compared to the diameter of pure titanium dioxide nanospheres of approximately 75 nm, the diameter of doped samples present the trend of increases which are 200, 250, 270, and 300 nm, respectively, corresponding to 0.5%, 1%, 1.5%, and 2% Ni/TiO2 samples. The data of EDX indicated that the nanocrystals were composed mostly of Ti, O and Ni elements (Fig. 2f). Detailed structural information on the Ni/TiO2 sample was investigated using TEM and HRTEM. The lattice fringes with an interplanar spacing of 0.35 nm correspond to the (101) crystal plane of TiO2 nanoparticles, and 0.354 nm for that of Ni/TiO2 samples; suggesting that Ni ions have influenced the environments of TiO2 lattice. The SAED patterns taken from an individual nanoparticle were shown in Fig. 3b6 and inset in Fig. 3a2. All diffraction rings on this pattern can be attributed to anatase tetragonal TiO2, suggesting the polycrystalline nature.
3. Microstructure of Ni/TiO2 photocatalyst The crystal structure and crystallinity of the samples were investigated using XRD. From Fig. 4a, it can be clearly seen that all the indexed peaks in the obtained spectra are well matched with that of anatase structure of TiO2 with lattice constants of a ¼ b ¼ 3.785 Å and c ¼ 9.514 Å (JCPDS 65-5714). No diffraction peaks corresponding to a nickel oxide phase were observed. In addition, an interesting situation displays in the inset of Fig. 4a, that is, the main peak of TiO2 at 2q ¼ 25.5 shifts slightly to lower angles with the increasing of Ni doping content, for the difference in size of the ionic radii of Ni2þ (0.074 nm) and Ti4þ (0.068 nm) ions [14], which indicating that there is an interaction between the Ni and Ti. Fig. 4b shows the XRD patterns of the 1% Ni/TiO2 calcined at different temperatures. No peaks of an impurity phase appeared even under the calcination temperature of 650 C, proving that the sample has unexceptionable thermal stability against phase transformation. However, some significant peaks of rutile phase appeared when the calcination temperature was reached to 750 C. This may be due to the rearrangement of the TiO2 octahedral 6 under high temperature [15,16]. The compositions and chemical states of Ni/TiO2 were further tested by XPS for comparison. As observed in Fig. 5a, the XPS survey
Fig. 1. Formation process of the TiO2 nanospheres.
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Fig. 2. SEM images of the as-prepared products with different Ni/Ti molar ratio calcinated at 450 C: (a1, a2) 0%, (b1, b2) 0.5%, (c1, c2) 1%, (d1, d2) 1.5%, (e1, e2) 2%. (f) EDX spectrum of 1% Ni/TiO2 sample.
Fig. 3. a1 and b1 are SEM images of TiO2 and 1% Ni/TiO2 samples, respectively; a2 and b2eb4 are TEM images of TiO2 and 1% Ni/TiO2 samples, respectively; a3 and b5 are HRTEM images of TiO2 and 1% Ni/TiO2 samples, respectively; b6 and inset in a2 are SAED patterns of 1% Ni/TiO2 and TiO2 samples, respectively.
spectra contains only Ti, O and C, but unconspicuous for element Ni. The peaks of C 1s are from to the adhesive hydrocarbon from the XPS instrument itself. Fig. 5b presents the XPS spectra of Ti 2p, two strong peaks of Ti 2p1/2 and Ti 2p3/2 centered at 464.74 and 459.04 eV for pure TiO2, respectively. This confirms that Ti is present as Ti4þ [17]. However, it is amazing to discovery that compared with the typical peak position of Ti 2p, all doped samples shift toward lower binding energies, which indicate that the chemical environment of Ti4þ has been altered [18,19]. Also, based on the publications have been reported, it can be ascribed to the formation defects such as oxygen vacancies [20]. The broad and asymmetric O
1s profiles are shown in Fig. 5c and can be fitted to three symmetrical peaks locating at 529.3, 530.2 and 531.8 eV, respectively, which present three different kinds of O species in the samples. As we all know, the O 1s peak is broad and complicated due to the nonequivalence of surface oxygen ions. The strong component at 530.2 eV can be attributed to TieO in TiO2; the peak at 531.8 eV may mainly be associated with the OH group absorbed on the surface; and finally, the component with lower binding energy at 529.3 eV are usually attributed to lattice oxygen (TieOeTi) [19,21,22]. In addition, the O 1s peak for pure TiO2 is located at 530.19 eV, while the O 1s peaks shift to lower energies for all the Ni doped TiO2
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Fig. 4. XRD patterns of (a) the as-prepared samples calcined at 450 C; (b) 1% Ni/TiO2 calcined at different temperatures.
Fig. 5. XPS spectra of Ni/TiO2 samples with different Ni/Ti molar ratio: (a), (b), (c) and (d) show high-resolution survey, Ti 2p, O 1s spectra, Ni 2p, respectively.
products. In summary, from both the shifting of the O 1s and Ti 2p peaks to lower energies, we can obtain that a strong interaction may be existed between Ni and TiO2. From Fig. 5d, it can be seen clearly that, after the increased Ni-dopant, the characteristic banding energy peaks of Ni 2p appear, and its intensities become stronger and stronger from 2% to 10% Ni/TiO2. It demonstrates the successful incorporation of the Ni ions into the TiO2 lattice [23]. This result is also confirmed by the elemental mapping pictures, which proved that Ni is uniformly doped on the surface of TiO2 (Fig. S1).
optical response of Ni/TiO2 samples has a red shift toward the longer wavelength region (l ¼ 400e520 nm), with no significant changes on the degree of absorption as the amount of Ni content increases. Some previous studies suggested that the red shift was likely attributed to the formation of a new dopant energy level within the band gap in TiO2 by the doping of Ni ions, thus, a visiblelight response was obtained [1,25]. From the absorption edge of the samples in Fig. 6b, we calculate the bands gaps are 3.34, 3.28, 3.29, 3.29 and 3.30 eV for TiO2, 0.5%, 1%, 1.5% and 2% Ni/TiO2, respectively.
3.2. Enhancement of visible-light photocatalytic performances 3.1. Extension of visible-light optical properties To investigate the optical properties and electronic band structure of Ni/TiO2, UVevis DRS characterization is performed, as is shown in Fig. 6. There is a strongly absorption at 325 nm, which related to the electronic excitation from O 2p valence band to Ti 3d conduction band [24]. Meanwhile, compared with pure TiO2, the
The photocatalytic activity of as-obtained specimens for RhB aqueous solution degradation under visible-light irradiation (l > 400 nm) are explored. The results were shown in Fig. 7a. The blank experiment in the absence of any catalyst shows that the degradation of RhB is negligible. Also it can be seen that all Ni/TiO2 samples have photocatalytic activities that are much higher than
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Fig. 6. (a) UVevis DRS of pure TiO2 and Ni/TiO2 nanospheres; (b) plots of the (ahn)1/2 vs photon energy (hn) for pure TiO2 and Ni/TiO2 nanospheres.
Fig. 7. (a) Photodegradation of RhB over Ni/TiO2 samples; (b) plot of ln(C0/C) versus irradiation time; (c) recycling photocatalytic test of 1% Ni/TiO2 for photodegradation of RhB dye three times under visible light irradiation (l > 400 nm); (d) the influence of calcination temperature on photocatalytic preference of 1% Ni/TiO2. Inset: calculated pseudo-first order constants, kapp (min1).
those of bare TiO2. The degradation efficiency of RhB are about 32, 60, 91, 74 and 86% for the TiO2, 0.5%, 1%, 1.5% and 2% Ni/TiO2 nanospheres after 45 min, respectively. This may be the results of the increased absorption in the visible light range after introducing Ni. However, pure TiO2 also has a little catalytic activity, which is insensitive to visible-light. We deduced that it is the photosensitization effect of RhB [26,27]. RhB molecules can absorb visible light, the excited electrons are injected into the conduction band of TiO2 and finally, the RhB will decompose due to the loss of electrons. The 1% Ni/TiO2 nanospheres show the highest photocatalytic activity, with about 91% of RhB degraded after 45 min. With further increase of molar ratio, photocatalytic degradation decreases. But it still maintains better photocatalytic activities compared to single TiO2, as well as P25. This indicates both that the doped Ni ions have remarkably impacted the photocatalytic activity of TiO2 and that there is an optimal dopant concentration of Ni ions in TiO2. For a better comparison of the photocatalytic efficiency of the TiO2 and Ni/TiO2 nanospheres, the photocatalytic degradation
kinetic was investigated. Fig. 7b shows the plot of ln(C0/C) versus irradiation time. The result indicates that the RhB photodegradation on each Ni/TiO2 nanospheres follows the pseudo-firstorder kinetics model, ln(C0/C) ¼ kappt, where C is the concentration of the RhB at time t, C0 is the initial concentration of the RhB solution, kapp is the apparent first-order rate constant (min1). The rate constant, k, of RhB photodegradation were derived from the ln(C/C0) ~ t plots and listed in Fig. 7b. The kapp value for the 1% Ni/ TiO2 nanosphere is estimated to be 0.05048 min1, which is 6.3 times larger than that of pure TiO2. In order to investigate the stability of the photocatalysts, experiments of visible-light-driven photodegradation of RhB were repeated for three times on the recycled Ni/TiO2 catalysts, and the results are shown in Fig. 7c. It can be seen that after three cycles of the photocatalytic reaction, there was no obvious reduction of activity for the Ni/TiO2 nanospheres, indicating that the photocatalysts were relatively effective and stable. And this result is also confirmed by the XRD patterns in Fig. S2.
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Fig. 7d presents the influence of calcination temperature on the photocatalytic activity of 1% Ni/TiO2 nanospheres. It is interesting to find that the photocatalytic activity reduced with the increased calcination temperature from 450 C to 650 C. However, the sample calcined at 750 C also possesses the excellent photocatalytic activity, same as calcined at 450 C. According to the results of XRD, we deduce that the phase structure significantly affects the photocatalytic activity of our catalysts. When TiO2 is the mixed-phase, there may be a built-in electric field formed between the anatase and rutile, which played an important role in promoting the separation of photogenerated electrons and holes pairs and improved the photocatalytic activity [28e30]. 3.3. Photovoltaic properties The good understanding of photogenerated charge separation and transition properties in the TiO2 system may provide useful information for understanding the better performance in the photocatalytic process. SPV is a technique for semiconductor characterization that relies on analyzing illumination-induced changes in the surface voltage [31,32]. Thus, SPV was employed to carefully investigate the photogenerated charge separation and transport properties of Ni/TiO2 photocatalysts. Fig. 8 presents the (a) SPV and (b) the corresponding phase spectra of the as-prepared samples. As presented in Fig. 8a, a pronounced SPV response was observed for all the Ni/TiO2 products, which can be attributed to the electron transitions from the valence band (VB) to the conduction band (CB) of TiO2, that is the intrinsic band-to-band transition (300e400 nm) [33]. Interestingly, in contrast to the spectrum of pure TiO2, the sub-band-gap transition of the Ni/TiO2 samples all significantly extended into the visible light region (400e480 nm) (Fig. 8a, inset). Referring to the SPV data and the reason that photocatalytic activity of Ni/TiO2 samples in the presence of visible light was enhanced, the exceptional transfer pathway of photogenerated charge carriers was discussed below. Compared with that of bulk TiO2 (~3.34 eV), the response for all doped samples in the range 400e480 nm belongs to the dopant energy levels of Ni in the band gap of TiO2. According to previous report, when doped with the transition metal of Ni, the 3d orbitals in the band gap split into two states, eg state in the higher energy region and t2g state in the lower energy region [7], seen in Fig. 9. Because there is
Fig. 9. Schematic illustration of the photogenerated charge transfer events in the Ni/ TiO2 system under visible light irradiation.
an interaction between the Ni species and the bulk TiO2, the photogenerated electrons can easily transfer from the valence band of TiO2 to the dopant energy level under the visible light irradiation, then we can ensure that Ni acts as the favorable effect in separation and transport of photogenerated electronehole pairs and inhibit their recombination. The corresponding phase spectra (Fig. 8b) show the statistic kinetic characteristics for each SPV with a distinguishable response. From the phase spectra (Fig. 8b) we could obtain that the phase values of all doped Ni/TiO2 present the retardation with respect to 180 , which means that the photogenerated electrons generally transport to the top electrode. This trend of phase retardation indicates that much more photogenerated charges accumulate on the sample surface and lead to recombination decrease of the photogenerated carriers [33]. Based on the above discussion, the improved photocatalytic activity of Ni/TiO2 samples can be ascribed to the electronic interaction between doped Ni and TiO2. The doping Ni sites act as trapping sites to capture the electrons, and effectively inhibit the recombination of electronehole pairs. As shown in Fig. 8a, the intensity of the photovoltage response reached a maximum when the Ni content was 1%, the maximum photocatalytic efficiency was, for the maximum photogenerated charge carriers. That is to say, the incorporation of Ni accelerates the charge separation and transport in TiO2 and thereby restrains the recombination and improves the visible light photocatalytic activity of TiO2. Nevertheless, Fig. 8a shows that when the Ni content exceeds 1%, the response intensity began to decrease. That is, the excessed doping Ni becoming the center of recombination of photogenerated charges, resulting in the observed decrease in photocatalytic activity.
3.4. Growth mechanisms of the titania spheres prepared by introducing of metal ions
Fig. 8. SPV spectra (a) and corresponding phase spectra (b) for pure TiO2 and Ni/TiO2 samples with different molar ratios; inset: the SPV in the region of 400e500 nm; schematic setup of SPV measurement.
To confirm the role of Ni ions for the synthesis of monodisperse titania spheres, which were proposed in the article, we conduct the confirmatory experiments by introducing another seven metal ions into the reaction system, respectively, as shown in Fig. 10. According to the SEM results above, we can find that all doped TiO2 samples are monodisperse titania spheres. We take Ni ions for example and detailed explanations are as follows: after introducing of nickel nitrate, the Ni2þ will adsorb onto the surface of the nucleus instantly and to form a positively charged layer; this charged layer can prevent aggregation under the action of electrostatic repulsion, which will finally induce the formation of the monodisperse titania spheres. The size of nanospheres could be tailored easily from approximately 70e420 nm by varying the metal ions added. That is, when Ba2þ, Sr2þ, Ni2þ, Mn2þ, Ca2þ, Bi3þ, Zn2þ and Pb2þ were introduced into the reaction system, ordered nanospheres with
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Fig. 10. SEM images of titania spheres synthesized by doping varying of metal ions (molar ratio: M/Ti ¼ 2%): (a) Ba, (b) Sr, (c) Ni, (d) Mn, (e) Ca, (f) Bi, (g) Zn, (h) Pb; (i) pure TiO2. Scale bar: 400 nm.
uniform diameters of 400, 420, 300, 300, 240, 110, 100 and 70 nm, respectively, were obtained. Fig. 11 shows the comparison photograph of titania solution whether or not introducing metal ions. It should be stressed that the stability of sol solution without introducing metal ions was poor, with some white precipitates gathered at the bottom of the bottle after aging 24 h. However, the sol solution with metal ion was extremely stable and almost no precipitate was observed, which provides evidence that metal ions serve as the stabilizing agent in the process of synthesis nanospheres. In summary, several conclusions are summarized as follows: (i) as for the diverse particle sizes of TiO2 nanospheres with different metal ions doping, it is explained that the different metal ions, the difference in electronic structure, resulting in the electrostatic interactions; (ii) the nanospheres exert excellent photocatalytic activities under visible light illumination, for an example of Ni/TiO2 photocatalysts above; (iii) this mechanism will initiate new preparation method of photocatalysts for synthesizing TiO2
nanospheres meaningfully; (iv) based on TEM, the doped TiO2 nanospheres is mesoporous structure, and this template-free method open a new approach for synthesizing mesoporous materials.
4. Conclusions A facile route is presented for the synthesis of metal-doped titania nanospheres with controllable sizes, which can be easily controlled between approximately 70 and 420 nm by varying the metal ions added. The probable formation mechanism has been discussed in detail and it was found that metal ions serve as the stabilizing agent in the process of synthesis nanospheres. Furthermore, the highly photocatalytic activity of Ni-doped TiO2 was explored for visible-light-driven RhB dye degradation. The surface photovoltage investigations showed that there is an electronic interaction between the Ni and TiO2, which plays a significant effect in improving the efficiency of photocatalysis.
Fig. 11. Photograph of titania solution with introducing varying of metal ions after aging for one day: (a) Ba, (b) Sr, (c) Ni, (d) Mn, (e) Ca, (f) Bi, (g) Zn, (h) Pb; (i) pure TiO2.
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5. Experimental section 5.1. Sample preparation All the chemicals were of analytical grade and were used as received without further purification. The experiment was conducted using a typical solegel method procedure according to our previous study [34]. A computed amount of Ni(NO3)2$6H2O was dispersed in 25 mL of ethylene glycol under vigorously stirred at room temperature, and then 1 mL of tetrabutyl titanate was added to above solution. After that, the system was sealed with Parafilm and stirred for 24 h. The resulting solution was then poured into 100 mL of acetone containing 1 mL of ultrapure water and stirred vigorously for 10 min. After aging for 24 h, the obtained precipitate was centrifugation and washed with acetone and ethanol several times to remove possible impurities, and subsequently dried for 8 h at 80 C vacuum circumstance. Finally, the products were calcined at 450 C for 2 h with a heating rate of 1 C min1. Undoped TiO2 were also synthesized using an identical procedure for comparison. The as-prepared samples were denoted as x% Ni/TiO2, where x% refers to the Ni/Ti molar ratio. The Ba-doped (Sr, Mn, Ca, Bi, Zn, Pb) nanospheres and undoped TiO2 were also synthesized using an identical procedure for comparison. 5.2. Characterization The resulting samples were characterized by several techniques. The crystalline phase was determined by powder X-ray diffraction (XRD) with a Rigaku D/Max-2550 diffractometer using Cu Ka1 radiation (l ¼ 1.54056 Å) at 50 kV and 200 mA in the 2q range 20e70 at a scanning rate of 10 min1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo VG Scientific ESCALAB 250 spectrometer using monochromatized Al Ka excitation. UVevis diffuse reflectance spectra (UVevis DRS) were measured on dispersions using a UVeviseNIR spectrophotometer (Shimadzu UV-3600) to detect absorption over the range 300e800 nm. SEM images were obtained on field emission scanning electron microscope (JSM-6700F, Japan). The transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images were performed on a TECNAIG2 TEM microscope (FEI Company). Energy-dispersive X-ray spectroscopy (EDX) was obtained on a microscope (IE350, Japan). The element mappings were applied on a HITACHI SU-8020 transmission electron microscopy. Surface photovoltage was measured with a lock-in based SPV measurement system, which was composed of a source of monochromatic light, a lock-in amplifier (SR830-D SP) with a light chopper (SR540), a sample cell, and a computer. A low chopping frequency of 24 Hz was used. A 500 W xenon lamp (CHFXQ 500 W, Global xenon lamp power) and a double-prism monochromator (Hilger and Watts, D 300) provided monochromatic light. The photovoltaic cell was a structure of indium tin oxide ITO-sample-ITO. 5.3. Photocatalytic tests The photodegradation efficiency of RhB was assessed in a cylindrical Pyrex flask (50 mL) at room temperature. A 500 W Xenon lamp (CHFXQ 500 W, Global Xenon Lamp Power) was employed as the visible light source and a cut-off filter was used to remove the UV light (l < 400 nm). Initially, 20 mL of aqueous dye solution (10 mg/L) and 20 mg of Ni/TiO2 photocatalyst were put into the reactor and sonicated for 5 min, then continuously stirred for 1 h in the dark to ensure the establishment of adsorptionedesorption equilibrium of RhB molecules on the nanospheres surface before illumination. Analytical samples were taken from the suspension at
intervals of 15 min under visible light, and then immediately centrifuged at 10,000 rpm for 5 min. The photodegradation efficiency was detected by measuring the absorption at 554 nm of the RhB, using a UVevis spectrometer (Maya 2000 Pro) at room temperature. Acknowledgments For financial support, we are grateful to the National Natural Science Foundation of China (20841003 and 20741001) and the Porous Material: Green Synthesis and Application (3A812E571461). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.01.001. References [1] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Nano-photocatalytic materials: possibilities and challenges, Adv. Mater. 24 (2012) 229e251. [2] Q. Zhang, E. Uchaker, S.L. Candelaria, G. Cao, Nanomaterials for energy conversion and storage, Chem. Soc. Rev. 42 (2013) 3127e3171. [3] P.D. Tran, L.H. Wong, J. Barber, J.S.C. Loo, Recent advances in hybrid photocatalysts for solar fuel production, Energy Environ. Sci. 5 (2012) 5902e5918. [4] H. Tada, T. Kiyonaga, S.-i. Naya, Rational design and applications of highly efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium(IV) dioxide, Chem. Soc. Rev. 38 (2009) 1849e1858. [5] M. Anpo, M. Takeuchi, The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation, J. Catal. 216 (2003) 505e516. [6] X. Li, N. Kikugawa, J. Ye, Nitrogen-doped lamellar niobic acid with visible light-responsive photocatalytic activity, Adv. Mater. 20 (2008) 3816e3819. [7] J.H. Ye, Z.G. Zou, Visible light sensitive photocatalysts In1-xMxTaO4 (M¼3d transition-metal) and their activity controlling factors, J. Phys. Chem. Solids 66 (2005) 266e273. [8] K. Obata, K. Kishishita, A. Okemoto, K. Taniya, Y. Ichihashi, S. Nishiyama, Photocatalytic decomposition of NH3 over TiO2 catalysts doped with Fe, Appl. Catal. B 160 (2014) 200e203. [9] L. Liu, Z. Ji, W. Zou, X. Gu, Y. Deng, F. Gao, C. Tang, L. Dong, In situ loading transition metal oxide clusters on TiO2 nanosheets as co-catalysts for exceptional high photoactivity, ACS Catal. 3 (2013) 2052e2061. [10] S. Tada, R. Kikuchi, A. Takagaki, T. Sugawara, S.T. Oyarria, K. Urasaki, S. Satokawa, Study of Ru-Ni/TiO2 catalysts for selective CO methanation, Appl. Catal. B 140 (2013) 258e264. [11] C. Yang, Z. Wang, T. Lin, H. Yin, X. Lu, D. Wan, T. Xu, C. Zheng, J. Lin, F. Huang, X. Xie, M. Jiangl, Core-shell nanostructured “Black” rutile titania as excellent catalyst for hydrogen production enhanced by sulfur doping, J. Am. Chem. Soc. 135 (2013) 17831e17838. [12] J.A.I. Joice, T. Sivakumar, R. Ramakrishnan, G. Ramya, K.P.S. Prasad, D.A. Selvan, Visible active metal decorated titania catalysts for the photocatalytic degradation of Amidoblack-10B, Chem. Eng. J. 210 (2012) 385e397. [13] W.-J. Ong, M.M. Gui, S.-P. Chai, A.R. Mohamed, Direct growth of carbon nanotubes on Ni/TiO2 as next generation catalysts for photoreduction of CO2 to methane by water under visible light irradiation, Rsc Adv. 3 (2013) 4505e4509. [14] M. Raileanu, M. Crisan, A. Ianculescu, D. Crisan, N. Dragan, P. Osiceanu, S. Somacescu, N. Stanica, L. Todan, I. Nitoi, The influence of Ni dopant on the structure and photocatalytic properties of sol-gel TiO2 nanopowders, Water Air Soil Pollut. 224 (2013) 1773e1782. [15] Y. Matsumoto, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 294 (2001), 1003e3. [16] S.C. Padmanabhan, S.C. Pillai, J. Colreavy, S. Balakrishnan, D.E. McCormack, T.S. Perova, Y. Gun'ko, S.J. Hinder, J.M. Kelly, A simple sol-gel processing for the development of high-temperature stable photoactive anatase titania, Chem. Mater. 19 (2007) 4474e4481. [17] W. Zhu, G. Wang, X. Hong, X. Shen, One-step fabrication of Ni/TiO2 Core/Shell nanorod arrays in anodic aluminum oxide membranes, J. Phys. Chem. C 113 (2009) 5450e5454. [18] H. Li, D. Wang, H. Fan, T. Jiang, X. Li, T. Xie, Synthesis of ordered multivalent Mn-TiO2 nanospheres with tunable size: a high performance visible-light photocatalyst, Nano Res. 4 (2011) 460e469. [19] W.J. Hong, M. Kang, The super-hydrophilicities of Bi-TiO2,V-TiO2, and Bi-VTiO2 nano-sized particles and their benzene photodecompositions with H2O addition, Mater. Lett. 60 (2006) 1296e1305. [20] S. Sharma, S. Chaudhary, S.C. Kashyap, S.K. Sharma, Room temperature ferromagnetism in Mn doped TiO2 thin films: electronic structure and Raman investigations, J. Appl. Phys. 109 (2011) 083905e083911. [21] N. Bahadur, R. Pasricha, Govind, S. Chand, R.K. Kotnala, Effect of ni doping on
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Please cite this article in press as: X. Yue, et al., Facile synthesis of metal-doped titania nanospheres with tunable size exhibiting highly efficient photoactivity for degradation, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.001