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The tunable photocatalytic activity of LN nanoparticles is a useful integratable function of LN-based lab-on-chip platform for unraveling the biological complexity in population. In this paper, Fe-Mn-codoped LiNbO3 nanoparticles were prepared by a soft-chemistry route. The nanoparticle absorption in UV-Vis range was modified through photochromic effect, and their influence on the photocatalytic activity of the nanoparticls was evaluated by the degradation of Rhodamine B under xenon lamp illumination. It was found that Fe-Mn-codoped LiNbO3 nanocrystals with less Vis absorption exhibited higher photocatalytic efficiency for photocatalytic dye solution. The mechanism of tuning the photocatalytic activity through photochromic effect was proposed for Fe-Mn-codoped LiNbO3 nanocrystals and the holes released through charge transfer from Mn3+ are emphasized for the explanation of the photocatalytic behavior.
© 2015 Optical Society of America
1. Introduction
Lithium niobate (LN) bulk crystal and nanoparticles (Nps) is considered as a promising substrate material for a medically used lab-on-chip platform because of its remarkable ferroelectric, piezoelectric and nonlinear optical properties [1–9 ]. In the past few years, the LN-base microfluidic SAW actuators, manipulator, sorter, and the multiphoton fluorescent prober were demonstrated in lab-on-chip platform for handling and analyzing small population of cells (including eukaryotes and prokaryotes) [2–4 ]. The photocatalytic activity of LN nanoparticles (Nps) is another useful integratable function of LN-based lab-on-chip platform for unraveling the biological complexity in population through degradation of the cells. For example, Gutmann et al investigated the impact of LN and LT (lithium tantalate) nanoparticles on the bacterium Escherichia coli in aqueous solutions and achieved a beautiful disinfection effect by using the high degradation ability of LN Nps within this population of bacterium [4]. However, the in situ tunability of photocatalytic activity of LN Nps, required by LN-based lab-on-chip platform, still lacks investigation so far.
For LN Nps, the relevant researches started intensively from this century. Liu et al. made a systematic investigation on the preparation of LN Nps, and some convenient soft-chemistry routes for Nps preparation and novel methods for tuning their shape were demonstrated successfully by them [5,6 ]. However, the photocatalytic activity of LN Nps was reported rarely in recent years. Stock and Dunn [7] carefully compared the photocatalytic activity of the p-type (Mg-doped) and n-type (Fe-doped) LN Nps and found that the rate of decolorization of the dye solutions is fastest over p-type material than with the n-type LN Nps. They attributed this unexpected result to the changes in the majority carrier and suggested that the majority carriers (holes) photoexcited in Mg-doped LN contributes a lot to the photocatalytic ability of the Nps. Although different types of LN Nps were studied in previous research, less effort was put on the UV-Vis absorption of LN Nps, which is widely considered as a quite important parameter for the tuning of photocatalytic activity of Nps.
The transition-element-impurities in LN crystals are the main cause for the optical absorption of the material. The types, concentrations and valence states of the transition-element-impurities may vary the absorption of LN significantly [10]. Fe-Mn-codoped LN (LN:Fe:Mn) crystal became famous because of its photochromic effect [11], by which the crystal absorption can be modified simply through in situ irradiation rather than through the complicated post-treatment such as oxidization and reduction annealing. Under strong irradiation the UV-Vis absorption of LN:Fe:Mn may change to some extent, depending the irradiation wavelength, intensity and duration. Thus, LN:Fe:Mn Nps are expected to be deposited or combined onto lab-on-chip platform for the cell degradation, with the in situ tunable photocatalytic activity.
In this paper, Fe-Mn-codoped LiNbO3 nanoparticles were prepared by a soft-chemistry route. It will be shown that the strong pre-irradiation can be applied to LN:Fe:Mn Nps for in situ tuning their photocatalytic ability through the Nps photochromic effect.
2. Experimental procedures
Niobium pentoxide, hydrofluoric acid, citric acid, lithium hydroxide, ammonium hydroxide and nitrate were used as starting materials without further purification. In a typical procedure, Nb2O5 was firstly dissolved in a proportion of HF acid at 80 °C for 0.5 h, forming a clear solution. Aqueous solution of ammonia was dropped into the solution till the white floccus precipitation (Nb2O5•nH2O) completely separates out. The precipitation was filtered, washed and dissolved in citric acid aqueous solution at 80 °C, and LiOH were added into the solution until neutral pH of solution was obtained under violent stirring. Fe(NO3)3 and Mn(NO3)3 were also added into the solution dropwise. Here in the solution the Li/Nb ratio is 1:1 and the Fe and Mn doping concentration is about 0.05 mol%. The transparent solution (sol) usually obtained after several-hour stirring was then dried at 70 °C for agitation. Finally, the gel was calcinated at 550 °C for obtaining Fe-Mn-codoped LN Nps.
The as-prepared Nps was characterized by using X-ray diffraction (XRD, Philips Xpert) with 2θ range of 10-80°. Scanning electron microscopy (SEM, S-4800) was used for the morphology acquisition of Nps. After the basic characterization, the Nps were spread onto a quartz wafer to receive strong uniform pre-irradiation. For increasing the Vis absorption of Nps, 365nm-UV light (Mercury Lamp L10862, 200W, HAMAMASTU) with an intensity of ~0.5 W/cm2 was used, while for decreasing the Vis absorption of Nps, the 532nm-green laser with an intensity of ~1W/cm2 was used. In order to achieve significant photochromic effect, the pre-irradiation duration was set as long as possible (typically > 6h). The UV-Vis absorption spectrum of as-prepared and irradiated LN Nps was measured on UV-Vis spectrumeter (U-3900h, HITACHI).
For the photocatalytic evaluation of LN Nps, a beaker was used to house the reaction and loaded with dye solution (Rhodamine b, Sigma) (4 ppm). The solution containing LN Nps (2 g/L) was then added into the dye solution. An ultrasonic machine (40KHz, JIEKANG) was used to make Nps fully dispersed in the solution. A quartz lid was used, allowing the transmission of incident light (its intensity after the quartz lid is ~600 mW/cm2) from xenon lamp (UXL-302-O, 300W, USHIO). The typical emission spectrum of the employed light source for photocatalytic (photo-degradation) experiment is plotted in the inset of Fig. 5. Note that a filter was used to suppress the infrared radiation from the xenon lamp. At predetermined interval (1 hour) the solution samples (2 mL) were collected. The same UV-Vis spectrometer (U-3900h, HITACHI) was used to collect absorption spectra and the degradation rates of dye solution over LN Nps were calculated.
3. Results and discussion
Figure 1 shows the XRD pattern of the as-prepared LN Nps. All the diffractions are assigned to the LiNbO3 crystalline phase. The XRD pattern is in excellent agreement with a reference pattern of LN (JCPDS file no.85-2456). No obvious peaks of other phase were detected, which implies that the products mainly are with single phase. The typical morphology for the as-prepared Nps is shown in Fig. 2 . It can be seen that Nps are close packed with a round shape and size ranging from tens to hundreds of nanometers.
Fig. 2 The typical morphology for the as-prepared Fe-Mn-codoped LN Nps: (a) Lower magnification (10.0 k) and (b) Higher magnification (50.0 k). Nps are close packed with a round shape and size ranging from tens to hundreds of nanometers.
After the UV irradiation the Nps demonstrates a pink color while the green irradiation may bleach the samples. The obvious photochromic effect indicates that Fe and Mn are indeed doped into the crystal lattice of Nps [12], i.e. Fe-Mn-doped LiNbO3 Nps were fabricated successfully by the soft chemical method. This photochromic effect can be shown quantitatively in the UV-Vis absorption spectrum of as-prepared and irradiated LN Nps in Fig. 3 . UV irradiation leads to an increased absorption in the whole wavelength. By contrary, the green irradiation can decrease the visible absorption but increase a little the absorption at deep UV range.
Photocatalytic (photo-degradation) experiments are performed respectively for the original, 365nm-irradiated, and 532nm-irradiated LN:Fe:Mn Nps. The normalized absorption spectra of the solution samples at 1h interval was shown in Fig. 4 for each samples. The temporal curves of calculated degradation rate are plotted for them in Fig. 5 . It can be seen in both figures that 365nm-irradiated LN:Fe:Mn Nps demonstrate the lowest photocatalytic ability among all the samples, indicating that the Vis absorption may suppress the photocatalytic process of Rhodamine b. In contrast with this, the bleached 532nm-irradiated LN:Fe:Mn Nps show much better photocatalytic ability. Thus, in the case of LN-based lab-on-chip platform, UV and Vis irradiation can be properly selected for in situ tuning of the photocatalytic ability of Nps.
Fig. 4 Normalized absorption spectra of the solution samples at 1h interval for a) original, b) 365nm-irradiated and c) 532nm-irradiated Fe-Mn-codoped LN Nps, repectively.
Fig. 5 The temporal curves of the degradation rate of dye for a) original, b) 365nm-irradiated and c) 532nm-irradiated Fe-Mn-codoped LN Nps, repectively.
The mechanism of tuning the photocatalytic activity through photochromic effect was shown in Fig. 6 . In general, photo-excited carriers play the key role in the photocatalytic process of dye. The electron transition from the filled traps Fe2+ (with intermediate energy level) to the conduction band usually induces the broad absorption in visible wavelength range. Our above result reveals that the visible absorption induced by Fe2+ may degenerate the photocatalytic ability of Nps. As a matter of fact, Stock et al. [7] suggested that the changes in the majority carrier may impact the photocatalytic ability of LN Nps and that the holes photo-excited in LN Nps contributes more to the photocatalytic process than photo-excited electrons. Under the illumination of xenon lamp both types of carriers may co-exist in Nps. Electrons are photo-excited directly from the filled traps Fe2+ while holes may be generated in two ways: one is from the intrinsic absorption of the material to the photons with very short wavelength, which produces photo-excited electron-hole pairs; another is from the empty traps of Mn3+ (with deep energy level), which locates usually near the valence band and may release holes to the valence band through charge transfer (CT) process under the visible illumination [13]. As compared with the photo-excited electrons from Fe2+ and holes released from Mn3+ through CT process, the electron-hole pairs produced by the intrinsic photon absorption are quite less because it requires the short-wavelength light whose intensity is usually very low in the spectrum of xenon lamp.
Fig. 6 The mechanism of tuning the photocatalytic activity through photochromic effect for Fe-Mn-codoped LN Nps.
In 365nm-irradiated Nps almost all of the traps are filled as Fe2+ and Mn2+. Under the illumination of xenon lamp a large amount of electrons are photo-excited from Fe2+ while only the intrinsic photo-excitation can produce some holes. In this situation, the combination of electrons and holes may leads to few holes left for helping the photocatalytic process. In other words, the electrons dominate the photo-excited carriers, resulting in the degeneration of the photocatalytic ability of 365nm-irradiated Nps. By contrary, in the bleached 532nm-irradiated Nps, Fe traps are completely empty as Fe3+ while a great number of Mn3+ may co-exist with a minority of Mn2+. Under the illumination of xenon lamp, a large amount of holes can be released from Mn3+ through CT process while electrons come mainly from the intrinsic photo-excitation. Even after the combination, holes are still sufficient for the photocatalytic process and they are the majority photo-excited carrier in this case. Therefore, the enhanced photocatalytic ability can be observed for 532nm-irradiated Nps. Note that a minority of Mn2+ can also provide the holes for dye degradation, considering they, as the photo-acceptors for visible photons, may reduce Fe3+ centers and finally being themselves oxidized to Mn3+, according to the photochromic response in Fe-Mn-doped LN as previously published [11,14,15 ].
Another effect which may be paid attention is that the degradation rate seems linear for 532nm-irradiated Nps but the other two, the original and 365nm-irradiated ones, clearly develop a non-linear trend. Indeed, this trend may result in a comparable performance of the original Nps with the 532nm-irradiated Nps after long-time photo-degradation. The possible explanation for this effect is that the photocatalytic activity of original and 365nm-irradiated Nps may change slowly with the photo-degradation duration. It should be noticed that a large part of xenon-lamp irradiation locates in the wavelength range around 500nm, which may lead to the gradual decrease of the Nps absorption in the visible range during the photo-degradation. The extent of the absorption decrease is different for Nps samples, depending on their initial absorption at the beginning of the photocatalytic experiment. As the original and 365nm-irradiated Nps show stronger initial absorption in the visible range, their absorption decrease are expected to be much more obvious than the case of 532nm-irradiated Nps. This gradual decrease of the Nps absorption in the visible range may induce the slow enhancement of their photocatalytic activity during the photo-degradation, resulting in the non-linear trend of the degradation rate for the original and 365nm-irradiated Nps.
4. Conclusion
In this paper, Fe-Mn-codoped LiNbO3 nanoparticles were prepared by a soft-chemistry route. The nanoparticle absorption in UV-Vis range was modified through photochromic effect, and their influence on the photocatalytic activity of the nanoparticls was evaluated by the degeneration of Rhodamine B under xenon lamp illumination. It was found that Fe-Mn-codoped LiNbO3 nanocrystals with less Vis absorption exhibited higher photocatalytic efficiency for photocatalytic dye solution. The mechanism of tuning the photocatalytic activity through photochromic effect was proposed and the holes released through charge transfer from Mn3+ under the illumination of xenon lamp are emphasized.
Acknowledgments
This work is partly supported by National Natural Science Foundation of China (NSFC) (No. 61108060), EYRF of HeBei EDP (No.YQ2013029), Key Project of MOE (No. 212016), Project-sponsored by SRF for ROS of MOE (2012), Hebei NSF (No. F2013202153), and SOCSF of MPC (No. CG2013003002).
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