b School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
Responsive compounds can easily fabricate the intelligent composite materials which possessed specially environmental response are increasingly interested by researchers, such as photosensitivity , pH response , temperature sensitivity . Among these compounds, temperature sensitive compound is easy to obtain and it is widely applied to fabricate the intelligent organicinorganic materials.
Poly N-isopropyl acrylamide (PNIPAM) is widely applied temperature sensitive polymer, it could reversibly switch the hydrophily to hydrophobicity at the lower critical solution temperature (LCST, value about 32 ℃ in aqueous solution). The temperature decreases below LCST, the hydrophily is improved and the PNIPAM exhibits the stretch structure, correspondingly the temperature increases above LCST, the hydrophobicity is enhanced and the PNIPAM shows shrink. According to this property, many temperature sensitive composite materials were prepared. Such as AuNP-NIPAM , SiO2-g-PAIPAM , etc. Which made these materials possess intelligent function, and it is really useful to understand the action of the composite materials with changing temperature. Therefore, PNIPAM coupled with the photocatalysts could fabricate the temperature sensitive photocatalysts which could effectively control the photodegradation processes through changing the reacted temperature [6, 7], it is also easy to understand the mechanism of photodegradation from the controllable photocatalytic processes.
Photocatalysis is interested due to that it can be widely used in synthesis and degradation [8, 9]. The photocatalytic efficiencies are influenced by many factors, and the most important is photocatalyst. ZnO is an n-type semiconductor and possesses high optical property, which could obtain various morphologies via different prepared methods, and it is widely used as the photocatalyst. Especially the nano-ZnO assemble structure is increasingly interested [10-14].
In this work, we successfully prepared the hamburger-like CZnO via the solvent thermal method, the processes followed: 0.966 g Zn(OAc)2·2CH2O and 0.4 g glucose were put into 40 mL ethanediol and stirred for 30 min, then transferred into the teflon reactor, and it was kept at 160 ℃ for 30 h. After cool down to room temperature, the precipitant was collected and wished with water and ethanol, the precipitant was dried at 60 ℃ for 12 h in the oven and the ZnO/C was obtained. In order to compare the performance of modified photocatalyst, the pure ZnO was obtained through the same processes without the glucose. The PNIPAM@ZnO/C temperature sensitive materials were prepared via the cross-linking polymerization technology, in a typical process, 0.5 g ZnO/C nanocomposite was put into 60 mL ethanol with 10 mL methacryloxy propyl trimethoxyl silane (MPS), the mixture was reacted at 50 ℃ for 12 h with the N2 protection and the surface modified ZnO/C was obtained. Then 0.2 g NIPAM and 0.03 g N, N'-methylenebisacrylamide (MBA) were dissolved into 50 mL deionized water, and the 0.2 g surface modified ZnO/C was added into above solution, subsequently 1 mL 0.01 mol/L ammonium peroxydisulfate solution was added the above solution, and the mixture was reacted at 60 ℃ for 12 h with N2 protection. Last, the precipitant was collected and washed with water and ethanol, the precipitant was dried at 50 ℃ for 10 h in the oven and the PNIPAM@ZnO/C was obtained.
Then, the physicochemical properties of as-prepared photocatalysts were characterized. The morphologies of samples (Fig. 1) were examined with S-4800 scanning electron microscopy (SEM) with the equipment of energy-dispersive X-ray analysis (EDS) (HITACHI, Japan). The nano-ZnO particles (Fig. 1a) are agglomerate bulk. ZnO/C (Fig. 1b) exhibits the hamburger-like structure. In order to confirm the carbon element in the ZnO/C, the EDS of ZnO/C was also analyzed (Fig. S1 in Supporting information). It is clearly found that the carbon element is successfully doped into ZnO. PNIPAM@ZnO/C (Fig. 1c) also exhibits hamburger-like structure. The elements of PNIPAM@ZnO/C were further investigated via EDS (Fig. 1d). It can be easy found Zn, C, N, O elements in the PNIPAM@ZnO/C photocatalyst.
|Fig. 1. SEM images of the as-obtained (a) ZnO, (b) ZnO/C, (c) PNIPAM@ZnO/C and (d) corresponding EDS spectrum of PNIPAM@ZnO/C.|
In order to further confirm the PNIPAM covered ZnO/C, Fourier transform infrared spectra (FT-IR) were used to analyze the surfaced groups of as-prepared samples by a Nicolet Nexus 470 FTIR spectrometer (Nicolet, USA) with KBr pellets in the 4000–400 cm-1 region (Fig. S2 in Supporting information). It can be clearly found three peaks at 2973 cm-1, 2930 cm-1 and 2867 cm-1 in PNIPAM@ZnO/C, and it can be attributed the stretching vibration of —CH3 and —CH2—. Also the peaks at 885 cm-1 and 1629 cm-1 were caused via bending vibration of N—H and stretching vibration of C＝O, respectively. It suggests that the temperature sensitive PNIPAM layer is successfully covered ZnO/C photocatalyst.
The crystal structure was analyzed by the powder X-ray diffraction (XRD model MAC Science, Japan) with Ni-filtrated Cu-Kα radiation. As shown in Fig. 2a, it can be obviously found the peaks of ZnO at 2θ = 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.3°, 68.0°, 69.1°, 72.6° and 77.1° which corresponding to the crystal face of 100, 002, 101, 102, 110, 103, 200, 112, 201, 004 and 202 (JCPDS No. 36-1451), respectively . There is a wide peak around 2θ = 12°, and the reason may be due to the amorphization of the samples which prepared via the hydrothermal method. After introducing carbon element into ZnO, it can be found the peaks of ZnO/C are wide and the intensities of these peaks are weak, indicating that the crystallinity of ZnO/C is obviously decreased. It further certifies that the carbon from glucose could prevent nano-ZnO agglomerate bulk. When grafted the PAIPAM, the XRD peaks of PNIPAM@ZnO/C have not changed, indicating the PNIPAM could not change the crystal structure.
|Fig. 2. XRD patterns (a) and photoluminescence (b) (Inset: Expanded photoluminescence excitation spectra of corresponding samples) of as-prepared ZnO, ZnO/C and PNIPAM@ZnO/C.|
It is widely known that the intensity of photoluminescence (PL) can revealed the recombination extent of photo-electrons and holes, the PL tests were obtained on a F4500 (Hitachi, Japan) photoluminescence detector. The results exhibit in Fig. 2b, which can be clearly seen that PNIPAM@ZnO/C has weakest intensity of PL, indicating that C-doping and PNIPAM graft could affect the PL of composite, the lower recombination of photo-electrons and holes, the higher transfer efficiency and long life of photo-electrons. Suggesting PNIPAM@ZnO/C possesses the higher photocatalytic activity [16, 17]. In order to understand the photoelectronic performance of as-prepared photocatalysts, photocurrents of ZnO, ZnO/C and PNIPAM@ZnO/C were tested via a CHI852C electrochemical station with a solar simulator (Newport 69920, 300 W Xenon lamp). The results were showed in Fig. S3 in Supporting information. All the samples exhibit photocurrents under the light irradiation and without photocurrents under dark, especially the intensity of PNIPAM@ZnO/C is the strongest and showed highest photoelectronic performance [18, 19]. It suggests that the photoelectrons possess long life time and high separation of photoelectrons and holes which corresponded to the results of PL, indicating the photocatalyst of PNIPAM@ZnO/C will possess highest photocatalytic performance.
In order to evaluate the controllable photocatalytic activity of PNIPAM@ZnO/C, the photocatalytic activity of as-prepared photocatalyst was investigated by degradation of tetracycline (TC). In a typical process, 0.04 g photocatalyst and 100 mL of 20 mg/L TC were put into the container and kept in a dark room under stirring for 30 min to reach adsorption equilibrium. 4 mL solution was taken out in the time before lamp was turn on. The solution in the container was irradiated by a 100 W tungsten lamp as visible light source. At every 10 min interval, 4 mL solution in photocatalytic reaction was taken out. The above solution was centrifuged and analyzed by UV–vis spectrophotometer. The removal rate was calculated using the following formula (1)
Where C0 denotes the initial concentration and Ci denotes concentration of t time.
The contrastive photocatalytic degradation TC experiments were investigated under 25 ℃ constant temperature with different photocatalysts. The results in Fig. 3a show that the degradation rate of ZnO/C is higher than that of ZnO, the reason may attribute to that the carbon element doping could enhance the adsorption of light and change the morphology of ZnO to the hamburger-like structure, which is better for photodegradation of TC. Comparing to ZnO/C and ZnO, PNIPAM@ZnO/C possesses highest photocatalytic degradation rate of TC. The reason may be due to that there were synergistic effect between PNIPAM (adsorption) and ZnO/C (photocatalysis) which could enhance the photocatalytic activity, also there are similar results have been previously reported [7, 20, 21]. In order to investigate the temperature sensitivity of PNIPAM@ZnO/C, the degradation of TC was carried out under different temperature (from 20 ℃ to 45 ℃). The results in Fig. 3b show that the temperature could seriously affect the photocatalytic degradation. Especially, at the lower temperature, the photocatalytic activity exhibits highest, the reason may be due to the high hydrophily at low temperature which formed the hydrogen bond on the PNIPAM chain, and it could expose more sites of ZnO/C. Indicating that the as-obtained PNIPAM@ZnO/C photocatalyst possesses excellent temperature sensitivity. In order to further confirm the controllable degradation of TC with temperature sensitive photocatalyst of PNIPAM@ZnO/C, the photodegradations of TC without photocatalyst were carried out under different temperatures, the result shows in the Fig. S4 in Supporting information. It can be clearly seen that the degradation rate of TC is virtual zero, indicating that there is no selfdegradation of TC at different temperatures. Therefore, the photocatalytic degradation process of TC is obviously controllable with PNIPAM@ZnO/C.
|Fig. 3. (a) Evolution of TC relative concentrations (C/C0) with irradiation time for blank, photolysis, ZnO, ZnO/C and PNIPAM@ZnO/C under constant temperature about 25 ℃; (b) Evolution of TC relative concentrations (C/C0) with irradiation time for PNIPAM@ZnO/C under different temperature conditions; (c) Recycled experiments of photocatalytic degradation of TC by PNIPAM@ZnO/C below LCST (20 ℃) and above LCST (45 ℃); (d) Effect of different scavengers on the degradation of TC using PNIPAM@ZnO/C at 25 ℃.|
In order to investigate the stability of PNIPAM@ZnO/C, the cycled experiments were carried out below LCST (20 ℃) and above LCST (45 ℃), respectively. The results shown in Fig. 3c can be easy to find the photocatalytic activity of fourth cycle is slight reduction, indicating the PNIPAM@ZnO/C photocatalyst possesses high stability. Moreover, the stability of ZnO was also investigated below and above LCST (Fig. S5 in Supporting information). The photocatalytic activity of ZnO is sharply decreased along with the cycled times. It further confirms that PNIPAM modified could enhance the stability of composite photocatalyst.
The different sacrificial reagents (Benzoquinone (BQ), for ·O2-; triethanolamine (TEOA) for h+; isopropanol (IPA) for ·OH) were added into the reacted system under the same conditions [22, 23] which could better to understand the photocatalytic processes. The results show in Fig. 3d, when added the sacrificial reagents of TEOA, IPA and BQ, the degradation rates decreased to 28.16%, 52.49% and 41.82%, respectively. It indicates that the species of ·O2- and h+ play key roles in the photocatalytic degradation processes, and ·OH also influences the photocatalytic degradation rate.
According to our experimental results, the mechanism of photocatalytic degradation with temperature sensitive photocatalyst is proposed. When the temperature decreases below the LCST, the N atom of —CO—NH— in the PNIPAM could form hydrogen bond with CH2O molecule, thus the surface of PNIPAM@ZnO/C exhibits hydrophily. Moreover, the PNIPAM chain extends in the solution, the surface of ZnO/C is also exposed to the solution which is better for the photocatalytic processes. Contrary to the below LCST, the temperature increases above LCST, hydrogen bond is destroyed, the lipophilicity of (CH3)2CH— is exposed, therefore the surface of PNIPAM@ZnO/C shows hydrophobicity. In this case, the ZnO/C is fully covered by the PNIPAM which limited the photocatalytic processes, as a result the photocatalytic degradation rate is decreased. And the controllable photocatalytic mechanism of degradation processes under different temperatures can be described in Fig. 4.
|Fig. 4. Schematic of the photocatalytic processes with PNIPAM@ZnO/C and the reversible temperature sensitive phase transitions of grafted PNIPAM chains.|
In summary, the temperature sensitive photocatalyst of PNIPAM@ZnO/C was successfully prepared via the cross-linking polymerization technology. The photocatalytic degradation of TC shows that the PNIPAM@ZnO/C has highest photocatalytic activity and stability. Moreover the PNIPAM@ZnO/C photocatalyst exhibits excellent temperature response, and possesses effectively controllable photocatalytic performance, the photocatalytic activity can be limited above LCST; and photocatalytic degradation rate can be improved below LCST.Acknowledgments
We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 21576125, 21407064), the Natural Science Foundation of Jiangsu Province (No. BKBK20151349), China Postdoctoral Science Founsation (Nos. 2017M611716 and 2017M611734), Six talent peaks project in Jiangsu Province (No. XCL-014), Zhenjiang Science & Technology Program (No. SCH2016012).Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.09.067.
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