初中英语人教版八年级上册Unit 1 Period 1.docx

AbstractIn this paper, we demonstrate the TiO2 microspheres via a facile solvothermal process using self-fabricated TiO2 precursor and a series of TiO2/carbon quantum dots(CQDs) hybrids via a in situ hydrothermal method from a green carbon source, glucose. XRD, HRTEM, FTIR and XPS analyses showed that carbon has doped in the as-synthesized TiO2 microspheres which led to local energy level in the band structure and a valence band tails to absorb visible light. Moreover, the photocatalytic activities of these products were evaluated by the photodegradation of Rhodamine B (RhB) under visible light irradiation. The improved photocatalytic activities were observed for the TiO2/CQDs samples under visible light degradation, in which the TiO2/CQDs-2 sample obtained with a glucose concentration of 0.02%wt exhibited the highest rate. It was attributed to the CQDs act as a charge separation center for impeding the recombination and prolonging the life time of electron and hole pairs, and an absorption site for the pollutant molecules.Keywords: carbon quantum dot (CQD), TiO2 microspheres, carbon-doped, photocatalytic activity.IntroductionIn recent years，the expanding organic environmental pollution has became the crucial issue which not only influence human beings healthy, but also in turn hinder the social and economic development. Therefore, several researches have been focused on and a great mount of semiconductors(such as TiO21, ZnO2, SnO23, ZnS, CdS4 and MoS25) have been carried out as efficient photocatalysts in environmental purification and energy conversion due to their unique chemical and electronic structures6. Among those semiconductors, titanium oxide (TiO2), n-type with the band gap of about 3.2 eV, attracts much attention and become one of the most promising candidates for dye degradation due to its low-cost, non-toxic property and chemical-photo stability7-9. Especially, TiO2 microspheres were fabricated to improve photocatalysis for its uniform morphology with less agglomeration and large surface area10, 11. However, the poor solar energy absorption and rapid recombination of photo-generated electrons(e-) and holes(h+) greatly limited the efficiency of photocatalytic activities7, 9. Therefore, energy band engineering such as doping with mental12 or nonmental elements9, hybrid and hetero-structure control such as modified with noble metals10, coupling with other semiconductors5 and supporting with other materials13, 14 were investigated to narrow the band gap and restrain the recombination of electron and hole pairs, respectively. Among these techniques, coupling TiO2 with carbon materials has been deemed to be an valid approach.8, 14, 15Owing to the performance that play an important role on strengthen the pollutant molecules absorption and suppress photo-generated carriers recombination, carbon materials, including carbon nanotubes16, 17, graphene2, 4, 18, fullerene19 and amorphous carbon14, 20 have been used to achieve TiO2/carbon materials hetero-structures in recent years. However, among these carbon materials, the drawbacks are high cost and complex preparation process. Therefore, carbon quantum dot (CQD) gradually becomes a rising star with size below 10 nm due to its high natural abundance, high aqueous solubility, environmentally benign and inexpensive nature7, 8, 15, 21-24. More importantly, there are researches reported that CQDs showing both down and up-conversion fluorescences and also performed as an electron reservoir21, 23, 24. Very recently, to achieve natural visible light exposed photocatalytic activity, many attempts were made to obtain CQDs nanostructures with other semiconductors. Although lots of researches established excellent visible-light-driven photocatalytic activities when hybrid with CQDs and found an optimum concentration of CQDs to achieve the best performance, however, in most reported systems, the critical role of CQDs in the enhanced photo degradation has not been studied in detail, and the mechanism of the degradation has also not been investigated25.Herein, this work, TiO2 microspheres have been synthesized via a facile solvothermal process using the self-fabricated TiO2 precursor made by sol-gel method. Moreover, different concentrations of CQDs have been applied to modify the TiO2 microspheres via a green in situ hydrothermal synthesis for the OH groups on the surface of TiO2 microspheres act as the nucleation of the carbon dot. The hydrothermal method using glucose as precursor may greatly influence its surface chemical states and defects, which in turn determines the optical and photocatalytic properties of different concentrations of CQDs modified TiO2 microspheres14. In this system, the as-prepared TiO2 microspheres turned out to be carbon-doped due to the sol-gel process, leading to effectively visible-light-response. Moreover, not only the optimum concentration of CQDs has been found out, but also the roles of different concentrations of CQDs influence on the microstructure and properties of the hybrids have been investigated.Experimental：1. Synthesis of TiO2 microspheres The TiO2 precursor was fabricated via a sol-gel method as follows. In detail, the 13.60 g of tetrabutyl titanate(TBOT), as the titanium source, was dissolved in absolute ethanol with 30 min of magnetic stirring, and marked as solution A; The 0.60 g of glacialaceticacid, 2.00 g of acetylacetone(AcAc) and 4.32 g of deionized water(DI) were put into some ethanol respectively, and dispersed with ultrasonic for 10 min, and noted as solution B; Then the solution B was added into solution A drop by drop. After magnetic stirring for several hours, the mixture was aged for 24 h and then dried at 80 for 12h, and was collected for further use. TiO2 microspheres were prepared via a typical hydrothermal method. 0.3 g as-prepared TiO2 precursor was dissolved into 60 mL of 3wt% HCl/EtOH aqueous solution under continuous stirring. Then the obtained mixture solution was transferred into a Teflon-lined autoclave with 100 mL capacity, which was heated at 150 for 6 h. When cooled to room temperature, the as-prepared yellowish products were washed with ethanol and DI several times and then centrifuged and dried at 80 for 12h. Finally, the TiO2 microspheres were obtained. 2. Preparation of TiO2 microsphere/CQDs nanocomposites0.1 g ground TiO2 microspheres were dispersed in 10 mL DI and stirred vigorously for 30 min. 15 mL aqueous solution of different concentration of glucose(0.01wt%, 0.02wt%, 0.05wt%, 0.1wt%) solutions were then added into the suspension and stirred for 30 min respectively. Then, the obtained mixed suspensions were transferred into a 50 mL Teflon-lined autoclave and maintained at 90 for 4 h. The following products were collected by centrifugation, washed with DI and ethanol several times, and then dried as the same to the TiO2 microspheres. The final product were noted as TiO2/CQDs-1, TiO2/CQDs-2, TiO2/CQDs-3 and TiO2/CQDs-4, respectively.3. Photocatalytic activityThe photocatalytic measurements were carried out at room temperature by the degradation of Rhodamine B (RhB) under visible light irradiation with a 500W Xeon-lamp equipped with a 420nm cut-off filter. In a typical experiment, 100mg of photocatalyst was dispersed into 50 mL of 10 mg/L RhB aqueous solution. Before light on, the suspension was kept in the dark under stirring for 30 min to reach an adsorption-desorption equilibrium. During illumination, a circulation of water through an external cooling coil was conducted to maintain the temperature at about 25. After irradiation for an appropriate interval, the reaction solution was centrifuged and concentration of RhB was determined by an UV-vis spectrophotometer for calculating the photocatalytic degradation rate of RhB 4. CharacterizationThe products were analyzed by X-ray diffraction(XRD, PANalytical Xpert MPD PRO) using Cu K radiation(=0.154 nm). The morphological and microstructural details of the samples were examined by using a field emission scanning electron microscope (SEM, JSM-7000F) and a transmission electron microscope (TEM, Tecnai F30 G2) images. Fourier transform infrared (FTIR) spectroscopy was acquired on a Bruker TENSOR27 FTIR spectrometer to confirm different functional groups on the surface. X-ray photoelectron spectroscope(XPS, Kratos AXIS Ultra DLD) was carried out to analyze surface chemical states of these composites. All binding energies were calibrated with surface adventitious carbon of 284.6 eV.Results and discussion Fig.1. XRD patterns of the as-prepared TiO2 and TiO2/CQDs hybrids with different CQD loading amountsThe crystal of TiO2 which is consisted of anatase or rutile may dramatically effect on the photocatalytic activity and definitively determine its practical activity. A systematic XRD study was carried out to analyze the crystal structure of the obtained samples and the results are shown in Fig.1. The distinctive peaks at 2=25.3, 37.8, 48.1, 53.9, 55.1, 62.7, 68.8 and 70.3 can be indexed to the (101), (004), (200), (105), (211), (204), (116) and (220) planes of the anatase phase TiO2 (JCPDS#21-1272), respectively. For the CQDs modified TiO2 samples, no characteristic peak of carbon (at about 26) can be detected, which may be attributed to the low CQD content in these composites. In addition, no other peaks are detected within the instrument scope, which indicating the high purity of the as-prepared samples.Fig.2. SEM(a) and EDS(b) images of the TiO2 microspheres. SEM images of TiO2/CQDs hybrids (c-f): TiO2/CQDs-1, TiO2/CQDs-2, TiO2/CQDs-3 and TiO2/CQDs-4Fig.2. presents the corresponding SEM images of the TiO2 microspheres and TiO2/CQDs composites in high magnifications. As shown in Fig.2a, the diameter of the TiO2 is of 5 m in size with a uniform morphology and a well dispersion, and its surface is very smooth. The energy dispersive X-ray spectroscopy (EDS) analysis of the as-prepared TiO2 (Fig.2b) shows that the sample consists of C, O, Ti elements, revealing that carbon has doped into TiO2 microspheres. Furthermore, when glucose, which was chosen as the green precursor for carbon dot source, was added, the products instantly turned from white to yellow or brown colour according to the glucose amounts after hydrothermal treatment, implying the yield of TiO2/CQDs composites, Fig.2c-2f show the morphologies of the carbon dot modified TiO2 samples (TiO2/CQDs-1, TiO2/CQDs-2, TiO2/CQDs-3 and TiO2/CQDs-4).Fig.3. Photodegradation of RhB by using different samples under visible light irradiation(a), and variations of ln(C/C0) versus irradiation time with different samples(b)The photocatalytic activities of these as synthesized samples were evaluated from the degradation of Rhodamine B (RhB) in solution under visible light irradiation, which is shown in Fig.3a. C0 is initial concentration of RhB while C is the corresponding degradative concentration. Considering that RhB may possibly self-degrade without catalyst, a blank experiment was established as a control line. Before light on, an adsorption-desorption equilibrium is carried out. The result reveals that nearly no degradation is detected in the control group, and all the obtained samples exhibit significant response to the visible light compared to the commercial TiO2(p25). Recently, in comparison to the as-prepared TiO2, the photocatalytic activities of hybrid samples are significantly improved, in which the TiO2/CQDs-2 shows the best performance: the photocatalytic activity of TiO2/CQDs-2 increase slightly when the mass ratio of carbon source glucose increase from 0.01%wt to 0.02%wt. However, the photocatalytic activity clearly decreases when the mass ratios further increase to 0.05%. To have a better understanding of the reaction kinetics of the RhB degradation, the experimental data were evaluated by a pseudo-first-order kinetic model: ln(C/C0)=-kt, the rate of photodegradation could be described by the slope coefficient k. As displayed in Fig.3b, the TiO2/CQDs-2 sample possesses the optimum value and was almost 3 times as high as that of as-prepared TiO2.Fig.4. TEM image (a) and High resolution TEM image (b) of TiO2/CQDs-2 sample.In order to further confirm the formation of the microstructure of the carbon dot modified TiO2, TEM images are explored. As displayed in Fig.4, it can be seen that some dark dots with an average size of dozens of nanometers are distributed on the relatively microspheres, implying that carbon dots are deposited on the surface of TiO2. Moreover, an obvious crystalline structure is observed, with an interplaner spacing of 0.21 nm which is assigned to in-plane (100) facet of disordered graphitic carbon8, 26. The observed lattice constant of the TiO2 crystallite was detected to be 0.34 nm corresponding to the (101) crystal plane of anatase TiO2 (JCPDS#21-1 272).Fig.5. FT-IR of as-prepared TiO2 and TiO2/CQDs-2Different functional groups on the surface of as-prepared TiO2 and TiO2/CQDs-2 samples were confirmed from FTIR. The band over the range 3200-3600 cm-1 is attributed to the O-H stretching of adsorbed water molecules and the surface hydroxyl groups on TiO2, which is determined to be crucial in the photocatalytic process of the catalyst due to these groups can inhibit the recombination of photo generated charges and holes to produce reactive oxygen species27, 28 . The absorption band at 1630 cm-1 and 1442 cm-1 were signed to stretching vibrations of C=C and C-O group respectively13, indicating the residual C elemental in the catalyst surface. The presence of characteristic bands between 1000-4000 cm-1 correspond to the asymmetric and symmetric stretching vibration of C-O-C21. As pure P25 powder showed a low frequency band around 690 cm-1, the existence of the broad absorption below 1000 cm-1 is confirmed to the synergistic effect of a combination of Ti-O-Ti and Ti-O-C vibrations29, 30, which is consistent with the XPS result.Fig.6. XPS survey scan of TiO2 and TiO2/CQDs-2(a), and high resolution XPS scan of Ti 2p(b), C 1s(c), O 1s(d) over TiO2 and TiO2/CQDs-2 samples.To further investigate surface chemical composition and bonding configuration of TiO2 and TiO2/CQDs-2 composites, the X-ray photoelectron spectroscopy (XPS) was employed and the results are shown in Fig.6. The full-scale XPS pattern of samples is observed in Fig.6a, When calibrated with surface adventitious carbon (C-C/C=C) of 284.6 eV, peaks responding to Ti 3s,Ti 3p, Ti 2s, Ti 2p, C 1s and O 1s were founded in both TiO2 and TiO2/CQDs-2 composites. Fig.6b-6d are the high-resolution XPS spectra of Ti 2p, C 1s and O 1s, respectively. Ti 2p peaks were located at 458.4 eV and 464.1 eV (Fig.6b) with a spin-orbital doublet splitting (Ti 2p3/2-Ti 2p1/2) of 5.7 eV in as-prepared TiO2 sample, which implies an oxidation state of Ti4+. As for the TiO2/CQDs-2 sample, the Ti 2p peaks exert a slight red shift of 0.2 eV compared to the TiO2, which conceivably indicates the coupling of TiO2 and carbon. It can be seen that C 1s peak of TiO2 could be resolved into three different parts using Shirley-type base line with a combination of a Gaussian (80%) curve and a Lorentzian (20%) curve (G-L), which were assigned to C-C/C=C bond (284.6 eV), C-O bond (286.2 eV) and C=O (288.5eV), respectively. When TiO2 was modified with carbon, an additional peak located at 282.7 eV was detected, which probably arise from the formation of Ti-C bonds28. To further investigate the oxide state, O 1s was measured (Fig.6d). Three peaks were observed at the binding energies of 529.7 eV, 531.5 eV and 533.1 eV in the spectrum of as-prepared C-TiO2 samples, which are thought to Ti-O bonds, Ti-O-C bonds along with the Ti-OH groups, respectively. The presence of Ti-O-C bonds indicated that partly carbon has existed in the lattice of TiO2 microspheres but not replace oxygen atom 27, which is correspond to the valence band VB XPS (Fig.7) result below. Furthermore, in the TiO2/CQDs-2 spectrum, a notable difference compared with the TiO2 sample is the presence of a peak located at 527.6 eV, which may be assigned to C-Ti-O. Fig.7. Valence band XPS spectra of (a) as-prepared TiO2, (b) TiO2/CQDs-2, the calculated band structures of (a) as-prepared TiO2, (b) TiO2/CQDs-2.VB-XPS was obtained to investigate the property of the band structure, which can probe the total density of state (DOS) distribution in the valence band31. The valence band maximum (VBM) of the obtained TiO2 is determined to be 2.12 eV, and the valence band tail of additional electronic states at about 0.25 eV is induced ascribed to the structural disorders32. Notably, the result indicate that carbon atoms have doped into TiO2 and induced a broadening of the valence band and lead to narrowing of the band gap. Thus, the obviously visible-light-response of the TiO2 on photodegradation can be associated to the carbon doped in lattice TiO2 corresponds to the C-Ti-O bond shown in XPS. Meanwhile, when theTiO2 microspheres hybrid with CQDs, the valence band maximum (VBM) consequently shift to 1.61 eV whereas the band tails is at about -0.26 eV. By analyzing the data, a shift of about 0.51 eV of the VBM was detected upon hybrid TiO2 with CQDs and consequently, the corresponding band gap narrowing values of both TiO2 and TiO2/CQDs-2 should be the same (1.87eV). Thus, CQDs make a further enhancement attributed to the adsorption on the dye and less recombination of the electron/hole pairs, which associated with the band structure discussed in Fig.7b. Scheme.1. Band structure of the as-prepared TiO2 and schematic of the separation and transfer of photogenerated charges in the TiO2/CQDs samples.Based on the above results, the schematic diagram (Scheme.1.) is carried out in which the band structure of as-prepared TiO2 and occurrence of carries transfer of TiO2/CQDs are s