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TiO2 heterostructure for efficient gas-solid phase CO2 photoreduction
Applied Catalysis B: Environmental 269 (2020) 118810
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Ultra-fast construction of plaque-like Li2TiO3/TiO2 heterostructure for efficient gas-solid phase CO2 photoreduction
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Wei Bi, Yanjie Hu*, Nan Jiang, Ling Zhang, Hao Jiang, Xing Zhao, Chengyun Wang, Chunzhong Li* Shanghai Engineering Research Center of Hierarchical Nanomaterials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-fast reaction process Ultra-high temperature Plaque-like Li2TiO3/TiO2 heterostructure Photocatalytic CO2 reduction Catalytic behavior and mechanism
Exploring a facile and universal strategy to fabricate an efficient heterostructure is a big challenge for achieving highly active and selective CO2 photoreduction. Herein, a plaque-like Li2TiO3/TiO2 heterostructure was in-situ constructed by the gradual intercalation of Li ions into the lattice of TiO2 from the outside and eventually formed the Li2TiO3 phase on the surface of TiO2 in an ultra-high temperature about 2000 K within several milliseconds. This novel structure has a significant improvement in the adsorption capacity of CO2 and reduction of the oxygen defect trap by introduction of Li species. Furthermore, owing to the built-in electric field directed from Li2TiO3 to TiO2, electrons are forced to move to the oxygen atom sites in Li2TiO3, which helps the plaque-like Li2TiO3/TiO2 heterostructure photocatalyst achieve nearly 3 times performance improvement with about 100 % selectivity towards CO. Theoretically and experimentally, we proposed a possible catalytic behavior and mechanism that Li2TiO3/TiO2 heterostructure will choose a shorter pathway through intermediate formate conversion to CO, as the free energy reduce by 26.53 % from 0.98 eV to 0.72 eV. This work gives a new insight for constructing heterostructure in an ultra-fast reaction process, and provides a rational reference for developing highly efficient photocatalysts towards CO2 reduction.
1. Introduction In recent years, photocatalysis has been a promising strategy to address the problems resulting from fossil energy consumption and global warming [1–4]. CO2 photoreduction is one of novel method could directly reduce atmospheric CO2 and H2O to compounds with high added value by the use of sunlight under the aid of a semiconductor material. Since no pollutants are involved in the reaction, no waste is generated, and the driving energy originates from the inexhaustible solar energy, this method can be recognized as an environmentally friendly chemical treatment [5–8]. In this process, TiO2 is considered as one of the most research-intensive nano-photocatalyst material because of its high chemical stability, thermal stability, nontoxicity, environmental friendliness and low cost [9–11]. However, the wide band gap of 3.2 eV causes the poor utilization of sunlight for TiO2, mainly in the visible region. Meanwhile, the nature of the intrinsic TiO2 also determines low charge separation efficiency and low charge utilization, which could also lead to severe electron-hole recombination. In order to solve such two key issues, in recent decades, several effective strategies have been developed to improve photocatalytic CO2 ⁎
reduction performance, including doping [12–14], morphological and structural design [15,16], multi-component coupling [17,18], etc. Among them, the construction of a reasonable heterogeneous structure is one of the most effective and promising strategies. However, how to design an effective heterostructure becomes a challengeable work to research efficient photocatalyst. A conventional gas phase synthesis method for preparing multifunctional nano-materials, FSP, has unique advantages in the preparation of heterostructure materials because of the ultra-high reaction temperature, which is conducive to element surface enrichment, or phase separation [19,20]. Li et al. [21] prepared a very unique Janusstructured Fe2O3/SiO2 heterostructure nanosphere by FSP and intensively explained the formation mechanism of the two phases combined with subtle experiment and the law of phase diagram. Gu et al. [22] by depositing SnO2 on TiO2 nanocrystals to form TiO2@SnO2 core/shell particles (TSN) with FSP, achieved a significant increase to an open circuit voltage of 0.59 V and an efficiency of 3.82 %, which proved to be attributed to the enhanced electron injection arising from decreased interfacial charge recombination losses and improved electron transport. Hu et al. [23] based on a MgO/TiO2 model, pointed out
Corresponding authors. E-mail addresses: [email protected] (Y. Hu), [email protected] (C. Li).
https://doi.org/10.1016/j.apcatb.2020.118810 Received 8 December 2019; Received in revised form 18 February 2020; Accepted 24 February 2020 Available online 25 February 2020 0926-3373/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (a) TEM images, (b) HR-TEM images of plaque-like Li2TiO3/TiO2 heterostructure, and (c) locally enlarged HR-TEM of heterostructure regions. Schematic illustration of (d) the synthesis process and the growth behavior of plaque-like Li2TiO3/TiO2 heterostructure by FSP.
2. Experimental
the formation mechanism of the outermost layer of TiO2 nanoparticles was located at the difference in reaction rate of precursors during the ultra-high temperature synthetic process. The surface MgO layer could effectively adjust the band structure of TiO2 nanoparticles while improving the maximum photocurrent and maximum photovoltage. This work revealed that improvements were largely attributed to red-shifted light absorption, and rapid photoelectron injection into the conduction band. In this paper, the plaque-like Li2TiO3/TiO2 heterostructure was insitu prepared in an ultra-high temperature (∼2000 K) reaction process. The introduction of Li2TiO3 increase the probability of contact between CO2 and the active sites on the catalyst, illustrated as a key intermediate step in photocatalytic reduction of CO2. On the other hand, Li species are attributed to the partial reduction of Ti4+ to Ti3+, which is beneficial to the passivation of oxygen vacancies, and result in improved conductivity and enhanced electrons separation and transmission. Owing to the rational surface modification, the optimal the plaque-like Li2TiO3/TiO2 heterostructure photocatalyst achieved nearly 3 times performance improvement of the intrinsic TiO2 with nearly 100 % selectivity. What is more, the catalytic behavior and mechanism of the plaque-like Li2TiO3/TiO2 heterostructure are intensively explained on the electronic level, by combining the first-principle calculation based on density functional theory (DFT) with systematic experiments. This work will provide an insight for development of high-performance CO2 photoreduction catalysts by effective heterostructure design.
2.1. Preparation The catalysts in this work were prepared by flame spray pyrolysis (FSP) method. Briefly, the tetrabutyl titanate (TBT) and the lithium acetate (1.5 wt% Li species) was dissolved in 100 mL of ethanol to obtain a uniform precursor solution at a concentration of 0.5 M. With the aid of the syringe pump, the precursor solution reached the tip of the nozzle at a flow rate of 5 mL min−1, and then, the precursor solution was rapidly dissipated into a micron-sized mist droplet cutting by a high-speed O2 airflow. Meanwhile, the atomized precursor droplets were ignited with the circular H2/O2 diff ;usion flame, wherein the flow rates of H2 and auxiliary air were controlled at 380 L h−1 and 1.0 m3 h−1, respectively. The resulting powder was collected by glass fiber, and the remaining conditions were consistent with the above experiments.
2.2. Characterization The Li2TiO3/TiO2 heterostructure was directly observed by the transmission electron microscopy (TEM, JEM-2100) and high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F20 S-Twin (FEI)). The lattice structure and the change in phase ratio were characterized by X-ray diff ;raction (XRD, Bruker D8 Avance) with Cu-Kα filament that X-ray wavelength of 1.5406 Å. The chemical state of Titanium (Ti 2p), Oxygen (O 1s) and Lithium (Li 1s) were further confirmed by the X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 X-ray photoelectron spectrometer, equipped with an Al 2
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peaks of anatase (JCPDS: 04-0477) and rutile (JCPDS: 04-0551) located at 25.35° and 27.46° correspond to the respective (101) and (110) crystal planes. Those characteristic peaks are observed in all as-prepared samples, indicating that the TiO2 prepared by FSP possesses a mixed phase structure. Importantly, as the content of Li species increase from 0 to 5 wt%, the composition of the anatase and rutile phases change significantly. The detailed variation data are listed in Table S2. Furthermore, the locally enlarged patterns indicate that the peak at 18.47° in region Ⅰ is ascribed to the (002) crystal plane of Li2TiO3, and the peak at 43.58° in region Ⅱ is ascribed to the (−133) crystal plane of Li2TiO3. Besides, a slight increase of diffraction angle 0.08°can be observed, which indicates the Li species do reduce the lattice spacing of TiO2. Moreover, those two peaks are gradually obvious with the increase of Li species content shown in Fig. S4. The results of XRD patterns show that the introduction of Li species not only promote the crystal phase transfer from anatase phase to rutile phase, but also form a coating layer of Li2TiO3 on the surface of TiO2 in combination with TEM images. Notably, the state of the coating, plaque-like shape or complete cover, is related to the introduction content of Li species. The XPS characterization was used to obtain information on the surface of the sample to clarify the compositional results of the element species in the sample as shown in Fig. 3. Fig. 3a shows the deconvolution of the Ti 2p XPS spectra for the pure TiO2 and plaque-like Li2TiO3/TiO2 heterostructure. For the pure TiO2, the binding energy at the position of 458.8 eV corresponds to Ti4+ 2p3/2, and the binding energy at the position of 464.6 eV corresponds to Ti4+ 2p1/2 [24]. However, the Ti 2p3/2 and Ti 2p1/2 peaks in Li2TiO3/TiO2 exhibit attractive negative shift of 0.2 eV with respect to that of the TiO2, which indicates that after the introduction of Li species, the original state of the electron cloud for Ti 2p orbit is changed, and the electron tends to flow around the Ti species, which is apparently reduced in binding energy [25]. In addition, it is observed in both sets of samples that a smaller shoulder peak belonging to Ti 2p3/2 appears at a binding energy of 460.2 eV, which corresponds to a non-stoichiometric TiOx [26]. And the relative ratio of peak integral area in Li2TiO3/TiO2 increased slightly compared with TiO2, indicating that Li species can also affect the valence state ratio of Ti species. The enlarged TiOx area is shown in Fig. S5 and the detailed peak parameters are listed in Table S3. Fig. 3b shows the deconvolution of the O 1s XPS spectra, from which similar changes in Ti 2p spectra can be observed. The peak at 530.2 eV corresponds to the TieO bond in the lattice structure. It can be found that Li species is resulting in the binding energy moving towards the low energy for about 0.15 eV, indicating that the lattice O atoms in the TiO2 will get electrons from nearby. In addition, there is a shoulder peak near the main peak at around 531.6 eV, which corresponds to the oxygen interaction with Li [25] and the adsorbed oxygen hydroxyl species [27,28] on the surface of TiO2. Comparatively, it is not difficult to find that the peak integral area of Li2TiO3/TiO2 is much larger, which is mainly the result of the synergistic effect of Li species on the catalyst surface, that is, the signal of oxygen interaction with Li will occupy part of the peak area. On the other hand, the modification of the surface of the catalyst by Li species can induce more adsorption sites of oxygen hydroxyl species, which together show a stronger signal peak. Intuitively, Li 1 s XPS spectra is shown in Fig. 3c. Clearly, the Li2TiO3/ TiO2 sample shows a broad peak at a binding energy of 54.9 eV, which is a characteristic peak of Li+ species [29], indicating that Li was successfully introduced into the surface of TiO2, and the characterization results are consistent to the TEM and XRD analysis mentioned above. Notably, information about Ti 3 s can also be observed in the spectrum. Literature indicates that the peak in the 51.5 eV–61.0 eV interval represents the presence of Ti3+, while the peak around 62.1 eV represents the presence of Ti4+ [24]. It can be seen from the spectra that the peak shape of Li2TiO3/TiO2 is wider than that of TiO2 as well as closer to the low energy region, indicating that the Li2TiO3/TiO2 possesses a higher ratio of Ti3+. This conclusion is consistent with the TiOx content analysis in Fig. 3a. More specifically, the content of low-valent
Kα source (1486.6 eV)). The binding energies were calibrated at the C 1s peak of 284.8 eV. Electron paramagnetic resonance (EPR, Bruker EMX-8/2.7 spectrometer operating at about 9.8 GHz.) test was carried out at 25 °C to obtain the information of the low-valent Ti species on catalyst surface. The UV–vis diffuse reflectance spectra (UV–vis DRS, U3900H Spectrophotometer) of the samples were acquired within the wavelength range of 200–800 nm. Raman spectroscopy (LabRAM HR Evolution) was used to analyze the effects of the optical properties, and ensure that there are no residual carbon species on the sample surface. The adsorption capacity of CO2 of the samples were obtained by CO2 adsorption-desorption isotherms at room temperature (RT) with a Micromeritics TriStar II 3flex system. Prior to the measurements, the samples were degassed at 120 °C for 12 h. The temperature-programmed CO2 desorption (CO2-TPD) was performed at Micromeritics AutoChem II 2920 system. The catalyst (100 mg) was firstly washed with Ar at 350 ℃ for 2 h, then cooled to RT, following that the CO2 (10 vol% CO2/Ar) gas was introduced for adsorption at a flow rate of 30 mL min−1 for 2 h and then flushed with Ar at a flow rate of 30 mL min−1 for 30 min to remove weakly adsorbed CO2, and then it was heated from RT to 650 ℃ at a rate of 10 ℃ min−1. The TPD spectra were recorded by the response of thermal conductivity detector. The insitu diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) was obtained on the spectrometer (Tensor II FT-IR, Bruker) equipped with a reaction chamber. 3. Results and discussion 3.1. Synthesis and characterizations of plaque-like Li2TiO3/TiO2 heterostructure Fig. 1a and b correspond to TEM and HR-TEM images of plaque-like Li2TiO3/TiO2 heterostructure catalyst, respectively. TEM image shows that the particle distribution of the nanoparticles is uniform with the size of about 20−30 nm. Moreover, HR-TEM image shows that the single nanoparticle is composed of the main body of TiO2 and some plaque-like protrusions, which is circled with the pink dotted line. Furthermore, we zoom in on the dotted square part in military blue, and the locally enlarged HR-TEM of heterostructure region is located in Fig. 1c. As can be seen from the image, two separated phases are clearly circle in yellow and in pink, respectively. Particularly, the lattice fringe spacing of 0.207 nm in region ① is ascribed to the (202) crystal plane of Li2TiO3. While the lattice fringe spacing of 0.351 nm in region ③ is ascribed to the (101) crystal plane of TiO2. Notably, an 1–2 nm heterojunction is figured out between those two phases, which is the direct evidence for forming Li2TiO3/TiO2 heterostructure. This result can be supported by the selected area FFT pattern in region ② showing an amorphous structure. Besides, the enrichment of the Li element on the surface of nanoparticles by the ICP results in Table S1, the lattice distortion of the heterojunction cross section in Fig. S1 and the mismatched lattice spacing in Fig. S2 also prove the successful construction of the plaque-like heterostructure. As a contrast, the pure TiO2 and overdosed Li species are shown in Fig. S3. In the previous work of our group, the dynamic process of nanoparticle formation in the flame field was deeply researched by Hu et al. [23] According to this guidance, it can be inferred that the TiO2 particles will preferentially nucleate due to the difference in decomposition temperature, followed by the intercalation of Li ions into the lattice of the TiO2 nanoparticles and finally formed the Li2TiO3 phase on the surface of the TiO2. The entire reaction process just takes place within milliseconds. This growth behavior of plaque-like Li2TiO3/TiO2 nanoparticles by FSP is clearly illustrated in Fig. 1d, which can be divided into 5 stages: (1) hydrolysis of TBT; (2) nucleation of TiO2; (3) thermal decomposition of lithium acetate; (4) intercalation of Li ions; and (5) formation plaque-like Li2TiO3/TiO2 heterostructure. In order to further determine the lattice structure of the as-prepared sample, the results of XRD are shown in Fig. 2. Obviously, the strongest 3
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Fig. 2. XRD patterns of plaque-like Li2TiO3/TiO2 heterostructure compared with pure TiO2 samples.
Ti species in xLi-TiO2 samples was explored by means of EPR. The results are shown in Fig. 3d. The position of g = 1.98 can provide information on the content of Ti3+ ions [30]. Both the samples have a fluctuating signal peak, indicating that there are Ti3+ ions in both, but the signal peak of Li2TiO3/TiO2 is more intense to a certain extent. Again, this demonstrates that Li species can induce the formation of low-valent Ti3+ species. The formation of Ti3+ species will help the
electrons avoid falling into the trap states or electronic defects that originate from oxygen vacancies (Vo) within the TiO2 lattice [31]. Accordingly, one of the reasons for the improved performance of the Li2TiO3/TiO2 catalyst is that the new structure facilitates the passivation of the electron trap, thereby increasing the mean free path of the semiconductor carriers. The overall element information can be obtained from full spectra of XPS in Fig. S5.
Fig. 3. XPS spectra of (a) Ti 2p, (b) O 1s and (c) Li 1s for the pure TiO2 and the plaque-like Li2TiO3/TiO2 heterostructure samples. (d) EPR spectra of the pure TiO2 and plaque-like Li2TiO3/TiO2 heterostructure catalysts. 4
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Fig. 4. (a) UV–vis diff ;use reflectance spectra with Tauc plots, (b) the transient photocurrent responses, (c) the Nyquist plots of electrochemical impedance spectra (EIS) spectra and the inset exhibits results from equivalent circuit fitting the Nyquist plots. (d) the Mott–Schottky curves for estimating CB potentials of the pure TiO2 and the plaque-like Li2TiO3/TiO2 catalysts.
determine the band gap energy of the catalyst shown in Fig. 4a. To further understand the type of semiconductor catalyst and the band structure characteristics, this work characterizes the Mott-Schottky plots of the TiO2 and the Li2TiO3/TiO2 samples at different frequencies, as shown in Fig. 4d. The results show that both have positive slopes, indicating that they are all typical n-type semiconductors [33]. The flat band potential can be estimated by the Mott-Schottky curve, that is, the intersection of the tangent of the curve obtained under the three test frequencies and the x-axis, −0.495 and −0.896 V versus the saturated Ag/AgCl reference electrode for the TiO2 and the Li2TiO3/TiO2, respectively. Nevertheless, it is well recognized that the conduction band potential (CB) for n-type semiconductors is more negative by 0.2 V than that of the flat band potential [34,35]. Thus, the CB potentials of samples TiO2 and the Li2TiO3/TiO2 were referred to be −0.695 and −1.096 V vs. Ag/AgCl, respectively. According to the equation ENHE = EAg/AgCl + 0.212 (where ENHE is the potential vs. NHE and EAg/AgCl is the measured potential vs. Ag/AgCl.), the obtained potentials could be converted to the normal hydrogen electrode (NHE) scale, −0.483 and −0.884 V for the TiO2 and the Li2TiO3/TiO2 vs. NHE, respectively [36]. Combined with the band gap energy obtained in Fig. 4a, the valence band (VB) of the TiO2 can be calculated to be 2.387 V and the VB of the Li2TiO3/TiO2 was 1.856 V.
3.2. Optical properties and photochemical measurements In order to further clarify the influence of Li species on the photochemical process of Li2TiO3/TiO2 photocatalyst, this research work carried out various photochemical measurements by means of electrochemical workstation, and the characterization results are shown in Fig. 4. Fig. 4a shows UV–vis DRS for the as-prepared TiO2 and Li2TiO3/ TiO2 samples. The Li2TiO3/TiO2 sample has a broader spectral absorption range, and apparently the absorption in the 400 nm–800 nm band has a macroscopic improvement, indicating that Li species can improve the visible light capturing ability on the surface of TiO2, that is, it is more efficient for the utility of the full spectrum of sunlight. According to the references [32], the two curves can be respectively tangent-treated. As shown in the figure, the intersection of the two tangent lines with the x-axis is 403.7 nm and 431.73 nm, respectively. Combined with the photoelectric effect equation, the band gap energy of TiO2 can be approximately calculated to be 2.87 eV, while the band gap energy of Li2TiO3/TiO2 is 2.74 eV. The reduction of the band gap energy will facilitate the efficient separation and transfer of electrons and holes in the intrinsic TiO2. Fig. 4b shows the transient photocurrent responses. The Li2TiO3/TiO2 has a fast and variable photocurrent response relative to TiO2, indicating that the Li2TiO3/TiO2 heterostructure has a more efficient electron-hole pairs separation and resistance to recombination. This result is attributed to the excellent electrical conductivity of the plaque-like Li2TiO3 formed on the surface, which is favorable for electron transport and enrichment. Similar test results can also be observed in the Nyquist plots of electrochemical impedance spectra (EIS) shown in Fig. 4c. As can be seen from the comparison, Li2TiO3/TiO2, the material pictured by the red dot has a smaller fitting radius, which indicate that it has a smaller internal resistance. Specifically, the approximate resistance value inside the material can be derived by simulating the circuit diagram. The optimized circuit diagram model and the calculation result have been embedded as part of the main diagram. The results show that the internal resistance of the original material is reduced by 39.89 % after Li modification. In the foregoing, the optical method has been used to
3.3. Photocatalytic activity and mechanism analysis Fig. 5a exhibits the CO generation rates for the pure TiO2 and Li2TiO3/TiO2 samples. As shown, the lowest photocatalytic activity is attributed to the pure TiO2, and the average conversion efficiency is only 0.6157 μmol g−1 h−1. With the increasing amount of Li introduction, the average conversion efficiency gradually increased from 0.9388 μmol g−1 h−1 to the optimal 1.7358 μmol g−1 h−1, and the CO2 photoreduction performance of Li2TiO3/TiO2 increased nearly three times. Then, the performance begins to decrease as the introduction of Li further increases. Eventually, the entire histogram exhibits a volcanic change trend of catalytic performance. Li2TiO3/TiO2 possesses a narrower band gap, which means that the electrons will be more easily 5
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Fig. 5. (a) Comparison of the CO generation rates for the pure TiO2 and heteroplaque-like Li2TiO3/TiO2 structure samples during 4 h CO2 photoreduction with water vapor under full spectrum light irradiation, and (b) the selectivity of the catalyst obtained in this work compared with the recently reported highly selective literatures.
observed in the strong basic site γ [37,38]. Fig. S7 shows the establish model 1 (M1) by using the (101) crystal plane of the main part of the anatase phase as the entry point. When the system reaches the final stable state, the CO2 adsorption energy is −0.27 eV in M1. On the other hand, Li2TiO3 has a much higher CO2 adsorption energy −1.29 eV shown in the Fig. S8, the model 2 (M2) of Li2TiO3 structure obtained from the results of DFT calculation. Hence, both theoretical calculations and experimental results have proved that the increase in CO2 adsorption capacity of Li2TiO3/TiO2 heterostructure is another important factor affecting the performance improvement of apparent photoreduction CO2. Fig. 7 shows a deep explanation on the structure formation mechanism based on computationally simulated Li2TiO3/TiO2 heterostructures. Fig. 7a shows a heterojunction model consisting the (101) crystal plane of TiO2 and the (−112) crystal plane of Li2TiO3. Fig. 7b shows the total density of states (TDOS) of Li2TiO3/TiO2 heterostructure and the pure TiO2. The results show that the valence band position of pure TiO2 is −0.66 eV, and the conduction band is at 1.24 eV, so the band gap is 1.90 eV. After the introduction of Li2TiO3, the valence band and the conduction band positions are shifted to the right, at 1.03 eV and 1.47 eV, respectively, as well as the band gap is reduced to 0.44 eV. This result is in good agreement with the trend of the results obtained from Fig. 4a and d. Fig. 7c shows the electron localization function (ELF) of the Li2TiO3/TiO2 heterostructure. It can be seen from the figure that the inner Li atoms in the Li2TiO3 phase will enter the lattice of TiO2, and the color change between Li atoms and Ti atoms indicates that Li may form a strong chemical bond with Ti atoms of TiO2 in the heterojunction region. In addition, Fig. 7d shows a charge density diff ;erence of the Li2TiO3/TiO2 heterostructure, where the blue electron cloud represents electrons accumulation and the yellow electron cloud represents electron depletion. It is not difficult to find from the figure that when Li2TiO3 and TiO2 form a heterostructure, the O atoms in Li2TiO3 and Ti atoms in TiO2 have strong chemical action, and the effect is derived from the Ti atoms in TiO2 to give electrons to the O
separated from the holes and transition to a higher energy, the activated valence band. Furthermore, the more negative potential makes it more excellent in reducing, which also clarify why such a Li2TiO3/TiO2 heterostructure can generally improve the performance of CO2 photoreduction. Notably, from 0.5 wt% to 1.0 wt% Li species introduction, the catalytic performance did not increase but decrease. We speculate that this is because the part of CO produced by 1.0 wt% Li species will continue to be converted into CH4, and this rate will exceed the CO generation rate in a short time, which is illustrated in Fig. S6. Similarly, this can also be the reason to illustrate why the photocatalytic performance of the catalysts 2.0 wt% and 5.0 wt% Li species are not decreased greatly. What is more, Fig. 5b shows the optimal catalyst obtained in this work has achieved a high selectivity compared with other recently reported highly selective literatures. On the one hand, the selectivity of the plaque-like Li2TiO3/TiO2 heterostructure for CO2 reduction is about 100 % that means the photogenerated electrons almost all obtained by CO2, and the reduction of H ions in water is suppressed. On the other hand, the methane content was barely monitored, indicating that the selectivity of CO is close to 100 %. The specific calculation process and selectivity values are listed in detail in Table S4. In the previous discussion, the work concluded that the formation of Ti3+ passivated the electron trap is one of the reasons for the performance improvement of plaque-like Li2TiO3/TiO2 catalyst. Besides, CO2 is one of the key factors to improve the catalytic activity. In this paper, the adsorption capacity of different kinds of catalyst surfaces for CO2 molecules was carried out by CO2 adsorption-desorption isotherms at RT. The experimental results in Fig. 6a clearly reveal that Li2TiO3/TiO2 has a better CO2 adsorption capacity than pure TiO2, and this is consistent with common sense. The results of CO2-TPD experiments shown in Fig. 6b show that the adsorption capacity of Li2TiO3/TiO2 catalyst for CO2 is generally improved, which can be judged from the comparison of peak area. On the other hand, the increase in CO2 adsorption capacity of Li2TiO3/TiO2 material by Li species is mainly reflected in the weak alkaline site α and the moderately basic site β, and almost no change is
Fig. 6. (a) CO2 adsorption curves and (b) CO2-TPD of the pure TiO2 and the plaque-like Li2TiO3/TiO2 heterostructure catalysts at RT. 6
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Fig. 7. (a) Calculation model of Li2TiO3/TiO2 heterostructure. The pink ball represents the Li atom; the blue ball represents the Ti atom; and the red ball represents the O atom. (b) Calculated total density of states (TDOS) for Li2TiO3/TiO2 heterostructure and pure TiO2. (c) Electron localization function (ELF) of Li2TiO3 constructed on the TiO2 and (d) charge density diff ;erence of Li2TiO3/TiO2 heterostructure. Electrons accumulation is in blue and depletion in yellow, respectively. The isosurfaces are set to 0.005eVÅ−3 (To explain the colors in this figure legend, the reader is referred to the web version.).
shown in Fig. 8c–d, obvious peaks of CO2- (1287 cm-1 and 1300 cm-1) start to appear in both TiO2 and Li2TiO3/TiO2 and the difference is that the signal in the latter is stronger [40], which indicates that CO2 can acquire electrons more rapidly on the surface of plaque-like Li2TiO3 structure. Notably, the formation of CO2- is a critical step in determining whether a subsequent reaction can proceed quickly. In general, the Li2TiO3/TiO2 is blue-shifted from the peak position of pure TiO2, indicating the efficient conversion and accumulation of different intermediates such as the formate, bicarbonate and carbonate on the surface. In addition, it is known that water plays an important role in the CO2 photoreduction process. During the experiment, with the increase of illumination time, the H2O (1665 cm-1) produced by the Li2TiO3/TiO2 sample weakens, indicating that Li2TiO3/TiO2 is more favorable for the conversion of reactant water into active radicals and thus enhances photocatalytic activity. Combined with the above in-situ DRFITS and related characterization analysis, the possible reaction pathways for this work are listed in Fig. 8e. Briefly, the approximate pathway is divided into two categories: (I) CO2- is combined with one H+ and e- to be converted into a formate intermediate, and then one molecule of water is removed to convert to CO; (II) CO2- is combined with one ·OH and converted into HCO3-, and then converted to CO32-. Finally, CO32- combines two H+ with two e- and simultaneously desorbs two OH- to produce CO. Intuitively, the conversion process of the first pathway is relatively easy to happen. Comparing the peak position and the curve shape of the two groups of catalysts after illumination, the Li2TiO3/TiO2 sample will experience both pathways at the same time. However, in combination with the strong HCOO- signals appearing at 1548 cm-1 in Fig. 8d, it is speculated that the catalyst is more prone to experience path (I). On the other hand, from the comparation of the 1622 cm-1 peak attributed to the bidentate carbonate (b-CO32-), it can be seen that the pure TiO2 is more inclined to experience the longer pathway of the path (II). The results of comprehensive analysis show
atoms in Li2TiO3. The electron transport direction is from Ti atoms to Li atoms. Therefore, the direction of the built-in electric field is directed from Li2TiO3 to TiO2, thus creating a force for facilitating electron hole separation in the Li2TiO3/TiO2 heterostructure system. In summary, the reason why Li2TiO3 and TiO2 form a heterostructure may be due to the strong interaction between Li atoms in Li2TiO3 and Ti atoms in TiO2, and the charge transfer mechanism between O atoms in Li2TiO3 and Ti atoms in TiO2. In order to explore the reaction mechanism, the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was adopted as a research tool. Firstly, in the adsorption stage shown in Fig. 8a–b, the absorption peak of water (1640 cm−1) starts to decrease in the process of adsorbing and activating water molecules by the pure TiO2 for about 1 h, while the Li2TiO3/TiO2 sample had a significant downward absorption peak at 10 min, which indicates the Li2TiO3/TiO2 TiO2 is more conducive to the activation and conversion of the reactant water molecules. As the adsorption time increases, pure TiO2 exhibits the chelated bicarbonate (HCO3-, 1435 cm−1) and the monodentate carbonate (m-CO32-, 1310 cm−1), while the Li2TiO3/TiO2 sample, the obvious blue shift of the chelated bicarbonate (HCO3-, 1452 cm−1) and the monodentate carbonate (m-CO32-, 1336 cm−1) occurred and showed a stronger peak signal, indicating that the stable combination of the intermediate species on the surface of Li2TiO3/TiO2. In addition, the formation of the formate species (HCOO-, 1537 cm−1) is also detected [39–43]. These results all indicate that the introduction of Li2TiO3 is attributed to increase in the conversion of reactant water molecules as well as the intermediate formate, bicarbonates, and carbonates species on the catalyst surface to a more stable state. From the analysis in the adsorption stage, it can be preliminarily judged that the Li2TiO3/TiO2 heterostructure had better CO2 conversion efficiency. Secondly, during radiation, minor changes were observed in the amidst the peak positions. It is not difficult to find that under the condition of illumination 7
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Fig. 8. In-situ DRIFT spectra of surface adsorbed species and photocatalytic CO2 reduction intermediates over the pure TiO2 and plaque-like Li2TiO3/TiO2 heterostructure samples during (a) and (b) the adsorption stage and (c) and (d) the reaction process. (e) The possible reaction pathway for photoreduction of CO2 with H2O by using Li2TiO3/TiO2 catalyst.
necessity of energy input and the simple reaction process endow the Li2TiO3/TiO2 heterostructure a doubling performance enhancement in CO2 photoreduction. The theoretical simulation verifies the correctness of the conjecture obtained from the experimental results. Hence, the catalyst behavior and mechanism of the novel plaque-like Li2TiO3/TiO2 heterostructure catalyst have been scientifically verified from both perspectives of experiment and theoretical simulation.
that Li2TiO3/TiO2 heterostructure can improve the adsorption and conversion ability of the catalyst for the intermediates of the reactants in the adsorption stage and reaction process, and finally show the outstanding activity of photocatalytic CO2 reduction. In order to verify whether the energy barrier of path (I) can be lower and quantitatively understand the reason for doubling the performance of the Li2TiO3/TiO2 heterostructure catalyst. We calculated the free energy of the transition state based on the assumption of the catalytic mechanism proposed in in-situ DRIFT. The results are shown in Fig. 9 and the optimized supercell are shown in Fig. S9. From the calculation results of the free energy of intermediates, the energy converted to formate is much lower than that conversion to bicarbonate, and the energy is reduced by 26.53 % from 0.98 eV to 0.72 eV, indicating path (I) has a lower energy barrier compared with path (II). The lower
4. Conclusions In summary, a novel Li2TiO3/TiO2 heterostructure was prepared in an ultra-fast reaction process by the gradual intercalation of Li ions into the lattice of TiO2 from the outside and eventually formed the Li2TiO3 phase on the surface of TiO2 in an ultra-high temperature about 2000 K 8
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Fig. 9. Proposed reaction mechanism for CO2 photoreduction on Li2TiO3/TiO2 heterostructure surface.
the Shanghai Pujiang Program (18PJ1402100), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutes of High Learning, the Basic Research Program of Shanghai (17JC1402300), the Social Development Program of Shanghai (17DZ1200900), the Shanghai City Board of education research and innovation project, and the Fundamental Research Funds for the Central Universities (222201718002).
within several milliseconds. The introduction of Li species not only improve the adsorption capacity of CO2, but also reduce the oxygen defect trap to reduce the resistivity. The built-in electric field directed from Li2TiO3 to TiO2 will be attributed to promote the separation of electron-hole pairs and forces electrons move to the oxygen atom sites in Li2TiO3. The optimized Li2TiO3/TiO2 catalyst exhibits a narrower band gap energy, a more negative conduction band position of −0.884 V and better reductive ability, eventually achieving nearly a 3 times performance enhancement with about 100 % selectivity towards CO. Notably, based on the in-situ DRFITS combined with transition state calculations by DFT simulation, we proposed an possible catalytic mechanism that the Li2TiO3/TiO2 catalyst will choose an easier pathway through the intermediate formate conversion to CO, of which free energy is reduced by 26.53 % from 0.98 eV to 0.72 eV. This work will provide a new insight into facile production of highly efficient CO2 photoreduction catalysts by constructing heterostructure in an ultrafast reaction process.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118810. References [1] [2] [3] [4]
CRediT authorship contribution statement
[5] [6] [7]
Wei Bi: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Yanjie Hu: Resources, Writing - review & editing, Data curation. Nan Jiang: Validation, Formal analysis, Visualization. Ling Zhang: Writing - review & editing, Supervision. Hao Jiang: Writing - review & editing, Supervision. Xing Zhao: Validation, Visualization. Chengyun Wang: Resources, Validation. Chunzhong Li: Writing - review & editing.
[8] [9] [10] [11] [12]
Declaration of Competing Interest
[13] [14]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[15] [16]
Acknowledgements
[17] [18] [19] [20]
This work was supported by the National Natural Science Foundation of China (21978088, 91534202, 51673063), Sponsored by 9
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Ultra-fast construction of plaque-like Li2TiO3/TiO2 heterostructure for efficient gas-solid phase CO2 photoreduction
T
Wei Bi, Yanjie Hu*, Nan Jiang, Ling Zhang, Hao Jiang, Xing Zhao, Chengyun Wang, Chunzhong Li* Shanghai Engineering Research Center of Hierarchical Nanomaterials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-fast reaction process Ultra-high temperature Plaque-like Li2TiO3/TiO2 heterostructure Photocatalytic CO2 reduction Catalytic behavior and mechanism
Exploring a facile and universal strategy to fabricate an efficient heterostructure is a big challenge for achieving highly active and selective CO2 photoreduction. Herein, a plaque-like Li2TiO3/TiO2 heterostructure was in-situ constructed by the gradual intercalation of Li ions into the lattice of TiO2 from the outside and eventually formed the Li2TiO3 phase on the surface of TiO2 in an ultra-high temperature about 2000 K within several milliseconds. This novel structure has a significant improvement in the adsorption capacity of CO2 and reduction of the oxygen defect trap by introduction of Li species. Furthermore, owing to the built-in electric field directed from Li2TiO3 to TiO2, electrons are forced to move to the oxygen atom sites in Li2TiO3, which helps the plaque-like Li2TiO3/TiO2 heterostructure photocatalyst achieve nearly 3 times performance improvement with about 100 % selectivity towards CO. Theoretically and experimentally, we proposed a possible catalytic behavior and mechanism that Li2TiO3/TiO2 heterostructure will choose a shorter pathway through intermediate formate conversion to CO, as the free energy reduce by 26.53 % from 0.98 eV to 0.72 eV. This work gives a new insight for constructing heterostructure in an ultra-fast reaction process, and provides a rational reference for developing highly efficient photocatalysts towards CO2 reduction.
1. Introduction In recent years, photocatalysis has been a promising strategy to address the problems resulting from fossil energy consumption and global warming [1–4]. CO2 photoreduction is one of novel method could directly reduce atmospheric CO2 and H2O to compounds with high added value by the use of sunlight under the aid of a semiconductor material. Since no pollutants are involved in the reaction, no waste is generated, and the driving energy originates from the inexhaustible solar energy, this method can be recognized as an environmentally friendly chemical treatment [5–8]. In this process, TiO2 is considered as one of the most research-intensive nano-photocatalyst material because of its high chemical stability, thermal stability, nontoxicity, environmental friendliness and low cost [9–11]. However, the wide band gap of 3.2 eV causes the poor utilization of sunlight for TiO2, mainly in the visible region. Meanwhile, the nature of the intrinsic TiO2 also determines low charge separation efficiency and low charge utilization, which could also lead to severe electron-hole recombination. In order to solve such two key issues, in recent decades, several effective strategies have been developed to improve photocatalytic CO2 ⁎
reduction performance, including doping [12–14], morphological and structural design [15,16], multi-component coupling [17,18], etc. Among them, the construction of a reasonable heterogeneous structure is one of the most effective and promising strategies. However, how to design an effective heterostructure becomes a challengeable work to research efficient photocatalyst. A conventional gas phase synthesis method for preparing multifunctional nano-materials, FSP, has unique advantages in the preparation of heterostructure materials because of the ultra-high reaction temperature, which is conducive to element surface enrichment, or phase separation [19,20]. Li et al. [21] prepared a very unique Janusstructured Fe2O3/SiO2 heterostructure nanosphere by FSP and intensively explained the formation mechanism of the two phases combined with subtle experiment and the law of phase diagram. Gu et al. [22] by depositing SnO2 on TiO2 nanocrystals to form TiO2@SnO2 core/shell particles (TSN) with FSP, achieved a significant increase to an open circuit voltage of 0.59 V and an efficiency of 3.82 %, which proved to be attributed to the enhanced electron injection arising from decreased interfacial charge recombination losses and improved electron transport. Hu et al. [23] based on a MgO/TiO2 model, pointed out
Corresponding authors. E-mail addresses: [email protected] (Y. Hu), [email protected] (C. Li).
https://doi.org/10.1016/j.apcatb.2020.118810 Received 8 December 2019; Received in revised form 18 February 2020; Accepted 24 February 2020 Available online 25 February 2020 0926-3373/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (a) TEM images, (b) HR-TEM images of plaque-like Li2TiO3/TiO2 heterostructure, and (c) locally enlarged HR-TEM of heterostructure regions. Schematic illustration of (d) the synthesis process and the growth behavior of plaque-like Li2TiO3/TiO2 heterostructure by FSP.
2. Experimental
the formation mechanism of the outermost layer of TiO2 nanoparticles was located at the difference in reaction rate of precursors during the ultra-high temperature synthetic process. The surface MgO layer could effectively adjust the band structure of TiO2 nanoparticles while improving the maximum photocurrent and maximum photovoltage. This work revealed that improvements were largely attributed to red-shifted light absorption, and rapid photoelectron injection into the conduction band. In this paper, the plaque-like Li2TiO3/TiO2 heterostructure was insitu prepared in an ultra-high temperature (∼2000 K) reaction process. The introduction of Li2TiO3 increase the probability of contact between CO2 and the active sites on the catalyst, illustrated as a key intermediate step in photocatalytic reduction of CO2. On the other hand, Li species are attributed to the partial reduction of Ti4+ to Ti3+, which is beneficial to the passivation of oxygen vacancies, and result in improved conductivity and enhanced electrons separation and transmission. Owing to the rational surface modification, the optimal the plaque-like Li2TiO3/TiO2 heterostructure photocatalyst achieved nearly 3 times performance improvement of the intrinsic TiO2 with nearly 100 % selectivity. What is more, the catalytic behavior and mechanism of the plaque-like Li2TiO3/TiO2 heterostructure are intensively explained on the electronic level, by combining the first-principle calculation based on density functional theory (DFT) with systematic experiments. This work will provide an insight for development of high-performance CO2 photoreduction catalysts by effective heterostructure design.
2.1. Preparation The catalysts in this work were prepared by flame spray pyrolysis (FSP) method. Briefly, the tetrabutyl titanate (TBT) and the lithium acetate (1.5 wt% Li species) was dissolved in 100 mL of ethanol to obtain a uniform precursor solution at a concentration of 0.5 M. With the aid of the syringe pump, the precursor solution reached the tip of the nozzle at a flow rate of 5 mL min−1, and then, the precursor solution was rapidly dissipated into a micron-sized mist droplet cutting by a high-speed O2 airflow. Meanwhile, the atomized precursor droplets were ignited with the circular H2/O2 diff ;usion flame, wherein the flow rates of H2 and auxiliary air were controlled at 380 L h−1 and 1.0 m3 h−1, respectively. The resulting powder was collected by glass fiber, and the remaining conditions were consistent with the above experiments.
2.2. Characterization The Li2TiO3/TiO2 heterostructure was directly observed by the transmission electron microscopy (TEM, JEM-2100) and high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F20 S-Twin (FEI)). The lattice structure and the change in phase ratio were characterized by X-ray diff ;raction (XRD, Bruker D8 Avance) with Cu-Kα filament that X-ray wavelength of 1.5406 Å. The chemical state of Titanium (Ti 2p), Oxygen (O 1s) and Lithium (Li 1s) were further confirmed by the X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 X-ray photoelectron spectrometer, equipped with an Al 2
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peaks of anatase (JCPDS: 04-0477) and rutile (JCPDS: 04-0551) located at 25.35° and 27.46° correspond to the respective (101) and (110) crystal planes. Those characteristic peaks are observed in all as-prepared samples, indicating that the TiO2 prepared by FSP possesses a mixed phase structure. Importantly, as the content of Li species increase from 0 to 5 wt%, the composition of the anatase and rutile phases change significantly. The detailed variation data are listed in Table S2. Furthermore, the locally enlarged patterns indicate that the peak at 18.47° in region Ⅰ is ascribed to the (002) crystal plane of Li2TiO3, and the peak at 43.58° in region Ⅱ is ascribed to the (−133) crystal plane of Li2TiO3. Besides, a slight increase of diffraction angle 0.08°can be observed, which indicates the Li species do reduce the lattice spacing of TiO2. Moreover, those two peaks are gradually obvious with the increase of Li species content shown in Fig. S4. The results of XRD patterns show that the introduction of Li species not only promote the crystal phase transfer from anatase phase to rutile phase, but also form a coating layer of Li2TiO3 on the surface of TiO2 in combination with TEM images. Notably, the state of the coating, plaque-like shape or complete cover, is related to the introduction content of Li species. The XPS characterization was used to obtain information on the surface of the sample to clarify the compositional results of the element species in the sample as shown in Fig. 3. Fig. 3a shows the deconvolution of the Ti 2p XPS spectra for the pure TiO2 and plaque-like Li2TiO3/TiO2 heterostructure. For the pure TiO2, the binding energy at the position of 458.8 eV corresponds to Ti4+ 2p3/2, and the binding energy at the position of 464.6 eV corresponds to Ti4+ 2p1/2 [24]. However, the Ti 2p3/2 and Ti 2p1/2 peaks in Li2TiO3/TiO2 exhibit attractive negative shift of 0.2 eV with respect to that of the TiO2, which indicates that after the introduction of Li species, the original state of the electron cloud for Ti 2p orbit is changed, and the electron tends to flow around the Ti species, which is apparently reduced in binding energy [25]. In addition, it is observed in both sets of samples that a smaller shoulder peak belonging to Ti 2p3/2 appears at a binding energy of 460.2 eV, which corresponds to a non-stoichiometric TiOx [26]. And the relative ratio of peak integral area in Li2TiO3/TiO2 increased slightly compared with TiO2, indicating that Li species can also affect the valence state ratio of Ti species. The enlarged TiOx area is shown in Fig. S5 and the detailed peak parameters are listed in Table S3. Fig. 3b shows the deconvolution of the O 1s XPS spectra, from which similar changes in Ti 2p spectra can be observed. The peak at 530.2 eV corresponds to the TieO bond in the lattice structure. It can be found that Li species is resulting in the binding energy moving towards the low energy for about 0.15 eV, indicating that the lattice O atoms in the TiO2 will get electrons from nearby. In addition, there is a shoulder peak near the main peak at around 531.6 eV, which corresponds to the oxygen interaction with Li [25] and the adsorbed oxygen hydroxyl species [27,28] on the surface of TiO2. Comparatively, it is not difficult to find that the peak integral area of Li2TiO3/TiO2 is much larger, which is mainly the result of the synergistic effect of Li species on the catalyst surface, that is, the signal of oxygen interaction with Li will occupy part of the peak area. On the other hand, the modification of the surface of the catalyst by Li species can induce more adsorption sites of oxygen hydroxyl species, which together show a stronger signal peak. Intuitively, Li 1 s XPS spectra is shown in Fig. 3c. Clearly, the Li2TiO3/ TiO2 sample shows a broad peak at a binding energy of 54.9 eV, which is a characteristic peak of Li+ species [29], indicating that Li was successfully introduced into the surface of TiO2, and the characterization results are consistent to the TEM and XRD analysis mentioned above. Notably, information about Ti 3 s can also be observed in the spectrum. Literature indicates that the peak in the 51.5 eV–61.0 eV interval represents the presence of Ti3+, while the peak around 62.1 eV represents the presence of Ti4+ [24]. It can be seen from the spectra that the peak shape of Li2TiO3/TiO2 is wider than that of TiO2 as well as closer to the low energy region, indicating that the Li2TiO3/TiO2 possesses a higher ratio of Ti3+. This conclusion is consistent with the TiOx content analysis in Fig. 3a. More specifically, the content of low-valent
Kα source (1486.6 eV)). The binding energies were calibrated at the C 1s peak of 284.8 eV. Electron paramagnetic resonance (EPR, Bruker EMX-8/2.7 spectrometer operating at about 9.8 GHz.) test was carried out at 25 °C to obtain the information of the low-valent Ti species on catalyst surface. The UV–vis diffuse reflectance spectra (UV–vis DRS, U3900H Spectrophotometer) of the samples were acquired within the wavelength range of 200–800 nm. Raman spectroscopy (LabRAM HR Evolution) was used to analyze the effects of the optical properties, and ensure that there are no residual carbon species on the sample surface. The adsorption capacity of CO2 of the samples were obtained by CO2 adsorption-desorption isotherms at room temperature (RT) with a Micromeritics TriStar II 3flex system. Prior to the measurements, the samples were degassed at 120 °C for 12 h. The temperature-programmed CO2 desorption (CO2-TPD) was performed at Micromeritics AutoChem II 2920 system. The catalyst (100 mg) was firstly washed with Ar at 350 ℃ for 2 h, then cooled to RT, following that the CO2 (10 vol% CO2/Ar) gas was introduced for adsorption at a flow rate of 30 mL min−1 for 2 h and then flushed with Ar at a flow rate of 30 mL min−1 for 30 min to remove weakly adsorbed CO2, and then it was heated from RT to 650 ℃ at a rate of 10 ℃ min−1. The TPD spectra were recorded by the response of thermal conductivity detector. The insitu diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) was obtained on the spectrometer (Tensor II FT-IR, Bruker) equipped with a reaction chamber. 3. Results and discussion 3.1. Synthesis and characterizations of plaque-like Li2TiO3/TiO2 heterostructure Fig. 1a and b correspond to TEM and HR-TEM images of plaque-like Li2TiO3/TiO2 heterostructure catalyst, respectively. TEM image shows that the particle distribution of the nanoparticles is uniform with the size of about 20−30 nm. Moreover, HR-TEM image shows that the single nanoparticle is composed of the main body of TiO2 and some plaque-like protrusions, which is circled with the pink dotted line. Furthermore, we zoom in on the dotted square part in military blue, and the locally enlarged HR-TEM of heterostructure region is located in Fig. 1c. As can be seen from the image, two separated phases are clearly circle in yellow and in pink, respectively. Particularly, the lattice fringe spacing of 0.207 nm in region ① is ascribed to the (202) crystal plane of Li2TiO3. While the lattice fringe spacing of 0.351 nm in region ③ is ascribed to the (101) crystal plane of TiO2. Notably, an 1–2 nm heterojunction is figured out between those two phases, which is the direct evidence for forming Li2TiO3/TiO2 heterostructure. This result can be supported by the selected area FFT pattern in region ② showing an amorphous structure. Besides, the enrichment of the Li element on the surface of nanoparticles by the ICP results in Table S1, the lattice distortion of the heterojunction cross section in Fig. S1 and the mismatched lattice spacing in Fig. S2 also prove the successful construction of the plaque-like heterostructure. As a contrast, the pure TiO2 and overdosed Li species are shown in Fig. S3. In the previous work of our group, the dynamic process of nanoparticle formation in the flame field was deeply researched by Hu et al. [23] According to this guidance, it can be inferred that the TiO2 particles will preferentially nucleate due to the difference in decomposition temperature, followed by the intercalation of Li ions into the lattice of the TiO2 nanoparticles and finally formed the Li2TiO3 phase on the surface of the TiO2. The entire reaction process just takes place within milliseconds. This growth behavior of plaque-like Li2TiO3/TiO2 nanoparticles by FSP is clearly illustrated in Fig. 1d, which can be divided into 5 stages: (1) hydrolysis of TBT; (2) nucleation of TiO2; (3) thermal decomposition of lithium acetate; (4) intercalation of Li ions; and (5) formation plaque-like Li2TiO3/TiO2 heterostructure. In order to further determine the lattice structure of the as-prepared sample, the results of XRD are shown in Fig. 2. Obviously, the strongest 3
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Fig. 2. XRD patterns of plaque-like Li2TiO3/TiO2 heterostructure compared with pure TiO2 samples.
Ti species in xLi-TiO2 samples was explored by means of EPR. The results are shown in Fig. 3d. The position of g = 1.98 can provide information on the content of Ti3+ ions [30]. Both the samples have a fluctuating signal peak, indicating that there are Ti3+ ions in both, but the signal peak of Li2TiO3/TiO2 is more intense to a certain extent. Again, this demonstrates that Li species can induce the formation of low-valent Ti3+ species. The formation of Ti3+ species will help the
electrons avoid falling into the trap states or electronic defects that originate from oxygen vacancies (Vo) within the TiO2 lattice [31]. Accordingly, one of the reasons for the improved performance of the Li2TiO3/TiO2 catalyst is that the new structure facilitates the passivation of the electron trap, thereby increasing the mean free path of the semiconductor carriers. The overall element information can be obtained from full spectra of XPS in Fig. S5.
Fig. 3. XPS spectra of (a) Ti 2p, (b) O 1s and (c) Li 1s for the pure TiO2 and the plaque-like Li2TiO3/TiO2 heterostructure samples. (d) EPR spectra of the pure TiO2 and plaque-like Li2TiO3/TiO2 heterostructure catalysts. 4
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Fig. 4. (a) UV–vis diff ;use reflectance spectra with Tauc plots, (b) the transient photocurrent responses, (c) the Nyquist plots of electrochemical impedance spectra (EIS) spectra and the inset exhibits results from equivalent circuit fitting the Nyquist plots. (d) the Mott–Schottky curves for estimating CB potentials of the pure TiO2 and the plaque-like Li2TiO3/TiO2 catalysts.
determine the band gap energy of the catalyst shown in Fig. 4a. To further understand the type of semiconductor catalyst and the band structure characteristics, this work characterizes the Mott-Schottky plots of the TiO2 and the Li2TiO3/TiO2 samples at different frequencies, as shown in Fig. 4d. The results show that both have positive slopes, indicating that they are all typical n-type semiconductors [33]. The flat band potential can be estimated by the Mott-Schottky curve, that is, the intersection of the tangent of the curve obtained under the three test frequencies and the x-axis, −0.495 and −0.896 V versus the saturated Ag/AgCl reference electrode for the TiO2 and the Li2TiO3/TiO2, respectively. Nevertheless, it is well recognized that the conduction band potential (CB) for n-type semiconductors is more negative by 0.2 V than that of the flat band potential [34,35]. Thus, the CB potentials of samples TiO2 and the Li2TiO3/TiO2 were referred to be −0.695 and −1.096 V vs. Ag/AgCl, respectively. According to the equation ENHE = EAg/AgCl + 0.212 (where ENHE is the potential vs. NHE and EAg/AgCl is the measured potential vs. Ag/AgCl.), the obtained potentials could be converted to the normal hydrogen electrode (NHE) scale, −0.483 and −0.884 V for the TiO2 and the Li2TiO3/TiO2 vs. NHE, respectively [36]. Combined with the band gap energy obtained in Fig. 4a, the valence band (VB) of the TiO2 can be calculated to be 2.387 V and the VB of the Li2TiO3/TiO2 was 1.856 V.
3.2. Optical properties and photochemical measurements In order to further clarify the influence of Li species on the photochemical process of Li2TiO3/TiO2 photocatalyst, this research work carried out various photochemical measurements by means of electrochemical workstation, and the characterization results are shown in Fig. 4. Fig. 4a shows UV–vis DRS for the as-prepared TiO2 and Li2TiO3/ TiO2 samples. The Li2TiO3/TiO2 sample has a broader spectral absorption range, and apparently the absorption in the 400 nm–800 nm band has a macroscopic improvement, indicating that Li species can improve the visible light capturing ability on the surface of TiO2, that is, it is more efficient for the utility of the full spectrum of sunlight. According to the references [32], the two curves can be respectively tangent-treated. As shown in the figure, the intersection of the two tangent lines with the x-axis is 403.7 nm and 431.73 nm, respectively. Combined with the photoelectric effect equation, the band gap energy of TiO2 can be approximately calculated to be 2.87 eV, while the band gap energy of Li2TiO3/TiO2 is 2.74 eV. The reduction of the band gap energy will facilitate the efficient separation and transfer of electrons and holes in the intrinsic TiO2. Fig. 4b shows the transient photocurrent responses. The Li2TiO3/TiO2 has a fast and variable photocurrent response relative to TiO2, indicating that the Li2TiO3/TiO2 heterostructure has a more efficient electron-hole pairs separation and resistance to recombination. This result is attributed to the excellent electrical conductivity of the plaque-like Li2TiO3 formed on the surface, which is favorable for electron transport and enrichment. Similar test results can also be observed in the Nyquist plots of electrochemical impedance spectra (EIS) shown in Fig. 4c. As can be seen from the comparison, Li2TiO3/TiO2, the material pictured by the red dot has a smaller fitting radius, which indicate that it has a smaller internal resistance. Specifically, the approximate resistance value inside the material can be derived by simulating the circuit diagram. The optimized circuit diagram model and the calculation result have been embedded as part of the main diagram. The results show that the internal resistance of the original material is reduced by 39.89 % after Li modification. In the foregoing, the optical method has been used to
3.3. Photocatalytic activity and mechanism analysis Fig. 5a exhibits the CO generation rates for the pure TiO2 and Li2TiO3/TiO2 samples. As shown, the lowest photocatalytic activity is attributed to the pure TiO2, and the average conversion efficiency is only 0.6157 μmol g−1 h−1. With the increasing amount of Li introduction, the average conversion efficiency gradually increased from 0.9388 μmol g−1 h−1 to the optimal 1.7358 μmol g−1 h−1, and the CO2 photoreduction performance of Li2TiO3/TiO2 increased nearly three times. Then, the performance begins to decrease as the introduction of Li further increases. Eventually, the entire histogram exhibits a volcanic change trend of catalytic performance. Li2TiO3/TiO2 possesses a narrower band gap, which means that the electrons will be more easily 5
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Fig. 5. (a) Comparison of the CO generation rates for the pure TiO2 and heteroplaque-like Li2TiO3/TiO2 structure samples during 4 h CO2 photoreduction with water vapor under full spectrum light irradiation, and (b) the selectivity of the catalyst obtained in this work compared with the recently reported highly selective literatures.
observed in the strong basic site γ [37,38]. Fig. S7 shows the establish model 1 (M1) by using the (101) crystal plane of the main part of the anatase phase as the entry point. When the system reaches the final stable state, the CO2 adsorption energy is −0.27 eV in M1. On the other hand, Li2TiO3 has a much higher CO2 adsorption energy −1.29 eV shown in the Fig. S8, the model 2 (M2) of Li2TiO3 structure obtained from the results of DFT calculation. Hence, both theoretical calculations and experimental results have proved that the increase in CO2 adsorption capacity of Li2TiO3/TiO2 heterostructure is another important factor affecting the performance improvement of apparent photoreduction CO2. Fig. 7 shows a deep explanation on the structure formation mechanism based on computationally simulated Li2TiO3/TiO2 heterostructures. Fig. 7a shows a heterojunction model consisting the (101) crystal plane of TiO2 and the (−112) crystal plane of Li2TiO3. Fig. 7b shows the total density of states (TDOS) of Li2TiO3/TiO2 heterostructure and the pure TiO2. The results show that the valence band position of pure TiO2 is −0.66 eV, and the conduction band is at 1.24 eV, so the band gap is 1.90 eV. After the introduction of Li2TiO3, the valence band and the conduction band positions are shifted to the right, at 1.03 eV and 1.47 eV, respectively, as well as the band gap is reduced to 0.44 eV. This result is in good agreement with the trend of the results obtained from Fig. 4a and d. Fig. 7c shows the electron localization function (ELF) of the Li2TiO3/TiO2 heterostructure. It can be seen from the figure that the inner Li atoms in the Li2TiO3 phase will enter the lattice of TiO2, and the color change between Li atoms and Ti atoms indicates that Li may form a strong chemical bond with Ti atoms of TiO2 in the heterojunction region. In addition, Fig. 7d shows a charge density diff ;erence of the Li2TiO3/TiO2 heterostructure, where the blue electron cloud represents electrons accumulation and the yellow electron cloud represents electron depletion. It is not difficult to find from the figure that when Li2TiO3 and TiO2 form a heterostructure, the O atoms in Li2TiO3 and Ti atoms in TiO2 have strong chemical action, and the effect is derived from the Ti atoms in TiO2 to give electrons to the O
separated from the holes and transition to a higher energy, the activated valence band. Furthermore, the more negative potential makes it more excellent in reducing, which also clarify why such a Li2TiO3/TiO2 heterostructure can generally improve the performance of CO2 photoreduction. Notably, from 0.5 wt% to 1.0 wt% Li species introduction, the catalytic performance did not increase but decrease. We speculate that this is because the part of CO produced by 1.0 wt% Li species will continue to be converted into CH4, and this rate will exceed the CO generation rate in a short time, which is illustrated in Fig. S6. Similarly, this can also be the reason to illustrate why the photocatalytic performance of the catalysts 2.0 wt% and 5.0 wt% Li species are not decreased greatly. What is more, Fig. 5b shows the optimal catalyst obtained in this work has achieved a high selectivity compared with other recently reported highly selective literatures. On the one hand, the selectivity of the plaque-like Li2TiO3/TiO2 heterostructure for CO2 reduction is about 100 % that means the photogenerated electrons almost all obtained by CO2, and the reduction of H ions in water is suppressed. On the other hand, the methane content was barely monitored, indicating that the selectivity of CO is close to 100 %. The specific calculation process and selectivity values are listed in detail in Table S4. In the previous discussion, the work concluded that the formation of Ti3+ passivated the electron trap is one of the reasons for the performance improvement of plaque-like Li2TiO3/TiO2 catalyst. Besides, CO2 is one of the key factors to improve the catalytic activity. In this paper, the adsorption capacity of different kinds of catalyst surfaces for CO2 molecules was carried out by CO2 adsorption-desorption isotherms at RT. The experimental results in Fig. 6a clearly reveal that Li2TiO3/TiO2 has a better CO2 adsorption capacity than pure TiO2, and this is consistent with common sense. The results of CO2-TPD experiments shown in Fig. 6b show that the adsorption capacity of Li2TiO3/TiO2 catalyst for CO2 is generally improved, which can be judged from the comparison of peak area. On the other hand, the increase in CO2 adsorption capacity of Li2TiO3/TiO2 material by Li species is mainly reflected in the weak alkaline site α and the moderately basic site β, and almost no change is
Fig. 6. (a) CO2 adsorption curves and (b) CO2-TPD of the pure TiO2 and the plaque-like Li2TiO3/TiO2 heterostructure catalysts at RT. 6
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Fig. 7. (a) Calculation model of Li2TiO3/TiO2 heterostructure. The pink ball represents the Li atom; the blue ball represents the Ti atom; and the red ball represents the O atom. (b) Calculated total density of states (TDOS) for Li2TiO3/TiO2 heterostructure and pure TiO2. (c) Electron localization function (ELF) of Li2TiO3 constructed on the TiO2 and (d) charge density diff ;erence of Li2TiO3/TiO2 heterostructure. Electrons accumulation is in blue and depletion in yellow, respectively. The isosurfaces are set to 0.005eVÅ−3 (To explain the colors in this figure legend, the reader is referred to the web version.).
shown in Fig. 8c–d, obvious peaks of CO2- (1287 cm-1 and 1300 cm-1) start to appear in both TiO2 and Li2TiO3/TiO2 and the difference is that the signal in the latter is stronger [40], which indicates that CO2 can acquire electrons more rapidly on the surface of plaque-like Li2TiO3 structure. Notably, the formation of CO2- is a critical step in determining whether a subsequent reaction can proceed quickly. In general, the Li2TiO3/TiO2 is blue-shifted from the peak position of pure TiO2, indicating the efficient conversion and accumulation of different intermediates such as the formate, bicarbonate and carbonate on the surface. In addition, it is known that water plays an important role in the CO2 photoreduction process. During the experiment, with the increase of illumination time, the H2O (1665 cm-1) produced by the Li2TiO3/TiO2 sample weakens, indicating that Li2TiO3/TiO2 is more favorable for the conversion of reactant water into active radicals and thus enhances photocatalytic activity. Combined with the above in-situ DRFITS and related characterization analysis, the possible reaction pathways for this work are listed in Fig. 8e. Briefly, the approximate pathway is divided into two categories: (I) CO2- is combined with one H+ and e- to be converted into a formate intermediate, and then one molecule of water is removed to convert to CO; (II) CO2- is combined with one ·OH and converted into HCO3-, and then converted to CO32-. Finally, CO32- combines two H+ with two e- and simultaneously desorbs two OH- to produce CO. Intuitively, the conversion process of the first pathway is relatively easy to happen. Comparing the peak position and the curve shape of the two groups of catalysts after illumination, the Li2TiO3/TiO2 sample will experience both pathways at the same time. However, in combination with the strong HCOO- signals appearing at 1548 cm-1 in Fig. 8d, it is speculated that the catalyst is more prone to experience path (I). On the other hand, from the comparation of the 1622 cm-1 peak attributed to the bidentate carbonate (b-CO32-), it can be seen that the pure TiO2 is more inclined to experience the longer pathway of the path (II). The results of comprehensive analysis show
atoms in Li2TiO3. The electron transport direction is from Ti atoms to Li atoms. Therefore, the direction of the built-in electric field is directed from Li2TiO3 to TiO2, thus creating a force for facilitating electron hole separation in the Li2TiO3/TiO2 heterostructure system. In summary, the reason why Li2TiO3 and TiO2 form a heterostructure may be due to the strong interaction between Li atoms in Li2TiO3 and Ti atoms in TiO2, and the charge transfer mechanism between O atoms in Li2TiO3 and Ti atoms in TiO2. In order to explore the reaction mechanism, the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was adopted as a research tool. Firstly, in the adsorption stage shown in Fig. 8a–b, the absorption peak of water (1640 cm−1) starts to decrease in the process of adsorbing and activating water molecules by the pure TiO2 for about 1 h, while the Li2TiO3/TiO2 sample had a significant downward absorption peak at 10 min, which indicates the Li2TiO3/TiO2 TiO2 is more conducive to the activation and conversion of the reactant water molecules. As the adsorption time increases, pure TiO2 exhibits the chelated bicarbonate (HCO3-, 1435 cm−1) and the monodentate carbonate (m-CO32-, 1310 cm−1), while the Li2TiO3/TiO2 sample, the obvious blue shift of the chelated bicarbonate (HCO3-, 1452 cm−1) and the monodentate carbonate (m-CO32-, 1336 cm−1) occurred and showed a stronger peak signal, indicating that the stable combination of the intermediate species on the surface of Li2TiO3/TiO2. In addition, the formation of the formate species (HCOO-, 1537 cm−1) is also detected [39–43]. These results all indicate that the introduction of Li2TiO3 is attributed to increase in the conversion of reactant water molecules as well as the intermediate formate, bicarbonates, and carbonates species on the catalyst surface to a more stable state. From the analysis in the adsorption stage, it can be preliminarily judged that the Li2TiO3/TiO2 heterostructure had better CO2 conversion efficiency. Secondly, during radiation, minor changes were observed in the amidst the peak positions. It is not difficult to find that under the condition of illumination 7
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Fig. 8. In-situ DRIFT spectra of surface adsorbed species and photocatalytic CO2 reduction intermediates over the pure TiO2 and plaque-like Li2TiO3/TiO2 heterostructure samples during (a) and (b) the adsorption stage and (c) and (d) the reaction process. (e) The possible reaction pathway for photoreduction of CO2 with H2O by using Li2TiO3/TiO2 catalyst.
necessity of energy input and the simple reaction process endow the Li2TiO3/TiO2 heterostructure a doubling performance enhancement in CO2 photoreduction. The theoretical simulation verifies the correctness of the conjecture obtained from the experimental results. Hence, the catalyst behavior and mechanism of the novel plaque-like Li2TiO3/TiO2 heterostructure catalyst have been scientifically verified from both perspectives of experiment and theoretical simulation.
that Li2TiO3/TiO2 heterostructure can improve the adsorption and conversion ability of the catalyst for the intermediates of the reactants in the adsorption stage and reaction process, and finally show the outstanding activity of photocatalytic CO2 reduction. In order to verify whether the energy barrier of path (I) can be lower and quantitatively understand the reason for doubling the performance of the Li2TiO3/TiO2 heterostructure catalyst. We calculated the free energy of the transition state based on the assumption of the catalytic mechanism proposed in in-situ DRIFT. The results are shown in Fig. 9 and the optimized supercell are shown in Fig. S9. From the calculation results of the free energy of intermediates, the energy converted to formate is much lower than that conversion to bicarbonate, and the energy is reduced by 26.53 % from 0.98 eV to 0.72 eV, indicating path (I) has a lower energy barrier compared with path (II). The lower
4. Conclusions In summary, a novel Li2TiO3/TiO2 heterostructure was prepared in an ultra-fast reaction process by the gradual intercalation of Li ions into the lattice of TiO2 from the outside and eventually formed the Li2TiO3 phase on the surface of TiO2 in an ultra-high temperature about 2000 K 8
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Fig. 9. Proposed reaction mechanism for CO2 photoreduction on Li2TiO3/TiO2 heterostructure surface.
the Shanghai Pujiang Program (18PJ1402100), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutes of High Learning, the Basic Research Program of Shanghai (17JC1402300), the Social Development Program of Shanghai (17DZ1200900), the Shanghai City Board of education research and innovation project, and the Fundamental Research Funds for the Central Universities (222201718002).
within several milliseconds. The introduction of Li species not only improve the adsorption capacity of CO2, but also reduce the oxygen defect trap to reduce the resistivity. The built-in electric field directed from Li2TiO3 to TiO2 will be attributed to promote the separation of electron-hole pairs and forces electrons move to the oxygen atom sites in Li2TiO3. The optimized Li2TiO3/TiO2 catalyst exhibits a narrower band gap energy, a more negative conduction band position of −0.884 V and better reductive ability, eventually achieving nearly a 3 times performance enhancement with about 100 % selectivity towards CO. Notably, based on the in-situ DRFITS combined with transition state calculations by DFT simulation, we proposed an possible catalytic mechanism that the Li2TiO3/TiO2 catalyst will choose an easier pathway through the intermediate formate conversion to CO, of which free energy is reduced by 26.53 % from 0.98 eV to 0.72 eV. This work will provide a new insight into facile production of highly efficient CO2 photoreduction catalysts by constructing heterostructure in an ultrafast reaction process.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118810. References [1] [2] [3] [4]
CRediT authorship contribution statement
[5] [6] [7]
Wei Bi: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Yanjie Hu: Resources, Writing - review & editing, Data curation. Nan Jiang: Validation, Formal analysis, Visualization. Ling Zhang: Writing - review & editing, Supervision. Hao Jiang: Writing - review & editing, Supervision. Xing Zhao: Validation, Visualization. Chengyun Wang: Resources, Validation. Chunzhong Li: Writing - review & editing.
[8] [9] [10] [11] [12]
Declaration of Competing Interest
[13] [14]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[15] [16]
Acknowledgements
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This work was supported by the National Natural Science Foundation of China (21978088, 91534202, 51673063), Sponsored by 9
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