Synthesis and Optimization of Ti/Li/Al Ternary Layered Double Hydroxides for Efficient Photocatalytic Reduction of CO2 to CH4

A series of Ti/Li/Al ternary layered double hydroxides (TiLiAl-LDHs) with different Ti:Li:Al molar ratios were prepared by a coprecipitation method for photocatalytic CO2 reduction. It was demonstrated that the contents of anions between the layers of Ti/Li/Al-LDHs greatly determined the photocatalytic activity for CO2 reduction. With Ti:Li:Al molar ratios optimized to be 1:3:2, the largest contents of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\bf{CO}}}_{{\bf{3}}}^{{\bf{2}}}$$\end{document}CO32− anion and hydroxyl group were obtained for the Ti1Li3Al2-LDHs sample, which exhibited the highest photocatalytic activity for CO2 reduction, with CH4 production rate achieving 1.33 mmol h−1 g−1. Moreover, the theoretical calculations show that Ti1Li3Al2-LDHs is a p-type semiconductor with the narrowest band gap among all the obtained TiLiAl-LDHs. After calcined at high temperatures such as 700 °C, and the obtained TiLiAl-700 sample showed much increased photocatalytic activity for CO2 reduction, with CH4 production rate reaching about 1.59 mmol h−1 g−1. This calcination induced photocatalytic enhancement should be related to the cystal structure transformation from hydrotalcite to mixed oxides containing high reactive oxygen species for more efficient CO2 reduction.

The greenhouse effect caused by greenhouse gases has seriously affected people's life and social development 1,2 . CO 2 has been believed as a major greenhouse gas, and its emission controlling has become a key problem faced by our human society 3,4 . Physical adsorption and photocatalytic reduction have been considered to be the most promising methods for CO 2 emission reduction 5,6 . However, photoreduction of CO 2 is a complex and difficult reaction with a lot of carbon species produced 7 , the searching for high efficiency adsorbents and photocatalysts is the key priority to the technological breakthrough 8 . Until now, different kinds of semiconductors, including oxides, sulfides and nitrides, have been developed for photocatalytic CO 2 reduction, and yet shown unsatisfying activity and selectivity 9 .
In recent years, hydrotalcite like compounds (i.e., layered double hydroxides, LDHs) have been widely used in the study of CO 2 adsorption and photocatalysis, because of its unique physical and chemical properties as well as excellent catalytic properties. Recently, the use of hydrotalcite as CO 2 adsorbent or photocatalyst has been reported in many literatures, aiming at the enhancement of adsorption and catalytic properties. For instance, Chang et al. 10 found that in the Ca/Al based hydrotalcite the highly dispersed inert alumina calcium oxide coated on the surface of calcium oxide could effectively prevent the aggregation of calcium oxide particles, thereby improving the stability of adsorbent, and enhancing the hydrotalcite adsorption performance. Iguchi et al. 11 prepared Al-LDHs composite by a co-precipitation method, which showed considerable photocatalytic activity for converting CO 2 to CO. Trough co-precipitation method, hydrothermal method and roasting recombination method, Zhao et al. 12 obtained a series of TiO 2 /MgAlTi-LDHs and found that the photocatalytic properties of the composite were related to the crystal shape and the crystal form of TiO 2 loaded on hydrotalcite and the adsorption surface area. Given the fact of low conversion and utilization of CO 2 , in our previsous study 13 , a new type of lithium aluminum hydrotalcite (Ti/Li/Al-LDHs) was developed for its utilization in efficient CO 2 adsorption. Ti/Li/Al-LDHs with chemical composition optimized showed quite high efficiency for CO 2 absorption and capture, and moreover the calcined Ti/Li/Al-LDHs exhibited much increased performance towards CO 2 adsorption. As inspired by these observations, herein, a series of Ti/Li/Al-LDHs with different Ti/Li/Al molar ratios were prepared by co-precipitation method and further calcined at different temperatures. It was found that the photocatalytic activity for CO 2 reduction was greatly on the dependence of the Ti/Li/Al molar ratios, and high temperature calcination could further increase the activity for photocatalytic conversion of CO 2 to CH 4 . In this study, the chemical compositions were optimized and the reaction active sites were modulated to elucidate the relationship between the structure and the photocatalytic activity, which provides new ideas and theoretical guides for the further design of high efficiency photocatalyst for CO 2 reduction.

Results
In this study, a series of Ti/Li/Al ternary hydrotalcites (TiLiAl-LDHs) with different Ti:Li:Al molar ratios was obtained by co-precipition method, and further calcined at different temperatures, with Ti/Li/Al molar ratios determined and summarized in Table 1. One will observe that with the increasing Al contents in the precursor solution, the Ti/Li/Al molar ratios in the obtained TiLiAl-LDHs samples could be well tuned, which display the same tendency as the Ti/Li/Al molar ratios in precursor solutions. Figure 1a shows the XRD patterns of Ti/Li/ Al-LDHs with different Ti:Li:Al molar ratios. One can easily observe that all samples exhibit sharp and clear peaks corresponding to the crystal indexes of (003), (006), (009), (105), (108), (110), and (113), respectively, which are matching well with the the layered structure of LDHs. No other peaks appear, indicating the single crystalline phase for these synthesized hydrotalcite samples. It is also easily noted that the (006) peak appears a doublet peak character, which is similar to Li/Al-LDHs reported in the previous literature 14 , indicating that monovalent Li + ions exist between the layers. However, because of the small quantity of Li + charge, the crystal cell structure is deformed due to the uneven charge density with Li + entering the laminate, leading to the splitting of the (006) peak 15 . Further comparison will find that the XRD peak intensity of Ti/Li/Al-LDHs decreases gradually with the Al 3+ content decreasing. It is clear that the decreased contents of interlayer Al 3+ will lead to the increased proportion of Ti 4+ . Thus, these Ti 4+ could produce more positive charges, which cause repulsion between the neighboured layers, resulting in the deformation of crystal structures and thus the decreased crystallinity 16 . Figure 1b shows the XRD patterns of Ti 1 Li 3 Al 2 -LDHs calcined at different temperatures. With the calcination temperatures increasing, the XRD peaks assigned to hydrotalcite show gradual decrease in peak intensity, and the the (003) and (110) diffraction peaks even disappear with the calcination temperature higher 300 °C. At the same time, some new peaks with crystal indexs of (211), (311), (400) and (440) emerge, which could could be assigned to mixed oxides such as Al 2 TiO 5 and spinel Li 4 Ti 5 O 12 . This observation demonstrates that high temperature calcination will destroy the layer structure of hydrotalcite, giving rise to the crystal structure transformation from hydrotalcite to oxides and spinel 17,18 .   19 . It can be seen that the hydroxyl peak decreases gradually with the increase in calcination temperatures, indicating the dehydration of the interlayer hydroxyl groups. The absorption peaks at 1389 cm −1 and 1038 cm −1 , which are characteristic of the telescopic − CO 3 2 carbon oxygen bond and carbon oxygen bond vibration absorption peak, respectively 20 , shows gradually weakened intensities and even disappears, implying the removal of CO 3 2− during high temperature calcination. The absorption peaks located at 540 cm −1 and 744 cm −1 should be related to the Ti-O bond and the Li-O bond 21 , both of which gradually fuse into a wide absorption peak with intensities weakened depending on the increasing calcination temperatures. All these observations suggest the destroyed layered structure of hydrotalcite and the crystal structure transformation from hydrotalcite to mixed oxides of Li 4 Ti 5 O 12 and spinel Al 2 TiO 5 during high temperature calcination 22 .
The morphology of Ti/Li/Al-LDHs was investigated by SEM images. With very similar morphology obtained for those Ti/Li/Al-LDHs with different Ti:Li:Al molar ratios, herein, the morphology of Ti 1 Li 3 Al 2 -LDHs was analyzed as the typical sample in details. Clearly, the Ti 1 Li 3 Al 2 -LDHs sample shows morphology in cluster-like coral reefs with serious particles agglomeration (Fig. 3a), due to the large free energy of the nanoparticles system 23 . One could also easily observe that the Ti 1 Li 3 Al 2 -LDHs particles are comprised of numerous petal-like nanosheets with thickness estimated to be tens of nanometers (Fig. 3b), which evidences the layered structure of the obtained tenary hydrotalcites. When calcined at different temperatures, the Ti 1 Li 3 Al 2 -LDHs shows a significant change in morphology, as shown in Fig. 3c-f. In comparison to Ti 1 Li 3 Al 2 -180 with layer structure well maintained ( Fig. 3c), Ti 1 Li 3 Al 2 -300 shows slight deformation in the lamellar structure (Fig. 3d), and further increase in calcination temperatures brings significant morphology change to Ti 1 Li 3 Al 2 -500 and Ti 1 Li 3 Al 2 -700 with lamellar structure almost completely destroyed (Fig. 3e,f). It is well known that the increase in free energy of nanoparticle system will lead to nanoparticles agglomeration. After dehydration, Ti 1 Li 3 Al 2 -LDHs particles are more easily to agglomerate, especially after high temperature calcination, as induced by the increasing free energy of nanoparticles, which should be the main reason for the morphology change. Moreover, high temperature calcination gives rise to the crystal structure transformation from hydrotalcite to LiO, Li 4 Ti 5 O 12 , Al 2 TiO 5 and other mixed oxides with low crystallization as supported by XRD and FTIR analysis.
The optical properties of Ti/Li/Al-LDHs with different Ti:Li:Al molar ratios were investigated by UV-vis diffuse reflectance spectra. As shown in Fig. 4a, all the Ti/Li/Al-LDHs samples show strong optical absorption in the ultraviolet light region. With optical absorption onsets (λ 0 ) determined to be 397 nm, 413 nm, 401 nm and 408 nm, respectively, the band gaps (E g ) could be calculated to be 3.23 eV, 3.10 eV, 3.14 eV and 3.13 eV for Ti 1 Li 3 Al 1 -LDHs, Ti 1 Li 3 Al 2 -LDHs, Ti 1 Li 3 Al 3 -LDHs and Ti 1 Li 3 Al 4 -LDHs, by the E g ~ λ 0 relationship (i.e., E g = hc/λ 0 = 1240/λ 0 ), herein, h is the Planck constant, and c is the speed of light 24 . It is clear that with the increase of Al 3+ content, Ti/Li/Al-LDHs shows very similar band gaps, which might not be determinative to their different photocatalytic activities. Given O 2p states mainly comprising the valence band of the hydrotalcites 25-27 , the conduction band of the Ti/Li/Al-LDHs samples should be negative enough for driving the CO 2 reduction reaction, as deduced from their band gaps (>3.0 eV) 28 . However, all the Ti/Li/Al-LDHs samples possess band gap much smaller than the previously reported (Cu)/Zn(Fe)/Al-LDHs (about 4.10-4.50 eV) 29,30 . Thus, it could be deduced that these obtained Ti/Li/Al-LDHs could efficiently utilize ultraviolet light to excite electrons from valence band to conduction band to trigger photocatalytic conversion of CO 2 to CH 4 . After calcined at different temperatures, www.nature.com/scientificreports www.nature.com/scientificreports/ the Ti 1 Li 3 Al 2 -LDHs shows obvious change in optical absorption. As shown in Fig. 4b, the Ti 1 Li 3 Al 2 -T (T = 180, 300, 500, and 700) samples show optical absorption onsets at 404 nm, 398 nm, 414 nm and 427 nm, respectively, with band gaps determined to be 3.07 eV, 3.12 eV, 3 eV and 2.90 eV. Comparative analysis demonstrates that the band gap of Ti 1 Li 3 Al 2 -T is widened first and then narrowed, depending on the increasing calcination temperature. These results imply that the conductivity of hydrotalcite decreases first and then increased, with the gradual deformation of the layered structure and the crystal structure transformation from hydrotalcite to mixed metal oxides during high temperature calcination. For instance, with the calcination temperature higher than 500 °C, the lamellar structure of Ti 1 Li 3 Al 2 -T (T = 500 and 700) collapses completely, with hydrotalcite transformed to Li 4 Ti 5 O 12 , Al 2 TiO 5 and other spinel mixed oxides, which should give rise to the obvious red shift in optical absorption and then benefit the photocatalytic CO 2 reduction reaction for the Ti 1 Li 3 Al 2 -700 sample as discussed in the following sections.
Given the narrowest band gap of Ti 1 Li 3 Al 2 -LDHs benefiting the photocatalytic process, the crystal structure and electronic structure of Ti 1 Li 3 Al 2 -LDHs were further optimized and calculated by Density Functional Theory 31 . As shown in Fig. 5a, the 2H model is adopted in the Al-LDHs structure and the [Al 8 (OH) 16 ] 2 CO 3 fundamental model is constructed. The CO 3 2− unit in the hcp-Al position is located in the middle of two parallel plates (The triangle formed by the three oxygen atoms on the top of the CO 3 2− is located in the interior of the three hydroxyl oxygen of the aluminum atom of the laminated plate). Based on the structure of Al-LDHs, three representative structures of Ti 1 Li 3 Al 2 -LDHs were constructed by atomic substitution, as shown in Fig. 5b-d. In all the three structures, the − CO 3 2 species tends to locate in the middle of the two adjacent layers, and does not appear to  The photocatalytic activity for the reduction of CO 2 with water vapor was measured on the obtained Ti/Li/ Al-LDHs under ultraviolet light irradiation. Without light irradiation or CO 2 , there is no product detected. As shown in Fig. 6a, all the Ti/Li/Al-LDHs samples display good photocatalytic activity for CO 2 reduction with CH 4 obtained as the main product; while other products can be hardly detected, meaning the high selectivity of CH 4 generation from CO 2 photoreduction. For all the samples, with the photocatalytic reaction proceeding, the CH 4 production rates increase first and then stabilize after ca. 2 hours. This could be explained by the adequate illumination condition which is necessary to ensure the photocatalytic CO 2 conversion reaction, and help Ti/Li/ Al-LDHs generate enough electrons to drive the reaction of CO 2 with H 2 O, producing H• and •CO 2 − as the two main intermediates for CH 4 generation 32 . It is observable that the photocatalytic activities for CO 2 reduction is of great dependence on the Ti:Li:Al molar ratios. In comparison, the Ti 1 Li 3 Al 2 -LDHs sample shows the highest photocatalytic activity for CO 2 reduction, with CH 4 production rate achieving as high as 1.33 mmol h −1 g −1 . As confirmed in our previous study 13 3 2 in Ti 1 Li 3 Al 2 -LDHs are the highest, which should contribute to the highest photocatalytic activity for CO 2 reduction, given the inherent adsorption of water and CO 3 2− between hydrotalcite layers for efficient photocatalytic CO 2 reduction reaction. The optical absorption property and the density functional calculation results suggest the characteristics of p-type semiconductor for Ti 1 Li 3 Al 2 -LDHs with the narrowest band gap among all the Ti/Li/Al-LDHs samples, which are also favorable for photocatalytic CO 2 reduction.
With the Ti 1 Li 3 Al 2 -LDHs sample undergoing high temperature calcination, the photocatalytic CO 2 reduction activity is decreased first and then increased, depending on the increasing calcination temperatures, as shown in Fig. 6b. The highest photocatalytic CO 2 reduction activity is achieved over the Ti 1 Li 3 Al 2 -700 sample, with CH 4 production rate reaching 1.59 mmol h −1 g −1 . As demonstrated by the XRD, FTIR and SEM analysis, high temperature calcination will cause the deformation of the layered structure and more importantly the crystal structure transformation from hydrotalcite to mixed metal oxides such as Li 4 Ti 5 O 12 and Al 2 TiO 5 . Especially for the samples calcined at temperatures higher than 500 °C, for instance, Ti 1 Li 3 Al 2 -700, the mixed oxides with high reactive oxygen species is generated in a large amount, which benefits the photocatalytic decomposition of H 2 O to produce active hydrogen species, activating the photocatalytic CO 2 reduction for CH 4 production 33 .
It is well recognized that photocatalytic CO 2 reduction process must be involved with several steps, including CO 2 adsorption, light absorption and photoexcitation, charge separation and transfer, surface CO 2 reduction reaction, and each step could determine the CO 2 photoreduction activity. In this study, Ti/Li/Al-LDHs was used as photocatalyst for CO 2 reduction. Given the almost the same band gaps of Ti/Li/Al-LDHs with different Ti:Li:Al molar ratios (Fig. 4a), light absorption and photoexcitation should not be the determinant for their different photocatalytic activities. Interestingly, as shown in Fig. 7a, with the increasing Ti:Li:Al molar ratios, both the surface area and the CO 2 adsorption capacity reach the highest for Ti 1 Li 3 Al 2 -LDH, which corresponds well with its highest photocatalytic activity. This observation implies that the CO 2 adsorption capacity should greatly determine the photocatalytic CO 2 reduction activities of these obtained Ti/Li/Al-LDHs samples. For Ti 1 Li 3 Al 2 -LDHs calcined at different temperatures, depending on the increasing calcination temperatures, one can hardly find any relationship between the surface area and the CO 2 adsorption capacity (Fig. 7b). It is somewhat surprising that Ti 1 Li 3 Al 2 -700 display the smallest CO 2 adsorption capacity, despite of the highest surface area. More importantly, www.nature.com/scientificreports www.nature.com/scientificreports/ Ti 1 Li 3 Al 2 -700 shows the highest photocatalytic activity for CO 2 reduction, which indicates that in this case some other reasons rather than the CO 2 adsorption capacity should be decisive to the CO 2 photoreduction activity. Given the band gap determining the light absorption ability and then the electron photoexcitation, the highest CO 2 photoreduction activity achieved over Ti 1 Li 3 Al 2 -700 should be then mainly due to its reduced band gap haversting more light for photoexcitation (Fig. 4b).
As an excellent photocatalyst, except for the high photocatalytic activity, good stability is also highly required. Herein, the photocatalytic stabilities of Ti 1 Li 3 Al 2 -LDHs and Ti 1 Li 3 Al 2 -700 were measured during a 10-cycle photocatalytic reaction. As shown in Fig. 6c and d, both Ti 1 Li 3 Al 2 -LDHs and Ti 1 Li 3 Al 2 -700 exhibit considerable photocatalytic stability during the 10-cycle photocatalytic measurement (ca. 2500 min), with photocatalytic CH 4 production rates decreased by only 19% and 12%, respectively. Given the almost unchanged XRD patterns of the samples after the CO 2 photoreduction reaction (data not shown), such decrease in photocatalytic activity  www.nature.com/scientificreports www.nature.com/scientificreports/ of Ti 1 Li 3 Al 2 -LDHs should be due to the inactivation of photoactive sites in the layered structure of hydrotalcite, while high temperature calcination will turn these active sites to high reactive oxygen species in mixed oxides, which should be more stable for photocatalytic CO 2 reduction, ensuring the better stability of Ti 1 Li 3 Al 2 -700. Therefore, to obtain a high efficiency LDHs photocatalyst, it is highly desired to optimize the cation molar ratio and then identify the photocatalytic active sites to elucidate the photocatalytic mechanism for CO 2 reduction with high activity, selectivity and stability.

Discussion
In this study, a series of tenary Ti/Li/Al-LDHs with different Ti:Li:Al molar ratios was synthesized by a simple co-precipitation method, and investigated for photocatalytic CO 2 reduction. It was found that the Ti/Li/Al-LDHs sample with Ti:Li:Al molar ratio optimized to be 1:3:2 displayed the highest photocatalytic activity for CO 2 reduction, with CH 4 production rate achieving 1.33 mmol h −1 g −1 , which should be related to the the interlayer anion content and the narrowest band gap, as supported by both experimental and theoretical evidences. After calcined at different temperatures, the photocatalytic activity for CO 2 reduction could be further improved. Especially, for the sample calcined at 700 °C, the photocatalytic CH 4 production rate was increased to be 1.58 mmol h −1 g −1 , and the activity was well maintained during a 10-cycle measurement. Such photocatalytic enhancement should be mainly attributed to the cystal structure transformation from hydrotalcite to mixed oxides containing high reactive oxygen species for more efficient CO 2 reduction. This study successfully developed a facile approach to prepare multiple cations contained LDHs as efficient and stable photocatalysts for CO 2 reduction, and demonstrated that the composition optimization and reactive site evolution could be pivotal to the high efficiency photocatalytic CO 2 reduction. Characterizations. The molar ratios of the samples were determined by Atomic Absorption Spectrometer (AAS Various 6, Analytik Jena AG, German). X-ray diffraction (XRD) patterns were collected on a MiniFlex600 desktop X-ray diffractometer operated at 30 kV and 10 mA using Cu Kα irradiation (Wavelength = 1.5406 Å). Scanning electron microscopy (SEM) images were recorded on a S-4800 scanning electron microscope at an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were recorded on a Brook Tensor 27 Fourier transform infrared spectrometer) using the KBr pellet technique. UV-vis diffuse reflectance spectra were recorded on a Perkin Elmer Lambda 950 ultraviolet spectrophotometer. N 2 adsorption-desorption isotherms were conducted at 77 K using an Accelerated Surface Area and Porosimetry Analyzer (ASAP2010, Micromeritics) after degassing the samples at 100 °C for 4 h. The specific surface areas were determined by the Brunauer-Emmett-Teller (BET) methods. The CO 2 adsorption capacity was calculated from the weights of the samples before and after CO 2 adsorption in a home-made quartz fixed bed reactor (Fig. 8). www.nature.com/scientificreports www.nature.com/scientificreports/ Computation methods. The crystal structure and electronic structure of Ti 1 Li 3 Al 2 -LDHs were optimized and calculated by Density Functional Theory. Based on the structure of Al-based hydrotalcite, the structure of Ti 1 Li 3 Al 2 -LDHs was constructed by atomic substitution, and built by Statistical Disorder included in Materials Studio. Statistical Disorder can build all possible structural models in an exhaustive way. The final structure model has 13186 structures, and each containing 144 atoms. Herein, we choose three representative structures, and calculated them in arithmetic average. Three structures are named as Ti 1 Li 3 Al 2 -LDHs-(I), Ti 1 Li 3 Al 2 -LDHs-(II), and Ti 1 Li 3 Al 2 -LDHs-(III), which will be described in Results and Discussion.

Materials preparation.
In order to predict the stable structure of hydrotalcite with specific measurement ratio, the structure is optimized. All calculations are performed using the CASTEP module in the Materials Studio package. By using Broyden-Fletcher-Goldfarb-Shanno structure optimization (BFGS) algorithm, the atomic positions and cell parameters are optimized at the same time, the convergence of standard energy is 5.0 × 10 −6 eV·atom −1 , the convergence criteria for each atomic force is less than 0.01 eV/Å, the displacement deviation is 5 × 10 −4 Å, and the pressure deviation is 0.02 GPa. The exchange correlation functionals are LDA-CA-PZ, the pseudopotential uses the OTFG form of the super soft pseudopotential, and the electron minimization method uses the Pulay density mixing method (Density, Mixing). The mixed error self-consistent field calculation is 5 × 10 −7 eV·atom −1 , the truncation kinetic energy is 630 eV, while the Brillouin zone K vector is selected for the 5 × 5 × 2. The total charge number is 0, and the inter layer has a weak interaction, such as van Edward force.
Photocatalytic CO 2 reduction. The activity of photocatalytic conversion of CO 2 to CH 4 was tested in a continuous entry type system (Fig. 8), including a home-made quartz fixed bed reactor equipped with an on-line gas detection system (Agilent 7890 A gas chromatograph). For photocatalytic CO 2 reduction, 1.0 g of photocatalyst was loaded in the middle part of the quartz tube reactor, with temperature heated up to 60 °C. CO 2 gas with the flow of 80 mL/min was continuously passed through the water vapor generator and then introduced into the photocatalytic reactor. CO 2 photocatalytic reduction was triggered under UV irradiation, with produced gas analyzed by on-line gas chromatograph with the temperature of the injector, column and detector set at 120 °C, 50 °C, 100 °C, respectively.