Probing Surface Structures of CeO2, TiO2, and Cu2O Nanocrystals

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Probing Surface Structures of CeO, TiO and CuO Nanocrystals with CO and CO Chemisorption 2

Shilong Chen, Tian Cao, Yuxian Gao, Dan Li, Feng Xiong, and Weixin Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06158 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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The Journal of Physical Chemistry

Probing Surface Structures of CeO2, TiO2 and Cu2O Nanocrystals with CO and CO2 Chemisorption Shilong Chen‡, Tian Cao‡, Yuxian Gao, Dan Li, Feng Xiong, and Weixin Huang* Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, P. R. China. AUTHOR INFORMATION ‡

: These authors contribute equally.

Corresponding Author *Phone number: +8655163600435. Email: [email protected]

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Abstract: CO and CO2 chemisorption on uniform CeO2, TiO2 and Cu2O nanocrystals with various morphologies were comprehensively studied with in-situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy. The formed adsorbates were observed to be morphologydependent. CO or CO2 chemisorbed at the metal cation sites and bidentate and bridged carbonates involving the O sites are sensitive to the surface composition and the local coordination environments of surface metal cations and O anions and can be correlated well with the surface structures of facets exposed on oxide nanocrystals. Carbonate and carbonite species formed by CO chemisorption can probe the different facets of CeO2. Carbonate species formed by CO chemisorption can probe the different facets of TiO2. Adsorbed CO and carbonate species formed by CO chemisorption can probe the different facets of Cu2O, and adsorbed CO2 formed by CO2 chemisorption can also probe the different facets of Cu2O. These results demonstrate chemisorption of probing molecules as a convenient technique to identify surface structures of different facets of oxide nanocrystals and lay the foundations of surface structures for the fundamental understanding of catalysis and other surface-mediated functions of CeO2, TiO2 and Cu2O nanocrystals.


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1. Introduction Oxides have wide applications in heterogeneous catalysis.1-3 Fundamental understanding of oxide catalysis is very important but remains great challenge due to the structural complexity and nonuiformness of oxide particles that make it unlikely to unambiguously identify the surface structure responsible for the catalytic performance. Oxide single crystals and single crystal thin films with well-defined surface structures have been used as models of oxide catalysts for the fundamental investigations,4-7 however, arguments exist on the “pressure gap” and “materials gap” between the surface science studies of single crystals-based model catalysts under UHV conditions and the catalysis studies of powder catalysts under atmospheric and elevated pressures. Recent advances in colloidal synthesis realize controlled preparations of catalytic oxide nanocrystals with various types of uniform morphologies. Morphology-dependent catalytic performances of oxide nanocrystals have been often observed and oxide nanocrystals are being explored as a novel type of model catalysts for fundamental studies of oxide catalysis without the “pressure gap” and “materials gap”.8-12 However, a prerequisite for applications of oxide nanocrystals as model catalysts is to well define surface structures of oxide nanocrystals with different morphologies. Presently, the morphology determined by microscopes and the Wulff rule are combined to identify the facets exposed on oxide nanocrystals.13-18 For examples, uniform cubic and octahedra nanocrystals of CeO2 with a cubic fluorite structure is enclosed with six {100} facets and eight {111} facets, respectively. But such an approach encounters some difficulty in determining the facets exposed on nanocrystals with low symmetries.19-22 For examples, strong debatements exist on the facets exposed on CeO2 rods synthesized with the same recipe but calcined at different temperatures. Moreover, facets exposed on oxide


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nanocrystals mostly synthesized by the wet-chemistry synthesis methods generally exhibit less perfect structures than corresponding single crystal surfaces and always contain surface contaminants such as surface hydroxyl groups and carbonates. In addition to terrace sites, threedimensional oxide nanocrystals expose edge sites and corner sites whose densities are few on the two-dimensional single crystal surface. Thus it is of great importance to develop convenient characterization techniques for the identification of surface structures of oxide nanocrystals. Chemisorption on solid surfaces is very sensitive to surface structures. CO chemisorption probed by infrared spectroscopy has been demonstrated as an effective method to probe structures of metal surfaces,4-6, 23 but this method has not been effectively extended to oxides because CO molecules adsorbed on surfaces of different oxide powders do not exhibit surface structure-dependent characteristic vibrational features. However, we consider that chemisorption of probing molecules should be able to characterize and identify surface structures of oxide nanocrystals with the same bulk composition/structure but different morphologies/facets. DFT calculations results demonstrated that CO and CO2 chemisorption on anatase TiO224, 25 and CO chemisorption on CeO226 were surface structure-dependent. Experimentally, CO and CO2 chemisorption on various oxides was previously studied to identify the surface species formed during relevant catalytic reactions.27-34 CO adsorption on truncated bipyramidal TiO2 anatase particles dominantly exposing {101} facets,35 hexagonal Al2O3 particles dominantly exposing (110) faces,36 and MgO particles with different morphologies37 was also previously studied. However, few systematic experimental study of CO and CO2 adsorption on oxide nanocrystals with different morphologies was reported with the aim at correlating the observed surface species with the structures of dominant facets exposed on the employed oxide nanocrystals and probing the morphology-dependent surface structures.


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With the above idea, CO and CO2 chemisorption on uniform CeO2, TiO2 and Cu2O nanocrystals with various morphologies were comprehensively studied with in-situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS). Morphology-dependent adsorbates were observed and correlated well with the surface structures of facets exposed on oxide nanocrystals. These results lay the foundations of surface structures for the fundamental understanding of catalysis and other surface-mediated functions of CeO2, TiO2 and Cu2O nanocrystals and validate chemisorption of probing molecules as a convenient technique to identify surface structures of oxide nanocrystals. 2. Experimental Section All chemical reagents with the analytical grade were purchased from Sinopharm Chemical Reagent Co. and all gases (> 99.9%) were purchased from Nanjing Shang Yuan Industrial Factory and purified with a deoxy tube (Dalian Samat Chemicals Co. Ltd., oxygen outlet < 0.1 ppm). Ultrapure water with resistance > 18 MΩ was used. Synthesis of CeO2 cubes (c-CeO2) and rods (r-CeO2) followed Mai et al.’s recipe.38 Typically, 1.96 g Ce(NO3)3·6H2O was dissolved in 40 mL water and 16.88 g NaOH was dissolved in 30 mL water. The NaOH solution was added dropwise into the Ce(NO3)3 solution under stirring at RT. The mixed solution was adequately stirred for additional 30 minutes at room temperature and then transferred into a 100-mL Teflon bottle. The Teflon bottle was tightly sealed and hydrothermally treated in a stainless-steel autoclave at 180 °C for 24 h. After cooling, the obtained white precipitate was collected, washed with water, and dried in vacuo at 80 °C for 16 hours. Then the acquired yellow powder was calcined in muffle oven at 500 °C for 4 hours to synthesize c-CeO2 nanocrystals. The synthesis procedure for r-CeO2 nanocrystals was the same


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as that for CeO2 cubes except that the hydrothermal treatment temperature was 100 °C. Assynthesized r-CeO2 nanocrystals was calcined at 500 and 700 °C for 4 hours respectively to acquire r-CeO2-500 and r-CeO2-700 nanocrystals. Synthesis of CeO2 octahedra (o-CeO2) followed Yan et al.’s recipe.39 Typically, 2 mmol Ce(NO3)3·6H2O was dissolved in 79 mL water and then 1 mL Na3PO4 (0.02 M) was added. The mixed solution was adequately stirred for 1 h at room temperature and then transferred into a 100-mL Teflon bottle. The Teflon bottle was tightly sealed and hydrothermally treated in a stainless-steel autoclave at 170 °C for 10 h. After cooling, the obtained white precipitate was collected, washed with water and ethanol several times, and dried in vacuo at 80 °C for 16 h. Then, the acquired white powder was calcined in muffle oven at 500 °C for 4 h to synthesize oCeO2 nanocrystals. Synthesis of anatase TiO2 nanocrystals dominantly enclosed with the {001} facets (TiO2{001}) followed a hydrothermal procedure.18 Typically, 25 mL Ti(OBu)4 and 3 mL HF aqueous solution (40 wt.%) were mixed under stirring at RT. The solution was then transferred into a 50 mL Teflon lined stainless steel autoclave and kept at 180 °C for 24 h. The resulted white precipitate was collected by centrifugation, washed repeatedly with ethanol and water, and dried at 70 °C for 12 h. The acquired powder was dispersed in 700 mL NaOH aqueous solution (0.1 mol/L), stirred for 24 h at RT, centrifuged, and washed repeatedly with water until the pH value of aqueous solution was of 7~8. Synthesis of anatase TiO2 nanocrystals dominantly enclosed with the {100} facets (TiO2{100}) and the {101} facets (TiO2-{101}) followed Liu et al.’s recipe.17 Typically, 6.6 mL TiCl4 was added dropwise into 20 mL HCl aqueous solution (0.43 mol/L) at 0 °C. After stirring for an


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additional 0.5 h, the solution was added dropwise into 50 mL NH3 aqueous solution (5.5 wt. %) under stirring at RT. Then the pH value of the solution was adjusted to between 6 and 7 using 4 wt. % NH3 aqueous solution, after which the system was stirred at RT for 2 h. The resulted precipitate was filtered, washed repeatedly with water until no residual Cl- could be detected, and then dried at 70 °C for 12 h to acquire Ti(OH)4 precursor. To prepare TiO2-{100} nanocrystals, 2.0 g Ti(OH)4 and 0.5 g (NH4)2SO4 were dispersed in a mixture of 15 mL H2O and 15 mL isopropanol under stirring at RT, then the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The obtained white precipitate was collected and washed repeatedly with water. To prepare TiO2-{101} nanocrystals, 2.0 g Ti(OH)4 and 0.2 g NH4Cl were dispersed in a mixture of 15 mL H2O and 15 mL isopropanol under stirring at RT, then the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The obtained white precipitate was collected and washed repeatedly with water. Synthesis of Cu2O cubes (c-Cu2O) and octahedra (o-Cu2O-PVP) followed the typical procedures40: 5.0 mL NaOH aqueous solution (2.0 mol/L) was added dropwise into 50 mL CuCl2 aqueous solution (0.01 mol/L) containing different amounts of PVP (MW = 30000) (c-Cu2O: 0 g; o-Cu2O-PVP: 2.0 g) at 55 °C. After adequately stirring for 0.5 h, 5.0 ml ascorbic acid aqueous solution (0.6 mol/L) was added dropwise into the solution. The mixed solution was adequately stirred (c-Cu2O: 5 h; o-Cu2O-PVP: 3 h) at 55 °C. The resulting precipitate was collected by centrifugation and decanting, then washed with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h.


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Synthesis of Cu2O rhombic dodecahedra (d-Cu2O-OA) followed the typical procedure41: 1 mmol CuSO4 was dissolved in 40 mL water to form a clear solution into which 4 mL oleic acid and 20 mL absolute ethanol were added successively with vigorous stirring. When the mixture was heated to 100 °C, 10 mL NaOH solution (8 mmol) was added. After 5 min, 30 mL aqueous solution containing 3.42 g D-(+)-glucose was added under constant stirring. The mixture reacted for additional 60 min and a brick-red color gradually appeared. The resulting precipitate was collected by centrifugation and decanting, then washed with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h. Capping ligands on o-Cu2O-PVP and d-Cu2O-OA were removed by controlled oxidation treatment to acquire capping ligand-free Cu2O octahedra (o-Cu2O) and rhombic dodecahedra (dCu2O).42 Typically, o-Cu2O-PVP and d-Cu2O-OA were treated in the stream of mixed gas (C3H6:O2:Ar=2:1:22) respectively at 200 and 215 °C for 120 min and then cooled down to room temperature in Ar stream. Powder X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (k = 0.15406 nm) operating at 40 kV and 50 mA. BET specific surface areas were measured using a Micromeritics Tristar II 3020M system. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were acquired with a JEOL JEM-2100F instrument at an acceleration voltage of 120 kV. Infrared spectra were measured on a Nicolet 8700 Fourier transform infrared spectrometer in a transmission mode with a spectral resolution of 4 cm–1. In-situ DRIFTS measurements of CO and CO2 chemisorption were performed on a Nicolet 6700 FTIR spectrometer equipped with an in-situ low temperature and vacuum DRIFTS reaction


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cell (Harrick Scientific Products, INC) in order to enhance the chemisorption with the minimum interference of gas-phase molecules. The DRIFTS spectra were measured with 256 scans and a resolution of 4 cm-1 using a MCT/A detector. 50 mg catalyst was loaded on the sample stage of the reaction cell. Prior to adsorption experiments, the samples were evacuated at 393 K for 1 h at a base pressure of 0.01 Pa. The samples were cooled to 120 K (TiO2) and 173 K (CeO2 and Cu2O) whose spectra were taken as the background spectra, and then CO was admitted into the reaction cell to desirable pressures via a leak valve and the DRIFTS spectra were recorded after the chemisorption reaches the steady-state. CO2 chemisorption experiments were carried out similarly to CO chemisorption experiments but the chemisorption temperature was room temperature. 3. Results and Discussion 3.1 CeO2 nanocrystals. XRD patterns (Figure S1A) demonstrate the cubic fluoride phase of all CeO2 nanocrystals and the improvement of the crystallinity of CeO2 rods with the calcination temperature. BET specific surface areas of c-CeO2, o-CeO2, r-CeO2, r-CeO2-500 and r-CeO2-700 were measured to be 23, 4, 101, 69 and 50 m2⋅g-1, respectively. CeO2 nanocrystals synthesized from the wet chemistry are inevitably contaminated by formate, bicarbonate, carbonate and adsorbed hydroxyl groups/water more or less (Figure S1B). TEM and HRTEM images (Figure 1) show that all CeO2 nanocrystals are quite uniform. The lattice fringes of 0.27, 0.31 and 0.19 nm correspond to those of {200}, {111}, and {110} planes of CeO2, respectively. c-CeO2 and o-CeO2 nanocrystals with highly symmetric morphologies are dominantly enclosed with the {100} and {111} facets respectively; however, the facets exposed on less-symmetric CeO2 rods are not unambiguously determined


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and are argued to vary with the calcination temperature. The HRTEM images identify the dominant lattice fringes of CeO2 {200} and {110} in the parallel direction with the side facets of r-CeO2 and r-CeO2-500 but of CeO2 {111} in the parallel direction with the side facets of rCeO2-700. We managed to acquire a HRTEM image of the cross section of r-CeO2-700 (Figure 1E3). Together with its Fast Fourier transformed image, it suggests that the side of r-CeO2-700 should consist of four dominant {111} facets and two minor {100} facets while the top and bottom should be the {110} facets. Figures 1 F-H schematically illustrate the surface structures of CeO2 (110), (100) and (111) previously proposed by DFT calculations.26, 43, 44 Ce is eightfoldcoordinated (Ce8C) in the bulk CeO2 and O is fourfold-coordinated (O4C). The CeO2(110) surface exhibits threefold-coordinated O (O3C) and sixfold-coordinated Ce (Ce6C) on the topmost layer (Figure 1F). The CeO2(100) surface has a repeating structure of the O-Ce bilayer with a surface dipole and is stabilized by moving half of the O from the top face to the bottom face (Figure 1G) exposing twofold-coordinated O (O2C) on the topmost layer and Ce6C on the second layer. The CeO2(111) surface expose O3C on the topmost layer and sevenfold-coordinated Ce (Ce7C) on the second layer. Figure 2 shows in-situ DRIFTS spectra of CO chemisorption on various CeO2 nanocrystals at 173 K. The assignments of observed vibrational bands based on previous literatures are summarized in Table 1.45-52 It is noteworthy that the vibrational features of various types of carbonates species can be well distinguished while those of various types of bicarbonates can not. Vibrational features of CO adsorbed at Ce(IV) and Ce(III) sites appear respectively at 21702175 and 2157cm-1 (Figure 2A). The slight difference among the vibrational wavenumbers of CO adsorbed at Ce(IV) of various CeO2 nanocrystals could be likely due to the different coverages of adsorbed CO(a). CO(a) at Ce(IV) sites were observed on all CeO2 nanocrystals


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whereas CO(a) at Ce(III) sites were observed on the CeO2 nanocrystals except o-CeO2. These results suggest the presence of Ce(III) species on the surfaces of CeO2 cubes and rods but not on the surface of o-CeO2. The Ce(III) species of CeO2 is generally related with the formation of oxygen vacancy, but it can also form at the edge/corner sites of CeO2 surface due to the very low-coordination number. Our previous Raman spectroscopic results demonstrate the presence of oxygen vacancies on all CeO2 nanocrystals.53 Therefore, oxygen vacancies are present on CeO2 (110) and (100) surfaces but dominantly in the subsurface/bulk regions of CeO2(111). The facet-dependent locations of oxygen vacancies in CeO2 probed by CO chemisorption agree with previous DFT calculation and experimental results that oxygen vacancies of CeO2 (110) and (100) are more stable on the surface than in the subsurface/bulk but those of CeO2(111) are more stable in the subsurface/bulk than on the surface.54, 55 Tiny vibrational bands of CO2 adsorbed at 2423 and 2349 cm-1 were also observed on all CeO2 nanocrystals except o-CeO2. This could also be associated to the presence of surface oxygen vacancies on all CeO2 nanocrystals except oCeO2. Thus minor oxygen species adsorbed at surface oxygen vacancy sites might remain on the as-synthesized CeO2 cubes and rods and react with CO to form adsorbed CO2 during CO adsorption. CO also adsorbs at oxygen sites of CeO2 surfaces to form carbonates, bicarbonates and carbonite species whose structures depend on the morphology of CeO2 nanocrystals (Figure 2B and Table 1). CO chemisorption on o-CeO2 enclosed with the {111} facets forms dominant bidentate carbonate (ν(CO3) at 1579 and 1296 cm-1) and minor bicarbonate (ν(CO3) at 1620 cm1

) while CO chemisorption on c-CeO2 enclosed with the {100} facets forms a variety of surface

species, including bridged carbonates (ν(CO3) at 1395 and 1218 cm-1), bidentate carbonate, polydentate carbonate (ν(CO3) at 1470 cm-1), unassigned carbonate/formate (ν(OCO) at 1326


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cm-1), bicarbonates (ν(CO3) at 1605 cm-1) and carbonite (ν(OCO) at 1277 cm-1). The great difference between the surface species formed on o-CeO2 and c-CeO2 upon CO chemisorption can be associated with the surface structures of CeO2 (111) and (100) surfaces. As illustrated in Figure 1H, adjacent O3C atoms on the topmost layer of CeO2(111) surface are all connected by the same Ce7C atom on the second layer, thus only bidentate species can form but the bridged species can not (Table 1). As illustrated in Figure 1G, adjacent O2C atoms on the topmost layer of CeO2(100) surface connected by the same Ce6C atom on the second layer or not are both present, thus both bidentate and bridged carbonates can form (Table 1). Meanwhile, the carbonite species forms due to the presence of surface oxygen vacancies on the CeO2{100} facets. Surface hydroxyl groups were also observed during CO chemisorption on c-CeO2 (Figure S2A), which could be attributed to the dissociation of residual water in CO on the surface oxygen vacancies. These results demonstrate that CO chemisorption can probe the different surface structures of the facets exposed on CeO2 nanocrystals with different morphologies. The facets exposed on CeO2 rods are less defined than on o-CeO2 and c-CeO2. It was proposed that the facets exposed on CeO2 rods vary with the final calcination temperatures during the synthesis. This is supported by our DRIFTS results of CO chemisorption (Figure 2B and Table 1). On r-CeO2 without additional calcinations, CO chemisorption leads to the formation of bidentate carbonate, bridged carbonate, bicarbonate and carbonite. On r-CeO2-500 calcined at 500 °C, these surface species remain, unassigned carbonate/formate emerges. On r-CeO2-700 calcined at 700 °C, bicarbonate and carbonite are overwhelming while bidentate carbonate and unassigned carbonate/formate are also present. Indicated from the relative intensities of various vibrational bands, increasing the calcination temperature of CeO2 rods results in the quick increase and dominance of bicarbonate and carbonite species on the surface but the decrease and


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varnish of bridged carbonate species. Comparing the dominant surface species formed on oCeO2 enclosed with {111} facets and c-CeO2 enclosed with {100} facets, the bicarbonate and carbonite species on r-CeO2-700 should form on the {110} facets. As illustrated in Figure 1F, adjacent O3C atoms on the topmost layer of CeO2(110) surface are all connected by the same Ce6C atom, thus the bridged carbonate species can not form (Table 1); meanwhile, the carbonite species forms due to the presence of surface oxygen vacancies on the CeO2{110} facets. Since the bridged carbonate species is a characteristic for CO chemisorption on the CeO2{100} facets, the above CO chemisorption results suggest that the fraction of {100} facets exposed on CeO2 rods should decrease with the calcination temperature increasing, in consistence with the HRTEM results. CO2 chemisorption on various CeO2 nanocrystals at RT was also studied with in-situ DRIFTS spectra (Figure 3). The assignments of observed vibrational bands based on previous literatures are summarized in Table 1.45-52 CO2 molecules adsorbed at Ce(IV) sites give vibrational features at 2339, 2313, 2380 cm-1 while those at Ce(III) sites give a vibrational feature at 2423 cm-1 (Figure 3A). The vibrational features of CO2 adsorbed on both Ce(IV) and Ce(III) sites were observed on CeO2 cubes and rods but barely on CeO2 octahedra. This could be associated with the weaker chemisorption ability of Ce7C sites on the {111} facets exposed on o-CeO2 than Ce6C sites on the {100} and {110} facets exposed on CeO2 cubes and rods, the much smaller specific surface area of o-CeO2 than CeO2 cubes and rods, and the absence of surface oxygen vacancies on the {111} facets exposed on o-CeO2 but the presence of surface oxygen vacancies on the {100} and {110} facets exposed on CeO2 cubes and rods. CO2 chemisorption at oxygen sites of CeO2 surfaces forms carbonates and bicarbonates species (Figure 3B and Table 1). Surface carbonates and bicarbonates species formed by CO2


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chemisorption only need one surface oxygen atom while those formed by CO chemisorption need two surface oxygen atoms. Meanwhile, CO2 chemisorption on CeO2 can not form carbonite species. On o-CeO2, CO2 chemisorption dominantly forms bidentate carbonate (ν(CO3) at 1579 and 1296 cm-1), but minor vibrational features for bridged carbonate (ν(CO3) at 1410 cm-1) and unassigned carbonate/formate (ν(CO3) at 1331 cm-1) not observed for CO chemisorption also appear. The bidentate carbonate species formed upon CO2 chemisorption directly involves the Ce7C sites, thus the formation of bidentate carbonate also likely suppresses the chemisorption of CO2 at the Ce7C sites. On c-CeO2, CO2 chemisorption forms bidentate carbonate, bridged carbonate (ν(CO3) at 1405 and 1216 cm-1), bicarbonate ((ν(CO3) at 1620 cm-1) and unassigned carbonate/formate. These surface species are the same as those formed by CO chemisorption, but the polydentate carbonate species observed upon CO chemisorption does not form by CO2 chemisorption. The carbonates and bicarbonates species formed by CO2 chemisorption on CeO2 rods vary with the calcination temperature. On r-CeO2, bridged carbonate and bicarbonate species form upon CO2 chemisorption. The formed monodentate bicarbonate (ν(CO3) at 1665 cm-1) was not observed upon CO chemisorption on r-CeO2 while the bidentate carbonate species formed upon CO chemisorption was not observed for CO2 chemisorption. On r-CeO2-500, CO2 chemisorption forms bidentate carbonate, bridged carbonate, bicarbonate and unassigned carbonate or formate. These carbonate and bicarbonate species are identical to those formed by CO chemisorption although the relative intensities among various species are different. On r-CeO2-700, CO2 chemisorption forms bidentate carbonate, bridged carbonate and bicarbonate. The bridged carbonate appears upon CO2 chemisorption but not upon CO chemisorption while the unassigned carbonate/formate appears upon CO chemisorption but not upon CO2 chemisorption.


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The above CO2 chemisorption results show that, comparing CO chemisorption, CO2 chemisorption is not so characteristic to the facets. The characteristic of CO2 chemisorption on the CeO2{111} facets can be considered as the formation of dominant bidentate carbonate species without the formation of bicarbonate, but no surface species can be used to distinguish between the CeO2 {100} and {110} facets. The monodentate bicarbonate species forms on rCeO2 enclosed with the {100} and {110} facets but not on c-CeO2 and r-CeO2-500; moreover, its formation is accompanied by a great decrease of surface hydroxyl groups on CeO2 (Figure S2B). Thus the monodentate bicarbonate species might be characteristic to the surface hydroxyl groups rather than the facets. 3.2 TiO2 nanocrystals. XRD patterns (Figure S3A) demonstrate the anatase phase of all TiO2 nanocrystals. BET specific surface areas of TiO2-{001}, TiO2-{100} and TiO2-{101} were measured to be 102, 99 and 108 m2⋅g-1, respectively. As-synthesized TiO2 nanocrystals are contaminated by formate, bicarbonate, carbonate and adsorbed hydroxyl groups/water more or less (Figure S3B). As shown in Figure 4, representative TEM and HRTEM images demonstrate that as-synthesized TiO2-{001}, TiO2-{100} and TiO2-{101} nanocrystals are of quite uniform morphologies. TiO2{001} nanocrystals (Figure 4A1) are of sizes between 40-60 nm, TiO2-{100} nanocrystals (Figure 4B1) are of a width distribution of 10-15 nm and a length distribution of 20-50 nm, and TiO2-{101} nanocrystals (Figure 4C1) are of sizes between 15-30 nm. The lattice fringes resolved in the HRTEM images (Figures 4 A2-C2) all arise from anatase TiO2, in which those of 0.24, 0.19, and 0.35 nm correspond to the lattice fringes of {004}, {200}, and {101} planes of TiO2, respectively. All these microscopic results agree with previously reported results.17, 18 Assynthesized TiO2 nanocrystals all expose the {001}, {100} and {101} facets while the {110}


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facets with the highest surface energy among the low-indexed facets of anatase TiO2 are not observed. According to the previously proposed procedure,56 the percentages of {001} facets in TiO2-{001}, {100} facets in TiO2-{100} and {101} facets in TiO2-{101} nanocrystals were estimated to be all around 80%. Surface structures of anatase TiO2 (001), (100), and (101) crystal planes without structural optimization are shown in Figure 4 A3–C3, respectively. In the bulk anatase TiO2, Ti is sixfold-coordinated (Ti6C) and O is threefold-coordinated (O3C). The TiO2(001) surface exhibits twofold-coordinated O (O2C) on the topmost layer and fivefoldcoordinated Ti (Ti5C) on the second layer; the TiO2(101) surface shows O2C on the topmost layer and fivefold-coordinated Ti (Ti5c) and O3C on the second layer; the TiO2(100) surface exposes O2C, O3C, and Ti5C on the topmost layer and O3C and Ti6C on the second layer.57-59 Figure 5 shows in-situ DRIFTS spectra of CO chemisorption on various TiO2 nanocrystals at 120 K. The assignments of observed vibrational bands based on previous literatures are summarized in Table 2.25, 30-32, 60-67 CO adsorbed at Ti5C(IV) sites of TiO2-{001}, TiO2-{100} and TiO2-{101} show vibrational bands at 2181, 2183, and 2179 cm-1, respectively (Figure 5A). It was demonstrated that CO adsorbed at Ti5C sites with a stronger strength exhibited a larger vibrational frequency.68, 69 Thus, the Ti5C sites on TiO2-{101} should bind CO most weakly. Although all TiO2 nanocrystals are with similar specific surface areas, the vibrational feature of adsorbed CO on TiO2-{001} is significantly weaker than those on TiO2-{101} and TiO2-{100}. In the zoomed-in DRIFTS spectra, tiny features of 13CO adsorbed at the Ti5C sites, in proportion with the 13C natural abundance, appear at 2131, 2133, and 2129 cm-1 on TiO2-{001}, TiO2-{100} and TiO2-{101}, respectively; moreover, tiny peaks for CO adsorbed at defective Ti4C sites were observed at 2216∼2220 cm-1 and their intensities vary with TiO2 morphology in a similar trend to those of CO adsorbed at Ti5C sites. These observations demonstrate the presence of surface


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oxygen vacancies on all TiO2 nanocrystals. Tiny CO2 formation was also demonstrated by the vibrational features at ∼2341 cm-1 upon CO chemisorption and could be attributed to the reaction of CO with minor oxygen species adsorbed at surface oxygen vacancy sites of TiO2 nanocrystals. The vibrational feature of CO2 is strongest on TiO2-{101}, in consistence with the strongest vibrational feature of CO adsorbed at defective Ti4C sites of TiO2-{101}. Surface hydroxyl groups were also observed to form during CO chemisorption on all TiO2 nanocrystals (Figure S4A), which could be attributed to the dissociation of residual water in CO on the surface oxygen vacancies. CO chemisorption on TiO2 nanocrystals also forms carbonates, bicarbonates and formate species (Figure 5B and Table 2). On TiO2-{001}, bidentate carbonate (ν(CO3) at 1564 and 1335 cm-1) and bicarbonate (ν(CO3) at 1657 cm-1) are dominant but bridged carbonate (ν(CO3) at 1438 cm-1) also appears. On TiO2-{100}, in addition to bridged carbonate and bicarbonate species, formate (ν(OCO) at 1587 cm-1) also forms. On TiO2-{101}, in addition to bridged carbonate, bicarbonate and formate species, monodentate carbonate (ν(CO3) at 1510 and 1310 cm-1) forms. These results demonstrate that CO chemisorption on TiO2 nanocrystals is quite characteristic to the facets. The surface species to distinguish the TiO2 {001} facets from the TiO2 {100} and {101} facets is bidentate carbonate whose formation needs the surface ensemble of adjacent O atoms connected by the same Ti atom. As shown in Figure 4 A3-C3, adjacent O2C atoms connected by the same Ti5C atom are present on the TiO2(001) surface while adjacent O2C and O3C atoms connected by the same Ti5C atom are present on the TiO2(100) and TiO2(101) surfaces. O3C is coordination-saturated and thus less active in binding CO than O2C. Thus, the bidentate carbonate species is facilitated upon CO chemisorption on the TiO2(001) surface but not on the TiO2 (100) and (101) surfaces. However, on the TiO2(100) and TiO2(101) surfaces,


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CO can chemisorb with C and O respectively binding with O2C and Ti5C, together with the abundant surface hydroxyl groups, to form the formate species, as observed experimentally. Our experimental results also show that monodentate carbonate is characteristic for TiO2{101} and thus can distinguish the TiO2 {101} facets from the TiO2 {001} and TiO2 {100} facets. CO2 chemisorption on various TiO2 nanocrystals at RT was also studied with in-situ DRIFTS spectra (Figure 6). The assignments of observed vibrational bands based on previous literatures are summarized in Table 2.25, 30-32, 61-64 CO2 molecules adsorbed at Ti(IV) sites exhibit vibrational features at 2339∼2345 cm-1 (Figure 6A), and CO2 molecules adsorbed at Ti(III) sites form CO2species with ν(OCO) at 1667 and 1248 cm-1 (Figure 6B). The formation of CO2- species was observed on TiO2-{100} and TiO2-{101} but not on TiO2-{001}, in consistence with the more surface oxygen vacancies on TiO2-{100} and TiO2-{101} than on TiO2-{001}. CO2 chemisorption at oxygen sites mainly forms carbonates and bicarbonates species (Figure 6B). On TiO2-{001}, CO2 chemisorption dominantly forms bidentate carbonate and bicarbonates. On both TiO2-{100} and TiO2-{101}, CO2 chemisorption forms dominant bicarbonates and minor unassigned carbonates. Thus the characteristic of CO2 chemisorption on TiO2-{001} is the formation of carbonate species but the absence of CO2- species. This can be used to distinguish the TiO2 {001} facets from the TiO2 {100} and {101} facets; however, the TiO2 {100} and {101} facets can not be distinguished with CO2 chemisorption. 3.3 Cu2O nanocrystals. XRD patterns (Figure S5A) demonstrate the cubic phase of all Cu2O nanocrystals. BET specific surface areas of c-Cu2O, o-Cu2O and d-Cu2O were measured to be 1.78, 2.50 and 1.07 m2⋅g-1, respectively. Cu2O nanocrystals synthesized from the wet chemistry are inevitably


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contaminated by formate, bicarbonate, carbonate and adsorbed hydroxyl groups more or less (Figure S5B). As shown in Figures 7A1-7C1, TEM images demonstrate that c-Cu2O, o-Cu2O and d-Cu2O are uniform and respectively with sizes of 400∼700, 300∼600 and 600∼900 nm. As shown in Figure 7A2-7C2, the lattice fringes of 0.43 and 0.30 nm respectively correspond to those of {001} and {110} planes of Cu2O structure, and the view directions of c-Cu2O, o-Cu2O, and d-Cu2O are along [001], [111], and [110] respectively. Therefore, HRTEM images (Figures 7A2-7C2) confirm that c-Cu2O, o-Cu2O and d-Cu2O selectively expose {100}, {111} and {110} facets, respectively. We previously optimized the surface structures of Cu2O (100), (111) and (110) with DFT calculations that are shown in Figures 7A3-7C3. Cu is twofold-coordinated (Cu2C) in the bulk Cu2O and O is fourfold-coordinated (O4C); the Cu2O(100) surface exhibits O2C on the topmost layer and Cu2C on the second layer; The Cu2O(111) surface exhibits O3C on the topmost layer and Cu2C (75%) and onefold coordinated Cu1C (25%) on the second layer; the Cu2O(110) surface exhibits O3C and Cu2C on the topmost layer and Cu2C on the second layer. Figure 8 shows in-situ DRIFTS spectra of CO chemisorption on various Cu2O nanocrystals at 173 K. The assignments of observed vibrational bands based on previous literatures are summarized in Table 3.42, 70-73 As shown in Figure 8A, on c-Cu2O, few CO adsorbed at Cu(I) sites could be observed. On o-Cu2O, CO molecules adsorbed at Cu(I) sites show vibrational band at 2106 cm-1; meanwhile, CO2 molecules adsorbed at Cu(I) sites with strong vibrational bands at 2338, 2220 and 2192 cm-1 appear. On d-Cu2O, CO molecules adsorbed at Cu(I) sites appear and vibrational peaks of adsorbed CO2 are also present. The CO2 formation can be attributed to the reaction of CO with oxygen species adsorbed on o-Cu2O and d-Cu2O respectively acquired by controlled oxidation of as-synthesized o-Cu2O-PVP and d-Cu2O-OA. CO adsorption on Cu2O nanocrystals also forms carbonates, bicarbonates, carboxylate and


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formate species (Figure 8B and Table 3). On c-Cu2O, bridge carbonate (ν(CO3) at 1697 and 1411 cm-1), monodentate carbonate (ν(CO3) at 1470 cm-1), unassigned carbonate (ν(CO3) at 1316 cm1

) and formate (ν(OCO) at 1731 and 1572 cm-1) form. On o-Cu2O, bidentate carbonate (ν(CO3)

at 1550 and 1276 cm-1), bridge carbonate, monodentate carbonate, unassigned carbonate, bicarbonate (ν(CO3) at 1637, 1407 and 1296 cm-1), carboxylate (ν(OCO) at 1499 and 1450 cm-1) and formate form. On d-Cu2O, monodentate carbonate, bicarbonate and formate form. The above DRIFTS results demonstrate that CO chemisorption on Cu2O nanocrystals is quite characteristic to the facets. The Cu2O(100) surface enclosing c-Cu2O exposes O2C, and no adjacent O2C atoms is connected by the same Cu2C atom (Figure 7A3), thus both CO chemisorbed at Cu(I) sites and bidentate carbonate species can barely form on c-Cu2O. The Cu2O(111) surface enclosing o-Cu2O exposes O3C, Cu1C and Cu2C, and no adjacent O3C atoms is connected by the same Cu2C atom (Figure 7B3), thus CO chemisorbed at Cu(I) sites forms; moreover, chemisorbed CO2 forms due to the reaction of CO with O2 molecules chemisorbed at the active coordination-unsaturated Cu1C sites. This also indicates the likely presence of oxygen adatoms on Cu1C, creating the adjacent O3C atom and O adatom connected by the same Cu atom for the formation of bidentate carbonate, as experimentally observed. The Cu2O(110) surface enclosing d-Cu2O exposes O3C and Cu2C, and adjacent O3C atoms along [1-11] are connected by the same Cu2C atom with an almost linear structure while those along [1-10] are not (Figure 7C3), thus CO chemisorbed at Cu(I) sites forms, but CO can not chemisorb on the linear O3CCu2C-O3C structure to form bidentate carbonate. Figure 9 shows in-situ DRIFTS spectra of CO2 chemisorption on various Cu2O nanocrystals at RT. The assignments of observed vibrational bands based on previous literatures are summarized


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in Table 3.42, 70-73 As shown in Figure 9A, no CO2 molecules chemisorbed at Cu sites forms on cCu2O, in consistence with the O2C-terminated Cu{100} facets; CO2 molecules chemisorbed at Cu1C and Cu2C sites of o-Cu2O exhibit vibrational features at 2338, 2220 and 2192 cm-1; CO2 molecules chemisorbed at Cu2C sites of d-Cu2O exhibit vibrational features at 2338 cm-1. Thus the vibrational band at 2220 and 2192 cm-1 can be assigned to CO2 chemisorbed at Cu1C sites. As shown in Figure 9B, CO2 chemisorbs on O-involved sites of c-Cu2O weakly to form bidentate carbonate, which can be attributed to the low activity of coordination-saturated Cu2C to bind O of CO2. The absence of other carbonates and bicarbonates species indicates the surface stoichiometric of the Cu2O{100} facets. This also agrees with the absence of CO and CO2 chemisorbed at the Cu sites of c-Cu2O. On o-Cu2O, the formed carbonate and bicarbonate species by CO2 chemisorption are identical to those by CO chemisorption due to the presence of Cu1C sites on Cu2O {111} facets. But the formate species requiring the bonding of two oxygen in CO2 to the Cu site does not form because the coordination saturated Cu of Cu2O is twofoldcoordinated. On d-Cu2O, CO2 chemisorbs weakly to form monodentate carbonate and bicarbonate, which can also be attributed to the low activity of coordination-saturated Cu2C to bind O of CO2. Thus CO2 chemisorption is quite characteristic to the Cu2O facets, particularly concerning the CO2 chemisorbed at the Cu sites. no CO2 molecules chemisorbed at Cu sites forms on c-Cu2O; CO2 molecules chemisorbed at Cu1C and Cu2C sites form on o-Cu2O; CO2 molecules chemisorbed at Cu2C sites form on d-Cu2O. These can be used to distinguish the Cu2O {100}, {111} and {110} facets. 3.4 General discussion. Our comprehensive in-situ DFIFTS results of CO and CO2 chemisorption on various CeO2, TiO2 and Cu2O nanocrystals validate that CO and CO2 chemisorption are capable of probing the


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surface structures of different facets of the same oxide. The most useful surface species include CO or CO2 chemisorbed at the metal sites and bidentate and bridged carbonates involving the O sites. These species are well assigned and sensitive to the surface composition and the local coordination environments of surface metal and O atoms. Although the formation of bicarbonate species also depend on the surface composition and structure, their assignments are ambiguous, which makes bicarbonate species not appropriate as the probe surface species. On the basis of our results, we propose the surface species formed upon CO and CO2 chemisorption to probe the different facets of CeO2, TiO2 and Cu2O (Figure 10). Carbonate and carbonite species formed by CO chemisorption can probe the different facets of CeO2 (Figure 10A). On CeO2 {100} facets, bridged and bidentate carbonates and carbonite form; on CeO2 {111} facets, only bidentate carbonate forms; on CeO2 {110} facets, carbonite forms but bridged carbonate does not. Carbonate species formed by CO chemisorption can probe the different facets of TiO2 (Figure 10B). On TiO2 {001} facets, bidentate and bridged carbonates form; on TiO2 {100} facets, bridged carbonate forms but bidentate carbonate does not; on TiO2 {101}, bridged and monodentate carbonates form. Adsorbed CO and carbonate species formed by CO chemisorption can probe the different facets of Cu2O (Figure 10C). On Cu2O {100} facets, adsorbed CO does not form, and bridged carbonate forms; on Cu2O {111} facets, adsorbed CO and bridged carbonate form; on Cu2O {110} facets, adsorbed CO forms, but no bridged carbonate forms. Adsorbed CO2 formed by CO2 chemisorption can also probe the different facets of Cu2O (Figure 10D). On Cu2O {100} facets, adsorbed CO2 does not form; on Cu2O {111} facets,


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adsorbed CO2 at Cu1C and Cu2C exhibiting different vibrational frequencies form; on Cu2O {110} facets, only adsorbed CO2 at Cu2C forms. The above judges from CO and CO2 chemisorption, combined with microscopic analysis, can be used to identify the facets and surface structures of various types of CeO2, TiO2 and Cu2O nanocrystals, providing the foundations of surface structures for the fundamental understanding of their catalysis and other surface-mediated functions. 4. Conclusions We have successfully demonstrated CO and CO2 chemisorption probed with in-situ DRIFTS spectroscopy as a convenient technique to identify surface structures of different facets of uniform CeO2, TiO2 and Cu2O nanocrystals. CO or CO2 chemisorbed at the metal cation sites and bidentate and bridged carbonates involving the O sites are sensitive to the surface composition and the local coordination environments of surface metal cations and O anions and can be correlated well with the surface structures of the facets exposed on oxide nanocrystals. Carbonate and carbonite species formed by CO chemisorption can probe the different facets of CeO2. Carbonate species formed by CO chemisorption can probe the different facets of TiO2. Adsorbed CO and carbonate species formed by CO chemisorption can probe the different facets of Cu2O, and adsorbed CO2 formed by CO2 chemisorption can also probe the different facets of Cu2O. These results lay the foundations of surface structures for the fundamental understanding of catalysis and other surface-mediated functions of CeO2, TiO2 and Cu2O nanocrystals.

ASSOCIATED CONTENT


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Supporting Information Available. XRD patterns and infrared spectra of various CeO2, TiO2, and Cu2O nanocrystals, In-situ DRIFTS spectra in the 3780-3500 cm-1 region (O-H vibrations) of CO and CO2 chemisorption on various CeO2 and TiO2 nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.. AUTHOR INFORMATION ‡

: These authors contribute equally.

Corresponding Author * Phone number: +8655163600435. Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by National Basic Research Program of China (2013CB933104), National Natural Science Foundation of China (21525313, 21373192, U1332113), MOE Fundamental Research Funds for the Central Universities (WK2060030017), Hefei Science Center of Chinese Academy of Sciences (2015HSC-UP014) and Collaborative Innovation Center of Suzhou Nano Science and Technology. REFERENCES (1) Trovarelli, A. Catalytic Properties of Ceria and CeO2-Containing Materials. Catal. Rev. 1996, 38, 439-520. (2) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891-2959. (3) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53-229.


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(48) Li, C.; Domen, K.; Maruya, K.-i.; Onishi, T. Spectroscopic Identification of Adsorbed Species Derived from Adsorption and Decomposition of Formic Acid, Methanol, and Formaldehyde on Cerium Oxide. J. Catal. 1990, 125, 445-455. (49) Binet, C.; Badri, A.; Boutonnet-Kizling, M.; Lavalley, J.-C. FTIR Study of Carbon Monoxide Adsorption on Ceria: CO Carbonite Dianion Adsorbed Species. J. Chem. Soc., Faraday Trans. 1994, 90, 1023-1028. (50) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Krohnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F.; Knopgericke, A.; Paal, Z. Preferential CO oxidation in Hydrogen (PROX) on Ceria-Supported Catalysts, Part I: Oxidation State and Surface Species on Pt/CeO2 under Reaction Conditions. J. Catal. 2006, 237, 1-16. (51) Vayssilov, G. N.; Mihaylov, M.; Petkov, P. S.; Hadjiivanov, K. I.; Neyman, K. M. Reassignment of the Vibrational Spectra of Carbonates, Formates, and Related Surface Species on Ceria: A Combined Density Functional and Infrared Spectroscopy Investigation. J. Phys. Chem. C 2011, 115, 23435-23454. (52) Chen, S.; Luo, L.; Jiang, Z.; Huang, W. Size-Dependent Reaction Pathways of LowTemperature CO Oxidation on Au/CeO2 Catalysts. ACS Catal. 2015, 5, 1653-1662. (53) Gao, Y.; Li, R.; Chen, S.; Luo, L.; Cao, T.; Huang, W. Morphology-Dependent Interplay of Reduction Behaviors, Oxygen Vacancies and Hydroxyl Reactivity of CeO2 Nanocrystals. Phys. Chem. Chem. Phys. 2015, 17, 31862-31871. (54) Yang, Z.; Woo, T. K.; Baudin, M.; Hermansson, K. Atomic and Electronic Structure of Unreduced and Reduced CeO2 Surfaces: A First-Principles Study. J. Chem. Phys. 2004, 120, 7741-7749.


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Table 1. Assignment of vibrational bands formed upon CO and CO2 chemisorption on various CeO2 nanocrystals. The red “O” atoms represent the oxygen atom from CO or CO2 molecules.

Assignment

Surface Species Adsorption probe CO

Structure

r-CeO2

o-CeO2 1579 1296 1579 1296 --

CO

1395 1218 1396 1218 --

1395-1406 1216 --

1572 1292 1579 1293 1395 1218 1405 1216 1470

CO2

--

--

--

--

--

Carbonite

CO

1279

1279

1277

1277

--

Bicarbonate

CO2 CO

---

-1609

-1600

-1600

-1605

-1620

CO2

--

1620

1620

1620

--

CO

--

1620 1665 --

1326

1326

1326

--

CO2

--

--

1326

--

1326

1331

2175 2158

2173 2156

2170 2158

2170 --

2422

2421

2421

--

Bridged carbonate

CO2 CO CO2

Polydentate carbonate

Unassigned carbonate or formate CO(a)-Ce(IV) CO(a)-Ce(III) CO2(a)-Ce(IV) CO2(a)-Ce(III)

Ce(IV)-C-O Ce(III)-C-O Ce(IV)-O-C-O Ce(III)-O-C-O

1572 1296 1585 1290 --

c-CeO2

1572 1294 1581 1293 1385-1410 1218 1395-1406 1216 --

Bidentate carbonate

1575 1294 --

Bands(cm-1) r-CeO2-500 r-CeO2-700

2173 2156 2313-2380 2425

1410 --


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Table 2. Assignment of vibrational bands formed upon CO and CO2 chemisorption on various TiO2 nanocrystals. The red “O” atoms represent the oxygen atom from CO or CO2 molecules.

Assignment Bidentate carbonate Bridged carbonate

Monodentate carbonate

Surface Species Adsorption probe CO

Structure

TiO2-{001}

Bands(cm-1) TiO2-{100}

TiO2-{101}

--

--

--

--

CO

1564 1335 1565 1318 1438

1438

1438

CO2

--

--

--

CO

--

--

CO2

--

--

1510 1310 --

--

--

--

--

1667 1248

1667 1248

CO2

CO

--

CO2CO2 CO

--

1657

1665

1665

CO2

--

1674 1642 1396

1667 1628 1409 1222

1667 1635 1422 1224

CO CO2 CO2

---

----

1587 -1367

1579 -1361

2181 2219 2131 2345

2183 2220 2133 2339

2179 2216 2129 2339

Bicarbonate

Formate Unassigned carbonate CO(a)-Ti5c CO(a)-Ti4c 13 CO(a)-Ti5c CO2(a)-Ti

Ti5c-C-O Ti4c-C-O Ti5c-13C-O Ti-O-C-O


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Table 3. Assignment of vibrational bands formed upon CO and CO2 chemisorption on various Cu2O nanocrystals. The red “O” atoms represent the oxygen atom from CO or CO2 molecules. Surface Species Assignment Adsorption probe CO Bidentate carbonate

Structure

--

--

CO2

1549 1263 1697 1411 --

1550 1276 1550 1276 1407 1407

--

CO

1470

1467

1469

CO2

--

1467

1469

CO

--

1499 1450

--

CO2

--

1499 1448

--

1637 1407 1296 1641 1407 1296 1716

1624

CO2 Bridged carbonate

CO

Monodentate carbonate

Carboxylate

Bicarbonate

Bands(cm-1) c-Cu2O o-Cu2O d-Cu2O

CO

--

--

CO2

--

--

Formate

CO

--

Unassigned carbonate

CO2 CO

---

1731 1572 -1316

CO2

--

--

CO(a)-Cu(I) CO2(a)-Cu2c CO2(a)-Cu1c

Cu(I)-C-O Cu2c-O-C-O Cu1c-O-C-O

2101 ---

-1368 1322 1358 1322 2106 2338 2220 2192

---

1638

1726 1577 ---2106 2338 --


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Figure captions Figure 1. TEM and HRTEM images of (A1, A2) c-CeO2-{100}, (B1, B2) o-CeO2-{111}, (C1C3) r-CeO2, (D1-D3) r-CeO2-500, and (E1, E2) r-CeO2-700. The insets schematically illustrate the crystal planes exposed on the CeO2 rods, cubes, and octahedra. (E3) the cross-sectional HRTEM image of r-CeO2-700, the inset shows the Fast Fourier-transformed pattern of the HRTEM image. Surface structures of (F) CeO2(110), (G) CeO2(100), (H) CeO2(111) crystal planes. Red, blue, cyan, white, yellow and green represent O4c, O2c, O3c, Ce8c, Ce7c and Ce6c atoms, respectively. Figure 2. In-situ DRIFTS spectra of CO chemisorption on various CeO2 nanocrystals at 173 K and PCO = 250 Pa: (A) CO and CO2 regions and (B) C-O vibration region. Figure 3. In-situ DRIFTS spectra of CO2 chemisorption on various CeO2 nanocrystals at RT and PCO2 = 300 Pa: (A) CO2 region and (B) C-O vibrations region. Figure 4. TEM and HRTEM images of (A1, A2) TiO2-{001}, (B1, B2) TiO2-{100} and (C1, C2) TiO2-{101}. The insets schematically illustrate the predominantly exposed facets on TiO2{001}, TiO2-{100}, and TiO2-{101}. Surface structures of anatase TiO2 (001) (A3), (100) (B3), and (101) (C3) crystal planes. Red, green, white and yellow balls represent O3c, O2c, Ti6c and Ti5c atoms, respectively. Figure 5. In-situ DRIFTS spectra of CO chemisorption on various anatase TiO2 nanocrystals at 120 K and PCO = 200 Pa: (A) CO and CO2 regions and (B) C-O vibrations region. The inset shows zoomed-in spectra.


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Figure 6. In-situ DRIFTS spectra of CO2 chemisorption on various anatase TiO2 nanocrystals at RT and PCO2 = 500 Pa: (A) CO2 region and (B) C-O vibrations region. Figure 7. TEM and HRTEM images of (A1, A2) Cu2O cubes, (B1, B2) capping-ligand-free Cu2O octahedra, (C1, C2) capping-ligand-free Cu2O rhombic dodecahedra. Surface structures of Cu2O (100) (A3), (111) (B3), and (110) (C3) crystal planes. Red, brick-red and green balls represent O, CuCSA and CuCUS atoms, respectively. Figure 8. In-situ DRIFTS spectra of CO chemisorption on various Cu2O nanocrystals at 173 K and PCO = 250 Pa: (A) CO region and (B) C-O vibration region. Figure 9. In-situ DRIFTS spectra of CO2 chemisorption on various Cu2O nanocrystals at RT and PCO2 = 400 Pa: (A) CO2 region and (B) C-O vibration region. Figure 10. Schematic illustrations of characteristic surface species formed on (A) CeO2 (100), (111), (110) surfaces via CO chemisorption, (B) anatase TiO2 (001), (100), (101) surfaces via CO chemisorption, (C) Cu2O (100), (111), (110) surfaces via CO chemisorption, and (D) Cu2O (100), (111), (110) surfaces via CO2 chemisorption.


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Figure 1. TEM and HRTEM images of (A1, A2) c-CeO2-{100}, (B1, B2) o-CeO2-{111}, (C1C3) r-CeO2, (D1-D3) r-CeO2-500, and (E1, E2) r-CeO2-700. The insets schematically illustrate the crystal planes exposed on the CeO2 rods, cubes, and octahedra. (E3) the cross-sectional HRTEM image of r-CeO2-700, the inset shows the Fast Fourier-transformed pattern of the HRTEM image. Surface structures of (F) CeO2(110), (G) CeO2(100), (H) CeO2(111) crystal planes. Red, blue, cyan, white, yellow and green represent O4c, O2c, O3c, Ce8c, Ce7c and Ce6c atoms, respectively.


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Figure 2. In-situ DRIFTS spectra of CO chemisorption on various CeO2 nanocrystals at 173 K and PCO = 250 Pa: (A) CO and CO2 regions and (B) C-O vibration region.


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Figure 3. In-situ DRIFTS spectra of CO2 chemisorption on various CeO2 nanocrystals at RT and PCO2 = 300 Pa: (A) CO2 region and (B) C-O vibrations region.


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Figure 4. TEM and HRTEM images of (A1, A2) TiO2-{001}, (B1, B2) TiO2-{100} and (C1, C2) TiO2-{101}. The insets schematically illustrate the predominantly exposed facets on TiO2{001}, TiO2-{100}, and TiO2-{101}. Surface structures of anatase TiO2 (001) (A3), (100) (B3), and (101) (C3) crystal planes. Red, green, white and yellow balls represent O3c, O2c, Ti6c and Ti5c atoms, respectively.


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Figure 5. In-situ DRIFTS spectra of CO chemisorption on various anatase TiO2 nanocrystals at 120 K and PCO = 200 Pa: (A) CO and CO2 regions and (B) C-O vibrations region. The inset shows zoomed-in spectra.


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Figure 6. In-situ DRIFTS spectra of CO2 chemisorption on various anatase TiO2 nanocrystals at RT and PCO2 = 500 Pa: (A) CO2 region and (B) C-O vibrations region.


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Figure 7. TEM and HRTEM images of (A1, A2) Cu2O cubes, (B1, B2) capping-ligand-free Cu2O octahedra, (C1, C2) capping-ligand-free Cu2O rhombic dodecahedra. Surface structures of Cu2O (100) (A3), (111) (B3), and (110) (C3) crystal planes. Red, brick-red and green balls represent O, CuCSA and CuCUS atoms, respectively.


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Figure 8. In-situ DRIFTS spectra of CO chemisorption on various Cu2O nanocrystals at 173 K and PCO = 250 Pa: (A) CO region and (B) C-O vibration region.


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Figure 9. In-situ DRIFTS spectra of CO2 chemisorption on various Cu2O nanocrystals at RT and PCO2 = 400 Pa: (A) CO2 region and (B) C-O vibration region.


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Figure 10. Schematic illustrations of characteristic surface species formed on (A) CeO2 (100), (111), (110) surfaces via CO chemisorption, (B) anatase TiO2 (001), (100), (101) surfaces via CO chemisorption, (C) Cu2O (100), (111), (110) surfaces via CO chemisorption, and (D) Cu2O (100), (111), (110) surfaces via CO2 chemisorption.


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TOC GRAPHICS


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Probing Surface Structures of CeO2, TiO2, and Cu2O Nanocrystals

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