Probing ligand-induced cooperative orbital redistribution that

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Probing ligand-induced cooperative orbital redistribution that dominates nanoscale molecule-surface interactions with one-unit-thin TiO2 nanosheets Guolei Xiang, Yan Tang, Zigeng Liu, Wei Zhu, Haitao Liu, Jiaou Wang, Guiming Zhong, Jun Li, and Xun Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03572 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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Probing ligand-induced cooperative orbital redistribution that dominates nanoscale molecule-surface interactions with one-unit-thin TiO2 nanosheets Guolei Xiang,*,#,† Yan Tang,#,‡ Zigeng Liu,*,§,□ Wei Zhu,‡ Haitao Liu,‖ Jiaou Wang,⊥ Guiming Zhong,▽ Jun Li,‡ Xun Wang*,‡ †State

Key Laboratory of Chemical Resource Engineering, School of Science, Beijing

University of Chemical Technology, Beijing 100029, P.R. China. ‡Department

of Chemistry, Tsinghua University, Beijing 100084, P.R. China.

§Forschungszentrum

Jülich GmbH, Institute of Energy and Climate Research Fundamental

Electrochemistry (IEK–9), Jülich, 52425,Germany. ‖Department

of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United

States. ⊥Beijing

Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy

of Science, Beijing 100049, P.R. China. ▽Xiamen

Institute of Rare Earth Materials, Chinese Academy of Sciences, Xiamen 361021,

Fujian, P.R. China. □ Max-Planck-Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim

an der Ruhr, Germany.

Table of Contents

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ABSTRACT: Understanding the general electronic principles underlying molecule-surface interactions at the nanoscale is crucial for revealing the processes based on chemisorption like catalysis, surface ligation, surface fluorescence, etc. However, the electronic mechanisms of how surface states affect and even dominate the properties of nanomaterials have long remained unclear. Here, using one-unit-thin TiO2 nanosheet as a model surface platform, we find that surface ligands can competitively polarize and confine the valence 3d orbitals of surface Ti atoms from delocalized energy band states to localized chemisorption bonds, through probing the surface chemical interaction at the orbital level with near-edge X-ray absorption fine structure (NEXAFS). Such ligand-induced orbital redistributions, which are revealed by combining experimental discoveries, quantum calculations and theoretical analysis, are cooperative with ligand coverages, and can enhance the strength of chemisorption and ligation-induced surface effects on nanomaterials. The model and concept of nanoscale cooperative chemisorption reveal the general physical principle that drives the coverage-dependent ligand-induced surface effects on regulating the electronic structures, surface activity, optical properties, and chemisorption strength of nanomaterials. KEYWORDS : nanosurface science, coverage effect, nanoscale cooperative chemisorption, TiO2 nanosheet, NEXAFS


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INTRODUCTION Heterogeneous molecule-surface interaction through chemisorption is a basic type of chemical interaction, and widely underlies catalysis, nanoscience, electrochemistry, crystal growth, surface electron transfer, etc1. At the nanoscale surface chemistry is affected by particle size, and surface ligation states can dominate the physical and chemical performances of nanomaterials2-5. For example, sub-5-nm nanomaterials can display size-dependent catalytic capabilities6-7; and both particle sizes and ligand coverages can regulate the optical properties and electronic structures of semiconducting nanoparticles and metallic nanoclusters8-15. In principle, the physical nature that drives these nano effects fundamentally lies in modified electronic structures, but cannot be simply explained by the geometrical parameters like decreased size, increased specific surface area or defect ratios2, 16-17. In practice, however, it is quite challenging to fully elucidate the electronic mechanisms of the unusual properties of nanomaterials, thus the physical nature of most nanoscale phenomena still remains mysterious. Experimentally, it is extremely difficult to specifically probe the orbital-level interactions of surface chemical bonds and the regulated electronic structures of solid surfaces18-19, because chemisorption can only perturb the local electronic structure of the surface-layer atoms, and the changes are highly interfered by the signals of the internal atoms20-21. Theoretically, most nanostructures are too large systems for quantum calculations now, which limits the insights from computational approaches. Therefore, revealing the general electronic principles dominating chemisorption, in particularly at the nanoscale, is crucial for both fundamental and applied surface science and nanoscience7, 22. Atomically thin two-dimensional (2D) materials have risen as ideal model platforms for studying nanoscale surface chemistry and heterogeneous catalysis due to the high ratios of surface atoms and confined dimensions23-25. Using one-unit-thin TiO2 nanosheets as a model platform,


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here we reveal the electronic mechanisms how surface ligands interact with nanomaterials and modify their electronic structures and properties. Through minimizing the interfering signals of the bulk atoms with the ultrathin 2D morphology, we realize direct probing of the orbital-level adsorbate-surface interactions with near edge X-ray absorption fine structure (NEXAFS), in which ethylene glycol and peroxide are used as the test ligands. The electronic interactions between ligands and surface Ti atoms are detected by probing the varied density of states (DOS) of unoccupied Ti 3d orbitals in t2g and eg energy bands. By combining the experimental discoveries and further theoretical calculations, we reveal that surface ligands modify the electronic structures of nanomaterials through competitively redistributing surface atomic orbitals from energy bands to surface bonds, which is cooperative with ligand coverages. RESULTS AND DISCUSSION Structure model of TiO2(B) nanosheet TiO2(B) nanosheets were used as the model platform (Figure 1a and S1), which were prepared by hydrolyzing TiCl4 in ethylene glycol (EG) following a solvothermal method we first reported in 2010 26. The exposed facet was determined by indexing the fast Fourier transformation (FFT) of a high-resolution transmission electron microscopy image (HRTEM). As shown in Figure 1b, the clear lattice fringe indicates that the nanosheets are highly crystalline. The FFT pattern of the lattice well matches the simulated reciprocal space of TiO2(B) along the * direction (Figure S2 and Table S1), indicating the exposed facet is (010). Thickness is another critical parameter of ultrathin 2D materials. However, it is challenging to measure the exact thickness of the TiO2(B) nanosheets via atomic force microscope (AFM), because the materials are soft and wrinkled caused by surface glycol ligands (Figure 1a and S1a). Alternatively, we determined the thickness through imaging the side views of the nanosheets with spherical aberration corrected TEM (Cs-


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TEM). As shown in Figure 1c, the Cs-TEM image shows that the thickness of the nanosheets is about 0.40 nm, which approximates the lattice parameter of b (0.37 nm). Surface chemical states of the nanosheets were determined with Fourier transform infrared spectra (FTIR) and solid-state nuclear magnetic resonance (ssNMR). FTIR results of EG and the nanosheets (Figure 1d and S3) show that the O-H modes at 3300 and 1400 cm-1 reduce and C-O modes shift towards larger wavenumbers by 42 and 34 cm-1, respectively. While the positions of C-C and CH2 modes remain unchanged. These results indicate that EG molecules bind to surface Ti sites by dissociating -OH groups, which can be further confirmed by 13C ssNMR spectra (Figure 1e). The ssNMR result shows that the 13C resonance of CH2 in EG shifts from 63.2 ppm to 78.0 ppm in nanosheets, which is in good agreement with the previous published results27. By comparing the deconvolution results we assign the 78.1-ppm resonance to the dissociative bidentate EG ligands (Ti-OCH2CH2O-Ti), the resonance at 64.8 ppm and the shoulder peak at 71.5 ppm to HOCH2- and -CH2O-Ti of monodentate EG ligands, respectively. The resonance at 63.2 ppm is assigned to the physisorptive EG molecules. The ratio of bidentate and monodentate EG ligands binding on TiO2(B) nanosheets (Figure S4) is determined through integrating their corresponding peak areas, which shows that about 91.6% surface Ti sites are occupied by bidentate EG (Figure 1f, Figure S4). Based on this coverage ratio and saturated adsorption model, we further calculate the weight ratios of EG to TiO2 for TiO2(B) nanosheets of varied thicknesses in the b direction (Figure S5, Table S2). The experimentally measured weight ratio of EG to TiO2 is ~27.3% as characterized by thermal gravimetric analysis (TGA, Figure S6), matching the calculated value of the 3-layer model (27.2%), which is also one-unit thick (0.37 nm). Therefore, as discussed above, we conclude that the TiO2(B) nanosheets expose (010) facet, are one-unit-thin in the b direction and are dominately saturated by bidentate EG ligands as shown in Figure 1g.


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Figure 1. Structure model of TiO2(B) nanosheets. (a) TEM image of TiO2(B) nanosheets. (b) HRTEM and FFT pattern to index the exposed (010) facet. (c) Side view of the nanosheets imaged via Cs-corrected TEM shows the thickness is 0.40 nm. (d) FTIR indicating dissociative bonding of ethylene glycol (EG) ligands. (e) Deconvoluted 13C ssNMR spectrum to calculate the ratio of bidentate to monodentate EG. (f) Scheme illustrates the bidentate and monodentate adsorption configuration of EG and their coverage. (g) Atomic structure model of TiO2(B) nanosheets, which expose (010) surface saturated by EG ligands and are one-unit thick (0.37 nm) in the b direction. In the structure the colors of Ti, O, C and H are green, red, purple and yellow, respectively. Coverage-dependent surface effects on optical properties We use optical properties to investigate surface ligation and coverage effects on modifying the electronic structures of TiO2(B) nanosheets. The optical absorption was characterized by solidstate UV-visible diffuse reflectance spectroscopy (DRS) and further transformed with Kubelka– Munk function (Figure 2a and Figure S7)28. A series of TiO2(B) nanosheet samples with varied EG coverages (θ) were prepared by re-adsorb different amounts of EG ligands onto the etched nanosheet samples. The operation is to first stir raw TiO2(B) nanosheets in 68% HNO3 at room temperature for 2 days to remove partial EG ligands. Then EG ligands are re-adsorbed on the


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etched samples by mixing the etched sample with EG in acetone, in which the coverages were tuned by controlling the reaction time. The shape, long-range ordered crystal structure and local atomic configurations in these nanosheet samples do not change, as characterized by TEM, X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) (Figure S8, S9). TiO2(B) nanowires (NWs, Figure S10) with diameters between 50 and 200 nm were chosen as the control. The electronic band gap of TiO2(B) NWs is 3.19 eV (Figure 2a), which is a typical value of TiO2. For raw TiO2(B) nanosheets saturated by EG ligands, the gap increases to 3.58 eV, but the value decreases to 3.19 eV after etching off 58% of EG (Figure 2a). The phenomena show that the band gaps of TiO2(B) nanosheets can be affected by EG coverages. We therefore performed more detailed studies by combining optical absorption and coverage quantification through quantifying the amounts of C in the samples (Figure S11 and Table S3). As shown in Figure 2b, the band gaps change from 3.19 eV at θ < 0.56 to 3.58 eV at θ = 1.00. At low coverages (θ < 0.56), etched TiO2(B) nanosheets show bulk-like band gaps, indicating surface ligation is not strong enough to modify the electronic structure. However, this surface effect can be enhanced effectively by increasing EG coverages, especially at θ > 0.90. At very high coverages, the band gaps become more sensitive to EG coverages, demonstrating that surface ligation states can effectively dominate the electronic structures and optical properties of the nanosheets.


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Figure 2. Surface ligation effects on the optical properties of TiO2(B) nanosheets and experimental probe of adsorbate-surface orbital interactions with NEXAFS. (a) Band gaps of TiO2(B) nanowire (1-NW, Eg=3.19 eV), nanosheet (2-NS, Eg = 3.58 eV, θEG = 1.00), etched nanosheet (3-ENS, Eg = 3.19 eV, θEG = 0.42) and the absorption spectra of peroxide modified etched nanosheet (4-PENS). (b) Dependence of TiO2(B) NSs band gaps upon EG coverages. (c) (d) NEXAFS data of Ti-L3 and O-K edges normalized to the pre-edges and eg peaks. e-g, Distribution configurations of surface Ti 3d orbitals in octahedral ligand fields with different top ligands in NWs (e), NSs (f), ENSs (g) and PENSs (h). e1-g1 show varied bonding features of σtype 3dz2; e2-g2 show varied bonding features of π-type 3dxz and 3dyz. The arrows indicate the relative delocalization tendencies of 3d orbitals. Probing ligand-surface orbital interactions with NEXAFS NEXAFS, a surface sensitive technology probing the density of states (DOS) of unoccupied electronic states based on synchrotron radiation, was used to reveal the adsorbate-surface orbital


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interactions and the electronic mechanisms underlying the above coverage-dependent surface effects (Figure 2c, 2d and Figure S12). For TiO2, the empty electronic states arise from the antibonding orbitals hybridized by Ti 3d, 4s, 4p with O 2p atomic orbitals (AOs) in octahedral [TiO6] coordination fields. These states locate in the conduction bands and further split into four sub-bands with the symmetries of t2g, eg, a1g and t1u. In an octahedral ligand field, the five-fold degenerate Ti 3d AOs split into two groups of molecular orbitals (MOs), σ-type 3dσ orbitals in eg sub-band evolved from dx2-y2 and dz2 AOs, and π-type 3dπ orbitals in t2g sub-band from dxy, dxz and dyz AOs (Figure S13)29. In the NEXAFS spectrum, Ti-L and O-K edges correspond to electronic dipole transitions of Ti 2p→Ti 3d and O 1s→O 2p, respectively. Changes in the peak width of eg band and the relative intensity ratio of t2g to eg (It2g/Ieg) indicates the varied local and extended bonding features of Ti 3d AOs and the ratios of Ti 3d and O 2p in the unoccupied electronic states30. As to the surface chemistry of TiO2 in this research, the narrowed width of eg peak and decreased intensity of t2g peak of Ti L3 line indicate the redistributions of Ti 3d AOs from energy bands to surface coordination bonds. For TiO2(B) nanowires, the Ti-L3 eg peak splits due to the geometrical distortion of [TiO6] octahedron (Figure 2c), and eg band is wider than t2g band, because σ bonds are more overlapped and delocalized (Figure 2e1) than π bonds (Figure 2e2). The It2g/Ieg ratio (3:2, Figure 2d) of the O-K edge in nanowire sample is consistent with the results of bulk TiO227. Therefore, TiO2(B) nanowires can be taken as a bulk standard to analyze the NEXAFS spectra, whose σ-type 3dσ orbitals (dz2) in eg band and π-type 3dπ orbitals (dxz and dyz) in t2g band distribute uniformly in the lattice (Figure 2e). The adsorbate-surface orbital interactions are revealed by comparing the NEXAFS results of nanowires (NW), raw nanosheets (NS, θEG = 1.00) and etched nanosheets (ENS, θEG = 0.42)


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(Figure 2c, 2d). Compared to NWs, the Ti-L3 eg peaks of NSs and ENSs obviously become narrower. According to the properties of energy bands, this narrowed energy band indicates reduced overlap and delocalization of dz2 orbitals in the lattices. This difference mainly results from size reduction, because the one-unit thickness removes long-range lattice periodicity and effectively suppresses the delocalization of dz2 orbitals to form eg bands. On the other hand, the TiL3 eg peak of ENSs with θEG = 0.42 is broader than that of NSs with θEG = 1.00 (Figure 2c). This suggests that surface ligands must also contribute to the suppressed delocalization of dz2 orbitals and narrowed eg bands. For raw NSs, the 3dz2 AOs of surface Ti atoms are partly polarized and localized into chemisorptive states with EG ligands, which leads to decreased delocalization and narrower eg band (Figure 2f1). While for ENSs, most five-fold coordinated surface Ti atoms (Ti5f) locate in pyramidal coordination configurations, thus their 3dz2 orbitals tend to extend into the lattice to enhance and broaden eg bands rather than polarize into chemisorption states(Figure 2g1)30. Coverage effects on  bonds are revealed by analyzing the varied It2g/Ieg of Ti-L3 and O-K edges. Compared to NWs, the Ti-L3 It2g/Ieg decreases from 0.96 to 0.90 for NSs, suggesting reduced ratios of unoccupied Ti 3dπ states in t2g bands, while the increased O-K It2g/Ieg from 1.32 to 1.64 suggests increased ratios of unoccupied O 2pπ states in t2g bands. The difference further indicates that EG ligands also bind to Ti atoms through overlapping their O 2pπ orbitals with Ti 3dπ orbitals, in which O 2pπ AOs shares electron pairs with Ti 3dπ. As a result, the Ti 3dπ AOs are polarized into chemisorption bonds, which decreases their delocalization into the lattices to form t2g bands (Figure 2f2). For ENSs, It2g/Ieg of Ti-L3 and O-K are close to that of NWs, suggesting they have similar t2g bands. The similar t2g states match their identical band gaps as shown in Figure


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2a. Therefore, the Ti 3dπ AOs also tend to extend into the lattices to enhance t2g bands in the absence of surface EG ligands (Figure 2g2). Peroxide (H2O2) can adsorb on TiO2(B) nanosheets to change the color from white to yellow as shown by the visible absorption mode and insets in Figure 2a 31. This surface modification cannot change the morphology and phase of TiO2(B) nanosheets (Figure S14). For the peroxide-modified etched nanosheets (PENS), the peak of Ti-L3 eg band is even broader than that of ENSs (Figure 2c), suggesting the dz2 AOs are even polarized into the lattices to enhance the eg states. While the least Ti-L3 It2g/Ieg (0.85) ratio indicates a further decrease of unoccupied Ti 3dxz and 3dyz AOs in t2g bands, which results in stronger surface bonds with peroxides instead. Therefore, we can reveal that peroxides bond to TiO2(B) nanosheets through strong π-π overlaps between their π*-type highest occupied molecular orbitals (HOMOs, Figure S15) and Ti 3dπ AOs (Figure 2h2). This strong orbital interaction is driven by their matched orbital symmetries (Figure S16). The result is to polarize and confine surface Ti 3dπ AOs into chemisorption states, leading to reduced delocalization of dxz and dyz AOs and further weakened t2g energy bands as shown in Figure 2c. Moreover, the polarized 3dπ AOs into surface chemisorption states can repel σ-type 3dz2 AOs towards the lattice (Figure 2h1), leading to enhanced delocalization into the lattices and further broadened eg band as shown in Figure 2c. This is a spatially repelling effect of orbitals on forming chemical bonds. Quantum calculations of coverage effects The above orbital-level ligand-surface interactions probed with NEXAFS show that surface ligands can effectively redistribute surface Ti 3d AOs through polarizing them from delocalized energy band states to localized surface chemical bonds. To reveal more features of such ligation effects, we further carry out quantum chemical calculations based on density functional theory


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(DFT) to analyze the tunable bonding interactions. The adsorption structures with EG coverages of 0.25, 0.50, 0.75 and 1.00 on TiO2(B) nanosheets were optimized (Figure 3a) as the models. We investigate the orbital configurations of surface Ti-O bonds, and coverage effects on the adsorption energies of surface Ti-EG complexes and the energy bands of the nanosheets. Periodic natural bond orbital (NBO) method was used to analyze the bonding configurations between surface Ti atoms and the O atoms in EG ligands32-33. NBO analysis of the θEG = 0.25 case shows that EG ligands bind to Ti sites through triple bonds, one σ and two π bonds (Table S4). The orbital configurations for σ-type surface chemical bonds is 0.13 Ti(s1p2.38d2.01) + 0.87 O(s1p0.72), and for π bonds are 0.08 Ti(s1p8.37d16.67) + 0.92 O(almost 100% p orbital) and 0.06 Ti(s1p8.54d9.40) + 0.94 O(almost 100% p orbital). The results indicate that σ bonds dominate the surface interactions of EG ligands with TiO2(B) nanosheets, which is consistent with dissociative bonding results of EG molecules.


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Figure 3. Quantum calculations of coverage-dependent chemisorption of EG on TiO2(B) nanosheets. (a) Calculation models with EG coverages of 0.25, 0.50, 0.75 and 1.00. (b) Calculated average Ti-O bonding energies (Eb) as a function of EG coverages. The zero point is set at θ = 0.25. (c) Calculated PDOS of EG-covered TiO2(B) nanosheet at θ = 0.75. (d) Calculated total DOSs of TiO2(B) nanosheets with EG coverages of 0.25, 0.50, 0.75 and 1.00, and the band gaps of dipole electron transfer from the π states at the tops of valence bands to the π* states (t2g) at the bottoms of conduction bands. The bonding energy between surface Ti atom and O atom in EG is a descriptor to evaluate the strength of chemisorption. As shown in Figure 3b and Table S5, we find that the average Ti-O bonding energies increase with increased EG coverages. Thus, increasing EG coverage can enhance the chemisorption of EG on the atomically thick TiO2(B) nanosheets, which is a kind of nano effects. We further calculated the coverage-dependent density of states (DOS) to understand coverage-dependent band gaps. Figure 3c displays the partial DOS of Ti 3d, lattice O 2p and EG O 2p orbitals with θEG = 0.75. The π*-type t2g states that mainly result from Ti 3d AOs locate at the bottom of the conduction band, while the π-type t1g state resulting from lattice O2p orbitals locate at the top of valence band (Figure S13). While the EG O 2p orbitals locate within the gap, which results from the σ bonds with Ti sites as shown in Figure 3c. Two types of electron transitions arise from this level, indirect approach from t1g to t2g and direct approach from EG O 2p to eg along the surface σ bonds, in which the indirect transition determines the band gap as indicated in Figure 3c. With increased EG coverages from 0.25 to 1.00, the calculated band gaps increase from 2.6 eV to 3.1 eV (Figure 3d). This evolution trend agrees with our experimental results. The concept of Nanoscale Cooperative Chemisorption (NCS)


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Our experimental and calculation results of the band gaps of TiO2(B) nanosheets and bonding energies of EG on the 0.37-nm-thick nanosheets suggest that chemisorption and the resulting electronic effects at the nanoscale can be cooperatively enhanced by increasing ligand coverages. Based on the discoveries, we establish a physicochemical model to describe nanoscale adsorbatesurface orbital interactions, to further understand the general features and physical nature of chemisorption, surface activity and ligation effect on nanomaterials. In general, chemisorptive strength is the central parameter describing adsorbate-surface interactions, and determines surface reactivity, ligand-induced surface fluorescence, interface charge transfer, etc. The basic features of solid electronic structures are their energy bands resulting from the extended overlaps of atomic orbitals within the whole lattices. Energy band theory suggests that the width of an energy band correlates to the delocalization degrees of its combinational atomic orbitals into the lattice. In general, more delocalized AOs lead to wider energy bands, while more localized AOs result in narrower energy bands. Our key discovery is how surface ligands modify the electronic structures of nanomaterials through forming surface chemical bonds. To understand the features and in-depth electronic principles dominating nanosurface chemistry, we introduce a parameter, the distribution fraction (f) of a surface atomic orbital (SAO), to establish our orbitallevel model of nanoscale chemisorption. For surface atoms, their valence atomic orbitals mainly distribute into two parts, confined into localized surface states (fS) or extended into the lattice to form Bloch states in energy bands (fB). Since AOs are normalized according to the basic postulate of quantum mechanics, thus fS + fB = 1. The quantum normalization feature of AOs implies fS and fB are competitive. Assuming the confined surface states correspond to chemisorption, then fS is a descriptor measuring surface reactivity and the strength of chemisorption, while fB represents the contribution to the energy bands and positively correlates to band width. Therefore, for fixed


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ligands and surfaces, greater fS denotes stronger chemisorption, while less fB means weaker delocalization of an AO into the lattice and narrower energy bands.

Figure 4. The model of coverage-dependent cooperative orbital redistributions at the nanoscale. (a) Physical model of cooperative ligand effect on redistributing surface atomic orbitals (SAOs). Without adsorbates (I), nanomaterials show intrinsic electronic structures, in which SAOs preferentially delocalize into the lattices. Adsorbates can induce SAO redistributions by increasing ligand coverages (I→II→III→IV). At high coverages (IV), SAOs are highly confined in surface chemisorption states. (b) The effects of cooperative orbital redistributions on electronic structures and surface activity.


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According to the above results, we propose an orbital-level physical model to describe such coverage-dependent molecule-surface interactions at the nanoscale as shown in Figure 4a. The increased bonding energies resulting from increased ligand coverage suggest that electronic nature underlying the coverage-dependent surface effects is to cooperatively increase fS and competitively decrease fB with increased ligand coverages. As illustrated from I→II→III→IV in Figure 4a, the total electronic effect is to decrease the delocalization of SAOs into the bulk phase but to increase the localization in chemisorption states. Such ligand-induced cooperative orbital redistributions simultaneously regulate the electronic structures of both nanomaterials and adsorbates. For nanomaterials with clean surfaces (I), their SAOs mainly delocalize into the lattices, and fB reaches the maximum value. In this case, nanomaterials display wider energy bands as illustrated by Figure 4b left. This effect can be supported by the wider eg bands of the etched TiO2(B) nanosheets we have shown in Figure 2C. At low coverages (II), adsorbates partly polarize SAOs into chemisorption states, but cannot effectively modify the total electronic structures of nanomaterials. But the perturbation effects can be enhanced by increasing adsorbate coverages (II→III), which confines more SAO fractions in surface states and decreases their delocalization through increasing fS while reducing fB. For nanostructures with extremely large surface-to-volume ratios, such as our TiO2(B) nanosheets, both fS and fB reach the extrema at highly saturated coverages (IV). The adsorbates can effectively polarize SAOs from energy bands to form strong chemisorption interactions, which leads to stronger chemisorption and narrowed energy bands of nanomaterials as shown by Figure 4b right and the narrowed eg band of raw TiO2(B) nanosheets in Figure 2C. Such coverage-dependent competitive and cooperative orbital redistributions generally underlie the nanoscale molecule-surface interactions, and the surface effects become increasingly dominant


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with increased surface-to-volume ratios and ligand coverages. In general, the orbital redistribution processes can cause two basic effects. For nanomaterials, the delocalization of SAOs into the lattices is decreased, leading to weakened state correlations and narrowed energy bands. This further shifts band edges, modifies the electronic structures, increases surface activity and changes other optical properties (Figure 4b). For adsorbates, the strength of chemisorption can be enhanced due to the increased fS, and the increased surface reactivity can further accelerate more chemisorption. In this case, fs is the descriptor of both surface reactivity and the stability or strength of surface complexes. As a result, increasing adsorbate coverages can enhance both surface reactivity and chemisorption strength, which is a positive feedback and eventually leads to saturated surface ligation states. This model accounts for the electronic mechanisms why size reduction can effectively activate the surface activities of nanomaterials, and why nanomaterials are saturated by ligands. We refer to such coverage-dependent adsorbate-surface interactions on nanomaterials as nanoscale cooperative chemisorption (NCS). CONCLUSION In summary, we have studied the electronic mechanisms of coverage-dependent adsorbatesurface interactions on nanomaterials by experimentally probing the ligand-regulated electronic structure of atomically thin TiO2 nanosheets with NEXAFS and theoretical studies with DFT calculations. We reveal the general electronic principle of how surface ligands modify the electronic structures and properties of nanomaterials, which is through competitively polarizing the surface valence atomic orbitals from energy bands to surface chemisorption bonds. In such orbital redistribution processes, increasing ligand coverage can cooperatively enhance both chemisorption and surface effects, which is a general phenomenon at the nanoscale. Our theory, nanoscale cooperative chemisorption (NCS), points out a new direction to generally explore the


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electronic mechanisms of size-dependent catalysis, ligand-induced surface effects on tuning electronic structures, optical properties, catalytic activity and device performance of nanomaterials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at Structural characterizations and determination of TiO2(B) nanosheets, including XRD, crystal information, FTIR, ssNMR, TGA, UV-vis data, EXAFS as well as analysis of electronic structures by NEXAFS and DFT calculations. (PDF) AUTHOR INFORMATION Corresponding Authors *G. X., E-mail: [email protected] *Z. L., E-mail: [email protected] *X. W., E-mail: [email protected] ORCID Guolei Xiang: 0000-0003-1221-1748 Zigeng Liu: 0000-0002-2955-5080 Xun Wang: 0000-0002-8066-4450 Author Contributions G. X. designed the research, prepared the materials, conceived the concepts and wrote the manuscript. Z. L., W. Z., H. L. and X. W. helped with designing the experiments. G. Z. carried out


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ssNMR measurement and elemental analysis. J. W. and G. X. carried out NEXAFS and EXAFS measurements and analyzed the results. Y. T. and J. L. carried out DFT calculations. All authors contributed to the discussion and preparation of the manuscript. #X. G. and Y. T. contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by Fundamental Research Funds for Central Universities of China (buctrc201812), National Natural Science Foundation of China (21431003, 21521091, 21590792, 91645203, 21801012). The calculations were performed by using supercomputers at Tsinghua National Laboratory for Information Science and Technology and the Supercomputing Center of Computer Network Information Center of the Chinese Academy of Sciences. We acknowledge the XAS beamtime at BSFC granted by 2018-BEPC-PT-001799. We appreciate the help from Dr. L. Gu and Q. Zhang with the Cs-TEM characterizations. We acknowledge the helps, discussions, and suggestions from H. Dong, Y. Wang, Y. Long, O. Scherman, J. Baumberg, C. Grey, K. Griffith, H. Wang and X. Sun. REFERENCES 1.

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Probing ligand-induced cooperative orbital redistribution that

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