surfaces: A density functional theory study

The adsorption of α-cyanoacrylic acid on anatase TiO2 (101) and (001) surfaces: A density functional theory study Jin-Gang Ma, Cai-Rong Zhang, Ji-Jun Gong, Bing Yang, Hai-Min Zhang, Wei Wang, You-Zhi Wu, Yu-Hong Chen , and Hong-Shan Chen Citation: The Journal of Chemical Physics 141, 234705 (2014); doi: 10.1063/1.4903790 View online: http://dx.doi.org/10.1063/1.4903790 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/23?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electronic band structures of Ge1−xSnx semiconductors: A first-principles density functional theory study J. Appl. Phys. 113, 063517 (2013); 10.1063/1.4790362 Ethanol adsorption on the Si (111) surface: First principles study J. Chem. Phys. 136, 114703 (2012); 10.1063/1.3691892 CO2 adsorption on TiO2(101) anatase: A dispersion-corrected density functional theory study J. Chem. Phys. 135, 124701 (2011); 10.1063/1.3638181 High field density-functional-theory based Monte Carlo: 4 H -SiC impact ionization and velocity saturation J. Appl. Phys. 105, 033703 (2009); 10.1063/1.3074107 Density functional study of the interaction between small Au clusters, Au n ( n = 1 – 7 ) and the rutile Ti O 2 surface. I. Adsorption on the stoichiometric surface J. Chem. Phys. 127, 084704 (2007); 10.1063/1.2770462

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THE JOURNAL OF CHEMICAL PHYSICS 141, 234705 (2014)

The adsorption of α-cyanoacrylic acid on anatase TiO2 (101) and (001) surfaces: A density functional theory study Jin-Gang Ma,1 Cai-Rong Zhang,1,2,a) Ji-Jun Gong,1 Bing Yang,3 Hai-Min Zhang,1 Wei Wang,1 You-Zhi Wu,2 Yu-Hong Chen,1,2 and Hong-Shan Chen4 1

School of Sciences, Lanzhou University of Technology, Lanzhou, Gansu 730050, China State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou, Gansu 730050, China 3 State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, Jilin 130012, China 4 College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China 2

(Received 27 August 2014; accepted 26 November 2014; published online 16 December 2014) The adsorption of α-cyanoacrylic acid (CAA) on anatase TiO2 (101) and (001) surfaces, including adsorption energies, structures, and electronic properties, have been studied by means of density functional theory calculations in connection with ultrasoft pseudopotential and generalized gradient approximation based upon slab models. The most stable structure of CAA on anatase TiO2 (101) surface is the dissociated bidentate configuration where the cyano N and carbonyl O bond with two adjacent surface Ti atoms along [010] direction and the dissociated H binds to the surface bridging O which connects the surface Ti bonded with carbonyl O. While for the adsorption of CAA on (001) surface, the most stable structure is the bidentate configuration through the dissociation of hydroxyl in carboxyl moiety. The O atoms of carboxyl bond with two neighbor surface Ti along [100] direction, and the H from dissociated hydroxyl interacts with surface bridging O, generating OH species. The adsorption energies are estimated to be 1.02 and 3.25 eV for (101) and (001) surfaces, respectively. The analysis of density of states not only suggests the bonds between CAA and TiO2 surfaces are formed but also indicates that CAA adsorptions on TiO2 (101) and (001) surfaces provide feasible mode for photo-induced electron injection through the interface between TiO2 and CAA. This is resulted from that, compared with the contribution of CAA orbitals in valence bands, the conduction bands which are mainly composed of Ti 3d orbitals have remarkable reduction of the component of CAA orbitals. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903790] I. INTRODUCTION

Titanium dioxide (TiO2 ) has been studied extensively in the past several decades due to its merits, such as nontoxicity, high stability, abundant resource, etc.1, 2 Correspondingly, TiO2 has been widely applied in many areas, including pigment, photocatalysis, production and storage of hydrogen, the conversion of solar energy to electricity, and so on.3–12 TiO2 has three kinds of natural polymorphs, named as rutile, anatase, and brookite.13 Anatase phase commonly exists in TiO2 nano-scale materials.11 It has been found that the (101) and (001) surfaces are most frequently exposed by anatase TiO2 crystals due to their low surface energy, low reactivity, and high stability.14, 15 One of the most important applications of TiO2 is dyesensitized solar cells (DSSCs), a promising photovoltaic device. The main components of DSSCs are dye sensitizers, wide band-gap semiconductor electrode, and electrolyte as redox shuttle. The semiconductor electrode is usually consisted of mesoporous TiO2 layer composed of nanometer-sized particles with anatase phase. The photon to current conversion mechanism of DSSC is as followings:16–21 the photon absorpa) Author to whom correspondence should be addressed. Electronic mail:

[email protected]. Tel.: +86 0931 2973780. Fax: +86 0931 2976040.

0021-9606/2014/141(23)/234705/10/$30.00

tions of dye sensitizer result into the electron injection from excited dyes to the conduction band of semiconductor electrode; the oxidized dye sensitizer is subsequently reduced by electron transfer from the electrolyte; the oxidized molecules of electrolyte are regenerated in turn by the reduction at the electrode. The optimization and development of dye sensitizers, electrode materials, and electrolyte are still ongoing in order to improve the efficiency of DSSCs. Dye sensitizers have significant influence on the efficiency of DSSCs. Apart from the wide absorption bands and suitable energy levels of dye sensitizers, the anchor group should be contained in dye sensitizers in order to strongly bind to semiconductor electrode surface (SEC).22 The stable adsorption of dye sensitizers on SEC through the anchor group leads to long-term thermal stability of DSSCs.23–25 Also the adsorption modes of anchor group determine the orientation of adsorbed dyes on SEC, and therefore affect the charge transfer rate, short-circuit current density (Jsc ), and open-circuit voltage (Voc ).26–29 The binding modes (molecular or dissociative, monodentate or bidentate, etc.) of anchoring group in dye sensitizers can significantly affect the electron injection times,30 which closely relate to Jsc . So, in the optimization and development of DSSCs, the fundamental step is to study the adsorption mode of the anchor moiety presented by the commonly employed dye sensitizers.

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The anchor groups in effective dye sensitizers are hydroxyl, carboxyl, etc. But, carboxyl group is the most common anchor group in effective dye sensitizers, for instance, porphyrin sensitizers YD2-o-C831 and SM315.32 Generally, dye sensitizers contain electron donor and acceptor parts in order to generate charge separated states in charge transfer (CT) excitations which are favorable for photo-induced electron injection in DSSCs. The anchor group should be very close to the dye acceptor moiety because of the enhancement of electronic coupling.22 Cyanoacrylic acid matches this condition very well. Many efficient dye sensitizers contain cyanoacrylic acid moiety. For these reasons, the investigation of cyanoacrylic acid adsorption on anatase TiO2 surface can help us to understand the mechanism of DSSCs based upon the family of dye sensitizers containing cyanoacrylic acid moiety. Based upon the investigation of the energetically favorable adsorption modes of acetic acid on TiO2 , the dissociated bridged bidentate adsorption mode was deduced to be the most stable structure for organic dyes bearing cyanoacrylic acid anchoring group on TiO2 .22 However, the adsorptions of NH3 ,33, 34 acetonitrile,35 azobenzene, and aniline36 on TiO2 surfaces support the effective interaction between N and surface Ti. In this work, α-cyanoacrylic acid (CAA) is presented as a prototype in order to understand the interaction between dye sensitizers containing CAA group and TiO2 electrode in DSSCs, and then we investigate the geometries, energies, and electronic properties of the adsorption of CAA on anatase TiO2 (101) and (001) surfaces with various monodentate, bidentate, and tridentate adsorption modes by means of density functional theory (DFT) calculations in connection with ultrasoft pseudopotential and generalized gradient approximation based upon slab models. The results of this work expose the DSSCs interaction mechanism between TiO2 anode and dye sensitizers containing CAA moiety, and also it would be helpful to extend the theoretical understanding of anatase TiO2 in view of the related technological applications.

II. COMPUTATIONAL METHOD

The DFT calculations in this work were performed by using CASTEP package.37 The electronic exchange and correlation interaction were described by using the PBE functional,38 a form of generalized gradient approximation (GGA). The ionic cores were represented by using ultrasoft pseudopotentials.39 The electronic wave functions were expanded in the plane wave basis sets with the cutoff kinetic energy 500 eV for all calculations. All of the relaxations of geometries were considered to be converged until the energy difference dropped below 1×10−5 eV/atom (the SCF tolerance was 1 × 10−6 eV/atom). The full optimization of pure anatase TiO2 crystal was carried out to evaluate the accuracy of the computational parameters. The optimized bulk lattice parameters of anatase TiO2 are a = b = 3.797 Å, and c = 9.721 Å, which agree well with other DFT calculations40–43 and the corresponding experimental values: a = b = 3.785 Å and c = 9.514 Å.44, 45 The agreement implies that our calculation method is appropriate to produce reliable results.

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The anatase TiO2 (101) and (001) surfaces were modeled by the periodic slabs in the supercells after cutting bulk crystal. Four O-Ti-O layers (about 6 Å and 8 Å for (101) and (001) surfaces, respectively) were adopted in this work because this layer amount is sufficient to produce good convergence of (101) and (001) surface structures, and it had been widely used in other previous studies of adsorption systems on both anatase (101) and (001) surfaces.43, 46–48 Different slabs were separated by 20 Å vacuum spaces in order to avoid the interaction between the slab and its repeated images. The unreconstructed (101) surface is the most stable surface for anatase TiO2 . The (2 × 4) supercell was used for anatase TiO2 (101) surface, corresponding the surface area of 10.436 Å × 15.188 Å. However, the anatase TiO2 (001) surface exists reconstruction.15, 49, 50 Up to now, the explanation of this surface structure is still under debate.49, 51–53 On the other hand, the unreconstructed (001) surface had been adopted to investigate the adsorptions of water,42, 49, 54 methanol,55 formic acid,48 and SF6 decomposed gas,56 etc. In this work, based upon the unreconstructed (001) surface, we adopted the (3 × 3) supercell which was applied to simulate the anatase (001)/liquid water interface.57 The corresponding surface area is about 11.391 Å ×11.391 Å. The meshes of K-points to sample the Brillouin zone for anatase (101) and (001) surfaces were 2 × 1 × 1 and 2 × 2 × 1, respectively. During the geometric optimization, the two upper O-Ti-O layers (six atomic layers) of the slabs were allowed to relax freely, while the rest atoms in bottom side were fixed at their initial theoretical bulk positions. The trends in adsorption energies, bond lengths, bond angles, and electronic properties as a function of adsorption modes were investigated. Here, the adsorption energies are calculated as following: Eads = Eadsorbate +Esurface − Eadsorbate/surface , where Eads is the adsorption energy, Eadsorbate is the total energy of an isolated CAA molecule, Esurface is the energy of the clean anatase (101) or (001) surface slab, and Eadsorbate/surface is the total energy of the adsorbed system. According to this definition, a positive value of Eads indicates an energetically favorable adsorption due to exothermic character. III. RESULTS AND DISCUSSION A. α-cyanoacrylic acid molecular structure

The optimized geometrical structures for the three isomers of CAA molecule are shown in Fig. 1. The selected bond lengths and bond angles are also labeled in Fig. 1. Isomer1 and isomer2 are planar structures, while the isomer3 is a stereo structure in which the torsion angle between the carboxylic plane and cyano plane is about 90◦ . Apparently, the corresponding bond lengths and bond angles of these isomers are quite similar due to the local character of chemical bonds. Also, the geometrical parameters of isomer1 agree well with that of CAA moiety in dye sensitizers for solar cells.58–61 The total energies of isomer2 and isomer3 are higher than that of isomer1 about 0.11 and 0.23 eV, respectively. The higher energy of isomer2 highlights the position effect of lone-pair


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Isomer1

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Isomer2

Isomer3

active sites due to the chemical activity of anatase TiO2 (001) surface, acting as the pair of basic anion and cation sites.65, 66

C. Adsorption on anatase (101) surface

FIG. 1. The optimized molecular structures of cyanoacrylic acid isomers (light gray circle: H atom; red circle: O atom; blue circle: N atom; and dark gray circle: C atom). This notation is used throughout this paper). The selected bond lengths (in Å) and bond angles (in ◦ ) are also shown in the figure.

electron in O1 , while the higher energy of isomer3 results from the broken of planar conjugation. The data of total energy supports that the isomer1 is the most stable one.

B. The models of anatase TiO2 (101) and (001) surfaces

The slab models of anatase TiO2 (101) and (001) surfaces (see Fig. 2) have sixfold and fivefold coordinated Ti atoms (Ti6c and Ti5c ) as well as threefold and twofold coordinated O atoms (O3c and O2c ). It should be mentioned that Ti5c and O2c exist in pairs at these surfaces. For anatase TiO2 (101) surface slab, the top-layer O3c and Ti6c atoms which are fully coordinated tend to loosen their bonds about 0.022 and 0.036 Å, respectively, relaxing to outward about 0.065 and 0.042 Å, respectively. Vice versa, the O2c and Ti5c atoms tend to shrink their bonds about 0.124 and 0.035 Å, respectively, and to relax inward about 0.003 and 0.015 Å, respectively. The average Ti5c -O2c bond length is about 1.831 Å, and the average bond angle of Ti-O2c -Ti is about 103.0◦ . The relaxation effects agree well with the previous theoretical work.14, 62 For anatase TiO2 (001) surface, the relaxation effects on atomic coordinates are tiny. For instance, the average Ti5c -O2c bond length and Ti-O2c -Ti bond angle are about 1.962 Å and 150.8◦ , respectively. The calculated surface energy (∼1.06 J/m2 ) agrees with the reported results (0.98 J/m2 ).48, 63 According to the reported work,64 the unsaturated O2c and Ti5c atoms are usually the source of catalytic

The adsorption mode determines the atomic configuration on surface. In terms of the O-H bond in carboxyl moiety is dissociated or not, the adsorption of CAA on anatase surface can be classified as dissociative or molecular adsorption. For CAA adsorbate, the N atom in cyano moiety, the O atom in carbonyl, and the O atom in hydroxyl can bond with Ti5c , as well as the H atom in carboxyl moiety can bind to surface bridge oxygen O2c . For both molecular and dissociated adsorption modes, the bonds between CAA molecule and anatase TiO2 surface can be either monodentate, bidentate, or tridentate, depending on how many of the atoms in CAA molecule bond with Ti5c . In this work, the bidentate chelating adsorption mode (two O atoms in carboxyl coordinated with one Ti5c ) was not considered, because the bidentate chelating mode had been demonstrated to be energetically unfavorable in the study of formic acid adsorption on dry TiO2 anatase (101) surface.67 The relaxed adsorption modes with larger adsorption energies, including MA1, MA2, BA1, BB1, BB2, BC1, and BC2, are shown in Fig. 3. The corresponding adsorption energies and the selected structural parameters are listed in Table I. Meanwhile, the structures of MB1, MB2, MC1, BA2, BB3, BC3, and TA1 are shown in Fig. S1 in the supplementary material68 due to their smaller adsorption energies, then their structural parameters and adsorption energies are listed in Table SI in the supplementary material.68

1. Atomic configurations of adsorption a. Monodentate adsorption. The molecular adsorption of monodentate mode means only one atom in CAA (oxygen or nitrogen) binds to the anatase (101) surface Ti5c atom, combining with the possibility of forming hydrogen bond between the H atom in hydroxyl group and the surface O2c atom. The dissociative adsorption means the O-H bond in hydroxyl is broken, and the oxygen atom binds with the surface Ti5c atom after the hydrogen transfers from the hydroxyl group to a surface O2c atom, forming another hydroxyl species. In Fig. 3,

FIG. 2. Optimized surface structures: (a) the clean anatase TiO2 (101) surface and (b) the clean anatase TiO2 (001) surface. Ti atoms are represented by light gray circles and O atoms are represented by red circles. This notation is used throughout this paper.


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FIG. 3. The adsorption of cyanoacrylic acid molecule on anatase TiO2 (101) surface with monodentate and bidentate modes. The atoms in bottom layers of slab model are omitted for clarity.

MA1 and MA2 are molecular adsorption. For MA1 mode, CAA keeps the planar structure. The O in carbonyl bonds with a surface Ti5c atom, and the H in hydroxyl group interacts with the surface O2c atom which also coordinated with Ti5c in same pair. The atomic distance between O2 and Ti5c is about 2.146 Å which is slightly longer than that in bulk anatase TiO2 (1.965 Å), and the bond length of H-O1 is elongated about 0.078 Å from 0.985 Å in isolated CAA molecule, while the distance between H and O2c is about 1.454 Å which belongs to hydrogen bond. The adsorption energy of MA1 is about 0.96 eV. MA2 is quite similar to MA1, except the hydrogen bond position where the H in CAA interacts with O2c which belongs to different pair of the Ti5c coordinated with

O2 . The Ti5c -O2 bond length and H-O2c distance are slightly longer than that in MA1. This configuration is very similar to the molecular adsorption of formic acid on anatase TiO2 (101) surface.67, 69 The slight smaller adsorption energy of MA2 (0.94 eV) is induced by hydrogen bond position. The Coulomb repulsion interaction between CAA and surface in MA2 might be stronger than that in MA1 due to the shorter distances between O in CAA and the negative charge layer of top O3c . The adsorption energies of MB1, MB2, and MC1 are smaller than that of MA1 about 0.40, 0.43, and 0.45 eV, respectively. Apparently, in terms of adsorption energy, the molecular adsorption mode MA1 is the most energetically favorable monodentate configuration.

TABLE I. The selected bond lengths or inter-atomic distances (Å), bond angles (◦ ), and adsorption energies (Eads , eV) for the adsorption of cyanoacrylic acid molecule on anatase TiO2 (101) surface with monodentate and bidentate modes.

b. Bidentate adsorption. All of the possible bidentate adsorption modes are dissociative. For BA1, the adsorption energy (about 1.02 eV) is larger than that of MA1 about 0.06 eV because of O-Ti5c and N-Ti5c bonds in BA1. The N and O atoms bond with two adjacent Ti5c atoms along [010] direction. The bond lengths of Ti-N and Ti-O2 are 2.328 and 2.016 Å, respectively. The N-Ti bond length is very close to bond length (∼2.342 Å) in Ti3 N4 crystal70 and that of acetonitrile adsorbed on the anatase TiO2 (101) surface,35 but it is longer than that of TiN diatomic data (∼1.796 Å) and Ti2 N2 (∼1.80 Å) due to different coordination environments.71 The slight shrink of C2 -N bond length after adsorption means that the bonding character of N-C2 is enhanced. This is similar to the case of acetonitrile adsorbed on the anatase TiO2 (101) surface.35 The hydrogen atom in hydroxyl moiety migrates to O2c which is adjacent to Ti5c bonded with O2 atom,

MA1

MA2

BA1

BB1

BB2

BC1

BC2

C1 -O1 1.306 1.304 1.243 1.309 1.314 1.278 1.286 1.256 1.249 1.297 1.228 1.226 1.278 1.269 C1 -O2 1.181 1.181 1.180 1.179 1.180 1.181 1.181 C2 -N 1.063 1.057 1.777 ... ... ... ... O1 -H 1.454 1.482 0.999 0.975 0.980 0.975 0.975 O2c -H ... ... ... 1.935 1.974 2.125 2.083 Ti5c -O1 2.164 2.149 2.016 ... ... 2.126 2.129 Ti5c -O2 ... ... 2.328 2.308 2.287 ... ... Ti5c -N 125.9 126. 9 126.4 127.0 126.9 127.6 O1 -C1 -O2 124.0 0.96 0.94 1.02 0.95 0.91 0.86 0.93 Eads


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corresponding H-O2c bond length about 0.999 Å. The interaction between the O1 and the dissociated H induces the broken of the planar structure of adsorbate, generating hydrogen bond with inter-atomic distance about 1.777 Å. BA1 structure is very similar to the stable model of dye on anatase TiO2 (101) surface with adsorption energy about 1.3 eV, depending on different dyes.72 The adsorption energy of BA2 is smaller than that of BA1 about 0.25 eV. The main atomic configuration difference from BA1 is that the dissociated H binds to O2c which does not directly interact with Ti5c connected O2 and N atoms in CAA. The bidentate adsorption modes BB1 and BB2 are quite similar. The N and hydroxyl O bond with two adjacent Ti5c atoms and the H in hydroxyl is dissociated, forming OH with different O2c . The main difference between BB1 and BB2 is the adsorption site of the dissociated H. For BB1 structure, the H adsorbed on O2c which is adjacent to Ti5c bonded with O1 atom is the most energetically favorable mode, corresponding adsorption energy about 0.95 eV. The adsorption energy of BB2 is smaller than that of BB1 about 0.04 eV. The carboxyl dissociated bidentate modes are labeled as BC1 and BC2, depending on which O2c interacts with the hydrogen from hydroxyl group. Also, the BC1 and BC2 structures are similar to the structure of acetic acid adsorption on anatase TiO2 (101) surface with a dissociated bidentate binding geometry where the acetate binds to two neighboring Ti5c atoms along the [010] direction.73 The adsorption energies of BC1 and BC2 (0.86 and 0.93 eV, respectively) again highlight that the hydrogen adsorbed on O2c which is adjacent to Ti5c bonded with O1 atom is energetically favorable mode. Furthermore, the adsorption energy differences between BB1 and BC2, BB2 and BC1 support that the interaction of N-Ti5c is stronger than that of O-Ti5c . Meanwhile, the adsorption energy differences between BB1 and BB2, BC2 and BC1 indicate that, the energetically favorable configuration is that the dissociated hydrogen from hydroxyl interacts with O2c which belongs to the same pair Ti5c bonded with carboxyl O. This is similar to the case of 2-propanol on anatase TiO2 (101) surface.46 c. Tridentate adsorption. The adsorption energy (about 0.55 eV) of TA1 (in Fig. S1 in the supplementary material68 ) is smaller about 0.47 eV than that of BA1 because of the torsion structure of adsorbate CAA.

In terms of adsorption energy, the bidentate dissociated adsorption mode BA1 is the most stable structure of CAA on anatase TiO2 (101) surface. To understand the adsorption strength of CAA on anatase TiO2 (101) surface, we compare the adsorption energies with other molecules on this surface. The calculated molecular adsorption energies of water on anatase TiO2 (101) surface are in the range of 0.56–0.85 eV, depending on computational methods.74 The phosphonic acid adsorption energy on anatase TiO2 (101) surface is about 2.87 eV (277 kJ/Mol) for two HPO(OH)2 with fully dissociated bidentate mode, forming doublet O-Ti bonds and one hydrogen bond for every HPO(OH)2 .75 The adsorption energy of HNO3 on the surface is about 0.58 eV with dissociated monodentate structure, corresponding one N-Ti bond, and the dissociated hydrogen bonded to a neighboring bridg-

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ing oxygen.76 The adsorption energy (about 0.75 eV) of acetonitrile adsorbed on the anatase TiO2 (101) surface suggests the effective interaction through N-Ti bond.35 The most stable adsorption structure for HCOOH on anatase TiO2 (101) surface, resembling MA2 in Fig. 3, is the molecular monodentate configuration with O-Ti bond and a hydrogen bond connected to surface bridging oxygen O2c , corresponding adsorption energy about 0.92 eV.67 Based upon the above analysis, the energetically favorable structures of these molecules adsorption on anatase TiO2 (101) surface are determined by the hydrogen bond, the binding position of dissociated hydrogen, the number and strength of formed bonds. Apparently, the adsorption energy of CAA on anatase TiO2 surface is larger than that of most above mentioned molecules. This ensures the stability and electronic coupling in DSSCs based upon dye sensitizers containing CAA moiety. Meanwhile, similar to the phosphonic acid anchoring group attached on anatase TiO2 (101) surfaces,30 this fully dissociated bidentate adsorption mode can also lead to faster electron injection, which is favorable to improve Jsc of DSSCs.

2. Electronic structures

To further understand the adsorption properties of CAA on anatase TiO2 (101) surface, the electronic density of states (DOS) analyses for the isolated CAA molecule, the slab of clean TiO2 (101) surface, and the most stable adsorption configuration BA1, are performed. In order to investigate the properties of bonds between adsorbate and surface, we focus on the projected DOS (PDOS) of the H, O and N atoms in adsorbate, as well as the surface O2c and Ti5c atoms which interact with the atoms from adsorbate. The DOS and PDOS of CAA molecule, the clean TiO2 (101) surface, and the most stable adsorbate-surface adsorption configuration BA1 are presented in Fig. 4. From the DOS and PDOS of CAA in Fig. 4(a), the orbital overlaps between 1s of H and 2p of O1 indicate the existence of orbital hybridization and O-H bond. The sp hybridizations of N and O are approved by the overlap of 2s and 2p orbitals. It also shows the HOMO is π orbital which is mainly composed by O 2p orbitals and the LUMO is π * orbital which is mainly contributed from 2p orbitals of O and N. The DOS and PDOS of TiO2 (101) surface are giving in Fig. 4(b). The valence bands of TiO2 (101) surface slab mainly consist of O 2p and Ti 3d, involving Ti-O d-p π bonds,69 while the conduction bands are mainly made up of Ti 3d, corresponding the localization of Ti cation. Fig. 4(c) shows the DOS and PDOS of CAA-TiO2 (101) surface system with BA1 structure. The OH in Fig. 4(c) stands for the surface O2c that connects hydrogen dissociated from hydroxyl. The TiN and TiO represent surface Ti bonded with N and O from CAA, respectively. The significant reduction of orbital overlap between 1s of H and 2p of O1 and the orbital energy resonance between 1s of H and 2p of OH indicate the dissociation of hydroxyl in CAA and formation of OH species at the surface. This is parallel with geometrical parameters. Fig. 4(c) also indicates that, the orbitals of the adsorbate O 2p and N 2p in the conduction band are nearly disappeared, and the energy range of N valence orbitals (2s and 2p) are extended, and the


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FIG. 4. The total DOS and PDOS of cyanoacrylic acid molecule (a), the slab model of clean TiO2 (101) surface (b), and the most stable configuration BA1 of the adsorption of cyanoacrylic acid on anatase TiO2 (101) surface (c).The panel (d) includes the PDOS of CAA and slab in CAA-slab system, the total DOS labeled as BA1.

lower energy boundary of N is below −6.0 eV (Fig. S2 in the supplementary material lists the PDOS of N2p68 ), resulting from N-Ti bond formation. The PDOS also shows the orbital overlap of TiO -O2 /TiN -N that suggests the formation of the O2 -Ti5c and N-Ti5c p-d bonds and the strong electronic coupling between CAA and anatase TiO2 (101) surface. Also, it is confirmed by Troisi’s group that77 the N-Ti coupling which exists in BA1 provides an efficient alternative pathway of fast electron injection. In Fig. 4(d), the PDOS of CAA and slab in CAA adsorbed (101) surface system are labeled with CAA and Sur., respectively. The total DOS of CAA adsorbed on (101) surface labeled as BA1 is also included in order to compare conveniently. The band gap of BA1 is slightly smaller (about 0.005 eV) than that of clean surface due to the downshift of conduction band edge after adsorption. The comparisons of total DOS between clean surface and BA1 (see Fig. S3 in the supplementary material68 ) also indicate that most of the BA1 DOS values in valence band are larger than that of clean surface. This means that the charge carrier density in valence band is increased after adsorption. The increasing of DOS in valence band will become more significant if complete dye sensitizer (containing electron donor and conjugated bridge) adsorb on this surface. Furthermore, we can find that, the CAA orbitals are presented in the valence band,

while the component of CAA orbitals in conduction band is significantly reduced, and the conduction band is mainly contributed by Ti 3d orbitals. All of these points indicate that the absorption transitions in UV/Vis region are CT excitations from electronic occupied-states (partially from CAA) to unoccupied-states (mainly from Ti). These CT excitations provide feasible channel for electron injection through the interface between TiO2 and CAA. The photo-induced electron injection is favorable for the generation of charge separated states and improvement of photocurrent in DSSCs.

D. Adsorption on anatase TiO2 (001) surface

1. Atomic configurations of adsorption

Similarly, the adsorption of CAA on anatase TiO2 (001) surface can be classified as monodentate, bidentate, or tridentate mode. The structures of bidentate modes are presented in Fig. 5, and their adsorption energies, as well as the selected geometrical parameters are listed in Table II. The other structures with smaller adsorption energies, including MA1, BA3, BD1 and TA1, are given in Fig. S4 in the supplementary material,68 and the selected geometrical parameters are listed in Table SII in the supplementary material.68


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FIG. 5. The adsorption of cyanoacrylic acid molecule on anatase TiO2 (001) surface with monodentate and bidentate modes. The atoms in bottom layers of slab model are omitted for clarity.

a. Monodentate adsorption. In MA1 configuration (see 68

Fig. S4 in the supplementary material ), CAA molecule attaches to anatase TiO2 (001) surface through the bond between carbonyl O and surface Ti5c . Meanwhile, the broken O-H bond of hydroxyl in CAA gives the H-O1 distance about 1.467 Å, and the dissociated H atom binds to surface O2c atom which is adjacent with Ti5c before adsorption, generating HO2c bond length about 1.033 Å. Because of the H-O2c bond formation, the O2c atom moves away from the surface plane and Ti5c -O2c bond is broken, elongating inter-atomic distance from 1.962 Å to 4.016 Å. b. Bidentate adsorption. BA1 and BA2 structures are bidentate dissociated adsorption modes through the dissociation of carboxyl. For BA1, the O atoms from dissociated carboxyl bond with two neighbor Ti5c along [100] direction, and the dissociated H interacts with O2c along [010] direction, generating OH species with O2c -H bond length about 1.003 Å. The stabilization of OH formation at this surface is same as TABLE II. The selected bond lengths or inter-atomic distances (Å), bond angles (◦ ), and adsorption energies (eV) for the adsorption of cyanoacrylic acid molecule on anatase TiO2 (001) surface with monodentate and bidentate modes.

C1 -O1 C1 -O2 C2 -N O1 -H O2c -H Ti5c -O1 Ti5c -O2 Ti5c -N TiO1 -O2c TiO2 -O2c TiN -O2c O1 -C1 -O2 Eads

BA1

BA2

BB1

BB2

BC1

BC2

1.279 1.277 1.181 ... 1.003 2.029 2.026 ... 3.549 3.594 ... 126.9 3.25

1.279 1.280 1.181 ... 0.975 2.078 2.001 ... 3.294 3.875 ... 126.6 3.02

1.338 1.220 1.179 1.659 1.000 1.960 ... 2.139 4.002 ... 3.964 125.1 3.19

1.324 1.227 1.184 1.969 0.987 1.946 ... 2.129 3.742 ... 4.107 122.4 2.99

1.332 1.220 1.177 1.716 0.990 1.956 ... 2.177 3.864 ... 3.500 126.0 3.19

1.331 1.221 1.178 1.765 0.989 1.956 ... 2.207 3.891 ... 2.737 126.0 3.01

the water dissociation on anatase TiO2 (001) surface.48 The molecular plane of CAA is not perpendicular to anatase TiO2 (001) surface. The two O2c atoms, which bond with the two Ti5c atoms where the CAA adsorbed on, move away from the surface, and the broken of O2c -Ti5c bonds results into the inter-atomic distances extend to 3.549 and 3.594 Å, respectively. The O2c -Ti5c bond broken is attributed to the fact that there is a high density of uncoordinated surface atoms which induces a very strained configuration of the surface atoms.46 The formation of OH species is approved by H-O2c bond length (∼1.003 Å). The adsorption energy of BA1 is 3.25 eV, which is 0.59 eV higher than that of MA1. The difference of BA2 from BA1 is the position of dissociated H at the surface. The adsorption energy of BA2 is less than that of BA1 about 0.23 eV. For BB1 and BB2 structures, the O1 and N in CAA bond with two Ti5c atoms where the line of these Ti5c locates between [010] and [100] directions. The distances of the Ti5c N and Ti5c -O bonds in BB1 are 2.122 and 1.959 Å. While in BB2 structure, the corresponding values are 2.129 Å and 1.946 Å. The calculated adsorption energies of BB1 and BB2 are about 3.19 eV and 2.99 eV, respectively. The difference between BB1 and BB2 structures is the position of dissociated H atom that binds to surface O2c atom and therefore to generate OH species. For BC1 and BC2, the N and O1 atoms in CAA coordinate two neighbor Ti5c along [100] direction. The difference between BC1 and BC2 is similar to that of BB1 and BB2. The Ti5c -N/Ti5c -O1 bond lengths in BB1, BB2, BC1, and BC2 are quite similar. The formation of OH species and the broken of O2c -Ti5c bond also exist in BC1 and BC2. The adsorption energies for BC1 and BC2 are 3.19 and 3.01 eV, respectively. The adsorption energy differences between BB1 and BB2, BC1 and BC2 underline the importance of the position to which the dissociated H binds. c. Tridentate adsorption. TA1 structure (see Fig. S4 in the supplementary material68 ) is dissociated tridentate adsorption mode. The O atoms from dissociated carboxyl bond with two neighbor Ti5c along [010] direction, while the N atom in CAA binds to another T5c which belongs to [100] direction


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FIG. 6. The total DOS and PDOS of cyanoacrylic acid molecule (a), the slab model of clean TiO2 (001) surface (b), and the most stable configuration of the adsorption of cyanoacrylic acid on anatase TiO2 (001) surface (c). The panel (d) includes the PDOS of CAA and slab in CAA-slab system, the total DOS labeled as BA1.

relative to Ti5c connected to O1 . The dissociated H binds to O2c which connects to Ti5c adsorbed O1 . The torsion (between the carboxylic plane and cyano plane) and the broken of several O2c -T5c bonds generate smaller adsorption energy (about 2.66 eV). In terms of adsorption energies, the dissociated bidentate adsorption mode BA1 is the most stable structure of CAA on TiO2 (001) surface due to the doublet O-Ti5c bonds and suitable dissociated hydrogen position. The quite large adsorption energy of CAA on anatase TiO2 (001) surface is resulted from the reactivity of (001) surface. The comparable adsorption energy of 2-propanol on anatase TiO2 (001) surface is about 3.10 eV through the dissociated mode.46 The similarity of BA1 structure with the ad-molecule mode49 implies a considerable structural arrangement. The adsorption energy difference between dissociated monodentate and bidentate modes is about 0.59 eV for CAA adsorbed on (001) surface. Furthermore, the adsorption energy of BA1 structure is about 1.57 eV larger than that of the most stable structure of formic acid on (001) surface which is similar to BA2 structure.48 Again, the quite larger adsorption energy of CAA on (001) surface is determined by the binding position of dissociated hydrogen, the

broken of O2c -Ti5c bonds, the number and strength of formed bonds, and thus generates the stability and electronic coupling of DSSCs. 2. Electronic structures

The DOS and PDOS of CAA, the slab of clean TiO2 (001) surface, and the most stable structure BA1 are giving in Fig. 6. The DOS and PDOS of CAA molecule are included in Fig. 6(a) in order to investigate the adsorption effects on electronic properties through the DOS variation before and after adsorption. The DOS and PDOS of TiO2 (001) surface (Fig. 6(b)) indicate that the valence bands of TiO2 (001) surface slab mainly consist of 2p of O and 3d of Ti, but the conduction bands are contributed by 3d of Ti. Compared with TiO2 (101) surface, the DOS of TiO2 (001) surface supports that the valence bands are broaden, and the conduction bands are shifted to lower energy, corresponding to smaller band gap. This tendency agrees with that of published work (PBE, DNP, Dmol3).56 The broadening of valence bands means the more electrons populated near Fermi energy level which are favorable for reaction activity. This again gives that the TiO2 (001) surface is more reactive than (101) surface.


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The OH in Fig. 6(c) stands for the surface O2c that connects the H from dissociated hydroxyl. The TiO1 and TiO2 represent surface Ti5c bonded with O1 and O2 from carbonyl in CAA, respectively. The complete disappear of orbital overlap between 1s of H and 2p of O1 indicates the dissociation of hydroxyl in CAA, whereas the significant orbital overlap between 1s of H and 2p of OH supports that exist of O-H bond at the surface. Compared with the PDOS of CAA, the PDOS of O1 in Fig. 6(c) becomes narrow, while the PDOS of O2 extends to the range of lower energy, and therefore the PDOS resemblance of O1 and O2 can be found due to the similar coordination of O1 and O2 with Ti5c . The orbital overlap between O1 /O2 2p and TiO1 /TiO2 3d, ranging from about −8.0 to 0.0 eV (see Fig. S5 in the supplementary material68 ), ensures the electronic coupling between CAA and anatase TiO2 (001) surface, and suggests the formation of O-Ti5c bonds. The shift of O2 PDOS to lower energy indicates the O-Ti5c bonds are stronger interaction which can induce electron localization. The PDOS of N still exhibits local character because of the invariant of CN bond. Compared with the deep energy level of N in CAA, the Fermi energy level of CAA-TiO2 (001) surface system has a slight up-shift. In Fig. 6(d), the labels of PDOS are same as that in Fig. 4(d). Similar to CAA adsorption on (101) surface, for CAA adsorbed on TiO2 (001) surface system, the valence bands contain the contributions from CAA orbitals, while the contribution of CAA orbitals is tremendously reduced in the conduction bands which are mainly composed of Ti 3d orbitals. This suggests that the transitions from the valence bands (occupied electronic states) to conduction bands (unoccupied states) within UV/Vis absorption region are CT excitations. These CT excitations pump the electrons populated in valence bands (partially contributed by CAA) to conduction bands that are mainly localized in TiO2 (001) surface slab. Therefore, CAA adsorption on TiO2 (001) surface also provides feasible channel for photo-induced electron injection from CAA to TiO2 . This is similar to the case of CAA adsorption on TiO2 (101) surface. Furthermore, the comparisons between Fig. 6(b) total DOS and Fig. 6(d) BA1 DOS (see Fig. S6 in the supplementary material68 ) indicate that the DOS near Fermi energy is slightly decreased after CAA adsorbed on TiO2 (001) surface, resulting into the increasing of band gap about 0.15 eV. This result points to the contrary effect of that on (101) surface. It underlines the possibility to tune electronic properties through the controlling TiO2 surface on which the dye sensitizers adsorbed in the fabrication of DSSCs. IV. CONCLUSIONS

In summary, the adsorption energies, stable structures, and related electronic properties of CAA on anatase TiO2 (101) and (001) surfaces have been studied by means of density functional theory calculations in connection with ultrasoft pseudopotential and PBE functional based upon periodical slab models. The most stable adsorption structure of CAA on anatase TiO2 (101) surface is the dissociated bidentate mode through the cyano N and carbonyl O atoms bond with two adjacent surface Ti atoms along the [010] direction, and the H from dissociated hydroxyl binds to the surface bridg-

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ing O2c which connects to the surface Ti atom bonded with carbonyl O. The adsorption of CAA on anatase TiO2 (001) surface, the most stable structure is the dissociated bidentate mode through the O atoms from dissociated carboxyl moiety. The O atoms of carboxyl bond with two neighbor surface Ti5c atoms along [100] direction, and the H from dissociated hydroxyl moiety interacts with the surface O2c , generating another OH species. The adsorption energies are estimated to be 1.02 and 3.25 eV for (101) and (001) surfaces, respectively. The quite larger adsorption energy ensures the electronic coupling between CAA and anatase TiO2 surfaces and also enhances the thermal stability of DSSCs based on dye sensitizers containing CAA moiety. The analysis of density of states not only suggests the bonds between CAA and TiO2 surfaces are formed but also indicates the most stable structures of CAA adsorbing on TiO2 (101) and (001) surfaces provide feasible mode for photo-induced electron injection from CAA to TiO2 . This work presents a perspective to understand the channel of photon-induced electron injection from the excited dye sensitizers that contain CAA moiety to semiconductor electrode. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant Nos. 11164016 and 11164015). 1 D.

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