Computational Studies of the Interaction of Carbon Dioxide with

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Computational Studies of the Interaction of Carbon Dioxide with Graphene supported Titanium Dioxide Julia Melisande Theresa Agatha Fischer, Marlies Hankel, and Debra J Searles J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10692 • Publication Date (Web): 07 Dec 2015 Downloaded from http://pubs.acs.org on December 12, 2015

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The Journal of Physical Chemistry

Computational Studies of the Interaction of Carbon Dioxide with Graphene supported Titanium Dioxide Julia M. T. A. Fischer,† Marlies Hankel,† and Debra J. Searles∗,†,‡ Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld 4072, Australia, and School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Qld 4072, Australia. E-mail: [email protected]

∗ To

whom correspondence should be addressed for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld 4072, Australia ‡ School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Qld 4072, Australia. † Centre

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Abstract The interaction of carbon dioxide (CO2 ) with titanium dioxide (TiO2 ) supported on graphene (GR) and epoxy-enhanced graphene (GR-O) was investigated using density functional theory (DFT) calculations, and compared with the interaction on unsupported TiO2 systems. Adsorption energies, charge density differences and activation barriers were calculated. TiO2 clusters, comprising two to four TiO2 units were considered. We show that the carbon support influences the binding energy of CO2 significantly when chemisorbed, and the molecule is bound in a bent configuration. The epoxy oxygen connection of GR-O with TiO2 leads to a further increase in the binding energy of CO2 , as does increasing the size of the TiO2 cluster, due to a higher charge delocalization on the GR-sheet.


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Introduction As a result of intensified industrialization, the amount of emitted carbon dioxide (CO2 ) is steadily increasing. Using CO2 in a sustainable way to produce value-added products, for example methane, is the main goal of many recent research studies. 1 In order to avoid CO2 production in the generation of energy to drive these processes, use of sunlight and photocatalysts offers much potential. A well known photocatalyst is titanium dioxide (TiO2 ). This is a semiconductor that can be excited with light radiation, and while the charge is successfully separated, the electron can be transported to the surface adsorbate for further reaction. Use of TiO2 to produce methane from hydrogen and carbon dioxide has been demonstrated using TiO2 pellets with UV radiation. 2 Experimental studies have subsequently shown an enhancement of CO2 reduction can be obtained when TiO2 is placed on graphene (GR). 3–5 This enhancement is argued to be due to a reduced band gap, 6 electric conduction of GR, 7 surface Ti3+ sites 8 and a reduced aggregation of both the GR layer and the TiO2 nanoparticles. 6–8 Key to the substrate significantly enhancing the effectiveness of TiO2 as a catalyst is its capacity to increase the binding of CO2 to the system. In a recent review by Liu and Li, it is noted that the adsorption step is frequently overlooked in considering new photocatalytic systems. 9 Therefore in this work, we will focus on determining effects of properties of the supported TiO2 on the binding energy of CO2 . The interaction of CO2 with different unsupported titanium dioxide phases and structures has been studied intensively. 10 It has been found, with thermal desorption spectroscopy, that the effect of oxygen defects (vacancies) in TiO2 (110) rutile surfaces on the CO2 adsorption dynamics are rather small. 11 Experimentally, the binding energy without defects has been determined to be 0.373 eV whereas with defects it rises to 0.497 eV with a 0.021 eV accuracy. Prior to that study, in another temperature-programmed desorption experiment, values of 0.5 eV without, and 0.56 eV with, oxygen vacancies were found. 12 Moreover it is observed that different structures and surface facets are more reactive and show a difference in band gaps and photoactivity. 9,13 Computational 3 ACS Paragon Plus Environment


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studies of adsorption of linear and bent conformations of the CO2 molecule on the anatase (001) surface gave binding energies of 0.23 eV and 1.31 eV respectively, while on a anatase (101) surface only the linear structure adsorbed in a stable configuration, and this gave a binding energy of 0.412 eV. 14 In addition, studies have been conducted on TiO2 clusters. Positively charged clusters with 1 to 8 units of TiO2 were produced by sputtering, and the binding energies were also calculated for systems with up to 4 units using a pair potential ionic model. 15 In other experiments with neutral clusters, a growth mechanism for (TiO2 )n + TiO2 was proposed. 16 In the DFT framework, the structure of titanium dioxide clusters of up to 3 or 9 units of TiO2 were studied with different functionals. 17,18 The adsorption of a few units of TiO2 on pristine GR and GR with defects such as edges, 19 vacancies 6 or with oxygen functionalisation, has been investigated using computational methods. Ayissi et al. 19 considered Ti2 O4 and Ti4 O8 with configurations corresponding to those in the rutile and anatase unit cells, respectively, and found that adsorption was strongest at carboxylate groups on the edges of the GR ribbons. On the other hand, rather than structures based on the bulk unit cell, Geng et al. 6 considered the most stable Ti3 O6 clusters. With density of states (DOS) plots, they observe a reduction in the band gap when TiO2 is adsorbed on GR and functionalised GR, and confirm its suitability as a visible light photocatalyst. Furthermore, they find the strongest binding is to the epoxy enhanced GR through a Ti-atom connection. The interfacial connection was also experimentally evaluated and it was found that TiO2 binds both through oxygen and titanium to the carbon support. 20 There are different routes to synthesis of TiO2 /GR. 5 In the most common method, graphene oxide (GO) is reduced to GR with titanium ions being adsorbed from the solution forming titanium dioxide nanoparticles. 21 It has been indicated that TiO2 nanoparticles have a stronger binding to epoxy enhanced GR than to a perfect GR sheets. Oxygen defects on GR serve as anchors for the formation of TiO2 particles, and are partially reduced in that progress. Since not all defects are removed, it is unclear how these defects influence the CO2 adsorption. According to Liang et al. 5


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defects in the GR reduce the efficiency of CO2 reduction, due to a reduced electric conductivity of the GR. Through these studies, information on the interaction of titanium dioxide and GR has been obtained and its enhanced photoactivity compared to TiO2 has been found in experiments as well as in theoretical studies. In addition, adsorption of CO2 on TiO2 has been modeled, however the reaction mechanism and the influence of the support on the CO2 reduction has not been addressed. We will address an important element of this process, the adsorption of CO2 , on the various systems. It is presumed that the reduction of CO2 occurs at the TiO2 particles and not on the GR. Hence in this study only the influence of the basal plane, not the edges, of GR on the TiO2 -CO2 interaction has been investigated. For the mechanism and the discovery of the active sites of this reduction, the first step, the adsorption of CO2 on this substrate plays an essential role. Here two cases are presented, the physisorption of CO2 where it remains in a linear conformation and the chemisorption of CO2 which results in a bent molecule. In this paper, different TiO2 clusters have been modeled, with and without GR or GR-O. Our results for the system configurations, adsorption sites and energies are generally consistent with previous studies. 6,19 New calculations on the direct influence of GR on the TiO2 -CO2 interaction have also been carried out using two to four units of TiO2 , as presented below.

Computational Methods All calculations were performed using density functional theory (DFT) calculations, with the Vienna Ab-initio Simulation Package (VASP). 22 The revised version of the Perdew-Burke-Ernzerhof functional (RPBE), 23 with the dispersion corrections DFT-D3, 24 was used for the electronic exchange and correlation effects. The basis set was constructed with plane waves in a periodic projection and the core electrons are approximated with the Projector Augmented Wave (PAW) method. 25 The basis set cutoff was set to 450 eV with 3 × 3 × 1 gamma centered k-points using a (6 × 6)-cell with lattice length of 12.82 Å. A 20 Å vacuum above the pristine GR was chosen.



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These values were tested for CO2 binding energy convergence, and an increase in cell size and other parameters did not change any trends or observations made in this paper. For the epoxy enhanced GR, an oxygen atom was adsorbed on the GR in the bridge position, i.e. above a carbon-carbon bond, forming an epoxy bond. To describe the adsorption of TiO2 , on the GR, we introduce some terminology: ontop site when an atom is directly above the C atom, hollow site which is above the center of a six atom carbon ring. The term distorted site it used when the atom is not exactly at one of these sites, but is close to it. The position of CO2 on the TiO2 can be described by they way in which it bonds. It can be due to coordination of a O atom of the CO2 to titanium (O-Ti), bonding of the C of the CO2 with surface-oxygen (C-O) or bonding through both these interactions. Distinctions between these are made in the manuscript by referring to these as singly coordinated (i.e. via O-Ti or C-O), or doubly coordinated (i.e. via O-Ti and C-O). The Ti in TiO2 or its clusters, before bonding to the CO2 or the substrate, is three-fold or four-fold coordinated. The former is often referred as ’undercoordinated’ whereas the latter has an almost symmetric, tetrahedral coordination. The oxygen will then be either be singly or doubly bonded to the Ti-atoms. The distance of the CO2 from the surfaces and the binding energy can be used to classify the adsorption as physisorption or chemisorption. Physisorption is associated with longer bond distances between the CO2 and surface atoms than chemisorption, and the binding is due to van der Waals forces. These are weaker than the bonding required for chemisorption. The chemisorption is due to chemical reaction of the adsorbate with the surface and will usually have an activation barrier. The decrease in the distance of the CO2 from the surface and increasing surface adsorbate interactions leads to changes in the bond lengths (and angle) in the CO2 molecules. This destabilization of the molecule is likely to reduce the activation barrier for reduction of CO2 . The binding energies Ebind can be defined as the negative of the adsorption energies, Ead , calculated using the total energy of the surface with the adsorbed molecule, ESF+mol , and the separate relaxed systems of the surface, ESF , and the molecule, Emol . For stable adsorption, the


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binding energy is positive and

Ebind = −Ead = (ESF + Emol − ESF+mol ).

(1)

In order to understand the nature of the binding between the CO2 and substrate, changes in the charge density ∆ρ on CO2 adsorption was obtained using the fully relaxed system comprised of the surface with an adsorbed CO2 molecule, (SF +CO2 ), and the surface and adsorbed CO2 molecule in their separated, unrelaxed geometries. We define the change in charge density at any position in space as: ∆ρ = ρSF+CO2 − ρSF − ρCO2 .

(2)

Transition states (TS) and barriers between the physisorption and chemisorption were calculated with the climbing image nudged elastic band (CI-NEB). 26 One image for the TS was used and two different local minima sites were considered.

Results and Discussion Titanium Dioxide on Graphene Firstly, different symmetries of titanium dioxide clusters with either 2, 3 or 4 units of TiO2 were geometry optimized without support, and then they were adsorbed in various positions and configurations on graphene (GR). The initial structures of the possible TiO2 clusters were chosen using chemical intuition (symmetry, coordination, reasonable bond length and angles) and up to 4 configurations were optimised by energy and force minimization. The adsorption sites on the GR were tested with different angles and atoms (either Ti or O or both) pointing towards the surface and further on different sites (bridge, ontop and hollow) with different orientations towards the hexagonal grid. Depending on the cluster, 5-12 different TiO2 /GR sites were tested. This process was also carried out for epoxy-enhanced graphene (GR-O), which contains an oxygen bonded to two adjacent graphene 7 ACS Paragon Plus Environment


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carbon-atoms. Our calculations resulted in minimum energy structures for the isolated clusters consisting of 2 and 3 units of TiO2 that are in agreement with those obtained previously using computational methods. 17,18 In Figure 1 the adsorption of Ti2 O4 and Ti3 O6 clusters on the two different supports, GR and GR-O, are shown from top and side views. The structures of the titanium dioxide clusters are very similar on both GR and GR-O, and it was found that these TiO2 clusters have almost the same most stable structure on GR or GR-O as they have without support, with only a minor variations of the angle and bond lengths. Therefore the unsupported clusters are not shown separately (see Figure 1). The most stable structures for the clusters on the supports are shown in (Figures 1 and 2), and were used for the next section of this work, where the adsorption of CO2 is considered on 10-17 different sites.

(a) Ti2 O4 /GR

(b) Ti3 O6 /GR

(c) Ti2 O4 /GR-O

(d) Ti3 O6 /GR-O

Figure 1: Two different perspectives of the most stable adsorption sites of Ti2 O4 and Ti3 O6 clusters on GR (Figures 1a and 1b) and GR-O (Figures 1c and 1d). Note that the epoxy-O becomes singly bound to the carbon in the Ti3 O6 /GR-O case, and in both the GR and GR-O cases, the graphene structure near the O atom is slightly buckled. In the view from above the graphene plane, eclipsed oxygen atoms are made larger so that their location can be seen. (Atoms are colored in red-oxygen, yellow-titanium and black-carbon support.)

The minimum energy structure of the Ti2 O4 cluster has a C2h symmetry. On GR (see Figure 1a), the configuration with one carbon underneath the TiO-ring and one Ti-atom located 2.52 Å above GR in the ontop position, was most stable. On GR-O, the titanium atom closest to the surface


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(a) Ti4 O8 (Cs ) molecule

(b) Ti4 O8 (C2v )/GR

(c) Ti4 O8 (C2v )/GR-O

Figure 2: wo different perspectives of the most stable configuration and adsorption site of the Ti4 O8 molecule unsupported (Figure 2a), on GR (Figure 2b) and on GR-O (Figure 2c). (Atoms are colored in red-oxygen, yellow-titanium and and black-carbon support.) bonds with the epoxy oxygen, as shown in Figure 1c. For the Ti3 O6 cluster, a structure with Cs symmetry was found to be the most stable configuration. When adsorbed on the GR, one Ti-atom is located 2.07 Å above the plain of the surface and in the hollow position, while the three oxygen nearest to the GR were all in distorted bridge positions, as seen in Figure 1b. This is in agreement with previous studies, except the distance between TiO2 and GR is smaller in our case as dispersion is included. 6 In the case of Ti3 O6 on GR-O (see Figure 1d), the oxygen of the epoxy group only forms a single bond with the GR sheet in the presence of the cluster (based on the distance between the atoms), in contrast to the case of Ti2 O4 where two bonds with the GR are maintained. In Figure 2 systems with Ti4 O8 without and with GR and GR-O support are shown from a side and top view. Although for clusters with 2 and 3 units of TiO2 the structures agree with those previously described in the literature, for Ti4 O8 another structure, which was not suggested by Qu et al., 18 was found to be more stable for the isolated molecule (see Figure 2a). Furthermore, the structure of the Ti4 O8 cluster is different when it is in its most stable state adsorbed on the GR or GR-O surface than when it is isolated, (see Figure 2). The Ti4 O8 unsupported structure has a Cs symmetry and has only four-fold coordinated Ti-atoms. It is broader than the C2v structure,



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which is more stable on GR and GR-O. The Ti4 O8 C2v structure has two three-fold coordinated Ti-atoms. The one closest to the surface is 2.60 Å above the plain of the surface and between the ontop and hollow position. Both oxygen atoms that are close to the surface are in a similar positions. The binding energies of the TiO2 clusters on the substrates can be seen in Table 1. Noticeable is the stronger interaction of the titanium oxide with the GR-O compared to GR. Additional stabilization occurs when the epoxy oxygen forms only a single bond to the GR-sheet, as in the case of Ti3 O6 and Ti4 O8 (seen Figures 1d and 2c). Table 1: Binding energies of TiO2 clusters on (GR) and (GR-O). Cluster Binding Energy/eV GR GR-O Ti2 O4 1.16 1.48 Ti3 O6 1.55 2.76 Ti4 O8 1.21 3.27 We found a further increase in binding energy of the TiO2 clusters to the carbon support is obtained when the bond between the epoxy oxygen and the GR is broken and oxygen enriched Tix Oy clusters (Ti3 O7 and Ti4 O9 ) are formed. As the binding energies of CO2 on these oxygen rich clusters are smaller than on stoichiometric clusters, they are not suitable for its reduction and were not further investigated. Nevertheless this result is consistent with the mechanism proposed in the most common method for synthesis of TiO2 clusters, in which the GR-O is reduced due to the formation of TiO2 . 21 This happens in a reducing environment, indicating that titanium dioxide nanoparticles can be formed that are not oxygen rich, but stoichiometric, as we have studied. We also note that not all defects on the GR-O are reduced in the synthetic process. It has been found that both C-Ti and Ti-O-C bonds are present in the composites produced, 20 indicating that systems with TiO2 on GR or GR-O are both of relevance (see Figures 1 and 2). The adsorption site of Ti3 O6 /GR agrees with previously published results, although the distance of the Ti3 O6 from the GR and the binding energies differ in this study. They are lower by 0.53 Å and higher by 0.08 eV respectively. 6 These discrepancies can be explained by the use of DFT-D3 methods 10 ACS Paragon Plus Environment

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in the current work, as tests without the use of dispersion correction give energies that coincide with the published results. This demonstrates the importance of including dispersion forces in the calculations.

Carbon Dioxide on Titanium Dioxide-Graphene composites Adsorption of CO2 on the TiO2 clusters in two different configurations physisorbed (linear) and chemisorbed (bent) have been considered in this work. The effects on the cluster size, introduction of a GR or GR-O support and type of binding are investigated. The most stable sites for adsorption of CO2 on Ti4 O8 for the three different systems (without and with GR or GR-O support) are shown in Figure 3. The stuctures for both physisorbed and chemisorbed CO2 are presented. In the latter case, the carbon forms a CO3 structure (see Figures 3d to 3f where the atoms that originated from the CO2 molecule are drawn larger for better visualisation). Formation of carbonates on metal oxides with chemisorption has often been observed in experimental and computational studies in the past, 27,28 and has been related to the basicity of the oxide. 29 An overview of structures of all systems including Ti2 O4 and Ti3 O6 , with and without carbon support, can be found in the Supporting Information (SI Figure 1) and the binding energies are listed in Table 2. Table 2: CO2 binding energies on TiO2 clusters, TiO2 /GR and TiO2 /GR-O. CO2 Structure

Cluster

Linear Linear Linear Bent Bent Bent

Ti2 O4 Ti3 O6 Ti4 O8 Ti2 O4 Ti3 O6 Ti4 O8

Binding Energy/eV No substrate GR GR-O 0.52 0.52 0.47 0.53 0.27 0.22 0.41 0.59 0.54 0.85 1.04 1.08 0.66 0.86 0.91 0.75 0.64 0.76

The first step in the catalytic interaction of CO2 and the surface is the physisorption of the molecule in a linear configuration. Without support, the linear structure is found to be most stable when two atoms from the CO2 can coordinate to under-coordinated titanium dioxide sites. This 11 ACS Paragon Plus Environment


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(a) Ti4 O8

(b) Ti4 O8 /GR

(c) Ti4 O8 /GR-O

(d) Ti4 O8

(e) Ti4 O8 /GR

(f) Ti4 O8 /GR-O

Figure 3: The most stable conformation of CO2 on unsupported and carbon supported Ti4 O8 . The left column shows the unsupported structures, the middle column shows the structure with a GR support and the right columns show the structure on a GR-O support. The physisorbed CO2 is shown in the first line (Figures 3a to 3c), where the CO2 molecule adopts a linear structure. The chemisorbed CO2 is shown in the second line (Figures 3d to 3f), where the CO2 molecule adopts a bent structure, reacting with an O on the Ti4 O8 surface to form a CO3 group. (Atoms are colored in red-oxygen, yellow-titanium and gray-carbon. The atoms that originated from the CO2 molecule atoms are drawn larger to allow better visualization.) means that the oxygen coordinates to a three-fold coordinated Ti-site (O-Ti) and additionally the carbon binds to a singly coordinated oxygen (C-O). This is not possible for the Ti4 O8 (Cs ) cluster, as all Ti-atoms are four-fold coordinated. In this case, when CO2 coordinates to the Ti-atom, the Ti-O bond length between a Ti a single-coordinate oxygen atom of the cluster increases. The binding energy of the CO2 is about 0.1 eV smaller to the Ti4 O8 (Cs ) cluster than to the Ti2 O4 cluster and Ti3 O6 clusters. For the adsorption of CO2 on TiO2 clusters with a GR support, the linear molecule generally prefers to singly coordinate via the O atom to a three-fold coordinated Ti-atom. The exception is Ti3 O6 , where all Ti-atoms are four-fold coordinated and the molecule weakly doubly-coordinates 12 ACS Paragon Plus Environment

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with (O-Ti) and (C-O). In this case, the binding energy is 0.26 eV smaller. Similar behavior is observed with the GR-O support.

(a) Linear CO2 adsorption

(b) Bent CO2 adsorption

(c) Ti4 O8 (C2 v) same configuration as on GR

(d) CO2 same site as on Ti3 O6 /GR

Figure 4: Binding energies of the linear (Figure 4a) and bent (Figure 4b) CO2 . The full lines connect the binding energies for the minimum energy structures for each case, while the dotted lines connect the binding energies obtained for the unsupported cluster when it adopts the same configuration (Figure 4c) and same adsorption site (Figure 4d) as most stable structure on GR. (Atoms are colored in red-oxygen, yellow-titanium and gray-carbon.) Figures 4a and 4b show how the binding energy of the structures with linear and bent CO2 configurations change with the addition of a support. Considering the physisorbed CO2 , it is noted that addition of a GR or GR-O support does not result in consistent changes to the binding energy (it increases in some cases, and decreases in others). This is because the sites available for adsorption on the TiO2 change when it is supported, and the TiO2 structure itself can change when GR or GR-O are added. These changes are different for the various cluster sizes. We therefore should also consider the changes in adsorption energies when CO2 is absorbed on isolated TiO2 13 ACS Paragon Plus Environment


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clusters that are assumed to have the same configuration as when they are adsorbed on GR, and by considering the same adsorption sites (Figures 4c and 4d). These structures are local minima, and the results obtained for the binding energies are shown by connection with dotted lines in Figures 4a and 4b. For Ti4 O8 , the C2v structure (Figure 4c) is observed when it is on the support, and the isolated cluster with this structure differs by only 0.035 eV in total energy from the global minima structure (Cs ) of the isolated cluster. Similarly, the most likely site of adsorption site of CO2 on the supported Ti3 O6 has the geometry shown in Figure 4d when it is on the unsupported Ti3 O6 cluster. As shown by the dotted lines in the figures, when we consider the energies of the same adsorption sites rather than just the minimum energy structures, we find a small but clear trend of reduced binding energies on addition of the GR support. Comparing the two carbon supports, GR and GR-O, all adsorption sites are the same and the binding energies are consistently 0.05 eV less on GR-O compared to GR. We now consider chemisorption of CO2 . In the case of the bent CO2 molecule, the adsorption sites are the same when 2 or 3 units of either supported or unsupported TiO2 are used. Energetically, GR increases the binding energies by about 0.2 eV (see the solid lines in Figure 4b). This is not the case for Ti4 O8 , since two different titanium dioxide configurations (Cs and C2v ) were obtained for the support and unsupported clusters. For comparison, we therefore also calculated the binding energy for adsorption of CO2 on Ti4 O8 with the C2v -symmetry, and found it to be 0.3 eV less stable than on the unsupported Cs configuration. Furthermore, when the same adsorption site as obtained for Ti4 O8 /GR is considered, the Ti4 O8 without support gives a binding energy of 0.45 eV. This is 0.19 eV lower than the GR supported adsorption. These results are shown as the green solid and dotted lines in Figure 4b. This allows us to conclude that there is a systematic binding energy increase with addition of a GR support, provided the same adsorption site is considered and the geometry of the cluster does not change significantly. As the cluster sizes increase, we expect that changes in the adsorption site and geometry is less likely to occur. Comparing the GR and GR-O support, the binding energy using a GR-O support is even great for all the clusters. This enhancement is greatest for the largest


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cluster as shown in Figure 4b. Binding distances between the CO2 molecule and the TiO2 clusters for all cases where physisorption occurs are in the range 2.23-2.53 Å (O-Ti) and 2.73-3.21 Å (C-O) when there is double coordination and are 2.22-2.35 Å(O-Ti) when there is single coordination. In contrast, when chemisorption occurs two atoms from the CO2 molecule are bound to the titanium dioxide and the bond lengths are (O-Ti) 1.94±0.05 Å and (C-O) 1.39±0.03 Å. These values are similar to the Ti-O bond lengths in the clusters and consistent with typical C-O single bonds.

(a) CO2 /Ti4 O8 -C2 v Transition state 1

(b) CO2 /Ti4 O8 -C2 v Transition state 2

Figure 5: Transition states for two different adsorption sites. The physisorbed and chemisorbed structures are shown colored in green and blue respectively. (Atoms are colored in red-oxygen, yellow-titanium and gray-carbon.) NEB calculations indicated that the activation barrier for chemisorption is approximately 0.290.32 eV. These results were obtained for Ti4 O8 without GR or GR-O support, and considering two different sites as shown in Figure 5. The same sites on Ti4 O8 /GR have an activation energy of 0.16 eV, which is half that without support, and with GR-O the same barrier is around 0.11-0.17 eV, depending on the site. For both the unsupported system and with the GR-O support, the CO2 on Ti4 O8 bound to a four-fold coordinate Ti-site with a single coordinated oxygen (see Figure 5a) has a higher barrier as when it is bound to the three fold coordinated Ti-site (see Figure 5b). We note that previous work has approximated an entropic contribution to the adsorption of CO2 on solid substrates of approximately 0.001 eV K−1 , 30 and therefore at 300 K a system with a binding energy of approximately 0.3 eV would be considered thermodynamically stable. As both 15 ACS Paragon Plus Environment


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CO2 and TiO2 bulk systems are rather inert, the efficiency of any reaction involving them will be correlated to the amount of CO2 adsorbed, hence their availability on the surface. In addition, a reduction in the stability of the CO2 will enhance its reactivity. As a result, an increase of binding energy can be seen as an enhancement on the CO2 reactivity, and is assumed to be favorable. In order to understand the binding that occurs when CO2 is adsorbed, we considered the charge density difference of CO2 adsorption for the different systems (see side view: Figure 6 and top view: SI Figure 2). The isosurface charge density is taken to be that at 0.0001 e/a30 where a0 is the Bohr-radius. The difference in the charge density when CO2 is separated and united with the supported/unsupported TiO2 clusters was considered, with regions of charge density accumulation and charge density depletion identified (colored green and blue, respectively, in Figure 6). When comparing the physi- and chemisorbed CO2 , it is clear that in the linear physisorbed case, the charge density on the CO2 oxygen atoms is depleted, compared to the separated CO2 . This is different in the bent structure, where charge density is greater on the CO2 oxygens, which is consistent with the formation of a carbonate group. In addition it can be seen that CO2 physisorption does not significantly change the electronic structure of the GR sheet in any of the cases considered (see Figures 6a, 6e and 6i). This was also seen with the binding energies, with little change observed. In contrast to this, the chemisorption clearly influences the whole system (see Figures 6b, 6f and 6j). In the direct vicinity of the CO2 an increase in the charge density on the GR sheet is observed, and further away from this region on the GR sheet the charge density is seen to be depleted. This suggests that for the bent structures, the charge relocation due to the GR-sheet is responsible for the increase in binding energy compared to the system with no substrate. Changes in the charge density due to adsorption of CO2 on the TiO2 clusters on GR-O were also considered. Similarly to using GR, when physisorption occurs, no charge difference of substrate could be found with GR-O (see Figures 6c, 6g and 6k). When chemisorption occurs on this system, which oxygen bond between the TiO2 molecule and the GR sheet, an increased charge relocation over the p-orbitals of the GR sheet is evident (see Figures 6d, 6h and 6l). The bigger the TiO2


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(a) linear Ti2 O4 /GR

(b) bent Ti2 O4 /GR

(c) linear Ti2 O4 /GR-O

(d) bent Ti2 O4 /GR-O

(e) linear Ti3 O6 /GR

(f) bent Ti3 O6 /GR

(g) linear Ti3 O6 /GR-O

(h) bent Ti3 O6 /GR-O

(i) linear Ti4 O8 /GR

(j) bent Ti4 O8 /GR

(k) linear Ti4 O8 /GR-O

(l) bent Ti4 O8 /GR-O

Figure 6: Isosurface of the charge difference for the CO2 adsorption on carbon supported TiO2 (GR and GR-O). CO2 in a linear configuration on GR (Figures 6a, 6e and 6i) and on GR-O (Figures 6c, 6g and 6k). CO2 in a bent configuration on GR (Figures 6b, 6f and 6j) and on GR-O (Figures 6d, 6h and 6l). (Atoms are colored in red-oxygen, yellow-titanium and black-carbon. Epoxy oxygen and CO2 are visualized with balls. Electron density depletion in blue and accumulation in green). molecule the greater is the charge density reduction that occurs on the carbon-base due to the CO2 adsorption. This trend is correlated with the increase of binding energies of CO2 on GR-O with increasing TiO2 size.

Conclusions Computational studies on novel nanostructured catalysts, TiO2 /GR and TiO2 /GR-O, which have been considered as a potential photocatalysts for the reduction of CO2 , have been conducted. The



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first step for a catalytic reaction, the adsorption of CO2 on potential sites was investigated and modeled with 2 to 4 units of TiO2 on graphene and epoxy enhanced graphene. Moreover a new minimum energy structure for Ti4 O8 with a Cs -symmetry was described. It is shown that GR significantly influences CO2 chemisorption on TiO2 , increasing the binding energies by about 0.2 eV. However the physisorption energy is slightly reduced. Further, an influence of the connection between the carbon base and titanium dioxide was noticed. GR-O increases the binding energies, and the increase is most significant for the largest TiO2 molecule considered. On charge density difference plots, it showed that this enhancement is due to an electronic charge relocation. Important for the adsorption of CO2 was the configuration of the adsorbed TiO2 clusters. For instance, a site with Ti and O-atoms sterically accessible is most favorable. Hence binding to a single coordinated oxygen close to a three-fold coordinated titanium atom was most stable, followed by a three-fold coordinated titanium and two-fold coordinated oxygen site. It was shown conclusively that GR influences CO2 adsorption and increases the chemisorbed binding strength, while reducing the activation barrier, hence improves the catalytic abilities of titanium dioxide clusters. By consideration of GR-O, we could show that the connection of the TiO2 cluster to the GR through a bridging O had an increasing influence for the adsorption of CO2 with increasing TiO2 size. The increases in binding energies for chemisorption is consistent with photocatalytic activity proposed in Ref. 3-5. The TiO2 clusters used for the calculations are not necessarily the systems that would be observed in experiment since the fabrication process would effect their geometries. However these results provide useful models to monitor the effect of GR and GR-O on the CO2 on titanium dioxide adsorption. It would be of interest to consider larger clusters to see if the observed size effect on the CO2 adsorption is still evident.


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Author Information Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgement We would like to thank the Australian Research Council for support of this project through the LIEF program. This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme supported by the Australian Government, support from the Queensland Cyber Infrastructure Foundation (QCIF) and the University of Queensland Research Computing Centre. We would also like to acknowledge Dr. Christoph Rohmann from the CTCMS, at the University of Queensland for the discussion of carbon dioxide on titanium dioxide. The first author would also like to thank the University of Queensland for the UQI scholarships for supporting her PhD.

Supporting Information Available Configurations of the most stable adsorption sites, and the isosurface of the charge difference, of CO2 in linear and bent configuration on two and three units TiO2 unsupported and carbon support (GR and GR-O). This material is available free of charge via the Internet at http://pubs.acs.org/.

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(a) Ti2 O4 /GR

(b) Ti3 O6 /GR

(d) Ti2 O4 /GR-O

(e) Ti3 O6 /GR-O

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(a) Ti4 O8 (Cs ) molecule

(b) Ti4 O8 (C2v )/GR

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(c) Ti4 O8 (C2v )/GR-O


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(a) Ti4 O8

(b) Ti4 O8 /GR

(c) Ti4 O8 /GR-O

(d) Ti4 O8

(e) Ti4 O8 /GR

(f) Ti4 O8 /GR-O

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(a) Linear CO2 adsorption

(b) Bent CO2 adsorption

(c) Ti4 O8 (C2 v) same configuration as on GR

(d) CO2 same site as on Ti3 O6 /GR

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(a) CO2 /Ti4 O8 -C2 v Transition state 1

(b) CO2 /Ti4 O8 -C2 v Transition state 2

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(a) linear Ti2 O4 /GR

(b) bent Ti2 O4 /GR

(c) linear Ti2 O4 /GR-O

(d) bent Ti2 O4 /GR-O

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(e) linear Ti3 O6 /GR

(f) bent Ti3 O6 /GR

(g) linear Ti3 O6 /GR-O

(h) bent Ti3 O6 /GR-O

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(i) linear Ti4 O8 /GR

(j) bent Ti4 O8 /GR

(k) linear Ti4 O8 /GR-O

(l) bent Ti4 O8 /GR-O

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Table of Contents Graphic:

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Computational Studies of the Interaction of Carbon Dioxide with

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