Surface Interfaced with Graphene, Reduced ... - ACS Publications

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Electronic Structures and Photocatalytic Responses of SrTiO3(100) Surface Interfaced with Graphene, Reduced Graphene Oxide and Graphane: Surface Termination Effect Yin-Cai Yang,† Liang Xu,† Wei-Qing Huang,∗,† Cai-Yun Luo,† Gui-Fang Huang,∗,† and Ping Peng‡ Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082,China, and School of Materials Science and Engineering, Hunan University, Changsha 410082, China E-mail: [email protected]; [email protected]

∗ To

whom correspondence should be addressed of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082,China ‡ School of Materials Science and Engineering, Hunan University, Changsha 410082, China

† Department

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

Abstract The enhanced photocatalytic activity of SrTiO3 (STO), a promising photocatalyst for decomposing organic compounds and overall water splitting for H2 /O2 evolution, has been experimentally demonstrated by coupling the graphene(GR) sheet. Here, we reveal the mechanism of the enhanced photocatalytic activity of STO/GR composites using ab initio calculations. Due to C 2p states forming the bottom of conduction band or the top of valence band, the band gap is reduced to about 0.6 eV, resulting into a strong absorption in the visible region. The composites of STO coupled with reduced graphene oxide(RGO) and graphane(GRH) are also explored to investigate their potential photocatalytic activity. We demonstrate that the surface termination layer of STO(100) surface plays an important role in determining the formation energy, interfacial distance, band gap and optical absorption of these composites. Moreover, the GR sheet is a sensitizer for STO with termination layer TiO2 , on the contrary, it is to be an electron shuttle to carry excited electrons from the STO with termination layer SrO. Interestingly, a type-II, staggered band alignment is formed in the interface, thus improving photoexcited charge separation. The negatively charged O atoms in the RGO are considered to be active sites in photocatalytic reactions, leading to the enhanced photocatalytic activity. The calculated results can rationalize the available experimental reports and provide design principles for optimizing the photocatalytic performance of the STO-based composites.

Keywords: Electronic structure, Photocatalytic response, Surface termination effect, STO/GR(RGO, GRH), Density functional theory


Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Photocatalysis is an attractive technology for solving energy crisis and environmental concerns without relying on fossil fuels and the emission of carbon dioxide. 1–3 Photocatalytic semiconductors are generally metal oxides, nitrides, or sulfides. 4–6 Due to its high photosensitivity and large band gap, which means high driving force for the reduction and oxidation processes, respectively, SrTiO3 (STO) has been proven to be a promising photocatalytic material in decomposing organic compounds and overall water splitting for H2 /O2 evolution. 7–9 However, with a large intrinsic band gap (3.2 eV), STO only responds to ultraviolet (UV) light, which greatly restricts its energy conversion efficiency of the incoming solar energy. 9 Several strategies have been proposed to extend its optical absorption edge to the visible (vis) light region, which possesses 43% of the solar energy, through modifying the energy band structure of STO. Doping with foreign atoms, 10–18 depositing noble metal, 19 integrating with other materials 20–23 have been demonstrated to be effective methods to enhance the photocatalytic performance of STO. In the photocatalytic process, the charge carrier recombination occurs within nanoseconds. To prolong the life-time of charge carriers, particular attention is recently devoted to the STO composites, in which the photogenerated electrons or holes can be trapped, resulting in the decrease of recombination. These STO-based composites show significantly enhanced photocatalytic activity. 20–29 Recently, graphene (GR), a two-dimensional carbon material with very high mobility of charge carriers, has been used to synthesize STO/GR nanocomposites as a photocatalyst by Xian et al. 30 The experimental results show that compared to the pure STO nanoparticles, the STO/GR composites exhibit significantly enhanced photocatalytic activity under ultraviolet (UV) light irradiation. It is speculated to photogenerated electrons captured by GR, leading to an increased separation and availability of electrons and holes for the photocatalytic reaction. 30 The density function calculations reveal, however, that the function of GR is generally to sensitize semiconductors, such as TiO2 , 31,32 Ag3 PO4 , 33 CeO2 , 34 and g-C3 N4 . 35 Moreover, the absorption edge of the STO/GR composite indicates that its band gap (Eg) is approximately 3.35 eV, 30 suggesting that the hybridization of GR leads to a slight larger band gap of STO. This is different from those of GR-based semi3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

conductor composites (for examples, TiO2/GR, 36 C3 N4 /GR, 37 ZnS/GR, 38 WO3 /GR 39 ), that their band gaps are decreased due to the hybridization of GR. The increase in band gap of STO/GR composite is unfavorable for absorbing sunlight. As is known, the band structure of GR-based composite largely depends on the interfacial structure. Moreover, the electron-hole recombination that occurs at the interface is crucial for photocatalytic activity of nanocomposites. 40–42 It is therefore of great importance to ascertain the mechanism of enhanced photocatalytic activity, and the correlation between photocatalytic performance and interfacial microstructure of STO/GR composite. In the present work, the structural and electronic properties of hybrid STO/GR composite have been investigated using large-scale density functional theory (DFT) computations to understand the mechanisms of its enhanced photocatalytic activity under UV irradiation. More importantly, we explore the possibility of vis-light response of STO/GR composite for its applications. Experimentally, the STO/GR nanocomposites were prepared by photocatalytic reduction of graphene oxide (GO), 30 which is very likely that there are oxygen atoms residue on the GR sheet. It is thus necessary to investigate the photocatalytic performance of STO/reduced oxide GR (RGO) composites. Due to its unique properties, graphane (hydrogenated graphene, GRH) is now attracted much attention. 43 It is tempting, for the first time, to predict the properties of STO/GRH composite, and compare it with STO/GR, STO/RGO composites. The results demonstrate that the surface termination layer of STO(100) surface can effectively tune the properties of the composite. The O atom on the RGO sheet leads to a stronger interfacial interaction in STO/RGO than that in STO/GR composites. A theoretical evidence for the termination effect on the photocatalytic activity of the STO-based composites is revealed. These results can rationalize the available experimental findings and are useful for designing and understanding STO-based photocatalysts.


Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. COMPUTATIONAL DETAILS Since STO can be viewed as a natural superlattice of SrO and TiO2 atomic layers, there are two possible terminations at the topmost layer of STO(100) surface. In the STO crystal, the TiO2 layer is part of the anatase crystal lattice, while the SrO layer is a kind of insulator. It is therefore essential to explore the effect of termination layer, SrO or TiO2 on the SrTiO3 (100) surface, on the photocatalytic activity of STO-based composites. For simplicity, the SrO (TiO2 )-terminated STO(100) surface is abbreviated to STO-Sr (Ti). Thus, The hybrid nanocomposite of the SrOterminated STO(100) surface and GR is named as STO-Sr/GR, and so on. To construct the hybrid of STO and GR, we choose a cubic STO(100) surface. In a supercell (8.16 × 7.59 × 25 Å3 ), a 4 × 3 single GR layer containing 24 carbon atoms sits on a 2 × 2 five atomic layer STO(100) surface slab with two bottom layers fixed at bulk position and is followed by a more than 12 Å thick vacuum layer to avoid artificial interaction (see Figure 1). DFT calculations is powerful to unravel the nature of the electronic interactions taking place at interfaces of composites. 44 In this work, the calculations are performed within the framework of the local density approximation (LDA) by using the Cambridge Serial Total Energy Package (CASTEP) 45 based on the plane-wave pseudopotential DFT methods. The LDA with inclusion of the van der Waals (vdW) interaction is chosen because long-range vdW interactions are expected to be significant in these complexes. This method has been successfully applied to the investigation on the nonbonding interaction between battery active organic molecules and pristine GR sheet. 46,47 However, LDA has been known generally to underestimate the energy gap of semiconductor, resulting into an overestimate for photoinduced electrons transfer in photocatalytic process. To correct this band gap problem, all of the theoretical calculations are performed using the DFT/LDA+U method. We have performed extensive tests to determine the appropriate U parameters for Sr 4d states, which reproduced the correct energy gap (3.2eV) 48 for cubic STO. The appropriate Hubbard U values for Sr 4d, O 2p, and Ti 3d are 5.0, 5.0, and 5.0 eV, respectively. The valence atomic configurations are C: 2s2 2p2 , H: 1s1 , O: 2s2 2p4 , Ti: 3s2 3p6 3d2 4s2 , and Sr: 4s2 4p6 5s2 , respectively. A MorkhostPack mesh of k points, 4 × 4 × 1 and 8 × 8 × 1 points, is used to sample the two-dimensional 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

Brillouin zone for geometry optimization and for calculating the density of states, respectively. The cutoff energy for plane wave is chosen to be 480 eV. All geometry structures are fully relaxed until the convergence criteria of energy and force are less than 10−6 eV/atom and 0.01 eV/Å, respectively. For a high-efficiency photocatalyst, one of the fundamental premises is the strong light absorption, especially within the vis-light range. The frequency-dependent dielectric matrix is calculated for pure STO, STO(100), and the hybrid STO/GR(RGO, GRH) nanocomposites by the Fermi golden rule within the dipole approximation. The imaginary part of the dielectric function due to direct interband transitions is given by the expression 49

ε2 (¯hω ) =

2e2 π |hψkc |u · r|ψkv|2 δ (Ekc − Ekv − E). ∑ Ωε0 k,v,c

(1)

where Ω, ω , u, v, and c are the unit-cell volume, photon frequency, the vector defining the polarization of the incident electric field, valence bands, and conduction bands, respectively. The real part of the dielectric function is obtained from ε2 by a Kramers-Kronig transformation. The absorption coefficient η (ω ) can be obtained based on ε1 and ε2 . 50

3. RESULTS AND DISCUSSION 3.1. Geometric Structure To explore the surface termination effect of the STO(100) surface, two kind terminations at the topmost layer are considered in each of STO-based composites. The structural and electronic properties, as well as the photocatalytic efficiency of the composites depend largely on the interaction between the STO and GR (RGO, GRH), which can be characterized by their geometries and separation. Figures 1(a1) and (a2) present the top and side views of the STO-Sr/GR interface model used in our calculations, respectively; and Figures 1(b1) and (b2) are for STO-Ti/GR model. Geometry optimizations have firstly been performed for the system using the conjugate


Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gradient method. For the STO-Sr/GR model, the equilibrium distance, d, between the GR sheet and the topmost SrO layer is calculated to be 2.99 Å, which is about equal to those between the GR sheet and other materials (3.042 Å for g-C3 N4 /GR, 35 2.96 Å for CeO2 (111)/GR). 34 While the d (2.92 Å) between the GR sheet and the topmost TiO2 layer is smaller in the STO-Ti/GR model. The similar change of d are found in the STO/RGO and STO/GRH composites: the d in the STO-Ti/RGO(GRH) is smaller than that in the STO-Sr/RGO(GRH), as shown in Table 2. The variation of d can be attributed to the surface termination effect of STO. Therefore, the interfacial interactions are stronger in the STO-Ti/GR(RGO, GRH) than in the STO-Sr/GR(RGO, GRH). After optimization, the C atom layers in the STO/GR and STO/RGO composites and H atom layer in STO/GRH composite are still flat, indicating that the STO-GR(RGO, GRH) interaction is indeed vdW rather than covalent, in accordance with the others’ results. 33–35 Closer inspection of the atomic position at each of the interfaces reveals that due to the interaction between the STO surface and the GR(RGO, GRH) sheet, the atoms at the top layer are shifted slightly. The variation values of each kind of atom at the top layer are summarized in Table I. Except that the O atom at the top layer slightly move upwards about 0.031 and 0.073 Å in the STO-Ti/RGO and STOTi/GRH composites respectively, the other top atoms in composites are pushed down by the GR (RGO, GRH) sheet. Interestingly, the vertical displacement of the Sr or Ti atom is much larger than that of the O atom, whether the termination layer of the STO(100) surface is SrO or TiO2. In particular, the Sr atom moves down approximately 0.275 Å in the STO-Sr/RGO composite. As a result, the STO(100) surface is an undulating one in the STO-based composites (Figures 1∼3). The interfacial interaction also affects the distance between two C atom layers in the GRH, which one is smaller (STO-Ti/GRH), while the other is bigger (STO-Sr/GRH) than in the pure GRH sheet (see Table 1), indicating that the interaction is much stronger in the STO-Ti/GRH composite due to its TiO2 termination. The rearrangements of atoms in the top layer of STO(100) indicate that the electron transfer occurs at the interface, which will be discussed later. The STO/GR nanocomposite has been synthesized by experiment. Therefore, we can assess the possibility and stability of the proposed STO/RGO and STO/GRH composites by comparing



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 34

the interface adhesion energy, Ead , which is defined as

Ead = Ecomb − EGR(RGO,GRH) − EST O(100)

(2)

where Ecomb , EGR(RGO,GRH) , and EST O(100) represent the total energy of the relaxed STO/GR(RGO, GRH), pure GR(RGO, GRH) sheet, and pure STO(100) surface, respectively. By this definition, negative Ead suggests that the adsorption is stable. The calculated Ead of these composites are summarized in Table 2. Just for comparison with the STO/GR composite, it is a little more difficult to form the STO/RGO composite, whereas much easier to form the STO/GRH composite. Table 2 clearly shows that the surface termination layer of the STO(100) surface also impacts the Ead : the Ead of the STO-Ti/GR (RGO, GRH) is smaller than that of the STO-Sr/GR(RGO, GRH) composite, suggesting that the layer TiO2 is more inclined to absorb onto the GR (RGO, GRH) sheet. Overall, the value of Ead is between -1.5 and 0.65 eV, indicating a rather strong interaction between GR(RGO, GRH) and STO(100) surface, and the high thermodynamical stability of these composites.

3.2. Density of State and Band Alignment The electronic structure of the nanocomposites mainly depends on the interfacial interaction of their components. The variation of electronic properties of the STO-Sr(Ti)/GR(RGO, GRH) composites can be evidenced by their total density of states (DOS) and partial density of states (PDOS), as shown in Figures 1(2,3) (a4) and (b4), respectively. For the bulk STO, its band gap is about 3.2 eV, and the valence band maximum (VBM) is mainly dominated by the O 2p and Sr 4p states and the conduction band minimum (CBM) is primarily contributed by Ti 3d states. 51 When the STO(100) surface is hybridized with GR(RGO, GRH) sheet, the band gap is significantly reduced and affected by the surface termination layer, as given in Table 2. The band gap of the STOSr/GR composite decreases to 0.633 eV, while the termination layer TiO2 leads to a further slight reduction of the band gap (0.603 eV) of the STO-Ti/GR composite. The surface termination ef-


Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fect on the band gap is more significant in the STO/RGO(GRH) composite: the band gap of the STO/RGO(GRH) composite decreases from 1.741 (2.100) to 0.004 (0.350) eV, as the termination layer changes from SrO to TiO2 . As a consequence, the photogenerated electrons transfer from the valence band (VB) to the conduction band (CB) of the STO/GR(RGO, GRH) composites become easier, leading to the red shift of the optical absorption edge. Therefore, the smaller band gap is beneficial to absorb the visible light for photocatalysis of the STO/GR(RGO, GRH) composites. Another important feature in Figures 1(2,3) (a4) and (b4) is the band alignment between the STO(100) surface and GR(RGO, GRH) sheet. One can find that the main shapes of the calculated DOSs projected on two different constituents in the composite are similar to those of the total DOSs of the STO(100) surface and GR(RGO, GRH) sheets, respectively (the DOSs of pure STO(100) surface and GR(RGO, GRH) sheet are given in Figure S1). This is due to the large separation space between the STO(100) surface and GR(RGO, GRH) sheet, suggesting that the STO-GR(RGO, GRH) interaction is weak due to the absence of covalent bonding upon formation between the interfaces. Figure 1 (a4) displays that the upper part of VB of the STO-Sr/GR composite is only consisted of O 2p states, while the bottom of CB is formed from C 2p orbits. This can be more clearly seen from the electron density distributions of the highest-occupied and lowest-unoccupied levels (HOL and LUL), respectively, as shown in Figure 1 (a2) and (a3). More importantly, the interface of STO-Sr/GR composite is type II (see top panel of Figure 1 (a4)), namely, with both the valence and conduction GR band edges below the corresponding STO-Sr counterparts, which significantly lowers the effective bandgap of composite, and facilitates efficient electron-hole separation. However, the case of STO-Ti/GR composite is different. The the upper part of VB of the STO-Ti/GR composite is only comprised of the C 2p states (Figure 1(b2)), while the bottom of CB is composed of the C 2p states mixed with a small Ti 3d states localized predominantly around the top Ti atoms (Figure 1(b3)). In photocatalysis, such band alignment is not beneficial for the separation of electron-hole pairs. These results indicate that different termination layers (SrO or TiO2) on the SrTiO3 (100) surface will lead to different types of band alignment of STO/GR composite.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 34

Interestingly, Figures 2 and 3 also clearly illustrate that a type II band alignment is present in the STO/RGO(GRH) composite, regardless of the surface termination layer is TiO2 or SrO on the SrTiO3 (100) surface. This indicates that the surface termination effect on STO/RGO(GRH) composite is different from STO/GR composite. For the STO/RGO composite, its upper part of VB is composed of the C 2p states mixed with O 2p orbits from RGO, while its bottom of CB is only contributed to Ti 3d orbits, as shown in Figure 2. The surface termination layer of STO(100) surface only affect the bottom of CB, which can be traced by the origin of Ti 3d orbitals. As the termination layer is TiO2, the Ti 3d orbitals at the bottom of CB come from all of the Ti atoms in the STO (Figure 2(b3)), whereas those predominantly originate from the first Ti layer just under the surface layer SrO (Figure 2(a3)). As for the STO/GRH composite, in which the surface termination effect is similar with that in the STO/RGO composite(Figure 3), the C 2p states constitutes the upper part of VB and the Ti 3d states are only one at the bottom of CB for the termination layer both TiO2 and SrO (Figure 3). The component variation of the bottom of CB due to different termination layers will influence the separation of photogenerated electron-hole pairs, thus affecting the photocatalytic performance of the STO/RGO(GRH) composites. Therefore, it can be concluded that this kind of the band alignment between the two components demonstrate that a type-II heterojunction is formed, which is key to elucidate the enhanced photocatalytic performance of the STO/GR(RGO, GRH) composite (except for the STO-Ti/GR composite). Under light irradiation, the electrons in the HOB can be directly excited to the CB of the composite, i.e., the photogenerated electrons will migrate from one component to another, thus producing well-spatial-separated electron-hole pairs.

3.3. Charge Transfer and Mechanism Analysis Such a strong change of the DOSs of the STO/GR(RGO, GRH) composites also indicates a substantial charge transfer between the involved constituents. This can be visualized (as shown in Figures 4 and 5) by three-dimensional charge density difference ∆ρ = ρSTO/GR(RGO,GRH) -ρST O(100) -

ρGR(RGO,GRH)), where ρST O/GR(RGO,GRH), ρSTO(100) and ρGR(RGO,GRH) are the charge densities of the composites, STO(100) surface, and free standing GR(RGO, GRH) sheet in the same con10 ACS Paragon Plus Environment

Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


figuration, respectively. We first discuss the composites with the termination layer SrO of the STO(100) surface. Figures 4(a-c) display that the charge redistribution mostly take place at the STO-Sr/GR(RGO, GRH) interface region, especially near the GR(RGO, GRH) sheet. A strong charge accumulation is found just above the Sr atoms, while the regions of charge depletion appear on the lower side of the GR (facing the surface) over the O atoms in the ground electronic state. This leads to the charge distribution fluctuations of GR sheet due to the interfacial interaction, analogous to those in other GR-semiconductor systems, such as Ag3 PO4 /GR 33 and TiO2 /GR. 31 Accordingly, the O atoms in the top layer of the STO-Sr/GR composite gain some electrons (Figure 4(a)). The gained charge amount of the O atoms in the STO is decreased gradually in the STO-Sr/RGO(GRH) composite (Figures 4 (b) and (c)). This can be attributed to the fact that the length of C-O bond in the RGO sheet is much smaller than the interfacial distance (i.e., the distance between C and O atom at the top layer of STO surface). Thus, The O atom in the RGO sheet exerts a greater attraction on the electrons of C atom than those O atoms in the STO surface. The situation is similar for the STO-Sr/GRH composite. The Sr and O atoms are arranged alternatively at the top layer of STO(100) surface. Therefore, we can see in Figure 4(b) that the charge accumulation and depletion of the O atom in the RGO alternately occur regularly, resulting in the electronand hole-rich O sites (as is denoted by green and black arrows, respectively). This indicates that the interfacial interaction results into the negatively and positively charged O atoms in the RGO, which are active sites and beneficial to improving the photocatalytic performance of the STO/RGO composite. Interestingly, the H atom (facing the surface) in the STO-Sr/GRH composite becomes a point-like electric dipole owing to the interfacial interaction. To quantitatively display the charge redistribution, part d of Figure 4 plots the planar averaged charge density difference along the direction perpendicular to the STO(100) surface. The positive(negative) values represent electron accumulation(depletion). One can see that, in the STOSr/GR composite, the largest efficient electron accumulation localized above the Sr atoms in the top layer is about 12.6×10−4 e/Å3 , the largest local efficient electron depletion above the O atoms in the top layer is about -18.3×10−4 e/Å3 , and the efficient electron accumulation above the GR



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 34

sheet is about 2.8×10−4 e/Å3 ). The efficient electron accumulation(depletion) becomes more and more large in the STO-Sr/RGO and STO-Sr/GRH composites, which is due to their smaller interfacial distance and much stronger interfacial interaction compared to the STO-Sr/GR composite. This clearly demonstrates that the electron transfer occurs in the interface due to the coupling of GR(RGO, GRH) and STO(100) surface. The three-dimensional charge density difference in the STO-Ti/GR(RGO,GRH) composites is rendered in Figure 5. The charge redistribution at the interface is more obvious in the STOTi/GR(RGO,GRH) composites than in the STO-Sr/GR(RGO,GRH) composite, owing to the smaller interfacial distance in the former. Comparing Figure 5 with Figure 4, it is clear that the charge redistribution at the interface also depends on the surface termination layer of the STO(100) surface. Because the termination layer is TiO2, a strong charge accumulation is localized just above the Ti atoms, while the regions of charge depletion appear on the lower side of the GR (facing the surface) over the O atoms in the ground electronic state, thus resulting to the charge distribution fluctuations of GR sheet (Figure 5(a) and (b)). Similarly, the interfacial interaction will push the charge from the C atom to the O atom in the RGO sheet. In the STO-Ti/GRH composite, only the H atom just above the Ti atom shows a point-like electric dipole (Figure 5(c)), which is different from the STO-Sr/GRH composite. Figure 5(d) shows that the efficient charge change along the direction perpendicular to the STO(100) surface in the STO-Ti/GRH composite is similar with that in the STO-Sr/GRH composite. To quantitatively analyze the charge variation at the interface, the Mulliken population analysis of the plane-wave pseudopotential calculations has been performed on the GR(RGO,GRH) sheet, STO(100) surface, and STO/GR(RGO,GRH) composites. Figures 6 and 7 show the results of the Mulliken charge on different atoms, in which several typical values are denoted. The O and Sr atoms in the top layer of the STO-Sr(100) surface have a Mulliken charge of - 0.96 and 1.46, respectively (Figure 6(a)). Coupling with the GR sheet, the Mulliken charges of the O and Sr atoms in the top layer become -0.93 and 1.56, respectively, while those of C atoms vary from -0.01 to 0.05. The charge variation demonstrates that some electrons transfer from the STO-Sr(100) surface


Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the GR sheet. Figure 6(c) displays that some electrons both of the O and Sr atoms in the top layer and of the C atom will transfer to the O atoms in the RGO sheet, resulting into the negatively charged O atoms. In the STO-Sr/GRH composite, the O and Sr atoms in the top layer also lose some electrons, and the H atom near the SrO layer and C atom gain some electrons, which the amount depends on their positions, as shown in Figure 6(d). This variation of Mulliken population can also be found in the STO-Ti/GR(RGO,GRH) composite (Figure 7), although the change values of each atom in both cases are not the same. It should be pointed out that the interaction results into the negatively charged O atoms in the RGO sheets, and C atoms in the GR and GRH sheets, which are active sites and beneficial to improving the photocatalytic performance of the STO composites. This is similar with the case of g-C3 N4 /RGO composites. 35 The effective net charge from one constituent to another in these composites can be analyzed based on the Bader method, 52 as listed in Table 2. Interestingly, the termination layer of STO(100) surface determines the transfer direction of net charge: some electron transfers from STO to GR(RGO, GRH) sheet in the STO-Sr/GR(RGO, GRH) composite, however, it is in the opposite direction in the STO-Ti/GR(RGO, GRH) composite. Moreover, the transfer amount of electron from STO-Sr to GR(RGO, GRH) sheet is larger than that from GR(RGO, GRH) sheet to STO-Ti. For example, 0.18 electron transfers from STO-Sr to GR sheet in the STO-Sr/GR composite, while 0.07 from GR to STO-Ti. To understand the origin of such an interface electron-transfer in these composites, work functions for the GR(RGO, GRH) sheet, STO-Sr and STO-Ti surfaces are calculated by aligning the Fermi level relative to the vacuum energy level. They are calculated to be 4.675, 5.246, 4.276, 2.930, and 6.564 eV for GR, RGO, GRH, STO-Sr, and STO-Ti surfaces, respectively. The first two values are consistent with previous studies. 34 The spontaneous interfacial charge transfer in the STO/GR(RGO, GRH) composites can be simply rationalized in terms of the difference of these work functions. Moreover, the larger difference in work functions, the more charge transfer. For instance, the work function difference (1.745 eV) between STO-Sr and GR is larger than that (1.589 eV) between STO-Ti and GR, thus the transfer amount of charge in former is bigger than in latter (0.18 vs 0.07).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

The interfacial charge transfer will alter the potential distribution at interface. Figure S2 shows that the planar average electrostatic potential at the GR(RGO, GRH) layer is higher than that at the top layer of STO-Sr, whereas it is lower than that the top layer of STO-Ti. As a consequence, a built-in potential is formed at the intereface, due to the appearance of electrostatic potential difference. Under light irradiation, the separation and migration of photogenerated carriers at the interface will be affected by this built-in potential. Based on these results, we reveal that the GR sheet is a sensitizer for STO with termination layer TiO2 , on the contrary, it is to be the electron shuttle to carry excited electrons from the STO with termination layer SrO. To take full advantage of GR’s excellent conductivity, it is therefore preferable to choose the layer SrO, rather than TiO2, interfaced with the GR sheet, thus accelerating the separation of photogenerated electron-hole pairs in the STO/GR composites.

3.4. Optical Property and its Mechanism For many semiconductors (for examples, TiO2, CeO2 , g-C3 N4 ) with wide band gap, the incorporated GR(RGO) sheet can extend their absorption edge to vis-light region. 33–35 The significantly enhanced photocatalytic activity of the STO/GR composite has only been experimentally demonstrated 30 under UV irradiation. To explore the mechanism of enhanced photocatalytic activity and the possibility of its vis-light response, the calculated UV-vis absorption spectra of bulk STO, pure STO-Sr and STO-Ti surfaces, and the hybrid STO/GR(RGO,GRH) composites are illustrated in Figure 8, respectively. The optical absorption of bulk STO occurs at about 3.2 eV due to the intrinsic transition from the O 2p and Sr 4p to Ti 3d orbitals, suggesting that the bulk STO can absorb only the UV part of the solar spectrum. For pure STO-Sr and STO-Ti surfaces, their absorption edges is respectively reduced to 2.09 eV and 3.02 eV due to the presence of surface states, also corresponding to the intrinsic transition. Figure 8 shows that the surface termination effect in the STO/GR(RGO,GRH) composites is also obvious in the adsorption spectra. Compared to those of the STO-Sr/GR(RGO,GRH) composites, the absorption spectra of the STO-Ti/GR(RGO,GRH) composites have apparent red shift owing to their smaller band gap. In particular, the strong ab14 ACS Paragon Plus Environment

Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sorption of the STO-Ti/GR composite can be observed in the entire vis-light region. Although the band gap of the STO-Ti/RGO composite is as low as 0.004 eV, the calculated absorption spectra in the vis-light region is still small. This can be understood as follows. Under irradiation of light, the normal photoexcitation in the STO-Ti/RGO composite is from the O 2p states of the RGO sheet to the Ti 3d orbitals of the STO. Because the O 2p states of the RGO sheet in the upper part of the VB (-2 ∼ 0 eV) are quite low (see bottom panel of Figure 2(b4)), the values of p-d matrix elements in Equation 2 are small, which results into very weak optical absorption in the vis-light region (Figure 8 (a)). After careful observation and comparison of the PDOS in the Figures 1-3, the low absorption in the vis-light region of other STO/GR(RGO,GRH) composites can similarly be reasonably interpreted. Among these STO composites, the STO/GRH composites have the weakest absorption. Thus, it is reasonable to conclude that the strong absorption in UV-vis region is one of the most important factors for enhanced photocatalytic activity of STO/GR composites. We now compare the theoretical calculation with the experimental results obtained by Xian. 30 The band gap of the STO/GR composite form its UV-vis diffuse reflectance spectra is approximately 3.35 eV, which is much larger than that of calculated value (about 0.6 eV). Thus, they only evaluated the photocatalytic activity of the STO/GR composite under UV light irradiation. First of all, it should be noted that the STO/GR nanocomposites were prepared via photocatalytic reduction of graphene oxide by UV light-irradiated STO nanoparticles. 30 The small absorption peak related to the C-O bond can also be traced in Figure 2 in Ref. 30. This indicates that some O atoms are very likely to still be on the GR sheet, i.e., some STO/RGO composites are also existed. Once the as-prepared composites are mainly the STO-Sr/RGO component, the blue shift of absorption edge obtained in experiment is consistent with that of calculation (see the solid rose-red and blue lines in Figure 8(a)). Therefore, the possibility of GR sheet containing O atom in the as-prepared composites is real high. Even if the as-prepared composites is only STO/GR, the UV-vis diffuse reflectance spectra might be rationally explained by considering the absorption of the STO/GR with termination layer SrO interfaced to the GR sheet (Figure 8(a)) and the strong light absorption of pure GR sheet in the UV-vis light region. Based on the above findings, we propose that using



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

solution method to synthesize the STO/GR nanocomposites by self-assembly is one of the most feasible methods. Especially, choosing the termination layer TiO2 as the top layer of the STO(100) surface interfaced with the GR sheet can lead to the strong light absorption of the composite in the whole vis-light region, thus effectively enhancing its photocatalytic activity. It is worth pointing out that the STO/GR systems have not only the enhanced photocatalytic activity, but also other excellent properties. For examples, GR shows strong and asymmetric hysteresis in resistivity when it is placed on STO. 53 Meanwhile, unconventional transport through GR on STO have also been found experimentally. 54 These investigations indicate that the STO/GR system is one kind of new materials, with promising application fields and unique properties. To understand in depth these interesting results, more experimental and theoretical studies are needed.

4. Summary The structural and electronic properties, and photocatalytic response have been investigated in the hybrid STO/GR(RGO,GRH) nanocomposites under the framework of DFT. The interfacial interaction in these composites is vdW rather than covalent, although the atoms in the top layer of STO(100) surface are rearranged due to the effect of GR(RGO,GRH). Owing to C 2p states forming the bottom of CB or the top of VB, the band gap of the STO/GR composite is reduced to about 0.6 eV, extending the absorption edge to the vis-light region. The STO/(RGO,GRH) composites are the first explored as their potential photocatalytic application. It is found that the surface termination layer of STO(100) surface plays an important role in determining the formation energy, interfacial distance, band gap and optical absorption of these composites. The GR sheet is a sensitizer for STO with termination layer TiO2, on the contrary, it is to be an electron shuttle to carry excited electrons from the STO with termination layer SrO. Moreover, a type-II, staggered band alignment is formed in the interface, thus improving photoexcited charge separation. Based on the results, we propose that using solution method to synthesize the STO/GR nanocomposites by self-assembly is one of the most feasible methods. Especially, choosing the termination layer TiO2 as the top layer of the STO(100) surface interfaced with the GR sheet is beneficial for the 16 ACS Paragon Plus Environment

Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photocatalytic response in the vis-light region. This work can rationalize the available experimental results and enrich our understanding of the STO-based composites for developing photocatalyst with high activity.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENTS This work is supported by Changsha Science and Technology Plan Projects (k1403067-11).

Supporting Information Calculated DOSs for STO-Sr, STO-Ti, RGO, and GRH, plots of the planar averaged self-consistent electrostatic potential for STO-Sr/GR(RGO,GRH) and STO-Ti/GR(RGO,GRH). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269. (2) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891. (3) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253. 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

(4) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nano-photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229. (5) Yi, Z. G.; Ye, J. H.; Kikugawa, N.; Kako, T.; Ouyang, S. X.; Stuart-Williams, H.; Yang, H.; Cao, J. Y.; Luo, W. J.; Li, Z. S.; Liu, Y.; Withers, R. L. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nat. Mater. 2010, 9, 559. (6) Chen, X. B.; Liu, L.; Yu, P. Y.;Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746. (7) Li, H.; Yin, S.; Wang, Y.; Sato, T. Microwave-Assisted Hydrothermal Synthesis of Fe2 O3 Sensitized SrTiO3 and its Luminescent Photocatalytic deNO(x) Activity with CaAl2 O4 :(Eu, Nd) Assistance. J. Am. Ceram. Soc. 2013, 96, 1258. (8) Kuang, Q.; Yang, S. Template Synthesis of Single-Crystal-Like Porous SrTiO3 Nanocube Assemblies and Their Enhanced Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Inter. 2013, 5, 3683. (9) Kato, H.; Kudo, A. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium. J. Phys. Chem. B 2002, 106, 5029. (10) Lehuta, K. A. Kittilstved, K. R. Speciation of Cr(III) in intermediate phases during the sol-gel processing of Cr-doped SrTiO3 powders. J. Phys. Chem. A 2014, 2, 6138. (11) Yu, H.; Wang, J.; Yan, S.; Yu, T.; Zou, Z. Elements doping to expand the light response of SrTiO3 . J. Photochem. Photobiol., A 2014, 275, 65. (12) Wang, Q.; Hisatomi, T.; Ma,S. S. K.; Li, Y.; Domen, K. Core/Shell Structured La- and RhCodoped SrTiO3 as a Hydrogen Evolution Photocatalyst in Z-Scheme Overall Water Splitting under Visible Light Irradiation. Chem. Mater. 2014, 26, 4144. (13) Wang, C.; Qiu, H.; Inoue, T.; and Yao, Q. Band gap engineering of SrTiO3 for water splitting under visible light irradiation. Int. J. Hydrogen Energy 2014, 39, 12507. 18 ACS Paragon Plus Environment

Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Tonda, S.; Kumar, S.; Anjaneyulu, O.; Shanker, V. Synthesis of Cr and La-codoped SrTiO3 nanoparticles for enhanced photocatalytic performance under sunlight irradiation. Phys. Chem. Chem. Phys. 2014, 16, 23819. (15) Modak, B.; Srinivasu, K.; Ghosh, S. K. A hybrid DFT based investigation of the photocatalytic activity of cation-anion codoped SrTiO3 for water splitting under visible light. Phys. Chem. Chem. Phys. 2014, 16, 24527. (16) Kawasaki, S.; Takahashi, R.; Akagi, K.; Yoshinobu, J.; Komori, F.; Horiba, K.; Kumigashira, H.; Iwashina, K.; Kudo, A.; Lippmaa, M. Electronic Structure and Photoelectrochemical Properties of an Ir-Doped SrTiO3 Photocatalyst. J. Phys. Chem. C 2014, 118, 20222. (17) Comes, R. B.; Sushko,P. V.; Heald, S. M.; Colby, R. J.; Bowden, M. E.; Chambers,S. A. Band-Gap Reduction and Dopant Interaction in Epitaxial La,Cr Co-doped SrTiO3 Thin Films. Chem. Mater. 2014, 26, 7073. (18) Kato, H.; Sasaki, Y.; Shirakura, N.; Kudo, A. Synthesis of highly active rhodium-doped SrTiO3 powders in Z-scheme systems for visible-light-driven photocatalytic overall water splitting. J. Mater. Chem. A 2013, 1, 12327. (19) Liu, L.; Li, P.; Adisak, B.; Ouyang, S.; Umezawa, N.; Ye, J.; Kodiyath, R.; Tanabe, T.; Ramesh, G. V.; Ueda, S.; Abe, H. Gold photosensitized SrTiO3 for visible-light water oxidation induced by Au interband transitions. J. Mater. Chem. A 2014, 2, 9875. (20) Liu, J.; Zhang, L.; Li, N.; Tian, Q.; Zhou, J.; Sun,Y. Synthesis of MoS2 /SrTiO3 composite materials for enhanced photocatalytic activity under UV irradiation. J. Mater. Chem. A 2015, 3, 706. (21) Zhang, Y., Lin, S.; Zhang, W.; Ge, H.; Li, G.; Qi, F. Y.; Song, X. M. Synthesis of a tailored SrTiO3 -TiO2 microspherical photocatalyst and its photogenerated charge properties. Rsc Adv. 2014, 4, 3226.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

(22) Kumar, S.; Tonda, S.; Baruah, A.; Kumar, B.; Shanker, V. Synthesis of novel and stable gC3 N4 /N-doped SrTiO3 hybrid nanocomposites with improved photocurrent and photocatalytic activity under visible light irradiation. Dalton T. 2014, 43, 16105. (23) Luo, J. H.; Maggard, P. A. Hydrothermal synthesis and photocatalytic activities of SrTiO3 coated Fe2 O3 and BiFeO3 . Adv. Mater. 2006, 18, 514. (24) Huang, J. R.; Tan, X.; Yu, T.; Zhao, L.; Xue, S.; Hu, W. L. Hetero-structured TiO2/SrTiO3 nanotube array film with highly reactive anatase TiO2 001 facets. J. Mater. Chem. A 2014, 2, 9975. (25) Guan, X.; Guo, L. Cocatalytic Effect of SrTiO3 on Ag3 PO4 toward Enhanced Photocatalytic Water Oxidation. ACS Catal. 2014, 4, 3020. (26) Zhang, Z., Liu, G.; Mao, Y. Improved separation efficiency of photogenerated carriers for Fe2 O3 /SrTiO3 heterojunction semiconductor. Int. J. Hydrogen Energy 2013, 38, 9349. (27) Jiao, Z.; Chen, T.; Xiong, J.; Wang, T.; Lu, G.; Ye, J.; Bi, Y. Visible-light-driven photoelectrochemical and photocatalytic performances of Cr-doped SrTiO3 /TiO2 heterostructured nanotube arrays. Sci. Rep. 2013, 3. (28) Jia, Y.; Shen, S.; Wang, D.; Wang, X.; Shi, J.; Zhang, F.; Han, H.; Li, C. Composite Sr2 TiO4 /SrTiO3 (La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation. J. Mater. Chem. A 2013, 1, 7905. (29) Ng, J.; Xu, S.; Zhang, X.; Yang, H. Y.; Sun, D. D. Hybridized Nanowires and Cubes: A Novel Architecture of a Heterojunctioned TiO2/SrTiO3 Thin Film for Efficient Water Splitting. Adv. Funct. Mater. 2010, 20, 4287. (30) Xian, T.; Yang, H.; Di, L.; Ma, J.; Zhang, H.; Dai, J. Photocatalytic reduction synthesis of SrTiO3 -graphene nanocomposites and their enhanced photocatalytic activity. Nanoscale Res. Lett. 2014, 9, 327. 20 ACS Paragon Plus Environment

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Du, A.; Ng, Y. H.; Bell, N. J.; Zhu, Z.; Amal, R.; Smith, S. C. Hybrid graphene/titania nanocomposite: interface charge transfer, hole doping, and sensitization for visible light response. J. Phys. Chem. Lett. 2011, 2, 894. (32) Li, X.; Gao, H.; Liu, G. A LDA+U study of the hybrid graphene/anatase TiO2 nanocomposites: Interfacial properties and visible light response. Comput. Theor. Chem. 2013, 1025, 30. (33) Xu, L.;Huang, W. Q.; Wang, L. L.; Huang, G. F.; Peng, P. Mechanism of Superior VisibleLight Photocatalytic Activity and Stability of Hybrid Ag3 PO4 /Graphene Nanocomposite. J. Phys. Chem. C 2014, 118, 12972. (34) Xu, L.; Huang, W. Q.; Wang, L. L.; Huang, G. F. Interfacial Interactions of Semiconductor with Graphene and Reduced Graphene Oxide: CeO2 as a Case Study. ACS Appl. Mater. Inter. 2014, 6, 20350. (35) Xu, L.; Huang, W. Q.; Wang, L. L.; Tian, Z. A.; Hu, W.; Ma, Y.; Wang, X.; Pan, A.; Huang, G. F. Insights into Enhanced Visible-Light Photocatalytic Hydrogen Evolution of g-C3 N4 and Highly Reduced Graphene Oxide Composite: The Role of Oxygen. Chem. Mater. 2015, 27, 1612. (36) Wang, W. S.; Wang, D. H.; Qu, W. G.; Lu, L. Q.; Xu, A. W. Large Ultrathin Anatase TiO2 Nanosheets with Exposed 001 Facets on Graphene for Enhanced Visible Light Photocatalytic Activity. J. Phys. Chem. C 2012, 116, 19893. (37) Xiang, Q.; Yu, J.; Jaroniec, M. Preparation and Enhanced Visible-Light Photocatalytic H-2Production Activity of Graphene/C3 N4 Composites. J. Phys. Chem. C 2011, 116, 7355. (38) Zhang, Y.; Zhang, N.; Tang, Z. R.; Xu, Y. J. Graphene Transforms Wide Band Gap ZnS to a Visible Light Photocatalyst. The New Role of Graphene as a Macromolecular Photosensitizer. ACS Nano. 2012, 6, 9777.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

(39) Wang, P. Q.; Bai, Y.; Luo, P. Y.; Liu,J. Y. Graphene-WO3 nanobelt composite: Elevated conduction band toward photocatalytic reduction of CO2 into hydrocarbon fuels. Catal. Commun. 2013, 38, 82. (40) Long, R.; English, N. J.; Prezhdo, O. V. Minimizing Electron-Hole Recombination on TiO2 Sensitized with PbSe Quantum Dots: Time-Domain Ab Initio Analysis. J. Phys. Chem. Lett. 2014, 5, 2941. (41) Long, R.; Prezhdo, O. V. Instantaneous Generation of Charge-Separated State on TiO2 Surface Sensitized with Plasmonic Nanoparticles. J. Am. Chem. Soc. 2014, 136, 4343. (42) Long, R.; English, N. J.; a Prezhdo, O. V. Photo-induced Charge Separation across the Graphene-TiO2 Interface Is Faster than Energy Losses: A Time-Domain ab lnitio Analysis. J. Am. Chem. Soc. 2012, 134, 14238. (43) Inagaki, M.; Kang, F. Graphene derivatives: graphane, fluorographene, graphene oxide, graphyne and graphdiyne. J. Mater. Chem. A 2014, 2, 13193. (44) Coropceanu, V.; Li, H.; Winget, P.; Zhu, L. Y.; Bredas, J. L. Electronic-Structure Theory of Organic Semiconductors: Charge-Transport Parameters and Metal/Organic Interfaces. Annu. Rev. Mater. Res. 2013, 43, 63. (45) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.-Condens. Mat. 2002, 14, 2717. (46) Yu, Y. X. A dispersion-corrected DFT study on adsorption of battery active materials anthraquinone and its derivatives on monolayer graphene and h-BN. J. Mater. Chem. A 2014, 2, 8910. (47) Yu, Y. X.Binding Energy and Work Function of Organic Electrode Materials Phenan-


Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thraquinone, Pyromellitic Dianhydride and Their Derivatives Adsorbed on Graphene. ACS Appl. Mater. Interfaces 2014, 6, 16267. (48) D′ Amico, N. R.; Cantele, G.; Ninno, D. First principles calculations of the band offset at SrTiO3 -TiO2 interfaces. Interfaces. Appl. Phys. Lett. 2012, 101, 141606. (49) Yu, PY.; Cardona, M. Fundamentals of Semiconductors; Springer- Verlag: Berlin, 1996. (50) Palik, E.D.; Ghosh, G. Handbook of Optical Constants of Solids, 3th ed.; Academic Press: London, U.K., 1998. (51) Wang, C.; Qiu, H.; Inoue, T.; Yao, Q. Band gap engineering of SrTiO3 for water splitting under visible light irradiation. Int. J. Hydrogen Energy 2014, 39, 12507. (52) Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm Without Lattice Bias. J. Phys. Condens. Matter 2009, 21, 084204. (53) Sachs, R.; Lin, Z.; Shi, J. Ferroelectric-like SrTiO3 surface dipoles probed by graphene. Sci. Rep. 2014, 4, 3657. (54) Saha, S.; Kahya, O.; Jaiswal, M.; Srivastava, A.; Annadi, A.; Balakrishnan, J.; Pachoud, A.; Toh, C. T.; Hong, B. H.; Ahn, J. H.; Venkatesan, T.; Oezyilmaz, B. Unconventional Transport through Graphene on SrTiO3 : A Plausible Effect of SrTiO3 Phase-Transitions. Sci. Rep. 2014, 4, 6173.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

Table 1: The displacement (in Å) of the Ti, Sr, and O atoms at the topmost layer of STO, and of carbon atoms (C) in the pure GR(RGO, GRH) sheet and (CST O/GRH ) in the optimized STO/GRH composites. Atom Sr/Ti O C CST O/GRH

STO- Sr/GR -0.184 -0.067 0 0

Ti/GR -0.128 -0.021 0 0

Sr/RGO -0.275 -0.022 0 0

Ti/RGO -0.123 0.031 0 0


Sr/GRH -0.243 -0.028 0.458 0.460

Ti/GRH -0.113 0.073 0.458 0.446

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Bandgap Egap , interface adhesion energy(Ead ), interfacial distance(d) and Bader charge analysis of optimized composites. Structure STO-Sr/GR STO-Ti/GR STO-Sr/RGO STO-Ti/RGO STO-Sr/GRH STO-Ti/GRH

Egap (eV)

Ead (eV)

d (Å)

0.633 0.603 1.741 0.004 2.100 0.350

-0.14 0.09 0.65 0.42 -0.85 -1.50

2.99 2.92 2.95 2.86 2.32 2.18


Bader charge (e) STO GR/RGO/GRH 0.18 -0.18 -0.07 0.07 0.13 -0.13 -0.12 0.12 0.06 -0.06 -0.13 0.13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

Figure 1: (a1) Top and (a2) side view of the simulated STO-Sr/GR composite; (b1) and (b2) for the STO-Ti/GR composite. Blue, red, gray, and light gray represent Sr, O, C, and Ti atoms, respectively. Maps of the electron and hole density distributions for the (a2), (b2) HOB and (a3), (b3) LUB with an isovalue of 0.005 e/Å3 for the hybrid STO-Sr/GR and STO-Ti/GR, respectively. Here, HOB and LUB denote the highest-occupied and lowest-unoccupied bands, respectively. DOS and PDOS for the (a4) STO-Sr/GR, (b4) STO-Ti/GR composite. The Fermi level is set to zero.


Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: (a1) Top and (a2) side view of the simulated STO-Sr/RGO composite; (b1) and (b2) for the STO-Ti/RGO composite. DOS and PDOS for the (a4) STO-Sr/RGO, (b4) STO-Ti/RGO composite. Others are the same as in Figure 1.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

Figure 3: (a1) Top and (a2) side view of the simulated STO-Sr/GRH composite; (b1) and (b2) for the STO-Ti/GRH composite. White sphere represents H atom. DOS and PDOS for the (a4) STO-Sr/GRH, (b4) STO-Ti/GRH composite. Others are the same as in Figure 1.


Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: 3D Charge density differences for (a) STO-Sr/GR, (b) STO-Sr/RGO, and (c) STOSr/GRH composites. The yellow and deep purple represent charge accumulation and depletion, respectively; the isovalue is 0.006 e/Å3 . Green (black) arrows in (b) denote the O atom with charge accumulation (depletion). (d) Planar averaged charge density difference for (red) STOSr/GR, (blue) STO-Sr/GRH, and (green) STO-Sr/RGO composites as a function of position in the z-direction. The black horizontal dashed line indicates the location of the topmost layer of the STO, color dashed lines represent the locations of C, O, and H atom layers, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

Figure 5: 3D Charge density differences for (a) STO-Sr/GR, (b) STO-Sr/RGO, and (c) STOSr/GRH composites. (d) Planar averaged charge density difference for (red) the STO-Ti/GR composite and (blue) the STO-Ti/GRH and (green) the STO-Ti/RGO composite as a function of position in the z-direction. Others are the same as in Figure 4.


Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: Charge distribution map of (a) STO-Sr, (b) STO-Sr/GR, (c) STO-Sr/RGO, and (d)STOSr/GRH, with a isovalue of 0.4 e/Å3 . Note that the transparency value of the isosurface is adjusted to an appropriate value, such that all of the atoms can be seen. Blue, red, gray, light gray and white represent Sr, O, C, Ti and H atoms, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

Figure 7: Charge distribution map of (a) STO-Ti, (b) STO-Ti/GR, (c) STO-Ti/RGO, and (d)STOTi/GRH, with a isovalue of 0.4 e/Å3 . Others are the same as in Figure 6.


Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8: Calculated absorption spectra of the bulk STO (blue solid line), STO-Sr surface (red solid line), STO-Ti surface (red dashed line), STO-Sr/GR (green solid line), STO-Ti/GR (green dashed line), STO-Sr/GRH (black solid line), STO-Sr/RGO (rose red solid line), and STO-Ti/RGO composite (rose red dashed line) for the polarization vector perpendicular to the surface.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Image


Page 34 of 34

Surface Interfaced with Graphene, Reduced ... - ACS Publications

Recommend Documents