Role of Heterojunction in Charge Carrier Separation in Coexposed

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Role of Heterojunction in Charge Carriers Separation in Coexposed Anatase (001)-(101) Surfaces Giovanni Di Liberto, Sergio Tosoni, and Gianfranco Pacchioni J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00504 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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The Journal of Physical Chemistry Letters

Role of Heterojunction in Charge Carriers Separation in Coexposed Anatase (001)-(101) Surfaces Giovanni Di Liberto*, Sergio Tosoni, and Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, Via Roberto Cozzi 55, I-20125 Milano, Italy Abstract A heterojunction made by co-exposed anatase (001)-(101) surfaces is studied using an explicit atomistic model of the interface via Density Functional Theory (DFT). High photoactivity for this system has been demonstrated recently. Usually, the nature of a semiconductor heterojunction is evaluated by looking at band edges of the separate, non-interacting units, thus neglecting interfacial effects. Our results show non-negligible structural and electronic effects occurring at the junction, but thanks to the cancelling nature of these effects the alignment of the bands is qualitatively similar for the real interface and for the separated, non interacting fragments. We also show from first principles that upon light absorption and electron excitation, the junction promotes charge carriers’ separation via localization of holes at O ions of the (001) side, and electrons at Ti ions of the (101) side of the junction. This hinders recombination, and is most likely the reason for high photoactivity.

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Titanium dioxide TiO2 is the most widely used oxide in applications in photocatalysis, environmental remediation, and sensing.1–5 Due to its quite large band gap (3.0-3.4 eV) a lot of effort was spent in the past years to enhance the visible light absorption capability, mainly introducing in the lattice doping species, both metals and/or non-metals.6–12 Nevertheless, even if a suitable apparent band gap decrease may be achieved in this way, when charge carriers are generated after light absorption, dopants do not hinder their recombination, and actually often increase this detrimental mechanism, thus resulting in less effective photocatalytic efficiency. Composite (or hybrid) materials, on the contrary, may separate charge carriers at opposite sides of the interface, thus effectively reducing the recombination.13,14 Hybrid materials are a wide class of systems, and TiO2-based composites play an important role. For instance, we can find structures made by TiO2 joined with a metal,15–17 graphene,18–20 or with other oxides.21,22 A peculiar case consists of combining two different polymorphs of the same oxide, in this case TiO2.23,24 The most common example is that of the P25 mixture of anatase (75%) and rutile (25%) used as gold standard to test the photocatalytic efficiency of new materials.25 In this respect, a heterostructure was proposed recently, where the two components are made by the same polymorph (anatase) but using different exposed faces. In this way one obtains a composite material made by anatase (101) and (001) faces in intimate contact.26,27 This system was recently shown to be highly active for photo-catalytic reactions, even more than the reference P25 and its precursors.26 It has been suggested that the reason behind the remarkable activity is the generation of a heterojunction between the two facets, promoting a separation of the photogenerated charge carriers.26,27 In particular, the electrons may go toward the (101) side, and the holes towards the (001) one. This interpretation, however, was exclusively based on Density Functional Calculations (DFT) calculations where the (101) and (001) moieties were considered as separated structures, without an explicit modelling of the junction. While this approach may be useful to provide qualitative hints on the nature of the heterojunction, it is a crude approximation which neglects effects due to the strain 2 ACS Paragon Plus Environment

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arising from the lattice mismatch, structural relaxations and reconstructions at the heterojunction, or electronic phenomena at the interface (including charge transfer, if any). This is particularly important in this case, since the bands position of the isolated slabs are close in energy, and the formation of an interface could result in important shifts in the band alignment. The aim of this work is twofold. First, we want to provide insight on the geometrical and electronic nature of the titania heterostructure by considering an explicit, realistic interface model. In this way we can have access to structural and electronic effects arising from the contact region. Second, we want to understand the origin of the high photo-activity. We provide robust evidence that this is due to the separation of the photogenerated charge carriers. The calculations have been performed at the Density Functional Theory (DFT) level by means of the localized basis set periodic CRYSTAL code.28 The approach allows to treat rather large supercells with hybrid functionals, which ensure a more accurate description of the electronic structure and band positions of semiconductors and insulators.29 The PBE0 parametrization of the exchange and correlation functional was adopted.30-31 Since the energy levels position is usually sensitive to the DFT functional employed, to strengthen results and to provide more solid outcomes, geometry optimization of TiO2 surfaces and of the heterojunction model were partly repeated using the fully ab-initio dielectric dependent (DD) self-consistent DFT approach32 which correlates the percentage of Fock exchange in the hybrid functional to the reciprocal of the static macroscopic dielectric function. For the case of anatase TiO2, a percentage of 18% of Fock exchange is obtained (this will be referred to as PBE0DD in the following).32 A detailed description of the computational setup is reported in the Supporting Information (SI) section S1. The anatase bulk was fully relaxed starting from the experimental structure. The calculated band gaps for bulk anatase are 4.46 eV and 3.82 eV for PBE0 and PBE0DD respectively, in agreement with 4.50 eV and 3.72 eV reported in the Literature.32,33 Moreover, the calculated gap for PBE0 is 3 ACS Paragon Plus Environment


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higher than PBE0DD according to the higher amount of Fock exchange in the functional. Both bulk values are overestimated compared to the experimental band gap of anatase, 3.2-3.4 eV. Then, six-layers thick slab models were cut along the (101) and (001) planes. For both faces, previous studies showed that six TiO2 layers are sufficient to reach reasonably accurate and converged surface properties.34–36 One may argue that periodic 2D slab models do not fully account for the TiO2 nanocrystals actually studied in the experiments. However, modeling oxides’ nanoparticles is a complicated task. For instance, edge states and undercoordinated sites at the particle’s corners introduce gap states in the electronic structure, which in turns affect the study of photoexcitation and charge carrier separation phenomena.37,38 All slab models have been fully optimized, relaxing both ionic positions and lattice parameters. The calculated cell parameters and bang gaps are reported in Table S1. Negligible differences occur comparing lattice parameters calculated at PBE0 and PBE0DD levels, with variations < 0.3% for both (101) and (001) slabs. Moreover, the computed surface energies are comparable: 1.06 Jm-2 (PBE0) and 1.01 Jm-2 (PBE0DD) for (001) surface, and 0.61 Jm-2 (PBE0) and 0.57 Jm-2 (PBE0DD) for (101), in agreement with previous calculations with different DFT functionals (0.98 Jm-2 and 1.36 Jm-2 for (001)34,39 and 0.49 Jm-2 and 0.64 Jm-2 for (101)

34,39).

The calculated band gaps with PBE0 are 4.79 eV and

4.31 eV for (101) and (001) surface, respectively (PBE0DD: 4.25 eV (101) and 3.68 eV (001), Table 1). A two-step process was carried on in order to design a realistic model of the heterojunction. A simple 1:1 epitaxial superposition of (101) and (001) anatase facets leads both to a large strain (4.5% and 10.5% along a and b directions, respectively), and a very poor Ti-O match at the interface. Thus, a proper superlattice was created minimizing the strain arising from the lattice mismatch between the two surfaces. Then, a convenient registry was defined, ensuring a good cation-anion match in the contact region. Recent experiments shed some light on the problem, since there are evidences suggesting that the co-growth of the two surfaces leads to a Moiré-like pattern 4 ACS Paragon Plus Environment

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with a reciprocal angle of 68.3°.26,27,40 We followed this hint by rotating the two surfaces one with respect to the other, finding that with an angle of 47° (reasonably close to the experimental value), the lattice strain is considerably reduced (6.3% and 0.9%) and there is a good cation-anion match in the contact region. Further details are reported in the SI. Eventually, a (TiO2)84 model (where 36 TiO2 formula units belong to the (101) moiety and 48 to the (001) one) was designed and fully optimized. The lattice parameters and band gaps of the isolated slabs and of the explicit heterojunction model are reported in Table 1. The rotation of the (001) moiety by 47° degrees and consequent structural optimization ensures a reasonably small lattice mismatch between the fully relaxed heterojunction and the isolated faces (4.3% and 0.9% for (001) face and 2.2% and 1.7% for (101) face, Table 1). The lattice parameters are almost unchanged comparing PBE0 and PBE0DD (differences: a = 0.2% b = 0.2%  = 0%). The interface band gap obtained is 3.48 eV with PBE0 and 2.85 eV with PBE0DD (Table 1). Figure 1(a) shows the optimized heterojunction model, where one can appreciate that the contact region is dominated by Ti-O bonds (Figures 1(b) and 1(c)). In Figure 1(c), one can identify six contacts between Ti ions from the (001) face and O ions from the (101) one, four with distance 1.946(1) Å (PBE0), 1.947(1) Å (PBE0DD) and two at 2.456(1) Å (PBE0), 2.464(3) Å (PBE0DD). At the same time, four Ti (101) are bound to O (001) at 1.869(10) Å (PBE0), 1.866(2) Å (PBE0DD), for a total of 10 Ti-O bonds per unit cell.



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Figure 1 (a) side view of the optimized (001)-(101) heterojunction model (PBE0). (b) and (c) top views at the contact region between the (001) and (101) surfaces. Green circles surround Ti (101) and O (001) contacts, while blue circles surround Ti (001) and O (101) ones. Orange arrows define the unit cell reported in Table 1. Ti (101) pink, O (101) red, Ti (001) light blue, O (001) purple. Table 1 Optimized parameters of the explicit heterojunction model (001_101) and (101) and (001) slabs. From left to the right: a and b lattice parameters, surface area, S, angle, , band gap, Eg. Values in parenthesis are the % strain of the lattice parameters with respect to optimized ones. Model

Functional

a/Å

b/Å

S / Å2

/°

Eg / eV

001_101

PBE0

10.902

10.527

114.7

88.5

3.48

101

PBE0

11.145 (2.2)

10.345 (1.7)

115.3 (0.5)

90 (1.7)

4.79

001rot

PBE0

10.434 (4.3)

10.435 (0,9)

108.6 (5.3)

85.6

4.31

001_101

PBE0DD

10.928

10.558

115.3

88.5

2.85

101

PBE0DD

11.172 (2.2)

10.367 (1.8)

115.8 (0.4)

90 (1.7)

4.25

001rot

PBE0DD

10.464 (4.2)

10.465 (0.8)

109.2 (5.3)

85.6 (3.2)

3.68


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The electronic structure of anatase (101) and (001) isolated faces is compared to that of the heterojunction in Figure 2, where the alignment of the Kohn Sham eigenvalues with respect to the vacuum level is shown.

Figure 2 Band alignment of coexposed (001)-(101) anatase surfaces (PBE0 results). (a): isolated, non-interacting (001) and (101) surfaces; (b) same as in (a) but after geometry optimization of isolated slabs at fixed heterojunction cell parameters; (c) fully optimized heterojunction. Blue: (101) surface, orange: (001) surface. Values are in eV with respect to the vacuum level. In Figure 2(a) the alignment of the levels of the non-interacting (101) and (001) faces is reported. One can see that the electronic levels of the (101) surface are lower in energy than those of the (001) surface, leading to a type II heterojunction. This picture has been used in the past to explain the experimental evidences, concluding that photogenerated electrons are favoured to move towards the (101) side of the interface and holes towards the (001) one. However, this picture lacks any kind of electronic interaction occurring between the two anatase facets, and it assumes that the two surfaces do not reconstruct upon interaction. Clearly, both assumptions represent crude approximations in the model. Indeed, we have shown above that, when the junction model is relaxed, new Ti-O bonds are formed. Figure 2(c) shows what happens when the (101) and (001) surfaces are interacting in the heterojunction model. The band edges have been obtained by employing the band alignment method based on the analysis of the plane-averaged electrostatic potential suggested originally by Conesa and described in the literature.41,42 In this approach, the 7 ACS Paragon Plus Environment


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band edges of independent slabs are aligned to a common reference, namely the stationary points (ETOP) of the plane-averaged electrostatic potential calculated for (101) and (001) isolated surfaces and in the heterojunction model. For further details look at section S3 of SI. As in Figure 2(a), a staggered type II heterojunction is formed, but it is worth noting how structural and electronic junction effects slightly enhance the band edges separation between (101) and (001) moieties, both for valence band (VB) (0.06 eV) and conduction band (CB) (0.06 eV). The valence band offset (VBO) is 1.02 eV while the conduction band offset (CBO) 0.54 eV. Both values are very close to the band offset computed with the non-interacting surfaces, Figure 2(a). This, however, is the fortuitous consequence of the occurrence of two effects that act in opposite directions. In order to disentangle the electronic from structural contributions, we have fully optimized the ionic positions of the isolated (101) and (001) surfaces, imposing the lattice parameters to be those of the simulated heterojunction, Figure 2(b). In this way, we are accounting for the structural contribution, but we are not considering the electronic interactions because the slab models are isolated from each other (no formation of the explicit interface). One important aspect is that the CB edges of the two oxides, Figure 2(b), are now at very similar energies, CBO being 0.17 eV only, while it is of 0.54 eV in the real interface, Figure 2(c). Thus, structural relaxation at the interface strongly reduces the driving force for the excited electrons to move towards the (101) side of the interface, an essential aspect to limit recombination. This also suggests that structural and electronic interactions work in opposite directions, almost outbalancing each other, explaining why the assumption of independent materials provide in some cases band offsets that are already similar to those of the coexposed surfaces, where the contribution of the heterojunction is explicitly accounted. The band alignment picture remains almost unchanged considering PBE0DD (see Section (S3) and Figure S2 of SI). Indeed, VBO moves from 1.02 eV to 0.97 eV, and CBO from 0.54 eV to 0.41 eV. Therefore, results show that both approaches lead to the comparable heterojunction band alignment pictures. 8 ACS Paragon Plus Environment

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The above discussion is entirely based on the Kohn-Sham energy levels of the ground state of the system. However, photoactivity is an excited state problem, and one needs to go beyond the oneelectron approximation. The fate of photogenerated electrons and holes in the junction model has been studied by computing explicitly the electronic vertical transition forcing the electronic structure of the system into a triplet state, that we assume to approximate an excited state of the system.37 This approximation is valid in the limit of weakly interacting electrons. We started from the ground state geometry and we optimized the electronic structure with the constrain to be a triplet state. Further details can be found in the SI file section S4. Since the band alignment results are very similar, we evaluated triplet states at PBE0. One electron has been promoted from the VB to the CB (vertical transition), then the heterojunction structure was relaxed. The energy cost for the singlettriplet vertical transition is 3.40 eV Figure 3(a). According to the spin density plot, Figure 3(a), in the vertical transition the excited electron is promoted to the CB of the (001) side, and the corresponding spin density is delocalized over several Ti atoms; the generated hole, in turn, is delocalized over several O ions of the same (001) moiety. Thus, light absorption and excitation occur at the (001) face. Then, if the geometric structure is relaxed, a polaron is spontaneously created in the lattice favouring the localization of the photogenerated hole at one O of the (001) side, while the electron remains delocalized over several Ti atoms of the same surface, Figure 3(b). It must be said, however, that creation of polarons and the consequent localization of electrons and holes depend also on the adopted functional, and many possible structures can be found in principle at similar energies.43,44 The result in which the hole localizes preferentially on the (001) surface (see the spin density plots in Figure 3(b)) agrees with the band alignment (Figure 2) and with previous works that showed that the (001) surface is suitable for allocating holes.

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The localization, and polaron formation of the hole on one O at the (001)

surface implies an energy gain of almost 2 eV (the singlet-triplet separation goes from 3.40 eV to 1.37 eV, Figure 3(b)). 9 ACS Paragon Plus Environment


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Polaron formation at a Ti site can also be obtained, with formation of a Ti3+ (3d1) center, where the electron localizes on a Ti ion of the (101) moiety, as depicted in Figure 3(c), and it implies an additional energy gain. Of course, this is only one of the several possible solutions for the electron localization on various Ti ions, but it indicates that the migration of the excited electron on the other side of the junction is a favourable process. Indeed, a further stabilization of the system leads to a decrease of the singlet-triplet energy separation from 1.37 eV to 0.88 eV, Figure 3(c). Once again, the model agrees with the Literature, indicating that the (101) surface may allocate electrons, while the (001) stabilizes the opposite charge carrier.

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The result, based on a fully first principles

approach, shows that the separation of photogenerated charge carriers is indeed favourable in this type of junction, and the interface plays a role in stabilizing separated charge carriers, as shown by the energy gain associated to charge localization, see Figure 3(b) to 3(c). This is fully coherent with the band alignment reported in Figure 2. In conclusion, the structural and electronic interactions between (101) and (001) anatase TiO2 surfaces lead to a stable heterojunction, and important but cancelling effects take place, where structural relaxation contributes to reduce and electronic effects to increase the band offset. The overall result is that the band edges are qualitatively similar to those of the separated, noninteracting units. This explains why by considering the band edges of the isolated fragments one obtains occasionally a qualitative understanding, despite this neglects the important role of the interface and the many body nature of the electron excitation. According to the model, the photogeneration of charge carriers takes place at the (001) surface with formation of a delocalized hole in the VB and a delocalized electron in the CB. Structural relaxation induces localization of the hole on a single O ions and migration of the excited electron at the (101) side of the system, where it localizes to form a Ti3+ ion. This picture, obtained by considering a realistic atomistic model of the heterojunction and the explicit electronic excitation, is fully consistent with the experimental evidences and confirms the 10 ACS Paragon Plus Environment

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hypothesis formulated in the literature about charge carriers localization and band alignment. Further work will include functionalization of the system with dopants in order to improve the light adsorption power of the (001) surface,46 as well as the introduction of a third semiconducting species acting as hole scavenger to be put into contact with the (001) side of the device.47 Both strategies are expected to improve the performances for photocatalysis under visible light irradiation.

Figure 3 Different triplet states configurations calculated with PBE0, with their corresponding energy with respect to ground state configuration (EGS) of Figure 1. In yellow are reported spin density iso-surfaces (0.005 |e|/(Bohr3)); in pink and red are shown Ti and O atoms. Orange and blue circles indicate hole and electron polarons respectively. (a) vertical transition, i.e. triplet charge distribution at the Ground State geometry. (b) and (c) two different configurations characterized by electron and hole in same and in separated surfaces respectively.



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Acknowledgements This work has been supported by the Italian MIUR through the PRIN Project 2015K7FZLH SMARTNESS "Solar driven chemistry: new materials for photo– and electro–catalysis" and the grant Dipartimenti di Eccellenza - 2017 "Materials For Energy". We acknowledge the Regione Lombardia and CINECA under the ISCRAB initiatives - projects IscrB-SEP-ZNO_0 and IscrB_WHPEM- for the availability of high performances computing resources and support. Author Information Corresponding author (*) [email protected] (G.D.L.) Notes The authors declare no competing financial interest. Supporting Information Available Supporting information: Computational setup and comments on the various quantum models.


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