Band-Edge Engineering at the Carbon DotâTiO2 Interface by

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Band-Edge Engineering at the Carbon Dot – TiO Interface by Substitutional Boron Doping 2

Dipayan Sen, Piotr Blonski, and Michal Otyepka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11554 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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The Journal of Physical Chemistry

Band-Edge Engineering at the Carbon Dot – TiO2 Interface by Substitutional Boron Doping Dipayan Sen†, Piotr Błoński†, Michal Otyepka*,† †Regional

Centre of Advanced Technologies and Materials, Department of Physical

Chemistry, Faculty of Science, Palacký University Olomouc, tř. 17, listopadu 12, 771 46 Olomouc, Czech Republic

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ABSTRACT: We investigated heterostructures of pristine and boron doped Carbon dots (CDs) with TiO2 using a first principles approach. Heterojunctions of CDs and TiO2 demonstrated type-II band alignments that promote spatial separation of charge carriers and were well suited for sensitizer configuration. However, large CD band gaps severely restricted their optical efficiencies. Substitutional boron doping of the CDs introduced new electronic states and pulled the CDs’ conduction band minimum (CBM) down, resulting in dramatic reduction of CD band gaps (by 48-57 %). The resulting band alignment of the boron doped CD - TiO2 heterostructures were found to be type-I, in which CBM and valence band maximum (VBM) of TiO2 straddled those of CDs and photoexcited carriers could migrate to CD side. This indicated better suitability for device configurations where conducting channel lies through the CDs, but elevated chance of recombination loss. However, the internal electric fields at these heterojunctions were found to selectively promote electron migration and hinder hole migration, which, we postulate could counter the recombination loss in these systems. The obtained results offer insights into designing a new generation of water splitting photocatalysts that would be able to utilize a broader spectrum of solar radiation than the current solutions.


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1. INTRODUCTION Fujishima et al.1 first demonstrated photocatalytic water splitting on single crystal TiO2, and since then significant research focus2-7 has been diverted to perfect this avenue of hydrogen fuel production. However, one of the key aspects that still significantly undermine the performances of TiO2 based systems is the wide band gap of TiO2 (~3.20 and ~3.05 eV for anatase and rutile polymorphs, respectively) that constricts the optical absorption window mostly in the ultraviolet (UV: ~5% of the total incoming solar radiation) region8-9. Concurrently, TiO2 also suffers from fast e-h recombination rate10-11 that significantly undermines its theoretical operating efficiency. Thus, surface augmentation of TiO2 is necessary12 to improve upon the optical absorption range and photocatalytic performance. Till date, one of the most effective strategies to address the aforementioned issues remains interfacing TiO2 with various forms of carbonaceous materials. Such composite systems could be devised using myriad different approaches, e.g., by a) loading TiO2 on carbon materials,13-14 b) doping TiO2 with elemental carbon,15-16 and c) coating TiO2 with various forms of carbon17-18. Moreover, the mechanism of carrier transport and separation in such a TiO2–carbon composites can also be distinctly different depending on the device configuration as well as morphology of the composite. It has been demonstrated that, carbon localization on TiO2 can lead to two distinct phenomena19: a) Carbon materials can donate electrons to the conduction band (CB) of TiO2; this constitutes the ‘classic’ sensitizer configuration where TiO2 constructs the carrier transport channel20-21 and b) Carbon materials can act as electron collector to capture photoexcited electrons from TiO2, in this case the carrier transport occurs through the carbon material22-23. These coexistent opposing phenomena may hinder or boost device performance; an interesting implementation of this has been demonstrated by Yu et al.24, in which, in a single system, the carbon material acts as electron reservoir under UV radiation and



as sensitizer under visible light. Thus, rational designing of the composite plays a very large role in dictating the final performance of such composite devices. Carbon dots25 - TiO2 heterostructures have recently garnered considerable attention, principally owing to CDs’ tunable electronic/optical properties, stability, economic and straightforward synthesis protocol and environment-friendly nontoxic nature26-30. Recent theoretical and experimental evidences indicate direct contact of extended π-electron systems of CDs with TiO2 may facilitate hot carrier injection31. Thus, various CD-TiO2 composites are currently finding themselves in the forefront of photocatalytic research. Herein lies an advantage that electronic properties of the CDs can be effectively tuned, e.g., by incorporating substitutional dopants, and recent reports demonstrate N-doped and S,N-co-doped CD-TiO2 composites32-33 are promising strategies for improving TiO2 performance. Substitutional B doping is an already established approach to modulate optical properties of CDs34, and thus intuitively may offer another pathway to enhance the performances of CD-TiO2 composites. However, the heterojunction of TiO2 with the B doped CDs has not been described in the literature yet. In the current work, we employed density functional theory (DFT) based first principles techniques to investigate electronic properties of heterostructures of various pristine/ substitutionally B doped CDs with TiO2. Pristine CD – TiO2 heterostructures demonstrated typical type-II band alignments. The substitutional B doping pulled down the CD CBMs and drastically reduced the CD band gaps. Consequently, the B doped CD -TiO2 heterostructures demonstrated type-I band alignments with significantly lower CD band gaps as well as electric field promoted reduced carrier recombination probabilities that are of interest from the perspective of photocatalytic water splitting. 2. THEORETICAL METHODS


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2.1 Computational Details. All computations were carried out using Vienna Ab-initio Simulation Package (VASP)35-38 which implements projector-augmented-wave (PAW) formulation39-40 with plane wave basis sets. Relaxation of isolated TiO2 (both cell parameters as well as the internal atomic positions) and CD models (only internal atomic positions) were carried out using Perdew– Burke–Ernzerhof (PBE) exchange-correlation functional41 under the generalized gradient approximation (GGA). For relaxing the CD-TiO2 heterostructures (only internal atomic positions), optB86b-vdW non-local correlation functional42 was used which provides a robust framework for integrating dispersion corrections. Electronic properties of the heterostructures were calculated using optB86b-vdW functional. The Hubbard U parameters: U-J = 7.51 eV for Ti d orbitals and U-J = 4.37 eV for O p orbitals, reported by Umezawa et al.43 were found to describe the closest match with HSE0644 bandgap of TiO2, and thus were adapted in all subsequent computations to account for the self-interaction error. HSE06 functional was used to assess the band gap underestimations of the isolated CDs under GGA and optB86b-vdW approximations. Plane wave basis up to an energy cut off 500 eV were utilized in all calculations and the ions were allowed to relax until all atomic forces were less than 0.01 eV/ Å. Brillouin zone integrations were carried out using Γ centred K-point grids (from 9×9×1 to 21×21×1, depending on the dimension of the supercell). All calculations were carried out in spin unrestricted manner and vacuum slabs of length ~12 Å were deployed along the perpendicular direction to ward off spurious interactions with the periodic images. Partial charges of various atomic species were computed by Bader analysis45. The relative stabilities of the various B doped CDs were estimated by computing their formation energies using the following expression: Efor = Ed ― CD ― ECD + nVμC ― nBμB (1)



where, Efor represents the computed formation energy of the B doped CD, Ed ― CD is the ground state total energy of the B doped CD, ECD is the is the ground state total energy of the pristine CD, nV is the number of substituted carbon atoms (2 in the current case), μC is the chemical potential of carbon atom computed using graphene as the reference state, nB is the number of doped boron atoms (2 in the current case) and μB is the chemical potential of the boron atom computed using β-rhombohedral boron as the reference state. Lower value of the formation energy, as calculated using equation 1, implies higher stability of the CD under consideration. Relative energetic stabilities of various CD – TiO2 heterostructures were estimated by computing the CD adsorption energies using the following expression: Eads = Ehet ― ETiO2 ― Ed ― CD (2) where, Eads represents the computed adsorption energy, Ehet is the ground state total energy of the B doped CD-TiO2 heterostructures, ETiO2is the ground state total energy of the TiO2 slab and Ed ― CD is the ground state total energy of the isolated, B doped CD. Lower value of the adsorption energy, as calculated using equation 2, implies higher energetic stability of the adsorbed CD on TiO2. 2.2 Modelling TiO2 slabs and CDs While designing TiO2 based surfaces and interfacial systems, from the perspective of energetic stability and practical viability, two systems are pertinent: rutile (1 1 0) and anatase (1 0 1) (technical difficulties associated with synthesising brookite strictly limits its applicability46; and high-pressure phases e.g. srilankite, cubic fluorite-type, pyrite-type, monoclinic baddeleyite-type and cotunnite-type polymorphs are constricted by their poor stability46). However, for photocatalytic applications, even though the rutile system demonstrates lower experimental band gap than the anatase system (~3.0 eV vs ~3.2 eV,


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respectively)46, the later bestow additional advantages such as slower carrier recombination and higher density of localised states46. In that light, in the current work, we cleaved the bulk, conventional unit cell of anatase TiO2 (space group: 141, I41/amd) to construct the models of 1-4 layer, stoichiometric, oxygen terminated slab surfaces. In 2 and 3 layered systems, only the bottom layers were kept constrained; whereas for the 4 layered systems, two of the bottom layers were kept constrained during relaxation. The relaxed slab geometries and their corresponding total density of states (TDOS) are shown in Figure S1 of the Supporting Information (SI)). Comparison of the TDOS indicate the 3 layered system to be an acceptable choice; and thus, in all subsequent calculations, appropriately proportioned supercells of this relaxed 3 layered geometry were used. To model small CDs, we opted for three representative single layer systems: a) 1 benzene, b) 4 pyrene, and c) coronene. Despite of the model proportions being rather small (~5-10 Å), such single layer small dots, in past, were successfully used47-48 to describe the salient features of optical and electronic properties of much larger systems. The coronene system was further utilized to investigate interactions between the B doped CDs and the TiO2 underlayer. Due to the small model dimension, it was computationally feasible to systematically explore all possible doping configurations. The benzene-TiO2 heterostructure was used namely for accessing quality of used electronic structure calculations. From the perspective of photocatalytic applications, the following criteria were utilized to search for optimum solutions: a) suitable CD band gap, which is necessary for harvesting visible light or even higher wavelength photons b) suitable relative band-edge alignments of CD and TiO2, which dictates the effective functionalities of CD (sensitizer vs. electron collector) c) suitable band-edge alignments of the CD-TiO2 heterostructure with respect to water redox potentials, which is necessary to drive the reaction forward and d) effective separation of e-h pairs as a counter to carrier recombination. 7 ACS Paragon Plus Environment


3. RESULTS AND DISCUSSION 3.1. Pristine CD-TiO2 heterostructures. As a precursor to the more complex B doped CD-TiO2 heterostructures, pristine CDTiO2 heterostructures were investigated first. The relaxed 3 layered anatase TiO2 (1 0 1) slab unitcell was used to construct the following models of pristine CD-TiO2 heterostructures: 1) Benzene on 2×2×1 TiO2 slab supercell. 2) Pyrene on 3×4×1 TiO2 slab supercell and 3) Coronene on 4×5×1 TiO2 slab supercell. The dimensions of the TiO2 slabs were chosen in line with prior reports on comparable systems49 to maintain sufficient separations between CDs’ periodic images. In all cases, the CDs models were placed ~3.5 Å above the TiO2 surface and were allowed to relax (the bottom layers of the 3 layered TiO2 slabs were kept constrained as described earlier). Each such systems were investigated using various starting orientations of the CDs and the lowest energy optimized structures were identified (Figure S2 of the SI). Following optimization, the CDs remained stacked ~2.7 Å - 2.9 Å above the TiO2 surface (Table S1 of the SI). Depending on the edge geometries of the individual CDs and their long range interaction with the underlying TiO2 substrate, minor inflection of the CD geometries could be noted; however, formation of strong chemical bonds was not observed in any cases. The respective lattice parameters and the computed total energies of the above configurations are listed in Table S1 of the SI for reference. Further we analysed electronic structures in terms of TDOS and projected (on TiO2 and CD atoms; and on 3d orbitals of Ti atoms, 2p orbitals of O atoms and 2p orbitals of C atoms respectively) DOS (PDOS) for all considered pristine CD-TiO2 heterostructures (Figure S3 and S4, respectively of the SI). The corresponding electronic states and CD band gaps were in well agreement with prior reports of similar small CD – TiO2 heterostructures50 that used comparable computational protocols. In general, the band gaps of the CDs (on heterostructure


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configuration, listed in Table S1 of the SI) were found to decrease with increasing number of carbon rings as expected. TiO2 and CD band gaps were clearly discernible for all considered systems, and absence of mid-gap trap states implied these systems might be suitable for minimizing recombination loss. However, it should be noted that these electronic states depict optB86b-vdW estimates; thus, the CD band gaps are likely underestimated and the CBMs most likely represent the lower bounds that are obtainable by ab initio protocols (e.g., for comparison benzene adsorbed on graphite has band-gap of 7.35 eV (using self-consistent GW calculations49) and 5.01 eV using optB86b-vdW method (Figure S5 of the SI)). Nevertheless, the PDOS plots depict relative positions of the conduction band minima (CBM) and valence band maxima (VBM) for both part of the heterostructure after junction formation i.e. Fermi level realignment had occurred. Energies of respective vacuum levels were used to rescale these energy levels, which is necessary for computing the absolute positions of the band-edges and for comparing to the water redox potentials. To this end, work functions (WF = Vacuum Level – Fermi Level) of all considered models were computed and are listed in Table S1 of the SI. The redox potentials for both half reactions of the overall water splitting reaction are pH dependent, and can be expressed in such a ‘vacuum scale’ as51: a) standard reduction potential for H+/H2 half reaction (Ered H + /H2) = ―4.44 eV + pH × 0.059 eV and b) standard oxidation potential for O2/H2O half reaction (Eox O2/H2O) = −5.67 eV + pH×0.059 eV. For simplification we assumed pH = 0, which presented us with the upper and lower bound of redox potentials as -4.44 eV and -5.67 eV respectively. For all considered pristine CD-TiO2 heterostructures, vacuum scale representation of the band diagrams along with overlay of water redox potentials are depicted in Figure 1ac, which shows that the CBM and VBM of CDs lie higher than those of TiO2 and all considered pristine CD-TiO2 heterostructures describe staggered i.e. type-II heterojunctions.



Figure 1. Band diagrams of a) 1 ring (benzene) b) 4 ring (pyrene) c) 6 ring (coronene) – TiO2 heterostructures; the blue highlighted regions represent the energy window 𝐄𝐫𝐞𝐝 𝐇 + /𝐇𝟐 𝐨𝐱 - 𝐄𝐎𝟐/𝐇𝟐𝐎.

Photoexcitation of these pristine CD-TiO2 heterostructures results in electrons jumping from valence band to conduction band in both CD and TiO2 sides. However, as CBM of TiO2 and VBM of CD offers the lowest energy states for e and h respectively, the charge carriers spontaneously accumulate on different side of the heterojunctions. This spatial separation of the charge carriers is highly beneficial as it can restrain recombination of electron−hole pairs


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and thereby tacitly improve the net photocatalytic performance. A few observations are of importance here: a) For all considered pristine CD-TiO2 heterostructures, CBM of TiO2 lie above Ered H + /H2; which indicate, in these systems, hydrogen evolution reaction can take place usually and they are ideal for sensitizer configuration. b) For practical deployment, photoinduced/ accumulated holes in the CDs may need to be quenched using an external agent (In typical experimental configurations52, electron donors such as ethylenediaminetetraacetic acid (EDTA), triethanolamine (TEOA) or ascorbic acid (AA) are used). c) As CD CBMs lie higher than those of TiO2 for all cases, the electron donating nature of the CDs portrayed by Figure 1 are adequately accurate and will persist even if more accurate computational protocols are deployed. Nevertheless, it should be stressed that the band gap of the pristine CDs in the heterostructure configurations are still on the higher side (e.g. 2.03 eV for coronene - TiO2, optB86b-vdW estimates). The above implies that the pristine CD-TiO2 heterostructures, despite having favourable band alignments, might fare poorly in practical applications and further lowering of the CD band gap is incumbent if we are to aim for higher efficiency. Table 1. (PBE Computed) Parameters of Substitutional Two B Atom Doping in Isolated Coronenea configuration BG (eV) Efor (eV) BG (%) 2B12 2.032 0.828 70.5 2B13 2.539 2.457 (up), 12.6 (up), 1.196 (down) 57.5 (down) 2B14 2.030 0.892 68.3 2B15 2.945 1.036 63.1 2B16 2.960 2.406 (up), 14.4 (up), 0.725 (down) 74.2 (down) 2B17 2.706 0.538 80.9 2B18 2.816 0.435 84.5 2B56 (ortho at centre) 4.037 2.462 (up), 12.4 (up), 0.552 (down) 80.4 (down) 2B57 (meta at centre) 3.422 2.410 (up), 14.3 (up), 0.454 (down) 83.8 (down) 2B58 (para at centre) 3.000 0.238 91.5 aE for is the computed formation energy, BG is the band gap, BG is the band gap reduction. The most stable configurations are highlighted in bold.



3.2. Substitutional B doping in CDs. Coronene, substitutionally doped by two B atoms, was used as a model of the B doped CDs. We considered B substitution at all possible sites to investigate the trends of band gap modulation. Doping B atoms at the edge sites of coronene was found to relax to non-planar geometries with (relatively) higher energies and these structures were not considered in further analysis. Possible configurations of two B atom doping in coronene are schematically shown in Figure S6 of the SI (the numbers in the nomenclature scheme denote the sites; e.g. for 2B12, two B atoms were substituted at sites 1 and 2, and for 2B14, two B atoms were substituted at sites 1 and 4; symmetric sites were considered as identical); and their respective (GGA-PBE computed) formation energies (Efor, computed using Equation 1) and band gaps (BG) are listed in Table 1. As mentioned earlier, the absolute values of these band gaps are likely to be underestimated, however such a scheme of computation facilitates estimating the relative modulation of band gap as a function of B doping when compared to the GGA-PBE gap of isolated, undoped coronene (2.811 eV). The computed relative reduction of band gaps (BG) for all configurations are also listed in the Table 1 for reference. Obtained results indicate that B dopants specifically favour the carbon sites of the outer C rings. The configurations: 2B12 and 2B14 were found to be energetically most favourable, with almost similar formation energies of 2.032 eV and 2.030 eV respectively. Most importantly, B doping was also found to modulate the band gaps of the coronene dots significantly; e.g. for 2B12 and 2B14, 70.5% and 68.3% reduction of the band gaps (in comparison to isolated, undoped coronene) were observed. Consistency of the observed trend of band gap reduction was also verified against HSE06 predictions (e.g. 1.408 eV and 1.506 eV band gaps were obtained for 2B12 and 2B14 respectively, which in comparison to isolated, undoped coronene (3.492 eV) yielded 59.6% and 56.8% band gap reduction respectively). This formidable band gap reduction, coupled with completed absence of mid-gap trap states in the 12 ACS Paragon Plus Environment

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DOS (Figure S7 of the SI), indicate B doped CDs might indeed present unprecedented opportunities for increasing their optical responses. 3.3. B doped CD-TiO2 heterostructures: structural properties. To construct models of B doped CD-TiO2 heterostructures, the most favourable 2 B atom doped coronene systems, i.e. 2B12 and 2B14 were used. The TiO2 underlayers were constructed akin to the manner described in 3.1 (4×5×1 supercell of 3 layered anatase TiO2 (1 0 1) slab, bottom layers were kept constrained). Each such systems were relaxed using various different starting orientation of the CDs. Unlike the case of pristine CDs, interactions B doped CDs with TiO2 were found to be strongly affected by the relative orientation of the CDs with respect to the upper oxygen layer of the TiO2 slab. Intuitively it can be argued that interactions

Figure 2. Optimized geometries of a) 2B12T-a b) 2B12T-b c) 2B14T-a and d) 2B14T-b: side and top views.



between the positively charged B atoms and the negatively charged O atoms may add additional forces which were not present in case of the pristine CDs; this is elaborated more thoroughly in the following sections of the manuscript. Both 2B12 and 2B14-TiO2 heterostructures, depending on the starting orientation of the CD, were found to relax into two possible configurations a) a (relatively) more distorted CD geometry (henceforth designated as 2B12T-a & 2B14T-a), in which strong B-O chemical bond (1.554 Å & 1.564 Å respectively) formation occurs and b) a (relatively) less distorted CD geometry (henceforth designated as 2B12T-b & 2B14T-b), in which the CDs and the TiO2 underlayers are comparatively weakly coupled. Side and top views of 2B12T-a, 2B12T-b, 2B14T-a and 2B14T-b geometries are shown in Figure 2ad. Adsorption energies (Eads, computed using Equation 2) and bond lengths/ shortest B doped CD-TiO2 distances (d) for 2B12T-a, 2B12T-b, 2B14T-a and 2B14T-b geometries are listed in Table 2 for reference. The lattice parameters as well as the total energies for these configurations are tabulated in Table S2 of the SI. Table 2. Parameters of B Doped CD – TiO2 Heterostructuresa configuration Eads d (Å) Q (e) BGCD BGCD WF (eV) (eV) (eV) (%) 2B12T-a -3.441 B-O non-bonded O: -0.968, 1.433 29.4 4.868 bond: bonded O: -1.186, bonded 1.554 B: +1.958, non-bonded B: +1.874 2B12T-b -2.196 2.584 O: -0.970, B: +1.842 and 0.873 57.0 4.875 +1.875 2B14T-a -2.816 B-O non-bonded O: -0.969, 1.233 39.3 4.868 bond: bonded O: -1.179, bonded 1.564 B: +1.977, non-bonded B: +1.873 2B14T-b -2.514 2.579 O: -0.971, B: +1.896 and 1.048 48.4 5.208 +1.867 aE ads is the CD adsorption energy, d is the bond length/ shortest B doped CD-TiO2 distance, Q is the Bader partial charge, BGCD is the CD band gap, BGCD is the CD band gap reduction and WF is the work function (upper side).


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Interactions between B doped CDs and TiO2 underlayers were critically examined by the means of Electron Localization Function (ELF) and Bader partial charge analyses. ELF (η(r)) is defined as: 1

η(𝐫) = 1+

(3)

D(𝐫) 2 Dh(𝐫)

( )

where, D(r) is the Pauli repulsion between two like-spin electrons and Dh(𝐫) is D(r) for uniform electron gas. The value of η(r) ranges between 0 to 1, and high η(r) at a certain r indicates that the electrons will be more localized at r in comparison to uniform electron gas of the same density. ELF isosurfaces (plotted for isovalue of 0.75) of 2B12T-a, 2B12T-b, 2B14T-a and 2B14T-b systems are depicted in Figure 3ad. Topological analyses of the flocalization domains indicate strong localization of electrons i.e. covalent bond formation

Figure 3: ELF isosurfaces of a) 2B12T-a b) 2B12T-b c) 2B14T-a and d) 2B14T-b (plotted for isovalue of 0.75).

along each three B-C directions for all four cases. For 2B12T-a and 2B14T-a, even though strong chemical bond formations occurred along the B-O directions (as evident by relaxed bond lengths), no evidence of electron localization could be discerned (Figures 3 (a) and 3 (c), inset); 15 ACS Paragon Plus Environment


which rules out the possibility of any significant covalent B-O interactions in the current case. On the other hand, we also computed Bader partial charges of the relevant atoms for 2B12T-a, 2B12T-b, 2B14T-a and 2B14T-b systems (listed in Table 2 for reference). This dataset show, for 2B12T-a and 2B14T-a, the chemically bonded B and O atoms accumulate higher positive charges and negative charges respectively. Consequently, it can be surmised that: a) For 2B12T-a and 2B14T-a, the ionic interactions between B doped CDs and TiO2 underlayer (on the top of van der Waals forces) play a significant role in determining their structural (and electronic) properties. and b) For 2B12T-b and 2B14T-b, B doped CDs and TiO2 underlayer occurs mostly through van der Waals forces.

Figure 4. TDOS and PDOS (projected on TiO2 and CD atoms) for a) 2B12T-a b) 2B12T-b c) 2B14T-a and d) 2B14T-b. The Fermi level is set at 0 eV.


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3.4. B doped CD-TiO2 heterostructures: electronic properties and photocatalytic response. OptB86b-vdW computed TDOS and Projected (on TiO2 and CD atoms respectively) DOS (PDOS) for 2B12T-a, 2B12T-b, 2B14T-a and 2B14T-b systems are shown in Figure 4ad. Introduction of the B atom was found to modulate their respective TDOS significantly (in comparison to the TDOS of pristine coronene-TiO2 heterostructure as shown in Figure S3 of the SI), especially in the CBM region where new energy levels appeared. CD projected PDOS show the new levels are principally contributed by the B doped CDs. As a direct consequence of B doping, the CD CBMs were pulled down and their bandgaps were significantly reduced. However, for both strongly coupled (2B12T-a and 2B14T-a) and weakly coupled (2B12T-b and 2B14T-b) cases, TiO2 and CD band gaps were observed to be clearly discernible and no mid-gap trap states were detected. The above implies that the current systems maintain the trait of their pristine coronene counterpart in this regard. Individual band gaps of the B doped CDs (BGCD) on TiO2 underlayer for all four cases are listed in Table 2. Here it should be stressed again that these band gaps are likely to be underestimated. However, comparing them to the same of pristine coronene’s on TiO2 underlayer (e.g. 2.03 eV under optB86b-vdW) facilitate a scheme for estimating the relative modulation of band gap as a function of B doping. Akin to the manner described in 3.2, the computed relative reduction of band gaps (BGCD) for 2B12T-a, 2B12T-b, 2B14T-a and 2B14T-b systems are also listed in Table 2. The obtained results reveal an interesting trend: In strongly coupled heterostructures, the ionic interaction between B doped CDs and TiO2 significantly modulated the electronic structures of the CDs and they displayed much larger CD band gaps (1.433 eV and 1.233 eV respectively for 2B12T-a and 2B14T-a) in comparison to the weakly coupled heterostructures (0.873 eV and 1.048 eV for 2B12T-b and 2B14T-b respectively). Consequently, 2B12T-b and 2B14T-b demonstrated significantly high 57.0% and 48.4% reduction of the CD band gap (in 17 ACS Paragon Plus Environment


comparison to pristine coronene - TiO2 heterostructures); whereas for 2B12T-a and 2B14T-a, the same was confined below 40%. To investigate this more closely, we computed PDOS (projected on 3d orbitals of Ti atoms, 2p orbitals of O atoms, 2p orbitals of C atoms and 2p orbitals of B atoms; and on p orbitals of individual B atoms, and individual O and C atoms bonded to them) (Figure S8 and S9, respectively of the SI) for all four heterostructures. They essentially reveal that the electronic levels near both CBM and VBM (of the strongly coupled cases) are dominated by the p states of the non-bonded B atom; consequently, the weakly coupled systems e.g. 2B12T-b and 2B14T-b in which both B atoms are unperturbed, were able to manifest greater modulation of the band gaps. The correlation between CD band gap and the B-O interaction is significant in the sense that it proactively allows to identify suitable heterostructures for specific target applications. In the current context, lowering of the bandgap is imperative to improve upon the optical absorption; hence it can be readily postulated that the weakly coupled (van der Waals heterostructures) of B doped CD and TiO2 would yield greater efficiency. Similar van der Waals heterostructures53-55, despite of their weak interlayer binding, has attracted phenomenal research interest in recent times owing to formidable optoelectronic properties and even ultrafast charge transfers in specific systems. We considered the B doped CD and TiO2 van der Waals heterostructures i.e. 2B12T-b and 2B14T-b (which showed maximum CD band gap reduction) and used their respective computed WF (listed in Table 2) to evaluate the absolute positions of their band-edges (following the methodology detailed in 3.1) in conjunction with the water redox potentials. The corresponding band diagrams are shown in Figure 5ab. In direct contrast to the trend observed for pristine CD-TiO2 heterostructures (which demonstrated type-II band alignment), both B doped CD-TiO2 van der Waals heterostructure models demonstrated straddling i.e. type-I band alignment. The above necessarily implies that, in these systems, upon photoexcitation, electrons jump from valence band to conduction band in both CD and TiO2 sides; however, 18 ACS Paragon Plus Environment

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since CBM of the CDs lie lower than those of TiO2, electrons (including those photogenerated in the CDs) accumulate on the CD side. Thus, these B doped CDs-TiO2 van der Waals heterostructures are best suited for the so called ‘electron collector’ configuration. A few points should be noted in this context: a) As evident from Figure 5, in these systems, the CBM of CD lies above Ered H + /H2; which indicates hydrogen evolution reaction can take place usually in these

Figure 5. Band diagrams of a) 2B12T-b b) 2B14T-b; the blue highlighted regions 𝐨𝐱 represent the energy window 𝐄𝐫𝐞𝐝 𝐇 + /𝐇𝟐 - 𝐄𝐎𝟐/𝐇𝟐𝐎. systems. b) Considering the energy gap between the CBMs of TiO2 and CDs in these systems, and taking the subsequent Fermi level realignment into account, it would require > 168% (2B12T-b) – 182% (2B14T-b) increase of their respective band gaps to lift the CD CBMs above 19 ACS Paragon Plus Environment


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TiO2 CBMs. As mentioned in 3.2 for isolated pristine and B doped coronene, the band gap underestimation under GGA (optB86b-vdW) are much lower than this limit. Thus, it can be argued that the type-I nature of the heterojunctions portrayed by Figure 5 are adequately accurate and will persist even if more extensive computational protocols are considered. Notwithstanding, there is another caveat that in these cases, that VBM of TiO2 lies lower than that of CDs; i.e. hole migration from TiO2 side to CD side is energetically permitted. Although in typical experimental condition, these photoinduced/ accumulated holes could be quenched using the strategy described in 3.1, it might imply greater recombination loss in comparison to a typical type-II band alignment. Nevertheless, it should also be noted that, in addition to the band-edge alignment, interfacial electric field and charge distribution also play great roles in dictating the carrier migration; thus, their effects in context of the current systems were also thoroughly explored. Charge transfer and separation at the B doped CD – TiO2 interfaces was investigated by the means of ground state charge density difference isosurfaces (plotted for isovalue of 0.03) and planar-averaged charge density differences (plotted along Z axis (off-planar direction)), and the corresponding plots for the 2B12T-b and 2B14T-b systems are shown in Figure 6ab. Charge density difference isosurfaces, as shown in Figure 6, readily supports the inferences obtained in 3.3 i.e. occurrence of no significant ionic bonding between CDs and TiO2 in these B doped CD – TiO2 van der Waals heterostructures. Even then, the planar-averaged charge density difference plots illustrate some amount of electron transfer from B doped CDs towards top-most oxygen layer of TiO2 for both cases. The net effect of the above is manifested as an internal electric field at the interface, from the direction of B doped CDs towards TiO2. This is of particularly importance; as this field, on one hand, promotes migration of photogenerated electrons from TiO2’s conduction band to CD’s conduction band ( ―

e―

+ ; i.e. in the direction

opposite to the internal electric field) but, on the other hand, restricts migration of 20 ACS Paragon Plus Environment

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photogenerated holes from TiO2’s valence band to CD’s valence band ( ―

h+

+ ). This internal

electric field assisted selective carrier migration across the heterojunction is analogous to the effects demonstrated in g-C3N4 - TiO2/ SnS2/ CdS heterostructures, where it promotes the

Figure 6. Charge density difference isosurfaces and planar-averaged charge density differences of a) 2B12T-b b) 2B14T-b electron accumulation to the VBM (for z-scheme systems56-57) or CBM58 of g-C3N4; and in blue phosphorene/ g-GaN van der Waals heterostructures59, where it assists the transfer of photogenerated electrons to the CBM of the blue phosphorene as well as the transfer of photogenerated holes to the VBM of the g-GaN. Consequently, it can be postulated that, in the present systems, this internal electric field also acts like a similar ‘charge valve’ and may potentially play a supportive role in mitigating the carrier recombination loss. 21 ACS Paragon Plus Environment


For the sake of completeness, H2O molecule adsorption on coronene–TiO2 and 2B12Tb (which demonstrated the lowest CD band gap) heterostructures was analysed in detail (D1 of the SI). Obtained results indicate B dopants can bind H2O molecules (adsorption energy: -0.232 eV) by a noncovalent interaction with small portion of charge transfer and can reduce the O-H bond dissociation energy by significant amount (from 5.310 eV in coronene–TiO2 to 3.678 eV in 2B12T-b). Thus, we conclude that B doping might indeed emerge as a highly beneficial strategy for not only reducing the CD band gaps, but also for engineering catalytically active sites into them. 4. CONCLUSIONS In the present work, first principles investigations were carried out to investigate a series of pristine and B doped CD - TiO2 heterostructures with the goal of improving their optical absorption range and photocatalytic water splitting performances. Heterostructures of pristine, CD models and 3 layered, stoichiometric, oxygen terminated anatase TiO2 (1 0 1) slabs were found to demonstrate a typical staggered i.e. type-II heterojunctions with CBMs and VBMs of CDs lying higher than those of TiO2. While these heterostructures showed favourable bandedge alignments for spontaneous, spatial carrier separation and hydrogen evolution; their band gaps were found to be too high for full spectrum optical absorption. Doping of two B atoms substitutionally in isolated, H terminated coronene reduced its band gap substantially by ~ 70 % for the most favourable doping configurations. Hence, electronic properties and photocatalytic responses of B doped CD-TiO2 heterostructures were probed in detail. The B doped CD – TiO2 heterostructures manifested lowest CD band gaps with ~48 – 57 % CD band gap reduction in comparison to their pristine CD- TiO2 counterpart. These systems demonstrated type-I band alignment with CBM and VBM of TiO2 straddling those of CDs, with CDs themselves having favourable band alignment for H evolution. In these heterostructures, even though both electron and hole migrations from TiO2 side to CD side 22 ACS Paragon Plus Environment

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were energetically permitted, the interfacial electric field was found to selectively promote electron migration and restrict hole migration by acting as a ‘charge valve’ and thereby countering recombination loss. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Optimized geometries and TDOS of TiO2 slabs. Optimized geometries/ parameters/ DOS of pristine CD-TiO2/ graphite heterostructures. Scheme of B doping sites in isolated coronene and their DOS (relevant configurations only). Parameters/ PDOS of all B doped CD-TiO2 heterostructures. H2O adsorption on pristine and B doped CD heterostructures. Optimized atomic positions of the relevant structures. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.O.) ORCID Dipayan Sen: 0000-0002-9864-0692 Piotr Błoński: 0000-0002-7072-232X Michal Otyepka: 0000-0002-1066-5677 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministry of Education, Youth and Sports of the Czech Republic (LO1305, CZ.1.05/2.1.00/19.0377, the Research Infrastructure NanoEnviCz: project No. LM2015073), the Operational Programme Research, Development 23 ACS Paragon Plus Environment


and

Education

–

European

Regional

Development

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Fund,

Project

No.

CZ.02.1.01/0.0/0.0/16_019/0000754) and the ERC (683024 from the European Union's Horizon 2020). REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (2) Tang, J.; Durrant, J. R.; Klug, D. R. Mechanism of Photocatalytic Water Splitting in TiO2. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for Four-Hole Chemistry. J. Am. Chem. Soc. 2008, 130, 13885-13891. (3) Yang, J.-S.; Wu, J.-J. Toward Eco-Friendly and Highly Efficient Solar Water Splitting Using In2S3/Anatase/Rutile TiO2 Dual-Staggered-Heterojunction Nanodendrite Array Photoanode. ACS Appl. Mater. Interfaces 2018, 10, 3714-3722. (4) Regonini, D.; Teloeken, A. C.; Alves, A. K.; Berutti, F. A.; Gajda-Schrantz, K.; Bergmann, C. P.; Graule, T.; Clemens, F. Electrospun TiO2 Fiber Composite Photoelectrodes for Water Splitting. ACS Appl. Mater. Interfaces 2013, 5, 11747-11755. (5) Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y. Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 3026-3033. (6) Zhang, Z.; Yang, X.; Hedhili, M. N.; Ahmed, E.; Shi, L.; Wang, P. Microwave-Assisted Self-Doping of TiO2 Photonic Crystals for Efficient Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2014, 6, 691-696. (7) Hu, Y. H. A Highly Efficient Photocatalyst—Hydrogenated Black TiO2 for the Photocatalytic Splitting of Water. Angew. Chem., Int. Ed. 2012, 51, 12410-12412. 24 ACS Paragon Plus Environment

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Figure 1: Band diagrams of a) 1 ring (benzene) b) 4 ring (pyrene) c) 6 ring (coronene) – TiO2 heterostructures; the blue highlighted regions represent the energy window EredH+/H2 - EoxO2/H2O 82x153mm (300 x 300 DPI)

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Figure 2. Optimized geometries of a) 2B12T-a b) 2B12T-b c) 2B14T-a and d) 2B14T-b: side and top views. 82x106mm (300 x 300 DPI)


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Figure 3: ELF isosurfaces of a) 2B12T-a b) 2B12T-b c) 2B14T-a and d) 2B14T-b (plotted for isovalue of 0.75). 82x82mm (300 x 300 DPI)



Figure 4. TDOS and PDOS (projected on TiO2 and CD atoms) for a) 2B12T-a b) 2B12T-b c) 2B14T-a and d) 2B14T-b. The Fermi level is set at 0 eV. 82x117mm (300 x 300 DPI)


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Figure 5. Band diagrams of a) 2B12T-b b) 2B14T-b; the blue highlighted regions represent the energy window EredH+/H2 - EoxO2/H2O 82x140mm (300 x 300 DPI)



Figure 6. Charge density difference isosurfaces and planar-averaged charge density differences of a) 2B12Tb b) 2B14T-b 82x130mm (300 x 300 DPI)


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TOC Graphic 82x44mm (300 x 300 DPI)


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