The photoexcited charge transport and accumulation in anatase TiO2

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The photoexcited charge transport and accumulation in anatase TiO2 Li Guan, and Xiaobo Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00944 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Applied Energy Materials

The photoexcited charge transport and accumulation in anatase TiO2

Li Guan,1 Xiaobo Chen2,* 1. Department of Physics, Hebei University, Hebei, the People’s Republic of China. 2. Department of Chemistry, University of Missouri – Kansas City, Missouri, 64110, the United States of America.

Corresponding Author: [email protected]

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Abstract Photogenerated charge separation and redox reactions on photoexcited semiconductors are of great significance in many photon excited systems, such as photocatalytic water splitting systems, photocatalytic pollutant removal systems, photovoltaic devices, etc. It is highly desirable if we know the charge separation directions inside the semiconductor crystals, the preferred crystal facets the charge accumulate after separation, and the preferred facets where redox reactions occur, so we can grow the semiconductor crystals accordingly with the desired facets exposed on the surface to have large numbers of the photoaccumulated electrons and holes on the facets in order to realize a high photocatalytic efficiency for both oxidation and reduction on the surface or to achieve a large photovoltaic efficiency. Here, we attempt to convert the information from the electronic band structure calculations in the k space into the real space in bulk crystals and combine the surface electronic band structures of various facets, in order to gain a comprehensive understanding of the charge transport inside the crystal and the preferred accumulations of electrons and holes on the surface facets, along with comparison with experimental observations. Anatase titanium dioxide (TiO2) is tried as an example with this strategy to elucidate such an effort.


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Semiconductors have been playing important roles in many optical, photoelectrical, photovoltaic, and photocatalytic applications. For example, titanium dioxide (TiO2) has been widely investigated as a promising photocatalyst for applications in photocatalytic water splitting, photocatalytic hydrogen generation, photocatalytic carbon dioxide (CO2) reduction to produce fuels, photocatalytic nitrogen (N2) reduction to produce ammonia, and photocatalytic decomposition of various organic pollutants to protect the environement.1-14 Generally, in the photocatalytic process, semiconductors act most frequently as both light absorbers and catalysts, or photocatalysts.1-8 In the first step, semiconductors absorb light with energy equal to or larger than their bandgaps to excite electrons from the valence band (VB) to the conduction band (CB) and leave holes in the VB, forming electron-hole pairs. Some of those electron-hole pairs overcome the exciton energy, get separated, and migrate to the surface. The electrons and holes reaching the surface can undergo chemical reduction and/or oxidation reactions with the chemicals adjacent. This is the simplest scenario for photocatalysis and the overall photocatalytic efficiency depends on the efficiency of each of those steps. In some cases, co-catalysts are used with semiconductors to help surface reactions and sometimes to enhance the light absorbing capability as well. In these cases, the photocatalytic processes are more complicated. Regardless of the specific photocatalytic processes, after light absorption, the charge transport in the bulk and the accumulation on the surface of semiconductors play critical roles, especially when the surface facets of the semiconductors are selectively constructed.1-14 Recent studies have shown that semiconductor photocatalysts with different surface facets possess distinct surface charge accumulation and photocatalytic activity properties where selective surface reduction or oxidation reactions are observed on different facets.15-59 Those exciting findings inspire us to speculate how



those phenomena can be connected with the structural and electronic properties of the semiconductor photocatalysts. In this work, we attempt to combine the bulk and surface electronic properties, in the hope to gain some understanding of the charge transport inside the crystals and the charge accumulations on the surface of the semiconductor photocatalysts. At this moment, we believe that our results in this work are somehow immature, far from satisfactory, and may be misleading or even wrong, partially due to our many technical and knowledge limitations and the only view angle of the charge separation and accumulation in this work. Meanwhile, we also realize that we may overlook some important references due to our limited knowledge in this field. Regardless of these shortcomings, we would like to share our thoughts and efforts, with the expectation to inspire more insightful studies to further improve our understanding in this area and promote the discovery and design of semiconductor photocatalysts with better activities. The links are frequently missing between the calculated electronic band structures in the k space and the charge separation and accumulation properties in crystals in real space, although both have been well investigated. In the following, we attempt to illustrate the logics for the possible predictions of charge separation and accumulation based first on the calculation of bulk electronic band structures. We know that in solid-state physics, the electronic band structure of a solid describes the range of energies that an electron within the solid may have (called energy bands) and may not have (called band gaps).60-65 Band theory derives these bands and band gaps by examining the allowed quantum mechanical wave functions for an electron in a large, periodic lattice of atoms or molecules. The single-electron Schrödinger equation is solved for an electron in a lattice-periodic potential, giving Bloch waves as solutions: nk(r) = eikrnk(r) 4 ACS Paragon Plus Environment

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where k is called the wavevector.60-65 For each k value, there are multiple solutions to the Schrödinger equation labelled by n, the band index, which numbers the energy bands. Each of these energy levels evolves smoothly with changes in k, forming a smooth band of states. For each band the function En(k) is the dispersion relation for electrons in that band. The wavevector takes on any value inside the Brillouin zone, a polyhedron in k space that is related to the crystal's lattice. Wavevectors outside the Brillouin zone correspond to states that are physically identical to those states within the Brillouin zone. Special high symmetry points/lines in the Brillouin zone are assigned labels like Γ, Δ, Λ, Σ (see Figure 1).

Figure 1. Brillouin zone of a face-centered cubic lattice showing labels for special symmetry points. There are two common definitions of wave vector, which normally differ by a factor of 2π in their magnitudes. One definition is preferred in physics and related fields as stated in the above paragraph, while the other definition is preferred in crystallography and related fields.60-64 In this article, it will be called the "physics definition" and the "crystallography definition", respectively. In the physics definition, a perfect one-dimensional traveling wave follows the equation: ψ(x , t ) = A cos(kx − ωt + φ ), where: x is position, t is time, ψ is the disturbance describing the wave, A is the amplitude of the wave, φ is a "phase offset", ω is the temporal angular frequency, k is the



spatial angular frequency (wavenumber). This wave travels in the +x direction with speed (more specifically, phase velocity) ω/k. In crystallography, the same wave is described as: ψ(x, t) = Acos(2π(kx − νt ) + φ). The main difference is that the frequency ν instead of angular frequency ω is used, and they are related by 2πν = ω. Thus, k is defined in a slightly different way in the physics and crystallography descriptions. In the crystallography description, k = | k | = 1 / λ, while in the physics definition, k = | k | = 2π/λ. Therefore, the wavevector points in the band theory in physics are the wavevector points in crystallography which correspond to the crystal planes in the crystal. Indeed, a Brillouin zone can be defined as a Wigner-Seitz cell in the reciprocal lattice.60 As the k in the crystallography can be converted directly to the directions and crystal planes in the crystal in real space, the k in the physics band structure can be connected to the directions and crystal planes in the crystal in real space as well. Therefore, the electronic band structure which is energy plotted against the wavevector actually indicates the plot of energy against the crystal plane in real space. Since the electronic band structure can provide information on the valence band maximum, conduction band minimum, the electron position and the hole position, then, in principle, we can predict which crystal plane/facet is the preferred plane/facet for electron or hole accumulation to conduct reduction or oxidation reactions. Specifically, the charge separation direction and accumulation in the bulk electronic band structure in the k space directly tells the charge separation direction and accumulation properties in the bulk crystal in the real space. For example, if electrons are preferred to accumulate on the A-point (111) in the k space in the band structure, then they are expected to move along the direction and accumulate in the (111) facet in the crystal in the real space; while if the electrons are preferred to accumulate on the G-point (000) in the k space, then they are expected to stay and accumulate in the center of the crystal in the real space.


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In this work, we calculated the bulk electronic band structures of bulk anatase TiO2 as follows. The exchange-correlation effects were treated by General Gradient Approximate (GGA), and a plane-wave expansion of the wave functions and ultrasoft pseudopotential formalism were selected. The Ti 3s23p63d24s2 electrons, O 2s22p4 electrons of cubic TiO2 were treated as the valence-electron configurations. A plane-wave cutoff energy of 380 eV and a 7×7×3 MonkhorstPack k-point grid for integration over the Brillouin zone were applied in calculations. The convergence in energy, maximum force, maximum stress, and maximum displacement tolerances were set as 5.0×10-6 eV/atom, 0.01 eV/Å, 0.02 GPa, and 0.0005 Å, respectively. (0.5,0.5,0) (0.5,0.5,0.5)

(0,0,0.5)

(0,0,0.5) (0,0,0)

(0,0.5,0) (0,0.5,0.5)

(0.5,0,0.5) (0.5,0,0)

(0,0,0)

(0,0,0.5)

3

4

Energy / eV

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


5

3

6

2

1

2

1

4

2

0

-1

Z

A M

1

3 5

6

R X

G Z

G

B C

Z

Figure 2. Calculated electronic structure of anatase TiO2 (Castep, GGA + PBE), the arrows and numbers in the graphs show the preferred electron and hole accumulation directions. Figure 2 shows the electronic structure of bulk anatase TiO2. For the conduction band, as pointed out by the arrows, the energy decreases in the order: point 6 - M (101), point 5 –R (011) 3.190 eV > point 4 – (112) – 2.909 eV > point 3 (110) – 2.897 eV > point 2- Z (001) – 2.191 eV > point 1 – G (000) – 2.121 eV; for the valence band, the energy increases in the order: point 6 – 7 ACS Paragon Plus Environment


(100) – (-0.414 eV) < point 5 – Z (001) – (-0.242 eV) < point 4- (445) – (-0.179 eV) < point 3 – G (000) – (-0.107 eV) < point 2 – M (110), point 1 – (110) – (0.000 eV). Thus, the excited electrons are thermodynamically preferably located in this order: (000) > (001) > (110) > (112) > (101), and the excited holes are preferably located in this order: (110) > (000) > (445) > (001) > (100). Therefore, the probability of the preferred facet for reduction reaction follows the order: (000) > (001) > (110) > (112) > (101) > other facets, and the probability of the preferred facet for oxidation reaction follows the order: (110) > (445) > (001) > (100) > other facets (including (101) – (-0.644 eV)), if those bulk planes are exposed on the surface without any changes of their electronic properties caused by surface relaxation. Experimentally, it has been found by Ohno et al. that during photocatalysis, the oxidation site is mainly on the (001) facet as evidenced by the dominant deposition of PbO2 nanoparticles and the reduction site is mainly on the (101) facet by the dominant deposition of Pt nanoparticles.19 It has been suggested by Zheng et al. that when (101) and (001) faceted are exposed on the surface and irradiated by UV light, the photogenerated holes will transfer to {001} facets, while the electrons will migrate to (101) facets, so that Ag nanoparticles are mainly deposited on the (101) facet of decahedral anatase,27 consistent with selective deposition of Pt nanoparticles on the (101) facet reported by Ohno et al.19,21 Apparently, there is some agreement and disagreement between the conclusions from the bulk electronic band structure and experimental observations of anatase TiO2. For the oxidation, it seemed to be consistent in that for anatase ending with (001) and (101) facets, oxidation prefers to occur on the (001) facet instead of the (101) facet. However, for the reduction, the calculation showed that for anatase ending with (001) and (101) facet, reduction prefers to occur on the (001) facet instead of the (101) facet, opposite to the above experimental observations. These 8 ACS Paragon Plus Environment

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discrepancies between the bulk electronic band structure calculation and the experimental observations are likely caused by many factors, including the limitations of the band-theory calculation and the experimental observation themselves. Band theory is only an approximation to the quantum state of a solid that consists of many identical atoms or molecules bonded together. The assumptions necessary for band theory to be valid are as follows. a). Electronic band theory is most suitable for an infinite-size system. For the electronic bands to be continuous, the material under consideration must consist of a large number of atoms, although with modifications, the concept of band structure can also be extended to systems which are only "large" along some dimensions, such as two-dimensional electron systems. For systems which are small along every dimension (e.g., a small molecule or a quantum dot), there is likely no continuous band structure. b). Band theory is valid for homogeneous system. Band structure is an intrinsic property of a homogeneous material. The bulk band structure is disrupted near surfaces, junctions, and other inhomogeneities. Local small-scale disruptions (e.g., surface states or dopant states inside the band gap) and charge imbalances have electrostatic effects that involve the physics of electrons passing through surfaces and/or near interfaces, which requires a rudimentary model of electron-electron interactions in a band structure picture. c). Band theory assumes non-interactivity. The band structure describes "single electron states", which assumes that the electrons travel in a static potential without dynamically interacting with lattice vibrations, other electrons, photons, etc. However, in practice, this assumption is hardly satisfied. Therefore, the electronic band structures may not provide useful information.60-65 In the preceding calculation, the effect of the surface are not taken into account. In order to consider the surface effect and overcome some of the aforementioned shortcomings of the bulk



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electronic property calculation, we further calculated the surface energy of anatase TiO2. The surface energy of TiO2 was defined as



1  slab 1  E  nTi ( Ti  Tibulk )  nO ( O  Ogas2 )   2A  2 

where E slab was the total energy of a relaxed slab with surface area A. of Ti and O atoms in the slab, respectively.

nTi and nO

(1)

were the numbers

Ti and  were the chemical potential of Ti and O O

atoms, respectively. Moreover, the sum of the chemical potentials satisfied the equilibrium condition:

2 H TiO  Ti  2O f

, where

bulk 2 H TiO  ETiO  ETibulk  EOgas2 f 2

was the formation energy of

equilibrium TiO2 bulk. Accordingly, Eq. (1) became the following form:



1  slab 1  bulk gas E  nTi ETiO  ( n  2 n )   ( n  2 n )  O Ti O O Ti O   2 2 2A  2

(2)

There were additional boundary conditions for the chemical potentials:

Ti  0 and O  0

H f  0 H f  Ti  0 1 H f  O  0 2

Table 1. Comparison of the calculated results based on the surface energy and bulk band structure (the numbers in parentheses).


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Surface energy /J/m2

Work function / eV

Affinity energy / eV

Band gap / eV

CBE / eV

VBE / eV

100

0.506

6.222 (4.097)

5.314 (2.220)

1.816 (3.754)

5.314 (2.220)

7.130 (5.970)

101

0.373

6.218 (4.279)

5.400 (2.353)

1.655 (3.851)

5.400 (2.353)

7.055 (6.204)

110

0.863

5.990 (4.112)

5.420 (2.663)

1.140 (2.897)

5.420 (2.663)

6.560 (5.560)

001

0.911

5.628 (4.586)

4.710 (3.369)

1.740 (2.433)

4.710 (3.369)

6.450 (5.802)

111

1.558

4.8125 (3.935)

4.085 (2.087)

1.455 (3.696)

4.085 (2.087)

5.540 (5.783)

Surface

The electron affinity of various surfaces of anatase TiO2 follows this order: (110) > (101) > (100) > (001) > (111). The order of electron affinity should correspond to the order of the reduction facet. That is (110) > (101) > (100) > (001) > (111). The work functions of various surfaces of anatase TiO2 follows this order: (100) > (101) > (110) > (001) > (111). The reversed order of work function should correspond to the order of the oxidation facet. That is: (111) > (001) > (110) > (101) > (100). Therefore, the most favorable facet for reduction is (110) and for oxidation is (111) based on the above surface calculation. This is not quite consistent with the current experimental observations where (001) is the oxidation facet and (101) is the reduction facet.19,21,27 The discrepancy may be understood as follows. In the surface calculation, the effects of the solvents are not taken into account, but the solvent are believed to have some influence on the surface electronic properties in the experimental observations. Meanwhile, the (111) and (110) facets are not well exposed on the surface in some experiments. Nevertheless, for anatase ending with (001) and (101) facets, reduction prefers to occur on the (101) facet and oxidation prefers to occur on 11 ACS Paragon Plus Environment


the (001) facet, which matches well with the experimental observations.19,21,27 Meanwhile, the results of the surface electronic structure are somehow different from to the results based on the bulk electronic structure calculation. Based on the bulk calculation, the reduction facet follows the order: (001) > (110) > (112) > (011) > (101) > other facets, and the oxidation facet follows the order: (110) > (445) > (001) > (100) > other facets (including (101) facet). The bandgaps of various surfaces of anatase TiO2 follows this order: (100) > (001) > (101) > (111) > (110), compared to the order based on the bulk calculation: (101) > (100) > (111) > (110) > (001). These comparisons are listed in Table 1 and Figure 3.

A

0

B

0 Electron affinity

Work function

based on bulk

2 based on bulk

4 Surface band bending

6

Energy / eV

Energy / eV

2

4

Surface band bending

based on surface

6

based on surface

8

(100)

(101)

(110)

(001)

8

(111)

(100)

Surface

C

(101)

(110)

(001)

(111)

Surface

D

5

0

Band gap based on bulk

4

2

Energy / eV

Energy / eV

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

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3

2

1

0

CBE

4 surface band bending

6 VBE

based on surface

(100)

(101)

(110)

(001)

(100) (101) (110) (001) (111)

8

(111)

Surface

Bulk

Position

Surface


Surface

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Figure 3. The electronic property changes of anatase TiO2 based on the bulk electronic structure calculation and surface energy calculation: (A) work function, (B) electron affinity and (C) band gap. (D) The illustration of the surface band bending. The work function and electron affinity energies for various facets based on the surface calculation are higher than those based on the bulk calculation, as can be seen from Figure 3A and Figure 3B, respectively, while the bandgap energies based on the surface calculation are smaller than those based on the bulk calculation as shown in Figure 3C. Specifically, as shown in Figure 3A, the work function energies based on the surface calculation for (101) and (110) increases much larger than other planes, while that for (111) and (001) increases much smaller. This suggests that when part of the surface effects is taken into account, (101) and (100) planes become much favorable for stronger oxidizing planes, while (111) and (001) becomes a much weaker oxidizing plane and the hole accumulation facet if exposed on the surface. Overall, the favored oxidizing or hole accumulating plane becomes the (111) plane based on the surface calculation instead of the (110) plane based on the bulk calculation. However, if the (111) plane is not exposed on the surface, then the (001) plane becomes the favored oxidizing plane if exposed on the surface. On the other hand, as shown in Figure 3B, the increased electron affinity energies based on the surface calculation are larger for the (101) and (100) planes than that for the (001) and (111) planes, and favored reducing or electron accumulating plane changes from (001) based on the bulk calculation to the (110) or (101) plane based on the surface calculation. Therefore, if the (110) is not exposed on the surface, then the (101) plane will become the favored plane for the electron accumulation. In addition, as shown in Figure 3C, the bandgap decreases much larger on the (111) and (101) planes than the (001) plane. Apparent, the surface relaxation largely alters the work function, electron affinity and bandgap of various planes from the bulk view to the surface exposure. Figure



3D illustrates the electronic band bending from the bulk to different surface facets. The CBEs for various surfaces bend towards higher energies compared to those of the bulk, while the VBEs for most surfaces bend towards higher energies except the (111) surface bends towards a lower energy than the bulk. Overall, the (110) and (101) surface have the largest CBE energies, while the (111) and (001) surface have the smallest VBE energies. It indicates that after light excitation, electrons are energetically favorable to move to the (110) or (101) surface, while the holes prefer to stay in the (001) plane unless the (111) plane is exposed on the surface. When compared to the experimental observations on the photoexcited charge accumulations on the surface of anatase TiO2, the above calculated results based on the surface energy had a better consistency than those based on the bulk calculation, and echoed with previous calculation results as well. For example, in the work by Zheng et al., a DFT calculation was conducted on the electronic structures of the (001) and (101) facets, the valence band of (001) extends to somewhat higher energies with respect to (101), while the conduction band minimum of these two faces locate at a similar position.27 Based on this calculation, oxidation should be on the (001) facet, while reduction is distributed almost equally on the (101) facet.27 Liu et al. provided a very straightforward proof of the facet-induced spontaneous separation of photoexcited electrons and holes by comparing the photocatalytic H2 productions of a group of bare anatase nanocrystals with tunable percentages of (001) facet area from 0 to 51.2% (the remaining surface is (101)).26 The highest activity is achieved for the sample with 14.9% (001) area as a result of the spatial separation of charge carriers on (001) and (101) facets. In contrast, the crystals with 100% (101) facets show negligible activity due to the appearance of photoexcited electrons and holes on the same surface.26 Ye et al. demonstrated effective charge carrier transfer between (001) and (101) facets but not between (001) and (010) facets.40 Roy et al. found that both the high-energy (001)


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oxidative and low-energy (101) reductive facets in an optimum ratio is necessary to reduce the charge recombination.30 Even though during the surface electronic structure calculation, the effects of solvents were not taken into account, the energy relaxation process of the surface did incorporate the surface lattice parameter changes compared to the bulk, which may explain the decent consistency between the surface calculation and the experimental observations. We would expect that a better consistency would be achieved if the effects of solvents were considered during the surface electronic structure calculation and the facets predicted in the calculation were fully exposed to the surface in the experiments.

Figure 4. Illustrations of the photoexcited electron and hole accumulation on various facets based on the bulk (a, c, d) and surface (e) calculations as well as experimental observations (b, e) of anatase TiO2. 15 ACS Paragon Plus Environment


Figure 4 shows a few scenarios for the comparisons of the bulk/surface calculations with experimental observations. Figure 4(a) illustrates that if all the related facets in the bulk calculation are exposed on the surface of the anatase TiO2, the photoexcited charges distribute mainly on the facets in the order of (110) > (445) > (001) for holes and on the facets in the order of (001) > (110) > (112) for electrons, while the experimental observation indicated that (001) is favorable for holes and (101) is for electrons, as shown in Figure 4(b), although the surface calculation shows that the (111) facet is the most favored oxidation facet and the (110) facet is the most favored reduction facet. Based on the bulk calculation, Figure 4(c) displays the favored facets ending on the surface for charge accumulations, the (001) facet is favored for electrons and the (110) facet is for holes, and if the surface is ended with (001) and (101) facets, (001) is favored for both electron and holes (expecting much recombination) as shown in Figure 4(d); while Figure 4(e) shows in experimental observation, if the surface is ended with (001) and (101) facets, the (001) facet is favored for holes and the (101) facet is favored for electrons (expecting good charge separation), consistent with the results based on the surface calculation. From the above results, it seemed that the results from the bulk calculation had a big discrepancy with the experimental observation, while the surface calculation had a better consistency. Although the discrepancy may be attributed to the many limitations based on the bulk calculation, there may exist many limitations in the experimental observation as well. For example, there are limitations in crystal growth. The facets favorable for charge accumulation from calculation may not expose on the surface in the experiment due to their high surface energies which are not favored during crystal growth, and therefore, those facets are not exposed on the surface and charge accumulations on them are not observed experimentally. In practical synthesis, 16 ACS Paragon Plus Environment

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the surface of crystals normally has a different bonding environment from the bulk, the facets exposed on the surface in experiment have different structural properties from those in the bulk or surface calculation. Meanwhile, in experiments, the facets exposed on the surface may face various redox environments with different redox molecules for reactions, therefore, the actual facets may possess different energies from those calculated and thus different charge accumulation behavior.

Figure 5. Illustrations of two scenarios for charge separation in the bulk based on the bulk electronic structure calculation and final accumulation on the surface based on the surface energy calculation of anatase TiO2. On the other hand, the discrepancy between the bulk/surface calculations with the experimental observation may also indeed suggest that the photoexcited charges may follow the results from the bulk calculation inside the particle, but change their directions near the surface and eventually end with different facets on the surface as expected by the surface energy calculation or the experimental observation. Figure 5 shows two possible scenarios for this speculations. For example, for anatase ending with the (001), (101), (100), (110), (111), (112) and (445) facets as shown in scenario (1), the photoexcited electrons may move towards the (001),



(110) and (112) facets, but make turn before they reach the surface and eventually ends on the (101) facet instead upon the favorable energy consideration on the surface; for anatase ending with (001) and (101) facets, both electrons and holes may move towards the (001) facet, however, before they reach the surface, the electrons change direction and eventually accumulate on the (101) facet due to the favorable energy consideration on the surface, as shown in scenario (2). However these speculation need experimental measurements to verify in future studies. In summary, in this study, we have attempted to combine the bulk electronic structure calculation with surface electronic structure calculation along with experimental observations to get some insights on the charge transport and accumulation properties on semiconductor photocatalysts using anatase TiO2 crystal as an example.66 Interestingly, the findings suggest that photoexcited charges may first transport to the planes near the surface following the order of the bulk electronic structures based on the thermodynamic energy consideration, while the alterations of the energy of different planes on the surface may change the direction of the charge transport near the surface and the final charge accumulation surface facets. Therefore, these findings may provide some insightful understanding on the charge transport and accumulation properties along with the related photocatalytic activity of semiconductor photocatalysts. Although in this work, we only demonstrate the envision of the charge transport inside and charge accumulation on the surface of TiO2 crystal, and the surface electronic band bending, by combining the bulk and surface electronic property calculations, we would expect the same strategy shown in this work can be applied to other materials or semiconductor systems to gain insightful information.

Acknowledgements


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X. Chen appreciates the support from the U.S. National Science Foundation (DMR1609061), and the College of Arts and Sciences, University of Missouri  Kansas City. L. Guan thanks the support from the Hebei Provincial Young Top-notch Talent Support Program (BJRC2016).

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TOC

Photoexcited charges in anatase TiO2 crystals transport in the bulk and accumulate on the surface after making turns near the surface.

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The photoexcited charge transport and accumulation in anatase TiO2

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