An Unusual Charge Distribution on the Facet of SrTiO3 Nanocube

Mar 1, 2019 - However, how the photogenerated charges distribute in one facet of a single crystal is still unknown. In this work, we found that the di...
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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

An Unusual Charge Distribution on the Facet of SrTiO Nanocube under Light Irradiation 3

Linchao Mu, Bin Zeng, Xiaoping Tao, Yue Zhao, and Can Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00243 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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The Journal of Physical Chemistry Letters

An Unusual Charge Distribution on the Facet of SrTiO3 Nanocube under Light Irradiation Linchao Mua, Bin Zengab, Xiaoping Taoa, Yue Zhaoab, Can Li*a State Key Laboratory of Catalysis, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, the Collaborative Innovation Center of Chemistry for Energy Materials, Zhongshan Road 457, Dalian, 116023, China. b University of Chinese Academy of Sciences, China. a

ABSTRACT: Spatial charge separation has already been realized between different facets of a single crystal. However, how the photogenerated charges distribute in one facet of a single crystal is still unknown. In this work, we found that the distribution of photogenerated charges in SrTiO3 nanocube varies with the light intensities. Photogenerated holes tend to transfer to the edges and corners of the crystal under weak illumination, indicating that a spatial charge separation takes place in the same facet of the SrTiO3 nanocube. Based on this effect, oxidation and reduction cocatalysts can be respectively photodeposited on the edges and central areas of one facet. The separated dual-cocatalysts lead to a remarkable enhancement in photocatalytic overall water splitting. These findings reveal that the spatial charge separation can happen even in the same facet.

strong illumination. Photogenerated charges can be spatially separated in one facet under weak illumination. Therefore, control the photon density to change the distribution of cocatalysts and further spatially separate the dual-cocatalysts could be adopted as a practically useful strategy. Strontium titanate (SrTiO3) is an n-type semiconductor, with perovskite structure and high symmetric microstructure.12,13 SrTiO3 nanocubes were synthesized with isotropic facet exposed (Figure S1a in Supporting Information).14 The as-synthesized SrTiO3 nanocubes show similar size (ca. 250 nm), smooth facets and well crystallized cubic phase (JAPDS No. 35-0734; Figure S1b in Supporting Information). The UV-vis diffuse reflectance spectra shows absorption edge at ca. 390 nm and the band energy structure of SrTiO3 nanocube is capable for overall water splitting (Figure S2 in Supporting Information).15

TOC Graphic

Charge separation is the key process in photocatalysis.1-4 Exposed anisotropic facets on a single nanocrystal has been demonstrated to be effective for the separation of photogenerated electrons and holes.5 Based on this understanding, dual-cocatalysts separated between anisotropic facets has been successfully achieved on many semiconductors, such as TiO26, BiVO47, Cu2O8 and SrTiO39 etc. Furthermore, a direct imaging on a single nanocrystal by a spatially resolved surface photovoltage spectroscopy supports this conclusion, which revealed the built-in electric field in space charge region between anisotropic facets according to the band bending theory.10,11 But till now, spatial charge separation is still limited to anisotropic facets and the distribution of photogenerated charges on a same facet of a high symmetry crystal is still unknown. In this work, taking high symmetry SrTiO3 nanocube with isotropic facet as a model, we investigated the photodepositing process in order to understand the charge separation on the isotropic facet. Interestingly, we observed a distribution change of photogenerated holes strongly depend on light intensities. Under weak illumination, the holes are accumulated at the edges and corners of the nanocube, and tend to a uniform distribution under

Figure 1. The location change of Co3O4 at different light intensities. Density of photon: (a) 5×1016 s-1·cm-2; (b) 1017 s-1·cm-2; (c) 5×1017 s-1·cm-2; (d) 1018 s-1·cm-2. SEM of Pt at different light intensities. Density of photon: (e) 5×1016 s-1·cm-2; (f) 1018 s-1·cm-2.

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Figure 2. (a) Schematic description of the surface energy band bending under weak illumination and under strong illumination on SrTiO3 nanocube. (b) Spatial separation of dual-cocatalysts at the photon density of 5×1016 s-1·cm-2 on SrTiO3 nanocube. (c) Random distribution of dual-cocatalysts on SrTiO3 nanocube by impregnation method. A photodeposition method is used to determine the distribution of photogenerated charges.16 Figure 1a-1d show the images of SrTiO3 nanocubes after photooxidation deposition at different light intensities. Interestingly, under weak illumination (Figure 1a; Density of photon: 5×1016 s-1·cm-2), the metal oxide particles selectively deposited on the edges and corners, while the facet center almost keep unchanged after photooxidation deposition, indicating that photogenerated holes selectively located at the edges and corners under weak illumination. When the light intensity increased, the distribution of the deposited metal oxide changed, from edges to facet center (Figure 1b-1c; Density of photon: 1017 s-1·cm-2 and 5×1017 s-1·cm-2). The metal oxide deposited uniformly on the entire surface under strong illumination (Figure 1d; Density of photon: 1018 s-1·cm-2). It should be noted that the metal oxide deposited on the whole surface of SrTiO3 nanocube in Figure 1d, which means that the precursor species can adsorb on the surface uniformly. Therefore, the selective adsorption has negligible influence in Figure 1a. This phenomenon clearly shows the distribution change of deposited metal oxide, indicating a distribution change of photogenerated holes at different light intensities in the same facet. According to the XPS of the photodeposited metal oxide at different light intensities, all the metal oxide species can be ascribed to Co3O4, the peaks of Co3O4 matched well with the literature,5,9 staying as the same species even at different locations and at different light intensities (Figure S3a in Supporting Information). High resolution TEM further confirmed the Co3O4 species on the surface of SrTiO3 (Figure S3b in Supporting Information). The distribution of cocatalysts changed with light intensity occurs in photooxidation deposition process. In photoreduction deposition process (Figure 1e-1f), Platinum (Pt) can be detected in EDX analysis (Figure S4 in Supporting Information) and the distribution of Pt particles exhibit uniform on every facet even at different light intensities. The difference between photoreduction and photooxidation deposition at different light intensities maybe due to the band bending at the surface of an n-type SrTiO3 semiconductor has different influence on photogenerated holes and electrons. The in-situ photodeposition process can be described as follows: For photo-reduction deposition,

𝑀𝑛 + + 𝑛𝑒 ― →𝑀0

(1)

For photo-oxidation deposition, 𝑀′ 𝑚 + +

(𝑛 + 𝑚) 2

𝐻 𝑂 + 𝑛ℎ + →𝑀′𝑂𝑛 + 𝑚 + (𝑛 + 𝑚)𝐻 + 2

2

(2)

Where, M is the noble metal for reduction, such as Pt. M’ is the metal for oxidation, such as Co.

Figure 2a schematically descripts the surface energy band bending to understand the charge separation under weak illumination and random distribution under strong illumination. For SrTiO3 nanocube, n-type semiconductor, the surface band tends to bend upward in space charge region and has different bend extents at different surface positions.17-23 Here the potential difference in conduction band or valance band between the edge and the center of facet is denoted by Δϕ, which can be taken as the driving force for spatial charge separation between the edge and the central part of facet. When illumination is applied, the band bending decrease under illumination and corresponding Δϕ becomes less than Δϕ0. Under strong illumination, the surface band is almost flattened, and Δϕ is close to zero. Under weak illumination, the reduced driving force Δϕ is still considerable, resulting in the spatial separation of holes in one facet of SrTiO3 nanocube. Less charge is generated and hardly changes the intrinsic surface band bending. The surface band bending at the edge and corner is higher than that of the center of facet. So the local built-in electrical field, served as a driving force, transfers the photogenerated holes to the edges and corners. Hence, dual-cocatalysts could be separated under weak illumination. Under strong illumination, the driving force Δϕ is almost zero, resulting in the random distribution of holes on SrTiO3 nanocubes. The density of photogenerated charges is much higher than that under weak illumination. Vast generated charges transfer to the surface, and the transferred charges weaken the surface band bending. Therefore, the disappearance of the driving force leads to a uniform distribution of photogenerated charges.

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The Journal of Physical Chemistry Letters Based on the findings mentioned above, rational constructing dualcocatalysts spatially separated on the surface of SrTiO3 nanocubes with a two-step photodeposition method becomes possible. First, photooxidation deposited cocatalysts under weak illumination (edges and corners), and then, followed by a photoreduction deposition of Pt (facet center). Figure 2b shows the SEM images of the two-step photodeposition process on SrTiO3 nanocubes. Oxidation cocatalysts selectively deposited on the edges and reduction cocatalysts selectively deposited on the center of facet in one facet of SrTiO3 nanocube. As a comparison, cocatalysts are randomly distributed on the surface prepared by impregnation method (Figure 2c).

Figure 4. (a-b) The average photocatalytic overall water splitting performances on Pt-Co3O4/SrTiO3 nanocube by different methods. The average photocatalytic half reaction performances of (c) HER of Pt-Co3O4/SrTiO3 nanocube with CH3OH as sacrificial reagent and (d) OER of Pt-Co3O4/SrTiO3 nanocube with NaIO3 as sacrificial reagent.

Figure 3. (a) Average photocatalytic water splitting activities of different samples on Pt-Co3O4/SrTiO3 nanocube. (b) HER and (c) OER half reactions of different samples on Pt-Co3O4/SrTiO3 nanocube. Density of photon for photodeposition method: Entry 1: 5×1016 s-1·cm-2; Entry 2: 1017 s-1·cm-2; Entry 3: 5×1017 s-1·cm-2; Entry 4: 1018 s-1·cm-2; Entry I: 5×1016 s-1·cm-2; Entry II: 1018 s-1·cm2; Entry III: 5×1016 s-1·cm-2; Entry IV: 1018 s-1·cm-2; CH OH as hole 3 sacrificial reagent and NaIO3 as electron sacrificial reagent.

Figure 3a shows the activities of photocatalytic overall water splitting on SrTiO3 nanocubes with Pt-Co3O4 dual-cocatalysts photo-deposited at different light intensities. The sample with dualcocatalysts separated (Entry 1) exhibits a 5 times enhancement of activities than those (Entry 2-4) with cocatalysts deposited under strong illumination. The activities of Entry 2-4 are almost at the same level due to the random distribution of cocatalysts. As a comparison, the dual-cocatalysts are also prepared on SrTiO3 nanocubes by an impregnation method, usually this method can make sure the cocatalysts deposited on the entire facet uniformly (Entry 5). The samples with cocatalysts photodeposited under strong illumination display a similar performance compared with impregnated nanocubes (Entry 2 ≈ Entry 3 ≈ Entry 4 ≈ Entry 5). The remarkable difference between Entry 1 and others is possibly due to some cocatalysts deposited on the wrong sites, e.g., the reduction cocatalyst deposited on the hole-accumulating site or the oxidation cocatalyst deposited on the electron-accumulating site, so that the cocatalysts are not fully functioned. The apparent quantum efficiency also increased from 0.16% (Entry 2-5; Average) to 0.79% (Entry 1).

In order to obtain kinetically intrinsic activity of reduction and oxidation reactions, hydrogen/oxygen evolution reactions (HER/OER) in the presence of CH3OH/NaIO3 as sacrificial agents were investigated (Figure 3b-3c). When photogenerated charges efficiently consumed by sacrificial agents, the HER and OER activities of two samples increase significantly. OER increase more than HER, suggesting HER may be a rate-determining step for water splitting on SrTiO3-based photocatalysts, possibly because the conduction band of SrTiO3 is very close to proton reduction potential (Figure S2b in Supporting Information). But the valence band is positive for water oxidation.24 In Figure 3b-3c, the sample with dual-cocatalysts separated, deposited under weak illumination (Entry I and III), exhibits a ca. 2 times enhancement of both HER and OER activities than the sample deposited under strong illumination with dual-cocatalysts distribute uniformly (Entry I ≈ 1.8×Entry II; Entry III ≈ 2.1×Entry IV). The apparent quantum efficiency increased from 0.58% to 1.06% for HER and from 0.67% to 1.42% for OER. This enhancement can be attributed to the promoted charge separation efficiency by the separated dualcocatalysts. It should be noted that when reverse reaction was avoided with sacrificial agents (Figure 3b-3c), the activity differences can be only attributed to the promoted charge separation efficiency by the separated dual-cocatalysts. Therefore, the 5 times enhancement in Figure 3a can be mainly attributed to enhanced charge separation efficiency by separated dualcocatalysts. The spatially separated dual-cocatalysts promote the efficiency of charge separation and catalyze HER/OER at different positions. The time-curve occurs almost linearly and the y-intercept almost at zero from the beginning of photocatalytic reactions for both overall water splitting and half reactions (Figure 4). The huge photocatalytic performance differences between Figure 4a and Figure 4b clearly show the positive effect of the spatially separated dual-cocatalysts in designing photocatalysts in the same facet. The random distribution property of photogenerated holes under strong illumination does not be affected by the photodeposited time (Figure S5 in Supporting Information) and the charge separation in the same facet is the main reason for this phenomenon. There are more SEM images of the selective distribution of Pt and Co3O4 on SrTiO3 nanocube under weak illumination (Figure S6 in

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Supporting Information). Under strong illumination, the Pt and Co3O4 randomly distribute on the surface of SrTiO3 nanocube (Figure S7 in Supporting Information). The size distribution of Pt and Co3O4 also be analyzed on SrTiO3 nanocube (Figure S8 in Support Information). The photocatalytic activities on different kinds of SrTiO3 samples and photocatalytic properties of only one cocatalyst (Pt) photodeposited on SrTiO3 nanocube under weak illumination are also investigated (Figure S9 in Supporting Information). There are no photocatalytic activities when only one cocatalyst (Co3O4) photodeposited on SrTiO3 nanocube. In summary, the distribution of photogenerated charges on the surface of SrTiO3 nanocube is found to be strongly dependent on the light intensities. Under weak illumination, photogenerated holes are distributed at the edges and corners. Under strong illumination, photogenerated holes are distributed randomly on the entire exposed surfaces. Spatial charge separation can be achieved on isotropic facet under weak illumination, owing to the surface energy band bending effect. Rational construction of dualcocatalysts on the SrTiO3 nanocube exhibits a remarkable enhancement in photocatalytic activities, indicating that the dual cocatalysts are spatially separated and located on the right positions where adaptable for photogenerated charges. This work will be instructive for constructing more efficient solar energy conversion systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthesis of SrTiO3 nanocubes, photo-deposition of noble metals and/or metal oxides, impregnation method, test of photocatalytic water splitting performances, characterizations, chemical reagents, more supporting data.

AUTHOR INFORMATION Corresponding Author *[email protected] (Can Li).

ORCID Can Li: 0000-0002-9301-7850

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB1700000).

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