Visualizing the Nano Cocatalyst Aligned Electric Fields on Single

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Visualizing the nano cocatalyst aligned electric fields on single photocatalyst particles Jian Zhu, Shan Pang, Thomas Dittrich, Yuying Gao, Wei NIe, Junyan Cui, Ruotian Chen, Hongyu An, Fengtao Fan, and Can Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02799 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 2017

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Visualizing the nano cocatalyst aligned electric fields on single photocatalyst particles Jian Zhu †, Shan Pang†, Thomas Dittrich§, Yuying Gao†‡, Wei Nie†‡, Junyan Cui†‡, Ruotian Chen †‡

†

, Hongyu An†‡ , Fengtao Fan*,† & Can Li*,†

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, The

Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China ‡

§

University of Chinese Academy of Sciences, Beijing 100049, China Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-

Photovoltaik, Kekuléstr. 5, D- 12489 Berlin, Germany

ACS Paragon Plus Environment

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ABSTRACT

The cocatalysts or dual cocatalysts of photocatalysts are indispensable for high efficiency in artificial photosynthesis for solar fuel production. However, the reaction activity increased by cocatalysts can not be directly ascribed to the accelerated catalytic kinetics, since photogenerated charges are involved in the elementary steps of photocatalytic reactions. To date, diverging views about cocatalysts show that their exact role for photocatalysis is not well understood yet. Herein, we image directly the local separation of photogenerated charge carriers across single crystals of the BiVO4 photocatalyst loaded locally with nanoparticles of a MnOx single cocatalyst or with nanoparticles of a spatially separated MnOx and Pt dual cocatalyst. The deposition of the single cocatalyst resulted not only in a strong increase of the interfacial charge transfer but also, surprisingly, in a change of the direction of built-in electric fields beneath the uncovered surface of the photocatalyst. The additive electric fields caused a strong increase of local surface photovoltage signals (up to 80 times) and correlated with the increase of the photocatalytic performance. The local electric fields were further increased (up to 2.5 kV·cm-1) by a synergetic effect of the spatially separated dual cocatalysts. The results reveal that, cocatalyst has a conclusive effect on charge separation in photocatalyst particle by aligning the vectors of built-in electric fields in the photocatalyst particle. This effect is beyond its catalytic function in thermal catalysis.


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KEYWORDS. Photocatalysis, Charge separation, Nano cocatalyst, Size dependent effect, Surface imaging, Built-in electric fields. Artificial photosynthetic systems based on semiconductor photocatalysts attracted worldwide interest for the development of innovative production technologies for fuels.1 In general, semiconductors with an appropriate bandgap can be applied for the generation of electron-hole pairs to in order to drive the photocatalytic water splitting2, 3. However, in order to achieve a high efficiency of water splitting, most of the systems require suitable cocatalysts or even dual cocatalysts.4-13 It is commonly believed that cocatalysts own the same features with the catalytic sites in thermal catalysis as well as Mn4Ca catalyst in PSII14. The function of a cocatalyst, in terms of an Arrhenius expression, is to decrease the activation energy for the catalytic reactions: 2H+ + 2e− → H2 and thus accelerate the reaction rates15-17. In contrast to thermal catalysis, elementary surface reactions of photocatalysis involve charge transfer processes.18, 19 For example, four holes are required to produce one O2 molecule in photocatalytic oxidation of water, 2H2O + 4h+ → 4H+ + O2.6,

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Thus, the photocatalytic

performance should strongly depend on the density of charge carriers at the surface.21 The loading of photocatalysts with cocatalysts inevitably results in the formation of interfaces and junctions between the semiconductor and the cocatalyst.22-24 However, charge transfer can be blocked at junctions between a semiconductor and a cocatalyst so that catalysts with a high catalytic performance may not be suitable for photocatalytic reactions in combination with certain cocatalysts. So far, the exact role which play cocatalysts in photocatalysis is not well understood yet. This issue becomes even more challenging for particulate photocatalysts,25 due to their feature size ranging from nano to micro meter scale.26-31


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Herein, with the visualizing of photogenerated charge redistribution on a BiVO4 single particle photocatalyst after deposition with nano cocatalyst. It is found that cocatalysts strongly improve the interfacial charge transfer and built-in electric fields and can even change the direction of built-in electric fields beneath the uncovered surface of a photocatalyst. The resulting additive electric fields directly influence the catalytic performance of a photocatalyst. Our findings open a new strategy to design highly efficient artificial photocatalysts by engineering the distribution of built-in electric fields with smart assemblies of cocatalysts. Well facetted BiVO4 micro-crystals can be selectively photodeposited with cocatalyst on certain facets while keeping other facets uncovered35. For this reason, BiVO4 microcrystals serve as an ideal model system for the investigation of local charge separation and transfer processes. In this work, nanoparticles of MnOx, which is a typical cocatalyst for oxygen evolution

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which mimics the function of the Mn4CaO5 OER catalyst in PSII system,37 were facet-selectively photodeposited onto the {011} facets of a BiVO4 single photocatalyst particle (Supplementary Information Figure 1 and Figure 2).31 In order to determine the distribution of photogenerated charges, SRSPS31, 38, 39 (Spatially Resolved Surface Photovoltage Spectroscopy) measurements was performed at a single BiVO4 photocatalyst particle before and after photodeposition of the MnOx cocatalyst (Figure 1a). The SPV (Surface Photovoltage) signals were positive on both of {011} and {010} facets, i.e. photogenerated holes are separated towards the surface of the single BiVO4 photocatalyst particle. This corresponds to charge separation in a surface SCR (Space Charge Region) of an n-type semiconductor in depletion.40 In addition, the amplitude of SPV signals reflects that the extent of upward band bending is higher at the {011} facets than at the {010} facet.(Supplementary Information Figure 3a)


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After selective deposition of MnOx cocatalyst at the {011} facets, the SPV signals at the {011} facets remained positive and increased by three times. In contrast, the SPV signals at the {010} facet are changed to negative whereas the absolute values of the SPV signals increased by a factor of three. Therefore, in contrast to the {011} facets, photogenerated electrons were separated towards the surface of the {010} facet after the selective deposition of MnOx cocatalyst at the {011} facets. The finding gives a clear evidence for upward and downward band bending at {011} and {010} facets concomitant with a strong increase of the built-in potentials after selective deposition of MnOx cocatalyst on the {011} facets. This result was also confirmed by KPFM (Kelvin probe Force Microscopy) images (Figure 1c, d), which showed that facet-selective photodeposition of the MnOx cocatalyst effectively changed the CPD (Contact Potential Difference) differences and hence the work function difference between the {010} and {011} facets from 30 to -70 mV (Figure 1e, f). As the Fermi level of the system is aligned, the relatively large CPD difference between the two facets reflects that a relatively strong built-in electric field is formed at the interface between the cocatalyst and the single BiVO4 photocatalyst particle. The CPD curve, indicated by the blue dotted line in Figure 1f was well fitted within the abrupt junction model (see Supplementary Information) in the part of BiVO4. Two important values can be obtained: the density of donors in the bulk of the single BiVO4 photocatalyst particles (Nd, equal to 1013 cm-3) and the width of the SCR (Xn, equals to 1 µm) (Figure1f, Supplementary Information Figure 3b). These data reveal that the synthesized BiVO4 crystals are lightly doped, which agrees with the reported single crystal data by Bard et al.41 The potential difference between {010} and {011} facets of a BiVO4 single photocatalyst particle can be well tuned by varying the size of the MnOx cocatalyst, which is controlled by the


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time of photodeposition (Figure 2a, Supplementary Information Figure 4). The CPD across {010} facets remained practically unchanged, in contrast to the {011} facets (Supplementary Information Figure 5). Therefore, the changes of the potential occurred on the {011} facets due to the facet-selective deposition of the MnOx cocatalyst. SRSPS measurements were performed for {011} facets after deposition of MnOx nanoparticles with different sizes (see the spectra in the Supplementary Information Figure 6; the cocatalyst sizes were measured by AFM, Supplementary Information Fig 4) and the corresponding SPV signals are summarized in Figure 2b as a function of the size of the cocatalyst. The SPV signal was about 7 mV for the bare BiVO4 single photocatalyst particle and increased to 12 and 17 mV after facet-selective deposition of MnOx nanoparticles with sizes of about 0.5 and 2 nm, respectively. The SPV signals increased further to about 32 and 70 mV after facet-selective deposition of MnOx nanoparticles with sizes of about 15 and 30 nm, respectively, and reached a saturation value of about 90 mV for cocatalyst sizes equal or larger than 50 nm. In the abrupt junction model, the drift-diffusion continuity equation42, the Poisson`s equation42, and the integration of the electric field from the {011} facets to the center point of the particle (estimated as 3.5 µm from Figure 1a) were numerically solved in order to obtain the SPV profiles for different cocatalyst sizes whereas the density of acceptors in the MnOx nanoparticles was varied (Supplementary Information). For a density of acceptors in the MnOx nanoparticles of 1015 cm-3, the dependence of the SPV signal on the cocatalyst size matched well with the measured values (Figure 2b, Supplementary Information Fig 7). The saturation of the SPV signals at about 40 nm means that the thickness of the SCR reached the maximum value. The O2 evolution rate was measured for powders of BiVO4 single photocatalyst particles with facet-selectively deposited MnOx nanoparticles with different sizes (Figure 2c,


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Supplementary Information Fig 8). The O2 evolution rate increased with increasing size of the MnOx particles and reached a maximum for the cocatalyst size of about 60 nm. The increase of the O2 evolution rate was, in relation to the bare BiVO4 photocatalyst particles, very similar to the increase of the SPV signals for cocatalyst sizes of up to about 20 nm. For larger cocatalysts, the increase of the O2 evolution rate was, in relation to the bare BiVO4 photocatalyst particles, more than two times smaller than for the SPV signals, an additional limiting process appeared for the O2 evolution rate for MnOx cocatalysts larger than about 30 to 40 nm. Usually, the reduction of the O2 evolution rate with increasing cocatalyst size above several tens of nm is ascribed to the increase of the blocking of light with the growing cocatalyst size. A further hint to this mechanism is given by the fact that the SPV signal is lower after deposition of MnOx nanoparticles with sizes of about 90 nm than with sizes of about 70 nm. The experiments described above clearly demonstrated that MnOx cocatalysts can significantly increase the strength of the built-in electric field depending on the particle size. The MnOx nanoparticles act as electron acceptors due to their work function difference (Figure 1f, Supplementary Information Figure 9). Therefore, the {011} facets of BiVO4 photocatalyst particles charge negatively whereas the negative charge stored in the MnOx nanoparticles increases with increasing size. As a result, photogenerated holes can be separated efficiently towards the surface of {011} facets of BiVO4 photocatalyst particles and participate in O2 evolution. As a further consequence, the reduced electron density in the bulk of single BiVO4 photocatalyst particle results in the decrease of Fermi level, EF in the bulk (Figure 2d). This leads to a change of the band bending at the {010} facets from upward to downward in the case that the Fermi-energy at the {010} facets is pinned at an intermediate energy43 (pinning follows from


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practically unchanged CPD of {010} facet; see Supplementary Information Figure 5). Thus, two electric fields with opposite sign were formed beneath the {011} facets of BiVO4 photocatalyst particles covered with MnOx cocatalysts and beneath uncovered {010} facets of BiVO4 photocatalyst particles. In addition, this leads to the formation of an additive strong built-in electric field across the BiVO4 photocatalyst particles. Actually, the formation of two built-in electric fields with opposite sign is decisive for reaching a very high photocatalytic activity (see, for example, the lower photocatalytic activity of BiVO4 photocatalyst particles with randomly impregnated cocatalyst in comparison to BiVO4 photocatalyst particles with facet-selectively photodeposited

cocatalysts; Supplementary Information Figure S10). These results clearly

reveal an essential role of the cocatalyst for aligning the built-in electric fields throughout the entire particles, i.e. the cocatalysts directly influence the catalytic performance. This role of the cocatalyst is beyond its sole surface catalytic function as in thermal catalysis. It was demonstrated that the photocatalytic performance BiVO4 photocatalyst particle can be further enhanced after the facet-selective photodeposition of Pt cocatalyst onto {010} facets.44 Thus, a BiVO4 particle was photodeposited with dual cocatalysts: MnOx on {011} and Pt on {010} facets (Supplementary Information Figure 11a). In order to identify the specific role of the dual cocatalysts, the cocatalyst size of MnOx nanoparticles photodeposited at {011} facets was kept constant in the following (30 nm as for Figure 3d; see also Supplementary Information Figure 11b, c). The corresponding KPFM images in Figure 3a and b indicate that under illumination, the CPD difference became negative at the {011} facets and positive at the {010} facet (Figure 3c, Supplementary Information Figure 11d). Furthermore, the SPV signals of the single BiVO4 photocatalyst particle with dual cocatalysts increased in total by about 40% in comparison to the BiVO4 single photocatalyst particle covered with only MnOx cocatalysts


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(Figure 3d). It is interesting to note that the SPV signals measured on the {011} facet after dual cocatalyst deposition increased to about 90 mV, which is close to the platform SPV value for the BiVO4 single photocatalyst deposited with sufficient large MnOx cocatalyst particles(Figure 2b). Therefore, the facet selective deposition of Pt cocatalyst further make the whole BiVO4 single photocatalyst particle in electron depletion state. Figure 3e displays the spatial distribution of the SPV signals of a single BiVO4 photocatalyst particle with selectively deposited MnOx and Pt cocatalyst at {011} and {010} facets, respectively. Across the whole single BiVO4 photocatalyst particle, photogenerated electrons and holes are separated towards the {010} and {011} facets, respectively, in a clear cut fashion. The distribution of CPD across the border between the {010} and {011} facets have been fitted with a sigmoid function which describes the potential distribution in a SCR: 45 y = /(1 + ^(− ⁄ ) ) From the fitted curve as labeled by the red curve in Figure 3f, a value a = 120 mV has been obtained, which represents the potential difference between the two facets, and b = 0.13, which reflects the relative width of SCR. By differentiating the fitting profile of the CPD, the maximum built-in electric field at the interface can be calculated to be 2.5 kV cm-1 (Figure 3f). The built-in electric field is of the same order as that of conventional silicon p–n junction, and provides a strong driving force to separate photogenerated electrons and holes between two facets. Incidentally, additional experiments with CoOx and Pt dual cocatalysts, which were selectively photodeposited onto {011} (CoOx nanoparticles) {010} (Pt nanoparticles) of a single BiVO4 photocatalyst particle, gave very similar results as the experiments with the MnOx and Pt dual cocatalysts (Supplementary Information Figure 12).


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From the above experiments, it is clear that the deposition of cocatalyst or dual cocatalyst can significantly align the direction of the built-in electric fields and enhance their amplitude in single BiVO4 photocatalyst particles. In order to summarize the observed effects, the SPV signals of the {011} and {010} facets, excited with photon of an energy above the bandgap of BiVO4, are depicted together with the difference of the SPV signals and the directions of the built-in electric fields in Figure 4a for a bare BiVO4 photocatalyst particle, for a BiVO4 photocatalyst particle with randomly deposited MnOx nanoparticles (single cocatalyst), for a BiVO4 photocatalyst particle with MnOx nanoparticles photodeposited selectively at {011} facets (single cocatalyst) and for a BiVO4 photocatalyst particle with MnOx and Pt nanoparticles photodeposited selectively at {011} and {010} facets (dual cocatalyst). For consideration of random impregnation, see also the Supplementary Information Figure 13a. The bare single BiVO4 photocatalyst particle has slight upward band bending of about 7.2 and 3.4 mV at the {011} and {010} facets, respectively (Figure 4b). After random deposition of MnOx cocatalysts (note that, the random deposited MnOx show the similar chemical states as the photodeposited one, as shown by XPS in Supplementary Information Figure 13b ), the SPV signals, in comparison to bare BiVO4 photocatalyst particle, are only slightly increased by 1.1 and 1.7 times at {011} and {010} facets, respectively. In these two cases, the vector directions of the built-in electric fields on the two facets are opposite to each other (Figure 4b) and the effective driving forces are reduced to low values of 3.8 and 2.1 mV, respectively. The result also reflects that the impregnated cocatalyst cannot efficiently change the built-in electric fields in photocatalyst The selective deposition of MnOx cocatalyst at {011} facets led to an increase of the upward band bending at the {011} facets to 65 mV and to a downward band bending at the


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{010} facets of about -35 mV. Note that the SPV signals measured at the {010} facet changed the sign. The vectors of the built-in electric fields are parallel and have the same direction on the two facets (Figure 4c), which results in an additive strong driving force of about 100 mV (Figure 4a). Figures 4b and c give an idea about the extension of the electric field between the {011} and {010} facets being of the order of 0.5-1 µm. The extension of the electric field between the {011} and {010} facets is much wider than the diffusion length of holes (about 70 nm) in BiVO4.41 Therefore, since the drift time of photogenerated charges in high electric fields is usually much shorter than the recombination time, the probability for the photogenerated holes to migrate to the surface reaction sites at the {011} facets is increased very much. Furthermore, the deposition of dual cocatalysts leads to an increase of the upward energy band bending at the {011} facets to about 95 mV and to a downward band bending at the {010} facets of about -79 mV (Figure 4c), resulting in an additive strong driving force to 174 mV. This synergetic effect of Pt and MnOx dual cocatalysts with a strong electric field up to 2.5 kV·cm-1 is about 40% increment of that with sole MnOx cocatalyst (about 1.5 kV·cm-1, as estimated from the SPV value) and about 80 times higher than that with impregnated cocatalyst. This work reveals an important function of the cocatalysts in photocatalysis with BiVO4 particles: to align the built-in electric fields at the interface between MnOx and BiVO4 and in the SCR of BiVO4 uncoated with a cocatalyst and, consequently, to form strong additive built-in electric fields, in which photo-generated charge carriers can migrate to active surface sites over distances that are much longer than their diffusion lengths. As the photocatalytic activity is the product of the surface density of accumulated charge carriers and the surface redox reaction rate22, the increased built-in electric fields influence directly the catalytic performance of


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photocatalysts by increasing the surface density of accumulated charge carriers, as shown by the significantly increased SPV signals. This understanding explains the fact that, for heterogeneous photocatalysts, the improper assembly of cocatalysts or dual cocatalysts could result sometimes in a reduced photocatalytic performance, which can be ascribed to a randomization at short distances and an overall reduction of built-in electric fields and consequently to an increase of recombination losses. Our finding is instructive to design highly efficient artificial photocatalysts by aligning the distribution of built-in electric fields into additive ones with smart cocatalysts assembly strategies. Another hypothesis obtained from this work is that, while the proper assembly of cocatalysts can help to improve the charge separation ability of the photocatalyst in the form of built-in electric fields, the charge separation ability has a maximum, which is also affected by the depletion layer length or crystal size of the photocatalyst. This finding provides an import factor for designing highly efficient photocatalysts: each photocatalyst has an optimum cocatalyst size which can be calculated and characterized, as demonstrated in this study.


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Figure 1. Facet-selective photodeposition of MnOx cocatalyst effectively changes the vector directions of built-in electric fields of different facets of a single BiVO4 photocatalyst particle. (a) Height image of a single BiVO4 photocatalyst particle with MnOx cocatalyst deposited selectively at the {011} facets. (b) Spatially resolved SPV spectra measured on {011} facet, P1 and {011} facet, P2 on a single BiVO4 photocatalyst particle before (dashed line) and after (solid line) photodeposition of MnOx cocatalyst; Dark state KPFM image of a single BiVO4 photocatalyst particle before (c) and after (d) selective photodeposition of MnOx nanoparticles on {011} facet; (e) and (f) their corresponding height and potential cross section profiles. The CPD value in (e) and (f) were calibrated based FTO substrate. The blue dotted line in f was fitted with abrupt junction model (see Supplementary Information)


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Figure 2. MnOx cocatalysts aligned the built-in electric fields throughout the entire particle and thus directly influence the catalytic performance. (a) Dark state KPFM images of single BiVO4 photocatalyst with different sizes of MnOx cocatalyst deposited selectively at the {011} facets; (b) Dependence of the SPV signals (excitation wavelength 443 nm) on the size of the MnOx nanoparticles; The dashed line shows the corresponding simulation for a density of acceptors in MnOx of 1015 cm-3; (c) Dependence of the photocatalytic O2 evolution rate of BiVO4 powder catalysts with selectively photodeposited MnOx nanoparticles on the cocatalyst size using NaIO3 as sacrificial agent; (d) Schematic band diagram showing the downward and upward band bending at {010} and {011} facets, respectively. The orange and black arrows indicate the direction of the electric field at the interface between MnOx and BiVO4 and in the SCR beneath bare {010} facet, respectively.


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Figure 3. A synergetic effect of spatially separated dual cocatalysts, MnOx and Pt on the further increase of the electric field strength. (a) Dark state KPFM image of a single BiVO4 photocatalyst particle with MnOx and Pt dual cocatalysts selectively deposited at the {011} and {010} facets, respectively, (b) corresponding KPFM image obtained under illumination excited at a wavelength of 443 nm. (c) CPD difference as the difference between the CPD in the dark and under illumination along the lines in shown (a) and (b) and corresponding height cross section profile. The dashed line was drawn across the zero point of ∆CPD. (d) Spatially resolved SPV spectra measured at the {011} and {010} facets (P1 and P2), respectively, see also to marks in (a). (e) Spatial distribution of the SPV signals. Pink color and green colors correspond to holes and electrons separated towards the external surface, respectively; (f) Fit of the potential distribution extracted in the dashed rectangle in (c). In order to obtain the strength of the electric field by fitting, the start point of the curve was vertically shifted to 0. The blue line shows the first derivative of the fit resulting in a maximum electric field of 2.5 kV·cm-1.


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Figure 4. Well aligned dual electric fields boost vectorial charge transfer across a single photocatalyst particle. (a) Summary of SPV signals (excitation wavelength 443 nm) measured at the {011} and {010} facets for a bare single BiVO4 photocatalyst particle and for single BiVO4 photocatalyst particles with differently deposited cocatalysts. The orange and black arrows mark the direction of built-in electric fields towards the external surface of {011} and {010} facets, respectively. Schematic band diagrams across the border between the {011} and {010} facets of a bare single BiVO4 photocatalyst particle (b) and of a single BiVO4 photocatalyst particle with MnOx cocatalyst selectively deposited at {011} facets (green line) and with MnOx and Pt nanoparticles selectively deposited at {011} and {010} facets, respectively (dashed pink line) (c).


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental details, theoretical details, supplementary Figures S1-S12 and Movie

AUTHOR INFORMATION Corresponding Authors Email: [email protected], [email protected]

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21633015, 21373212, 21773228), the National Key Basic Research Program of China (973 Program, Grant No. 2014CB239403), and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB01020300.

REFERENCES (1) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Nat. Photon. 2012, 6, 511-518. (2) Lin, Y.; Yuan, G.; Liu, R.; Zhou, S.; Sheehan, S. W.; Wang, D. Chem. Phys. Lett. 2011, 507, 209-215. (3) Kitano, M.; Matsuoka, M.; Ueshima, M.; Anpo, M. Applied Catalysis A: General 2007, 325, 1-14.


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