Facet-Dependent Enhancement in the Activity of Bismuth Vanadate

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Facet-Dependent Enhancement in the Activity of Bismuth Vanadate Microcrystals for the Photocatalytic Conversion of Methane to Methanol Wenlei Zhu, Meikun Shen, Guozheng Fan, Alicia Yang, James R. Meyer, Yining Ou, Bo Yin, John D. Fortner, Marcus B. Foston, Zhaosheng Li, Zhigang Zou, and Bryce Sadtler ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01490 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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ACS Applied Nano Materials

Facet-Dependent Enhancement in the Activity of Bismuth Vanadate Microcrystals for the Photocatalytic Conversion of Methane to Methanol Wenlei Zhu1, Meikun Shen1, Guozheng Fan2, Alicia Yang1, James R. Meyer3, Yining Ou3, Bo Yin4, John Fortner3, Marcus Foston3, Zhaosheng Li2, Zhigang Zou2, Bryce Sadtler1,4* 1

Department of Chemistry, Washington University, St. Louis, Missouri 63130, United States

2 Collaboration

Innovation Center of Advanced Microstructures, National Laboratory of Solid

State Microstructures and College of Engineering and Applied Sciences, Nanjing University, 22 Hankou Road, Nanjing, 210093, People’s Republic of China 3 Department

of Energy, Environmental & Chemical Engineering, Washington University, St.

Louis, Missouri 63130, United States 4 Institute

of Materials Science & Engineering, Washington University, St. Louis, Missouri

63130, United States * To whom correspondence should be addressed. Email: [email protected]

Abstract Photocatalysis provides a route to convert methane into an energy-dense, liquid fuel, such as methanol, using only natural gas, sunlight, water (or oxygen), and the catalyst. In this report, we compare the photocatalytic activity and selectivity for bismuth vanadate (BiVO4) microcrystals with different morphologies to partially oxidize methane to methanol. Bipyramidal BiVO4 microcrystals comprised of {102} and {012} surface facets were found to be both more active and more selective for methane to methanol conversion compared to platelet microcrystals that expose {001} facets as their top and bottom surface.

The selectivity of the bipyramidal BiVO4

microcrystals for methanol production was over 85% for reaction times between 60 and 120 1 ACS Paragon Plus Environment

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minutes with mass activity between 112 and 134 mol-h-1-g-1 during this period. These activities are among the highest reported for photocatalytic methane to methanol conversion using illumination conditions comparable to solar irradiation and without the need for sacrificial reagents. Photochemical deposition of metal salts indicates that photoexcited electrons and holes react selectivity at different facets of the platelet and bipyramidal BiVO4 microcrystals. Combining the photodeposition results with surface energy calculations, we propose that the high selectivity for methanol observed using bipyramidal BiVO4 microcrystals arises from efficient extraction of photoexcited holes from surfaces that have intermediate reactivity for oxidation.

Keywords Photocatalysis, methane oxidation, methanol, bismuth vanadate, photodeposition, surface energy

Introduction Methane gas is an abundant fuel source that can be used to generate electricity and provides a source of hydrogen via steam reforming. The direct conversion of methane to a partially oxidized liquid product, such as methanol or formaldehyde, at mild temperatures and near ambient pressure would facilitate on-site conversion of methane found in natural gas reserves and its transportation using the existing infrastructure for liquid fossil fuels.1-3 In particular, the oxidation of CH4 to CH3OH is an attractive target as CH3OH can be used directly as a transportation fuel and chemical feedstock. Currently, identification of catalysts that possess both high activity and selectivity for the partial oxidation of CH4 to either CH3OH or CH2O is a major challenge. While C–H bonds are relatively difficult to activate in non-polar CH4, these bonds become increasingly easier to

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break as the central carbon atom becomes more oxidized.4-6 Thus, as the overall yield of CH4 conversion increases, the product tends towards complete oxidation to CO2. Highly dispersed vanadium and molybdenum oxo groups supported on a mesoporous oxide substrate, such as Al2O3, ZrO2, or SiO2 have been studied for the oxidation of CH4 and other hydrocarbons.7-15 These catalysts are typically thermally activated at temperatures ranging from 300 to 800C. The transition metal oxo catalysts also absorb UV light enabling photoactivation of CH4 at temperatures below 100C. Photoactivation can provide higher selectivity for CH3OH and CH2O over CO2 but typically at the cost of lower conversion yields.8-15 In addition to supported metal oxo catalysts, several metal oxide and nitride semiconductors, including WO3, TiO2, NiO, Bi2WO6, BiVO4, ZnO, Ga2O3, CaTiO3, NaTaO3, and GaN have been used as photocatalysts for CH4 conversion including the partial oxidation of CH4, the non-oxidative coupling of CH4 to make C2H6 and H2, steam reforming to convert CH4 and H2O into H2 and CO, and the aromatization of CH4 to make benzene.1, 16-29 The majority of these semiconductors only absorb UV light. Among the metal oxo and semiconductor photocatalysts, only WO3, Bi2WO6, and BiVO4 have electronic band gap energies smaller than 3.0 eV and absorb light at wavelengths longer than 400 nm. The development of photocatalysts for CH4 conversion that absorb a significant fraction of the solar spectrum would enable the energy needed to activate C–H bonds to come from a freely available energy source, the sun.1-3 BiVO4 is an n-type semiconductor with a band gap energy between 2.4 and 2.5 eV, such that it absorbs visible light near 500 nm where solar radiation reaching the Earth is most intense.30-31 In addition to photocatalytic CH4 oxidation, dispersions of BiVO4 microcrystals have been shown to photocatalyze the oxidation of water and other organic molecules.20-21, 31-36 Polycrystalline thin films of BiVO4 have also been used as photoanodes in photoelectrochemical cells for water



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oxidation.37-42 Control over the crystal facets exposed at the surface of BiVO4 particles and the orientation of crystallites in thin films has provided significant improvements in this material’s activity for water oxidation.33-37 However, the effect of different morphologies of BiVO4 crystals has not yet been examined for photocatalytic CH4 oxidation. The crystal facets that comprise the surface of a semiconductor photocatalyst or photoelectrode have several important consequences for the resulting energetics and kinetics of photochemical reactions.43 The specific arrangement of atoms at the surface of a heterogeneous catalyst affect the adsorption strength of reactant and product molecules.

For several semiconductor

photocatalysts, such as TiO2, Ag3PO4 and AgBr, exposed crystals facets with higher calculated surface energies have been correlated with higher photocatalytic activity compared to facets with lower calculated surface energies.44-46 Different crystal facets will also possess different types and densities of unpassivated surface atoms as well as surface defects, such as ion vacancies. Defects that introduce isolated states within the electronic band gap of the semiconductor can have a deleterious effect on photocatalytic activity by facilitating the surface recombination of photoexcited charges. On the other hand, surface oxygen vacancies in metal oxide semiconductors can act as preferential adsorption sites for reactants and lead to increased photocatalytic activity.4748

Finally, the dispersion of electronic bands in a semiconductor determines the effective mass of

charge carriers. In a crystal with an anisotropic unit cell, like monoclinic BiVO4, the mobility of electrons and holes are also anisotropic for different crystallographic directions. If the diffusion length of photoexcited carriers is comparable to the physical dimensions of the photocatalyst, the yield of carriers that reach different surfaces of the semiconductor will depend on their directiondependent mobilities.37, 43, 49


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Previous reports have studied the amount of oxygen produced during photocatalytic water oxidation using BiVO4 platelet microcrystals of different aspect ratios in the presence of AgNO3 as a sacrificial electron acceptor.32-35 The activity of BiVO4 platelets for water oxidation can be further enhanced through judicious placement of metal and metal oxide co-catalysts on the surface of the microcrystals.50 In CH4 oxidation, the figures of merit are both the amount of CH3OH produced as well as the selectivity for producing CH3OH rather than CO2. To study the role of surface faceting in BiVO4 photocatalysts for CH4 oxidation, we synthesized thick platelet microcrystals of similar morphology to those that have been previously studied for CH4 oxidation,20 thin platelet microcrystals that have shown high activity for water oxidation,34-35 and bipyramidal microcrystals that have been used for oxidative dye degradation (Scheme 1).51 To compare the native facets of BiVO4 microcrystals with different shapes, we did not use any cocatalysts or sacrificial reagents in our study. We found that morphologies of BiVO4 that are highly active for water oxidation lack selectivity during CH4 oxidation. Thin platelets produced a large amount of CO2 rather than CH3OH. On the other hand, BiVO4 bipyramids, which have not been studied previously for either water oxidation or methane oxidation, gave both the highest activity and selectivity for CH3OH. We discuss the observed differences in the activity and selectivity of these different samples for generating CH3OH in terms of the relative surface energies of the exposed facets and the selective extraction of photogenerated charge carriers at different surfaces.



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Scheme 1. Shape control of BiVO4 semiconductor microcrystals and their use as photocatalysts to transform CH4 into CH3OH. Results and Discussion I. Synthesis and shape control of BiVO4 microcrystals. Several previous reports have demonstrated that both the solution pH and the addition of organic surfactants can be used to direct the morphology of BiVO4 microcrystals prepared by hydrothermal synthesis.33-36,

51-52

We adapted these recipes to synthesize bipyramidal

microcrystals of BiVO4 as well as platelet microcrystals of different thicknesses. The synthetic procedures are described in detail in the Supporting Information. Scanning electron microscopy (SEM) images of the BiVO4 microcrystals are shown in Figure 1.

To synthesize BiVO4

microcrystals with a bipyramidal shape, we used a 1:1:1 ratio of Bi(NO3)3, NH4VO3, and sodium dodecylbenzene sulfonate (SDBS) with the pH of the growth solution adjusted to -0.3 using nitric acid (Figure 1a, d), following a procedure previously reported by Han.51 The formation of bipyramidal BiVO4 crystals was only observed at very low pH (< 0). The typical growth time for the bipyramids was 5 hours, as the crystals were observed to slowly transform into platelet crystals 6 ACS Paragon Plus Environment

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at longer reaction times (> 10 hrs.) (Figure S1). BiVO4 platelets were prepared using a 1:1:1 ratio of Bi(CH3COO)3, NH4VO3, and tetramethylammonium chloride (Me4NCl) for thick platelets (Figure 1b, e) and a 1:1:10 ratio of these reagents for thin BiVO4 platelets (Figure 1c, f). The pH of the solution was adjusted to 2 in both cases, and the reaction time was 20 hrs. Absorption spectra of the BiVO4 microcrystals deposited onto quartz substrates and measured in transflectance mode using an integrating sphere showed that the absorption onset was between 525 and 575 nm for each sample (Figure S2a), which matches the band gap energy range of 2.4 to 2.5 eV for the monoclinic phase of BiVO4.30-31 X-ray diffraction (XRD) patterns of the three samples matched the standard pattern for the monoclinic scheelite structure of BiVO4 (pdf # 0140688) (Figure S2b). Raman spectra of the different samples also matched the expected spectrum for monoclinic BiVO4 with no observable differences between the platelet and bipyramidal samples (Figure S3a).37, 53-54 The ratio of Bi:V measured for the bipyramidal microcrystals by energy dispersive spectroscopy was 1:1. X-ray photoelectron spectroscopy (XPS) gave a surface Bi:V ratio of 3.1:1 for the bipyramids and a ratio of 3.6:1 for the thick platelets. An excess of Bi on the surface of BiVO4 polycrystalline films has been observed by other groups and has been attributed to the presence Bi-rich phases (e.g. Bi2VO5.5, Bi2O3, or BiVO4-x) on the surface of the films.37, 55 The Bi 4f core level peaks obtained by XPS can be assigned to Bi3+. The V 2p3/2 peaks are assigned to V5+, but possess a small tail at lower binding, indicative of a small amount of V4+ present on the surface of the microcrystals (Figure S3).37, 47, 56 The binding energy regions for Bi 4f and V 2p electrons in the XPS spectra of bipyramidal and thick platelet microcrystals showed no significant differences.

However, the bipyramidal microcrystals exhibited a greater

contribution from surface adsorbed oxygen species in the O 1s region compared to the thick



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platelets (see Figure S3d). The combination of V4+ and adsorbed oxygen species for BiVO4 has been previously attributed to the presence of oxygen vacancies.47, 54 The shape of the BiVO4 microcrystals dictates the crystallographic facets that are exposed at their surface. The facets that comprise the surface of both BiVO4 platelets and bipyramids have been previously assigned.34-35, 51, 57 Several different unit cells are used to describe the monoclinic phase of BiVO4. We use the body-centered, monoclinic unit cell described by Sleight and coworkers, which possesses a space group of I2/b and the following lattice parameters: a = 5.196, b = 5.094, c = 11.704, and  = 90.38 (see Supporting Information for further discussion of the BiVO4 monoclinic crystal structure).58 Based on this unit cell, the thin and thick BiVO4 platelets expose {001} facets as their top and bottom surface. The thick platelets possess well-defined sides, which are comprised of {101}, {011}, and {112} facets. The edges of the thin platelets appear rough, and specific facets are not discernible using SEM. The surfaces of the bipyramidal microcrystals are comprised of {102} and {012} facets. The Supporting Information shows SEM images of BiVO4 microcrystals with the facets indexed (Figure S4).


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Figure 1. SEM images of BiVO4 microcrystals. (a-c) Low magnification images of BiVO4 (a) bipyramids, (b) thick platelets, and (c) thin platelets. The scale bar of 5 m applies to the top row of images. (d-f) High magnification images of BiVO4 (d) bipyramid, (e) thick platelets, and (f) thin platelets. The scale bar of 2 m applies to the bottom row of images.

II. Photocatalytic methane oxidation with BiVO4 microcrystals. Previous studies have shown that BiVO4 particles exhibit morphology-dependent reactivity for both photocatalytic water oxidation and dye degradation.33-36, 51-52 Similar to water oxidation, the oxidation of CH4 in aqueous solution is mediated via hydroxyl radicals.1, 18, 20 Semiconductor photocatalysts, such as BiVO4 and TiO2, with a valence band edge that is sufficiently positive in



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energy can generate hydroxyl radicals via the transfer of a photogenerated hole in the valence band .59

of the semiconductor to water

We used

benzoic acid to trap hydroxyl radicals generated by the different BiVO4 samples and measured the concentration of the product, 4-hydroxybenzoic acid, by liquid chromatography.60-61 We found that all three morphologies of monoclinic BiVO4 generated 4-hydroxybenzoic acid when suspended in aqueous solution and irradiated with a 350 W Xe lamp (Figure 2). The bipyramids and thick platelets produced a higher concentration of 4-hydroxybenzoic acid than the thin platelets. BiVO4 microcrystals possessing the tetragonal phase (see Figure S5), which has been reported to be less photocatalytically active,32, 52 did not produce measurable amounts of hydroxyl radicals. Irradiation of an aqueous solution of benzoic acid without monoclinic BiVO4 particles (labeled as Visible h, 65C and UV + Vis h, 65C in Figure 2) did not produce measurable amounts of 4-hydroxybenzoic acid.

Figure 2. The rate of photocatalytic hydroxyl radical production using different BiVO4 samples as measured by the reaction between OH and benzoic acid to produce 4-hydroxybenzoic acid.


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Only points at 120 minutes of reaction time are shown for irradiation with either visible light (green diamond) or ultraviolet and visible light (purple triangle) of an aqueous solution of benzoic acid at 65C without monoclinic BiVO4 particles present.

We next measured the activity and selectivity of the BiVO4 microcrystals to convert methane to methanol (Figure 3). Thick platelet microcrystals of BiVO4, similar to the ones shown in Figure 1b have been previously used to photocatalytically oxidize CH4 when dispersed in water and produced a mixture of CH3OH, C2H6, and CO2.20-21 The reaction pathway to form these products via semiconductor photocatalysis has been previously described and is shown in Scheme 2.1, 18, 20 Hydroxyl radicals can abstract a hydrogen atom from CH4 to generate a methyl radical. The methyl radical can then react with water or another hydroxyl radical, resulting in CH3OH. If the concentration of hydroxyl radicals is high enough, two hydroxyl radicals can couple to form hydrogen peroxide. Likewise, two methyl radicals can couple to make ethane. Methanol in solution can undergo further oxidation with hydroxyl radicals to form formaldehyde, formic acid, CO and CO2. Water is a relatively milder oxidant compared to O2, which can improve the selectivity for CH3OH over these further oxidized products.8-9 The mole fraction solubility of CH4 in water at our reaction temperature of 65C is approximately 1.5 10-5 (i.e. 15 ppm).62 The amounts of CH3OH, CO2, and H2 produced at different reaction times were measured by gas chromatography (see Supporting Information for further details). At a reaction temperature of 65C, only small amounts of CH3OH were detected in solution by NMR at the end of the reaction (see Figure S9). The CH3OH in solution was not included in determining the activity of the different BiVO4 samples. As shown in Figure 3a, the mass activities of the BiVO4 bipyramids,



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thick platelets, and thin platelets for CH3OH production were 111.9, 79.2, and 65.7 mol-h-1-g-1, respectively, after 1 hour of the reaction. The bipyramidal and thick platelet microcrystals showed similar selectivities of 85.0% and 85.7%, respectively, for CH3OH production after 1 hour of the reaction (Figure 3c). Thin platelets produced a greater amount of CO2 and had a selectivity of 58.2% for CH3OH after 1 hour of reaction. The surface areas for the three samples were measured using the Brunauer–Emmett–Teller (BET) method and were determined to be 3.2 m2-g-1 for the bipyramids, 3.6 m2-g-1 for the thick platelets, and 3.6 m2-g-1 for the thin platelets. The bipyramidal BiVO4 microcrystals displayed the highest activity normalized for both the mass (Figure 3a) and the surface area of the particles (Figure 3b).

 1) hVB  H 2O  OH  H 

2) OH  CH 4  CH 3  H 2O 3) CH 3  H 2O  CH 3OH   H  4) eCB   H  H 2O  H 2  OH _  5) eCB  H    H ads

6) OH  OH  H 2O2  7) 2eCB  H 2O2  2H   2H 2O

8) CH 3  CH 3  C2 H 6 9) CH 4  H 2O  CH 3OH  H 2 10) CH 4  2H 2O  CO2  4H 2

Scheme 2. Possible reactions between photoexcited electrons and holes in BiVO4 with H2O and CH4 leading to their conversion to CH3OH and CO2. The sum of reactions 1 through 4 leads to reaction 9. The sum of reactions 1(2), 6, and 7 produces no net transformation of reactants. The subscripts in the scheme have the following definitions: CB = conduction band, VB = valence band, and ads = adsorbed species.


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Figure 3. (a) Mass activity for the conversion of CH4 to CH3OH and CO2 measured for different BiVO4 morphologies after 60 minutes of reaction. (b) The specific activity for the same samples using the BET method to measure the surface area for powders of each sample. (c) The selectivity for producing CH3OH and CO2 after 60 minutes of reaction for the same samples.



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Figure 4. (a) Mass and (b) specific activity for the conversion of CH4 to CH3OH and CO2 measured for bipyramidal BiVO4 microcrystals at different reaction times.

(c) The selectivity for

bipyramidal BiVO4 microcrystals to produce CH3OH and CO2 at different reaction times.


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Figure 4 shows the activity and selectivity of the bipyramidal BiVO4 microcrystals at different reaction times. The mass activities of this sample were 111.9, 128.2 and 134.2 mol-h-1-g-1 for reaction times of 60, 90, and 120 minutes (Figure 4a). The surface-area normalized, specific activities were 34.6, 39.7 and 41.5 mol-h-1-m-2 at these reaction times (Figure 4b). A greater amount of CO2 was produced at the beginning of the reaction, such that the selectivity for CH3OH was only 48.8% after the first 30 minutes of CH4 oxidation (Figure 4c). Oxidation of surfaceadsorbed species, such as residual surfactant molecules (Me4NCl or SDBS) that remain following annealing of the microcrystals may contribute to the higher CO2 production during the first 30 minutes of the reaction. The selectivity for CH3OH production increased to over 85% for reaction times between 1 and 2 hours. The highest activity we achieved among different samples of the BiVO4 bipyramids was 151.7 mol-h-1-g-1 at 2 hours of reaction time. The activity of this sample remained above 100 mol-h1-g-1

during the first 5 hours of reactions, but dropped to 34.8 mol-h-1-g-1 after 10 hours of

continuous CH4 oxidation (Figure 5). One reason for the loss in activity is that the microcrystals do not form a stable colloidal solution and eventually precipitate around the walls and bottom of the reaction vessel during the course of the reaction. As the solution is illuminated through a flat quartz window at the bottom of the cell (see Figure S6 for a diagram of the experimental setup), flocculation of the microcrystals over time both lowers the total surface area available for catalysis and blocks the incident light. One way to improve the reaction design in future work could be to support the particles on a high surface area and transparent matrix. The amount of H2 produced during the reaction nearly balances the amount of CH4 oxidation according to reactions 9 and 10 shown in Scheme 2. For example, we observed that 37.9 moles of H2 gas was produced after 1 hour of CH4 oxidation using 0.2 g of BiVO4 microcrystals, whereas



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the expected amount is 38.4 moles of H2 for a CH3OH activity of 111.9 mol-h-1-g-1 and a CO2 activity of 19.7 mol-h-1-g-1 at 1 hour of reaction (see Figure 2). In the absence of CH4, the direct reduction of aqueous protons to form H2 using photogenerated electrons is typically not observed for BiVO4 as the position of the conduction band is very close to the proton reduction potential.31, 63

However, in the presence of CH4, one half-equivalent of H2 can be generated by the reaction

between a methyl radical and water during the oxidation of CH4 (reaction 3 in Scheme 2). One possibility is that the transfer of an electron from the conduction band of BiVO4 to a proton (reaction 5 in Scheme 2, known as the Volmer step in electrocatalytic hydrogen evolution) is the rate-determining step for H2 production in the absence of CH4.64 If H atoms are instead supplied by the reaction of methyl radicals and water, then reaction 4 in Scheme 2 (known as the Heyrovsky step in the Volmer–Heyrovsky mechanism for electrocatalytic hydrogen evolution) can produce H2 without the need for the Volmer step. We observed CH3OH and CO2 as the only carbon-based products in our reaction. Intermediates, such as formaldehyde and formic acid, during the stepwise oxidation from CH3OH to CO2 are likely not observed as the remaining C–H bonds become more reactive with increasing oxidation of the carbon atom.4-6 The production rate of OH radicals is also important in determining the product ratio. From Figure 2, the amount of OH radicals produced after 1 hour of illumination using 0.23 g of bipyramidal BiVO4 microcrystals was 520.7 umol. However, the amount of OH radicals needed based on the amount of MeOH and CO2 produced after 1 hour of reaction for 0.2 g of bipyramidal BiVO4 microcrystals is only 38.4 umol. As the rate at which OH radicals are produced is greater than the rate at which CH3OH and CO2 are produced, some photogenerated electrons likely quench intermediates along the oxidative pathway of CH4. Hydroxyl radicals not used in CH4 conversion can react directly with photogenerated electrons to


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produce hydroxide. Alternatively, two hydroxyl radicals can couple to make hydrogen peroxide, which can then be reduced by photogenerated electrons (Reactions 6 and 7 in Scheme 2).

Figure 5. Mass activity for CH3OH production during continuous CH4 oxidation over a 10-hour period using a sample of bipyramidal BiVO4 microcrystals.

We found that the activity of BiVO4 microcrystals for methanol production could be improved compared to previous reports20-21 by using a reaction temperature of 65C and a 350 W xenon lamp as the illumination source in which we partially filtered both the UV (< 350 nm) and NIR (> 800 nm) components of the spectral output (see Figure S7 for the spectrum of the Xe lamp after filtering). In the original reports of CH4 oxidation using BiVO4 microcrystals, the reaction temperature was 55C, and a 450 W, medium-pressure, Hg-vapor immersion lamp was used that emits both UVC (i.e. 100 – 280 nm) and visible radiation.20-21 The deep UV light is able to directly photolyze water and produce hydroxyl radicals in solution, such that CH4 oxidation was observed in the absence of a catalyst.20 We believe the exclusion of UVC light in our system prevents the production of hydroxyl radicals in solution via direct water photolysis, which helps to inhibit further oxidation of CH3OH. To our knowledge, the only studies that have reported higher 17 ACS Paragon Plus Environment


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activities for photocatalytic CH4 to CH3OH conversion than the activities we report here used conditions that include irradiation from a UV source (e.g. a Hg lamp or Nd:YAG laser), sacrificial chemical reagents, and/or a pressured reaction vessel.1, 12-15, 18-24 Table S4 of the Supporting Information provides a comparison of these photocatalyst systems and their activities for CH3OH production. The advantages of our system include the use of a Xe lamp source with a comparable spectral irradiance to that of sunlight (see Figure S7) without the need for a pressurized reaction vessel or sacrificial chemical reagents to achieve the observed photocatalytic activities.

III. Surface energy calculations for BiVO4 platelets and bipyramids. Multiple groups have calculated the relative surface energies of the facets that make up BiVO4 platelets.49, 57, 65-66 These calculations indicate that the {001} facets that comprise the top and bottom of BiVO4 platelets possess a lower surface energy than the {101}, {011}, and {112} facets that make up the sides of the platelets.67 We used density functional theory (DFT) to compare the stability of various surface terminations for the {102} and {012} facets that comprise the bipyramidal BiVO4 microcrystals and calculate the surface energies of the most stable terminations (see Supporting Information for a description of the computational methods). The surface energies for the most stable terminations of the {102} and {012} facets were 0.662 J/m2 and 0.660 J/m2, respectively. Using the same methods, we calculated the surface energy of the {001} facet to be 0.349 J/m2. These calculations, as well as the previous ones for the surface energies of BiVO4 platelets, were performed with the crystal surfaces interfaced with vacuum. They do not take into account how the surface termination may change with pH in aqueous solution or with the adsorption of different passivating agents.66, 68-69 Experimental evidence that the surfaces of the BiVO4 bipyramids are higher in energy than those of the platelets include the observed


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transformation of the bipyramids to platelet crystals at longer growth times (Figure S1) and the greater amount of adsorbed oxygen species on the surface of the bipyramids (Figure S3d).47, 54 Our calculations as well as previous ones indicate that the {001} facets possess the lowest surface energy among different facets of monoclinic BiVO4.57, 65, 66 Multiple groups have observed that thin BiVO4 platelets, similar to those in Figure 1c, f, are highly active for photocatalytic water oxidation and that the activity increases as the relative surface area of {001} facets increases.34-35 This observation appears to contrast other semiconductor photocatalysts where catalytic activity is generally found to be enhanced when the particles expose facets with higher surface energy.4446

However, in these prior examples of photocatalytic water oxidation using BiVO4 platelets, silver

nitrate (AgNO3) was added to the reaction mixture to serve as a scavenger for photogenerated electrons. As described further below, when BiVO4 crystals are irradiated in the presence of AgNO3, Ag+ is photoreduced to deposit Ag particles preferentially onto the {001} facets of BiVO4. The presence of Ag particles on the surface of BiVO4 likely changes its photocatalytic activity similar to other catalysts, such as iron and nickel oxyhydroxides, that have been used to improve the activity of BiVO4 photoanodes for water oxidation.39-41 Thus, it is not expected that the activity trends for the BiVO4–Ag composite should correlate with calculated energies for bare BiVO4 surfaces.

IV. Photochemical deposition on BiVO4. Photochemical deposition has been used to image the fate of photoexcited charge carriers in semiconductor particles and nanostructured films.50, 70 For BiVO4 platelets of similar morphology to those shown in Figure 1 b, e, the reduction of HAuCl4, H2PtCl6, or AgNO3 by photoexcited electrons in the conduction band of BiVO4 was shown to deposit Au, Pt, or Ag metal nanoparticles



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preferentially on the {001} facets of the platelets.50 On the other hand, Pb(NO3)2 and MnSO4 were both oxidized by photoexcited holes to deposit lead oxide or manganese oxide nanoparticles preferentially around the side facets of the platelets.50 Facet-selective photodeposition has been attributed both to the surface termination of different facets leading to energetic differences in the positions of the conduction and valence band edges at the surface of the crystals as well as to the anisotropic mobilities of charge carriers along different crystallographic directions.44,

49-50, 70

Similar to these previous reports, we observed that Ag deposited preferentially on the top and bottom facets of BiVO4 platelets while manganese oxide deposited around the perimeter of these microcrystals (Figure 6a-c). Photodeposition has not been previously studied on bipyramidal BiVO4 microcrystals. We observed that photooxidation of MnSO4 produced uniform coverage of manganese oxide over the {102} and {012} facets of the bipyramids (Figure 6e).

Some

bipyramids possessed truncated apexes to reveal {001} facets. The manganese oxide coverage was lower on the {001} facets compared to the {102} and {012} facets (Figure 6f). We found that we could increase the truncation of the bipyramids by growing them for a longer time (Figure S1). Photoreduction of HAuCl4 on the truncated bipyramids led to selective deposition of Au particles on the {001} facets at the apexes of the microcrystals (Figure 6d).


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Figure 6. SEM images of BiVO4 microcrystals after photodeposition with different metal salts. (a) Thick platelets after photodeposition of AgNO3 to deposit Ag. (b, c) Thick platelets after photodeposition of MnSO4 to deposit manganese oxide. (d) Truncated bipyramids after photodeposition of HAuCl4 to deposit Au. (e, f) Bipyramids after photodeposition of MnSO4 to deposit manganese oxide.

V. Summary of facet effects in the selectivity and activity of BiVO4 microcrystals. Combining results for surface energy calculations of different BiVO4 facets and the selective facet reactivity observed during the photodeposition of metal salts, we rationalize the observed



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facet-effects in the activity and selectivity for CH4 to CH3OH conversion by BiVO4 microcrystals of different shapes in the following manner.

The thin BiVO4 platelets possessed the highest

activity for total oxidative events during photocatalytic CH4 conversion, albeit at the expense of an increased yield of CO2 and reduced selectivity for making CH3OH. Photoexcited holes are expected to possess higher mobility parallel to the {001} platelet facets than perpendicular to these facets based on previous charge transport studies in BiVO4 films and single crystals (see the Supporting Information for further discussion of charge mobility in BiVO4).37-38 The extraction of photoexcited holes around the perimeter of the platelets is supported by our photodeposition studies, while photogenerated electrons are extracted from the top and bottom facets. As the thickness of the thick platelets (~ 1 m) is larger than the estimated majority carrier diffusion length for photoexcited electrons (< 10 nm for intrinsic BiVO4),39 some electrons likely recombine before reaching the surface in these microcrystals. Inefficient extraction of either photoexcited carrier will reduce the frequency of reaction events.47 Photoexcited electrons can be collected more efficiently from the top and bottom facets of thinner platelets due to the shorter distances they need to travel to reach these surfaces. Furthermore, the perimeters of the thin platelets do not consist of well-defined facets. The increased surface roughness of these edges likely contributes to the higher reactivity of the thin platelet crystals.

From the photodeposition results on

bipyramids, photoexcited holes are extracted from the majority of the surface of these microcrystals, while electrons are extracted selectively at the apexes. Thus, a bipyramidal microcrystal provides more surface area for photoexcited holes to be extracted compared to the perimeter of a thick platelet, which is consistent with the higher activity of the bipyramids for both CH3OH and CO2 compared to the thick platelets. We hypothesize that the {102} and {012} facets of the bipyramids are less active surfaces compared to the rough edges of the thin platelets, which


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is consistent with the higher selectivity for CH3OH observed for the bipyramids. The longer distance that photoexcited electrons need to travel to be extracted at the apexes of the bipyramids may also contribute to the lower overall number of oxidative turnovers observed for the bipyramids compared to the thin platelets. Another factor that may contribute to differences in activity observed for the platelets and bipyramids is their susceptibility to form surface oxygen vacancies. These defects have been correlated with increased photocatalytic activity in BiVO4 as well as other metal oxides.47-48 The bipyramids show a greater contribution of surface adsorbed oxygen species (see Figure S3d) that have been proposed to bind at oxygen vacancies on the surface of the crystal.54 A microkinetic analysis of the free energy profiles for CH4 oxidation on different BiVO4 surfaces, including how surface oxygen vacancies alter the adsorption of reactants and products, may provide deeper insight into the mechanism by which the further oxidation of CH3OH is inhibited on the bipyramids, leading to high selectivity for CH3OH production.48, 66, 68, 71

Conclusions The photocatalytic activity of BiVO4 microcrystals for CH4 to CH3OH conversion has been enhanced by controlling the shape of the microcrystals. Bipyramidal microcrystals were found to possess the highest activity for CH3OH with a mass activity above 100 mol-h-1-g-1 and a selectivity above 80% during the first two hours of CH4 oxidation. Notably, these activities were obtained using illumination conditions comparable to solar irradiation and without the need for sacrificial reagents. We propose that thin platelets produce more CO2 because photoexcited holes are extracted from the rough edges of the platelets, which are highly reactive surfaces. The bipyramids combine efficient extraction of holes from nearly the entire surface of the microcrystal



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and intermediate surface reactivity (i.e. lower than the rough edges of the thin platelets but higher than the side facets of the thick platelets) leading to high selectivity for partial oxidation. In the future, partial ion substitution and the deposition of nanoparticle cocatalysts, which have been used to increase the activity for water oxidation in BiVO4 photoanodes may help to further improve the CH4 oxidation activity of the bipyramidal microcrystals.39-42

Associated content. Supporting Information Available: Materials and Methods, calculated surface energies for various terminations of BiVO4, additional discussion of the structure and charge transport in BiVO4, a summary of previous photocatalysts for CH4 to CH3OH conversion from the literature, Supporting Figures, and Supporting References.

Author information. *email: [email protected] Conflicts of interest. The authors declare no competing financial interest Acknowledgements. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (award # PRF58165-DNI10). The synthesis and characterization of the BiVO4 samples was supported by a grant from the International Center for Energy, Environment and Sustainability (InCEES) at Washington University. J. R. M was supported by the National Science Foundation Science and Technology Center for Engineering Mechanobiology under CMMI-1548571. Electron microscopy was performed at the Institute of Materials Science & Engineering at Washington University. X-ray diffraction was performed in


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the Department of Earth and Planetary Sciences at Washington University. Measurements of the catalyst surface area were performed at the Nano Research Facility at Washington University. S. Singamaneni and Z. Wang are acknowledged for use of their Raman spectrometer.

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Table of Contents Abstract Graphic


Facet-Dependent Enhancement in the Activity of Bismuth Vanadate

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