Spray Pyrolysis as a Combinatorial Method for the Generation of

Research Article Cite This: ACS Comb. Sci. 2019, 21, 489−499

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Spray Pyrolysis as a Combinatorial Method for the Generation of Photocatalyst Libraries Jordan S. Compton,† Christi A. Peterson,‡ Dilek Dervishogullari,§ and Lee R. Sharpe* Department of Chemistry, Grinnell College, Grinnell Iowa 50112, United States

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ABSTRACT: An inexpensive, combinatorial method to evaluate an array of metal oxide materials as photocatalysts for solar fuel production utilizing spray pyrolysis is presented. This new approach capitalizes on the inherent properties of spray pyrolysis. We take advantage of the natural lateral gradient produced in a spray cone to fashion four-metal-threeat-a-time compositional triangular patterns on conductive glass substrates from simple nitrate salt precursor solutions. Subsequent annealing produces thin-film electrodes that are readily screened for photochemical activity using a simple laser scanner system. The apparatus employed is constructed from readily available commercial components, making it accessible to a wide number of laboratories. Our method complements other combinatorial methods in that it provides a chemically different environment for the formation of materials that might produce different morphologies and metal oxidation states and it allows for easy evaluation of layered structures, as well single-phase materials, thereby expanding the number of unique materials tested as potential photocatalysts. As a proof of principle, the discovery and optimization of a new Na-doped CuBi2O4 photocatalyst is described. KEYWORDS: combinatorial materials science, photocatalyst, solar fuel, photochemical water splitting, spray pyrolysis, metal oxides, CuBiO4, copper bismuth oxide

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INTRODUCTION The goal of producing a renewable fuel with sunlight has been an aspiration of the scientific community for almost 50 years ever since Fujishima and Honda demonstrated that light could be used to split water in a photoelectrochemical cell. Unfortunately, their photocatalytic electrode material, nTiO2, has a bandgap too large to efficiently utilize the solar spectrum.1 In the intervening years, a wide range of semiconducting materials have been investigated, yet not one has met all the requirements to be a commercially viable photocatalyst: (1) a bandgap (1.5−2.0 eV) that overlaps well with the solar spectrum; (2) band edges that straddle the oxidation and reduction potentials required; (3) stability to photocorrosion in aqueous solution; (4) catalytic for the formation of the solar fuel; and (5) composed of Earthabundant elements.2,3 Potentially, a photoanode and photocathode could be used simultaneously, both contributing to the required photovoltage and allowing each to have a smaller bandgap (0.8−1.2 eV), allowing better overlap with the solar spectrum.4 Because of the their inherent stability and ease of preparation, metal oxides are actively being investigated as potential photocatalysts.3 However, with ∼60 useable metals in the periodic table and the likelihood that a high-performing photocatalyst will require multiple metals with specific atomic ratios, surveying all possible permutations becomes challenging. This inherent difficulty is augmented when you consider that many metals can have multiple possible oxidation states © 2019 American Chemical Society

and specific metal oxides can have several morphologies. In addition, the best photocatalysts might be composed of a layered structure forming a heterojunction composite. These have been shown to both improve photocatalytic activity and improve light absorption.5,6 Because of the staggering number of possibilities, combinatorial methods are best suited as a highly effective tools for screening potential photocatalysts. The most common method of synthesizing libraries of materials is by inkjet printing or jet dispensing of either aqueous metal ion solutions7 (usually as the nitrate and sometimes acidified) or metal salts dissolved in more viscous solvents, such as ethylene glycol,8−10 3:1 water/glycerol,11 water/∼30% diethylene glycol12 or ∼30% diethylene glycol with 1% diethylene glycol monobutyl ether.13,14 Higher viscosities serve to control the spread of material on the substrate. The Sayama group used butyl acetate with 0.2 M ethyl cellulose (to increase viscosity) to dissolve the metal salts and then mixed combinations of the different metal precursor solutions prior to printing.15,16 This addresses the concern with printing individual solutions over one another that they may not be completely mixed to yield a homogeneous solution before being oxidized.15 Metal salt “inks” have also been prepared by dissolving the metal ions in a mixture of 0.80 g of Received: March 6, 2019 Revised: May 3, 2019 Published: May 30, 2019 489

DOI: 10.1021/acscombsci.9b00042 ACS Comb. Sci. 2019, 21, 489−499

Research Article

ACS Combinatorial Science

parameters, and if these parameters are varied during deposition, depth composition gradients and layered structures can be created.27 The amount of material deposited by spray pyrolysis also varies laterally with distance as one moves away from the center of the spray cone. This can be exploited to deposit overlapping gradients of different elements to explore a range of compositions as well as layered composites. What material is produced will depend on the ability of the components to interdiffuse or remain as individual layers. We report the development of a combinatorial method for the creation of libraries of potential photocatalysts by spray pyrolysis in which gradients of three different solutions are layered over individual triangles on a conductive fluorine doped tin oxide (FTO) glass substrate. Our apparatus is built with readily available materials typically found around a lab and at a local hardware/toy store. This spray pyrolysis method complements other combinatorial methods in that it can be used to explore layered structures as well as single phase materials. The example we detail in this work results in a layered material. We have previously reported on the preparation and characterization of single phase materials for oxygen reduction catalysis prepared with this apparatus.28 In addition, this method is likely to create unique morphologies and possibly different oxidation states compared to materials prepared by other combinatorial methods, expanding the number of materials that can be evaluated for photocatalysis.

Pluronic F127, 1.0 mL of glacial acetic acid, 0.40 mL of concentrated HNO3, and 30 mL of absolute ethanol.17,18 Complexation of metal ions via metal salt “inks” has the advantage of maintaining a homogeneous mixture during the drying process as opposed to the possible sequential deposition of the different metals based on their relative solubility. Chemical vapor deposition provides an alternative option to form libraries of materials. Molybdenum- and tungsten-doped BiVO4 films have been prepared with a range of dopant concentrations by simultaneous vapor deposition of the elements onto FTO substrates and then converted to the corresponding oxide in air at 500 °C.19 Similarly, cobalt-doped WO3 films were grown on ITO substrates with the cobalt concentration ranging up to 30%. WO3 powder and cobalt metal sources were used and the resulting films were annealed at 400 °C under oxygen.20 Magnetron sputtering of the pure elements has been used to create ternary gradients of Cu, Si, and Ti in a triangle on gold-coated Al2O3, followed by annealing in air at 600 °C.21 Another method, combinatorial aerosol-assisted chemical vapor deposition, was used to produce a film that varied from SnO2 on one side to TiO2 on the other with a range of mixed compositions in the middle. Titanium tetra-isopropoxide and tin butyl trichloride were individually dissolved in ethyl acetate, aerosolized, and carried to the heated substrate in N2 gas.22 Rühle et al. utilized a twostep method combining two different deposition techniques. A thickness gradient of TiO2 was achieved using spray pyrolysis by controlling the number of layers of material being deposited. This was accomplished by moving the spray nozzle with an xyz controller in such a way that thickness steps were created through a series of spray cycles with a successively decreasing scan area. The Cu2O layer was then deposited over the TiO2 thickness gradient by pulsed laser deposition (Cu2O target) forming a Cu2O island with a bell-shaped profile. This provided a range of TiO2−Cu2O heterojunctions of different thicknesses.23 Combinatorial methods involving deposition from solution include both electrochemical deposition and spin coating. The McFarland group has prepared libraries of WO3 films doped with a variety of transition metals, where different compositions and structures are deposited by varying both the electrolyte compositions and the applied potential.24 The advantage of this technique is that the materials produced have a direct electrical contact with the substrate.25 Spin coating using a home-built spin coater wedge allows the formation of a compositional gradient over a surface. Using this approach, Lu et al. produced a thickness gradient of TiO2, which was partially converted by a hydrothermal reaction to grow a density gradient of TiO2 nanorods. These nanorods were subsequently coated with a graphene oxide gradient layer.26 We were interested in developing a combinatorial technique that is inexpensive to implement so that it is accessible to a large number of laboratories. This is important given the number of possible materials needed to be screened. We also were interested in a technique that is complementary to the existing combinatorial approaches and is readily applicable to large scale manufacture of the resulting materials. Spray pyrolysis is an ideal method to grow thin films because of the simplicity of the required equipment and the scalability of the technique. In addition, it offers an effective way to grow and dope films with almost any element and does not require a vacuum or high-quality targets/substrates. Deposition rate and film thickness can easily be tailored by changing spray

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EXPERIMENTAL METHODS Preparation of Metal Oxide Test Patterns. Our combinatorial pattern, adapted from Woodhouse et al.,7 has the advantage of combining four-metals-three-at-a-time and includes both n and p-type references. Some rearrangement was needed to reduce the number of spray passes required to complete the pattern (Figure 1). Metal nitrate solutions with a concentration of 0.15 M dissolved in 10% ethanol/deionized water were used as precursor solutions.

Figure 1. Four-metals-three-at-time spray pattern. Different colors and the corresponding letters indicate the four metals. Copper oxide and iron oxide reference triangles are at the top left and right, respectively.

Some metals, including tungsten and molybdenum, require the addition of an acid or complexing ammonia solution to solubilize the metal ion. The addition of ethanol reduces the surface tension of the solution, resulting in a finer spray and thus a more uniform film.27 The solutions were deposited onto 76 mm (3 in.) square FTO conductive glass substrate plates (Hartford Glass). Plates were first cleaned with detergent (Fisherbrand Versa-Clean, 04−342), then rinsed with deionized water, followed by 99% ethanol, and finally wiped dry with Kimwipes. Next, they were rinsed with 99% methanol and wiped dry with Kimwipes, followed by a lint-free cotton cloth. 490


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Figure 2. U bracket holding the triangular pattern template over a piece of paper, where the substrate is usually held to visually highlight the triangles. The second and forth pins are removable allowing the bracket to rotate left and right (b and c) or be held perpendicular when both pins are in place (a and d). Numbers and letters correspond to the spray order and specific metal, respectively, and are positioned in the corner of highest coverage.

The FTO substrate is held in place by a U bracket (conductive side facing the sprayer) mounted on an aluminum base plate, Figure 2a, which is attached to a hot plate. The hot plate is set to 500 °C, which heats the substrate to ∼170 °C (measured using an IR thermometer (Fluke 62 MAX)). A template is placed over the substrate to define the triangle pattern and various masks can be placed above the template to select the particular one or two triangles to be sprayed. The base plate has 5 pins. The middle pin allows the U bracket to rotate. The second and fourth pins are removable, allowing the U bracket to rotate to the right or left or be held perpendicular when both pins are in place (Figure 2a and d). The outside two pins stop the rotation at ±60° (Figure 2b and c). To complete a pattern, 11 passes of the sprayer are required. The numbers in Figure 2 correspond to the spray order and the letters correspond to the metal ion being sprayed. The top left and right triangles are reserved for the copper oxide and iron oxide references on each plate, respectively. Individual triangles are sprayed by covering the other 4−5 triangles with one of six masks, then aiming the center of the spray cone approximately

1 cm outside of the triangle corner to have the highest coverage. The trolley holding the sprayer goes back and forth over the substrate parallel to the triangle side (leg) opposite from the corner to be coated but again aimed outside of the triangle near the corner to be coated. This allows a thickness gradient to form over the triangle from high thickness at the corner to virtually no material on the far side of the triangle. Note the position of the numbers/letters in Figure 2 represent the high coverage corner of the specific metal and the coverage decreases as you go to the opposite side of the triangle. Initially, all the triangles are covered except the top left one using mask (I), allowing copper nitrate solution to be sprayed only in that triangle with the higher coverage of copper solution at the bottom, Figure 2a. The set of six masks is shown in Figure 3. The trolley stops on either side of the substrate, where the spray is collected and channeled to waste containers by a trough formed using four inch dryer vent tubes (Figure S1). Deposition on the substrate only occurs when the sprayer passes over the substrates between the trolley stops. 491


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the wood track allows for precise repositioning. The sprayer is mounted on a LEGO trolley that moves back and forth horizontally at 9 cm/s under the control of a LEGO Mindstorms motor (Figure S1). The sprayer, whose end is 11.5 cm above the substrate surface, is mounted perpendicular to the surface on the trolley moving in the X direction, while the substrate is mounted on the hot plate moving in the Y direction. Together, this enables us to spray any part of the substrate. The custom glass sprayer (Figure 4) was constructed from a standard 10.00 mL glass pipet by removing the bulb and

Figure 4. Glass sprayer with internal capillary removed. Capillary fits into long tube of the sprayer and N2 flows into side arm. The small piece of white rubber tubing shown on the capillary makes an airtight seal between the capillary and body of the sprayer.

adding a side arm for the flow of N2 gas. The metal ion solution is drawn into tubing (Tygon 2375) that connects to a homemade internal capillary tube, which has an outer diameter of 0.5 mm on the spray end and 1.5 mm on the tubing end. This is made by heating an 8 mm piece of Pyrex tubing and pulling it quickly while rotating it to get a straight capillary. It is then scored and cut to yield the desired dimensions with the tubing end being ∼1.7 mm, which is then heated to thicken and shrink to the 1.5 mm diameter. A small piece of rubber tubing creates a seal between the capillary and the glass sprayer. It also allows slight movement of the capillary for fine adjustment of the spray. The flow rate through the sprayer was 6.3 mL/min at a N2 pressure of 17 psi, though different capillary tip diameters yield different flow rates. The complete apparatus is mounted on the angle-iron frame shown in Figure S1. Once a promising composition is discovered, the composition is refined by spraying specific concentration ratios of the metals. To accomplish this, a template with a 5 × 5 grid pattern of 3 mm diameter holes spaced 12 mm apart is placed over the prepared substrate and a different set of masks are used to select a particular row or column (Figure 5). One metal is sprayed along the horizontal rows, another along the vertical columns (90° counter clockwise rotation), and if needed, a third one on the diagonal (a spacer is attached to left

Figure 3. (Top six) Photos of the set of masks used to select certain triangles. Some can be flipped to access other triangles. (Bottom) Mask VI covering the triangle template showing how specific triangles are selected.

Mask I is then flipped over to cover all the triangles except for the top right one and iron nitrate is sprayed into that triangle, again with the higher amount of iron at the bottom. A second mask (II) is placed over the template to cover all but the top middle triangle, where we spray metal D with the sprayer aiming to have the highest amount of metal at the top. A third mask (III) is placed that covers four of the six triangles, allowing us to spray metal A into both of the lower left and right triangles with the higher amount of metal in the top of those triangles. Finally, the forth mask (IV) is placed covering all but the lower middle triangle so we can spray metal A with the higher concentration into the bottom corner of the triangle. To access a new set of corners of the triangles, the pin just right of the center pin is removed, allowing the U bracket to rotate right until it comes to rest on the far right pin (Figure 2b). A fifth mask (V) is placed over the template, and metal B is sprayed into the bottom of the bottom triangle, then the sixth mask (VI) is placed, and the corners marked as 7B are sprayed. Next, mask IV is used again, and metal D is sprayed with the highest coverage at the corner labeled 8D. In a similar way, the last set of corners is sprayed by removing the pin just left of the center pin and rotating the U bracket to the left (Figure 2c). Using mask V for corner 9D, mask VI for corners 10C, and mask IV for corner 11C completed the pattern with metals C and D. The complete pattern is shown in Figure 2d. The order of metals can be rearranged to access the effects of the layering of the metal oxides. The required up and down positioning is accomplished by moving the hot plate/substrate assembly up and down on a three-wheeled wood block that runs on a grooved track. The assembly is positioned with a lab jack and a ruler mounted on

Figure 5. Twenty five dot pattern template (left) with one example mask (right) that is used to select the second row. 492


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photocurrent output of the cell and the optical power (Newport, model 1917-R) were measured from 400 to 800 nm in 20 nm increments. The IPCE at each wavelength was calculated according to the eq 1.

most pin on the base plate in Figure 2 to make the required 45° angle). The two holes at the right in their own column are used for reference materials. This template is also used to optimize the thickness of the metal oxide films by varying the number of spray passes over a series of rows or columns (one “pass” is the trolley going back and forth over the substrate or two layers of material). After spraying, the substrates are allowed to slowly cool on the hot plate and, then, were placed in an oven and heated to 500 °C at 15 °C min−1 to form the oxides and anneal the films for 2−4 h. Photoelectrochemical Screening. An annealed pattern on a substrate is placed into a glass electrochemical cell filled with 0.1 M Na2SO4 electrolyte solution and purged with N2 gas if testing for p-type materials. The substrate is then connected in a two-electrode configuration and placed in an apparatus supplied by the Parkinson group29 that applies a specified potential, pulses a 532 nm pen laser (20 mW, Wicked Lasers), and measures the current between the working electrode (coated substrate) and the counter electrode (a platinum mesh). A Lab View based program records the difference in current (between light on and off (background current measurement)) to determine the amount of photocurrent that is produced at a given spot on the sprayed substrate. The software also moves two mirrors in the Lego Mindstorms assembly that rotate on axes 90° from each other to sequentially raster the laser spot in a two-dimensional pattern over the substrate. The current difference measurements at each spot on the substrate are used to create a falsecolor image that displays the photocurrent as a function of the Cartesian position using ImageJ software. The spots with higher photoactivity are displayed as brighter spots in the image. The contrast and brightness of the false color image are adjusted for each substrate so that the highest photocurrent is not “washed out”, while the lowest is still visible. As a result, there is not an absolute color/brightness to current scale, photocurrent (brightness) must be compared to the internal references on each substrate and that difference compared between substrates. We choose to use a neutral electrolyte to minimize potential dissolution of the material at either high or low pH. Many metal oxides dissolve in acidic solution but some also can dissolve in basic solution. These amphoteric metal oxides form soluble hydroxo complexes at high pH which can lead to the deterioration of the material. Photoelectrochemical Characterization. Absorbance spectra were measured with an Agilent 8453 diode-array spectrophotometer using the bare FTO substrate as a blank. Electrodes were prepared by spraying the material of interest across the bottom 1 cm of a FTO substrate and then cutting the substrate into several 1 × 7.5 cm strips so that the bottom square centimeter was coated with the material. The middle half of the conductive surface of the electrode was covered with clear fingernail polish (Rimmel 581 Clear) so that only the film was exposed to the electrolyte solution and the top is left uncoated allowing electrical connection. The cell was made from the bottom three-fourths of a Corning polystyrene tissue culture flask filled with 0.1 M Na2SO4 electrolyte purged with N2 gas. Photoaction spectra were measured in a three-electrode configuration with a SCE reference electrode and Pt wire counter electrode. The photoelectrode was illuminated by a modified ISS PC1 spectrofluorometer (1 mm slits) equipped with a 300 W Xe arc lamp. The potential of the electrode was controlled with a Pine model AFRDE5 bipotentiostat. The

IPCE (%) =

photocurrent (mA) × 100% power (mW) × wavelength (nm) (1)

I−V measurements were performed using a 300 W Xe arc lamp (Oriel Corporation) to illuminate a four-electrode electrochemical cell filled with 0.1 M Na2SO4 electrolyte purged with N2 gas. The light was filtered with an Air-Mass 1.5 Global Filter (Oriel Instruments). The four-electrode arrangement allows for the simultaneous measurement of two photoelectrodes, ensuring both electrodes are tested under identical conditions (Figure S2). Light intensity incident on the photoelectrodes was ∼100 mW cm−2, measured with an optical power meter (Newport, model 1917-R) and to make certain the light intensity was the same on both electrodes. The applied potential was scanned at 6 mV s−1 with a Pine Instruments RDE 3 bipotentiostat while the light was “chopped” manually with a piece of black metal at 5 s intervals in order to compare the behavior of the electrodes in the dark and under illumination. Stability Testing. The I−V curve setup was used for the determination of the thin films’ stability. An applied potential of −0.2 V vs SCE in 0.1 M Na2SO4 electrolyte purged and blanketed with N2 gas was maintained, while the photocurrent produced by each electrode was measured under constant illumination. Materials Characterization. SEM-EDS and XRD measurements were performed at Iowa State University’s Materials Analysis and Research Laboratory. XRD was measured on a Siemens D 500 diffractometer with a Cu X-ray tube operated at 45 kV and 30 mA with medium resolution slits and a diffracted beam monochromator. The SEM data was collected on a FEI Quanta FEG 250 scanning electron microscope with a field emission gun. All images were collected using the backscattered electron signal. The EDS was acquired with an Oxford Aztec energy-dispersive spectrometer system with an X-Max 80 detector having light element capability. Samples were examined at 10 kV to limit the excitation volume to about 500 nm in Bi carbonate. Still, it was difficult to get clean analyses due to the thin layers. Beam current was about 1.5 nA, which yielded a count rate of about 15 kcps. Elemental maps were collected over about 10 min at 256 × 224 pixels.

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RESULTS AND DISCUSSION Growing good quality thin films using spray pyrolysis requires the optimization of the following deposition conditions: solution concentration, solution flow rate, N2 pressure, spray nozzle scanning speed, distance between the spray nozzle and substrate, and substrate temperature. Using standard ink jet printing conditions as a guide, we started with a solution concentration of 0.15 M,30 and a substrate surface temperature of ∼170 °C. We adjusted the spray nozzle scan speed such that the solution evaporated quickly enough off the substrate surface so that no liquid would flow over or drip down the surface during deposition. The distance between the spray nozzle and substrate, as well as the N2 pressure (which determines the solution flow rate) were adjusted to achieve an even gradient over the area of a triangle (∼2 cm) and a fine 493


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A template that covers the substrate was used to define six triangles and a collection of masks that confine the spray into only one or two triangles was used to create the combinatorial library (see Experimental Section). The different corners of the triangles can be accessed by moving the template/mask/ substrate assembly up and down and ±60° to the left or right. Four of the triangles are used to mix four-metals (A−D in Figure 1) three-at-a-time. The top left and right triangles are reserved for the two references, copper and iron oxide, which serve as p- and n-type references, respectively, to provide a comparison to the photoactivity of the new mixed metal oxide combinations/layers. This pattern differs slightly from what Parkinson et al.7 used for ink jet printing in that the positions for the metals in two of the triangles is rearranged affording more efficient spraying (note that two triangles can be sprayed in one pass for a couple of template positions). Our strategy is to spray three gradients of three metals, a different metal in each corner. Ideally, the three individual metal oxides will be formed in each corner, binary mixtures/layers along the edges of the triangle and ternary mixtures/layers of varying composition/thickness fill the center. A complete pattern can be accomplished in 11 passes of the sprayer. Since layering of the metal oxides is the most likely outcome (unless the materials interdiffuse) testing different metal spray orders is important. In between spraying each metal solution a 5% acetic acid solution is run through the sprayer. This serves two purposes: first to clean the sprayer and second to act as a safety check on the hood. If one smells the acetic acid then the spray is not being properly ventilated. Once sprayed, the substrate is cooled, transferred to a furnace, and heated to 500 °C to convert the metal salts to the corresponding oxides. Screening of the resulting plates is accomplished using a LEGO Mindstorms-based laser scanner system.29 This approach was used in the discovery of a sodium-doped CuBi2O4 photocatalyst as a proof of principle. We were interested in improving the photocatalysis of CuBi2O4 films by incorporating monovalent ions. Berglund et al. tested 22 different metals with Cu and Bi finding that Ag, the only monovalent metal tested, improved the photoactivity the most

spray arriving at the surface of the substrate. The initial solution concentration of 0.15 M yielded a good surface coverage of the metal salt on the substrate. SEM-EDS was used to verify that the spray apparatus produced films with a good concentration gradient across the triangle. To produce our gradient, the substrate was first covered with a template that had a triangular opening and the spray cone was positioned so it would pass over the template ∼1 cm from the corner of the triangle being coated with the largest amount of material as the trolley moves back and forth running parallel to the side (leg) of the triangle opposite of the corner to be coated. Taking advantage of the natural decrease in spray proportional to the distance from the center of the cone, we sprayed a gradient of a 0.15 M Fe(NO3)3 solution across a triangle so that the highest concentration of Fe would be in a corner of the triangle and the concentration would drop off approaching the opposite side (leg). The iron signal vs distance from the corner of the triangle was then measured using SEM-EDS. Figure 6 shows that a reasonably linear

Figure 6. Plot of Fe deposited in a triangle vs distance from the corner of the triangle. Inset is a photograph of the iron oxide gradient on the FTO substrate. Counts are from the integrated Fe L-line peak at 0.7 keV corrected for both background and peak shape.

elemental gradient over the length of the triangle is observed. The less than expected intensity at 0.5 mm is most likely due to the “shadow” caused by the template.

Figure 7. Elemental spray patterns and false color images of (a) a BiCuZnK plate in which the bismuth was dissolved in glacial acetic acid; (b) a CuBiKZn plate where the bismuth was dissolved in 1.5 M HNO3; and (c) a BiCuNa plate with no fourth metal. Again, bismuth was dissolved in 1.5 M HNO3. (d) BiCuNaLi plate. In this case, bismuth was dissolved in 2.5 M HNO3. All solutions were diluted with 10% ethanol-deionized water. All photocurrent scans were done in N2 purged 0.1 M Na2SO4 solution. 494


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Figure 8. False color images of dot patterns to (a) compare the different alkali metals (solution concentrations given below figures), (b) determine of the best Bi to Cu ratio, (c) determine the best spray order and mixing, and (d) determine the best Na concentration. All photocurrent scans were done in N2 purged 0.1 M Na2SO4 solution.

while maintaining stability in neutral electrolyte.30 As a result, we investigated the effect of adding monovalent alkali metals to CuBi2O4. We initially sprayed Bi as Bi(NO3)3 dissolved in glacial acetic acid.31 The resulting material was only slightly photoactive, showing a weak response from a BiCuK triangle and slightly better photocurrents from a BiCuZn triangle (Figure 7a). Berglund et al. also observed that Zn improved the photoactivity of CuBi2O4.30 Significant improvements were observed relative to the copper oxide reference when the Bi(NO3)3 was dissolved in increasing concentrations of HNO3. Interestingly, the BiCuK significantly outperformed the BiCuZn triangle when the Bi(NO3)3 was dissolved in 1.5 M HNO3 in 10% ethanol-deionized water solution (Figure 7b). This is very likely due to the spray order of metal solutions (see below). To establish if the improvement in performance is due to the higher acid concentration or the oxidizing strength of the nitrate, we added additional nitrate to the bismuth solution in one experiment and dissolved the bismuth in HCl in a second. We observed no change in performance with the addition of NH4NO3 to the Bi(NO3)3/1.5 M HNO3 solution, and we saw similar performance when the Bi(NO3)3 was dissolved in 3 M HCl (minimum concentration of HCl required to dissolve the Bi(NO3)3) instead of HNO3. This suggests that the bismuth solution must be highly acidic but an oxidizing counterion is not required. Figure 7c shows the results from the combination of bismuth, copper, and sodium with the bismuth nitrate dissolved in 1.5 M HNO3. The resulting false color image shows a large bright area in the bottom right triangle indicating that the active material is a mix of all three metals and could fall within a wide range of compositional ratios of Bi, Cu, and Na. Note that the binary mixture of Bi and Cu does not yield as high a photocurrent as

the ternary mixture of bismuth, copper, and sodium. When the concentration of HNO3 in the bismuth solution was increased to 2.5 M, the photoactivity again increases (Figure 7d), as evidenced by the two large bright areas on the BiCuNa and BiCuLi triangles, which are now much brighter than the copper oxide reference. Note the difference between the bright triangles compared to the copper reference in Figure 7d and c and also note the slightly better performance of sodium over lithium. We did not observe by SEM-EDS any aluminum from the template being “washed” off into our film even with the highly acidic spray solution. Next, we compared alkali metals Li, Na, K, and Rb using four rows of our 5 × 5 grid-dottemplate. All rows were first sprayed with 0.08 M Cu(NO3)2, then 0.16 M Bi(NO3)3 dissolved in 2.5 M HNO3, and then a 0.075 M alkali metal ion (all Cu, Bi, and alkali metal solutions contained 10% EtOH) with the top row LiNO3, the next NaNO3, then KNO3, and the bottom row RbNO3. Sodium worked the best by a small amount (Figure 8a), with lithium and potassium having similar performance and rubidium clearly exhibiting the poorest. Interestingly, the ionic radius of Ag+ is similar to sodium, 1.15 and 1.13 Å (four coordinate) or 1.29 and 1.16 Å (six coordinate) respectively with K+ being larger, 1.51 and 1.52 for the two different coordination environments.32 It appears that a monovalent ion with an ionic radius around 1.1 Å seems to be ideal. Kang et al. argues that Ag+ has a similar size to Bi3+ and can substitutionally replace bismuth in the CuBi2O4 structure, which leads to a higher hole concentration and thus improved hole transport properties.33 The alkali metals may behave in a similar way. BiCuNa Film Optimization. The large active area on the scan of the Bi, Cu, and Na plate indicates a wide range of compositions that yield photocurrent. To find the optimum 495


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rows on the grid. The optimal thickness was achieved by spraying three passes of the copper solution, followed by three passes of mixed bismuth/sodium solution (Figure S5). To test how the addition of Na compares to Ag, we compared our optimized Bi:Cu:Na material to the best Bi:Cu:Ag composition reported by Berglund et al., 22:11:3.30 A plate with both materials was prepared by spray pyrolysis. The spray pattern and false color image of the scan are shown in Figure S6. The rows containing Na show significantly greater photoactivity than those with Ag. One can get a sense of the reproducibility of the method by looking especially at Figures S6 and 8a, c, and d. In each of these figures, we are changing one thing at a time and all five spots in each row are produced with the same spray composition so one can compare the color intensity (photocurrent) of the five spots to get a measure of reproducibility. In Figure S6, rows 1 and 3 are the same composition, and rows 2 and 4 are another unique composition. Here one can compare reproducibility across different spray heights, spray times, and screening times. The reproducibility is very good for our relatively simple apparatus, but not perfect. Note that in Figure 8D, the third spot in row 4 is slightly dimmer than the other 4 spots. Materials Characterization. The SEM of a freshly broken edge (Figure 9) reveals a layered structure. The glass substrate

composition, the dot-grid pattern was used to spray different concentrations in rows and columns. The copper was sprayed in the horizontal rows, bismuth in the vertical rows, and a single concentration of sodium (0.075 M) was sprayed over the whole plate. The spray pattern and false color image of the scan measured in 0.1 M Na2SO4 solution are shown in Figure 8b. Many dots on this scan show great promise. The spot representing a 2:1 ratio of Bi to Cu shows exceptional photoactivity; however, several other spots richer in copper are even better. Berglund et al. also observed this but found that when these Cu rich materials were used as photocathodes they had higher dark currents and were less stable than materials composed with a 2:1 Bi to Cu ratio. In addition, the largest peaks in the XRD of the copper rich films matched Kusachiite, CuBi2O4.30 Thus, in this work we focused on materials with the 2:1 Bi:Cu ratio by spraying concentrations of 0.16 and 0.08 M, respectively. Note the current for the 2:1 spot is ∼4 times larger than the current from the pure copper oxide reference dot (top right). We have found that the mixing and/or spray order (layering) of the metal solutions can greatly affect the photoactivity. For example, is the best film prepared by spraying a homogeneous mixture of the metal ions or by spraying the metal solutions individually and, if so, in what order? Therefore, to determine the optimal layering and mixing, several plates were prepared to test all combinations. For example, Figure 8c shows the result of a plate in which the top row was prepared by spraying one pass of copper, then bismuth followed by sodium. In all cases, the Bi:Cu:Na concentrations were 0.16:0.08:0.075 M. The next row was one pass of copper, followed by one pass of a mixture of bismuth and sodium. Third row was one pass of bismuth followed by one pass of a mixture of copper and sodium. The forth row was one pass of bismuth, then copper, followed with sodium. The last row is one pass of bismuth followed by one pass of copper and no sodium. The other combinations can be seen in Figure S3. Generally, spraying copper first gives the best results. Spraying a mixed copper/bismuth solution also works well, but the solution is not stable over extended periods. Since row one and two yield similar results in Figure 8c and row two required less passes of the sprayer, for the remainder of the study, Cu was deposited first followed by a mixed Bi/Na solution. Additionally, when spraying thicker films, it was found that alternating between single passes of Cu and Bi/Na solution yields the same photoactivity as spraying 2−3 copper solution passes first followed by 2−3 passes of mixed bismuth and sodium solution (Figure S4). Various concentrations of sodium were tested by spraying the whole plate (dot-grid pattern) with copper (0.08 M), then bismuth (0.16 M), and then varying the concentration of sodium in horizontal rows. The false color image of the scan at −0.6 V in a 0.1 M Na2SO4 solution is shown in Figure 8d. The optimal composition ratio of the three metals was determined to be 0.16 M: 0.08 M: 0.068 M Bi:Cu:Na. Compared to the reference dot (top right), the average maximum intensity for the row is ∼7 times that of the pure copper oxide reference. Optimizing the thickness of the film is also important. The film should be sufficiently thick to absorb as much light as possible. However, if the film is too thick the minority carriers can be trapped in the material before being swept to the surface to reduce the electrolyte.7 To optimize the film thickness, the number of passes of the copper solution and the mixed bismuth and sodium solution were varied in horizontal

Figure 9. Freshly broken edge showing the depth profile of the CuBiNa film: magnification of 15 000× at a pressure of 40 Pa.

on the bottom is coated with the 370 nm thick FTO. The EDS spectra (Figure S7) indicate that on the surface of the FTO is a 200 nm sodium doped copper bismuth oxide-rich layer and the outermost layer is a 670 nm bismuth sodium oxide-rich film. The edge was investigated using the variable-pressure mode on the SEM. At this angle the insulating glass substrate comes into the field of view, causing significant charging. Variable pressure mode was used to alleviate charging, but the residual atmosphere results in scattering of the electron beam. As a result, the precise determination of the elemental composition is not possible in VP mode. Also, the limited thickness of the layers leads to the EDS being a superposition of signals from the adjacent layers. The observation of the copper/bismuth rich layer indicates that bismuth has diffused into the copper layer which was sprayed first, followed by the mixed bismuth/sodium solution. This is supported by the XRD (Figure S8), which shows the presence of kusachiite, CuBi2O4 (PDF no. 00-042-0334). The XRD also indicates the presence of bismite, Bi2O3 (PDF no. 00-041-1449), and possibly sodium bismuth oxide, Bi7.38Na.62O11.38 (PDF no. 00-050-0371), as well as the underlying cassiterite, SnO2 (PDF no. 00-041-1445). These four phases match all the major peaks in the XRD, leaving very few residual lines. SEM images of the surface (Figure S9) show a relatively smooth surface with no cracking, but there are 496


Research Article

ACS Combinatorial Science some round openings in the outer bismuth-rich layer exposing the BiCu rich layer below. The surface contains different sized particles, which could be either different size particles of the same phase or a mix of distinct bismite and sodium bismuth oxide phases. Sodium bismuth oxide has overlapping peaks with bismite and kusachiite, so its presence cannot be ruled out. The morphology of the underlying BiCu rich phase is similar to that observed for jet deposited BiCu films.30 The SEM also shows occasional long crystals on the surface. These are composed of mostly sodium, carbon, and oxygen as shown by the EDS elemental maps (Figure S10), but the XRD does not confirm the presence of a Na2CO3 phase. The underlying BiCu rich phase can clearly be seen through the openings in the surface BiNa rich phase. EDS elemental maps, Figure S11, show that, in places where top bismuth layer is absent, the copper bismuth layer can be plainly observed. Na is seen throughout the material but it is more concentrated in the bismuth phase. In order to sort out the overlapping reflections in our XRD, we sprayed thick films by spray pyrolysis on FTO of both of the individual materials. For the bismuth oxide film, we sprayed a thick film using a 0.16 M Bi(NO3)3 mixed with 0.068 M NaNO3 in a 2.5 M HNO3, 10% ethanol solution. The resulting film was white and the measured XRD matches all peaks of the indexed of bismite (PDF 00-041-1449) with no residual peaks (Figure S12). The above bismuth/sodium solution was mixed with a 0.80 M Cu(NO3)3 dissolved in 10% ethanol and a thick film was sprayed to produce a brown film. The XRD (Figure S13) matched all the peaks for CuBi2O4 kusachite (PDF no. 00-042-0334); however, there are some small residual peaks that do not match anything in the ICDD database that contain any combination (including the elements, binary, and ternary compounds) of Na, Cu, Bi, and O, which were the only elements observed in the film by SEM-EDS. This may represent a new phase or the presence of the sodium atoms taking up regular positions in the kusachite lattice. The SEM of a freshly cleaved edge, Figure S14, shows the film to be just over 4 μm thick. Sodium bismuth oxide was not observed in either film. The photoelectrochemical characterization of the BiCuNa film compared to both an optimized BiCu and Cu oxide films are shown in the Supporting Information. The BiCu films were prepared the same way as the BiCuNa except that the bismuth solution contained no sodium ions. The Cu oxide films were optimized for thickness by running a series of different thicknesses on the 25 dot template finding that 2.5 passes produced the best photocurrent. To summarize, the absorbance spectrum of the BiCuNa film compared to the BiCu reference film, Figure S15, shows that the presence of Na increases the absorbance in the 600−800 nm range. However, the indirect bandgap Tauc plots for both materials are very similar when calculated from the absorption spectra and suggest an unrealistic band gap of 1.23 eV. Tauc plots generated from the IPCE34 yield more realistic indirect band gaps of 1.69 and 1.87 eV for the BiCuNa and BiCu oxides respectively (Figure S15). Kang et al. report 1.83 eV direct band gap for electrochemically deposited CuBi2O4.33 Berglund et al. did not observe a straight line with a sharp onset in a Tauc plot for either direct or indirect band gap for their drop cast or spray pyrolysis CuBi2O4 films.35,36 The onset of photocurrent, Figure 10, produced by the BiCuNa material occurs around 750 nm, almost 100 nm further into the red than the BiCu material, which is consistent with the absorption spectrum.

Figure 10. Comparison of the photocurrent action spectra of the BiCuNa oxide material (green), BiCu oxide (red), and Cu oxide (blue) with an applied voltage of −0.2 V verse SCE. Backside illumination in N2 purged 0.10 M Na2SO4 solution.

When the material is illuminated from the front side, the copper reference material shows a much greater IPCE than both the BiCuNa and BiCu materials. We believe that the layering of the films is responsible for this effect. In the preparation of both materials, the Bi layer which is white, is sprayed last, giving the film a top layer that is very efficient in scattering the light. When the films are illuminated from the back, there is a decrease in IPCE for all three materials, but the BiCuNa film has the best performance. The overall reduction in IPCE for all three films is most likely due to the poor carrier mobilities typically observed in oxides, which causes the photogenerated carriers produced near the substrate surface and far away from the solution interface to be lost to recombination.35 Unlike front side illumination, the copper oxide film IPCE drops off at lower wavelengths. This is most likely due to the decreasing penetration depth of the light at lower wavelengths, thus resulting in the creation of photogenerated carriers further and further from the solution interface. The optimum thickness of a film is determined by both its ability to absorb light and conduct the photogenerated carriers through the material before they recombine. The optimum thickness of the copper film is thinner than that of the BiCuNa film, suggesting that BiCuNa is either better at conducting carriers or has less recombination sites than copper oxide. Competitive, chopped current−voltage (IV) curves, Figure S16, are measured simultaneously in the same cell, under the same conditions using a bipotentiostat and duel electrode clamp (see Experimental Section). We observed that at low applied potentials the photocurrent produced by the BiCuNa material was more than twice as large as that produced by the Cu oxide reference electrode; however, at high negative applied potentials the Cu electrode outperforms the BiCuNa electrode. This photocurrent performance, 0.2 mA/cm2, is very similar to what Berglund had observed for both silver doped CuBi2O4,30 0.18 mA/cm2, and backside illumination of a drop-cast CuBi2O4,0.24 mW/cm2, under the same illumination intensity of 100 mW/cm2.35 Also shown in Figure S16 is the comparison of the first two IV scans of freshly prepared BiCuNa and BiCu films, again measured simultaneously. During the first scan the photocurrent produced by the BiCu film decreases relative to the BiCuNa electrode over the scan and is significantly smaller, about one-third that the photocurrent produced by the BiCuNa film by the time the second scan was recorded. Figure S17 shows the photocurrent versus time produced by both a BiCuNa and Cu oxide 497



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electrode simultaneously measured over 8 h. Both photocurrents initially drop quickly and then partially recover, with the copper oxide electrode leveling off after 4 h and then declining while the BiCuNa electrode continues to slowly improve over the 8 h of testing. The presence of Na appears to improve the stability of the film under constant applied potential and illumination. Both the Cu and BiCu oxides film demonstrated significant decline in photocurrent compared to BiCuNa oxide. Kang et al. report that the monovalent Ag+ also significantly suppressed photocorrosion for the photoreduction of O2 in an electrochemically deposited Ag-doped CuBi2O4 film electrode.33

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jordan S. Compton: 0000-0001-7099-3456 Dilek Dervishogullari: 0000-0002-2596-627X Lee R. Sharpe: 0000-0002-0875-5286 Present Addresses †

J.S.C.: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. ‡ C.P: Materials Department, College of Engineering, University of California, Santa Barbara, CA 93106-5050. § D.D.: Department of Chemistry, Vanderbilt University, Nashville, TN 37212.

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CONCLUSION A new inexpensive and practical combinatorial method to discover photocatalysts for solar fuel production utilizing spray pyrolysis has been developed. Spray pyrolysis has the advantage of being a simple and scalable technique for the growth of thin films of any element that can be dissolved in solution. This method allows the preparation of layered structures, depth gradients, and single phase materials. The natural lateral gradient produced in the spray allows the fabrication of four-metals-three-at-a-time triangular patterns to test a compositional array of potential photocatalysts. The apparatus is constructed inexpensively from readily available components, making it accessible to a wide number of laboratories. This is important because of the multitude of possible materials that need to be evaluated to find suitable photocatalysts for solar fuel production. To highlight our methodology, we detailed the discovery and optimization of a sodium-doped copper bismuth oxide photocatalyst. Other alkali metals also work but sodium, possibly due to its ionic radius, results in the highest photoactivity. Using a dot matrix template we were able to spray specific solutions in order to refine the composition, solution spray order/mixing, and thickness. SEM-EDS shows that the optimized film has a layered structure with a sodium-doped bismuth copper oxide layer covered with a sodium-doped bismuth oxide layer. The material was found to perform better than both copper oxide and bismuth copper oxide specifically in its durability under illumination in a photoelectrochemical cell. While more work needs to be done to fully understand and refine the preparation of this new material, it does provide a nice example on how a new photocatalyst can be discovered by combinatorial spray pyrolysis. Our method complements other combinatorial methods in that it allows one to easily evaluate layered structures and single-phase materials, as well as providing a chemically different environment for the formation of materials that might produce different morphologies and metal oxidation states, expanding the number of unique materials tested as potential photocatalysts.

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Research Article

Author Contributions

All authors have given approval to the final version of this manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grinnell College Mentored Advanced Project program. We would like to thank members of the SHArK Project and specifically Bruce A. Parkinson at the University of Wyoming for suppling the laser scanning system whose funding was provided by the National Science Foundation Center for Chemical Innovation in Solar Fuels (grant CHE - 1305124). We would also like to thank Warren Straszheim and Scott Schlorholtz at the Materials Analysis and Research Laboratory, Iowa State University, for materials characterization and helpful discussions.

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ABBREVIATIONS FTO, fluorine-doped tin oxide; SCE, saturated calomel electrode; IPCE, incident photon to current efficiency

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscombsci.9b00042. Photos of the experimental apparatus; additional false color image data; SEM images, elemental maps, and EDS spectra; XRD data; absorption spectra and Tauc plots; and photocurrent versus voltage/photocurrent versus time plots (PDF) 498


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Spray Pyrolysis as a Combinatorial Method for the Generation of

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