Polypyrrole

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Flexible Photocatalytic Composite Film of ZnO-microrods/Polypyrrole Bingxi Yan, Yongchen Wang, Xinyi Jiang, Kaifeng Liu, and Liang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08462 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Flexible Photocatalytic Composite Film of ZnO-Microrods/Polypyrrole Bingxi Yan†#, Yongchen Wang⊥#, Xinyi Jiang†, Kaifeng Liu§, and Liang Guo†‡*

†

Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH 43210, USA

⊥Department

of Biomedical Engineering, The Ohio State University, Columbus, OH 43210, USA

§

Department of Physics, The Ohio State University, Columbus, OH 43210, USA

‡

Department of Neuroscience, The Ohio State University, Columbus, OH 43210, USA

#

Equal Contribution

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ABSTRACT: Zinc oxide (ZnO)-based photoanodes with sunlight photocatalytic activity are widely used in dye-sensitized solar cells. Presently, most of such electrodes are inflexible due to the rigidness of ZnO and substrate, thus hindering their application in flexible electronics. Here, we report a flexible composite film of ZnO microrod arrays and polypyrrole (PPy) featuring significant flexibility, durability, and photocatalytic capability under visible light. In this composite film, the upper section of the ZnO microrods is coated with an approximately 400 nm thick PPy shell, and the lower section of the ZnO microrods is tightly embedded into an underlying PPy base layer, creating an integrated heterogeneous structure. The upper PPy coating shell serves as a photosensitizer for the ZnO-based photocatalysis, while the lower PPy base layer facilitates electron transport to the substrate and mechanically reinforces the ZnO microrod arrays. Under visible light, this facile structure achieves much higher photocatalytic efficiency in comparison to pure ZnO microrod arrays or PPy film, degrading methylene blue at a rate of 0.22% per min. This photocatalytic composite film may find promising applications in flexible solar cells to power stretchable and wearable electronics.

Keywords: polypyrrole, ZnO microrod arrays, photocatalysis, core/shell, carrier recombination


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1. INTRODUCTION Thin solid films of wide bandgap semiconductors, such as zinc oxide (ZnO, band gap ~ 3.0 eV) and titanium dioxide (TiO2, band gap ~ 3.25 eV for anatase), have been widely used in dyesensitized solar cells, self-cleaning glasses, and sunlight-driven water splitters for hydrogen production owing to their sunlight-driven photocatalytic properties.1-4 However, their limited flexibility remains a challenge to applications in flexible electronics. Although ZnO nanorods have been grown on flexible substrates to attain flexibility,5-7 free-standing nanorods easily fell off during recurrent substrate deformation. ZnO nanorods/conducting polymer composites were designed to overcome this challenge.8 In addition to mechanical enforcement, conducting polymers also enhanced the conductivity and photocatalytic efficiency.9-12 Under visible light, conducting polymers as photosensitizers donate photon-induced electrons to ZnO, preserving holes in conducting polymers thus reducing carrier recombination.9,10 To create a flexible photocatalytic film, this study presents a structure comprising vertically aligned ZnO microrod arrays and electrodeposited polypyrrole (PPy). In the composite film, the upper section of the microrods is coated with a thin PPy shell, and the lower section of the microrods is embedded into a thick PPy base layer. In this design, the upper PPy shell serves as a photosensitizer, absorbing visible light and converting photons into free carriers (i.e. electrons and holes),13-16 while the lower PPy base layer prevents the microrods from delaminating from the flexible substrate during deformation and also facilitates electron transport to the substrate.17 The large interface between ZnO microrods and PPy photosensitizer accelerates carrier separation near the interface region and thus enhances the sunlight photocatalytic property. Such films are particularly promising as photoanodes in flexible solar cells to power stretchable electronics.18-19

2. EXPERIMENTAL 2.1. Synthesis of ZnO Microrod Arrays All chemicals were purchased from Sigma-Aldrich and used without further purification unless otherwise specified. ZnO microrod arrays were synthesized via a solution-based method.2 10 nm thick titanium adhesion layer and 50 nm thick gold layer were sequentially deposited onto a 25 µm thick polyester film using an electron beam evaporator (DV-502A, Denton Vacuum). With 3 ACS Paragon Plus Environment


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the coating in contact with the solution, coated polyester films were gently placed to float on an aqueous

solution

containing

25

mM

zinc

nitrate

hexahydrate

(98%),

12.5

mM

hexamethylenetetramine (≥99.0%), and 0.70 M ammonium hydroxide (28.0-30.0% ammonia basis) in an 80 °C oven for 24 hours. The ZnO microrod arrays were washed with distilled water and dried in air. 2.2. Electropolymerization of PPy 6 g polyethylene glycol (PEG, Mw ~400) was dissolved in 45 mL of 2-propanol (anhydrous, 99.5%). Thereafter, 18 mL of boron trifluoride diethyl etherate (≥46.5 % boron trifluoride basis) was slowly added, followed by 220 µL of pyrrole. The solution was degassed on a rotary evaporator (RV10, IKA) for 3 minutes at 150 mbar. ZnO microrod arrays grown on a goldcoated polyester film were immersed into the solution as the working electrode. A silver/silver chloride (Ag/AgCl) wire (4 cm in length, 2 mm in diameter) was used as the quasi-reference electrode,

and

a

platinum

sheet

(30

cm2) was

used

as

the

counter electrode.

Electropolymerization was conducted potentiostatically at 1 V versus the reference electrode on an electrochemical workstation (CHI 760E, CH Instruments) in an ice bath, giving an average deposition rate of 2 nm∙s-1. The film thickness was varied by controlling the deposition time. ZnO-microrods/PPy films obtained were sequentially rinsed by 2-propanol and distilled water for 1 minute, respectively. 2.3. Fabrication of Photocatalytic Molybdenum (Mo)-doped TiO2 films A tiny Mo tablet (3 mm2) was embedded in a TiO2 ceramic target (99.99%, Alluter). Before sputtering, the vacuum chamber was evacuated to 2.0×10-3 Pa. Sputtering was conducted under a pressure of 0.6 Pa (O2 (14.3%)/ Ar (85.7%)). By modulating the sputtering power on the Moembedded TiO2 target and the TiO2 target separately, doping content was adjusted to 0.9 at.% Mo.1 After sputtering, all samples were annealed in a furnace (SX2-8-10, Tianye) under 550 °C for 2 hours. A three-dimensional (3D) optical profiler (Zeta-20, Zeta Instruments) indicated the Mo-doped film had a thickness around 300 nm. 2.4. Characterization of Morphology and Composition The surface morphology of the composite film was characterized by a scanning electron microscope (SEM) (Helios NanoLab 600, FEI). The fourier transform infrared (FTIR) spectra 4 ACS Paragon Plus Environment

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were collected on an FTIR spectrometer (Cary 660, Agilent Technologies) in the wavenumber range from 500 cm-1 to 2000 cm-1. Raman spectra were collected on a handheld spectrometer (ProgenyTM, Rigaku Analytical Devices) in the wavenumber range from 600 cm-1 to 2000 cm-1. 2.5 Characterization of Photocatalytic Properties The ultraviolet-visible (UV-Vis) absorption spectra of PPy films of different thicknesses were collected on a UV-Vis spectrometer (Cary 50, Agilent Technologies). Samples were illuminated by a xenon lamp (CHF-XM35-500W, Beijing Trusttech Technology) with a UV filter (Kopp 3850, Newport Industrial Glass) cutting off light wavelengths below 380 nm. Photocatalytic activity under visible light (20 mW·cm-2) was assessed by measuring the UV-Vis absorbance of methylene blue (MB) solution degraded by the composite films at a wavelength of 660 nm.

3. RESULTS AND DISCUSSIONS ZnO-microrods/PPy films were fabricated and synthesized following the scheme in Figure 1. Vertically aligned ZnO microrod arrays were grown on a 25 µm thick polyester film pre-coated with 10 nm thick titanium and 50 nm thick gold, and PEG-borate ester doped PPy (PPy/PEGborate) was thereafter electropolymerized onto the as-synthesized microrod arrays, forming an upper PPy shell and a lower PPy base layer. An approximately 3 µm thick PPy base layer was attained at an optimized density of microrods as shown in a 3D optical image (Figure S2b) reveals forest-like arrays, and the average height of the microrods was 6.5 ± 0.012 µm. Similar to other polyol-borate esters doped PPy films,20 the composite films presented a black color with high flexibility. Delamination of ZnO microrods was not observed from the composite films even after continuous twists and rolls, but it consistently occurred for pure ZnO microrod arrays grown on a soft substrate without PPy (Figure S1).



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Figure 1.

A schematic illustration of the fabrication and synthesis of a flexible ZnO-

microrods/PPy composite film (not to scale). a) A 10 nm thick layer of titanium and a 50 nm thick layer of gold were sequentially deposited onto a 25 µm thick polyester film. b) ZnO microrod arrays were grown on the gold-coated polyester film. c) PPy/PEG-borate was electrodeposited onto the ZnO microrod arrays. d) A photo showing a highly flexible ZnOmicrorods/PPy composite film (20 mm × 20 mm).

Figure 2 demonstrates the surface morphology of a ZnO-microrods/PPy composite film characterized by SEM. As electropolymerization proceeded, PPy was continuously deposited onto the uncovered gold surface at an average rate of 2 nm·s-1 forming a thick base layer, whilst a thin PPy shell wrapping the upper section of microrods was simultaneously formed, yet at a much smaller rate of 5 nm·min-1 due to the limited conductivity of ZnO. Noteworthy, the distribution density of microrods remarkably affected the conductivity of the substrate and thus the electropolymerization rate of PPy, i.e., too densely packed microrods severely impeded the deposition of PPy (Figure S2 shows an optimized density of ZnO microrods). As seen in the SEM image (Figure 2b), the upper section of ZnO microrods was coated by a thin PPy shell. The average diameter of these coated microrods was measured as approximately 1.8 ±0.25 µm, whereas the average diameter of the bare microrods was measured as approximately 1.0 ±0.25 µm (Figure 2a). Thus, the thickness of the upper PPy shell was estimated to be 400 nm. Interestingly, as electrochemical deposition of PPy progressed, the thicknesses of upper PPy shell and PPy base layer further grew, eventually merging into a monolithic PPy top layer as 6 ACS Paragon Plus Environment

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shown in Figure2c and Figure 2d. The cross-sectional view of a composite film with a 11 µm thick PPy top layer was shown in Figure 2c, where ZnO microrods were buried in the PPy top layer. The top view of the composite film (Figure 2d) reveals a granular surface.



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Figure 2. Surface morphology of ZnO-microrods/PPy composite film (all scale bars refer to 10 µm). a) SEM image of ZnO microrod arrays. b) SEM image of ZnO microrods encapsulated by a 400 nm thick PPy shell. c) SEM image shows cross-sectional details of the ZnO microrod arrays which are buried into 11 µm thick PPy top layer. d) SEM overview reveals a granular morphology of PPy top cover.

Carrier density in a photocatalytic film depends on the generation of electron-hole pairs as well as the recombination ratio. Thus, the thickness of the upper PPy shell needs to be carefully controlled, since it remarkably affects the absorption of visible light and carrier generation. To 8 ACS Paragon Plus Environment

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determine a suitable thickness of the PPy shell for adequate light absorption, UV-Vis absorption spectra of the PPy/PEG-borate films of a series of thicknesses, i.e. 50 nm, 200 nm, 400 nm, and 800 nm were collected (Figure 3). These films were potentiostatically deposited on transparent indium tin oxide (ITO)-coated glass slides. At 1V (vs Ag/AgCl), an average deposition rate of 2 nm s-1 was achieved and different thicknesses were obtained by varying deposition time, and the actual thicknesses were measured by a non-contact optical profiler as 42 nm (T1), 220 nm (T2), 410 nm (T3), and 860 nm (T4).

Figure 3. UV-Vis absorption spectra of PPy/PEG-borate films of different thicknesses (an arbitrary unit was used for the absorbance; T1 = 42 nm, T2 = 220 nm, T3 = 410 nm, and T4 = 860 nm).

In Figure 3, thicker PPy films demonstrated higher absorption at all wavelengths in the visible light range (380-760 nm). From T4 to T1, films turned from completely black to nearly transparent. The characteristic absorption peak of PPy corresponding to a band gap of 2.3 eV was pronounced at 546 nm on the spectrum of T4, and was associated with the excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in PPy.27 Interestingly, this peak shifted left as the thickness decreased, i.e. a blue shift effect as a result of a widened band gap. This was similar to the quantum effect in thin solid semiconductor films, i.e. strong constraints on the electron motion along the perpendicular direction appear as film thickness decreases to tens of nanometers, while the loss of 9 ACS Paragon Plus Environment


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microstructure regularity in thinner films also accounted for the smaller peak intensity.28 As energy bands represent delocalized electrons in a periodic potential field, when the dimension along a certain direction is reduced—for example, limited thickness of PPy films—more localized electrons eventually result in increased band gaps.29 The peaks at 730 nm on the spectra of T3 and T4 correspond to a band gap of 1.7 eV, indicating the energy required to delocalize polarons and bipolarons into free carriers,30 and the peaks at 470 nm and 390 nm are a result of bipolaron absorption in thin PPy films.31-32 Based on such a result, T3 and T4 were chosen as the thicknesses of the PPy shells encapsulating the upper section of ZnO microrod arrays, and the composite films were referred as ZnO-microrods/PPy(400) and ZnO-microrods/PPy(800), respectively.


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Figure 4. FTIR a) and Raman b) spectra of a PPy/PEG-borate film (11 µm thick) and a flexible ZnO-microrods/PPy(400) composite film. FTIR and Raman spectra of a pure 11 µm thick PPy film and a ZnO-microrods/PPy(400) composite film were given in Figure 4. Both the FTIR and Raman spectra of the PPy film agreed well with previous reports, validating the efficient doping of PEG-borate macromolecular counterions in PPy.

20-21

Specifically, in the FTIR spectrum of PPy, the peaks at 1572 cm-1 and

1478 cm-1 were ascribed to the C=C stretching and vibration of pyrrole rings, and the peak at 1336 cm-1 was ascribed to the anti-symmetrical C−N stretching.10 The peaks at 1048 cm-1 (=C−H) and 917 cm-1 (C−C out of phase) were correlated to the doping state of PPy.16 These characteristic peaks were reserved in the FTIR spectrum of the composite film ZnOmicrorods/PPy(400), but shifting slightly to a lower wavenumber. In the composite film, characteristic peaks of ZnO at 1750 cm-1, 1362 cm-1, 1017 cm-1 and 729 cm-1 were observed, while PPy peaks at 1572 cm-1 and 1478 cm-1 were reduced due to breached conjugation and molecular order in PPy shell.10 In addition, the shift of N−C structure from 1206 cm-1 to 1195 cm-1, as well as the enhanced peak intensity of the peaks at 790 cm-1 and 917 cm-1, suggested increased dipole moment from localized electrons in the composite film due to interaction between ZnO microrods and PPy shell.22-23 Additional evidence could be found in the Raman spectra in Figure 4b: the peaks at 1323 cm-1 and 1598 cm-1 were ascribed to the ring stretching and C=C stretching,24 the peak at 1475 cm-1 was ascribed to the C−N stretching,25 and the peaks at 918 cm-1 and 1040 cm-1 were ascribed to the deformation of bipolaron ring and the C−H bonds in plane vibration.26



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Figure 5. EIS spectra of ZnO microrod arrays a) and ZnO microrods/PPy(400) b) scanned at 105-10-2 Hz under open circuit potential in phosphate buffered saline (PBS) solution. The insert in a) zooms in the high-frequency response of ZnO microrod arrays, and the insert in b) represents an equivalent dual-RC circuit for the core/shell structure in ZnO-microrods/PPy(400) composite film.

Electrochemical impedance spectroscopy (EIS) spectra of ZnO microrod arrays (Figure 5a) and ZnO-microrods/PPy(400) composite film (Figure 5b) were collected with the same electrode area of 25 mm2 to probe the conductivity, structure and charge transport at the electrode/film interface. ZnO microrods showed large impedance at small frequency below 0.1 Hz, and thus for 12 ACS Paragon Plus Environment

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clarity Figure 5a zoomed in the range from 105 to 10-1 Hz, with an insert further revealing its high frequency response in comparison to the composite film in Figure 5b. Both EIS plots started from 63.3 Ω rather than zero due to the resistance of electrolyte solution (Rs). The impedance of pure ZnO microrod arrays reached 215.7 kΩ at the frequency of 0.1 Hz, whereas a much smaller impedance of the composite film (866.7 Ω) was observed at the same frequency, indicating rapid carrier exchange across the interface in the composite film. In the insert of Figure 5a, the frequency at the curve apex (fmax) was found 2.15 kHz. Given time constant ߬ = 1/߱ = ܴ௖௧ ‫ܥ‬ௗ , where ‫ܥ‬ௗ is the capacitance of a double layer, and ܴ௖௧ is the charge transfer resistance that was 280 Ω (diameter of the curve), ߬ and ‫ܥ‬ௗ could be calculated as 0.07 ms and 0.26 µF, respectively. The impedance plot of ZnO-microrods/PPy(400) in Figure 5b reveals a typical dual-resistorcapacitor (dual-RC) circuit for core/shell microstructures.33-34 Given that the frequency at the first curve apex (fmax1) was 2.6 kHz, and the frequency at the second curve apex (fmax2) was 97.7 Hz, τ1 and τ2 were calculated as 0.06 ms and 1.6 ms, respectively. Given that Rct1 was 100 Ω and Rct2 was 250 Ω, Cd1 and Cd2 were obtained as 0.6 µF and 6.4 µF, respectively. It is noteworthy that the curve shifted downward to a more negative Z´´ range leaving a flattened semicircle above the Z´-axis, which was the dispersion effect due to nonuniform distribution of electric field in the double layer at a rough interface.35 As photon-induced electrons in the upper PPy shell flowed to ZnO and were readily transferred to the substrate via the conducting PPy base layer, the impedance analysis suggested the upper thin PPy shell efficiently accelerated carrier transport while maintaining large surface roughness, which is favorable for the photocatalytic process by offering large specific surface area.



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Figure 6. Photocatalytic activity and mechanism of carrier transport. a) Degradation rates of methylene blue by photolysis, PPy/PEG-borate film, ZnO microrod arrays,

ZnO-

microrods/PPy(400) composite film, and ZnO-microrods/PPy(800) composite film under visible light. b) A schematic illustration of carrier separation and transport mechanism at the ZnO/PPy interface.

Figure 6a presents degradation rates of MB by photolysis, PPy/PEG-borate film, ZnO microrod arrays, ZnO-microrods/PPy(400) composite film, and ZnO-microrods/PPy(800) composite film under visible light (20 mW·cm-2 irradiance). Each sample had a surface area of 2 cm2. Before illumination, all samples were immersed in 10 mL of methylene blue solution (10-5 M) for 1 hour to ensure that absorption equilibrium was achieved at the interface. After two hours illumination, ZnO-microrods/PPy(400) composite film demonstrated the highest photocatalytic capability (27.2%), ZnO-microrods/PPy(800) composite film showed the second highest photocatalytic capability (17.4%), while pure photolysis and the as-deposited PPy/PEG14 ACS Paragon Plus Environment

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borate film only demonstrated limited photocatalysis. Interestingly, samples covered by an 11 µm thick PPy top layer did not show detectable photocatalysis, similar to the PPy/PEG-borate film of the same thickness. This could be explained by the fact that photoinduced carriers suffer from high recombination ratio in PPy and cannot reach the surface, leading to reduced carrier density. On the other hand, when the thickness of the upper PPy shell was 200 nm or smaller, the composite film also demonstrated very limited photocatalytic efficiency under visible light. Thus, either a thick (above 10 µm) or an ultrathin (below 200 nm) PPy shell should be avoided in the photocatalytic composite film. Noteworthy, under visible light, unannealed ZnO microrod arrays didn’t present detectable photocatalytic efficiency, but a notable degradation rate was observed from ZnO microrod arrays annealed at 300° for 2 hours (Figure 6a) owing to enhanced crystallinity.1,28 Undoped ZnO can only absorb UV light of a wavelength below 400 nm due to a wide band gap around 3.0 eV,3-4 and is mostly transparent to visible light. Although thick PPy (thickness larger than 1 µm) with a much smaller band gap can generate carriers effectively under visible light, photoinduced electrons and holes dissipate rapidly as a result of fast recombination. This was confirmed by the observation that thick PPy/PEG-borate films didn’t show detectable catalytic efficiency. The combination of a thin PPy shell and ZnO microrod arrays achieved a more significant photocatalytic property than either material alone owing to high specific surface area as well as reduced recombination ratio. Its unique forest-like arrays offered a much higher specific surface area than conventional thin solid films. More importantly, the accelerated carrier separation across the large interface between ZnO and PPy shells, together with the rapid dissipation of electrons via the PPy base, reduced the carrier recombination in the PPy shell effectively, thereby enhancing its photocatalytic property (Figure 6b). Specifically, photoexcited electrons in the conduction band (CB) of PPy transferred to the CB of ZnO microrods of lower energy, whereas the holes in the valence band (VB) of ZnO microrods flowed to the VB of PPy as holes preferred higher energy levels.10 As a result, in the PPy shell, electron density near the interface was smaller than that in regions further. This density gradient accelerated electron diffusion to the interface, facilitated carrier separation across the interface, and eventually raised the collection ratio of carriers in the near-interface-region.28 In this way, holes in the thin PPy shell could reach the surface with minimum recombination, while electrons could be rapidly delivered to substrate via the thick conducting PPy base layer rather than accumulated in the 15 ACS Paragon Plus Environment


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lower section of ZnO microrods.

11,16

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When the PPy shell became too thick, however, most

electrons within it recombined with holes before reaching the interface. Thus, the thickness of the upper PPy shell needs to be optimized, and a thickness around 400 nm was found more efficient than a thickness around 800 nm. Under natural sunlight, ZnO-microrods/PPy(400) achieved 85.95% degradation of MB after three hours illumination, giving a rate of 0.48% min-1 which is higher than 0.21% min-1 achieved by a Mo-doped TiO2 film of the same surface area (Figure S3).36 The degradation rate of this flexible composite is comparable to the reported degradation rates of 0.44% min-1 by PPy/TiO2 nanocomposites37 and 0.49% min-1 by ZnO/TiO2 nanotube arrays under natural light,38-39 yet smaller than the degradation rates by aqueous dispersion solution of PPy-encapsulated nanoparticles such as PPy/TiO2 (0.74% min-1) and PPy/ZnO (0.54% min-1).40-41 Another advantage of this flexible composite film is the enhanced robustness against mechanical failures during continuous deformation, otherwise ZnO microrod arrays easily fell off from the soft substrate, as shown in Figure S1. This unique structure integrating flexibility, sunlight-driven photocatalytic property, and high mechanical strength is particularly promising for its application in flexible electronics.

4. CONCLUSIONS A photocatalytic flexible ZnO-microrods/PPy composite film was attained by electrodepositing PPy onto a ZnO microrod array grown on a gold-coated flexible polymer substrate, forming an upper PPy shell and a lower PPy base layer. The upper PPy shell served as a photosensitizer, while the lower PPy base layer dissipated electrons readily to the gold-coated substrate and also made the composite film robust against delamination. The accelerated carrier separation at the ZnO/PPy interface enabled a much higher photocatalytic activity than that of pure ZnO microrod arrays or PPy films. This flexible photocatalytic composite film will find promising applications as photoanodes in flexible solar cells to drive wearable and stretchable electronics.

ASSOCIATED CONTENT 16 ACS Paragon Plus Environment

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Supporting Information Delamination of pure ZnO microrods after recurrent deformation, microscopy images, and a comparison of sunlight-driven photocatalytic activity with conventional solid films (Mo-doped TiO2) are provided. This material is available free of charge via the internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This research was partially supported by the Juvenile Diabetes Research Foundation (JDRF) through the Award No. 2-SRA-2016-237-Q-R and the 2016-2017 OSU IMR Facility Grant through the Award No. IMR-FG0183. Y. Wang was supported by the 2015-2016 OSU HHMI Med into Grad Scholarship. We sincerely thank Drs. Luis Rodriguez-Saona, Mei-Ling Shotts, and Didem Peren Aykas for help with the UV-Vis, Raman, and FTIR spectroscopy.



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Table of Contents Graphic


Polypyrrole

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