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Hybrid (Organic/Inorganic) Electrodes From Bacterially Precipitated CdS for PEC/Storage Applications Yaying Feng, Edgard Ngaboyamahina, Katherine E. Marusak, Yangxiaolu Cao, Lingchong You, Jeffrey T. Glass, and Stefan Zauscher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11387 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry

Hybrid (Organic/Inorganic) Electrodes from Bacterially Precipitated CdS for PEC/Storage Applications Yaying Feng,†,a Edgard Ngaboyamahina,‡,a Katherine E. Marusak,† Yangxiaolu Cao,§ Lingchong You,§ Jeffrey T. Glass,‡ Stefan Zauscher*,† †

Department of Mechanical Engineering & Materials Science, Duke University, Durham NC, 27708 USA ‡ Department of Electrical & Computer Engineering, Duke University, Durham NC, 27708 USA § Department of Biomedical Engineering, Duke University, Durham NC, 27708 USA * Corresponding author. Department of Mechanical Engineering and Materials Science, Duke University, FCIEMAS 3385, 101 Science Drive, Durham NC 27708 USA. E-mail: [email protected], Phone: +1(919) 660-5360, Fax: +1(919) 660-8963 a These authors contributed equally to this paper

Abstract Hybrid organic-inorganic compounds are receiving increasing attention for photoelectrochemical (PEC) devices due to their high electron transport efficiency and facile synthesis. Biosynthesis is a potentially low-cost and eco-friendly method to precipitate transition metal-based semiconductor nanoparticles (NPs) in an organic matrix. In this work we examine the structure and composition of bacterially precipitated (BAC) cadmium sulfide (CdS) NPs using electron microscopy, and we determine their PEC properties and the energy band structure by electrochemical measurements. In addition, by taking advantage of the organic matrix, which is residual from the biosynthesis process, we fabricate a prototype photo-charged capacitor electrode by incorporating the bacterially-precipitated CdS with a reduced graphene oxide sheet. Our results show that the hydrophilic groups associated with the organic matrix make BAC CdS NPs a potentially useful component of PEC devices with applications for energy conversion and storage.

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Introduction Cadmium sulfide (CdS) is one of the broadly studied semiconductor material due to its wide, direct bandgap (~ 2.4 eV) in bulk.1–7 CdS is of interest for planar photovoltaic systems as the window layer in the form of polycrystalline thin films,8–16 for water splitting catalysts in the form of nanoparticles (NPs),17–20 for electrochemical photoanodes in the form of NPs or polycrystalline thin films,21–30 and for photodegradation catalysts in the form of NPs to degrade large organic molecules into smaller units.31–35 This wide variety of applications has led to intensive studies on how to synthesize and deposit CdS, including chemical bath deposition (CBD),36–42 electrodeposition,43–47 infrared pulsed laser deposition,48–50 and a range of other physical deposition methods including magnetron sputtering51–55 to form CdS thin film layers. These methods are generally simple and cost efficient (in the case of CBD and electrodeposition), and are able to produce high quality CdS crystals (in the case of infrared pulsed laser deposition and magnetron sputtering),51,56–62 and thus have been well-studied.57,59,63,64 However, these processes all require additional processing steps such as heating, pulling a vacuum, or generating high voltages.65 The biosynthesis of transition metal compounds, such as CdS, is a relatively new method, first reported in 1989,66 and has seen increasing attention.66–70 As an alternative synthesis method, biosynthesis is simple and occurs at low temperature (37 °C). Furthermore, the crystal size can be controlled through the reaction conditions, and the residual organic matrix, typically associated with biosynthesized NPs69–73 may enable new device applications for CdS.74 However, the photoelectrochemical (PEC) properties of biosynthesized CdS, the focus of this work, have not yet been explored in detail. We previously showed that genetically engineered Escherichia coli (E. coli) can precipitate CdS NPs with control over particle size and thus bandgap.75 By manipulating the concentration of 2 ACS Paragon Plus Environment

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Cd2+ and the cysteine precursor added into the culture medium, the crystal size of the precipitated CdS can be controlled from 3-15 nm. In the present work we studied the physical and PEC properties of these bacterially precipitated (BAC) CdS NPs. In addition, taking advantage of the residual organic matrix associated with the biosynthesized CdS nanocrystals, we integrated these hybrid organic-inorganic nanoparticles with reduced graphene oxide (RGO) sheets, and fabricated a photoelectrode that can store charge, demonstrating the concept of a simple, multifunctional device made from biosynthesized materials.

Experimental Section CdS precipitation. For a 500 ml culture (precipitating ~ 40 mg CdS), an isolated E. coli colony was taken from an agar plate and inserted into 5 ml of Miller’s LB broth containing 75 mg/mL carbenicillin and shaken for 24 hours in a 37 °C incubator. After growth, the 5 ml bacterial suspension was mixed with 500 ml modified M9 medium containing 75 mg/mL carbenicillin, 0.2 mM K2SO4, 100 µM IPTG, and 1 mM cysteine. This 500 mL culture was subsequently shaken for a total of 24 hrs, however after 10 hrs of growth, CdCl2 was added to the culture to a final concentration of 0.2 mM. After 24 hrs of growth yellow precipitate (CdS) was formed and fell out of solution. To harvest the CdS, the medium with CdS was centrifuged at 2800 rpm for 20 min to isolate the solid material from the medium. The solid material was then suspended into 5 ml QIAGEN P2 buffer and 5 ml deionized (DI) water simultaneously. The new mixture was sonicated for 20 min to ensure a fully lysed bacterial culture. Then the bacterial debris (suspended in the buffer) was removed by centrifuging at 14000 rpm for 15 min. To further remove the buffer from the CdS



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NPs, we added DI water followed by another round of centrifugation. Finally, the CdS NPs were resuspended into DI water and stored in a plastic tube covered with foil to avoid light exposure, at 4 °C. Before use, the CdS NPs were resuspended by shaking for 1 min. Scanning electron microscopy (SEM)/Electron dispersive X-ray spectroscopy (EDX). For SEM/EDX analysis, 40 µl of the CdS suspension was drop casted onto a cleaned, 1.27 cm × 1.27 cm silicon wafer chip and dried in the chemical hood overnight at room temperature (~ 25 °C), covered with aluminum foil to avoid light exposure. A FEI XL30 SEM-FEG and a FEI XL30 ESEM with Bruker XFlash 4010 EDS detector were used for SEM/EDX characterization. For SEM imaging, the acceleration voltage was set to 20 kV and the working distance was set to 15 mm. For elemental composition measurements using EDX, the working distance was set to 12 mm, with a magnification of 800×. To obtain representative values for the CdS concentration, we averaged the measurements from three randomly chosen points for each of the 5 different samples. Transmission electron microscopy (TEM). For TEM imaging, the samples were grown and purified as described above, however, the solution was diluted further in DI water (10-fold) and 2 µl of the solution was drop casted onto Formvar-coated, Cu TEM grids. The samples were then covered in aluminum foil and allowed to air-dry overnight. TEM images were acquired on a FEI CM12 with 200 kV accelerating voltage. Particle diameters were determined by manual measurement on 10 TEM images using ImageJ.75 X-ray diffraction analysis (XRD). 50 µl of the CdS suspension were drop casted on a cleaned 1.27 cm × 2.54 cm glass slide and heat treated at 450 °C for 15 mins in a tube furnace under ambient conditions. XRD measurements were performed on a Panalytical X’Pert PRO MRD HR X-Ray Diffraction System, with 1/2° divergence slit, a step size of 0.02°, and an acquisition 4 ACS Paragon Plus Environment

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time/step of 0.75 s. The pre-set count was 10000, and the scan angles ranged from 20° to 60° (2θ). The diffraction spectra were smoothed and peaks were fitted manually using OriginPro 8.5 software. UV-vis spectroscopy. The samples used for UV-vis spectral analysis were identical to those for SEM and EDS experiments. A Shimadzu UV-3600 UV-Vis-NIR Spectrophotometer with a reflectance attachment was used to test the absorption performance of the BAC CdS NPs. During the measurement, a medium scan speed was used with a 1 nm sampling interval and a 0.5 nm slit width. After background subtraction, spectral intensities were normalized. The onset of the absorbance was determined via MATLAB, by fitting the spectrum to the left and right of the absorbance region by straight lines, and calculating the intercept of the lines.75 BAC CdS NP electrode preparation. 40 µl of CdS (52.8 µg) suspension were drop casted on cleaned 1.27 cm × 1.27 cm ITO (703192 ALDRICH) glass slides and dried in the chemical hood overnight at room temperature (~25 °C), while being covered with aluminum foil to avoid light exposure. The CdS covered only ~2/3 of the total ITO glass area. After ~8 hr drying in the chemical hood, the CdS/ITO sample was ready to be used in the assembly of the test electrode. To this end, 1/3 of the ITO that was not covered by CdS, was covered by an indium gallium (InGa, liquid metal) layer, and a flat-end copper (Cu) wire, connecting to the InGa layer, fixed in place with Epoxy resin. The copper wire was isolated from the electrolyte solution with a glass tube that was also connected to the Cu/InGa connection with Epoxy. For the CBD CdS control sample, an amount of CdS (52.8 µg) equivalent to that used in the preparation of the BAC CdS NP thin film was drop-casted onto a cleaned ITO glass substrate. After drying, the electrode preparation followed the procedure described above for the BAC CdS NPs.



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Electrochemical measurements. Photoelectrochemical (PEC) tests. A Bio Logic SP-200 electrochemistry workstation was used for photoelectrochemical testing. In these tests, a 150 W Xenon lamp (DRIEL-66001 with 68805 universal power supply) was used as the light source, with a wavelength filter simulating sunlight. The distance between the light source and the working electrode surface was around 15 cm to ensure a 100 mW/cm2 light intensity (1 sun) in the electrochemical cell. A Platinum Counter Electrode (23 cm, by BAS Inc., Cat. No. 012961), Ag/AgCl reference electrode (012167 RE-1B Reference Electrode, by ALS Co., Ltd), and the CdS working electrode (or control ITO working electrode) were assembled as a three-electrode testing system. The electrolyte was 0.5 M Na2SO4 dissolved in DI water with pH = 6.8. All photoelectrochemical tests were carried out at room temperature. Before testing, the electrolyte was perfused with nitrogen (N2) gas for 20 min, and during testing N2 was continually bubbled into the electrolyte to minimize dissolved oxygen. Transient photocurrent responses. The transient photocurrent was measured at 0 V vs. open circuit potential (OCP) bias. During the test, the sample was intermittently irradiated with light every 60 s for a total of 300 s, and the corresponding current was measured for a total of 11 min. The electrode working area was calculated from optical images using ImageJ software, and was used to convert the measured current into current density. In addition, the specific current generated by both bacterially-precipitated (BAC) CdS and chemical bath deposited (CBD) CdS samples was calculated by dividing the measured current by the total sample weight (180 µg for the BAC CdS NPs, and 103 µg for the CBD CdS NPs).


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Cyclic Voltammetry (CV) measurements. The measurement potential range was set from -1.0 V to +0.5 V vs. Ag/AgCl with a sweep rate of 1 mV/s. Measurements were taken in both, dark and illuminated conditions. EIS measurements. A potentiostat equipped with a frequency response analyzer (Biologic SP-200) was used under potentiostatic conditions over a frequency range from 0.1 Hz to 200 kHz, using a 10 mV sinusoidal potential modulation. The potential range was from 0.4 to 1.0 V vs. EAg/AgCl. The selected potential was applied for 2 minutes to reach a steady state before the EIS measurements. The impedance parameters involved in the selected EIS equivalent circuit model were obtained by fitting the experimental data with EC-Lab software (Bio-Logic). Photo-charged capacitor electrode fabrication. The BAC CdS was drop-casted onto a cleaned ITO glass substrate using the same procedure as described for PEC measurements. PVA-electrolyte gel preparation. 3 g Polyvinyl alcohol (PVA) powder (MW 146000 - 186000) (Sigma Aldrich, USA) was dissolved in 30 ml DI water and heated at 90 °C for 1 hr with vigorous stirring. After cooling the solution to room temperature (25 °C), the PVA crosslinked into a hydrogel. 1 M NaCl aqueous solution was prepared separately as the electrolyte. Reduced graphene oxide (RGO) sheet preparation. Following previously published procedures,76,77 10 ml of highly concentrated graphene oxide (GO) solution (0.6 mg/ml) (Graphene Supermarket, USA) was transferred into 90 ml DI water. Then 0.2 ml hydrazine monohydrate (35% in aqueous solution) and 0.35 ml ammonia (27.5% wt. % in aqueous solution) were added to the suspension, and the mixture was heated to 95 °C for 2 hrs with vigorous stirring. After the mixture was air-cooled, the black RGO suspension was ready for deposition. 30 ml of the RGO suspension were mixed with 200 ml DI water, and filtered through a



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nitrocellulose filter membrane (0.05 µm pore size, 47 mm in diameter, EMD Millipore, USA). After the filtration, the RGO sheet attached to the filter membrane was formed. Hybrid electrode assembly. The hybrid electrode refers to the integration of CdS NPs with PVA and RGO, as shown in Figure 4 and the TOC image. A thin layer of PVA was doctorbladed onto an ITO glass slide, previously coated with CdS. Immediately after coating, 40 µl of a 1 M NaCl solution were dropped onto the PVA layer, and allowed to soak in for 1 hr until the NaCl solution was absorbed by the PVA hydrogel. Before the PVA was dried, a piece of RGO sheet was prepared as described above, and attached to the PVA coated sample with the RGO side facing the PVA. After the PVA dried, the filter membrane was peeled off, leaving the RGO sheet attached to the PVA. This hybrid electrode was then connected to a Cu wire and the edges were covered by epoxy, as described above. Electrochemical measurements of the hybrid electrode. The transient current, CV and EIS tests followed the procedure described in the Electrochemical Measurements section above. For the OCP tests, measurements were recorded at every 10 mV (or every 0.5 s) for the first three steps (stabilization, photo-charging, and charge storage). For the first step, the OCP of the electrode was measured until it reached a steady value. Once a steady-state OCP was obtained, the electrode was charged by illumination by 1 sun. After 90 s of charging, the light was turned off, and the OCP was measured for another 10 s. As the fourth step, a 0.9 µA discharge current was applied until the OCP dropped back to 0 V.


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Results and Discussion 1. Characterization of the biosynthesized CdS 1.1 Compositional and physical characterization The PEC electrode performance is not only affected by the size and crystalline structure of the CdS NPs,

78

but also by the residual organic matrix (arising from the bacterial precipitation

process) in which the crystallites are embedded. We assume i) that the residual organic materials will affect the interactions of the NPs to the substrate and determine the charge transfer characteristics at the interface between the CdS crystallites and the matrix, and ii) that the CdS crystallite size will primarily influence the bandgap. To determine the composition and crystallite size of BAC CdS we used energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction analysis (XRD). The EDS elemental mapping (Figure 1a and b) indicates that the CdS crystallites are homogeneously distributed within the carbonaceous organic matrix. Figure S1 shows that other than C, also nitrogen (N) and oxygen (O) are present in the samples. These elements are typical components of proteinaceous materials present in the bacterial culture and the P2 lysis buffer used in the particle harvesting process.79 Using EDS data, we estimated the weight fraction of CdS to be around 26% (Table S1). Figure 1c shows the cross-sectional view of a flat, ~7 µm thick, CdS NP film coated onto a silicon (Si) substrate. The TEM image (Figure 1d) shows typical CdS crystallites (quantum dots) embedded in the organic matrix of the precipitated NPs. The crystallites have an average diameter of about 7 nm with narrow size distribution (Figure S2), and are well dispersed in the matrix.75 XRD analysis yielded four main peaks, labeled 1 to 4 in Figure 1e. Also shown are the locations of the characteristic peaks for the face-centered cubic (FCC, yellow lines, ICDD ref #: 01-075-0581) and hexagonal close packed (HCP, blue lines, ICDD ref #: 9 ACS Paragon Plus Environment


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01-074-9663) structures. The diffraction intensity envelope shown in Figure 1e suggests that an

unequivocal assignment to one particular crystal structure is not possible, as the intensities and full widths at half maximum (FWHM) of peaks 1, 2, and 4 reflect a composite of multiple peaks related to the HCP and FCC structures. There is a small shoulder on peak 1, located at about 32 deg., which is not accounted for by the fitted XRD spectrum. We attribute this peak to the reflections from the {200} associated with the FCC crystal structure. This suggests that BAC CdS nanocrystals occur in two crystalline habits, which is similar to what is observed for CdS crystals synthesized by traditional methods that also produce mixtures of HCP and FCC crystalline structures.80,81

Figure 1: Compositional and structural characterization of the BAC CdS NPs. (a) EDS elemental mapping of a typical CdS thin film coated on a silicon substrate. The green dots and red dots represent cadmium (Cd) and sulfur (S), respectively, and the pink dots represent silicon (Si). (b) EDS elemental mapping of the same CdS thin film on a Si substrate. The blue dots represent carbon (C), and the pink dots represent Si. (c) SEM side view of the CdS thin film coated on a Si substrate. The thickness of the


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film is ~ 7 µm. (d) TEM image of CdS nanocrystals, embedded in the carbonaceous matrix. (e) XRD analysis of the BAC CdS NPs. The green horizontal line is the baseline. Above the baseline, the light gray curve is the XRD raw intensity, and the red curve is the fitted XRD spectrum, in which four peaks can be seen. Below the XRD spectrum baseline, the characteristic peak positions of the face centered cubic (FCC, yellow lines, ICDD ref#: 01-075-0581) and the hexagonal close packed (HCP, blue lines, ICDD ref#: 01074-9663) structures are shown.

1.2 Electrochemical characterization To understand photoelectrochemical response, it is beneficial to know the bandgap of the BAC CdS NPs, which was determined from UV-vis spectroscopy data.82,83 Figure 2a shows that the absorption edge of the CdS NPs is around 483 nm, which by the Planck relation, translates to a bandgap of ~ 2.57 eV. This bandgap value is close to the upper limit of ideal PEC materials.84 Furthermore, the crystalline size calculated from the bandgap value, assuming a relative dielectric constant of 5.23,85–87 is about 7 nm, which is in good agreement with the TEM measurements shown in Figure 1d.



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Figure 2: UV-vis spectrum and EC characterization of BAC CdS. (a) The UV-vis spectrum of the CdS NPs shows an absorption peak at ~ 483 nm, which indicates a bandgap of ~ 2.57 eV. (b) Mott-Schottky flat-band potential of the BAC CdS NPs. (c) CV curves of BAC CdS in the dark (black) and under illumination (red). (d) Schematic representation of the band structure of the CdS at the interface of the electrolyte.

Electrochemical measurements were conducted to evaluate how the carbonaceous matrix surrounding CdS crystallites affects the electrochemical properties of the CdS NPs, e.g., the flatband potential and the surface state energy. For the electrochemical and photoelectrochemical (PEC) tests, all potentials are reported with respect to a KCl-saturated silver/silver chloride reference electrode (0.197 V vs. NHE). From electrochemical impedance 12 ACS Paragon Plus Environment

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spectroscopy (EIS) measurements, values of the semiconductor space charge layer capacitance were extracted as a function of the applied voltage. The slope of the Mott-Schottky plot (Figure 2 b) indicates that BAC CdS NPs are an n-type material, with a charge carrier density of 5.6 × 1016/cm3, in agreement with reported values for semiconductors used in photovoltaic applications.88 Additionally, the intercept of the sloped line with the horizontal line at 1/C2 = 0, indicates a flat band potential of -0.58 V.89 While lower values have been reported elsewhere,90,91 a direct comparison cannot be performed, because the observed difference in flat band potentials may arise from differences at the semiconductor-electrolyte interface caused by different preparation and measurement conditions. For example, we used an organic-inorganic CdS compound surface instead of freshly cleaved single crystal surface (surface chemistry),91 and we used an aqueous Na2SO4 solution as the electrolyte instead of a sodium sulfide (Na2S) solution (composition of the electrolyte).90 These factors can have a strong influence on the ion adsorption and hence the value of the flat band potential. Compared to oxide or III-V semiconductor materials, these effects are likely more pronounced with BAC CdS NPs as their surface is a mixture of crystalline CdS surfaces and organic matrix materials.92 The electron transport performance, which is influenced by surface states and the onset potential, was characterized by cyclic voltammetry (CV) measurements.93 CV measurements performed in the dark (Figure 2c), show a cathodic current with a bias voltage lower than -0.7 V, which is likely related to hydrogen evolution.91 The small shoulder in the measured data at ~ -0.9 V likely is associated with Cd2+ reduction, while the anodic peak at ~ -0.7 V may arise from either adsorbed H2, or S2- oxidation.94,95 When the sample is illuminated, the CV curve shows a higher cathodic current for voltages below -0.7 V, which suggests that this potential is close to the Fermi level. This value also agrees with the flat band potential value obtained from EIS



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measurements. For voltages higher than -0.3 V, the CV curve under illumination starts to separate from that obtained in the dark. The difference between the onset potential and the flatband potential indicates the presence of shallow surfaces states lying just below the conduction band.84 Surface states are usually attributed to impurities or abrupt terminations in the crystal that trap electrons and holes, and thus reduce the charge transport efficiency inside the material.93 As a result, this implies that the band structure at the interface of the BAC CdS and the electrolyte has the form illustrated schematically in Figure 2d. To show the functionality of the BAC CdS, the transient current of these NPs was measured and compared to that of chemical bath deposited (CBD) CdS NPs. The transient currents were measured at open circuit potential (OCP) to evaluate the charge separation induced by illumination. Figure 3 shows that BAC CdS displays a photocurrent (8 mA/g) that is over 2× higher than that of CBD CdS (3 mA/g). We attribute this higher specific photocurrent to the presence of an organic matrix associated with BAC CdS, which likely gives a better contact to the indium

tin

oxide (ITO)

glass

substrate.

Fourier Transform

Infrared

(FT-IR)

photospectroscopy of BAC CdS reveals signals we attribute to carboxyl (-COOH) and amine (NH2) functional groups (Figure S3). These functional groups are present in the proteins that are part of the organic matrix, and render the NPs hydrophilic.96,97 We surmise that these functional groups are ionized in aqueous environments and thus give rise to electrosteric repulsion between the NPs when drop-casting from aqueous suspension. This effect contributes to the formation of uniform films (Figure S4). On the contrary, CBD CdS NPs tend to sediment in aqueous suspension, and when drop-casted on the ITO substrate, they do not form a uniform film (Figure S4). However, successive photocurrent measurements of BAC CdS NPs exhibit an exponential decay as depicted in Figure S5. The likely explanation for this behavior is the loss of particles


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from the substrate surface. This is reasonable because after drop-casting the aqueous CdS particle suspensions onto the substrates, they were simply air-dried without any further heat treatment, or chemical bonding. Despite the obvious stability problems, the unique chemical composition, and, when compared to CBD CdS, enhanced electrochemical properties, make BAC CdS an interesting candidate for integration with other electronic materials.

Figure 3: Transient current of the BAC CdS (red) at OCP, compared to the transient current of same concentration of CBD CdS (black).

2. Application of the biosynthesized CdS-RGO hybrid material We sandwiched BAC CdS NPs as a photo-active layer between an ITO top layer and a hydrogel electrolyte layer, supported on reduced graphene oxide (RGO) (Figure 4). We used a polyvinyl alcohol (PVA) hydrogel layer,77,98 swollen with an aqueous sodium chloride (NaCl) electrolyte, to integrate the CdS layer with the RGO sheet.98 To collect the current we attached a copper wire



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to the ITO layer. The edges of the electrode as well as the copper wire were covered with Epoxy resin (not shown in the figure) to prevent direct contact with the electrolyte when in operation. We hypothesized that this layered hybrid electrode is able to convert solar energy into electricity and to store it for later use; i.e., when light transmits through the ITO layer, electrons will be excited to the conduction band in the CdS layer, and the resulting holes will polarize the RGO sheet, which subsequently will store charges at the RGO/electrolyte interface.

Figure 4: Schematic showing the multi-layer CdS-PVA-RGO hybrid electrode.

Figure 5a shows the transient current generated by the hybrid electrode when exposed continuously to light for 10 min at OCP. The hybrid electrode exhibits a higher and more stable photocurrent compared to that produced by a bare BAC CdS layer containing the same amount of CdS. Given the good mechanical properties of polymer gel electrolytes,99 the PVA-electrolyte gel is believed to act as a buffer layer leading to a better stability of the hybrid electrode compared to the configuration without the polymer. To further investigate the performance of the hybrid electrode, we performed EIS measurements and compared the results to those obtained on bare BAC CdS NP electrode. From the Mott-Schottky plots, we estimate the apparent flat-band


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potential to be ~ -1.09 V (Figure S6), and the corresponding charge carrier density to be ~ 3.3 × 1017/cm3. Compared with BAC CdS NP electrode, we attribute the negative shift of the flat band potential to charge injection and the blocking of surface states following the deposition of PVA. Consequently, the charge carrier density increased by one order of magnitude.

Figure 5: PEC tests of the photo-charged supercapacitor electrode. (a) The transient current generated by the CdS-PVA-RGO hybrid electrode and by a BAC CdS NP electrode without PVA and RGO. (b) CV curves of CdS in the dark and light. (c) CV curves of the CdS-PVA-RGO hybrid electrode in the dark and light. (d) OCP test of the hybrid electrode. The four areas indicated in the figure represent four steps: 1) stabilization, 2) photo-charging, 3) charge storage, and 4) discharge. The inset shows the comparison of



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the photo-voltage and discharge performance at the same discharge rate for the BAC CdS NP electrode (black) and the hybrid electrode (red).

To show the ability of the hybrid electrode to store charges, we performed CV measurements in the dark and under illumination, and compared these to those of a bare BAC CdS NP layer. As seen in Figures 5b and c, and in contrast to the BAC CdS NP electrode, the hybrid electrode exhibits an obvious capacitive behavior, which is enhanced under illumination. We hypothesize that the improved capacitance (i.e. charge storage properties) results from an enhanced dielectric constant of the hybrid structure following RGO deposition.100 Additionally, the photo-current generated by the hybrid electrode is higher than that of the BAC CdS NP electrode. This difference arises from the negative shift of the flat-band potential which allows a greater band bending and thus a better photo-generated charge separation. To show the possibility of storing the photo-generated charges in the hybrid electrode, we performed photo-charge/galvanostatic discharge measurements at open circuit (Figure 5d). The process is divided into four steps as marked in the figure. First, the OCP is measured in the dark. Second, the light is switched on for 90 seconds until the OCP reaches a steady value. At this point, the electrode generates a photo-voltage of ~600 mV, which is higher than that for the BAC CdS NP electrode (Figure 5d) and comparable to values found in the literature.101,102 Third, after switching the light off the OCP was measured in the dark for 10 seconds. The only small potential drop, which is common for capacitors, suggests that the hybrid electrode can store the charges that were generated during the photo charging step. Finally, the electrode is discharged at a constant specific current of 10 mA/g in the dark. The discharge process lasted longer than that for the BAC CdS NP electrode (inset Figure 5d), and shows that the energy stored in this electrode can be extracted by connection to an external circuit. During the galvanostatic 18 ACS Paragon Plus Environment

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discharge process, the slope of the voltage-current curve is proportional to the capacitance according to Equation 1: ‫ܫ=ܥ‬

ௗ௏ ௗ௧

Equation 1

The inset of Figure 5d shows that under our operating conditions, the hybrid electrode is able to store approximately twice the charge compared to the amount stored by the pristine BAC CdS layer. This experiment proves the concept that our hybrid electrode can convert photon energy into electric charge and store it. To further improve the device performance, the BAC CdS particle size and the film thickness, as well as the electrolyte composition, all need to be optimized. Conclusions In this work we investigated the physical and PEC properties of bacterially precipitated (BAC) CdS NPs, and applied these NPs in a proof-of-concept, multifunctional device. Our characterization showed that BAC CdS nanocrystallites (25%) are embedded into an organic, hydrophilic matrix (75%), and have a band gap of around 2.57 eV. Transient current tests showed that BAC CdS NPs can generate higher photo-current than chemical bath deposited (CBD) CdS. We attribute this behavior to the presence of the organic matrix which contains -COOH and -NH2 functional groups. These groups likely improve contact between the BAC CdS NPs and the ITO substrate, while also preventing agglomeration. Additionally, we integrated BAC CdS NPs with a PVA-electrolyte hydrogel layer and a RGO sheet, to build a photo-charged, hybrid supercapacitor electrode. A significantly higher photocurrent was generated by the hybrid electrode compared to that produced by bare BAC CdS



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NP electrode. We attribute this improved performance largely to the deposition of PVA/RGO, which lowers the number of recombination sites and prevents the organic matrix from dissolving. Additional electrochemical tests showed that the hybrid electrode is able to store the photogenerated charges. We believe that device performance can be improved by several means, including the use of BAC CdS NPs containing larger CdS crystallites, i.e., NPs with smaller bandgaps, and the optimization of the CdS film thickness. To our knowledge, this is the first time that biosynthesized hybrid materials have been shown as promising candidates for PEC energy conversion and energy storage in multifunctional devices.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website EDS spectrum and quantitative analysis, TEM images for CdS crystal size distribution, FT-IR spectrum showing the chemical bonds, photos of BAC CdS thin film and CBD CdS thin film, and the Mott-Schottky plot of the photo-charged electrode. (PDF) Author Information Corresponding Author *E-mail: [email protected]. Tel.: +1 (919) 660-5360

Acknowledgements


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S. Zauscher and L. You acknowledge the support from the Office of Naval Research (ONR, N00014-15-1-1-2508). J. T. Glass acknowledges the support from National Science Foundation (NSF, ECCS-1344745). Dr. Charles Parker is acknowledged for helpful discussions and assistance with experimental/laboratory operations. The authors thank Luis M. Navarro, Evann Wu, Travis E. Morgan for helpful discussion and experimental assistance. Y. Feng thanks Isvar Cordova for PEC characterization training. We acknowledge Dr. Mark D. Walters and Michelle G. Plue at Duke’s Shared Materials Instrumentation Facility (SMiF) for training on SEM/EDS, XRD, and UV-vis characterization.

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