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Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin
international journal of hydrogen energy xxx (xxxx) xxx
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Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin Pooja Devi a,**, Anupma Thakur a,b, Dibyendu Ghosh c, E. Senthil Prasad d,***, S.M. Shivaprasad e, R.K. Sinha a, Praveen Kumar c,* a
Central Scientific Instruments Organization, Sector-30C, Chandigarh, 160030, India AcSIR, Council of Scientific and Industrial Research, Ghaziabad, 201002, India c School of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700032, India d Institute of Microbial Technology, Sector 39A, Chandigarh, 160036, India e Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, 560064, India b
highlights
graphical abstract
First report on GaN NWN as photoanode with bR into electrolyte. Measured photocurrent density is 28.74 mA/cm2 at 1.0 V (vs RHE). Largely
enhanced
applied
photon-to-current
bias
efficiency
(ABPE) ~7.8%.
article info
abstract
Article history:
The ever-increasing demand for renewable and clean energy sources has prompted the
Received 7 June 2019
development of novel materials for photoelectrochemical (PEC) water splitting, but effi-
Received in revised form
cient solar to hydrogen conversion remains a big challenge. In this work, we report a bio-
25 September 2019
nanohybrid strategy in a photo-system to simultaneously enhance the charge separation
Accepted 24 October 2019
and water splitting efficiency of photoanode (PA) by introducing Bacteriorhodopsin (bR), a
Available online xxx
natural proton pumping photosystem and GaN nanowall network (NWN), a direct band gap and corrosion-resistant semiconductor. The experimental study reveals that this combi-
Keywords:
nation of bR and GaN NWN has huge potential as a light-activated sensitizer as well as
Bio-nanohybrid
proton pumping source to achieve enhance photocurrent density in hydrogen evolution
Hydrogen
reaction (HER). Consequently, this synergistic effect in bR/GaN NWN PA gives rise to largely
Nanowall network
enhanced applied bias photon-to-current efficiency (ABPE) ~7.8% and photocurrent density (28.74 mA/cm2 at 1.0 V vs RHE). It is worth mentioning that the photocurrent density of bR/
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (P. Devi), [email protected] (E. Senthil Prasad), [email protected] (P. Kumar). https://doi.org/10.1016/j.ijhydene.2019.10.184 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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international journal of hydrogen energy xxx (xxxx) xxx
Proton-pumping
GaN NWN, to the best of our knowledge, is superior to previously reported bR-based PAs
Photoanode
and bio-photoelectric devices reported till today for solar-to-hydrogen fuel generation. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The up surged environmental pollution owing to existing fossil fuels followed by increasing energy crisis, demand exploration of alternate sustainable and renewable clean energy sources. The direct solar-to-hydrogen conversion by efficient, environment-friendly and inexpensive means is one of the potential alternates of fossil fuel, however, lacking with the desired efficiency. Following the first report on PEC water splitting using TiO2 photoelectrodes (PEs), a huge number of attempts have been made to enhance PEs performance by sensitizing them with visible components such as quantum dots (QDs), dyes, low bandgap semiconductors, doping etc., however due to low absorption the overall efficiency remains low [1]. Further, due to visible light responsivity, natural abundance, and non-toxicity, biological entities such as nanobio hybrid catalyst, hydrogenase enzyme, phycocyanins, etc., were also investigated in combination with TiO2 PEs for the efficiency enhancement, however, suffers from stability issues [2]. Bora et al. [3] reported nano bio-hybrid photoelectrode combining hematite thin-film and light-harvesting membrane protein C-Phycocyanin from the phycobilisome family like blue-green algae (cyanobacteria). Tsui et al. [4] reported bacteriochlorophyll modified TiO2 nanotubes for photoelectrochemical application whereas Wang et al. [5] reported nano-bio architecture through noncovalent assembly of a cell-free expressed transmembrane proton pump and TiO2 semiconductor nanoparticles as an efficient nanophotocatalyst for H2 evolution. But in present scenario, only ~4% hydrogen is produced by water electrolysis, the rest 96% production is again involved in fossil fuels. Therefore, it is an utmost need to find a combination of suitable photoelectrodes (PEs) materials with stable biological entities to enhance the overall efficiency. Amongst several carbides, oxides, sulphides, nitrides, borides, and metals based PEs, III-nitrides semiconductors, indeed is a promising material to perform solar water splitting due to their advanced properties such as higher absorption coefficient, stability in aqueous solution, direct and appropriate band gap to split H2O and high corrosion resistance [6e8]. It paves the promising path for the commercial realization of PEC devices for hydrogen fuels. However, due to unintentional n-type nature, the upward band bending of these III-nitrides PEs at electrolyte interface limits overall efficiency. On the other hand bR, a natural photosynthetic system has emerged out as a candidate for solar energy conversion owing to its proton pumping function upon visible light photoexcitation. It converts solar energy to chemical energy in the form of adenosine triphosphates (ATPs), where the retinal component of bR pump, upon photoexcitation under visible light undergoes photoisomerization (~sub
picoseconds) and proton is released by Schiff base. The photocycle of bR proteins is a measurement of its proton pumping efficiency [9]. Besides, being a key component of extremophiles, It also exhibits exceptional thermal, chemical & photochemical stability and can withstand high ionic strength (3 M NaCl), temperature (80 C in water, 140 C in dry condition), wide pH range (511) and protease insulation without loss of activity [10]. In earlier reports, bR has been demonstrated in its dual role with several nanostructured materials such TiO2 nanotube array [11], Pt/TiO2NPs [12] TiO2 nanoparticulate films [13], reduced graphene oxide (rGO) [14] etc. Recently, silver NPs and bR based novel hybrid catalysts are reported to exhibit enhanced hydrogen evolution performance (733 mA/cm2) with a lower overpotential of 63 mV due to synergetic effects upon visible light irradiation [15]. Guo et al. [16] demonstrate bR/ gold nanoparticles (AuNPs) multilayers as a mimic to the stack structure of granum to perform as a novel photovoltaic stack system with improved photoelectric performance. Chellamuthu et al. [17] reported 0.49% conversion efficiency in a bio sensitized solar cells (BSSCs) using BR into TiO2 photoanode with acetamide based gel electrolytes. Johnson et al. [18] reported bR encapsulated amorphous titanium dioxide, or titania, gels for the production of hydrogen through water splitting under white light irradiation. Despite all these investigations, the obtained photocurrent with bR sensitized PEs system due to less absorption is still low, and requires improvements in terms of PEs characteristics in hybrid catalysts as a possible solution. Therefore, the combination of an efficient high-quality GaN PE with highly stable and naturally abundant bR protein has the capability to enhanced hydrogen evolution in PEC water splitting [20,21]. No attempt has been reported to the best of our knowledge on the use of biological entities; in particular, bR as a sensitizer component in IIInitrides based PEs during PEC water splitting. Therefore, we explored bR not only as a light-activated sensitizer but also as proton pumping source with robust GaN NWN PEs to achieve enhance photocurrent density in HER. The hybrid catalyst is fully characterized and demonstrated towards PEC water splitting under dark, as well white light illumination under optimized pH condition.
Experimental section Growth and characterization of GaN NWN PE The GaN NWN samples were grown directly on a c-Al2O3 substrate using a radio frequency plasma-assisted molecular beam epitaxy (SVTA). After ex-situ chemically cleaning of cAl2O3, it was inserted into the introduction chamber for initial degasing followed by high temperature cleaning in the growth
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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chamber to get the atomically clean surface. The temperature of the gallium (Ga) effusion cell was kept at 1030 C. A constant nitrogen low rate of 8 scc/m (standard cubic centimeter per minute), substrate temperature of 630 C, plasma forward power of 375 W and growth duration of 4 h were maintained for all of the films. The GaN NWNs were formed spontaneously under nitrogen-rich growth condition without using any external catalyst or buffer layers. The details of the growth conditions, fluxes were given in Ref. [19]. The grown NWNs exhibit excellent structural, optical and electrical quality, with the absence of defect-related yellow luminescence peak in PL spectrum.
Extraction and purification of bR Wild-type bR purple membrane protein namely ET001 was isolated from Halobacterium salinarium ET001 strain from lab scale shake flask culture as portrayed in Figs. S1 and S2. All operations were carried out at room temperature and centrifugation at 4 C. Briefly, Halobacterium ET001 cells were harvested after reaching stationary phase at optical density OD 1.26. The cell separation was performed by centrifugation at 7000g and 4 C for 30 min. ET001 Cell Pellets obtained were washed twice with 200 ml of basal salt media without peptone in 500 mL centrifuge bottles. Final wet cell weight obtained is in the range of 3.4e3.8 g/L after basal salt washing. Each Centrifuge bottles of 1.68 g pellet wet weight suspended in 30 mL sterile distilled water and then added with 50 mL of DNase (Stock - 5 mg DNase/mL, 2000 units/mg, Pierce Biotech, USA) to reduce the excess viscosity liberated during cell lysis by osmolysis, which was performed by dialyzing the abovesaid cell suspension of 30 mL in 10 kDa snake skin dialysis membrane (Thermo Scientific, USA) at 4 C against deionized water overnight. The deep coloured purple suspension obtained was centrifuged at 21,000 g to remove red carotenoid pigments in the supernatant. The purple pellet was dissolved in 30 mL of distilled water and the procedure was repeated three times at 21,000 rpm for 40 min until the supernatant becomes colourless. Finally the purple pellet was suspended in 2 mL of distilled water and layered over 10 ml of 20%e60% for sucrose density gradient separation (Thermo Scientific, Sorval WX100 Ultracentrifuge, USA) in swinging out rotor (Sorval swinging bucket TH - 641) for 17 h at 100,000 g, 4 C. The small red band and the large purple band appeared at the top and bottom of 45% sucrose density gradient. The purple membrane around 1.5 mL collected was diluted with 1.5 mL water (1:1) was dialyzed in 10e12 kDa semi-permeable membrane sac called as small wonder lyzer (Innovative dialysis membrane sac, US Patent 6,368,509 B1) sold by Excellon Innovations, Punjab, India against milliQ water at 4 C for removal of sucrose. The purified purple membrane obtained was stored at 20 C deep freezer for further use.
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spectrophotometrically using molar extinction coefficient of 63,000 M1 cm1. The maximal absorption at 280 nm corresponding to the total protein content of the sample and the maximal absorption at 570 nm indicated bR protein [22]. The quality of isolated ET001 Bacteriorhodopsin containing purple membrane was estimated by A 280 nm/A 570 nm ¼ 3.326/ 1.679 ¼ 1.98 found to be equal to the reported ratio values by Cassim JY 1975. The ratio values less than 2.0 indicates the high quality of bacteriorhodopsin purple membranes extracted from Halobacterium salinarium ET001 cells. The optical and structural characteristic of bR modified electrolyte were probed by UVevis (Hitachi Ltd., Japan), FT-IR (670 IR, Varian, United States) and Raman inVia instrument (Reninshaw) at lex ¼ 785 nm. The GaN NWN PAs were ex-situ characterized by a field emission scanning electron microscope (FEI and high resolution), X-ray diffractometer (Discover D8 Bruker) with a Cu Ka X-ray source that had a wavelength of 1.5406 A. The electronic structure of the PAs were probed by Xray photoelectron spectroscopy (Omicron) by recoding valence band spectra. The optical properties were studied by photoluminescence spectroscopy (PL, Horiba Jobin Yvon) using a xenon lamp source and 325 nm excitation.
Photo-electrochemical characterization of hybrid catalyst PEC characterizations were performed current densityvoltage (J-V) curves in three different set of experiments. 1) using GaN NWN as PE in a 0.1 M Na2SO4/H2SO4 (NH) electrolyte solution, 2) bR/GaN NWN hybrid PE in a 0.1 M Na2SO4/H2SO4 (NH) electrolyte solution and 3) using GaN NWN as PEs and adding different concentration of bR in electrolyte to measure the effect on overall hydrogen evolution efficiency. The preparation of 0.1 M Na2SO4/H2SO4 (NH) electrolyte consists of firstly preparing 0.1 M Na2SO4 in deionized water, then balancing pH of this solution to 7.2 by adding 0.1 M H2SO4. bR was adsorbed onto NWN PEs directly by overnight incubation, whereas it was added in standard addition approach into electrolyte before photo measurements (AUTOLAB Metrohm electrochemical workstation) under dark and illuminated condition. A solar simulator equipped with 300 W Xe lamp and AM 1.5G filter was used as light source. The intensity of the source was calibrated and controlled at 100 mW/cm2. The current density versus the potential of PEs was measured under dark and light conditions by linear scan voltammetry (LSV) and cyclic voltammetr (CV) at a constant scan rate of 10 mVs1. Impedance spectra were recorded with frequency response analyzer and Nova software at zero bias in the 0.1e105 Hz frequency range under 10 mV ac perturbation signals in light and dark condition. All pH measurements were made using digital pH-meter (LMPH 10, Labman Scientific Instruments Ltd).
Characterization
Results and discussion
The quality of the isolated bacteriorhodopsin containing purple membrane was checked by scanning between 200 and 800 nm in UVeVis spectrophotometer (Hitachi 2390 UV spectrophotometer, Japan) and measuring absorbance ratio of A280nm/A570nm. bR protein concentration was estimated
Radio frequency plasma-assisted molecular beam epitaxial GaN NWN has been thoroughly characterized (Fig. 1) via Field Emission Scanning Electron Microscopy (FESEM), High Resolution X-ray Diffraction (HRXRD), X-ray photoelectron spectroscopy (XPS), and Photoluminescence (PL). The FESEM
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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Fig. 1 e FESEM micrographs of GaN NWN (a) Top view and the inset shows a cross-sectional view. (b) HRXRD rocking curve of GaN NWN corresponding to the reflection for (002) plane, (c) Valance band spectra and (d) PL spectra for GaN NWN.
image (Fig. 1a) depicts homogeneous network morphology consisting of wedge shaped nanowalls surrounding the voids formed by open screw dislocations. The average tip width and average cavity size of nanowall were measured to be ~10 nm and 56 nm, respectively, with the corresponding surface coverage of 56%. The cross-sectional image (inset of Fig. 1a) reveals the presence of gaps in-between the wall which manifests that the pore as seen in the plan view percolate all the way down to the bottom, hence making it more effective for light absorption by multiple scattering, which will enhance the overall PAs efficiency. Further, the structural quality of these grown PAs was confirmed by the rocking curve of (0002) plane from HRXRD (Fig. 1b). The sharp FWHM (~2600 arc.sec) of the rocking curve confirms the low defects and high structural quality of the grown PAs. To understand the effect of the bend bending behaviour at the surface of PA, XPS valence band (VB) spectra were measured and presented in Fig. 1c. As XPS gives the information about the occupied states therefore 0 eV is considered to be as Fermi level whereas extrapolated edge five the position of VB maxima. Figure shows the VB maxima position at 3.05 eV, which manifests n-type characteristic of at PA surface. In order to have more insight on the optical quality of as-grown GaN NWN PAs, LT-PL measurements were performed to identify the luminescence originating from defects and band edge. Generally defects are created in between conduction and valence band due to the presence of Ga and N
vacancies, oxygen or deep level impurities and possess broadening of the PL peak towards low bandgap (higher wavelength). The sharp and strong emission peak at 3.19 eV and absence of the defects related peak as shown in Fig. 1d, confirm the high quality of the MBE grown GaN-NWN PEs. The extraction of the purple membrane was performed from Halobacterium ET001 Cell pellets. The Osmolysis of the cell lysates was carried out on 10 kDa snake skin dialysis membrane against deionized water. After the differential centrifugation of purple lysates at 21,000 rpm the sucrose density fractionation was peformed to separate the red carotenoids and purple bacteriorhodopsin bands. The Purple bands were obtained at 45% fraction of sucrose density layer. The obtained crude purple suspensions were further dialysed against deionized water to remove residual sucrose. The quality of isolated purple membrane protein was checked by measuring the absorbance ratio at A280 nm/A570 nm (Fig. 2 Supplementary Data). The structural characterizations of the bR were performed using FTIR and Raman spectroscopy. The UVevis spectra shows the characteristic absorption peaks at 280 and 570 nm, which are assigned to aromatic residues of bR including 8-tryptophan and 11-tyrosine and the P-P* transition in the retinal chromophore between two energy levels upon exposure to the light as shown in Fig. 2a [23]. Fig. 2b portrays the Raman spectra of bR with a laser of 514 nm excitation wavelength. The vibrational spectra show characteristic peaks at 1528 cm1 corresponding to C]C stretching
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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Fig. 2 e (a) UVeVis absorption, (b) Raman and (c) FTIR spectra for the bR with different concentration as well as pure bR.
and fair stretch is observed in the range of 1150e1220 cm1 which attributes to CeC stretching [24]. Fig. 2c shows the FTIR spectra of bR with characteristic peak attributing to the vibrational stretch of hydrogen out-of-plane (HOOP) at 1050 cm1, which corresponds to the twist in the skeleton of the protein. The other comparatively weak vibrations within 1100e3500 cm1 could be attributed to CeOeC asymmetric stretching, OeH bending, C]O stretching, CeH bending of the retinal chromophore and the amide vibrational stretch at 1540 cm1 due to NeH bending, which reveals the secondary structure of the protein [25]. After asserting the quality of the GaN NWN PAs, PEC experiments were performed under three electrode setup in 0.1 M Na2SO4/H2SO4 (NH) electrolyte (pH 7.2) under solar light illumination. The PEC results for GaN NWN PAs with or without bR are shown in Fig. 3. To understand the role of bR as a visible light harvesting molecule in addition to working as a proton pump, the two-fold PEC measurements were performed. In the first set of experiments, GaN NWN PAs were modified by adsorbing bR directly on to it and PEC performance was checked under 0.1 M Na2SO4/H2SO4 (NH) electrolyte (pH 7.2) solution. For the experimental optimized conditions (effect of electrolyte (Fig. S3) and effect of pH of electrolyte (Fig. S4) maximum current density of 10.12 mA/ cm2 with the on-set potential of 1.0 V vs RHE was observed for bare GaN NWN PAs. Futher, in the second set, the PEC
experiments were performed using GaN NWN PAs in the absence and presence of bR in the electrolyte solution. Fig. 3a shows a LSV under dark and white light illumination. Under dark condition the GaN NWN PA doesn't exhibit any current as the potential is swept from 0.4 V to 1.4 V vs RHE, even with the presence of bR in solution, revealing chemical inertness of PAs under a dark condition in the selected potential range. On solar light illumination, the PAs showed an onset photocurrent at 0.65 V vs RHE and, which continues to increase until reaching a steady-state value of 25.4 mA/cm2. Upon adsorption of bR, the modified GaN NWN PAs show the decrease in the PEC performance with the maximum current density of 12.3 mA/cm2 (Fig. S5), which can be subjected to low transfer of electrons from GaN to bR. However, the optimization for the thickness of bR on NWNs for hybrid PAs is still underway for the enhancement of overall efficiency. On the addition of bR (0.5 mLe1.5 mL in 100 mL of electrolyte) into the electrolyte solution, the photocurrent density increased to 28.5 mA/cm2, which is ~3 fold enhancement over GaN NWN PA without bR in the electrolyte (10.8 mA/cm2), at the potential of 1.0 V vs RHE (Table 1) indicating more solar light harvesting in the presence of bR, thereby, leading to effectual separation of photogenerated electronehole pairs as compared to bare GaN PA. The role of bR in decreasing/enhancing the photocurrent density can be explained as follows. The principal scheme of
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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Fig. 3 e (a) J-V characteristics for the bare GaN NWN under dark and 1 sun illumination along with 1.5 mL bR modified electrolyte under1 sun illumination. (b) The schematic illustration of the electron transfer from bR to GaN NWN PAs. EIS spectra for NWN (c) without bR and (d) with 1.5 mL bR.
Table 1 e PEC and EIS parameter for bare GaN NWN and bR modification. Experimental Conditions 0.1 M Na2SO4/H2SO4 electrolyte 0.2 (pH 7.2) Xe lamp (AM 1.5 filter) of 100 mW/cm2 power intensity.
PAs
Jphoto (mA/cm2) at 1.0 V vs RHE
Rs (U)
Rp (U)
ABPE (%)
TOF at 1.0 V vs RHE (S1)
GaN NWN GaN NWN_1.0 mL bR GaN NWN_1.5 mL bR
10.80 21.24 28.74
15.3 7.41 20.0
68.1 47.0 26.5
2.55 4.86 7.85
1.4 2.26 3.66
enhanced light-driven water splitting by hybrid system employs the light-harvesting proton pump bR as a key building block responsible for coupling the visible light reactivity of GaN NWN PA and for driving proton towards counter platinum counter electrode for subsequent reduction to hydrogen gas. When decorated onto GaN NWN directly by overnight incubation, the photocurrent density reduced in comparison to bare PAs (Fig. S5). This may be due to the larger size of bR (~800 nm) in comparison to GaN NWN (~50e100 nm). Hence bR might have attached on the top surface of GaN NWN films, which led to the covering of effective surface and therefore, suppressing the photoresponse of hybrid PEs [10]. The presence of bR at the surface completely masks the light harvesting effect of the underlying GaN NWN films and significantly turns into its negligible photoresponse towards PEC water-splitting. The bR-concentration dependency on the GaN NWN was observed clearly with varying concentrations opted for adsorption namely 1:10, 1:5 and 1:1 dilutions of
bR:PBS quoted as very less bR/GaN NWN, less bR/GaN NWN and more bR/GaN NWN, respectively as presented in Fig. S6. Therefore, to exploit bR advantages, it was added into solution directly during PEC water splitting. The effect of bR concentration on photocurrent density was also studied and shown in Fig. S7a. The results portray the increase in the photocurrent density with the increasing bR concentration, which can be attributed to the proton-pumping effect of bR in the electrolyte. The maximum photocurrent density of 28.74 mA/cm2 was observed with an optimal concentration of 1.5 mL bR. Upon increasing bR concentration, the effect similar to direct loading of bR onto GaN NWN PA via overnight incubation is observed, which could be understood in terms of a hindrance in catalytic performance of GaN NWN PAs. As, the assessment of the PEC activity is realized from the generalized hv water-splitting reaction i.e. 2H2 O/ 2H2 þ O2 , which is clearly expressed by the observed effectual photo-anodic current behaviour explicitly coming from the GaN PA in the presence
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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of bR under light irradiation. This highlights to hypothesize the influence of the absorption of green light by bR which might have caused excitation of electrons from HUMO to LUMO and injection of same from protein retinal complex to the low energy conduction band of GaN NWN photoanodes, which is energetically favourable (Fig. 3b) as reported earlier [8]. These electrons complete the loop through an external circuit, causing reduction of Hþ ions (from bR and catalytic water splitting) at Pt electrode and well supported by the previous reports [8e12]. As the proton pumping process of bR continues with the light irradiation and the generated protons gets drifted from the anode to the cathode. As the proton concentration at the cathode is different from that at the anode results in different local pH environments. The difference in pH values at anode and cathode due to the presence of bR protein is also proposed as one of the reasons behind bR enabled enhanced photocurrent in PEC cell. The two different pH values at electrodes result in the different pH local environment, which may impose a chemical bias to help in charge separation and more photocurrent generation [26e28]. The corresponding CV analysis shows the enhanced current characteristic of PA in presence of bR and cathodic shift of reduction peak responsible for high H2 evolution which indicates that a more effective separation of photogenerated electron-hole pairs and a faster interfacial charge transfer had occurred as compared to bare GaN PA (Fig. S7b). The impedance characteristic of PA in presence and absence of bR under
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white light illumination further supports above observations. The figure is associated with impedance response in frequency range and associated Randel's equivalent circuit, where, Rs, Rp, CPE, and W correspond to the solution series resistance, polarization (charge transfer) resistance and constant phase element and Warburg impedance respectively. It can be seen that for the bR adsorbed PAs the polarization resistance, which is associated with charge transfer characteristic at electrode/electrolyte interface, increases from 68.1 U to 139 U, whereas on the addition of bR into electrolyte it decreases to 26.5 U for optimized bR concentration (Table 1), which further supports the LSV and CV observations. These charge-carrier transport properties of the GaN PA in presence and absence of bR is well reconnoitred by electrochemical impedance spectra (EIS) as a function of light irradiation and dark in the presence and absence of bR, also lays the foundation to understand the mechanism for the PEC performance of the proposed bio-nanohybrid PA. As, the radius of the arc on the EIS spectra reveals the reaction rate occurring at the surface of the electrode, which states that the smaller arc radius of GaN PA in the presence of bR indicates that a more effective separation of photogenerated electronehole pairs and a faster interfacial charge transfer had occurred as compared to bare GaN PA. The EIS-Nyquist plots of GaN PA with bR portrays that the bio-nanohybrid PA acts as solar light harvester and absorbed photons at the surface which promotes an electronexcitation from the ground state of the bR i.e. p-orbital
Fig. 4 e Cyclic photocurrent measurement under chopped light for bare GaN NWN and after 1.5 mL bR in the electrolyte solution at 0.4 V vs RHE bias potential and the corresponding LSV is shown in (b) for NWN_1.5 mL bR. (c) ABPE AND (d) TOF plots for bare NWN and NWN_1.5 mL bR. Blue correspond to NWN and red correspond to NWN_1.5 mL bR. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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(HOMO) to the excited state i.e. p*-orbital (LUMO) which enhance the separation efficiency of photo generated electronehole pairs and indorse water splitting reactions. Fig. 4 is associated with transient short circuit photoresponse of PAs in absence and presence of bR protein, which was carried out under intermittent solar light illumination using potentiostatic condition at 0.4 V vs RHE bias. As can be seen very clearly, that under illumination, photocurrent increases rapidly and reaches a steady-state current density of 70 (mA/cm2) and 290 (mA/cm2) with absence and presence of bR protein, respectively. The photocurrent increases rapidly and reaches steady-state values, respectively. Once the illumination is turned off, the photocurrent returns to the background level, and this process can be repeated many times. In the dark conditions, the JeV curve shows typical rectifying behaviour, with a weak current density. The portrayed rectangular signal revealing the transient short circuit photoresponse of PAs in absence and presence of bR protein under light irradiation and dark, implies to the non-equilibrium state amidst the boosted rate of charge carrier generation in the presence of light and vice-versa. With these observed high PEC performance, the potential of the proposed system is highly efficient amongst bio-photoelectric devices reported till today for solar-to-hydrogen fuel generation. Further, the ABPE for optimized bR concentration in the electrolyte and without bR is calculated to be 8% and 2.3%, respectively, (Fig. 4c), which is the highest reported value for such a bio-nanohybrid system till now in our knowledge best. Additionally, to explicate the boosted PEC water splitting performance of PAs, we thoroughly investigated the amount of active catalytic active sites which was estimated by turnover frequency (TOF). The number of active sites (n) is examined using CVs at a scan rate of 10 mV/s. When the number of voltammetric charges (Q) is obtained after deduction of the blank value, n (mol) can be calculated with the equation of n ¼ Q/2F, where F is Faraday constant (96,485 C mol1). TOF, calculated with the equation of TOF ¼ I/(2Fn) where I (A) is the current of the polarization curve obtained from the LSV. Fig. 4d shows the polarization curves which has been normalized by the active sites and expressed in terms of TOF. It can be seen that to achieve a TOF of 3.625 s1, sample NWN_bR needs an overpotential of 230 mV. Also, Tafel polarization curve (Fig. S8) reveals the catalytic performance of GaN NWN_1.5 mL bR is more superior to GaN NWN without bR and it drives 30 mA/cm2 at an overpotential of 124 mV. The enhanced HER activity is further illustrated by comparing the Tafel slopes to analyze the quantifiable charge kinetics of the water-splitting process. As shown in Fig. S8, the Tafel slope of 124 and 126 mV/dec for NWN-bR and NWN suggest typical Volmer-Heyrovsky characteristics.
Conclusion In summary, a new system based on GaN NWN and bR protein are investigated and characterized for PEC water splitting for hydrogen production. GaN NWN array was grown by epitaxial growth approach and used as PAs in presence of bR protein in the electrolyte, which observed to
cause ~3-fold enhancement in photocurrent density. The observed high PEC performance is assigned to band alignment of bR system with GaN NWN PAs, enhanced light absorption by bR along with proton releasing property of bR protein. The proposed bR/GaN NWN hybrid catalyst as PEs provides a novel perspective approach for emerging versatile bio-photoelectric devices for solar-to-hydrogen fuel generation.
Acknowledgement The authors thank Prof. S Bhattacharya, Director IACSKolkata for his encouragement and support. The authors wish to thank DST, Govt. of India for the financial grant (DST INPIRE Faculty & DST/TSG/PT/2012/66). AT acknowledges DST-INSPIRE fellowship for doctoral study. SK acknowledges Prof. Mark T. Facciotti, University of California, for providing Halobacterium ET001 cells and Devendar Singh, Vinod Kumar, Shivani Sharma, Riya Sharma, Sushmita, IMTECH, Chandigarh for BR isolation and characterization.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.184.
references
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[9] Chu LK, Yen CW, El-Sayed MA. Carbon nanotubes-based chemiresistive biosensors for detection of microorganisms. Biosens Bioelectron 2010;26:620e6. [10] Royant A, Edman K, Ursby T, Pebay-Peyroula E, Landau EM, Neutze R. Helix deformation is coupled to vectorial proton transport in the photocycle of bacteriorhodopsin. Nature 2000;406:645e8. [11] Allam NK, Yen CW, Near RD, El-Sayed MA. Bacteriorhodopsin/TiO2 nanotube Arrays hybrid system for enhanced photoelectrochemical water splitting. Energy Environ Sci 2011;4:2909e14. [12] Balasubramanian S, Wang P, Schaller RD, Rajh T, Rozhkova EA. High-performance bioassisted nanophotocatalyst for hydrogen production. Nano Lett 2013;13:3365e71. [13] Naseri N, Janfaza S, Irani R. Visible light switchable bR/TiO2 nanostructured photoanodes for bio-inspired solar energy conversion. RSC Adv 2015;5:18642e6. [14] Wang P, Dimitrijevic NM, Chang AY, Schaller RD, Liu Y, Rajh T, Rozhkova EA. Photoinduced electron transfer pathways in hydrogen-evolving reduced graphene oxideboosted hybrid nano-bio catalyst. ACS Nano 2014;8:7995e8002. [15] Zhao Z, Wang P, Xu X, Sheves M, Jin Y. Bacteriorhodopsin/Ag nanoparticle-based hybrid nano-bio electrocatalyst for efficient and robust H2 evolution from water. J Am Chem Soc 2015;137:2840e3. [16] Guo Z, Liang D, Rao S, Xiang Y. Heterogeneous bacteriorhodopsin/gold nanoparticle stacks as a photovoltaic system. Nano Energy 2015;11:654e61. [17] Chellamuthu J, Nagaraj P, Chidambaram SG, Sambandam A, Muthupandian A. Enhanced photocurrent generation in bacteriorhodopsin based bio-sensitized solar cells using gel electrolyte. J Photochem Photobiol B 2016;162:208e12. [18] Johnson KE, Gakhar S, Risbud SH, Longo ML. Controlled wrinkling of gradient metal films. Langmuir 2018;34:7488e96.
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[19] Kesaria M, Shetty S, Shivaprasad SM. Evidence for dislocation induced spontaneous formation of GaN nanowalls and nanocolumns on bare C-plane sapphire. Cryst Growth Des 2011;11:4900e3. [20] Alvi NH, Rodriguez PEDS, Kumar P, Gomez VJ, Aseev P, Alvi AH, Alvi MA, Willander M, Notzel R. Photoelectrochemical water splitting and hydrogen generation by a spontaneously formed InGaN nanowall network. Appl Phys Lett 2014;104:223104. [21] Ebaid M, Kang JH, Lim SH, Ha JS, Lee JK, Cho YH, Ryu SW. Enhanced solar hydrogen generation of high density, high aspect ratio, coaxial InGaN/GaN multi-quantum well nanowires. Nano Energy 2015;12:215e23. [22] Becher B, Tokunaga F, Ebrey TG. Molecular order in cis and trans unsaturated phospholipid bilayers. Biochemistry 1978;17:2293e300. [23] Kumar S, Bagchi S, Prasad S, Sharma A, Kumar R, Kaur R, Singh J, Bhondekar AP. BacteriorhodopsineZnO hybrid as a potential sensing element for low-temperature detection of ethanol vapour. Beilstein J Nanotechnol 2016;7:501e10. [24] Moltke S, Heyn MP. Voltage sensing by fluorescence resonance energy transfer in single cells. Biophys J 1995;69:2066e73. [25] Lemon BI, Hupp JT. Electrochemical quartz crystal microbalance studies of electron addition at nanocrystalline tin oxide/water and zinc oxide/water interfaces: evidence for band-edge-determining proton uptake. J Phys Chem B 1997;101:2426. [26] Bak T, Nowotny J, Rekas M, Sorrell CC. Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects. Int J Hydrogen Energy 2002;27:991e1022. [27] She C, Anderson N, Guo J, Liu F, Goh W, Chen D, Mohler D, Tian Z, Hupp J, Lian T. pH-dependent electron transfer from Re-bipyridyl complexes to metal oxide nanocrystalline thin films. J Phys Chem B 2005;109:19345e55. [28] Nollert P. Lipidic cubic phases as matrices for membrane protein crystallization. Methods 2004;34:348e53.
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Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin Pooja Devi a,**, Anupma Thakur a,b, Dibyendu Ghosh c, E. Senthil Prasad d,***, S.M. Shivaprasad e, R.K. Sinha a, Praveen Kumar c,* a
Central Scientific Instruments Organization, Sector-30C, Chandigarh, 160030, India AcSIR, Council of Scientific and Industrial Research, Ghaziabad, 201002, India c School of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700032, India d Institute of Microbial Technology, Sector 39A, Chandigarh, 160036, India e Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, 560064, India b
highlights
graphical abstract
First report on GaN NWN as photoanode with bR into electrolyte. Measured photocurrent density is 28.74 mA/cm2 at 1.0 V (vs RHE). Largely
enhanced
applied
photon-to-current
bias
efficiency
(ABPE) ~7.8%.
article info
abstract
Article history:
The ever-increasing demand for renewable and clean energy sources has prompted the
Received 7 June 2019
development of novel materials for photoelectrochemical (PEC) water splitting, but effi-
Received in revised form
cient solar to hydrogen conversion remains a big challenge. In this work, we report a bio-
25 September 2019
nanohybrid strategy in a photo-system to simultaneously enhance the charge separation
Accepted 24 October 2019
and water splitting efficiency of photoanode (PA) by introducing Bacteriorhodopsin (bR), a
Available online xxx
natural proton pumping photosystem and GaN nanowall network (NWN), a direct band gap and corrosion-resistant semiconductor. The experimental study reveals that this combi-
Keywords:
nation of bR and GaN NWN has huge potential as a light-activated sensitizer as well as
Bio-nanohybrid
proton pumping source to achieve enhance photocurrent density in hydrogen evolution
Hydrogen
reaction (HER). Consequently, this synergistic effect in bR/GaN NWN PA gives rise to largely
Nanowall network
enhanced applied bias photon-to-current efficiency (ABPE) ~7.8% and photocurrent density (28.74 mA/cm2 at 1.0 V vs RHE). It is worth mentioning that the photocurrent density of bR/
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (P. Devi), [email protected] (E. Senthil Prasad), [email protected] (P. Kumar). https://doi.org/10.1016/j.ijhydene.2019.10.184 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
2
international journal of hydrogen energy xxx (xxxx) xxx
Proton-pumping
GaN NWN, to the best of our knowledge, is superior to previously reported bR-based PAs
Photoanode
and bio-photoelectric devices reported till today for solar-to-hydrogen fuel generation. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The up surged environmental pollution owing to existing fossil fuels followed by increasing energy crisis, demand exploration of alternate sustainable and renewable clean energy sources. The direct solar-to-hydrogen conversion by efficient, environment-friendly and inexpensive means is one of the potential alternates of fossil fuel, however, lacking with the desired efficiency. Following the first report on PEC water splitting using TiO2 photoelectrodes (PEs), a huge number of attempts have been made to enhance PEs performance by sensitizing them with visible components such as quantum dots (QDs), dyes, low bandgap semiconductors, doping etc., however due to low absorption the overall efficiency remains low [1]. Further, due to visible light responsivity, natural abundance, and non-toxicity, biological entities such as nanobio hybrid catalyst, hydrogenase enzyme, phycocyanins, etc., were also investigated in combination with TiO2 PEs for the efficiency enhancement, however, suffers from stability issues [2]. Bora et al. [3] reported nano bio-hybrid photoelectrode combining hematite thin-film and light-harvesting membrane protein C-Phycocyanin from the phycobilisome family like blue-green algae (cyanobacteria). Tsui et al. [4] reported bacteriochlorophyll modified TiO2 nanotubes for photoelectrochemical application whereas Wang et al. [5] reported nano-bio architecture through noncovalent assembly of a cell-free expressed transmembrane proton pump and TiO2 semiconductor nanoparticles as an efficient nanophotocatalyst for H2 evolution. But in present scenario, only ~4% hydrogen is produced by water electrolysis, the rest 96% production is again involved in fossil fuels. Therefore, it is an utmost need to find a combination of suitable photoelectrodes (PEs) materials with stable biological entities to enhance the overall efficiency. Amongst several carbides, oxides, sulphides, nitrides, borides, and metals based PEs, III-nitrides semiconductors, indeed is a promising material to perform solar water splitting due to their advanced properties such as higher absorption coefficient, stability in aqueous solution, direct and appropriate band gap to split H2O and high corrosion resistance [6e8]. It paves the promising path for the commercial realization of PEC devices for hydrogen fuels. However, due to unintentional n-type nature, the upward band bending of these III-nitrides PEs at electrolyte interface limits overall efficiency. On the other hand bR, a natural photosynthetic system has emerged out as a candidate for solar energy conversion owing to its proton pumping function upon visible light photoexcitation. It converts solar energy to chemical energy in the form of adenosine triphosphates (ATPs), where the retinal component of bR pump, upon photoexcitation under visible light undergoes photoisomerization (~sub
picoseconds) and proton is released by Schiff base. The photocycle of bR proteins is a measurement of its proton pumping efficiency [9]. Besides, being a key component of extremophiles, It also exhibits exceptional thermal, chemical & photochemical stability and can withstand high ionic strength (3 M NaCl), temperature (80 C in water, 140 C in dry condition), wide pH range (511) and protease insulation without loss of activity [10]. In earlier reports, bR has been demonstrated in its dual role with several nanostructured materials such TiO2 nanotube array [11], Pt/TiO2NPs [12] TiO2 nanoparticulate films [13], reduced graphene oxide (rGO) [14] etc. Recently, silver NPs and bR based novel hybrid catalysts are reported to exhibit enhanced hydrogen evolution performance (733 mA/cm2) with a lower overpotential of 63 mV due to synergetic effects upon visible light irradiation [15]. Guo et al. [16] demonstrate bR/ gold nanoparticles (AuNPs) multilayers as a mimic to the stack structure of granum to perform as a novel photovoltaic stack system with improved photoelectric performance. Chellamuthu et al. [17] reported 0.49% conversion efficiency in a bio sensitized solar cells (BSSCs) using BR into TiO2 photoanode with acetamide based gel electrolytes. Johnson et al. [18] reported bR encapsulated amorphous titanium dioxide, or titania, gels for the production of hydrogen through water splitting under white light irradiation. Despite all these investigations, the obtained photocurrent with bR sensitized PEs system due to less absorption is still low, and requires improvements in terms of PEs characteristics in hybrid catalysts as a possible solution. Therefore, the combination of an efficient high-quality GaN PE with highly stable and naturally abundant bR protein has the capability to enhanced hydrogen evolution in PEC water splitting [20,21]. No attempt has been reported to the best of our knowledge on the use of biological entities; in particular, bR as a sensitizer component in IIInitrides based PEs during PEC water splitting. Therefore, we explored bR not only as a light-activated sensitizer but also as proton pumping source with robust GaN NWN PEs to achieve enhance photocurrent density in HER. The hybrid catalyst is fully characterized and demonstrated towards PEC water splitting under dark, as well white light illumination under optimized pH condition.
Experimental section Growth and characterization of GaN NWN PE The GaN NWN samples were grown directly on a c-Al2O3 substrate using a radio frequency plasma-assisted molecular beam epitaxy (SVTA). After ex-situ chemically cleaning of cAl2O3, it was inserted into the introduction chamber for initial degasing followed by high temperature cleaning in the growth
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
international journal of hydrogen energy xxx (xxxx) xxx
chamber to get the atomically clean surface. The temperature of the gallium (Ga) effusion cell was kept at 1030 C. A constant nitrogen low rate of 8 scc/m (standard cubic centimeter per minute), substrate temperature of 630 C, plasma forward power of 375 W and growth duration of 4 h were maintained for all of the films. The GaN NWNs were formed spontaneously under nitrogen-rich growth condition without using any external catalyst or buffer layers. The details of the growth conditions, fluxes were given in Ref. [19]. The grown NWNs exhibit excellent structural, optical and electrical quality, with the absence of defect-related yellow luminescence peak in PL spectrum.
Extraction and purification of bR Wild-type bR purple membrane protein namely ET001 was isolated from Halobacterium salinarium ET001 strain from lab scale shake flask culture as portrayed in Figs. S1 and S2. All operations were carried out at room temperature and centrifugation at 4 C. Briefly, Halobacterium ET001 cells were harvested after reaching stationary phase at optical density OD 1.26. The cell separation was performed by centrifugation at 7000g and 4 C for 30 min. ET001 Cell Pellets obtained were washed twice with 200 ml of basal salt media without peptone in 500 mL centrifuge bottles. Final wet cell weight obtained is in the range of 3.4e3.8 g/L after basal salt washing. Each Centrifuge bottles of 1.68 g pellet wet weight suspended in 30 mL sterile distilled water and then added with 50 mL of DNase (Stock - 5 mg DNase/mL, 2000 units/mg, Pierce Biotech, USA) to reduce the excess viscosity liberated during cell lysis by osmolysis, which was performed by dialyzing the abovesaid cell suspension of 30 mL in 10 kDa snake skin dialysis membrane (Thermo Scientific, USA) at 4 C against deionized water overnight. The deep coloured purple suspension obtained was centrifuged at 21,000 g to remove red carotenoid pigments in the supernatant. The purple pellet was dissolved in 30 mL of distilled water and the procedure was repeated three times at 21,000 rpm for 40 min until the supernatant becomes colourless. Finally the purple pellet was suspended in 2 mL of distilled water and layered over 10 ml of 20%e60% for sucrose density gradient separation (Thermo Scientific, Sorval WX100 Ultracentrifuge, USA) in swinging out rotor (Sorval swinging bucket TH - 641) for 17 h at 100,000 g, 4 C. The small red band and the large purple band appeared at the top and bottom of 45% sucrose density gradient. The purple membrane around 1.5 mL collected was diluted with 1.5 mL water (1:1) was dialyzed in 10e12 kDa semi-permeable membrane sac called as small wonder lyzer (Innovative dialysis membrane sac, US Patent 6,368,509 B1) sold by Excellon Innovations, Punjab, India against milliQ water at 4 C for removal of sucrose. The purified purple membrane obtained was stored at 20 C deep freezer for further use.
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spectrophotometrically using molar extinction coefficient of 63,000 M1 cm1. The maximal absorption at 280 nm corresponding to the total protein content of the sample and the maximal absorption at 570 nm indicated bR protein [22]. The quality of isolated ET001 Bacteriorhodopsin containing purple membrane was estimated by A 280 nm/A 570 nm ¼ 3.326/ 1.679 ¼ 1.98 found to be equal to the reported ratio values by Cassim JY 1975. The ratio values less than 2.0 indicates the high quality of bacteriorhodopsin purple membranes extracted from Halobacterium salinarium ET001 cells. The optical and structural characteristic of bR modified electrolyte were probed by UVevis (Hitachi Ltd., Japan), FT-IR (670 IR, Varian, United States) and Raman inVia instrument (Reninshaw) at lex ¼ 785 nm. The GaN NWN PAs were ex-situ characterized by a field emission scanning electron microscope (FEI and high resolution), X-ray diffractometer (Discover D8 Bruker) with a Cu Ka X-ray source that had a wavelength of 1.5406 A. The electronic structure of the PAs were probed by Xray photoelectron spectroscopy (Omicron) by recoding valence band spectra. The optical properties were studied by photoluminescence spectroscopy (PL, Horiba Jobin Yvon) using a xenon lamp source and 325 nm excitation.
Photo-electrochemical characterization of hybrid catalyst PEC characterizations were performed current densityvoltage (J-V) curves in three different set of experiments. 1) using GaN NWN as PE in a 0.1 M Na2SO4/H2SO4 (NH) electrolyte solution, 2) bR/GaN NWN hybrid PE in a 0.1 M Na2SO4/H2SO4 (NH) electrolyte solution and 3) using GaN NWN as PEs and adding different concentration of bR in electrolyte to measure the effect on overall hydrogen evolution efficiency. The preparation of 0.1 M Na2SO4/H2SO4 (NH) electrolyte consists of firstly preparing 0.1 M Na2SO4 in deionized water, then balancing pH of this solution to 7.2 by adding 0.1 M H2SO4. bR was adsorbed onto NWN PEs directly by overnight incubation, whereas it was added in standard addition approach into electrolyte before photo measurements (AUTOLAB Metrohm electrochemical workstation) under dark and illuminated condition. A solar simulator equipped with 300 W Xe lamp and AM 1.5G filter was used as light source. The intensity of the source was calibrated and controlled at 100 mW/cm2. The current density versus the potential of PEs was measured under dark and light conditions by linear scan voltammetry (LSV) and cyclic voltammetr (CV) at a constant scan rate of 10 mVs1. Impedance spectra were recorded with frequency response analyzer and Nova software at zero bias in the 0.1e105 Hz frequency range under 10 mV ac perturbation signals in light and dark condition. All pH measurements were made using digital pH-meter (LMPH 10, Labman Scientific Instruments Ltd).
Characterization
Results and discussion
The quality of the isolated bacteriorhodopsin containing purple membrane was checked by scanning between 200 and 800 nm in UVeVis spectrophotometer (Hitachi 2390 UV spectrophotometer, Japan) and measuring absorbance ratio of A280nm/A570nm. bR protein concentration was estimated
Radio frequency plasma-assisted molecular beam epitaxial GaN NWN has been thoroughly characterized (Fig. 1) via Field Emission Scanning Electron Microscopy (FESEM), High Resolution X-ray Diffraction (HRXRD), X-ray photoelectron spectroscopy (XPS), and Photoluminescence (PL). The FESEM
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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Fig. 1 e FESEM micrographs of GaN NWN (a) Top view and the inset shows a cross-sectional view. (b) HRXRD rocking curve of GaN NWN corresponding to the reflection for (002) plane, (c) Valance band spectra and (d) PL spectra for GaN NWN.
image (Fig. 1a) depicts homogeneous network morphology consisting of wedge shaped nanowalls surrounding the voids formed by open screw dislocations. The average tip width and average cavity size of nanowall were measured to be ~10 nm and 56 nm, respectively, with the corresponding surface coverage of 56%. The cross-sectional image (inset of Fig. 1a) reveals the presence of gaps in-between the wall which manifests that the pore as seen in the plan view percolate all the way down to the bottom, hence making it more effective for light absorption by multiple scattering, which will enhance the overall PAs efficiency. Further, the structural quality of these grown PAs was confirmed by the rocking curve of (0002) plane from HRXRD (Fig. 1b). The sharp FWHM (~2600 arc.sec) of the rocking curve confirms the low defects and high structural quality of the grown PAs. To understand the effect of the bend bending behaviour at the surface of PA, XPS valence band (VB) spectra were measured and presented in Fig. 1c. As XPS gives the information about the occupied states therefore 0 eV is considered to be as Fermi level whereas extrapolated edge five the position of VB maxima. Figure shows the VB maxima position at 3.05 eV, which manifests n-type characteristic of at PA surface. In order to have more insight on the optical quality of as-grown GaN NWN PAs, LT-PL measurements were performed to identify the luminescence originating from defects and band edge. Generally defects are created in between conduction and valence band due to the presence of Ga and N
vacancies, oxygen or deep level impurities and possess broadening of the PL peak towards low bandgap (higher wavelength). The sharp and strong emission peak at 3.19 eV and absence of the defects related peak as shown in Fig. 1d, confirm the high quality of the MBE grown GaN-NWN PEs. The extraction of the purple membrane was performed from Halobacterium ET001 Cell pellets. The Osmolysis of the cell lysates was carried out on 10 kDa snake skin dialysis membrane against deionized water. After the differential centrifugation of purple lysates at 21,000 rpm the sucrose density fractionation was peformed to separate the red carotenoids and purple bacteriorhodopsin bands. The Purple bands were obtained at 45% fraction of sucrose density layer. The obtained crude purple suspensions were further dialysed against deionized water to remove residual sucrose. The quality of isolated purple membrane protein was checked by measuring the absorbance ratio at A280 nm/A570 nm (Fig. 2 Supplementary Data). The structural characterizations of the bR were performed using FTIR and Raman spectroscopy. The UVevis spectra shows the characteristic absorption peaks at 280 and 570 nm, which are assigned to aromatic residues of bR including 8-tryptophan and 11-tyrosine and the P-P* transition in the retinal chromophore between two energy levels upon exposure to the light as shown in Fig. 2a [23]. Fig. 2b portrays the Raman spectra of bR with a laser of 514 nm excitation wavelength. The vibrational spectra show characteristic peaks at 1528 cm1 corresponding to C]C stretching
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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Fig. 2 e (a) UVeVis absorption, (b) Raman and (c) FTIR spectra for the bR with different concentration as well as pure bR.
and fair stretch is observed in the range of 1150e1220 cm1 which attributes to CeC stretching [24]. Fig. 2c shows the FTIR spectra of bR with characteristic peak attributing to the vibrational stretch of hydrogen out-of-plane (HOOP) at 1050 cm1, which corresponds to the twist in the skeleton of the protein. The other comparatively weak vibrations within 1100e3500 cm1 could be attributed to CeOeC asymmetric stretching, OeH bending, C]O stretching, CeH bending of the retinal chromophore and the amide vibrational stretch at 1540 cm1 due to NeH bending, which reveals the secondary structure of the protein [25]. After asserting the quality of the GaN NWN PAs, PEC experiments were performed under three electrode setup in 0.1 M Na2SO4/H2SO4 (NH) electrolyte (pH 7.2) under solar light illumination. The PEC results for GaN NWN PAs with or without bR are shown in Fig. 3. To understand the role of bR as a visible light harvesting molecule in addition to working as a proton pump, the two-fold PEC measurements were performed. In the first set of experiments, GaN NWN PAs were modified by adsorbing bR directly on to it and PEC performance was checked under 0.1 M Na2SO4/H2SO4 (NH) electrolyte (pH 7.2) solution. For the experimental optimized conditions (effect of electrolyte (Fig. S3) and effect of pH of electrolyte (Fig. S4) maximum current density of 10.12 mA/ cm2 with the on-set potential of 1.0 V vs RHE was observed for bare GaN NWN PAs. Futher, in the second set, the PEC
experiments were performed using GaN NWN PAs in the absence and presence of bR in the electrolyte solution. Fig. 3a shows a LSV under dark and white light illumination. Under dark condition the GaN NWN PA doesn't exhibit any current as the potential is swept from 0.4 V to 1.4 V vs RHE, even with the presence of bR in solution, revealing chemical inertness of PAs under a dark condition in the selected potential range. On solar light illumination, the PAs showed an onset photocurrent at 0.65 V vs RHE and, which continues to increase until reaching a steady-state value of 25.4 mA/cm2. Upon adsorption of bR, the modified GaN NWN PAs show the decrease in the PEC performance with the maximum current density of 12.3 mA/cm2 (Fig. S5), which can be subjected to low transfer of electrons from GaN to bR. However, the optimization for the thickness of bR on NWNs for hybrid PAs is still underway for the enhancement of overall efficiency. On the addition of bR (0.5 mLe1.5 mL in 100 mL of electrolyte) into the electrolyte solution, the photocurrent density increased to 28.5 mA/cm2, which is ~3 fold enhancement over GaN NWN PA without bR in the electrolyte (10.8 mA/cm2), at the potential of 1.0 V vs RHE (Table 1) indicating more solar light harvesting in the presence of bR, thereby, leading to effectual separation of photogenerated electronehole pairs as compared to bare GaN PA. The role of bR in decreasing/enhancing the photocurrent density can be explained as follows. The principal scheme of
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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Fig. 3 e (a) J-V characteristics for the bare GaN NWN under dark and 1 sun illumination along with 1.5 mL bR modified electrolyte under1 sun illumination. (b) The schematic illustration of the electron transfer from bR to GaN NWN PAs. EIS spectra for NWN (c) without bR and (d) with 1.5 mL bR.
Table 1 e PEC and EIS parameter for bare GaN NWN and bR modification. Experimental Conditions 0.1 M Na2SO4/H2SO4 electrolyte 0.2 (pH 7.2) Xe lamp (AM 1.5 filter) of 100 mW/cm2 power intensity.
PAs
Jphoto (mA/cm2) at 1.0 V vs RHE
Rs (U)
Rp (U)
ABPE (%)
TOF at 1.0 V vs RHE (S1)
GaN NWN GaN NWN_1.0 mL bR GaN NWN_1.5 mL bR
10.80 21.24 28.74
15.3 7.41 20.0
68.1 47.0 26.5
2.55 4.86 7.85
1.4 2.26 3.66
enhanced light-driven water splitting by hybrid system employs the light-harvesting proton pump bR as a key building block responsible for coupling the visible light reactivity of GaN NWN PA and for driving proton towards counter platinum counter electrode for subsequent reduction to hydrogen gas. When decorated onto GaN NWN directly by overnight incubation, the photocurrent density reduced in comparison to bare PAs (Fig. S5). This may be due to the larger size of bR (~800 nm) in comparison to GaN NWN (~50e100 nm). Hence bR might have attached on the top surface of GaN NWN films, which led to the covering of effective surface and therefore, suppressing the photoresponse of hybrid PEs [10]. The presence of bR at the surface completely masks the light harvesting effect of the underlying GaN NWN films and significantly turns into its negligible photoresponse towards PEC water-splitting. The bR-concentration dependency on the GaN NWN was observed clearly with varying concentrations opted for adsorption namely 1:10, 1:5 and 1:1 dilutions of
bR:PBS quoted as very less bR/GaN NWN, less bR/GaN NWN and more bR/GaN NWN, respectively as presented in Fig. S6. Therefore, to exploit bR advantages, it was added into solution directly during PEC water splitting. The effect of bR concentration on photocurrent density was also studied and shown in Fig. S7a. The results portray the increase in the photocurrent density with the increasing bR concentration, which can be attributed to the proton-pumping effect of bR in the electrolyte. The maximum photocurrent density of 28.74 mA/cm2 was observed with an optimal concentration of 1.5 mL bR. Upon increasing bR concentration, the effect similar to direct loading of bR onto GaN NWN PA via overnight incubation is observed, which could be understood in terms of a hindrance in catalytic performance of GaN NWN PAs. As, the assessment of the PEC activity is realized from the generalized hv water-splitting reaction i.e. 2H2 O/ 2H2 þ O2 , which is clearly expressed by the observed effectual photo-anodic current behaviour explicitly coming from the GaN PA in the presence
Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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of bR under light irradiation. This highlights to hypothesize the influence of the absorption of green light by bR which might have caused excitation of electrons from HUMO to LUMO and injection of same from protein retinal complex to the low energy conduction band of GaN NWN photoanodes, which is energetically favourable (Fig. 3b) as reported earlier [8]. These electrons complete the loop through an external circuit, causing reduction of Hþ ions (from bR and catalytic water splitting) at Pt electrode and well supported by the previous reports [8e12]. As the proton pumping process of bR continues with the light irradiation and the generated protons gets drifted from the anode to the cathode. As the proton concentration at the cathode is different from that at the anode results in different local pH environments. The difference in pH values at anode and cathode due to the presence of bR protein is also proposed as one of the reasons behind bR enabled enhanced photocurrent in PEC cell. The two different pH values at electrodes result in the different pH local environment, which may impose a chemical bias to help in charge separation and more photocurrent generation [26e28]. The corresponding CV analysis shows the enhanced current characteristic of PA in presence of bR and cathodic shift of reduction peak responsible for high H2 evolution which indicates that a more effective separation of photogenerated electron-hole pairs and a faster interfacial charge transfer had occurred as compared to bare GaN PA (Fig. S7b). The impedance characteristic of PA in presence and absence of bR under
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white light illumination further supports above observations. The figure is associated with impedance response in frequency range and associated Randel's equivalent circuit, where, Rs, Rp, CPE, and W correspond to the solution series resistance, polarization (charge transfer) resistance and constant phase element and Warburg impedance respectively. It can be seen that for the bR adsorbed PAs the polarization resistance, which is associated with charge transfer characteristic at electrode/electrolyte interface, increases from 68.1 U to 139 U, whereas on the addition of bR into electrolyte it decreases to 26.5 U for optimized bR concentration (Table 1), which further supports the LSV and CV observations. These charge-carrier transport properties of the GaN PA in presence and absence of bR is well reconnoitred by electrochemical impedance spectra (EIS) as a function of light irradiation and dark in the presence and absence of bR, also lays the foundation to understand the mechanism for the PEC performance of the proposed bio-nanohybrid PA. As, the radius of the arc on the EIS spectra reveals the reaction rate occurring at the surface of the electrode, which states that the smaller arc radius of GaN PA in the presence of bR indicates that a more effective separation of photogenerated electronehole pairs and a faster interfacial charge transfer had occurred as compared to bare GaN PA. The EIS-Nyquist plots of GaN PA with bR portrays that the bio-nanohybrid PA acts as solar light harvester and absorbed photons at the surface which promotes an electronexcitation from the ground state of the bR i.e. p-orbital
Fig. 4 e Cyclic photocurrent measurement under chopped light for bare GaN NWN and after 1.5 mL bR in the electrolyte solution at 0.4 V vs RHE bias potential and the corresponding LSV is shown in (b) for NWN_1.5 mL bR. (c) ABPE AND (d) TOF plots for bare NWN and NWN_1.5 mL bR. Blue correspond to NWN and red correspond to NWN_1.5 mL bR. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184
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(HOMO) to the excited state i.e. p*-orbital (LUMO) which enhance the separation efficiency of photo generated electronehole pairs and indorse water splitting reactions. Fig. 4 is associated with transient short circuit photoresponse of PAs in absence and presence of bR protein, which was carried out under intermittent solar light illumination using potentiostatic condition at 0.4 V vs RHE bias. As can be seen very clearly, that under illumination, photocurrent increases rapidly and reaches a steady-state current density of 70 (mA/cm2) and 290 (mA/cm2) with absence and presence of bR protein, respectively. The photocurrent increases rapidly and reaches steady-state values, respectively. Once the illumination is turned off, the photocurrent returns to the background level, and this process can be repeated many times. In the dark conditions, the JeV curve shows typical rectifying behaviour, with a weak current density. The portrayed rectangular signal revealing the transient short circuit photoresponse of PAs in absence and presence of bR protein under light irradiation and dark, implies to the non-equilibrium state amidst the boosted rate of charge carrier generation in the presence of light and vice-versa. With these observed high PEC performance, the potential of the proposed system is highly efficient amongst bio-photoelectric devices reported till today for solar-to-hydrogen fuel generation. Further, the ABPE for optimized bR concentration in the electrolyte and without bR is calculated to be 8% and 2.3%, respectively, (Fig. 4c), which is the highest reported value for such a bio-nanohybrid system till now in our knowledge best. Additionally, to explicate the boosted PEC water splitting performance of PAs, we thoroughly investigated the amount of active catalytic active sites which was estimated by turnover frequency (TOF). The number of active sites (n) is examined using CVs at a scan rate of 10 mV/s. When the number of voltammetric charges (Q) is obtained after deduction of the blank value, n (mol) can be calculated with the equation of n ¼ Q/2F, where F is Faraday constant (96,485 C mol1). TOF, calculated with the equation of TOF ¼ I/(2Fn) where I (A) is the current of the polarization curve obtained from the LSV. Fig. 4d shows the polarization curves which has been normalized by the active sites and expressed in terms of TOF. It can be seen that to achieve a TOF of 3.625 s1, sample NWN_bR needs an overpotential of 230 mV. Also, Tafel polarization curve (Fig. S8) reveals the catalytic performance of GaN NWN_1.5 mL bR is more superior to GaN NWN without bR and it drives 30 mA/cm2 at an overpotential of 124 mV. The enhanced HER activity is further illustrated by comparing the Tafel slopes to analyze the quantifiable charge kinetics of the water-splitting process. As shown in Fig. S8, the Tafel slope of 124 and 126 mV/dec for NWN-bR and NWN suggest typical Volmer-Heyrovsky characteristics.
Conclusion In summary, a new system based on GaN NWN and bR protein are investigated and characterized for PEC water splitting for hydrogen production. GaN NWN array was grown by epitaxial growth approach and used as PAs in presence of bR protein in the electrolyte, which observed to
cause ~3-fold enhancement in photocurrent density. The observed high PEC performance is assigned to band alignment of bR system with GaN NWN PAs, enhanced light absorption by bR along with proton releasing property of bR protein. The proposed bR/GaN NWN hybrid catalyst as PEs provides a novel perspective approach for emerging versatile bio-photoelectric devices for solar-to-hydrogen fuel generation.
Acknowledgement The authors thank Prof. S Bhattacharya, Director IACSKolkata for his encouragement and support. The authors wish to thank DST, Govt. of India for the financial grant (DST INPIRE Faculty & DST/TSG/PT/2012/66). AT acknowledges DST-INSPIRE fellowship for doctoral study. SK acknowledges Prof. Mark T. Facciotti, University of California, for providing Halobacterium ET001 cells and Devendar Singh, Vinod Kumar, Shivani Sharma, Riya Sharma, Sushmita, IMTECH, Chandigarh for BR isolation and characterization.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.184.
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Please cite this article as: Devi P et al., Boosting photoelectrochemical performance of GaN nanowall network photoanode with bacteriorhodopsin, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.184