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Bio-template assisted hierarchical ZnO superstructures coupled with graphene quantum dots for enhanced water oxidation kinetics
Solar Energy 199 (2020) 39–46
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Bio-template assisted hierarchical ZnO superstructures coupled with graphene quantum dots for enhanced water oxidation kinetics
T
Suhaib Alama, Tushar Kanta Sahua, Devipriya Gogoib, Nageswara Rao Peelab, ⁎ Mohammad Qureshia, a b
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam 781039, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Hierarchical ZnO superstructures Photoelectrochemical water oxidation Polygalacturonic acid Hole extracting agent Graphene quantum dots
Due to anisotropic growth behavior and tunable electrical properties, ZnO nanostructures having dimensions such as 0-D, 1-D, 2-D and 3-D are actively studied for their optoelectronic properties. However, ZnO based photoanodes suffer from unfavorable recombination of electron hole pair, which hinders its use in photoelectrochemical (PEC) water oxidation. Herein, we demonstrate a strategy to enhance the PEC performance using bio-template assisted in-situ grown hierarchical ZnO superstructures directly over fluorine-doped tin oxide (FTO) modified by graphene quantum dots (GQDs). GQDs decorated hierarchical ZnO superstructures displayed a significant increment of ~77% in photocurrent density value compared to pristine ZnO with an impressive carrier density of 3.19 × 1020 cm−3, which is ~1.8 orders of magnitude higher than that of pristine ZnO. It is observed that GQDs acts as an efficient hole extractor, which improves the carrier separation on ZnO surface and reduces the hole trapping probability.
1. Introduction Splitting of water into hydrogen and oxygen is an attractive alternative to harvest solar energy for future energy crisis. In this regard, photo electrochemical (PEC) water splitting has received much attention to effectively couple solar irradiation with the electrochemical processes (Ma et al., 2018; Li et al., 2013). During the past few decades, several metal oxides such TiO2, ZnO, WO3, Fe2O3, BiVO4, etc have been used as photo-electrodes for PEC water splitting (Zhang et al., 2014; Zhang et al., 2018; Tang et al., 2019; Marelli et al., 2014; Hegner et al., 2017). Among these n-type metal oxides, Zinc oxide (ZnO), a wide bandgap (~3.2 eV) semiconductor photoanode has attracted much attention due to its attractive optical properties, high electron mobility, low toxicity and its anisotropic growth behavior (Zhan et al., 2018). Although ZnO has many advantages over similar n-type materials, still the PEC activity is inferior to many photoanodes due to its sluggish water oxidation kinetics. Recently, ZnO has been modified with different strategies to overcome the above drawbacks (Chen et al., 2014). Recent developments in the synthetic strategies of inorganic semiconductors having different morphologies with controlled shape and size have gained significant interest due to their catalytic and charge transport properties (Li and Yu, 2019; Dong et al., 2012). The
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significant scientific and technological importance of nanomaterials and their synthetic procedures are still challenging and require additional efforts for the complete utilization of their potential for application (Shi et al., 2013; Guo et al., 2008; Shevchenko et al., 2006; Liu et al., 2013). Different structures of ZnO such as 3D branched nanowires (Kargar et al., 2013), star like structures (Marlinda et al., 2019), ZnO Nano tree and nano cluster structures (Ren et al. 2016), nanowires (Li et al., 2015a, 2015b), nanorods (Wang et al., 2015), nanoplates assemble sphere like structures (Emil et al., 2018), have been utilized for PEC water oxidation. Semiconductors having hierarchical morphologies can be an effective way to enhance the PEC performance because of their improved charge transfer, surface area and sensitizer loading capabilities apart from their strong light scattering effects and efficient electron transport (Memarian et al., 2011; Ko et al., 2011). Compared to the 1D nanostructures, hierarchical superstructures are more advantageous as they provide long optical pathways for efficient light absorption and multiple reflections, short channels for faster charge transport and a large interfacial area for water redox reactions (Li et al., 2015a, 2015b). Several growth patterns based on their crystal growth behavior have been used to synthesize a regular ordered structure such as template derived homo or heteroepitaxial growth behavior (Xu et al., 2009). Here, we have utilized a naturally occurring
Corresponding author. E-mail address: [email protected] (M. Qureshi).
https://doi.org/10.1016/j.solener.2020.02.015 Received 22 November 2019; Received in revised form 17 January 2020; Accepted 4 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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down at room temperature and obtained solution was filtered through 0.22 μm syringe filter, in order to separate the larger particles. This filtered solution was then dialyzed through a dialysis bag (Da = 1000) and water for 24 h. The water used in this process was changed after every 6 h. This dialyzed solution was dried at 100 °C for 48 h to obtain the desired graphene quantum dots.
biomass-derived templating agent as an alternative to conventional templating agents. Use of such templating agent is not only economical, but also environmentally benign under moderate conditions. Templating agent with several functionalities is accountable for the alteration of nanostructure morphologies by adsorbing at the surface of the crystal and confine the crystal growth in a specific crystal plane (Chetia et al., 2015). An essential requirement to enhance the PEC water oxidation performance is the efficient separation of photogenerated electron-hole pair. For this, 2D materials such as reduced graphene oxide (RGO) has been widely used to assist the photogenerated electron-hole separation (Sun et al., 2017). Notably, quantum dots due to their size-dependent electronic and optical properties have been attracting substantial attention. The recombination of photogenerated electron-hole pairs can be greatly reduced with quantum dots sensitization. Moreover, due to the large specific surface area provided by quantum dots, a number of surface-active sites will be available for efficient PEC water splitting. Quantum dots are also known to be passivating surface states by massassembly over the photoanode surface (Xie et al., 2017). Despite having several physical-co-chemical properties of GQDs, reports on the rational design of semiconductor/GQDs nanocomposites are still rare (Zeng et al., 2016; Ghosh et al., 2016).
2.3. Preparation of hierarchical ZnO superstructures/graphene quantum dots composites For the fabrication of hierarchical ZnO superstructures and Graphene Quantum Dots hybrid device, a measured amount of GQDs (1.5 mg/mL) was dissolved in deionized water under stirring for overnight. This GQDs solution was then spin-coated over as-synthesized hierarchical ZnO superstructures at 3000 rpm for 30 sec and heated at 120 °C for 10 min to complete one cycle and named as ZnO/GQDs-1. This process of GQDs spin coating was repeated and named hereafter ZnO/GQDs-2, and ZnO/GQDs-3 respectively. Finally, all the samples were heated at 150 °C for 60 min. 2.4. Photo-electrochemical measurements An electrochemical analyzer (model-CHI1120B) with three-electrode configuration, was used to carry out the photo-electrochemical measurements of as-synthesized photoanodes. For the measurements, 1 M NaOH solution was used as an electrolyte. Before the measurements, this electrolyte solution was deaerated by nitrogen gas purging for 30 min. The linear sweep voltammetry was performed between scan ranges of −0.8 to 0.6 V vs Ag/AgCl with a scan rate of 10 mV/sec. As synthesized photoanodes, a Pt wire and Ag/AgCl were used as working electrode, counter electrode, and reference electrode respectively. The applied potential was then converted into reversible hydrogen electrode (RHE) potential by using the following equation.
2. Experimental section 2.1. In-situ growth of hierarchical ZnO superstructures on-to fluorine doped tin oxide substrate Bio-template (polygalacturonic acid) assisted in-situ growth of hierarchical ZnO superstructures, directly over the FTO was carried out by the hydrothermal method. In a typical synthetic procedure, a calculated amount of zinc nitrate hexahydrate (1 mmol, 297 mg) was dissolved into 25 ml of deionized water with continuous stirring for 10 min. After this 0.1 wt% of polygalacturonic acid (1 mg/ml) was added to the above solution and stirred for another 15 min. This solution became more viscous after the addition of polygalacturonic acid, due to the interaction of inorganic salt and bio template (Fig. 3). After this 0.25 M of hexamethylenetetramine was added to the above solution and leave the solution for 30 min. After continuous stirring, this solution became light yellow. This yellow-colored solution was then transferred into 50 ml Teflon lined stainless steel autoclave and precleaned FTO substrates were immersed into the solution by facing down the conductive side. After this, the autoclave was sealed tightly and maintain the temperature at 140 °C for 4 h for the growth of hierarchical ZnO superstructures. After the completion of the reaction, autoclave was allowed to cool down at room temperature and the substrates were cleaned with plenty of water and ethanol for the complete removal of residual from the surface and then these substrates were dried at 100 °C for 12 h. In last these substrates were annealed at 500 °C for 1 h in electric furnace with heating rate of 10°/min. In order to understand the effect of hexamethylenetetramine additive, we have also studied the reaction in detail by varying the amount of hexamethylenetetramine keeping the other reaction conditions same. In addition, we also studied the reaction by changing the reaction temperature as well as the reaction time to understand the growth process of hierarchical ZnO superstructures.
°
ERHE = EAg/AgCl + 0.0591 ∗ pH + EAg/AgCl
(1)
where ERHE = RHE converted potential EAg/AgCl = experimentally measured potential vs Ag/AgCl E°Ag/AgCl = standard potential of Ag/AgCl reference electrode against RHE (0.1976) pH = pH of electrolyte used Tungsten halogen lamp of 300 W with light intensity calibrated at 100 mW/cm2 was used as a light source. The electrochemical impedance spectroscopy (EIS) measurements were performed using the CH Instruments model CHI680E, Inc., Austin, TX, in the frequency range of 0.1–106 Hz under light illumination. To measure the amount of gas evolved and to calculate the Faradaic efficiency, online Gas chromatography (GC) (model-7820A, Agilent Technologies) was used. 3. Results and discussions 3.1. X-ray diffraction analysis To elucidate the formation and crystal phase purity of as-synthesized hierarchical ZnO superstructures, X-ray diffraction (XRD) analysis was performed. Fig. 1 represents the X-ray diffraction pattern of assynthesized pristine and graphene quantum dots coated ZnO thin films, recorded for 2θ value of 20°–80° with a scan speed of 3°/min. From the Fig. 1, the diffraction peaks for all samples at 2θ values of 31.82°, 34.39°, 36.23°, 47.63°, 56.63°, 62.88°, 66.38°, 68.03° and 69.13° with their respective crystal planes (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1) are indexed to hexagonal wurtzite phase with P63mc space group symmetry according to JCPDS card no. 36-1451. Based on this observation, it is clear that all films are polycrystalline in nature. From the Fig. 1, it is also evident that there is no
2.2. Synthesis of graphene quantum dots (GQDs) Water-soluble graphene quantum dots were synthesized by a facile hydrothermal route. A calculated amount of citric acid (2.1 g) was dissolved in 20 ml of deionized water with continuous stirring. After the complete suspension of citric acid, 1.8 g of urea was added to the above solution. This solution was then transferred into 50 ml Teflon lined stainless steel autoclave and heated at 200 °C for 5 h. After the completion of the hydrothermal reaction, this autoclave was allowed to cool 40
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Fig. 1. X-ray diffraction pattern of as-synthesized (a) Hierarchical ZnO superstructures with polygalacturonic acid, (b) ZnO without polygalacturonic acid and (c) Hierarchical ZnO superstructures coated with Graphene Quantum Dots (GQDs), and corresponding FESEM images.
Fig. 2. (a) Depicts FESEM images of as-synthesized hierarchical ZnO superstructures at 140 °C for 4 h, (b) TEM image of as synthesized hierarchical ZnO superstructures, (c) TEM image of GQDs coated hierarchical ZnO superstructures and (d) HRTEM image of GQDs coated hierarchical ZnO superstructures.
diffraction peak related impurity has been observed. From Fig. 1 trace c, there is no peak of graphene quantum dots, which is due to the low content and scattering cross-section of graphene quantum dots as well as the high crystallinity of ZnO as compared to graphene quantum dots. It is important to mention that the intensity ratio of (1 0 0) to (0 0 2) crystal planes changed significantly, indicative of different tropism of products under different reaction conditions. Because of the anisotropic property of ZnO, nano-cones systematize themselves and attached to each other along the c-axis to minimize the surface energy. Therefore, varying the surface energy of crystal planes could be an effective way to modulate the crystal growth behavior in order to acquire different morphologies in ZnO. Polygalacturonic acid was used to reduce the surface energy of the crystal planes because of the electrostatic interaction between the polar surface of ZnO and the ions of polygalacturonic acid (Sun et al., 2006). A smaller value of intensity ratio suggested the crystal growth along the c-axis while a higher value specified retarded growth along c-axis (Huang et al., 2014). From the diffraction pattern the intensity ratios of (1 0 0) to (0 0 2) crystal planes were found to be 0.48 and 0.84 for ZnO superstructures (with PGA) and ZnO nano-cones (w/o PGA, only hexamine) respectively. A smaller value of intensity ratio was achieved when hierarchical ZnO superstructures were synthesized in presence of PGA and hexamine specified anisotropic growth along c-axis. In contrast to this higher value of intensity ratio was obtained when ZnO (nano-cones) were synthesized with only hexamine, suggested retarded growth along c-axis.
to optical E2g phonons at Brillouin zone center. The degree of disorder can be determined by the intensity ratio of ID to IG (ID/IG ratio), which is inversely proportional to the average size of the crystallite size domain. Average crystallite size can be determined using the following equation (Lin et al., 2016; Gokus et al., 2009).
ID /IG = C(λ )/La
(2)
where ID is the intensity of D band, IG is the intensity of G band, C(λ) is the empirical constant depends on the wavelength of the excitation laser and La is the crystallite size. Using above Eq. (2), the calculated average crystallite size was found to be ~4.1 nm which is in close agreement to the size of GQDs determined by TEM analysis. Observed ID/IG valued is greater than 1, indicative of several defects present at GQDs surface (Ahirwar et al., 2017). 3.4. Morphological analysis Morphological features of in-situ grown hierarchical ZnO superstructures were evaluated by field emission scanning electron microscope (FESEM) analysis as depicted in Figs. 2 and S3. From the figure, it is clear that as grown hierarchical ZnO superstructures were uniformly distributed throughout the FTO substrate. The superiority of these structures is that, these were grown directly over FTO without any seed layer and displayed robust adhesion and good Ohmic contact to the FTO substrates. The average dimension of as-grown ZnO superstructures was observed in the micron range. From the figure (2 a), it is clear that these structures were formed by connecting different nanocones. The surface functionalities of polygalacturonic acid such as hydroxyl and carboxylic groups were accountable for the oriented assembly of the ZnO nano-cones to form hierarchical ZnO superstructures due to the interaction of the ZnO polar surfaces. Polygalacturonic acid was used as a bio-template to govern the growth as well as an accumulating agent to construct the hierarchical ZnO superstructures. It is evident from the Fig. 2(a) that these hierarchical ZnO superstructures were closely connected, due to which the inter-particle connectivity was increased which helps in effective charge transfer by minimizing the surface traps for electron-hole recombination. In addition to the top view, cross-sectional analysis was also carried out for better understanding, depicted in Fig. S4(a, b). The surface morphology of as-synthesized ZnO in the absence of polygalacturonic acid was also performed presented in Fig. S4(c, d). A systematic approach has been used
3.2. FT-IR analysis Formation and presence of different functional groups in as-synthesized graphene quantum dots were confirmed by FT-IR analysis, presented in Fig. S1. Broad peak present at 3230–3600 cm−1 could be attributed to the stretching vibration of eOeH. The peaks at 1660 cm−1 and 1558 cm−1 exhibited the vibrational stretching of eC] O and eNeH bending. Peaks appeared at 1390 cm−1 was due to the symmetric stretching of eCOOH group while peak at 1175 cm−1 was due to the eCeOeC group. It is noteworthy to mention that the presence of hydroxyl and carboxyl groups, ensured high dispersion of GQDs in aqueous medium (Yang et al., 2014; Chen et al., 2016). 3.3. Raman analysis Graphene quantum dots were further characterized by Raman analysis, depicted in Fig. S2. Raman spectra of as-synthesized GQDs showed two bands: D band at 1356 cm−1 and G band at 1548 cm−1. D band can be attributed to the bonding and anti-bonding orbitals i.e. defect-induced breathing mode of A1g phonons while G band arises due 41
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integration of ZnO nanocrystals by considering the assembling function of the polysaccharide. It is traditional that the growth process of the hierarchical ZnO superstructures can be controlled by two steps viz. nucleation and the crystal growth at the assembling stage. The growth mechanism for assembling of small nano-cones into microstructures can be described based on crystal aggregation (Wang et al., 2006; Mo et al., 2005). Here polygalacturonic acid was first dissolved in the aqueous medium and generated negatively charged ions in the solution due to the presence of hydroxyls and carboxylate groups. Therefore, there is an electrostatic interaction between the negatively charged bio-template and the inorganic ions (positively charged Zn2+) to form adduct (Zn2+–Polygalacturonic acid) in the reaction mixture (Sun et al., 2006). The addition of hexamine is responsible to maintain the pH of the reaction mixture and instantaneously generated a light yellow color, probably related to Zn(OH)2. Zn(OH)42− and Zn(NH3)42+ ions acted as the crystal growth seed units for the formation of ZnO building blocks (Wang et al., 2004). Under the hydrothermal reaction, as obtained growth units were transformed into ZnO nuclei, considered as the growth units. Generally, the crystal growth units of ZnO have the tendency to grow along the c-axis by minimizing the surface energy. It is established that ZnO crystal possesses the {+0 0 0 1} positive crystal surface and {−0 0 0 1} negative crystal surface, attributed to the Zn2+ populated and O2− populated high energy polar facets respectively. So, the crystal growth units i.e., zincate ions (Zn(OH)42−) are favorably attached to the {+0 0 0 1} positive surface of ZnO crystal, which encourages the formation of ZnO nano-cones along the c-axis direction (Sun et al., 2002). Therefore, the growth rate of ZnO crystal is considered to be faster in {+0 0 0 1} positive crystal surface as compared to {−0 0 0 1} negative crystal surface, indicative of Zn(OH)42− ions are responsible major growth units for the ZnO crystal (Govender et al., 2004). In this growth mechanism, negatively charged bio-template i.e., polygalacturonic acid is accountable for the suppressed growth of ZnO nano-cones along the c-axis, resulted in the formation of non-uniform dimensions of nanocrystals. During the synthetic procedure, polygalacturonic acid not only acts as a soft template for the formation of ZnO nano-cones but also assists as an assembling agent to fabricate hierarchical ZnO superstructures. In the absence of soft template, only nano-cones were formed, depicted as the FESEM images in Fig. S4(c and d). After the complete nucleation of crystal from the precursor, the negatively charged bio-template initiates the assembling of building blocks and arrange them in an array to form hierarchical ZnO superstructures. As the reaction proceeds, nano-cones and connected cones
to understand the growth mechanism and evolution of hierarchical ZnO superstructures under controlled experimental conditions by changing the concentration of hexamine as well as by varying the reaction time and temperature. Morphological changes under different reaction conditions were characterized by performing FESEM analysis (Figs. S5–S7 supporting information). 3.5. Structural analysis To further corroborate the structural features of as-synthesized hierarchical ZnO superstructures and modified with GQDs, transmission electron microscopy (TEM) analysis was carried out as depicted in Figs. 2 and S3. From Fig. 2 trace (b) it is evident that these hierarchical ZnO superstructures were constructed by joining a number of small nano-cones, which is in agreement with FESEM analysis. Fig. 2 trace (c) and (d) represented the TEM and HRTEM images of ZnO coated with GQDs. From the fig., it is clear that the quantum dots were affixed over ZnO surface. From HRTEM images the interplanar distance of 0.216 nm for GQDs is correspond to (1 1 0) crystal plane while interplanar distance of 0.265 nm for ZnO is correspond to (0 0 2) crystal plane. Fig. S3 trace (c) and (d) presented the TEM and HRTEM images of as-synthesized GQDs. The average diameter of these quantum dots was found to be 4–8 nm with an inter-planar distance of 0.217 nm correspond to (1 1 0) plane of graphite (Fan et al., 2016; Lai et al., 2015; Thuan et al., 2017). To confirm the presence of all constituent elements, TEM elemental mapping of GQDs modified hierarchical ZnO superstructures were performed and Fig. S8 represents the same. As obtained from the figure that all the elements i.e. Zn, O, C and N are uniformly distributed in the scan area. 3.6. Growth mechanism of hierarchical ZnO superstructures On the basis of the above discussion and as per the morphological analysis, the plausible growth mechanism for the synthesis of hierarchical ZnO superstructures under controlled reaction conditions is illustrated in Fig. 3. In this synthetic procedure, we have used a soft biotemplate “polygalacturonic acid” as an assembling agent of basic building blocks to fabricate hierarchical ZnO superstructures. Polygalacturonic acid is a naturally occurring anionic polysaccharide, extracted from plant cell walls and the chemical structure of this homopolymer comprises a backbone chain of 1,4-linked D-galacturonic acid units. Hierarchical ZnO superstructures were synthesized via the
Fig. 3. Step by step possible growth mechanism for bio-template assisted synthesis of hierarchical ZnO superstructures. 42
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GQDs attributed to the presence of negatively charged oxygen functional groups (Carboxyl and Hydroxyl groups). In case of ZnO/GQDs-3, photocurrent density decreases and found to be 0.51 mA/cm2. This decrease in current density can be attributed to the fact that when ZnO surface is covered with excess quantum dots, chances of electron-hole recombination increased because of the decrease in effective surface area participating in oxidation process which in turn in lower photocurrent value. Fig. 4(B) exhibits the transient current responses of all the photoanodes under chopped light illumination in a regular interval as a function of time with a biased potential of equivalent to 1.23 V vs RHE. In high accordance with LSV curves ZnO/GQDs-2 showed highest photocurrent density without attenuation, followed by ZnO/GQDs-3, ZnO/GQDs-1, and ZnO. Also, under irradiation cycles, prompt and reproducible current responses were obtained for all photoanodes. Photocurrent improvement was further confirmed by IPCE measurements. To evaluate the External Quantum Efficiency (EQE), which is a measure of how efficiently the device converts incident light into electrical energy at a given wavelength, IPCE spectra were recorded for devices with the wavelength range from 330 nm to 600 nm, displayed in Fig. 5(A). In general, EQE of a solar device is indicative of its lightharvesting efficiency, photo-excited electrons injection efficiency, and charge collection efficiency. In Fig. 5(A), ZnO/GQDs-2 exhibited a higher EQE value viz. ~13% as compared to pristine ZnO based photoanode i.e., ~8%. Observed large EQE value of best-performed photoanode validates the higher Jsc value obtained from the linear sweep voltammetry curve. Improvement in photon-to-electron conversion is mainly related to better electron injection, reduced recombination of the photo generated excitons. The amount of oxygen evolved was measured to calculate the Faradaic Efficiency on the photoanode, which is the confirmation that the generated photocurrent is only because of the water oxidation, illustrated in Fig. 5(B). Faradaic efficiency measurements were carried out using online GC at a fixed potential of 1.23 V vs RHE under 1 Sunlight illumination in 1 M NaOH electrolyte solution for 60 min. Basically, Faradaic efficiency is the ratio of experimental oxygen production rate to that of the theoretically calculated oxygen production rate. Experimental amount of oxygen evolved and theoretical evolved oxygen were comparable and an impressive average Faradaic efficiency of ~83% was found after 1 h, indicative of that the photocurrent is only because of water oxidation. Electron transport and recombination kinetics, which occurred at the various interfaces of as-fabricated photo electrochemical photoanodes were analyzed using electrochemical impedance spectroscopy (EIS) under light irradiation in the frequency range of 0.1–105 Hz. Fig. 6(A) represents the Nyquist plot for the fabricated photoanodes, plotted between the imaginary part versus the real part of the complex impedance. The figure comprises the semicircle for all photoanodes at
like nanostructures were formed and appear to be attached to a single point. We assumed the highly directional assembling of nano-cones, was determined by the surface carboxylate and hydroxyl groups of the bio-template. Additionally, formation of integrated assemblies of ZnO was mainly due to anisotropic growth behavior of ZnO crystal along {0 0 0 1} direction and the dipole-dipole induced oriented attachment of building blocks, as discussed in previous literatures (Barick et al., 2008; Gasparotto et al., 2012, Tian et al., 2018). 3.7. BET surface area analysis A key requirement to assess the PEC water oxidation is to achieve a high surface area. N2 adsorption and desorption isotherms for as-synthesized hierarchical ZnO superstructures and coated with GQDs, represented by Fig. S9. As obtained surface area for ZnO superstructures is ~32.0 m2/g, while for GQDs coated ZnO its value is ~45 m2/g. 3.8. Photo electrochemical analysis One of the manifestation of such hierarchical superstructures having better Ohmic response and surface area is the usability for its photo electrochemical water splitting properties. Photo electrochemical activity of as-synthesized pristine hierarchical ZnO superstructures and GQDs modified photoanodes were characterized using a three-electrode system with Ag/AgCl and Pt as a reference and counter electrodes respectively in 1 M NaOH electrolyte solution by sweeping the potential between −0.8 V to 0.6 V vs Ag/AgCl. This potential was then converted into RHE using Eq. (1). PEC performance of ZnO/GQDs superstructures with different reaction times and different concentrations of GQDs were systematically explored to decide the optimal condition in preparing ZnO/GQDs photoanodes, as represented in Fig. S10(A) and (B) respectively. Fig. 4(A) represents the linear sweep voltammetry curves of as optimized photoanodes as a function of applied bias voltage. As applied potential increases, photocurrent density of all photoanodes increases, indication of a typical n-type semiconductor (Zeng et al., 2016). Based on result, it is clear that there is a gradual increase in the photocurrent density and a maximum of 0.61 mA/cm2 was found in case of ZnO/GQDs-2. The photocurrent density for bare ZnO superstructures was found to be 0.35 mA/cm2. Photoanodes fabricated with ZnO/GQDs showed higher photocurrent density as compared to pristine ZnO. After the modification with GQDs, the device showed an overall improvement of ~77% in the photocurrent density. This increase in the current density is mainly due to the reduced recombination of the photogenerated excitons, faster electron transport kinetics, and better charge extraction efficiency. The higher surface area provided by the modified ZnO structures ensures exposure to abundant sites and efficient charge transfer. In addition to the surface area this increase in the photocurrent density is due to the remarkable hole extraction ability of
Fig. 4. (A) Linear Sweep Voltammetry Curves, (B) Chronoamperomettry curves for both ZnO and GQDs modified hierarchical ZnO superstructures. 43
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Fig. 5. (A) IPCE and (B) Faradaic Efficiency Curves for both ZnO and GQDs modified hierarchical ZnO superstructures.
lower frequency regions due to charge transfer resistance (RCT) at the photoanode/electrolyte interface, crucial in revealing the interfacial properties. The equivalent circuit shown in the inset of Fig. 6(A) was used to fit all the impedance parameters. As obtained fitted parameters such as RS, RCT, are summarized in Table 1. RS represents the Ohmic resistance between the transparent conductive oxide (TCO) and photoanode, whereas, charge transport resistance RCT specifies the charge transport processes across photoanode/electrolyte interface. It is clear that the device with pristine ZnO shows the highest charge transfer resistance of 31.26 KΩ, which indicates the poor charge transport properties reflected in low photocurrent value (LSV curve). The value of charge transfer resistance is found to be decreased significantly with GQDs modification. In case of ZnO/GQDs-2, the charge transfer resistance is lowest having the value of 15.55 KΩ, suggesting a better interfacial charge transport with low contact resistance. Therefore, GQDs modification can effectively enhance the charge transport from photoanode to electrolyte by reducing the electron transfer resistance. Consequently, the recombination rate is reduced, making the PEC water oxidation more efficient. Further verification for improved charge transfer kinetics after GQDs modification was confirmed by Mott-Schottky analysis. Fig. 6(B) represents Mott-Schottky plots for pristine and GQDs modified ZnO under dark conditions in 1 M NaOH with an applied frequency of 1 kHz. Positive slop for all the photoanodes confirms n-type behavior. Charge carrier density (ND) and flat band potential for as-fabricated photoanodes were calculated using the following equation.
1 1 kT = 2 [E − EFB − ] C2 A NDeεε0 e
Table 1 Obtained parameters such as RS, RCT from EIS fitting.
Colored parameters are related to optimized nanocomposite
photoanode, ND denotes charge carrier density of photoanode, e is electron charge, ɛ0 represents vacuum permittivity, ɛ denotes dielectric constant of semiconductor, E is applied bias, EFB is flat band potential, K represents Boltzmann constant, and T is the temperature. Charge carrier density values calculated from the slopes of Mott-Schottky plots for ZnO, ZnO/GQDs-1, ZnO/GQDs-2 and ZnO/GQDs-3 were found to be 1.83 × 1020 cm−3, 2.65 × 1020 cm−3, 3.19 × 1020 cm−3 and 3.13 × 1020 cm−3 respectively. From the observations, it is clear that there is a significant improvement in the charge carrier density of GQDs modified ZnO which is in close agreement with the photocurrent values shown in LSV curves. This increase in the carrier density is mainly attributed to the fact that with GQDs modification, the surface trap sites got decreased. To confirm the reduced recombination of photogenerated excitons, we have carried out the hole scavenger test in the presence of 0.20 M H2O2. Fig. 7(A) represent the current-voltage curves for bare and GQDs modified hierarchical ZnO superstructures with and without H2O2, whereas Fig. 7(B) represents the charge injection efficiency. From figure it is clear that hierarchical ZnO superstructures gives lower
(3)
where C represents semiconductor capacitance, A is the surface area of
Fig. 6. (A) EIS and (B) Mott-Schottky plots for bare and GQDs modified hierarchical ZnO superstructures. 44
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Fig. 7. (A) Current-Voltage curves with and without hole scavenger (B) Charge injection efficiency.
growth mechanism in detail by varying the reaction conditions such as varying the reaction temperature, time and Hexamine concentration. Further modification of these hierarchical ZnO superstructures was done by Graphene Quantum Dots, which acts as hole extracting agent. It was revealed that well defined hierarchical ZnO superstructures coated with GQDs showed significant enhancement in PEC water splitting performance. After modification there is ~ 77% increment in photocurrent response, which is due to the efficient charge separation as well as reduced electron hole recombination. Presented approach can be extrapolated as a model system in development of novel photoanodes for efficient extraction of holes. This work will also be useful to design and development of different morphological structures of metal oxides directly over conductive glass substrate by using bio-mass derived templating agents. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 8. Operational stability test for both pristine and GQDs modified hierarchical ZnO superstructures under 1 Sun illumination at 1.23 V vs RHE.
current value in absence of H2O2, mainly due to recombination of photogenerated excitons. On the other hand, in the presence of H2O2, current obtained was comparable to that of current obtained in GQDs modified hierarchical ZnO superstructures. Because of the faster kinetics of H2O2 reduction reaction, reducing H2O2 is more favorable process than injection of photogenerated electrons to the conduction band of ZnO. This reduction of H2O2 is responsible for suppressed recombination of excitons and improved photocurrent value throughout the sweeping voltage to get higher PEC activity (Sreedhar et al., 2019; Zhu et al., 2017). From Fig. 7(B) charge injection efficiency for GQDs modified hierarchical ZnO superstructures was found to be ~84% which is higher than that of bare hierarchical ZnO superstructures (~60%). This analysis further proves that GQDs are superior hole extractor which can extracts more number of holes from the ZnO surface as compared to the hole scavenging action of H2O2. Also, we have carried out the operational stability test at fixed potential of 1.23 V vs RHE, depicted in Fig. 8. From the figure it is clear that after the continuous light irradiation for 1 hr, there is negligible change in current value. So, from this stability test we can say that both the photoanodes i.e. pristine ZnO and graphene quantum dots modified hierarchical ZnO superstructures are quite stable.
Acknowledgements The authors sincerely acknowledge the Department of Science and Technology, India through project no. SERB/EMR/2016/005123. The Infrastructure and instrumentation help from IIT Guwahati and CIF, IIT Guwahati, are hereby acknowledged. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.02.015. References Ahirwar, S., Mallick, S., Bahadur, D., 2017. Electrochemical method to prepare graphene quantum dots and graphene oxide quantum dots. ACS Omega 2, 8343–8353. https:// doi.org/10.1021/acsomega.7b01539. Barick, K.C., Aslam, M., Dravid, V.P., Bahadur, D., 2008. Self-aggregation and assembly of size-tunable transition metal doped ZnO nanocrystals. J. Phys. Chem. C 112, 15163–15170. https://doi.org/10.1021/jp802361r. Chen, H., Wei, Z., Yan, K., Bai, Y., Zhu, Z., Zhang, T., Yang, S., 2014. Epitaxial growth of ZnO nanodisks with large exposed polar facets on nanowire arrays for promoting photo electrochemical water splitting. Small 10 (22), 4760–4769. https://doi.org/10. 1002/smll.201401298. Chen, W., Li, F., Ooi, P.C., Ye, Y., Kim, T.W., Guo, T., 2016. Room temperature pHdependent ammonia gas sensors using graphene quantum dots. Sens. Actuat., B 222, 763–768. https://doi.org/10.1016/j.snb.2015.09.002. Chetia, T.R., Ansari, M.S., Qureshi, M., 2015. Ethyl cellulose and cetrimonium bromide assisted synthesis of mesoporous, hexagon shaped ZnO nanodisks with exposed ± {0001} polar facets for enhanced photovoltaic performance in quantum dot sensitized solar cells. ACS Appl. Mater. Interfaces 7, 13266–13279. https://doi.org/10.
4. Conclusions In this work, In-situ growth of hierarchical ZnO superstructures directly over FTO have been carried out under controlled hydrothermal conditions using a naturally abundant bio-template. We also studied the 45
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Bio-template assisted hierarchical ZnO superstructures coupled with graphene quantum dots for enhanced water oxidation kinetics
T
Suhaib Alama, Tushar Kanta Sahua, Devipriya Gogoib, Nageswara Rao Peelab, ⁎ Mohammad Qureshia, a b
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam 781039, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Hierarchical ZnO superstructures Photoelectrochemical water oxidation Polygalacturonic acid Hole extracting agent Graphene quantum dots
Due to anisotropic growth behavior and tunable electrical properties, ZnO nanostructures having dimensions such as 0-D, 1-D, 2-D and 3-D are actively studied for their optoelectronic properties. However, ZnO based photoanodes suffer from unfavorable recombination of electron hole pair, which hinders its use in photoelectrochemical (PEC) water oxidation. Herein, we demonstrate a strategy to enhance the PEC performance using bio-template assisted in-situ grown hierarchical ZnO superstructures directly over fluorine-doped tin oxide (FTO) modified by graphene quantum dots (GQDs). GQDs decorated hierarchical ZnO superstructures displayed a significant increment of ~77% in photocurrent density value compared to pristine ZnO with an impressive carrier density of 3.19 × 1020 cm−3, which is ~1.8 orders of magnitude higher than that of pristine ZnO. It is observed that GQDs acts as an efficient hole extractor, which improves the carrier separation on ZnO surface and reduces the hole trapping probability.
1. Introduction Splitting of water into hydrogen and oxygen is an attractive alternative to harvest solar energy for future energy crisis. In this regard, photo electrochemical (PEC) water splitting has received much attention to effectively couple solar irradiation with the electrochemical processes (Ma et al., 2018; Li et al., 2013). During the past few decades, several metal oxides such TiO2, ZnO, WO3, Fe2O3, BiVO4, etc have been used as photo-electrodes for PEC water splitting (Zhang et al., 2014; Zhang et al., 2018; Tang et al., 2019; Marelli et al., 2014; Hegner et al., 2017). Among these n-type metal oxides, Zinc oxide (ZnO), a wide bandgap (~3.2 eV) semiconductor photoanode has attracted much attention due to its attractive optical properties, high electron mobility, low toxicity and its anisotropic growth behavior (Zhan et al., 2018). Although ZnO has many advantages over similar n-type materials, still the PEC activity is inferior to many photoanodes due to its sluggish water oxidation kinetics. Recently, ZnO has been modified with different strategies to overcome the above drawbacks (Chen et al., 2014). Recent developments in the synthetic strategies of inorganic semiconductors having different morphologies with controlled shape and size have gained significant interest due to their catalytic and charge transport properties (Li and Yu, 2019; Dong et al., 2012). The
⁎
significant scientific and technological importance of nanomaterials and their synthetic procedures are still challenging and require additional efforts for the complete utilization of their potential for application (Shi et al., 2013; Guo et al., 2008; Shevchenko et al., 2006; Liu et al., 2013). Different structures of ZnO such as 3D branched nanowires (Kargar et al., 2013), star like structures (Marlinda et al., 2019), ZnO Nano tree and nano cluster structures (Ren et al. 2016), nanowires (Li et al., 2015a, 2015b), nanorods (Wang et al., 2015), nanoplates assemble sphere like structures (Emil et al., 2018), have been utilized for PEC water oxidation. Semiconductors having hierarchical morphologies can be an effective way to enhance the PEC performance because of their improved charge transfer, surface area and sensitizer loading capabilities apart from their strong light scattering effects and efficient electron transport (Memarian et al., 2011; Ko et al., 2011). Compared to the 1D nanostructures, hierarchical superstructures are more advantageous as they provide long optical pathways for efficient light absorption and multiple reflections, short channels for faster charge transport and a large interfacial area for water redox reactions (Li et al., 2015a, 2015b). Several growth patterns based on their crystal growth behavior have been used to synthesize a regular ordered structure such as template derived homo or heteroepitaxial growth behavior (Xu et al., 2009). Here, we have utilized a naturally occurring
Corresponding author. E-mail address: [email protected] (M. Qureshi).
https://doi.org/10.1016/j.solener.2020.02.015 Received 22 November 2019; Received in revised form 17 January 2020; Accepted 4 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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down at room temperature and obtained solution was filtered through 0.22 μm syringe filter, in order to separate the larger particles. This filtered solution was then dialyzed through a dialysis bag (Da = 1000) and water for 24 h. The water used in this process was changed after every 6 h. This dialyzed solution was dried at 100 °C for 48 h to obtain the desired graphene quantum dots.
biomass-derived templating agent as an alternative to conventional templating agents. Use of such templating agent is not only economical, but also environmentally benign under moderate conditions. Templating agent with several functionalities is accountable for the alteration of nanostructure morphologies by adsorbing at the surface of the crystal and confine the crystal growth in a specific crystal plane (Chetia et al., 2015). An essential requirement to enhance the PEC water oxidation performance is the efficient separation of photogenerated electron-hole pair. For this, 2D materials such as reduced graphene oxide (RGO) has been widely used to assist the photogenerated electron-hole separation (Sun et al., 2017). Notably, quantum dots due to their size-dependent electronic and optical properties have been attracting substantial attention. The recombination of photogenerated electron-hole pairs can be greatly reduced with quantum dots sensitization. Moreover, due to the large specific surface area provided by quantum dots, a number of surface-active sites will be available for efficient PEC water splitting. Quantum dots are also known to be passivating surface states by massassembly over the photoanode surface (Xie et al., 2017). Despite having several physical-co-chemical properties of GQDs, reports on the rational design of semiconductor/GQDs nanocomposites are still rare (Zeng et al., 2016; Ghosh et al., 2016).
2.3. Preparation of hierarchical ZnO superstructures/graphene quantum dots composites For the fabrication of hierarchical ZnO superstructures and Graphene Quantum Dots hybrid device, a measured amount of GQDs (1.5 mg/mL) was dissolved in deionized water under stirring for overnight. This GQDs solution was then spin-coated over as-synthesized hierarchical ZnO superstructures at 3000 rpm for 30 sec and heated at 120 °C for 10 min to complete one cycle and named as ZnO/GQDs-1. This process of GQDs spin coating was repeated and named hereafter ZnO/GQDs-2, and ZnO/GQDs-3 respectively. Finally, all the samples were heated at 150 °C for 60 min. 2.4. Photo-electrochemical measurements An electrochemical analyzer (model-CHI1120B) with three-electrode configuration, was used to carry out the photo-electrochemical measurements of as-synthesized photoanodes. For the measurements, 1 M NaOH solution was used as an electrolyte. Before the measurements, this electrolyte solution was deaerated by nitrogen gas purging for 30 min. The linear sweep voltammetry was performed between scan ranges of −0.8 to 0.6 V vs Ag/AgCl with a scan rate of 10 mV/sec. As synthesized photoanodes, a Pt wire and Ag/AgCl were used as working electrode, counter electrode, and reference electrode respectively. The applied potential was then converted into reversible hydrogen electrode (RHE) potential by using the following equation.
2. Experimental section 2.1. In-situ growth of hierarchical ZnO superstructures on-to fluorine doped tin oxide substrate Bio-template (polygalacturonic acid) assisted in-situ growth of hierarchical ZnO superstructures, directly over the FTO was carried out by the hydrothermal method. In a typical synthetic procedure, a calculated amount of zinc nitrate hexahydrate (1 mmol, 297 mg) was dissolved into 25 ml of deionized water with continuous stirring for 10 min. After this 0.1 wt% of polygalacturonic acid (1 mg/ml) was added to the above solution and stirred for another 15 min. This solution became more viscous after the addition of polygalacturonic acid, due to the interaction of inorganic salt and bio template (Fig. 3). After this 0.25 M of hexamethylenetetramine was added to the above solution and leave the solution for 30 min. After continuous stirring, this solution became light yellow. This yellow-colored solution was then transferred into 50 ml Teflon lined stainless steel autoclave and precleaned FTO substrates were immersed into the solution by facing down the conductive side. After this, the autoclave was sealed tightly and maintain the temperature at 140 °C for 4 h for the growth of hierarchical ZnO superstructures. After the completion of the reaction, autoclave was allowed to cool down at room temperature and the substrates were cleaned with plenty of water and ethanol for the complete removal of residual from the surface and then these substrates were dried at 100 °C for 12 h. In last these substrates were annealed at 500 °C for 1 h in electric furnace with heating rate of 10°/min. In order to understand the effect of hexamethylenetetramine additive, we have also studied the reaction in detail by varying the amount of hexamethylenetetramine keeping the other reaction conditions same. In addition, we also studied the reaction by changing the reaction temperature as well as the reaction time to understand the growth process of hierarchical ZnO superstructures.
°
ERHE = EAg/AgCl + 0.0591 ∗ pH + EAg/AgCl
(1)
where ERHE = RHE converted potential EAg/AgCl = experimentally measured potential vs Ag/AgCl E°Ag/AgCl = standard potential of Ag/AgCl reference electrode against RHE (0.1976) pH = pH of electrolyte used Tungsten halogen lamp of 300 W with light intensity calibrated at 100 mW/cm2 was used as a light source. The electrochemical impedance spectroscopy (EIS) measurements were performed using the CH Instruments model CHI680E, Inc., Austin, TX, in the frequency range of 0.1–106 Hz under light illumination. To measure the amount of gas evolved and to calculate the Faradaic efficiency, online Gas chromatography (GC) (model-7820A, Agilent Technologies) was used. 3. Results and discussions 3.1. X-ray diffraction analysis To elucidate the formation and crystal phase purity of as-synthesized hierarchical ZnO superstructures, X-ray diffraction (XRD) analysis was performed. Fig. 1 represents the X-ray diffraction pattern of assynthesized pristine and graphene quantum dots coated ZnO thin films, recorded for 2θ value of 20°–80° with a scan speed of 3°/min. From the Fig. 1, the diffraction peaks for all samples at 2θ values of 31.82°, 34.39°, 36.23°, 47.63°, 56.63°, 62.88°, 66.38°, 68.03° and 69.13° with their respective crystal planes (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1) are indexed to hexagonal wurtzite phase with P63mc space group symmetry according to JCPDS card no. 36-1451. Based on this observation, it is clear that all films are polycrystalline in nature. From the Fig. 1, it is also evident that there is no
2.2. Synthesis of graphene quantum dots (GQDs) Water-soluble graphene quantum dots were synthesized by a facile hydrothermal route. A calculated amount of citric acid (2.1 g) was dissolved in 20 ml of deionized water with continuous stirring. After the complete suspension of citric acid, 1.8 g of urea was added to the above solution. This solution was then transferred into 50 ml Teflon lined stainless steel autoclave and heated at 200 °C for 5 h. After the completion of the hydrothermal reaction, this autoclave was allowed to cool 40
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Fig. 1. X-ray diffraction pattern of as-synthesized (a) Hierarchical ZnO superstructures with polygalacturonic acid, (b) ZnO without polygalacturonic acid and (c) Hierarchical ZnO superstructures coated with Graphene Quantum Dots (GQDs), and corresponding FESEM images.
Fig. 2. (a) Depicts FESEM images of as-synthesized hierarchical ZnO superstructures at 140 °C for 4 h, (b) TEM image of as synthesized hierarchical ZnO superstructures, (c) TEM image of GQDs coated hierarchical ZnO superstructures and (d) HRTEM image of GQDs coated hierarchical ZnO superstructures.
diffraction peak related impurity has been observed. From Fig. 1 trace c, there is no peak of graphene quantum dots, which is due to the low content and scattering cross-section of graphene quantum dots as well as the high crystallinity of ZnO as compared to graphene quantum dots. It is important to mention that the intensity ratio of (1 0 0) to (0 0 2) crystal planes changed significantly, indicative of different tropism of products under different reaction conditions. Because of the anisotropic property of ZnO, nano-cones systematize themselves and attached to each other along the c-axis to minimize the surface energy. Therefore, varying the surface energy of crystal planes could be an effective way to modulate the crystal growth behavior in order to acquire different morphologies in ZnO. Polygalacturonic acid was used to reduce the surface energy of the crystal planes because of the electrostatic interaction between the polar surface of ZnO and the ions of polygalacturonic acid (Sun et al., 2006). A smaller value of intensity ratio suggested the crystal growth along the c-axis while a higher value specified retarded growth along c-axis (Huang et al., 2014). From the diffraction pattern the intensity ratios of (1 0 0) to (0 0 2) crystal planes were found to be 0.48 and 0.84 for ZnO superstructures (with PGA) and ZnO nano-cones (w/o PGA, only hexamine) respectively. A smaller value of intensity ratio was achieved when hierarchical ZnO superstructures were synthesized in presence of PGA and hexamine specified anisotropic growth along c-axis. In contrast to this higher value of intensity ratio was obtained when ZnO (nano-cones) were synthesized with only hexamine, suggested retarded growth along c-axis.
to optical E2g phonons at Brillouin zone center. The degree of disorder can be determined by the intensity ratio of ID to IG (ID/IG ratio), which is inversely proportional to the average size of the crystallite size domain. Average crystallite size can be determined using the following equation (Lin et al., 2016; Gokus et al., 2009).
ID /IG = C(λ )/La
(2)
where ID is the intensity of D band, IG is the intensity of G band, C(λ) is the empirical constant depends on the wavelength of the excitation laser and La is the crystallite size. Using above Eq. (2), the calculated average crystallite size was found to be ~4.1 nm which is in close agreement to the size of GQDs determined by TEM analysis. Observed ID/IG valued is greater than 1, indicative of several defects present at GQDs surface (Ahirwar et al., 2017). 3.4. Morphological analysis Morphological features of in-situ grown hierarchical ZnO superstructures were evaluated by field emission scanning electron microscope (FESEM) analysis as depicted in Figs. 2 and S3. From the figure, it is clear that as grown hierarchical ZnO superstructures were uniformly distributed throughout the FTO substrate. The superiority of these structures is that, these were grown directly over FTO without any seed layer and displayed robust adhesion and good Ohmic contact to the FTO substrates. The average dimension of as-grown ZnO superstructures was observed in the micron range. From the figure (2 a), it is clear that these structures were formed by connecting different nanocones. The surface functionalities of polygalacturonic acid such as hydroxyl and carboxylic groups were accountable for the oriented assembly of the ZnO nano-cones to form hierarchical ZnO superstructures due to the interaction of the ZnO polar surfaces. Polygalacturonic acid was used as a bio-template to govern the growth as well as an accumulating agent to construct the hierarchical ZnO superstructures. It is evident from the Fig. 2(a) that these hierarchical ZnO superstructures were closely connected, due to which the inter-particle connectivity was increased which helps in effective charge transfer by minimizing the surface traps for electron-hole recombination. In addition to the top view, cross-sectional analysis was also carried out for better understanding, depicted in Fig. S4(a, b). The surface morphology of as-synthesized ZnO in the absence of polygalacturonic acid was also performed presented in Fig. S4(c, d). A systematic approach has been used
3.2. FT-IR analysis Formation and presence of different functional groups in as-synthesized graphene quantum dots were confirmed by FT-IR analysis, presented in Fig. S1. Broad peak present at 3230–3600 cm−1 could be attributed to the stretching vibration of eOeH. The peaks at 1660 cm−1 and 1558 cm−1 exhibited the vibrational stretching of eC] O and eNeH bending. Peaks appeared at 1390 cm−1 was due to the symmetric stretching of eCOOH group while peak at 1175 cm−1 was due to the eCeOeC group. It is noteworthy to mention that the presence of hydroxyl and carboxyl groups, ensured high dispersion of GQDs in aqueous medium (Yang et al., 2014; Chen et al., 2016). 3.3. Raman analysis Graphene quantum dots were further characterized by Raman analysis, depicted in Fig. S2. Raman spectra of as-synthesized GQDs showed two bands: D band at 1356 cm−1 and G band at 1548 cm−1. D band can be attributed to the bonding and anti-bonding orbitals i.e. defect-induced breathing mode of A1g phonons while G band arises due 41
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integration of ZnO nanocrystals by considering the assembling function of the polysaccharide. It is traditional that the growth process of the hierarchical ZnO superstructures can be controlled by two steps viz. nucleation and the crystal growth at the assembling stage. The growth mechanism for assembling of small nano-cones into microstructures can be described based on crystal aggregation (Wang et al., 2006; Mo et al., 2005). Here polygalacturonic acid was first dissolved in the aqueous medium and generated negatively charged ions in the solution due to the presence of hydroxyls and carboxylate groups. Therefore, there is an electrostatic interaction between the negatively charged bio-template and the inorganic ions (positively charged Zn2+) to form adduct (Zn2+–Polygalacturonic acid) in the reaction mixture (Sun et al., 2006). The addition of hexamine is responsible to maintain the pH of the reaction mixture and instantaneously generated a light yellow color, probably related to Zn(OH)2. Zn(OH)42− and Zn(NH3)42+ ions acted as the crystal growth seed units for the formation of ZnO building blocks (Wang et al., 2004). Under the hydrothermal reaction, as obtained growth units were transformed into ZnO nuclei, considered as the growth units. Generally, the crystal growth units of ZnO have the tendency to grow along the c-axis by minimizing the surface energy. It is established that ZnO crystal possesses the {+0 0 0 1} positive crystal surface and {−0 0 0 1} negative crystal surface, attributed to the Zn2+ populated and O2− populated high energy polar facets respectively. So, the crystal growth units i.e., zincate ions (Zn(OH)42−) are favorably attached to the {+0 0 0 1} positive surface of ZnO crystal, which encourages the formation of ZnO nano-cones along the c-axis direction (Sun et al., 2002). Therefore, the growth rate of ZnO crystal is considered to be faster in {+0 0 0 1} positive crystal surface as compared to {−0 0 0 1} negative crystal surface, indicative of Zn(OH)42− ions are responsible major growth units for the ZnO crystal (Govender et al., 2004). In this growth mechanism, negatively charged bio-template i.e., polygalacturonic acid is accountable for the suppressed growth of ZnO nano-cones along the c-axis, resulted in the formation of non-uniform dimensions of nanocrystals. During the synthetic procedure, polygalacturonic acid not only acts as a soft template for the formation of ZnO nano-cones but also assists as an assembling agent to fabricate hierarchical ZnO superstructures. In the absence of soft template, only nano-cones were formed, depicted as the FESEM images in Fig. S4(c and d). After the complete nucleation of crystal from the precursor, the negatively charged bio-template initiates the assembling of building blocks and arrange them in an array to form hierarchical ZnO superstructures. As the reaction proceeds, nano-cones and connected cones
to understand the growth mechanism and evolution of hierarchical ZnO superstructures under controlled experimental conditions by changing the concentration of hexamine as well as by varying the reaction time and temperature. Morphological changes under different reaction conditions were characterized by performing FESEM analysis (Figs. S5–S7 supporting information). 3.5. Structural analysis To further corroborate the structural features of as-synthesized hierarchical ZnO superstructures and modified with GQDs, transmission electron microscopy (TEM) analysis was carried out as depicted in Figs. 2 and S3. From Fig. 2 trace (b) it is evident that these hierarchical ZnO superstructures were constructed by joining a number of small nano-cones, which is in agreement with FESEM analysis. Fig. 2 trace (c) and (d) represented the TEM and HRTEM images of ZnO coated with GQDs. From the fig., it is clear that the quantum dots were affixed over ZnO surface. From HRTEM images the interplanar distance of 0.216 nm for GQDs is correspond to (1 1 0) crystal plane while interplanar distance of 0.265 nm for ZnO is correspond to (0 0 2) crystal plane. Fig. S3 trace (c) and (d) presented the TEM and HRTEM images of as-synthesized GQDs. The average diameter of these quantum dots was found to be 4–8 nm with an inter-planar distance of 0.217 nm correspond to (1 1 0) plane of graphite (Fan et al., 2016; Lai et al., 2015; Thuan et al., 2017). To confirm the presence of all constituent elements, TEM elemental mapping of GQDs modified hierarchical ZnO superstructures were performed and Fig. S8 represents the same. As obtained from the figure that all the elements i.e. Zn, O, C and N are uniformly distributed in the scan area. 3.6. Growth mechanism of hierarchical ZnO superstructures On the basis of the above discussion and as per the morphological analysis, the plausible growth mechanism for the synthesis of hierarchical ZnO superstructures under controlled reaction conditions is illustrated in Fig. 3. In this synthetic procedure, we have used a soft biotemplate “polygalacturonic acid” as an assembling agent of basic building blocks to fabricate hierarchical ZnO superstructures. Polygalacturonic acid is a naturally occurring anionic polysaccharide, extracted from plant cell walls and the chemical structure of this homopolymer comprises a backbone chain of 1,4-linked D-galacturonic acid units. Hierarchical ZnO superstructures were synthesized via the
Fig. 3. Step by step possible growth mechanism for bio-template assisted synthesis of hierarchical ZnO superstructures. 42
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GQDs attributed to the presence of negatively charged oxygen functional groups (Carboxyl and Hydroxyl groups). In case of ZnO/GQDs-3, photocurrent density decreases and found to be 0.51 mA/cm2. This decrease in current density can be attributed to the fact that when ZnO surface is covered with excess quantum dots, chances of electron-hole recombination increased because of the decrease in effective surface area participating in oxidation process which in turn in lower photocurrent value. Fig. 4(B) exhibits the transient current responses of all the photoanodes under chopped light illumination in a regular interval as a function of time with a biased potential of equivalent to 1.23 V vs RHE. In high accordance with LSV curves ZnO/GQDs-2 showed highest photocurrent density without attenuation, followed by ZnO/GQDs-3, ZnO/GQDs-1, and ZnO. Also, under irradiation cycles, prompt and reproducible current responses were obtained for all photoanodes. Photocurrent improvement was further confirmed by IPCE measurements. To evaluate the External Quantum Efficiency (EQE), which is a measure of how efficiently the device converts incident light into electrical energy at a given wavelength, IPCE spectra were recorded for devices with the wavelength range from 330 nm to 600 nm, displayed in Fig. 5(A). In general, EQE of a solar device is indicative of its lightharvesting efficiency, photo-excited electrons injection efficiency, and charge collection efficiency. In Fig. 5(A), ZnO/GQDs-2 exhibited a higher EQE value viz. ~13% as compared to pristine ZnO based photoanode i.e., ~8%. Observed large EQE value of best-performed photoanode validates the higher Jsc value obtained from the linear sweep voltammetry curve. Improvement in photon-to-electron conversion is mainly related to better electron injection, reduced recombination of the photo generated excitons. The amount of oxygen evolved was measured to calculate the Faradaic Efficiency on the photoanode, which is the confirmation that the generated photocurrent is only because of the water oxidation, illustrated in Fig. 5(B). Faradaic efficiency measurements were carried out using online GC at a fixed potential of 1.23 V vs RHE under 1 Sunlight illumination in 1 M NaOH electrolyte solution for 60 min. Basically, Faradaic efficiency is the ratio of experimental oxygen production rate to that of the theoretically calculated oxygen production rate. Experimental amount of oxygen evolved and theoretical evolved oxygen were comparable and an impressive average Faradaic efficiency of ~83% was found after 1 h, indicative of that the photocurrent is only because of water oxidation. Electron transport and recombination kinetics, which occurred at the various interfaces of as-fabricated photo electrochemical photoanodes were analyzed using electrochemical impedance spectroscopy (EIS) under light irradiation in the frequency range of 0.1–105 Hz. Fig. 6(A) represents the Nyquist plot for the fabricated photoanodes, plotted between the imaginary part versus the real part of the complex impedance. The figure comprises the semicircle for all photoanodes at
like nanostructures were formed and appear to be attached to a single point. We assumed the highly directional assembling of nano-cones, was determined by the surface carboxylate and hydroxyl groups of the bio-template. Additionally, formation of integrated assemblies of ZnO was mainly due to anisotropic growth behavior of ZnO crystal along {0 0 0 1} direction and the dipole-dipole induced oriented attachment of building blocks, as discussed in previous literatures (Barick et al., 2008; Gasparotto et al., 2012, Tian et al., 2018). 3.7. BET surface area analysis A key requirement to assess the PEC water oxidation is to achieve a high surface area. N2 adsorption and desorption isotherms for as-synthesized hierarchical ZnO superstructures and coated with GQDs, represented by Fig. S9. As obtained surface area for ZnO superstructures is ~32.0 m2/g, while for GQDs coated ZnO its value is ~45 m2/g. 3.8. Photo electrochemical analysis One of the manifestation of such hierarchical superstructures having better Ohmic response and surface area is the usability for its photo electrochemical water splitting properties. Photo electrochemical activity of as-synthesized pristine hierarchical ZnO superstructures and GQDs modified photoanodes were characterized using a three-electrode system with Ag/AgCl and Pt as a reference and counter electrodes respectively in 1 M NaOH electrolyte solution by sweeping the potential between −0.8 V to 0.6 V vs Ag/AgCl. This potential was then converted into RHE using Eq. (1). PEC performance of ZnO/GQDs superstructures with different reaction times and different concentrations of GQDs were systematically explored to decide the optimal condition in preparing ZnO/GQDs photoanodes, as represented in Fig. S10(A) and (B) respectively. Fig. 4(A) represents the linear sweep voltammetry curves of as optimized photoanodes as a function of applied bias voltage. As applied potential increases, photocurrent density of all photoanodes increases, indication of a typical n-type semiconductor (Zeng et al., 2016). Based on result, it is clear that there is a gradual increase in the photocurrent density and a maximum of 0.61 mA/cm2 was found in case of ZnO/GQDs-2. The photocurrent density for bare ZnO superstructures was found to be 0.35 mA/cm2. Photoanodes fabricated with ZnO/GQDs showed higher photocurrent density as compared to pristine ZnO. After the modification with GQDs, the device showed an overall improvement of ~77% in the photocurrent density. This increase in the current density is mainly due to the reduced recombination of the photogenerated excitons, faster electron transport kinetics, and better charge extraction efficiency. The higher surface area provided by the modified ZnO structures ensures exposure to abundant sites and efficient charge transfer. In addition to the surface area this increase in the photocurrent density is due to the remarkable hole extraction ability of
Fig. 4. (A) Linear Sweep Voltammetry Curves, (B) Chronoamperomettry curves for both ZnO and GQDs modified hierarchical ZnO superstructures. 43
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Fig. 5. (A) IPCE and (B) Faradaic Efficiency Curves for both ZnO and GQDs modified hierarchical ZnO superstructures.
lower frequency regions due to charge transfer resistance (RCT) at the photoanode/electrolyte interface, crucial in revealing the interfacial properties. The equivalent circuit shown in the inset of Fig. 6(A) was used to fit all the impedance parameters. As obtained fitted parameters such as RS, RCT, are summarized in Table 1. RS represents the Ohmic resistance between the transparent conductive oxide (TCO) and photoanode, whereas, charge transport resistance RCT specifies the charge transport processes across photoanode/electrolyte interface. It is clear that the device with pristine ZnO shows the highest charge transfer resistance of 31.26 KΩ, which indicates the poor charge transport properties reflected in low photocurrent value (LSV curve). The value of charge transfer resistance is found to be decreased significantly with GQDs modification. In case of ZnO/GQDs-2, the charge transfer resistance is lowest having the value of 15.55 KΩ, suggesting a better interfacial charge transport with low contact resistance. Therefore, GQDs modification can effectively enhance the charge transport from photoanode to electrolyte by reducing the electron transfer resistance. Consequently, the recombination rate is reduced, making the PEC water oxidation more efficient. Further verification for improved charge transfer kinetics after GQDs modification was confirmed by Mott-Schottky analysis. Fig. 6(B) represents Mott-Schottky plots for pristine and GQDs modified ZnO under dark conditions in 1 M NaOH with an applied frequency of 1 kHz. Positive slop for all the photoanodes confirms n-type behavior. Charge carrier density (ND) and flat band potential for as-fabricated photoanodes were calculated using the following equation.
1 1 kT = 2 [E − EFB − ] C2 A NDeεε0 e
Table 1 Obtained parameters such as RS, RCT from EIS fitting.
Colored parameters are related to optimized nanocomposite
photoanode, ND denotes charge carrier density of photoanode, e is electron charge, ɛ0 represents vacuum permittivity, ɛ denotes dielectric constant of semiconductor, E is applied bias, EFB is flat band potential, K represents Boltzmann constant, and T is the temperature. Charge carrier density values calculated from the slopes of Mott-Schottky plots for ZnO, ZnO/GQDs-1, ZnO/GQDs-2 and ZnO/GQDs-3 were found to be 1.83 × 1020 cm−3, 2.65 × 1020 cm−3, 3.19 × 1020 cm−3 and 3.13 × 1020 cm−3 respectively. From the observations, it is clear that there is a significant improvement in the charge carrier density of GQDs modified ZnO which is in close agreement with the photocurrent values shown in LSV curves. This increase in the carrier density is mainly attributed to the fact that with GQDs modification, the surface trap sites got decreased. To confirm the reduced recombination of photogenerated excitons, we have carried out the hole scavenger test in the presence of 0.20 M H2O2. Fig. 7(A) represent the current-voltage curves for bare and GQDs modified hierarchical ZnO superstructures with and without H2O2, whereas Fig. 7(B) represents the charge injection efficiency. From figure it is clear that hierarchical ZnO superstructures gives lower
(3)
where C represents semiconductor capacitance, A is the surface area of
Fig. 6. (A) EIS and (B) Mott-Schottky plots for bare and GQDs modified hierarchical ZnO superstructures. 44
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Fig. 7. (A) Current-Voltage curves with and without hole scavenger (B) Charge injection efficiency.
growth mechanism in detail by varying the reaction conditions such as varying the reaction temperature, time and Hexamine concentration. Further modification of these hierarchical ZnO superstructures was done by Graphene Quantum Dots, which acts as hole extracting agent. It was revealed that well defined hierarchical ZnO superstructures coated with GQDs showed significant enhancement in PEC water splitting performance. After modification there is ~ 77% increment in photocurrent response, which is due to the efficient charge separation as well as reduced electron hole recombination. Presented approach can be extrapolated as a model system in development of novel photoanodes for efficient extraction of holes. This work will also be useful to design and development of different morphological structures of metal oxides directly over conductive glass substrate by using bio-mass derived templating agents. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 8. Operational stability test for both pristine and GQDs modified hierarchical ZnO superstructures under 1 Sun illumination at 1.23 V vs RHE.
current value in absence of H2O2, mainly due to recombination of photogenerated excitons. On the other hand, in the presence of H2O2, current obtained was comparable to that of current obtained in GQDs modified hierarchical ZnO superstructures. Because of the faster kinetics of H2O2 reduction reaction, reducing H2O2 is more favorable process than injection of photogenerated electrons to the conduction band of ZnO. This reduction of H2O2 is responsible for suppressed recombination of excitons and improved photocurrent value throughout the sweeping voltage to get higher PEC activity (Sreedhar et al., 2019; Zhu et al., 2017). From Fig. 7(B) charge injection efficiency for GQDs modified hierarchical ZnO superstructures was found to be ~84% which is higher than that of bare hierarchical ZnO superstructures (~60%). This analysis further proves that GQDs are superior hole extractor which can extracts more number of holes from the ZnO surface as compared to the hole scavenging action of H2O2. Also, we have carried out the operational stability test at fixed potential of 1.23 V vs RHE, depicted in Fig. 8. From the figure it is clear that after the continuous light irradiation for 1 hr, there is negligible change in current value. So, from this stability test we can say that both the photoanodes i.e. pristine ZnO and graphene quantum dots modified hierarchical ZnO superstructures are quite stable.
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