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Modulating water oxidation kinetics utilizing h-BN quantum dots as an efficient hole extractor on fluorine doped hematite photoanode
Journal of Power Sources 445 (2020) 227341
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Modulating water oxidation kinetics utilizing h-BN quantum dots as an efficient hole extractor on fluorine doped hematite photoanode Tushar Kanta Sahu, Manoj Kumar Mohanta, Mohammad Qureshi * Department of Chemistry, Indian Institute of Technology, Guwahati, 781039, Assam, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� First study utilizing BNQDs as an effi cient hole extractor in hematite photoanode. � Selectfluor as a source of fluorine has been used for ex-situ doping of fluorine. � Superior enhancement in charge carrier density was achieved with comodification. � A high photocurrent density (2.24 mA/ cm2) was achieved with comodification.
A R T I C L E I N F O
A B S T R A C T
Keywords: Boron nitride quantum dots Hole extractor Selectfluor Non-metal dopant Hematite Water oxidation
The sluggish water oxidation kinetics in hematite photoanode is dominated by unfavorable recombination of photogenerated holes which restrict the efficiency of photoelectrochemical (PEC) water splitting. Herein, we demonstrate a superior photoelectrochemical performance using boron nitride quantum dots (BNQDs), which is incorporated over fluorine doped hematite (F–Fe2O3) photoanode. An impressive carrier density of 2.08 � 1020 cm 3 is achieved, which is two orders of magnitude higher than that of the bare α-Fe2O3 photo anode, resulting in a photocurrent density of 2.24 mA cm 2 at 1.23 V vs reversible hydrogen electrode (RHE) for F–Fe2O3-BNQDs, which is six-fold higher than bare hematite. It is found that BNQD acts as an efficient hole extractor, which enhances the carrier separation on hematite surface and decreases the hole trapping probability. Present work emphasizes on two important parameters, i.e. (i) Enhancement in carrier density via fluorine doping directly onto the substrate using soluble precursor, organic Selectflour (ii) Modification with BNQDs to provide a simple, novel and effective strategy for the design and development of more efficient PEC water splitting systems.
1. Introduction Photoelectrochemical water splitting (PEC) provides an attractive and alternate pathway to generate clean hydrogen utilizing solar energy to overcome future energy demand. In PEC process, multi-electron process at the anodic half reaction makes water oxidation more
difficult compared to its counterpart and has received significant attention to expedite various approaches [1–5]. However, developing the photoanodes with outstanding PEC performance is challenging because of its must meet parameters such as ideal bandgap for effective light absorption, effective minority carrier transfer to minimize recom bination, anodic stabilities at various pH conditions and low-cost
* Corresponding author. E-mail address: [email protected] (M. Qureshi). https://doi.org/10.1016/j.jpowsour.2019.227341 Received 20 September 2019; Received in revised form 15 October 2019; Accepted 19 October 2019 Available online 24 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 445 (2020) 227341
fabrication for possible commercialization [6–8]. In this regard, many n-type semiconductor photoanodes have been studied for their water oxidation, including BiVO4, WO3, Fe2O3, TiO2 and ZnFe2O4 [9–16]. Among the photoanodic materials explored, α-Fe2O3 has received increasing attention as a promising photoanode due to its suitable visible absorbing bandgap (1.9–2.2 eV), excellent chemical stability under various pH conditions, natural abundance and low-toxicity [11]. How ever, practical difficulties in reaching the theoretical photocurrent density (solar-to-hydrogen conversion efficiency of 16% at 1.23 V vs RHE) due to technical bottlenecks of hematite photoanode such as sluggish water-oxidation kinetics, short hole diffusion length, poor conductivity and short lifetime of photogenerated charge carriers limits the PEC performance [17–21]. To overcome the abovementioned drawbacks such as carrier con centration and poor conductivity, elemental doping is reported to be an effective strategy to enhance the PEC performance of hematite photo anode with increasing conductivity and charge carrier density. In this regard, metallic dopants such as Sn, Ti, Mn, Co have been used exten sively [22–25]. Recently, non-metallic dopants such as P, S, and Si have been used to enhance the water-oxidation performance of hematite owing to their involvement in increasing carrier density and in addition greater carrier mobility [26–28]. Zhang et al. reported that non-metal P-dopant offers more number of valence electrons as compared to many metallic dopants which competently facilitate charge separation by eluding the surface electron trapping sites in hematite [26]. Recently, van de Krol et al. re-stated that water oxidation capability of a photo anode is strongly limited by its insufficient hole injection property [29]. Zhang et al. demonstrated that incorporation of black phosphorene layer onto BiVO4 can accelerate the hole transport from the semiconductor to co-catalyst [30]. Therefore, promoting hole extraction from photoanode surface holds broader interest for significant improvement in PEC per formance. In the recent past, boron nitride, also known as “white gra phene” has received much attention due to its chemical and thermal stability and conductivity [31]. Intrinsically, boron nitride nano structures were always negatively charged which makes them a good hole carrier acceptor, and could be used to improve the electron-hole carrier separation [32]. In this work, for the first time, we incorporated BNQDs as a simple and effective strategy to overcome the main drawbacks of hematite based photoanodes. The water oxidation of hematite photoanode was intended to be improved by fluorine doping and adsorption of BNQDs. Photoelectrochemical oxidation of water was studied at a voltage which corresponds to the thermodynamic limit of water electrolysis, hence water was just not electrolyzed in the dark but oxidation set in if the hematite anode had been illuminated. For doping non-metal fluorine, a simple organic precursor Selectfluor has been utilized by an ex-situ method which helps to achieve higher charge carrier density with preservation of morphological features. Hexagonal BNQDs have been synthesized in a relatively low-temperature hydrothermal method which effectively acts as an efficient hole extractor for tuning the PEC performance with better charge separation. With optimized doping amount and quantum dots incorporation, the photocurrent density of hematite photoanode is enhanced by six-fold as compared to bare counterpart due to the synergistic effect of both fluorine doping and BNQDs incorporation. The superior PEC activity of modified hematite is supported by electrochemical impedance spectra (EIS) measurement, Mott-Schottky analysis. The present form of modification with BNQDs provides a simple and effective strategy for the design and development of more efficient photoelectrochemical water splitting systems.
Boric acid were purchased from Merck. Iron chloride (97%), Melamine (99%) and Selectfluor (95% Fþ active) were purchased from SigmaAldrich. 2.1.1. Fabrication of α-Fe2O3 and F-doped Fe2O3 The α-Fe2O3 nanorod structures were directly grown over a conductive substrate by following earlier reports [26]. In a typical synthesis, FeCl3 (0.15 M) and NaNO3 (1 M) were added to 60 mL distilled water with stirring. To maintain the pH at 1.5, concentrated HCl was added to the above solution. The solution was then transferred to a 100 mL Teflon lined stainless steel vessel. Four cleaned FTO (2.5 cm � 1.00 cm) with polyimide tape masking were put into the bottom of the solution such that the conductive side faced towards up. The autoclaved solution was put in a pre-heated hot-air oven for 6 h at 100 � C. After the reaction, the autoclave was naturally cooled to room temperature and as synthesized β-FeOOH thin films were cleaned several times with distilled water and ethanol to remove any residue present over the films. The cleaned films were then dried overnight in a hot-air oven at 60 � C. As obtained β-FeOOH thin films were annealed at 800 � C for 15 min (20 � C/min) to obtained α-Fe2O3. F-doped Fe2O3 films were prepared by dipping as prepared β-FeOOH films in an aqueous solution of Selectfluor for 15 min and annealed at 800 � C (20 � C/min) for 15 min. 2.1.2. Synthesis of boron nitride quantum dots (BNQDs) Boron nitride quantum dots (BNQDs) were synthesized by hydro thermal method [33]. For synthesis, boric acid (100 mg) was dissolved in deionized water (10 mL). To the above solution, melamine (34 mg) was added under stirring. After stirring for 30 min, the white suspension was transferred to a 23 mL Teflon lined stainless steel vessel and heated in a hot-air oven at 200 � C for 15 h. A different set of quantum dots were synthesized by keeping reaction time for 20 h to get different sized quantum dots. After cooling, the product was filtered with a 0.22 μm syringe filter membrane to remove large particles. Finally, the solution was diluted to 40 mL with the addition of distilled water and used as a stock solution of BNQDs. 2.1.3. Fabrication of Fe2O3-BNQDs and F–Fe2O3-BNQDs As-prepared Fe2O3 and F–Fe2O3 films were dipped in different con centrations of BNQDs solution for 6, 9, 12 and 15 h at 60 � C to fabricate Fe2O3-BNQDs and F–Fe2O3-BNQDs. After the sensitization process, the films were cleaned with DI water to remove the excess BNQDs at the surface. Then the films were annealed at 400 � C for 1 h. 2.2. Photoelectrochemical measurements A three-electrode configuration was used for the photo electrochemical measurement of all the samples with a potentiostat (model-CHI1120B). For the measurements, 1 M sodium hydroxide (NaOH) electrolyte solution was used. The electrolyte solution was purged with nitrogen gas for 20 min before PEC measurements to remove any dissolved oxygen present in the electrolyte solution. As prepared photoanodes, standard Ag/AgCl (aqueous) electrode with saturated KCl as the electrolyte and Pt wire were used as working electrodes, reference electrode and counter electrode, respectively. All the potentials applied with respect to Ag/AgCl(aqueous) electrode were converted into reversible hydrogen electrode (RHE) potential by the following expression: ERHE ¼ EAg/AgCl þ 0.059 pH þ EoAg/AgCl
2. Experimental
(1)
where EAg/AgCl is the obtained potential vs. Ag/AgCl, EoAg/AgCl ¼ 0.1976 V (saturated KCl) at 25 � C and pH is the pH of the electrolyte. Epoxy adhesive (Araldite®) was used to cover the photoanodes leaving an exposed thin film area of 0.20 cm2. For light source, a 300 W tungsten halogen (OSRAM SYLVANIA, USA) lamp was used and the light intensity
2.1. Materials All the chemicals purchased were used as received without further purification. Sodium nitrate (99%), Hydrochloric acid (35–37%) and 2
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was calibrated at 100 mW/cm2 (1 Sun illumination) with the help of a Si photodiode. An electrochemical work station (Model CHI760D, Inc., Austin, TX) was used to perform the Electrochemical impedance spec troscopy (EIS) measurement in 1 M NaOH aqueous solution at 1 V vs. RHE with an AC amplitude of 10 mV under light illumination (frequency range ¼ 0.1 Hz-10kHz). Mott–Schottky plots were obtained in a po tential window of 1.0 V to þ1.0 V vs. Ag/AgCl under dark conditions (at a constant frequency of 1 kHz). Gas evolution test was analysed by online Gas Chromatograph (model- Agilent 7820A).
charge carriers. Fluorine was doped to Fe2O3 by soaking β-FeOOH films in Selectfluor solution for 15min before calcination at high temperature. Selectfluor used as a source of fluorine which is known to be a good fluorinating agent and offers solubility in common organic solvents as well as water and shows less reactivity towards glass. Ex-situ doping helps to achieve higher charge carrier density with preservation of morphological features. BNQDs was incorporated over Fe2O3 and F–Fe2O3 films by dipping the films in BNQDs solution at 60 � C and keeping it for different times. The BNQDs used for this purpose was synthesized by low-temperature hydrothermal method by using boric acid and melamine as a precursor. Thin film XRD of α-Fe2O3, Fe2O3-BNQDs, F–Fe2O3, and F–Fe2O3BNQDs photoanodes are shown in Fig. 1a. All the peaks correspond to α-Fe2O3 (JCPDS-33-0664) and SnO2 (from FTO substrate). The strong diffraction peak for (110) plane for the samples indicate that the nanostructures are oriented preferentially in the (110) direction over the FTO substrate [22]. No peaks for BNQDs were identified in the com posite samples due to a very low concentration of BNQDs. To confirm the dopant incorporation in hematite, Raman analysis was performed as shown in Fig. 1b. The Raman spectra of all the samples show the char acteristic vibrations of hematite comprising two A1g modes (225 and 500 cm 1) and four Eg modes (245, 293, 412 and 612 cm 1). The Raman spectra of F–Fe2O3 and F–Fe2O3-BNQDs samples shows a new peak at 660 cm 1 which is absent in bare Fe2O3 and Fe2O3-BNQDs, could be attributed to the disorder induced by doping of fluorine [34]. The optical absorption properties of all the photoanodes were analysed through UV–visible absorption spectra as shown in Fig. S1a. BNQDs absorbs in the UV region (inset to Fig. S1a). Similar optical curves and band gaps estimated from Tauc analysis (Fig. S1b) shows that F-doping and BNQDs does not influence the absorption edge of Fe2O3 as well as band gaps [35]. The absorbance and photoluminescence spectra (Fig. S2) of BNQDs are consistent with reported literature of BNQDs [36]. Fig. S3 shows the FT-IR spectra of F–Fe2O3, BNQDS, and F–Fe2O3-BNQDs. The FT-IR spectrum of F–Fe2O3 shows two strong absorption peaks at 483 and 579 cm 1 which can be assigned to Fe–O band. The broad peak at 3436 cm 1 could be attributed to hydroxyl group stretching. The FT-IR spectrum of BNQDs shows peaks at 807 and 1448 cm 1 corresponds to B–N bending and stretching modes, respectively [37]. The peak at 1194 cm 1 corresponds to B–O band [37]. In addition, peaks at
2.3. Material characterization A Rigaku X-ray diffractometer (model-SmartLab9kW) with Cu- Kα (λ ¼ 1.54 Å) as the source with 9 kW power was used for X-ray diffrac tion (XRD) analysis of thin films. A JASCO (Model V-650) UV–vis spectrophotometer was used for the measurement of light absorption spectra. Horiba Jobin Vyon Laser Micro Raman System (Model- LabRam HR) with 488 nm laser excitation was used to analyze Raman spectra. A PerkinElmer (Spectrum-II) instrument was used to obtain the FT-IR spectra in KBr pellets. Field emission scanning electron microscope (FESEM) of Zeiss (model- Sigma-300) instrument operated at 5 kV was used to investigate the surface morphology. Field emission transmission electron microscope (FETEM) of JEOL (JEM-2100F) instrument with an operating voltage of 200 kV was used to analyze the morphology of the samples. ZETASIZER Nano series (Malvern, Nano-ZS90) instrument was used for the measurement of zeta potential. Atomic Force Microscope (AFM) from Oxford (model ¼ Asylum Cypher) instruments was used to analyze the surface area and roughness of the thin films. 3. Results and discussion 3.1. Fabrication and material characterizations Scheme 1 depicts the step-wise fabrication of F–Fe2O3-BNQDs pho toanode. First, β-FeOOH nanorods were directly grown over FTO sub strate by the hydrothermal method which were transformed to α-Fe2O3 after annealing at 800 � C for 15 min. This direct growth of oriented nanorods over the conductive substrate enables not only the smoother transport of carriers, but also minimizes the diffusion length of minority
Scheme 1. Schematic representation of the fabrication process of F–Fe2O3-BNQDs. 3
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Fig. 1. (a) Thin film X-ray Diffraction analysis and (b) Raman spectra of Fe2O3, Fe2O3-BNQDs, F–Fe2O3, and F–Fe2O3-BNQDs photoanodes.
3224 cm 1 corresponds to B–OH stretching [37]. The FT-IR spectrum of F–Fe2O3-BNQDs shows all the peaks correspond to both F–Fe2O3 and BNQDs. Figure S4shows the FESEM images of all the photoanodes. It can be seen that the morphology of all the photoanodes are same after modi fication and the hematite nanorod structures are uniformly distributed over the conductive substrate. The morphological features of hematite are found to be intact after the doping of fluorine. The cross-sectional FESEM image of F–Fe2O3-BNQDs confirms the vertical growth of nanorods over the conductive substrate as shown in Fig. 2a. The lengths of hematite nanorods over the conductive substrate were found to be ~450 nm. The FETEM of boron nitride quantum dots synthesized hy drothermally for 15 h is shown in Fig. S5a, confirms the size of the quantum dots which are found to be around ~5 nm. Inset to Fig. S5a shows the HR-TEM of BNQDs with a d-spacing of 0.21 nm corresponds to (100) plane of BNQDs [33]. Fig. S5b shows the FETEM of BNQDs syn thesized hydrothermally for 20 h which shows that size of the BNQDs increases with increase in synthesis time. The FETEM of F–Fe2O3-BNQDs shown in Fig. 2b display the presence of BNQDs over hematite nanorods. As shown in Fig. 2c, the HR-TEM of F–Fe2O3-BNQDs shows d-spacing of
0.21 nm and 0.25 nm corresponds to (100) plane of BNQDs and (110) plane of Fe2O3, respectively [27,33]. The STEM elemental mapping shows the presence of Fe (e), O (f), F (g), B (h), N (i). 3.2. Photoelectrochemical characterizations Zeta potential measurement was carried out to know the surface charge over BNQDs, to prove its hole extraction ability. A highly nega tive zeta potential value of 35.8 mV was found for BNQDs as shown in Fig. S6, which shows its ability to extract positive ions (holes). To evaluate the effects of F-doping and BNQDs decoration on the water oxidation activity of Fe2O3 photoanodes, photocurrent was measured in 1 M NaOH electrolyte under light illumination. For comparison, the bare Fe2O3, F–Fe2O3, and Fe2O3-BNQDs photoanodes have also been measured under the same conditions. As shown in Fig. 3a, bare Fe2O3 photoanode exhibits a low photocurrent density of 0.35 mA cm 2 at 1.23 V vs. RHE. Fluorine was doped to Fe2O3 with different concentra tion of Selectfluor solution and the corresponding photocurrent densities are shown in Fig. S7a. With ex-situ fluorine doping, the optimized photocurrent density of F–Fe2O3 could reach up to 1.45 mA cm 2, which
Fig. 2. (a) Cross-sectional FESEM image of F–Fe2O3-BNQDs, (b) FETEM image of F–Fe2O3-BNQDs, (c) HRTEM of F–Fe2O3-BNQDs and (d) STEM-EDX elemental mapping of F–Fe2O3-BNQDs showing the uniform distribution of Fe(e), O(f), F(g), B(h) and N(i). 4
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Fig. 3. (a) Linear sweep voltammograms of Fe2O3 (black, ), Fe2O3-BNQDs (magenta, ), F–Fe2O3 (red, ), and F–Fe2O3-BNQDs (olive, ) photoanodes under light illu mination and (b) corresponding chronoamperometry of all the photoanodes at 1.23V vs. RHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
is four times higher than the bare Fe2O3. This enhancement in the photocurrent density could be attributed to the increase in charge car rier density and mobility with F-doping [27]. Decoration of BNQDs with Fe2O3 exhibit a photocurrent density of 1.1 mA cm 2, which is three times higher than that of Fe2O3. The enhancement in photocurrent density due to BNQDs incorporation could be accredited to better charge separation induced by BNQDs. It is known in the literature that hexag onal boron nitride has high negative charge density, particularly on the edge geometry [38], due to which it shows exceptional hole extraction properties, extracting holes from the surface of hematite, resulting in efficient charge separation [32]. The hole extracting ability of BNQDs by virtue of its charges located on the edge geometry is further proved by its highly negative zeta potential value of 35.8 mV (Fig. S6), as results it can readily extract the holes from the surface of hematite. BNQDs incorporation to F–Fe2O3 was optimized with different chemical bath deposition times and the corresponding photocurrent densities are shown in Fig. S7b. To know the effect of the size of BNQDs on the photoelectrochemical performance of F–Fe2O3, BNQDs synthesized with different time periods are also studied as shown in Fig. S7c. The BNQDs synthesized hydrothermally with 15 h are found to be more effective. The decrease in the photoelectrochemical performances with increase in sizes may be attributed the decrease of negative charges over the sur face/edge geometry. With both F-doping and BNQDs incorporation, the photocurrent density reaches up to a remarkable value of 2.24 mA cm 2. These results clearly reveal a significant improvement of the PEC per formance should mainly be attributed to the synergistic effects of F-doping and the BNQDs incorporation. This enhancement in photo current density is due to increase in charge carrier density and mobility as a results of non-metal F-doping as well as efficient hole extraction by negatively charged BNQDs at the interface which supress the recombi nation by rapid oxidation of holes. Fig. 3b shows the transient photo current measurement of all the photoanodes at 1.23 V vs. RHE under chopped light illumination. The sharp photocurrent spikes demonstrate the rapid recombination of charge carriers during their transportation to the semiconductor surface. With both F-doping and BNQDs modifica tion, the photocurrent spikes diminished due to increase in electrical conductivity induced by F-doping and efficient hole extraction by BNQDs for faster water oxidation, indicating that charge recombination is suppressed with dual modification [39]. To explore the interfacial charge separation and charge transfer process, electrochemical impedance spectroscopy (EIS) measurements were carried out for all these samples. Fig. 4a shows the Nyquist plots derived from EIS for bare and modified photoanodes. An equivalent circuit model was used (inset to Fig. 4a) to fit all the Nyquist plot and the results listed in Table 1. Rs expresses the series resistance at the interface
between semiconductor/FTO substrate. The Nyquist plot consists of two semicircles, where in the arch in the low-frequency region corresponds to charge transfer resistance (Rct) at the photoelectrode/electrolyte interface and the arch in the high-frequency region related to charge transfer resistance (Rtrap) at the bulk of photoelectrode [40,41]. With F-doping, the Rct value (441.9 Ω) decreases due to increase in electrical conductivity of hematite as compare to its bare counterpart (5061 Ω) [26]. In case of Fe2O3-BNQDs, due to hole extraction ability of BNQDs the Rct (500 Ω) decreases as compared to bare hematite (5061 Ω) [30]. With incorporation of BNQDs over F–Fe2O3 the Rct value (130.8 Ω) significantly decreases as compared to bare Fe2O3, F–Fe2O3 and Fe2O3-BNQDs at the bulk as well as in the interface of F–Fe2O3-BNQDs due to increase in electrical conductivity and carrier density induced by F-doping and enhanced hole extraction by BNQDs at the photo electrode/electrolyte interface. Furthermore, the increase in the capac itance (Ctrap) value at photoelectrode/electrolyte interface of Fe2O3-BNQDs and F–Fe2O3-BNQDs signifies better hole extraction properties of BNQDs as compared to bare Fe2O3 and F–Fe2O3. The better hole extraction property of BNQDs leads to accumulation of charges on the photoanode/electrolyte interfaces which results in increasing value of capacitance value at photoanode/electrolyte interface [30]. The Mott–Schottky (M S) measurements were carried out to calculate the charge carrier densities of different photoanodes as shown in Fig. 4b. From the M S plot, charge carrier density (ND) can be calculated from the following equations: � � 1 1 kT E E (2) ¼ FB C2 A2 ND eεε0 e where C represents the capacitance of the semiconductor/electrolyte interface, A is the surface area of the photoanode, e is the charge of an electron, ϵ is the dielectric constant of the semiconductor, ϵ0 is the vacuum permittivity, E is the applied potential, k is the Boltzmann constant, and T is the temperature [42]. To minimizes the error in the calculation of charge carrier density, the actual surface area of the photoanodes are obtained from AFM analysis as shown in Fig. S8. Pos itive slopes for all the samples including F-doping indicates that fluorine is an n-type dopant and the significant increase in carrier density is primarily accredited to doping of fluorine [27]. BNQDs incorporation also responsible for enhanced carrier density due to an efficient charge separation and transport as a result of effective hole extraction by BNQDs as compared to bare Fe2O3 [43]. With dual modification, the carrier density further increases due to combined effect of both F-doping and BNQDs. More specifically, the carrier densities calculated from the slopes of the Mott–Schottky plots of all the photoanodes are shown in 5
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Fig. 4. (a) The Nyquist plots for Fe2O3 (black, ), Fe2O3-BNQDs (magenta, ), F–Fe2O3 (red, ), and F–Fe2O3-BNQDs (olive, ) photoanodes under light illumination and their equivalent circuit used for fitting and (b) corresponding Mott–Schottky plots. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Fitting results of the Nyquist plots and M-S plots of bare Fe2O3, F–Fe2O3, Fe2O3BNQDs and F–Fe2O3-BNQDs photoanodes. Photoanode
α- Fe2O3
Fe2O3-BNQDs
F– Fe2O3
F– Fe2O3-BNQDs
RS/Ω Rtrap/Ω Cbulk/μF Rct/Ω Ctrap/μF ND(cm¡3)
37.69 3051 2.75 5061 7.9 4.57 � 1018
34.88 529.5 3.74 500 19.1 9.27 � 1018
36.13 443.7 3.59 441.9 12.1 2.02 � 1019
40.09 441.3 4.73 130.8 23.4 2.08 x 1020
ηinj ¼
JH2 O2 Jabs
(4)
where JH2 O is the measured photocurrent density, JH2 O2 is the photo current density in the presence of hole scavenger H2O2, Jabs is the maximum theoretical water oxidation photocurrent density [27]. As shown in Fig. 5a, a significant improvement in the ηsep at 1.23 VRHE has been achieved for F–Fe2O3-BNQDs (37%) as compared to F–Fe2O3 (31%), Fe2O3-BNQDs (24%) and bare Fe2O3 (14%). The ηinj , as shown in Fig. 5b, is higher for F–Fe2O3-BNQDs (64%) as compared to F–Fe2O3 (48%), Fe2O3-BNQDs (45%) and bare Fe2O3 (27%). The enhancement in ηsep and ηinj suggests that both F-doping and BNQDS incorporation has positive impacts in the enhancement in the photocurrent density. Fig. 6a shows the operational stability of bare hematite and comodified hematite with 1 h continuous illumination. F–Fe2O3-BNQDs shows only 2% decrease in photocurrent density after 1 h of illumina tion, confirm the stability of as synthesized material with modification. Oxygen gas evolved was measured in order to confirm that the photo current observed was really due to PEC water oxidation. For the mea surement of gas, a closed configuration of PEC cell was used. Before the experiment, the cell was de-aerated with N2 gas for an hour. A blank test was done before irradiation to demonstrate that there was no oxygen evolution. After irradiation, with regular interval of time the gas evolved was measured with the help of Gas Chromatograph. A theoretical gas evolution rate was calculated from the current (I) vs. time (t) curve
Table 1. To quantify the effect of F-doping and BNQDs incorporation, charge separation efficiency (ηsep Þ and charge injection efficiency ðηinj Þ were employed with the help of a hole scavenger (0.5 M H2O2) in the elec trolyte (1.0 M NaOH) [44]. Fig. S9 shows the photocurrent densities of bare and modified photoanodes with and without H2O2 in the NaOH electrolyte under light illumination. The ηsep and ηinj are calculated based on the following equations:
ηsep ¼
JH2 O JH2 O2
(3)
and
Fig. 5. (a) Charge separation and (b) charge injection efficiencies of all the photoanodes with respect to applied potential. 6
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Fig. 6. (a) Operational stability of Fe2O3 and F–Fe2O3-BNQDs at 1.23v vs RHE under continuous light illumination and (b) Faradic efficiency test of F–Fe2O3-BNQDs for oxygen evolution.
measured at 1.23 V vs. RHE. The faradaic efficiency, which is the ratio between measured and calculated gas evolution, was found to be ~90% after 1 h of irradiation as shown in Fig. 6b. Thus, most of the photo generated charges were utilized for water oxidation. To show the sig nificance of present work, a comparison study of similar non-metal doped hematite systems were tabulated and presented in Table S1.
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4. Conclusions In conclusion, we have successfully doped fluorine onto hematite using Selectfluor as dopant source and incorporated low temperature synthesized BNQDs for enhanced photoelectrochemical water oxidation. It is apparent that F-doping on Fe2O3 nanorods facilitate charge carrier transport by providing excellent electrical conductivity and charge carrier density. As a result, the one-dimensional Fe2O3 nanorod arrays can quickly transfer the separated electrons to the FTO substrate. At the same time, the BNQDs acts as an efficient hole extractor and can endorse surface charge separation, so that the holes on the surface can be utilized for water oxidation. An excellent photocurrent density of 2.24 mA cm 2 was achieved with the synergistic effects of both F-doping and BNQDs modification, which is six-fold higher than bare hematite. Present strategy can be extrapolated as a model system in designing novel photoanodes for efficient extraction of holes. 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. Acknowledgements Financial support from Department of Science and TechnologyScience and Engineering Research Board, India through project no. SERB/EMR/2016/005123 is duly acknowledged. The authors thank IIT Guwahati and CIF, IIT Guwahati for infrastructure and instrumentation facilities. Dr C. V. Sastri and Professor Parameswar K Iyer are acknowledged for providing cyclic voltammetric measurement facility. Authors thank Devipriya Gogoi and Dr. Nageswara Rao Peela for their help in gas evolution test. Appendix ASupplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227341. 7
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Modulating water oxidation kinetics utilizing h-BN quantum dots as an efficient hole extractor on fluorine doped hematite photoanode Tushar Kanta Sahu, Manoj Kumar Mohanta, Mohammad Qureshi * Department of Chemistry, Indian Institute of Technology, Guwahati, 781039, Assam, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� First study utilizing BNQDs as an effi cient hole extractor in hematite photoanode. � Selectfluor as a source of fluorine has been used for ex-situ doping of fluorine. � Superior enhancement in charge carrier density was achieved with comodification. � A high photocurrent density (2.24 mA/ cm2) was achieved with comodification.
A R T I C L E I N F O
A B S T R A C T
Keywords: Boron nitride quantum dots Hole extractor Selectfluor Non-metal dopant Hematite Water oxidation
The sluggish water oxidation kinetics in hematite photoanode is dominated by unfavorable recombination of photogenerated holes which restrict the efficiency of photoelectrochemical (PEC) water splitting. Herein, we demonstrate a superior photoelectrochemical performance using boron nitride quantum dots (BNQDs), which is incorporated over fluorine doped hematite (F–Fe2O3) photoanode. An impressive carrier density of 2.08 � 1020 cm 3 is achieved, which is two orders of magnitude higher than that of the bare α-Fe2O3 photo anode, resulting in a photocurrent density of 2.24 mA cm 2 at 1.23 V vs reversible hydrogen electrode (RHE) for F–Fe2O3-BNQDs, which is six-fold higher than bare hematite. It is found that BNQD acts as an efficient hole extractor, which enhances the carrier separation on hematite surface and decreases the hole trapping probability. Present work emphasizes on two important parameters, i.e. (i) Enhancement in carrier density via fluorine doping directly onto the substrate using soluble precursor, organic Selectflour (ii) Modification with BNQDs to provide a simple, novel and effective strategy for the design and development of more efficient PEC water splitting systems.
1. Introduction Photoelectrochemical water splitting (PEC) provides an attractive and alternate pathway to generate clean hydrogen utilizing solar energy to overcome future energy demand. In PEC process, multi-electron process at the anodic half reaction makes water oxidation more
difficult compared to its counterpart and has received significant attention to expedite various approaches [1–5]. However, developing the photoanodes with outstanding PEC performance is challenging because of its must meet parameters such as ideal bandgap for effective light absorption, effective minority carrier transfer to minimize recom bination, anodic stabilities at various pH conditions and low-cost
* Corresponding author. E-mail address: [email protected] (M. Qureshi). https://doi.org/10.1016/j.jpowsour.2019.227341 Received 20 September 2019; Received in revised form 15 October 2019; Accepted 19 October 2019 Available online 24 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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fabrication for possible commercialization [6–8]. In this regard, many n-type semiconductor photoanodes have been studied for their water oxidation, including BiVO4, WO3, Fe2O3, TiO2 and ZnFe2O4 [9–16]. Among the photoanodic materials explored, α-Fe2O3 has received increasing attention as a promising photoanode due to its suitable visible absorbing bandgap (1.9–2.2 eV), excellent chemical stability under various pH conditions, natural abundance and low-toxicity [11]. How ever, practical difficulties in reaching the theoretical photocurrent density (solar-to-hydrogen conversion efficiency of 16% at 1.23 V vs RHE) due to technical bottlenecks of hematite photoanode such as sluggish water-oxidation kinetics, short hole diffusion length, poor conductivity and short lifetime of photogenerated charge carriers limits the PEC performance [17–21]. To overcome the abovementioned drawbacks such as carrier con centration and poor conductivity, elemental doping is reported to be an effective strategy to enhance the PEC performance of hematite photo anode with increasing conductivity and charge carrier density. In this regard, metallic dopants such as Sn, Ti, Mn, Co have been used exten sively [22–25]. Recently, non-metallic dopants such as P, S, and Si have been used to enhance the water-oxidation performance of hematite owing to their involvement in increasing carrier density and in addition greater carrier mobility [26–28]. Zhang et al. reported that non-metal P-dopant offers more number of valence electrons as compared to many metallic dopants which competently facilitate charge separation by eluding the surface electron trapping sites in hematite [26]. Recently, van de Krol et al. re-stated that water oxidation capability of a photo anode is strongly limited by its insufficient hole injection property [29]. Zhang et al. demonstrated that incorporation of black phosphorene layer onto BiVO4 can accelerate the hole transport from the semiconductor to co-catalyst [30]. Therefore, promoting hole extraction from photoanode surface holds broader interest for significant improvement in PEC per formance. In the recent past, boron nitride, also known as “white gra phene” has received much attention due to its chemical and thermal stability and conductivity [31]. Intrinsically, boron nitride nano structures were always negatively charged which makes them a good hole carrier acceptor, and could be used to improve the electron-hole carrier separation [32]. In this work, for the first time, we incorporated BNQDs as a simple and effective strategy to overcome the main drawbacks of hematite based photoanodes. The water oxidation of hematite photoanode was intended to be improved by fluorine doping and adsorption of BNQDs. Photoelectrochemical oxidation of water was studied at a voltage which corresponds to the thermodynamic limit of water electrolysis, hence water was just not electrolyzed in the dark but oxidation set in if the hematite anode had been illuminated. For doping non-metal fluorine, a simple organic precursor Selectfluor has been utilized by an ex-situ method which helps to achieve higher charge carrier density with preservation of morphological features. Hexagonal BNQDs have been synthesized in a relatively low-temperature hydrothermal method which effectively acts as an efficient hole extractor for tuning the PEC performance with better charge separation. With optimized doping amount and quantum dots incorporation, the photocurrent density of hematite photoanode is enhanced by six-fold as compared to bare counterpart due to the synergistic effect of both fluorine doping and BNQDs incorporation. The superior PEC activity of modified hematite is supported by electrochemical impedance spectra (EIS) measurement, Mott-Schottky analysis. The present form of modification with BNQDs provides a simple and effective strategy for the design and development of more efficient photoelectrochemical water splitting systems.
Boric acid were purchased from Merck. Iron chloride (97%), Melamine (99%) and Selectfluor (95% Fþ active) were purchased from SigmaAldrich. 2.1.1. Fabrication of α-Fe2O3 and F-doped Fe2O3 The α-Fe2O3 nanorod structures were directly grown over a conductive substrate by following earlier reports [26]. In a typical synthesis, FeCl3 (0.15 M) and NaNO3 (1 M) were added to 60 mL distilled water with stirring. To maintain the pH at 1.5, concentrated HCl was added to the above solution. The solution was then transferred to a 100 mL Teflon lined stainless steel vessel. Four cleaned FTO (2.5 cm � 1.00 cm) with polyimide tape masking were put into the bottom of the solution such that the conductive side faced towards up. The autoclaved solution was put in a pre-heated hot-air oven for 6 h at 100 � C. After the reaction, the autoclave was naturally cooled to room temperature and as synthesized β-FeOOH thin films were cleaned several times with distilled water and ethanol to remove any residue present over the films. The cleaned films were then dried overnight in a hot-air oven at 60 � C. As obtained β-FeOOH thin films were annealed at 800 � C for 15 min (20 � C/min) to obtained α-Fe2O3. F-doped Fe2O3 films were prepared by dipping as prepared β-FeOOH films in an aqueous solution of Selectfluor for 15 min and annealed at 800 � C (20 � C/min) for 15 min. 2.1.2. Synthesis of boron nitride quantum dots (BNQDs) Boron nitride quantum dots (BNQDs) were synthesized by hydro thermal method [33]. For synthesis, boric acid (100 mg) was dissolved in deionized water (10 mL). To the above solution, melamine (34 mg) was added under stirring. After stirring for 30 min, the white suspension was transferred to a 23 mL Teflon lined stainless steel vessel and heated in a hot-air oven at 200 � C for 15 h. A different set of quantum dots were synthesized by keeping reaction time for 20 h to get different sized quantum dots. After cooling, the product was filtered with a 0.22 μm syringe filter membrane to remove large particles. Finally, the solution was diluted to 40 mL with the addition of distilled water and used as a stock solution of BNQDs. 2.1.3. Fabrication of Fe2O3-BNQDs and F–Fe2O3-BNQDs As-prepared Fe2O3 and F–Fe2O3 films were dipped in different con centrations of BNQDs solution for 6, 9, 12 and 15 h at 60 � C to fabricate Fe2O3-BNQDs and F–Fe2O3-BNQDs. After the sensitization process, the films were cleaned with DI water to remove the excess BNQDs at the surface. Then the films were annealed at 400 � C for 1 h. 2.2. Photoelectrochemical measurements A three-electrode configuration was used for the photo electrochemical measurement of all the samples with a potentiostat (model-CHI1120B). For the measurements, 1 M sodium hydroxide (NaOH) electrolyte solution was used. The electrolyte solution was purged with nitrogen gas for 20 min before PEC measurements to remove any dissolved oxygen present in the electrolyte solution. As prepared photoanodes, standard Ag/AgCl (aqueous) electrode with saturated KCl as the electrolyte and Pt wire were used as working electrodes, reference electrode and counter electrode, respectively. All the potentials applied with respect to Ag/AgCl(aqueous) electrode were converted into reversible hydrogen electrode (RHE) potential by the following expression: ERHE ¼ EAg/AgCl þ 0.059 pH þ EoAg/AgCl
2. Experimental
(1)
where EAg/AgCl is the obtained potential vs. Ag/AgCl, EoAg/AgCl ¼ 0.1976 V (saturated KCl) at 25 � C and pH is the pH of the electrolyte. Epoxy adhesive (Araldite®) was used to cover the photoanodes leaving an exposed thin film area of 0.20 cm2. For light source, a 300 W tungsten halogen (OSRAM SYLVANIA, USA) lamp was used and the light intensity
2.1. Materials All the chemicals purchased were used as received without further purification. Sodium nitrate (99%), Hydrochloric acid (35–37%) and 2
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was calibrated at 100 mW/cm2 (1 Sun illumination) with the help of a Si photodiode. An electrochemical work station (Model CHI760D, Inc., Austin, TX) was used to perform the Electrochemical impedance spec troscopy (EIS) measurement in 1 M NaOH aqueous solution at 1 V vs. RHE with an AC amplitude of 10 mV under light illumination (frequency range ¼ 0.1 Hz-10kHz). Mott–Schottky plots were obtained in a po tential window of 1.0 V to þ1.0 V vs. Ag/AgCl under dark conditions (at a constant frequency of 1 kHz). Gas evolution test was analysed by online Gas Chromatograph (model- Agilent 7820A).
charge carriers. Fluorine was doped to Fe2O3 by soaking β-FeOOH films in Selectfluor solution for 15min before calcination at high temperature. Selectfluor used as a source of fluorine which is known to be a good fluorinating agent and offers solubility in common organic solvents as well as water and shows less reactivity towards glass. Ex-situ doping helps to achieve higher charge carrier density with preservation of morphological features. BNQDs was incorporated over Fe2O3 and F–Fe2O3 films by dipping the films in BNQDs solution at 60 � C and keeping it for different times. The BNQDs used for this purpose was synthesized by low-temperature hydrothermal method by using boric acid and melamine as a precursor. Thin film XRD of α-Fe2O3, Fe2O3-BNQDs, F–Fe2O3, and F–Fe2O3BNQDs photoanodes are shown in Fig. 1a. All the peaks correspond to α-Fe2O3 (JCPDS-33-0664) and SnO2 (from FTO substrate). The strong diffraction peak for (110) plane for the samples indicate that the nanostructures are oriented preferentially in the (110) direction over the FTO substrate [22]. No peaks for BNQDs were identified in the com posite samples due to a very low concentration of BNQDs. To confirm the dopant incorporation in hematite, Raman analysis was performed as shown in Fig. 1b. The Raman spectra of all the samples show the char acteristic vibrations of hematite comprising two A1g modes (225 and 500 cm 1) and four Eg modes (245, 293, 412 and 612 cm 1). The Raman spectra of F–Fe2O3 and F–Fe2O3-BNQDs samples shows a new peak at 660 cm 1 which is absent in bare Fe2O3 and Fe2O3-BNQDs, could be attributed to the disorder induced by doping of fluorine [34]. The optical absorption properties of all the photoanodes were analysed through UV–visible absorption spectra as shown in Fig. S1a. BNQDs absorbs in the UV region (inset to Fig. S1a). Similar optical curves and band gaps estimated from Tauc analysis (Fig. S1b) shows that F-doping and BNQDs does not influence the absorption edge of Fe2O3 as well as band gaps [35]. The absorbance and photoluminescence spectra (Fig. S2) of BNQDs are consistent with reported literature of BNQDs [36]. Fig. S3 shows the FT-IR spectra of F–Fe2O3, BNQDS, and F–Fe2O3-BNQDs. The FT-IR spectrum of F–Fe2O3 shows two strong absorption peaks at 483 and 579 cm 1 which can be assigned to Fe–O band. The broad peak at 3436 cm 1 could be attributed to hydroxyl group stretching. The FT-IR spectrum of BNQDs shows peaks at 807 and 1448 cm 1 corresponds to B–N bending and stretching modes, respectively [37]. The peak at 1194 cm 1 corresponds to B–O band [37]. In addition, peaks at
2.3. Material characterization A Rigaku X-ray diffractometer (model-SmartLab9kW) with Cu- Kα (λ ¼ 1.54 Å) as the source with 9 kW power was used for X-ray diffrac tion (XRD) analysis of thin films. A JASCO (Model V-650) UV–vis spectrophotometer was used for the measurement of light absorption spectra. Horiba Jobin Vyon Laser Micro Raman System (Model- LabRam HR) with 488 nm laser excitation was used to analyze Raman spectra. A PerkinElmer (Spectrum-II) instrument was used to obtain the FT-IR spectra in KBr pellets. Field emission scanning electron microscope (FESEM) of Zeiss (model- Sigma-300) instrument operated at 5 kV was used to investigate the surface morphology. Field emission transmission electron microscope (FETEM) of JEOL (JEM-2100F) instrument with an operating voltage of 200 kV was used to analyze the morphology of the samples. ZETASIZER Nano series (Malvern, Nano-ZS90) instrument was used for the measurement of zeta potential. Atomic Force Microscope (AFM) from Oxford (model ¼ Asylum Cypher) instruments was used to analyze the surface area and roughness of the thin films. 3. Results and discussion 3.1. Fabrication and material characterizations Scheme 1 depicts the step-wise fabrication of F–Fe2O3-BNQDs pho toanode. First, β-FeOOH nanorods were directly grown over FTO sub strate by the hydrothermal method which were transformed to α-Fe2O3 after annealing at 800 � C for 15 min. This direct growth of oriented nanorods over the conductive substrate enables not only the smoother transport of carriers, but also minimizes the diffusion length of minority
Scheme 1. Schematic representation of the fabrication process of F–Fe2O3-BNQDs. 3
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Fig. 1. (a) Thin film X-ray Diffraction analysis and (b) Raman spectra of Fe2O3, Fe2O3-BNQDs, F–Fe2O3, and F–Fe2O3-BNQDs photoanodes.
3224 cm 1 corresponds to B–OH stretching [37]. The FT-IR spectrum of F–Fe2O3-BNQDs shows all the peaks correspond to both F–Fe2O3 and BNQDs. Figure S4shows the FESEM images of all the photoanodes. It can be seen that the morphology of all the photoanodes are same after modi fication and the hematite nanorod structures are uniformly distributed over the conductive substrate. The morphological features of hematite are found to be intact after the doping of fluorine. The cross-sectional FESEM image of F–Fe2O3-BNQDs confirms the vertical growth of nanorods over the conductive substrate as shown in Fig. 2a. The lengths of hematite nanorods over the conductive substrate were found to be ~450 nm. The FETEM of boron nitride quantum dots synthesized hy drothermally for 15 h is shown in Fig. S5a, confirms the size of the quantum dots which are found to be around ~5 nm. Inset to Fig. S5a shows the HR-TEM of BNQDs with a d-spacing of 0.21 nm corresponds to (100) plane of BNQDs [33]. Fig. S5b shows the FETEM of BNQDs syn thesized hydrothermally for 20 h which shows that size of the BNQDs increases with increase in synthesis time. The FETEM of F–Fe2O3-BNQDs shown in Fig. 2b display the presence of BNQDs over hematite nanorods. As shown in Fig. 2c, the HR-TEM of F–Fe2O3-BNQDs shows d-spacing of
0.21 nm and 0.25 nm corresponds to (100) plane of BNQDs and (110) plane of Fe2O3, respectively [27,33]. The STEM elemental mapping shows the presence of Fe (e), O (f), F (g), B (h), N (i). 3.2. Photoelectrochemical characterizations Zeta potential measurement was carried out to know the surface charge over BNQDs, to prove its hole extraction ability. A highly nega tive zeta potential value of 35.8 mV was found for BNQDs as shown in Fig. S6, which shows its ability to extract positive ions (holes). To evaluate the effects of F-doping and BNQDs decoration on the water oxidation activity of Fe2O3 photoanodes, photocurrent was measured in 1 M NaOH electrolyte under light illumination. For comparison, the bare Fe2O3, F–Fe2O3, and Fe2O3-BNQDs photoanodes have also been measured under the same conditions. As shown in Fig. 3a, bare Fe2O3 photoanode exhibits a low photocurrent density of 0.35 mA cm 2 at 1.23 V vs. RHE. Fluorine was doped to Fe2O3 with different concentra tion of Selectfluor solution and the corresponding photocurrent densities are shown in Fig. S7a. With ex-situ fluorine doping, the optimized photocurrent density of F–Fe2O3 could reach up to 1.45 mA cm 2, which
Fig. 2. (a) Cross-sectional FESEM image of F–Fe2O3-BNQDs, (b) FETEM image of F–Fe2O3-BNQDs, (c) HRTEM of F–Fe2O3-BNQDs and (d) STEM-EDX elemental mapping of F–Fe2O3-BNQDs showing the uniform distribution of Fe(e), O(f), F(g), B(h) and N(i). 4
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Fig. 3. (a) Linear sweep voltammograms of Fe2O3 (black, ), Fe2O3-BNQDs (magenta, ), F–Fe2O3 (red, ), and F–Fe2O3-BNQDs (olive, ) photoanodes under light illu mination and (b) corresponding chronoamperometry of all the photoanodes at 1.23V vs. RHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
is four times higher than the bare Fe2O3. This enhancement in the photocurrent density could be attributed to the increase in charge car rier density and mobility with F-doping [27]. Decoration of BNQDs with Fe2O3 exhibit a photocurrent density of 1.1 mA cm 2, which is three times higher than that of Fe2O3. The enhancement in photocurrent density due to BNQDs incorporation could be accredited to better charge separation induced by BNQDs. It is known in the literature that hexag onal boron nitride has high negative charge density, particularly on the edge geometry [38], due to which it shows exceptional hole extraction properties, extracting holes from the surface of hematite, resulting in efficient charge separation [32]. The hole extracting ability of BNQDs by virtue of its charges located on the edge geometry is further proved by its highly negative zeta potential value of 35.8 mV (Fig. S6), as results it can readily extract the holes from the surface of hematite. BNQDs incorporation to F–Fe2O3 was optimized with different chemical bath deposition times and the corresponding photocurrent densities are shown in Fig. S7b. To know the effect of the size of BNQDs on the photoelectrochemical performance of F–Fe2O3, BNQDs synthesized with different time periods are also studied as shown in Fig. S7c. The BNQDs synthesized hydrothermally with 15 h are found to be more effective. The decrease in the photoelectrochemical performances with increase in sizes may be attributed the decrease of negative charges over the sur face/edge geometry. With both F-doping and BNQDs incorporation, the photocurrent density reaches up to a remarkable value of 2.24 mA cm 2. These results clearly reveal a significant improvement of the PEC per formance should mainly be attributed to the synergistic effects of F-doping and the BNQDs incorporation. This enhancement in photo current density is due to increase in charge carrier density and mobility as a results of non-metal F-doping as well as efficient hole extraction by negatively charged BNQDs at the interface which supress the recombi nation by rapid oxidation of holes. Fig. 3b shows the transient photo current measurement of all the photoanodes at 1.23 V vs. RHE under chopped light illumination. The sharp photocurrent spikes demonstrate the rapid recombination of charge carriers during their transportation to the semiconductor surface. With both F-doping and BNQDs modifica tion, the photocurrent spikes diminished due to increase in electrical conductivity induced by F-doping and efficient hole extraction by BNQDs for faster water oxidation, indicating that charge recombination is suppressed with dual modification [39]. To explore the interfacial charge separation and charge transfer process, electrochemical impedance spectroscopy (EIS) measurements were carried out for all these samples. Fig. 4a shows the Nyquist plots derived from EIS for bare and modified photoanodes. An equivalent circuit model was used (inset to Fig. 4a) to fit all the Nyquist plot and the results listed in Table 1. Rs expresses the series resistance at the interface
between semiconductor/FTO substrate. The Nyquist plot consists of two semicircles, where in the arch in the low-frequency region corresponds to charge transfer resistance (Rct) at the photoelectrode/electrolyte interface and the arch in the high-frequency region related to charge transfer resistance (Rtrap) at the bulk of photoelectrode [40,41]. With F-doping, the Rct value (441.9 Ω) decreases due to increase in electrical conductivity of hematite as compare to its bare counterpart (5061 Ω) [26]. In case of Fe2O3-BNQDs, due to hole extraction ability of BNQDs the Rct (500 Ω) decreases as compared to bare hematite (5061 Ω) [30]. With incorporation of BNQDs over F–Fe2O3 the Rct value (130.8 Ω) significantly decreases as compared to bare Fe2O3, F–Fe2O3 and Fe2O3-BNQDs at the bulk as well as in the interface of F–Fe2O3-BNQDs due to increase in electrical conductivity and carrier density induced by F-doping and enhanced hole extraction by BNQDs at the photo electrode/electrolyte interface. Furthermore, the increase in the capac itance (Ctrap) value at photoelectrode/electrolyte interface of Fe2O3-BNQDs and F–Fe2O3-BNQDs signifies better hole extraction properties of BNQDs as compared to bare Fe2O3 and F–Fe2O3. The better hole extraction property of BNQDs leads to accumulation of charges on the photoanode/electrolyte interfaces which results in increasing value of capacitance value at photoanode/electrolyte interface [30]. The Mott–Schottky (M S) measurements were carried out to calculate the charge carrier densities of different photoanodes as shown in Fig. 4b. From the M S plot, charge carrier density (ND) can be calculated from the following equations: � � 1 1 kT E E (2) ¼ FB C2 A2 ND eεε0 e where C represents the capacitance of the semiconductor/electrolyte interface, A is the surface area of the photoanode, e is the charge of an electron, ϵ is the dielectric constant of the semiconductor, ϵ0 is the vacuum permittivity, E is the applied potential, k is the Boltzmann constant, and T is the temperature [42]. To minimizes the error in the calculation of charge carrier density, the actual surface area of the photoanodes are obtained from AFM analysis as shown in Fig. S8. Pos itive slopes for all the samples including F-doping indicates that fluorine is an n-type dopant and the significant increase in carrier density is primarily accredited to doping of fluorine [27]. BNQDs incorporation also responsible for enhanced carrier density due to an efficient charge separation and transport as a result of effective hole extraction by BNQDs as compared to bare Fe2O3 [43]. With dual modification, the carrier density further increases due to combined effect of both F-doping and BNQDs. More specifically, the carrier densities calculated from the slopes of the Mott–Schottky plots of all the photoanodes are shown in 5
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Fig. 4. (a) The Nyquist plots for Fe2O3 (black, ), Fe2O3-BNQDs (magenta, ), F–Fe2O3 (red, ), and F–Fe2O3-BNQDs (olive, ) photoanodes under light illumination and their equivalent circuit used for fitting and (b) corresponding Mott–Schottky plots. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Fitting results of the Nyquist plots and M-S plots of bare Fe2O3, F–Fe2O3, Fe2O3BNQDs and F–Fe2O3-BNQDs photoanodes. Photoanode
α- Fe2O3
Fe2O3-BNQDs
F– Fe2O3
F– Fe2O3-BNQDs
RS/Ω Rtrap/Ω Cbulk/μF Rct/Ω Ctrap/μF ND(cm¡3)
37.69 3051 2.75 5061 7.9 4.57 � 1018
34.88 529.5 3.74 500 19.1 9.27 � 1018
36.13 443.7 3.59 441.9 12.1 2.02 � 1019
40.09 441.3 4.73 130.8 23.4 2.08 x 1020
ηinj ¼
JH2 O2 Jabs
(4)
where JH2 O is the measured photocurrent density, JH2 O2 is the photo current density in the presence of hole scavenger H2O2, Jabs is the maximum theoretical water oxidation photocurrent density [27]. As shown in Fig. 5a, a significant improvement in the ηsep at 1.23 VRHE has been achieved for F–Fe2O3-BNQDs (37%) as compared to F–Fe2O3 (31%), Fe2O3-BNQDs (24%) and bare Fe2O3 (14%). The ηinj , as shown in Fig. 5b, is higher for F–Fe2O3-BNQDs (64%) as compared to F–Fe2O3 (48%), Fe2O3-BNQDs (45%) and bare Fe2O3 (27%). The enhancement in ηsep and ηinj suggests that both F-doping and BNQDS incorporation has positive impacts in the enhancement in the photocurrent density. Fig. 6a shows the operational stability of bare hematite and comodified hematite with 1 h continuous illumination. F–Fe2O3-BNQDs shows only 2% decrease in photocurrent density after 1 h of illumina tion, confirm the stability of as synthesized material with modification. Oxygen gas evolved was measured in order to confirm that the photo current observed was really due to PEC water oxidation. For the mea surement of gas, a closed configuration of PEC cell was used. Before the experiment, the cell was de-aerated with N2 gas for an hour. A blank test was done before irradiation to demonstrate that there was no oxygen evolution. After irradiation, with regular interval of time the gas evolved was measured with the help of Gas Chromatograph. A theoretical gas evolution rate was calculated from the current (I) vs. time (t) curve
Table 1. To quantify the effect of F-doping and BNQDs incorporation, charge separation efficiency (ηsep Þ and charge injection efficiency ðηinj Þ were employed with the help of a hole scavenger (0.5 M H2O2) in the elec trolyte (1.0 M NaOH) [44]. Fig. S9 shows the photocurrent densities of bare and modified photoanodes with and without H2O2 in the NaOH electrolyte under light illumination. The ηsep and ηinj are calculated based on the following equations:
ηsep ¼
JH2 O JH2 O2
(3)
and
Fig. 5. (a) Charge separation and (b) charge injection efficiencies of all the photoanodes with respect to applied potential. 6
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Fig. 6. (a) Operational stability of Fe2O3 and F–Fe2O3-BNQDs at 1.23v vs RHE under continuous light illumination and (b) Faradic efficiency test of F–Fe2O3-BNQDs for oxygen evolution.
measured at 1.23 V vs. RHE. The faradaic efficiency, which is the ratio between measured and calculated gas evolution, was found to be ~90% after 1 h of irradiation as shown in Fig. 6b. Thus, most of the photo generated charges were utilized for water oxidation. To show the sig nificance of present work, a comparison study of similar non-metal doped hematite systems were tabulated and presented in Table S1.
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4. Conclusions In conclusion, we have successfully doped fluorine onto hematite using Selectfluor as dopant source and incorporated low temperature synthesized BNQDs for enhanced photoelectrochemical water oxidation. It is apparent that F-doping on Fe2O3 nanorods facilitate charge carrier transport by providing excellent electrical conductivity and charge carrier density. As a result, the one-dimensional Fe2O3 nanorod arrays can quickly transfer the separated electrons to the FTO substrate. At the same time, the BNQDs acts as an efficient hole extractor and can endorse surface charge separation, so that the holes on the surface can be utilized for water oxidation. An excellent photocurrent density of 2.24 mA cm 2 was achieved with the synergistic effects of both F-doping and BNQDs modification, which is six-fold higher than bare hematite. Present strategy can be extrapolated as a model system in designing novel photoanodes for efficient extraction of holes. 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. Acknowledgements Financial support from Department of Science and TechnologyScience and Engineering Research Board, India through project no. SERB/EMR/2016/005123 is duly acknowledged. The authors thank IIT Guwahati and CIF, IIT Guwahati for infrastructure and instrumentation facilities. Dr C. V. Sastri and Professor Parameswar K Iyer are acknowledged for providing cyclic voltammetric measurement facility. Authors thank Devipriya Gogoi and Dr. Nageswara Rao Peela for their help in gas evolution test. Appendix ASupplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227341. 7
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