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Plasmonic nanometal decorated photoanodes for efficient photoelectrochemical water splitting
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Plasmonic nanometal decorated photoanodes for efficient photoelectrochemical water splitting Palyam Subramanyam, Bhagatram Meena, Duvvuri Suryakala1, Melepurath Deepa, Challapalli Subrahmanyam* Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, 502285, Sangareddy, Telangana India
A R T I C LE I N FO
A B S T R A C T
Keywords: Photoelectrochemical cell Bismuth sulfide Bismuth nanoparticles Surface plasmon Resonance Charge transportation Water splitting
Plasmonic metal nanoparticles containing photoanodes are known to exhibit stable photoelectrochemical (PEC) performance due to their optical and electronic properties. In this work, we report the application of plasmonic Bi nanoparticles supported over a g-C3N4/Bi2S3 photoanode for PEC water splitting. Typical results indicated that g-C3N4/Bi2S3/BiNPs ternary composite photoanode showed a high photo-current density of 7.11 mA cm−2 at 1.23 V under solar irradiation, which was ∼ 5 and 10 times higher than g-C3N4/Bi2S3 and g-C3N4 photoanodes, respectively. Further, the composite electrode also demonstrated superior solar to hydrogen efficiency and long-term stability. It was concluded that Bi nanoparticles play a major role in enhancing the PEC performance for hydrogen evolution reaction. Thus, g-C3N4/Bi2S3/BiNPs has superior PEC performance and proved to work as an alternative to noble metal based photo-electrodes for solar-water splitting reactions.
1. Introduction Over several decades, photoelectrochemical (PEC) water splitting has been tested for hydrogen production by conversion of solar energy to chemical energy [1–3]. In a typical PEC cell, semiconducting materials like TiO2, Fe2O3, ZnO and BiVO4 have been used for splitting water [4,5]. Until now, several semiconductors have been designed and developed to address the issues that affect PEC water splitting such as suitably positioned valence/conduction band, tailoring of bandgap to harvest the visible light thus allowing fast charge carrier separation and transportation. Recently, graphitic carbon nitride (g-C3N4) has been reported to be an efficient photoanode due to its low bandgap of 2.7 eV, it’s metal-free nature, low-toxicity and ease of preparation [6–8]. In addition, the band-edge positions of the g-C3N4 are aligned favorably with respect to the redox potentials of water [9]. However, their role in solar to hydrogen conversion is limited due to fast recombination of photogenerated charge carriers. Several attempts were made for the modification of g-C3N4 to attain high PEC efficiency, including heterojunction formation with narrow band gap semiconductors [10,11], co-catalyst-loading [12], elemental-doping [13] and plasmonic enhancement [14,15] etc. In principle, the formation of heterojunction with binary or ternary materials in photo-electrodes is the most effective way to increase the
charge-separation efficiency as well as to extend the light absorption range. For instance, Alam et al., fabricated g-C3N4-BiOI heterojunction which delivered a high PEC performance for water splitting compared to pristine electrodes (such as g-C3N4 and BiOI) [16]. Ye et al., reported a S-doped g‑C3N4/MoS2 film with an enhanced visible light photo-response as well as charge separation efficiency than S-doped g‑C3N4 [17]. Therefore, modifying the g‑C3N4 based heterojunction photoelectrode for PEC water splitting is useful for improving the PEC performance. Among different metal chalcogenides, Bi2S3 has emerged as an efficient material in the field of PECs, due to the following factors: high optical absorption coefficient, small band gap of 1.3–1.7 eV and favorable band alignment with wide bandgap semiconductors [18,19]. Furthermore, to improve charge carrier transfer, plasmonic behavior of metal nanoparticles like noble metals (Ag and Au) and non-noble metals (Bi and Cd) can be tapped for they contribute by reducing the charge recombination rate while in-contact with suitable semiconductors [20–22]. Also, they enhance the photo-response through surface plasmon resonance (SPR) effects. In order to showcase its’ SPR property, a support system is needed. Recently, bismuth nanoparticles (BiNPs) are reported to exhibit unique optical-electronic properties, ease of availability with low cost and SPR property for the fabrication of photo-electrode [23,24]. Many studies have reported BiNPs-based
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Corresponding author. E-mail address: [email protected] (C. Subrahmanyam). 1 Department of Chemistry, GITAM University, Visakhapatnam, India. https://doi.org/10.1016/j.cattod.2020.01.041 Received 27 October 2019; Received in revised form 19 December 2019; Accepted 28 January 2020 0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Palyam Subramanyam, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.01.041
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Fig. 1. Absorption spectra of (a) BiNPs (b) g-C3N4 and (c) Bi2S3 QDs, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs and (d) XRD pattern of g-C3N4, Bi2S3 and BiNPs.
glass used for fabrication of electrode has a sheet resistance of 13 Ω /cm2 and is purchased from Aldrich. All the FTO glass plates are precleaned with HCl solution (35%), followed by DI water and acetone.
heterojunction systems. Toudert et al. reported that BiNPs owes to have tunable SPR phenomena in near-UV to NIR region [24]. The combination of Bi and BiVO4 photoanode was reported by Wang et al. achieved excellent PEC hydrogen evolution activity than the pure BiVO4 [25]. Wulan et al., reported Bi/BiVO4 heterojunction as a photoanode which delivered two times increment in photocurrent density for water splitting [26]. Recently, Fan et al., reported Bi/BiOCl photocathode for PEC water splitting, which exhibited high photocurrent density and H2 evolution rate of 2.4 mol h−1 due to the SPR of BiNPs [27]. Wang et al., fabricated plasmonic Bi metal based photocatalyst such as g-C3N4/Bi/Bi2WO6 which demonstrated high photocatalytic activity than those of the g-C3N4 and g-C3N4/Bi2WO6 samples [28]. Alongside noble metals, decorating semiconductors with BiNPs would also extend the optical absorption spectral range while reducing the electron-hole recombination. In the present work, we focused on improving the g-C3N4/Bi2S3 heterojunction by incorporation of BiNPs to form a g-C3N4/Bi2S3/BiNPs ternary composite. Plasmonic BiNPs were synthesized by a chemical solution method. The preparation of g-C3N4/Bi2S3/BiNPs is carried out by a simple drop casting and successive ionic layer adsorption and reaction (SILAR) methods. A systematic study was carried out to ensure the phase purity and formation of the composite material. The fabricated g-C3N4/Bi2S3/BiNPs was used as a photoanode for PEC water splitting.
2.2. Fabrication of g-C3N4/Bi2S3/BiNPs photoanodes The detailed synthesis of BiNPs and g-C3N4 are shown in supporting information (SI). The g-C3N4/Bi2S3/BiNPs photoanode was prepared by doctor blading technique and SILAR methods followed by drop-casting. The photoanode is fabricated as follows: the g-C3N4 paste was prepared by a mixing 100 mg g-C3N4, 160 μl of NMP and 2.5 ml of ethanol and sonication for 15 min. The paste was coated onto the pre-cleaned FTO substrates using a doctor blading method, followed by drying at 80 °C for 30 min, and the FTO/g-C3N4 film was obtained. Further, Bi2S3 quantum dots (QDs) were grown on g-C3N4 by using a SILAR process to form g-C3N4/Bi2S3. In brief, a g-C3N4 film was first vertically immersed in a bismuth precursor solution (0.1 M of Bi (NO3)3.3H2O in acetone) at 25 °C for 15 s, rinsed in acetone and dried on hot plate at 60 °C. Then, the film was dipped in a sulfide precursor solution (0.1 M of Na2S in methanol) at 25 °C for 15 s and dried on hot plate at 60 °C via rinsed in methanol to remove excess ions. This remained one SILAR cycle for Bi2S3 QDs. Thoroughly, eight more SILAR cycles were done resultant in dark-brown color of FTO/g-C3N4/Bi2S3 films. Finally, synthesized BiNPs were deposited onto the FTO/g-C3N4/Bi2S3 films via drop-casting 1 ml of the BiNPs/ethanol solution and dried at room temperature. The deposition scheme of g-C3N4/Bi2S3/BiNPs photoanode as presented in the supporting information.
2. Experimental 2.1. Materials
2.3. Characterizations Bismuth nitrate trihydrate (Bi(NO3)3.3H2O), bismuth acetate (Bi (CH3COO)3), sodium sulfide (Na2S), acetic acid (CH3COOH), ethanol (C2H5OH), N-methyl pyrrolidone (NMP), methanol (CH3OH), sodium borohydride (NaBH4), hydrochloric acid (HCl) and melamine. FTO
Shimadzu UV-3600 instrument was used for collecting UV–vis spectra for synthesized photoanodes, whereas the phase purity and presence of consistent materials was confirmed by Powder X-ray 2
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Fig. 2. FE-SEM images of (a) BiNPs, (b) g-C3N4, (c) g-C3N4/Bi2S3/BiNPs and (d) EDAX image of g-C3N4/Bi2S3/BiNPs (e) TEM and (f) HR-TEM images of g-C3N4/ Bi2S3/BiNPs.
measured in aqueous electrolyte containing of 0.1 M Na2SO4 and Na2SO3.
diffraction (XRD) by using PANalytical, XpertPRO. Surface morphologies of the samples were measured by using a FESEM (Zeiss supra 40), whereas, TECNAI G-2 FEI (300 kV) was use to collect HR-TEM images.
3. Results and discussion 2.4. Photoelectrochemical measurements 3.1. Optical and XRD studies Linear sweep voltammetric (I-V) studies were performed on LOTOriel with a 150 W Xe that provides ∼100 mW cm−2. Chronoamperometric (I-t) studies, electrochemical impedance spectroscopic (EIS) and cyclic voltammograms (CV) studies were performed on an Autolab PGSTAT 302 N using NOVA 2.1 software. A Trace-1310 GC fitted with a TCD was used to quantify hydrogen gas. The samples were
BiNPs display strong absorption due to its plasmonic behavior in the range of 300−680 nm (Fig. 1a). From the Fig. 1b, the g-C3N4 has narrow absorption spectral range with an absorption edge at 460 nm which was attributed to the bandgap for g-C3N4 (2.69 eV). For Bi2S3 QDs, an absorption edge at 765 nm (Fig. 1c) and the calculated bandgap 3
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Fig. 3. (a) LSV plots and (b) STH efficiency (c) stability of pure g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs nanocomposite photoelectrodes under solar radiation (d) H2 evolution of g-C3N4/Bi2S3/BiNPs at 1.23 V vs RHE over 2 h under light illumination.
and 20−30 nm respectively. The High resolution image (Fig. 2b) shows the lattice fringes of BiNPs and Bi2S3. Orthorhombic Bi2S3 has an interfringe spacing of 0.35 nm belongs to (310) (JCPDS: 170320) and the plasmonic BiNPs have a lattice spacing of 0.32 nm that belongs to (012) (JCPDS-851331).
for Bi2S3 is 1.62 eV. The absorption edge extends to near infrared region by decoration of Bi2S3 QDs on g-C3N4 can be seen in Fig. 1c. Finally, the absorption intensity of g-C3N4/Bi2S3/BiNPs composite is slightly higher due to the introduction of plasmonic effects of BiNPs (Fig. 1c). The phase purity and crystal structure of g-C3N4, Bi2S3 and BiNPs were examined by XRD and results are presented in the Fig.1d. The g-C3N4 show peaks at 13.1 and 27.7° which corresponds to the (100) and (002) planes respectively (JCPDS-87-1526), which are the distinctive peaks of g-C3N4 [29]. The former, which are corresponds to d-spacing of 6.7 and 3.21 Å, which were indicates the in-plane structural repeating packing motif and interlayer stacking of aromatic systems respectively. Bi2S3 has orthorhombic primitive lattice with (130), (211), (312) and (242) planes corresponding to d-spacing of 3.56, 3.11, 1.57 and 1.53 Å respectively (JCPDS: 170320). Finally, the BiNPs possess rhombohedral structure with (012), (104), (110) and (202) planes corresponding to dspacing of 3.25, 2.34, 2.24 and 1.83 Å respectively (JCPDS: 851331).
3.3. Photoelectrochemical studies All the PEC studies were performed under visible light illumination. The LSV plots (current versus potential, I vs V) curves are displayed in Fig. 3a. The electrolyte used for the experiment is 0.1 M of Na2SO4 and Na2SO3. In typically, presence of Na2SO3 in the electrolyte solution act as a hole scavenger, where SO32− is oxidized to SO42- that preventing the anodic photo-corrosion of Bi2S3, which significantly increase the amount of hydrogen evolution at the Pt counter electrode. The measured potential with reference Ag/AgCl was converted to RHE as shown in supporting information. Upon light on and off mode (chopped illumination), the observed photocurrent density for g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs nanocomposite is 0.74, 3.54 and 7.11 mA/cm2 at 1.23 V, respectively. However, all the studied photoelectrodes does not show photocurrent under dark conditions. g-C3N4/Bi2S3/BiNPs composite produced the highest photocurrent density, which is twotimes and six-times higher than for g-C3N4/Bi2S3 and g-C3N4, respectively. The g-C3N4/Bi2S3/BiNPs has the onset potential of 0.11 V, which is lower than pristine g-C3N4 (0.42 V). The decrement of onset potential and enhanced PEC performance of g-C3N4/Bi2S3/BiNPs for water splitting can be explained by ability of Bi2S3 as an efficient visible light absorber and BiNPs support increased charge carrier separation and mobility due to SPR effect. Solar-to-hydrogen conversion (STH) for gC3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs nanocomposite has been calculated using the equation (2) (SI), which was found to be 0.05, 1.23 and 3.51 % at 0.4 V respectively (Fig. 3b). This improved activity of gC3N4/Bi2S3/BiNPs suggests that chemical stability of BiNPs have been improved on g-C3N4/Bi2S3. The stability of the photoanodes was examined through chronoamperometric studies. The photocurrent for g-
3.2. Electron microscopy studies FE-SEM image of synthesized Bi nanoparticles displayed flower like morphology with average particle size is around few hundred nanometers (Fig. 2a). From Fig. 2b, g-C3N4 revealed irregular stacking flat sheets with aggregated morphologies. The g-C3N4/Bi2S3/BiNPs composite also displayed irregular shape with high roughness and porosity, where Bi2S3 QDs and Bi nanoparticles are embedded onto the g-C3N4 as shown in the Fig. 2c. Further, the composites have been quantitative analyzed by energy dispersive analysis of X-rays (EDAX). The constitute elements present in the samples are shown in Fig. 2d and this confirms that BiNPs are well formed in the composite. The SEM images of Bi2S3 and g-C3N4/Bi2S3 are provided in Fig. S1. Fig. 2a reveals an overview of the transmission electron microscopic image of g-C3N4/Bi2S3/BiNPs composite, where the grey color part of sheets was assigned to g-C3N4 and the dark particles were assigned to Bi2S3 and Bi nanoparticles, which were dispersed on the surface of g-C3N4. The diameter of nanoparticles such as Bi metal and Bi2S3 to be in the range of 10−15 nm 4
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maximum H2 evolution over 2 h. Thus, high photocurrent and H2 quantification details for g-C3N4/Bi2S3/BiNPs are in good agreement and which suggest that g-C3N4/Bi2S3/BiNPs is a promising material for PEC. The IPCE response of the pristine g-C3N4 and g-C3N4/Bi2S3/BiNPs photoelectrodes as shown in Fig. 4a. Pristine g-C3N4 and g-C3N4/Bi2S3/ BiNPs photoelectrodes reveals maximum IPCE values are 10.8 and 24.5 % at 360 nm, respectively. Impressively, the ternary nanocomposite exhibited higher IPCE than the pristine g-C3N4 electrode. The best performance of g-C3N4/Bi2S3/BiNPs concludes that synergetic effect of Bi2S3 QDs and BiNPs which inhibit the charge recombination and also facilitates higher charge injection in the composite. This is also supported by the maximum photocurrent at around 310 nm due to the formation of energetic hot electrons by plasmonic excitation of BiNPs. Therefore, it is realized that g-C3N4/Bi2S3/BiNPs composite enhance light harvesting and the overall current generation of the photoelectrode. In order to validate the ease of charge transfer at the electrodeelectrolyte interface, EIS experiments were performed. The Nyquist plots obtained for g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs photoanodes over the frequency range (1 kHz-1 Hz) under light irradiation are shown in Fig. 4b. In typical, large diameter of semicircle indicates higher Rct value at the electrode-electrolyte interface. From the Fig. 4b, g-C3N4 shows high Rct due to large diameter of semicircle that reflected low charge separation and weak carrier transportation which were the main reasons for its poor water oxidation performance. Impressively, gC3N4/Bi2S3/BiNPs exhibit low Rct (21 Ω) than other electrodes such as g-C3N4/Bi2S3 (46 Ω) and g-C3N4 (278 Ω), resulting indicates that the facile transfer of charge carriers that is reflected in its superior PEC performance. In simple terms, it could be explained as photogenerated electrons from photosensitizers (g-C3N4 and Bi2S3 QDs) can be effectively collected with the help of BiNPs and consolidated electrons are transferred to counter electrode. Therefore, g-C3N4/Bi2S3/BiNPs exhibit dominated PEC response. Furthermore, Mott-Schottky (M-S) analysis was used to study the conducting behavior of the as-prepared samples such as g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs under light illumination, at a constant ac frequency of 1 kHz and corresponding plots are shown in Fig. 4c. All g-C3N4 samples show the positive slopes, thus confirming that they are n-type semiconductors. By extrapolating the xintercept in M-S plots, the flat band potential (VFB) at the photoelectrodes was calculated. The VFB was found to be 0.408, 0.364 V and 0.301 versus RHE respectively for pristine g-C3N4, g-C3N4/Bi2S3 and gC3N4/Bi2S3/BiNPs composites. These results suggest that BiNPs act as extra charge carriers in the composites, influences the work function of the g-C3N4/Bi2S3/BiNPs composite and provides a facile charge transport at the electrode-electrolyte interface. Interestingly, the composite exhibited negative shift of VFB compared to that pristine g-C3N4, gC3N4/Bi2S3 photo-electrode, which implies that a better transport of photo-generated carriers in the composite, which reduces the charge recombination and thereby exhibited higher PEC performance for hydrogen evolution. Fig. S3 reveals the M-S plot of Bi2S3 film, which confirms that Bi2S3 has a n-type semiconductor with positive slope. The mechanistic details explaining the electron transfer process in gC3N4/Bi2S3/BiNPs composite photoelectrodes could be explained through schematic representation (Fig. 5). The Fermi level of BiNPs, VB and CB positions for g-C3N4 and Bi2S3 QDs are obtained from cyclic voltammetry (Fig. S2). The procedure for calculation of band positions in the energy level diagram is presented in (SI). Firstly, due to absorption of solar radiation, excitons are generated in Bi2S3 and g-C3N4. Originally, the photogenerated electrons are transferred from CB of Bi2S3 to CB of g-C3N4 due to the favorable band-edge positions between them as shown in Fig. 5 [20,30]. Furthermore, the surface plasmons is facilitated by BiNPs which enables the generated charge carriers are efficiently transferred to the contact FTO. The photogenerated holes created in the VB of Bi2S3 react with electrolyte to drive water oxidation. On the other hand, the photogenerated electrons are transferred to external circuit lead to HER at the counter electrode. Therefore, we
Fig. 4. (a) IPCE of pristine g-C3N4 and g-C3N4/Bi2S3/BiNPs photoelectrodes under monochromatic light radiation. (b) Nyquist plots, (c) M-S plots for gC3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs photoelectrodes.
C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs electrodes at 0.4 V vs RHE as a function of time is shown in Fig. 3c. Remarkable long term stability has been exhibited by g-C3N4/Bi2S3/BiNPs nanocomposite over 100 min highlighted its best performance. The current density value from LSV curve and chronoamperometric curves are consistent and gC3N4/Bi2S3/BiNPs composite shows higher current than pristine g-C3N4 and g-C3N4/Bi2S3. The H2 evolution is quantified using gas chromatography and the H2 evolved is plotted as a function of time at 1.23 V as shown in Fig. 3d. The g-C3N4/Bi2S3/BiNPs photoanode showed the 5
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Fig. 5. Schematic diagrams of energy alignment and charge transfer in g-C3N4/Bi2S3/BiNPs photoanode under solar light illumination.
References
could clearly say that the presence of BiNPs accelerates the overall transportation and efficiency of electron injection resulting into the enhanced PEC activity for hydrogen generation.
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4. Conclusions To summarize, g-C3N4/Bi2S3/BiNPs photoanode was fabricated and used for photoelectrochemical water splitting reaction. This ternary photoanode displayed the maximum photocurrent density of 7.11 mA/ cm2 at 1.23 V over g-C3N4 and g-C3N4/Bi2S3 with STH efficiency of 3.51 % at 0.4 V. The low charge transfer resistance and best IPCE for g-C3N4/ Bi2S3/BiNPs improves the charge separation and transportation. This enhanced PEC performance is due to the combined action of SPR property of BiNPs that improves light harvesting as well as charge carrier density and Bi2S3 QDs which increases the light absorption range and reduce the charge carrier recombination. This composite has potential for best PEC performance over expensive noble metal based photoelectrodes for hydrogen generation. Author contributions Palyam Subramanyam is the first author. He was planned to design the scheme, did work and finally wrote the manuscript. Bhagatram Meena helped to characterize the fabricated samples. Duvvuri Suryakala and Melepurath Deepa, both helped to check the typo errors and English grammar in the manuscript. Challapalli Subrahmanyam is the corresponding author. He was helped to design the work and wrote the manuscript. Declaration of Competing Interest There is no conflict of interest. Acknowledgements PS thanks CSIR, India for a senior research fellowship. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2020.01.041.
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Plasmonic nanometal decorated photoanodes for efficient photoelectrochemical water splitting Palyam Subramanyam, Bhagatram Meena, Duvvuri Suryakala1, Melepurath Deepa, Challapalli Subrahmanyam* Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, 502285, Sangareddy, Telangana India
A R T I C LE I N FO
A B S T R A C T
Keywords: Photoelectrochemical cell Bismuth sulfide Bismuth nanoparticles Surface plasmon Resonance Charge transportation Water splitting
Plasmonic metal nanoparticles containing photoanodes are known to exhibit stable photoelectrochemical (PEC) performance due to their optical and electronic properties. In this work, we report the application of plasmonic Bi nanoparticles supported over a g-C3N4/Bi2S3 photoanode for PEC water splitting. Typical results indicated that g-C3N4/Bi2S3/BiNPs ternary composite photoanode showed a high photo-current density of 7.11 mA cm−2 at 1.23 V under solar irradiation, which was ∼ 5 and 10 times higher than g-C3N4/Bi2S3 and g-C3N4 photoanodes, respectively. Further, the composite electrode also demonstrated superior solar to hydrogen efficiency and long-term stability. It was concluded that Bi nanoparticles play a major role in enhancing the PEC performance for hydrogen evolution reaction. Thus, g-C3N4/Bi2S3/BiNPs has superior PEC performance and proved to work as an alternative to noble metal based photo-electrodes for solar-water splitting reactions.
1. Introduction Over several decades, photoelectrochemical (PEC) water splitting has been tested for hydrogen production by conversion of solar energy to chemical energy [1–3]. In a typical PEC cell, semiconducting materials like TiO2, Fe2O3, ZnO and BiVO4 have been used for splitting water [4,5]. Until now, several semiconductors have been designed and developed to address the issues that affect PEC water splitting such as suitably positioned valence/conduction band, tailoring of bandgap to harvest the visible light thus allowing fast charge carrier separation and transportation. Recently, graphitic carbon nitride (g-C3N4) has been reported to be an efficient photoanode due to its low bandgap of 2.7 eV, it’s metal-free nature, low-toxicity and ease of preparation [6–8]. In addition, the band-edge positions of the g-C3N4 are aligned favorably with respect to the redox potentials of water [9]. However, their role in solar to hydrogen conversion is limited due to fast recombination of photogenerated charge carriers. Several attempts were made for the modification of g-C3N4 to attain high PEC efficiency, including heterojunction formation with narrow band gap semiconductors [10,11], co-catalyst-loading [12], elemental-doping [13] and plasmonic enhancement [14,15] etc. In principle, the formation of heterojunction with binary or ternary materials in photo-electrodes is the most effective way to increase the
charge-separation efficiency as well as to extend the light absorption range. For instance, Alam et al., fabricated g-C3N4-BiOI heterojunction which delivered a high PEC performance for water splitting compared to pristine electrodes (such as g-C3N4 and BiOI) [16]. Ye et al., reported a S-doped g‑C3N4/MoS2 film with an enhanced visible light photo-response as well as charge separation efficiency than S-doped g‑C3N4 [17]. Therefore, modifying the g‑C3N4 based heterojunction photoelectrode for PEC water splitting is useful for improving the PEC performance. Among different metal chalcogenides, Bi2S3 has emerged as an efficient material in the field of PECs, due to the following factors: high optical absorption coefficient, small band gap of 1.3–1.7 eV and favorable band alignment with wide bandgap semiconductors [18,19]. Furthermore, to improve charge carrier transfer, plasmonic behavior of metal nanoparticles like noble metals (Ag and Au) and non-noble metals (Bi and Cd) can be tapped for they contribute by reducing the charge recombination rate while in-contact with suitable semiconductors [20–22]. Also, they enhance the photo-response through surface plasmon resonance (SPR) effects. In order to showcase its’ SPR property, a support system is needed. Recently, bismuth nanoparticles (BiNPs) are reported to exhibit unique optical-electronic properties, ease of availability with low cost and SPR property for the fabrication of photo-electrode [23,24]. Many studies have reported BiNPs-based
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Corresponding author. E-mail address: [email protected] (C. Subrahmanyam). 1 Department of Chemistry, GITAM University, Visakhapatnam, India. https://doi.org/10.1016/j.cattod.2020.01.041 Received 27 October 2019; Received in revised form 19 December 2019; Accepted 28 January 2020 0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Palyam Subramanyam, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.01.041
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Fig. 1. Absorption spectra of (a) BiNPs (b) g-C3N4 and (c) Bi2S3 QDs, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs and (d) XRD pattern of g-C3N4, Bi2S3 and BiNPs.
glass used for fabrication of electrode has a sheet resistance of 13 Ω /cm2 and is purchased from Aldrich. All the FTO glass plates are precleaned with HCl solution (35%), followed by DI water and acetone.
heterojunction systems. Toudert et al. reported that BiNPs owes to have tunable SPR phenomena in near-UV to NIR region [24]. The combination of Bi and BiVO4 photoanode was reported by Wang et al. achieved excellent PEC hydrogen evolution activity than the pure BiVO4 [25]. Wulan et al., reported Bi/BiVO4 heterojunction as a photoanode which delivered two times increment in photocurrent density for water splitting [26]. Recently, Fan et al., reported Bi/BiOCl photocathode for PEC water splitting, which exhibited high photocurrent density and H2 evolution rate of 2.4 mol h−1 due to the SPR of BiNPs [27]. Wang et al., fabricated plasmonic Bi metal based photocatalyst such as g-C3N4/Bi/Bi2WO6 which demonstrated high photocatalytic activity than those of the g-C3N4 and g-C3N4/Bi2WO6 samples [28]. Alongside noble metals, decorating semiconductors with BiNPs would also extend the optical absorption spectral range while reducing the electron-hole recombination. In the present work, we focused on improving the g-C3N4/Bi2S3 heterojunction by incorporation of BiNPs to form a g-C3N4/Bi2S3/BiNPs ternary composite. Plasmonic BiNPs were synthesized by a chemical solution method. The preparation of g-C3N4/Bi2S3/BiNPs is carried out by a simple drop casting and successive ionic layer adsorption and reaction (SILAR) methods. A systematic study was carried out to ensure the phase purity and formation of the composite material. The fabricated g-C3N4/Bi2S3/BiNPs was used as a photoanode for PEC water splitting.
2.2. Fabrication of g-C3N4/Bi2S3/BiNPs photoanodes The detailed synthesis of BiNPs and g-C3N4 are shown in supporting information (SI). The g-C3N4/Bi2S3/BiNPs photoanode was prepared by doctor blading technique and SILAR methods followed by drop-casting. The photoanode is fabricated as follows: the g-C3N4 paste was prepared by a mixing 100 mg g-C3N4, 160 μl of NMP and 2.5 ml of ethanol and sonication for 15 min. The paste was coated onto the pre-cleaned FTO substrates using a doctor blading method, followed by drying at 80 °C for 30 min, and the FTO/g-C3N4 film was obtained. Further, Bi2S3 quantum dots (QDs) were grown on g-C3N4 by using a SILAR process to form g-C3N4/Bi2S3. In brief, a g-C3N4 film was first vertically immersed in a bismuth precursor solution (0.1 M of Bi (NO3)3.3H2O in acetone) at 25 °C for 15 s, rinsed in acetone and dried on hot plate at 60 °C. Then, the film was dipped in a sulfide precursor solution (0.1 M of Na2S in methanol) at 25 °C for 15 s and dried on hot plate at 60 °C via rinsed in methanol to remove excess ions. This remained one SILAR cycle for Bi2S3 QDs. Thoroughly, eight more SILAR cycles were done resultant in dark-brown color of FTO/g-C3N4/Bi2S3 films. Finally, synthesized BiNPs were deposited onto the FTO/g-C3N4/Bi2S3 films via drop-casting 1 ml of the BiNPs/ethanol solution and dried at room temperature. The deposition scheme of g-C3N4/Bi2S3/BiNPs photoanode as presented in the supporting information.
2. Experimental 2.1. Materials
2.3. Characterizations Bismuth nitrate trihydrate (Bi(NO3)3.3H2O), bismuth acetate (Bi (CH3COO)3), sodium sulfide (Na2S), acetic acid (CH3COOH), ethanol (C2H5OH), N-methyl pyrrolidone (NMP), methanol (CH3OH), sodium borohydride (NaBH4), hydrochloric acid (HCl) and melamine. FTO
Shimadzu UV-3600 instrument was used for collecting UV–vis spectra for synthesized photoanodes, whereas the phase purity and presence of consistent materials was confirmed by Powder X-ray 2
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Fig. 2. FE-SEM images of (a) BiNPs, (b) g-C3N4, (c) g-C3N4/Bi2S3/BiNPs and (d) EDAX image of g-C3N4/Bi2S3/BiNPs (e) TEM and (f) HR-TEM images of g-C3N4/ Bi2S3/BiNPs.
measured in aqueous electrolyte containing of 0.1 M Na2SO4 and Na2SO3.
diffraction (XRD) by using PANalytical, XpertPRO. Surface morphologies of the samples were measured by using a FESEM (Zeiss supra 40), whereas, TECNAI G-2 FEI (300 kV) was use to collect HR-TEM images.
3. Results and discussion 2.4. Photoelectrochemical measurements 3.1. Optical and XRD studies Linear sweep voltammetric (I-V) studies were performed on LOTOriel with a 150 W Xe that provides ∼100 mW cm−2. Chronoamperometric (I-t) studies, electrochemical impedance spectroscopic (EIS) and cyclic voltammograms (CV) studies were performed on an Autolab PGSTAT 302 N using NOVA 2.1 software. A Trace-1310 GC fitted with a TCD was used to quantify hydrogen gas. The samples were
BiNPs display strong absorption due to its plasmonic behavior in the range of 300−680 nm (Fig. 1a). From the Fig. 1b, the g-C3N4 has narrow absorption spectral range with an absorption edge at 460 nm which was attributed to the bandgap for g-C3N4 (2.69 eV). For Bi2S3 QDs, an absorption edge at 765 nm (Fig. 1c) and the calculated bandgap 3
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Fig. 3. (a) LSV plots and (b) STH efficiency (c) stability of pure g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs nanocomposite photoelectrodes under solar radiation (d) H2 evolution of g-C3N4/Bi2S3/BiNPs at 1.23 V vs RHE over 2 h under light illumination.
and 20−30 nm respectively. The High resolution image (Fig. 2b) shows the lattice fringes of BiNPs and Bi2S3. Orthorhombic Bi2S3 has an interfringe spacing of 0.35 nm belongs to (310) (JCPDS: 170320) and the plasmonic BiNPs have a lattice spacing of 0.32 nm that belongs to (012) (JCPDS-851331).
for Bi2S3 is 1.62 eV. The absorption edge extends to near infrared region by decoration of Bi2S3 QDs on g-C3N4 can be seen in Fig. 1c. Finally, the absorption intensity of g-C3N4/Bi2S3/BiNPs composite is slightly higher due to the introduction of plasmonic effects of BiNPs (Fig. 1c). The phase purity and crystal structure of g-C3N4, Bi2S3 and BiNPs were examined by XRD and results are presented in the Fig.1d. The g-C3N4 show peaks at 13.1 and 27.7° which corresponds to the (100) and (002) planes respectively (JCPDS-87-1526), which are the distinctive peaks of g-C3N4 [29]. The former, which are corresponds to d-spacing of 6.7 and 3.21 Å, which were indicates the in-plane structural repeating packing motif and interlayer stacking of aromatic systems respectively. Bi2S3 has orthorhombic primitive lattice with (130), (211), (312) and (242) planes corresponding to d-spacing of 3.56, 3.11, 1.57 and 1.53 Å respectively (JCPDS: 170320). Finally, the BiNPs possess rhombohedral structure with (012), (104), (110) and (202) planes corresponding to dspacing of 3.25, 2.34, 2.24 and 1.83 Å respectively (JCPDS: 851331).
3.3. Photoelectrochemical studies All the PEC studies were performed under visible light illumination. The LSV plots (current versus potential, I vs V) curves are displayed in Fig. 3a. The electrolyte used for the experiment is 0.1 M of Na2SO4 and Na2SO3. In typically, presence of Na2SO3 in the electrolyte solution act as a hole scavenger, where SO32− is oxidized to SO42- that preventing the anodic photo-corrosion of Bi2S3, which significantly increase the amount of hydrogen evolution at the Pt counter electrode. The measured potential with reference Ag/AgCl was converted to RHE as shown in supporting information. Upon light on and off mode (chopped illumination), the observed photocurrent density for g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs nanocomposite is 0.74, 3.54 and 7.11 mA/cm2 at 1.23 V, respectively. However, all the studied photoelectrodes does not show photocurrent under dark conditions. g-C3N4/Bi2S3/BiNPs composite produced the highest photocurrent density, which is twotimes and six-times higher than for g-C3N4/Bi2S3 and g-C3N4, respectively. The g-C3N4/Bi2S3/BiNPs has the onset potential of 0.11 V, which is lower than pristine g-C3N4 (0.42 V). The decrement of onset potential and enhanced PEC performance of g-C3N4/Bi2S3/BiNPs for water splitting can be explained by ability of Bi2S3 as an efficient visible light absorber and BiNPs support increased charge carrier separation and mobility due to SPR effect. Solar-to-hydrogen conversion (STH) for gC3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs nanocomposite has been calculated using the equation (2) (SI), which was found to be 0.05, 1.23 and 3.51 % at 0.4 V respectively (Fig. 3b). This improved activity of gC3N4/Bi2S3/BiNPs suggests that chemical stability of BiNPs have been improved on g-C3N4/Bi2S3. The stability of the photoanodes was examined through chronoamperometric studies. The photocurrent for g-
3.2. Electron microscopy studies FE-SEM image of synthesized Bi nanoparticles displayed flower like morphology with average particle size is around few hundred nanometers (Fig. 2a). From Fig. 2b, g-C3N4 revealed irregular stacking flat sheets with aggregated morphologies. The g-C3N4/Bi2S3/BiNPs composite also displayed irregular shape with high roughness and porosity, where Bi2S3 QDs and Bi nanoparticles are embedded onto the g-C3N4 as shown in the Fig. 2c. Further, the composites have been quantitative analyzed by energy dispersive analysis of X-rays (EDAX). The constitute elements present in the samples are shown in Fig. 2d and this confirms that BiNPs are well formed in the composite. The SEM images of Bi2S3 and g-C3N4/Bi2S3 are provided in Fig. S1. Fig. 2a reveals an overview of the transmission electron microscopic image of g-C3N4/Bi2S3/BiNPs composite, where the grey color part of sheets was assigned to g-C3N4 and the dark particles were assigned to Bi2S3 and Bi nanoparticles, which were dispersed on the surface of g-C3N4. The diameter of nanoparticles such as Bi metal and Bi2S3 to be in the range of 10−15 nm 4
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maximum H2 evolution over 2 h. Thus, high photocurrent and H2 quantification details for g-C3N4/Bi2S3/BiNPs are in good agreement and which suggest that g-C3N4/Bi2S3/BiNPs is a promising material for PEC. The IPCE response of the pristine g-C3N4 and g-C3N4/Bi2S3/BiNPs photoelectrodes as shown in Fig. 4a. Pristine g-C3N4 and g-C3N4/Bi2S3/ BiNPs photoelectrodes reveals maximum IPCE values are 10.8 and 24.5 % at 360 nm, respectively. Impressively, the ternary nanocomposite exhibited higher IPCE than the pristine g-C3N4 electrode. The best performance of g-C3N4/Bi2S3/BiNPs concludes that synergetic effect of Bi2S3 QDs and BiNPs which inhibit the charge recombination and also facilitates higher charge injection in the composite. This is also supported by the maximum photocurrent at around 310 nm due to the formation of energetic hot electrons by plasmonic excitation of BiNPs. Therefore, it is realized that g-C3N4/Bi2S3/BiNPs composite enhance light harvesting and the overall current generation of the photoelectrode. In order to validate the ease of charge transfer at the electrodeelectrolyte interface, EIS experiments were performed. The Nyquist plots obtained for g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs photoanodes over the frequency range (1 kHz-1 Hz) under light irradiation are shown in Fig. 4b. In typical, large diameter of semicircle indicates higher Rct value at the electrode-electrolyte interface. From the Fig. 4b, g-C3N4 shows high Rct due to large diameter of semicircle that reflected low charge separation and weak carrier transportation which were the main reasons for its poor water oxidation performance. Impressively, gC3N4/Bi2S3/BiNPs exhibit low Rct (21 Ω) than other electrodes such as g-C3N4/Bi2S3 (46 Ω) and g-C3N4 (278 Ω), resulting indicates that the facile transfer of charge carriers that is reflected in its superior PEC performance. In simple terms, it could be explained as photogenerated electrons from photosensitizers (g-C3N4 and Bi2S3 QDs) can be effectively collected with the help of BiNPs and consolidated electrons are transferred to counter electrode. Therefore, g-C3N4/Bi2S3/BiNPs exhibit dominated PEC response. Furthermore, Mott-Schottky (M-S) analysis was used to study the conducting behavior of the as-prepared samples such as g-C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs under light illumination, at a constant ac frequency of 1 kHz and corresponding plots are shown in Fig. 4c. All g-C3N4 samples show the positive slopes, thus confirming that they are n-type semiconductors. By extrapolating the xintercept in M-S plots, the flat band potential (VFB) at the photoelectrodes was calculated. The VFB was found to be 0.408, 0.364 V and 0.301 versus RHE respectively for pristine g-C3N4, g-C3N4/Bi2S3 and gC3N4/Bi2S3/BiNPs composites. These results suggest that BiNPs act as extra charge carriers in the composites, influences the work function of the g-C3N4/Bi2S3/BiNPs composite and provides a facile charge transport at the electrode-electrolyte interface. Interestingly, the composite exhibited negative shift of VFB compared to that pristine g-C3N4, gC3N4/Bi2S3 photo-electrode, which implies that a better transport of photo-generated carriers in the composite, which reduces the charge recombination and thereby exhibited higher PEC performance for hydrogen evolution. Fig. S3 reveals the M-S plot of Bi2S3 film, which confirms that Bi2S3 has a n-type semiconductor with positive slope. The mechanistic details explaining the electron transfer process in gC3N4/Bi2S3/BiNPs composite photoelectrodes could be explained through schematic representation (Fig. 5). The Fermi level of BiNPs, VB and CB positions for g-C3N4 and Bi2S3 QDs are obtained from cyclic voltammetry (Fig. S2). The procedure for calculation of band positions in the energy level diagram is presented in (SI). Firstly, due to absorption of solar radiation, excitons are generated in Bi2S3 and g-C3N4. Originally, the photogenerated electrons are transferred from CB of Bi2S3 to CB of g-C3N4 due to the favorable band-edge positions between them as shown in Fig. 5 [20,30]. Furthermore, the surface plasmons is facilitated by BiNPs which enables the generated charge carriers are efficiently transferred to the contact FTO. The photogenerated holes created in the VB of Bi2S3 react with electrolyte to drive water oxidation. On the other hand, the photogenerated electrons are transferred to external circuit lead to HER at the counter electrode. Therefore, we
Fig. 4. (a) IPCE of pristine g-C3N4 and g-C3N4/Bi2S3/BiNPs photoelectrodes under monochromatic light radiation. (b) Nyquist plots, (c) M-S plots for gC3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs photoelectrodes.
C3N4, g-C3N4/Bi2S3 and g-C3N4/Bi2S3/BiNPs electrodes at 0.4 V vs RHE as a function of time is shown in Fig. 3c. Remarkable long term stability has been exhibited by g-C3N4/Bi2S3/BiNPs nanocomposite over 100 min highlighted its best performance. The current density value from LSV curve and chronoamperometric curves are consistent and gC3N4/Bi2S3/BiNPs composite shows higher current than pristine g-C3N4 and g-C3N4/Bi2S3. The H2 evolution is quantified using gas chromatography and the H2 evolved is plotted as a function of time at 1.23 V as shown in Fig. 3d. The g-C3N4/Bi2S3/BiNPs photoanode showed the 5
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Fig. 5. Schematic diagrams of energy alignment and charge transfer in g-C3N4/Bi2S3/BiNPs photoanode under solar light illumination.
References
could clearly say that the presence of BiNPs accelerates the overall transportation and efficiency of electron injection resulting into the enhanced PEC activity for hydrogen generation.
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4. Conclusions To summarize, g-C3N4/Bi2S3/BiNPs photoanode was fabricated and used for photoelectrochemical water splitting reaction. This ternary photoanode displayed the maximum photocurrent density of 7.11 mA/ cm2 at 1.23 V over g-C3N4 and g-C3N4/Bi2S3 with STH efficiency of 3.51 % at 0.4 V. The low charge transfer resistance and best IPCE for g-C3N4/ Bi2S3/BiNPs improves the charge separation and transportation. This enhanced PEC performance is due to the combined action of SPR property of BiNPs that improves light harvesting as well as charge carrier density and Bi2S3 QDs which increases the light absorption range and reduce the charge carrier recombination. This composite has potential for best PEC performance over expensive noble metal based photoelectrodes for hydrogen generation. Author contributions Palyam Subramanyam is the first author. He was planned to design the scheme, did work and finally wrote the manuscript. Bhagatram Meena helped to characterize the fabricated samples. Duvvuri Suryakala and Melepurath Deepa, both helped to check the typo errors and English grammar in the manuscript. Challapalli Subrahmanyam is the corresponding author. He was helped to design the work and wrote the manuscript. Declaration of Competing Interest There is no conflict of interest. Acknowledgements PS thanks CSIR, India for a senior research fellowship. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2020.01.041.
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