- Email: [email protected]
Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Decoration of plasmonic Cu nanoparticles on WO3/ Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting Palyam Subramanyam 1, Bhagatram Meena 1, Gudipati Neeraja Sinha, Melepurath Deepa, Challapalli Subrahmanyam* Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, 502285, Sangareddy, Telangana, India
article info
abstract
Article history:
We report a WO3/Cu/Bi2S3 wherein incorporation of Cu nanoparticles (Cu NPs) to enhance
Received 16 March 2019
the photoelectrochemical activity over WO3/Bi2S3. Cu NPs effectively harvest the light
Received in revised form
energy upon plasmon excitation and transfer the energy to contacted WO3, thereby
9 May 2019
improving the photoelectrochemical (PEC) performance. The WO3/Cu/Bi2S3 composite was
Accepted 20 May 2019
characterized by scanning electron microscopy (SEM), Transmission electron microscopy
Available online xxx
(TEM) and X-ray diffraction (XRD) to analyze the morphology and interfacial contact between the semiconductors. The photocurrent density and Solar-to-Hydrogen conversion
Keywords:
efficiency for this composite is 10.6 mA cm2 at 1.23 V (versus RHE) and 3.21% at 0.81 V
Photoelectrochemical cell
(versus RHE), which are much higher than WO3/Bi2S3 with 4.02 mA cm2 at 1.23 V (versus
Copper nanoparticles
RHE) and 2.46% at 0.81 V (versus RHE) respectively. Moreover, the stability and photo-
Bismuth sulfide
response of WO3/Cu/Bi2S3 were carried out through chronoamperometric studies. The
Surface plasmon resonance
composite retained its stability over 50 cycles without decay in PEC performance. High
Water splitting
incident photon conversion efficiency (IPCE) value of about 51% is achieved which is
Charge transportation
evident from the high photocurrent density. Incorporation of Cu NPs increase the photoactivity which is evident from the photocurrent value. The increased activity of Cu NPs sandwiched composite is attributed for the quick electron transfer to semiconductor due to surface plasmon resonance (SPR) effect. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photoelectrochemical (PEC) water splitting has received great attention as it is a promising route for solar-to-chemical energy conversion and can contribute as a solution to deal with the ever-growing global energy requirements [1e3]. Different metal oxide materials including TiO2, ZnO, WO3, a-Fe2O3 and
BiVO4 have been used for the development of photoanodes in PEC water splitting [4e9]. Among them, WO3 is attractive as a photoanode due to its unique optical and electrical properties such as its strong optical absorption, high electron mobility, moderate hole diffusion length, charge-carrier transport properties and good electrochemical stability [10e15]. However, the PEC performance of WO3-based photoanode is poor due to weak absorption in the visible region due to its wide
* Corresponding author. E-mail address: [email protected] (C. Subrahmanyam). 1 Contributed equally. https://doi.org/10.1016/j.ijhydene.2019.05.168 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
2
international journal of hydrogen energy xxx (xxxx) xxx
bandgap (2.6e3.0 eV), high charge recombination and poor charge transfer kinetics at the electrode/electrolyte interface. Many efforts were put forth to enhance the PEC performance of WO3 via heterojunction formation, metal/non-metal doping, co-catalyst loading and morphology control nanoarchitecture [16e21]. In the recent past, heterojunction formation has been extensively used to improve the PEC activity of WO3 for this approach can enhance its response to visible light and minimize the recombination of photo-generated charge carriers. Narrow bandgap semiconductors like Bi2S3 is used to enhance photo-activity [18,22,23]. For example, Wang et al., showed good activity for WO3/Bi2S3 with a current density of 5.95 mA cm2 at 0.9 V (vs RHE) which supports the injection of photogenerated electrons from Bi2S3 to WO3 [24]. Liu et al., have also reported WO3/Bi2S3 as a photoanode with different synthetic approach showing considerable current densities [25]. Bi2S3 nanobelt/WO3 nanoplate array composite reported by Liu et al. showed current density of 8.91 mA cm2 at 0.1 V vs. Ag/AgCl [26]. Also, photo-stability is another major issue. This can be addressed by introducing a plasmonic metal nanostructure [27e32]. These structures help for better utilization of solar energy to improve the overall efficiency of PEC. One of the main fundamental forms of surface plasmon resonance (SPR) is localized SPR. Surface plasmons (SPs) are coherent delocalized electron oscillations that occur at the interface of two materials. A SP is generated in the nanoparticle when light is incident on the material. The wavelength of light must be smaller or comparable to that of the nanoparticle so as to generate a plasmon. The frequency of the incident light and that of the surface free electrons in nanoparticles (NPs) meet the resonance condition thereby increasing the optical absorption of the nanoparticle. In an earlier study, plasmonic Cu nanoparticles have been attracted research interest due to their high conductive properties as well as excellent photo-response in near-infrared region (~800e900 nm) and superior photocatalytic performance. Moreover, it is a low cost effective material compared to Au and Ag nanoparticles and Cu NPs have been considered for hydrogen evolution reaction (HER) [33e35]. Recently, Zhang et al., reported plasmonic Cu NPs decorated with TiO2 nanotube arrays (TNAs) as photocatalyst, which exhibited high PEC performance as well as excellent chemical stability for HER under visible light than the pure TNAs [36]. Yang et al., fabricated sandwiched-Ag NPs based nanocomposite like ZnO/Ag/ CdS photo-electrode for PEC water splitting showed a photocurrent density of 4 mA cm2. This results suggest that Ag NPs act as electron transfer mediators which trigger the electron transfer chain reaction at the interface between the ZnO and CdS semiconductor nanostructures [37]. Liu et al., reported plasmonic Ag NPs decorated with WO3/CdS NRs as photoanode, which exhibited high PEC performance for hydrogen production than the pure WO3 and WO3/CdS films [38]. On considering the benefits of SPR effect of Cu NPs and Bi2S3 QDs, the optical absorption properties in the visible region and the effect of the same on the PEC activity of WO3 film is studied. The incorporation of Cu leads to generation of plasmons on its surface thereby, electron relays between Bi2S3 and WO3 thus increasing the photocurrent. In this work, we present low cost Cu nanostructures based WO3/Cu/Bi2S3 composite as photoanode for PEC water
splitting. Here, the WO3 and Cu NPs were prepared by hydrothermal process and Cu NPs were decorated via electrophoresis deposition. Further, Bi2S3 QDs layers deposited by successive ionic adsorption reaction (SILAR) process. In the work presented, case study and comparisons were provided for various heterojunctions of the synthesized composite for better PEC activity. Among them, the WO3/Cu/Bi2S3 showed maximum PEC activity which is consistent with the plasmonic behavior of Cu NPs that were incorporated.
Experimental section Chemicals Sodium tungsten dihydrate (Na2WO4.2H2O), sodium sulfide (Na2S), oxalic acid (H2C2O4) and bismuth nitrate trihydrate (Bi(NO3)3.3H2O) were purchased from Merck. Triton-X 100, hydrochloric acid (HCl), hydrazine hydrate (N2H4.H2O), ethylene diamine (C2N2H8) and sodium hydroxide (NaOH), copper nitrate (Cu(NO3)3) from sigma Aldrich. FTO (13 U/cm2) glass purchased from Aldrich. FTO glass pre-cleaned with 35% HCl solution, followed by distilled water and acetone.
Synthesis of copper nanoparticles (Cu NPs) Cu NPs were synthesized by hydrothermal process containing 7 M NaOH solution (25 mL), 0.1 M Cu (NO3)3.3H2O (0.15 mL), C2N2H8 (150 mL) as capping agent and N2H4.H2O (60 mL) as reducing agent [39]. To this solution, excess of NaOH solution was added to maintain a pH of 14. The reaction mixture was transferred to a Teflon lined autoclave, sealed and maintained at 150 C for 3 h. The product obtained was separated by centrifugation followed by drying in oven at 60 C to obtain Cu NPs that can be stored in dark for further use.
Synthesis of WO3 platelets WO3 platelets were synthesized via hydrothermal process. Briefly, Na2WO4.2H2O (0.25 g) was dissolved in 4 mL of deionized water, followed by addition of 0.7 mL conc. HCl under stirring. Further, 62 mg of H2C2O4 was added and continued stirring for 1 h. Then, a mixture of ethanol and water (30 mL in 16:14 ratio) was added to the reaction mixture. The contents are then transferred to a Teflon lined autoclave and maintained at 100 C for 16 h. The product was filtered and dried in oven at 60 C to obtain WO3.
Preparation of photoanodes Preparation of FTO/WO3/Bi2S3 photoanode The grinded powder of WO3 was coated on pre-cleaned FTO using acetyl acetone: water (1.5: 8.5 v/v) solution and 0.2 g of Triton X-100 as binder via doctor blade technique. The FTO/ WO3films were annealed at 70 C for 0.5 h followed by calcination at 450 C for 0.5 h. Further, Bi2S3 QDs were deposited on FTO/WO3 using SILAR method with 0.1 M Bi (NO3)3.3H2O in acetone and 0.1 M Na2S in methanol as precursor solutions
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
[40]. Totally, eight SILAR cycles were performed resulting in dark brown FTO/WO3/Bi2S3 films.
Preparation of Cu-based photoanodes Cu-based films such as WO3/Cu, WO3/Cu/Bi2S3 and WO3/Bi2S3/ Cu were prepared using electrophoretic deposition method. In the electrophoretic deposition, two electrode system is used with WO3 or WO3/Bi2S3 films and Pt rod as the working and counter electrode respectively while the solution of Cu NPs (10 ml THF) served as the electrolyte for Cu deposition [39]. Further, these plates are decorated with Bi2S3 QDs via SILAR process to form FTO/WO3/Cu/Bi2S3. The deposition was carried out at 30 V applying a dc voltage for 5 min. It could be explained that the Cu NPs were directed by the electric field through the pores of the WO3 or WO3/Bi2S3 assembly, resulting in brownish colored WO3/Cu and WO3/Bi2S3/Cu. This makes the strong contact of Cu NPs with the semiconductors. Further, these electrodes were washed in THF, dried in oven at 45 C.
Characterization UVeVis spectra for synthesized photoanodes recorded on Shimadzu UV-3600 instrument. Powder X-ray diffraction (XRD) patterns of samples were measured by PANalytical, XpertPRO instrument. Surface morphologies of the samples were measured using field emission scanning electron microscope (FESEM-Zeiss supra 40). High resolution transmission electron microscopy (HR-TEM) recorded for photoanodes using a TECNAI G-2 FEI (300 kV).
3
Photoelectrochemical measurement Linear sweep voltammetric (IeV) studies were performed on Autolab (LOT-Oriel) with a 150 W Xe arc lamp light source (100 mW cm2). Chronoamperometric (I-t) studies, electrochemical impedance spectroscopic (EIS) and Mott-Schottky studies were performed on an Autolab PGSTAT 302 N using NOVA 2.1 software. The samples were measured in aqueous electrolyte containing 1:1 mixture of 0.1 M Na2SO3 and Na2SO4.
Results and discussion The absorption spectra of the synthesized samples such as WO3, WO3/Cu, WO3/Bi2S3 and WO3/Cu/Bi2S3 are shown in Fig. 1. Cu NPs show a strong surface plasmon resonance peak centered at 580 nm as shown in Fig. 1a [41e44]. WO3 has a narrow absorption with an absorption edge at 450 nm and bandgap of 2.75 eV (Fig. 1b). While for the WO3/Cu sample, the absorption band edge was found to be at 510 nm, this signifies a red shift in the absorption range (Fig. 1b). A peak in the spectrum of WO3/Cu around 580 nm is attributed to the SPR mode of Cu NPs, thus indicating the symbiotic effect in increasing the photo-activity through coating of Cu NPs on WO3. Additionally, there is a decrease in the absorption for WO3/Cu as compared to WO3 alone which clearly states the presence of SPR effect of the Cu NPs in combination with WO3. To further increase the photo-response of WO3/Cu, it was decorated with Bi2S3 QDs. Bi2S3 QDs show absorption in the 300e800 nm with a bandgap of 1.55 eV (Fig. 1c). As compared to WO3/Bi2S3, Cu NPs embedded composite WO3/Cu/Bi2S3
Fig. 1 e Absorption spectra of the (a) Cu NPs (b) WO3 and WO3/Cu (c) Bi2S3 QDs, WO3/Bi2S3 and WO3/Cu/Bi2S3 samples (d) PL spectra of WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 at an excitation wavelength of 350 nm. Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e XRD patterns of the WO3, Cu NPs, Bi2S3 QDs and WO3/Cu/Bi2S3 samples.
showed enhanced light absorption. This indicates that Cu NPs form good interfacial contact with semiconductors to have efficient light harvesting [41,42,44].
The photoluminescence (PL) study for as-prepared samples such as WO3, WO3/Cu, WO3/Bi2S3 and WO3/Cu/Bi2S3 at an excitation wavelength of 350 nm and corresponding plots are shown in Fig. 1d. Typically, PL spectra is used to evaluate the separation ability of the photogenerated charge carriers where low PL emission intensity indicates less recombination of photogenerated electrons. The WO3, WO3/Cu, WO3/Bi2S3, WO3/Cu/Bi2S3 and WO3/Bi2S3/Cu samples also have a broad emission peaks at 426 nm. However, in Cu NPs and Bi2S3 QDs introduced electrodes namely, WO3/Cu/Bi2S3 and WO3/Bi2S3/ Cu heterojunctions, the PL intensity is quenched. Impressively, the WO3/Cu/Bi2S3 heterostructure exhibited lowest emission, which would suggest that it has the most efficient charge carrier separation among all electrodes and significantly inhibited the charge recombination in the heterojunction. This quenching in PL emission reveals excited state interaction between WO3 and Bi2S3 QDs where photogenerated electrons can be transferred from Bi2S3 QDs to WO3 via Cu NPs and this led to the increase in PEC performance of the ternary heterostructure. Fig. 2 shows the XRD patterns of WO3, Cu NPs Bi2S3 and WO3/Cu/Bi2S3. WO3 has a monoclinic crystal lattice (JCPDS830950) with XRD peaks of (002), (200), (112), (202), (122), (400) and (420) which are attributed to d ¼ 4.05, 3.67, 3.09, 2.62, 2.53, 1.83 and 1.65 A respectively. Cu NPs shows planes at (111), (200) and (220) of the face centered cubic lattice (JCPDS-892838) which corresponding to d ¼ 2.09, 1.82 and 1.28 Å. Bi2S3
Fig. 3 e FE-SEM images of the (a) Cu NNs, (b) WO3, (c) WO3/Cu and (d) WO3/Cu/Bi2S3samples.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e (aed) HR-TEM images of WO3/Cu/Bi2S3 composite.
oriented along (120), (220), (101), (310), (211), (221), (311), (240), (141), (421), (431), (251), (312), (242), (152), (721), (651) and (811) planes with an orthorhombic primitive crystal lattice (JCPDS170320) which attributed to d ¼ 5.03, 3.96, 3.56, 3.11, 2.81, 2.71, 2.52, 2.23, 2.11, 1.99, 1.95, 1.83, 1.73, 1.56, 1.48, 1.44, 1.39 and 1.28 A respectively. The XRD pattern of WO3/Cu/Bi2S3 shows the characteristic peaks of Cu NPs and Bi2S3 in blue and red respectively. This indicates the formation of the WO3/Cu/Bi2S3 composite.
The morphology of the samples were analyzed by FE-SEM and are displayed in Fig. 3. The synthesized Cu NPs have needle shaped morphology (Fig. 3a). WO3 formed appear as nanoplatelets with average diameter of 150e200 nm as shown in Fig. 3b. Fig. 3c displays the presence of Cu NPs on WO3 nanoplatelets. The composite WO3/Cu/Bi2S3 shows a combination of the individual morphologies of WO3/Cu and aggregate randomly with no specific shape displaying high roughness. FE-SEM images of WO3/Bi2S3, WO3/Bi2S3/Cu and Bi2S3 samples are shown in S1.
Fig. 5 e (a) LSV plots and (b) STH efficiency of pristine WO3 and WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes under solar radiation (intensity of 100 Wm-2). Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 6 e LSV of (a) pristine WO3 and WO3/Cu, (b) WO3/Bi2S3 (c) WO3/Bi2S3/Cu, and (d) WO3/Cu/Bi2S3 photoanodes under solar radiation exposure for 50 repeated cycles of operation.
The HR-TEM images of WO3/Cu/Bi2S3composite are shown in Fig. 4. The distance between the consecutive fringes in WO3, Cu NPs and Bi2S3 respectively are shown in the figure. The dspacing of 0.43 nm relates to (002) of monoclinic WO3 (JCPDS: 830950). The d-spacing of 0.42 nm is due to the (220) of orthorhombic Bi2S3 (JCPDS: 170320) and the d-spacing of 0.24 nm corresponds to the (200) of Cu (JCPDS: 892838). The TEM images of Bi2S3 QDs as shown in the Fig. S2a. Furthermore, the spots in the SAED pattern confirms the presence of WO3, Cu NPs and Bi2S3 in the WO3/Cu/Bi2S3 composite (Fig. S2b).
Photoelectrochemical studies The PEC activity of photo-electrodes were analyzed under dark and light conditions and the current-potential (IeV) behavior are presented in the Fig. 5a. The measurements were performed using a three electrode system where prepared composites, Pt and Ag/AgCl are working, counter and reference electrodes respectively. Equi-molar (0.1 M) mixture of Na2SO3 and Na2SO4 is used as the electrolyte. The following relation is used to convert the measured potential versus Ag/ AgCl into RHE:
ERHE ¼ EAg=AgCl þEAg=AgCl þ0:059pH
(1)
where standard electrode potential of Ag/AgCl (E Ag/AgCl)¼ 0.197 V at 25 C with electrolyte pH ¼ 12.7. Under dark conditions, the photoanodes: pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Cu/Bi2S3 and WO3/Bi2S3/Cu exhibit nearly negligible currents. Under illumination, the measured photocurrent densities for WO3, WO3/Cu, WO3/Bi2S3, WO3/Cu/ Bi2S3 and WO3/Bi2S3/Cu composite photoanodes are 0.92, 1.85, 4.02, 10.6 and 8.46 mA cm2 at 1.23 V versus RHE respectively. The same photoanodes exhibited an onset potentials of 0.32, 0.30, 0.29, 0.21 and 0.25 V versus RHE respectively. Among these photo-electrodes, sandwiched type WO3/Cu/Bi2S3 composite exhibited highest photocurrent density with a low onset potential as compared to the other photo-electrodes. This illustrates that incorporation of Cu NPs in WO3/Cu/Bi2S3 composite leads to high current at a 40 mV less potential than WO3/Bi2S3/Cu. The high photocurrent exhibited by WO3/Cu/ Bi2S3 indicates its ability to mitigate charge recombination. The sandwiched Cu NPs in WO3/Cu/Bi2S3 ternary composite showed superior PEC performance among the prepared photoanodes. Bi2S3 improves the charge separation and transport of electrons while Cu NPs due to SPR generated surface plasmons increase charge separation in Bi2S3, improve charge transfer and reduce the charge recombination rate. Compared to previous reports on WO3 and Bi2S3 based photo-electrodes, our reported composite exhibits higher photocurrent density for water oxidation reaction.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
The efficiency of the photo-electrodes for PEC watersplitting is calculated using the following equation: hð%Þ ¼ Jð1:23 VÞ=Plight
(2)
where J is photocurrent density at a measured potential, V is applied bias (versus RHE) and Plight is power density of the source (Plight ¼ 100 Wm-2). The evaluated STH (solar to hydrogen) conversion efficiencies of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 composite photo-electrodes are 0.14, 0.57, 1.23, 2.46 and 3.21% at 0.81 V versus RHE respectively as shown in Fig. 5b. As expected, the WO3/Cu/Bi2S3 photoanode achieved a maximum STH over other photo-electrodes. WO3/Cu/ Bi2S3 show enhanced efficiency over WO3/Bi2S3/Cu, thus indicating Cu NPs coated act as recombination sites for the carriers. The stability of prepared pristine WO3, WO3/Cu, WO3/ Bi2S3, WO3/Cu/Bi2S3 and WO3/Bi2S3/Cu nanostructured photo-electrodes were examined through linear sweep voltammetry (LSV) measurements under light illumination for 50 repeated cycles of operation (Fig. 6). Fig. 6a shows comparison between WO3 and WO3/Cu, WO3 shows very low photo-activity after 50 cycles while WO3/Cu showed great activity even -after 50 cycles. This indicates good interfacial contact and the ability of Cu NPs to enhance photo-activity. Fig. 6b shows activity of WO3/Bi2S3 for 1st and 50th cycle, indicating slight loss in photo-response due
7
to corrosion with electrolyte. Fig. 6c and d shows photoactivities of WO3/Bi2S3/Cu and WO3/Cu/Bi2S3, a significant difference is observed in the current densities after 50 cycles. Therefore, it is concluded that Cu plasmons increase the stability of the WO3/Bi2S3. Chronoamperometric (I-t curves) studies reveal instant photo-response and stability response of the photoanodes (Fig. 7). The photocurrent response of the anodes were tested under repeated on-off cycles at 1.23 V versus RHE. Under switch on/off, all the photo-electrodes exhibit an excellent photo-response. The values of photocurrent are consistent with that obtained from LSV. It reveals that the WO3/Cu/Bi2S3 composite electrode exhibited higher transient photocurrent than the other photo-electrodes. The stability for photoelectrodes was tested over a period of 5000 s under light illumination as shown in Fig. 7b. For pristine WO3 photoelectrode, photocurrent density decreases continuously with time, and a nearly 60% decrement was observed after 3200 s. For the WO3/Cu photo-electrode, no significant decrement of photocurrent density was observed, thus demonstrating superior stability of WO3/Cu photo-electrode due to SPR effect and chemical stability of Cu NPs in an aqueous medium. The WO3/Bi2S3 shows nearly 25% decay over 4500 s since sulfide based QDs are less stable in aqueous solution. Finally, Cubased composite photoanodes, WO3/Cu/Bi2S3 and WO3/Bi2S3/ Cu showed significant long-term stability without decay in performance. This suggests that decoration with Cu NPs can
Fig. 7 e (a) Transient photo-response curves and (b) stability curves of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes under solar radiation exposure at 1.23 V vs RHE (c) H2 evolution over prepared samples under solar light irradiation at 1.23 V vs RHE in 0.1 M of Na2SO4 and Na2SO3 solution during period time 2 h and (d) IPCE data of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes under monochromatic solar radiation. Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
8
international journal of hydrogen energy xxx (xxxx) xxx
enhance stability of the photo-electrode and reduce the onset potential as well as increase the photocurrent density. The hydrogen evolution activity of all photoelectrodes were performed as a function of time at 1.23 V vs RHE in 0.1 M of Na2SO4 and Na2SO3 solution and results are presented in the Fig. 7c. During the 2 h period, the hydrogen evolution of the WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes were 0.32, 0.61, 1.54, 2.09 and 2.36 mmol respectively. The above results confirms that the higher hydrogen evolution was achieved for WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes than other electrodes. The higher hydrogen evolution for WO3/Cu/Bi2S3 could be attributed to Cu NPs which enhanced the photocurrent and the solar to-hydrogen efficiency of the WO3-based photoanodes. The IPCE response of the fabricated photoelectrodes as shown in Fig. 7d are evident to state that WO3/Cu/Bi2S3 exhibited high photo conversion. For the WO3 photoanode, the maximum IPCE of 8.5% observed at 350 nm and photo-response observed up to 455 nm due to the wide bandgap (~2.7 eV). On introduction of Cu NPs on WO3, IPCE maximum improved from 8.5 to 13.1% at 350 nm with photoresponse extended up to 700 nm, which is due to SPR effect of Cu NPs that could be enhance the visible light absorption. For the WO3/Bi2S3 photo-electrode, the highest IPCE value is 21% in 350e750 nm region, the contribution of Bi2S3 QDs gives the additional enhancement. The highest IPCE of 51% is achieved at 350 nm with spectral range of UV-NIR region for WO3/Cu/Bi2S3 photoanode after decoration of Bi2S3 QDs. The high IPCE for WO3/Cu/Bi2S3 can be explained by the dual effect of Bi2S3 QDs that enhance the interfacial active sites due to high surface area and Cu NPs improving the light harvesting in the visible region due to the SPR effect. Usually, Cu NPs display SPR peak in the wavelength range of 550e650 nm. Upon excitation, the appearance of a plasmonic peak of Cu NPs at ~580 nm indicates the formation of plasmon induced photo-excitation which contributed to the enhanced photocurrent.
Electrical impedance spectroscopy (EIS) experiments were performed for the kinetic study of composites at photoanode/electrolyte interface. The photoanodes were tested in the electrolyte (0.1 M Na2SO3 and 0.1 Na2SO4 mixed solutions) under light illumination with frequency range of 10 KHz to 1 Hz. The Nyquist plots for WO3, WO3/ Cu, WO3/Bi2S3, WO3/Cu/Bi2S3, and WO3/Bi2S3/Cu composite photoanodes are shown in Fig. 8. The diameter of the semicircle represents charge transfer resistance (Rct) in the Nyquist plot. Lower the Rct, better the charge transfer process at the electrode-electrolyte interface. Lowest Rct is observed for WO3/Cu/Bi2S3 among the synthesized photoanodes (inset Figure). By incorporation of Cu NPs between the WO3 and Bi2S3, the Rct value was further lowered resulting in the enhancement of PEC performance compared to that of WO3/Bi2S3 photo-electrode. This suggests that the plasmons generated on Cu NPs facilities low charge transfer resistance which is also evident from high current density, leading to a significant enhancement in the photoelectrochemical performance for HER. The CV plots of WO3 and Bi2S3 and Cu NPs films are displayed in the supporting information (Fig. S3). The conduction band (CB) and valence band (VB) positions of WO3 are 3.69 eV and 6.44 eV at pH 12.7 respectively, which are calculated from the reduction and oxidation peak potentials. For Bi2S3, the CB and VB are 3.62 eV and 5.17 eV respectively. For Cu NPs, the work function or Fermi level (EF) is 4.32 eV which is obtained from the oxidation potential value (Fig. S3c). These values are obtained by the CV and optical studies and this values are used in the energy band diagram of the photoanode for PEC water splitting. The possible reason for the electron transfer process in Cu NPs-Bi2S3 QDs sensitized WO3 system could be explained via a schematic representation as shown in Fig. 9. The exposure of Bi2S3 to solar light generates excitons due to its high absorption in the visible region. The photogenerated electrons are introduced from the CB of Bi2S3 and are injected onto Cu, where the SPR effect results in efficient transfer of carriers to the CB of WO3, finally to the contact (FTO). While, the holes generated in the VB of Bi2S3 react with electrolyte and drive the water oxidation to produce oxygen gas and hydrogen ions.
Fig. 8 e Nyquist plots of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes at open circuit potential.
Fig. 9 e Schematic of energy alignment of and charge transfer in the WO3/Cu/Bi2S3 photoanode under solar light illumination.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
While the electrons transfer to counter electrode (Pt) via external circuit leads to hydrogen evolution. The Cu NPs embedded between Bi2S3 and WO3 facilitate efficient charge transfer and improves the PEC performance for hydrogen production.
[6]
[7]
Conclusions In summary, a WO3/Cu/Bi2S3 composite was synthesized via hydrothermal process, followed by electrophoresis and SILAR methods. The prepared composite served as photoanode for PEC water splitting with photocurrent density of 10.6 mA cm2 at 1.23 V (versus RHE) and solar to hydrogen conversion efficiency of 3.21% at 0.81 V (versus RHE), which is higher than WO3/Bi2S3, WO3/Bi2S3/Cu, WO3/Cu and pristine WO3 films. This improved photocurrent is mainly due to plasmons generated on the surface of Cu NPs that are in conjugation with WO3 and Bi2S3. The effect of Cu NPs is clearly visible in terms of photo-stability and low charge transfer resistance. Overall, this improved PEC performance of the composite is possibly due to high optical absorption by Bi2S3 while Cu NPs serving as electron relays from Bi2S3 to WO3. This synergic charge transfer explains the low charge recombination in the composite that led to the improvement in the number of photoexcited carriers for water splitting efficiency.
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgements PS thanks CSIR for a senior research fellowship. G.N.S thanks DST-Inspire (IF170949), New Delhi, India for the award of research fellowship.
[14]
Appendix A. Supplementary data
[15]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.05.168. [16]
references [17] [1] Chen X, Zhang Z, Chi L, Nair AK, Shangguan W, Jiang Z. Recent advances in visible-light-driven photoelectrochemical water splitting: catalyst nanostructures and reaction systems. Nano-Micro Lett 2016;8:1e12. [2] Sun J, Zhong DK, Gamelin DR. Composite photoanodes for photoelectrochemical solar water splittingEnergy. Environ Sci 2010;3:1252e61. [3] Li Y, Zhang JZ. Hydrogen generation from photoelectrochemical water splitting based on nanomaterials. Laser Photonics Rev 2010;4:517e28. [4] Cui H, Zhao W, Yang C, Yin H, Lin T, Shan Y, Xie Y, Gu H, Huang F. Black TiO2 nanotube arrays for high-efficiency photoelectrochemical water-splitting. J Mater Chem 2014;2:8612e6. [5] Wolcott A, Smith WA, Kuykendall TR, Zhao Y, Zhang JZ. Photoelectrochemical study of nanostructured ZnO thin
[18]
[19]
[20]
[21]
9
films for hydrogen generation from water splitting. Adv Funct Mater 2009;19:1849e56. Han H, Riboni F, Karlicky F, Kment S, Goswami A, Sudhagar P, Yoo J, Wang L, Tomanec O, Petr M, Haderka O. aFe2O3/TiO2 3D hierarchical nanostructures for enhanced photoelectrochemical water splitting. Nanoscale 2017;9:134e42. Feng X, Chen Y, Qin Z, Wang M, Guo L. Facile fabrication of sandwich structured WO3 nanoplate arrays for efficient photoelectrochemical water splitting. ACS Appl Mater Interfaces 2016;8:18089e96. Liu X, Wang F, Wang Q. Nanostructure-based WO3 photoanodes for photoelectrochemical water splitting. Phys Chem Chem Phys 2012;14:7894e911. Subramanyam P, Vinodkumar T, Nepak D, Deepa M, Subrahmanyam C. Mo-doped BiVO4@ reduced graphene oxide composite as an efficient photoanode for photoelectrochemical water splitting. Catal Today 2019;325:73e80. Zhou J, Lin S, Chen Y, Gaskov AM. Facile morphology control of WO3 nanostructure arrays with enhanced photoelectrochemical performance. Appl Surf Sci 2017;403:274e81. s L, Selim S, Sachs M, Blackman C, Kafizas A, Corby S, Franca Durrant JR. Water oxidation and electron extraction kinetics in nanostructured tungsten trioxide photoanodes. J Am Chem Soc 2018;140:16168e77. Nayak AK, Sohn Y, Pradhan D. Facile green synthesis of WO3$H2O nanoplates and WO3 nanowires with enhanced photoelectrochemical performance. Cryst Growth Des 2017;17:4949e57. Zhao Y, Balasubramanyam S, Sinha R, Lavrijsen R, Verheijen MA, Bol AA, Bieberle-Hutter A. Physical and chemical defects in WO3 thin films and their impact on photoelectrochemical water splitting. ACS Appl Energy Mater 2018;1:5887e95. Cai M, Fan P, Long J, Han J, Lin Y, Zhang H, Zhong M. Largescale tunable 3D self-supporting WO3 micro-nano architectures as direct photoanodes for efficient photoelectrochemical water splitting. ACS Appl Mater Interfaces 2017;9:17856e64. Song K, Gao F, Yang W, Wang E, Wang Z, Hou H. WO3 mesoporous nanobelts towards efficient photoelectrocatalysts for water splitting. ChemElectroChem 2018;5:322e7. Li L, Zhao X, Pan D, Li G. Nanotube array-like WO3/W photoanode fabricated by electrochemical anodization for photoelectrocatalytic overall water splitting. Chin J Catal 2017;38:2132e40. Xu S, Fu D, Song K, Wang L, Yang Z, Yang W, Hou H. Onedimensional WO3/BiVO4 heterojunction photoanodes for efficient photoelectrochemical water splitting. Chem Eng J 2018;349:368e75. Liu C, Yang Y, Li W, Li J, Li Y, Shi Q, Chen Q. Highly efficient photoelectrochemical hydrogen generation using ZnxBi2S3þx sensitized platelike WO3 photoelectrodes. ACS Appl Mater Interfaces 2015;7:10763e70. Xu Z, Li X, Li J, Wu L, Zeng Q, Zhou Z. Effect of CoOOH loading on the photoelectrocatalytic performance of WO3 nanorod array film. Appl Surf Sci 2013;284:285e90. Dong G, Zhang Y, Bi Y. The synergistic effect of Bi2WO6 nanoplates and Co3O4 cocatalysts for enhanced photoelectrochemical properties. J Mater Chem 2017;5:20594e7. Jeon D, Kim N, Bae S, Han Y, Ryu J. WO3/conducting polymer heterojunction photoanodes for efficient and stable photoelectrochemical water splitting. ACS Appl Mater Interfaces 2018;10:8036e44.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
10
international journal of hydrogen energy xxx (xxxx) xxx
[22] Park M, Seo JH, Kim JH, Park G, Park JY, Seo WS, Song H, Nam KM. Effective formation of WO3 nanoparticle/Bi2S3 nanowire composite for improved photoelectrochemical performance. J Phys Chem C 2018;122:17676e85. [23] Liu C, Yang Y, Li W, Li J, Li Y, Chen Q. Construction of novel Bi2S3 nanobelt@ WO3 nanoplate arrays on FTO glass with high photoelectrochemical activity. Int J 2016;41:5878e86. [24] Wang Y, Tian W, Chen L, Cao F, Guo J, Li L. Threedimensional WO3 Hydrog Energy nanoplate/Bi2S3 nanorod heterojunction as a highly efficient photoanode for improved photoelectrochemical water splitting. ACS Appl Mater Interfaces 2017;9:40235e43. [25] Liu C, Yang Y, Li W, Li J, Li Y, Chen Q. In situ synthesis of Bi2S3 sensitized WO3 nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performance. Sci Rep 2016;6:23451. [26] Liu C, Yang Y, Li W, Li J, Li Y, Chen Q. In situ synthesis of Bi2S3 sensitized WO3 nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performance. Sci Rep 2016;6:23451. [27] Hu D, Diao P, Xu D, Wu Q. Gold/WO3 nanocomposite photoanodes for plasmonic solar water splitting. Nano Res 2016;9:1735e51. [28] Naseri N, Qorbani M, Kim H, Choi W, Moshfegh AZ. To what extent can surface morphology influence the photoelectrochemical performance of Au:WO3 electrodes? J Phys Chem C 2015;119:1271e9. [29] Li H, Gao Y, Zhou Y, Fan F, Han Q, Xu Q, Wang X, Xiao M, Li C, Zou Z. Construction and nanoscale detection of interfacial charge transfer of elegant Z-scheme WO3/Au/In2S3 nanowire arrays. Nano Lett 2016;16:5547e52. [30] Kong Y, Sun H, Fan W, Wang L, Zhao H, Zhao X, Yuan S. Enhanced photoelectrochemical performance of tungsten oxide film by bifunctional Au nanoparticles. RSC Adv 2017;7:15201e10. [31] Karbalaei Akbari M, Hai Z, Wei Z, Detavernier C, Solano E, Verpoort F, Zhuiykov S. ALD-developed plasmonic twodimensional Au-WO3-TiO2 heterojunction architectonics for design of photovoltaic devices. ACS Appl Mater Interfaces 2018;10:10304e14. [32] Xu F, YaoY Bai D, Xu R, Mei J, Wu D, Gao Z, Jiang K. A significant cathodic shift in the onset potential and enhanced photoelectrochemical water splitting using Au nanoparticles decorated WO3 nanorod array. J Colloid Interface Sci 2015;458:194e9.
[33] Guo X, Hao C, Jin G, Zhu HY, Guo XY. Copper nanoparticles on graphene support: an efficient photocatalyst for coupling of nitroaromatics in visible light. Angew Chem Int Ed 2014;53:1973e7. [34] Zhang P, Song T, Wang T, Zeng H. In-situ synthesis of Cu nanoparticles hybridized with carbon quantum dots as a broad spectrum photocatalyst for improvement of photocatalytic H2 evolution. Appl Catal, B 2017;206:328e35. [35] Mahanti M, Basak D. Cu/ZnO nanorods' hybrid showing enhanced photoluminescence properties due to surface plasmon resonance. J Lumin 2014;145:19e24. [36] Zhang S, Peng B, Yang S, Wang H, Yu H, Fang Y, PengF. Nonnoble metal copper nanoparticles-decorated TiO2 nanotube arrays with plasmon-enhanced photocatalytic hydrogen evolution under visible light. Int J Hydrogen Energy 2015;40:303e10. [37] Yang X, Li H, Zhang W, Sun M, Li L, Xu N, Wu J, Sun J. High visible photoelectrochemical activity of Ag nanoparticlesandwiched CdS/Ag/ZnO nanorods. ACS Appl Mater Interfaces 2017;9:658e67. [38] Liu Z, Wu J, Zhang J. Quantum dots and plasmonic Ag decorated WO3 nanorod photoanodes with enhanced photoelectrochemical performances. Int J Hydrogen Energy 2016;41:20529e35. [39] Kumar PN, Deepa M, Ghosal P. Low-cost copper nanostructures impart high efficiencies to quantum dot solar cells. ACS Appl Mater Interfaces 2015;7:13303e13. [40] Subramanyam P, Kumar PN, Deepa M, Subrahmanyam C, Ghosal P. Bismuth sulfide nanocrystals and gold nanorods increase the photovoltaic response of a TiO2/CdS based cell. Sol Energy Mater Sol Cells 2017;159:296e306. [41] Xu X, Luo F, Tang W, Hu J, Zeng H, Zhou Y. Enriching hot electrons via NIR-photon-excited plasmon in WS2@Cu hybrids for full-spectrum solar hydrogen evolution. Adv Funct Mater 2018;28:1804055. [42] Bhardwaj S, Pal B. Photodeposition of Ag and Cu binary cocatalyst onto TiO2 for improved optical and photocatalytic degradation properties. Adv Powder Technol 2018;29:2119e28. [43] Bhavani K, Naresh G, Srinivas B, Venugopal A. Plasmonic resonance nature of Ag-Cu/TiO2 photocatalyst under solar and artificial light: synthesis, characterization and evaluation of H2O splitting activity. Appl Catal, B 2016;199:282e91. [44] Mahanti M, Basak D. Cu/ZnO nanorods' hybrid showing enhanced photoluminescence properties due to surface plasmon resonance. J Lumin 2014;145:19e24.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Decoration of plasmonic Cu nanoparticles on WO3/ Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting Palyam Subramanyam 1, Bhagatram Meena 1, Gudipati Neeraja Sinha, Melepurath Deepa, Challapalli Subrahmanyam* Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, 502285, Sangareddy, Telangana, India
article info
abstract
Article history:
We report a WO3/Cu/Bi2S3 wherein incorporation of Cu nanoparticles (Cu NPs) to enhance
Received 16 March 2019
the photoelectrochemical activity over WO3/Bi2S3. Cu NPs effectively harvest the light
Received in revised form
energy upon plasmon excitation and transfer the energy to contacted WO3, thereby
9 May 2019
improving the photoelectrochemical (PEC) performance. The WO3/Cu/Bi2S3 composite was
Accepted 20 May 2019
characterized by scanning electron microscopy (SEM), Transmission electron microscopy
Available online xxx
(TEM) and X-ray diffraction (XRD) to analyze the morphology and interfacial contact between the semiconductors. The photocurrent density and Solar-to-Hydrogen conversion
Keywords:
efficiency for this composite is 10.6 mA cm2 at 1.23 V (versus RHE) and 3.21% at 0.81 V
Photoelectrochemical cell
(versus RHE), which are much higher than WO3/Bi2S3 with 4.02 mA cm2 at 1.23 V (versus
Copper nanoparticles
RHE) and 2.46% at 0.81 V (versus RHE) respectively. Moreover, the stability and photo-
Bismuth sulfide
response of WO3/Cu/Bi2S3 were carried out through chronoamperometric studies. The
Surface plasmon resonance
composite retained its stability over 50 cycles without decay in PEC performance. High
Water splitting
incident photon conversion efficiency (IPCE) value of about 51% is achieved which is
Charge transportation
evident from the high photocurrent density. Incorporation of Cu NPs increase the photoactivity which is evident from the photocurrent value. The increased activity of Cu NPs sandwiched composite is attributed for the quick electron transfer to semiconductor due to surface plasmon resonance (SPR) effect. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photoelectrochemical (PEC) water splitting has received great attention as it is a promising route for solar-to-chemical energy conversion and can contribute as a solution to deal with the ever-growing global energy requirements [1e3]. Different metal oxide materials including TiO2, ZnO, WO3, a-Fe2O3 and
BiVO4 have been used for the development of photoanodes in PEC water splitting [4e9]. Among them, WO3 is attractive as a photoanode due to its unique optical and electrical properties such as its strong optical absorption, high electron mobility, moderate hole diffusion length, charge-carrier transport properties and good electrochemical stability [10e15]. However, the PEC performance of WO3-based photoanode is poor due to weak absorption in the visible region due to its wide
* Corresponding author. E-mail address: [email protected] (C. Subrahmanyam). 1 Contributed equally. https://doi.org/10.1016/j.ijhydene.2019.05.168 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
2
international journal of hydrogen energy xxx (xxxx) xxx
bandgap (2.6e3.0 eV), high charge recombination and poor charge transfer kinetics at the electrode/electrolyte interface. Many efforts were put forth to enhance the PEC performance of WO3 via heterojunction formation, metal/non-metal doping, co-catalyst loading and morphology control nanoarchitecture [16e21]. In the recent past, heterojunction formation has been extensively used to improve the PEC activity of WO3 for this approach can enhance its response to visible light and minimize the recombination of photo-generated charge carriers. Narrow bandgap semiconductors like Bi2S3 is used to enhance photo-activity [18,22,23]. For example, Wang et al., showed good activity for WO3/Bi2S3 with a current density of 5.95 mA cm2 at 0.9 V (vs RHE) which supports the injection of photogenerated electrons from Bi2S3 to WO3 [24]. Liu et al., have also reported WO3/Bi2S3 as a photoanode with different synthetic approach showing considerable current densities [25]. Bi2S3 nanobelt/WO3 nanoplate array composite reported by Liu et al. showed current density of 8.91 mA cm2 at 0.1 V vs. Ag/AgCl [26]. Also, photo-stability is another major issue. This can be addressed by introducing a plasmonic metal nanostructure [27e32]. These structures help for better utilization of solar energy to improve the overall efficiency of PEC. One of the main fundamental forms of surface plasmon resonance (SPR) is localized SPR. Surface plasmons (SPs) are coherent delocalized electron oscillations that occur at the interface of two materials. A SP is generated in the nanoparticle when light is incident on the material. The wavelength of light must be smaller or comparable to that of the nanoparticle so as to generate a plasmon. The frequency of the incident light and that of the surface free electrons in nanoparticles (NPs) meet the resonance condition thereby increasing the optical absorption of the nanoparticle. In an earlier study, plasmonic Cu nanoparticles have been attracted research interest due to their high conductive properties as well as excellent photo-response in near-infrared region (~800e900 nm) and superior photocatalytic performance. Moreover, it is a low cost effective material compared to Au and Ag nanoparticles and Cu NPs have been considered for hydrogen evolution reaction (HER) [33e35]. Recently, Zhang et al., reported plasmonic Cu NPs decorated with TiO2 nanotube arrays (TNAs) as photocatalyst, which exhibited high PEC performance as well as excellent chemical stability for HER under visible light than the pure TNAs [36]. Yang et al., fabricated sandwiched-Ag NPs based nanocomposite like ZnO/Ag/ CdS photo-electrode for PEC water splitting showed a photocurrent density of 4 mA cm2. This results suggest that Ag NPs act as electron transfer mediators which trigger the electron transfer chain reaction at the interface between the ZnO and CdS semiconductor nanostructures [37]. Liu et al., reported plasmonic Ag NPs decorated with WO3/CdS NRs as photoanode, which exhibited high PEC performance for hydrogen production than the pure WO3 and WO3/CdS films [38]. On considering the benefits of SPR effect of Cu NPs and Bi2S3 QDs, the optical absorption properties in the visible region and the effect of the same on the PEC activity of WO3 film is studied. The incorporation of Cu leads to generation of plasmons on its surface thereby, electron relays between Bi2S3 and WO3 thus increasing the photocurrent. In this work, we present low cost Cu nanostructures based WO3/Cu/Bi2S3 composite as photoanode for PEC water
splitting. Here, the WO3 and Cu NPs were prepared by hydrothermal process and Cu NPs were decorated via electrophoresis deposition. Further, Bi2S3 QDs layers deposited by successive ionic adsorption reaction (SILAR) process. In the work presented, case study and comparisons were provided for various heterojunctions of the synthesized composite for better PEC activity. Among them, the WO3/Cu/Bi2S3 showed maximum PEC activity which is consistent with the plasmonic behavior of Cu NPs that were incorporated.
Experimental section Chemicals Sodium tungsten dihydrate (Na2WO4.2H2O), sodium sulfide (Na2S), oxalic acid (H2C2O4) and bismuth nitrate trihydrate (Bi(NO3)3.3H2O) were purchased from Merck. Triton-X 100, hydrochloric acid (HCl), hydrazine hydrate (N2H4.H2O), ethylene diamine (C2N2H8) and sodium hydroxide (NaOH), copper nitrate (Cu(NO3)3) from sigma Aldrich. FTO (13 U/cm2) glass purchased from Aldrich. FTO glass pre-cleaned with 35% HCl solution, followed by distilled water and acetone.
Synthesis of copper nanoparticles (Cu NPs) Cu NPs were synthesized by hydrothermal process containing 7 M NaOH solution (25 mL), 0.1 M Cu (NO3)3.3H2O (0.15 mL), C2N2H8 (150 mL) as capping agent and N2H4.H2O (60 mL) as reducing agent [39]. To this solution, excess of NaOH solution was added to maintain a pH of 14. The reaction mixture was transferred to a Teflon lined autoclave, sealed and maintained at 150 C for 3 h. The product obtained was separated by centrifugation followed by drying in oven at 60 C to obtain Cu NPs that can be stored in dark for further use.
Synthesis of WO3 platelets WO3 platelets were synthesized via hydrothermal process. Briefly, Na2WO4.2H2O (0.25 g) was dissolved in 4 mL of deionized water, followed by addition of 0.7 mL conc. HCl under stirring. Further, 62 mg of H2C2O4 was added and continued stirring for 1 h. Then, a mixture of ethanol and water (30 mL in 16:14 ratio) was added to the reaction mixture. The contents are then transferred to a Teflon lined autoclave and maintained at 100 C for 16 h. The product was filtered and dried in oven at 60 C to obtain WO3.
Preparation of photoanodes Preparation of FTO/WO3/Bi2S3 photoanode The grinded powder of WO3 was coated on pre-cleaned FTO using acetyl acetone: water (1.5: 8.5 v/v) solution and 0.2 g of Triton X-100 as binder via doctor blade technique. The FTO/ WO3films were annealed at 70 C for 0.5 h followed by calcination at 450 C for 0.5 h. Further, Bi2S3 QDs were deposited on FTO/WO3 using SILAR method with 0.1 M Bi (NO3)3.3H2O in acetone and 0.1 M Na2S in methanol as precursor solutions
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
[40]. Totally, eight SILAR cycles were performed resulting in dark brown FTO/WO3/Bi2S3 films.
Preparation of Cu-based photoanodes Cu-based films such as WO3/Cu, WO3/Cu/Bi2S3 and WO3/Bi2S3/ Cu were prepared using electrophoretic deposition method. In the electrophoretic deposition, two electrode system is used with WO3 or WO3/Bi2S3 films and Pt rod as the working and counter electrode respectively while the solution of Cu NPs (10 ml THF) served as the electrolyte for Cu deposition [39]. Further, these plates are decorated with Bi2S3 QDs via SILAR process to form FTO/WO3/Cu/Bi2S3. The deposition was carried out at 30 V applying a dc voltage for 5 min. It could be explained that the Cu NPs were directed by the electric field through the pores of the WO3 or WO3/Bi2S3 assembly, resulting in brownish colored WO3/Cu and WO3/Bi2S3/Cu. This makes the strong contact of Cu NPs with the semiconductors. Further, these electrodes were washed in THF, dried in oven at 45 C.
Characterization UVeVis spectra for synthesized photoanodes recorded on Shimadzu UV-3600 instrument. Powder X-ray diffraction (XRD) patterns of samples were measured by PANalytical, XpertPRO instrument. Surface morphologies of the samples were measured using field emission scanning electron microscope (FESEM-Zeiss supra 40). High resolution transmission electron microscopy (HR-TEM) recorded for photoanodes using a TECNAI G-2 FEI (300 kV).
3
Photoelectrochemical measurement Linear sweep voltammetric (IeV) studies were performed on Autolab (LOT-Oriel) with a 150 W Xe arc lamp light source (100 mW cm2). Chronoamperometric (I-t) studies, electrochemical impedance spectroscopic (EIS) and Mott-Schottky studies were performed on an Autolab PGSTAT 302 N using NOVA 2.1 software. The samples were measured in aqueous electrolyte containing 1:1 mixture of 0.1 M Na2SO3 and Na2SO4.
Results and discussion The absorption spectra of the synthesized samples such as WO3, WO3/Cu, WO3/Bi2S3 and WO3/Cu/Bi2S3 are shown in Fig. 1. Cu NPs show a strong surface plasmon resonance peak centered at 580 nm as shown in Fig. 1a [41e44]. WO3 has a narrow absorption with an absorption edge at 450 nm and bandgap of 2.75 eV (Fig. 1b). While for the WO3/Cu sample, the absorption band edge was found to be at 510 nm, this signifies a red shift in the absorption range (Fig. 1b). A peak in the spectrum of WO3/Cu around 580 nm is attributed to the SPR mode of Cu NPs, thus indicating the symbiotic effect in increasing the photo-activity through coating of Cu NPs on WO3. Additionally, there is a decrease in the absorption for WO3/Cu as compared to WO3 alone which clearly states the presence of SPR effect of the Cu NPs in combination with WO3. To further increase the photo-response of WO3/Cu, it was decorated with Bi2S3 QDs. Bi2S3 QDs show absorption in the 300e800 nm with a bandgap of 1.55 eV (Fig. 1c). As compared to WO3/Bi2S3, Cu NPs embedded composite WO3/Cu/Bi2S3
Fig. 1 e Absorption spectra of the (a) Cu NPs (b) WO3 and WO3/Cu (c) Bi2S3 QDs, WO3/Bi2S3 and WO3/Cu/Bi2S3 samples (d) PL spectra of WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 at an excitation wavelength of 350 nm. Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e XRD patterns of the WO3, Cu NPs, Bi2S3 QDs and WO3/Cu/Bi2S3 samples.
showed enhanced light absorption. This indicates that Cu NPs form good interfacial contact with semiconductors to have efficient light harvesting [41,42,44].
The photoluminescence (PL) study for as-prepared samples such as WO3, WO3/Cu, WO3/Bi2S3 and WO3/Cu/Bi2S3 at an excitation wavelength of 350 nm and corresponding plots are shown in Fig. 1d. Typically, PL spectra is used to evaluate the separation ability of the photogenerated charge carriers where low PL emission intensity indicates less recombination of photogenerated electrons. The WO3, WO3/Cu, WO3/Bi2S3, WO3/Cu/Bi2S3 and WO3/Bi2S3/Cu samples also have a broad emission peaks at 426 nm. However, in Cu NPs and Bi2S3 QDs introduced electrodes namely, WO3/Cu/Bi2S3 and WO3/Bi2S3/ Cu heterojunctions, the PL intensity is quenched. Impressively, the WO3/Cu/Bi2S3 heterostructure exhibited lowest emission, which would suggest that it has the most efficient charge carrier separation among all electrodes and significantly inhibited the charge recombination in the heterojunction. This quenching in PL emission reveals excited state interaction between WO3 and Bi2S3 QDs where photogenerated electrons can be transferred from Bi2S3 QDs to WO3 via Cu NPs and this led to the increase in PEC performance of the ternary heterostructure. Fig. 2 shows the XRD patterns of WO3, Cu NPs Bi2S3 and WO3/Cu/Bi2S3. WO3 has a monoclinic crystal lattice (JCPDS830950) with XRD peaks of (002), (200), (112), (202), (122), (400) and (420) which are attributed to d ¼ 4.05, 3.67, 3.09, 2.62, 2.53, 1.83 and 1.65 A respectively. Cu NPs shows planes at (111), (200) and (220) of the face centered cubic lattice (JCPDS-892838) which corresponding to d ¼ 2.09, 1.82 and 1.28 Å. Bi2S3
Fig. 3 e FE-SEM images of the (a) Cu NNs, (b) WO3, (c) WO3/Cu and (d) WO3/Cu/Bi2S3samples.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e (aed) HR-TEM images of WO3/Cu/Bi2S3 composite.
oriented along (120), (220), (101), (310), (211), (221), (311), (240), (141), (421), (431), (251), (312), (242), (152), (721), (651) and (811) planes with an orthorhombic primitive crystal lattice (JCPDS170320) which attributed to d ¼ 5.03, 3.96, 3.56, 3.11, 2.81, 2.71, 2.52, 2.23, 2.11, 1.99, 1.95, 1.83, 1.73, 1.56, 1.48, 1.44, 1.39 and 1.28 A respectively. The XRD pattern of WO3/Cu/Bi2S3 shows the characteristic peaks of Cu NPs and Bi2S3 in blue and red respectively. This indicates the formation of the WO3/Cu/Bi2S3 composite.
The morphology of the samples were analyzed by FE-SEM and are displayed in Fig. 3. The synthesized Cu NPs have needle shaped morphology (Fig. 3a). WO3 formed appear as nanoplatelets with average diameter of 150e200 nm as shown in Fig. 3b. Fig. 3c displays the presence of Cu NPs on WO3 nanoplatelets. The composite WO3/Cu/Bi2S3 shows a combination of the individual morphologies of WO3/Cu and aggregate randomly with no specific shape displaying high roughness. FE-SEM images of WO3/Bi2S3, WO3/Bi2S3/Cu and Bi2S3 samples are shown in S1.
Fig. 5 e (a) LSV plots and (b) STH efficiency of pristine WO3 and WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes under solar radiation (intensity of 100 Wm-2). Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 6 e LSV of (a) pristine WO3 and WO3/Cu, (b) WO3/Bi2S3 (c) WO3/Bi2S3/Cu, and (d) WO3/Cu/Bi2S3 photoanodes under solar radiation exposure for 50 repeated cycles of operation.
The HR-TEM images of WO3/Cu/Bi2S3composite are shown in Fig. 4. The distance between the consecutive fringes in WO3, Cu NPs and Bi2S3 respectively are shown in the figure. The dspacing of 0.43 nm relates to (002) of monoclinic WO3 (JCPDS: 830950). The d-spacing of 0.42 nm is due to the (220) of orthorhombic Bi2S3 (JCPDS: 170320) and the d-spacing of 0.24 nm corresponds to the (200) of Cu (JCPDS: 892838). The TEM images of Bi2S3 QDs as shown in the Fig. S2a. Furthermore, the spots in the SAED pattern confirms the presence of WO3, Cu NPs and Bi2S3 in the WO3/Cu/Bi2S3 composite (Fig. S2b).
Photoelectrochemical studies The PEC activity of photo-electrodes were analyzed under dark and light conditions and the current-potential (IeV) behavior are presented in the Fig. 5a. The measurements were performed using a three electrode system where prepared composites, Pt and Ag/AgCl are working, counter and reference electrodes respectively. Equi-molar (0.1 M) mixture of Na2SO3 and Na2SO4 is used as the electrolyte. The following relation is used to convert the measured potential versus Ag/ AgCl into RHE:
ERHE ¼ EAg=AgCl þEAg=AgCl þ0:059pH
(1)
where standard electrode potential of Ag/AgCl (E Ag/AgCl)¼ 0.197 V at 25 C with electrolyte pH ¼ 12.7. Under dark conditions, the photoanodes: pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Cu/Bi2S3 and WO3/Bi2S3/Cu exhibit nearly negligible currents. Under illumination, the measured photocurrent densities for WO3, WO3/Cu, WO3/Bi2S3, WO3/Cu/ Bi2S3 and WO3/Bi2S3/Cu composite photoanodes are 0.92, 1.85, 4.02, 10.6 and 8.46 mA cm2 at 1.23 V versus RHE respectively. The same photoanodes exhibited an onset potentials of 0.32, 0.30, 0.29, 0.21 and 0.25 V versus RHE respectively. Among these photo-electrodes, sandwiched type WO3/Cu/Bi2S3 composite exhibited highest photocurrent density with a low onset potential as compared to the other photo-electrodes. This illustrates that incorporation of Cu NPs in WO3/Cu/Bi2S3 composite leads to high current at a 40 mV less potential than WO3/Bi2S3/Cu. The high photocurrent exhibited by WO3/Cu/ Bi2S3 indicates its ability to mitigate charge recombination. The sandwiched Cu NPs in WO3/Cu/Bi2S3 ternary composite showed superior PEC performance among the prepared photoanodes. Bi2S3 improves the charge separation and transport of electrons while Cu NPs due to SPR generated surface plasmons increase charge separation in Bi2S3, improve charge transfer and reduce the charge recombination rate. Compared to previous reports on WO3 and Bi2S3 based photo-electrodes, our reported composite exhibits higher photocurrent density for water oxidation reaction.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
The efficiency of the photo-electrodes for PEC watersplitting is calculated using the following equation: hð%Þ ¼ Jð1:23 VÞ=Plight
(2)
where J is photocurrent density at a measured potential, V is applied bias (versus RHE) and Plight is power density of the source (Plight ¼ 100 Wm-2). The evaluated STH (solar to hydrogen) conversion efficiencies of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 composite photo-electrodes are 0.14, 0.57, 1.23, 2.46 and 3.21% at 0.81 V versus RHE respectively as shown in Fig. 5b. As expected, the WO3/Cu/Bi2S3 photoanode achieved a maximum STH over other photo-electrodes. WO3/Cu/ Bi2S3 show enhanced efficiency over WO3/Bi2S3/Cu, thus indicating Cu NPs coated act as recombination sites for the carriers. The stability of prepared pristine WO3, WO3/Cu, WO3/ Bi2S3, WO3/Cu/Bi2S3 and WO3/Bi2S3/Cu nanostructured photo-electrodes were examined through linear sweep voltammetry (LSV) measurements under light illumination for 50 repeated cycles of operation (Fig. 6). Fig. 6a shows comparison between WO3 and WO3/Cu, WO3 shows very low photo-activity after 50 cycles while WO3/Cu showed great activity even -after 50 cycles. This indicates good interfacial contact and the ability of Cu NPs to enhance photo-activity. Fig. 6b shows activity of WO3/Bi2S3 for 1st and 50th cycle, indicating slight loss in photo-response due
7
to corrosion with electrolyte. Fig. 6c and d shows photoactivities of WO3/Bi2S3/Cu and WO3/Cu/Bi2S3, a significant difference is observed in the current densities after 50 cycles. Therefore, it is concluded that Cu plasmons increase the stability of the WO3/Bi2S3. Chronoamperometric (I-t curves) studies reveal instant photo-response and stability response of the photoanodes (Fig. 7). The photocurrent response of the anodes were tested under repeated on-off cycles at 1.23 V versus RHE. Under switch on/off, all the photo-electrodes exhibit an excellent photo-response. The values of photocurrent are consistent with that obtained from LSV. It reveals that the WO3/Cu/Bi2S3 composite electrode exhibited higher transient photocurrent than the other photo-electrodes. The stability for photoelectrodes was tested over a period of 5000 s under light illumination as shown in Fig. 7b. For pristine WO3 photoelectrode, photocurrent density decreases continuously with time, and a nearly 60% decrement was observed after 3200 s. For the WO3/Cu photo-electrode, no significant decrement of photocurrent density was observed, thus demonstrating superior stability of WO3/Cu photo-electrode due to SPR effect and chemical stability of Cu NPs in an aqueous medium. The WO3/Bi2S3 shows nearly 25% decay over 4500 s since sulfide based QDs are less stable in aqueous solution. Finally, Cubased composite photoanodes, WO3/Cu/Bi2S3 and WO3/Bi2S3/ Cu showed significant long-term stability without decay in performance. This suggests that decoration with Cu NPs can
Fig. 7 e (a) Transient photo-response curves and (b) stability curves of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes under solar radiation exposure at 1.23 V vs RHE (c) H2 evolution over prepared samples under solar light irradiation at 1.23 V vs RHE in 0.1 M of Na2SO4 and Na2SO3 solution during period time 2 h and (d) IPCE data of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes under monochromatic solar radiation. Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
8
international journal of hydrogen energy xxx (xxxx) xxx
enhance stability of the photo-electrode and reduce the onset potential as well as increase the photocurrent density. The hydrogen evolution activity of all photoelectrodes were performed as a function of time at 1.23 V vs RHE in 0.1 M of Na2SO4 and Na2SO3 solution and results are presented in the Fig. 7c. During the 2 h period, the hydrogen evolution of the WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes were 0.32, 0.61, 1.54, 2.09 and 2.36 mmol respectively. The above results confirms that the higher hydrogen evolution was achieved for WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes than other electrodes. The higher hydrogen evolution for WO3/Cu/Bi2S3 could be attributed to Cu NPs which enhanced the photocurrent and the solar to-hydrogen efficiency of the WO3-based photoanodes. The IPCE response of the fabricated photoelectrodes as shown in Fig. 7d are evident to state that WO3/Cu/Bi2S3 exhibited high photo conversion. For the WO3 photoanode, the maximum IPCE of 8.5% observed at 350 nm and photo-response observed up to 455 nm due to the wide bandgap (~2.7 eV). On introduction of Cu NPs on WO3, IPCE maximum improved from 8.5 to 13.1% at 350 nm with photoresponse extended up to 700 nm, which is due to SPR effect of Cu NPs that could be enhance the visible light absorption. For the WO3/Bi2S3 photo-electrode, the highest IPCE value is 21% in 350e750 nm region, the contribution of Bi2S3 QDs gives the additional enhancement. The highest IPCE of 51% is achieved at 350 nm with spectral range of UV-NIR region for WO3/Cu/Bi2S3 photoanode after decoration of Bi2S3 QDs. The high IPCE for WO3/Cu/Bi2S3 can be explained by the dual effect of Bi2S3 QDs that enhance the interfacial active sites due to high surface area and Cu NPs improving the light harvesting in the visible region due to the SPR effect. Usually, Cu NPs display SPR peak in the wavelength range of 550e650 nm. Upon excitation, the appearance of a plasmonic peak of Cu NPs at ~580 nm indicates the formation of plasmon induced photo-excitation which contributed to the enhanced photocurrent.
Electrical impedance spectroscopy (EIS) experiments were performed for the kinetic study of composites at photoanode/electrolyte interface. The photoanodes were tested in the electrolyte (0.1 M Na2SO3 and 0.1 Na2SO4 mixed solutions) under light illumination with frequency range of 10 KHz to 1 Hz. The Nyquist plots for WO3, WO3/ Cu, WO3/Bi2S3, WO3/Cu/Bi2S3, and WO3/Bi2S3/Cu composite photoanodes are shown in Fig. 8. The diameter of the semicircle represents charge transfer resistance (Rct) in the Nyquist plot. Lower the Rct, better the charge transfer process at the electrode-electrolyte interface. Lowest Rct is observed for WO3/Cu/Bi2S3 among the synthesized photoanodes (inset Figure). By incorporation of Cu NPs between the WO3 and Bi2S3, the Rct value was further lowered resulting in the enhancement of PEC performance compared to that of WO3/Bi2S3 photo-electrode. This suggests that the plasmons generated on Cu NPs facilities low charge transfer resistance which is also evident from high current density, leading to a significant enhancement in the photoelectrochemical performance for HER. The CV plots of WO3 and Bi2S3 and Cu NPs films are displayed in the supporting information (Fig. S3). The conduction band (CB) and valence band (VB) positions of WO3 are 3.69 eV and 6.44 eV at pH 12.7 respectively, which are calculated from the reduction and oxidation peak potentials. For Bi2S3, the CB and VB are 3.62 eV and 5.17 eV respectively. For Cu NPs, the work function or Fermi level (EF) is 4.32 eV which is obtained from the oxidation potential value (Fig. S3c). These values are obtained by the CV and optical studies and this values are used in the energy band diagram of the photoanode for PEC water splitting. The possible reason for the electron transfer process in Cu NPs-Bi2S3 QDs sensitized WO3 system could be explained via a schematic representation as shown in Fig. 9. The exposure of Bi2S3 to solar light generates excitons due to its high absorption in the visible region. The photogenerated electrons are introduced from the CB of Bi2S3 and are injected onto Cu, where the SPR effect results in efficient transfer of carriers to the CB of WO3, finally to the contact (FTO). While, the holes generated in the VB of Bi2S3 react with electrolyte and drive the water oxidation to produce oxygen gas and hydrogen ions.
Fig. 8 e Nyquist plots of pristine WO3, WO3/Cu, WO3/Bi2S3, WO3/Bi2S3/Cu and WO3/Cu/Bi2S3 photoanodes at open circuit potential.
Fig. 9 e Schematic of energy alignment of and charge transfer in the WO3/Cu/Bi2S3 photoanode under solar light illumination.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
international journal of hydrogen energy xxx (xxxx) xxx
While the electrons transfer to counter electrode (Pt) via external circuit leads to hydrogen evolution. The Cu NPs embedded between Bi2S3 and WO3 facilitate efficient charge transfer and improves the PEC performance for hydrogen production.
[6]
[7]
Conclusions In summary, a WO3/Cu/Bi2S3 composite was synthesized via hydrothermal process, followed by electrophoresis and SILAR methods. The prepared composite served as photoanode for PEC water splitting with photocurrent density of 10.6 mA cm2 at 1.23 V (versus RHE) and solar to hydrogen conversion efficiency of 3.21% at 0.81 V (versus RHE), which is higher than WO3/Bi2S3, WO3/Bi2S3/Cu, WO3/Cu and pristine WO3 films. This improved photocurrent is mainly due to plasmons generated on the surface of Cu NPs that are in conjugation with WO3 and Bi2S3. The effect of Cu NPs is clearly visible in terms of photo-stability and low charge transfer resistance. Overall, this improved PEC performance of the composite is possibly due to high optical absorption by Bi2S3 while Cu NPs serving as electron relays from Bi2S3 to WO3. This synergic charge transfer explains the low charge recombination in the composite that led to the improvement in the number of photoexcited carriers for water splitting efficiency.
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgements PS thanks CSIR for a senior research fellowship. G.N.S thanks DST-Inspire (IF170949), New Delhi, India for the award of research fellowship.
[14]
Appendix A. Supplementary data
[15]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.05.168. [16]
references [17] [1] Chen X, Zhang Z, Chi L, Nair AK, Shangguan W, Jiang Z. Recent advances in visible-light-driven photoelectrochemical water splitting: catalyst nanostructures and reaction systems. Nano-Micro Lett 2016;8:1e12. [2] Sun J, Zhong DK, Gamelin DR. Composite photoanodes for photoelectrochemical solar water splittingEnergy. Environ Sci 2010;3:1252e61. [3] Li Y, Zhang JZ. Hydrogen generation from photoelectrochemical water splitting based on nanomaterials. Laser Photonics Rev 2010;4:517e28. [4] Cui H, Zhao W, Yang C, Yin H, Lin T, Shan Y, Xie Y, Gu H, Huang F. Black TiO2 nanotube arrays for high-efficiency photoelectrochemical water-splitting. J Mater Chem 2014;2:8612e6. [5] Wolcott A, Smith WA, Kuykendall TR, Zhao Y, Zhang JZ. Photoelectrochemical study of nanostructured ZnO thin
[18]
[19]
[20]
[21]
9
films for hydrogen generation from water splitting. Adv Funct Mater 2009;19:1849e56. Han H, Riboni F, Karlicky F, Kment S, Goswami A, Sudhagar P, Yoo J, Wang L, Tomanec O, Petr M, Haderka O. aFe2O3/TiO2 3D hierarchical nanostructures for enhanced photoelectrochemical water splitting. Nanoscale 2017;9:134e42. Feng X, Chen Y, Qin Z, Wang M, Guo L. Facile fabrication of sandwich structured WO3 nanoplate arrays for efficient photoelectrochemical water splitting. ACS Appl Mater Interfaces 2016;8:18089e96. Liu X, Wang F, Wang Q. Nanostructure-based WO3 photoanodes for photoelectrochemical water splitting. Phys Chem Chem Phys 2012;14:7894e911. Subramanyam P, Vinodkumar T, Nepak D, Deepa M, Subrahmanyam C. Mo-doped BiVO4@ reduced graphene oxide composite as an efficient photoanode for photoelectrochemical water splitting. Catal Today 2019;325:73e80. Zhou J, Lin S, Chen Y, Gaskov AM. Facile morphology control of WO3 nanostructure arrays with enhanced photoelectrochemical performance. Appl Surf Sci 2017;403:274e81. s L, Selim S, Sachs M, Blackman C, Kafizas A, Corby S, Franca Durrant JR. Water oxidation and electron extraction kinetics in nanostructured tungsten trioxide photoanodes. J Am Chem Soc 2018;140:16168e77. Nayak AK, Sohn Y, Pradhan D. Facile green synthesis of WO3$H2O nanoplates and WO3 nanowires with enhanced photoelectrochemical performance. Cryst Growth Des 2017;17:4949e57. Zhao Y, Balasubramanyam S, Sinha R, Lavrijsen R, Verheijen MA, Bol AA, Bieberle-Hutter A. Physical and chemical defects in WO3 thin films and their impact on photoelectrochemical water splitting. ACS Appl Energy Mater 2018;1:5887e95. Cai M, Fan P, Long J, Han J, Lin Y, Zhang H, Zhong M. Largescale tunable 3D self-supporting WO3 micro-nano architectures as direct photoanodes for efficient photoelectrochemical water splitting. ACS Appl Mater Interfaces 2017;9:17856e64. Song K, Gao F, Yang W, Wang E, Wang Z, Hou H. WO3 mesoporous nanobelts towards efficient photoelectrocatalysts for water splitting. ChemElectroChem 2018;5:322e7. Li L, Zhao X, Pan D, Li G. Nanotube array-like WO3/W photoanode fabricated by electrochemical anodization for photoelectrocatalytic overall water splitting. Chin J Catal 2017;38:2132e40. Xu S, Fu D, Song K, Wang L, Yang Z, Yang W, Hou H. Onedimensional WO3/BiVO4 heterojunction photoanodes for efficient photoelectrochemical water splitting. Chem Eng J 2018;349:368e75. Liu C, Yang Y, Li W, Li J, Li Y, Shi Q, Chen Q. Highly efficient photoelectrochemical hydrogen generation using ZnxBi2S3þx sensitized platelike WO3 photoelectrodes. ACS Appl Mater Interfaces 2015;7:10763e70. Xu Z, Li X, Li J, Wu L, Zeng Q, Zhou Z. Effect of CoOOH loading on the photoelectrocatalytic performance of WO3 nanorod array film. Appl Surf Sci 2013;284:285e90. Dong G, Zhang Y, Bi Y. The synergistic effect of Bi2WO6 nanoplates and Co3O4 cocatalysts for enhanced photoelectrochemical properties. J Mater Chem 2017;5:20594e7. Jeon D, Kim N, Bae S, Han Y, Ryu J. WO3/conducting polymer heterojunction photoanodes for efficient and stable photoelectrochemical water splitting. ACS Appl Mater Interfaces 2018;10:8036e44.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168
10
international journal of hydrogen energy xxx (xxxx) xxx
[22] Park M, Seo JH, Kim JH, Park G, Park JY, Seo WS, Song H, Nam KM. Effective formation of WO3 nanoparticle/Bi2S3 nanowire composite for improved photoelectrochemical performance. J Phys Chem C 2018;122:17676e85. [23] Liu C, Yang Y, Li W, Li J, Li Y, Chen Q. Construction of novel Bi2S3 nanobelt@ WO3 nanoplate arrays on FTO glass with high photoelectrochemical activity. Int J 2016;41:5878e86. [24] Wang Y, Tian W, Chen L, Cao F, Guo J, Li L. Threedimensional WO3 Hydrog Energy nanoplate/Bi2S3 nanorod heterojunction as a highly efficient photoanode for improved photoelectrochemical water splitting. ACS Appl Mater Interfaces 2017;9:40235e43. [25] Liu C, Yang Y, Li W, Li J, Li Y, Chen Q. In situ synthesis of Bi2S3 sensitized WO3 nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performance. Sci Rep 2016;6:23451. [26] Liu C, Yang Y, Li W, Li J, Li Y, Chen Q. In situ synthesis of Bi2S3 sensitized WO3 nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performance. Sci Rep 2016;6:23451. [27] Hu D, Diao P, Xu D, Wu Q. Gold/WO3 nanocomposite photoanodes for plasmonic solar water splitting. Nano Res 2016;9:1735e51. [28] Naseri N, Qorbani M, Kim H, Choi W, Moshfegh AZ. To what extent can surface morphology influence the photoelectrochemical performance of Au:WO3 electrodes? J Phys Chem C 2015;119:1271e9. [29] Li H, Gao Y, Zhou Y, Fan F, Han Q, Xu Q, Wang X, Xiao M, Li C, Zou Z. Construction and nanoscale detection of interfacial charge transfer of elegant Z-scheme WO3/Au/In2S3 nanowire arrays. Nano Lett 2016;16:5547e52. [30] Kong Y, Sun H, Fan W, Wang L, Zhao H, Zhao X, Yuan S. Enhanced photoelectrochemical performance of tungsten oxide film by bifunctional Au nanoparticles. RSC Adv 2017;7:15201e10. [31] Karbalaei Akbari M, Hai Z, Wei Z, Detavernier C, Solano E, Verpoort F, Zhuiykov S. ALD-developed plasmonic twodimensional Au-WO3-TiO2 heterojunction architectonics for design of photovoltaic devices. ACS Appl Mater Interfaces 2018;10:10304e14. [32] Xu F, YaoY Bai D, Xu R, Mei J, Wu D, Gao Z, Jiang K. A significant cathodic shift in the onset potential and enhanced photoelectrochemical water splitting using Au nanoparticles decorated WO3 nanorod array. J Colloid Interface Sci 2015;458:194e9.
[33] Guo X, Hao C, Jin G, Zhu HY, Guo XY. Copper nanoparticles on graphene support: an efficient photocatalyst for coupling of nitroaromatics in visible light. Angew Chem Int Ed 2014;53:1973e7. [34] Zhang P, Song T, Wang T, Zeng H. In-situ synthesis of Cu nanoparticles hybridized with carbon quantum dots as a broad spectrum photocatalyst for improvement of photocatalytic H2 evolution. Appl Catal, B 2017;206:328e35. [35] Mahanti M, Basak D. Cu/ZnO nanorods' hybrid showing enhanced photoluminescence properties due to surface plasmon resonance. J Lumin 2014;145:19e24. [36] Zhang S, Peng B, Yang S, Wang H, Yu H, Fang Y, PengF. Nonnoble metal copper nanoparticles-decorated TiO2 nanotube arrays with plasmon-enhanced photocatalytic hydrogen evolution under visible light. Int J Hydrogen Energy 2015;40:303e10. [37] Yang X, Li H, Zhang W, Sun M, Li L, Xu N, Wu J, Sun J. High visible photoelectrochemical activity of Ag nanoparticlesandwiched CdS/Ag/ZnO nanorods. ACS Appl Mater Interfaces 2017;9:658e67. [38] Liu Z, Wu J, Zhang J. Quantum dots and plasmonic Ag decorated WO3 nanorod photoanodes with enhanced photoelectrochemical performances. Int J Hydrogen Energy 2016;41:20529e35. [39] Kumar PN, Deepa M, Ghosal P. Low-cost copper nanostructures impart high efficiencies to quantum dot solar cells. ACS Appl Mater Interfaces 2015;7:13303e13. [40] Subramanyam P, Kumar PN, Deepa M, Subrahmanyam C, Ghosal P. Bismuth sulfide nanocrystals and gold nanorods increase the photovoltaic response of a TiO2/CdS based cell. Sol Energy Mater Sol Cells 2017;159:296e306. [41] Xu X, Luo F, Tang W, Hu J, Zeng H, Zhou Y. Enriching hot electrons via NIR-photon-excited plasmon in WS2@Cu hybrids for full-spectrum solar hydrogen evolution. Adv Funct Mater 2018;28:1804055. [42] Bhardwaj S, Pal B. Photodeposition of Ag and Cu binary cocatalyst onto TiO2 for improved optical and photocatalytic degradation properties. Adv Powder Technol 2018;29:2119e28. [43] Bhavani K, Naresh G, Srinivas B, Venugopal A. Plasmonic resonance nature of Ag-Cu/TiO2 photocatalyst under solar and artificial light: synthesis, characterization and evaluation of H2O splitting activity. Appl Catal, B 2016;199:282e91. [44] Mahanti M, Basak D. Cu/ZnO nanorods' hybrid showing enhanced photoluminescence properties due to surface plasmon resonance. J Lumin 2014;145:19e24.
Please cite this article as: Subramanyam P et al., Decoration of plasmonic Cu nanoparticles on WO3/Bi2S3 QDs heterojunction for enhanced photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.05.168