g-C3N4 nanosheets functionalized silicon nanowires hybrid photocathode for efficient visible light induced photoelectrochemical water reduction

Journal of Power Sources 413 (2019) 293–301

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g-C3N4 nanosheets functionalized silicon nanowires hybrid photocathode for efficient visible light induced photoelectrochemical water reduction

T

S. Gopalakrishnana, G.M. Bhaleraob, K. Jeganathana,∗ a b

Centre for Nanoscience and Nanotechnology, Department of Physics, Bharathidasan University, Tiruchirappalli, 620 024, Tamil Nadu, India UGC-DAE Consortium for Scientific Research (CSR), Kalpakkam Node, Kokilamedu, 603 104, Tamil Nadu, India

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

Si NWs@g-C N NSs photocathode is • fabricated using facile technique. photocathode shows the en• Hybrid hanced PEC water reduction perfor3

4

mance.

photocurrent density of • A22maximum is achieved. mA cm low charge transfer resistance • Exhibits with high life time of excited elec−2

trons.

A R T I C LE I N FO

A B S T R A C T

Keywords: Si nanowires g-C3N4 nanosheets Photocathode Interface Solar water reduction

We report the fabrication of hybrid Si nanowires @ g-C3N4 nanosheets based photocathode using metal assisted chemical etching and facile liquid exfoliated process. The g-C3N4 nanosheets on Si nanowires form hybrid heterojunction photocathode, which exhibits an enhanced photon induced water reduction activity enabling higher photocurrent density of 22 mA cm−2 with applied bias photocurrent conversion efficiency of 4.3% under visible light irradiation. The onset potential of cathodic photocurrent is positively shifted from 41 to 420 mV vs. RHE with the short circuit current density, Jsc of 0.50 mA cm−2 owing to superior charge transport in hybrid photocathode as compared to pristine Si nanowires for hydrogen evolving reaction at pH∼7. The electrochemical impedance spectroscopy measurement elucidates the interface layer of g-C3N4 nanosheets form hybrid heterojunction with Si nanowires that result significant increment in solar water reduction activity owing to low charge transferred resistance with high life time of excited electrons in conduction band. This strategy may open to design a new low cost stable hybrid heterostructure photocathode for solar induced water reduction.

1. Introduction Hydrogen energy is a most appealing way to overcome the global energy economy crisis and is considered to be future clean energy carrier without the byproduct of desired pollute. One of the great efforts to produce hydrogen is photocatalyst based water splitting process by utilizing of solar energy and abundant water source [1–3]. Fujishima

∗

and Honda were first demonstrated photoelectrochemical (PEC) water splitting with help of TiO2 as photoanode in 1972 [4]. The PEC water splitting process involves absorbance of sunlight and generation of electron-hole pairs in semiconductor/electrolyte interface to drive the reduction-oxidation reaction process. During reduction process, photon induced species (H+) turn into H2 by interaction of electron in the photocathode surface whereas oxidation reaction takes place in the

Corresponding author. E-mail addresses: [email protected], [email protected] (K. Jeganathan).

https://doi.org/10.1016/j.jpowsour.2018.12.034 Received 2 August 2018; Received in revised form 17 October 2018; Accepted 14 December 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

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diffusion technique. The crystal structure and morphology of Si NWs@ g-C3N4 NSs were examined by powder X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy analysis. The influence of g-C3N4 NSs as interface layer in heterojunction of hybrid Si photocathode was investigated for PEC water reduction process in neutral aqueous solution (pH∼7) under visible light irradiation. The Si NWs@g-C3N4 NSs based photocathode exhibit a highest PEC water reduction activity with higher photocurrent density (22 mA cm−2 at 1.23 V vs. RHE bias) with applied bias photon to current conversion efficiency (ABPE) of 4.3% at 0.80 V vs. RHE bias. At, optimum content of g-C3N4 NSs functionalized hybrid heterostructure photocathode results ∼420 mV vs. RHE anodic shift for proton reduction process and even photocurrent is observed at zero bias, quantitatively measured in terms of short circuit current (Jsc-0.50 mA cm−2) with considerable solar to hydrogen conversion (STH) efficiency is 0.61%. In hybrid photocathode, interface layers provide high density of charge carrier separation and thereby photon induced charge carrier lifetime is significantly improved. Further, the possible mechanism has been proposed for the enrichment of PEC water reduction with g-C3N4 NSs as an interface on heterojunction photocathode/electrolyte. Hence, the fabricated Si NWs@g-C3N4 NSs hybrid photocathode is expected to outperform in terms of PEC water reduction activity with excellent stability.

photoanode side [4–8]. So far, numerous number of metal oxide semiconductors such as TiO2 and ZnO have been used as photoelectrode. The absorbance of these semiconductors falls approximately 5% of solar energy region near to UV light absorbance because of their band gap [9–13]. Therefore, an enormous effect has been paid to develop the suitable visible or entire solar light active semiconductor for efficient PEC hydrogen generation. However, reported solar conversion (ƞ) efficiencies are unsatisfactory. The PEC efficiency factors depend on; (i) good light harvesting of solar spectrum (ii) suitable band gap and band position for reduction-oxidation reaction (iii) charge separation & recombination lifetime in electrode/electrolyte interface and (iv) high photostability during PEC operation [14–17]. As an earth abundant and low cost semiconductor, p-type silicon (Si) is an interesting candidate as photocathode material due to unique many advantages. In particular, band gap is Eg = 1.1 eV which enables to cover major portion of solar spectrum and conduction band alignment of Si suitable for water reduction process [18,19]. Through band bending alignment, p-type Si nanostructure favor for hydrogen evolving reaction (HER) at semiconductor/electrolyte interface is still do not produce considerable efficiency due to its inherent property of slow HER kinetics and photo instability [20,21]. Besides, pristine p-type silicon nanostructure photocathode produces very limited photocurrent density with large negative onset potential owing to poor charge transfer and separation. In recent years, p-type Si nanostructure with buried homo- (n+Si layers on p-Si) and formation of heterojunction using SrTiO3, Co-P and Ta3N5 that alter band bending alignments are widely employed to improve photon absorption and fast charge carrier transport for enhanced water reduction activities at semiconductor heterojunction/electrolyte interface [22–26]. Further, a lot of efforts have been made to develop the earth abundant metal free and nonprecious materials as heterojunction organic interface with p-Si nanostructure materials, which represents a new class of hybrid heterostructure photocathode towards efficient proton reduction process. In the recent past, metal free polymeric organic semiconductor of layered graphitic carbon nitride (g-C3N4) has been widely used as cocatalyst to improve the photocatalytic activity of semiconductor. The g-C3N4 has visible light absorbance (Eg = 2.7 eV) and conduction band edge more negative than reduction band edge of water and hence an electron donor in HER [27–29]. In general, the bulk g-C3N4 structure provides the limited photocatalytic/water reduction process, the obstacle of rapid recombination of photon induced charge carriers can be resolved by down scale of g-C3N4 which consists of stacked few layered geometry to planar structure namely nanosheets (NSs) [30,31]. On the other hand, g-C3N4 NSs provide new physicochemical properties owing to large surface area and improved transport ability along the planar direction with fast charge carrier transport. The functionalizing of such gC3N4 NSs on one dimensional (1D) Si nanostructures would form a hybrid heterojunction photocathode/electrolyte interface system where electron conduction towards electrolyte (the holes travel towards the counter electrode) take place for the proton reduction without any impediment [29–33]. The proposed hybrid (g-C3N4 NSs) functionalized 1D nanostructure system could be highly profitable new class of heterojunction photocathode similar to Si nanostructure with high class noble metals. However, to the best of our knowledge, there was no report on silicon nanowires functionalized with graphitic carbon nitride nanosheets (Si NWs@g-C3N4 NSs) as hybrid heterojunction photocathode for PEC water reduction at pH∼7. Therefore, developing Si NWs heterojunction integrating with g-C3N4 NSs as interface layer would maximize the photon conversion efficiency. Motivated by unique properties, we have successfully fabricated gC3N4 NSs functionalized Si NWs hybrid heterostructure as photocathode for enhanced water reduction process under solar irradiation. In this process, Si NWs were fabricated by simple two step metal assisted chemical etching (MACE) techniques using p-Si (100) substrate. Subsequently, the liquid exfoliated g-C3N4 NSs was introduced as interface layer onto the surface of Si NWs via spin coating by wet

2. Experimental details 2.1. Fabrication of Si nanowires The simple metal assisted chemical wet etching route was utilized for the fabrication of the Si NWs. The polished single crystalline of boron doped Silicon (p-type) with the orientation of (100) was used as starting wafer. Prior to etching process, Si wafer cut into 1 × 1 cm2 was cleaned successively using acetone and ethanol by ultrasonication bath followed by and rinsed in deionized water respectively. Further, Si substrates were cleaned by standard Radio Corporation of America (RCA) process to remove unwanted residual oxides and other contamination. Then, the Si substrates were dipped into a solution of AgNO3/HF (0.01 M/4.8 M) for the deposition of Ag NPs on Si substrate for 60 s. Subsequently, Ag NPs coated Si substrates are immersed into etching solution containing H2O2/HF (0.6 M/4.8 M) for 60 min of optimal duration and etched Si substrates were rinsed with deionized water. The Si substrates were subsequently immersed in concentrated nitric acid for several hours to remove etching traces and finally the samples are dried with gentle N2 gas. 2.2. Preparation of bulk g-C3N4 In typical procedure, a g-C3N4 bulk material was synthesized using simple thermal treatment of melamine (C3H6N6) powder. The melamine powder was placed in a crucible and then heated at 550 °C for 2 h. After cooled down to room temperature naturally, the resultant sample color turns to light yellow which evidences the formation of g-C3N4 in bulk form.

2.3. Preparation of g-C3N4 NSs network The g-C3N4 NSs were obtained by liquid exfoliation technique of assynthesized bulk g-C3N4 in isopropanol. Briefly, 10 g of bulk g-C3N4 was dispersed in 100 mL of isopropanol and ultrasonicated for about 10 h. The resultant suspension was centrifuged at about 5000 rpm for 15 min to remove residual of unexfoliated g-C3N4 and supersaturated solution treated with rotary evaporator to evaporate the solvent. Finally, the obtained pale yellow colored g-C3N4 represents the formation of g-C3N4 NSs network. 294

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are orientated along the (004) plane with cubic crystal structure (JCPDS card no:75-0589) as shown in Fig. 1b. After the introduction of g-C3N4 NSs on Si NWs, the Si (004) peak is slightly down shifted (0.05°) to lower angle side (supporting information S3) that confirms a strong interaction between Si NWs and g-C3N4 NSs, leading to formation of hybrid heterostructure [33]. Further, no characteristics diffraction peaks of g-C3N4 NSs could be observed in Si NWs @ g-C3N4 NSs due to low concentration of functionalized layer on the surface of Si NWs. The surface features of Ag NPs deposited on bare Si substrate shows uniform deposition with an average size distribution of 63 nm (in supporting information S4). Fig. 2a shows AFM image of g-C3N4 NSs on bare Si substrate and 3D view of corresponding AFM image (in supporting information S5). The thickness of exfoliated g-C3N4 NSs measured by AFM height profile measurement is ∼3.42 nm which suggest that NSs consist of a few stack of C-N sheets as evidenced from Fig. 2b. Fig. 2c shows the TEM image evidencing the 2D structural description of as-exfoliated g-C3N4 NSs and the stacked nature of g-C3N4 NSs with high transparency, which indicates NSs are ultrathin. The dark portion of nanosheets is attributed to overlap of few NSs after liquid exfoliation process. Fig. 3 (a&e) depict the tilted view of Si NWs and Si NWs@gC3N4 NSs hybrid heterostructure, respectively. The FESEM micrograph shows vertically aligned nanowires perpendicular to Si substrate with apex of NWs congregate together due to electrostatic interactions (inset of Fig. 3a&e) and g-C3N4 NSs are well anchored on Si NWs surface. The cross sectional view of NWs clearly shows that etched NWs are well separated and vertical standing with an average length of 35 μm (in supporting information S6). The EDX spectra of Si NWs@g-C3N4 NSs show a low intensity peak associated with the coexistence of C, N with Si. Further, impurity elements pertaining to etching process such as Ag is absent in the heterostructure (inset of Fig. 3e) and enlarged view of EDX spectra with atomic percentage are also depicted in supplementary S7. Further, pristine Si NWs and Si NWs@g-C3N4 NSs hybrid samples surface morphology and crystal structure were examined using TEM and HR-TEM analysis where samples prepared by scratching NWs from etched substrate. Fig. 3b exhibit the typical TEM image of single Si NWs and its lattice fringes with spacing 3.29 Å calculated using line profile (Fig. 3c) which can be assigned to (100) plane of cubic structure. In Fig. 3f, TEM image of single hybrid nanowire clearly reveals that gC3N4 NSs are uniformly wrapped on Si NWs. The HRTEM images of hybrid NWs displayed in Fig. 3g provide more details pertaining to interfacial layer structure of hybrid nature and good crystalline Si NWs functionalized by g-C3N4 NSs. However, the lattice fringes are not seen due to honeycomb porous structure [34]. The selected area electron diffraction (SAED) pattern of Si NWs shown in Fig. 3d reveals the bright electron diffraction spots confirming the crystalline nature of NWs and the corresponding electron diffraction spots are indexed as (400), (222) and (311). From Fig. 3h, it is evident that SAED pattern of hybrid Si NWs exhibit single crystalline nature and g-C3N4 NSs offer no effect on SAED due to its amorphous nature (in supporting information S8), further it suggested that the hybrid nature of the Si NWs functionalized with g-C3N4 NSs. The functionalized g-C3N4 NSs avoids direct contact of Si NWs to electrolyte and prevents the photocorrosion of core material. The etched Si NWs samples have better anti reflectance properties than planer surface owing to unique morphology which appeared to be black. The difference between anti reflectance behaviors of pristine Si NWs and Si NWs@g-C3N4 NSs are confirmed by their diffusion reflectance spectra as shown in Fig. 4. The pristine Si NWs and hybrid Si NWs show the superior anti reflectance behaviors due to strong photon trapping capability attributed to multiple internal reflectance within vertical nanowire in entire visible region. It should be noted that few order of decrement observed in reflectance spectra after introducing of g-C3N4 NSs onto Si NWs because of increasing surface roughness of hybrid NWs which leads to enhancing light trapping behavior. Moreover, Si NWs@g-C3N4 NSs sample could provide the large number of photon absorption that induced electron-hole pair generation in hybrid nanostructure to achieve the enhanced PEC water reduction process.

2.4. Fabrication of Si NWs@g-C3N4 NSs hybrid heterostructure The Si NWs@g-C3N4 NSs nanostructure was obtained by spin coating process as wet diffusion technique. First, Si NWs samples are treated with HF solution for the removal of native oxide layer on surface of samples. In a typical spin coating process, g-C3N4 NSs (0.018 wt %) was dispersed in isopropanol with aid of ultrasonication for 30 min. The net resultant solution was dropped onto Si NWs substrate and spun at 3500 rpm for 15 s. The above optimized parameters are sufficient to ensure that homogenous functionalized of g-C3N4 on Si NWs samples. The schematic diagram for the preparation of g-C3N4 NSs functionalized on Si NWs shown in supporting information S1. 2.5. Characterization methods The crystalline structure of the samples was examined by X-ray diffractometer (XRD PAN analytical X′ Pert PRO diffractometer) with Cu Kα radiation (λ = 1.5406 Å). The surface topography of samples was evaluated using atomic force microscopy (AFM-Agilent −5500) in noncontact mode. The surface morphology and elemental analysis were intensively investigated using field emission scanning electron microscopy (FESEM, Carl Zeiss, Sigma) equipped with energy dispersive Xray (EDX) spectroscopy (Oxford instruments) at an accelerating electron beam of 10 keV. The atomic structure and interface was exclusively analyzed by high resolution transmission electron microscopy (HRTEMLEBRA 200 TEM- Carl Zeiss) and selective area electron diffraction (SAED) mode. The anti-reflectance spectra of samples were recorded using UV-Vis reflectance spectroscopy (SHIMADZU 2450 UV- Vis absorption) in the wavelength region of 200–1000 nm. 2.6. Photoelectrochemical measurement The photocathode Si NWs @ g-C3N4 NSs with surface area of 0.25 cm2 was fabricated by connecting copper wire on the back side of samples with In-Ga alloy and sealed with epoxy resin to avoid electrolyte contacting back side of Si substrate. The PEC water splitting measurements were performed by three electrode configuration system embedded with standard electrochemical analyzer (Biologic SP 150), the prepared samples were employed as photoactive working electrode, Ag/AgCl and Pt wire acts as reference electrode and counter electrode respectively. The tested potential was converted from Ag/AgCl to Reversible Hydrogen Electrode (RHE) scale using Nernst equation (1)

ERHE = Eo Ag/AgCl + 0.059pH + EAg/AgCl

(1)

EoAg/AgCl

= 0.197 V and EAg/AgCl is experiment measurement powhere tential versus Ag/AgCl reference, electrolyte pH∼7 [24]. All the measurements were carried out using 0.5 M Na2SO4 (pH∼7) aqueous electrolyte solution under solar light illumination having intensity of 100 mW cm−2 (Oriel) with AM 1.5G. The electrochemical impedance spectroscopy (EIS) measurement were performed in the frequency range from 10 Hz to 1 MHz with amplitude of AC is 5 mV. The MottSchottky plots were carried at 1 kHz in a three electrode system under the dark condition. 3. Results and discussion Fig. 1a shows XRD pattern of as-prepared bulk g-C3N4 & g-C3N4 NSs, where the pronounced diffraction peaks appear at 13.2° and 27.6° attributed to the (100) and (002) plane. The predominant diffraction peak at 27.6° reveals characteristic of interlayer stacking reflectance of an aromatic conjugated system, whereas weaker peak at 13.2° reveals that presence of inplane order of tri-s-triazine units. In g-C3N4 NSs diffraction pattern, (002) peak was slightly experienced a lower angle shift as compared to bulk which confirms the presence of few layered NSs with increasing inter stacking distance [32]. The enlarged XRD peak of gC3N4 NSs is shown in supporting information S2. Etched Si nanowires 295

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Fig. 1. XRD patterns of (a) bulk g-C3N4 and g-C3N4 NSs and (b) Pristine Si NWs and Si NWs @ g-C3N4 NSs.

Fig. 2. AFM image of (a) g-C3N4 NSs, (b) Corresponding height profile of AFM image and (C) TEM image of g-C3N4 NSs.

measurement shows small ground current density. During the photocurrent density - voltage (J-V) measurement, the galore of bubbles can be found to evolve from photocathode which is direct evidence of HER process. The J-V portrait of hybrid heterojunction photocathode clearly exhibits incredible enhancement of HER performance that can be attributed to high density of the photocurrent and strongly key influence of g-C3N4 NSs content as interface layer in PEC performance. In short, Si NWs@g-C3N4 NSs hybrid heterostructure photocathode provides the high photocurrent density of 22 mA cm−2 at 1.23 V vs. RHE and g-C3N4

The PEC water reduction performance of as prepared hybrid photocathodes were evaluated by linear sweep voltammetry (LSV) measurement under the simulated solar illumination with A.M 1.5G filter against with pristine Si NWs as shown in Fig. 5a. To further reveal that the merits of g-C3N4 NSs as an interface layer on Si NWs photocathode for solar induced water reduction performance. Upon illumination, as compared with pristine Si NWs photocathode, the hybrid heterojunction photocathode is produced the drastically enhance the photocurrent density due to photocatalytic activity while dark voltammetry scan 296

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Fig. 3. FESEM images of (a) Si NWs (Inset represents the corresponding EDX spectra), (b) TEM image of Si NWs, (c) HR-TEM image of Si NWs, (d) SAED pattern of Si NWs (e) FESEM images of Si NWs@g-C3N4 NSs (Inset represents the corresponding EDX spectra), (f) TEM image of Si NWs@g-C3N4 NSs, (g) HR-TEM image of Si NWs@g-C3N4 NSs and (h) SAED pattern of Si NWs@g-C3N4 NSs.

compared to pristine Si NWs. Fig. 5b shows, photocurrent transient behavior of photocathode with switching time 30 s at 0.80 V vs. RHE applied bias and spike in photo response which quickly returns to a steady states are observed for all prepared photocathode. The hybrid photocathode exhibits highest photocurrent of 4.7 mA cm−2 as compared to pristine Si NWs (2.2 mA cm−2) which is consistent with the results of J-V profile. The presence of g-C3N4 NSs on Si NWs hybrid cathode is primarily for the increase in light absorption and charge transfer in electrode/electrolyte interface, and accounting for enhanced PEC behavior. The photo-stability of photocathodes were evaluated by chronoamperometric (J-t) measurement using 0.5 M Na2SO4 electrolyte (pH∼7) at 0 V vs. RHE bias under the illumination (100 mW cm−2) for 16 min of continuous operation. Fig. 5c shows that J-t curve of hybrid Si NWs@g-C3N4 NSs photocathode with improved photocurrent at 0 V vs. RHE bias with substantial stability than pristine Si NWs. The g-C3N4 NSs aided photocathode holds remarkable constant stability and along preventing of Si NWs containing isolated from electrolyte which forbidden the oxidation of Si under the PEC operation without scarifying the water reduction process. The enhancement of PEC activities benefits from its unique feature of decorated g-C3N4 NSs as protective layer of hybrid photocathode. In comparison, pristine Si NWs has produced low photocurrent at 0 V vs. RHE due to poor charge separation and oxidation of NWs by direct contacting with electrolyte. The evolution of hydrogen from as a function of time at zero bias (vs. RHE) is calculated using Faraday law of electrolysis (given in supplementary S13). The theoretically estimated H2 evolution rates are 9.29 and 1.40 μmol h−1 cm−2 for Si NWs@g-C3N4 NSs and pristine Si NWs photocathodes respectively. The photoconversion efficiency namely applied bias photon to current conversion efficiency (ABPE) to evaluate PEC water reduction performance of prepared photocathode can be estimated by eqn. (2) [21].

Fig. 4. UV-Vis diffusion reflectance spectra of Si NWs and Si NWs@g-C3N4 NSs hybrid photocathode.

NSs promoting the PEC water reduction with suppressed recombination life time of charge carriers at Si NWs/g-C3N4 NSs/electrolyte interface photocurrent and strongly key influence of g-C3N4 NSs content as interface layer in PEC performance. In short, Si NWs@g-C3N4 NSs hybrid heterostructure photocathode provides the high photocurrent density of 22 mA cm−2 at 1.23 V vs. RHE. Here, g-C3N4 NSs is accountable for the PEC water reduction with suppressed recombination life time of charge carriers at Si NWs/g-C3N4 NSs/electrolyte interface. The observed maximum photocurrent in this system is few orders higher than other reported samples with heterojunction of p-Si NWs/NiMoZn, Au-WS2/Si, NiMoO4-xSx NSs/Si [35–37]. It is worthwhile to note that the onset potential of proton reduction process was positively shifted (420 mV vs. RHE) for Si NWs@g-C3N4 NSs photocathode that could be credited to the quick separation of photon induced electron-hole pair as compared to pristine Si NWs photocathode onset potential (41 mV vs. RHE) [21]. The proton reduction of hybrid photocathode at zero bias can be quantitatively described by short circuit current density (Jsc) of 0.50 mA cm−2. The higher Jsc of hybrid photocathode can be ascribed to superior photo-absorption and better charge carrier transport as

1.23 − Vb ⎞ ABPE (%) = Jpc × ⎜⎛ ⎟ × 100 ⎝ Plight ⎠

(2) −2

Where Jpc is photocurrent density (in mA cm ) under simulated sunlight, Vb is the applied bias potential (V vs. RHE) and Plight is intensity of illumination light (mW cm−2). Fig. 6 depicts the photoconversion efficiency versus applied potential profile of hybrid and pristine Si NWs photocathodes under the light irradiation. The hybrid heterostructure photocathode shows the maximum photoconversion efficiency of 4.3% at 0.80 V vs. RHE which is 4.3 times higher than 297

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Fig. 5. (a) Photocurrent density Vs. Potential curve of control Si NWs and Si NWs@g- NSs photocathodes under illumination, (b) Transient behavior of photocurrent density Vs. Time curve of control Si NWs and hybrid Si NWs photocathodes under illumination and (c) Amperometric (J-t) curves for hydrogen evolution on Si NWs and Si NWs@g-C3N4 NSs under visible solar illumination in a solution of 0.5 M Na2SO4 at constant applied potential of 0 V vs. RHE (inset photograph shows hydrogen evolution (bubbles) on photocathode surface during the amperometric measurement).

pristine Si NWs photocathode (1% at 0.80 V vs. RHE). On the other hand, maximum STH of 0.61% for Si NWs@g-C3N4 NSs photocathode at pH ∼7 (supporting information S14) is observed, whereas pristine Si NWs produce poor STH efficiency (0.09%). Electrochemical kinetic properties of photocathodes such as charge carrier separation and recombination life time features at photocathode/electrolyte interface have been analyzed using EIS measurement. Fig. 7 (a) shows Nyquist plot of hybrid heterostructure and pristine Si NWs based photocathodes measured under solar irradiation at −0.5 V vs. RHE applied potential. The measured plot was fitted with two different equivalent circuit models as displayed in inset of Fig. 7 (a). Typically, series resistance (Rs) is influenced by photoelectrode internal resistance and electrolyte Ohmic resistance. The hybrid photocathode exhibit very low Rs of 18 Ω which can be related to the improved conductivity after the functionalization of g-C3N4 NSs on Si NWs surface. Whereas, the pristine photocathode exhibits Rs of 370 Ω and the expended view of Nyquist plot at higher frequency region is shown in inset of Fig. 7 (a). The Nyquist plot for hybrid photocathode shows two semicircles in low and intermediate frequency regions which reveal the existence of interfacial charge transfer resistances at semiconductor/electrolyte (R1 = 652 Ω) and Si NWs/g-C3N4 NSs interfaces (R2 = 75 Ω). The Si NWs@g-C3N4 NSs photocathode having low interfacial charge transfer resistance with small semicircle radius reflects the enhanced electron-hole pair separation as compared to pristine Si NWs (4260 Ω) [38–40]. Further, the presence of g-C3N4 NSs on the surface of

Fig. 6. Variations of ABPE (%) versus Potential plot of Si NWs and Si NWs@gC3N4 NSs photocathodes.

298

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Fig. 7. (a) EIS Nyquist polts, (b) Bode plots and (c) Mott-Schottky plots of Si NWs and Si NWs@g-C3N4 NSs.

potential (Efb) is estimated by extrapolating the X-intercepts (capacitance (C−2)) of the liner region in Mott-Schottky plot [41]. The hybrid photocathode exhibit Efb of 0.40 V (vs. RHE) which is more positive than pristine Si NWs (0.041 V vs. RHE). The observed Efb value is nearly close to onset potential of hybrid Si NWs (0.041 V vs. RHE) and pristine Si NWs (0.420 V vs. RHE) from J-V profile (Fig. 5a). Based on equation Eb = Ea-Efb, magnitude of Eb and Efb are determined by the applied potential in the semiconductor. The increment of Efb leads to large band bending of Eb in depletion region of semiconductor near to electrode/ electrolyte interface than pristine Si NWs photocathode where the Ea is always negative for photocathode [42]. As consequence, large band bending promotes the fast-kinetic reaction steps occur in photon induced charge carriers separation with suppressed charge recombination and quick charge transport. Thus, the hybrid photocathode enlarged band bending in semiconductor depletion layer also make a positive contribution for the improved photocurrent density towards enhanced PEC water reduction process. The above results demonstrate that hybrid heterojunction photocathode system is promising towards the PEC water reduction. The possible mechanism for improved charge separation and transfer is schematically shown in Fig. 8 (a&b). Typically, the band alignments structure between the parent materials and photocatalytic properties of interface layer as co-catalyst are decisive factors addressing to the enhancement of PEC water reduction performances [43,44]. In general, the large conduction band (CB) between the silicon and g-C3N4 discontinuity present in heterostructure could create an additional barrier at heterojunction/electrolyte interface [45]. A short onset potential of water reduction process with enhanced life time of solar induced charge carriers can be ascribed to the presence of interfacial dipole at Si NWs/

Si NWs significantly improved the interfacial charge transfer with reduced electrochemical reaction resistance as evidenced by the reduced onset potential for water reduction process (Fig. 5a). Thus the enhancement can intimately be associated with the internal electric field at Si NWs@g-C3N4 NSs heterojunction which boost up the solar induced charge carrier separation with reduced interface barrier potential for electrochemical reaction. Furthermore, the carrier lifetime of photoelectrode can be calculated from Bode plot using following equation (3) [38].

τ=

1 2πfmax

(3)

Where τ is lifetime of carrier, fmax = maximum peak frequency. Fig. 7 (b) shows the fmax of heterostructure photocathode is down shifted to 11 from 2137 Hz as compared to Si NWs photocathode that evidences the improved charge separation due to interface charge transfer at Si NWs@g-C3N4 NSs heterojunction. The lifetime of carriers in hybrid photocathode is substantially enhanced to 14 ms as compared to 0.07 ms for pristine Si NWs photocathode. The results show that the proposed hybrid heterostructure photocathode holds the several merits such as effective electron-hole pair separation, minimum interface charge transfer resistance with longer life time of electron which resultant improved PEC water reduction performance. To further understand the intrinsic properties of photoelectrode, Mott-Schottky plots have been derived as shown in Fig. 7c. The measurement was carried out for the prepared pristine and hybrid photocathodes as a function of sweep potential from −0.3 V to 0.7 V vs. RHE under dark conditions. In Fig. 7c, the extracted negative slope indicates that the p-type nature of the prepared photocathodes. The flat band 299

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4. Conclusion In summary, we have successfully designed and developed a novel system of Si NWs@g-C3N4 NSs as hybrid heterostructure photocathode for high performance PEC water reduction process at neutral pH condition. The optimal content of g-C3N4 NSs (0.018 wt%) covered on vertical Si NWs form hybrid heterojunctions and exhibits the remarkable photocatalytic performance. The maximum cathodic photocurrent of 22 mA cm−2 at 1.23 V vs. RHE with ABPE efficiency of 4.3% under solar irradiation is observed with notable shift in anodic onset potential. The improved conversion efficiency via onset anodic potential shift can be ascribed to the successful infusion of g-C3N4 NSs as interface layer inducing an effective separation and migration of electron hole pairs with longer life time as evidenced by EIS study. Further, the g-C3N4 NSs assisted hybrid photocathode provides enhanced water reduction owing to the inhibited surface recombination with enriched charge transfer at electrode/electrolyte. Thus, the present work could provide an important pathway to design a facile protocol for semiconductor/organic polymer photocathodes to realize improved PEC performance under visible solar irradiation. Acknowledgement K.J. acknowledges Defence Research and Development Organisation, Department of Science and Technology, Government of India under the contract no. ERIP/ER/1403174/M01/1593-2015 and SR/NM/NS-1202/2014 respectively for financial support. K.J also acknowledges the Improvement of Science and Technology Infrastructure in Universities and Higher Educational Institutes (FIST) for financial support to develop the infrastructure facilities. The authors K.J. and S.G. thank UGC-DAE consortium for scientific research, Kalpakkam Node, Kokilamedu for TEM measurements. The author S.G. would like to thank, Dr. P. Justin Jesuraj, Mr. M. Gopalakrishnan and Miss. T.S. Sheena, Centre for Nanoscience and Nanotechnology, Bharathidasan University, Tiruchirappalli, India for their fruitful scientific discussions and technical help. S.G. acknowledges the award of UGC Research Fellowship in Sciences for Meritorious Students (RFSMS), UGC, Govt. of India. Fig. 8. (a) Schematic diagram for photon induced electron-hole pair separation and water reduction process of Si NWs@g-C3N4 NSs heterojunction photocathode and (b) Band structure of Si NWs and g-C3N4 NSs with the presence of interfacial dipole of single nanowire.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.12.034.

g-C3N4 NSs heterojunction. The interfacial dipole reduces the potential of electrochemical reaction, opposing additional barrier in heterojunction, owing to the produced continuity in CB edges. As shown in Fig. 8 (a), the high density of electron-hole pair generation with light irradiation. The Si NWs@g-C3N4 NSs primarily contribute to the total photocurrent density. Besides, photon induced electron is accelerated towards the g-C3N4 NSs/electrolyte interface as the two systems attain the equilibrium itself by new Fermi level (EF new) formation assuming a flat band bending occur in g-C3N4 NSs/electrolyte. Consequently, builtin potential (E) form an interfacial dipole which induces fast transfer of electron-hole pair with prolonged lifetime of light induced charge carriers. Hence, the electron from silicon transfer along with perpendicular to organic polymer sheets due to nature of aromatic pi orbital of g-C3N4 NSs [45] as shown in Fig. 8 (b). The electrons reacting with H+ turns into H2 via reduction process and meanwhile hole migrated towards the Pt electrode for oxidation process. Moreover, interfacial dipole effects at Si NWs/g-C3N4 NSs hybrid heterojunction photocathode provide superior charge carrier transfer with less recombination lifetime that substantially enhances overall PEC water splitting process.

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