1. Introduction
The precipitous rise in worldwide energy consumption has shifted the focus of research towards finding environment-friendly energy sources [
1]. The exploration of alternatives for sustainable energy production is progressing—photoelectrochemical (PEC) splitting of water is one such alternative, as it is considered a primary clean source of energy [
2,
3]. Hydrogen production by water splitting is crucial as a clean energy source because water splitting has zero contribution to CO
2 generation [
4]. Being an earth-abundant material, silicon (Si) has also gained massive attention in the area of photon-assisted light harvesting, owing to its narrow band gap, which allows the absorption of solar radiation ranging from the ultraviolet region to the visible light region [
5]. At the same time, one-dimensional nanostructures demonstrate promising applications in the area of photovoltaics, owing to their remarkable high optical absorption compared with bulk and zero-dimensional materials [
6,
7,
8,
9].
However, using a silicon electrode is highly challenging due to the high overpotential required for the generation of hydrogen. Thus, developing highly efficient electrodes, which can overcome the drawbacks posed by silicon, is crucial. Various approaches, such as surface modifications considering the cost and natural abundance of the electrocatalyst, have been explored to improve the PEC response of one-dimensional silicon nanostructures, especially that of silicon nanowires (SiNWs). Owing to their narrow band structure, silicon-based photoelectrodes are employed as both photocathode and photoanode for water splitting reactions [
8,
10,
11]. A fundamental problem observed with a low-band gap Si as a photocathode for H
2 generation is that the difference between the Si valence band edge and the H
+/H
2 redox level is rather narrow, which limits the generated photovoltage [
12,
13].
As an emerging class of two-dimensional material, graphene has attracted significant attention in numerous applications owing to its stability, higher surface area, better optical transparency, and high carrier mobility. Reduced graphene oxide (rGO), which is a carbon-based material, is often used for these applications, owing to its exceptional PEC water splitting capability and fast-interfacial charge transfer between the electrode surface and the electrolyte [
14,
15,
16]. Considering these advantages, we used rGO as a co-catalyst for PEC water splitting reactions.
Scheme 1 illustrates the functionalization of rGO on SiNW photocathode.
Herein, we report a simple, but convenient fabrication of an rGO-SiNW composite to enhance the PEC response to a hydrogen evolution reaction (HER). In order to minimize the recombination rate on the electrode surface and thus increase efficiency, we employed a SiNW structure, which contributes to higher light absorption and charge carrier collection [
17,
18]. The PEC performance could be further controlled by transforming the plane surface of the Si electrode to the shape of a wire. Moreover, in addition to its unique optoelectronic properties, the high surface-to-volume ratio can be advantageous for various applications. This distinctive surface property also enables the reduction of light being reflected from the surface of a planar silicon electrode. Another important factor enhancing the efficiency is the length of the Si nanowire [
19,
20,
21]; thus, we used a 22 μm long Silicon nanowire. We also conducted studies for optimization of photoelectrode through surface treatment for a better PEC response towards HER. The cyclic voltammetry studies and the transient photo response for the Si electrode yielded large onset potential value and displayed a strong band bending for enhanced HER [
18]. Considering the excellent optical absorption capability of the rGO-SiNW composite and the innumerous possibilities of chemically modifying graphene and the fabrication method followed here, we believe it is a step forward in discovering and employing new materials and synthesis approaches towards photoelectrochemical hydrogen production.
3. Results and Discussion
A silicon nanowire (SiNW) structure was fabricated by the metal-catalyzed electroless chemical etching method. The length of the nanowire was controlled by controlling the etching time.
Figure 1 shows the Scanning Electron Microscope (SEM) images of SiNW photoelectrode deposited with reduced graphene oxide (rGO), and energy-dispersive X-ray spectroscopy (EDS) shows the presence of each element on the photoelectrode. The elements of silicon, oxygen and carbon make up the constituents of rGO, which was deposited uniformly on the SiNW photoelectrode through the EDS image. Cross-sectional SEM images also showed uniform coating of rGO on the SiNW with average length of 22 μm.
To provide more insights into the photocathodic efficiency of the SiNW photoelectrode, the inherent current density was measured in the potential window ranging from 0.4 V to −0.7 V vs. RHE using a three-electrode cell. In our measurement setup, a 300 W Xe lamp was used with a light intensity of 100 mW/cm
2 and the light passed through an air mass 1.5 global filter, in 1 M of aqueous perchloric acid (HClO
4) under acidic condition (pH = 0). In
Figure 2, a voltammogram with a linear sweep under light illumination is presented, which shows the current density according to the specific applied potential. First, the reductive current density starts to increase from a specific point as the applied potential increased negatively. The current density saturates at a higher applied potential, which typically is described as limiting current density or saturation current density. The onset potential is better described as a potential at which a sharp increase in current density (−1 mA/cm
2) is observed. The onset potential of SiNW electrode under illumination is 0.179 V vs. RHE, while bare planar silicon electrode only shows −0.106 V vs. RHE. (
Figure 2 and
Table 1). Saturation current density also increased when surface nanostructuring was implemented. The saturation current density of SiNW showed 38.2 mA/cm
2, which is approximately 1.2 times higher than that of bare planar Si (32.8 mA/cm
2). To calculate the solar-to-hydrogen conversion efficiency (STH), the concept of applied bias photon-to-current efficiency (ABPE) was introduced, according to the following equation [
22,
23]:
where j
ph is the photocurrent density obtained under the applied potential V
b, V
redox is the redox potential required for hydrogen production (0 V vs. RHE), V
bias is the applied external bias potential that is typically necessary to achieve reasonable photocurrents, and P
light is the intensity of the applied incident light under AM 1.5 G conditions (100 mW/cm
2). Half STH can be defined as the STH in the 3-electrode cell system, which is also the maximized value of ABPE and can also be calculated using the following equation [
23]:
where j
sc is the current density at the reversible potential (the density at the 0 V vs. RHE), V
oc is the potential at the zero-current density (the open circuit potential), and FF is the fill factor in the J-E curve in the 4-quadrant region. In principle, same photoelectrode materials, despite the difference in surface structures, show similar open circuit potential (V
oc). Bare Si and SiNW showed an almost same value for V
oc, indicative that no other metal residue participated during the photoelectrochemical measurements. Consequently, the SiNW electrode system exhibited 46 times higher half STH compared to bare planar Si electrode, which mainly contributes to the enhanced current density (
Table 1). The enhanced current density, including limiting current density, is better explained by increased surface area by surface nanostructuring. The nanowire structure in turn increases the current density of the Si photocathode, thus increasing the number of effective reaction sites on the illuminated surface. Moreover, the reduced reflection also increases the possibility of enhanced incident light collection. It is well known that the bare planar Si reflects about 25% of the incident light [
24]; however, the nanostructured surface decreases photon absorption significantly due to the its low reflectance. Thus, the SiNW structure electrode exhibits a higher light-trapping effect, which contributes to a higher photon-to-current conversion efficiency than that of bare planar Si electrode. Moreover, orthogonalization reduces the recombination rate between the electron-hole pair, generated at the photoelectrode. The nanowire structure has a relatively shorter horizontal length compared with its length in the vertical direction, and this structure enables the generated minority carrier (electrons) to move faster in the lateral direction of the electrode and to collide with a proton in the electrolyte to produce hydrogen. Consequently, the potential for the water splitting reaction approaches the theoretical value with a decreased kinetic barrier for hydrogen generation, resulting in the increased onset potential, as shown in
Figure 2a. Under dark conditions, the onset potential for the bare planar Si electrode was −0.656 V vs. RHE and SiNW electrode exhibits −0.448 V vs. RHE. It is noteworthy to mention that the degree of onset potential shift from bare Si to SiNW in both light and dark conditions is almost the same (approximately 0.2 V vs. RHE). From the results, the photovoltage of bare Si and SiNW electrodes are 0.55 V vs. RHE and 0.62 V vs. RHE respectively, which indicate that with the nanowire structure, an increased photovoltage under both light and dark conditions and a decrease in the kinetic barrier for hydrogen production is observed. rGO catalyst further enhances the PEC property of the Si photocathode.
Figure 2 shows the photoelectron-catalytic performance of rGO on SiNW toward HER. The onset potential of rGO deposited on the SiNW photocathode is 0.326 V vs. RHE, which is significantly higher than that of the bare SiNW photoelectrode (0.147 V vs. RHE). When the rGO co-catalyst was deposited on the Si photoelectrode, V
oc slightly increased up to 0.04 V for rGO-bare planar Si and 0.08 V for rGO-SiNW, respectively. In the dark condition, the difference in the overpotential between rGO-SiNW and SiNW was 66 mV. The saturation current density of rGO-SiNW somewhat decreased (25 mA/cm
2), compared to the bare SiNW, which is attributed to the light hindrance by opaque rGO. To further study the effect of loading amount on rGO, we varied the loading amount of rGO and found that 0.8 mg of rGO showed the best efficiency for photoelectrochemical response. (
Figure 2b). It is also important to mention that as the loading amount of rGO increased, the saturation current density invariably decreased.
To gather more details on the optical reflection property of SiNW and rGO functionalized SiNW, we measured optical reflectance in the wavelength range of 200–2000 nm with SiNW as control. As displayed in
Figure 3, when SiNW is fabricated on rGO-SiNW, the light reflectance is much lesser than compared to bare SiNW. The reflectance of rGO on SiNW and bare SiNW is almost 2% and 6% in the visible light region. This low reflectance can be attributed to the increased loading amount of rGO; thus, we believe that rGO absorbs a part of the incident light and reduces the absorption of light on the Si photoelectrode, which is followed by a decrease in current density with an increase in the loading amount of rGO. To clarify the absorption property, we also measured absorbance (
Figure 3b) on applying the following equation [
25]:
assuming that transmittance is almost zero in the visible region, since rGO-SiNW happens to be a non-transparent wafer type sample. Nevertheless, the catalytic activity of rGO co-catalyst highly increased to a positive onset potential; eventually, the overall solar-to-current conversion efficiency increased on the introduction of rGO co-catalyst. rGO-deposited SiNW showed better photon to current conversion efficiency of 3.16%, which is 3.47 times higher than that of bare SiNW (
Table 1). As a result, rGO works as an effective HER co-catalyst on the Si electrode, owing to the synergistic effects between rGO and SiNW, and is solely responsible for the improved photocatalytic performance.
To provide a deeper understanding of the electrocatalytic activity of the rGO catalyst, capacitance studies were performed using a rotating disk electrode (RDE) system, as shown in
Figure 4. A catalyst ink comprising rGO was drop-casted on to the glassy carbon (GC) tip. The significant rise in the current density at the same potential for rGO on GC compared to that of the bare GC can be attribute to the electrochemically active catalyst (rGO), which can further improve the water splitting efficiency towards HER. When considering the resistance between electrolyte and the electrode interface, it is necessary to consider internal resistance (iR) compensation. Following iR compensation, the onset potential of rGO decreased significantly. To study the catalytic effects of rGO, Tafel plots for rGO-SiNW were obtained, as shown in
Figure 4b. The Tafel slope provides the appropriate pathways suitable for HER [
26]:
In the above process, Hads is the absorbed H, (4) is a discharge step (the Volmer reaction), (5) is the desorption of the H-atom (the Heyrovsky reaction), and (6) is the recombination step (the Tafel reaction). If the recombination of Hads is the rate determining step for the hydrogen evolution (the Tafel reaction), the measured Tafel slope was 30 mV per decade. In the Tafel reaction, the Hads coverage on the surface is almost 1 (θH = 1). If the electrochemical desorption step (the Heyrovsky reaction) can be regarded as the rate-determining step here, a Tafel slope of 40–118 mV per decade is measured, which is dependent on the value observed for θH (θH = 0 to 1). The Tafel slope of 45 mV. per decade in our work indicates that the kinetics of the HER on the rGO catalyst was found to follow the Heyrovsky reaction.
Comprehensive electrochemical measurements on the rGO-SiNW system also show the efficient roles of the rGO catalyst (as shown in
Figure 5). The transient photoelectrochemical behavior of the rGO-SiNW is also dependent on limiting the illumination time range.
Figure 5a shows the transient photoresponse of the rGO-SiNW and bare SiNW electrode. With a 10-s interval, the light was turned on and off, while the potential was maintained at open circuit potential (OCPT) vs RHE. As shown in
Figure 5a, the initial rise in the photocurrent density (
Jin) was 0.70 mA/cm
2 for rGO-SiNW and 0.45 mA/cm
2 for SiNW, respectively, caused by the abrupt turning on of the light source. After taking the peak, the current density saturated at a certain point called the saturated photocurrent density (
Jst). Under illumination, the value of
Jst was 0.19 mA/cm
2 for the rGO-SiNW, which is higher than that of bare SiNW. This shows that fewer carriers were trapped and that less recombination occurred at the surface state of the electrode surface. Simultaneously, when the light was turned off, the current density showed a shift in the direction opposite to
Jin, which was designated as
Joff. The
Joff values of the rGO-SiNW and SiNW electrodes were 0.18 mA/cm
2 and 0.16 mA/cm
2, respectively.
Capacitance measurements also showed that the rGO catalyst is much more effective for electrochemical hydrogen production. Potential sweep ranging from 0.4 V to −0.3 V vs. RHE under the dark condition, as shown in
Figure 5b, and the flat band potential was calculated using the Mott-Schottky equation [
27]:
where
Csc is the capacitance in space charge region,
E is the applied potential,
Efb is the flat band potential that can be determined from the extrapolation to zero capacitance (
Figure 5b),
k is the Planck constant,
T is the temperature,
e is the electron charge,
ε is the dielectric constant of the Si semiconductor (photoelectrode),
ε0 is the permittivity of the free space,
N is the hole acceptor concentration for the p-type Si semiconductor, calculated from the slope in the figure plot shown above. On the basis of these relations, rGO-SiNW exhibits an
Efb of 0.128 V vs. RHE, which is 88 mV higher than the
Efb for the bare SiNW. The higher
Efb indicates that the band bending near the depletion of the semiconductor at the electrode/electrolyte interface is higher. Based on this equation, the magnitude of the band bending (
Eb) is equals to
E −
Efb [
28]. The band bending at the boundary between the electrode surface and electrolyte accelerates for rapid separation of charge carriers and increase in recombination of protons to generate hydrogen [
29].
To sum up, the results of the capacitance and transient photoresponse measurements substantiate that the rGO-SiNW system is highly active in terms of decrease in recombination of charge carriers, and reduces the energy barrier for HER along the electrode/electrolyte interface. The long-term stability test of the 0.8 mg rGO-SiNW was implemented for 2 h at 0.25 V vs. RHE, showing a stable current density, as shown in
Figure 6. The photocathode maintains a current density of approximately −2 A/g and more than 92% of the current, even after 2 h. Our stability testing result implies that the rGO co-catalyst efficiently suppresses the degradation of photoelectrochemical performance.