CoSe2 Embedded in C3N4: An Efficient Photocathode for

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CoSe2 Embedded in C3N4: An Efficient Photocathode for Photoelectrochemical Water Splitting Mrinmoyee Basu,† Zhi-Wei Zhang,‡ Chih-Jung Chen,† Tzu-Hsiang Lu,† Shu-Fen Hu,*,‡ and Ru-Shi Liu*,†,§ †

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan § Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan ‡

S Supporting Information *

ABSTRACT: An efficient H2 evolution catalyst is developed by grafting CoSe2 nanorods into C3N4 nanosheets. The as-obtained C3N4−CoSe2 heterostructure can show excellent performance in H2 evolution with outstanding durability. To generate phatocathode for photoelectrochemical water splitting CoSe2 grafted in C3N4 was decorated on the top of p-Si microwires (MWs). p-Si/C3N4−CoSe2 heterostructure can work as an efficient photocathode material for solar H2 production in PEC water splitting. In 0.5 M H2SO4, p-Si/C3N4−CoSe2 can afford photocurrent density −4.89 mA/cm2 at “0” V vs RHE and it can efficiently work for 3.5 h under visible light. Superior activity of C3N4−CoSe2 compared to CoSe2 toward H2 evolution is explained with the help of impedance spectroscopy.

KEYWORDS: C3N4−CoSe2, heterostructure, hydrogen evolution, photocathode, nanorod

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can work as alternative to novel metal Pt.10,11 Sulfides, selenides, phosphides of Co, Mo, W, Ni, and Fe were studied as the most efficient catalyst for the same reaction.12,13 Previously, we have decorated CoS2 and CoSe2 on Si microwires (MWs) as an efficient cocatalyst for PEC water splitting.14 At the same time we have also introduced CoTe2 as a proficient electrocatalyst for H2 evolution. There are some important issues which must be carefully handled before the practical application of a material in solar water splitting. In our previous case, CoSe2 decorated on Si MWs can generate high photocurrent density under PEC water splitting condition but high charge accumulation during water splitting restricts it to work for long time. So, another important issue is the accelerated transportation of the photogenerated charge carriers from the cocatalyst to electrolyte.15−18 Otherwise photogenerated carriers accumulated on the electrode surface may cause corrosion and reduce the durability. To improve the stability of the efficient cocatalyst (CoSe2), CoSe2 was grafted onto C3N4 nanosheets, which reduces the charge accumulation on CoSe2 providing higher stability. Therefore, in our present work initially nanosheets of C3N4 were synthesized following a combustion technique using urea as the source of “C’ and ‘N”. Then C3N4 nanosheets were used

INTRODUCTION Increasing energy demand and rapid depletion of the fossil fuels leads to develop alternative energy source which can fulfill the daily energy requirement.1−3 Hydrogen generation from water splitting may be one of the efficient alternative approach. Solar energy irradiated per day on earth is nearly four orders in magnitude than the daily energy requirement. Following photoelectrochemical (PEC) solar water splitting utilizing only solar light and water, H2 can be produced which can further work as sustainable alternative energy source.4 Photoelectrode is mainly composed of two parts: light absorber and other is the cocatalyst. Si, having band gap of 1.1 eV poses suitable band alignment for H2 evolution.5,6 Being the earth abundant semiconductor it is explored as an efficient light absorber material from past. But, the stability and sluggish surface reaction restricts the practical applicability of the material. So, it is very likely to provide protection toward Si so that it can function properly with enhanced durability. Other important part in PEC cell is suitable cocatalyst, which helps in faster charge carrier transportation from semiconductor or the light absorber surface to electrolyte. In this regards, earthabundant 3d transition metal compounds are getting explored as efficient and stable material.7−9 In the past few years, people were trying to develop noble electrocatalyst on 3d transition metals for H2 and O2 evolution reaction. Various 3d transition metal compounds were already established as efficient electrocatalyst for H2 evolution, which © 2016 American Chemical Society

Received: June 10, 2016 Accepted: September 16, 2016 Published: September 16, 2016 26690

DOI: 10.1021/acsami.6b06520 ACS Appl. Mater. Interfaces 2016, 8, 26690−26696

Research Article

ACS Applied Materials & Interfaces

After that solution A and B mixed properly through ultrasonnication for 15 min. Finally the solution was transferred in 100 mL Teflon lined autoclave and simultaneous hydrothermal reaction was carried out at 180 °C for 18 h.14 Black precipitate was collected and washed with DI water and dried for further characterization and application. Synthesis of CoSe2 Nanostructure. A simple hydrothermal route was followed to synthesize CoSe2 composite. Solution A was prepared by adding 5 mL 0.4 M Co(II)-chloride hexahydrate in 5 mL 0.5 M aqueous solution of ethylene diamine tetraacetic acid ligand. On the other hand solution B was prepared by dissolving 0.32 g of Se powder in 30 mL 3.3 M NaOH. After that solution A and B mixed properly through ultrasonnication for 15 min. Finally the solution was transferred in 100 mL Teflon lined autoclave and simultaneous hydrothermal reaction was carried out at 180 °C for 18h.14 Black precipitate was collected and washed with DI water and dried for further characterization and application. Decoration of C3N4−CoSe2 on Si MWs. C3N4−CoSe2 ink was prepared by dispersing 10 mg of the compound in 1 mL of isopropanol and sonnicated for 4h. The as synthesized C3N4−CoSe2 composite was decorated on Si MW by drop-casting technique. 50, 80, 100, and 140 μL of C3N4−CoSe2 ink was dropped on Si MW arrays and dried in air and denoted as “Si/C3N4−CoSe2−V”, in which V represents volume of C3N4−CoSe2 inl (μL). For the electrochemical study ink on C3N4−CoSe2 and CoSe2 was prepared by dispersing 10 mg of sample in 1 mL of isopropanol and 500 μL of 0.5% Nafion.

as substrate to grow CoSe2 nanorods (Scheme 1). Finally, C3N4−CoSe2 heterostructure was decorated on the top of Si Scheme 1. Schematic Representation of the Synthesis of CoSe2 Nanorods on C3N4 Substrate

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MW arrays and applied as photocathode in PEC water splitting. C3N4 accelerates the charge transportation which is further asserted with the help of impedance spectroscopy.

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RESULTS AND DISCUSSION

Crystallinity of C3N4−CoSe2 deposited on Si MWs was confirmed by Raman spectroscopy (Figure 1a). Raman spectra shows a sharp peak located at 520 cm−1 which is assigned to Si MWs and other two peaks at 174 and 676 cm−1 are due to the Se−Se stretching of CoSe2 and C3N4, respectively.20,21 Inset of Figure 1a clearly shows the peak for Se−Se stretching and C− N stretching of C3N4. Peak centered at 950 cm−1 originated from Si wafer which is in agreement with the literature.22 Powder X-ray diffraction (PXRD) helps to determine phase purity of C3N4−CoSe2 and C3N4. Figure 1b shows the typical PXRD pattern of C3N4−CoSe2 which matches clearly with the JCPDS no 89-2003. Peaks observed in this pattern are assigned to the orthorhombic crystal phase of CoSe2. No other impurity was detected from XRD which further confirms that the synthesized CoSe2 is phase pure. C3N4 was not observed through XRD, which further depicts the amorphous nature of C3N4. SEM images of C3N4−CoSe2 in different magnifications are shown in Figures 2a and S1. Low magnification SEM images

EXPERIMENTAL SECTION

Synthesis of Si MWs. The Si MWs (resistivity = 1−15 Ω·cm) were synthesized following our previously reported method.5 Synthesis of C3N4. C3N4 was synthesized following combustion technique. Ten grams of urea was placed in a silica crucible and then calcined at 550 °C for 2 h.19 Then the yellow powder of C3N4 was collected, washed, and dried in an air oven. Synthesis of C3N4−CoSe2 Nanostructure. A Simple hydrothermal route was followed to synthesize C3N4−CoSe2 composite. Solution A was prepared by adding 50 mg of C3N4 in 5 mL of 0.5 M aqueous solution of ethylene diamine tetraacetic acid ligand and 5 mL 0.4 M Co(II)-chloride hexahydrate. On the other hand Solution B was prepared by dissolving 0.32 g of Se powder in 30 mL of 3.3 M NaOH.

Figure 1. (a) Raman spectra of Si/C3N4−CoSe2 showing the peaks of Si, C3N4, and CoSe2 and (b) PXRD pattern of C3N4−CoSe2. 26691

DOI: 10.1021/acsami.6b06520 ACS Appl. Mater. Interfaces 2016, 8, 26690−26696

Research Article

ACS Applied Materials & Interfaces

Figure 2. SEM images of (a) C3N4−CoSe2: (b) top view and (c) cross-sectional view of C3N4−CoSe2.

Figure 3. TEM image of (a, b) C3N4−CoSe2 and (d) HR-TEM image of C3N4−CoSe2, which is the enlarged part of encircled portion of panel c. Panel c clearly shows the interface of amorphous C3N4 and crystalline CoSe2.

clearly show nanorods of CoSe2 were synthesized following our method. High magnification image of C3N4−CoSe2 confirms that the length and width of CoSe2 nanorod is ∼300 nm and ∼50 nm, respectively. Figure S2 shows typical SEM images (top view and cross-sectional view) of bare Si MWs. SEM images show that Si MWs having length of ∼10 μm and ∼1 μm width, respectively. Electrodes were prepared by drop-casting ink of C3N4−CoSe2 on the top of Si MWs. SEM images of Si/C3N4− CoSe2 confirm that the top of Si MWs are fully covered with C3N4−CoSe2 (Figures 2b and S3) nanorods. Cross sectional view also shows that the top of Si MWs are covered with the nanorods of C3N4−CoSe2 (Figures 2c and Figure S4). Morphology and the crystal structure of the synthesized C3N4−CoSe2 were also checked with the help of TEM and HRTEM analysis, respectively. TEM images of C3N4−CoSe2 clearly depict that CoSe2 nanorods are grafted on the thin sheets of C3N4 and the length of CoSe2 nanorod is ∼300 nm with width about 50 nm (Figures 3a, b and S5a and b). Observed TEM images are good in accordance with the SEM images. HRTEM images clearly show the interface between the crystalline CoSe2 and amorphous C3N4 (Figures 3c and S5c) which confirms that crystalline CoSe2 is embedded within amorphous C3N4. The resolved lattice spacing of CoSe2 is ∼0.258 nm which corresponds to the spacing between (111) crystal plane of orthorhombic CoSe2 (Figure 3d).7 EDS analysis clearly confirms the presence of N, Co, Se, and C elements (Figure S5d). Electrocatalytic Activity. Previously, we have unveiled that CoSe2 is semimetallic in nature.14 CoSe2 can efficiently snatch photogenerated electrons from Si surface and effectively transfer to electrolyte. Before checking the PEC performance, electrocatalytic activity of C3N4−CoSe2 and CoSe2 was ensured by drop-casting the ink on glassy carbon (GC) electrode. Electrocatalytic activity of C3N4−CoSe2 and CoSe2 were investigated in 0.5 M H2SO4 solution using a three-electrode set up. Linear sweep voltamogram (LSV) study shows that CoSe2 can generate 10 mA/cm2 current density at potential −0.232 V vs RHE where as C 3 N 4 −CoSe 2 can show photocurrent density 10 mA/cm2 at potential −0.212 V vs RHE. C3N4−CoSe2 shows little anodic shift in the onset

potential (Figure S6). This superior performance in electrocatalytic behavior of C3N4−CoSe2 is due to the faster electron transportation. A positive shift of the onset potential was observed through the assistance of CoSe2 and CoSe2/C3N4 cocatalysts, which function as electron collectors. C3N4 present in CoSe2/C3N4 helps in faster electron transportation from CoSe2 to electrolyte and further decreased the recombination which further results in anodic shift in the onset potential.5 To check the electrocatalytic stability of CoSe2 and C3N4−CoSe2, 1000 continuous cycles were supposed to run. But for CoSe2, a successive decay in the electrocatalytic performance was observed. Current density decreases within 300 consecutive cycles and up to 500 cycles it decays more seriously (Figure 4a). Whereas C3N4−CoSe2 can grip its performance up to 1000 cycles without showing any observable fluctuations (Figure 4b). It is because the C3N4 present in the composite of C3N4− CoSe2 helps in faster electronic transportation which further lowers the corrosion and provides long-term stability. Following chrono-amperometric study, electrocatalytic stability of C3N4−CoSe2 has been checked in different potential windows. C3N4−CoSe2 can show long-term stability up to 48h upon applying potential −0.284 V vs RHE (Figure S7a). At the time of long-term stability of C3N4−CoSe2, it was observed that initially there was little decrease in the current density and then with time photocurrent density further increases successively. This phenomenon can be ascribed due to the initial destruction of little amount of CoSe2 which further provides more interface for contact of electrolyte and increase the current density further.23 C3N4−CoSe2 can also show longterm (10h) stability even if under high potential like −0.434 V vs RHE (Figure S7b). Electrochemical and Photoelectrochemical Impedance Study. With the help of electrochemical impedance measurement, feasibility of charge transportation from CoSe2 and C3N4−CoSe2 toward electrolyte was examined. Nyquist impedance was measured under −0.185 V vs RHE. An equivalent circuit shown in the inset of Figure 5 was used to fit the observed data which consists of constant phase elements 26692

DOI: 10.1021/acsami.6b06520 ACS Appl. Mater. Interfaces 2016, 8, 26690−26696

Research Article

ACS Applied Materials & Interfaces

Figure 4. Stability tests (a) CoSe2 following 500 cycles, (b) C3N4−CoSe2 following 1000 cycles. Very negligible cathodic current density is lost for C3N4−CoSe2 through this 1000 cycling.

(resistance), where R1 is the resistance of the material itself and R2 is the resistance of the charge transfer between the electrolyte and the electrode, and constant phase element (CPE). In both the cases CPE value resistance R1 remain nearly same. CoSe2 (R2 = 170 ohm) shows increased charge transfer resistance compared to the C3N4−CoSe2 (R2 = 107 ohm) (fitted data; Table S1). Decreased charge transfer resistance in C3N4−CoSe2 further reflects about the faster charge transportation due to the presence of C3N4 in C3N4−CoSe2 composite. Further photoelectrochemical impedance study also supports that C3N4 promotes in faster charge transportation from electrode to electrolyte (Figure S8). Literature supports the fact that C3N4 support helps in faster charge transportation.24,25 Photoelectrochemical Activity. Efficient electocataytic performance and long-term stability of C3N4−CoSe2 suggest that this material can function as an efficient cocatalyst material for PEC water splitting. Photoresponse of C3N4 was checked by depositing C3N4−CoSe2 on fluorine-doped tin oxide (FTO) substrate. C3N4−CoSe2 deposited on FTO shows the nearly similar response both under illumination and also in dark which further suggests that in this scanned potential range C3N4 does not show any photoresponse (Figure S9). Photoelectrodes were prepared by drop-casting varying amount of C3N4−CoSe2 ink on p-Si MWs. Details of the experimental procedure is

Figure 5. Nyquist impedance plots of C3N4−CoSe2 and CoSe2 measured in 0.5 M H2SO4. The solid black line traces correspond to the fitting using the equivalent circuit shown in inset.

(CPE) for CoSe2 and C3N4−CoSe2 (inset of Figure 5). Nyquist circuit is fitted with the corresponding elements like R1, R2

Figure 6. (a) Photocurrent versus potential of Si/C3N4−CoSe2 of various deposited amount under dark and under illumination. (b) Plots of variation of photocurrent density at ‘0’ V and onset potential with the deposited amount of cocatalyst on Si MWs. 26693

DOI: 10.1021/acsami.6b06520 ACS Appl. Mater. Interfaces 2016, 8, 26690−26696

Research Article

ACS Applied Materials & Interfaces

Figure 7. (a) Transient photocurrent densities of Si/C3N4−CoSe2 of various deposited amount of C3N4−CoSe2 in 0.5 M H2SO4. (b) Optical absorbance of Si and different amount of C3N4−CoSe2 deposited on Si.

Figure 8. (a) Current density measured at −0.234 V vs RHE up to 1 h for Si/C3N4−CoSe2-50, 80, 100, and 140. (b) J vs t plot measured at −0.234 V vs RHE up to 3.5h for Si/C3N4−CoSe2-100.

shift in onset potential and photocurrent density decreases. Onset potential reaches 0.114 V vs RHE and the photocurrent density was 2.29 mA/cm2 at −0.289 V vs RHE. Like our previous report, onset potential and photocurrent density at 0 V vs RHE both follow the volcano type trend (Figure 6b) with the successive increase in the amount of cocatalyst decoration.7 Decrease in photocurrent density may be attributed due to the accumulation of more charges on the electrode surface which work as the recombination center. The transient behavior of all the electrodes was measured for 400 s under applied bias of −0.234 V vs RHE (Figure 7a). All the photoelectrode can show the transient “on” and “off” behavior with illumination of light and dark with unaltered photocurrent density suggesting the high stability. No sharp current spikes are observed upon illumination which further indicates that C 3 N 4 −CoSe 2 facilitates the rapid electron transfer.11 Stability in acidic media is an important criterion for a good catalyst. Stability of all the photocathodes of Si/C3N4−CoSe2 (50, 80, 100 and 140) were also checked for 1h (Figure 8a). All the electrodes are very stable which suggests that C3N4−CoSe2 can function as stable cocatalyst to generate unaltered photocurrent density. Si/ C3N4−CoSe2−100 can work as an efficient photocathode up to 3.5 h generating photocurrent density nearly ∼8.25 mA/cm2 upon applied potential −0.234 V vs RHE (Figure 8b). The

described in the Experimental Section. Si MWs were etched in 10% aqueous HF solution before drop-casting C3N4−CoSe2 ink on it. For convenience we abbreviated Si MWs with different loading amounts of C3N4−CoSe2 ink as “Si/C3N4−CoSe2−V”, in which “V” represents volume of C3N4−CoSe2 in microliters (μL). Figure 6a shows that a very small amount (50 μL) of C3N4−CoSe2 can successively shift the onset potential up to 0.134 V vs RHE having photocurrent density 3.19 mA/cm2 at −0.289 V vs RHE. At “0” V it can achieve photocurrent density nearly 1.12 mA/cm2. All the prepared electrodes of Si/C3N4− CoSe2−V (V= 50, 80, 100, and 140 μL) show a very negligible response in dark. In this measured potential window Si MWs does not show any PEC response. Freshly etched Si MWs can achieve an 8 mA/cm2 photocurrent density at an applied potential −0.69 V vs RHE (−0.95 V vs Ag/AgCl) with an onset potential −0.49 V vs RHE (−0.75 V vs Ag/AgCl) (Figure S10). Successive increase in the loading amount of C3N4−CoSe2 on Si MWs further increases the photocurrent density and also shifts the onset potential. Si/C3N4−CoSe2−100 (100 μL) can achieve the optimum onset potential 0.194 V vs RHE with photocurrent density 8.4 mA/cm2 at −0.289 V vs RHE. At 0 V vs RHE Si/C3N4−CoSe2-100 can generate 4.89 mA/cm2 photocurrent density. With further increase in the loading amount (140 μL) of C3N4−CoSe2 on Si MWs, there is cathodic 26694

DOI: 10.1021/acsami.6b06520 ACS Appl. Mater. Interfaces 2016, 8, 26690−26696

ACS Applied Materials & Interfaces

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hydrogen gas evolution rate of Si/C3N4−CoSe2 was 1.77 μmol/ min. The Faradaic efficiency of hydrogen evolution was 80% under the applied potential of −0.184 V vs RHE in 1 h (Figure S11). We have previously synthesized Pt nanoparticles of the surface of Si MWs through electroless deposition technique to check its photo electrochemical performance.14 Now the photoelectrochemical performance of Si/C3N4−CoSe2 was compared with Si/Pt. It is observed that the Pt decorated Si MWs can show photocurrent density 13.64 mA/cm2 at −0.147 V vs RHE, whereas Si/C3N4−CoSe2 can generate only 6.8 mA/ cm2 (Figure S12). Performance of Si/C3N4−CoSe2 is little worse than that of Si/Pt. Previously it was observed that efficiency of Si/CoSe2 is comparable with Si/Pt.14 This decrease in efficiency may be attributed to the different arrangement of C3N4−CoSe2 on Si surface compared to CoSe2. UV−vis absorption spectroscopy revealed that the visible and near-infrared illumination was absorbed by Si MWs and the shorter wavelength mostly reflected. After decoration with CoSe2/C3N4 on Si MWs helps to reduce reflectance of Si MWs which further implies increase in absorbance. Optical absorbance profile depicts that C3N4−CoSe2 decorated on Si MWs does not hinder the light absorbance of Si MWs (Figure 7b). When the catalyst is fully screening the semiconductor’s top there is the possibility of blocking light absorbance.5 As in our present case catalyst is not fully covering the Si MW top, so it can not block the light absorbance of Si even though having little high catalyst loading in case of Si/C3N4−CoSe2−140.

CONCLUSIONS In conclusion, C3N 4 embedded CoSe2 nanorods were successfully synthesized following a two step method; combustion technique followed by a simple hydrothermal route. C3N4−CoSe2 can function as an efficient H2 evolution electrocatalyst. C3N4−CoSe2 decorated on p-Si MWs can function as stable and efficient photocathode in PEC H2 evolution reaction. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06520. Photoelectrochemical measurement, electrochemical study, impedance spectroscopy, characterization of materials, FESEM images, TEM, HRTEM images, EDS, and electrochemical analysis data (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the Ministry of Science and Technology of Taiwan (Contract Nos. MOST 104-2113-M-002-012-MY3 and MOST 103-2112-M003-009-MY3 and), Academia Sinica (Contract No. AS-103TPA06) and National Taiwan University (104R7563-3). 26695

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DOI: 10.1021/acsami.6b06520 ACS Appl. Mater. Interfaces 2016, 8, 26690−26696