Graphdiyne Nanowall for Enhanced Photoelectrochemical

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Graphdiyne Nanowall for Enhanced Photoelectrochemical Performance of Si Heterojunction Photoanode Suicai Zhang,†,‡ Chen Yin,†,§,⊥ Zhuo Kang,†,‡ Pingwei Wu,‡ Jing Wu,‡ Zheng Zhang,‡ Qingliang Liao,‡ Jin Zhang, *,§ and Yue Zhang*,‡,∥

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/11/18. For personal use only.

‡

State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, and ∥Beijing Municipal Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, P. R. China § Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, and ⊥Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: Graphdiyne (GDY), a new member of 2D carbon material family, was introduced into a Si heterojunction (SiHJ)-based photoelectrochemical water splitting cell. With assistance of magnetron-sputtered NiOx, the plateau photocurrent density of SiHJ/GDY/NiOx-10 nm with optimized NiOx film thickness was twice higher than that of SiHJ/NiOx-10 nm, demonstrating the catalytic function of GDY itself as well as the synergistic effect between GDY and NiOx. The results verified that GDY is a promising photoelectrode material candidate to realize highly efficient PEC performance, and pave a novel pathway to further improve Si-based PEC system. KEYWORDS: graphdiyne, Si heterojunction, photoelectrode

A

s one of the most promising strategies to deal with severe problems of energy scarcity, water splitting with hydrogen evolution has been intensively investigated to supplement clear energy source.1 Hydrogen, a low-cost, renewable, and neutral source chemical fuel, has played an important role in lots of industries as an energy carrier,2 and is expected to be massively generated utilizing natural energy. A typical scenario to realize this aim is the photoelectrochemical (PEC) cell, which takes full advantage of the perpetuate and inexhaustible solar resource and transforms solar energy into chemical fuels.3,4 Such a PEC cell consists of two electrodes (photocathode and photoanode), which is able to generate electrons and holes to participate in water splitting reaction process driven by harvesting solar energy.5 One great challenge is further exploration of earth-abundant materials, which possess ideal properties of high photoresponse and long-term stability to serve in harsh aqueous environment.6 Silicon, a semiconductor widely used in electronics industry, can also be utilized as the photoelectroode in PEC cells, due to its excellent ability to absorb visible light.7 To further enhance the performance of Si in PEC cells, nanostructure designing with diverse surface morphologies like porous, pyramid, or nanowires is an effective way to trap the light, reduce the reflection, and increase the active area for electrochemical reaction.8 © XXXX American Chemical Society

However, application of Si in photoanode is still restricted because its band gap is smaller than 1.23 eV, to which combining other electro/photoactive materials is an alternative solution.9 Also, although Si-based electrodes possess advantages like high crystallinity, light absorbance, charge carrier mobility, etc.,8 photoanodic corrosion poses a challenge for their wider application.10 Various solutions have been proposed to deal with this problem, including morphology optimization, surface passivation, catalyst decoration, and alternative electrolyte.11 Among them, passivating the surface stands out as a both cheap and beneficial way. The extensively investigated metal oxides introduced in PEC cells, such as TiO2,12 NiOx,13 NiRuOx,14 and CoOx,15 not only protect photoelectrodes from photocorrosion but also function as catalyst or form heterostructures to promote the oxygen evolution reaction (OER). Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: April 19, 2018 Accepted: August 1, 2018 Published: August 1, 2018 A

DOI: 10.1021/acsami.8b06382 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Morphologic characterizations of the photoanode. SEM images of (a) micropyramidal structure of silicon, (b) silicon substrate loaded with GDY at low magnification, (c) porous structure of the GDY at higher magnification, and (d) GDY loaded with NiOx catalyst. (e) HR-TEM image of NiOx catalysts on the GDY. (f) Raman spectra of GDY, GDY/NiOx-5 nm, GDY/NiOx-10 nm, and GDY/NiOx-15 nm.

NiOx modification might unexpectedly impede such strengthening effect. The saturated photocurrent density reaching up to 39.1 mA cm−2 indicated the superiorities of GDY introduction to Si-based PEC system, and again verified the universal significance of GDY in solar energy harvesting and other related energy conversion applications. The scanning electron microscopy (SEM) images (Figure 1a) showed that silicon substrates possessed micropyramidal structure, which would supply more room for catalysts to accommodate. Bayeta-like GDY (Figure 1b), were loaded on the surface of the micropyramidal silicon substrates. The amplified porous structures were obtained in the image Figure 1c. The NiOx catalysts were magnetron sputtered on the surface of GDY without changing its nanowall structure (Figure 1d) and could offer larger active area for splitting water and generating oxygen. The high-resolution TEM (HRTEM) image (Figure 1e) indicated the small particles on the surface of the GDY are the NiOx catalysts. The Raman spectrum, as an effective way to investigate Raman-active C−C triple bonds, exhibited four characteristic peaks around 1390.1, 1565.9, 1933.3, and 2177.9 cm−1 (Figure 1f) which all matched previous report well.25 The peak located at 1390.1 and 1565.9 cm−1 corresponded to the D band and G band, respectively. Besides, the two peak around 1933 and 2177 cm−1 were ascribed to the vibration of conjugated diyne links.25 In the case of GDY with different thickness of NiOx samples, the G band with a higher frequency region was observed, indicating that the interaction remained between GDY and NiOx. Similar phenomenon also took place in the sp C peak, referred to the previous literature on linear carbon chains, it concluded that coupling with electron-withdrawing roles may attribute to the blue shift.23,26 Especially, for the GDY/NiOx-10 nm, the intensity ratio of D and G bands (ID/IG = 0.91) was larger compared with pristine GDY (ID/IG = 0.79) sample, which was also the largest among the different thickness of GDY/NiOx samples, indicating an increased active

GDY, a new carbon family member, is a metal-free carbonbased two-dimensional material, and its sp- and sp2- hybridized carbon atoms alignment gives rise to unique electronic properties and outstanding stability.16 It has been demonstrated to be promising in research fields like catalysis, nonlinear optics, gas separation, electronic devices, and energy storage devices.17−19 This is because of its unique properties including high thirdorder nonlinear optical susceptibility, extreme hardness, uniformly distributed pores, low thermal conductivity, and high charge carrier mobility.20 In terms of water splitting application, GDY with high hole mobility, could also be easily prepared and integrated with other catalysts,21 thus greatly improving the PEC performance as a coating layer of photoanode.22 Li group developed a GDY-based hydrogen evolution cathode (Cu@GDY) that exhibited higher catalytic activity than most non-noble metal electrocatalysts.21 GDY on BiVO4 was reported to obtain a photocurrent density nearly two times that of bare BiVO4.22 The photoelectrode with CdSe quantum dots sensitized GDY was designed to exhibit −70 μA cm−2 photocurrent density at 0 V vs NHE in neutral aqueous solution.23 Especially, considering that the hole mobility (1 × 104 cm2 V−1 S−1) and conductivity (2.516 × 10−4 S m−1) of GDY are of similar magnitude with those of silicon, it was expected that taking GDY and silicon into combination as photoelectrodes might boost even higher PEC efficiency.23,24 In this study, for the first time, we successfully introduced GDY into the Si-based photoelectrochemical system for water splitting. The GDY nanowalls and subsequent magnetron sputtered NiOx nanoparticles were employed on the top of SiHJ (Si heterojunction) for photoinduced hole transfer facilitation and oxygen evolution catalysis. NiOx and GDY have been characterized to be respectively capable to catalyze water oxidization as previously reported.13,21−23 Meanwhile, the chemical bonds between GDY and NiOx led to the synergistic effect, which contributed to the further catalytic enhancement only with proper NiOx thickness. On the contrary, the excessive B

DOI: 10.1021/acsami.8b06382 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 2. (a) Schematic of the SiHJ and (b) energy band diagrams of the SiHJ photoanode.

Figure 3. XPS spectra of GDY/NiOx (a) Ni 2p3/2, (b) O 1s, and (c) C 1s.

magnetic sputtering. The chemical and structural information are further examined by XPS. Ni 2p3/2 peaks of NiOx were shown in Figure 3a, which can be separated into three distinct peaks, consistent with previous works of NiOx films prepared through sputtering methods.29 The peak located at 853.5 eV matched the classic Ni−O octahedral bonding configuration of the cubic NiOx, which pertains to Ni2+. Another peak center at 855.5 eV was associated with vacancy-induced Ni3+ ions, and the shakeup process could account for the broad peak at 860.9 eV. Further evidence about octahedral bonding of Ni−O was exhibited in Figure 3b, the XPS spectra of O 1s in NiOx, with a peak at 529.7 eV. The peak at 531.6 eV may be related to nickel hydroxides, containing defective nickel oxides with hydroxyl groups which were possibly adsorbed onto the surface.29 The GDY’s C 1s XPS spectrum was presented in Figure 3c. After deconvolution, the peak of C 1s could be divided into four subpeaks at 284.4 eV (C−C (sp2)), 285.0 eV (C−C (sp)), 286.0 eV (C−O) and 288.2 eV (CO). The small peaks of oxygen-containing groups can be explained as some oxidized terminal alkyne. The OER performance was first characterized based on a three-electrode cell (SiHJ/NiOx, Pt wire, and Ag/AgCl were used as photoanode, counter electrode (CE), and reference electrode (RE), respectively) in 1 M KOH under the illumination of 100 mW cm−2. Polarization curves in Figure 4a demonstrated that,the SiHJ photoanode barely showed photocurrent density in the range of 0.5−2 V (vs RHE), after the deposition of NiOx, the J−V curves of SiHJ/NiOx samples all shifted negatively compared with that of the bare SiHJ photoanode, and exhibited a light-limited photocurrent density under illumination. Especially, the onset potential of SiHJ/ NiOx was even below the thermodynamic oxidation level of water (EH2O/O2, 1.23 V, vs RHE), which was attributed to the photovoltage generated by the SiHJ and the catalytic effect of NiOx catalyst. In particular, the SiHJ/NiOx-15 nm sample exhibited an OER onset potential (1.129 V) lower than those of SiHJ/NiOx-10 nm (1.142 V), SiHJ/NiOx-5 nm (1.168 V), and SiHJ (1.932 V) samples. This indicated that the OER

sites, and it was fairly beneficial for the enhancement of OER performance. Here we selected silicon heterojunction (SiHJ) as the substrate to realize further photocatalyst modification. As can be seen in the structure diagram (Figure 2a), the thickness of n-type single crystalline silicon was 280 μm, both sides of which were of micropyramidal structure. Both sides were coated by an ultrathin (7 nm in thickness) intrinsic a-Si:H layer, then by an 8 nm thick n-type a-Si: H layer and 15 nm thick p-type a-Si: H coating, respectively on each side. At last, conducive ITO layers were sputtered on its both sides, and a silver electrode was also added on the backside. Micropyramidal structure on the two sides of the n-type crystalline silicon we utilized here were obtained by KOH anisotropic etching, and was expected to enhance the light harvest ability and quantity of active sites for surficial reaction. Such structure significantly enlarged the interface area between the semiconductor and electrolyte, so that more electrocatalyst could be effectively loaded. However, the number of surficial defects can be also increased along with the increase of the electrode surface area. Unfortunately, the electron−hole recombination will be favored and the density of photocurrent will be lowered consequently. To take advantage of the strong incident light absorption of micropyramidal structure and its large surface area, the passivation layer was introduced to decrease the density of surficial defect states. Ultrathin intrinsic a-Si:H composed passivation layers on the two sides of n-type crystalline silicon, and effectively reduced dangling bonds on the surface of n-type crystalline silicon.27 Meanwhile, a-Si passivation layer formed a junction with crystalline silicon, resulting in interfacial band bending of ΔEv and ΔEc correspondingly to valence band and conduction band. This energy barrier brought about field effect passivation and impeded charge carrier recombination as well.28 The ITO layers on each side of a-Si contributed to the a-Si protection and were beneficial for charge transfer with the least electronic loss by ohmic contact (Figure 2b). GDY was introduced onto the obverse side of SiHJ, and the catalyst layer NiOx was then deposited on the GDY layer via C

DOI: 10.1021/acsami.8b06382 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) J−V characteristics of the SiHJ photoanodes with varying thickness of NiOx as OER catalysts; (b) J−V characteristics of GDY modified SiHJ photoanodes with varying thickness of NiOx as OER catalysts; (c) J−V characteristics of the SiHJ photoanodes loaded with 10 nm thick NiOx catalysts with/without GDY modifying; (d) Chronopotentiometry curves of the SiHJ photoanodes loaded with 10 nm thick NiOx catalysts with/without GDY modifying at potential of 1.4 V. All the experiments were conducted in 1 M KOH under the illumination of 100 mW cm−2 regularly.

properties of SiHJ increased along with the increasing thickness of NiOx catalysts, which was also verified by the saturated photocurrent density comparison. Specifically, the champion saturated photocurrent density based on SiHJ/NiOx-15 nm is 22.7 mA cm−2, whereas SiHJ/NiOx-10 nm is 18.7 mA cm−2, SiHJ/NiOx-5 nm is 10.9 mA cm−2, and SiHJ is only 4.2 mA cm−2. For the samples introduced with GDY in Figure 4b, after the deposition of NiOx, the J−V curves of SiHJ/GDY/NiOx samples greatly shifted negatively, a saturated photocurrent density was obtained with the increase of anodic potential, and the onset potential was below EH2O/O2. Similar phenomena can be found in Figure 4a. To be more specific, again, with varied NiOx thickness, the SiHJ/GDY/NiOx-10 nm exhibited the best onset potential of 1.082 V and saturated photocurrent density of 39.1 mA cm−2 compared with SiHJ/GDY, SiHJ/GDYNiOx-5 nm and SiHJ/GDY-NiOx-15 nm samples. In these scenario, NiOx effectively boosted the accumulation of photoinduced holes, thus improving the interfacial catalytic dynamics and simultaneously facilitating the photoinduced charge separation inside the photoelectrode.30 As is demonstrated by Figure 4c, the onset potential of SiHJ/GDY/NiOx-10 nm was negatively shifted compared with that of SiHJ/NiOx-10 nm. The saturated photocurrent density was approximately two times higher than that of SiHJ/NiOx-10 nm after the introduction of GDY. On the basis of this, it can be concluded that, when GDY was introduced between Si and NiOx, the OER performance was further elevated. Besides the demonstrated excellent conductivity and catalytic activity of GDY, the chemical bonds between GDY and NiOx induced synergistic effect should also be responsible for such improvement.23

It is worth noting that the 15 nm thick NiOx was no longer the optimal choice after GDY introduction in Figure 4b. Interestingly, the SiHJ/GDY/NiOx-10 nm sample exhibited the champion performance, indicating that excessive NiOx modification might unexpectedly impede the synergistic effect. A further comparative study was conducted in Figure 4d to highlight the superiorities of the combination of GDY and optimized NiOx. In summary, we successfully introduced GDY nanowalls to the Si-based PEC system for significantly enhanced water splitting performance. With further modification of varied NiOx thickness, the optimized SiHJ/GDY/NiOx-10 nm exhibited a saturated photocurrent density reaching up to 39.1 mA cm−2 due to the superiorities of GDY and NiOx themselves, as well as the chemical interaction induced synergistic effect. We also noticed that such strengthening effect might be impeded with excessive NiOx modification. This work supplied a feasible route for Si-based PEC performance enhancement, and again demonstrated the unique superiority and significant role of GDY in energy conversion applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06382. Experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Z.). *E-mail: [email protected] (Y.Z.). D

DOI: 10.1021/acsami.8b06382 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ORCID

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Jin Zhang : 0000-0003-3731-8859 Yue Zhang: 0000-0001-7772-3280 Author Contributions †

S.Z., C.Y., and Z.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks Prof. Yuliang Li’s great support and instruction on this work. This work was supported by the National Key Research and Development Program of China (2016YFA0202701), Overseas Expertise Introduction Projects for Discipline Innovation (111 project, B14003), the National Natural Science Foundation of China (Nos. 51527802, 51232001, 51702014, 51372020 and 51602020), National Major Research Program of China (2013CB932602).

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DOI: 10.1021/acsami.8b06382 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX