Functionalized Phosphorene Quantum Dots as Efficient Electrocatalyst for Oxygen Evolution Reaction Ranjith Prasannachandran,† Thazhe Veettil Vineesh,† Athira Anil,† B. Murali Krishna,‡ and Manikoth M. Shaijumon*,† ACS Nano Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/26/18. For personal use only.
†
School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala PO, Thiruvananthapuram, Kerala 695551, India ‡ Department of Chemistry, CMS College, Kottayam, Kerala 686001, India S Supporting Information *
ABSTRACT: Phosphorene has attracted great interest in the rapidly emerging field of two-dimensional layered nanomaterials. Recent studies show promising electrocatalytic activity of few-layered phosphorene sheets toward the oxygen evolution reaction (OER). However, controllable synthesis of mono/fewlayered phosphorene nanostructures with a large number of electrocatalytically active sites and exposed surface area is important to achieve significant enhancement in OER activity. Here, a novel strategy for controlled synthesis and in situ surface functionalization of phosphorene quantum dots (PQDs) using a single-step electrochemical exfoliation process is demonstrated. Phosphorene quantum dots functionalized with nitrogen-containing groups (FPQDs) exhibit efficient and stable electrocatalytic activity for OER with an overpotential of 1.66 V @ 10 mA cm−2, a low Tafel slope of 48 mV dec−1, and excellent stability. Further, we observe enhanced electron transfer kinetics for FPQDs toward the Fe2+/Fe3+ redox probe in comparison with pristine PQDs. The results demonstrate the promising potential of phosphorene as technologically viable OER electrodes for water-splitting devices. KEYWORDS: black phosphorus, quantum dots, electrochemical exfoliation, oxygen evolution reaction, electrocatalysis gap (0.3 to 2 eV), high carrier mobility (∼1000 cm2 V−1 s−1), and numerous projected applications in nanoelectronics, optoelectronics, photovoltaics, and energy storage.9−13 Mechanical and liquid phase exfoliation techniques have been mostly used for the synthesis of few-layered phosphorene sheets.6,7,14−16 While scalability remains a problem with the former approach, sonication-assisted liquid phase exfoliation results in poor quality of the products. Recently, the electrochemical exfoliation method has been successfully demonstrated to achieve scalable growth of good quality fewlayered phosphorene sheets.17−19 Recent studies on electrocatalytic activity of black phosphorus and few-layered phosphorene sheets for the oxygen evolution reaction (OER) have shown exciting potential of these unique 2D layered material.20,21 In order to achieve enhanced electrocatalytic activity, it is of great significance to increase the number of active sites and exposed surface area by achieving controllable growth of
he discovery of graphene by Novoselov, Geim, et al.1 inspired researchers to explore similar layered materials and has led to the emergence and development of atomically thin two-dimensional layered nanomaterials, such as hexagonal boron nitride (hBN),2,3 transition metal dichalcogenides (TMDCs),4,5 phosphorene, and their heterostructures.6,7 With their intriguing layer-dependent physicochemical properties, the recently discovered 2D layered materials have emerged as a fascinating platform to explore both fundamental and device properties and have opened up a wide range of applications in the field of nanoscience and engineering. The recent discovery of phosphorene, a unique 2D material with a monolayer or few layers of black phosphorus (BP), has generated considerable excitement in the rapidly emerging field of 2D layered materials.8 Phosphorus is one of the most earth-abundant elements and exists in three main forms: white, red, and black allotropes. Black phosphorus, with a bond angle of 96.33° and 103.09°, close to the tetrahedral bond angle of 109.5°, exhibits better stability over other allotropes of phosphorus. Widespread interest in phosphorene results from its remarkable properties, which include a unique puckered layer structure, tunable band
T
© XXXX American Chemical Society
Received: August 31, 2018 Accepted: October 22, 2018
A
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
www.acsnano.org
Cite This: ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 1. Schematic representation of the electrochemical synthesis of few-layered PQDs and FPQDs from bulk BP crystals.
RESULTS AND DISCUSSION Synthesis and Characterization of FPQDs and PQDs. Phosphorene quantum dots are prepared from their bulk crystals by using a custom-designed electrochemical exfoliation setup, as detailed in the Materials and Methods section. Briefly, the experiment consists of applying a 2 V potential to the anode (black phosphorus crystal) placed in an electrolyte solution consisting of 0.1 wt % of LiClO4 (supporting electrolyte) in propylene carbonate against a platinum counter electrode. Propylene carbonate is found to be a better candidate for this reaction mechanism, as it has a wide operational potential window and high dielectric constant.25 Its nonaqueous nature further prevents the chance of degradation of the material. An applied potential to the electrolyte beyond its stable potential window can result in its decomposition, which in turn results in the formation of charged functional groups that further attack the anode of the electrolytic cell.32 With appropriate choice of electrolyte, the process can lead to in situ functionalization of the exfoliated products, as previously demonstrated for GNRs.33 In view of improving the conductivity and increasing the number of active sites toward electrocatalytic activity of PQDs, we further chose 0.1 wt % LiClO4 in formamide as the electrolyte for the electrochemical exfoliation process. FPQDs are obtained under controlled experimental conditions under an applied potential of 3 V, which was fixed based on the decomposition voltage of formamide (Figure S1). A schematic representation of the reaction pathway and the exfoliation mechanism is shown in Figure 1. An electrochemically exfoliated solution consisting of FPQDs showed an intense greenish color even after centrifugation at 7000 rpm for 20 min, which upon UV irradiation emitted a highly fluorescent bluish color (Figure S2). The observation of fluorescence could be attributed to the presence of different functional groups, similar to the mechanism proposed for functionalized carbon nanoparticles.34
mono/few-layered phosphorene with reduced dimensions, as demonstrated for graphene and TMDCs.22−24 Our group has previously demonstrated size-controlled synthesis of quantum dots of TMDCs using electrochemical etching of the bulk material.25−27 Synthesis of black phosphorus quantum dots following liquid phase exfoliation techniques has recently been reported.28−30 Achieving phosphorene quantum dots (PQDs) through a controlled electrochemical exfoliation method would be of great interest for their electrocatalytic studies. PQDs, with a large number of active edge sites and exposed surfaces, are expected to exhibit enhanced OER activity. Further, surface functionalization of these materials can lead to improved electrocatalytic properties,31 owing to the presence of an increased number of active sites. Few-layer phosphorene is found to be more prone to degradation under exposure to oxygen. As it is one of the major issues that restrict the semiconducting applications of phosphorene, several efforts are being explored to address this, and recently it has been reported that functionalization improves the stability and thus prevents the rate of degradation.32 However, to date, there is no report on in situ functionalization of phosphorene quantum dots for OER applications. Herein, we demonstrate a novel strategy for size-controlled synthesis and in situ surface functionalization of PQDs using a single-step electrochemical exfoliation process. Nitrogen-functionalized few-layer phosphorene quantum dots (FPQDs) are found to exhibit remarkable enhancement in electrocatalytic properties for the oxygen evolution reaction, compared with the other reported BP-based catalysts,20,21 with an overpotential of 1.66 V (@ 10 mA cm−2) and a Tafel slope of 48 mV dec−1. Density functional theory (DFT) calculations confirm the enhanced OH− adsorption on FPQDs leading to enhanced OER activity. The obtained results are promising and show exciting potential of this unique 2D material for OER applications. B
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 2. (A) TEM and (B) HRTEM images of FPQDs. The inset of (A) shows the histogram representing the average particle size distribution of FPQDs, and the inset of (B) shows the diffraction pattern of FPQDs. A lattice-resolved HRTEM image of the selected particle revealing different crystal planes is also shown as an inset in (B). (C) TEM and (D) HRTEM images of PQDs. The inset of (C) shows a histogram representing the average particle size distribution of PQDs. Diffraction pattern of PQDs and a magnified TEM image of the particle showing lattice fringes are given as insets in (D). (E) AFM image of FPQDs and (F) the corresponding height profiles from two different spots as marked in (E).
2C. The lattice fringes found to be 0.22 nm apart correspond to the (0 2 0) plane of black phosphorus crystal (inset of Figure 2D).6,35 The elements present in FPQDs are also confirmed from TEM-EDS analysis, as shown in Figure S3. The topographic morphology of the FPQDs characterized by using AFM show a uniform distribution of particles (Figure 2E) with measured heights of 2−3.5 nm (Figure 2F), corresponding to ∼3−6 layers. PQDs exhibit similar morphology as revealed from AFM studies (Figure S4). The increased height of FPQDs (∼3.5 nm) compared to that of PQDs (∼3 nm) can be due to the presence of attached functional groups.36
Employing transmission electron microscopy (TEM) and atomic force microscopy (AFM), the morphologies of asprepared PQDs and FPQDs were characterized. Figure 2A shows the TEM image of FPQDs, and the histogram drawn from the same shows an average particle size of 2−3 nm (inset of Figure 2A). Figure 2B depicts the high-resolution TEM (HRTEM) image of FPQDs, and the HRTEM image shown in the inset of Figure 2B shows an interplanar distance of 0.25 and 0.38 nm, corresponding to the (1 1 1) and (0 1 2) planes of crystalline black phosphorus.35 The as-synthesized PQDs have an average size between 1 and 2 nm (Figure 2C,D). The particle size distribution of PQDs is shown as an inset in Figure C
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 3. (A) IR and (B) Raman spectra of PQDs and FPQDs compared with bulk BP. (C) XPS survey spectra of various samples. (D, E, and F) Deconvoluted P 2p, C 1s, and N 1s spectra of FPQDs, respectively.
spectrum (Figure 3C). The additional peak observed around 400 eV in the case of FPQDs in comparison with PQDs and bulk BPs clearly confirms the presence of nitrogen-containing functionalizing groups. The high-resolution P 2p XPS spectra of FPQDs (Figure 3D) clearly indicate that phosphorus oxides are also present along with the elemental phosphorus as reported by Pumera et al., for the synthesis of few-layered BP nanosheets.19 The peak observed at ∼132 eV for P 2p spectra can be assigned as a P−N bond, and further, highly electronegative functional groups (O-containing) are attached to the phosphorus atoms during electrochemical exfoliation, which allows the P 2p peak to be shifted toward the higher binding energy region. Thus, we observe an intense peak at 135.4 eV.41 The N 1s peak can be deconvoluted into two, one peak centered at 400.2 eV and the other at 402.5 eV (Figure 3F), which can be attributed to the N−P bond and oxidized-N, respectively.42,43 Deconvoluted high-resolution C 1s spectra show three peaks. A signal at 286.1 eV represents the O−C−O bond, the peak at the 288.4 eV signal corresponds to the attached nitrogen group (N−CO), and the peak observed at 290.7 eV represents OC functionalization in phosphorenes (Figure 3E).42,44 For a comparison, the deconvoluted P 2p spectra of PQDs and bulk BP are shown in Figure S5. The deconvoluted high-resolution P 2p spectra of bulk BP crystal
The Fourier transform infrared (FTIR) spectra of FPQDs (Figure 3A) show peaks at 3386, 2070, 1683, 1390, 1089, and 910 cm−1, corresponding to −NH2, OP−OH, CO−NH2, C−N, PO, and P−N, respectively. The peak at ∼910 cm−1, corresponding to the P−N bond indicating the presence of nitrogen-containing functional groups attached to phosphorus atoms, was absent for both PQDs and bulk BP. Raman spectroscopy is a powerful characterization technique, and a typical Raman spectrum of bulk BP comprises three prominent phonon vibration modes, 362.6, 439.6, and 466.3 cm−1, when excited with a green laser of 532 nm (Figure 3B). Raman peaks observed for PQDs and FPQDs are broadened and are found to be shifted toward higher wavenumber due to their structural anisotropy. It has been well studied that when the bulk black phosphorus gets exfoliated into its layered counterparts, the covalently bonded P atoms oscillate together with more energy as the interlayer stacking van der Waal force weakens with the decrease in layer number. The peaks observed for PQDs and FPQDs were found to be less intense compared to bulk BP, owing to reduced thickness of the exfoliated sample, like in other 2D systems.37−40 X-ray photoelectron spectroscopy (XPS) was carried out to investigate the chemical composition of the as-prepared materials. The electrochemically imposed chemical modifications were well seen in the XPS survey D
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 4. Cyclic voltammograms of PQDs and FPQDs in 5 mM ferricyanide/0.1 M KCl solution. (A) Comparison of the cyclic voltammetric response of PQDs and FPQDs at a scan rate of 10 mV s−1. (B) Cyclic voltammetric response at sweep rates of 10 and 100 mV s−1 for FPQDs.
Figure 5. (A) LSV for the OER activity of PQDs, FPQD (2V), and FPQDs in 1 M NaOH solution. (B) Tafel plots obtained from the corresponding LSVs in (A). (C) Impedance spectra of PQDs and FPQDs with the corresponding equivalent circuit. The inset shows the expanded portion of the FPQD impedance spectra. (D) Chronoamperometric plot showing the stability of FPQDs.
Zhang et al.30 The functionalized quantum dots are found to be more fluorescent under UV irradiation (Figure S2 and Figure S7 inset). When excited over a range of 340−400 nm, the PL emission is found to be more or less similar at ∼430 nm, confirming the presence of a direct band gap (Figure S7 A), and when excited with a green diode laser of 532 nm, the PQDs showed a band gap of 1.79 eV, whereas FPQDs showed a band gap of 1.87 eV (Figure S7B). Electrochemical Studies. In order to find out the electrode kinetics, cyclic voltammetric (CV) measurements for both PQD and FPQD electrodes are carried out in a
showed 2P1/2 at 130.2 eV and 2P3/2 at 129.3 eV. In PQDs, in addition to the peak observed at 130.3 eV representing a P−P bond, the intense peak centered around 134.1 eV can be ascribed to the presence of PxOy, as few-layered BP in nanometric dimensions is prone to surface oxidation.19,30,45 The nitrogen content in FPQDs was calculated to be 4.7% from XPS data. UV−vis NIR absorption spectroscopy is used to unravel the optical and electronic properties of both PQDs and FPQDs (Figure S6). The absorption spectrum of PQDs has peak positions at 320 and 380 nm, which are in agreement with the studies on phosphorene quantum dots reported by E
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
synthesized using the same formamide-based electrolyte under an applied potential of 2 V. Reduced applied voltage would lead to a lesser extent of electrolyte decomposition, resulting in a lower amount of nitrogen-containing functional groups attached to the PQDs. We obtained PQDs with an average size of ∼6 nm (Figure S10), with a smaller amount of nitrogen-containing functional groups (FPQDs-2V), as revealed from the XPS survey scan (Figure S11). The amount of nitrogen content was calculated to be ∼3.2%. For comparison, OER activity of FPQDs-2V was studied under similar experimental conditions. FPQDs-2V exhibited improved OER activity compared to PQDs; however, it was found to be less active compared to FPQDs (Figure S12). FPQDs thus exhibit excellent OER activity, one of the best among BP-based catalysts reported so far (Table S1) and which is on par with benchmarked IrO2 based OER catalyst (Figure S13), with low electrical resistivity, low Tafel slope, higher current density, and appealing stability, making it an efficient anode material for electrochemical water splitting. The nitrogen functionality can induce a charge separation in the lattice due to the electronegativity difference between the phosphorus atom and the nitrogen. This in turn will enhance the OH− adsorption on the phosphorus atoms. In order to verify our hypothesis, DFT calculations have been carried out and the P− O bond distance in phosphorene- and amine-functionalized phosphorene is found to be 1.94 and 1.68 Å, respectively (Figures S20 and S23). Further we have calculated the binding energies of OH− on the BP and BP-NH2 at the geometryoptimized structures with the supramolecular approach including the Boys−Bernadi counterpoise correction, which were found to be −63.66 and −112.39 kcal/mol, respectively. Thus, the decrease in P−O bond distance as well as the increase in binding energies for nitrogen-functionalized phosphorene clearly confirms the enhanced −OH adsorption, leading to enhanced OER activity.
solution containing 5 mM potassium ferricyanide as redox probe and 0.1 M KCl as supporting electrolyte. The CV responses of PQDs and FPQDs are shown in Figure 4A for a scan rate of 10 mV s−1. The anodic and cathodic peaks observed in both cases correspond to an Fe3+/Fe2+ redox couple. From the CV curves it is clear that the peak potential separation (ΔEp) is larger in the case of PQDs compared to FPQDs. FPQDs show a ΔEp of 70 mV at a sweep rate of 100 mV s−1, while PQDs show a ΔEp of 183 mV. This indicates the faster electron transfer kinetics for the FPQD-modified electrodes. For quasi-irreversible systems the current is controlled by mass transfer and charge transfer. The standard heterogeneous rate constant (k0) for quasi-irreversible systems can be calculated by the method proposed by Nicholson.46,47 Both samples exhibit the characteristics of quasi-reversibility as shown in the CV curves at sweep rates of 10 and 100 mV s−1 for FPQDs and PQDs, as shown in Figure 4A and Figure S8, respectively. At faster sweep rates ΔEp increases as shown in Figure 4B. The rate constant values calculated using the Nicholson method for FPQDs and PQDs were 1.305 × 10−3 and 4.34 × 10−4 cm s−1, respectively. As this indicates much faster electron transfer kinetics for FPQDs, the k0 is therefore approximately 1 order of magnitude higher for FPQDs compared to PQDs. The oxygen evolution reaction activity of the as-prepared samples was evaluated using a rotating disc electrode (RDE) system, which consists of glassy carbon coated with different samples, a Pt wire, and Ag/AgCl electrodes as working electrode, counter electrode, and reference electrode, respectively. All the electrode potentials were converted to reversible hydrogen electrode (RHE) for better comparative study unless otherwise stated. Linear sweep voltammetry was performed in 1 M NaOH solution with a sweep rate of 10 mV/s in the RDE system with a rotation speed of 1600 rpm. Figure 5A shows the comparison of the linear sweep voltammetric analysis for the OER activity of FPQDs and PQDs. FPQDs generated a current density of 10 mA cm−2 at 1.66 V vs RHE, which is far better than PQDs and other reported BP-based materials (PQDs show a current density of 0.2 mA cm−2 at 1.66 V). The Tafel slope is the other important parameter to analyze the electrocatalytic activity of the material. The Tafel slope can be calculated from a Tafel plot in which the overpotential (η) is logarithmically related to the current density (−j). A catalytic material having a small Tafel slope is considered to be highly active. Tafel slopes were obtained for FPQDs and PQDs and show a value of 48 and 103 mV dec−1, respectively, as shown in Figure 5B. Impedance spectra of FPQDs and PQDs (Figure 5C) reveal a charge transfer resistance of 295 Ω for FPQDs, which is much smaller than that of PQDs (impedance analyses were performed at an overpotential of 1.585 V vs RHE). In order to check the stability of the material, a chronoamperometry test was performed on FPQDs for >10 h at 1.65 V, and the current density was found to be extremely stable, indicating the long-term viability of this material. Various ex situ techniques48,49 can be employed to understand the stability of the catalyst after water electrolysis, in which XPS measurement is found to be a better tool.50 There is no observable change in deconvoluted P 2p XPS spectra of FPQDs before and after OER experiments, which indicates the stability of FPQDs toward OER (Figure S8). In order to further confirm the significance of nitrogen-containing functional groups on the OER activity of PQDs, we prepared PQDs with varying nitrogen content. For this, PQDs were
CONCLUSIONS In summary, we have for the first time demonstrated a singlestep procedure for the synthesis and in situ functionalization of phosphorene quantum dots by using a simple electrochemical approach, providing rich active sites for enhanced oxygen evolution. The nitrogen functionality can induce a charge separation in the lattice due to the electronegativity difference between a phosphorus atom and the nitrogen, which in turn enhances the OH− adsorption on the phosphorus atoms, leading to improved OER activity. The heterogeneous electron transfer kinetics of the as-prepared materials (PQDs and FPQDs) was studied using a standard redox couple (Fe2+/Fe3+ system). The electrochemical measurements showed enhanced electron transfer kinetics of FPQDs in comparison with PQDs. The materials were further tested for their oxygen evolution activity in 1 M NaOH solution. The higher current density (10 mA cm−2 at 1.66 V), low Tafel slope (48 mV dec−1), low electrical resistance (295 Ω), and higher stability (stable for >10 h) of FPQDs in comparison with PQDs and other reported works make them a promising anode material for alkaline water splitting. MATERIALS AND METHODS Synthesis of PQDs and FPQDs. Bulk black phosphorus crystals (purchased from Smart Elements GmbH, Vienna) and lithium perchlorate (Sigma-Aldrich) were used directly as received. Functionalized phosphorene quantum dots were synthesized using a basic twoF
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano ORCID
electrode electrochemical exfoliation process. In a typical experiment an electrochemical cell was designed with black phosphorus crystal flakes clamped onto stainless steel as the anode material against a platinum counter electrode. These electrodes were placed in an ionic electrolyte (comprising 0.1 wt % LiClO4 in formamide) 1 cm apart. The process comprises applying a positive potential of 3 V to the anode at room temperature. To reduce the chances of degradation due to air exposure, we carried out the reaction in an argon-filled glovebox. The schematic representation of FPQD synthesis is shown in Figure 1. The applied potential decomposes formamide as well as carries out the exfoliation process, resulting in the synthesis of functionalized phosphorene quantum dots. The resultant electrolyte containing FPQDs having an intense greenish color was collected after 48 h and centrifuged using Vivaspin 2 centrifugal concentrators at 7000 rpm for 30 min to remove any traces of the bulk material. Phosphorene quantum dots were synthesized using the same protocol as mentioned above with the electrolyte replaced with propylene carbonate with an applied potential of 2 V for 48 h. Calculations. DFT calculations were performed using the generalized gradient approximation (GGA) using the parametrization of Perdew, Burke, and Ernzerhof (PBE)48 with an added fraction of Hartree−Fock exact exchange, choice of α = 0.25, in the “PBE0” functional.49 Since phosphorene is a large system, at the optimized geometries, total energy was calculated taking into account the dispersion contribution using Grimme’s dispersion correction D3.50 The calculations used the density fitting correlation consistent ccpVDZ-JKFIT and cc-pVDZ-MP2FIT basis sets.51 The calculations were run using the implementation in the MOLPRO2015 suite of programs.52 Electrode Preparation and Electrochemical Characterization. The as-synthesized FPQD/PQD solution was taken, and after centrifugation, 5 μL of the solution was drop casted onto a glassy carbon (4 mm) RDE setup (ALS Instruments). The electrode was then kept for drying inside a vacuum desiccator under normal room temperature. A 1 mL amount of binder solution was prepared by adding 50 μL of Nafion 117 (Sigma Aldrich) to 950 μL of ethanol. A 3 μL amount of the aforementioned solution was drop casted on top of the catalyst drop casted RDE electrode and was kept for drying. The loading amount of PQDs/FPQDs on the working electrode was measured to be ∼0.2 mg/cm2. All the electrochemical measurements were done with a standard RDE setup, which is a three-electrode system consisting of a rotating glassy carbon working electrode along with a Ag/AgCl reference electrode and a platinum counter electrode. Linear sweep voltammetry was performed in a 1 M NaOH (Sigma Aldrich) (pH = 14) solution with a sweep rate of 10 mV/s in the RDE system with a rotation speed of 1600 rpm. EIS measurements were performed in a frequency range of 50 kHz to 500 mHz. The electrode potential versus Ag/AgCl was converted to RHE with the help of the Nernst equation:
Manikoth M. Shaijumon: 0000-0001-5745-4423 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS M.M.S. acknowledges financial support from Science and Engineering Research Board (SERB), Department of Science & Technology, Govt. of India (EMR/2017/000484), and Indian Institute of Science Education and Research (IISER), Thiruvananthapuram, Kerala, India. We acknowledge the computational resources of the IDRIS computer center of France and of the ROMEO computer center at Reims Champagne−Ardenne. REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Xue, J.; Sanchez-Yamagishi, J.; Bulmash, D.; Jacquod, P.; Deshpande, A.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; LeRoy, B. J. Scanning Tunnelling Microscopy and Spectroscopy of Ultra-Flat Graphene on Hexagonal Boron Nitride. Nat. Mater. 2011, 10, 282−285. (3) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722−726. (4) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (5) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699− 712. (6) Brent, J. R.; Savjani, N.; Lewis, E. A.; Haigh, S. J.; Lewis, D. J.; O’Brien, P. Production of Few-Layer Phosphorene by Liquid Exfoliation of Black Phosphorus. Chem. Commun. 2014, 50, 13338− 13341. (7) Yasaei, P.; Kumar, B.; Foroozan, T.; Wang, C.; Asadi, M.; Tuschel, D.; Indacochea, J. E.; Klie, R. F.; Salehi-Khojin, A. HighQuality Black Phosphorus Atomic Layers by Liquid-Phase Exfoliation. Adv. Mater. 2015, 27, 1887−1892. (8) Carvalho, A.; Wang, M.; Zhu, X.; Rodin, A. S.; Su, H.; Castro Neto, A. H. Phosphorene: From Theory to Applications. Nat. Rev. Mater. 2016, 1, 16061. (9) Xia, F.; Wang, H.; Jia, Y. Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 2014, 5, 1−6. (10) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033−4041. (11) Sun, J.; Lee, H. W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y. A Phosphorene-Graphene Hybrid Material as a HighCapacity Anode for Sodium-Ion Batteries. Nat. Nanotechnol. 2015, 10, 980−985. (12) Li, W.; Yang, Y.; Zhang, G.; Zhang, Y. W. Ultrafast and Directional Diffusion of Lithium in Phosphorene for High-Performance Lithium-Ion Battery. Nano Lett. 2015, 15, 1691−1697. (13) Sibari, A.; El Marjaoui, A.; Lakhal, M.; Kerrami, Z.; Kara, A.; Benaissa, M.; Ennaoui, A.; Hamedoun, M.; Benyoussef, A.; Mounkachi, O. Phosphorene as a Promising Anode Material for (Li/Na/Mg)-Ion Batteries: A First-Principle Study. Sol. Energy Mater. Sol. Cells 2017, 180, 253−257. (14) Zhang, S.; Yang, J.; Xu, R.; Wang, F.; Li, W.; Ghufran, M.; Zhang, Y. W.; Yu, Z.; Zhang, G.; Qin, Q.; Lu, Y. Extraordinary Photoluminescence and Strong Temperature/angle-Dependent
E(vs RHE) = E(vs Ag/AgCl) + E 0(Ag/AgCl) + 0.059 × pH
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b06671. Additional information on instrumentation; additional characterization of PQDs and FPQDs including CVs, LSVs, XPS, AFM, EDX, HR-TEM, photoluminescence of PQDs, and FPQDs; details on ECSA and Faradaic efficiency calculation; detailed information on computational studies; comparison of OER activity of FPQDs with reported works (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. G
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano Raman Responses in Few-Layer Phosphorene. ACS Nano 2014, 8, 9590−9596. (15) Cui, S.; Pu, H.; Wells, S. A.; Wen, Z.; Mao, S.; Chang, J.; Hersam, M. C.; Chen, J. Ultrahigh Sensitivity and Layer-Dependent Sensing Performance of Phosphorene-Based Gas Sensors. Nat. Commun. 2015, 6, 1−9. (16) Hanlon, D.; Backes, C.; Doherty, E.; Cucinotta, C. S.; Berner, N. C.; Boland, C.; Lee, K.; Harvey, A.; Lynch, P.; Gholamvand, Z.; Zhang, S.; Wang, K.; Moynihan, G.; Pokle, A.; Ramasse, Q. M.; McEvoy, N.; Blau, W. J.; Wang, J.; Abellan, G.; Hauke, F.; et al. Liquid Exfoliation of Solvent-Stabilized Few-Layer Black Phosphorus for Applications Beyond Electronics. Nat. Commun. 2015, 6, 8563. (17) Erande, M. B.; Suryawanshi, S. R.; More, M. A.; Late, D. J. Electrochemically Exfoliated Black Phosphorus Nanosheets Prospective Field Emitters. Eur. J. Inorg. Chem. 2015, 2015, 3102− 3107. (18) Erande, M. B.; Pawar, M. S.; Late, D. J. Humidity Sensing and Photodetection Behavior of Electrochemically Exfoliated Atomically Thin-Layered Black Phosphorus Nanosheets. ACS Appl. Mater. Interfaces 2016, 8, 11548−11556. (19) Ambrosi, A.; Sofer, Z.; Pumera, M. Electrochemical Exfoliation of Layered Black Phosphorus into Phosphorene. Angew. Chem., Int. Ed. 2017, 56, 10443−10445. (20) Jiang, Q.; Xu, L.; Chen, N.; Zhang, H.; Dai, L.; Wang, S. Facile Synthesis of Black Phosphorus: An Efficient Electrocatalyst for the Oxygen Evolving Reaction. Angew. Chem. 2016, 128, 14053−14057. (21) Ren, X.; Zhou, J.; Qi, X.; Liu, Y.; Huang, Z.; Li, Z.; Ge, Y.; Dhanabalan, S. C.; Ponraj, J. S.; Wang, S.; Zhong, J.; Zhang, H. FewLayer Black Phosphorus Nanosheets as Electrocatalysts for Highly Efficient Oxygen Evolution Reaction. Adv. Energy Mater. 2017, 7, 1700396. (22) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis. Nat. Mater. 2012, 11, 963−969. (23) Gopalakrishnan, D.; Damien, D.; Shaijumon, M. M. MoS2 Quantum Dot-Interspersed Exfoliated MoS2 Nanosheets. ACS Nano 2014, 8, 5297−5303. (24) Sarma, P. V.; Tiwary, C. S.; Radhakrishnan, S.; Ajayan, P. M.; Shaijumon, M. M. Oxygen Incorporated WS2 Nanoclusters with Superior Electrocatalytic Properties for Hydrogen Evolution Reaction. Nanoscale 2018, 10, 9516−9524. (25) Valappil, M. O.; Anil, A.; Shaijumon, M.; Pillai, V. K.; Alwarappan, S. A Single-Step Electrochemical Synthesis of Luminescent WS2 Quantum Dots. Chem. - Eur. J. 2017, 23, 9144−9148. (26) Gopalakrishnan, D.; Damien, D.; Li, B.; Gullappalli, H.; Pillai, V. K.; Ajayan, P. M.; Shaijumon, M. M. Electrochemical Synthesis of Luminescent MoS2 Quantum Dots. Chem. Commun. 2015, 51, 6293− 6296. (27) Damien, D.; Anil, A.; Chatterjee, D.; Shaijumon, M. M. Direct Deposition of MoSe2 Nanocrystals onto Conducting Substrates: Towards Ultra-Efficient Electrocatalysts for Hydrogen Evolution. J. Mater. Chem. A 2017, 5, 13364−13372. (28) Vishnoi, P.; Mazumder, M.; Barua, M.; Pati, S. K.; Rao, C. N. R. Phosphorene Quantum Dots. Chem. Phys. Lett. 2018, 699, 223−228. (29) Sun, Z.; Xie, H.; Tang, S.; Yu, X.-F.; Guo, Z.; Shao, J.; Zhang, H.; Huang, H.; Wang, H.; Chu, P. K. Ultrasmall Black Phosphorus Quantum Dots: Synthesis and Use as Photothermal Agents. Angew. Chem. 2015, 127, 11688−11692. (30) Zhang, X.; Xie, H.; Liu, Z.; Tan, C.; Luo, Z.; Li, H.; Lin, J.; Sun, L.; Chen, W.; Xu, Z.; Xie, L.; Huang, W.; Zhang, H. Black Phosphorus Quantum Dots. Angew. Chem. 2015, 127, 3724−3728. (31) Shao, L.; Sun, H.; Miao, L.; Chen, X.; Han, M.; Sun, J.; Liu, S.; Li, L.; Cheng, F.; Chen, J. Facile Preparation of NH2 -Functionalized Black Phosphorene for the Electrocatalytic Hydrogen Evolution Reaction. J. Mater. Chem. A 2018, 6, 2494−2499. (32) Tang, X.; Liang, W.; Zhao, J.; Li, Z.; Qiu, M.; Fan, T.; Luo, C. S.; Zhou, Y.; Li, Y.; Guo, Z.; Fan, D.; Zhang, H. Fluorinated Phosphorene: Electrochemical Synthesis, Atomistic Fluorination, and Enhanced Stability. Small 2017, 13, 1702739.
(33) Jaison, M. J.; Narayanan, T. N.; Prem Kumar, T.; Pillai, V. K. A Single-Step Room-Temperature Electrochemical Synthesis of Nitrogen-Doped Graphene Nanoribbons from Carbon Nanotubes. J. Mater. Chem. A 2015, 3, 18222−18228. (34) Jin, S. H.; Kim, D. H.; Jun, G. H.; Hong, S. H.; Jeon, S. Tuning the Photoluminescence of Graphene Quantum Dots through the Charge Transfer Effect of Functional Groups. ACS Nano 2013, 7, 1239−1245. (35) Hultgren, R.; Gingrich, N. S.; Warren, B. E. The Atomic Distribution in Red and Black Phosphorus and the Crystal Structure of Black Phosphorus. J. Chem. Phys. 1935, 3, 351−355. (36) Ryder, C. R.; Wood, J. D.; Wells, S. A.; Yang, Y.; Jariwala, D.; Marks, T. J.; Schatz, G. C.; Hersam, M. C. Covalent Functionalization and Passivation of Exfoliated Black Phosphorus via Aryl Diazonium Chemistry. Nat. Chem. 2016, 8, 597−602. (37) Castellanos-Gomez, A.; Vicarelli, L.; Prada, E.; Island, J. O.; Narasimha-Acharya, K. L.; Blanter, S. I.; Groenendijk, D. J.; Buscema, M.; Steele, G. A.; Alvarez, J. V.; Zandbergen, H. W.; Palacios, J. J.; van der Zant, H. S. J. Isolation and Characterization of Few-Layer Black Phosphorus. 2D Mater. 2014, 1, 25001. (38) Lu, W.; Nan, H.; Hong, J.; Chen, Y.; Zhu, C.; Liang, Z.; Ma, X.; Ni, Z.; Jin, C.; Zhang, Z. Plasma-Assisted Fabrication of Monolayer Phosphorene and Its Raman Characterization. Nano Res. 2014, 7, 853−859. (39) Gan, Z. X.; Sun, L. L.; Wu, X. L.; Meng, M.; Shen, J. C.; Chu, P. K. Tunable Photoluminescence from Sheet-like Black Phosphorus Crystal by Electrochemical Oxidation. Appl. Phys. Lett. 2015, 107, 21901. (40) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts; Wiley, 2001. (41) Sofer, Z.; Luxa, J.; Bouša, D.; Sedmidubský, D.; Lazar, P.; Hartman, T.; Hardtdegen, H.; Pumera, M. The Covalent Functionalization of Layered Black Phosphorus by Nucleophilic Reagents. Angew. Chem., Int. Ed. 2017, 56, 9891−9896. (42) Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chem. Mater. 2005, 17, 1290−1295. (43) Wu, J.; Zheng, X.; Jin, C.; Tian, J.; Yang, R. Ternary Doping of Phosphorus, Nitrogen, and Sulfur into Porous Carbon for Enhancing Electrocatalytic Oxygen Reduction. Carbon 2015, 92, 327−338. (44) Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.; Kim, B. S.; Hammond, P. T.; Shao-Horn, Y. High-Power Lithium Batteries from Functionalized Carbon-Nanotube Electrodes. Nat. Nanotechnol. 2010, 5, 531−537. (45) Mayorga-Martinez, C. C.; Mohamad Latiff, N.; Eng, A. Y. S.; Sofer, Z.; Pumera, M. Black Phosphorus Nanoparticle Labels for Immunoassays via Hydrogen Evolution Reaction Mediation. Anal. Chem. 2016, 88, 10074−10079. (46) Nicholson, R. S. Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics. Anal. Chem. 1965, 37, 1351−1355. (47) Aristov, N.; Habekost, A. Cyclic Voltammetry - A Versatile Electrochemical Method Investigating Electron Transfer Processes. World J. Chem. Educ. 2015, 3, 115−119. (48) Chen, G.; Guo, Y.; Miao, S.; Sun, H.; Gu, B.; Li, W.; Zhong, Y.; Zhou, W.; Shao, Z. Constructing Self-Standing and Non-Precious Metal Heterogeneous Nanowire Arrays as High-Performance Oxygen Evolution Electrocatalysts : Beyond the Electronegativity E effect of the Substrate. J. Power Sources 2018, 396, 421−428. (49) Chen, G.; Hu, Z.; Zhu, Y.; Chen, Z.; Zhong, Y. UltrahighPerformance Tungsten-Doped Perovskites for the Oxygen Evolution Reaction. J. Mater. Chem. A2018, 6, 9854−9859. (50) Zhu, Y.; Chen, G.; Chen, Y.; Ma, N.; Zhou, W. A SurfaceModified Antiperovskite as an Electrocatalyst for Water Oxidation. Nat. Commun. 2018, 9, 2326. (51) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. H
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano (52) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0Model. J. Chem. Phys. 1999, 110, 6158−6170.
I
DOI: 10.1021/acsnano.8b06671 ACS Nano XXXX, XXX, XXX−XXX
