Porous Two-Dimensional Nanosheets Converted ... - ACS Publications

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Chemistry of Materials

Porous Two-Dimensional Nanosheets Converted from Layered Double Hydroxides and Their Applications in Electrocatalytic Water Splitting Hanfeng Liang,†,‡ Linsen Li,† Fei Meng,† Lianna Dang,† Junqiao Zhuo,†,# Audrey Forticaux,† Zhoucheng Wang,‡,* and Song Jin†,* †

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States; ‡College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China; #Institute of Analytical

Chemistry, College of Chemical and Molecular Engineering, Peking University, Beijing 100871, China.

ABSTRACT: Porous materials are of particular interest due to their high surface area and rich edge sites, which are favorable for applications such as catalysis. Although there are well-established strategies for synthesizing porous metal oxides (e.g. by annealing the corresponding metal hydroxides), facile and scalable routes to porous metal hydroxides and metal chalcogenides are lacking. Here, we report a simple and general strategy to synthesize porous nanosheets of metal hydroxides by selectively etching layered double hydroxide (LDH) nanoplate precursors that contain amphoteric metal, and 1 ACS Paragon Plus Environment


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to further convert them into porous metal chalcogenides by a solution method. Using NiGa LDH as an example, we show that the thin nanoplates with high surface accessibility facilitate the topotactic conversion of NiGa LDH to β-Ni(OH)2 and further to NiSe2 with porous texture while preserving the sheet-like morphology. The converted β-Ni(OH)2 and NiSe2 are highly active for electrocatalytic oxygen evolution reaction and hydrogen evolution reaction (HER), respectively, which demonstrates the applications of such high surface area porous nanostructures with rich edge sites. Particularly, the porous NiSe2 nanosheets exhibited excellent catalytic activity toward HER with low onset overpotential, small Tafel slope, and good stability under both acidic and alkaline conditions. Overall electrochemical water splitting experiments using these porous Ni(OH)2

and

NiSe2

β-

nanosheets were further demonstrated. Our work presents a new

strategy to prepare porous nanomaterials, and to further enhance their catalytic and other applications.

KEYWORDS: porous nanosheets, layered double hydroxide (LDH), solution conversion, β-Ni(OH)2, NiSe2, oxygen evolution reaction (OER), hydrogen evolution reaction (HER)

INTRODUCTION Porous nanomaterials have found many applications in energy conversion and storage, catalysis, and drug delivery because of their high surface area, rich edge sites, and generally good strain accommodation.1-3 The common route to fabricate porous 2 ACS Paragon Plus Environment

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nanostructures involves the use of hard templates, such as anodic aluminum oxide, mesoporous silica and carbon, or soft templates such as surfactants.4,5 Despite the wide adoption, templated syntheses require selective removal of the templates and often suffer from tedious procedures.5 Alternatively, thermal decomposition of precursor materials (e.g. metal hydroxides) can be an efficient route to introduce pores when water or other volatile species leave the solid precursors,6-11 but this approach is generally limited to the preparation of metal oxides, while the facile and scalable preparation of other porous nanomaterials, such as metal hydroxides and metal chalcogenides, is more difficult and highly desired. We suggest that layered double hydroxides (LDHs)12 can serve as versatile precursors for porous nanomaterials of other metal compounds. LDHs are a class of twodimensional (2D) materials with unique structure consisting of metal hydroxide layers and inorganic/organic gallery anions/molecules.12-16 The general formula of LDHs can be expressed as [MII1-xMIIIx(OH)2]x+(An-)x/n·mH2O, where MII (M = Mn, Fe, Co, Ni, Cu, Zn, etc.) and MIII (M = Al, Ga, Ti, Cr, Fe, Co, etc.) are di- and trivalent metal cations, respectively, and An- is a charge-balancing anion intercalated between the brucite-like metal hydroxide layers. The easily tailored properties, composition versatility, and low cost of LDHs have led to surging interest in these materials and many applications, such as adsorption,16,17 photochemistry,14,18,19 and electrocatalysis.20-24 Moreover, LDHs can be readily synthesized in aqueous solutions and can serve as versatile precursors to produce a variety of porous (mixed) metal oxides.8,14 In this work, we describe an off-the-beatenpath strategy by selectively etching nanoplates of LDH precursors that contain amphoteric metals (e.g. Ga, Al) to prepare porous nanosheets of metal hydroxides with 3 ACS Paragon Plus Environment


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layered structures, and further to porous metal chalcogenide nanosheets via simple solution chemical conversions. Chemical conversion offers a versatile route to convert nanostructures of one composition into another.25,26 Generally, the starting material also serves as a template, whose size and morphology could be well preserved in a properly designed conversion reaction. Hence the precursors should have a well-defined morphology, and at least one nanoscale dimension and high surface accessibility to facilitate the conversion, as well as to relieve the structural strain during phase transition. Therefore, nanowires and nanosheets are ideal starting materials. Usually high temperature vapor phase conversions are used to convert metal hydroxides or other precursor phases to metal chalcogenides,27 for example, as previously shown in the conversion of β-Co(OH)2 to CoS2 micro-/nanowires and FeF3 to FeS2 nanowires.28,29 Solution phase conversion at a much milder temperature avoids the use of vapor conversion at higher temperatures and can facilitate large-scale preparation with lower cost and higher throughput. Herein, we choose thin nanoplates of NiGa LDH (Figure 1A) as the precursor to demonstrate this general conversion strategy. The Ga(OH)3 is amphoteric while Ni(OH)2 can only be dissolved in acids. Therefore, by selectively etching Ga3+ ions away from the NiGa LDH in alkaline solution (Figure 1, step I), porous β-Ni(OH)2 nanosheets (Figure 1B) can be readily obtained. Note that both NiGa LDH and β-Ni(OH)2 are layered materials with similar Brucite-like crystal structures (Figure S1, Supporting Information), which also facilitates the smooth topotactic conversion of NiGa LDH into β-Ni(OH)2 while preserving the overall morphology. The porous β-Ni(OH)2 can be further converted

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into porous NiSe2 nanosheets (Figure 1C) via a simple hydrothermal selenization reaction (Figure 1, Step II).

Figure 1. Schematic illustration of the crystal structures represented by ball-and-stick unit cell and the solution conversion processes from NiGa LDH nanoplates to porous βNi(OH)2 then porous NiSe2 nanosheets. (A) NiGa LDH, (B) β-Ni(OH)2, and (C) NiSe2. Step I: the Ga3+ ions in NiGa LDH layers react with OH- under hydrothermal conditions to form soluble Ga[(OH)4]-, resulting in the formation of porous β-Ni(OH)2 nanosheets. Step II: solution conversion of β-Ni(OH)2 into porous NiSe2 nanosheets by selenization under hydrothermal conditions. The schemes on top illustrate the (A, B) oxygen evolution reaction (OER) and (C) hydrogen evolution reaction (HER) catalytic processes. These porous nanostructures hold the advantages of high surface area, rich edge sites, and generally good stability, which could enhance many applications such as electrocatalysis, lithium-ion batteries, and supercapacitors.1,3,7,9,27,29 As a demonstration of these benefits, the as-converted porous β-Ni(OH)2 and NiSe2 nanosheets were evaluated for electrocatalytic water splitting. Efficient splitting of water into O2 (oxygen evolution reaction, OER) and H2 (hydrogen evolution reaction, HER) can potentially fulfill future demand of clean and renewable energy.30-35 Precious metals/metal oxides are 5 ACS Paragon Plus Environment


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currently the most active water splitting catalysts, but their scarcity and high cost greatly limit their practical use in large-scale applications. Apart from searching for highly active and low cost new catalysts that contain earth-abundant transition metal elements, nanostructuring of existing catalysts also provides an efficient route to enhance the catalytic performance.31,36 Porous nanostructures are particularly attractive because their pores not only expose more active sites for catalysis but also offer more contact with electrolyte and promote the transfer of ionic species.1 As a result, these porous nanostructures exhibit high electrocatalytic OER activity over β-Ni(OH)2 (Figure 1B, top panel) and HER activity over NiSe2 (Figure 1C, top panel). Particularly, the porous NiSe2 nanosheets were found to be an excellent HER catalyst in both acidic and alkaline solutions. The low overpotentials (135 mV in 0.5 M H2SO4 and 184 mV in 1 M KOH) needed to achieve a catalytic current density of 10 mA cm-2, small Tafel slopes (37 and 77 mV dec-1 in 0.5 M H2SO4 and 1 M KOH, respectively), and good stability, place porous NiSe2 nanosheets among the most active non-noble metal HER electrocatalysts. The superb catalytic activity could be attributed to their high surface areas and rich edge sites of the porous structures. This work presents a new and general strategy to prepare porous 2D nanomaterials of layered or non-layered compounds, and to further enhance their catalytic and other applications.

EXPERIMENTAL SECTION All chemical reagents were purchased from Sigma-Aldrich and used as received.


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Hydrothermal Continuous Flow Synthesis of NiGa Layered Double Hydroxide (LDH) Nanoplates and β-Ni(OH)2 Microplates. The NiGa LDH nanoplates and βNi(OH)2 microplates were synthesized using a home-built high temperature high pressure hydrothermal continuous flow reactor (HCFR). This system contains five major components: a precursor solution reservoir, a high performance liquid chromatography (HPLC) pump, a Teflon-lined 316 stainless-steel reaction column, a home-built tube furnace with temperature control (RT-300 ℃) and a back pressure regulator (0-500 psi). The setup of the HCFR has been previously detailed,20 thus, here we will only describe the important steps. 0.7 g (2.4 mmol) of Ni(NO3)2·6H2O and 0.31 g of Ga(NO3)3·xH2O (from 1.2 mmol with x = 0 to 1.1 mmol with x = 1) were dissolved in 352 mL of nanopure water (18.2 MΩ cm), yielding a clear, light green solution. Then 48 mL of concentrated NH3·H2O (28.0-30.0% NH3 basis) was added to the above solution, which resulted in a color change from light green to light blue. This mixture was used as the precursor solution for continuous flow reaction. 5% Teflon treated carbon paper (FuelCellEarth, TGP-H-060) with 10 cm in length and 1 cm in width attached onto a glass sheet using adhesive carbon tape was placed in the center of the HCFR. The reaction column with preloaded precursor solution (~30 mL) was placed through the tube furnace and connected with the HPLC pump and the back pressure regulator. Then the furnace was turned on to heat up the reaction solution to 160 °C in 10 min and kept at that temperature for another ~20 min to allow the seed growth before the feeding precursor solution was flowed at a speed of 1.0 mL min-1. The pressure was set to ~160 psi and normally stayed constant during the reaction if there was no leakage or clogging. The heating and the flow were stopped after 7 ACS Paragon Plus Environment


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4 h. After the column was cooled down, the pressure was released by completely opening the back pressure regulator valve, and the column could be uninstalled. The substrate (carbon paper) was then removed from the column, rinsed with nanopure water and ethanol and dried in a flow of N2 gas. For synthesis of β-Ni(OH)2 microplates, the synthetic procedure is the same as that for NiGa LDH mentioned above except for the stating materials: 0.7 g of Ni(NO3)2·6H2O and 0.31 g of Ga(NO3)3·xH2O were replaced by 1.05 g Ni(NO3)2·6H2O. Conversion of NiGa LDH Nanoplates into Porous β-Ni(OH)2 Nanosheets. The carbon paper (1 cm × 6 cm) covered with NiGa LDH nanoplates was immersed in 60 mL of 1.0 M KOH solution in a Teflon-lined stainless steel autoclave with a capacity of 80 mL. The autoclave was sealed and heated at 160 °C for 20 h. The pressure inside the autoclave was estimated to be 90 psi. After the reaction, the substrate was rinsed with nanopure water and ethanol and dried in a flow of N2 gas. Conversion of Porous β-Ni(OH)2 Nanosheets into Porous NiSe2 Nanosheets. A piece of carbon paper (1 cm × 4 cm) covered with porous β-Ni(OH)2 nanosheets was immersed into 60 mL aqueous solution containing 39 mg (0.5 mmol) Se powder and 38 mg (1.0 mmol) NaBH4. After aged for 30 min, the above mixture was transferred into an 80 mL autoclave and heated at 180 °C for 20 h. The pressure inside the autoclave was estimated to be 145 psi. After the reaction, the substrate was rinsed with nanopure water and ethanol and dried in a flow of N2 gas. Direct Conversion of NiGa LDH Nanoplates into Porous NiSe2 Nanosheets. The synthetic procedure is the same as that for the conversion of β-Ni(OH)2 into NiSe2


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mentioned above except for the carbon paper (1 cm × 4 cm) covered with NiGa nanoplates was used as starting material. Structural Characterization. Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Advance powder X-ray diffractometer using Cu Kα radiation. For scanning electron microscopy (SEM), samples were imaged using a LEO Supra 55 VP field emission SEM. For transmission electron microscopy (TEM) observation, the carbon paper covered with products was immersed into ethanol and sonicated for 5 min. Then a few drops of suspension were casted onto lacey carbon supported TEM grids. TEM was carried out on a FEI Titan transmission electron microscope operated at an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping was performed on the same instrument in scanning TEM (STEM) mode. Electrochemical Measurements. The oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) measurements were performed using a threeelectrode cell using a Bio-Logic SP-200 potentiostat at room temperature (25 °C). The OER measurements were performed in 1 M KOH using the carbon paper covered with products as the working electrode, a platinum wire (Kurt J. Lesker, 99.99%; 0.50 mm in diameter) as the counter electrode, and a saturated calomel electrode (SCE, CH Instruments) as the reference electrode. Linear sweep voltammograms (LSV) were measured from 1.0 V to 1.8 V vs. reversible hydrogen electrode (RHE) at a scan rate of 0.5 mV s-1. Electrochemical impedance spectroscopy (EIS) was performed at open circuit potential over a frequency range from 5 MHz to 10 mHz with a 10 mV AC dither. The HER measurements were performed using the carbon paper covered with products as the 9 ACS Paragon Plus Environment


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working electrode, a graphite rod (National Carbon Co, AGKSP Spectroscopic Electrode) as the counter electrode, and a SCE as the reference electrode. 1 M KOH solution or 0.5 M H2SO4 solution were used as electrolytes. LSV were measured from +0.2 V to −0.4 V vs. RHE at a scan rate of 3 mV s-1. After voltammetric characterization, EIS was performed in potentiostatic mode at −0.2 V vs. RHE over a frequency range from 5 MHz to 10 mHz with a 10 mV AC dither. The overall water splitting was performed in 1 M KOH using a two-electrode cell, where porous β-Ni(OH)2 nanosheets act as the positive electrode for OER and porous NiSe2 nanosheets act as the negative electrode for HER. The EIS measurements of the two-electrode full cell was performed under 1.7 V. The catalyst loading was calculated by comparing the mass of carbon paper before and after nanostructure growth. All polarization curves were corrected for background current and iR losses as described in detail previously.20,28

RESULTS AND DISCUSSION NiGa LDH Nanoplates. The NiGa LDH nanoplates was synthesized using a recently developed high temperature high pressure hydrothermal continuous flow reactor (HCFR, Figure 2A).20 Compared to the conventional static hydrothermal reaction normally carried out in a sealed autoclave, a continuous flow reaction can deliver and maintain a constant precursor concentration (i.e., supersaturation) during the reaction, which has been demonstrated to be crucial to the formation of well-defined onedimensional (1D)6,37-39 and 2D nanostructures.20 We recently demonstrated the controlled synthesis of thin NiCo LDH nanosheets using such HCFR.20 Here, we took advantage of


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the HCFR and developed the synthesis of thin NiGa LDH nanoplates in high density directly on conducting substrates. In a typical synthesis, a solution of Ni(NO3)2·6H2O, Ga(NO3)3·xH2O, and NH3·H2O precursors was continuously flowed through the HCFR heated at 160 °C for 4 h (see details in the Method section). After the flow reaction, a green film covered the substrates, indicating the formation of NiGa LDH. Figures 2B and 2C show the scanning electron microscopy (SEM) images of NiGa LDH grown on fluorine-doped tin oxide (FTO) and carbon cloth substrates, respectively. On both substrates, well-defined nanoplates about 2 µm wide and 25 nm thick with smooth surface are interconnected and sometimes interpenetrated with each other to form a hierarchical network. Using the HCFR, such uniform, well-defined plate-like structure can also grow in high density on other conductive substrates, such as carbon paper. Here, we chose the thin NiGa LDH nanoplates grown on carbon paper as the precursor for the conversion reaction, because the high surface area of carbon paper is beneficial to the conversion and the electrocatalysis to be demonstrated later.

Figure 2. The HCFR setup and NiGa LDH nanoplate products. (A) Schematic of the HCFR components (1. Reservoir; 2. HPLC pump; 3. Reaction column; 4. Tube furnace; and 5. Back pressure regulator). (B, C) SEM images of NiGa LDH nanoplates grown on (B) fluorine-doped tin oxide (FTO) and (C) carbon cloth (CC) substrates. 11 ACS Paragon Plus Environment


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The NiGa LDH grown on carbon paper was first characterized by powder X-ray diffraction (PXRD). The PXRD pattern (Figure 3A) of the product consists of regularly spaced peaks at low 2θ angles and weaker ones at high 2θ angles, which are characteristic of layered materials.40 The sharp diffraction peaks associated with (003) and (006) basal planes indicate these NiGa LDH nanoplates are highly crystalline. The d (003) spacing of 7.72 Å determined from PXRD is consistent with previous reports (7.71 Å).41 The SEM image of the as-prepared NiGa LDH on carbon paper (Figure 3B) shows thin vertically standing nanoplates with ~25 nm in thickness, similar to those grown on FTO and carbon cloth. Furthermore, these nanoplates uniformly and fully cover the carbon paper substrate (Figure S2A, Supporting Information). To further confirm the composition, we performed energy-dispersive X-ray spectroscopy (EDS) and elemental mapping in highangle annular dark-field scanning transition electron microscopy (HAADF-STEM). The elemental maps (Figure 3C) clearly show that Ni and Ga are homogeneously distributed in a nanoplate, which confirms the product is indeed NiGa LDH rather than separated Ni and Ga hydroxides. The EDS spectrum (Figure 3D) also shows the final molar ratio of Ni:Ga (1.99:1) is almost the same as the ratio (2:1) of the stating precursors used.


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Figure 3. Characterization of the as-prepared or as-converted products on carbon paper. (A, E, I) PXRD patterns, (B, F, J) SEM images, and (D, H, K) the corresponding EDS spectra of (left panels) NiGa LDH nanoplates, (middle panels) porous β-Ni(OH)2 nanosheets, and (right panels) directly grown β-Ni(OH)2 microplates. The asterisks (*) in A, E and I mark the diffraction peaks from carbon paper substrate. (C) Typical HAADFSTEM image and the corresponding EDS mapping recorded from an individual NiGa LDH nanoplate showing the distribution of Ni and Ga elements. (G) TEM image and corresponding SAED pattern of an individual nanoplate of the converted porous βNi(OH)2 nanosheets. The scale bars in C and G are 200 nm. Conversion of NiGa LDH to Porous to β-Ni(OH)2 Nanosheets. The chemical conversion of NiGa LDH to β-Ni(OH)2 was then carried out by reacting the NiGa LDH with 1 M KOH solution at 160 °C under hydrothermal conditions (see details in Method section). The PXRD pattern of the as-converted product (Figure 3E) shows that all the diffraction peaks can be indexed to the hexagonal β-Ni(OH)2 phase (space group: P3m1 ) with lattice parameters a = 3.126 Å and c = 4.605 Å (JCPDS No. 14-0117). No additional 13 ACS Paragon Plus Environment


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peaks from NiGa LDH or Ga(OH)3 can be observed. Representative SEM image (Figure 3F) clearly shows that many holes are introduced to the surface, even through the overall interconnected nanoplate network-structure on carbon paper is well preserved (Figure S2B, Supporting Information). Further investigation by TEM (Figure 3G) reveals that the converted β-Ni(OH)2 nanosheets consist of many loosely aligned smaller single crystalline nanoplates with an average size of 300 nm. This indicates the large plates (up to several microns) observed by SEM result from the reorganization and aggregation of these small nanosheets and the overlapping of these nanosheets further leads to the formation of the hierarchically porous β-Ni(OH)2 structure. After conversion, the Ga peaks are barely observed in the EDS spectrum (Figure 3H). This and the PXRD confirm that Ga (~97% based on EDS analysis) is selectively etched from the NiGa LDH precursor by the base during the conversion. Such smooth topotactic conversion from NiGa LDH to β-Ni(OH)2 could be due to their similar layered crystal structures (Figure S1, Supporting Information). As a comparison, we also directly synthesized non-porous β-Ni(OH)2 using the HCFR. The phase of the product (Figure 3I) can also be identified as hexagonal β-Ni(OH)2 phase. The strong diffraction peaks of the (001) and (101) planes suggest the crystals preferentially grow along the (001) (c-axis) direction, which can be further confirmed by the SEM image (Figure 3J). The microplate products are much thicker (~0.8 µm) and are composed of vertically stacked nanoplates. These microplates also fully cover the whole surface of the carbon paper substrate in high density (Figure S2C, Supporting Information). It is interesting that the thickness of β-Ni(OH)2 microplates (~0.8 µm) is much larger than that of NiGa LDH nanoplates (~25 nm) grown using very similar HCFR conditions while their lateral dimensions are similar, which


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suggests that β-Ni(OH)2 grows faster only along c direction. This may be because the βNi(OH)2 without anions intercalated within the interlayers has stronger bonding between layers and consequently faster growth rate. These results show that the selective etching of NiGa LDH nanoplates can produce thin, porous β-Ni(OH)2 nanosheets with welldefined morphology that are otherwise difficult to synthesize directly. Note that the Ga3+ ions can be recycled, or we can alternatively use cheaper Al3+. This could further reduce the cost and make this route comparable to the direct hydrolysis of metal precursors in cost, but enable us to prepare unique porous metal hydroxide nanostructures. OER Catalytic Performance of NiGa LDH and β-Ni(OH)2 Micro/Nanostructures. We then evaluated the electrocatalytic activity of the NiGa LDH nanoplates, the as-converted porous β-Ni(OH)2 nanosheets, and the as-grown β-Ni(OH)2 microplates toward OER to demonstrate the advantage of porous structures. The OER measurements were performed in 1 M KOH using a standard three-electrode system, where a Pt wire and a saturated calomel electrode (SCE) served as counter electrode (CE) and reference electrode (RE), respectively, and the carbon paper covered with micro/nanostructures was directly employed as the working electrode (WE) without using any extra binders (e.g. Nafion) (see details in Method section). Figure 4A shows the iRcorrected and background subtracted polarization curves that were recorded at a low scan rate of 0.5 mV s-1. The bare carbon paper substrate has no measurable OER activity within the potential window investigated, whereas the three Ni-based catalysts clearly show good catalytic activity for the OER. For β-Ni(OH)2, an anodic peak that could be associated with the oxidation of NiII to NiIII is observed around 1.4 V vs. reversible hydrogen electrode (RHE).42 Interestingly, this anodic peak observed for NiGa LDH 15 ACS Paragon Plus Environment


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shifts to higher overpotential, suggesting that the electronic structure of Ni2+ ions is likely altered by the presence of Ga3+ ions. A similar phenomenon was also reported for Fe3+ substituted NiOOH.43 The porous β-Ni(OH)2 nanosheets, even with a much less mass loading (~0.22 mg cm-2), show better performance than NiGa LDH nanoplates (~1.02 mg cm-2) and β-Ni(OH)2 microplates (~24.48 mg cm-2), as indicated by the sharp increase in the anodic current density with increasing overpotential. Specifically, the porous βNi(OH)2 nanosheets require an applied overpotential as low as 415 mV to achieve significant O2 evolution (j = 10 mA cm-2). In comparison, the NiGa LDH nanoplates and β-Ni(OH)2 microplates require 450 and 541 mV, respectively.

Figure 4. Electrochemical properties of β-Ni(OH)2 microplates, NiGa LDH nanoplates, and porous β-Ni(OH)2 nanosheets for OER. (A) iR-corrected and background subtracted polarization curves of various catalysts in 1 M KOH at a scan rate of 0.5 mV s-1. (B) The corresponding Tafel slopes derived from polarization curves. (C) Plots showing the extraction of the double-layer capacitances (Cdl) for the estimation of relative


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electrochemically active surface area. (D) Long-term stability test carried out under a constant current density of 10 mA cm-2. Samples: (I, black trace) carbon paper; (II, blue trace) β-Ni(OH)2 microplates; (III, green trace) NiGa LDH nanoplates, and (IV, red trace) porous β-Ni(OH)2 nanosheets. Catalyst loading: β-Ni(OH)2 microplates, ~24.48 mg cm-2; NiGa LDH nanoplates, ~1.02 mg cm-2; and porous β-Ni(OH)2 nanosheets, ~0.22 mg cm2

. The difference in catalytic activity of the three Ni-based electrocatalysts was

further verified by their corresponding Tafel slopes (Figure 4B). The Tafel slope of NiGa LDH nanoplates (117 mV/dec) is much higher than those of β-Ni(OH)2 structures, which clearly reveal that the β-Ni(OH)2 exhibits higher intrinsic catalytic activity than NiGa LDH toward OER. Since the Tafel slopes for porous β-Ni(OH)2 nanosheets and βNi(OH)2 microplates are very close (60 and 68 mV/dec, respectively), which suggests that the different synthetic routes and morphologies do not affect their intrinsic activity, the observed disparity in performance has to be due to other factors, such as active surface area and exposed active sites. We thus measured the capacitive current density versus scan rate to extract the double-layer capacitance (Cdl), which can be used to estimate the electrochemically active surface area (EASA).28 The comparison of Cdl values (Figure 4C) reveals that the active surface area of porous β-Ni(OH)2 nanosheets is about four times higher than that of β-Ni(OH)2 microplates. This huge change in active surface area is primarily responsible for their different performance. Indeed, when normalized by relative electrochemically active surface area, they yield almost the same normalized exchange current densities (j0,normalized, see Table S1 in the Supporting Information). The porous structure caused by the conversion from the NiGa LDH would 17 ACS Paragon Plus Environment


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also increase the number of edge sites thus leading to more open coordination sites exposed as catalytically active sites, which could also contribute to the high performance of the as-converted β-Ni(OH)2 nanosheets. Evidently, the area of anodic peak around 1.4 V vs. RHE of porous β-Ni(OH)2 nanosheets is also much larger than that of β-Ni(OH)2 microplates (Figure 4A), suggesting more Ni2+ sites are involved in the porous nanosheets during the OER reaction. The EASA of β-Ni(OH)2 nanosheets is also higher than that of NiGa LDH nanoplates, which could be due to the formation of pores in the surface caused by the dissolution of Ga3+ ions. It should be mentioned that despite the fact that β-Ni(OH)2 has a smaller basal plane spacing than NiGa LDH (Figure S1, Supporting Information), the porous βNi(OH)2 nanosheets exhibit better OER performance than the NiGa LDH nanoplates. This is interesting because normally larger basal plane spacing would enable more accessible active sites and less structural rearrangement, thus leading to enhanced catalytic activity.44,45 Moreover, they are of similar plate-like morphologies and the amount of active material (Ni in this case) of the porous β-Ni(OH)2 nanosheets is supposed to be equal to or even less than that of the NiGa LDH nanoplates given the mass loss during the etching process. Therefore, the difference in their performance should be attributed to the different properties and different porosities, as confirmed by their Tafel slopes and EASAs discussed earlier. We also carried out the chronopotentiometric measurements to evaluate the durability of these three Ni-based catalysts. As shown in Figure 4D, the porous βNi(OH)2 nanosheets (red trace) can maintain a constant current density (j = 10 mA cm-2) with minimal change (~31 mV, from 1.636 V to 1.667 V vs. RHE) in operating potential 18 ACS Paragon Plus Environment

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over a 10 h-test. In sharp contrast, the applied potential required for β-Ni(OH)2 microplates (Figure 4D, blue trace) to achieve a current density of 10 mA cm-2 increases from 1.757 V to 1.921 V vs. RHE (~164 mV). It is worth mentioning that the required overpotential for NiGa LDH (Figure 4D, green trace) gradually decreases during the stability test (i.e. the catalytic performance improved), which could be due to the slow dissolution of Ga3+ ions in the KOH electrolyte under the electrochemical conditions at room temperature. This observation further explains our motivation for intentionally making porous β-Ni(OH)2 nanosheets by selectively etching NiGa LDH nanoplates for OER. In general, the porous β-Ni(OH)2 nanosheets exhibit a higher catalytic activity toward OER than the NiGa LDH nanoplates and β-Ni(OH)2 microplates. Conversion of β-Ni(OH)2 to Porous NiSe2 Nanosheets. We then further converted the porous β-Ni(OH)2 nanosheets to porous NiSe2 nanosheets in solution. For a typical reaction, the porous β-Ni(OH)2 nanosheets grown on carbon paper were immersed into an aqueous solution containing Se powder and NaBH4 and hydrothermally heated at 180 °C for 20 h (see details in Method section). After conversion, the substrate turned black (Figure S3A, Supporting Information). The PXRD pattern (Figure 5A) clearly identifies the product as the cubic pyrite-phase of NiSe2 (space group: Pa3 , lattice parameter a = 5.991 Å, JCPDS No. 41-1495). A putative chemical equation for this conversion reaction could be: Ni(OH)2 + 2 Se + NaBH4

NiSe2 + NaBO2 + 3 H2

(1)

The porous nanosheet structure of the β-Ni(OH)2 precursor is well preserved (Figure 5B and more images in Figure S3B and 3C, Supporting Information). The porous



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structure could originate from the pores in the β-Ni(OH)2 precursor (Figure 3F) but may also be caused by strain release due to the crystal mismatch after phase conversion. The EDS spectrum (Figure 5C) shows the molar ratio of Ni:Se is 1:2.05, consistent with the stoichiometry of NiSe2. Together with the PXRD, it is clear that the porous and thin βNi(OH)2 nanosheets were fully converted to NiSe2. In contrast, the β-Ni(OH)2 microplates shown in Figure 3I could not be fully converted to NiSe2 even after a longer reaction time (Figure S4, Supporting Information). This could be due to the fact that the vertically stacked thick plates (~0.8 µm in thickness) prevent the Se atoms from diffusing and reacting with material inside thus only the external surface can be converted to NiSe2. Therefore, thin β-Ni(OH)2 nanosheets are essential to the complete conversion into porous NiSe2 nanosheets. TEM (Figure 5D) further verifies the porous structure throughout the converted NiSe2 nanosheets. The selected area electron diffraction (SAED) pattern (Figure 5E) and HRTEM (Figures 5F and G) indicate that the porous NiSe2 nanosheets are polycrystalline and composed of crystalline nanoparticles. The EDS elemental maps (Figure 5H) reveal the Ni and Se are distributed homogeneously in an individual nanoplate. These results show that low temperature solution conversion is equally effective in producing metal chalcogenides as conversion conducted at high temperature (500 °C) with vapor precursors.28,29,46 Moreover, this route can be made even more efficient by directly converting NiGa LDH nanoplates into NiSe2 nanosheets in one pot using NaBH4 and Se reagents (Figure S5, Supporting Information).


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Figure 5. Structural characterization of the as-converted porous NiSe2 nanosheets. (A) PXRD pattern, (B) SEM image, (C) the corresponding EDS spectrum, (D) TEM image, (E) the corresponding SAED pattern, and (F, G) high-resolution TEM images of the nanoparticles that make up the NiSe2 nanosheets. (H) HAADF-STEM image of an individual NiSe2 nanosheet and the corresponding EDS mapping showing the distribution of Ni and Se elements. The scale bar in H is 100 nm. HER Catalytic Performance of Porous NiSe2 Nanosheets. Compared to bulk NiSe2, the porous NiSe2 nanosheets should exhibit higher catalytic activity toward HER because they have higher surface area and their porous structure could promote the diffusion of ionic species thus allow the efficient utilization of active sites. Furthermore, the direct growth of NiSe2 on current collectors (i.e. carbon paper) also ensures good electrical conductivity and contact. We first performed the electrochemical measurements in 0.5 M H2SO4 electrolyte using a three-electrode setup, where a graphite rod and a SCE electrode served as CE and RE, respectively, and the as-converted porous NiSe2



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nanosheets on carbon paper was directly used as the WE (see details in Method section). Figure 6A shows the polarization curves of porous NiSe2 nanosheets (catalyst loading: ~0.46 mg cm-2) and bare carbon paper as a control with a scan rate of 3 mV s-1. The carbon paper shows negligible HER activity within the measurement potential. The porous NiSe2 nanosheets exhibit superior HER activity as indicated by the low onset overpotential (90 mV) and the rapidly rising cathodic current at more negative potentials. The overpotential required to drive current densities of 10, 100, and 200 mA cm-2 are 135, 183, and 202 mV, respectively. These overpotentials compare favorably to those recently reported for the most active noble metal-free HER electrocatalysts and are the best among the metal pyrite and other metal chalcogenide HER catalysts (see comparison in Table S2, Supporting Information).28,31,36,46-53 The high catalytic activity is further confirmed by the low Tafel slope of 37 mV dec-1 (Figure 6B), which suggests that the HER over porous NiSe2 nanosheets in acidic conditions proceeds via a Volmer-Tafel mechanism,54 where the Tafel reaction is the rate-limiting step. We further calculated the exchange current density (j0) by fitting the Tafel plot according to the Tafel equation (Figure S6, Supporting Information). The j0 of 6.46 µA cm-2 is among the best values reported for metal chalcogenides and other high performance HER catalysts (see Table S1, Supporting Information).28,31,36,46-53 In addition to excellent catalytic activity, the porous NiSe2 nanosheets also show good stability upon the chronopotentiometric measurements (Figure 6C). The overpotential required to achieve a constant current density of 10 mA cm-2 only slightly increases by 22 mV after a 20 h test. The SEM image of NiSe2 nanosheets after the stability test (Figure 6C inset) further confirms that there is no significant structural distortion during the measurement and the plate-like morphology


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is well maintained. We further investigated the HER performance of the porous NiSe2 nanosheets in alkaline solution (1 M KOH), and the results show that the catalyst is also highly active. The overpotential required to reach a significant H2 evolution (j = 10 mA cm-2) is as low as 184 mV (Figure 6D), 201 mV to reach 20 mA cm-2, and 219 mV to reach 50 mA cm-2. The porous NiSe2 nanosheets also present a small Tafel slope of 77 mV dec-1 (Figure 6E), accompanied by a high exchange current density of 38.9 µA cm-2 (Figure S6, Supporting Information). These values represent superb catalytic activity toward HER in alkaline solution. The NiSe2 was also found to be stable under alkaline conditions (Figure 6F). After a 20 h operation, the potential required to drive a current density of 10 mA cm-2 only slightly increases from 192 to 210 mV and the plate-like structure is well preserved (Figure 6F inset). Overall, the high current density, small Tafel slope, and good stability place the porous NiSe2 nanosheets among the most active nonnoble metal HER catalysts in both acidic and alkaline conditions.

Figure 6. Electrochemical properties of porous NiSe2 nanosheets for HER in both acidic and alkaline solutions. (A, D) iR-corrected and background subtracted polarization curves 23 ACS Paragon Plus Environment


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of porous NiSe2 nanosheets (circles) and carbon paper (squares) at a scan rate of 3 mV s1

, (B, E) the corresponding Tafel plots, and (C, F) chronopotentiometry at a constant

current density of 10 mA cm-2 in 0.5 M H2SO4 (A, B, C) and 1 M KOH (D, E, F) solutions. The insets in C and F are the SEM images of the NiSe2 nanosheets after HER measurements. Catalyst loading: ~0.46 mg cm-2. Furthermore, due to the stability of the NiSe2 HER catalyst in alkaline conditions, together with the other half reaction of OER catalyzed by porous β-Ni(OH)2 nanosheets, the overall electrochemical water splitting in an alkaline solution (1 M KOH) could be achieved by a two-electrode full cell using these two materials (Figure 7A). The overall water splitting voltage (at joverall = 1 mA cm-2) is measured to be 1.7 V (Figure 7B), corresponding to an overpotential of 470 mV, which is slightly larger than that observed for OER catalyzed by porous β-Ni(OH)2 nanosheets (350 mV) using three-electrode system (Figure 4A). In addition, the long-term stability test showed the voltage required to achieve 5 mA cm-2 of constant overall water splitting current increases slightly from initial 1.76 V to 1.8 V after 2-h operation (Figure 7B), suggesting the good stability of the catalysts.


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Figure 7. (A) Schematic illustration of the overall water splitting by a two-electrode full cell using porous β-Ni(OH)2 and NiSe2 nanosheets as positive and negative electrodes, respectively. (B) iR-corrected and background subtracted polarization curve at a scan rate of 1 mV s-1. Inset: long-term stability test carried out under a constant overall water splitting current of 5 mA cm-2. CONCLUSION In summary, we present a general strategy for the preparation of porous metal hydroxide and metal chalcogenide nanosheets using the example of synthesizing βNi(OH)2 nanosheets by selectively etching NiGa LDH nanoplates and their further solution conversion to porous NiSe2 nanosheets. The utilization of a recently developed HCFR enables controlled synthesis of thin NiGa LDH nanoplates with well-defined morphology in high density. Such thin nanoplates with high surface accessibility facilitate the diffusion and reaction with the reactive species therefore allow the facile topotactic conversions of LDH to metal hydroxides and further to metal chalcogenides, while maintaining the plate-like nanostructures. Selective etching of the amphoteric Ga3+ ions results in the formation of porous β-Ni(OH)2 nanosheets. The advantage of the porous nanostructures was demonstrated by the excellent OER and HER catalytic activities of porous β-Ni(OH)2 and porous NiSe2 nanosheets, respectively. Particularly, the porous NiSe2 nanosheets exhibit excellent and stable catalytic activity toward HER under both acidic and alkaline conditions as confirmed by the high electrocatalytic current density, small Tafel slope, and high exchange current density. The overall electrochemical water splitting experiments using porous β-Ni(OH)2 and NiSe2 nanosheets further illustrates the advantages of these porous materials. 25 ACS Paragon Plus Environment


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Moreover, due to the diverse compositions of LDHs with many divalent and trivalent metals to choose from the periodic table,13,15 this facile route could in principle be extended to synthesize nanosheets of a wide variety of (mixed) metal hydroxides with porous nanostructures with high surface area. For example, porous Mg(OH)2 might be synthesized by etching MgAl LDH55 or porous NixFe1-x(OH)2 by etching NiFeAl LDH.56 Furthermore, through a simple solution conversion reaction, the porous metal hydroxides can serve as precursors to further synthesize various porous metal chalcogenides or even other compounds, which will find more applications in fields such as electrocatalysis, photoelectrochemistry, and electrochemical energy storage.

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Schematic illustration of the crystal structures, additional SEM images and PXRD patterns, optical photographs of the samples, Tafel plots, and comparison of catalytic performance, as noted in the main text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] (S.J.); [email protected] (Z.W.) Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This research is supported by NSF Grant DMR-1106184. S.J. also thanks the Research Corporation SciaLog Award and UW-Madison H. I. Romnes Faculty Fellowship for support. H.L. and J. Z. thank the China Scholarship Council for support. Z.W. thanks the National Natural Science Foundation of China (grant no. 51372212) for support.

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Porous Two-Dimensional Nanosheets Converted ... - ACS Publications

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