Ultrasmall Ni/NiO Nanoclusters on Thiol-Functionalized and -Exfoliated Graphene Oxide Nanosheets for Durable Oxygen Evolution Reaction

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The demand of high anodic potential for oxygen evolution reaction (OER) because of its sluggish kinetics limits the overall efficiency and practical applications of electrochemical water splitting process. Though metal oxides are envisioned as the
  Ultrasmall Ni/NiO Nanoclusters on Thiol-Functionalized and-Exfoliated Graphene Oxide Nanosheets for Durable OxygenEvolution Reaction  Akhtar Munir, † Tanveer ul Haq, †  Ahsanulhaq Qurashi, ‡ Habib ur Rehman, †  Anwar Ul-Hamid, § and Irshad Hussain *  , † † Department of Chemistry and Chemical Engineering, SBA School of Science and Engineering, Lahore University of ManagementSciences (LUMS), DHA, Lahore 54792, Pakistan ‡ Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia § Center of Engineering Research, King Fahad University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia * S  Supporting Information  ABSTRACT:  The demand of high anodic potential foroxygen evolution reaction (OER) because of its sluggishkinetics limits the overall e ffi ciency and practical applicationsof electrochemical water splitting process. Though metaloxides are envisioned as the potential contenders in this questdue to their high redox potential, nevertheless their low conductivity and instability are among the formidablechallenges that need to be addressed. Here, we demonstratethe synthesis and electrochemical applications of covalently linked ultrasmall Ni/NiO NCs (about  ∼ 2 nm) with theexfoliated thiol-functionalized graphene (G-SH) nanosheetsas a highly e ffi cient and durable electrocatalyst for OER. Ni/NiO@G-SH nanohybrid showed a very sharp onset potentialof 1.46 V, Tafel slope of 46 mV/dec, turnover frequency (TOF) of 245 s − 1 @1.72 V and a steady-state current response at 10 mA/cm 2 for more than 3 days in 0.1 M KOH solution. We believe that the active redox couple of Ni 2+/3+ in nanoscale Ni/NiO at equilibrium  fl uctuates periodically for the expectedsustained OER process. Moreover, the synergistic e ff  ect between NCs-GO-SH nanosheets together with the slightly reducingenvironment due to the strong electron donor thiol groups facilitate the dynamics of the released O 2  as a  fi nal product and thusencourage the recycling potential of such nanohybrid materials at low anodic bias. KEYWORDS:  graphene oxide, thiolation, Ni/NiO nanoclusters, hybrid materials, oxygen evolution reaction (OER) 1. INTRODUCTION The rapid depletion of unwarranted and exhaustible fossil fuelsis a serious threat to the environment endowing a substantialamount of CO 2  (surpassing 400 ppm) at an alarming rateresponsible for global warming. To counter these formidableenergy and environmental challenges, the demand forsustainable energy production is on rise globally. 1 In thisregard, water being an abundant and widely distributed sourceof energy and its ability for the production of green electrons,protons (H + ), and oxygen (O 2 ) is an attractive candidate toexploit safe, clean, and sustainable H 2 -based economy. 2 ,3 Electrochemical water splitting is among the holy grails of thechemistry to meet the future energy demand. However, theoverall e ffi ciency of water splitting is still limited due to thedemand of high anodic potential for the half-cell oxygenevolution reaction (OER). During the recent past, seriouse ff  orts have been made to enable this process kinetically feasible at a potential reasonably close to its thermodynamiclimit (1.23 V). RuO 2  and IrO 2  are still the benchmark catalysts, which can potentially catalyze the multistep OER process in wide pH range, and even conventional electrolysis relies onthese precious catalysts. 4 However, their scarcity and high costlimit their large-scale applications. The design of cost-e ff  ectiveearth abundant and more e ffi cient catalyst is, therefore, highly desired to trim down the high anodic bias for OER in the watersplitting process. 3 ,5 In addition to the high anodic potential and the high cost of noble metal-based catalysts, it has recently been established, both experimentally and theoretically, that the high Gibbs freeenergies of di ff  erent intermediates (M − OH, M − O, MOOH)are indeed the major kinetic and thermodynamic barriers inOER process. Moreover, the high nucleophilic character of the Received:  August 20, 2018  Accepted:  December 10, 2018 Published:  December 10, 2018 Articlewww.acsaem.org Cite This:  ACS Appl. Energy Mater.  2019, 2, 363 − 371 © 2018 American Chemical Society  363  DOI:10.1021/acsaem.8b01375  ACS Appl. Energy Mater.  2019, 2, 363 − 371 This article is made available for a limited time sponsored by ACS under the ACS Free toRead License, which permits copying and redistribution of the article for non-commercialscholarly purposes.    D  o  w  n   l  o  a   d  e   d  v   i  a   K   I   N   G   F   A   H   D   U   N   I   V   P   E   T   R   O   L   E   U   M    &   M   I   N   E   R   A   L   S  o  n   N  o  v  e  m   b  e  r   4 ,   2   0   1   9  a   t   1   0  :   5   4  :   2   7   (   U   T   C   ) .   S  e  e   h   t   t  p  s  :   /   /  p  u   b  s .  a  c  s .  o  r  g   /  s   h  a  r   i  n  g  g  u   i   d  e   l   i  n  e  s   f  o  r  o  p   t   i  o  n  s  o  n   h  o  w   t  o   l  e  g   i   t   i  m  a   t  e   l  y  s   h  a  r  e  p  u   b   l   i  s   h  e   d  a  r   t   i  c   l  e  s .  “ M  O ”  bond drastically inhibits the attack of the incomingnucleophile (OH − ) and thus impedes the chain reaction forthe ultimate production of O 2  molecules. 6 Geologically abundant metal oxides, sul fi des, selenides, phosphides, anddouble layer hydroxide (LDH) have been widely studied inthis regard. 7 − 9  Among these, metal oxides particularly Nioxides/hydroxide are still the leading candidate to catalyze theOER process at minimum overpotential due to their optimalredox potential and active sites. 10 ,11 Moreover, their alloy andnanohybrid forms can further encourage their inherent redox properties. 12 However, their poor conductivity, less number of active sites, and poor durability of catalyst are still the commonimpediments which seriously hamper the overall water splittingprocess. An attractive solution to address many suchformidable challenges is the nanostructuring/nanoscaling of metal oxides to ensure their high electrocatalytic activity,selectivity, durability, and maximum exposure of activesites. 13 ,14 For instance, the use of metal nanoclusters (NCs, ≤ 2 nm)) with drastically reduced dimensions relative to their bulk analogues is an emerging area of interest having a numberof unique and exciting size-dependent properties. Moving from bulk to NPs and then to the subnanometric regime, uniquecatalytic and electronic properties have emerged due to thequantum con fi nement and high surface-to-volume ratio. 15 ,16 Moreover, the less number of core atoms with openelectrochemical accessibility further enhances their dynamicredox potential for a challenging reaction like OER. 17 In thisregard. Frei et al. have recently demonstrated that the Co 3 O 4 NCs supported on mesoporous silica (SBA-15) are highly active for water oxidation under mild conditions with highturnover frequency. 18 Similarly, Fominykh et al. reportedultrasmall and well-dispersed colloidal NiO nanocrystals as ane ffi cient electrocatalyst for water oxidation with high electro-chemically active surface area. 19 Recently, Gong et al. reporteda decent Ni/NiO@Cr 2 O 3  triphase electrocatalyst where Cr 2 O 3 decreases the oxidation of nanoscale Ni core retaining theabundant Ni/NiO interphases for long-term water splittingprocess. 20 In our previous work, we have reported ultrasmall(1 − 2 nm) and monodisperse Ni 6  and Ni 4  nanoclusters torationally induce the redox   fl exibility for OER by controllingthe numbers of core atoms with maximum exposed activesites. 21 However, the uniform dispersion of such ultrasmall andchemically stable nanoclusters/nanoparticles on highly con-ducting surfaces, like carbon nanotubes or graphene, is amongthe daunting challenges to ensure the practical use of alikenanoscale materials in nanocatalysis. Therefore, serious e ff  ortsare being made to exploit the inherent potential of metal- andgraphene-based nanohybrid as highly e ff  ective materials withultimately tuned electronic and catalytic properties desired forOER. 22 Recently, Razmjooei et al. reported the e ff  ect of di ff  erent hetero atoms (N, S, and P) doped graphene oxide(rGO) hybrid materials for OER. 23 They found that thedoping of heteroatoms, particularly the  “ S ”  atom, is very usefulto improve the interaction of active catalytic centers with theadsorbed intermediates and thus facilitate the OER process.Moreover, the conductivity and physicochemical properties of the GO can also be signi fi cantly enhanced by chemicalexfoliation and selective functionalization using wet chem-istry. 24 Taking into account the recent trends in the designing of potential electrocatalysts and nanoscale materials for OER,herein we report the e ff  ective decoration of thiolated grapheneoxide nanosheets with ultrasmall and uniform Ni/NiO NCs tostudy the synergistic role of such stable electrocatalysts towardthe sustainable OER process. The regioselective functionaliza-tion of graphene oxides via wet chemistry is one of the prudentapproaches for the controlled and selective incorporation of thiol functionality ( − SH). Besides, chemically linked Ni/NiONCs with the thiolated and well exfoliated 2D graphene oxidenanosheets was expected to result in superior catalytic activity for OER because of better conductivity, stability, high surface-to-volume ratio and reasonably enhanced electronic density around the NCs to eventually facilitate the heterogeneouselectron transfer (HET) at the graphene − S − Ni interphase. 25 To the best of our knowledge, the ultrasmall and uniformdistribution of Ni/NiO NCs particularly on the thiolatedgraphene oxide nanosheets is the  fi rst ever report for OER, and we believe that these  fi ndings will open up new researchdirections toward the better design of highly stable ande ff  ective functionalized graphene-based nanohybrid to make water splitting a practically viable process for the sustainableproduction of hydrogen. 2. EXPERIMENTAL SECTION 2.1. Chemicals.  Graphite powder (325 mesh,  ≥ 80%, Alfa Acer),potassium permanganate (KMNO 4  ,  ≥ 98%, Sigma-Aldrich), potassi-um phosphate (H 3 PO 4  ,  ≥ 98%, Sigma-Aldrich), sulfuric acid (H 2 SO 4  ,nickel nitrate hexahydrate (Ni(NO 3 ) 2 .6H 2 O,  ≥ 98%, Sigma-Aldrich),hydrobromic acid (HBr, 48%, Sigma-Aldrich), potassium hydroxide(KOH,  ≥ 85%, Sigma-Aldrich), potassium phosphate (KH 2 PO 4  , ≥ 99%, Sigma-Aldrich), sodium borohydride (NaBH 4  ,  ≥ 96% Sigma- Aldrich), thiourea (CH 4 N 2 S,  ≥ 99%, Sigma-Aldrich), diethyl ether[(CH 3 CH 2 ) 2  ,  ≥ 99%, Sigma-Aldrich)], iridium(IV) oxide (IrO 2  , ≥ 99.9%, Sigma-Aldrich), and polypropylene (average  M  n  ∼ 5000, ≥ 99%, Sigma-Aldrich). All the remaining solvents (methanol, ethanol, water) were used after double distillation. 2.2. Synthesis of Graphene Oxide (GO).  Graphene oxide (GO) was synthesized by using improved Hummer method. 26 Brie fl  y, 1 g of graphitic powder was dispersed in 9/1 ratio of concentrated H 2 SO 4 /H 3 PO 4  (120 mL/13 mL) in a 500 mL beaker. After 30 min of stirring,6 mg of KMNO 4  was added and the temperature raised up to 50  ° Cfor 18 h to ensure the complete oxidation of graphene. One hundred fi fty milliliters (150 mL) of ice-cold water was then added to stop theoxidation reaction followed by the addition of 30% H 2 O 2  (3 mL). After stirring (1 h), the appearance of brown color indicates theformation of GO. The reaction mixture was centrifuged (4000 rpm)for 1 h to obtain the solid product. The solid product was then washed thrice each with HCl, H 2 O, and ethanol to remove theorganic/inorganic impurities, which are formed during oxidationreaction. Finally, the product was washed many times with deionized water and dried at 60  ° C for further use. 2.3. Chemical Functionalization of GO (G-SH).  The synthe-sized GO was functionalized using a modi fi ed method recently reported (Scheme S1). 27 Thirty milligrams (30 mg) of GO was washed twice with ether and was ultrasonically dispersed in 30 mL of deionized water (DIW) for 2 h. HBr (1.5 mL) was then addeddropwise and further sonicated for 30 min. The reaction mixture wasthen stirred for 45 min followed by the addition of 1.5 g of thiourea atroom temperature (RT). The temperature was raised to 70  ° C for 24h under normal stirring. The reaction was then stopped and cooleddown to RT under natural cooling. Fifteen milliliters (15 mL) of aqueous solution of KOH (3 M) was then added and the temperaturedecreased to 4  ° C. The reaction was kept under vigorous stirring for45 min. After stopping the reaction, the black suspension wascollected via centrifugation at 8000 rpm. The solid product was washed thrice each with ethanol, ether, dimethylfarmamide (DMF),and  fi nally the solid product was ultrasonically dispersed in DIW (5mL) and dried under vacuum at room temperature for 3 days toobtain the thiolated graphene oxide nanosheets (G-SH). ACS Applied Energy Materials  Article DOI:10.1021/acsaem.8b01375  ACS Appl. Energy Mater.  2019, 2, 363 − 371 364  2.4. Synthesis of Ni/NiO@G-SH.  Ni/NiO NCs were fabricatedon thiolated graphene by using simple reduction method (SchemeS1) providing open atmosphere. Twenty milligrams of G-SH wasultrasonically dispersed in 20 mL of distilled water for 45 min in a 50mL round-bottom  fl ask (RBF). The dispersed G-SH was furtherstirred for 30 min at room temperature. Ten milligrams of Ni(NO 3 ) 2 · 6H 2 O (2 mg of Ni metal) was then added and the RBF containing thereaction mixture was placed in an ice-bath under normal stirring. After45 min, 3 mL of NaBH 4  (8 times to Ni atom) was added dropwiseunder vigorous stirring and the reaction was allowed to complete for24 h at RT. Finally, the product was collected via centrifugation (8000rpm), washed with water, methanol, and dried at RT under vacuum. 2.5. Synthesis of NiO@GO.  The same procedure was followedfor the synthesis of NiO@GO as mentioned above. The only di ff  erence was to take GO instead of G-SH and  fi nally the product was calcined to obtain the NiO@GO following the reportedmethod. 28 2.6. Characterization Techniques.  The synthesized samples were characterized using Fourier transform infrared spectroscopy (FT-IR), Raman microscope (Renishaw), scanning electron micro-scope (NOVA FEISEM-450 equipped with EDX detector), X-ray photon spectroscopy (XPS) using PHI 500 Versa Probe IIspectrometer (UIVAC-PHI), and high-resolution transmissionelectron microscope HR-TEM (JEM2100F, JEOL at 200 kV). 2.7. Electrochemical Measurements.  All the electrochemicalmeasurements were performed in basic medium (0.1 M KOH) usingGamry potentiostat 600 in a three-electrode setup with scan rate of 5mA/cm 2 . Fluorinated tin oxide (FTO,  ≥ 7 Ω ), Pt wire, and Ag/AgCl were employed as a working electrode, counter electrode, andreference electrode, respectively. Catalysts were deposited (0.5 mg/cm 2 ) on FTO by a simple drop-casting method followed by a gentlespin coating to obtain a homogeneous thin layer covering 1 cm 2 area.Potential reported in all our experiments was converted to thereversible hydrogen electrode (RHE) using (eq 1, SI). The possiblee ff  ect of iron impurities in KOH was also monitored by taking thepolarization curves before and after the removal of impurities. 29 Double layer capacitance (C DL ) was calculated by taking polarizationcurve in the non-Faradaic region (1.07 − 1.17 V vs RHE) at di ff  erentscan rate (5 − 25 mV/s). Other electrochemical parameters andcatalytic activities, that is, electrochemically active surface area(EASA), mass activity, exchange current density, and turnoverfrequencies (TOFs) were calculated by using standard equations(eqs 2 − 7, SI). 30 ,31 Electrochemically active sites were calculated fromthe integrated area of reduction peak of Ni 2+/3+ redox couple in theoptimum potential range of 1.191 to 1.328 V. Electrochemicalimpedance spectroscopy (EIS) was acquired in the frequency range of 0.1 Hz to 1 MHz at 5 mV potential. However, to calculate the Tafelslope from EIS, the data for the  fi rst Nyquist plot was collected at theoverpotential of 120 mV followed by incremental increase of 15 mV in the overpotential. 3. RESULTS AND DISCUSSION 3.1. Composition and Structural Characterization. Graphene oxide (GO) is an oxygen-enriched material entirely consisting of aldehyde, ketone, carboxylic acid, epoxide, andalcohol functional groups. Among these functional groups,epoxide and alcoholic groups can be selectively modi fi ed withreactive thiol group (for detail, see the SupportingInformation). 27 Epoxide ring with electrophilic carbon (nextto O) is very reactive functional group due to its high ring andangle strain. However, the reactivity of electrophilic carbon andring opening reaction of epoxide present in GO is very selective and preferably takes place with HBr among the wholeseries of halogen acids. 32  After bromination, the substitution of  bromide is usually very easy, even in the presence of a weak nucleophile, because of its appropriate electronegativity andleaving group ability. Similarly, the hydroxyl groups ( − OH) bound with sp 3 carbon in GO may also undergo nucleophilicsubstitution reaction (S N 2) in the presence of acid catalysts. Asevident from the synthetic Scheme S1 , after the dispersion of GO, HBr was added and the reaction mixture sonicated toenhance the probability of bromide ion to attack on theepoxide ring, which is thermodynamically more feasible thanthe bromination of alcohol at room temperature. However, athigh temperature the bromination and the attack of thenucleophile (thiourea) can take place simultaneously. Beforethe addition of KOH, the reaction mixture was placed in an ice bath to facilitate the exothermic acid −  base neutralizationreaction to yield su ffi cient thiol-modi fi ed GO for subsequentdeposition of ultrasmall metal/metal oxide NCs.During the loading of the metal/metal oxide NCs, thereaction was deliberately carried out in aqueous medium tomediate the outer thin layer of oxide formation. Before theaddition of reducing agent, the temperature of reaction mixture was maintained at 0  ° C to ensure the formation of uniform andsmaller NCs. Under these reaction conditions, there is achance of adsorption of unhybridized thiourea- and sulfur-containing byproducts on the surface of chemically function-alized GO which may impede the formation of metal NCs and become di ffi cult to control over their size, morphology, andcatalytic activity. Therefore, before the formation of Ni/NiONCs, modi fi ed thiolated-GO was repeatedly washed withdimethylfarmamide (DMF) to ensure the removal of theunwanted adsorbed species on modi fi ed GO as con fi rmed by  XPS. The overall structural framework, incorporation of heteroatoms and the electronic network due to  π  − π   bondsin the nanostructure of GO, is known to have uniquecontribution in the development of GO-based nanocompositesin numerous application. 33 Moreover, the selective function-alization of GO with ( − NH 2  , − SH, − OR) can further enhanceits physicochemical properties, that is, electrical conductivity,surface area, exfoliation, extended  π  − π   networking, and morespeci fi cally their covalent interaction with metals. 34 − 36 Thiolfunctionality stands tall for its strong interaction with metalsdue to their high polarizability and strong electron donatingability that can signi fi cantly alter the surface chemistry of thenanomaterials. 37 To this end, we employed thiolated and well-exfoliated  fl ower-like graphene nanosheets (Figure 1a) asconducting organic support to generate/immobilize ultrasmallNi/NiO NCs to address the controversial report about theconductivity and instability issues of transition metal/metaloxide for sluggish OER process (discussed in the last section).It is noteworthy that the characteristics peak of   “ SH ”  is not visible in FT-IR spectrum, which may be due to the insu ffi cientnumber of the thiol functional group below the limit of detection or inherently low IR absorption ability of thiol.However, it can be seen from the comparative FT-IR spectra(Figure S1) that after functionalization and loading of metal/metal oxide NCs the partial reduction of graphene takes placeand decrease in the peak intensity of carbonyl and alcoholfunctionality is observed.Because of the chemical functionalization and partialreduction of GO, the exfoliation and defect sites are generated which can be probed from the appearance of the D and G bands in the Raman spectrum. Typically, the two prominentpeaks, i.e., G band and D bands in the range of 1570 − 1600cm − 1 (sp 2C − C  , E 2g  symmetric mode) and 1350 − 1370 cm − 1 (defects and sp 3C − C  , A  1g  symmetric mode), respectively, are thecharacteristics peaks. The  “ D ”  band of GO is very sensitive tothe e ff  ect of perturbation, doping, and functional group.Therefore, the exfoliation and extent of defective sites can be ACS Applied Energy Materials  Article DOI:10.1021/acsaem.8b01375  ACS Appl. Energy Mater.  2019, 2, 363 − 371 365  quanti fi ed from the intensity ratio (I D /I G ) of these two bands. 38 The I D /I G  ratio of 0.73, 0.80, and 1.02 was observedfor GO, G-SH, and Ni@NiOG-SH respectively as shown inthe Figure S2. The increasing order in the intensity ratioindicates the sensitivity of the D band with functionalizationand disorder in the structure. 39 Moreover, the appearance of 2D band at  ∼ 2700 cm − 1 (two phonons double resonancephenomenon) at slightly higher intensity for G-SH and Ni/NiO@G-SH indicates the few layers of graphene oxidenanosheets. 40 The synthesized Ni/NiO@G-SH nanohybrid was alsocharacterized with EDX for elemental composition. Thepresence of C, O, and S in G-SH, while C, O, S, and Ni inthe EDX spectrum of Ni/NiO@G-SH indicates the expectedchemical composition of the nanohybrid (Figures S4 and S5).The SEM images of GO before and after the functionalizationare given in Figures S3 and S4 , respectively, in comparison with NiO@GO (Figure S6). From the SEM image, the highly dense and layered structure of the GO can be seen, which isdue to the interlayer stacking phenomenon. After functional-ization, the expected well-exfoliated structure of G-SHnanosheets is evident from the SEM image, which shows thedecent control over restacking phenomenon. Moreover, theelemental mapping of G-SH and Ni/NiO@G-SH shows theuniform distribution of sulfur and Ni over the whole thiolatedgraphene oxide in G-SH and Ni/NiO@G-SH (Figures S4 andS5 in the Supporting Information). The TEM images of G-SHfurther indicate the thin layers of nanosheets with an averagethickness size of  ∼ 2 to 3 nm (Figure S7). Similarly, Ni/NiO@G-SH nanohybrid was carefully examined under the highresolution transmission electron microscope (HRTEM) asshown in Figure 1. At low magni fi cation, the particles are not very clear due to their ultrasmall size and inherently low contrast of Ni metal. The thin layers of the functionalized GOcan be seen extended in two-dimensional (2D) manner andlook like the  fl ower ’ s petals (Figure 1a). At highermagni fi cation, the ultrasmall particles with an average size of  ∼ 2 nm (calculated via image j) can be clearly seen uniformly distributed over the thiolated graphene oxide nanosheets with d  spacing of 2 Å (Figure 1 b). The intercrossed fringes can beseen in the randomly taken image, which obviously indicate thecrystalline nature of Ni/NiO NCs and look like a part of graphene network (Figure S8). Typically, the equivalentinterplanar spacing of 2 Å can be indexed to (111) of 100%relative intensity plane found in the Ni and NiO cubicstructure. 41 The corresponding fast Fourier transform (FFT)pattern is shown in the inset of  Figure 1d. Moreover, theselected area electron di ff  raction (SAED) image (Figure S9)suggests the proper crystalline structure with intense di ff  ractionfrom the (111) plane of Ni/NiO nanocrystal (inset, Figure 1c).However, to determine the perfect core − shell structure andelemental mapping is very challanging in the case of NCs toshow the superposition of metal.The exact composition, chemical nature, functionalization,and the possible oxidation states of the metal in Ni/NiOG-SH was examined with X-rays photon spectroscopy (XPS) incomparison with G-SH. The C 1s (  E  = 284.5 eV) was used asthe internal standard. The XPS survey spectrum of G-SHshows the presence of C, O, and S at the correspondingpositions (Figure 2a). The XPS C 1s deconvoluted corespectrum shows peaks at 284.63, 285.17, 285.54, and 169.19eV corresponding to the C  C, C  C, C  O/C  S, and C  O respectively (Figure 2 b). But due to the close bindingenergy values, the C 1s core spectrum cannot be precisely usedto di ff  erentiate between the C  O, C  S, and the desired C  S  H bond. However, for high accuracy the S 2p corespectrum of   “ S ”  (Figure 3) can be used that shows a peak at163.71 eV with a splitting factor of 1.18 eV, which can beclearly assigned to R   C  SH bond. 27 In addition, thepresence of OH peak indicates that complete removal of HOfrom the GO surface is very di ffi cult (Figure S11).The XPSsurvey spectrum after the metal loading also shows thesignatures of C, O, S, and Ni atoms (Figure 2a). The C 1s andNi 2p deconvoluted core spectra of C and Ni are shown inFigure 2 b − d. In the 2p core spectrum of the Ni, the intensepeaks at 852.83 eV (2p 3/2 ) and 871.24 eV (2p 1/2 ) with 2:1ratio represent the metallic nickel (Ni 0 ) while peaks at 855.11eV (2p 3/2 ) and 873.15 eV (2p 1/2 ) correspond to Ni 2+  withsplitting factor of 18  ±  2 eV. We have also recorded the XPS Figure 1.  Transmission electron microscope (TEM) images of Ni/NiOG-SH nanohybrid at (a) low-magni fi cation, (b) high-magni fi ca-tion, (c) HRTEM image of Ni/NiO@G-SH nanohybrid. Inset showsthe SAED pattern, (d) magni fi ed HRTEM image of the fringesappearing in  “ c ”  from the di ff  raction of Ni(111) and thecorresponding FFT pattern. Figure 2.  (a) XPS survey scan of thiol functionalized graphene oxide(G-SH) and Ni/NiO@G-SH nanohybrid, (b) C 1s core spectrum of G-SH, (c) C 1s core spectrum of Ni/NiO@G-SH nanohybrid, and(d) Ni 2p core level spectrum on Ni/NiO@G-SH. ACS Applied Energy Materials  Article DOI:10.1021/acsaem.8b01375  ACS Appl. Energy Mater.  2019, 2, 363 − 371 366  spectrum after etching the surface oxides and found that bothspectra are exactly similar which validate the presence of nickelin two di ff  erent oxidation states (Ni 0/+2 ) as an intrinsicconstituent of these nanohybrids. The additional andunwanted peak at 169.51 eV in the core spectrum of G-SHand Ni/NiO@G-SH can be assigned to S  O/O  S  O, which may be produced due to the formal oxidation of sulfurduring analysis. The absence of peak at 169.51 eV in S 2p corelevel spectrum after etching the surface validate the aboveobservations (Figure S12). The C 1s core level spectrum of Ni/NiO@G-SH was also collected after etching, where thepeak positions were observed slightly shifting toward lower binding energy. In contrast, after the loading NCs the peak at163.71 eV corresponds to S 2p shifts to 164.13 eV, whichshows the possible transfer of electron from thiol to metal. Thesomewhat lower binding energies of Ni(0) and Ni(II) than theexpected values further validate the above observation. 41 The characterization techniques demonstrate the successfulfunctionalization of GO and subsequent decoration of thiolated graphene with Ni/NiO NCs. The TEM character-ization is inconsistent with Raman data showing a few layers of GO nanosheets which can enhance the conductivity andpotentially mediate the cross synergistic e ff  ect betweenchemically introduced impurities and abundantly availability empty   π  *  orbitals. 25 Moreover, the well-exfoliated thin layersof GO nanosheets show that chemical functionalizationincreases their stability and in-turn their availability for theuniform distribution of crystalline metal NCs. Such morphol-ogy and uniform NCs with drastic decrease in their dimensionsgreatly enhance the surface and interfacial structure for theoptimum catalysis process. From the XPS data, the reshu ffl ingof the electronic density can be critically probed. Usually, theelectronic density is shifted from the metal to the vacant  π  * orbital abundantly available on the GO, which can be seenfrom the slightly lower binding energy of C 1s core spectrumcollected after loading the NCs (Figure S10). In contrast, aninteresting trend was observed after the metal deposition thatcan be seen in the 2p core spectrum of thiol (Figure 3 b). The S2p peak at 163.71 eV shifts to 164.13 eV after the metalloading with the same splitting factor (1.18 eV), which showsthe possible transfer of electronic density from thiol to metal toestablish the covalent interaction with eventual enhancementof electronic density in the vicinity of NCs. These resultssupport the synergistic e ff  ect between the thiolated-GOnanosheets and metal NCs that is worth exploring in varioussuch catalytic reactions. 42 ,43 3.2. Electrochemical Evaluation.  Electrocatalytic activity of all the synthesized compounds (Ni/NiO@G-SH, NiO@GO, G-SH, IrO 2 ) were initially evaluated for OER by takingcyclic voltammogram (CV) Figure 4a. The appearance of redox peak of Ni 2+/3+ in the CV shows the electrochemicalaccessibility of Ni as an active catalytic site. 44 Interestingly, water oxidation was initiated at an onset potential of 1.46 V  with minimum overpotential (230 mV) and highest currentdensity of 140 mA/cm 2 at 1.7 V was achieved by Ni/NiO@G-SH nanohybrid showing superior activity among the Ni- andgraphene-based nanocomposite materials reported so far andeven better than the state-of-the-art catalyst (IrO 2 ) for wateroxidation (Table 1 and Table S2). A 10 mA/cm 2 currentdensity was achieved at  η  = 270 mV with an incrementalincrease in the overpotential after the starting point of wateroxidation, which shows the potential role of catalyst to harnessthe solar light in the photovoltaic cell with 10% photonconversion e ffi ciency (PCE). The ameliorated current density of 180 mA/cm 2  was achieved at 2 V (Figure S13) and thislarge in fl ation in current density per small change of potentialindicates the fast heterogeneous electron transfer (HET)process which is very impressive for the supportedheterogeneous electrocatalysts.The sluggish OER process at low overpotential can only bedriven by the support of intrinsic catalytic behavior to easily facilitate the 4-electron transfer process between the catalystand the adsorbed species. 45 However, the fast heterogeneouselectron transfer as well as easy adsorption and desorption of di ff  erent intermediates (OH, OOH, and O * ) from the surfaceof the catalyst depends upon the energy level of d orbital of themetal and the physicochemical properties of support. 46  Amongthe transition metals, the energy of the d orbital of the Ni(particularly oxide) is comparable to the 2p orbital of theoxygen, which can e ff  ectively overlap with optimum adsorptionof hydroxyl specie (OOH) and ultimately facilitate thedynamic release of O 2  molecules. 47 Moreover, this intrinsic behavior can further be improved by providing a synergisticenvironment where Ni can not only settle in a stable electronicenvironment but can maintain the optimal redox couple(Ni 2+/3+ ) for OER in a cyclic manner. The surfacefunctionalization of graphene with thiol as a strong electrondonor and Ni-based nanohybrid was, therefore, proven as the judicious selection to ful fi ll the aforementioned requirementsof the competent catalyst for OER. Figure 3.  Sulfur 2p core level XPS spectra of (a) G-SH and (b) Ni/NiO@G-SH. Figure 4.  Electrocatalysis (a) cyclic voltammogram (CV) of Ni/NiO@G-SH, G-SH, Ni@GO, and IrO 2  in 0.1 M KOH with scan rateof 5 mV/s (10%  i R corrected). (b) Corresponding Tafel plots and (c)controlled potential electrolysis (CPE) of Ni/NiO@G-SH at 10 mA/cm 2 in 0.1 M KOH with incremental change in the overpotential. ACS Applied Energy Materials  Article DOI:10.1021/acsaem.8b01375  ACS Appl. Energy Mater.  2019, 2, 363 − 371 367
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