Fabrication and applications of nickel selenide

Journal of Solid State Chemistry 277 (2019) 316–328

Contents lists available at ScienceDirect

Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Fabrication and applications of nickel selenide Raja Azadar Hussain a, *, Iqtadar Hussain b a b

Department of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan Department of Mathematics, Statistics and Physics, Qatar University, Doha, 2713, Qatar

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrode material Hydrothermal Nickel selenide Solar cell

This article presents synthetic strategies of different phases and morphologies of nickel selenide in terms of solid state reactions, solvothermal and hydrothermal processes, electrodeposition, precipitation reactions, colloidal synthesis, chemical bath deposition, chemical vapor deposition, solid liquid solution method and composite salt mediated methods. An insight for the applications of nickel selenide in solar cells, water splitting, super capacitors, urea conversion and glucose sensing has also been presented.

1. Introduction

1.1. Crystal structure, phases and chemical states

Depletion of fossil fuels and global warming are two main problems which risk the existence of human race with peace on earth [1]. These two problems are related to increasing demands of energy due to exponential increase in world population. It is need of the time to have green energy sources which will decrease the use of fossil fuels and ultimately global warming issue will be tackled. In this regard huge research has been carried out on solar cells [2], batteries [3], fuel cells [4,5], water splitting [6] and catalysis [7]. Best results have been achieved with noble metals [8] (details are with each subheading) which make their industrial use impossible because of financial and availability constraints. Metal chalcogenides have potential to replace the noble metals (Pt [9], Ir [10], Ru [11], Au [12] etc.) due to their manageable stoichiometries, versatile electronic and optical properties, unique crystalline phases, easy fabrication, comparatively low costs and easy availability of required metals in earth crust [13]. Metal selenides have higher electrical conductivities than corresponding sulfides and oxides and lower band gaps due to high metallic character of selenium [14–16]. Synthesis and applications of transition metal chalcogenides have been reviewed generally [15,17] but full fledge review on nickel selenide materials is not available. Nickel is comparatively less toxic than cadmium and lead and it is cheap to synthesize nickel selenides materials. This review article discusses in detail the crystalline structure, energy states, synthetic methodologies, solar cell applications, water splitting capabilities, supercapacitive applications, glucose sensing and urea conversion of nickel selenide materials.

Nickel selenide forms different homogeneous and stable crystalline phases at different atomic ratios [18]. NiSe and Ni(1-x)Se mostly exist as hexagonal phase [19–32] but NiSe has also been reported to have orthorhombic phase [33–35]. At 50% Ni, NiSe is formed whereas at 33% Ni, NiSe2 is the predominant phase. NiSe2 generally has cubic crystal system [19,22,27,32,33,36] but orthorhombic has also been reported [34]. Between NiSe and NiSe2 another stable phase is rhombohedral Ni3Se2 [18–22]. In this rhombohedral system each selenium atom is surrounded by six nickel atoms and each nickel atom is present at the corners of distorted triangular prism [18]. Less famous orthorhombic Ni6Se5 and monoclinic Ni3Se4 are also a part of scientific literature [31]. Ni3Se2 is converted from rhombohedral phase to face centred cubic phase at 620
C and tetragonal phase is observed at 650
C. When cooled back, face centred cubic structure persists down to 560
C and then is transformed to a metastable body centred tetragonal form. Between 550
C and 510
C this metastable form is converted to stable rhombohedral Ni3Se2 [37]. Ni3Te2 and Ni3(Se1.6Te0.4) behave very similar to Ni3S2 and Ni3Se2 at high temperature (Fig. 1) and have face centred cubic structure. However, below 770
C a defective Cu2Sb type phase of Ni3Te2 exists which converts to superstructures at 300
C. Fig. 1 shows common phases of nickel selenide and phase diagram [19]. In pure nickel selenide, oxidation states of nickel and selenium are 2þ and 2- respectively although some investigations by XPS analysis provide the peaks for surface oxidation of nickel and selenium as well. Nickel 2p spectrum is resolved into 2P3/2 and 2P1/2 and their peaks appear at 855.4 and 873.2 eV respectively due to spin orbit coupling.

* Corresponding author. E-mail address: [email protected] (R.A. Hussain). https://doi.org/10.1016/j.jssc.2019.06.015 Received 11 April 2019; Received in revised form 8 June 2019; Accepted 9 June 2019 Available online 15 June 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

Fig. 1. a) PXRD patterns of cubic NiSe2, hexagonal Ni(1-x)Se, and rhombohedral Ni3Se2. Reproduced from Reference [19], Copyright Wiley 2006, b) Phase transitions in nickel chalcogenides at cooling rate of 20
C per hour. Modified from Reference [37], Copyright Recueil 1968. XPS spectra of c) Ni 2p, d) Se 3d of Ni3Se2/NF electrode. Reproduced from Reference [21]. Copyright Elsevier 2017.

NiSe2 [27]. By changing the mole ratio of Ni/Se to 3:1 and continuing the reaction for 7 h at pH 14 and 180
C Ni3Se2 is the final product. At mole ratio of Ni/Se 1:1 on 180
C, reaction time governs the phase transformation of hexagonal NiSe to rhombohedral NiSe. After 4 h, reaction mixture is purely hexagonal NiSe. Rhombohedral NiSe appears after 20 h and after 65 h main product is rhombohedral NiSe. Increasing the reaction time does not eliminate the hexagonal NiSe from the reaction mixture. In reaction mixtures where Se contents are higher (Ni/Se, 1: 3) phase transformation phenomenon is not observed [19]. When Na2SeO3 is used as a source of Se its anion SeO2 3 is reduced to elemental Se under alkaline conditions via following reaction;

Selenium 3d has binding energies of 53.9 and 54.9 eV due to 3d5/2 and 3d3/2 respectively [21,23,24,38,39]. 2. Synthesis of nickel selenide Nickel selenide has been fabricated by versatile methods such as solid state reactions [18,37,40], electrodeposition [20,22,41,42], solvothermal methods [25,29,31,38,39,43–45], hydrothermal methods [19,21,24,27,30,34–36,43,46–50] precipitation method [33], colloidal synthetic method [51], solid liquid solution methods [23], chemical bath deposition [52], composite salt mediated method [53], chemical vapor deposition [54] and single source precursor method. In all these methods different types of nickel sources, selenium sources, solvents, reductants, reaction times and reaction temperatures have yielded different phases and formulae of nickel selenide which have been mentioned with detail in Table 1. Synthesis of nickel selenide is effected by changes in pH, complexing agents, Ni/Se ratios, reaction intermediates, reactions time, reaction temperature, type of catalyst and reaction media. Differences exist for all these parameters with different type of precursors and no generalization can be drawn sometimes. Therefore, we have tried to provide all possible details below for the effects of all these parameters.

SeO2 3 þ 3H2Oþ 4e → Se þ 6OH

(1) 2

This elemental Se is converted to Se reaction

following a disproportionation

3Se þ 6OH- → 2Se2 þ SeO2 3 þ 3H2O

(2)

So by controlling the pH we are basically controlling the concentration of Se2 which in turn controls the formation of mono or diselenide [19]. In the presence of EDTA (ethylenediamine tetracetate ion) cationic Ni is present in complexed form and solution concentration of Ni2þ is low. This cationic nickel is reduced by hydrazine hydrate to elemental Ni for Ni(1-x)Se at pH 14 and NiSe2 at pH 10. Morphologies of nickel selenide nanocrystals are dependent on the morphologies of intermediate elemental Se. In first step Ni(1-x)Se nanocrystals precipitate on elemental Se templates and then due to strength of alkaline solution are converted to Se2. Interaction of Se with different exposed surfaces having different orientations governs a road map for different morphologies. Ni3Se2 has been reported to produce via an intermediate Ni(1-x)Se. In

2.1. NiSO4.7H2O and Na2SeO3 In hydrothermal methods where NiSO4.7H2O and Na2SeO3 have been used as a precursor for Ni and Se respectively; when Ni/Se ratio is 1:3 and time is 5 h Ni(1-x)Se is the main product with a check on temperature between 100 to 180
C. At 180
C NiSe is formed and for other compositions of Ni(1-x)Se (x ¼ 0-0.15) lower temperatures are required. For these two compositions pH is maintained at 14. Keeping the mole ratio same (1:3) and maintaining the pH at 10 on 140
C for 20 h will yield 317

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

Table 1 Details of methods used for the fabrication of different phases and morphologies of nickel selenide portraying different Ni sources, selenium sources, reaction times, reaction temperatures and reaction conditions. Method

Ni source

Se source

Time

Temp (
C)

Solvent/Reductant

Product

Ref.

SSR

Ni powder Ni powder Ni foil Ni-EDTA Ni(CH3CO2)2.4H2O

Se powder

350-400 600 B.P of Se 27 25

Nil

Na2SeO3 SeO2, LiCl

Water Water

Ni3Se2 Ni3Se2 NiSe2 Ni3Se2 Ni3Se2

[18] [37] [40] [41] [20]

ED/EP

NiSO4.7H2O Ni foam, NiCl2, NH4Cl

Na2SeO35H2O NaHSe, polyaniline

several hrs one week NA 40, 130, 300, 600 s 30, 90, 180 s -

RT -

-

[22] [42]

HT

NiCl2.6H2O

Se powder

NiSO4.7H2O

Na2SeO35H2O

Na2SeO35H2O Se powder NaHSe Se powder Se powder Ni3Se2 Se powder Se powder Se powder Na2SeO35H2O Se/C Se powder, g-C3N4 Se powder SeO2, graphite boxes

120 100 180 100 100-180 180 140 90 180 140 160 160 90 180 180 180 120, 180 160 160 180 180

water, hydrazine, sodium dodecyl sulphate water, hydrazine water, hydrazine, EDTA

Nickel acetate Nickel foam Nickel foam Nickel foam Nickel foam Ni(OH)2 Nickel foam NiCl2.6H2O NiCl2.6H2O

12 h 12-72 h 5h 5h 5h 7h 20 h 6h 1h 16 h 25 h 5h 4h 2h 12 h 12 h 24, 6h 20 h 12 h 15 h 2.5 h 1h

160

1h 1h 3h 6h 24 h 1h 1h 12 h 12 h 12 h 6h 24 h 70 h 12 h 12 h 24 h

160 40

trioctylphosphine, hexadecylamine, sodium borohydride trioctylphosphine, hexadecylamine sodium borohydride, water

NiSe2 NiSe/polyaniline/ NF NiSe, NiSe2 Ni0.85Se NiSe Ni0.85Se Ni(1-x)Se x ¼ 0-0.15 Ni3Se2 NiSe2 NiSe2 NiSe2/NF NiSe/NF Ni3Se2/NF Ni3Se2/NF Ni3Se2/Ni(OH)2 NiSe NiSe0.85/rGO Ni0.85Se Se/C Ni0.85Se/C NiSe2/g-C3N4 NiSe2 NiSe/graphite boxes HDA Capped NiSe

ED

NiCl2.6H2O NiCl2.6H2O Ni(OH)2 nanosheets NiCl2.6H2O NiCl2(SeC6H5)2

ST

NiCl2.6H2O NiCl2.6H2O

NiCl2.6H2O

Ni(NO3)2.6H2O

SLS Col.S CVD CSM

TOPSe Se powder

SeCl4

Se Powder

Nickel foam Nickel foam Nickel foil Nickel foil Nickel acetate Nickel acetate SeO2 Nickel foam Se powder Nickel acetate TOPSe NiCl2.6H2O NiCl2.6H2O {Ni[iPr2P(S)NP(Se)iPr2]2} Nickel acetate Se powder

water, hydrazine, glycerol water , hydrazine, dimethyl formamide Water, NaBH4 water, etylenrdiamine Water, NaBH4 ethanol, water water, hydrazine hydrate water, polyvinylpyrrolidone, NaBH4 water, water, water, water, water,

SDS, ascorbic acid, hydrazine NaBH4 NaBH4 hydrazine, ammonia

ehtylenediamine

pyridine water, hydrazine, CTAB

80 110 180 180 180 180

HDA Capped NiSe Ni0.95Se Ni0.95Se Ni3Se4 Ni3Se4 Ni6Se5 Ni5Se5 NiSe

[27] [54] [19]

[21, 47] [24] [21] [48] [43] [45] [49] [30] [46] [50] [35] [33]

[31]

[25, 32]

water, Zn, CTAB water, KBH4, CTAB

160

water, hydrazine, octanol, oleic acid

Ni0.85Se

4h 24 12 h 12 h 1hr, 2hr 15 h

160 160 130 130 140 180

12-48 h 4h 60 min 3h 6h 12 h 18 h 24 h 6h 12 h 24 h

150 60 375-400 210 210 210 210 210 180 180 180

ethylene glycol, ethylene diamine ethylene glycol, ethylene diamine water, NaBH4 water, hydrazine oleylamine, tetralin benzyl alcohol Water, NaBH4 phenyl ether, oleic acid water, ammonia

NiSe/NF Ni3Se2/NF NiSe/Ni NiSe/Ni NiSe NiSe/NiSe2 NiSe/NF NiSe2 NiSe thin film NiSe2 NiSe2 NiSe2, Ni1-xSe NiSe2, Ni1-xSe NiSe2, Ni1-xSe Ni1-xSe Se, NiSe2 NiSe2 NiSe2

ethylenediamine, LiNO3, KNO3

[29, 62] [39] [23] [38] [73] [23] [51] [54] [53]

Solid state reaction ¼ SSR, Electrodeposition ¼ ED, Electropolymerization ¼ EP, Hydrothermal ¼ HT, Solvothermal ¼ ST, Chemical bath deposition ¼ CVD, Composite salt mediated ¼ CSM, Solution-liquid-solid ¼ SLS, Precipitation ¼ PR, Colloidal synthesis ¼ Col.S, Room temperature ¼ RT, ip ¼ isopropyl.

318

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

Fig. 2. SEM images of sea urchin like nickel selenide fabricated at a) 12 h, b) 24 h, c) 48 h and 72 h. Modified from Reference [26] Copyright Elsevier 2007.

fact Se source acts as a limiting reactant in the presence of excess Ni2þ which is reduced by hydrazine hydrate to elemental nickel during reaction. Following reactions explain the process 2Ni2þ þ N2H4þ 4OH- → 2Ni þ N2 þ 4H2O

(3)

2 Ni(1-x)Se þ (1þ2x)Ni → Ni3Se2

(4)

agent but also as dissolving media for Se inside carbon walls [46]. Electrodeposition is an established technique for the fabrication of metal chalcogenide films. Conventionally a three electrode system consisting of working, reference and counter electrode is used. For the deposition of thin films deposition voltage, deposition time, pH of the deposition bath and atmosphere of deposition are important. Table 2 presents all these parameters for NiSe electrodeposition determined and used by different workers. Formation of nickel selenide films via electrodeposition proceeds via following reactions [22];

Morphologies of Ni3Se2 are entirely dependent on intermediate Ni(1[5–7]. Competition between kinetic parameters and thermodynamic parameters is the reason for different phases. At the start of reaction NiSe concentration keeps on increasing fast and this production is kinetically controlled which favors the formation of hexagonal phase. When all the precursor in the reaction vessel is consumed then thermodynamic parameters control the formation of rhombohedral nickel selenide [19]. Similarly Xiaohe Liu and coworkers [55] have reported the formation of hexagonal NiSe at low temperature (100
C) and change of mole ratio has yielded cubic NiSe2. According to this study reaction time is the core parameter which changes the morphologies and size of NiSe (Fig. 2). After successful synthesis and characterization of any metal chalcogenide next step is the modulation of nanocrystals into desired morphologies for enhanced applications. Nanowires (NWs) and nanotubes have high surface area and therefore better activities. Carbon supported hexagonal Ni0.85Se nanoplates have been reported by Xuming et al. via simple hydrothermal method for battery application. Instead of intermediate of reactions as template this group has used Se/C nanowires as a template. Hydrazine in this reaction has not only acted as a reducing x)Se

H2SeO3 þ 4Hþ þ 4e- ⇆ Se þ 3H2O þ

(5)

-

H2SeO3 þ 6H þ 6e ⇆ H2Se þ 3H2O 2þ

Se þ xNi

(6)

-

þ 2xe ⇆ NixSe

(7)

H2Se þ xNi2þ ⇆ NixSe þ 2Hþ

(8)

H2SeO3 is reduced to form either elemental Se or hydrogen selenide which then reacts with cationic nickel to form nickel selenide films. Continuous voltage electrodeposition produces Ni3Se2 whereas pulsed voltage deposition (PVD) yield NiSe2. In both the cases thickness of films increases by increasing the deposition time and morphology of film is better for PVD i.e. without cracks [22]. 2.1.1. Reaction time reduction To avoid the requirements of harsh reaction conditions for much longer times N. Moloto et al. have reported the production of cubic

Table 2 Different parameters for electrodeposition of nickel selenide. Ref. electrode

Working electrode

Counter electrode

Deposition potential (V)

pH

Atmosphere

Ref.

Ag/AgCl SCE Ag/AgCl Ag/AgCl Ag/AgCl

Ti Mild steel Au coated glass and Si FTO glass, ITO glass FTO glass

Pt Ti Pt Pt Pt mesh

-0.5 -0.6 -0.8 -0.8 -0.8

2.5 3 2.5-4 2.5-4 3

Nitrogen Air Nitrogen Nitrogen Air

[41] [28] [20] [20] [22]

319

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

produces thin layer of NiSe2 on the surface of nickel hydroxide nanosheets. Due to lattice mismatch between nickel hydroxide and NiSe2, pores are formed and Kirkendall effect leads to the formation of hollow structures which after prolonged exposure results in the formation of nanosheets. Most plausible anion exchange reaction is as follow [36,57];

NiSe2, orthorhombic NiSe and monoclinic Ni3Se4 by decomposition of single source precursor in hexadecylamine (HDA) at 160
C by reacting NiCl2 with TOPSe (Reaction 9), and by reacting NaBH4 and NiCl2 in methanol and water (Reaction 10) separately, respectively [8]. All these reactions are completed in less than 1 h. HDA; 160
C

2NiCl2 þ TOPSeƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!HAD  NISe

Ni(OH)2þ 2Se þ NaBH4 → NiSe2 þ B(OH)-2Naþ þ 2H2

(9)

(11)

Same publication [36] has reported the formation of NiSe by direct selenization of Ni foam in the presence of hydrazine hydrate via flowing reactions;

MeðOHÞ; H2 O

NaBH4 þ Se þ NiCl2 ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ ƒ!NiSe þ NaCl þ B2 H6 þ H2

Se þ N2H4.H2O → H2Se þ 4Hþ þ N2

(10)

þ

H2Se þ Ni (Foam) → NiSe þ 2H

In another attempt oleic acid have been used to produce NiSe2 nanorods by the reaction of nickel acetate and TOPSe without any reducing agent [51]. This attempt however requires much longer times (12-48 h) as compared to similar scheme in which NaBH4 was used as reducing agent which requires ~1 h for reaction completion. Reaction time is directly proportional to particle size according to this study as well [51] (Scheme 1).

(12) (13)

Reaction of NaHSe with Ni foam according to Scheme 2 produces NiSe NW films (Fig. 3) [24]. Se powder in Scheme 2 acts as a source of selenium as well as a nonmetal molecular catalyst to assist the NW growth [23]. Generally in solution liquid solid (SLS) methods metallic catalysts are used which have to be removed from final NW products. This makes the process cumbersome [23]. Use of nonmetal catalysts will offer lower boiling point which will assist the growth of NWs in wide variety of solutions and removal of nonmetal catalyst from final product is also comparatively easy [58]. In a similar type of attempt Ni foam has been replaced by Ni foil as Ni source in two different methods [38]. In first method Ni foil (1.5  3 cm2) was dipped in the solution of Se powder, NaBH4 and deionized water. All these things were placed in an autoclave at 130
C for 12 h to have NiSe/Ni counter electrode. In second method NaBH4 was replaced with hydrazine hydrate [38]. Both these methods provide hexagonal NiSe but with different morphologies. This difference may be attributed to different reactivities of reducing agents according to following reactions [38];

2.2. NiCl2/Ni foam/Ni foil/Ni acetate and Se powder In all hydrothermal strategies where NiCl2 is used as a source of Ni, Se powder as a source of Se and NaBH4 as a reducing agent following are the interesting points to be considered;  At mole ratio of Ni/Se 1:1, particles are agglomerated and are of larger size than the particles synthesized at Ni/Se ratio of 1:2 which are well dispersed. At mole ratio of Ni/Se 2:1 and 1:3 polydispersed particles are formed which are of larger size than 1:1 and 1:2 particles. This may be because of secondary core formation of excessive Ni on already formed NiSe for 2:1 and 3:1 particles. This happens due to excessive Se which cannot be removed owing to its poor solubility in water [31].  For Ni/Se ratios 1:1, 1:2, and 2:1, resulting material is Ni0.95Se with hexagonal phase. 1:3 produces monoclinic phase Ni3Se4 with elemental Se on it.  Reaction time is directly proportional to the size of particles. At low reaction times (~< 3 h) particles show narrow size distribution without any surface defects. As the reaction time increases (6-24 h) different populations of particles emerge. This may be because of accumulation of monomers on already synthesized NiSe materials. This phenomenon is also reported for CdS particles and is known as Ostwald ripening [31,56].  Except for pH ¼ 1 which yields monoclinic Ni3Se4 with cubic morphology all other values (6, 7, 8, 14) yield hexagonal Ni0.95Se. This means pH has a very minor effect on phase and morphology which is a contradiction to work reported above. This may be because of different behaviors of Ni and Se sources in these two studies [31].

NaBH4 þ Se þ 3H2O → NaHSe þ 3H2 þ H3BO3

(14)

3NaHSe þ 3Ni þ H3BO3 → 3NiSe þ Na3BO3 þ 3H2

(15)

5N2H4 þ 4H2O þ N2 þ 2Ni ⇆ 2[Ni(N2H4)3]2þ þ 4OH-

(16)

SeO2 3

-

2

þ

þ 3H2O

(17)

2þ

2

→ NiSe þ 3N2H4

(18)

3Se þ 6OH → 2Se [Ni(N2H4)3]

þ Se

Ni foam has also been used as a source of Ni for the synthesis of Ni/ Ni3Se2 NW array in nonaqueous conditions [43]. Instead of deionized water 66.6% ethylenediamine and 33.3% ethylene glycol have been used as a reaction medium. Then this array has been decorated with Ni(OH)2 for supercapacitor applications [43]. Epitaxial growth in one of the methodologies for the preparation of chalcogenide films in micro and nano regime [40]. Haoling Sun et al. have fabricated 80 nm thick NiSe2 films by thermally evaporating high purity Se onto Ni foil after proper precleaning steps. Then the deposited Se films on Ni substrate were annealed at 373K, 423K and 52 K for 30 min in ultrahigh vacuum to have the final products [40]. Different morphologies and phases have recently been reported by composite salt hydroxide mediated method and solvothermal method [53]. Details of different factors which effect the product have been mentioned in Table 1.

In another attempt Ni(OH)2 nanosheets have been used as a template to synthesize NiSe2 nanosheets via hydrothermal method. In the presence of sodium borohydride exchange of hydroxyl ion from Ni(OH)2 with Se

Scheme 1. Synthesis of hexadecylamine capped monoclinic Ni3Se4 by thermolysis of single source precursor manufactured by the reaction of diphenyldiselenide, sodium borohydride and nickel dichloride [33].

Scheme 2. Schematic diagram presenting the synthesis of NiSe/NF (nickel foam) [24]. 320

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

Fig. 3. a) SEM images of bare NF. b) Low- and c) high-magnification SEM images of NiSe/NF. d) TEM image of a NiSe nanowire (inset: HRTEM image). Reproduced with permission from Reference [24]. Copyright Wiley-VCH 2015.

(composition, size and morphology) that a generalization is almost impossible to draw. However, solid state reactions for the synthesis of nickel selenide require higher temperature and reaction times relative to all other synthetic methods. Both solvothermal and hydrothermal methods work under almost similar conditions of reaction times and reaction temperatures (Table 1) and these two methods have been used the most by the researchers for the fabrication of nickel selenide materials. Slight modification of Ni and Se sources in these two methods yield different phases and morphologies of nickel selenide which are then used for device applications. Chemical vapor deposition and epitaxial growth are high temperature methods like solid state reactions whereas composite salt mediated method, colloidal synthesis methods and solid liquid solution methods need more work to achieve synthetic excellence. Electrodeposition is the best in terms of less time requirement at room temperature for nickel selenide materials. Despite of all the extensive work for synthesis of nickel selenide we are far behind technical mass production and nanoscience is the appropriate term compared with nanotechnology.

2.2.1. Effect of solvent Solvent not only provides reaction medium but also act as surface active agent to prevent the aggregation and agglomeration. Three different solvents (water, ehtylenediamine and pyridine) have been compared by using the similar precursors. Water provides hexagonal Ni0.95Se with agglomeration of the particles. Ehthylenediamie yields orthorhombic Ni6Se5 with impurity phase and pyridine gives larger sized orthorhombic Ni5Se5 particles without any impurity. To us, this difference in behavior is firstly because of difference in chelating and surfactant ability of ethylenediamine with water and pyridine. Secondly, water and pyridine are behaving differently due to difference in electronegativity of nitrogen and oxygen. Ethylenediamine and water also have the ability to form hydrogen bonds [31]. 2.3. NiCl2 and SeCl4 When SeCl4 was used instead of Se powder and Na2SeO3 as a source of Se in aqueous media with nickel chloride hexahydrate as a source of Ni, hydrazine as reducing agent and CTAB as a surfactant following were the outcomes;

3. Applications of nickel selenide 3.1. As counter electrode in solar cells

 There is no change in phase of nickel selenide from 12 h reaction time to 70 h. These results are a contradiction to previously reported time effects on phase [25,27]. Changes in sizes were observed in hexagonal nickel selenide which were similar to the previous findings [31].  Effect of reducing agents i.e. hydrazine, NaBH4 and Zn were investigated but clear cut changes of morphology and shape were not observed [25].  These precursors produce hexagonal NiSe at mole ratio Ni/Se 1:1 and 3:2. Although the precursors are different but the effects of mole ratio for 1:1 is similar to the previously reported results [31,32]. At mole ratio of Ni/Se 1:2 hexagonal and cubic phases are present in the product mixture. For pure cubic phase reaction temperature has to be reduced to 120
C [32].

Dye sensitized solar cells (DSSC) are cheap, environmental friendly, easy to fabricate and have excellent performance. A DSSC mainly consists of a dye adsorbed on TiO2 as photoanode, a catalyst coated substrate (conductive fluorine doped tin oxide etc) as a counter electrode (CE) and an electrolyte comprising of a redox couple (I-/I-3). CE performs two main functions, first is the collection of electrons from external circuit and second is to catalyze the reduction of redox couple [29,59,60]. An acceptable CE will be the one which has good electrocatalytic activity, high conductivity and stability. Pt CEs are being used in DSSCs but efforts are in progress to replace them because of their high cost, decomposition byproducts (PtI4 and H2PtI6) poor long term stability against iodine based electrolyte and difficult manufacturing (sputtering and high temperature hydrolysis). Therefore first row transition metal chalcogenides are a perspective material for CE in DSSCs [29]. Metal oxides, metal sulfides and metal selenides have extensively been studied in this regard [15]. To check the

Synthesis of different phases and morphologies of nickel selenide is dependent on type of methodology, type of solvent/reductant, different types of Ni and Se sources, time, temperature, catalyst and pH. All these parameters are so tightly involved for the fate of final product 321

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

the intercept of semicircle on x-axis represents Rs. The difference between the two intercepts of semicircle represents RCT. These parameters are determined with Z-view software and both the electrodes (Ni0.85Se and Pt) show nearly same results. This infers that effect of Rs on photovoltaic performance of both the electrodes can be neglected. RCT value of Ni0.85Se is 4.49 Ωcm2 and Pt electrode is 6.17 Ωcm2 which confirms better performance of Ni0.85Se electrode for catalytic reduction of triiodide [29]. Tafel polarization curves (Fig. 4d) of symmetrical cells as used in EIS measurements show that anodic and cathodic current for Ni0.85Se have higher slope values than Pt electrode due to high Jο on Ni0.85Se according to Tafel equation;

electrocatalytic activity, electrochemical impedance spectroscopy (EIS) of dummy cell consisting of two identical CEs having electrolyte in between is employed. EIS study provides information about series resistance (Rs), charge transfer resistance (RCT), constant phase element (CPE) at electrolyte electrode interface and Nernst diffusion impedance (ZN) from fitted EIS spectra. Results from EIS are supplemented by cyclic voltammetry (CV) and Tafel curves. A good catalyst has smaller RCT, large CV current density, smaller potential separation between CV peaks and higher exchange current density (Jο). Jinbio Jia et al. have reported that DSSC having Ni0.85Se CE possess an open circuit voltage (Voc) of 0.780 mV, short circuit current density (Jsc) of 16.39 mAcm-2 and power conversion efficiency (PCE) of 8.88% relative to DSSC with Pt CE which has Voc of 0.777 mV, Jsc of 15.55 mAcm-2 and PCE of 8.13% (Fig. 4a) [29]. By placing a reflect mirror at the bottom of Ni0.85Se CE, Voc of 0.791 mV, Jsc of 18.44 mAcm-2 and PCE of 10.19% is achieved. Superior values of these parameters are attributed to transparency of Ni0.85Se in the visible region and utilization of extra incident light reflected from mirror. There is a direct proportionality between the electrocatalytic activity of CE and photocurrent production. The difference of Jsc between Pt and Ni0.85Se can be explained by comparing their CV behavior (Fig. 4b) [29]. When CV of Pt and Ni0.85Se is carried out by using Ag/AgCl reference electrode in iodide/triiodide electrolytic media, a couple of well-defined redox peaks is observed owing to following reactions; I-3 þ 2e- ⇆ 3I-

3I2 þ 2e ⇆

2I-3

Jο ¼ RT/nFRCT

(21)

Where R is the general gas constant, F is the Faraday’s constant and n is the number of electrons involved in the reduction of triiodide ion. Ni0.85Se on the basis of RCT has better Jο values than Pt electrode. Limiting current density (Jlim) of Ni0.85Se is also greater than Pt electrode according to following equation Jlim ¼ 2nFCDn/I

(22)

Where I is the distance between the electrodes, D is the diffusion current and C is the concentration of triiodide ion. These results were consistent with the first ever finding on Ni0.85Se (PCE ¼ 8.32%) and Co0.85Se (PCE ¼ 9.40%) [29,60]. In another attempt 1D single crystal NiSe/Ni CEs have shown better fill factor and Jsc than DSSC having Pt/FTO CE. PCE of NiSe/Ni is 6.75% which is comparatively better than 6.13% value of Pt/FTO [38]. In a similar type of study Ni0.85Se/reduced graphene oxide has provided a PCE of 9.75% relative to DSSC based on Pt CE which has PCE of 8.15% [49]. Still another experimental effort reports PCE of 8.96% for Ni0.85Se as compared to 8.44% of Pt CE [30,49]. Other crystalline phases of nickel selenide such as NiSe2 and Ni3Se2 are also useable as CEs. NiSe2 and Ni3Se2 have PCEs of 4.64% and 2.63% in comparison to 2.62% value of Pt [22,61]. Composite of NiSe with cube shaped lidless graphite boxes have

(19) (20)

Keeping in view the function of CE, Eq. (20) is important. Greater is the value of reduction current higher is the catalytic activity and smaller value of potential difference between the peaks means smaller value of opposing potential against the catalytic function. When data is compared according to Fig. 4c Ni0.85Se has higher value of reduction current (1.15 mAcm-2) than Pt (0.78 mAcm-2) however, overpotential value of Ni0.85Se (440 mV) is higher than Pt (348 mV). In EIS measurements for Nyquist plot (Fig. 4c) a semicircle and a sloping straight line is obtained. In the equivalent circuit (inset Fig. 4c)

Fig. 4. a) Photocurrent–voltage curves of the DSSCs with different CEs, irradiating from the front, b) Cyclic voltammograms for Pt and Ni0.85Se electrodes, c) Nyquist plots of Pt and Ni0.85Se., d) Tafel polarization curves of the dummy cells with different CEs. Reprinted with permission from Reference [29]. Copyright Elsevier 2015. 322

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

high covalent nature of nickel selenium bond. It has been stated that catalytic activity is directly proportional to the covalency of bond [73]. HER is favorable under acidic conditions due to easy availability of protons but kinetics of OER is dependent on the type of material which is being used as a catalyst. Ir, Ru and their compounds are useful in acidic conditions but Fe, Co and Ni provide best results in basic media for OER. This difference of OER is due to different mechanistic approaches followed by two set of metals for coordination of hydroxide to the active site firstly and sluggish kinetics of rate determining step secondly [74]. Traditionally efficiency of OER is measured at overpotentials which yield a current density of 10 mAcm-2. OER performance is measured with linear scan voltammetry (LSV) and stability of the catalyst is determined with chronoamperometry. Electrochemical measurements are generally performed on three electrode system consisting of reference (Ag/AgCl), counter (Pt mesh) and working electrodes. Electrochemical surface area (ECSA) of catalyst is measured by determining the electrochemical capacitance of electrolyte electrode interface. Cyclic voltammograms are recorded at different scan rates in inert atmosphere using a basic electrolyte (0.3 M KOH). Capacitive current (iDL) is measured from non-faradic double layer region and involves accumulated charge instead of charge transfer or chemical reaction. iDL is directly proportional to scan rate [20];

shown a PCE of 8.26% which is better than PCE of graphitic boxes (2.01%) and NiSe CEs (7.45%). This higher PCE is attributed to better three dimensional electron transfer property and high surface area of the composite relative to the pure materials (NiSe and graphitic boxes) separately [35]. NiSe/NiSe2 hybrid nanomaterial CEs have shown a PCE of 7.78% which is better than Pt CE (7.09%) [44]. Better performance of NiSe/NiSe2 has been related to its greater number of active sites, strong charge separation and transfer [44,45]. More recently Yadav et al. have synthesized rhombohedral Ni3Se2 and cubic Ni3Se4 via hot injection method for DSSCs applications. Ni3Se2 and Ni3Se4 CEs have shown a PCE of 6.40% and 5.22% respectively, relative to 6.62% PCE of Pt electrode [62].

3.2. In water splitting Splitting of water into its constituent gases hydrogen and oxygen is need of the time for renewable carbon free and environmental friendly energy sources. However there are two main problems, first is that two half reactions in water electrolysis are sluggish and especially oxygen evolution reaction is kinetically hindered and second is the production of pure hydrogen. To achieve a current density of 10 mAcm-2 noble metal catalysts such as Pt (cathode), Ir/IrO2 and RuO2 (anode) are required which make the process economically unfavorable. Basic role of these catalysts is to reduce the amount of overpotential at respective electrodes and increase the rate of conversion. These catalysts should also have the ability to sustain the corrosive oxidizing conditions and should have low cost. Extensive effort has been carried out to use 3d transition metals instead of noble elements because of their abundance and low cost. Ni in particular has been studied due to its earth abundance and high potential in water. Transition metal oxides (Mn-Cu) [20,63], Ni2P [64], iron selenide, iron sulfide, cobalt sulfide and selenide, nickel sulfide and selenide have also been evaluated for oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and oxygen reduction reaction [65, 66]. These transition metal chalcogenides are important due to their interesting optical and electronic properties not only in water electrolysis but many other applications as well [14,67–72]. They can be a replacement of Pt based catalysts in HER and Ir/Ru based catalyst in OER [20]. Focus on nickel selenide as compared to oxides and sulfides is because of

iDL ¼ CDL x v

(23) -2

where CDL is specific capacitance of double layer in Fcm (electrode). A plot of iDL vs. v gives a straight line and slope of this line gives CDL. ECSA is determined by ECSA ¼ CDL/Cs

(24)

Where Cs is the specific capacitance. Fig. 5a presents the LSV of OER with Ni3Se2 at 40, 130, 300 and 600 s at onset overpotential of 220 mV, 220 mV, 210 mV and 250 mV respectively relative to reversible hydrogen electrode (RHE) [20]. These evolution activities are far much better than blank Au electrode. 30 mV shift in the onset potential is attributed to better deposition after longer time. Best activity was for 300 s with much lower overpotential values. Higher contents of Ni3Se2 have positive impact on catalytic activity initially but this effect depletes afterward. Enhanced activity of Ni3Se2 can be achieved by maintaining and attaining a fine balance of catalytic loading, surface roughness, film

Fig. 5. a) Cyclic voltammograms measured for the Ni3Se2 catalyst (electrodeposited for 300 s) in N2 saturated 0.3 M KOH solution at different scan rates from 2.5 to 40 mV s-1. The inset shows a plot of anodic current measured at _0.14 V as a function of scan rate. b) LSVs measured for catalysts electrodeposited for different time periods in N2 saturated 0.3 M KOH solution at a scan rate of 0.01 Vs-1. The dotted line shows the current density of 10 mA cm-2. (b-1) Tafel plots of catalysts. Reproduced with permission from Reference [20]. Copyright RSC 2016. 323

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

[77], NiSe2 nanosheets at 198 mV [36] and NiSe nanoflakes at 217 mV [36].

properties i.e. thickness and particle size [20]. Greater is the amount of catalyst greater will be the population of active sites and higher will be the activity however amount of catalyst on films increases by increasing the deposition time and enhanced deposition time increases the thickness of films which blocks some of the active sites to reduce the catalytic performance [20]. Greater thickness also plays a negative role in the movement of charge carriers and electrical conductivity of baseline material (Au). High roughness is directly proportional to higher catalytic activities due to high active sites on rough surface. Better catalytic activities will be obtained by designing an electrode with thin deposition, maximum loading and high roughness. Overpotential of 300 s deposited Ni3Se2 catalyst is 310 mV which is comparable to IrOx (320 mV) and RuO2 (390 mV) [20]. Specific activity of a catalyst is its intrinsic property which determines the comparison between catalyst concentration and deposition time. Specific current densities of the catalyst provide OER activities at constant overpotential (310 mV) [20]. Ratio of current density per geometric area (Jg) at overpotential with RF is called specific current density (Js); Js ¼ Jg/RF

3.3. As electrode in supercapacitors and batteries Energy conversion and storage are main research focus due to depletion of fossil fuel and environmental concerns. Supercapacitors or electrochemical capacitors are one of the options for power devices due to their cost effectiveness, environmental friendly concern, high power density and long cycle life [24]. Supercapacitors store energy via two methods; 1) ion adsorption (electrochemical double layer capacitors (ECDLCs)) and 2) due to faradic reactions at the metal surface (Pseudo capacitors) [24]. Metal chalcogenides have been under investigation extensively due to similar properties of chalcogens in VIA group. Replacement of metal oxides and metal sulfides with metal selenides as a research interest is favored by more metallic character of Se which enhances its optoelectronic properties. Pseudocapacative properties of nickel selenide were expected to be promising due to its availability in different oxidation states, good conductivity and charge transfer [60,78, 79]. Chung Tang et al. have used NiSe/NF NW as working electrode, graphite plate as CE and SCE as reference electrode in CV for the determination of its supercapacative behavior (Fig. 6a) [24]. Specific capacitance of an electrode is determined by area under the CV curve and Fig. 6a shows that NF has negligible electrochemical activity and almost all the contribution in specific capacitance is due to NiSe/NF nanowires. In the CV profile oxidation maximum is at 0.38 V and reduction maximum is at 0.22 V. At different scan rates current intensities of oxidation and reduction maxima increase which shows that the process is diffusion controlled (Fig. 6b). These oxidation and reduction maxima were attributed to following reactions;

(25)

On Js basis also, Ni3Se2 deposited at 300 s was the best catalyst. Fig. 5b presents the Tafel plots which are an important tool for kinetics of OER. Linear dependence of overpotential vs. Log J with different slopes was achieved. Lowest values of Tafel slopes for 300 s deposited catalyst show better OER catalytic activities [20]. Substrates also have effect on OER activities. Onset overpotentials and overpotentials to achieve current density of 10 mAcm-2 change by changing the substrate. Among Au-Si, glassy carbon, Au-glass, Ni foam, FTO and ITO, lowest overpotential value of 0.270  0.20 was observed for Ni foam [20]. NiSe/NF has been demonstrated to achieve 10 mAcm-2 at a cell voltage of 1.63 V for OER [75]. Ni foam however is highly porous and has high surface area therefore use of its geometric surface area to calculate current density for comparison with planner substrate requires second thoughts. Table 3 presents comparative performances of different Ni based catalysts [20]. Arumugum Sivanantham et al. have reported 100 mAcm-2 current density at 315 mV. This activity remains almost constant in terms of applied potential even after 285 h [21]. On the other hand Ni3S2/NF and NiSe/NF have been reported to have overpotentials of 123 mV and 137 mV for HER and 222 mV and 271 mV for OER respectively to achieve a current density of 10 mAcm-2. Li et al. have reported OER performace of NiSe@NiOOH/NF with 332 mV at 50 mAcm-2 [76]. In other experimental efforts 10 mAcm-2 current density is afforded by NiSe2 at 299 mV

NiSe þ H2O þ 1/2O2 → Ni(OH)2 þ Se -

Ni(OH)2 þ OH → NiOOH þ H2O þ e

IrOx RuO2 Ni-Fe oxide nanotubes Ni@Ni[(Ni2þ/3þ) Co2(OH)6-7]x nanotube arrays α-Ni(OH)2 Ni3S2-Ni foam NiCo2O4 Ni0.9Fe0.1Ox NiCo-LDH Ni3Se2-Au@glass Ni3Se2-Au@glass annealed Ni3Se2-Au@Si Ni3Se2-GC Ni3Se2-Ni foam

Onset overpotential vs. RHE

Overpotential vs. RHE required to get 10 mAcm-2

0.22 0.28

0.32 0.39 0.38 (for 5 mAcm-2)

105

[81] [82] [83]

0.31

0.46

65

[84]

0.31 0.157 0.26 0.25 0.29 0.22 0.21

0.331 0.187 0.34 0.336 0.41 0.32  0.20 0.29  0.10

42 159.3 75 30

[85] [64] [86] [87] [62] [20]

0.21 0.22 0.23

0.3  0.10 0.31  0.20 0.27  0.20

122 79.5 142.8

Tafel slope (mVdec-1)

97.1 97.2

(26) (27)

CV results were confirmed by charge-discharge curves at different current densities [Fig. 6c]. Specific capacitance is calculated by Cspe ¼ IΔt/ΔVm

(28)

Where I is the current density, Δt is the charge discharge time, ΔV is the potential window and m is the mass of active material. For areal capacitance m is equal to the area of electrode. NiSe/NF electrode provided a specific capacitance of 1790 Fg-1 at a current density of 5 Ag-1 and even at 20 Ag-1, 840 Fg-1 was retained [24]. According to authors this decrease is because of increase in potential drop which happened due to less active material on the electrode at high current density. This study showed that areal capacitances are 5.01. 3.87, 2.35, 2.07 and 1.82 Fcm-2 at current densities of 5, 10, 20, 30, and 50 Ag-1 respectively (Fig. 6e) [24]. Repeated charge discharge at a current density of 10 Ag-1 for 1000 cycles retains 70% capacitance (Fig. 6f). These high supercapacitive performances of NiSe/NF were because of good mechanical adhesion and better electrical contact between NiSe NWs and NF as a result of direct growth of NiSe NWs on NF firstly, more catalytic site availability between 3DNF and 1D NWs secondly and absence of polymer binders lastly. Table 4 presents the comparison between the performance of NiSe and other Ni based materials reported in the literature [24]. In another attempt Ni3Se2 NWs@Ni(OH)2 nanoflakes have yielded a specific capacitance of 1689 μAhcm-2 (281.5 mAhg-1) at a discharge current of 3 mAcm-2. By using Ni3Se2 NWs@Ni(OH)2 nanoflakes architecture as positive electrode and activated carbon as negative electrode a cycling stability of 83.6% after 10,000 cycles with 59.47 WhKg-1 energy density and 100.5 WKg-1 power density was achieved [43]. This performance was further enhanced by replacing activated carbon with metal hydride electrode [43]. Ni0.85Se/C nanocomposites have provided a reversible capacity of

Table 3 Comparative performances of different nickel based catalysts. Catalyst

-

Ref.

324

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

Fig. 6. a) CV curves of bare NF (nickel foam) and NiSe/NF in the range of 0–0.6 V at 5 mVs-1. b) CV curves of NiSe/NF in the range of 0–0.6 V at various scan rates. c) Galvanostatic charge–discharge curves of NiSe/NF at various current rates in the range of 0–0.5 V. d) Specific capacitance and e) areal capacitance of NiSe/NF at various current densities. f) Cycling performance of NiSe/NF at a current rate of 10 Ag-1 (inset: the last five cycles of the galvanostatic charge–discharge curves). Reproduced with permission from Reference [24]. Copyright Wiley-VCH 2015.

390 mAhg-1 with only 3% loss of cycling stability after 100 cycles when used as an anode material in sodium ion battery. This system delivers a capacity of 219 mAhg-1 even when current rate is increased to 5C. However, practical application of Ni0.85Se/C nanocomposites is limited due to lower theoretical capacity and high carbon contents [46]. Hollow NiSe/CoSe sample with NiSe to CoSe ratio of 4:2 provides a high specific value of 584 Cg-1 at 1 Ag-1. Assembly of this hybrid supercapacitor with reduced graphene oxide gives a specific capacity of

201 Cg-1 at a current density of 1 Ag-1. This NiSe-CoSe system delivers 41.8 WhKg-1 at 750 WKg-1 and 20.3 WhKg-1 at 30 KWKg-1 [80].

3.4. In urea conversion Urea is present in human urine at a level of about 2-2.5% by weight. About 240 million tons of urine is added by human in the environment per day. Agriculture adds about 200 million tons of urea as nitrogen release fertilizer. Urea is converted to ammonia in water and its incomplete degradation can produce nitrites, nitrates and nitric oxides. One solution to this problem is biodegradation but it is not cost effective. Electro-oxidation of urea at anode surface produces molecular nitrogen and carbon dioxide is released. On the cathode surface hydroxyl ions and hydrogen is produced which may be used as fuel after industrial excellence of the process. However, oxidation of urea at anode surface is a six electron transfer process and is kinetically slow. Pt, platinized titanium, ruthenium coated titanium oxide and iradium oxide have been tried for electroxidation of urea. Due to cost constraints these noble metal catalysts have not been commercialized. Efforts have been directed in the recent years to use first row transition elements especially Ni-based materials for urea electroxidation [47,81–85]. Pei Xiong et al. have used NiSe2 nanoflakes for urea oxidation which showed decent catalytic performance relative to nickel oxide and hydroxide. At a current density of 10 mAcm-2, NiSe2 presents a potential of 324 mV against Ag/AgCl which is lower than precursor (347 mV), Pt/C (377 mV) and nickel foam (378 mV). Authors relate this performance to

Table 4 Supercapacitive performances of nickel based materials. Electrode structure

Specific capacitance (Fg-1)

Current density Ag-1

Ref.

NiSe/NF NiO nanosheets Nanoporous NiO film Ni(OH)2/NiO/Ni Ni(OH)2 nanoflakes array/ NF Ni(OH)2 coated-ZnO array Ni3S2 nanosheets/CNTs NiS/GNS/CNTs 3D Ni3S2 nanosheets array/ NF NiS/rGO Nickel sulfides/rGO NiSe/MoSe2 Ni0.85Se/MoSe2

1384 354 1400 1070 1125

10 10 10 15 10

[24] [88] [89] [90] [91]

2028 450 1621 1100

10 8 9 10

[92] [93] [94] [95]

579 1000 223 774

5 10 1 1

[96] [97] [86] [87]

325

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328

Fig. 7. a) CVs of Ni3Se2 NS/NF and bare NF in 0.5 M NaOH in the absence (dash curves) and presence (solid curves) of 1.0 mM glucose at a scan rate of 20 mV s1., b) High-resolution SEM images of Ni3Se2 NS/NF. Reprinted with permission from Reference [48]. Copyright Elsevier 2019.

Another aim of new research in the field of nickel selenide may be to synthesize new morphologies such as core shell materials (spheres, rods, tubes, wires) with other nickel and first row transition metal chalcogenides for all the applications which have been discussed in this article. Use of nickel selenides as non-homogeneous catalysts for the conversion of atmospheric pollutants to useful products has not been investigated yet. Nickel selenide based materials have provided good performances in DSSCs but mechanism of catalytic action is not yet clear. Effect of stoichiometric ratios and morphologies separately on the catalytic performance will also be interesting. Nickel selenide materials should also be evaluated for iodine free electrolyte systems in future investigations. Despite of all the best results for OER performances of nickel selenide materials, sluggish kinetics of counter HER in alkaline media cannot be improved. This kinetics is slow for Pt based catalysts as well. Possible solution to this problem may be to find a catalyst which performs well in the acidic media but even this will not eliminate the requirement of Pt in HER. Another choice may be to design a bimetallic bifunctional catalyst which performs well without any separator. We suggest the synthesis of materials such as NiSe/S, NiSe/P, NiSe/CoS, NiSe/Ti mesh etc with different morphologies for water electrolysis. Complex phase changes without exact information of mechanism and high charge voltage performance hinder the use of metal chalcogenides in the market products generally but lower theoretical capacities of nickel selenides (NiSe2, 495 mAhg1; Ni0.85Se, 416 mAhg1; NiSe, 397 mAhg1) and higher carbon contents limit their use in practical applications. By decreasing the carbon contents, capacity may be improved in future investigations. Glucose sensing is at initial stage for nickel selenide materials. There is enough space in this domain to check other nickel selenide morphologies, formulae and phases and there combination with other materials such as graphene, CNTs and metals. Emphasis should be to understand the exact mechanism of glucose electro-oxidation at first part and then improve the selectivity and sensitivity of electrocatalysts.

increased surface area which supports charge transfer capacity via more exposed active sites. NiSe2 anode have better catalytic activities in terms of lower Tafel slopes and high current densities [47]. 3.5. Glucose sensing Glucose sensing is important in medicinal diagnostics, biotechnology and food industry [48]. Enzymatic glucose sensing is interfered by thermal and chemical instability of glucose oxidase. Non enzymatic sensing is a direct method which detects the glucose on anode surface. CV with three electrode system consisting of Pt wire as a counter electrode, Ag/AgCl as reference electrode and Ni3Se2/NF as working electrode has been used by Min Ma et al. [48] for glucose sensing. Ni3Se2/NF provides a couple of well-defined redox peaks which are enhanced in the presence of 0.1 mM and 1 mM glucose via diffusion controlled process (Fig. 7a) [48]. In this process glucose is oxidized to gluconoacetone whereas reduction of Ni3þ to Ni2þ takes place. This electrode system senses glucose between 0.25 μM to 6.335 mM concentrations. This performance of Ni3Se2/NF is attributed to following.  Nanosheet morphology (Fig. 7b) which allows good penetration of electrolyte, good diffusion and efficient catalysis.  Faster electron transfer in Ni3Se2 is due to synergistic interaction between Ni3Se2 and Ni foam couple.  Absence of polymer binder in the electrode system which eliminates opposing potential at catalyst/substrate interface.  Direct contact and good binding between Ni foam and Ni3Se2 nanosheets arrays. 4. Conclusions and future horizons Different morphologies and phases of nickel selenide can be synthesized by changing the reaction type, precursors, reaction media, reducing agents, mole ratios, pH, reaction time and reaction temperatures. In solar cells PCEs of CEs made up of Ni0.85Se, NiSe/Ni, Ni0.85Se/reduced graphene oxide, NiSe2, Ni3Se2, NiSe/NiSe2 and NiSe/lidless graphitic boxes are better than PCEs of Pt CE. Water splitting activities of Ni3Se2, NiSe2, NiSe2 nanoflakes and nanosheets are better than Au electrode and overpotentials are even lower than IrOx and RuOx. Due to more metallic character of Se than its group members (S, O), nickel selenides have better supercapacitive properties than other nickel compounds. Urea oxidation of NiSe2 is better than Pt/C and Ni foam and Ni3Se2/NF also senses glucose. Nickel selenides are emerging chalcogenides both synthetically and application wise. Applications of nickel selenides for sensing of chemicals such as organic contaminants, DNA, medical diagnostics, aqueous pollutants, industrial waste and non-biodegradable chemical/dyes is the future horizons of research.

Acknowledgements Publication of this manuscript is not funded by any organization. It is a self effort of authors. References [1] G. McDowell, C. Huggel, H. Frey, F.M. Wang, K. Cramer, V. Ricciardi, Adaptation action and research in glaciated mountain systems: are they enough to meet the challenge of climate change? Glob. Environ. Chang. 54 (2019) 19–30. [2] M. Urbani, M.-E. Ragoussi, M.K. Nazeeruddin, T. Torres, Phthalocyanines for dyesensitized solar cells, Coord. Chem. Rev. 381 (2019) 1–64. [3] L. Chen, X. Guo, W. Lu, M. Chen, Q. Li, H. Xue, H. Pang, Manganese monoxidebased materials for advanced batteries, Coord. Chem. Rev. 368 (2018) 13–34.

326

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328 [36] K.S. Bhat, H. Nagaraja, Nickel selenide nanostructures as an electrocatalyst for hydrogen evolution reaction, Int. J. Hydrogen Energy 43 (2018) 19851–19863. [37] A. Stevels, J. Bouwma, G. Wiegers, F. Jellinek, Phase transitions in the nickel selenide Ni3Se2 and related phases, Recl. Trav. Chim. Pays-Bas 87 (1968) 705–708. [38] C. Bao, F. Li, J. Wang, P. Sun, N. Huang, Y. Sun, L. Fang, L. Wang, X. Sun, One-pot solvothermal in situ growth of 1D single-crystalline NiSe on Ni foil as efficient and stable transparent conductive oxide free counter electrodes for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 8 (2016) 32788–32796. [39] H. Ren, Z.-H. Huang, Z. Yang, S. Tang, F. Kang, R. Lv, Facile synthesis of freestanding nickel chalcogenide electrodes for overall water splitting, J. Energy Chem. 26 (2017) 1217–1222. [40] H. Sun, Z. Liang, K. Shen, M. Luo, J. Hu, H. Huang, Z. Zhu, Z. Li, Z. Jiang, F. Song, Fabrication of NiSe2 by direct selenylation of a nickel surface, Appl. Surf. Sci. 428 (2018) 623–629. [41] K. Anuar, Z. Zainal, N. Saravanan, A. Kartini, Cathodic electrodeposition and characterization of Ni3Se2 thin films, Asian J. Sci. Technol. (ASTD) 21 (2004) 19–25. [42] P.F. Liu, L. Zhang, L.R. Zheng, H.G. Yang, Surface engineering of nickel selenide for an enhanced intrinsic overall water splitting ability, Mater. Chem. Frontiers 2 (2018) 1725–1731. [43] X. Shi, J. Key, S. Ji, V. Linkov, F. Liu, H. Wang, H. Gai, R. Wang, Ni(OH)2 nanoflakes supported on 3D Ni3Se2 nanowire array as highly efficient electrodes for asymmetric supercapacitor and Ni/MH battery, Small 0 (2018) 1802861. https://d oi.org/10.1002/smll.201802861. [44] Q. Jiang, R. Chen, H. Chen, J. Jiang, X. Yang, Y. Ju, R. Ji, Y. Zhang, Improved performance in dye-sensitized solar cells via controlling crystalline structure of nickel selenide, J. Mater. Sci. 53 (2018) 7672. [45] H. Wu, Y. Wang, L. Zhang, Z. Chen, C. Wang, S. Fan, Comparison of two nickel selenides materials with different morphologies as counter electrodes in dyesensitized solar cells, J. Alloy. Comp. 745 (2018) 222–227. [46] X. Yang, J. Zhang, Z. Wang, H. Wang, C. Zhi, D.Y.W. Yu, A.L. Rogach, Carbonsupported nickel selenide hollow nanowires as advanced anode materials for sodium-ion batteries, Small 14 (2018) 1702669. [47] P. Xiong, X. Ao, J. Chen, J.-G. Li, L. Lv, Z. Li, M. Zondode, X. Xue, Y. Lan, C. Wang, Nickel diselenide nanoflakes give superior urea electrocatalytic conversion, Electrochim. Acta 297 (2019) 833–841. [48] M. Ma, W. Zhu, D. Zhao, Y. Ma, N. Hu, Y. Suo, J. Wang, Surface engineering of nickel selenide nanosheets array on nickel foam: an integrated anode for glucose sensing, Sensor. Actuator. B Chem. 278 (2019) 110–116. [49] J. Dong, J. Wu, J. Jia, L. Fan, J. Lin, Nickel selenide/reduced graphene oxide nanocomposite as counter electrode for high efficient dye-sensitized solar cells, J. Colloid Interface Sci. 498 (2017) 217–222. [50] S. Wang, P. He, L. Jia, M. He, T. Zhang, F. Dong, M. Liu, H. Liu, Y. Zhang, C. Li, J. Gao, L. Bian, Nanocoral-like composite of nickel selenide nanoparticles anchored on two-dimensional multi-layered graphitic carbon nitride: a highly efficient electrocatalyst for oxygen evolution reaction, Appl. Catal. B Environ. 243 (2019) 463–469. [51] StephenO’Brien MingYin, Synthesis and characterization of nanostructured nickel diselenide NiSe2 from the decomposition of nickel acetate, (CH3CO2)2Ni, J. Nanomater. 2014 (2014) 1. [52] P.A. Nwofe, R.A. Chikwenze, P.E. Agbo, H.U. Igwe, Deposition and characterization of nickel selenide thin films for applications in optoelectronic devices, Asian J. Sci. Res. 10 (2017) 43–49. [53] C. Zhao, Y. Zhu, W. Liu, Z. Chen, Z. Wang, P. Feng, A simple way to prepare nickel selenide from NiSe2 to Ni1-xSe and their formation mechanism, J. Solid State Chem. 266 (2018) 37–43. [54] A. Panneerselvam, M.A. Malik, M. Afzaal, P. O’Brien, M. Helliwell, The chemical vapor deposition of nickel phosphide or selenide thin films from a single precursor, J. Am. Chem. Soc. 130 (2008) 2420–2421. [55] X. Liu, N. Zhang, R. Yi, G. Qiu, A. Yan, H. Wu, D. Meng, M. Tang, Hydrothermal synthesis and characterization of sea urchin-like nickel and cobalt selenides nanocrystals, Mater. Sci. Eng. B 140 (2007) 38–43. [56] Y. Mastai, R. Polsky, Y. Koltypin, A. Gedanken, G. Hodes, Pulsed sonoelectrochemical synthesis of cadmium selenide nanoparticles, J. Am. Chem. Soc. 121 (1999) 10047–10052. [57] K. Wang, Z. Ye, C. Liu, D. Xi, C. Zhou, Z. Shi, H. Xia, G. Liu, G. Qiao, Morphologycontrollable synthesis of cobalt telluride branched nanostructures on carbon fiber paper as electrocatalysts for hydrogen evolution reaction, ACS Appl. Mater. Interfaces 8 (2016) 2910–2916. [58] K. Xu, H. Ding, K. Jia, X. Lu, P. Chen, T. Zhou, H. Cheng, S. Liu, C. Wu, Y. Xie, Solution-liquid-solid synthesis of hexagonal nickel selenide nanowire arrays with a nonmetal catalyst, Angew. Chem. Int. Ed. 55 (2016) 1710–1713. [59] Z. Jin, M. Zhang, M. Wang, C. Feng, Z.-S. Wang, Metal selenides as efficient counter electrodes for dye-sensitized solar cells, Acc. Chem. Res. 50 (2017) 895–904. [60] F. Gong, H. Wang, X. Xu, G. Zhou, Z.-S. Wang, In situ growth of Co0. 85Se and Ni0. 85Se on conductive substrates as high-performance counter electrodes for dyesensitized solar cells, J. Am. Chem. Soc. 134 (2012) 10953–10958. [61] Y. Duan, Q. Tang, B. He, R. Li, L. Yu, Transparent nickel selenide alloy counter electrodes for bifacial dye-sensitized solar cells exceeding 10% efficiency, Nanoscale 6 (2014) 12601–12608. [62] S. Yadav, M. Hassan, P. Verma, S. Sapra, Nickel selenide nanoparticles as a cheap alternative for Pt-counter electrode in dye-sensitized solar cells, J. Nanosci. NanoTech. 19 (2019) 375–382. [63] M.D. Merrill, R.C. Dougherty, Metal oxide catalysts for the evolution of O2 from H2O, J. Phys. Chem. C 112 (2008) 3655–3666.

[4] M. Kim, C. Lee, S.M. Ko, J.-M. Nam, Metal alloy hybrid nanoparticles with enhanced catalytic activities in fuel cell applications, J. Solid State Chem. 270 (2019) 295–303. [5] Y.-J. Wang, J. Qiao, R. Baker, J. Zhang, Alkaline polymer electrolyte membranes for fuel cell applications, Chem. Soc. Rev. 42 (2013) 5768–5787. [6] Z. Wang, C. Li, K. Domen, Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting, Chem. Soc. Rev. (2019), https://doi.org/ 10.1039/C8CS00542G. [7] G. Zhang, Y. Li, Z. Hou, J. Xv, Q. Wang, Y. Zhang, Research on the synergistic doped effects and the catalysis properties of Cu2þ and Zn2þ co-doped CeO2 solid solutions, J. Solid State Chem. 264 (2018) 148–155. [8] M.N. Sokolov, P.A. Abramov, Chalcogenide clusters of Groups 8–10 noble metals, Coord. Chem. Rev. 256 (2012) 1972–1991. [9] O.A. Shromova, N.M. Kinnunen, T.A. Pakkanen, M. Suvanto, Promotion effect of water in catalytic fireplace soot oxidation over silver and platinum, RSC Adv. 7 (2017) 46051–46059. [10] M.-L. Cui, Y.-S. Chen, Q.-F. Xie, D.-P. Yang, M.-Y. Han, Synthesis, properties and applications of noble metal iridium nanomaterials, Coord. Chem. Rev. 387 (2019) 450–462. https://doi.org/10.1016/j.ccr.2018.12.008. [11] F.B. Hamad, T. Sun, S. Xiao, F. Verpoort, Olefin metathesis ruthenium catalysts bearing unsymmetrical heterocylic carbenes, Coord. Chem. Rev. 257 (2013) 2274–2292. [12] N. Li, Y. Chen, Q. Shen, B. Yang, M. Liu, L. Wei, W. Tian, J. Zhou, TS-1 supported highly dispersed sub-5 nm gold nanoparticles toward direct propylene epoxidation using H2 and O2, J. Solid State Chem. 261 (2018) 92–102. [13] J.-R. Xiao, S.-H. Yang, F. Feng, H.-G. Xue, S.-P. Guo, A review of the structural chemistry and physical properties of metal chalcogenide halides, Coord. Chem. Rev. 347 (2017) 23–47. [14] R.A. Hussain, A. Badshah, B. Lal, Fabrication, characterization and applications of iron selenide, J. Solid State Chem. 243 (2016) 179–189. [15] T. Lu, S. Dong, C. Zhang, L. Zhang, G. Cui, Fabrication of transition metal selenides and their applications in energy storage, Coord. Chem. Rev. 332 (2017) 75–99. [16] Um-e-Habiba, A. Badshah, R.A. Hussain, Synthesis of iron chalcogenides from single source precursors, Appl. Organomet. Chem. 30 (2016) 783–795. [17] U. Shamraiz, R.A. Hussain, A. Badshah, Fabrication and applications of copper sulfide (CuS) nanostructures, J. Solid State Chem. 238 (2016) 25–40. [18] Z. Anorg. Allg. Chem. 289 (1957) 203–206. [19] Z. Zhuang, Q. Peng, J. Zhuang, X. Wang, Y. Li, Controlled hydrothermal synthesis and structural characterization of a nickel selenide series, Chem. Eur J. 12 (2006) 211–217. [20] A.T. Swesi, J. Masud, M. Nath, Nickel selenide as a high-efficiency catalyst for oxygen evolution reaction, Energy Environ. Sci. 9 (2016) 1771–1782. [21] A. Sivanantham, S. Shanmugam, Nickel selenide supported on nickel foam as an efficient and durable non-precious electrocatalyst for the alkaline water electrolysis, Appl. Catal. B Environ. 203 (2017) 485–493. [22] Y.-H. Lee, Y.-H. Yun, V.H.V. Quy, S.-H. Kang, H. Kim, E. Vijayakumar, K.-S. Ahn, Preparation of nickel selenide by pulsed-voltage electrodeposition and its application as a highly-efficient electrocatalyst at counter electrodes of quantumdot sensitized solar cells, Electrochim. Acta 296 (2019) 364–371. [23] K. Xu, H. Ding, K. Jia, X. Lu, P. Chen, T. Zhou, H. Cheng, S. Liu, C. Wu, Y. Xie, Solution-liquid-solid synthesis of hexagonal nickel selenide nanowire arrays with a nonmetal catalyst, Angew. Chem. 128 (2016) 1742–1745. [24] C. Tang, Z. Pu, Q. Liu, A.M. Asiri, X. Sun, Y. Luo, Y. He, In situ growth of NiSe nanowire film on nickel foam as an electrode for high-performance supercapacitors, Chem Electro Chem 2 (2015) 1903–1907. [25] A. Sobhani, F. Davar, M. Salavati-Niasari, Synthesis and characterization of hexagonal nano-sized nickel selenide by simple hydrothermal method assisted by CTAB, Appl. Surf. Sci. 257 (2011) 7982–7987. [26] X. Liu, X. Liang, N. Zhang, G. Qiu, R. Yi, Selective synthesis and characterization of sea urchin-like metallic nickel nanocrystals, Mater. Sci. Eng. B 132 (2006) 272–277. [27] X. Liu, G. Qiu, J. Wang, Y. Li, Formation of hollow submicrometer spheres of nickel selenides, Chem. Lett. 33 (2004) 852–853. [28] S. Gohar, R. Pathak, Potentiostatic Electrochemical Preparation and Characterisation of aluminium containing nickel selenide, Orient. J. Chem. 29 (2014) 1469–1474. [29] J. Jia, J. Wu, Y. Tu, J. Huo, M. Zheng, J. Lin, Transparent nickel selenide used as counter electrode in high efficient dye-sensitized solar cells, J. Alloy. Comp. 640 (2015) 29–33. [30] J. Dong, J. Wu, J. Jia, J. Ge, Q. Bao, C. Wang, L. Fan, A transparent nickel selenide counter electrode for high efficient dye-sensitized solar cells, Appl. Surf. Sci. 401 (2017) 1–6. [31] N. Moloto, M.J. Moloto, N.J. Coville, S.S. Ray, Synthesis and characterization of nickel selenide nanoparticles: size and shape determining parameters, J. Cryst. Growth 324 (2011) 41–52. [32] A. Sobhani, M. Salavati-Niasari, Synthesis and characterization of a nickel selenide series via a hydrothermal process, Superlattice. Microst. 65 (2014) 79–90. [33] N. Moloto, M.J. Moloto, N.J. Coville, S. Sinha Ray, Optical and structural characterization of nickel selenide nanoparticles synthesized by simple methods, J. Cryst. Growth 311 (2009) 3924–3932. [34] S. Lee, S. Cha, Y. Myung, K. Park, I.H. Kwak, I.S. Kwon, J. Seo, S.A. Lim, E.H. Cha, J. Park, Orthorhombic NiSe2 nanocrystals on Si nanowires for efficient photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 10 (2018) 33198–33204. [35] C. Feng, Z. Jin, M. Zhang, Z.-S. Wang, Nanocomposites of nickel selenide supported on cube-shaped lidless graphitic boxes as efficient counter electrodes for quasisolid-state dye-sensitized solar cells, Electrochim. Acta 281 (2018) 237–245.

327

R.A. Hussain, I. Hussain

Journal of Solid State Chemistry 277 (2019) 316–328 [76] X. Li, G.-Q. Han, Y.-R. Liu, B. Dong, W.-H. Hu, X. Shang, Y.-M. Chai, C.-G. Liu, NiSe@ NiOOH core–shell hyacinth-like nanostructures on nickel foam synthesized by in situ electrochemical oxidation as an efficient electrocatalyst for the oxygen evolution reaction, ACS Appl. Mater. Interfaces 8 (2016) 20057–20066. [77] C. Cai, Y. Mi, S. Han, Q. Wang, W. Liu, X. Wu, Z. Zheng, X. Xia, L. Qiao, W. Zhou, Engineering ordered dendrite-like nickel selenide as electrocatalyst, Electrochim. Acta 295 (2019) 92–98. [78] L. Mi, H. Sun, Q. Ding, W. Chen, C. Liu, H. Hou, Z. Zheng, C. Shen, 3D hierarchically patterned tubular NiSe with nano-/microstructures for Li ion battery design, Dalton Trans. 41 (2012) 12595–12600. [79] M.-R. Gao, Z.-Y. Lin, T.-T. Zhuang, J. Jiang, Y.-F. Xu, Y.-R. Zheng, S.-H. Yu, Mixedsolution synthesis of sea urchin-like NiSe nanofiber assemblies as economical Ptfree catalysts for electrochemical H2 production, J. Mater. Chem. 22 (2012) 13662–13668. [80] H. Chen, M. Fan, C. Li, G. Tian, C. Lv, D. Chen, K. Shu, J. Jiang, One-pot synthesis of hollow NiSe–CoSe nanoparticles with improved performance for hybrid supercapacitors, J. Power Sources 329 (2016) 314–322. [81] Y. Tong, P. Chen, M. Zhang, T. Zhou, L. Zhang, W. Chu, C. Wu, Y. Xie, Oxygen vacancies confined in nickel molybdenum oxide porous nanosheets for promoted electrocatalytic urea oxidation, ACS Catal. 8 (2017) 1–7. [82] R.P. Forslund, J.T. Mefford, W.G. Hardin, C.T. Alexander, K.P. Johnston, K.J. Stevenson, Nanostructured LaNiO3 perovskite electrocatalyst for enhanced urea oxidation, ACS Catal. 6 (2016) 5044–5051. [83] C.C. Jara, S. Di Giulio, D. Fino, P. Spinelli, Combined direct and indirect electroxidation of urea containing water, J. Appl. Electrochem. 38 (2008) 915–922. [84] J. Wright, A. Michaels, A. Appleby, Electrooxidation of urea at the ruthenium titanium oxide electrode, AIChE J. 32 (1986) 1450–1458. [85] V. Amstutz, A. Katsaounis, A. Kapalka, C. Comninellis, K.M. Udert, Effects of carbonate on the electrolytic removal of ammonia and urea from urine with thermally prepared IrO2 electrodes, J. Appl. Electrochem. 42 (2012) 787–795. [86] H. Peng, J. Zhou, K. Sun, G. Ma, Z. Zhang, E. Feng, Z. Lei, High-Performance asymmetric supercapacitor designed with a novel NiSe@MoSe2 nanosheet array and nitrogen-doped carbon nanosheet, ACS Sustain. Chem. Eng. 5 (2017) 5951–5963. [87] H. Peng, C. Wei, K. Wang, T. Meng, G. Ma, Z. Lei, X. Gong, Ni0.85Se@ MoSe2 nanosheet arrays as the electrode for high-performance supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 17067–17075.

[64] J. Jiang, A. Zhang, L. Li, L. Ai, Nickel–cobalt layered double hydroxide nanosheets as high-performance electrocatalyst for oxygen evolution reaction, J. Power Sources 278 (2015) 445–451. [65] W. Xu, Z. Lu, X. Lei, Y. Li, X. Sun, A hierarchical Ni–Co–O@ Ni–Co–S nanoarray as an advanced oxygen evolution reaction electrode, Phys. Chem. Chem. Phys. 16 (2014) 20402–20405. [66] W. Zhou, X.-J. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, J. Wang, H. Zhang, Ni3S2 nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution, Energy Environ. Sci. 6 (2013) 2921–2924. [67] R.A. Hussain, A. Badshah, M.D. Khan, F. Ahmad, Morphological changes under the influence of surfactants of FeSe fabricated from single source precursor via AACVD with mechanism and photocatalytic activity, Res. Develop. Mater. Sci. 9 (2019) 1–7. [68] R.A. Hussain, A. Badshah, N. Haider, M.D. Khan, B. Lal, Effect of surfactants on the morphology of FeSe films fabricated from a single source precursor by aerosol assisted chemical vapour deposition, J. Chem. Sci. 127 (2015) 499–507. [69] R.A. Hussain, A. Badshah, M.D. Khan, N. Haider, S.I. Khan, A. Shah, Comparative temperature and surfactants effect on the morphologies of FeSe thin films fabricated by AACVD from a single source precursor with mechanism and photocatalytic activity, Mater. Chem. Phys. 159 (2015) 152–158. [70] R.A. Hussain, A. Badshah, S. Marwat, F. Yasmin, M.N. Tahir, Iron selenide nanoparticles coated on carbon nanotubes from single source ferrocene incorporated selenourea precursor for fuel cell and photocatalytic applications, J. Organomet. Chem. 769 (2014) 58–63. [71] R.A. Hussain, A. Badshah, F. Yasmin, M.D. Khan, M.N. Tahir, Aerosol-Assisted chemical vapour deposition for iron selenide thin films from single source ferrocene-incorporated selenourea precursor in the presence of surfactants, Aust. J. Chem. 68 (2015) 298–306. [72] R.A. Hussain, A. Badshah, A. Younis, M.D. Khan, J. Akhtar, Iron selenide films by aerosol assisted chemical vapor deposition from single source organometallic precursor in the presence of surfactants, Thin Solid Films 567 (2014) 58–63. [73] J. Suntivich, K.J. May, H.A. Gasteiger, J.B. Goodenough, Y. Shao-Horn, A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles, Science 334 (2011) 1383–1385. [74] S. Anantharaj, S.R. Ede, K. Sakthikumar, K. Karthick, S. Mishra, S. Kundu, Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: a review, ACS Catal. 6 (2016) 8069–8097. [75] C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, NiSe Nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting, Angew. Chem. Int. Ed. 54 (2015) 9351–9355.

328