Indian Journal of Engineering & Materials Sciences Vol. 7, October-December 2000, pp. 404-410

Room temperature preparation of NiFe204 thin films by electrochemical route

s 0 Sartale & C 0 Lokhande

Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India

Received 25 February 2000; accepted 27 September 2000

Depos ition of polycrystalline nickel iron oxide (NiFe204) spinel thin films on various conducting substrates has been carried out using electrochemical processes at room temperature. The electrodeposited NiFe2 alloy films, when electrochemica lly oxidized in IN KOH e lec trol yte at room temperature, result in NiFe20 4 spinel thin films. Electrochemical deposition of NiFe2 alloy films was carried out on various conducting substrates in sulfate electrolyte. Deposition potential of NiFe2 alloy was determined from the polariza tion curves. The film composition was determined using atomic absorption spectroscopy (AAS) technique. The structure of alloy and oxide films was studied using X-ray diffraction and optical microscopy techniques. Orientat ion along (400) NiFe20 4 plane was observed for both alloy and oxide films on various substrates.

The spinel metal iron oxides MFe20.j (M is divalent metal ion) have wide range of applications in various technical fields due to their interesting electrical and magnetic properties I. Among which, NiFe204 is a ferromagnetic material having potential applications in magnetic cores, opotomagnetic devices, bubble memory devices and vertical recording magnetic materials in thin film form2.

Several approaches to the synthesis of spinel MFe20 4 films have proven successful including pul sed laser deposition3

.4, sputtering5.6

, electron beam . . 7 . 8

reactive evaporation, vacuum arc evaporation , non aqueous sol method9

, spray pyrolysis 1o•ll , etc. All

these methods often require a high degree sophistication and control and substrate heating during and/or after deposition of the films to obtain the desired characteristics properties in thin film form.

Abe and Tamaura l2 have prepared CoFe20 4 thin films on copper, stainless steel and PET substrates. Surve and Puri 13 have prepared CoFe204 thin films on copper oxide coated alumina at room temperature by electrochemical route.

In this paper, we report galvanostatic electrodeposition of NiFe2 alloy films on a variety of conducting substrates from simple sulfate bath containing no additives or complexing agents. Effect of various substrates on deposition potentials of Ni, Fe and NiFe2 is also reported.

Experimental Procedure Film preparation -Aqueous solutions in double

distilled water of Analar NiS04 (0.05 M) and FeS04 (0.1 M) were used. Variety of conducting substrates,

viz., stainless steel, brass, copper, titan ium and soda lime glass coated with conductive (FTO) fluorine doped tin oxide (-10 Q;[] sheet res istance) were used as cathodes. Prior to each electrodeposi tion, the metal substrates were mirror polished by zero grade polish paper, cleaned with detergent and finally cleaned ultrasonically, while the conducting glass substrates were cleaned by 50% dilute HCL and by double distilled water. The economic and inert polished graphite plate was used as an anode. The distance between anode and cathode was 0.5 cm. The bath composition was optimized by the rat io of Ni and Fe in the deposit determined using AAS technique. The polarization curves were recorded to determine deposition potentials for the reducti on of Ni, Fe and NiFe2 alloy from bath composition of 0.05 M NiS04

and 0.1 M FeS04 in the proportion of 20:0,0:20 and 13:7, respectively. Electrodeposi tion was carried out galvanostatically using a constant current source (0-200 mA) without stilTing.

The deposition conditions were optimized to get good quality NiFe2 alloy films with maximum thickness. After every deposition, the films were washed with doubly distilled water and preserved in dissector.

The alloy films deposited at optimized deposition conditions were electrochemically oxidized at room temperature (27°q. The NiFe2 alloy fi lm acts as an anode, while the graphite acts as a cathode. IN KOH aqueous solution was used as an electrolyte to intercalate oxygen species into NiFe2 alloy films to form NiFe20 4 thin films at room temperature.

SARTALE & LOKHANDE: NICKEL IRON OXIDE TH IN FILMS 405

The current density for electrochemical oxidation was optimized by keeping oxidation time (10 min) constant until the film remained adherent to the substrates. Using same current density for different time periods, the optimized current density and oxidation time are 5 mA/cmz and 30 min, respectively.

Characterization - The Ni:Fe composition in the alloy film deposited from the baths (20 ml) of various ratios of 0.05 M NiS04 and 0.1 M FeS04 was determined using AAS with a Perkin Elmer model 3030B spectrometer. The uncertainty in the measurement was ±3 % for analytical analysis of the solution concentration range 1-5 ppm.

The film thickness of NiFez alloy was determined by gravimetric weight difference method. For this, a sensitive microbalance was used and film density was assumed to be as bulk density of NiFez.

The X-ray diffraction patterns of alloy and oxide films were recorded with Philips PW 3710 X-ray diffractometer with a copper anticathode without monochromator CUkal (/.,,= 1.54056A) and CUkaz (A=1.54439 A) radations operated at 25 kY and 20 rnA. In order to find grain size of the film, a slow scan X-ray diffraction pattern with step width of the diffractometer 0.020 28/min. was taken and the grain size was calculated using Scherre's formula.

The microstructural studies of alloy and oxide film deposited on to FrO coated glass substrates were carried out with Orthoplan optical microscope (Leitz, Germany, 160X).

Results and Discussion

Effect of bath composition percentage of iron in deposit Prior to each deposition, the mirror polished and

ultrasonically cleaned stainless steel substrates are given electroforming pretreatment l4 with potassium femocyanide (10 gIL) and potassium hydroxide (10 giL) for 1 min. It enables the alloy films to be easily detached from the substrates for AAS analysis.

Ni-Fe alloy was galvanostatically electrodeposited at room temperature (27°C) on stainless steel substrates. The amount of Fe deposited from the 20 mL bath solution at 8 mNcm2 current density for 10 min. of deposition time is shown in Fig. 1. From the figure, the reduction of reaction rate of more noble metal (Ni) and an increase of the reaction rate of less noble metal (Fe) is observed, indicating the anomalous behavior of Ni-Fe codeposition for different volume ratios of the Fe and Ni solutions.

Cathodic polarization Effect of substrates - The cathodic deposition

potential depends upon bath temperature, nature of substrates, metal ion concentration, complexing agent and its concentration, etc. 15 Fig. 2 shows polarization curves for reduction of NiFe2 alloy onto various substrates at room temperature using 13 mL and 7 mL of the Ni and Fe solutions. Table 1 shows the estimated deposition potentials of Ni, Fe and NiFez alloy from their respective baths at room temperature. In all cases, hydrogen evolution was observed above the optimum potentials during deposition resulting into nonhomogeneous film formation. It is observed that the deposition potentials of Ni, Fe and NiFez alloy are different for different substrates Also, the deposition potential of NiFe2 alloy is in between the deposition potentials of Ni and Fe. Different deposition potentials observed for different substrates may be attributed to the substrates on to which deposition was carried out. The substrates are polycrystalline (as evidenced from XRD study) in nature and orientation of planes varies from substrates to substrate. Al so, the order of matching and different affinity between the depositing material s and substrates gives different values of deposition potential 16.17.

Effect of bath temperature - The effect of bath temperature on deposition potential of NiFez alloy was studied for all substrates. Fig. 3 shows polarization curves for stainless steel substrate at different bath temperatures using 13 mL and 7 mL of

100

90

80

~ 70

t .~ 60 0 c>.

" 'C

.5 50

" "" .... 40 0

E " 0

30 ~ 20

10

0

0

Fe

5 . 15

10 : 10 15 : 5

Composition (0.05 M NiSO. 0.1 M FcSO,)

20 : 0

Ni

Fig. I - Amount of Fe in the Ni-Fe alloy deposited on stainless steel substrate

406 INDIAN J. ENG. MATER. SCI. , OCTOBER-DECEMBER 2000

Deposition potential vs. SeE (volt)

o -OJ -0.6 -0.9 -1.2 -1.5

2

""' "'s u 4

~ '-" ;>., .....

6 -"'-ss .;; t: Q) -cu ~

1:: -.-bs Q)

t:: 8 -o-ti ::::l

U -...-ft

10

16

Fig. 2 - Polarization curves for reduction of NiFe2 alloy on ss~ stainless steel; bs~ brass; cu~copper; ti~titanium ; and ft-FTO coated glass substrates at room temperature

Table I-Estimated deposition potentials for reduction of Ni , Fe and NiFe2 alloy from their respective baths for different substrates and temperature.

Bath composition

0.05 M NiS04

0. 1 M FeS04

( 13 mL) 0.05 M NiS04 + (7 mL) O. I M FeS04

(13 mL) 0.05 M NiS04 + (7 mL) 0.1 M FeS04

(13 mL) 0.05 M NiS04 + (7 mL) 0. 1 M FeS04

13 mL) 0.05 M NiS04 + (7 mL) 0.1 M FeS04

Bath

temperature (OC)

27

27

27

50

70

90

the Ni and Fe solutions. The estimated deposition potentials are listed in Table 1. It is observed that as bath temperature increases the deposition potential decreases. An increase in bath temperature increases the concentration of metal in the cathodic diffusion

Estimated deposition potenti al vs SCE (volt)

Stainless Copper Brass Titanium FTO coated steel glass

-0.89 -0.74 -0.76 -0.84 - 0.86

-0.94 -0.96 -1 .00 -1 .08 - 1.06

-0.93 -0.87 -0.89 -0.90 -1.04

-0.84 -0.75 -0.77 -0.86 -0.94

-0.78 -0.68 -0.70 -0.78 - 0.88

-0.62 -0.53 -0.59 -0.65 -0.79

because the rates of diffusion and of convection increase with temperature affecting the composition of deposited alloy. The decrease in deposition potential might be due to increase in the content of more noble metal (Ni) in the deposited alloy'8.

SART ALE & LOKHANDE: NICKEL IRON OXIDE THIN FILMS 407

Deposition potential vs. SeE (volt)

0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 -1.4 -1.6

0

5

10 ,-...

N

E 15 c.>

~ 20 -- -+-a >-. ......

'r;) ---b ~ 25 Q)

"'0 -'-c 1:: Q) 30 ~

-+-d ~ ;:::s u

35

40

45

50

Fig. 3 - Polarization curves for reduction of NiFe2 alloy at different bath temperature: a~30°C; b~50°C; c~70°C; and d~90°C

At room temperature, deposited films are thin adhesive, homogeneous and coherent. However, rise in bath temperature results in less adherent and non homogeneous film formation due to instability of the bath at high temperature.

NiFe2 alloy film thickness

Film thickness of NiFe2 alloy was measured by grave metric weight difference method using the relation:

m t=-

Ap ... (1)

where A is surface area of the film, m is the mass of the film and P is the density of the film material expressed as:

... (2)

where PI and P2 and XI and X2 are densities and atomic fractions of Ni (33 %) and Fe (67 %) elements in NiFe2 alloy. Before and after NiFe204 deposition on stainless steel substrate, the samples were precisely weighed using sensitive microbalance. The difference of masses was used to determine the effective deposited thickness using Eq.(l).

NiFe2 alloy films were deposited on the stainless steel substrates for 10 mjn at room temperature with different current densities. Fig. 4 shows the variation of film thickness with current density using 13 mL and 7 mL of the Ni and Fe solutions. It is observed that initially as current density increases, NiFe2 alloy thickness increases, attains maximum thickness (1.3 fJ.m) for 8 mNcm2 current density. For, further increase in current density, the film peels off from the substrate due to porous, foggy and less adherent film formation.

The variation of NiFe2 alloy film thickness with deposition time, deposited at 8 mNcm2 current density at room temperature was studied using 13 mL

408 INDIAN J. ENG. MATER. SCI., OCTOBER-DECEMBER 2000

1.4 ,------------------,

1.2

0.4

0.2

o ~--~--~---~---L __ -~ o 6 10

Current densi ty (mNcm')

Fig. 4 - Variation of NiFe2 alloy film thickness with different currcnt densities. (dotted line indicates peeling off the film from substrate)

and 7 mL of the Ni and Fe solutions, as shown in Fig. 5. The maximum film thickness (-311m) was attained for 20 min. deposition period. Further increase in deposition time, film thickness was decreased for 30 min deposition and peeled off from the substrate for further deposition. This is attributed to the increase in rate of dissolution than the rate of deposition after attending the maximum thickness l9

•

Further studies were carried out at optimized deposition conditions (8 mA/cm2 current density, 20 min deposition time and room temperature) on various substrates. Alloy films on all substrates were smooth, homogeneous and well adherent to the substrates. However, on removal from the bath, they quickly oxidized and became blackish. Similar observation has been reported for CuFe alloy deposited from cyanide bath 18.

Electrochemical ox idation of electrodeposited NiFe2 alloy film occurs according to the reaction (3) :

... (3)

Current efficiency In the present case, it is assumed that both metals

are reduced in two consecutive steps as described by others20,2 1:

Ni (II) + e- ~ Ni (I)ads ... (4)

Ni (I)ads + e-~ Ni (S) ... (5)

3.5 r-----------------,

2.5

0.5 \

\ 0

0 10 15 20 25 30 35

Deposition time (min.)

Fig. 5 - Variation of NiFc2 alloy film thickness with deposition time with 8 mAlcm2 current densi ty

Fe (II) + e-~ Fe (I)ads .. . (6)

Fe (I)ads + e-~ Fe (S) .. . (7)

In these equations, Ni(II) and Fe(II) are dissolved metal ions, Ni(l)ads and Fe(l)ads are monovalent adsorbed reaction intermediates that mayor may not contain a hydroxyl group and Ni(S) and Fe(S) are the deposited metals.

It is assumed that pH dependent hydrolysis equilibrium does not directly influence the reduction rate of metallic species. However, the kinet ic of deposition and dissolution of Fe are pH dependent21 .22

, a simple approach was preferred here. Reduction of protons and dissociation of water

molecule gives hydrogen evolution may occur as a side reaction21 .

... (8)

... (9)

... (10)

Let iNi , iFe and iside be the partial CUITents for Ni, Fe and side reaction, respectively. The total current is equal to sum of partial currents and the current efficiency is given by 21:

iNi + iFe TJ = -'-"-. --.:~ ... (1\)

Ltotal

SARTALE & LOKHANDE: NICKEL IRON OXIDE THIN FILMS 409

It was observed that efficiency remains almost constant (56%) for all current densities.

X-ray diffraction X-ray diffractograms of NiFe2 alloy and

electrochemically oxidized NiFe2 alloy films deposited on stainless steel, copper, titanium and fluorine doped tin oxide (FTO) coated glass substrates were studied. Table 2 shows the comparison of observed 'd' values of alloy and oxide films with standard23 'd' values of NiFe20 4 phase with cubic spinel structure onto different substrates. It is observed that the alloy and oxide films deposited on all substrates are polycrystalline. For all substrates, orientations along (400) NiFe20 4 plane for alloy and oxide films are observed. After oxidation, the peak intensity of NiFe204 increases suggesting the improvement in crystallinity after oxidation of the alloy films. Fig. 6 shows a typical X-ray

2500

400 0

- (400) ::i ~ 1'500

b

~ ' ';; c .. 1000 C

a ....

500

o tz:::=======::::===~:::::!::-:::::/ 10 20 1.,0 60 80 100

29 (de<;!)

Fig. 6 - X-ray diffraction patterns of (a) alloy and (b) oxide thin films on copper substrate

diffractogram of a) alloy and b) oxide films on copper substrates.

In order to study the average grain size of alloy and oxide particles, thin films of alloy and oxide films were studied by taking a slow scan X-ray diffraction pattern around the (400) peak with a step width of the diffractometer of 0.002 28 min- I. The grain size was calculated by using the full width at half maximum of peak and by using Scherre's relation:

d = A Dcose

... (12)

where D is full width of half maximum of the peak and A = 1.5406 A is the wavelength of CUka

radiations. Table 2 shows the grain size of alloy and oxide films on various substrates. After oxidation the grain size increases indicating improvement in the crystallinity.

Optical microscopy The alloy and oxide films deposited on FTO coated

glass substrates were used to study the surface morphology. Fig. 7 shows optical micrographs at 160X magnification for: (a) alloy, and (b) oxide films . It is observed that the alloy films are homogeneous having uniform surface without cracks and holes. Oxidation results into formation of porous and relatively rough surface.

Conclusions Spinel cubic NiFe204 films have been formed using

electrochemical processes at room temperature on various conducting substrates from their simple sulfate baths with 56% current efficiency. The Alloy films of thickness as about 3 ~m could be obtained.

Table 2 - X-ray diffraction studies for alloy and oxide films on different substrates

Substrate Standard 'd' Observed 'd' values (A) Planes Grain size (A)

values (A) Alloy Oxide (hkl) Alloy Oxide (NiFe204)

Stainless steel 2.08 2.07 2.08 400 273 290 1.09 1.08 1.09 553/731

Copper 2.08 2.09 2.10 400 287 357 1.09 1.09 1.09 5531731

Titanium 2.08 2.06 2.07 400 1.48 1.47 1.48 440 270 295 1.32 1.33 1.33 620

FTO coated glass 2.08 2.07 400 250 285 1.28 1.29 1.27 533

410 INDIAN J. ENG. MATER. SCI. , OCTOBER-DECEMBER 2000

Fig. 7 - Optical micrographs of: (a) alloy and (b) oxide thin films on FTO coated glass substrate (magnification 160X)

These films are bright, smooth and well adherent to the substrates. However, on removal from the bath, the films get quickly oxidized and become blackish. The NiFe2 alloy films are electrochemically oxidized in IN KOH electrolyte at room temperature. XRD studies show that both alloy and oxide films consist of NiFe204 phase, and the crystallinity and grain size improve after oxidation.

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