Delafossite CuFeO2 thin films electrochemically grown from a DMSO based solution

Electrochimica Acta 164 (2015) 297–306

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Delafossite CuFeO2 thin films electrochemically grown from a DMSO based solution G. Riveros a, * , C. Garín b , D. Ramírez a , E.A. Dalchiele c , R.E. Marotti c, C.J. Pereyra c , E. Spera c , H. Gómez d, P. Grez d, F. Martín e , J.R. Ramos-Barrado e a

Instituto de Química y Bioquímica, Facultad de Ciencias, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile Departamento de Física, Universidad Técnica Federico Santa María, Avda. España 1680, Valparaíso, Chile Instituto de Física, Facultad de Ingeniería, Universidad de la República, Herrera y Reissig 565, C.C. 30, Montevideo 11000, Uruguay d Instituto de Química, Pontificia Universidad Católica de Valparaíso, Avda. Universidad 330, Curauma, Valparaíso, Chile e Laboratorio de Materiales y Superficies (Unidad Asociada al CSIC). Departamentos de Física Aplicada & Ing. Química, Universidad de Málaga, E29071 Málaga, Spain b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 December 2014 Accepted 27 February 2015 Available online 28 February 2015

This study shows the results obtained in the direct electrodeposition of CuFeO2 thin films from a DMSO based solution. First, a detailed electrochemical study was carried out in order to determinate the best condition for the CuFeO2 electrodeposition. The films were obtained potentiostatically from a 0.01 M CuCl2 + 0.005 M Fe(ClO4)3 + 0.1 M LiClO4 solution in the presence of molecular oxygen at 50
C onto FTO/ glass substrates. In all cases, the time of electrodeposition was 1000 s. The grown films presented a yellow-reddish color and exhibit an homogeneous aspect. Analyses of composition carried out through EDS, shown that a stoichiometric composition (atomic relation Cu:Fe = 1:1) is obtained at a potential of –0.6 V. However, as-grown films analyzed through XRD experiences did not evidence the presence of CuFeO2 compound presumably because it is amorphous. An annealing treatment at 650
C for 30 minutes in an argon atmosphere was necessary to transform the solid phase of the as grown films in crystalline CuFeO2. Furthermore, the presence of CuFeO2 has been confirmed through XPS analyses. UV-vis analyzes shown a ladder-like appearance due to the presence of several absorption edges from the IR to the UV spectrum region. The most clearly determined were a direct-like edge at the IR: Eg(IR) = 1.64 eV. However, several absorption edges in the visible (Egdir(vis) = 2.35 eV, and Egind(vis) = 2.03 eV) and UV beginning (Egind(UV) = 3.37 eV) spectrum region also were observed. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Delafossite CuFeO2 electrodeposition

1. Introduction Metal oxide semiconductors are very attractive for optoelectronic devices as they are chemically stable, and many oxides are non-toxic, abundant, and fulfill the requirements for low-cost manufacturing at ambient conditions [1]. Therefore, devices made of metal oxide semiconductors can be very inexpensive, stable, and environmentally safe [1]. In the last years, metal oxide semiconductors have been widely used in several technological domains such as transparent conducting oxides (TCOs) [2,3], solar cells [1,4], electrochromic devices [5], gas [6,7] and chemical sensors [8], solar water-splitting [9–13], light emitting devices [14], field emission [15,16], electrochemical energy storage [17,18] and piezoelectronics [19]. Within the most widely used and studied semiconducting metal oxides are ZnO, TiO2, In2O3, SnO2

* Corresponding author. Tel.: +56 32 2508175; fax: +56 32 2508062. E-mail address: [email protected] (G. Riveros). http://dx.doi.org/10.1016/j.electacta.2015.02.226 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

and a-Fe2O3 [2], all of them exhibiting an n-type conductivity. On the other hand, the Cu-based delafossite ternary oxides CuMO2 (where M is Al, Y or a tri-valent 3d cation) [20], had received relatively little attention [21]. However, since the discovery of p-type conductivity on CuAlO2 thin films, the delafossite-based materials have attracted considerable scientific and technological attention due to their novel optoelectronic properties [22]. In fact, one of the main proposed applications of these films is in transparent p-n junction semiconductor devices; thus, it can be used as a transparent functional window in microelectronic devices [23,24]. Moreover, in particular, cuprous delafossites have attracted significant interest due to their exceptional (thermo) electric, magnetic and optical properties, finding diverse technological applications, namely, in the fields of optoelectronic devices, field electron emitters, light emitting diodes, laser diodes, solar cells, functional windows and thermoelectric materials [25]. Among these Cu-based delafossites, CuFeO2 as the archetypal delafossite is a well-known p-type semiconductor with the largest conductivity at room temperature among the delafossites,

298

G. Riveros et al. / Electrochimica Acta 164 (2015) 297–306

excluding CuCrO2 [20,25–27]. Several and different applications of CuFeO2 thin films have been reported in the literature such as anode material for lithium-ion batteries [28], active layer in Al/pCuFeO2/p-Si/Al metal/transparent conducting oxide-semiconductor (MTCOS) Schottky photodiodes [29], in n-CdS/p-CuFeO2 heterojunction diodes [30], a promising photocatalyst for the reduction of heavy metals [21] and a photocathode in photoelectrochemical cells for water splitting and hydrogen evolution [31,32]. Delafossite CuFeO2 thin films have been prepared using different techniques such as pulsed laser deposition [33], radiofrequency (RF) sputtering [34,35], sol–gel methods [20,23,27,29] and electrochemical deposition [31]. The advantages of electrodeposition compared with other techniques include low process temperature, low cost and capability of controlling morphology and orientation of the deposited films [14,36]. Read et al. [31] have reported the electrodeposition of CuFeO2 thin films using a DMSO based solution, that to our knowledge is the only work reporting an electrochemical route for CuFeO2 preparation. The use of a non-aqueous media exhibits several advantages and attributes, i.e.: a wider electrochemical potential window than that accessible in aqueous media, deposition of metal hydroxide does not normally occur in a non-aqueous bath, therefore high deposition temperature can be used, usually leading to a better crystallinity of the deposit compared to the material obtained by electrodeposition from aqueous solutions [37]. A difference of that reported by Read et al. [31], here an exhaustive electrochemical study in order to obtain the better conditions for obtaining a stoichiometric CuFeO2 phase are presented, and exhaustive structural (XRD, XPS) and optical characterization (UV–vis) of the obtained CuFeO2 thin films, is also presented. For the electrochemical growth of the CuFeO2 thin films a DMSO based solution containing the copper, iron and oxygen precursors, i.e.: copper ion, Cu(II); iron ion, Fe(III) and molecular oxygen, O2, respectively, has been used. An investigation of the influence of different anion counterions (Cl–, ClO4–and Cl– + ClO4–) in the electrochemical behaviour and then in the final film stoichiometry is reported. 2. Experimental Section Salts and solvents were employed as they were received and without previous treatment. In every case shown in this study, dimehyl sulfoxide (DMSO, Merck) was used as a solvent. All the electrochemical experiences were carried out in a conventional three electrode glass cell: working electrode, counter electrode (platinum wire) and reference electrode (Ag/AgCl(sat), E = 0.195 V vs NHE). All the potentials in this study are referred to this reference electrode. Two substrates were used as working electrode: glassy carbon disc (Ø 3 mm, CH Instrument) and FTO conducting glass (TEC 15, XOP Glass, 12–14 V/sq). Because of its reproducibility, glassy carbon was employed in the electrochemical study of the processes involved in the electrodeposition of CuFeO2. Prior to each measure, this electrode was polished with an alumina suspension (0.05 mm) in order to renew its surface. On the other hand, FTO substrates (2.5 cm  1 cm) were employed in the electrodeposition of the films. Before using these substrates as electrodes, these were successively cleaned with acetone and ethanol (each one for 10 minutes in an ultrasonic cleaner). Every electrochemical experience was carried out in a potentiostat–galvanostat CH Instruments model 660D. In order to keep a constant temperature within the system (50
C), the cell was placed in a glycerin bath where the temperature was controlled by means of a digital hot plate. For the electrochemical study, different solutions of CuCl2 (Merck), FeCl3 (Aldrich), Cu(ClO4)2 (Merck) and Fe(ClO4)3 (Aldrich) were employed, using LiClO4 (Aldrich) or LiCl (Merck) as a support electrolyte. Molecular oxygen (O2) was employed as an oxygen

precursor. This way, before the electrodeposition, the molecular oxygen was bubbled through the solution for 20 minutes. Following this and during the electrodeposition an oxygen stream was kept over the solution in order to assure its saturation. On the other hand, in those experiences where oxygen absence was required, the solutions were bubbled with Ar during 20 minutes. Subsequently, the solutions were place in an Ar atmosphere so as to avoid the presence of air (molecular oxygen) in the systems. The films were potentiostatically obtained during 1000 s at different potentials. The as–grown films obtained were thick and amorphous. Thus, in order to improve the crystalline character, the films were annealed at 650
C for 30 minutes under an Ar atmosphere. Afterwards, the films were analyzed through different techniques. The morphology of the films was observed through SEM images which were obtained by a Carl Zeiss EVO MA 10 scanning electron microscope model, at 25 KV. This electron microscope is equipped with an EDS detector provided by Oxford Instruments, model X-Act which is used in the composition analyses of the films. The crystalline character was established by X-ray diffraction (XRD) which was performed in standard theta –2 theta scans on a model D8 Advanced Bruker equipment, employing CuKa radiation (l = 1.5406 Å) and a glancing angle incidence of 0.5
. The CuFeO2 diffraction peaks were indexed in reference to the JCPDS powder diffraction file (JCPDS File No. JCPDS 75-2146, Rhombohedral Structure of delafossite CuFeO2). X-Ray photoelectron spectra were recorded on a Physical Electronics PHI 5700 spectrometer, using non-monochromated Mg Ka (1253.6 eV,15 kV, 300 W) and 4 KeV Ar ion etching was carried out with a PHI 5700 equipment recording survey spectra and high resolution multiregional spectra. The pressure in the analysis chamber was about 107 Pa. Binding energies were corrected against that for C 1s at 284.8 eV before etching with Ar ions, and for C 1s 284.8 eV and O 1s at 530 eV as recommend by Yamashita and Hayes [38] after Ar etching. Samples were never Ar etched for more than 0.5 min. Spectral data were processed using PHIESCA V8.0c and Multipack software, both from Physical Electronics. XPS atomic concentrations were calculated from Fe 2p, O 1s, C 1s, and Cu 2p photoelectron peak areas using Shirley background subtraction and sensitivity factors supplied by the spectrometer manufacturer (PHI). The optical properties of the samples were studied by optical spectroscopy at different spectral regions. In the visible region an S2000 Ocean Optics (OO) spectrophotometer was used coupling the light through a 50 mm optical fiber. The illumination was an HL2000 OO tungsten halogen light source through a 600 mm OO optical fiber. Either, a substrate or the bare lamp spectrum was used as a reference to avoid any influence of interference fringes originated in the transparent conductive oxide film of the substrate [39]. These measurements were extended into the near infrared (NIR) and ultraviolet (UV) region with the use of an ORIEL 77250 monochromator, an SRS SR540 chopper, and an SRS SR530 lock-in amplifier. Another lock-in (EG&G 5209) was used for light intensity fluctuation correction. For UV measurements a 1000 W electric power Xe lamp (ORIEL 6271), was used with an UDT 11-09-001-1 silicon detector and an ORIEL 77298 diffraction grating in the monochromator. For NIR measurements a 20 W tungsten filament lamp, a Newport 71616 InGaAs thermoelectrically cooled detector and an ORIEL 77299 diffraction grating were used instead. All measurements were done at room temperature. 3. Results and Discussion 3.1. Electrochemical study of the Cu + Fe + O phases formation In order to obtain the electrochemical growth of the CuFeO2 thin films, a DMSO based solution containing copper, iron and

G. Riveros et al. / Electrochimica Acta 164 (2015) 297–306

oxygen precursors (i.e.: copper ion, Cu(II); iron ion, Fe(III) and molecular oxygen, O2, respectively) was used. Lithium ion based solutions were employed as a supporting electrolyte. In order to find the best condition for the electrodeposition of stoichiometric CuFeO2 films, different solutions were assayed as a function of the employed anion for both the precursor and supporting electrolyte. These solutions are named, according to the anion employed: perchlorate solution, chloride solution and perchlorate-chloride solution. Each of these was analyzed through cyclic voltammetry with and without molecular oxygen in order to establish the processes involved in the electrodeposition of the films. Fig. 1(a) shows the voltammetric response of a glassy carbon electrode in an iron (III) perchlorate and copper (II) perchlorate solution without molecular oxygen, using LiClO4 as a supporting electrolyte. Previous studies [39,40] have shown that the processes assigned as C1, C2 and C3 are clearly defined as the reduction of Fe(III) to Fe(II), the reduction of Cu(II) to Cu(I) and the reduction of Cu(II) to metallic copper, respectively. C1 and C2 processes are linked to the A1 and A2 anodic processes, respectively, as shown in the inset of Fig. 1(a). There, the cathodic limit in the cyclic voltammogram is fixed at a potential value previous to the electrodeposition of metallic copper on the electrode. Thus, the reoxidation of the electroactive species Fe(II) and Cu(I) in the electrochemical interphase is observed, with a quasi-reversible

3 j (mA cm )

-2

j (mA cm )

2

0.0

1

A1

A2

A3

a)

C1 -0.4

electrochemical behavior. The above proves that there is no interaction between the electrogenerated ionic species in the interphase (Fe(II) and Cu(I)), but both ions are stable in a DMSO solution. Then again, the A3 process is the re-oxidation of the metallic copper previously deposited in the C3 process. The reactions involved in the previously described processes are the following: 3þ 0 C1=A1 : Fe3þ ðsolÞ þ 1e$FeðsolÞE ¼ 0:330V

(1)

þ 0 C2=A2 : Cu2þ ðsolÞ þ 1e$CuðsolÞ E ¼ 0:100V

(2)

0 C3=A3 : Cu2þ ðsolÞ þ 2e$CuðsÞE ¼ 0:225V

(3)




where E ' is the formal potential of each electrochemical process in DMSO solution. In the presence of molecular oxygen, drastic changes in the voltammogram can be observed (Fig. 1(b)). No changes are observed in both C1 and C2 processes which correspond to the electrogeneration of Fe(II) and Cu(I), respectively. However, following C2, new reduction processes can be observed. These cathodic processes are a consequence of the molecular oxygen reduction, according to our previous studies [39–42] and confirmed by other authors [43]. The first process produces a stable oxide phase on which the molecular oxygen is again reduced. The global processes for each ionic species in the interphase are the following: 2Cuþ ðsolÞ þ

1 O þ 2e ! Cu2 OðsÞ 2 2ðsolÞ

(4)

2Fe2þ ðsolÞ þ

3 O þ 4e ! Fe2 O3ðsÞ 2 2ðsolÞ

(5)

C2

-0.8 0.0

0.4

E (V vs Ag/AgCl

0.8

)

Reaction (4) and (5) represent simplified processes that involve more complex mechanisms which include different electrochemical and chemical steps (see references above). Thus, the simultaneous co-deposition of Cu2O and Fe2O3 will allow the formation of CuFeO2 according to the following equation:

0

C1 C2

-1

C3 -2 -0.8

299

-0.4

0.0

0.4

0.8

Cu2 OðsÞ þ Fe2 O3ðsÞ ! 2CuFeO2ðsÞ

(6)

E (V vs Ag/AgCl(sat)) 0.5

b) -2

j (mA cm )

0.0

C1

-0.5

C2 -1.0 -1.5

Oxygen reduction -2.0 -0.8

-0.4

0.0

0.4

0.8

E (V vs Ag/AgCl(sat)) Fig. 1. (a) Cyclic voltammetry of a 0.01 M Cu(ClO4)2 + 0.005 M Fe(ClO4)3 + 0.1 M LiClO4 DMSO solution on a glassy carbon electrode in the absence of molecular oxygen at 50
C. Inset shows the quasi-reversible processes assigned to the couples Fe(III)/Fe(II) (C1/A1) and Cu(II)/Cu(I) (C2/A2). (b) The same system as above but in the presence of molecular oxygen in solution. In all cases the scan rate was 10 mV s1.

The films potentiostatically obtained in this solution, between –0.2 and –0.6 V, were analyzed by EDS, showing that in all the potentials studied, the films were mainly comprise by copper (vide infra,Table 2, perchlorate solution), presumably Cu2O due to the yellow-brownish colour of the films. This condition was maintained even when the iron (III) concentration in the solution was increased (results not shown). The above can be explain because the Cu2O electrodeposition is kinetically favored than the corresponding iron oxide, considering that FeO is firstly deposited and subsequently transformed in Fe2O3 [40]. Thus, the preferential deposition of Cu2O at the expense of FeO is observed. Films obtained at a more cathodic potential than –0.6 V, were black, due presumably to the CuO deposition. One way in order to assist the iron deposition in the films is to stabilize the Cu(I) ion in the electrolytic bath through a complexing agent, i.e. chloride anion. Both ions, Cu(II) and Cu(I) form stable Table 1 Stability constant of Cu(II) and Cu(I) ions with Chloride in DMSO solution, according to reference [45]. Cathion in solution

log b11

log b21

log b31

Cu(II) Cu(I)

4.50 6.00

7.50 11.95

9.10 –

300

G. Riveros et al. / Electrochimica Acta 164 (2015) 297–306

Table 2 Composition analyses of thin films obtained potentiostatically at different potentials from different solutions, at 50
C. Solution

Ratio Cu:Fe (atomic percentage) for different solution at different potential of electrodeposition (in Volts)

Perchlorate Chloride Perchlorate + Chloride

–0.2 Cu:Fe

–0.3 Cu:Fe

–0.4 Cu:Fe

–0.5 Cu:Fe

–0.6 Cu:Fe

–0.7 Cu:Fe

–0.8 Cu:Fe

–0.9 Cu:Fe

98:2 – –

– – –

96:4 – 29:71

– 0:100 33:67

94:6 1:99 50:50

– 0:100 59:41

– 6:94 –

– 25:75 –

complexes with chloride ion in a DMSO solution whose stability constants have been previously reported [44,45] and are summarized in Table 1. According to these values, the C2 process (reduction of Cu(II) to Cu(I)) must be displaced to more positive potentials than when in the absence of chloride ion. On the contrary, the C3 process (reduction of Cu(II) to elemental Cu) must be displaced to a more cathodic potential. However, both Fe(III) and Fe(II) do not form complexes with chloride ions in DMSO, thus the process associated to the iron reduction is not influence by the presence of this anion. The above is confirmed in Fig. 2a), in which the voltammetric response of a glassy carbon electrode in a CuCl2 + FeCl2 + LiCl solution without molecular oxygen can be observed.

ð2nÞ

-2

j (mA cm )

j (mA cm )

A1

-0.5 -1.0

a)

C2'

A2'

C1 -0.7

0

A3'

A2'

0.0

0.0

0.7

E (V vs Ag/AgCl

)

A1 C2'

-1

C1

-2

C3' -1.5

-1.0

-0.5

0.0

0.5

1.0

E (V vs Ag/AgCl(sat)) 2

b) 1

A''

0

-2

j (mA cm )

ð2nÞ

C2'

-1

C1 -2 -3

Oxygen reduction -1.5

-1.0

-0.5

0.0

0.5





C20 =A20 : CuClnðsolÞ þ 1$CuCl2ðsolÞ þ ðn  2ÞClðsolÞ E0 ¼ 0:370V



C30 =A30 : CuClðsolÞ þ 2$CuðsÞ þ nClðsolÞE 0 ¼ 1:480V

2

1

Here, the C20 and C30 processes are associated to the Cu(II) reduction to Cu(I) and elemental Cu respectively, both in the presence of chloride ion in solution. The C1 process (reduction of Fe (III) to Fe(II)) does not change and appears in the same potential as when chloride ion is absent. On the other hand, the whole process named A30 is linked to the re-oxidation of elemental copper deposited in C30 . According to the high free chloride concentration in solution and to the stability constant values in Table 1, all the Cu (I) found in the electrochemical interphase is CuCl2– ion. Furthermore, the C1 and C20 processes display a quasi-reversible response, similar to those obtained in a perchlorate media. The above can be summarized with the following reactions:

1.0

E (V vs Ag/AgCl(sat)) Fig. 2. (a) Cyclic voltammetry of a 0.01 M CuCl2 + 0.005 M FeCl3 + 0.1 M LiCl DMSO solution on a glassy carbon electrode in the absence of molecular oxygen at 50
C. Inset shows the quasi-reversible processes assigned to the couples Fe(III)/Fe(II) (C1/ A1) and Cu(II)-chloride/Cu(I)-chloride (C20 /A20 ). b) The same system as above but in the presence of molecular oxygen in solution. In all cases the scan rate was 10 mV s1.

(7)

(8)

where “n” can be 1, 2 or 3. The C1 and A1 processes are the same shown in the equation (1). In the presence of molecular oxygen (Fig. 2(b)), the C20 and C1 processes are not affected by the presence of this element in solution. However, new cathodic processes at more negative potentials than the corresponding C20 are observed. These processes are attributed to the molecular oxygen reduction with a stable oxide phase formation, similar to the one observed in perchlorate solution. The absence of any anodic process between –1.5 and 0.0 V (Fig. 2(b)) confirms the above mentioned. A small anodic process named A” can be caused by of the reoxidation of an oxide product previously formed during the reduction of molecular oxygen. The formation processes of Cu2O and Fe2O3 are the same as those described in Eqs. (4) and (5), but in the case of Cu(I) it includes the corresponding chloride complex. Films obtained from a chloride solution between –0.5 V and –0.9 V where yellowreddish. When the potential was more cathodic than –0.9 V, elemental copper was observed in the film. EDS analyses of these films (see Table 2, chloride solution) show that these are mainly form by iron (presumably Fe2O3) confirming our previous hypothesis. According to these results, it is clear that the presence of a complexing agent is a decisive factor in order to control the quantity of copper in the films. This way, controlling the concentration of chloride in solution can result in the codeposition of stoichiometric films in order the produce the CuFeO2 phase. Taking into account what has been previously discussed, a mixture of chloride–perchlorate was employed in order to control the copper deposition. Thus, a 0.01 M CuCl2 + 0.005 Fe(ClO4)3 + 0.1 M LiClO4 based DMSO solution was studied in order to obtain stoichiometric films. Fig. 3(a) shows the voltammetric response of this solution on a glassy carbon electrode in the absence of molecular oxygen. The processes assigned in Fig. 3(a) are the same as those previously mentioned in Fig. 2 (a), but with a lower free chloride concentration in solution. The same situation can be observed in the presence of molecular oxygen in solution (Fig. 3(b)). Therefore, C20 and C1 processes are not affected and new cathodic waves

G. Riveros et al. / Electrochimica Acta 164 (2015) 297–306

A3'

a)

-2

1

C3'

C2'

-1

-0.5

0.0

0.5

1.0

E (V vs Ag/AgCl(sat))

D ¼ 0:94l=b cosu

1

b) A'' -2

j (mA cm )

0

C2' C1

-1

-2 -1.5

Oxygen Reduction -1.0

-0.5

0.0

0.5

1.0

E (V vs Ag/AgCl(sat)) Fig. 3. (a) Cyclic voltammetry of a 0.01 M CuCl2 + 0.005 M Fe(ClO4)3 + 0.1 M LiClO4 DMSO solution on a glassy carbon electrode in the absence of molecular oxygen at 50
C. (b) The same system as above but in the presence of molecular oxygen in solution. In all cases the scan rate was 10 mV s1.

linked to the molecular oxygen reduction, can be observed. Films electrodeposited in this solution, show to be stoichiometric (Cu:Fe ratio 1:1) when they were obtained at –0.6 V (see Table 2, perchlorate + chloride solution). Films obtained at other potentials were not stoichiometric. Thus, a controlled concentration of chloride ion in solution allows the electrodeposition of films with a stoichiometric ratio Cu:Fe (1:1) in order to form thin films of CuFeO2. All these results are summarized in Table 2, where the ratio Cu:Fe of the films obtained from different solutions at different potentials are compared. However, films obtained in these conditions resulted amorphous (vide infra). In order to obtain crystalline films with a delafossite structure, these need to be annealed at 650
C for 30 minutes under argon atmosphere. 3.2. Structural and morphological characterization. In order to study the structural properties of the CuFeO2 thin films, i.e. to investigate the crystallographic phase, the overall crystalline quality, and the possible texture of the electrochemically grown thin films, X-ray diffraction experiments have been carried out. This X-ray diffraction characterization showed that the as-electrodeposited CuFeO2 thin films were either amorphous or with very small crystallite size, which normally occurs when metal oxides are formed at relatively low temperatures. Thus, to improve their crystalline character they were subjected to thermal

(9)

where l is the X-ray wavelength, u is the Bragg angle, and b is the FWHM of the diffraction peak. By applying the above mentioned Scherrer’s equation, a mean typical value of 18 nm crystallite size have been estimated from the (0 1 2) diffraction peak for the delafossite films annealed at 650
C, corroborating the nanocrystalline character of the obtained delafossite thin films. Fig. 5(a), (b) and (c) show SEM images of a FTO substrate and thin films electrodeposited at –0.6 V before and after annealing, respectively. In the first case, the image shows pyramidal grains typical of the FTO surface. Nevertheless, the surface of the CuFeO2 film is completely different, being this smooth and compact, with an homogeneous grain size. After annealing, the surface is more irregular than in the absence of this treatment. However, the film is equally compact, which reflects the good film quality as it was confirmed by X-ray diffraction measurements. 3.3. XPS study of delafossite-CuFeO2 thin films. Fig. 6 shows the XPS regions of the CuFeO2 electrodeposited at –0.6 V from a perchlorate + chloride solution and after annealing (650
C for 30 minutes). The Fe 2p3/2 and Fe 2p1/2 peaks (Fig. 6(a)) have associated satellite peaks that are located approximately from 6 to 8 eV higher than the main Fe 2p3/2 peaks [38]. Nevertheless the presence of satellites in the 2p region makes difficult the deconvolution, for this it is advisable to use the Fe 3p region. The Fe 3p XPS region shows only a peak due to that the XPS

800

*

* 600

400

200

0 25

30

*

* 35

CuFeO2(015)

-1.0

CuFeO2(104)

-2 -1.5

C1

CuFeO2(101) CuFeO2(012)

0

A2'

CuFeO2(006)

j (mA cm )

2

treatment. Fig. 4 shows a typical grazing incidence diffraction pattern (GI-X-ray) of an annealed (at 650
C and under Ar atmosphere) electrodeposited copper–iron–oxide thin film. It can be seen that all diffraction peaks except the peaks of the substrate marked as (*) can be indexed to the rhombohedral structure of CuFeO2 (delafossite), and are in agreement with standard reported values, see JCPDS pattern at the bottom of Fig. 4. The presence of diffraction peaks corresponding to (0 0 6), (1 0 1), (0 1 2), (1 0 4) and (0 1 5) crystallographic planes of delafossite CuFeO2 can be appreciated. Then, GI-X-Ray diffraction results indicate that the single delafossite CuFeO2 phase is well-defined, the samples are polycrystalline and randomly oriented. Moreover, the broadening of the diffraction peaks demonstrates the nanocrystalline character of these delafossite CuFeO2 thin films. An average crystallite size was obtained using the Scherrer’s formula [46] for the crystallite size broadening of diffraction peaks:

Intensity (arb. units)

3

301

40

45

50

2 theta (degrees) Fig. 4. GI-X-ray diffraction pattern of a delafossite CuFeO2 thin film electrodeposited from a DMSO based solution at 50
and after annealing treatment at 650
C under an Ar atmosphere. Rhombohedral delafossite CuFeO2 JCPDS pattern is also shown for comparison in the bottom of the figure. (*) indicates the peaks originating from the SnO2:F substrate.

302

G. Riveros et al. / Electrochimica Acta 164 (2015) 297–306

a)

Fe 2p Fe 2p1/2

730

Fe 2p3/2

720

Intensity (arb. units)

Intensity (arb. units)

Fig. 5. SEM images of a (a) FTO substrate (as comparison) and thin films electrodeposited at –0.6 V onto FTO substrate from a 0.01 M CuCl2 + 0.005 M Fe(ClO4)3 + 0.1 M LiClO4 DMSO based solution in the presence of molecular oxygen at 50
C (b) before annealing and (c) after annealing.

710

960

945

930

Binding energy (eV)

Fe 3p3/2

56

52

Binding energy (eV) Intensity (arb. units)

Intensity (arb. units)

Cu 2p

Fe 3p

Fe 3p1/2

60

Binding energy (eV)

c)

b)

d)

536

copper is very sensitive to radiation being easily reduced, for this the XPS copper regions were firstly recorded, and the samples were never etched for more of 0.5 min using low energy Ar+ ions. CuO 2p spectra show very characteristic skake-up satellites, the recorded spectrum (Fig. 6(c)) does not show these shake-up satellites and the binding energy of Cu 2p3/2 at 932.5 eV is corresponding to Cu+ [23]. The binding energy for O 1 s is expected at 530.3 eV and 530.2 eV for Cu1+ and Fe3+ as Cu2O and Fe2O3 [47] and for CuFeO2 [23], as is shown in Fig. 6(d). Thus, these results coincide with the results expected for CuFeO2, confirming the presence of this compound.

O1s 3.4. Optical properties

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Binding energy (eV)

Fig. 6. XPS regions of the CuFeO2 electrodeposited at –0.6 V after annealing. (a) Fe 2p region. (b) Fe 3p region, showing the fitting of contributions of Fe 3p1/2 and Fe 3p3/2, with the experimental result (open circles). (c) Cu 2p region. (d) O 1s region.

spectrometer has not the resolution to resolve the 3p3/2 and the 3p1/2 peaks (Fig. 6(b)). The deconvolution of the 3p region could be carried out considering the peaks 3p3/2 of Fe2+ and Fe3+ at 53.7 eV and 55.5 eV respectively as proposed by Yamashita [38]. The Fe 3p region can be fitted by a gauss peak at 55.5 eV as expected for Fe3+ [38]. But we have also fitted the experimental Fe 3p peak (50 to 62 eV) by using Shirley background subtraction and Gauss–Lorentz curves (Gauss/Lorentz = 60/40), FWHMs of 2.6, 0.5 for 3p1/2/3p3/2 area ratio, and a DE = 1.5 eV between the 3p3/2 and the 3p1/2. The region Fe 3p shows that peaks Fe 3p3/2 and Fe 3p1/2 at 710.5 eV and 723.7 eV respectively as reports for CuFeO2 [26]. On the other hand

The optical properties of semiconductor metal oxides are usually due to wide band gaps that give them their almost transparent aspect [3]. Many different reports on the position of the absorption edges of CuFeO2 may be found in the literature, typically they are found not only in the UV and blue region, but also in the visible and IR. In effect, while many authors report on an indirect low laying transition in the NIR region (between 1.0 eV and 1.2 eV [20,48,49], and even at 1.63 eV [49]), others report on direct absorption edges close to these values (1.32 eV [32], 1.55 eV [31]), in the visible (2.0 eV [20,34,48,49], 2.82 eV [29]) or UV region (between 3.0 eV and 3.4 eV [20,23,26,27,48,49]). In fact several authors report simultaneously at least three of these edges (one in the NIR, another in the visible and a last one in the UV) [20,48] that results in a transmittance spectrum that has a ladder-like aspect with several absorption edges [23,26,27,48,49]. As its delafossite structure consists in octahedral structures (formed by Fe atoms surrounded by six O atoms [34,50]) where layers of Cu coordinated to two O atoms are intercalated, it was suggested that the optical properties may be due to resemblance of CuFeO2 between Cu2O and Fe2O3 optical properties. In effect Fe2O3 has an absorption edge close to 1.9–2.2 eV [39], whose direct or indirect nature is still not well understood [51]. However, this resemblance must be

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Fig. 7. Transmittance spectra in the IR region for whole sample (dot-dashed curve), FTO/Glass substrate (short dashed curve) and the CuFeO2 film only (full curve). The scatter dots in the lower part are the first derivative of previous one. The inset shows Tauc plot for band gap energy determination.

830 nm is originated at interference effects it should be due to the CuFeO2, and not an artifact due to the use of FTO/Glass as substrate [65]. However it is seen that as the maximum in the CuFeO2 film spectrum has an almost 100% transmittance (equal values of transmittance of whole sample and substrate), a perfect coupling of light is being obtained here due to interference effects and/or refractive index matching. In spite of these effects the transmittance of CuFeO2 decreases into the visible, which may reveal the presence of an absorption edge. The first derivative (scatter curve in Fig. 7) of the transmittance has a clear peak at 739 nm (1.68 eV) that may correspond to an abrupt absorption edge close to this value, typical of direct transitions [60,66–67]. The Tauc plot for the as obtained aexp absorptance is seen in Fig. 7 inset. A clear linear region is obtained which corresponds to a first direct band gap energy in the IR at Eg(IR) = 1.64 eV. This value is very close to the one reported by other authors [31,32,49]. Moreover, no clear absorption edge can be found at lower energies. Fig. 8 shows the same spectra as Fig. 7 but in the visible region. Once again the structures in the 600 nm to 800 nm are clearly seen (i.e. the absorption edge close to 1.64 eV, the almost coincidental

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Transmittance (%)

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dT/dλ (arb. units)

coincidental [48]. Moreover, while the complicated Cu2O absorption edge [52,53] is dipole forbidden the corresponding edge in CuFeO2 is dipole allowed [54]. The optical properties of delafossite oxides show unique properties that are sometimes against usual trends of other semiconductors [55]. It was shown that in some cases they can be understood by almost flat valence band energy dispersion. Moreover, as the transition to the conduction band minimum in the center of the Brillouin zone (G point) is indirect forbidden, the sharp absorption edge is originated in direct transitions from the valence band maximum (close to F point) and from the Brillouin zone edge (L point) [55]. Specific theoretical calculations using local spin density calculation (LSDA) gives for CuFeO2 a metal nature, although LSDA + U calculations give it a semiconductor behavior with band gap energy of 2.0 eV [50]. More recent calculations have confirmed these results but have refined the position of the optical gaps [56]. They give place to different optical gaps in the NIR (1.04 or 1.30 eV), visible (1.80, and 2.20 or 2.06 eV) and UV (3.10 and 3.5 or 3.20 eV), their exact positions depends on the calculation procedure [56]. The presence of these many absorption transitions was reported not only for CuFeO2, but also for CuMnO2 and CuCoO2 [54] (i.e. other delafossite oxides where Fe is substituted by Mn and Co, whose atomic numbers differs in that of Fe by 1). It was interpreted that these several absorption edges are due to the appearance of narrow bands due to Fe 3d states inside the band gap of the typical delafossite electronic structure. The calculated absorption structures were found at 1.55, 2.80, 3.85 and 4.38 eV [54]. The last three ones are absorption peaks appearing at higher energies than the ones reported in the literature, while the lowest laying one at 1.55 eV is due to the optical absorption edge between the typical flat valence band of delaffosite structure and the lowest states of the Fe 3d arising bands [54]. In view of this general context, the optical properties of present samples were studied at different spectral regions (NIR, visible, and UV). And as a general rule the measurements were analyzed just to search for direct band edges, like the ones reported from theoretical calculation of absorption spectra [55,56]. The performed analysis started by calculating the experimental absorptance aexp from the measured optical transmittance T as aexp =–ln T. Then Tauc like plots (aexp hn)n against photon energy hn are done [57], where n = 2 corresponds to direct allowed edges [58]. Eventually n = 1/2 would correspond to indirect allowed edges [57,59]. The bangap energy Eg is obtained from the extrapolation of a linear fitting to the zero absorptance line [60] (or eventually a measurement of the absorption edge position in case the fitting is not good). As in previous papers, for determining the higher energy absorption edges, the corresponding absorptance is corrected by the next low laying edge: acorr = aexp–aback, where aback is a background absorptance obtained in general from the linear fitting of Tauc plots with either direct or indirect edges [61–64]. The dash dotted line in Fig. 7 shows the IR transmittance for a representative sample (CuFeO2 film grown onto FTO/glass substrate). The curve is almost flat in the longer wavelength region, has a maximum around 830 nm and decrease into the visible (shorter wavelengths). To try to understand this behavior the spectrum of the FTO/Glass substrate was obtained (short dashed curve). Although this spectrum is almost flat, it has an oscillatory behavior with a relative minimum at 880 nm, and maxima close to 680 nm and between 1200 nm to 1400 nm (almost flat this last one). These structures may be due to interference effects in the FTO film [65]. The spectrum of the CuFeO2 film was calculated using the previous one as a reference in former measurement. The result is the full line in Fig. 7. This curve has a similar behavior than the one of the whole sample and the maximum is almost unchanged. Therefore if the maximum at

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0 400

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600

700

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Wavelength (nm) Fig. 8. Transmittance spectra in the visible region for whole sample (dot-dashed curve), FTO/Glass substrate (short dashed curve) and the CuFeO2 film only (full curve). The scatter dots in the lower part are the first derivative of previous one. The inset shows the same spectra in the UV region.

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value of transmittance for sample and substrate, and the peak in the first derivative). Moreover, the substrate spectrum continues to be almost flat, although it has oscillations typical of interference [65]. In spite of that, the spectra of the sample (Glass/FTO/CuFeO2) and just CuFeO2 film (i.e. sample but using substrate as reference) have a similar shape. These spectra are very similar to the ones reported in the literature, with the ladder-like shape caused by the several absorption edges [23,26,27,49]. In addition in the 400–600 nm region the first derivative has at least three peaks, which may correspond to more than one absorption edge. To search for these absorption edges, the absorptance was calculated aexp and the corresponding corrections acorr = aexp–aback (as previously described), where the aback was obtained from the fitting that gives place to the IR band gap energy determination. These spectra are seen in Fig. 9. In the full curve (aexp) it is clearly seen the abrupt step-like increase of the absorptance that corresponds to the direct Eg(IR) = 1.64 eV. In the dashed curve (acorr) this absorption is no more present and a smooth increase form 2.0 eV can be seen. As this increase is not as step-like as previous one both Tauc plots for direct and indirect absorption edges are seen in the insets of Fig. 9. Linear fittings can be done in both cases giving place to at least two different values: a direct like edge at Egdir(vis) = 2.35 eV, and an indirect like edge at Egind(vis) = 2.03 eV. These values are close to the ones reported in the literature, either experimentally [20,34,48,49] or theoretically [56]. However, from the insets of Fig. 9 it cannot be concluded the nature of these edges. This may be due to the almost linear dependence of the acorr spectrum of main Fig. 9. For other metal oxides it was reported that such linear dependence of absorptance against photon energy may be due to the coincidental superposition of several absorption edges of different nature [52,62]. In present case it agrees with the theoretical calculation that assigns these transitions to excitation from valence band originated in Cu 3d states to flat band originated in Fe 3d states [56]. Finally, the inset of Fig. 8 shows the transmittance at the UV region, where there is another clear absorption edge. However the interpretation of the data in this region is complicated by the appearance of an absorption edge of the FTO/Glass substrate close to 350 nm (3.54 eV). Fig. 10 shows the same spectra as in Fig. 9 but in this UV region: aexp and the corresponding correction acorr = aexp–aback, where aback was obtained from a indirect-like fitting from the absorptance in the visible region, but near to the UV region. A different fitting than the ones in the inset of Fig. 9 was done due to the complex nature of this spectrum. The proper correction of this background is seen in the fact that acorr is almost

Fig. 9. Absorptance spectrum aexp (full curve) obtained from main Fig. 8 and corresponding correction after low energy absorption subtraction acorr (dashed curve). Insets are direct gap (left) and indirect gap (right) determination.

Fig. 10. Absorptance spectrum aexp (full curve) obtained from the inset of Fig. 8 and corresponding correction after low energy absorption subtraction acorr (dashed curve). Insets are direct gap (left) and indirect gap (right) determination.

null in the 3.0 eV to 3.5 eV region where aexp is almost flat. A smooth increase is observed staring from 3.5 eV which may correspond to an indirect like absorption with a band gap energy at Egind(UV) = 3.37 eV (see right inset in Fig. 10). A more abrupt increase at higher energy is also seen, which may correspond to a direct-like absorption at Egdir(UV) = 3.61 eV (see left inset in Fig. 10). This last value is too close to the absorption edge of the substrate, but the presence of an absorption edge in the UV (specially previous one that is clearly below the one of the FTO/ Substrate) is usually reported for CuFeO2 [20,23,26,27,48,49]. It is commonly seen in several delafossite metal oxides due to valence to conduction band transition [56]. In spite of the nature, direct or indirect, of all these transitions, the presence of so many absorption edges is commonly reported in this material, which may be due to the appearance of an intermediate band due to Fe 3d states in the wide band gap usually present between the flat valence band originated in Cu 3d sates and corresponding high energy conduction band [56] of typical delafossite oxides [55]. 4. Conclusions Thin films of CuFeO2 with delafossite structure where electrodeposited from a 0.01 M CuCl2 + 0.005 M Fe(ClO4)3 + 0.1 M LiClO4 DMSO based solution employing molecular oxygen as oxygen precursor at –0.6 V. An intensive electrochemical study based on the chloride concentration in solution was carried out in order to set up the best condition for the film electrodeposition. However, as-grown films were thick and amorphous. Rhombohedral delafossite structure of CuFeO2 was obtained after of a thermal treatment at 650
C for 30 minutes under Ar atmosphere, confirming the formation of this compound. Furthermore, the presence of CuFeO2 was verified through of XPS analyses on different XPS regions of the CuFeO2. The optical transmittance of the films shows a ladder-like appearance due to the present of several absorption edges from the IR region into the UV. The most clearly determined were a direct-like edge at the IR: Eg(IR) = 1.64 eV. Several edges are present in the visible (Egdir(vis) = 2.35 eV, and Egind(vis) = 2.03 eV) which gives the absorptance spectrum a complicate dependence. Finally another edge in the UV beginning at Egind(UV) = 3.37 eV was observed. These results are in agreement with previous reports and theoretical calculations that assign them to an intermediate band originated in Fe 3 d states inside the wide band gap of delafossite oxides.

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