Dye Regeneration Kinetics of Sensitized Nickel Oxide Films under Illumination Investigated by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) feedback mode has been used to investigate regeneration kinetics on P1 (4-(bis-{4-[5-(2,2-dicyanovinyl) thiophene-2-yl] pH-enyl} amino) benzoic acid) dye sensitized nickel oxide (NiO) electrodes in contact with reduced iodide liquid electrolyte in different electrolyte solvents. We were used acetonitrile, ethanol, methanol and propylene carbonate solvents for comparison under illumination of different wavelengths. We found significant variation of regeneration kinetics parameters such as regeneration rate constant ( k_eff), the reduction rate constant ( k_red) and absorption cross-section (Φhv) in different illumination intensity and different solvents.

• Scanning Electrochemical Microscopy
• Nickel Oxide
• Regeneration Kinetics

1. Introduction

Over the past decades, dye-sensitized solar cells (DSSCs) emerged as a new innovative technology for development of environmentally clean and low-cost energy source. Dye sensitized solar cells (DSSCs) provide a technically trustworthy alternative concept for p-n junction photovoltaic devices [1] [2] [3]. Efficient charge separation in DSSCs is achieved by photo induced electron injection from light absorbing sanitizer into the conduction band of nanocrystalline electrode [4]. Dye sensitized solar cells composed of mesoporous nanocrystalline films modified sensitizing dyes as photo electrodes, electrolytes and counter electrodes [5]. In past decades, the enormous attention has been given on n-type based DSSCs (TiO₂ or ZnO nanoparticles based solar cells) [6]; relatively few works have been reported on sensitization of p-type semiconductors DSSCs [7] [8]. More recently, dye sensitization p-type semiconductors with mesoporous, NiO-based protective electrode get much interest research due to the development of new materials. Dye sensitization of the p-type semiconductors happens with a hole injection mechanism, from the illumination excited dye to the semiconductor. The reported efficiency for p-type DSSCs is significantly lower than n-type DSSCs solar cell [9] [10] [11] [12]. We used the p-type semiconductor NiO, which has a band gap of ∼3. 5 eV and has the advantage of stability and transparency. But the efficiency of P-type semiconductor not based DSSCs is still low due to a number of reasons, such as the low potential difference, insufficient driving force, fast charge recombination between reduced dye and holes in p-type semiconductors.

Upon light absorption of the dye, the frequency of the charge-transfer reaction of the photo chemically reduced dye molecules to the electrolyte must be competitive with the tremendously fast charge recombination process between the reduced dye molecules and injected holes in the NiO electrode. Dye regeneration reaction to P-type dye sensitized solar cells the reaction of the excited dye reduced form with one iodide ion that might lead to the formation of an intermediate complex. The dye regeneration kinetics, process significantly affected by electrolyte solvent, dye solvent and dye concentration and dye loading time [13] [14] [15]. In past decades, investigation of dye regeneration on DSSCs has been performed by spectroscopy, photo electrochemistry characterization of individual electrode and by nanosecond transient absorption spectroscopy (TAS), which provides valuable information about the regeneration of sensitized solar cells [16] [17] [18]. In this study, we used SECM approach curve measurement to investigate kinetics of dye regeneration. We explore the effects solvent on the regeneration kinetics of P1 dye (molecular structure as shown in Figure 1(a)) with reduced iodide electrolyte (I⁻) in different illumination in different electrolyte solvents, such as acetonitrile, methanol, ethanol and propylene carbonate respectively.

2. Experimental Section

2.1. Preparation of the P1/NiO Film

The NiO paste was prepared as fellow 4 gm of NiO nanoparticles (particle size ~20 nm), were ball-milled in 50 mL ethanol overnight. The mixture of above colloidal solution, 4.4 gm EC (30 - 60 mPas) and 5.6 gm EC (5 - 15 mPa) ethyl cellulose and 10 gm triponal were sonicated and stirred overnight alternatively to obtain a fine dispersion. A paste was made by evaporating the ethanol from the mixture on a rotary evaporator. FTO glass were coated with nickel acetate ethanol solution (0.05 M) by dip-coating and subsequently dried before screen print. The photocathode films were screen-printed with the NiO paste and dried for 5 min at 125˚C [19] [20] [21].

2.2. SECM Apparatus and Procedure

SECM apparatus for dye sensitized solar experiments is described elsewhere [22]. SECM experiments were performed on the CHI920C electrochemical workstation (CH Instruments, Shanghai). A homemade Teflon cell with a volume of 2 ml was used to hold a pot wire counter electrode, an Ag/Ag⁺ reference electrode. The dye sensitized NiO film coded as no/P1 electrodes were placed at the bottom sealed with an O ring. An extra PT wire connected the back contact of the NiO/P1 sample with reduced (I⁻) electrolyte to operate the photo electrochemical cell in a short-circuit setup. The LEDs were placed close to the dye coated sample on the back side and focused on film electrodes by an objective lens incident light power on the illuminated area 0.0785 cm² and photon flux density J_hv of J_hv of 2.2 × 10⁻⁹ to 22.4 × 10⁻⁹ mol cm²∙s⁻¹ for blue LED and 2.2 × 10⁻⁹ to 14.9 × 10⁻⁹ mol∙cm² s -mol∙cm²∙s⁻¹ for red LED respectively. A 25 µm diameter PT wire (Good fellow, Cambridge, UK) was sealed into a 5 cm glass capillary prepared by Vertical pull pin instrument (PC-10, Japan). The ultramicroelectrode (UME) was polished by a grinding instrument (EG-400, Japan) and micro-polishing cloth with 1.0, 0.3 and 0.05 µm alumina powder. Then the UME was sharpened conically to an RG of 10, where RG is the ratio between the diameters of the glass sheath and the PT disk. The approach curves are given as normalized UME current IT vs. normalized distance L. the position Zmax at which mechanical contact of the UME with the sensitized sample and distance does the active electrode area of the sample at Zmax. L is obtained from the vertical position z increasing with an approach to NiO/P1 sample. There for the possible distance between the sub-straight and tip 200 μm ± do, potentials at the tip ET = 0.7 V. The potential of UME was selected after recording a cyclic voltammogram ET was taken well in the region of the diffusion-controlled [I⁻] oxidation current such that insignificant variations of the reference electrode potential would not change the UME Current [23] [24].

3. Results and Discussion

3.1. SECM Feedback Mode of P1 Sensitized NiO Film

In feedback mode ultra-micro electrode (UME) immersed in an electrolyte solution and operated as working electrode in the system [25]. We were measured SECM approach curve of P1/NiO sample at different illumination intensities to explore the influence of light wavelength on dye regeneration. Figure 1(b) shows absorption spectra of P1 the UV-vis absorption spectra, show one minor absorption band looking at 300 - 420 nm and one prominent band at 400 - 700 nm, respectively, which are ascribed to π-π* transitions of the conjugated chromophores. Figure 2(a) shows theoretical approach curves of UME over sensitized nickel oxide, it represents the finite kinetic reaction at UME and diffusion-controlled re-action on the surface of sensitized nickel oxide (P1/NiO). Curve#1 represents the electrochemical conversion of I⁻3 to I⁻ at the surface of sensitized nickel oxide (P1/NiO) called positive feedback, curve#2 represents the current response to finite reaction kinetics on illuminated sensitized nickel oxide.

In dark the approach curve#3 of dye sensitized nickel oxide (P1/NiO) identical to insulating surface, which hinders the diffusion of I⁻ towards the UME, named as negative feedback [26] [27] [28].

Figure 2(b) shows approach curves of blue LED (13.9 × 10⁻⁹ mol∙cm⁻²∙s⁻¹) and red LED, (J_hv = 2.2 × 10⁻⁹ mol∙cm⁻²∙s⁻¹) which represent the depends of UME current on the flux density of illuminated light. There are number of processes that affect charge transfer kinetics in NiO/P1/electrolyte/UME interface such as light absorption and hole injection from excited dye to valence band potential of the nickel oxide, diffusion of the I⁻ redox mediator, ET at the illuminated dye-sensitized NiO/electrolyte interface, hole conduction across the nano porous NiO film and ET at the NiO film/FTO interface.

Figure 3 shows that charge transport process in illuminated NiO/dye/electrolyte interface. When excited dye injected holes into valence band of nickel oxide (NiO) schemes 1 and 2 charge separation and hole injection. The reaction of ground state dye reduced with electrolyte lead electron transport across nickel oxide (NiO) film to back contact and lead to charge regeneration scheme 3 and 4 [29] [30]. The dye regeneration reaction rate constant k_eff at NiO/P1/UME interface related to electron transfer kinetics on the basis of SECM feedback mode expressed as function of, k_red, Ф_hv, Γ D o , J_hv and [I⁻] given by Equation (1) [31] [32].

k eff = 3 Γ D o Φ hv J hv k red 6 k red [ I − ] − 2 Φ hv J hv (1)

where Ф_hv, absorption cross section,k_inj the rate constant of hole injection, J_hv the photon flux, Γ D o dye content at ground level and k_red extracted reduction rate constant.

In order to investigate dependence of dye regeneration on light wavelength we have carried approach curve measurement of sensitized nickel oxide film. Figure 4(a) and Figure 4(b) show normalized approach curve of UME on P1/NiO film in blue and red LED illuminations. As photon density J_hv increased from 2.2 × 10⁻⁹ to 22.4 × 10⁻⁹ mol∙cm⁻²∙s⁻¹ k_eff increased from 6.02 × 10⁻³ cm∙s⁻¹ to 20.22 × 10⁻³ cm∙s⁻¹ in blue illumination. Similarly, as red LED J_hv increased from 2.2 × 10⁻⁹ to 14.68 × 10⁻⁹ mol∙cm⁻²∙s⁻¹ red illumination k_eff increased from 3.54 × 10⁻³ to 10.01 × 10⁻³ cm∙s⁻¹. For example, at J_hv = 2.2 × 10⁻⁹ mol∙cm⁻²∙s⁻¹ its k_eff = 6.02 × 10⁻³ cm∙s⁻¹ and at J_hv = 22.4 × 10⁻⁹ mol∙cm⁻²∙s⁻¹ its k_eff = 20.22 × 10⁻³ cm∙s⁻¹ in blue LED illumination. This means when J_hv increased ten times keff increased more than three times in blue illumination.

In the same way for J_hv = 2.2 × 10⁻⁹ mol∙cm⁻²∙s⁻¹ its, k_eff = 3.54 × 10⁻³ cm∙s⁻¹, at J_hv = 14.68 × 10⁻⁹ mol∙cm⁻²∙s⁻¹, its k_eff = 3.54 × 10⁻³ cm∙s⁻¹ in red illumination as shown (Table 1). This variation of rate constant k_eff with the variation of J_hv clearly shows that remarkable influence of the wavelength and light intensity on dye regeneration kinetics. Another finding elucidates that k_eff with blue LED more than two times higher than k_eff of red LED. The higher value of k_eff for blue LED lead to the maximum absorption of P1 dye around 490 nm as shown Figure 1(b).

3.2. SECM Approach Curves on P1/NiO Electrodes: Effect of Solvent

The overall performance dye sensitized solar cell strongly affected by the numbers of parameters such as nature of solvent, solvent permittivity and dye extinction coefficient [33]. Figure 5 shows normalized approach curves at different photon flux density J_hv in (a) methanol (b) propylene carbonate for electrolyte

Table 1

Normalized apparent rate constants k and apparent rate constants k eff = kD/r T obtained for the reduction of photo excited D − by I − , D = 8.61 × 10 −5 cm∙s −1

Jhv/10−9 mol∙cm−2∙s−1	Curve#	κ	keff /10−3 cm∙s−1
(a) Blue
2.2	6	0.087	6.02
6.1	5	0.178	12.31
11.8	4	0.242	16.71
13.9	3	0.255	17.62
19.8	2	0.283	19.51
22.4	1	0.294	20.22
(b) Red
2.2	6	0.053	3.54
4.19	5	0.081	5.55
6.81	4	0.106	7.32
9.44	3	0.123	8.52
12.06	2	0.137	9.41
14.68	1	0.241	10.01

[I⁻] of 1 mM. When sensitized nickel oxide illuminated in back side the current was significantly higher than dark (curves #1-6) as shown in Table 2. For blue illumination as J_hv increased from 2.2 × 10⁻⁹ to 22.4 × 10⁻⁹ mol∙cm⁻²∙s⁻¹ as k_eff increased from 6.02 × 10⁻³ to 22 × 10⁻³ cm∙s⁻¹ in acetonitrile, k_eff increased from 0.97 × 10⁻³ to 3.13 × 10⁻³ cm∙s⁻¹ in ethanol, k_eff increased from 1.73 × 10⁻³ to 5.54 × 10⁻³ cm∙s⁻¹ in methanol and k_eff increased from 0.838 × 10⁻³ to 2.69 × 10⁻³ cm∙s⁻¹ in propylene carbonate s respectively as shown in Figure 2(a) and Figure 2(b). Approach curves in acetonitrile and ethanol solvents documented in supporting information SI Figure 3(a) and Figure 3(b). In this experiment regeneration parameters reduction rate constant and absorption cross-section (k_red, Ф_hv) of (7.96 × 10⁵ mol⁻¹∙cm³∙s⁻¹, 7.98 × 10⁶ cm²∙mol⁻¹) in acetonitrile (1.24 × 10⁵ cm³∙s⁻¹, 3.57 × 10⁵ cm²∙mol⁻¹) in ethanol (2.18 × 10⁵ mol⁻¹∙cm³∙s⁻¹, 4.19 × 10⁶ cm²∙mol⁻¹) in methanol and (1.02 × 10⁵ mol⁻¹∙cm³, 1.53 × 10⁵ cm²∙mol⁻¹) in propylene carbonate respectively (Table 3). These considerable variations of reduction rate constant k_red and absorption cross-section Ф_hv shows clear evidence of effect of solvent on dye regeneration process.

Figure 6(a) shows Plot of k_eff vs J_hv of four different solvents in blue illumination. As our experiment shown the steady-state currents and diffusion coefficient in acetonitrile solvent was significantly higher than, methanol, ethanol and propylene carbonate. The diffusion coefficient D and tip current [I_T] of five times higher than tip current of propylene carbonate, four times of ethanol and two times of methanol. Figure 6(b) shows plot of k_eff vs η shows the effect of solvent on dye regeneration. The experimental observations clearly show significant impact of solvent viscosity on regeneration parameters such as k_red, D, I_T and ϕ_hv [34] [35] [36].

This implies that regeneration parameters in acetonitrile are greater than methanol, ethanol and propylene carbonate. As shown in experimental reduction rate constant k_red in acetonitrile seven times higher than k_red of propylene carbonate, six times higher than k_red of ethanol and k_red of two times higher than of methanol. This realizes that acetonitrile is preferable solvent for dye regeneration and significant impact of solvent nature on regeneration kinetics (Table 3).

Figure 6(b) shows that when viscosity of solvent increases, dye regeneration parameters such as k_eff, k_red,D and ϕ_hv decrease inversely. This finding explored

Table 2

Normalized rate constants k and apparent rate constants k eff = kD/r T obtained for the reduction of photo excited D − by I − , D = 8.61 × 10 −5 cm 2 ∙s −1 in acetonitrile, D = 3.19 × 10 −5 cm 2 ∙s −1 methanol, D = 1.85 × 10 −5 cm 2 ∙s −1 Ethanol, D = 1.67 × 10 −5 cm 2 ∙s −1 in different solvent

Jhv/10−9 mol∙cm−2∙s−1	Curve#	κ	keff /10−3 cm∙s−1
(a) acetonitrile
2.2	1	0.087	6.02
6.1	2	0.178	12.31
11.8	3	0.242	16.71
13.9	4	0.255	17.62
19.8	5	0.283	19.51
22.4	6	0.294	20.22
(b) Methanol
2.2	1	0.0067	1.73
6.1	2	0.0132	3.38
11.8	3	0.0178	4.56
13.9	4	0.0189	4.83
19.8	5	0.0214	5.37
22.4	6	0.0219	5.54
(c) Ethanol
2.2	1	0.0655	0.97
6.1	2	0.129	1.91
11.8	3	0.174	2.58
13.9	4	0.185	2.73
19.8	5	0.205	3.04
22.4	6	0.213	3.13
(d) poly carbonate
2.2	1	0.0062	0.838
6.1	2	0.0122	1. 64
11.8	3	0.0165	2.21
13.9	4	0.0175	2.34
19.8	5	0.0194	2.61
22.4	6	0.0213	2.69

with the diffusion coefficient of the reactants, D, k_eff, κ normalized rate constant varied with solvent viscosity, η as expressed by stokes-Einstein equation.

k eff = K r T ( κ T 6 π r η ) (2)

where K is Boltizmann’s constant, r_T radius of UME and r the hydrodynamics radius of the diffusing species, T temperature [37] [38].

According to photo modulated voltammeter the viscosity of the solvent clearly affects diffusion coefficient and diffusion length of redox intermediates. When dye illuminated, it led charge separation, hole injection into NiO valence band, regenerate to ground state as Equation 3(a), and iodine-based intermediates which accountable for dye regeneration processes Equation 3(b) [39].

I 3 − + e − → I 2 · − + I · 3(a)

I 2 · − + ( e − ) NiO → 2 I − + NiO ( + ) 3(b)

Δ G reg 0 = e ( E o ( D / D − ) ) − E o ( I 3 − / I 2 · − ) 3(c)

This implies that regeneration parameters in acetonitrile greater than methanol, ethanol and propylene carbonate. As shown in experimental reduction rate constant k_red in acetonitrile seven times higher than k_red of propylene carbonate, six times higher than k_red of ethanol and k_red of two times higher than of methanol. This realizes that acetonitrile is preferable solvent for dye regeneration and significant impact of solvent nature on regeneration kinetics (Table 3).

The driving force is difference of the reduction potential of dye and electron reduction potential of I 3 − in solvents has significant impact on regeneration kinetics. The standard potentials of E˚ ( I 3 − / I 2 · − ) redox couple in different solvents have been investigated in literature; for acetonitrile E˚ ( I 3 − / I 2 − ) of (−0.58 eV) and propylene carbonate E˚ ( I 3 − / I 2 − ) of (−0.64 eV). From previous report estimated value of E˚ (P1/P1⁻) of (−3.13 eV) for P1 dye. The driving force according to (Equation 3(c)), in acetonitrile Δ G reg o of (−2.55 eV) higher than propylene carbonate Δ G reg o of (−2.49 eV). Therefore, driving force in different solvents show that dye−electrolyte interaction plays significant role on dye regeneration kinetics.

Table 3

Feedback model reduction rate k red , the regeneration rate, k eff normalized rate constant k, and absorption cross-section ϕ hv of P1-sensitized NiO photo electrochemical electrodes in different electrolyte solvent] k red reduction rate constant and Φ hv .

Solvent (η)	D/10−5 cm−2∙s	IT/10−8 A	kred/105 mol−1∙cm3∙s−1	Φhv cm2∙mol−1
Acetonitrile (0.34)	8.61	8.235	7.96	7.98 × 106
Methanol (1.074)	3.19	3.078	2.18	4.19 × 106
Ethanol (0.544)	1.85	1.771	1.24	3.57 × 105
P/carbonate (2.51)	1.67	1.596	1.02	1.53 × 105

4. Conclusion

Scanning electrochemical microscopy has been powerful technique for electron transfer reactions in sensitized electrode/electrolyte interface. The SECM study shows that the significant difference in the regeneration parameters of illuminated dye sensitized NiO samples with different illumination intensity and wavelength of blue and red LEDs, with different solvent. We found significant variation of regeneration kinetic parameters such as regeneration rate constant (k_eff), reduction rate constant (k_red) and absorption cross section (Φ_hv) in different illumination intensity and different solvent. We found, k_red 7.96 × 10⁵ mol⁻¹∙cm³∙s⁻¹ for acetonitrile, 2.18 × 10⁵ mol⁻¹∙cm³∙s⁻¹ for methanol, 1.24 × 10⁵ mol⁻¹∙cm³∙s⁻¹ for ethanol and 1.24 × 10⁵ mol⁻¹∙cm³∙s⁻¹ for P/carbonate. Therefore, SECM method is new dimension to study dye regeneration factor to improve energy conversion efficiencies for the P-type DSSC.

Acknowledgements

A.G.A. gratefully acknowledges the Chinese Scholarship Counsel (CSC) for the Ph.D. fellowship.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Supporting Information

SI⁻1 SECM Measurement

Normalized heterogeneous rate constants κ have been extracted from experimental approach curves by fitting them to an analytical approximation of simulated data evaluated by Amphlett and Denuault.

The formula of Amphlett and Denuault given for RG = 10.2/10. Constants for other selected RG are also available. Normalized approach curves I_T vs. L have been calculated from each experiment approach curves i_T(z) using I_T = i_T/i_T,∞ and L = d/r_T. The analytical approximation of Amphlett and Denuault was used for calculating a theoretical current IT for each experimental, normalized distance L [1] [2] [3] [4]

I T ( L ) = I T , i n s ( L ) + ( 1 − I T , i n s ( L ) I T , c o n d ( L ) ) I S ( L ) (SI−1)

I T , c o n ( L ) = 0.78337 L + 0.331 exp ( − 1.0672 L ) + 0.68 (SI−1a)

I T i n s = 1 0.40472 + 1.60185 L + 0.58819 exp ( − 2.372294 L ) (SI−1b)

I S ( L ) = 0.78377 1 + 1 k L + 0.68 + 0.3315 exp ( − 1.0672 L ) 1 + 11 / k L + 7.3 110 − 40 L (SI−1c)

Feedback Mode

For measurements in FB mode, we used electrolyte solution of reduced iodide (I⁻) electrolyte as a mediator. At the UME, a potential E_T is applied at which diffusion-controlled conversion of themediator occurs according to Equation (SI⁻3) and thus a steady.

Diffusion-controlled reaction on the surface of sensitized nickel oxide as eq (SI⁻4) respectively.

3 I − − 2 e − → I 3 − (SI−3)

I 3 − + 2 e − → 3 I − (SI−4)

Diffusion-limited electrolysis of the I⁻ mediator at the Pt UME in the bulk solution according to reaction Equation 1(a) results in a steady-state limiting current I_T,∞ at negative potentials which is given by Equation (SI⁻5).

I T , ∞ = 4 n F D C r T (SI−5)

where n is number of electrons transferred in reaction Equations 1(a) and 1(b), D is diffusion coefficient, C is concentration of electrolyte and r_T is UME radius

SI-2. Determining diffusion coefficient of I⁻ in different solvent

The voltammogram show a two sigmodal waves, corresponding to the oxidation and reduction of I⁻ to I 3 − and I 2 · − , respectively. Accordingly, it is considered that this property reduces the mobility of ions and hence the steady-state currents at the UME. When the steady state condition is obtained, the diffusion coefficients D of I⁻ in different solvent could be determined from diffusion-limited UME-currents using the following equation

D = I T , ∞ 4 n F [ I − ] r T (SI−6)

FigureS1 SECM approach curves in different solvent in blue illumination

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