Characterization and Analysis of Self-Assembly of a Highly Active

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J. Phys. Chem. C 2008, 112, 3774-3779

Characterization and Analysis of Self-Assembly of a Highly Active Colloidal Catalyst for Water Oxidation onto Transparent Conducting Oxide Substrates Takayuki Kuwabara, Emi Tomita, Sayaka Sakita, Daisuke Hasegawa, Koji Sone, and Masayuki Yagi* Faculty of Education and Human Sciences, Center for Transdisciplinary Research, Niigata UniVersity, 8050 Ikarashi-2, Niigata 950-2181, Japan ReceiVed: October 9, 2007; In Final Form: NoVember 22, 2007

An IrO2 colloid stabilized by citrate ions was self-assembled on an indium tin oxide (ITO) electrode when it was immersed in the colloid solution at pH 3.5. The IrO2 colloid on the ITO surface was characterized using electrochemical, inductively coupled plasma mass spectroscopic, X-ray diffraction spectroscopic, scanning electron microscopic, and atomic force microscopic techniques. The self-assembly was promoted steeply at pH 3.5 to 4.1, although it hardly occurred at pH 5.3 to 9.7. It is considered to be caused by chemical interaction between carboxylic groups on the citrate stabilizer and hydroxyl groups of the ITO surface. The adsorption isotherm of the IrO2 colloid onto the ITO surface was analyzed by a Langmuir adsorption isotherm to provide the maximum coverage and an adsorption equilibrium constant Γmax ) 1.1 × 10-8 mol cm-2 and Kads ) 1.8 × 104 M-1 at 25 °C, respectively. The Kads value increased from 6.7 × 103 to 1.8 × 104 M-1 with a temperature increase from 5 to 25 °C. The temperature dependence of Kads gave ∆H° ) 36.5 kJ mol-1, ∆G° ) -24.4 kJ mol-1, and ∆S° ) 204 J mol-1 K-1 at 25 °C. The positive ∆H° and ∆S° values are explained by the rearrangement of solvating water molecules and counter cations surrounding the IrO2 colloid that is involved in its assembly on the ITO surface. In electrocatalytic water oxidation, the maximum turnover frequency of the IrO2 catalyst was 23 600 h-1 under potential static conditions at 1.3 V versus Ag/AgCl.

Introduction Much attention has recently been paid to clean and safe alternative energy sources due to recent problems with energy and the environment. Artificial photosynthesis is expected to be one of the promising energy-providing systems in the future from the possibility of producing O2 and H2 (as clean energy) from water and solar energy. Since it consequently requires an active catalyst for water oxidation to evolve O2 (eq 1), development of a potential catalyst for water oxidation is attracting much interest in related research fields.1-3

2H2O f O2 + 4H+ + 4e-

(1)

It is generally known that metal oxides such as RuO2, IrO2, Co3O4, Rh2O3, and Mn2O3 have catalytic activities for water oxidation.4-8 An efficient catalyst, RuO2, has been wellresearched as a colloidal and heterogeneous catalyst in electrochemical and photochemical systems.5,6 However, it has been pointed out that RuO2 easily converts to unstable RuO4.9 IrO2 is more stable than RuO2, and its activity is comparative with the latter. Harriman and Thomas reported that the IrO2 colloid is stabilized by citrate ions.10 A citrate-stabilized IrO2 colloid has been successfully utilized as an efficient catalyst for photochemical water oxidation.11-13 However, it has never been applied to electro- and photoelectrochemical systems except in our recent preliminary study on electrocatalytic water oxidation by a self-assembled IrO2 colloid on an indium tin oxide (ITO) electrode.14 * Corresponding author. E-mail: [email protected]; fax and tel.: +81-25-262-7151.

It has been extensively reported that functional molecules with linkage groups such as thiol,15,16 alkoxysilyl,17,18 carboxylic,19-21 and phosphonic groups22,23 are self-assembled onto surfaces of Au, Ag, and ITO electrodes by a chemical interaction between the linkage groups and the electrode surfaces. We found that a citrate-stabilized IrO2 colloid can be self-assembled on the ITO surface and that it works as an efficient catalyst for electrochemical water oxidation. This result illustrates that the electrode surface can be modified by the assembly of not only small molecules but also relatively large objects such as colloids. It could expand the possible modification of the electrode surface by a self-assembly technique to yield new electrochemical functions. The preliminary data on the behavior of the selfassembly of IrO2 colloids were reported elsewhere.14 However, the detailed characteristics and important factors of the selfassembly remain still open. Herein, we studied the assembly of citrate-stabilized IrO2 onto the ITO surface to reveal its mechanism and characteristics and to provide thermodynamic insight into the assembly. The analysis of the assembly of IrO2 colloids, characterization, and electrocatalytic activity of assembled IrO2 colloids will be reported. Experimental Procedures Materials. Potassium hexachloroiridate(IV) (K2IrCl6) was purchased from Wako Pure Chemical Ind., Ltd. Sodium hydrogencitrate sesquihydrate (HOC(COOH)(CH2COONa)2‚ 1.5H2O) and anion-exchange resin (DOWEX 2×8-50) were purchased from Aldrich Chemical Co., Inc. The purest grades of all the chemicals were used as received. ITO-coated glass (10 Ω/0) and fluorine-doped tin oxide (FTO)-coated glass (8.6 Ω/0) were purchased from Kinoene Kogaku Kogyo Co., Ltd.

10.1021/jp7098416 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/15/2008

Self-Assembly of Highly Active Colloidal Catalyst and Asahi Glass Co., Ltd., respectively. Deionized water was used for all the experiments. Preparations. IrO2 colloid was prepared according to the procedure reported elsewhere.10 Potassium hexachloroiridate(IV) (30 mg, 6.2 × 10-5 mol) was added to an aqueous solution (50 mL) of sodium hydrogencitrate (50 mg, 1.9 × 10-4 mol). The red-brown solution was adjusted to pH 7.5 with a 0.25 M NaOH solution and then refluxed in an oil bath with constant stirring for 30 min. The pH of the solution was again adjusted to 7.5 after the solution was cooled to room temperature. The pH adjustment, followed by reflux for 30 min, was repeated until the pH maintained a value of 7.5. The solution was additionally refluxed for 2 h with oxygen bubbling through the solution to yield a deep blue solution of IrO2 colloid. The solution was stirred with anion exchange resin (DOWEX 2×8-50, 10 mL) for 30 min to remove excess citrate ions. The solution was diluted to 100 mL after filtration and then stocked in a glass-stoppered flask. The stocked solution is stable for several months. The UV-vis absorption spectrum of the IrO2 colloid solution gave a broad absorption band at 590 nm that is characteristic of IrO2 colloids.10 The light scattering measurement indicated that the size distribution of the IrO2 colloid in the solution ranged from 60 to 100 nm with the mode at 75 nm. The concentration of the Ir atom in the stocked colloid solution was measured to be 6.0 × 10-4 M by an inductively coupled plasma mass spectrometry (ICP-MS) technique. Measurements. The particle size of the IrO2 colloid in the solution was calculated from dynamic light scattering data using a particle size analyzer (Nicomp Instrument Corp., Nicomp Submicron Particle Sizer Model 370), equipped with an autocorrelator and a small He-Ne laser (639.0 nm). The light scattering data were collected at 90° to the incident beam in the drop-in cell mode.24 The X-ray diffraction (XRD) measurement was carried out using an X-ray diffractometer (MAC Science, MX labo). Scanning electron microscopy (SEM) data were recorded using a field emission SEM apparatus (JEOL, JSM-7400F) operated at an accelerating voltage of 2 kV. Atomic force microscopy (AFM) data were recorded using a nanoscale hybrid microscope (Keyence, VN-8000). The ICP-MS measurement was conducted using an ICP mass spectrometer (Yokogawa, HP4500). Cyclic voltammograms (CV) were measured in a 0.1 M KNO3 aqueous solution (pH 5.3) at 20 mV s-1 in a conventional single-compartment electrochemical cell equipped with a saturated calomel reference electrode (SCE) and a platinum wire counter electrode. Electrochemical water oxidation was conducted in a 0.1 M KNO3 aqueous solution (pH 5.3) under potentiostatic conditions at 1.3 V versus Ag/AgCl using a gastight single-compartment electrochemical cell. Gas evolved in the electrochemical reaction was analyzed on a gas chromatograph (Shimadzu, GC-8A) equipped with a molecular sieve 5A column using an argon carrier gas (flow rate is 40 cm3 min-1) at 50 °C. All the electrochemical experiments were implemented under argon atmosphere at 25 °C using an electrochemical analyzer (Hokuto Denko, HZ-3000). Results and Discussion Characterization of Self-Assembled IrO2 Colloid on an ITO Electrode. After an ITO electrode was immersed in the IrO2 solution (0.28 mM, pH 3.5) for 24 h and then washed with water, CV of the treated electrode was measured in a 0.1 M KNO3 aqueous solution at pH 5.3. It gave redox responses at -0.02 and 0.46 V in the potential range of -0.3 to 0.7 V versus SCE (inset of Figure 1), which are assigned to IrIII/IrIV and IrIV/IrV of the IrO2 colloid, respectively.25 This result shows

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Figure 1. Cyclic voltammograms of (a) an IrO2 colloid-coated ITO electrode (Γcov ) 1.0 × 10-8 mol cm-2), (b) an IrO2 colloid-coated FTO electrode (Γcov ) 4.7 × 10-10 mol cm-2), (c) a bare ITO electrode, and (d) a bare FTO electrode, dipped in a 0.1 M KNO3 aqueous solution (pH 5.3) as measured in the potential range of 0.0-1.3 V vs SCE at 20 mV s-1. Inset illustrates the corresponding CV in the potential range of -0.3 to 0.7 V.

that the IrO2 colloid is self-assembled onto the ITO electrode from its aqueous solution. The assembled IrO2 colloid was not desorbed when it was immersed in water in a wide pH range of 1 to 13, demonstrating that the assembly of the IrO2 colloid is basically irreversible. (The ITO substrate is damaged at pH < 1 and pH > 13.) In the extended CV from 0 to 1.3 V, the anodic current increased above 1.0 V concomitantly with the assembly of the IrO2 colloid, and it reached 3.4 mA cm-2 at 1.3 V (Figure 1a), which is higher than that measured at the untreated ITO electrode (Figure 1c) by 3 orders of magnitude. This was due to water oxidation because the O2 gas evolved with the anodic current was detected by a gas chromatograph. This result indicates that the assembled IrO2 colloid works efficiently as an electrocatalyst for water oxidation. The IrO2 colloid was also self-assembled onto the FTO electrode (Figure 1b) when it was immersed in the same IrO2 colloid solution for 24 h. However, the amount of the self-assembled IrO2 colloid on the FTO electrode was significantly lower than that on the ITO electrode (compare the anodic wave at 0.46 V between both electrodes). The ITO surface was more favorable for the IrO2 colloid to be assembled than the FTO surface. This might be due to either the different preparation or material of the conducting oxide substrate, including the possibility that the SnO2-based surface is more favorable than the In2O3-based surface. The amount (Γea (mol cm-2), based on the molar unit) of electroactive IrO2 was calculated from the anodic peak area (excluding the capacitive current) at 0.46 V on the CV data. Figure 2 displays the relationship between Γea and coverage (Γcov (mol cm-2)) of IrO2 on the ITO electrode measured by an ICP-MS technique. A linear relationship was given in the range of Γcov ) 0 to 1.0 × 10-8 mol cm-2, showing that the fraction of the electroactive IrO2 is independent of Γcov and that the average value was given to be 16% of Γcov from the slope.26 The XRD pattern of assembled IrO2 on the ITO substrate did not show any peak for crystalline IrO2 ((110) at 2θ ) 27.7° and (101) at 2θ ) 34.5°),27 indicating that IrO2 is amorphous. The SEM image of the IrO2 colloid-assembled surface showed that it was densely covered with IrO2 colloid particles of ca. 50 to 100 nm size, as shown in Figure 3. This is consistent with its size distribution in the solution provided by the light scattering measurement (vide supra). It was also observed using an AFM technique to evaluate the surface roughness. The AFM

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Figure 5. Imaged illustration of the chemical interaction of the citratestabilized IrO2 colloid with the ITO surface.

Figure 2. Relationship between the amount (Γea) of electroactive IrO2 and the coverage (Γcov) of IrO2 adsorbed on an ITO electrode.

Figure 3. SEM image of IrO2 colloid-assembled ITO electrode.

Figure 4. Dependence of the coverage ((Γcov)eq) of the IrO2 colloid adsorbed on an ITO electrode at equilibrium on the pH of the solution for its assembly. Solid circles indicate that precipitation is partially formed in the solution.

data displayed that the arithmetical mean surface roughness is 3.54 nm, consistent with the dense surface suggested by the SEM measurement. Analysis of Self-Assembly of IrO2 Colloid onto an ITO Surface. Γcov significantly increased with time and saturated at about 24 h as the ITO electrode was immersed in the IrO2 colloid solution at pH 3.5. The saturated Γcov ((Γcov)eq) in adsorption equilibrium increased steeply at pH 3.5 to 4.1, although the IrO2 colloid was hardly adsorbed from the colloid solution at pH 5.3 to 9.7 (Figure 4). This demonstrates that the adsorption of the IrO2 colloid is significantly promoted at pH 3.5 to 4.1. However, it decreased particularly at pH < 3 because precipitation was formed by the aggregated IrO2 colloid. This result

shows that narrow and critical pH conditions (pH 3.5 to 4.1) exist for the assembly of the IrO2 colloid. The pH dependence of (Γcov)eq is ascribable to a character of the citrate stabilizer because (Γcov)eq did not depend on pH at the same conditions when using the IrO2 colloid prepared by other stabilizers such as polyvinyl sulfonic acid and poly(sodium 4-styrenesulfonate) instead of citrate. Moreover, the assembly of the IrO2 colloid could be related to the dissociation of carboxylic groups on the citrate stabilizer since pKa values (pKa ) 3.13, 4.76, and 6.40 corresponding to three carboxylic groups) of citric acid are close to the critical pH conditions. The pH-dependent and irreversible adsorption of the IrO2 colloid suggests a chemical interaction rather than physical interaction between the carboxylic groups and the ITO surface. Meyer et al. reported that [Ru(bpy)2(dcbpy)]2+ (bpy: 2,2′bipyridine and dcbpy: 4,4′-dicarboxy-2,2′-bipyridine) is attached to the ITO surface by ester linkages between carboxylic groups and hydroxyl groups on the surface that are suggested by resonance Raman spectroscopy.19 Zotti et al. reported that ferrocene derivatives functionalized by carboxylic groups are adsorbed on the ITO surface from their acetonitrile or chloroform solutions.20 This was interpreted by electrostatic interaction between carboxylate groups and cationic defect sites of ITO, based on the presence of the carboxylate form suggested from the negatively shifted redox potential of the adsorbed ferrocene derivatives as compared to that in solution. Armstrong et al. reported that ferrocene dicarboxylic acid and 3-thiophene acetic acid are chemically adsorbed on the ITO surface from their ethanol solutions possibly by hydrogen bond formation between carboxylic groups and hydroxyl groups on the ITO surface as well as chelation of the cationic defect site of ITO by carboxylate groups.21 Since the present adsorption of the IrO2 colloid (promoted at pH 3.5 to 4.1) favors a (protonated) carboxylic form rather than a (deprotonated) carboxylate form, interactions involving carboxylate forms are unlikely. Chemical interactions by ester linkages and hydrogen bonds do not conflict with the pH dependence of (Γcov)eq. (Ester bond formation is known to be catalyzed by acidity.) The IrO2 colloid is supposed to be adsorbed by a dual-mode of ester linkage and hydrogen bond between carboxylic groups and hydroxyl groups of the ITO surface, as illustrated in Figure 5. Lateral electrostatic repulsion between IrO2 colloidal particles must be suppressed by partial protonation of carboxylate groups under acidic conditions so that IrO2 colloid particles can be densely assembled on the ITO surface. The synergetic effects subjected to chemical interaction between colloid particles and surface as well as lateral electrostatic repulsion between the colloid particles are a possible explanation of the critical pH dependence (at pH 3.5 to 4.1) for IrO2 colloid adsorption. The adsorption isotherm of the IrO2 colloid onto the ITO surface was examined to reveal the adsorption mechanism. (Γcov)eq increased with the IrO2 concentration (ceq (M)) in the solution at equilibrium and tended to be saturated at high ceq conditions, as shown in Figure 6. (Γcov)eq significantly increased

Self-Assembly of Highly Active Colloidal Catalyst

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Figure 6. Adsorption isotherm of IrO2 colloid onto the ITO surface from the solution at pH 3.5 as measured with the temperature changed. Solid lines are simulated based on the Langmuir adsorption isotherm. Temperatures are (b) 5 °C, (O) 10 °C, (9) 15 °C, and (0) 25 °C.

Figure 7. Plots of 1/(Γcov)eq vs 1/ceq. Solid lines are simulated based on eq 3. Temperatures are (b) 5 °C, (O) 10 °C, (9) 15 °C, and (0) 25 °C.

at the same ceq values as the temperature increases from 5 to 25 °C. The Langmuir adsorption equation (eq 2) assuming a monolayer adsorption mechanism was applied to analyze the adsorption isotherms

(Γcov)eq )

KadsΓmaxceq 1 + Kadsceq

(2)

where Γmax (mol cm-2) and Kads (M-1) are the maximum coverage and adsorption equilibrium constant, respectively. Eq 2 can be transformed to eq 3

1 1 1 1 ) + × (Γcov)eq Γmax KadsΓmax ceq

(3)

The plots of 1/(Γcov)eq versus 1/ceq in Figure 7 gave a straight line for each temperature, meaning that the adsorption can be adequately analyzed using the Langmuir adsorption isotherm. This suggests that the monolayer film of the IrO2 colloid is formed on the ITO surface. It is consistent with chemical interaction between carboxylic groups and the ITO surface suggested by pH-dependent adsorption (vide supra). The intercept and slope of the straight line provided the Γmax and Kads values at each temperature, as summarized in Table 1. The Γmax values (7.1 to 11.2 × 10-9 mol cm-2) were nearly the same under the temperature employed. The Kads values increased from 6.7 × 103 to 1.84 × 104 M-1 with a temperature increase from 5 to 25 °C. The Van’t Hoff plot related to Kads gave a straight line under the temperature employed, as shown in Figure

Figure 8. Van’t Hoff plot of the equilibrium constant (Kads (M-1)) for IrO2 colloid adsorption.

TABLE 1: Summary of Γmax and Kads Values Given in Analysis Based on The Langmuir Adsorption Isotherm temp (°C)

Γmax (10-9 mol cm-2)

Kads (103 M-1)

5 10 15 25

7.1 ((0.4) 9.4 ((1.4) 10.2 ((1.4) 11.2 ((0.6)

6.7 ((0.7) 7.8 ((2.3) 11.9 ((3.8) 18.4 ((3.3)

8. The standard enthalpy change for adsorption of the IrO2 colloid was calculated to be ∆H° ) 36.5 ((3.8) kJ mol-1 from the slope of the line. The corresponding standard Gibbs free energy change (∆G° ) -24.4 ((7.8) kJ mol-1) and the standard entropy change (∆S° ) 204 ((13.3) J K-1 mol-1) were given at 25 °C using ∆G° ) -RT ln Kads and ∆G° ) ∆H° - T∆S°, where R and T are a gas constant and an absolute temperature, respectively. The positive ∆H° value indicates that the adsorption of the IrO2 colloid is an endothermic process. However, it is entropically favored at 25 °C (suggested from the positive ∆S° value). The contribution of the entropy factor to ∆G° exceeds that of the enthalpy factor to result in a negative ∆G° value, which is consistent with the spontaneous assembly of the IrO2 colloid at 25 °C. The IrO2 colloid particles must be solvated by water molecules and electrostatically interacted with counter cations in the solution. The solvating water molecules and counter cations could be rearranged by the attachment of the IrO2 colloid onto the ITO surface and the lateral interaction between the particles on the surface, consequently resulting in a significant dissociation of water molecules and counter cations. The positive ∆H° and ∆S° values can be explained by the dissociation of water molecules and counter cations surrounding the IrO2 colloid that is involved in its assembly, as illustrated in Figure 9. Electrocatalytic Water Oxidation by Self-Assembled IrO2 Colloid. Electrocatalytic water oxidation was conducted under the potentiostatic conditions to evaluate the catalytic activity and stability of the assembled IrO2 colloid on the electrode. A steady anodic current was kept during electrocatalysis at 1.3 V versus Ag/AgCl for 1 h using the IrO2 colloid-assembled ITO electrode, as shown in Figure 10. The anodic current density at 1 h was 0.88 mA cm-2, which is higher than that (0.46 µA cm-2) of a bare ITO electrode by 3 orders of magnitude. The amount (nO2 (mol cm-2 h-1)) of O2 evolved during the electrocatalysis was 7.6 µmol cm-2 h-1. This result shows that the assembled IrO2 colloid works actively and stably as an electrocatalyst for water oxidation. nO2 increased linearly with Γea in the range of 0.60 to 6.8 × 10-10 mol cm-2, as shown in Figure 11. This means that the catalytic activity of the IrO2 colloid does not depend on the amount of the IrO2 colloid adsorbed on the electrode. The turnover frequency (TOF (h-1)) of the IrO2 colloid was provided to be 23 600 ( 560 h-1 from

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Figure 11. Plot of the amount (nO2 (mol cm-2 h-1)) of O2 evolved vs Γea on the IrO2 colloid-assembled ITO electrode for electrochemical water oxidation in a 0.1 M KNO3 aqueous solution (pH 5.3) at 1.3 V vs Ag/AgCl for 1 h.

Figure 9. Imaged illustration of IrO2 colloid adsorption involving desolvation and dissociation of counter cations.

electrode surfaces is well-known. The assembly of the IrO2 colloid was promoted steeply under critical pH conditions (pH 3.5 to 4.1). It turns out to be an endothermic process in contrast to an exothermic process for the general adsorption of small gas molecules (such as N2 and O2) to adsorbents. This could be attributed to the rearrangement of solvating water molecules and counter cations surrounding the IrO2 colloid that is involved in its assembly. The assembled IrO2 colloid worked as an efficient catalyst for electrochemical water oxidation. It is expected to be an anodic electrode for industrial electrochemical processes as well as a catalyst for artificial photosynthetic systems. Acknowledgment. Research was partially supported by a Grant-in-Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology (16750113) and a Grant for Promotion of Niigata University Research Projects. A fellowship grant was provided by The Niigata Engineering Promotion, Inc. (T.K.) References and Notes

Figure 10. Current-time curves during electrochemical water oxidation in a 0.1 M KNO3 aqueous solution (pH 5.3) under potentiostatic conditions at 1.3 V vs Ag/AgCl using (a) an IrO2 colloid-assembled ITO electrode (Γea ) 3.3 × 10-10 mol cm-2) and (b) a bare ITO electrode.

the slope. It was reported that a tri-nuclear ruthenium complex ([(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+) works efficiently for electrocatalytic water oxidation when it is adsorbed on a platinum black electrode, and its TOF was 1500 h-1 at 1.3 V versus Ag/AgCl.28 The TOF of IrO2 electrodeposited on the ITO electrode was reported to be 12 700 to 16 400 h-1 at 1.3 V versus Ag/AgCl.29 The present TOF (23 600 h-1) is the highest value in hitherto reported catalysts for electrochemical water oxidation. Conclusion The IrO2 colloid stabilized by citrate ions was self-assembled on the ITO electrode by chemical interaction between the carboxylic groups of citrate and the ITO surface from the aqueous solution (pH 3.5) to form its monolayer film. This is a rare illustration that colloidal material is self-assembled on the electrode surface by chemical interaction, although the selfassembly of functional molecules with linkage groups on the

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