Efficient Photoelectrolysis of Water using TiO2 Nanotube Arrays by

J. Phys. Chem. C 2008, 112, 11007–11012

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Efficient Photoelectrolysis of Water using TiO2 Nanotube Arrays by Minimizing Recombination Losses with Organic Additives S. K. Mohapatra, K. S. Raja, V. K. Mahajan, and M. Misra* Chemical and Metallurgical Engineering, MS 388, UniVersity of NeVada, Reno, NeVada 89557 ReceiVed: October 16, 2007; ReVised Manuscript ReceiVed: May 2, 2008

Electron-hole (e-h) recombination loss is a major practical problem in using TiO2 as a photocatalyst. This paper describes the use of organic additives to reduce e-h recombination losses which significantly improved the photocurrent density of the integrated TiO2 nanotube/Ti photoanode. Studies on photoelectrochemical hydrogen generation using nanotubular arrays of TiO2 photoanodes were carried out in 1 M KOH with the addition of three different organic additives, namely, methanol (one hydroxyl group), ethylene glycol (two hydroxyl groups), and glycerol (three hydroxyl groups) using a simulated solar light. Ethylene glycol was found to be the best among the investigated organic additives to reduce electron hole recombination. The addition of ethylene glycol produced a photocurrent density of 3.3 mA/cm2 at 0.2 VAg/AgCl compared to 0.87 mA/cm2, using a 87 mW/cm2 light intensity in 1 M KOH solution. On the other hand, methanol and glycerol showed a photocurrent density of 2.43 and 2.55 mA/cm2 under the same experimental conditions. A high charge carrier density and reduction of recombination losses were observed in the organically modified electrolytes as compared to that in aqueous basic solution. 1. Introduction TiO2 has been used as a photocatalytic material for splitting of water and oxidation of organic pollutants.1 It has a great role to achieve the hydrogen economy2 by replacing the use the fossil fuels. Photoelectrolysis of water using solar light is one of the promising methods for the production of hydrogen from the sustainable sources like water and sun.3 However, since the band gap of the TiO2 is in the range of 3.0-3.2 eV, only the UV region of the solar light is effective for inducing photo activity. The photo conversion efficiency of TiO2 can be improved by reducing the band gap, decreasing the electron-hole recombination losses, and increasing the charge mobility. A substantial amount of research work has been focused on improving the absorption of light by TiO2 in the visible region by doping with C, N, S, etc.4 Incorporation of carbon and nitrogen in TiO2 has been reported either to reduce the band gap marginally or to introduce mid gap levels. Both cases increase the light absorption in the visible region.4d,5 Even though the visible light absorption of the modified TiO2 results in charge carrier generation, the redox process of the electrolyte proceeds only if the holes possess sufficient energy. When the valence band maximum of the modified TiO2 is above the O2/H2O energy level, hole relaxation and recombination occur that results in lower photo current than that of unmodified TiO2. Therefore, the reduction of the band gap of the TiO2 alone is not a sufficient condition to increase the photo activity. Electron-hole (e-h) recombination loss is another major practical problem in using TiO2 as a photocatalyst. Addition of alcohols such as methanol, propanol and ethylene glycol is reported to help in the reduction of electron-hole recombination losses in rutile and polycrystalline TiO2 nanoparticles and films.6 These alcohols not only help to minimize e-h recombination losses, but also modify the energy levels of the electrolyte compared to the O2/H2O energy level (which enhances the * To whom correspondence should be addressed. Phone: 775 784 1603. E-mail: [email protected].

photoactivity of the TiO2-based photoelectrodes).7 In this study, we have investigated the effect of these organic additives on the photoactivity of a new type (self-assembled nanotubular titania) of photoanode. Currently, TiO2 nanotubular arrays are intensively studied due to the exceptional photoelectrochemical properties they exhibit.8 Good light harvesting properties and high corrosion resistance render titania nanotubes as one of the most stable and promising photoelectrochemical materials available today.9 There are various techniques reported that enhance the photoactivity of the titania nanotube arrays by modifying their semiconducting properties. However, there is no report available on the modification of electrode/electrolyte interface in these types of nanotubular electrodes. In this study, the effect of organic compounds as additives to reduce the e-h recombination losses is discussed. Various hydroxyl organic compounds such as methanol, ethylene glycol, glycerol, sugar, etc. were investigated to achieve the best photoactivity. A comparison of nanotubular photoanodes with thin films is also described. 2. Minimization of Recombination Losses Titania as a semiconductor photocatalyst is mostly used for water/air purification (photo degradation of pollutants) as well as for photocatalytic hydrogen production.10 Both the systems require the photogeneration of hole/electron pairs. However, their use and mechanism are different. In photocatalytic degradation of organics, the holes are key species in the overall activity so addition of electron-sink (e.g., molecular oxygen, metal, etc.)11 help to reduce the recombination losses. On the other hand, in a photoelectrochemical system the electron is key to reduce protons to hydrogen molecules so hole traps (e.g., methanol) help to reduce the electron-hole recombination losses. This discussion will be restricted into the latter system. Figure 1 shows a schematic view of a water splitting mechanism using oxygen annealed TiO2 (O2-TiO2) nanotube arrays as a photoelectrode in the presence of organic additives.

10.1021/jp7100539 CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

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Mohapatra et al. (eq 5) and detrapped to react with the absorbate or electrolyte (eq 6).

SCHEME 1 hVB+ + R f R*

(4)

hVB+ + ss f ss+

(5)

SCHEME 2 +

+

ss + R f ss + R

Figure 1. Schematic illustrating the process of hydrogen generation using oxygen annealed TiO2 (O2-TiO2) nanotube arrays using methanol as an organic additive in 1 M KOH solution. The other parallel reactions are eqs 2 (2h+ + 2H2O f 2O2 + 4H+) and 3 (2e- + 2H+ f H2), where hydrogen also evolves from water and oxygen generates in the photoanode.

O2-TiO2 nanotubes are n-type semiconductors with a band gap (Eg) of 3.0-3.1 eV. Exposure of the nanotubular photoanode to simulated sunlight results in generation of electron (e′) and hole (ho) pairs from the valence band (VB) of the TiO2 nanotube (Equation 1). As the band gap is in the higher range, mostly the UV components of the solar spectrum (which contribute 4-5% of the total spectrum) help to excite the electrons to jump from the VB to the conduction band (CB). The electron from the CB moves to the external circuit and reaches the cathode (Pt). Meanwhile, the hole comes out from the nanotube surface to the electrolytic solution and oxidizes water to generate the proton (H+). In this process, oxygen (O2) is evolved in the anodic compartment of the PEC cell (eq 2). The external potential applied through the potentiostat to the cell helps the proton to reach the cathodic surface, where it combines with the electron to evolve hydrogen (H2, eq 3).

(6)

It should be noted that according to scheme 1, the holes will be at the valence band maximum and energetically more favorable to participate in the oxidation reaction. When the holes are trapped at the intra band, the difference between the energy level of the redox couple in the electrolyte and the hole will be less than that of the scheme 1, and energetically less favorable. In the O2-TiO2 nanotube arrays the photo oxidation reaction presumably occurs through scheme 1. The trapped holes have a negative impact on the efficiency of the photoelectrode in an aqueous electrolyte as they again combine with electrons (i.e., electron-hole recombination losses). In metal oxide semiconductors (due to the large defect density) electron-hole recombination losses are high in alkaline aqueous solution. However, the addition of organic additives to the alkaline aqueous solution helps the separation of the holes and electrons. The adsorption of organic additives on the defect sites and the consumption of the hole by the organic additives help to reduce the recombination losses. An organic additive reacts with the hole in the presence of water to generate protons, which are then potentially driven to the cathode where they get reduced to evolve hydrogen. An example is given below with methanol as an organic additive (eqs 7 and 8).

CH3OH + H2O + 6h+ f CO2gv + 6H+aq (anodic reaction) (7)

TiO2 + hV f (TiO2 · hVB+) + (TiO2 · eCB-)

(1)

6H+ + 6e- f 3H2gv (cathodic reaction)

2h+ + 2H2O f 2O2 + 4H+

(2)

2e- + 2H+ f H2

(3)

Similarly, one mole of ethylene glycol and glycerol should give four and seven moles of hydrogen, respectively, if a complete oxidation of the organic additives takes place. The effect of organic additives on the photoactivity can be explained by the electrochemical Mott-Schottky measurement. This measurement is widely used for semiconductor materials characterization for photoelectrocatalysis.14 The efficiency of a photoanode depends upon many factors, such as the number of incident photons absorbed, potential distribution at the TiO2 electrode, conductivity, and type of defects, etc. Such information can be obtained from the C-V (capacity vs voltage) plots obtained from the impedance measurements. Since the space charge region on a n-type semiconductor electrode is a depletion layer, its capacity is described by the well-known Mott-Schottky equation (eq 9):

The efficiency of a photoelectrode (i.e., solar-to-hydrogen conversion efficiency) is decided mostly by two factors, first, how efficiently it can absorb the photons to generate electron and hole pairs, and second, how efficiently it utilizes the generated photoelectrons and holes. Whereas the first factor mostly depends on the surface area, light absorbance coefficient, band gap, etc., the second factor depends on the defect density present in the material. The defects in a semiconductor material are of two types, i.e., point defects and bulk defects. Irrespective of the morphology and preparation methods, TiO2 is known to contain a large number of defects.12 These defects act as charge carrier traps and recombination centers, which reduce the photoactivity of the material. The goal of this work is reduce the recombination losses by utilizing the holes before they recombine by addition of organic compounds to the electrolyte. Two schemes are proposed for the photo generated holes to participate in the oxidation reaction of the electrolyte. In Scheme 1, the holes could directly oxidize the absorbate, R as given in eq 4. Alternately, depending on the energy level of the defect states, the holes could be trapped at the surface states (ss)13

Csc ) (εrε0Ne ⁄ 2)1⁄2(∆φsc - kT ⁄ e)-1⁄2

(8)

(9)

where Csc is the space charge capacity, ∆φsc the potential drop across the space charge layer (may also be expressed as ∆φsc ) V - Vfb), εr the dielectric constant of TiO2, ε0 the permittivity in vacuum, e is the elementary charge, and N is the donor concentration. All other terms have the usual meanings. Since the space charge capacitance (Csc) is in series with the large

Photoelectrolysis of Water using TiO2 Nanotube Arrays

J. Phys. Chem. C, Vol. 112, No. 29, 2008 11009

electrolytic double layer capacitance (Cdl), the contribution of Cdl on the total measured capacity (C) could be ignored only for large values of φsc (i.e., for large anodic bias). Under these assumptions, a linear relation could be found between the measured values of C-2 and the electrode potential V as expressed under

C-2 ) (2 ⁄ εrε0Ne)(V - Vfb - kT ⁄ e)

(10)

A Mott-Schottky plot (inverse square of space charge layer capacitance versus semiconductor electrode potential, eq 10) gives flatband potential by extrapolating the linear region to intercept the potential axis at 1/C2 ) 0, and doping density is calculated from the slope of the linear region.15 A lower flat band potential and higher charge carrier density are signs of an efficient photoelectrochemical system. The effect of organic additives on these factors will be discussed later in the article. 3. Materials and Methods Chemicals. Phosphoric acid (H3PO4, Sigma-Aldrich, 85% in water), sodium fluoride (NaF, Fischer), ethylene glycol (C2H6O2, Fischer), Ti foil (0.2 mm thick, 99.9%, ESPI-metals, USA), potassium hydroxide (KOH, Fischer, 86%), titanium tetrachloride (TiCl4, Fischer, 99.9%) methanol (CH4O, Sigma-Aldrich), sucrose (C12H22O11), ethanol (C2H6O, Sigma-Aldrich), isobutyl alcohol (C4H9OH, Sigma-Aldrich), and glycerol (C3H8O3, Fischer) were used as received in this study. Preparation of TiO2 Nanotube Arrays by the Anodization Method. Electrochemical anodization of titanium (Ti) was carried out using an aqueous solution containing 0.5 M H3PO4 + 0.14 M NaF (pH 2.1). A polytetrafluoroethylene (PTFE) holder exposing 0.7 cm2 area of Ti to the electrolyte was used as anode and a flag shaped platinum (Pt) electrode (thickness, 1 mm,; area, 3.75 cm2) as a cathode. The distance between the two electrodes was kept at 4.5 cm. An external potential of 20 VDC was applied for 45 min under ultrasonic waves (100 W, 42 kHZ, Branson 2510R-MT) to synthesize the titania nanotubes on the titanium disk. A detailed discussion of the above process with experimental setup was discussed previously.16 The asprepared titania nanotubes are amorphous in nature, and therefore annealed to convert them to crystalline nanotubes. The annealing of the above prepared titania nanotubes was carried out at 500 °C for 6 h under an oxygen atmosphere. For a comparison, a thin layer of TiO2 was prepared on a Ti disk using TiCl4. For this purpose, a few drops of TiCl4 were dispersed over a Ti disk, dried at room temperature, and sintered at 500 °C for 6 h under an oxygen atmosphere. The thickness of the TiO2 film prepared by this method was kept almost similar to the TiO2 nanotubular layer, prepared by the anodization method. Characterization. A field emission scanning electron microscope (FESEM; Hitachi, S-4700) was used to analyze the morphology of the nanotubes. Electronic band gap values of the TiO2 samples were measured from the optical absorption spectra using a UV-vis spectrometer (Model: UV-2401 PC, Shimadzu Corporation, Kyoto, Japan). The Mott-Schottky analysis was carried out by conducting a standard electrochemical impedance spectroscopy at 3000 Hz in 1 M KOH solution by scanning the potential from positive to negative direction in steps of 50 mV/s in both dark (without light illumination) and illuminated conditions (by the simulated solar light using AM 1.5 filter). Flat band potential (UB) and charge carrier density (ND) of the TiO2 nanotubular anode were calculated from the Mott-Schottky plots with and without the addition of organic additives.

Figure 2. FESEM image of O2-TiO2 nanotubular arrays surface. The inset shows the cross sectional view of the nanotubes.

Photoelectrolysis of Water. Experiments on hydrogen generation by photoelectrolysis of water were carried out in a glass cell with separated photoanode (O2-TiO2 specimen) and cathode (Platinum foil) compartments. The compartments were connected by a fine porous glass frit. The reference electrode (Ag/AgCl) was placed closer to the anode using a salt bridge (saturated KCl)-Luggin probe capillary. The cell was provided with a 60 mm diameter quartz window for light incidence. The following electrolytes were investigated 1. 1 M KOH; 2. 1 M KOH + 5 vol % methanol; 3. 1 M KOH + 1-10 vol % ethylene glycol (EG); 4. 1 M KOH + 5 vol % glycerol. In addition to the above organic additives, ethanol, isobutyl alcohol, and sucrose were also tested for this purpose. A computer-controlled potentiostat (Model: SI 1286, Schlumberger, Farnborough, England) was employed to control the potential and record the photocurrent generated. A 300 W solar simulator (Model: 69911, Newport-Oriel Instruments, Stratford, CT, USA) was used as a light source. An AM 1.5 filter was used to obtain one sun intensity, which was illuminated on the photoanode (87 mW/cm2). The samples were anodically polarized at a scan rate of 5 mV/s under illumination and the photocurrent was recorded. 4. Results and Discussion Characterization of the Photoanode. Self-organized and vertically oriented TiO2 nanotube arrays were obtained on a Ti disk (TiO2/Ti) after a sonoelectrochemical anodization method. The TiO2/Ti sample was then annealed under an oxygen atmosphere to make them crystalline. The characterization techniques used to evaluate the physical properties of the titania nanotubes are discussed below. Figure 2 shows the SEM images (surface and cross sectional views) of O2-TiO2 nanotubular arrays. The average diameter of these nanotubes was found to be ∼80 nm and the tube length in the range of ∼600 nm. The wall thickness of the titania nanotubes was found to be in the range of 15-20 nm. It was also observed from Figure 2 that TiO2 nanotubes were one-dimensional, compact (nanotubes were well attached to each other) and vertically oriented (straight). The morphologies of the O2-TiO2 nanotubes were similar to the as-anodized TiO2 nanotubes. X-ray diffraction (figure is not shown here) and TEM studies (figure is not shown here) show the crystallization of the titania nanotubes in anatase and rutile phases.16 DRUV-vis studies (figure is not shown here) show that O2-TiO2 nanotube arrays absorb in the 200-400 nm region with a band gap of 3.1 eV. A detailed description of the material properties was discussed previously.9,16,17 Photoelectrochemical Studies. The above-discussed photoanode (O2-TiO2/Ti) containing self-organized titania nanotube

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Mohapatra et al.

Figure 3. Potentiodynamic plot of O2-TiO2 photoanode in 1 M KOH solution containing (a) without any organic additives, (b) 1% EG, (c) 2% EG, (d) 5% EG and (e) 10% EG as organic additive.

Figure 4. A potentiostatic (I-t) plot of O2-TiO2 photoanode in 1 M KOH + 5% EG under illumination. The photocurrent density came down from 2.75 mA/cm2 to almost zero when the illumination was interrupted. It showed the current observed was due to photocurrent.

TABLE 1: Electrochemical Measurements of O2-TiO2 With and Without the Addition of Ethylene Glycol as Organic Additive conditions

NDa

1 M KOH dark 8.91 × 1017 1M KOH bright 1.92 × 1019 1 M KOH + 5 vol % 6.01 × 1020 EG dark 1 M KOH + 5 vol % 1.54 × 1022 EG bright

OCPc Jp UBb (VAg/AgCl) (VAg/AgCl) (mA/cm2)d -0.725 -0.725 -1.34

-0.04 -0.93 -0.36

0.001 0.87 0.0012

-1.34

-1.38

3.40

a Charge carrier density. b Flat band potential. potential. d Photocurrent density.

c

Open circuit

arrays was used for the photoelectrochemical studies in 1 M KOH solution with and without adding organic additives under the illumination of simulated solar spectrum. Figure 3 shows the results of the photocurrent generated by the O2-TiO2 nanotubes in 1 M KOH solution with variable ethylene glycol concentration. The photocurrent was recorded as a function of applied potential. The potential was increased gradually (5 mV/ s) from the open circuit condition until 0.2 VAg/AgCl. The maximum photocurrent density (Jp) obtained from O2-TiO2 photoanode in 1 M KOH solution was 0.87 mA/cm2. More than 150% increase in the photo current density was observed with the addition of 1 vol % EG (Figure 3). A maximum photocurrent density (3.97 mA/cm2 at 0.2 VAg/AgCl) was obtained using 10% ethylene glycol. The increase in the photocurrent density could be correlated to the photo induced open circuit potential which became more negative with the addition of EG (Table 1) as well as attributed to the generation of more charge carrier density (Table 1). In the case of 1 and 2 vol % EG addition, the photo current reached almost a plateau value at potentials more anodic than -0.6 VAg/AgCl. However, with a further addition of EG, the current profile showed a continuous increase in the photocurrent in the entire range of applied potentials. The OCP was also observed to shift to negative value with an increase in the ethylene glycol amount in the electrolyte solution. The plateau current behavior as well as the increasing current with applied potential indicated that the applied electric field played an influential role in charge separation. Without light illumination, the dark current density remained constant at very low values (∼1.2 µA/cm2) in the 1 M KOH, as well as in the mixed electrolyte solution (figure is not shown here). Therefore, the observed photocurrent was due to the photoactivity of the TiO2 photoanode. Further potentiostatic (current vs time) experiments were carried out to confirm that there was no significant contribution of the dark current to the above photocurrent. Figure

Figure 5. Potentiodynamic plot of O2-TiO2 photoanode in 1 M KOH solution containing (a) without organic additives (b) 5% methanol, (c) 5% glycerol and (d) 5% ethylene glycol as organic additive.

4 shows the photocurrent and dark-current transients obtained with the oxygen annealed titania nanotubular sample in 1 M KOH + 5 vol % EG solution at -0.3 VAg/AgCl. When the illumination was interrupted, the current rapidly dropped to almost zero (1 µA as against 2.75 mA). Upon illumination, the photo current reverted back to the original steady state value within a couple of seconds (>90% of the steady state value was obtained instantaneously). This result indicated that the observed photocurrent density was due to the photoinduced charge separation in the titania nanotubes, and the charge transportation process was very rapid. Furthermore, the experiments with addition of other organic additives were also carried out under the same conditions (Figure 5). Addition of 5 vol % methanol in 1 M KOH solution moved the OCP to -1.02 VAg/AgCl and increased the photocurrent density to 2.43 mA/cm2. On the other hand, addition of 5 vol % of glycerol shifted the OCP to a more negative potential (-1.26 VAg/AgCl) and showed a better photocurrent density (2.55 mA/cm2) compared to methanol. It was found that all three alcohols acted effectively to enhance the photoactivity of the TiO2 nanotubes. Other hydroxyl organic additives, such as ethanol (2.1 mA/cm2), isobutyl alcohol (2.6 mA/cm2) and sucrose (2.25 mA/cm2) showed photocurrent densities lower than ethylene glycol at 0.2 VAg/AgCl. The effect of various organic additives on the reduction of the e-h recombination losses (and thus to enhance the photocurrent density) depends on their redox potential and reactivity in a particular solution.18 For a comparison, a thin film of TiO2 was prepared on a Ti disk and tested for photoelectrolysis of water under similar experimental conditions (5 vol % ethylene glycol in 1 M KOH) as discussed for the O2-TiO2 photoanode. Figure 6 shows the

Photoelectrolysis of Water using TiO2 Nanotube Arrays

J. Phys. Chem. C, Vol. 112, No. 29, 2008 11011 band potential; the conduction band stayed above the H2/H2O energy level, and this facilitated easy electron transfer for hydrogen reduction.19 This also helped the system to generate the same amount of hydrogen by applying lesser external bias. The higher charge carrier density obtained using ethylene glycol additive could be attributed to reduced recombination losses. The results of the Mott-Schottky analyses supported the higher photocurrent density observed with organic additives in the solution. 5. Conclusions

Figure 6. Potentiodynamic plot obtained using the photoanode containing a thin film of TiO2 on Ti (prepared from TiCl4) in 1 M KOH solution containing (a) and 5% ethylene glycol as organic additive (b).

In conclusion, ethylene glycol was found to be a potential organic additive to reduce electron-hole recombination losses and enhance the photoactivity of titania nanotubes. 1-10 vol % of ethylene glycol addition to the 1 M KOH solution was observed to enhance the photocurrent from 0.87 to 3.97 mA/ cm2 (a 4.5 times enhancement). The other hydroxyl organic additives such as methanol, and glycerol were also found to effectively to enhance the photocurrent density of the titania nanotubes. The effect of ethylene glycol addition to the 1 M KOH solution was observed to cause a large negative shift in the open circuit potential, a negative shift in the flat band potential, and an increase in the charge carrier density. This combination of titania nanotube arrays and organic additives in 1 M KOH electrolyte could be useful to increase the overall solar-to-hydrogen efficiency of the photoelectrochemical systems. It is also observed that changing electrode/electrolyte interface chemistry is more substantial in these types of nanotubular photoanodes compared to thin films. Acknowledgment. This work was sponsored by the U.S. Department of Energy through DOE Grant No. DE-FC3606GO86066.

Figure 7. Mott-Schottky plot for n-type O2-TiO2 semiconductor nanotube arrays in 1 M KOH solution containing 5% ethylene glycol at OCP.

photocurrent density obtained with and without the addition of ethylene glycol. It can be seen from the figure that the photoactivity of this thin film photoanode was also enhanced by the addition of the organic additives. However, the activity of this photoanode was found to be lower than the nanotubular O2-TiO2 photoanode (Figures 3 and 5). This might be due to the higher surface and better charge transport properties of 1D nanotubes as compared to the thin film.8g The above results strongly indicated that the organic additives enhanced the photoactivity of the TiO2 photoanode irrespective of the titania morphology. Ethylene glycol was found to be the best among all the organic additives tested. These organic additives helped to reduce recombination losses, and thus enhanced the photoactivity of the material. Electrochemical measurements (Mott-Schottky) were carried out to investigate the role of these organic additives to enhance the photoactivity. A typical Mott-Schottky plot obtained from the O2-TiO2 using 1 M KOH containing 5% ethylene glycol solution with and without illumination is shown in Figure 7. All the plots obtained from the O2-TiO2 nanotubes showed n-type behavior. The photoactivity of the O2-TiO2 photoanode was associated with a two order increase in the charge carrier density when light was illuminated on the nanotubes. This increase in charge carrier density manifested into a large negative shift in the OCP, upon illumination (Figure 5 and Table 1). There was also a large negative shift in the flat band potential observed when organic additives were used (Table 1). The more negative was the flat

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