ARTICLE pubs.acs.org/JPCC
Mechanism of O2 Production from Water Splitting: Nature of Charge Carriers in Nitrogen Doped Nanocrystalline TiO2 Films and Factors Limiting O2 Production Junwang Tang,*,†,§ Alexander J. Cowan,† James R. Durrant,† and David R. Klug† †
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom
bS Supporting Information ABSTRACT: The low efficiency of the extensively investigated visible light photocatalyst N-TiO2 has been widely assumed to be determined by the dynamics of the charge carriers. The nature of the photoelectrons and photoholes produced on the nanostructured (nc) N-TiO2 film has been systematically investigated in this work by the use of time-resolved absorption spectroscopy. Here the fingerprints of the two distinct photohole populations on nc-N-TiO2 films are reported and the reaction between these photoholes and water has been examined. The origin of the low efficiency of the visible-driven material for water oxidation was explored and rapid electron hole decay following visible excitation is believed to be a key factor. Pt deposition on nc-N-TiO2 resulted in an 80% enhancement of the quantum yield for O2 production under UV light. Finally, it has been summarized that the oxygen production on the nc-N-TiO2 film requires photoholes with lifetimes of ∼0.4s.
’ INTRODUCTION The amount of solar energy incident on the earth’s surface in a single hour is in excess of that generated annually by fossil fuel consumption, nuclear power, and other renewable energy technologies.1 However to harness this abundant energy supply, it is necessary to convert and store the solar energy, ideally into a chemical fuel. The photochemical splitting of water can produce hydrogen, which could be utilized directly as a fuel and coupled to other chemical processes such as CO2 reduction to produce carbon-based fuels such as methanol in artificial photosynthesis. Pioneering research performed by Fujishima et al. demonstrated that UV light driven photocatalytic water oxidation can occur on single crystals of TiO2 photoannodes, with hydrogen production occurring at a platinum counter electrode.2 Since this initial report, there has been intense research into developing new efficient semiconductor photocatalysts for the oxidation of water.3 While considerable numbers of new materials have been developed for photochemical water splitting, to date there is not a material that meets the proposed minimum commercial requirements.4 Mechanistic research of current materials offers a route to obtain important design parameters for the next generation of water photo-oxidation catalysts.5 TiO2 has been widely studied as a model system for a range of photocatalytic processes; however, the large band gap of TiO2 limits its photoactivity to the UV region of the solar spectrum. A clear way to enhance the efficiency of TiO2 would be to extend its photoactivity into the visible region of the spectrum. Over the past twenty years, there has been extensive r 2011 American Chemical Society
research into extending the absorption profile of TiO2 into the visible region by doping with main group elements, in particular with nitrogen.6-15 Nitrogen doping is one of the simplest synthetic methods to engineer a visible photocatalyst, and it can narrow the band gap of TiO2 to ∼2.6 eV which consequently gives rise to visible light-driven photocatalytic activity. Asahi et al. first reported the use of N-TiO2 for the decomposition of acetaldehyde and methylene blue under visible light.6 There are two differing viewpoints regarding the electronic structure and origin of visible light absorption of N-TiO2. Asahi proposed a mixed valence band of the N 2p orbital and O 2p orbital, indicating a delocalized N state leading to a rise in the top of the valence band based DFT calculations.6 While Beranek et al. have recently used wavelength-dependent photocurrent and photovoltage measurements to suggest the presence of a strong electronic coupling between the N 2p and O 2p states.16 However, some experimental evidence supporting a localized N 2p state has been reported by several groups,8,9,17 and this is in agreement with further theoretical calculations.18,19 Following the initial reports on N-TiO2, the effects of dopant level, defect density and synthesis method on the activity of N-TiO2 for the photocatalytic decomposition of organic materials have been studied.9,20 However the efficiency of the nitrogen-doped TiO 2 for organic degradation is often observed to be lower than that of undoped TiO 2 . To our knowledge there has Received: August 24, 2010 Revised: December 21, 2010 Published: February 1, 2011 3143
dx.doi.org/10.1021/jp1080093 | J. Phys. Chem. C 2011, 115, 3143–3150
The Journal of Physical Chemistry C also been no reported measurement of O 2 production by photocatalytic water splitting over N-TiO 2 under visible illumination, although incident photon to current efficiency (IPCE) measurements have indicated this may be occurring under visible illumination with a very low efficiency. 8 A systematic and fundamental investigation into the poor activity of this low-cost, nontoxic and visible-light-driven material has implications for both the study of organic degradation reactions and for the photolysis of water under visible irradiation. In this work, we examine the lack of visible light driven water splitting on a nanocrystalline (nc) N-TiO2 by using transient absorption spectroscopy (TAS) to monitor the dynamics of the photoelectrons and photoholes produced under both UV and visible illumination. TAS has been previously used to study nanocrystalline (nc) TiO2. Following UV excitation initial trapping of photoelectrons and photoholes at the surface of the ncTiO2 particles occurs in less than 500 ps.21-23 The trapped charge carriers can then participate in reduction and oxidation chemistry such as water oxidation, oxygen reduction, or organic material oxidation. We have recently reported that the rate of water oxidation by photoholes on nc-TiO2 is very slow at neutral pH’s (∼0.3 s)5 and that electron-hole recombination occurs very rapidly with large losses occurring on the submillisecond time scale, even in positively biased nc-TiO2 films,24 leading to low efficiencies for photocatalytic water oxidation on nc-TiO2. Nakamura et al., investigated the activity of the visible light generated photoholes on N-TiO2 using IPCE measurements and concluded that the photoholes were trapped at N-induced mid gap level from which direct oxidation of organic materials such as methanol could not occur.8 Instead, it was proposed that the observed organic degradation occurred via reactions with either reduced oxygen or surface intermediates from the photo-oxidation of water. Tachikawa et al. have examined the activity of visible light generated photoholes on doped TiO2 powders25 and have examined the activity of N-TiO2 toward ethylene glycol oxidation using TAS.26 TAS spectra recorded in acetonitrile in the presence and absence of ethylene glycol were only slightly different and a lack of change in the photoelectron dynamics led the authors to conclude that although trapped photoholes were generated under visible light these were not able to directly oxidize the ethylene glycol. Instead, it was proposed that the mechanism of ethylene glycol degradation was also via reaction with reduced oxygen or water splitting intermediates, in agreement with the work of Nakamura et al.8 Despite the importance of the potential role of water oxidation in the activity of N-TiO2 for both organic decomposition and solar hydrogen production, the reaction of visible light excited photoholes with water has not been directly investigated yet. In this paper, the charge carriers’ dynamics on the synthesized nc-N-TiO2 films are measured and compared to the undoped nc-TiO2 films in an attempt to explore the nature of the factors limiting the efficiency of photochemical processes. By correlating the activity and lifetime of the observed photoholes to measured O2 production yields over nc-N-TiO2 important conclusions regarding the photocatalytic oxidation of both organic materials and water on nc-N-TiO2 can be drawn. Furthermore, we demonstrate how these mechanistic findings can be used to make a simple material modification that leads to a large enhancement in photocatalytic water oxidation efficiency on nc-N-TiO2.
ARTICLE
’ EXPERIMENTAL SECTION Preparation and Physical Characterization of nc-N-TiO2 Films. nc-N-TiO2 films were made by high-temperature nitridi-
zation of nc-TiO2 films. nc-TiO2 suspensions were prepared by aqueous hydrolysis of titanium isopropoxide as previously described.5,27 All regents were commercial grade and used as received from Sigma-Aldrich or their subsidiaries, unless stated otherwise. Microscope glass slides, used as the substrate for these films, were cleaned by sonication in acetone for 30 min, then in deionized water for 30 min. nc-TiO2 films were prepared by doctor blading the prepared TiO2 paste using “magic tape” (3M) as spacers. Following initial drying in the air for 20 min, the films were sintered at 723 K for 30 min. The resulting films were placed into a tubular furnace and calcined at a range of different temperatures (723-873 K) for 30 min under an ammonia gas flow (BOC, flow rate 60 mL/min,) and then cooled to 673 K under ammonia prior to being cooled to room temperature under a nitrogen gas flow. The crystal structure of the sample was determined by X-ray diffraction (JEOL JDX-3500 Tokyo, Japan). UV-visible diffuse reflectance spectra of the samples were measured with a Shimadzu UV-1601. The morphology of the samples was observed with a Field Emission Scanning Electron Microscope (FE-SEM) operated at 15 kV. For all studies under UV irradiation, the films were made by one layer of precursor paste, which produced a film of ∼4 μm thickness that was measured by a profilometer (Alpha-Step 200, Tencor Instruments). For oxygen production studies under visible light excitation, two layers of precursor paste were used to produce ∼8 μm films. The glass slides were cut into 1.5 2.7 cm2 pieces with a TiO2 coated area of 1 1.5 cm2 in the slide center. Prior to each measurement, all samples were pretreated at 673 K for a further 30 min in air to remove any absorbed organic substances. Time-Resolved Absorption Spectroscopy. Time-resolved absorption spectroscopy (TAS) was utilized to directly observe the dynamics of electrons and holes in doped nc-TiO2 films. Details of the TAS system employed in this work have been previously reported and we include only a brief description here.24 The third harmonic of a Nd:YAG laser (Continuum, Surelite I-10) was employed as the UV excitation source (355 nm, ∼200 μJ/cm2/pulse, typically 1 Hz repetition rate and 6 ns pulse width). An OPO (Surelite OPO PLUS) pumped by the third harmonic of a Nd:YAG laser was utilized as the visible light excitation source (420 nm, ∼318 μJ/cm2/pulse, 1 Hz repetition rate). A 75 W Xe lamp (Hamamatsu) provided the probe light. A liquid light guide was utilized to transmit the laser pulse to the sample. The wavelength of the probe light was controlled by monochromators before and after the sample. The light intensity was measured using a Si-PIN photodiode (Hamamatsu), and these data were transferred for computer analysis in custom built software (LabVIEW, National Instruments). All TAS traces were obtained following averaging of 200-500 pulses. Pt and 2 mM aqueous AgNO3 solutions were employed (separately) to scavenge photoelectrons while ethanol was used to scavenge photoholes. Pt nanoparticles were coated onto the TiO2 films in the manner previously reported.5 A detailed discussion on the choice of chemical scavengers employed can be found in the Supporting Information. TAS studies were carried out in sealed quartz cuvettes and solutions and samples were thoroughly degassed with argon prior to use. Oxygen Production from Water. The measurement of dissolved oxygen has been previously descirbed.5 An oxygen 3144
dx.doi.org/10.1021/jp1080093 |J. Phys. Chem. C 2011, 115, 3143–3150
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Dissolved O2 production measurement following the irradiation of nc-TiO2, nc-N-TiO2, and Pt-nc-N-TiO2 samples in 2 mM AgNO3 (aq) using (a) UV light (355 nm pulsed laser, 1 Hz, 200 μJ cm-2) and (b) visible light (filtered λ > 395 nm 150 W Xe lamp).
membrane polarographic electrode system (Rank Brothers Ltd., Cambridge, UK), which was calibrated using O2-saturated distilled water before each run, was used. Sample films were studied in a temperature controlled chamber (293 K) filled with ∼6.5 mL of 2 mM AgNO3 aqueous solution. For qualitative measurements, an Xe lamp (150 W) was used as the light source and a KG1 short pass filter (Schott, T350-630 nm g 80%) was employed to remove infrared and far UV light and a further long pass filter (395 nm) was used to asses activity under visible light. For quantitative measurements, the third harmonic of a pulsed and frequency-controlled Nd:YAG laser (Continuum, Surelite I-10) operated at 355 nm. The photon flux intensity was measured (1 Hz laser repetition rate, 200 μJ cm-2) by a laser energy meter (meter: model 1918-C, sensor: 818E-10-25L, Newport).
’ RESULTS The nc-N-TiO2 films were synthesized by thermal treatment of the prepared nc-TiO2 films under an ammonia flow at a range of temperatures (723-823 K). The doped samples reveal a visible absorption that appears as a shoulder in the UV-vis spectrum extending to 500 nm and the strongest visible absorption can be seen in the sample prepared at 823 K. The increased level of doping at higher temperatures is in good agreement with many previous publications.9,20 Above 823 K, glass substrate deformation occurs, therefore samples prepared at 823 K are used in all further studies (Figure S1, of the Supporting Information). All of the doped samples show identical XRD patterns as the precursor anatase nc-TiO2 films and the morphologies of the nc-N-TiO2 and nc-TiO2 films observed by FE-SEM are similar with an average particle size of 15-20 nm, (Figure S7, of the Supporting Information). Oxygen Evolution under Visible and UV Light. In the absence of a suitable electron scavenger or an applied bias, water oxidation on nc-TiO2 does not occur due to rapid electron-hole recombination.5,23,24 Therefore, in order to assess the activity of the nc-N-TiO2 and nc-TiO2 samples toward photochemical water oxidation we have examined the activity of our samples in the presence of Agþ ions, a widely used, efficient, electron scavenger.28-32 Quantitative measurement for O2 evolution using a controlled photon flux emitted by a pulsed 355 nm laser system (1 Hz, 200 μJ cm-2) have been carried out, Figure 1a. The dissolved O2 concentration is seen to increase for both the doped and undoped samples under UV laser illumination. When
the laser light is turned off, the oxygen concentration stabilizes, demonstrating that the rise in dissolved oxygen concentration is due to photochemical water oxidation and not from leaks from the surrounding atmosphere. Despite having a slightly lower optical absorption at 355 nm (Figure S2, of the Supporting Information), nc-TiO2 has a much higher rate of oxygen production and this is reflected in it is higher quantum yield (QY) for O2 production than nc-N-TiO2 (nc-TiO2 = 16%, nc-N-TiO2 = 5.5%), which was calculated using the procedure described in a previous paper.5 In an attempt to enhance the low quantum yield for oxygen production over nc-N-TiO2, we have examined the effect of the addition of Pt to the surface of the nc-N-TiO2. Pt is known to act as an electron sink lowering electron-hole recombination and its addition results in a ∼80% increase in O2 production under UV irradiation when compared to the unmodified nc-N-TiO2, Figure 1a. The potential mechanistic implications of this significant enhancement are discussed in the latter sections of this work. The potential evolution of O2 over N-doped TiO2 was also measured under visible light irradiation (Xe lamp λ > 395 nm) in 2 mM AgNO3 aqueous solution, Figure 1b. To improve the possibility of observing any visible light driven water oxidation, we prepared films of 8 μm thick nc-N-TiO2 for this measurement. The lack of an intrinsic optical absorption in the visible region means that no noticeable O2 consumption or production occurs over an 8 μm nc-TiO2 film (S3, of the Supporting Information). Irradiation of the nc-N-TiO2 film in AgNO3 solution with visible light actually leads to a lowering of the dissolved oxygen concentration, which is assigned to the reduction of oxygen by photoelectrons, a previously observed phenomena over TiO2 photocatalysts under UV illumination.5 The addition of Pt to the nc-N-TiO2 surface is seen to prevent the previously observed decrease in oxygen concentration; however, no clear increase in dissolved oxygen was detected. Figure 1b clearly demonstrates that although the nc-N-TiO2 film can absorb visible light to create active charge carriers, net production of oxygen through water oxidation is not observed. Transient Absorption Spectrum of Charge Carriers. In order to understand the lack of activity of nc-N-TiO2 for photocatalytic water oxidation under visible light, we have studied the dynamics of the photoelectrons and photoholes using TAS spectroscopy. It is known that in nc-TiO2, the rate of electronhole recombination is strongly dependent on the UV light intensity,5,21 therefore, we have used relatively weak laser excitation 3145
dx.doi.org/10.1021/jp1080093 |J. Phys. Chem. C 2011, 115, 3143–3150
The Journal of Physical Chemistry C
Figure 2. (a) Transient absorption spectra of nc-N-TiO2 following UV excitation (355 nm, 1 Hz 200 μJ cm-2) in the presence of ethanol (circles), Pt (squares), and the absence of a chemical scavenger (triangle), (b) Spectra of Pt treated nc-N-TiO2 and nc-TiO2 following UV excitation (355 nm, 1 Hz 200 μJ cm-2), spectra are normalized by dividing by ΔO.D.max (c) Spectra of nc-N-TiO2 recorded following excitation with visible light (420 nm, 1 Hz, 318 μJ cm-2) in the presence of 2 mM AgNO3 (squares) and the absence of chemical scavengers. The absorption spectra of photoholes are enhanced in the presence of an electron scavenger (Pt or AgNO3) and the spectra of photoelectrons are enhanced in the of a hole scavenger (ethanol). All spectra are recorded 20 μs after the laser flash.
energies (200-320 μJ cm-2) at very low repetition rates (1 Hz) which are more comparable to solar excitation fluxes. Figure 2a shows the transient absorption spectra recorded at 20 μs after the UV excitation (355 nm, 200 μJ cm-2) of nc-N-TiO2 in the presence of both hole (ethanol) and electron (Pt) scavengers. The oxidation of ethanol on the undoped nc-TiO2 following UV excitation occurs with a lifetime of 1 ns with a very high efficiency and the resultant TAS spectrum is assigned to photoelectrons.22 The TAS spectrum recorded following the UV excitation of ncN-TiO2 in ethanol shows an increase in absorption with wavelength and is in good agreement with the spectrum of photoelectrons on nc-TiO2, allowing us to assign this spectrum to trapped photoelectrons on nc-N-TiO2. It has been shown on ncTiO2 that trapping of photoelectrons and photoholes occurs within 500 ps and 200 fs, respectively21,23,33 The deposition of Pt on undoped nc-TiO2 leads to the scavenging of photoelectrons and the photohole spectrum on nc-TiO2, which peaks at 460 nm is observed, Figure 2(b).5 The photoholes on the platinized ncN-TiO2 film exhibit a broad absorption from 450 to 600 nm (Figure 2(a)), which is significantly different from the undoped nc-TiO2, Figure 2(b).34 It is apparent that there is a significant shoulder, peaking at 550 nm, in the absorption spectrum of UV excited nc-N-TiO2. This can be assigned to either (i) the presence of a new form of trapped holes leading to a superposition of two TAS hole signals or (ii) a broadening of a single hole population on nc-N-TiO2.
ARTICLE
The transient absorption spectroscopy data under UV excitation suggest there may be two distinct photohole populations present on the nc-N-TiO2 films. One type of hole would be the same as those formed on nc-TiO2, whereas the second hole type would be distinct to the doped sample and would be solely formed following visible light excitation. The TAS spectrum of ncN-TiO2 following visible (420 nm) laser excitation in the presence of an efficient electron scavenger (Agþ) is shown in Figure 2(c). The absorption peak is observed at ∼550 nm. A superposition of the TAS spectrum of the photoholes on UV excited nc-TiO2 and of visible excited nc-N-TiO2 approximately reproduces the photohole spectra observed following the UV excitation of nc-N-TiO2. However, the limited resolution of this data prevents definitive assignment. A more definitive assignment of the TAS spectrum in Figure 2(c) to the formation of holes at trap states introduced by nitrogen doping is achieved in the following section by analysis of the recombination kinetics of the photoholes on nc-N-TiO2. This is in agreement with previous research on nitrogen doped nc-TiO2 powders,26 which reported a red shifting in the broad hole absorption spectrum upon doping, although a distinct second hole population was not reported. In the presence of ethanol, a hole scavenger, no clear TAS signals were observed between 550 and 800 nm on nc-N-TiO2 following visible light excitation, confirming our assignment of the photohole spectrum. Scavenging of the photoholes would be expected to lead to long-lived photoelectron signals. Visible excitation of N-nc-TiO2 generates a much lower concentration of charge carriers than UV excitation due to the lower absorption of the sample at visible wavelengths; therefore, it is possible that we are unable to detect the potentially weak absorption spectrum of the visible light generated photoelectrons. The lack of a TAS signal for the visible light generated phototelectrons is discussed fully in the following sections. Dynamics of Electron-hole Recombination. When experiments are carried out under argon in the absence of any chemical scavengers, the transient absorption decays on both doped and undoped TiO2 are attributed to electron-hole recombination. The decay of the signal at 465 nm on nc-N-TiO2 following UV excitation (355 nm, 200 μJ cm-2) is clearly slower (t50% = 220 μs)35 than that on undoped nc-TiO2 (t50% = 40 μs), Figure 3. This difference in the hole lifetime is further investigated by comparing the TAS decay traces at wavelengths where only the photoholes absorb (420-460 nm) to 800 nm, a wavelength where trapped photoelectrons absorb, Figure 4. It should be noted that in the absence of chemical scavengers, the TAS signal at 800 nm may have a contribution from the tail of the photohole absorption; however, by measuring the differences between these two spectral regions we can evaluate the differences in the rates of decays of photoelectrons and photoholes. On nc-TiO2, the transient absorption signal decay is independent of probe wavelength, indicating that the charge carrier populations probed are directly recombining with each other. We also observe an independence of the rate of decay of the TAS traces on the wavelength studied for nc-TiO2 samples that have been thermally treated to 823 K in the absence of ammonia (Figure S6, of the Supporting Information). On nc-N-TiO2, the TAS decay at shorter wavelengths (420-460 nm), is slower than that measured at 800 nm, and the decay rate in this region varies with the wavelength probed, Figure 4. The TAS spectrum of the photoholes on UV excited nc-NTiO2 indicated that either (i) two distinct hole populations can 3146
dx.doi.org/10.1021/jp1080093 |J. Phys. Chem. C 2011, 115, 3143–3150
The Journal of Physical Chemistry C
Figure 3. TAS decay traces recorded following the UV excitation (200 μJ cm-2, 355 nm, 1 Hz) of nc-N-TiO2 and nc-TiO2 observed at 465 nm under argon in the absence of chemical scavengers.
ARTICLE
Figure 5. TAS decay traces recorded at 550 nm on nc-N-TiO2 following visible light excitation (420 nm, 318 μJ cm-2, 1 Hz) in the presence and absence of a hole scavenger (ethanol), (420 nm, 318 μJ cm-2, 1 Hz). The transient absorption signal at 550 nm is quenched by ethanol (a hole scavenger) in
