Tuning the Fermi Level and the Kinetics of Surface States of TiO2

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Tuning the Fermi Level and the Kinetics of Surface States of TiO2 Nanorods by Means of Ammonia Treatments. Cristian Fabrega, Damián Monllor-Satoca, Santiago Ampudia, Andres Parra, Teresa Andreu, and Joan Ramon Morante J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp407167z • Publication Date (Web): 06 Sep 2013 Downloaded from http://pubs.acs.org on September 14, 2013

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The Journal of Physical Chemistry

Tuning the Fermi Level and the Kinetics of Surface States of TiO2 Nanorods by Means of Ammonia Treatments. Cristian Fàbrega,1,* Damián Monllor-Satoca,1 Santiago Ampudia,1 Andrés Parra,1 Teresa Andreu1 and Joan Ramón Morante1,2. 1

Catalonia Institute for Energy Research (IREC), Advanced Materials for Energy Department,

Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain. 2

Departament d’Electrònica, Universitat de Barcelona, Martí i 45 Franquès 1, 08028 Barcelona,

Spain. AUTHOR INFORMATION Corresponding Author *Cristian Fàbrega, e-mail: [email protected], Telf: +34933562615

ACS Paragon Plus Environment

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ABSTRACT

Ammonia-induced reduction treatment of titanium dioxide rutile nanorods has been performed, where the treatment triggered a synergistic surface modification of titania electrodes that enhanced its overall photoelectrochemical performance, besides introducing a new absorption band in the 420-480 nm range. A physical model has been proposed to reveal the role of each fundamental interfacial property on the observed behavior. On the one hand, by tuning the Fermi level position, charge separation was optimized by adjusting the depletion region width to maximize the potential drop inside titanium dioxide and also filling the surface states, which in turn decreased electron-hole recombination. On the other hand, by increasing the density of surface holes traps (identified as surface hydroxyl groups) the average hole lifetime was extended, depicting a more efficient hole transfer to electrolyte species. The proposed model could serve as a rationale for controlled interfacial adjustment of nanostructured photoelectrodes tailoring them for the required application.

KEYWORDS Hydrogen; depletion region; Fermi level; titanium dioxide; surface states.


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INTRODUCTION

Over the last decades, photocatalysts have aroused a growing interest because of their ability to harvest light and subsequently induce chemical reactions on the interface of the material surface and the surrounding homogeneous phase.1, 2 Given the nature of a photocatalytic reaction, the catalyst surface properties become crucial to improve the overall catalytic features. In addition, it is equally important to improve the material morphology and electronic structure for both effective photoinduced charge separation and transportation.3, 4 Inasmuch as the use of nanosized materials has captured all the attention in this field, this latter strategy has undergone lesser relevance since nanoparticles behave very differently from their macroscopic analogues.5-7 However, the increasing capability of researchers to create higher quality nanosized materials with excellent crystallographic properties8,

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has awaken again the significance to govern and

control materials properties such as charge carrier density, space charge layer width, and Fermi level position, among others.3, 10 Among the diverse applications of the photocatalytic materials, photoelectrochemical water splitting11,

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has recently undergone special awareness by virtue of the actual and future

energetic juncture. Titanium dioxide is considered the material par excellence in water splitting applications13 despite its light absorption limitation caused by its large band gap (3.0 eV). Many attempts have been tried in order to circumvent such limitation and improve its overall photocatalytic efficiency.13 Among them, tailoring the interface through the control of the electrode nanostructure has shown promising results.8 In this regard, single crystalline TiO2 nanorods have been successfully grown on FTO substrates through a simple hydrothermal route.14 Since then, the number of publications based on this material that attempt to improve its


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photocurrent has increased, reaching values very next to the theoretical efficiency values for TiO2.15 However, the grounds of the photoactivity improvement of TiO2 nanorods have not been fully elucidated nor profoundly treated. Other approximations such as doping with transition metals have been explored with relative success.16, 17 On the other hand, doping with non-metals, particularly nitrogen, has been shown to be the best option for the improvement of titanium dioxide efficiency. Notwithstanding the large efforts that have been spent in studying this topic, the origin of such enhancement is still in question. While some authors argued that this improvement lies on the bandgap narrowing caused by substitution of nitrogen on oxygen sites,18 others consider that vacancy formation inherent to nitrogen doping19 is responsible for such activity.20 In this work, we present an ammonia-induced reduction treatment of titanium dioxide rutile nanorods that triggered a surface modification of titania electrodes. A model, based on fundamental physical parameters (e.g. donor density, Fermi level position, depletion width and surface states) is proposed to disclose the relevance of each interfacial property on the observed photoefficiency modification. This model provides a logic that can be used for designing a more rational and tailored material modification that can be also extended to other oxides.

EXPERIMENTAL METHODS

Materials preparation Titanium dioxide rutile nanorods were grown by a synthetic route recently published.8 60 mL of 6 M HCl (37% Panreac) solution and 1 mL of titanium butoxide (Fluka) as titania precursor were introduced in a teflon-lined stainless steel autoclave (125 mL, Parr Instrument Co.). A piece


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of FTO glass (4 x 3 cm2) was placed with the FTO side downwards against the wall, and then the reactor was heated up at 200ºC and kept at this temperature for 4 hours. A controlled cooling process to room temperature was held by using a water bath. In order to get subsequent electrical access to the FTO, substrates were partially covered with Kapton tape. The as-prepared samples were cleaned by sonication in water, dried under nitrogen stream and finally treated at 450ºC in air in order to remove any residues from the prior synthetic procedure. The sample was finally cut in four small pieces (1 x 3 cm2) in order to ensure that no morphological differences affected our results. One of the pieces was kept as a reference sample and the other were treated at different temperatures (300, 400 and 500ºC) in a pure ammonia atmosphere for 2 hours with a 5ºC/min ramp and a gas flow of 100 mL/min.

Characterization The morphology of TiO2 nanorod arrays was observed with a Zeiss Serie Auriga field emission scanning electron microscope (FESEM). Optical transmission spectra were recorded with a UVvis-NIR Lambda Spectrometer 950 from Perkin Elmer. Structural characterization was carried out by X-ray diffraction (XRD) in a D8 Advance Brucker equipment with a Cu Kα radiation source working at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) measurements were done with a PHI equipment 5500 Multitechnique model with the Al Kα radiation (1486.6 eV). For high resolution XPS a pass energy of 0.1 eV was adopted.21 The potentiodynamic (I–E) behavior was measured with a PARSTAT 2273 (Princeton Applied Research) potentiostat. All electrochemical experiments were performed in a 1 M NaOH electrolyte using a platinum mesh as counter electrode and an Ag/AgCl/KCl (3.5 M) electrode as reference inside a quartz cell and a 150 W Xenon lamp equipped with AM.1.0 and AM.1.5 filters


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were used as light source. A Bentham Tmc – 300 V- U – LS with triple grating and 300 mm focal length were used as monochromatic light source to perform the Incident Photon-to-electron Conversion Efficiency (IPCE) measurements. Electrochemical Impedance Spectroscopy (EIS) was also conducted with the same potentiostat and cell within a frequency range from 10-1 Hz to 106 Hz and an AC voltage below 10 mV. Impedance spectra were acquired with ZPlot and analysed with ZView software. The data was fitted to two ZARC elements connected in series (see S.I)

RESULTS AND DISCUSSION

Uniform and highly crystalline nanorods layers of 200 nm average width and 3 microns length (Figure 1a) were obtained by hydrothermal growth over FTO substrates. After that, a surface post-treatment was held by heating the samples in an ammonia atmosphere at different temperatures (see S.I.).


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Figure 1. Top and cross sectional SEM images of titanium dioxide nanorods (a) and UV-vis transmittance spectra of the ammonia treated samples at 300, 400 and 500ºC together with the non-treated sample (b).

XRD spectra (see SI, Fig. S1) from all four samples showed that no remarkable crystallinity differences were found after the annealing process in ammonia atmosphere. The rutile phase polymorph was obtained with some small traces of anatase and TiO2-SnO2 solid solution, the latter indicating the good quality of the contact between the substrate and TiO2 nanorods. In addition, UV-vis spectra (Figure 1b) showed the expected absorption band from bare TiO2 rutile phase and a new absorption band induced by both the nitridation of the sample and the presence of intra bandgap states.18


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Ammonia treatments on TiO2 not only introduce new visible absorption bands22-24 by the creation of Ti-O-N bonds but also increase the n-doping character thanks to the reductive character of ammonia (i.e. likewise a hydrogen treatment), as it is partially decomposed to hydrogen and nitrogen at high enough temperatures. Mott-Schottky plots were performed in order to approximately determine the number of free carriers (donor density, ND). As standard Mott-Schottky measurements are known to be reliable only in almost ideal semiconductor metal oxides with flat interfaces, no grain boundaries and no surfaces defects, electrochemical impedance spectroscopies (EIS) have been performed (Fig. S2) in order to get the point-by-point Mott-Schottky plots by fitting the obtained Nyquist plots at different applied potentials to an appropriate equivalent circuit (two ZARC’s elements connected in serial). From these MottSchottky plots (Fig. 2), we estimated the donor density from the slope of the linear region.

Figure 2. Mott-Schottky plots represented with the data obtained from the impedance spectroscopy measurements as function of the applied potential versus reference electrode and the corresponding linear fit.

As it is hard to discern the real active area of such a nanostructured electrode, we assigned a donor density of ND = N

1.5 × 1017 cm-3 to the non-treated sample, which is a typical

reported value for undoped rutile.25 Considering that all samples have a similar active surface


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area (as all of them came from the same batch), we can extrapolate the donor density value using the relative Mott-Schottky slopes (eq. 1):

where S

and S

N

S S

N

1

correspond to the slopes of the non-treated and treated samples,

respectively, and N refers to the donor density of the treated sample. In Table I the donor density and the charge carrier diffusion length (Debye length, LD) for each sample are gathered: 2

where ND is the concentrations of ionized electron donor; εr and ε0 are the relative dielectric constant of the semiconductor (100 for rutile TiO2) and the vacuum permittivity (8.85 × 10-12 N1 2

C m-2), respectively; e is the fundamental electric charge (1.602 × 10-19 C); k is the Boltzmann

constant (1.38 × 10-23 J·K-1); and T is the absolute temperature (K). We also calculated the depletion region width (W) for each sample at different applied potentials for further discussion (eq. 3, see SI for calculation details):

2 | | 3 where ≡ # $ #%& is the maximum potential drop in the depletion layer. All ammonia treated samples presented an increasing value of donor density, reaching up to two orders of magnitude larger than the non-treated sample. Very few attempts have been made


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in order to determine donor densities in nanostructured TiO2 photoelectrodes. Among them, Wang et al.15 determined in similar titania nanorods photoelectrodes treated under hydrogen atmosphere a donor density around 1018 cm-3 for pristine TiO2 and 1022 cm-3 for hydrogen treated sample. However, as they pointed out, they didn't consider the roughness of the surface and so, these donor density values are overestimated.

Table 1. Fundamental parameters calculated and obtained from the Mott-Schottky plots: donor density (ND), Debye length (LD) and depletion region width at different applied potentials (W). Efb was estimated as -0.8 V vs. Ag/AgCl.

ND

Slopea

a b

b

(cm-3)

LD

W (nm) at (E-Efb)b

b

(nm)

0.2 V

0.4 V

0.6 V

0.8 V

Bare TiO2

2.81(20) × 106

1.50(11) × 1017

30(2) 121(9) 171(13) 210(15) 242(18)

NH3 - 300ºC

2.55(36) × 105

1.65(23) × 1018

9(1)

36(5)

51(7)

63(9)

73(10)

NH3 - 400ºC

1.61(13) × 105

3.63(29) × 1018

6.2(5)

24(2)

34(3)

42(3)

49(4)

NH3 - 500ºC

1.35(04) × 104

3.12(09) × 1019

2.1(1) 8.4(2)

11.9(3)

14.6(4) 16.8(5)

Standard error from the linear fit of the Mott-Schottky plots. Propagation of error from the linear fit of the Mott-Schottky plots.

Photoelectrochemical measurements (Figure 3a) were conducted to evaluate the performance of the ammonia treated samples. Results showed that ammonia treatments increased the photocurrent density up to two-fold with respect to bare TiO2 (NH3 300ºC). However, at higher temperatures, the effect was reversed, eventually resulting in a lower photocurrent density than the non-treated sample (NH3 500ºC). No significant changes were observed on the photocurrent onset potentials.


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Figure 3. Voltammogram under illumination (a) and Incident Photon-to-electron Conversion Efficiency (IPCE) taken at 0 V vs. Ag/AgCl (b) of the ammonia treated samples at 300, 400 and 500ºC together with the non-treated sample.

IPCE measurements (Figure 3b) revealed that the increasing photocurrent density was mainly due to a more efficient photocurrent response of the UV part of the spectrum, and to a lesser extent in the band gap region also. Again, at 500ºC the tendency was reversed and it started to lose response in the band gap region. Logarithmic representation of the IPCE results (Inset in Figure 3b) showed that, at 300ºC and 400ºC, samples contained some response below the theoretical band gap.


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The origin of this sub band gap photoactivity relies on the visible absorption band aforementioned. However, this absorption band does not necessarily implies further photocatalytic activity as it has been stated.15 In our particular case, IPCE measurements showed that samples treated at 300ºC and 400ºC actually presented photocatalytic activity under subband gap excitation to some extent. In any case, nitrogen doping can induce intra bandgap levels in the form of N2p levels near the valence band that have shown photocatalytic activity.23, 26, 27 The most relevant about IPCE results is the huge improvement on the activity in the UV region. Recently, Wang et al.15 found similar results by treating TiO2 nanorods under hydrogen atmosphere, nonetheless the reasons for such behavior were still not clear. The role of free carriers density (ND) and the absorption coefficient (α) is fundamental to understand and to improve photogenerated carrier collection efficiency. In this regard, Gärtner model28 is a useful starting point despite it is assuming that every hole entering or generated at the depletion layer does not recombine neither at the surface nor in the space charge layer. Some improvements to the basic Gärtner equation were found29 but they are not strictly required for the present discussion and are beyond the scope of this letter. The Gärtner equation can be written in normalized form as (eq. 4): Φ ≡

()* ,- $. 1$ 4 + 1 / .

where Φ is the effective quantum yield, given by the ratio of the photocurrent density jph (A·cm2

) to the incident light flux, I0 (photons·s-1·cm-2); e is the elementary charge (1.602 × 10-19 C); α

is the wavelength-dependent material absorption coefficient (cm-1); and W is the space charge layer width (cm). Gärtner model provides a good enough theoretical basis to partially explain and justify the increasing quantum efficiency (IPCE) by modifying the carrier density and consequently tuning


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the depletion region width (W) in order to efficiently separate electrons and holes. Indeed, from the estimated value of ND for the non-treated sample and the potential at which IPCE data was collected, we determined that the width of the depletion region was around 243 nm. Considering that nanorods mean width is ca. 150-200 nm, then the radius is too small for the potential to fully drop within the semiconductor and thus the assumption of a planar electrode-type potential distribution cannot be considered herein.30 Although in those conditions a nanorod would be fully depleted, the potential difference would be lower and consequently, the electric field.30 Effectively, the maximum potential drop for a flat electrode is given by (eq. 5): 1 12 3 4 5 2

where W is the depletion layer width, LD is the Debye length and θ is a dimensionless variable that is related to the potential drop measured with respect to the center of the semiconductor and is given by 1 ⁄, where k is the Boltzmann constant (1.38 × 10-23 J·K-1) and T is the absolute temperature (K). In the case of a fully depleted region we can use the parallelism of the spherical particle approximation (eq. 6):30 1 89 1 3 4 6 6 where rs is the particle radius. It is then apparent that in fully depleted nanorods the electric field is small and high dopant levels are required to produce a significant band bending inside the semiconductor. While adherence to the Gärtner model is satisfactory for large values of φSC (i.e. large band bending, E - Efb> 0), the model fails when being closer to the flat band potential (E ≈ Efb). Interestingly, this problem is exacerbated as the semiconductor excitation wavelength becomes shorter, since absorption coefficient steeply increases with decreasing wavelengths,31 meaning


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that electron-hole pairs are generated near the surface and so their probability for being trapped by surface states increases. Photocurrent-time transient behaviors of the samples as a function of the potential and incident light wavelength were performed to highlight the aforementioned problem. Figure 4 shows the potential dependence of the ratio between the stationary photocurrent reached after a long enough time of illumination (jss) and the initial photocurrent spike, just obtained at the moment the light is switched on (j0). The origin of such behavior has to be explained in terms of surface electron-hole recombination processes which results in a back reaction (i.e. cathodic electron transfer) that is superimposed to the anodic photocurrent.32 Electrons promoted to the conduction band from the valence band are trapped at intrinsic surface states and/or as photogenerated species (extrinsic surface states) at the semiconductor-electrolyte interface and then recombined with photogenerated holes or chemisorbed molecules such as H2O2, which is known as an intermediate step of O2 evolution.33

Figure 4. Plots of the photocurrent ratios jss/ j0 against potential at different incident light wavelengths (300, 320, 340, 360, 380 and 400 nm) of the ammonia treated samples at 300, 400 and 500ºC together with the non-treated sample.


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For shorter wavelengths, it is easy to appreciate that for ammonia treated samples the ratio jss/j0 reaches its maximum at lower applied potentials, while the non-treated sample needs higher applied potentials for reaching similar ratio values. This is particularly evident for the case of 300 nm illumination. On the contrary, for longer wavelengths the tendency is reversed, and the non-treated sample reach a ratio close to 1 at lower applied potentials. To understand this trend, we propose an effect related to the increasing donor density obtained by ammonia treatments. On the one hand, by decreasing the depletion region we ensure that nanorods are not fully depleted and then the potential drop at the electrolyte interface is higher, which results in an improvement of the separation of the photogenerated electrons from the surface. This is especially important for shorter wavelengths because the absorption coefficient is higher and then photogenerated electrons are formed very close to the surface. For instance, at 300 nm the absorption coefficient of TiO2 is around 106 cm-1, which means that most of the photons are absorbed within the first 10 nm. On the contrary, at longer wavelengths, photons are absorbed deeper in the semiconductor and thus far away from the surface, avoiding any back reaction. On the second hand, by increasing the donor density we are also shifting the Fermi level close to the conduction band and consequently, filling surface states located below the conduction band edge, which act as electron traps. However, increasing the donor density too much would be deleterious because at higher wavelengths, photons are absorbed out from the depletion region. This limitation was clearly proved for the 500ºC treated sample, in which the IPCE was maximum for shorter wavelengths, but for longer the response was almost annihilated. Besides, this behavior under long wavelengths could be understood as an apparent blue shift of the band gap material. At a heavy


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doping level, the Fermi level eventually lies inside the conduction band by a certain quantity (:). Since states below this energy are filled, electronic transitions to states below Eg+: are forbidden; hence, the absorption band edge should shift to higher energies (Burstein-Moss effect).34 Another important feature in the water splitting kinetics that involves photogenerated holes is the frequently reported surface states right above the valence band. While electron kinetics is very important in the overall performance of a photoanode, holes lead the main role in the water splitting process and usually are the overall rate limiting factor.35 As Bahnemann36 and others stated on the basis of flash photolysis and TAS experiments, water oxidation takes nearly 1 s to be completed, which is taken as an indication of long-lived oxidation intermediates. Some models37 have related these surface states to the presence of hydroxyl groups that acts as hole traps to form a new radical (eq. 7): ;9? → ;

Tuning the Fermi Level and the Kinetics of Surface States of TiO2

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