Photocatalysis and Hydrogen Evolution of Al- and Zn-doped TiO2

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Photocatalysis and Hydrogen Evolution of Al- and Zn-doped TiO2 Nanotubes Fabricated by Atomic Layer Deposition Chung-Yi Su, Li-Chen Wang, Wei-szu Liu, Chih-Chieh Wang, and Tsong-Pyng Perng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12299 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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ACS Applied Materials & Interfaces

Photocatalysis and Hydrogen Evolution of Al- and Zn-doped TiO2 Nanotubes Fabricated by Atomic Layer Deposition Chung-Yi Su 1, Li-Chen Wang 1, Wei-Szu Liu 1, Chih-Chieh Wang 2, and Tsong-Pyng Perng*1 1

Department of Materials Science and Engineering, National Tsing Hua University, 101 Section

2, Kuang-Fu Rd., Hsinchu 30013, Taiwan. 2

Department of Materials Science and Engineering, Feng Chia University, 100 Wenhwa Rd.,

Seatwen, Taichung 40724, Taiwan Keywords: atomic layer deposition, TiO2, photocatalysis, water splitting, hydrogen, electron paramagnetic resonance

ABSTRACT

Highly homogeneous Al- and Zn-doped TiO2 nanotubes were fabricated by atomic layer deposition (ALD) via nanolaminated stacks of binary layers of TiO2/Al2O3 and TiO2/ZnO, respectively. The bilayers were alternately deposited on the polycarbonate (PC) membrane template by ALD with various cyclic sequences. The nanotubes in a length of 20 µm and a diameter of 220 nm were obtained after removal of the PC membrane by annealing at 450 oC.

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The effects of doping composition on the photocatalytic and photoelectrochemical (PEC) activities were investigated. Increasing the Al doping reduced the photocatalytic activity of TiO2 due to formation of charge recombination sites and reduction of hydroxide radicals. In contrast, there was an optimal range of Zn doping to get enhanced photocatalytic activity and higher PEC efficiency. With a doping ratio of 0.01, the hydrogen production rate from water splitting was 6 times higher than that of commercial P25 TiO2. The energy-band diagram of Zn-doped TiO2 determined by ultraviolet photoelectron spectroscopy revealed shift up of the Fermi level to provide more electrons to the conduction band. The photoinduced trapped electrons and holes were detected in Zn-doped TiO2 by in-situ electron paramagnetic resonance spectroscopy, which revealed that Ti3+ sites on the surface and surface oxygen vacancies played a key role in promoting the photocatalytic process.

INTRODUCTION Utilization of semiconductor materials for hydrogen generation from photocatalytic water splitting has attracted much attention since hydrogen is considered as a clean and renewable energy. Among the photo-responsive materials, TiO2 is regarded as a promising material with versatile applications, e.g., optical devices, photocatalysis, and dye sensitized solar cell, because of its relatively low cost, nontoxicity, good chemical stability, and high photo-corrosion resistance.1 However, it is well-known that the application of TiO2 as the photocatalyst is strongly influenced by the transport and recombination rate of photo-generated electrons and holes.1,2 With a wide band gap (Eg = 3.2 eV), it is photoactive only in the UV range, which limits the use of TiO2. In order to address these problems, several strategies have been adopted to improve the photocatalytic activity, including energy band modulation by doping some anions or transition metal ions that have the unique d electronic configuration.3-6 In general, introducing


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doping ions would increase the formation of a large amount of surface oxygen vacancies (SOVs) which are beneficial for improvement of photochemical and photophysical properties. It also extends the photo-response to visible light by formation of an alternative energy level within the band gap of TiO2.5-8 Various techniques have been used to prepare metal ion-doped TiO2, such as physical mixing, vapor-phase doping, and solution based synthesis.2-5 The major problem of those methods is that they provide little or no control over the distribution of doping elements, and it is not easy to get conformal morphology of desired complex geometry. Additionally, two separate phases are easily formed during the synthesis process to reduce incorporation of metal ions into the TiO2 lattice. Recently, there is a growing attention in fabricating nanostructured semiconductors for photocatalysis, including nanosheet, nanotube, and nanorod, because their special geometries provide large surface area, small lateral diffusion resistance, and low reflectivity compared to their bulk phase.3,5,9 A large variety of fabrication routes for TiO2 nanotubes have been proposed, such as hydrothermal, anodization, and template methods.9-15 Among these methods, the template method is a simple and facile way to produce nanotubes with high aspect ratio in a large quantity. Additionally, it is easy to decompose or dissolve the template by solvent or post annealing. ALD has been widely applied to prepare nanostructured materials by using various templates, such as biological nanofibrillar aerogel,10 carbon nanotubes,11 and ZnO nanowires.12 The layer-by-layer deposition feature of ALD allows straightforward fabrication of artificial nanolaminates by varying the cycle ratio of the binary constituents. Previously, we reported the preparation of smooth and conformal alumina nanotubes by using a low temperature ALD process combined with tris-(8-hydroxyquinoline) gallium (GaQ3) nanowire template.13 We have also demonstrated that TiO2 nanotubes fabricated by using GaQ3 nanowire or anodized alumina


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oxide (AAO) as the template had better photocatalytic and PEC properties.14-16 However, doped nanotubes prepared by nanolamination of ALD for phtoelectrochemical reaction and water splitting have been rarely reported so far. Therefore, it is of great interest to fabricate highly efficient doped TiO2 nanotubes with a uniform and continuous composition control. Recently, we have reported the fabrication of Al- and Zn-doped TiO2 by using nanolamination of ALD combined with PC membrane as the template.17,18 This method allows arbitrary control of the pore size and doping level. In this study, the effect of doping ions on photocatalyic activity, PEC efficiency, and water splitting performance of these two doped TiO2 nanotubes were examined in detail. EXPERIMENTAL 1. Fabrication and characterization of Al- and Zn-doped TiO2 nanotubes The fabrication process and characterization of the nanotubes have been presented previously.17,18 Briefly, commercial PC membrane (Millipore GTTP02500) with the pore diameter of 220 nm and the thickness of 20 µm was used as the template. Titanium tetrachloride (TiCl4), diethyl zinc (DEZ, Zn(C2H5)2), and trimethylaluminum (TMA, Al(CH3)3) were used as the precursors of titanium, zinc, and aluminum, respectively, and deionized water was used as the oxygen source in the ALD reaction. The ALD apparatus and technique have also been presented previously.13-16 Because of the layer-by-layer deposition nature of ALD, incorporating Al or Zn into the film was performed by alternating deposition of some Al2O3 or ZnO layers onto the TiO2 layers, i.e., the cycles of TiCl4/ H2O were substituted by specified cycles of TMA/H2O or DEZ/H2O to grow Al2O3/TiO2 or ZnO/TiO2 nanolaminates, respectively. The Al and Zn concentrations or precursor cycle ratios (XA and XZ) in the films were controlled by the fractions of Al2O3 and ZnO cycles in the nanolaminate. A total of 400 ALD cycles were conducted. The


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PC membrane was removed by annealing at 450 oC for 2 h to obtain the Al- and Zn-doped TiO2 nanotubes with the anatase phase. Pure TiO2 nanotubes were also prepared for comparison. Photoluminescence (PL) spectra were taken with a Hitachi F-4500 spetrophotometer equipped with a Xe lamp. To determine the work function (Φ) and band edges of the sample semiconductors, ultraviolet photoelectron spectroscopy (UPS, PHI 5000 Versaprobe II, UlvacPHI Inc.) was performed using He I (21.22 eV) as the excitation source at an applied bias voltage of 5 eV for films fabricated on Si and annealed at 450 oC for 2 h. X-band mode electron paramagnetic resonance (EPR) spectra were recorded in situ on a Brucker EMX spectrometer with a high-sensitivity cavity connected to a 250 W Hg lamp (Moritex, MUV-250U-L) with the major wavelength at 365 nm via an optical fiber. The g values were calibrated by a standard of DPPH (1,1-diphenyl-2-picrylhydrazyl, g =2.0037). The spectra were obtained at 77 K both in dark and under 30 min of UV irradiation. The g values and unresolved EPR signals were determined by using the Brucker WINEPR SimFonia simulation software. 2. Photocatalytic measurement and detection of hydroxyl radicals TiO2 nanotubes with various doping ratios were tested for photocatalytic activity. 1 mg of sample was dispersed in 90 ml aqueous methyl blue (MB) solution at a concentration of 2×10-5 M. The solution was transferred to a cubic quartz cell with a small capped opening at the top. Before irradiation, the cell was continuously stirred for 30 min in the dark to reach adsorption equilibrium between the dye molecules and the sample surface. The cell was then irradiated with light from an Oriel 150 W high-pressure Xe lamp placed at a distance of 15 cm. UV-vis absorption spectra of the samples were taken before and after every 20 min of irradiation for up to 100 min.


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The formation of hydroxyl radicals (OH•) on the surface of photo-irradiated doped and undoped TiO2 nanotubes were detected by the terephthalic acid (TA) fluorescence probe method. The experimental procedure was similar to photocatalytic activity measurement except that the MB dye was replaced by an aqueous solution containing 0.01 M NaOH and 0.5 mM TA. The production of highly fluorescent 2-hyroxyterephthalic acid (TAOH) was monitored by a luminescence spectrofluorometer (Horiba, Fluoromax-4). 3. Photoelectrochemical analysis and hydrogen production measurement The PEC measurement was carried out in a typical three-electrode electrochemical cell under irradiation with an Oriel 150 W high-pressure Xe lamp by a potentiostat (Solartron 1286). The electrolyte was 0.5 M KOH which was bubbled with argon for 30 min before the test. The Pt foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The working electrode was prepared by spin coating the nanotube sample on fluorine-doped tin oxide (FTO) glass with an active area of 1 cm2. Linear sweep voltammagrams were obtained by sweeping from -1.0 V to +0.4 V (vs. SCE) at a rate of 10 mV/ s. The hydrogen production was evaluated at room temperature in a side irradiation quartz cell. 20 mg of nanotube sample was suspended in the cell containing 25 ml of 20 vol % methanol solution. The light source was an Oriel 150 W high-pressure Xe lamp. In order to evaluate the efficiency in visible light region, the light source was also equipped with a filter which allowed only wavelengths higher than 420 nm to pass through. Before illumination, argon gas was purged into the solution and whole quartz cell for 1 h to remove the dissolved oxygen and nitrogen. The amount of evolved H2 was quantified every 1 h by gas chromatography (Shimadzu 2014) equipped with a thermal conductivity detector using a molecular sieve 5A in the column and argon as the carrier gas.


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RESULTS and DISCUSSION 1.

Photocatalytic performance of Al- and Zn-doped TiO2 nanotubes Scanning electron micrographic examination of the doped TiO2 nanotubes reveals that

homogeneous nanotubes with conformal deposition inside the PC pores are formed by a combination of ALD and template methods.17,18 The Al and Zn dopings exhibit no much influence on the morphology of the nanotubes (Figure S1). The surface of Al- and Zn doped TiO2 nanotubes remained smooth and conformal, similar to that of pure TiO2 nanotubes. (Figure S2). The depth profiles of Al and Zn along the thickness direction of laminated films on Si substrate by previous secondary ion mass spectrometric (SIMS) analysis, as shown in Figure S3, reveal continuous wave-shaped Zn distribution and inclined Al profile with a slope toward the growing surface. Therefore, by conducting the nanolamination process of ALD, the initial nonequilibrium multi-layered coating with the individual constituents in the nanoscale would trigger the diffusion of doping ions in the host lattice. Because a large ratio of surface-to-volume free energy is achieved in the structure of nanolaminates, interface equilibrium would be changed and phase transformation would take place to attain the lower energy state with a certain amount of doping. Even though the phase diagrams do not indicate any solubility of Al2O3 and ZnO in TiO2 in those two binary systems,19,20 homogeneous doping concentrations of as high as ~7 at.% for Al and ~8 at.% for Zn in TiO2 with both XA and XZ=0.04, respectively, were obtained. The detailed characterization and mechanism regarding the formation and crystallization of Al- and Zn-doped TiO2 nanotubes have been reported previously.17,18 The photocatalytic activities of all samples prepared by 400 cycles of ALD were examined for degradation of MB, as shown in Figures 1 (a) and (b). For comparison, the activity of undoped TiO2 nanotubes is also included as a reference. The catalytic kinetics of heterogeneous


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photocatalysis is usually described by the Langmuir-Hinshelwood kinetic model with the assumption of pseudo-first-order reaction and relatively low initial concentration,14 ஼

ln ቀ ቁ = −݇௔ ‫ݐ‬ ஼

(1)

బ

where ka is apparent reaction rate constant and t is reaction time (irradiation time). The photocatalytic performance can be quantitatively compared based on the values of ka which can be derived from the plot of ln(C/C0) versus reaction time t. The kinetic data of the MB degradation for all samples are well-fitted by the first-order reaction model (Figure S4). Figures 1 (c) depicts the derived photocatalytic reaction rate constants of the Al- and Zn-doped TiO2 samples as a function of doping level. Under light irradiation, all Al-doped TiO2 nanotubes exhibit lower photocatalytic activities than undoped TiO2, and the activity decreases with the increase of doping level of Al. For Zn-doped TiO2 nanotubes, it is seen that the order of degradation rate constant is 0.01 > 0.02 > 0.005 > undoped > 0.04. Utilization of Zn as a doping ion with a certain concentration seems beneficial to the photocatalytic activity. Generally speaking, the photocatalytic reaction is strongly related to the amount of reactive hydroxyl radicals (OH•) whose production rate is highly dependent on charge separation efficiency of the photocatalyst. The TA fluorescence probe method was then adopted to quantify the photocatalytic capability, since hydroxyl radicals can react with the TA to form TAOH which emits fluorescence at 426 nm on the excitation of 315 nm.21 As shown in Figure 1 (d), the fluorescence emission spectra associated with TAOH were generated by using Zn-doped TiO2 (XZ=0.01) as a photocatalyst under light irradiation, and the fluorescence intensity dramatically increased as the TA solution containing the photocatalyst was continuously exposed to light. It implies that most hydroxyl groups (OH‒) in water can be captured by the photogenerated holes to form hydroxyl radicals. The inset of Figure 1 (d) shows the comparison of fluorescence


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intensities of pure TiO2 and Al- and Zn-doped TiO2 (XA and XZ = 0.01) as a function of light irradiation time. The intensities of Al-doped TiO2 are consistently lower than those of pure TiO2, which agree well with the poorer photocatalytic performance shown in Figure 1 (a). Therefore, reduced amount of hydroxyl radicals by the Al doping in TiO2 may cause decreased activity. Conversely, the intensities of Zn-doped TiO2 are consistently higher than those of pure TiO2, which is also consistent with the results of photodegradation test shown in Figure 1 (b). This indicates that the lifetime of photoinduced charge carriers is indeed enhanced by doping of Zn, thereby promoting the amount of hydroxyl radicals for better photocatalytic performance. The mechanism will be discussed in detail later. To further investigate the influence of doping element and doping ratio on the photocatalytic property, systematic PEC measurements for Al- and Zn-doped TiO2 nanotubes were performed. Figure 2 shows a set of linear sweep voltammagrams upon illumination with a Xe lamp for these nanotubes from -1.0 V to +0.4 V (vs. SCE). Starting from -0.9 V, for undoped TiO2 nanotubes the photocurrent starts to appear and continues to increase to 2.4 mA/cm2 at 0.4 V. In comparison to undoped nanotubes, the Al-doped TiO2 nanotubes showed lower photoresponse, as shown in Figure 2 (a). The reduction of photocurrent density follows the order of Al doping ratio, which is consistent with the catalytic activities for decomposition of MB. For Zn-doped TiO2 nanotubes, as shown in Figure 2 (b), they generally show enhancement in photoresponse, and the photocurrent of the sample with XZ=0.01 exhibits the highest value of 4.2 mA/cm2 at 0.4 V. For XZ=0.04, however, the current density decreases to below that of undoped TiO2. The result is consistent with the order of photocatalytic activities for decomposition of MB as well. Therefore, the electron carrier density may be increased as TiO2 is doped with a small quantity of Zn, but excess doping may generate more defects such as recombination sites for


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photogenerated charges, which would decrease the photocurrent.4,22 The detailed explanation will be given later. In addition, the open circuit potential (OCP) for undoped TiO2 electrode was found to be about -0.8 V., and a slight shift toward negative potential was observed for Zn-doped TiO2. The higher OCP value means that the undoped TiO2 has more recombination sites than Zndoped TiO2, which would result in lower photocurrent density.14,23 Besides, there is no significant saturated photocurrent in all nanotubes at more positive potentials, which indicates efficient charge separation in the nanotubes under light irradiation.24,25 The photoconversion efficiency (η) of a photoelectrode for the conversion of light energy to chemical energy under an applied external potential can be determined according to the following expression: 26 ηሺ%ሻ = ‫ܬ‬௣ [

బ ିหா ாೝ೐ೡ ೌ೛೛ ห

ூబ

] × 100

(2)

where Jp is the photocurrent density in mA/cm2, and Eorev is the standard reversible potential for water splitting, 1.23 V. The intensity of the incident light (Io) in the present study is 50 mW/cm2. Eapp is the applied potential which equals to Emeas-Eocp, where Emeas is the electrode potential (vs. SCE) of the working electrode at which photocurrent is measured under illumination, and Eocp is the open circuit potential (vs. SCE) of the working electrode under the same illumination in the electrolyte. The corresponding photoconversion efficiencies of Al- and Zn-doped TiO2 were derived as a function of applied potential, as shown in Figures 2 (c) and (d), respectively. As expected, the maximum photoconversion efficiency of 4.88% was obtained for Zn-doped TiO2 nanotubes (XZ=0.01), which is improved by a factor of 2.4 in comparison with undoped TiO2 (2.01%). On the contrary, the conversion efficiency of Al-doped TiO2 decreases gradually from 2.01 % to 0.21% as the Al doping level increases to XA=0.04. It is noteworthy that the maximum conversion value is one order of magnitude higher than the reported values for anodized TiO2


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nanotubes and TiO2 nanowires.23,27,28 It is suggested that efficient separation of photogenerated charges could be achieved by a synergistic effect of uniform distribution of Zn doping and one dimensional structure of nanotubes. An additional experiment was conducted to evaluate the activity of Zn-doped TiO2 nanotubes for production of hydrogen by means of water splitting. The experiment was performed in a side irradiation cell, which contained 20 vol % of methanol to serve as a sacrificial hole scavenger. Figure 3 shows the hydrogen production rate of Zn-doped TiO2 with a doping level XZ=0.01. Remarkably, the doped TiO2 nanotubes exhibited six times higher hydrogen production rate than commercially available P25 powder. The specific surface area was estimated by using the Brunauer-Emmett-Teller (BET) method. From the BET analysis, the obtained surface area of the P25 is 38.0 m2g-1, whereas for the Zn-doped TiO2 nanotubes (XZ=0.01) it is 45.3 m2g-1. Therefore, the enhancement of hydrogen production rate can be ascribed to the efficient separation of photo-induced charges and high reactive surface area of the nanostructured catalyst. When the light was filtered to allow only visible and infrared light to reach the sample, the hydrogen production rate of Zn-doped TiO2 nanotubes dropped from 2.66 mmol/g·h to 0.75 µmol/g·h. Even so, the hydrogen production rate is still significantly higher than that of P25 powder. There is almost no hydrogen generated under visible light for P25. The promotion of visible light absorption of the catalyst could be ascribed to the Zn doping that induces the formation of defective surface oxygen vacancies and Ti3+, resulting in the sub-band transitions.7,29 2.

Mechanism for enhanced catalytic activity of TiO2 by Zn doping EPR spectroscopy was carried out to evaluate the effect of Zn doping on photo-generated

electrons and holes in TiO2 nanotubes at 77K, as shown in Figure 4. In principle, the process of


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photocatalytic reaction is triggered by the absorption of photons with the energy larger than the band gap of the material. The electrons are excited to jump from the valance band to conduction band, and electron-hole pairs are generated. The photogenerated electrons and holes can either recombine to dissipate the photon energy as heat or get trapped by surface states, i.e., electron acceptors or electron donors adsorbed on the surface. For the UV irradiated pristine and Zndoped TiO2 (XZ=0.01), there are two sets of g values marked as A (g1= 1.958 and g2= 1.991) and B (g1=2.002, g2=2.012, and g3=2.016), which could be assigned to surface electron trapped site (Ti3+sur) and surface hole trapped site (Ti4+O•‒Ti4+OH‒), respectively.30,31 Accordingly, the positive photogenerated holes can be trapped by surface OH‒ at the subsurface of hydrated lattice oxygen, and the photogenerated electrons may also be stabilized by the titanium atoms on the surface through the following reactions:32 Ti4+[O2‒] latticeTi4+OH‒ + h+ Ti4+[O•‒] latticeTi4+OH‒ Ti4+ + e‒ Ti3+sur

(3)

(4)

Besides, for the Zn-doped TiO2 a fairly weak and broad hole signal represented by signal C appears at 2.034, which is corresponding to O2H• radicals.33 The formation of these radicals is through the reaction of electron-trapped superoxide ions (O2•‒) with protons that are from the hydrated surface: Ti3+sur + O2 Ti4+ + O2•‒

(5)

O2•‒ + H+ O2H•

(6)

A set of g values marked as D (g1=2.005, g2=2.015, and g3=2.026) begin to emerge that are originated from the formation of radical sites (Ti4+O2‒Ti4+O•‒) by trapping the holes into the lattice oxygen ions on the surface of catalyst via the following reaction: 33,34 Ti4+O2‒Ti4+[O2‒]lattice + h+ Ti4+O2‒Ti4+O•‒

(7)


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Note that the intensities of signals A and B for Zn-doped TiO2 are larger than that for pristine TiO2. The presence of signals from Ti3+sur (signal A), Ti4+[O•‒] latticeTi4+OH‒ (signal B), O2H• (signal C), and Ti4+O2‒Ti4+O•‒ (signal D) implies that with the proper amount of Zn doping the separation efficiency of charge carriers is enhanced to generate Ti3+sur and electron- or holetrapped sites which are responsible for the improvement of the photocatalytic activity. They play an essential role in creating highly active radicals for oxidative decomposition of organic pollutants. 29-35 When the doping ratio further increases to 0.04, however, two additional signals denoted as E (g1=1.984 and g2=1.997) appear, which suggest an increase in the concentration of latticetrapped electrons.36 Basically, the interior electron traps usually act as a recombination center for photogenerated electron-hole pairs, which is detrimental to the photocatalytic activity. Moreover, a noticeable signal at g=2.004 labeled with an asterisk is observed, which could be assigned to the oxygen vacancies. It has been reported that the excessive oxygen vacancies also hamper the separation of photoinduced charges, hence the lifetime of electron-hole pairs is reduced.7 The PL spectroscopy was employed to investigate the Zn doping effect on the capability of charge separation, as shown in Figure S5. The efficiency of photo-generated charge carrier trapping and separation is associated with the PL emission from the recombination of carriers. In Figure S5, all the samples show similar PL peaks from 350 to 500 nm, and the main peak at approximately 392 nm is attributed to the emission of band gap transition of TiO2 (i.e., 3.16 eV of band gap energy). In addition, those small peaks in the wavelength range from 420 to 500 nm are due to the excitonic PL caused by defects and surface oxygen vacancies.37 The band-to-band PL intensity follows the order of photocatalytic activity results. The PL intensity for doping ratio XZ=0.04 is the highest one, suggesting that the unwanted recombination center, interstitial


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interior electron traps, or excessive oxygen vacancies to make a non-negligible contribution to lower the charge separation efficiency. 31,33 This again concludes that when a certain amount of Zn ions are incorporated into TiO2 the electron trapping centers would be generated to reduce the recombination rate of electrons and holes. The UPS and UV-vis absorbance spectra were used to investigate the Φ, band edges, and electronic band structure of the pristine and Zn-doped TiO2. The value of Φ was calculated by using the equation Φ = hν – Ecut + Eonset, where hν (21.22 eV) represents the incident photoenergy from He I excitation source, Ecut is the cutoff region of the secondary electron, and Eonset is the onset of the Fermi edge, as shown in Figure 5 (a). The values of Φ of the pristine and Zn-doped TiO2 (Xz=0.01) were estimated to be 4.51 and 4.25 eV, respectively. Furthermore, from the linear intersection in the inset of UPS-valence band (UPS-VB) spectrum (Figure 5 (b)), the distances between the valence band edge (EV) and the Fermi level (EF) for the pristine and Zndoped TiO2 were determined to be 2.72 and 3.02 eV, and their EV were then calculated to be 7.23 and -7.27 V, respectively. The bandgap energy (Eg) of these two samples were obtained from the UV-vis absorption spectrum (Figure 5 (c)), and both of their conduction band level (EC) were estimated to be -4.07 V by using the equation EC = EV + Eg. The same measurement and calculation could also be applied to the Al-doped TiO2, as shown in Figures S6 (a)-(c). Accordingly, as summarized in Table 1 and depicted in Figure 5 (d) and Figure S6 (d), the energy-band diagrams of the pristine, Zn-doped2, and Al-doped TiO2 could be proposed. From the UV-vis absorbance analysis, it is revealed that the Eg increases from 3.16 to 3.20 eV after the Zn-doping (XZ=0.01). This blue shift of Eg by Zn-doping in TiO2 suggests an increase in the concentration of n-type carrier, and most of the Zn ions are incorporated into the crystal structure as interstitial donors. These calculated results are also similar to other studies


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which reported the tuning of the Eg of metal oxides by metal ion doping.38 In addition, it is demonstrated that the amount of donor states in anatase TiO2 increases with the concentration of oxygen vacancies. Therefore, the energy-band diagram shows a substantial shift of the Fermi level of TiO2 toward higher energy by formation of oxygen vacancies. The Zn-doped TiO2 with higher Fermi level could provide more electrons to transfer to conduction band than the pristine TiO2, thus promoting photocatalytic reaction for water splitting.7 With Al-doping (XA=0.01), the Eg increases from 3.16 to 3.22 eV and the energy-band diagram indicates a substantial shift of the Fermi level of TiO2 toward lower energy. The larger bandgap and lower Fermi level than pristine TiO2 could hinder electrons to transfer to conduction band, thus hampering the photocatalytic activity. Based on the above observation, a reasonable conclusion and mechanism can be drawn for the activity enhancement by an optimal doping of Zn to TiO2, as shown in Figure 6. The number of considerable electron trapped states Ti3+ on the surface of catalyst can be raised by Zn doping. The presence of surface Ti3+sur centers formed as the shallow donor levels just below the conduction band allows the chemisorption of oxygen molecules to form superoxide ions (O2•‒) and O2H• groups, both of which are responsible for mineralization or degradation of organic pollutants.39 When the doping level is increased to 0.04, some interstitial interior electron traps may act as unwanted bulk recombination sites, resulting in an opposite contribution to photocatalytic activity and PEC response. Furthermore, when a host of Ti4+ is replaced by Zn2+ ion, the local electrostatic balance is broken. The charge compensating oxygen vacancies would be introduced as the localized donor subbands below the conduction band, which can easily capture or bind photoinduced electrons.7 In the meantime, the trapped holes could be stabilized by surface or lattice oxygen or hydrated subsurface lattice oxygen [O2‒]lattice to form O•‒ species


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in the valence band. Besides, some trapped holes are captured by surface hydroxyl groups OH‒ to produce hydroxyl radicals (OH•). Consequently, those highly reactive trapped holes and electrons could facilitate photocatalysis or water splitting reactions. It is noteworthy to mention that both surface oxygen vacancies and Ti3+ defective states in TiO2 can extend the absorption to visible light region.7,29,36 On the contrary, Al-doped TiO2 showed decreased photocatalytic activities, although a previous study has reported that TiO2 and Al2O3 composite showed improved activity for decomposition of salicylic acid due to more adsorption of acidic species by Al2O3.40 Chu et al. have reported that nanoporous TiO2/Al2O3 film exhibited high photocatalytic activity in decomposing acetaldehyde under UV irradiation, 13 times in reaction rate and 7.8 times in quantum yield higher than those of commercial P25.41 In the present case, the decreased activity by doping Al is ascribed to the AlOx species formed on the TiO2 surface that may prevent adsorption of reactants on TiO2 active sites and to the Al ions distributed in the bulk that may provide more recombination sites within the TiO2 matrix.18 The declined SIMS depth profiles of Al shown in Figure S2 (b) also indicate that Al ions tend to situate on the TiO2 surface. It is in agreement with the report by Gesenhues where the photocatalytic activity was suppressed by the doping of Al2O3 in TiO2 white pigment.42

CONCLUSION Al- and Zn-doped TiO2 nanotubes with tunable atomic-scale wall thickness and doping concentration were fabricated by ALD using polycarbonate membrane as the template. Zn-doped TiO2 nanotubes with optimal doping levels displayed higher photocatalytic activities than pure TiO2, which is consistent with the PEC results. On the other hand, the photocatalytic activities of


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Al-doped TiO2 nanotubes systematically decreased with increasing the Al concentration. Compared with P25 powder, TiO2 nanotubes with XZ=0.01 show 6 times higher hydrogen production rate under irradiation with a 150 W Xe lamp. EPR measurement suggests that Zndoped TiO2 nanotubes contain a certain amount of electron trapped Ti3+ surface states and surface oxygen vacancies. Both of which contribute significantly to visible light absorption and photocatalytic performance. Excess Zn doping would, however, assist formation of interstitial inner electron traps or more excess oxygen vacancies, which are unfavorable charge recombination centers. Al-doped TiO2 exhibited decreased activity owing to the surface AlOx species that may prevent adsorption of reactants and to the Al ions distributed in the bulk that may provide more charge recombination sites. Therefore, application of ALD to fabricate nanolaminted photocatalysts with precise composition control opens a new direction for the design and application of novel TiO2-based semiconductors.


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ASSOCIATED CONTENT Supporting Information Additional SEM figures, ToF-SIMS elemental profiles and the PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Tel: (886)-3-5742634, Fax: (886)-3-5723857 ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of Taiwan under the Contract Nos. NSC 99-2221-E-007-066-MY3 and NSC 102-2120-M-007-007. REFERENCES 1.

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Table 1 Summary of the values obtained from UPS, UPS-VB, and UV-vis spectra. |EV-EF| (eV)

Eg (eV)

EV (V)

-3.10 13.61 4.51

2.72

3.16

-7.23 -4.07

Zn-doped TiO2

-2.95 14.02 4.25

3.02

3.20

-7.27 -4.07

Al-doped TiO2

-2.51 14.08 4.63

2.46

3.22

-7.09 -3.87

Samples

Eonset Ecut (eV) (eV)

pristine TiO2

Φ (eV)

EC (V)


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Figure 1. Photocatalytic decomposition of MB by (a) Al- and (b) Zn-doped TiO2 nanotubes. (c) Photocatalytic reaction rate constants of Al- and Zn-doped TiO2 as a function of precursor cycle ratio. (d) Fluorescence spectra of the light irradiated Zn-doped TiO2 (XZ=0.01) in 0.5 mM TA solution with different lengths of irradiation time. The inset is the time dependence of fluorescence intensity of Al- and Zn-doped TiO2 nanotubes at 426 nm. Both XA and XZ are 0.01..


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Figure 2. Current-potential curves for (a) Al-doped and (b) Zn-doped TiO2 nanotubes by potentiodynamic scan at 10 mV/s under a 150W Xe lamp, and variation of photoconversion efficiency with applied potential for (c) Al-doped and (d) Zn-doped TiO2 photoelectrodes..


27


24 H2 (µ mol /g )

5

20

H2 (mmol /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Zn-doped TiO2

λ > 420 nm

4

P25

3 2 1 0 0

12

1 2 3 4 5 6 Irradiation time (h)

8 4 0

0

1

2

3

4

5

6

Irradiation time (h) Figure 3. Hydrogen evolution from water splitting by Zn-doped TiO2 (XZ=0.01) nanotubes and P25 irradiated with a 150 W Xe lamp without filter. The inset shows hydrogen evolution under visible light irradiation.


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Figure 4. EPR spectra of pristine and Zn-doped TiO2 nanotubes (XZ=0.01 and 0.04) after 30 min of UV illumination at 77 K.


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Figure 5. (a) UPS, (b) UPS-VB, (c) UV-vis absorbance spectra, and (d) proposed energy-band diagram for the pristine and Zn-doped TiO2 (XZ=0.01).


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Figure 6. Proposed mechanism for charge separation and formation of reactive radicals in Zndoped TiO2 nanotubes.


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TOC


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Photocatalysis and Hydrogen Evolution of Al- and Zn-doped TiO2

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