Rutile {111} Faceted TiO2 Film with High Ability for Selective

Article pubs.acs.org/JPCC

Rutile {111} Faceted TiO2 Film with High Ability for Selective Adsorption of Aldehyde Tao Sun,† Yun Wang,*,† Mohammad Al-Mamun,† Haimin Zhang,‡ Porun Liu,† and Huijun Zhao*,†,‡ †

Centre for Clean Environment and Energy, and Griffith School of Environment, Gold Coast Campus, Griffith University, Gold Coast QLD 4222, Australia ‡ Centre for Environmental and Energy Nanomaterials, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China S Supporting Information *

ABSTRACT: Selective adsorption is an important approach to separate organic molecules. In this study, an extraordinary selective adsorption capability of the rutile TiO2 (111) surface toward aldehyde over alcohol and carboxylic acid has been demonstrated on the basis of in situ photoelectrochemical (PEC) measurements. The adsorption strength of benzaldehyde on the rutile (111) surface has been investigated through the analysis of thermodynamic and kinetic properties of photodegradation processes using ex situ PEC measurements. The comparative results with rutile {111} and anatase {101} faceted electrodes demonstrate that there is a strong adsorption of benzaldehyde on the rutile (111) surface. The high ability of the rutile (111) surface for selective adsorption of aldehyde can therefore be utilized as a new approach to separate and purify aldehyde in industry.

1. INTRODUCTION Selective adsorption is an important approach which has been widely used in organic and biological system where high purity separation is required, primarily due to cost-effectiveness.1−7 Aldehydes are one of the important precursors to synthesize acids, polyurethanes for drugs, plasticizers, and detergents.8 As a result, highly selective adsorption is a doable method for separation and purification of aldehydes. Previous experimental and theoretical results have demonstrated that organic molecules can be well adsorbed on the surfaces of titanium dioxides (TiO2).9−12 TiO2 is a good adsorbent because it is reactive, stable, nontoxic, and abundant.13−15 However, few studies focused on selective adsorption phenomena regarding organics on TiO2 surfaces. Specifically, to the best of our knowledge, there is no study regarding selective adsorption of aldehydes on TiO2 surfaces. Therefore, to find a facile method to understand the selectivity of adsorption on the TiO2 surface becomes essentially important. There have been many methods developed to understand the adsorption properties of organics on TiO2 surfaces.9,16,17 However, it is challenging to robustly measure adsorption properties under operational conditions. Recently, a simple, rapid, and effective photoelectrochemical (PEC) measurement method developed in our group has successfully been employed to understand the adsorption properties of organic species on the surface of TiO2 electrodes.18−20 First, TiO2 photocatalysts are immobilized on conductive substrate, such as fluorinedoped tin oxide (FTO), which are excited under ultraviolet (UV) light irradiations to generate electron−hole pairs. These electron−hole pairs dissociate into free photoelectrons in the © XXXX American Chemical Society

conduction band and photoholes in the valence band. The photohole formed at TiO2 is a very powerful oxidizing agent (E0 = +3.1 V versus NHE) that can mineralize almost all organic molecules. In this way, direct oxidation of organic species and/or solvent molecules can take place at the TiO2− liquid interface.21,22 When the applied potential bias (E) is high enough to fully suppress the recombination of photogenerated electron−hole pairs, photocurrent (Iph) can directly represent the rate of the photocatalytic oxidation, which reflects the rate of photohole capture by organics at the interface.19,21,23,24 The intrinsic reaction rates on the TiO2 surface obtained from PEC measurement can therefore be used to investigate the adsorption properties of adsorbates under operational conditions.20 Recently, some rutile TiO2 crystals with exposed special facets have been gaining increasing research interest.25,26 Herein, we demonstrate that the rutile TiO2 (111) surface has high ability for selective adsorption of aldehyde over alcohol and carboxylic acid based on our in situ PEC measurements. The ex situ PEC method is also used to quantitatively analyze the adsorption strength of benzaldehyde on the rutile (111) surface through the analysis of thermodynamic and kinetic properties during the photocatalytic processes. Received: May 6, 2015 Revised: July 13, 2015

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DOI: 10.1021/acs.jpcc.5b04363 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION Fabrication of R111 and A101 Photoanodes. In this study, two types of TiO2 photoanodes with different exposed facets at the liquid/solid interface were fabricated. One is capped by rutile {111} facets (denoted as R111), and another one is mainly exposed by anatase {101} facets (denoted as A101). Herein, the A101 photoanode was used as a reference. The R111 photoanode was synthesized using a facile one-pot hydrothermal method. In the typical synthesis, 0.03 g of titanium nitride (TiN, >95%, Sigma-Aldrich) was added to 27 mL of 6 M hydrochloric acid (HCl, 32%, Sigma-Aldrich) and 3 mL of hydrogen peroxide (H2O2, 30%, Sigma-Aldrich). After magnetic stirring for 60 s, the reaction solution was transferred into a Teflon-lined stainless steel autoclave (80 mL in volume). A piece of cleaned conductive FTO glass (30 mm × 15 mm × 2 mm) was immersed into the reaction solution with the conductive side facing up. The subsequent hydrothermal reaction was carried out at 200 °C for 24 h. After that the autoclave was naturally cooled to room temperature where the FTO conducting glass was then taken out and rinsed thoroughly with deionized water and dried in a nitrogen stream. The resulting material was calcinated in a tube furnace at 450 °C for 2 h in air, with a heating rate of 5 °C/min. The A101 photoanode was fabricated by a dip-coating method. An aqueous TiO2 gel was first synthesized through hydrolysis of titanium butoxide. The resultant colloidal solution contained ca. 60 g/dm3 of TiO2 solid with particle sizes ranging from 8 to 10 nm. Carbowax (30% w/w based on the solid weight of the TiO2 colloid) was then added to increase the porosity of the final dense TiO2 film. The FTO glass was cleaned by deionized water, 2-propanol, and acetone washing solution before it was dip-coated in the TiO2 colloidal solution. To control the thickness of the dense TiO2 film on the support substrate, the FTO glass was lifted out of the colloidal solution at 2 mm/s. Finally, the coated electrode was calcinated at 450 °C for 2 h in a muffle furnace. The thickness of the A101 film was quantified to be ca. 600 nm based on the measurement by a surface profilometer. Characterization. The structural characteristics of the R111 photoanodes and A101 photoanodes were investigated by scanning electron microscopy (SEM, JSM-6300F), transmission electron microscopy (TEM, Philips CM120), and Xray diffraction (XRD, Shimadzu XRD-6000 diffractometer) techniques. Photoelectrochemical Measurements. All PEC measurements were carried out in a three-electrode PEC cell with a quartz window for ultraviolet (UV) illumination. The fabricated TiO2 photoanode was used as a working electrode with a platinum mesh and a saturated Ag/AgCl as a counter electrode and a reference electrode, respectively. The electrolyte was 0.1 M NaNO3 solution with a pH value of 4.0. A voltammograph (CV-27, BAS) was employed for the application of potential bias. The photocurrent signals and potential were recorded using a Maclab 400 interface (AD Instruments). The photocatalyst illuminated area was 0.785 cm2. UV illuminations were carried out by a 150 W xenon arc lamp light with focusing lenses (HF-200W-95, Beijing Optical Instruments). To avoid a temperature change in the electrolyte by the infrared light, a UV-band-pass filter (UG 5, Avotronics Pty. Ltd.) was utilized. The UV light intensity was regulated and carefully measured at 400 nm.

The in situ light-on/-off experiments were conducted using linear sweep voltammetry (LSV) experiments in 0.1 M NaNO3 solution with a given concentration of analytes. The ex situ PEC measurements were performed to analyze thermodynamic and kinetic properties during photoelectrocatalytic oxidation of benzaldehyde by A101 and R111 photoanodes, respectively. Before the ex situ PEC measurements, the photoanode was immersed in the 0.1 M NaNO3 solution with a given concentration of benzaldehyde at pH 4.0. After preadsorption of organic species reached equilibrium, the photoanode was removed, rinsed with a 0.1 M NaNO3 solution, and then immediately transferred into the PEC cell under UV illuminations with the light intensity of 5.0 mW/cm2 for measurements.

3. RESULTS AND DISCUSSION R111 Photoanodes. Rutile TiO2 photoanodes were synthesized using a facile hydrothermal method based on our previous reports.27−29 The scanning electron microscopy (SEM) image of the electrodes in Figure 1a shows the well-

Figure 1. (a) Surface SEM image of the R111 photoanodes, insets showing the high-magnification SEM image (top) and cross-section SEM (bottom). (b) TEM image of an individual rod-like structure, insets showing the SAED pattern (top) and HRTEM image (bottom).

defined pyramid-shaped crystal facets. The angle between the bottom and lateral edged of the pyramid crystal facets is 53.6°, matching the theoretical value between the [110] and [011] directions for a tetragonal rutile TiO2.29,30 The high-resolution transmission electron microscopy (HRTEM) image in Figure 1b demonstrates the fringe spacing of 3.23 and 2.95 Å, respectively, which are in good agreement with the d values of the (110) and (001) planes of rutile TiO2. The SEM and TEM images demonstrate that the exposed facets at the solid−liquid interface are {111}. In Situ PEC Measurements. To do in situ PEC measurements, one key step is to utilize a high applied potential bias (E) to fully suppress the recombination of photogenerated electron−hole pairs. The required potential bias can be determined by LSV experiments. The LSV Iph−E profiles with R111 photoanodes originating from the oxidation of water under different UV and visible light intensities are shown in Figures 2a and 2b, respectively. It can be found that photocurrents increase near to a linear relationship with the potential bias in the low potential region (−0.15 V < E < 0.1 V vs Ag/AgCl). When the bias further increases to higher potential region (E > 0.2 V vs Ag/AgCl), the photocurrent reaches a saturated level, which suggests that the applied potential can completely suppress the recombination of photogenerated electron−hole pairs.18,20,22 For better confidence in the protocol, a +0.40 V potential bias was selected for all subsequent experiments. From Figures 2c and B


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in situ light-on/-off experiment, the change of Iph is determined by the interfacial transport and mass transfer.11,22,34 If the adsorbates are preadsorbed on the surface of photoanodes during the dark (light-off) period, a Iph spike can be observed when the light is turned on due to the photooxidation of preadsorbed organics and/or solvent.18,31,32,34 Because of this occurrence, the peak area should increase with respect to the length of dark period.18 Then, photocurrent will quickly decay back to a steady current (Isph). The Isph is ascribed to the Faradaic photoelectrochemical process caused by oxidation of organic species and/or solvent in the solution,18 which is proportional to their concentration (C).18,27,35,36 In this study, in situ light-on/-off experiments were conducted to understand the adsorption properties of the organics on surfaces of R111 photoanodes.28,37 Given that R111 photoanodes are fully capped by {111} facets at the solid−liquid interface (Figure 1a), the results reflect the adsorption behavior of organic molecules on the rutile TiO2 (111) surface. The Iph−time profile of water in 0.1 M NaNO3 (denoted as blank) by using the R111 electrode is shown in Figure 3b. Comparatively, the Iph−time profile of water by A101 is also illustrated from previous studies regarding the adsorption properties of water and other organics on the surface of A101 photoanodes (Figure S2).18,19,38,39 No photocurrent spike was found with the R111 photoanode when the UV light is turned on (Figure 3b). However, such a spike can be observed with the A101 photoanode. Such difference indicates a weak preadsorption of water on the rutile (111) surfaces and a strong

Figure 2. LSV Iph−E profiles of the R111 photoanode under (a) UV and (b) visible light with different light intensities. Relationship of saturated photocurrent Isaph against different light intensities of (c) UV and (d) visible light at the applied potential bias as +0.4 V.

2d, the linear relationships between light intensities and the saturated photocurrents (Isaph) at the potential bias of +0.4 V are observed, which further confirm that the R111 photoanode can satisfy the requirements of the PEC measurements for this study. The corresponding SEM, TEM, and LSV profiles for the A101 photoanode are given in Figure S1 as a reference. Under a constant applied potential bias, a typical PEC reaction includes three major steps: electron transport, interfacial transport, and mass transfer (Figure 3a).31−33 In an

Figure 3. (a) Scheme of three major steps of a typical PEC oxidation process and a typical Iph−time profile obtained in in situ light-on/-off experiments; in situ Iph−time profiles of (b) water, (c) phenol, (d) benzyl alcohol, (e) benzoic acid, and (f) benzaldehyde with R111 and A101 photoanodes in a solution of 0.1 M NaNO3 at pH 4.0. Black bar represents light-off period, and orange bar is the light-on period. C


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possess the high ability for selective adsorption of aldehydes over aromatics with other functional groups, such as hydroxyl and carboxyl. To validate the selective adsorption capacity of the rutile (111) surface toward aldehydes, in situ light-on and -off experiments for other molecules with the hydroxyl, carboxyl, or formyl group were also conducted with R111 photoanodes. The Iph−time profiles with photocurrent spikes can only be observed for the molecules with formyl groups (Figure 5c),

adsorption on the anatase (101) surface, which matches the theoretical prediction of such an interaction.29 To understand the effect of functional groups on the adsorption properties of organics on the rutile (111) surface, in situ light-on and -off experiments for a range of aromatics were performed by using both A101 and R111 electrodes. Aromatics were chosen as analytes because benzene rings can reduce the small molecule influencers during PEC measurements.21,38 Photocurrent spikes are found for all selected aromatics when A101 electrodes are used (Figure 3c−f). The increase of spike areas with the dark period can also be observed with A101 photoanodes. These Iph−time profiles agree with previous results that the anatase (101) surface has the ability to adsorb most organic molecules with various functional groups.17,40 On the contrary, photocurrent spikes were only found for benzaldehyde when the R111 photoanodes were used (Figure 3f), and its spike areas in the Iph−time profile increase with the dark period. In contrast, no spikes are observed in Iph−time profiles of other aromatics with hydroxyl and carboxyl groups. The lack of spikes can be ascribed to two factors. One is the weak adsorption of analytes. Another one is the inertness of photoanodes in regard to the oxidation of organic species. To understand which factor is the determinative one, we further analyzed the relationship between the Isph and C of aromatics. A linear Isph−C relationship should be found if organics can be oxidized by the photocatalysts.18,27,35 Figure 4 illustrates the

Figure 5. In situ transient Iph−time profiles of different (a) alkanol, (b) carboxylic acid, and (c) aldehyde using R111 photoanodes with the concentration as 1.00 mM in the solution of 0.1 M NaNO3. Black bar represents light-off period, and orange bar is the light-on period.

supporting the selective adsorption of formyl groups on the rutile (111) surface. It can also found that the photocurrent shapes of carboxyl and hydroxyl group organics are similar as that of water, which indicates that there is no preadsorption process of organic species with hydroxyl and carboxyl group on the rutile TiO2 (111) surface. In addition, the benzaldehyde has the obviously photocurrent peak, which suggests that it has the strongest interaction with the rutile TiO2 (111) surface among all the considered organic species. The slope of Isph−C lines of benzaldehyde (Figure 4e) is larger than those of other aromatics, further indicating stronger reactivity of R111 photoanodes toward aldehyde over other aromatics. Ex Situ PEC Measurement. The adsorption strength of aldehyde on the rutile (111) surface can be quantitatively analyzed based on their thermodynamic and kinetic properties during the photodegradation process using the ex situ PEC measurements.18−20 The ex situ PEC method was adopted because the effects of mass transfer of organic species in solution can be removed. Previous studies have demonstrated that the adsorption equilibrium constant (K) and the degradation rate constant (k) are determined by the adsorption strength.41 Thus, these constants can be used to understand the adsorption properties of organic species. The K values can be obtained by fitting the Langmuir adsorption model. The Langmuir adsorption equation can be rearranged as follows:

Figure 4. In situ Iph−time profiles of (a) phenol, (b) benzyl alcohol, (c) benzoic acid, and (d) benzaldehyde with various concentrations using R111 photoanodes in a solution of 0.1 M NaNO3 at pH 4.0; (f) Isph−C relationships of different aromatics.

Iph−time profiles of aromatics with different C with the dark period as 140 s. It can be found that the Isph values increase with the concentration of aromatics. Figure 4e further demonstrates the linear Isph−C relationship for all aromatics. The same linear relationship is also observed with the A101 photoanode (see Figure S2). The linear Isph−C relationships confirm that all aromatics studied here can be fully oxidized by R111 photoanodes. Therefore, the lack of photocurrent spikes in the aromatics with hydroxyl and carboxyl functional groups can only be ascribed to their weak adsorption strength on the rutile TiO2 (111) surface. Therefore, the rutile TiO2 (111) surface

C 1 1 1 = C+ Q Q max Q max K

(1)

Here, Q is the net charge caused by the oxidation of analytes (Q can be calculated by the shaded area in Figure S3a); Qmax is the D


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on the curve-fitting results using eq 2, the values of I0phs, I0phf, ks, and kf for the PEC oxidations of adsorbed benzaldehyde on R111 and A101 were calculated (see Table S2). Using the R111 electrode, the average value of kf is 1.019 s−1 with the deviation less than 0.044 s−1 over the entire range of considered concentrations of benzaldehyde. The corresponding average value of kf with the A101 electrode is 0.415 ± 0.018 s−1, which is 59.3% lower than that with the R111 electrode (see Figure 7). Thus, both thermodynamic and kinetic properties based on

maximum Q, which is determined by the saturated adsorption coverage of adsorbates on surfaces. The fitted parameters with R111 and A101 photoanodes are listed in Table S1. Figure 6

Figure 6. Plot of Q and Q/C versus C of benzaldehyde and data fitting according to the Langmuir adsorption model.

shows the Q−C and C/Q−C relationships of benzaldehyde. The good linear C/Q−C regression (R2 = 0.998) demonstrates that the adsorption of benzaldehyde on the rutile (111) surface follows the Langmuir adsorption model under this concentration range. As a reference, the fitting was also carried out using the A101 electrode. The K value with the R111 electrode is 1.561 × 103 M−1; ca. 13% higher than that with A101 photoanodes (Figure S4). The larger K value demonstrates the higher reactivity of R111 photoanodes toward the adsorption of aldehyde. The degradation rate constants (k) can be attained by curvefitting of the photocurrent decay profiles. Although heterogeneous photocatalytic reactions should be a first-order reaction in the case of only one type of adsorbates,42 experimental results often deviate from a first-order reaction with respect to the concentration of adsorbates, namely due to the existence of more than one type of adsorbates, which itself may exist at different concentrations and adsorption strengths.19,43 Previous studies on the oxidation of oxalic acid by A101 photoanodes via PEC methods have revealed that the overall photocurrent decay profiles can be curve-fitted into a two-exponential expression due to the surface-adsorbed complexes simultaneously undergoing oxidations by two different first-order intrinsic kinetic processes.20 One is a fast process (denoted as f) and related to the strongly adsorbed surface complex; another is a slow process (denoted as s) which caused by the photocatalytic degradation of the intermediately and weakly bound surface complex.19,20,36 These two distinct decay processes represent two different adsorbed processes. Thus, the overall photocurrent decay profiles can be fitted through eq 2 0 −kst 0 −k f t Iph = Iblank + Iphs e + Iphf e

Figure 7. Profiles of fast process rate constants (kf) versus the concentration of benzaldehyde using the R111 and A101 photoanodes.

the ex situ photoelectrocatalytic degradation of benzaldehyde supports that their adsorption strength on the rutile (111) surface is stronger than that on the anatase (101) surface.

4. CONCLUSIONS In summary, an extraordinarily highly selective adsorption behavior of aldehydes over alkanols and carboxylic acids on the rutile TiO2 (111) surface has been demonstrated by our in situ PEC measurements. By fitting the Langmuir adsorption model, it was found that the adsorption equilibrium constant of benzaldehyde on the rutile (111) surface is 13% larger on that on the anatase (101) surface. The analysis of kinetics reveals that photodegradation rate constant of strongly adsorbed benzaldehyde on the anatase (101) surface is 59.3% lower than that on the rutile (111) surface. Therefore, the ex situ PEC measurements support that there is a stronger adsorption on the rutile (111) surface. Such strong adsorption indicates that R111 electrodes can be used to efficiently photodegrade aldehyde for environmental remediation. More importantly, our results supplies a path to separate aldehyde from other organic species through the selective adsorption approach by nanomaterials capped by rutile TiO2 {111} facets.

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(2)

where Iblank is the steady photocurrent corresponding to the 0 0 continuous photocatalytic oxidation of water. Iphs and Iphf present the initial photocurrent generated from slow and fast process, respectively. ks and kf are the rate constants for the fast and slow processes. The kf directly reflects the adsorption strength of strongly adsorbed organic species on the TiO2 surfaces, which can be used to understand the adsorption strength of organic molecules on the surface. In this study, the benzaldehyde was preadsorbed on the surface of electrodes in 0.1 M NaNO3 solution with the concentration (C) ranged from 0.3 to 5.0 mM at pH 4.0. Based

ASSOCIATED CONTENT

S Supporting Information *

Properties characterization and in situ PEC measurement results of A101 photoanodes (Figures S1 and S2), schematic of ex situ Iph−time profile and benzaldehyde experiment results (Figure S3); ex situ PEC measurements results of A101 photoanodes (Figure S4); Langmuir isotherm fitting results based on eq 1 (Table S1); values of I0phs, I0phf,ks, and kf based photocurrent decay fitting by eq 2 (Table S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04363. E


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AUTHOR INFORMATION

Corresponding Authors

*(Y.W.) E-mail yun.wang@griffith.edu.au; Tel +61-755528456; Fax +61-7-55528067. *(H.Z.) E-mail h.zhao@griffith.edu.au; Tel +61-7-55528261; Fax +61-7-55528067. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Research Council (ARC) and the Natural Science Foundation of China (Grant No. 51372248) for funding.

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Rutile {111} Faceted TiO2 Film with High Ability for Selective

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