Energy-Efficient Photodegradation of Azo Dyes with TiO2

Dec 29, 2009 - With typical catalyst (TiO2 P25), the UVLAP of commercial azo dyes (Reactive Brilliant Red K-2G, denoted as K-2G below, Figure S1 of th...
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Environ. Sci. Technol. 2010, 44, 1107–1111

Energy-Efficient Photodegradation of Azo Dyes with TiO2 Nanoparticles Based on Photoisomerization and Alternate UV-Visible Light HAO ZHANG, DA CHEN, XIAOJUN LV, YING WANG, HAIXIN CHANG, AND JINGHONG LI* Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, China

Received September 24, 2009. Revised manuscript received November 21, 2009. Accepted December 9, 2009.

Herein, we demonstrated a UV-vis light alternate photocatalysis (UVLAP) strategy in the photodegradation of azo dyes. The UVLAP of azo dyes over TiO2 catalysts exhibited significantly higher energy efficiency than the conventional UV process by 40%, which was attributed to the photoisomerization of azo dyes and the resulting diversity of dyes’ cis and trans states in interfacial properties, including conductance and spatial effects. This UVLAP strategy could contribute to the energysaving photodegradation of azo dyes and other pollutants with photoisomerization properties and facilitate the practical application of TiO2 in the environmental remediation.

Introduction Azo dyes constitute a considerable portion (ca. 50%) in the overall category of industrial dye-stuffs that represent an increasing environmental danger all over the world (1, 2). Most of them are carcinogenic, nonbiodegradable, and of consequence harmful to human health and ecosystem stability (3). Therefore, the decomposition of azo dyes has drawn great attention (1-3). From the viewpoint of green chemistry, photodegradation process of azo dyes based on semiconductors such as TiO2 is considered as a promising way to conduct complete mineralization of dyes (4). However, a critical drawback for TiO2 catalyst is that it absorbs only UV light, which occupies a small portion (5%) of the solar spectrum (2, 4). Thus, in the practical degradation of dyes, artificial UV light should be used to facilitate an efficient degradation of azo dyes over TiO2, which means a large quantity of energy cost (defined as the cost for UV irradiation from the artificial light source and proportional to the UV irradiation time). One strategy to reduce the energy cost and improve the efficiency in the photocatalysis is based on the modification of TiO2, including phase and morphological control, doping, sensitizations, and semiconductor coupling (5). Upon modification, the photoresponding range is expected to extend to the visible light region, providing the possibility for more efficient utilization of the solar spectrum and reduction of energy cost. However, in these material modification based strategies, problems, such as the cost of time and energy, instability, strong dependence on the preparation methods, * Corresponding author e-mail: [email protected]; phone and fax: +86-10-62795290. 10.1021/es9029123

 2010 American Chemical Society

Published on Web 12/29/2009

and even the introduction of undesirable toxic materials, not only considerably hinder the practical and sustainable applications of these strategies in the photocatalysis (6) but also contradict with the original notion of green chemistry and energy efficiency. Herein, we demonstrated a nonmaterial modification based strategy in the photodegradation of azo dyes, using UV-vis light alternate photocatalysis (UVLAP) rather than conventional UV irradiation photocatalysis. With typical catalyst (TiO2 P25), the UVLAP of commercial azo dyes (Reactive Brilliant Red K-2G, denoted as K-2G below, Figure S1 of the Supporting Information) showed a similar photodegradation rate with the conventional one but a significant reduction in the energy cost (more than 40%). In other words, the degradation efficiency with this new approach was much higher than that of the conventional UV photocatalysis at the same energy cost. This significant energy efficiency could be attributed to the photoisomerization of azo dyes under UV-vis light and the resulting diversity of cis and trans states in the photodegradation rate. Furthermore, the UVLAP strategy does not contradict with the aforementioned material modification based one but could be a powerful supplement. As a result, a proper combination of the two strategies is expected to achieve a desirable rate and remarkable energy efficiency at the same time. This work also provides new insights into the energy-saving attempts in the photodegradation of azo dyes and promotes the practical application of TiO2 in the decontamination of pollutants with photoisomerization properties.

Experimental Section Material and Reagents. TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa. The typical nonbiodegradable azo dye Brilliant Red K-2G (denoted as K-2G below) was purchased from Shanghai Chemical Company. Unless otherwise specified, all other reagents involved were obtained commercially from the Beijing Chemical Reagent Plant (Beijing, China) and used as received without further purification. Fluorine-doped SnO2 (FTO, 15 Ω/ square) glasses were chosen as the electrode substrates. Ultrapure water (resistivity g 18 MΩ cm) was used during the experimental process. The experiments were carried out at room temperature and humidity. Fabrication of TiO2 Films. TiO2 film electrode was used as the photocatalyst in the degradation of azo dyes, and the fabrication of TiO2 film was achieved in the way described in our previous work (7). TiO2 P25 nanoparticles and ethanol were mixed homogeneously (150 mg/mL), and the obtained paste was then spread on the conducting FTO glass substrate with a glass rod using adhesive tapes as spacers. Finally, the resultant TiO2 films with a ca. 4 µm thickness and 1 cm2 active area were calcinated at 450 °C for 2 h to achieve good electronic contact between the TiO2 nanoparticles. UV-vis Absorption Spectra and Electrochemical Impedance Spectra (EIS) Measurements. To study the transition between trans and cis states of azo dyes and the resulting influence in the interfacial properties, the UV-vis absorption spectra and electrochemical impedance spectra (EIS) measurements were conducted. In these measurements, the concentration of K-2G solution is ca. 5 wt %, and the adsorption of K-2G molecules on the surface of TiO2 was accomplished by immersing the TiO2 film in the dye solution for 1 h to achieve adsorption/desorption equilibrium. UV and visible light were used to induce the isomerization of azo dyes (K-2G) in solution or adsorbed on TiO2 surface. For K-2G solution, a high-pressure Hg lamp (100 W) was used VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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as the UV light source, whereas for the K-2G adsorbed TiO2 (denoted as K-2G/TiO2 below), a single-wavelength UV light at around 365 nm with a much lower intensity (12 W) was adopted to avoid the photodegradation of dyes by TiO2. In both measurements, the backward reverse transition (cis to trans) was accomplished under the ambient visible light irradiation, that is exposing the K-2G solution or K-2G/TiO2 dispersion to the indoor natural light in our lab (ca. 50 W/m2, measured by a radiometer, Photoelectric Instrument Factory of Beijing Normal University, China). The UV-vis absorption spectra of K-2G solution and K-2G/TiO2 or blank TiO2 dispersion (ca. 0.1 mg/mL in water) were measured with a UV-vis spectrophotometer (UV 2100, Shimadzu). The EIS measurements were carried out on a PARSTAT 2273 Potentiostat/Galvanostat (Advanced Measurement Technology Inc., USA) by using three-electrode cells. A TiO2 film was immersed in the K-2G solution for 1 h and then rinsed with deionized water and dried in N2. The resultant electrode served as the work electrode, with a platinum wire as the counter and a Ag/AgCl (saturated KCl) electrode as the reference electrode, which were performed in the presence of a 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture as a redox probe in 0.1 M KCl solution. The impedance spectra were recorded with the help of ZPlot/ ZView software under an ac perturbation signal of 5 mV over the frequency range of 1 MHz to 100 MHz. Photocatalytic Measurements. The photodegradation of azo dyes was observed based on the absorption spectroscopic technique. An aqueous dyes solution of K-2G (ca. 5 wt %) and the TiO2 film were placed in a 5 mL quartz glass vessel. After 1 h of immersion to establish the dye adsorption/ desorption equilibrium status, the film was irradiated by UV or visible light on the whole immersed surface of the samples. The photodegradation measurements were conducted under ambient conditions and the temperature was kept at 20-25 °C. A high-pressure Hg lamp (100 W) was used as the UV light source in both conventional UV and UV-vis light alternate photocatalysis (UVLAP) and all other lights were insulated during the UV irradiation. Besides, in the UVLAP, during the visible light (Vis) period, the quartz glass vessel with TiO2 film and dye solution was exposed to ambient indoor natural light in our lab (ca. 50 W/m2) to induce the transition of dyes from cis to trans state. In each hour of photocatalysis, a continual UV irradiation was adopted in the conventional mode, whereas an alternate irradiation of UV and visible light was introduced in the UVLAP (15 min UV-10 min Vis-15 min UV-10 min Vis-10 min UV). The degradation rates of the dyes were determined by monitoring the changes in absorption profile (λ ) 510 nm) at given time interval (1 h) with a UV-vis spectrophotometer (UV 2100, Shimadzu). The photodegradation rate constants k were acquired from the absorption profile changes and presented in two forms, versus time and versus the energy cost, respectively. As the visible light used in this work comes merely from the sun and requires no extra energy cost, the “Energy Cost” mentioned here is defined as the energy demand for the UV light irradiation by the artificial light source, that is the Hg lamp, and is proportional to the actual UV irradiation time. Hence, the UVLAP costs 2/3 of the energy of the conventional UV strategy with the same time period of photocatalysis.

Results and Discussion Isomerizations of Azo Dyes during the Photocatalysis. In the photocatalysis of K-2G over TiO2 film, a continual UV irradiation was adopted in the conventional mode, whereas an alternate irradiation of UV and visible light (Vis) was introduced in the UVLAP process. As is known that azobenzene and its derivatives show unique cis-trans photoisomerization property (8), the conformations of azo dyes 1108

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SCHEME 1. Schematic Illustration of the Photoisomerization of Azo Dyes (K-2G) on TiO2 Surface and the Resultant Influence in the Interaction between the Electrons and Electron Acceptorsa

a (Vis: visible light) In conventional UV photocatalysis, the azo dyes stay in the compact cis state, resulting in the reduction of free volume near the surface. Therefore, the interaction between the photogenerated electrons from TiO2 and electron acceptors such as O2 becomes difficult (right part of the scheme). However, in the UVLAP, due to the visible light irradiation, the dyes come back to the extended trans state and facilitate a much better interaction between electrons and electron acceptors during the photocatalysis (left part of the scheme).

should differ under UV and visible light irradiation during the photocatalysis, as shown in Scheme 1. In the case of conventional UV photocatalysis, the azo dyes were initially in the thermodynamically stable trans state, and trans to cis transition occurred within the first 10 to 15 min. Thereafter, most of the azo dyes stayed in the cis state and were gradually decomposed. Nevertheless, in the case of UVLAP, similar trans to cis transitions took place in each UV irradiation period, whereas the backward reverse was achieved in the following visible light irradiation period. Evidence of the isomerizations of azo dye K-2G in solution and adsorbed on the catalysts’ surface induced by UV-vis irradiation was acquired by UV-vis absorption spectra measurements, as shown in Figure 1. For K-2G solution (ca. 5 wt %), a notable decrease in the absorption signature (at about 510 nm) was observed after irradiated under UV light for 15 min, whereas after a following visible light (Vis) irradiation for 10 min, the absorbance increased again (part a of Figure 1). During the three cycles of UV and Vis alternate irradiation, the absorption peak (λ ) 510 nm) decreased and recovered periodically, as shown in part b of Figure 1. Simultaneously with the isomerization, slow photodegradation of K-2G solution also occurred, and thereby the absorption could not recover to the original value. Whatever, the decrease and increase in the absorbance should be attributed to the photoisomerization of K-2G via different kinds of light irradiation, and the transition from trans to cis state induced a decrease in the absorption spectra. Parts c and d of Figure 1 provided sufficient evidence for the successful adsorption of K-2G on the surface of TiO2 after dark adsorption for 1 h because the differential absorption spectrum in part d of Figure 1 showed similar absorption signature with the K-2G dye solution (the concentration K-2G solution in part c of Figure 1 is 1 wt % for the convenience to compare to the K-2G/TiO2 dispersion). Moreover, from the absorption spectra of K-2G/TiO2 dispersion (parts e and f of Figure 1), it was observed that the absorbance near the absorption signature of K-2G (about 510 nm) increased after 15 min of visible light irradiation. This increase should be ascribed to the transition of the adsorbed K-2G molecules from cis to trans states under visible light. The above results in Figure 1 revealed the relationship between the conformational transition and the changes in absorbance for K-2G both in solution and adsorbed on TiO2. Furthermore, these results also supported the assumption that the K-2G molecules would undergo conformational transition during the UVLAP shown by Scheme 1.

FIGURE 1. UV-vis absorption spectra of K-2G solution and K-2G/TiO2 dispersion under UV or visible light (Vis) irradiation. (a) Changes of absorption spectra of K-2G solution (ca. 5 wt %) after three cycles of alternate UV-vis irradiation. In each cycle: UV for 15 min (dash line, from top to bottom correspond to the absorption spectra obtained after the first, second and third UV irradiation period), and then Vis for 10 min (solid line, from top to bottom correspond to the absorption spectra of the original dye solution and those obtained after the first, second and third Vis irradiation period); (b) Changes in the dye’s absorption profile (λ ) 510 nm) in (a); (c) absorption spectra of K-2G solution (ca. 1 wt %) (1), dispersion of K-2G/TiO2 (2) and blank TiO2 (3); (d) differential absorption spectrum of K-2G/TiO2 and blank TiO2 dispersion (Curve (2) subtracts (3) in (c), smoothed); (e) absorption spectra of K-2G/TiO2 dispersion under irradiation: under UV for 30 min (1), and then Vis for 15 min (2); (f) differential absorption spectrum of K-2G/TiO2 dispersion under UV and Vis irradiation (Curve (2) subtracts (1) in (e), smoothed). Photocatalytic Results. The photoisomerization of azobenzene derivatives and the resulting changes in various properties have been considered to induce a notable effect in the “nanoscale environments” (9), which could have an influence in the photocatalysis. Therefore, the isomerization of azo dyes adsorbed on the photocatalyst’s surface could be taken advantage for their photodegradation. Temporal changes in the absorption spectra of K-2G during the photocatalysis in conventional and UVLAP modes are shown in parts a and b of Figure S2 in the Supporting Information, respectively. The normalized concentration of the dye solution (C/C0) is proportional to the normalized maximum absorbance (A/A0), and therefore the temporal concentration changes of K-2G during the photodegradation can be derived

from the changes in the dye’s absorption profile (λ ) 510 nm) at given time interval. Accordingly, the absorption changes of K-2G due to photocatalytic degradation over TiO2 films versus time and versus energy cost in the aforementioned modes were derived, as shown in parts a and b of Figure 2, respectively. As shown in Figure 2, all modes of photocatalysis followed first-order kinetics (-ln(C/C0) ) kt, where k is the photodegradation rate constant), which was in accordance with the widely used Langmuir-Hinshelwood kinetic model (10). From part a of Figure 2, it was clear that the conventional and UVLAP photocatalysis showed similar reaction rate (kconventional ) 17.6% h-1, k UVLAP ) 16.9% h-1), whereas almost no photodegradation was observed with visible light (indoor natural light, 50 W/m2) as the only light VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Absorption changes (λ ) 510 nm) plots for the photocatalytic degradation of K-2G over TiO2 films (a) vs time and (b) energy cost (the energy cost is defined to be the cost for the UV light irradiation and thus proportional to the actual UV light irradiation time). Each one involves the photocatalytic modes of conventional (9) and UVLAP (O). Control experiments were conducted under visible light (indoor natural light) (2) and dark (0) conditions.

TABLE 1. Photodegradation Rate Constants k in the Photodegradation of K-2G in Different Modes, Including Conventional, UVLAP, Visible Lighta and Dark Conditions photodegradation rate constants k

vs time (% h-1)

kconventional kUVLAP kvisible light kdark kUVLAP/kconventional

17.6 16.9 0.36 0.30 96.0%

a

vs energy cost (%/unit energy) 17.6 25.3 143.8%

Visible light was from the ambient indoor natural light.

source or under dark conditions. The UVLAP strategy showed a similar rate constant with the conventional mode but with only 2/3 of UV irradiation time and the energy cost required in the conventional photocatalysis. Thus, when the energy cost for the UV irradiation was taken into account (the energy cost for 1 h actual UV irradiation was defined as one energy cost unit), the UVLAP showed significantly higher energy efficiency than the conventional mode by more than 40% (part b of Figure 2). More detailed results are shown in Table 1. Importantly, it should be noted that with optimizations in irradiation conditions, such as the adjustment of irradiation time of UV and visible light, both higher rate and energy efficiency could be expected in the UVLAP. Moreover, it is noteworthy that the UVLAP process cannot be considered as a simple combination of the conventional UV photoca1110

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talysis based on the direct TiO2 band gap photoexcitation during the UV irradiation period and the dye-sensitized photocatalysis, triggered by electron transfer from the visiblelight-excited dye molecules to the TiO2 conduction band during the visible light irradiation period. Otherwise, the photodegradation rate constant kUVLAP should be equal or approximate to the value of 11.9% h-1, which is much lower than the actual one. Detailed discussion is listed in Supporting Information. Possible Mechanism. The improved performance of UVLAP was attributed to the diversity of the cis and trans dyes in the photocatalysis. The dyes were predominantly in the cis state in the conventional mode, while in the trans state along with a simultaneous trans to cis transition during each of the UV irradiation periods in the UVLAP. Two major differences in the nanoscale environment between the two states could be responsible for the observed results in photocatalysis. The first one is the conductance effect. It has been reported that isomers of azo dyes show different conductance when adsorbed on a surface, and the trans isomer has a better conductance likely due to its more favorable conjugated plane than that of a cis isomer (11). In photocatalysis, the interfacial electron transfer rate is crucial to the overall quantum efficiency (12). A higher conductance on the surface of catalyst (TiO2) facilitates faster interfacial electron transfer, and therefore higher energy efficiency is expected in the photodegradation of the trans dyes. The other one is the spatial effect. The photoisomerization of azo dyes adsorbed on the TiO2 surface, from an extended trans to a compact cis conformation, resulted in a concomitant change in the free volume and porosity of the dyes layer (13, 14), as shown in Scheme 1. According to the Langmuir-Hinshelwood kinetic model, the photocatalytic reaction takes place on the surface but also near the surface (15), where the interaction between the generated electrons and electron acceptors such as O2 is important not only in the suppression of charge recombination but also in the formation of reactive oxygen species (16). In the conventional UV photodegradation of azo dyes, the compact cis state dominated during the photocatalysis and induced a reduction in the free volume and porosity of the dye layer. Consequently, the concentration of electron acceptors on and near the surface decreased and the interaction between electrons and electron acceptors was also hindered (as illustrated in Scheme 1), which would aggravate the recombination of photogenerated electron-hole pairs and thus decrease the photocatalytic activity of TiO2 in the degradation of azo dyes. Besides, the reduction in the free volume could also hinder the transfer and adsorption of more dye molecules from solution to the catalysts’ surface, and thereby decrease the photodegradation rate, because the adsorption is a prerequisite for good performance in the photocatalysis (17). Overall, both the conductance and spatial effects induced by cis and trans isomerization could introduce significant changes in the nanoscale environment and contribute to their different performances in the photodegradation; a higher conductance and a larger free volume on/ near the photocatalyst surface could lead a higher rate in the photodegradation of trans dyes compared to the cis ones, and thereby a higher energy efficiency in the UVLAP. As shown in Figure 3, the typical electrochemical impedance spectra (EIS) of K-2G/TiO2 film were presented as Nyquist plots, and it was observed that after visible light irradiation, the semicircle in the plot became shorter, which indicated a decrease in the solid state interface layer resistance and the charge transfer resistance on the surface (18). These results were in line with the above explanation that a photoisomerization on the surface of K-2G/TiO2 films from cis to trans state could facilitate fast charge transfer and efficient separation in photocatalysis and improve the performance in photodegradation.

FIGURE 3. EIS changes of K-2G/TiO2 film electrode under UV for 15 min (0) and then visible light (Vis) for 10 min (9). The EIS measurements were performed in the presence of a 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture as a redox probe in 0.1 M KCl aqueous solution. In conclusion, we demonstrated a UV-vis light alternate photocatalysis for the degradation of azo dyes with TiO2. This new approach availed of the photoisomerization and the resulting changes in the nanoscale environment of azo dyes and showed significant enhancement in the energy efficiency compared to conventional UV photocatalysis. This work is anticipated to open new possibilities in reducing the energy cost and promoting photodegradation of azo dyes and other pollutants with isomerization properties. Ongoing work is focused on further improving the UVLAP activity through optimization of experimental parameters.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20975060) and the National Basic Research Program of China (No. 2007CB310500).

Supporting Information Available Absorption spectra results, control experiments in photocatalysis, and expanded discussion. This material is available free of charge via the Internet at http://pubs.acs.org.

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