ARTICLE pubs.acs.org/IECR
Energy Efficient UV-LED Source and TiO2 Nanotube Array-Based Reactor for Photocatalytic Application Thillai Sivakumar Natarajan, Kalithasan Natarajan, Hari C. Bajaj, and Rajesh J. Tayade* Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR), G. B. Marg, Bhavnagar-364 021 Gujarat, India
bS Supporting Information ABSTRACT: The present study focuses on the development and feasibility of ultraviolet light emitting diode (UV-LED) source and TiO2 nanotube array (TNA)-based photocatalytic reactor for Congo red (CR) dye degradation. Highly ordered TNA was synthesized by the anodization method. The synthesized highly ordered TNA was characterized by X- ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscope (AFM), and electronic impedance spectroscopy (EIS) techniques. The percentage degradation was determined using a UVvisible spectrophotometer, while the mineralization of CR dye was further confirmed by chemical oxygen demand (COD) and kinetic analysis. The effect of operational parameters such as initial concentration of dye and pH on the degradation of CR dye has been studied to determine the optimum conditions. A possible degradation mechanism based on the electrospray ionization mass spectrometry (ESI-MS) has been suggested. The results demonstrated that CR dye was completely degraded in 5 h using the designed photocatalytic reactor. The electrical energy per order (EEo) was calculated for estimating the electrical energy efficiency. The result demonstrated that highly adhered nanotube array can effectively be used for photocatalytic degradation of CR dye in the presence of UV-LED light irradiation.
1. INTRODUCTION Water pollution is a major concern throughout the world in the present scenario. The treatment of wastewater has been extensively studied by several conventional remediation techniques such as physical methods, biological methods, thermal and chemical methods, and high energy UV light. These techniques need a high operating cost over long-term, consume a large amount of energy, are nondestructive in nature, and transfer the organic pollutant from one phase to another phase resulting in the formation of a secondary pollutant, which requires further necessary treatment. To overcome these difficulties, heterogeneous photocatalysis using semiconductor materials has emerged as a potential technique for the purification of a wide variety of aqueous organic contaminants or air pollutants.18 Among various oxide semiconductor photocatalysts, titanium dioxide (TiO2) has been used as an excellent photocatalytic material for the degradation of hazardous pollutant due to its strong oxidizing power, high photocatalytic activity, chemical and biological stability, relatively low-cost, nontoxicity, and long-term photostability. TiO2 with different morphologies such as nanosphere, nanotube, nanorod, nanofiber, and nanowire has been reported for removal of pollutant from water, but in the present scenario the synthesis of different morphology on solid supports offers useful application for the development and fabrications of photocatalytic reactors. Furthermore, the next generation of photocatalytic systems needs a flexible, lightweight, and easily portable reactor, and the usage of energy efficient sources instead of classical ultraviolet (UV) excitation source for the production of renewable energy and self-cleaning system. However, the designing of photocatalytic reactor for environmental remediation is hampered due to various factors such as its designing, size, effective r 2011 American Chemical Society
cost, process time, and the light source.911 The photocatalytic reactor based on classical ultraviolet (UV) excitation sources has limited applications due to the harmful side effect of UV sources, its power instability during long time operation, low photonic efficiency, operating conditions such as high voltage at initial stage, cooling requirement, high vapor pressure, and usage of hazardous mercury metal; it has a shorter lifetime and broader spectral wavelength.1214 However, the mercury metal is one of the hazardous air pollutants (HAP) specified by the U.S. Environmental Protection Agency. Also, it is very harmful to the human eye, kidneys, brain, and skin. The efficient use of solar light-based photocatalytic reactors is as a hygienic, renewable, and sustainable energy source. However, commercialization seems limited because of its high cost and large area for installation, and the efficiency of the reactor depends upon the direction, intensity, and availability of solar light.15,16 To triumph over the difficulties associated with conventional light sources, the new and energy efficient alternative for the gas discharge sources is ultraviolet light emitting diodes (UV-LED). UV-LEDs are semiconductor pn junction devices, which are made up of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), gallium phosphide (GaP), or indium gallium nitride (InGaN). The flow of current in LED is onedirectional (forward biased), and it emits UV light in a narrow spectrum in the form of electroluminescence. Another advantage is that their lifetime is 100 000 h, whereas in the case of gas Received: December 7, 2010 Accepted: May 11, 2011 Revised: April 18, 2011 Published: May 11, 2011 7753
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Industrial & Engineering Chemistry Research discharge sources it is 1000 h. UV-LED sources have high robustness, minimum heat generation, good linearity of the emitted light intensity with current, suitability for operation in a pulsed regime at high frequencies, are easily portable, and have small size compatible with the modern trend in the design of miniaturized photocatalytic reactor.17,18 They are commercially used for a wide range of applications, such as in the development of photocatalytic reactor for environmental remediation,1925 including fluorescent detection of fraudulent documents, forensic investigations, antique identification, and disinfecting devices.17 TNA arrays fabricated by electrochemical anodization method have attracted tremendous importance in recent years because of their remarkably enhanced photoelectric properties and technological significance for various applications such as solar cells, photocatalysis, gas sensors, functional surface devices, and water splitting.2628 Recently, TNA-based photocatalytic reactor has been reported for various applications because it possesses scattering nature of free electrons, high surface area, better light absorption efficiency, and enhances electron mobility, which offers superior charge transport leading to higher photocatalytic efficiency.29 TNA has been used as a photocatalyst for degradation of pentachlorophenol, phenols, rhodamine B dye, tetracycline, and aromatic amine using photocatalytic and photoelectrocatalytic process under irradiation of classic UV light source.3035 In this present work, we have proposed a facile photocatalytic reactor composed of UV-LED source and TiO2 nanotube array. TNA was synthesized by anodization method and characterized by XRD, SEM, TEM, AFM, and EIS techniques. The photocatalytic application of designed reactor was studied by the degradation of CR dye solution. The electrical energy per order (EEo) was calculated for degradation studies. The effects of initial dye concentration and pH on degradation of CR dye have been investigated to find optimum conditions. Degraded samples were analyzed by ESI-MS analysis, and possible intermediates for the photocatalytic degradation of CR dye were suggested. The degradation studies were further confirmed by COD and kinetic analysis. Until today, CR dye has been degraded with high power conventional UV light as an irradiation source.3642 To the best of our knowledge, this is the first report based on the combined use of the UV-LED source/TiO2 nanotube array system for the development of photocatalytic reactor.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Titanium foils with thickness of 0.25 mm and 99.7% purity and counter Pt wire electrode were purchased from Aldrich, India. Ethanol, isopropanol, acetone, ethylene glycol, sodium hydroxide, hydrochloric acid, and ammonium fluoride (NH4F) were purchased from s.d. Fine Chem Ltd. and CDH Private Limited, India. Congo red (CR) A.R. dye was purchased from CDH Private Limited, New Delhi, India. Congo red is an anionic azo dye with molecular formula of C32H22N6Na2O6S2. The molecular structure is given in Figure 1. The absorption maxima wavelength of CR dye (λmax = 500 nm) was used for the analysis during the photocatalytic degradation reaction. COD standard chemical reagents for chemical oxygen demand (COD) measurements were purchased from E. Merck India Limited, Mumbai, India. The double distilled water was used to prepare experimental solutions. UVLight emitting diodes used for this work were pn junction devices made of indium gallium nitride (InGaN). The
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Figure 1. Molecular structure of Congo red (CR) dye.
diameter of UV-LED was 5 mm having irradiation wavelength in the range of 390410 nm. They emit blue UV light, and the operating temperature range of UV-LED was 25 to 85 °C. The optical rising time for this UV-LED was 30 ns. The luminous intensity of each LED was 350 mcd, and radiant flux was in the range of 1012 mW at 20 mA. The switch mode power of 030 V, 06 A, manufactured by Thurlby Thandar Instruments, England, and resistor of 47 ohm were procured from the local market. 2.2. TiO2 Nanotube Array Preparation. The TiO2 nanotube array (TNA) on the surface of titanium metal plate (0.25 mm, 99.7% purity, Aldrich, India) was grown by anodization method.43,44 Prior to anodization, the titanium metal plate was polished with abrasive paper and degreased by sonication in acetone, ethanol, and distilled water, respectively. The titanium metal plate was dried under nitrogen atmosphere, and it was used as an anode; Pt wire was used as a counter electrode. The distance between two electrodes was 1 cm, and the electrolyte was composed of 0.5 wt % NH4F and 3 vol % of H2O in ethylene glycol as electrolyte. The anodic oxidation was carried out by applying the voltage at the rate of 0.3 V/min up to 40 V, then maintaining at 40 V for 4 h. After anodization, the TNA surface was washed with deionized water, dried, and calcined at 300, 450 °C for 1 h under air atmosphere. The photocatalytic surface was denoted as TNA-300 and TNA-450. 2.3. Characterization. The X-ray diffraction was carried out to study the presence of anatase phase in the anodized titanium metal plate by using a Philips X’pert MPD system with Cu KR1 radiation (λ = 1.54056 Å) in a 2θ range of 580° at ambient temperature. The operating voltage and current were 40 kV and 30 mA, respectively. A step size of 0.033° with a step time of 1 s was used for data collection. The data were processed using the Philips X’Pert (version 1.2) software. The morphology of synthesized highly ordered TNA surface was analyzed by scanning electron microscopy (SEM) (Leo Series VP1430) equipped with INCA, energy dispersive system (EDX). The highly ordered TNA grown on the titanium metal plate was supported over aluminum stubs and then sputter coated with gold (by using Polaris sputter coater model Polaron SC7620, Quantum Technologies). The morphology of the grown nanotube was further confirmed by transmission electron microscopy (TEM) using a JEOL JEM-2010 electron microscope. To carry out the TEM analysis, the nanotube was taken out from the anodized titanium metal plate by scratching the surface. The material scratched from the titanium metal plate was loaded on the grid for further analysis. The surface morphology of TNA was measured by tapping mode atomic force microscope (AFM) analysis using Ntegra Aura SPM with a scan rate of 1 Hz. Electrochemical impedance spectroscopy (EIS) of TNA and Ti plate was determined using a potentiostat/galvanostat frequency response analyzer (EcoChemie, B.V. Utrecht, The Netherlands Auto Lab, model PGSTAT 30). 7754
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Figure 2. Photocatalytic reactor setup.
2.4. UV-LED/TNA-Based Photocatalytic Reactor. The photocatalytic activities of TNA surface were determined by the degradation of an aqueous solution of Congo red dye in locally developed photocatalytic reactor as shown in Figure 2. The photocatalytic reactor (85 40 45 mm) consists of a rectangle quartz cell (12.5 12.5 45 mm) that was kept at the center of photocatalytic reactor and four UV-LED (two on each side) that were kept close to the wall of quartz cell. The UV-LEDs were connected in series and attached to a d.c. power supply through a current limiting resistor. Titanium metal plate (30 10 mm) having highly ordered TNA was kept at the center of the quartz cell. 2.5. Photocatalytic Degradation Studies. The photocatalytic activity of designed reactor was studied by degradation of CR dye solution. CR dye solution (1.4354 105 M) was taken in quartz cell, and highly ordered TNA was immersed at the center of quartz cell in such a way that it can be irradiated by UV-LED light from both the side. The quartz cell containing the sample was taken out for every 1 h interval after switching off the UVLED, and the concentration of CR dye (λmax = 500 nm) in the solution was determined using a calibration curve of CR dye (concentration vs absorbance) prepared with known concentrations using Cary 500 UVvis spectrophotometer (Varian, Palo Alto, CA). All photocatalytic experiments were carried out by keeping the same voltage and current under constant stirring. The possible intermediates of CR dye degradation were studied using electrospray ionization mass spectra (ESI-MS) experiments performed on a Water Q-TOF micro Y A-260 (Micromass) tandem quadruple orthogonal TOF instrument fitted with a lock spray source. 2.6. Chemical Oxygen Demand (COD) Analysis. The oxygen equivalent of the organic matter of each sample, that is, chemical oxygen demand (COD), was measured using a Spectroquant NOVA 60 photometer (Merck KGaA, Darmstadt, Germany). The reagents for COD analysis and a 3 mL sample were mixed together in a glass cell and digested in a Spectroquant TR 320 Thermo digester for 2 h at 150 °C. After digestion, the mixture was cooled to room temperature, and the COD was measured using the photometer.
3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis. The photocatalytic properties of the TNA depend on the crystallinity and crystal structure (anatase phase and rutile phase).29 It is well-known that the anatase phase of TiO2 is more photocatalytically active than the rutile phase. Figure 3 shows the XRD patterns of the assynthesized and TNA annealed at two different temperatures for 1 h in air atmosphere. The result demonstrated that the annealing temperature could significantly affect the crystalline phase of the TiO2 nanotubes. From Figure 3b, it is clear that the as-synthesized sample is amorphous in nature with reflection of Ti foil only. It is further observed that the anatase phase occurred
Figure 3. XRD pattern of (a) titanium plate, (b) TNA before calcination, (c) TNA-300, (d) TNA-450 (A, anatase; Ti, titanium; *, Ti3O phase (CSD-36055-ICSD)).
after calcinations of TNA grown Ti metal plate at 300 °C (TNA300, Figure 3c). The anatase phase occurs with a low and broad characteristic peak corresponding to the anatase (101) plane at 2θ of 25.35. This broad peak may be due to the lower crystalline nature of TiO2 nanotube. The peak around 20° corresponds to Ti3O phase (CSD-36055-ICSD). When the calcination temperature increased to 450 °C (TNA-450, Figure 3d), the Ti3O phase vanished, and the intensity of the anatase phase at 25.35° increased, indicating an improvement in crystallinity. Also, it can be seen from the XRD pattern that the TNA-450 sample possesses characteristic peaks at 25.35° (101), 38.1° (004), 48.2° (200), 54.02° (105), 55.12° (211), and 62.08° (213) for the anatase phase. Similar observations were reported by Zou and Shankar et al.45,46 for nanotube array developed by anodization method and calcined at different temperatures. 3.2. SEM and TEM Analysis. The diameter and length of titanium nanotube array highly depend upon the operating conditions of the anodization such as applied potential, time, and electrolyte. To study the morphology of the developed nanotube array, the surface was scanned under the scanning electron microscope without removing the nanotube from the Ti metal plate. The scanning electron microscopy (SEM) image (Figure 4a and b) depicted the uniform distribution of TNA having hexagonal order on the surface of titanium metal plate. Further measurement of the length and diameter of the TNA was carried out by cross-sectional SEM images (Figure 4 c and d). The result demonstrated that the length of the TiO2 nanotube on the surface of titanium metal plate was ca. 11 μm and diameter (Figure 4e and f TEM images) was ca. 165 nm in 40 V anodization potential in 4 h duration. Both ends of TiO2 nanotubes were open as the tube was removed from the titanium metal plate (Figure 4f). Similarly, Shankar et al.46 synthesized nanotube on Ti metal plate by applying an anodization potential of 60 V and 17 h duration of time to achieve 142 μm of length and 160 nm diameter. They further reported that the tube length and diameter can be increased by increasing anodization potential and anodization time. 3.3. Photocatalytic Degradation of CR Dye. Our earlier study demonstrated the photocatalytic degradation of methylene blue and rhodamine B dye solution using suspended TiO2 particles in the presence of UV-LED light irradiation.19,20 Here, 7755
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Figure 4. (a,b) Top view, (c,d) cross-sectional SEM images of TNA, and (e,f) TEM images of TNA. Inset: Open end of TNA.
we have tried to use the TNA grown on the surface of the Ti plate for the photocatalytic application in the presence of UV-LED light irradiation to explore the feasibility toward the development of UV-LED-based photocatalytic reactor. The photocatalytic activity of the TNA surface (TNA-300 and TNA-450) was evaluated by the photocatalytic degradation of the CR dye using designed reactor. The results are shown in Figure 5. In the presence of photocatalyst without UV-LED light irradiation, about 24% decrease in concentration was observed from absorbance measurements. This is due to the adsorption of dye molecule on the surface of photoactive surface. Figure 5A shows the UVvisible spectra of degradation of CR dye with TNA-450 under UV-LED light irradiation taken at different time intervals. It is clearly observed that the dye was completely degraded after 5 h reaction. This may be due to increasing the cacination temperature to 450 °C, which increases the crystallinity of TNA surface and intensity of anatase phase, and in turn increased the process of charge separation. However, in the case of TNA300 photocatalyst and only UV-LED light, 95% and 5% of CR dye was degraded after 6 h photocatalytic reaction (Figure 5B). A similar result on the photocatalytic activity of the TNA has been reported on the degradation of rhodamine B dye in the presence of a 300 W mercury lamp,32 which clearly indicates that the TNA can be utilized for the photocatalytic degradation of organic contaminant present in water. After the photocatalytic degradation studies, TNA-450 was chosen for further photocatalytic reaction to study the effect of pH, dye concentration, and mineralization studies. 3.4. Effect of Dye Concentration. The concentration of dye solution plays an important role in the photocatalytic reaction studies. The dye concentration varied in the range of
Figure 5. (A) UVvisible spectra of CR dye degradation with TNA450 under irradiation of UV-LED light at different time interval. (B) Percent degradation of CR dye with TNA-300, TNA-450, and UV-LED light only.
0.86252.9695 105 M. Photocatalytic reaction was carried out with TNA-450 photocatalyst surface under the irradiation of UV-LED light. The results are shown in Figure 6. The results demonstrated that the complete degradation of CR dye was observed in 0.8265 and 1.4354 105 M concentration, whereas the concentration was increased from 1.4354 105 to 2.9695 105 M, and the percentage degradation of CR dye was reduced from 100% to 76%. The decrease in degradation of CR dye with an increase in dye concentration can be attributed to the greater amount of dye competing for degradation and the reduction in the light intensity that reaches the TNA surface. At very high concentrations, much of the light is screened by the solution, and fewer photons are able to reach the TNA surface. Thus, the generation of electronhole pairs is greatly reduced, and the dye degradation is reduced due to the absence of oxidizing species. A similar trend was observed by Smith and Zang et al., for the photocatalytic degradation of methyl orange dye using a TiO2 nanotube array. 47,48 3.5. Effect of pH. Industrial wastewater discharged with a wide range of pH values. Photocatalysis is a surface phenomenon; the performance of a photocatalyst can be highly inclined by the pH of wastewater, nature of the dye, and its ability to absorb onto the photocatalyst surface. It is very difficult to determine the optimum pH because it is related to ionization state of the 7756
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Figure 8. Kinetics of CR dye degradation reaction.
Figure 6. Photocatalytic degradation of different concentrations of CR dye: (A) 0.8265 105 M, (B) 1.4354 105 M, (C) 2.1707 105 M, and (D) 2.9695 105 M.
Table 1. Kinetic Data and Electric Energy (EEo) for Photocatalytic Removal Reaction of CR Dye
photocatalyst surface. The ionization states of the TiO2 surface are as follows: (i) positively charged in acidic media (attributed to Hþ ions), and (ii) negatively charged under alkaline medium (attributed to OH).49,50 TiOH þ Hþ T TiOH2 þ
TiOH þ OH T TiO þ H2 O
ð1Þ ð2Þ
To determine the optimal pH for the degradation of CR dye, the pH of the reaction mixture was varied from 4.00 to 8.67 by keeping the CR dye concentration (1.4354 105 M) and TNA-450 catalyst under the irradiation of UV-LED light. The pH of the dye solution was adjusted by the addition of appropriate amounts of NaOH or HCl solution. The initial pH of the dye solution was 6.50. The results are shown in Figure 7. The results demonstrated that when the pH of the solution increased from 6.50 to 8.67, the percentage degradation was reduced to 90%. This may be due to the anionic nature of dye, and the negatively charged catalyst surface in alkaline
rate constant
107
kapp, 102
1
1
EEo (kW h m3 2
order1)
(mol L )
(min )
R
TNA-300 TNA-450
0.48 0.77
0.9 1.25
0.965 0.993
317 228
UV-LED light only
0.03
0.02
0.956
14 285
catalyst
Figure 7. Effect of pH on the degradation of CR dye: (A) pH = 4.00, (B) pH = 6.50, (c) pH = 8.67.
initial rate,
medium leads to lower adsorption of dye molecules on the TNA surface. The CR molecule with two sulphuric groups ionized easily even in acidic media and became a soluble CR anion. At the same time, the pH was reduced from 6.50 to 4.00; the percentage degradation of CR dye was reduced to 82%. This may be due to the higher adsorption of dye molecules and reducing light intensity that reaches on the TNA surface. Thus, the formation of oxidizing species is less, which leads to a decrease in the percentage degradation of CR dye. The results indicated that the pH value of the solution was the key factor for dye degradation and pH 6.50 is an optimum for higher percentage of degradation. 3.6. Kinetic Analysis of CR Dye Degradation. In heterogeneous photocatalysis, kinetic analysis is one of the most important factors to determine the reaction mechanism. Photocatalytic degradation of CR dye follows pseudofirst-order kinetics in agreement with the LangmuirHinshelwood mechanism. The LangmuirHinshelwood model of Congo red dye degradation can be written as follows: r ¼ kKCt =1 þ KCt kapp Ct
ð3Þ
where the rate r is proportional to the concentration Ct at time t, kapp is the reaction rate constant, and K is the reactant adsorption constant. The straight line confirms that the degradation of Congo red dye follows the pseudofirst-order kinetics (Figure 8). The regression coefficient R2 was 0.993, which suggested the photodegradation of CR by the TNA and the UV-LED light fit the LangmuirHinshelwood kinetic model. The initial rates and apparent rate constants of photocatalytic degradation were calculated, and the results are shown in Table 1. From the initial rate, it was further confirmed that the presence of TNA-450 under UVLED light irradiation showed a higher percentage of degradation than that in presence of TNA-300 and only UV-LED light 7757
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Scheme 1. Possible Degradation Pathway of CR Dye
irradiation (without TNA). Kinetic studies also confirm that the presence of highly crystalline anatase phase is the most important factor for a higher percentage of photocatalytic activity. 3.7. Photocatalytic Mineralization of CR Dye. In semiconductor photocatalytic degradation, mechanistic elucidation is one of the most important processes. When a TiO2 catalyst absorbs a light energy equal to or higher than the band gap energy (Eg), the electron of the valence band of titanium dioxide becomes excited, and the excited electron is promoted to the conduction band of titanium dioxide, therefore creating the negative-electron (e) and positive-hole (hþ) pair as shown in eq 4. TiO2 þ hν f TiO2 ðecb þ hvb þ Þ
ð4Þ
hydroxyl and superoxide radical anions as shown in eqs 57. TiO2 ðhvb þ Þ þ H2 O f TiO2 þ Hþ þ OH 3
ð5Þ
TiO2 ðhvb þ Þ þ OH f TiO2 þ OH 3
ð6Þ
TiO2 ðecb Þ þ O2 f TiO2 þ O2 3
ð7Þ
The dyes then react with generated radicals, producing a range of intermediates including radical and radical cations to reach complete mineralization with the formation of carbon dioxide, water, and inorganic nitrogen with ammonium and nitrate ion. CR dye þ OH 3 f intermediate products f CO2 þ H2 O
þ
The formed e and h pairs moved to catalyst surface, where they react with water and surface hydroxyl group to create
þ NO3 þ NH4 7758
ð8Þ
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Industrial & Engineering Chemistry Research To confirm the degradation, the reaction mixture taken at different time interval was analyzed by LCMS. It is understood from the literature that the degradation of CR dye may takes place in different ways:51 (i) cleavage of the azo (NdN) double bond, (ii) cleavage of the sulfonate group from the aromatic ring, and (iii) direct cleavage of the benzene ring. From LCMS results (Supporting Information Figure S3), we have interpreted possible intermediate compounds during the degradation pathway for CR dye shown in Scheme 1. These are the possible intermediate species formed after the irradiation over CR dye in the presence of TNA-450 surface under UV-LED light at different time interval. First, the cleavage of two sulfonate groups or the direct cleavage of azo bond leads to the formation of intermediate with m/z values of 491 and 387. On the other hand, the removal of two sulfonate groups and one or two benzene ring leads to the formation of intermediate with m/z of 443, 392, or it can directly fragment into possible intermediates corresponding to mass values of 288, 245, 211, 197, 185, and 143. The formed intermediates then were fragmented into aniline (m/z = 93), naphthalene-1,4-dione (m/z = 158), 2,5-cyclohexadiene-1,4-dione (m/z = 108), benzene (m/z = 78), and benzene-1-ylium (m/z = 77). This can further oxidized by formed OH 3 radicals into phthalic acid (m/z = 167), 2-hydroxybenzoic acid (m/z = 139), malonate (m/z = 102), alcohols, and other low molecular weight intermediates and finally mineralized into CO2, H2O, NO3, and NH4þ as shown in eq 8. The photocatalytic degradation was further confirmed by COD analysis. Results demonstrated that 100% of COD was reduced using TNA-450 photocatalyst after 5 h irradiation of UV-LED light (Figure 9), whereas in the case of TNA-300 and only UV-LED light irradiation, 92% and 11% of COD was reduced after reaction. The complete reduction in COD value obtained using TNA-450 also confirms that the highly crystalline anatase phase was more photocatalytically active. The reduction in COD values also confirms the photocatalytic degradation of CR dye using the TNA photocatalyst under irradiation of UVLED light. The decrease in COD shows the same trend as that observed by a UVvisible spectrophotometer. 3.8. Recyclability of Photocatalyst. The evaluation of the reusability of TNA surface was carried out by the degradation of CR dye up to five cycles. After every cycle, the TNA-450 surfaces were washed with water, and the photocatalytic activity of surface was determined up to five cycles (Figure S4). The result demonstrated that the photocatalytic activity of TNA surface was not reduced after five cycles, but the percentage degradation of CR dye was decreased from 100% to 95%. This decrease in percentage degradation may be the adsorption of dye molecules on the surface of TNA. This adsorbed dye reducing the light intensity that reaches the TNA surface and decreaing the percentage degradation was observed after five cycles of photocatalytic reaction. From the recyclability test, it was further confirmed that no leaching of TiO2 nanotube from the surface of titanium metal plate was detected after five cycles. To study the adhesion property, TNA surface was analyzed by three tests such as the adhesive tape test, and testing under the flow of tap and hot water. First, adhesive tape was applied on the TNA surface, and the tape was quickly removed from it, which showed the nonstickiness of TNA. Second, the TNA surface was kept under the rapid flow of tap water, and, third, TNA was immersed in hot water for 24 h followed by sonication for 5 min. The results demonstrated that there was no leaching of TNA from the surface in the entire test.
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Figure 9. Percent reduction of COD values.
3.9. Electrical Energy Calculation. The selection of technology for wastewater treatment depends upon a number of important factors such as economy of scale, economics, regulations, effluent quality goals, operation (maintenance, control, safety), and robustness (flexibility to change/upsets). Among these major factors, economics is the most important one. Because photocatalytic degradation of aqueous organic pollutant is an electric-energy-intensive process, and electric energy can represent a major fraction of the operating costs, simple figuresof-merit based on electric energy consumption can be very useful and informative. Recently, the International Union of Pure and Applied Chemistry (IUPAC) has proposed two figures-of-merit for advanced oxidation processes (AOPs) on the use of electrical energy. In the zero-order range, the appropriate figure-of-merit is the electrical energy per mass (EEm) defined as the kW h of electrical energy required for the removal of one kilogram of the pollutant.52,53 In the case of low pollutant concentrations, the appropriate figure-of-merit is the electrical energy per order (EEo), defined as the number of kW h of electrical energy required to reduce the concentration of a pollutant by 1 order of magnitude (90%) in 1 m3 of the contaminated water. The EEo (kW h m3 order1) can be calculated from the following equations:
EEo ¼ P t 1000=V 60 lnðC0 =Ct Þ
ð9Þ
lnðC0 =Ct Þ ¼ k1 t
ð10Þ
where P is the rated power (kW) of the AOP system, t is the irradiation time (h), V is the volume (L) of the water in the reactor, C0 and Ct are the initial and final pollutant concentrations, and k1 is the pseudofirst-order rate constant (min1) for the decay of pollutant concentration. The results were shown in Table 1, and it justified that 228 and 317 kW h m3 order1 energy was consumed during the degradation of Congo red dye in the presence of TNA-450, TNA-300, and UV-LED light, respectively, whereas in the case of UV-LED light only (without TNA) it was 14 285 kW h m3 order1. In the case of UV-LED light only, the EEo was increased; this may be due to more absorption of UV light by the dye molecules. In the presence of TNA, the EEo was decreased, because of absorption of UV light by TNA and generating hydroxyl radical and increasing the percentage degradation of dye molecules.53 Similar results are 7759
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Table 2. Comparison of Degradation Percentages of Congo Red Dye with Literature serial no.
catalyst
light source
time (min)
degradation (%)
1
chitosan/nano-CdS
xenon lamp (300 W)
180
2
TiO2Degussa P-25
mercury lamp UV
480
100
37
100
38
85.9
ref 36
3
AgTiO2
HPMV lamp (400 W)
30150
4
WO3TiO2/AC
HPMV lamp (500 W)
120
5
ZnIn2S4
tungstenhalogen lamp (500 W)
300
100
40
6
ZrMo2O8
HPMV lamp (125 W)
120
100
41
7
TiO2
HPMV lamp (125 W)
180
100
42
8
TNA-450
UV-LED- 400 nm (0.6 W)
300
100
this study
reported by various researchers, in the presence of photocatalyst and UV-LED light lower energy consumed in the photocatalytic reaction.21,54 From this significant observation, it is expected that the decomposition efficiency and evaluation of the treatment costs for the photocatalytic system in industrial applications may be markedly improved in the future through advances in UVLEDs and photocatalytic reactors. 3.10. Comparison of Degradation Percentages of CR Dye. To the best of our knowledge, until today there have been no report based on the combined use of TNA photocatalyst and UV-LED light irradiations for the degradation of CR dye. So we compared this study with the earlier reported work based on classical UV light as a source for photocatalytic degradation of CR dye. The comparison results are given in Table 2. It is clear from Table 2 that the reported studies have been carried out on photocatalytic degradation of CR dye using different lamps such as xenon, tungstenhalogen, and high/low pressure mercury vapor, as a source for UV light irradiation.3642 In comparison to all of the sources used previously for photocatalytic degradation of CR dye, UV-LEDs are the lowest power consuming source. The increase in the titanium nanotube area and number of UVLED may reduce the time of degradation. From Table 2, it seems that UV-LED sources may be a good alternative for photocatalytic wastewater treatment.
4. CONCLUSION A facile TiO2 nanotube array, UV-LED source-based photocatalytic reactor has been designed and successfully applied for the degradation of CR dye due to the stable light emission of UV-LEDs and high adherence of TNA on the titanium metal plate. The initial rate of degradation was 0.77 107 mol L1 obtained for CR dye in the presence of UV-LED source/TNA450. The percentage degradation of CR dye decreased with an increase in dye concentration. The optimum concentration and pH were 1.4354 105 M and 6.5, respectively. Furthermore, the photocatalytic degradation of dye was confirmed by COD analysis. The photocatalytic degradation of CR dye was confirmed by kinetics studies, and it was observed that they follow the LangmuirHinshelwood kinetics model. The electrical energy for photocatalytic degradation was calculated, and it demonstrated that very low power has been utilized for the degradation of CR dye using the designed photocatalytic reactor. A possible degradation pathway of CR dye degradation was proposed using ESImass analysis. UV-LED sources may be good alternative sources to high power irradiation sources. The results suggested that photocatalytic degradation of dyes using energy efficient UV-LED source and TiO2 system is an economically viable and feasible process for future application. Further studies on the
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optimization of different factors in the designing of photocatalytic reactor such as growing of TNA at different temperatures, size of the TNA metal plate, increase in the number of UV-LEDs, and study of different parameters for photocatalytic degradation are in progress.
’ ASSOCIATED CONTENT
bS
Supporting Information. AFM, electronic impedance spectra analysis figure, ESI-mass spectra of degraded samples of CR dye, and recyclability of TNA photocatalyst surface. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ91 278 2567760, ext 718. Fax: þ91 278 2567562/ 2566970. E-mail:
[email protected],
[email protected].
’ ACKNOWLEDGMENT We acknowledge the Department of Science and Technology (DST), New Delhi, India, for financial assistance under “Fast Track Proposals for Young Scientists Scheme” (SR/FT/CS027/2009) and CSIR under Network Project (NWP 044). We also thank the Analytical Science discipline of the institute and Dr. D. N. Srivastava, Dr. Pragnya Bhatt, Mr. Jayesh C. Chaudhari, Dr. Babulal Rebary, Mr. Arun Kumar Das, Mr. Pradip Parmar, and Mr. Mahesh Sanghani for analytical support. ’ REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on TiO2 surfaces: Principles, mechanisms, and selected results. Chem. Rev. 1995, 95, 735–758. (3) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modification, and applications. Chem. Rev. 2007, 107, 2891– 2959. (4) Mills, A.; Lee, S. K. A web-based overview of semiconductor photochemistry-based current commercial applications. J. Photochem. Photobiol., A 2002, 152, 233–247. (5) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1–21. (6) Zhao, J.; Yang, X. Photocatalytic oxidation for indoor air purification: a literature review. Build. Environ. 2003, 38, 645–654. (7) Lee, S. K.; Mills, A. Detoxification of water by semiconductor photocatalysis. J. Ind. Eng. Chem. 2004, 2, 173–187. 7760
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