8436
J. Phys. Chem. 1996, 100, 8436-8442
Environmental Photochemistry on Semiconductor Surfaces. Visible Light Induced Degradation of a Textile Diazo Dye, Naphthol Blue Black, on TiO2 Nanoparticles Chouhaid Nasr,† K. Vinodgopal,*,‡ Luke Fisher,‡ Surat Hotchandani,† A. K. Chattopadhyay,§ and Prashant V. Kamat*,⊥ Department of Chemistry, Indiana UniVersity Northwest, Gary, Indiana 46408; Centre de Recherche en Photobiophysique, UniVersite´ du Que´ bec a` Trois RiVie` res, Trois RiVie` res, Que´ bec, G9A 5H7, Canada; ICI Canada Incorporated, 701 Richelieu BouleVard, McMasterVille, Quebec, J3G 6N3, Canada; and Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: NoVember 30, 1995; In Final Form: February 20, 1996X
Visible light induced degradation of the textile diazo dye Naphthol Blue Black (NBB) has been carried out on TiO2 semiconductor nanoparticles. Diffuse reflectance transient absorption and FTIR techniques have been used to elucidate the mechanistic details of the dye degradation. The failure of the dye to degrade on insulator surfaces such as Al2O3 or in the absence of oxygen further highlights the importance of semiconducting properties of support material in controlling the surface photochemical processes. The primary event following visible light excitation is the charge injection from the excited dye molecule into the conduction band of the semiconductor TiO2, producing the dye cation radical. This was confirmed by diffuse reflectance laser flash photolysis. The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the dye cation radical and photoinjected electrons. Diffuse reflectance FTIR study facilitated identification of reaction intermediates and end products of dye degradation. By comparison with the degradation products from other azo dyes such as Chromotrope 2B and Chromotrope 2R we conclude that the NBB is degraded to a colorless naphthaquinone-like end product.
Introduction Over the past few years, a large amount of work has been done on the photosensitization of semiconductors such as TiO2 by a variety of colored organic and inorganic compounds (see for example, refs 1-6). One reason for this interest stems from the ability of these sensitizers to extend the photoresponse of large bandgap semiconductors into the visible region. This has led to increased applications in photoelectrochemical cells,5,6 imaging science,7,8 and more recently the degradation of colored organic contaminants.9-15 The principle of photosensitized degradation on a semiconductor particle is illustrated in Scheme 1. Following initial excitation of the adsorbed dye, electron is then injected from the excited dye into the semiconductor particle.1-3,5,6 The driving force for this charge injection process is the energy difference between the oxidation potential of the excited sensitizer and the conduction band of the semiconductor. By carefully eliminating the regeneration step, it is possible to initiate oxidative degradation of the dye. A variety of colored compounds have been examined to investigate the photosensitized degradation on semiconductor surfaces.16-24 In all of the cases referenced above, the colored compound (in the absence of a redox couple), when adsorbed on the semiconductor surface, is bleached following steady state photolysis. The irreversible bleaching results in the destruction of the chromophore. Textile dyes and other commercial colorants have emerged as a focus of environmental remediation efforts.25-27 These efforts have largely been targeted at removing colorants from wastewater effluents of textile mills and other colorant manu†
Universite´ du Que´bec. Indiana University Northwest (email:
[email protected]. edu). § ICI Chemicals. ⊥ University of Notre Dame (email:
[email protected]). X Abstract published in AdVance ACS Abstracts, April 15, 1996. ‡
S0022-3654(95)03556-8 CCC: $12.00
SCHEME 1: A Photosensitization Approach for Degrading Dye Molecules on a Semiconductor Surfacea
a The charge injection from excited sensitizer (S*) into conduction band (CB) of a large bandgap semiconductor initiates its oxidation.
facturers. Incomplete decolorization of the effluent prior to discharge shifts the burden of treatment to publicly owned water treatment facilities (POWT’s). In the POWT, these dyes often end up as sludges, which are then dewatered and eventually deposited in landfills.25 Within the overall category of dyestuffs, azo dyes constitute a significant portion and probably have the least desirable consequences in terms of the surrounding ecosystem. In recent review articles it has been suggested that photosensitized degradation on semiconductor surfaces can have important applications for the remediation of a particular class of colored organic pollutants such as textile dyes.9-11 The photosensitization process of degradation has certain inherent advantages in that it utilizes visible light for degrading these colored compounds and can find applications in potentially difficult matrices such as sludge cakes containing dyestuffs from treatment plants. In an earlier work,28 we showed that a simple monoazo dye such as Acid Orange 7 can be oxidized to a colorless carboxylic acid end product by photosensitization on a TiO2 semiconductor surface. To the best of our knowledge, the photosensitized degradation of larger dyes has not been reported. Oxidative decolorization of these compounds by conventional methods such as ozonation is expected to be © 1996 American Chemical Society
Environmental Photochemistry on Semiconductor Surfaces significantly harder than the simple monoazo dyes. We present here results of the photosensitized degradation of the diazo dye Naphthol Blue Black on TiO2 particles. Time-resolved transient
absorption studies using diffuse reflectance laser flash photolysis have been carried out to elucidate the kinetic and mechanistic aspects of the NBB photodegradation on TiO2 while diffuse reflectance absorption and Fourier transform infrared (DRIFT) spectroscopies have been used to investigate the end products of this degradation. We show that the degradation albeit slower than that of Acid Orange 7 nevertheless results in destruction of the visible chromophore. Experimental Section The NBB dye obtained from Aldrich is only about 80% pure. It was purified by column chromatography using a neutral silica column and ethyl acetate, ethanol, and water as eluents. The dyes Chromotrope 2B and Chromotrope 2R were obtained from Aldrich and used as is without purification. TiO2 (product name P-25) was a gift sample from Degussa Corp. TiO2 has a particle size of 30 nm and surface area 50 m2/g. Samples of NBB on TiO2 were prepared on the basis of adsorption isotherm measurements. The adsorption isotherm of NBB on TiO2 was determined by mixing 100 mL aliquots of aqueous solutions of the dye at various initial concentrations Ci at natural pH with a fixed weight (0.2 g) of Degussa P25 TiO2. The suspensions were stirred overnight in the dark and then filtered. The absorbance of the filtrate was then measured at 618 nm to determine the dye concentrations in it. The decrease in the dye concentration (∆C) measured following filtration was used to determine the extent of equilibrium adsorption. From the adsorption-induced decrease in the molarity (∆C) and V, the volume of dye solution aliquot, n2s, the number of dye moles adsorbed per gram of TiO2, can be determined, where n2s ) (V∆C)/W and W is the weight of TiO2 in grams. All the isotherm measurements were made in the dark. For the diffuse reflectance FTIR and steady state photolysis studies, samples of dye adsorbed TiO2 were prepared in the same manner described above such that the coverage on the samples amounted to 4 µmol of NBB/g of TiO2. The samples were prepared on the day previous to the measurements and stored in the dark in a vacuum desiccator to prevent degradation. (Long-term exposure to room light can result in significant degradation.) Optical Measurements. The diffuse reflectance absorption spectra of azo dye coated oxide samples were recorded with a Milton Roy 3000 array spectrophotometer with a diffuse reflectance attachment. The measurements on degassed samples were carried out in a vacuum-tight 6 × 3 × 40 mm3 rectangular quartz cell which was custom designed for the optical measurements.24 Steady state photolysis was executed with visible light from a 30 W halogen lamp (Fiber-Lite Model 190 Fiber Optic Illuminator). The white light output was 70 mW/cm2 with approximately 9.3 × 1014 photons/s at 615 ((5) nm at the sample position. The sample was either pressed between the
J. Phys. Chem., Vol. 100, No. 20, 1996 8437 two microscope slides or filled in a high-vacuum spectrophotometer cell described above. Diffuse Reflectance Laser Flash Photolysis Experiments. The details of the experimental setup for the time-resolved diffuse reflectance laser flash photolysis experiments have been described earlier.18,29 The sample was excited with the 532 nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta Ray DCR-1 Nd:YAG laser system. The monitoring source was a 1000 W xenon lamp. The diffusely reflected monitoring light from the sample was collected and focused onto a monochromator which was fitted to a photomultiplier tube and the photomultiplier output was input to a Tektronix 7912A digitizer. The cell was shaken to expose a fresh surface for excitation, prior to triggering each laser pulse. The principle of diffuse reflectance laser flash photolysis is described by Wilkinson and his collaborators.30-32 Diffuse Reflectance FTIR Experiments. All FTIR experiments were performed with the samples exposed to air. The diffuse reflectance spectra of these air-equilibrated samples were measured in the region 4000-400 cm-1, at a resolution of 8 cm-1 using a Bio-Rad FTS 165 or FTS 175 FTIR spectrometer and the Bio-Rad diffuse reflectance accessory. In most cases no enhancement in spectral features was observed when the resolution was increased from 8 to 4 cm-1. Hence, the spectra were all obtained at 8 cm-1 resolution. Depending on the particular sample, KBr or TiO2 was used as the background. Because of strong absorbance by oxides, TiO2, spectral data below 1100 cm-1 for the adsorbed species were not measurable. Similarly, information above 2500 cm-1 was also difficult to obtain because of adsorbed water or surface OH groups on the oxide surfaces. The diffuse reflectance FTIR spectrum of the pure dye was obtained by mixing it with KBr powder. The diffuse reflectance FTIR spectra of the NBB dye adsorbed on TiO2 particles as also the Chromotrope 2B and Chromotrope 2R dyes on TiO2 were monitored while undergoing steady-state photolysis using visible light from a Fiber-Lite Model 190 Fiber Optic Illuminator. Results Excited Singlet State of NBB. Textile azo dyes are usually not reactive under visible light because of their short-lived excited states. Deactivation of the excited singlet state usually occurs via a nonradiative internal conversion. Dyes such as Acid Orange 7 have failed to yield triplet excited states by direct laser pulse excitation, thus confirming poor intersystem crossing for this class of dyes.33 NBB has a strong absorption in the visible region with a broad maximum at 618 nm in water and 619 nm in ethanol. The molecule does not fluoresce in solutions or on TiO2 surface. This makes excited state characterization rather challenging. Picosecond transient laser spectroscopy was used to investigate the excited singlet state of NBB in aqueous and ethanolic media. The transient absorption spectrum recorded following 532 nm laser pulse excitation of NBB in ethanol is shown in Figure 1. The difference absorption spectrum shows both the characteristic bleaching of the ground state at 620 nm and a positive absorption at 500 nm region. The bleaching at 620 nm indicates the depletion of ground state molecules as they populate the excited singlet state. The positive absorption band at 500 nm corresponds to the absorption of the excited singlet state. A first-order kinetic analysis of the transient absorption decay at 500 nm (inset in Figure 1) suggests that the excited singlet state is short-lived with a lifetime of 25-30 ps. Similar excited state behavior of NBB was also observed in water. Absence of any other long-lived transients suggests that the dye does not undergo intersystem crossing to generate an excited
8438 J. Phys. Chem., Vol. 100, No. 20, 1996
Nasr et al. 1, a plot of n2s vs Ceq should be linear with a slope equal to nsK. In the initial low concentration regimes, this expected linearity is borne out in the plots. But at higher concentrations of NBB, the number of moles of adsorbed material reaches a plateau. Assuming a value of ns ) 4 × 10-4 mol of sites/g of TiO2,37 the adsorption isotherm equilibrium constant is deduced from the slope of the best fit line in Figure 2 to be 4.125 × 103 dm3/mol for NBB on TiO2. However, Cunningham and others37 have argued that due to competition for the available sites with solvent molecules it is unrealistic to expect that all the available sites on TiO2 are occupied by the dye as is implicit when we use the value of ns ) 4 × 10-4 mol/g of TiO2. They have recommended linearizing the adsorption data by using either of the following equations
Figure 1. Spectrum a shows the ground state absorption spectrum of NBB in ethanol. Transient absorption spectrum of singlet excited NBB in ethanol (spectrum b) was recorded immediately after excitation of the sample with a 532 nm laser pulse (pulse width 18 ps). The inset shows the decay of the excited singlet state at 500 nm.
Ceq s
n2
Ceq n2s
Figure 2. Adsorption isotherm for NBB on TiO2. Plot of the n2s, the number of moles of NBB adsorbed per gram of TiO2, vs Ceq, the equilibrium concentration of NBB remaining in solution. Also plotted on the right-Y axis is the quantity Ceq/n2s as defined in eqs 4 and 5.
triplet state. Thus one can conclude that the excited singlet state is likely to be the major reactive state that can participate in the charge injection process (reactions 1 and 2). hν
NBB 98 1NBB* f NBB
(1)
NBB* + TiO2 f TiO2(e) + NBB•+
(2)
1
It has been shown earlier that charge injection is an ultrafast process and can be completed within a few picoseconds.34-36 Thus, the charge injection process can compete with other nonradiative deactivation processes. Adsorption Isotherms. We have determined the adsorption isotherm for NBB on TiO2 so as to assure optimum coverage of the dye on TiO2. The dye adsorbs strongly on to TiO2 particles from aqueous solution with a visible decrease in the concentration of the dye solutions subsequent to equilibration overnight with TiO2 powder in the dark. The adsorption isotherm for Naphthol Blue Black is shown in Figure 2. The behavior of n2s, the number of dye moles adsorbed per gram of TiO2 as a function of the equilibrium solute (dye) concentration, Ceq can be expressed by the equation
n2s ) (nsKCeq)/(1 + KCeq)
(3)
where ns is the total number of adsorption sites and K represents the equilibrium constant for the process of dye adsorption. In the limit of low equilibrium concentrations such that KCeq ,
)
)
Ceq s n2(max)
(4)
NAσ0 NAσ0 C + AspK Asp eq
(5)
1 s Kn2(max)
+
where ns2(max) in eq 4 represents the limiting number of moles of NBB that can be adsorbed onto a gram of TiO2 while σ0 is the average area that each NBB molecule occupies on the surface monolayer, Asp is the specific surface area of the adsorbent, and NA is Avogadro’s number in eq 5. Our plot of Ceq/n2s vs Ceq displays the expected linearity giving us from eq 4 a value of 5.96 × 10-6 for ns2(max) the limiting number of moles of NBB that can be adsorbed for a monolayer per gram of TiO2 and a value of 4.13 × 105 dm3/ mol for the equilibrium constant K. It is expected that only a small number of moles of NBB are adsorbed on the TiO2 surface, consistent with the former’s size. In fact, assuming a specific surface area of 50 m2/g for the TiO2, we calculate the average area, σ0, for the NBB molecule on the TiO2 surface to be ∼14 nm2. This suggests that along with the NBB molecule, a sizable number of solvent water molecules surrounding the dye molecule are coadsorbed on the surface. The presence of these coadsorbed water molecules could play an important part in the decay mechanism for NBB on the surface of TiO2. Steady State and Transient Flash Photolysis of NBB on Oxide Surfaces. When air-equilibrated samples of the dye on TiO2 were irradiated with visible light (λ > 380 nm), it underwent degradation and the colored titania powder was completely bleached. Figure 3 shows the diffuse reflectance spectra of aerated samples of NBB on TiO2. As is characteristic for most of these solid adsorbed samples, the NBB absorption band on the metal oxide surfaces is quite broad due to interaction between dye and the surface of the support material. Such an interaction is expected to alter the energetics of the electronically excited molecule.4,38 The dye degradation on the semiconductor surface is relatively slow compared to monoazo and other nonazo dyes.20,28 Complete bleaching of the absorption band at 615 nm for these low-coverage samples requires nearly 42 h. In these experiments, the coverage of NBB on TiO2 corresponds to submonolayer amounts and therefore one expects all the dye molecules to interact with the semiconductor surface and participate in the charge injection process (reaction 2). The degassed samples of NBB on TiO2 behave quite differently. The observed degradation in this case (Figure 4) is marginal and certainly does not proceed to completion over
Environmental Photochemistry on Semiconductor Surfaces
Figure 3. Diffuse reflectance absorption spectra of NBB on TiO2 following steady state photolysis of air-equilibrated samples of NBB on TiO2 with visible light. Diffuse reflectance spectra were recorded using plain TiO2 as reference. The ordinate scale is expressed in Kubelka-Munk units. Dye coverage was 4 µmol of NBB/g of TiO2 and the spectra were recorded at (a) 0, (b) 3.5, (c) 21, and (d) 42 h of irradiation.
Figure 4. Steady state photolysis of degassed samples of NBB on TiO2 with visible light. Diffuse reflectance spectra were recorded using plain TiO2 as reference at different photolysis times: (a) 0, (b) 3.5, (c) 21, and (d) 42 h of irradiation. The ordinate scale is expressed in Kubelka-Munk units. The dye coverage was 4 µmol of NBB/g of TiO2.
the same 42 h period. Spectrum c in Figure 4 which was obtained after 21 h of steady state photolysis indicates some bleaching but not to the extent that is observed in the aerated samples. The small amount of degradation that is observed probably arises from the residual oxygen which is bound to the surface of TiO2. Once this oxygen is consumed, the degradation ceases to proceed. This provides good evidence for the necessity of oxygen to scavenge photoinjected electrons during the photosensitization process (reaction 6). The reduced oxygen
TiO2(e) + O2 f TiO2 + O2-
(6)
can also play a role in the overall decay mechanism. In the absence of oxygen, the recombination between injected electrons and the cation radical of the dye results in the regeneration of the sensitizer (reaction 7).
TiO2(e) + NBB•+ f NBB + TiO2
(7)
To ensure that the visible light induced degradation of NBB is a TiO2-mediated phenomenon, we also obtained the diffuse reflectance spectra of degassed and air-equilibrated samples of NBB adsorbed on an insulator surface such as alumina. The absorption spectrum shows no change following prolonged steady state illumination with visible light. (Since no changes were observed, spectra of NBB on alumina before and after
J. Phys. Chem., Vol. 100, No. 20, 1996 8439
Figure 5. Normalized decay traces representing (a) the degradation of NBB on alumina (air-equilibrated), (b) NBB on TiO2, and (c) NBB on TiO2. Sample b was degassed while the others were equilibrated in air before the photolysis. The coverage of NBB was 4 µmol of NBB/g of TiO2.
photolysis are not shown.) The differing rates of decay of NBB on oxide surfaces as cited in the experiments above are compared in Figure 5. A first-order exponential fit of the decay for the aerated sample of NBB on TiO2 gives k a value of 8.8 × 10-4 min-1. The steady state photolysis results illustrate that degradation occurs rapidly following charge injection from the dye into the semiconductor. The electron from the excited NBB dye molecule is injected into the conduction band of the TiO2 and the cation radical formed at the surface quickly undergoes degradation to yield stable products. This charge injection process was probed by diffuse reflectance laser flash photolysis of NBB on TiO2. The transient absorption spectrum was recorded by monitoring the changes at different wavelengths in the fractional absorption, Ad, of the transient formed at the surface. Ad ) ln(I°a/It), where I°a and It are the incident and unabsorbed fraction, respectively, of analyzing light intensities at the solid sample.30-32 The time-resolved transient absorption spectra recorded 10 µs after 532 nm laser pulse excitation of degassed and air-equilibrated NBB/TiO2 samples are shown in Figure 6, A and B, respectively. The transient absorption spectrum of the degassed sample (Figure 6A) shows a broad bleaching in the 600 nm region, suggesting the depletion of the ground state dye molecules. The inset in Figure 6A shows the recovery of the bleaching at 600 nm in the degassed sample as the dye cation radical recombines with the injected electrons (reaction 7). The air-equilibrated sample (Figure 6B) shows increased bleaching in the 600 nm band. The recombination reaction between injected electron and dye cation radical is suppressed as the surface-adsorbed oxygen scavenges away the trapped electrons. The absorption-time profiles recorded at 600 nm (insets in Figure 6A,B) show the difference in the bleaching recovery of the two samples (degassed and air-equilibrated). The depletion of ground state dye in air-equilibrated sample indicates permanent changes following charge injection into the semiconductor and irreversible degradation of NBB. The reduced oxygen species could further promote degradation of the dye on the semiconductor surface. The degassed samples do show a transient absorbance at ∼470 nm and a broad absorbance at longer wavelengths above 670 nm. Pulse radiolysis studies of NBB in aqueous solution confirm that the 470 nm peak arises from the cation radical.39 Diffuse Reflectance FTIR. In order to probe the surface photochemical events of the dye NBB adsorbed on TiO2 powder and to identify the products arising from such a self-sensitized
8440 J. Phys. Chem., Vol. 100, No. 20, 1996
Nasr et al.
Figure 8. In situ diffuse reflectance FTIR spectra of NBB on TiO2 surface (4 µmol of NBB/g of TiO2) during steady state photolysis. The spectra were recorded at time intervals of (a) 0 and (b) 36 h of irradiation with visible light. Also shown (spectrum c) is the diffuse reflectance FTIR spectra of 1,2-naphthaquinone on TiO2 surface.
Figure 6. (A) Time-resolved transient absorption spectra of degassed samples of NBB on TiO2 recorded following 532 nm laser pulse excitation at 10 µs. Inset shows absorption-time profiles recorded at 600 nm of degassed samples. The dye coverage was 4 µmol of NBB/g of TiO2. (B) Time-resolved transient absorption spectra of airequilibrated samples of NBB on TiO2 recorded following 532 nm laser pulse excitation at 10 µs. Inset shows absorption-time profiles recorded at 600 nm of air-equilibrated samples. The dye coverage was 4 µmol of NBB/g of TiO2.
Figure 7. The diffuse reflectance FTIR spectra in the region 12002000 cm-1 of NBB on (a) KBr, (b) TiO2. (The dye coverage was 4 µmoles of NBB/g of TiO2).
degradation, a diffuse reflectance FTIR study of the powders was carried out at various irradiation times. The FTIR spectrum of NBB on TiO2 closely parallels that on KBr (Figure 7 a and b, respectively). In both cases, characteristic vibrations are observed at 1153, 1182, 1236, 1290, 1344, 1433, 1511, and 1583 cm-1. The band at 1344 cm-1 is by far the most intense and we attribute it to the NO2 symmetric stretching vibration by comparison with other azo dyes that contain nitro groups. The smaller peak to the red at ∼1290 cm-1 is the C-N stretch from the aromatic amine substituent on the naphthalene ring. Comparison with infrared spectrum
of other similar azo dyes suggests that the band at 1511 cm-1 is the phenyl-N stretch, while that at 1433 cm-1 is the azo NdN vibration. For symmetry reasons, the intensity of the NdN vibration is expected to be weak in an infrared spectrum but para substituents to the azo group are expected to enhance its intensity. The 1583 cm-1 is almost as intense as the 1344 cm-1 vibration and can be assigned to an aromatic ring stretch. Of all the above vibrations, those at 1511 and 1433 cm-1 are the ones that are expected to be the most sensitive to any oxidative cleavage of the azo bond. The IR spectra of NBB on both TiO2 and alumina were recorded following steady state photolysis at various time intervals. Substantial changes are observed upon photolysis of the TiO2 sample. No spectral changes are observed with alumina sample, since alumina does not possess semiconducting properties like TiO2. Only the FTIR spectrum of NBB on TiO2 is shown in Figure 8. The biggest decrease is observed in the phenyl-N vibration at 1511 cm-1 as also in the putative azo bond vibration at 1433 cm-1 which is also sharply reduced. Other changes that appear in the vibrational spectrum include the disappearance of the C-N stretch at 1290 cm-1 presumably due to oxidation of the amine substituent on the naphthalene moiety. The NO2 stretch at 1344 cm-1 is also reduced, although a distinct band persists at that frequency. All of these changes suggest that the molecule is undergoing dramatic irreversible chemical changes. Along with the decrease of these abovementioned vibrations, the spectra in Figure 8 also show a new broad band centered at 1650 cm-1 which is characteristic of a quinone functional group. Spectrum b was obtained after 36 h of irradiation and shows no change on further irradiation. Only two peaks are observed at ∼1650 and 1344 cm-1. The constancy of the 1344 cm-1 peak suggests that a nitro group persists in the photoproduct on the TiO2 surface. To further verify our vibrational assignments and to aid in the identification of the intermediates we also carried out FTIR studies of the two dyes Chromotrope 2B and Chromotrope 2R
adsorbed on TiO2 particles. These dyes shown below can be considered to be smaller versions of NBB less one azo bond link.
Environmental Photochemistry on Semiconductor Surfaces
J. Phys. Chem., Vol. 100, No. 20, 1996 8441 The pattern of behavior for the Chromotrope dyes on TiO2 is different from that of NBB but is similar to Acid Orange 7. The end product of photolysis on the semiconductor surface for both the Chromotrope dyes and the Acid Orange 7 is clearly a carboxylic acid as evidenced by the growth of a new band at 1700 cm-1. These results suggest that the simple monoazo dyes such as the Chromotrope dyes can be oxidized with relative ease to a carboxylic acid but for a complex diazo dye such as NBB on the TiO2 surface, oxidation appears to terminate at the quinone intermediate. Discussion
Figure 9. (A) In situ diffuse reflectance FTIR spectra of Chromotrope 2R on TiO2 surface (0.10 µmol of 2R/g of TiO2) during steady state photolysis. The spectra were recorded at time intervals of (a) 0 and (b) 36 h of steady state photolysis. (B) In situ diffuse reflectance FTIR spectra of Chromotrope 2B on TiO2 surface (0.10 µmol of 2R/g of TiO2) during steady state photolysis. The spectra were recorded at time intervals of (a) 0 and (b) 36 h of steady state photolysis.
Our prime motivation for choosing these two compounds was (i) to assist in the assignment of the vibrational peaks of NBB and (ii) to establish whether the degradation of NBB was a sequential cleavage of the two azo bonds so that a Chromotrope type moiety could be an intermediate in the degradation process. The FTIR spectra of the dyes Chromotrope 2B and Chromotrope 2R on TiO2 are shown in Figure 9, A and B, respectively. The major structural difference between Chromotrope 2R and the other dyes investigated in this work is the absence of a NO2 group para to the azo bond in the former. Predictably, the Chromotrope 2R dye on TiO2 does not show the characteristic NO2 symmetric stretching band observed at 1344 cm-1 in NBB and Chromotrope 2B. Both Chromotrope 2B and Chromotrope 2R exhibit the phenyl-N vibration at 1510 cm-1. We observe a strong band at this frequency for a variety of azo dyes adsorbed on TiO2 including 4-phenylazophenol, Acid Orange 7, Chicago Sky Blue, etc.39 Given the reproducibility of this band in all of these azo dyes, we believe that it is an identifying vibration for these azo dyes. The changes observed following photolysis on the TiO2 surface of the Chromotrope dyes are in some respects similar to NBB, but the end product of photolysis of the Chromotrope dyes on TiO2 is different from NBB. The characteristic 1510 cm-1 vibration disappears almost completely and a new band appears at 1700 cm-1. Oxidative cleavage in this class of azo dyes is expected to occur at the point where the azo bond is attached to the naphthalene moiety giving rise to a naphthaquinone type intermediate.40,41 This intermediate is then expected to be oxidized subsequently to a carboxylic acid derivative. In contrast, the FTIR spectrum of the product resulting from steady state photolysis of the NBB on TiO2 surfaces does not show the characteristic carboxylic acid vibration at 1700 cm-1 but instead displays the characteristic quinone type vibration at 1650 cm-1. Assuming on the basis of the FTIR spectrum that the oxidative pathway in the photosensitized degradation of NBB proceeds only as far as the quinone intermediate, we prepared samples of 1,2-naphthaquinone on TiO2. The FTIR spectrum of 1,2-naphthaquinone on the TiO2 surface is shown as spectrum c in Figure 8. The spectrum of the photolyzed NBB is relatively similar, especially in the area of 1400-1700 cm-1, to the spectra of the naphthaquinone samples. It is reasonable to conclude therefore that some form of quinone is the end product of the photosensitized degradation of NBB on TiO2.
The steady state diffuse reflectance absorption and laser flash photolysis studies presented above confirm that the NBB dye from its excited singlet state injects electrons into the semiconductor particle and that such a charge injection process is rapid occurring within the lifetime of the excited singlet state (2530 ps). Events consequent to charge injection are dependent on whether coadsorbed oxygen and water molecules are present or not. In degassed samples, the cation radical of the dye formed at the TiO2 surface quickly recombines with trapped electrons thereby averting any degradation, whereas in the air-equilibrated samples the cation radical decays to a quinone like end product. The laser flash photolysis data supports such a charge injection mechanism. Our FTIR studies provide us some important insight into the nature of the products in so far as we are able to pinpoint the ultimate product in the case of NBB to be a quinone. What is intriguing, however, is that the smaller Chromotrope dyes do degrade to a carboxylic acid type product. In the case of NBB, the evidence from the bleaching on the TiO2 surface suggests that oxidative cleavage is occurring around both azo bonds. If this was not the case, a Chromotrope type moiety would result and our FTIR data do not support such a conclusion. We can only conclude therefore that subsequent to cleavage of the two azo bonds, the quinone like molecule that is formed on the surface is incapable of undergoing further oxidation. The FTIR results seem to suggest that the amino substituent on the naphthalene ring in NBB is oxidized to a nitro group. This would account for the residual intensity of the NO2 vibration at 1344 cm-1. Nitro groups by virtue of their electronwithdrawing capabilities would impede any further oxidation of the quinone that is formed. Such a mechanism is the best possible explanation for the formation of a quinone as the end product of oxidative degradation of NBB on TiO2 surfaces. Other analytical techniques are currently being attempted to identify precise structure of the end product. Conclusions Visible light induced degradation of a complex diazo dye, Naphthol Blue Black, has been carried out on TiO2 particles. Diffuse reflectance measurements carried out with these samples confirm the degradation to proceed via the photosensitization pathway and could provide a unique way to decolorize these dyes in difficult matrices. While complete mineralization of the dye has not been achieved on the surface, it is likely that such a degradation can be achieved by using broad band illumination that would combine both the conventional photocatalytic and photosensitized degradations. Acknowledgment. We thank Degussa Corp. for supplying us the gift sample of TiO2 and Al2O3 powders. K.V. acknowledges the support of Indiana University Northwest through a Grant-In-Aid. P.V.K. acknowledges the support of the Office of Basic Energy Sciences of the U.S. Department of Energy.
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