- Email: [email protected]
Photocatalytic degradation of X-3B dye by visible light using lanthanide ion modified titanium dioxide hydrosol system
Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Photocatalytic degradation of X-3B dye by visible light using lanthanide ion modified titanium dioxide hydrosol system Yibing Xiea,b,∗ , Chunwei Yuana , Xiangzhong Lib b
a Key Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, China Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong
Received 12 March 2004; accepted 7 October 2004
Abstract The present work was focused on photocatalytic activity of sol photocatalysts. The lanthanide neodymium ion modified titania sol was prepared by chemical coprecipitation–peptization method and its photoactivity was studied by investigating the photodegradation efficiency of active brilliant red dye X-3B in hydrosol reaction system. It was noted that pure TiO2 and Nd3+ –TiO2 sol particles had anatase crystalline structure, uniform nanoparticles distribution and spheral particle morphology, which were prepared at low temperature (70 ◦ C), ambient pressure and acidic condition (pH 1.5). This preparation method was much better than traditional high temperature calcination process to fabricate crystal TiO2 . The average size was 10 nm for Nd3+ –TiO2 and 25 nm for TiO2 sol particles. The sol photocatalysts in hydrosol system demonstrated better interfacial adsorption effect and photoactivity than commercial P25 TiO2 powder in suspension system, which was due to small particle size and well nanoparticles dispersion. Moreover, under visible light irradiation (vis, λ > 400 nm), Nd3+ –TiO2 sol showed higher photocatalytic activity than TiO2 sol, which was ascribed to the electron trapping effect of modified Nd3+ ion on TiO2 sol particles. Additionally, photosensitization-photocatalysis mechanism was discussed in the VIS/X-3B/Nd3+ –TiO2 hydrosol reaction system. © 2004 Elsevier B.V. All rights reserved. Keywords: TiO2 ; Sol photocatalyst; Visible light; Photoactivity; Lanthanide ion modification
1. Introduction Dyes wastewater released into environment mainly by dyestuff and textile industry cause severe ecological problems. These compounds are highly colored and can heavily contaminate water source. Azo dye, comprising various synthetic dyestuffs, even can be potentially reduced into carcinogenic aniline. Furthermore, they are recalcitrant to microbial decomposition due to various chemical structures. The traditional technologies are not practically feasible. The chemical oxidation by chemicals is too costly and physical adsorption by active carbon often causes secondary pollution [1]. Many attempts have been carried out to develop efficient biological methods to decolorize these effluents. But these methods ∗
Corresponding author. E-mail address: [email protected] (Y. Xie).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.10.061
have still not been very successful. Some techniques about advanced oxidation processes (AOPs), such as Fenton’s reagent oxidation (Fe2+ (or Fe3+ )–H2 O2 ), photo-Fenton oxidation (UV/Fe2+ (or Fe3+ )–H2 O2 ), ozone–hydrogen peroxide oxidation (O3 –H2 O2 ) and photo-ozone oxidation (UV/O3 ), was applied to degrade various organic dyes [2–4]. These AOPs techniques are very effective and total abatement of dyes can achieved in a short time. But high cost limits its wide application in the area of organic pollutants treatment. The heterogeneous photocatalysis appears as another promising oxidation technology leading to the total mineralization of most of organics. Semiconductor TiO2 as a photocatalyst has also been deeply investigated and can successfully degrade various organic pollutants [5–7]. TiO2 semiconductor has high bandgap (3.2 eV), which limits its wide application in visible light range of solar spectrum. So, it only absorbs in the ultraviolet range, and efficient
88
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
collection of the solar spectrum requires sensitization by a modification molecule absorbing in the visible range. Therefore, in order to realize the solar decontamination process, many efforts are contributed to develop the second-generation photocatalysts that can drive the photodegradation reaction under visible light [8]. The photosensitizer modification and transition metals doping are usually adopted methods on the base of crystal TiO2. However, many researchers have disclosed uncertain doping effects (even controversial results) about transition metal ion dopants [9]. As one of main pollutants, the dye molecule and dyestuff intermediate themselves have good absorption in visible range. So, photodegradation reaction of dye wastewater can be achieved under visible light illumination by taking good advantage of visible absorption of object molecules. Although the oxidation methods by TiO2 /visible light do not show so effective as Fenton reagent and TiO2 /UV, this method is still very attractive due to potential application of realizing solar decontamination. Another problem is how to overcome the growth of TiO2 nanocrystallites during calcining process at high temperature (>400 ◦ C) and serious aggregation of prepared nanoparticles when dispersed in aqueous solution. Regarding the titania phase state, amorphous TiO2 system was seldom studied due to its photocatalytic inactivity [10]. Many attempts have been made to prepare crystal structure TiO2 at low temperature in order to overcome the traditional high temperature calcination process. The previous related studies showed that the formation of anatase crystal structure was regarded as a unique phenomenon to the SiO2 –TiO2 system, not to pure TiO2 system. The former researching works mainly focused on the fabrication of sol–gel-derived anatase nanocomposite SiO2 –TiO2 film at low temperature, whose photocatalytic activity was not investigated [11,12]. However, pure TiO2 sol particles with anatase crystalline structure was prepared at low temperature and TiO2 sol directly acted as photocatalyst, which, to the best of our knowledge, was not reported before. In this paper, the lanthanide neodymium ion was selected as a TiO2 dopant to investigate its unique physicochemical property due to its ability to form complexes with various Lewis bases in the interaction of these functional groups with the f-orbitals of the lanthanide metal. Thus, incorporation of lanthanide ion in a TiO2 matrix could provide an effective method to concentrate the organic pollutant at the semiconductor surface [13–15]. The pure TiO2 and lanthanide metal neodymium ion (Nd3+ ) modified TiO2 sol samples were prepared at low temperature, respectively. The characteristics and properties of these sol photocatalysts were studied. The photocatalytic reaction was carried out in the novel hydrosol system, which was very different from traditional suspension or immobilization reaction system. The photoactivity was evaluated by measuring photodegradation degree of representative azo dye (reactive brilliant red dye X-3B) as a probe molecule under visible light irradiation. The possibility of the sol photocatalyst in visible region was investigated. The photocatalysis mechanism of TiO2 sol system was also discussed.
2. Materials and methods 2.1. Materials Titanium dioxide powder sample used in the experiment was commercial Degussa P25 TiO2 with 80% anatase, 20% rutile and BET area of ca. 50 m2 g−1 , which was obtained from Degussa AG Company in Germany. Titanium tetrachloride (TiCl4 ) and neodymium oxide (Nd2 O3 ) were pure reagent grade and obtained from J&K China Chemical Ltd. Ammonium hydroxide (NH4 OH), nitric acid (HNO3 ), hydrochloric acid (HCl) and other chemicals were analytical reagent grade quality and all obtained from Shanghai reagent company. The high purity water used in the experiment was double distilled and then purified with the Milli-Q system. The substrate of X-3B dye was obtained from Shanghai Dyestuff Chemical Plant and used without further purification. Fig. 1 displays the structure of reactive brilliant red dye X-3B. 2.2. Experimental procedure Nd3+ modified TiO2 sol was prepared by chemical coprecipitation–peptization method [16]. Firstly, 50 mL TiCl4 was diluted and hydrolyzed with 100 mL deionized distilled frozen water (0 ◦ C). Nd2 O3 was added to the above solution to produce transparent Nd3+ aqueous solution according to required doping amount of neodymium (Nd3+ ion dopant equivalent to 3.0 atom percent of Ti4+ in bulk solution) under vigorous stirring. In order to ensure its complete hydrolysis reaction, a diluted NH4 OH aqueous solution (10%) was added dropwise into transparent TiCl4 aqueous solution to obtained a grey precipitate, giving an ultimate suspension of pH 10. In order to remove residual NH4 + and Cl− ions, the precipitate was adequately wished with deionized water till the pH value of filtrated water was below 7.5. Then, the acquired amorphous Nd3+ –TiO2 was well dispersed into water and added with nitric acid solution (20%) in corresponding amount as a peptization catalyst and phase-transfer accelerant. The above suspension was adjusted to pH 1.5 and stirred for 4 h at room temperature, then peptized at 70 ◦ C for 24 h in airproof condition. Finally, 3.0 atom percent Nd3+ ion modified TiO2 sol was formed with uniform,
Fig. 1. Molecule structure C19 H10 O7 N6 Cl2 S2 Na2 ).
of
dye
X-3B
(chemical
formula =
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
stable, and semitransparent characteristics. The obtained sol can maintain homogenous distribution for quite a long time without sedimentation and delamination phenomena. The Nd3+ modified titania xerogel powder was prepared by aging, gelation and vacuum drying treatment of sol sample at 70 ◦ C for 8 h. The same procedure was adopted to prepare TiO2 sol. The experiment of photocatalytic reaction was conducted in a cylindrical quartz photoreactor with an effective cubage of 40 mL. Nd3+ –TiO2 and TiO2 sol were acted as photocatalyst and visible light as illuminating light source. The aqueous reactants mixture was vertically irradiated under the visible light with constant stirring speed at room temperature. Reaction system was setup by adding right-amount of sol photocatalyst (adding amount equivalent to 1.0 g L−1 TiO2 in solution) into 20 mL 100 mg L−1 X-3B solution. The pH value of X-3B/sol photocatalyst mixture was adjusted to 5.0 for hydrosol system. Prior to photocatalytic reaction, the dark adsorption experiment was firstly carried out under airproof condition. The mixture of X-3B and sol photocatalyst was sealed and continuously stirred in darkness for 30 min to establish an adsorption–desorption equilibrium. Degussa P25 TiO2 powder was also used as a standard photocatalyst to compare the catalytic activity with pure TiO2 sol and Nd3+ –TiO2 sol. 2.3. Analytic methods The particulate morphology of photocatalysts was observed on atom force microscope (AFM, Nanoscope III System, American Digital Corporation, USA) and scanning electron microscope (SEM, LEO 1530VP Field Emission Scanning Electron Microscope, LEO Electron Microscopy Inc., Germany). The phase state of samples were identified by X-ray diffractometer (XRD, XD-3A, Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu K␣) at 40 kV, 40 mA over the diffraction angle (2θ) range 10–90◦ . The cold light source acts as visible light illumination (LGY-150, 150w, halogen–tungsten lamp with UV and IR cut-off filter). Photoemission spectrum was measure by Optical Fiber Spectrometer (Model S2000, Ocean Optic Inc., USA), which mainly emits spectrum in the range of 400–800 nm and the main wavelength locates at 540 nm (see Fig. 2). Absorption spectrum of dye X-3B was also measured on UV–vis Recording Spectro-Photometer (UV-2201, Shimadazu Corporation, Japan). By comparison, the maximum emission and absorption spectrum was well matched in the range of visible light. The pH value of reaction medium was controlled by acidimeter (pHS-2, Shanghai Rex Instruments Factory). The photodegrated product was collected every 15 min. The X3B/P25 TiO2 suspension sample was filtered through two layers of Millipore 0.22 m films. The X-3B/TiO2 hydrosol mixture was centrifuged at 10 000 rpm for 15 min and then filtered by a 0.22 m Millipore filter. X-3B concentration was
89
Fig. 2. Photoemission spectrum of visible light source.
determined by measuring characteristic absorption intensity in X-3B absorption spectrum (Fig. 3).
3. Results and discussion 3.1. AFM and SEM analysis The sol particle morphology and particle size were investigated by AFM and SEM. The samples were fabricated on silica glass by means of spin-coating. AFM micrograph shows that two types of sol particles all had spheroidal shape and were uniformly spread out on the support. The average size were about 10 nm for Nd3+ –TiO2 sol particles and 25 nm for TiO2 sol particles (Fig. 4(A) and (B)). However, compositive single nanoparticle of P25 TiO2 was about 45 nm. These particles in aqueous suspension tended to aggregate and finally formed congeries with hundreds of nanometer in mean size (Fig. 4(C)). SEM micrograph shows that well-developed Nd3+ –TiO2 sol particles dispersed in colloidal system and continuously close-packed particles formed an alveolate network structure (Fig. 5). The above result means that sol particles obviously have better particles distribution in aqueous medium than P25 TiO2 powder. More-
Fig. 3. UV–vis absorption spectrum of 100 mg L−1 X-3B.
90
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Fig. 5. SEM micrograph of Nd3+ –TiO2 sol sample.
over, the difference of two sorts of sol particles indicates that Nd3+ ion modification preferably retarded the aggregation and growth of well-dispersed sol particles, which resulted in smaller grain size of Nd3+ –TiO2 sol particles. 3.2. XRD analysis The XRD patterns of Nd3+ modified TiO2 sol and bare TiO2 sol samples all show the presence of peaks (2θ = 25.4◦ , 38.0◦ , 48.0◦ , 54.6◦ ), which was regarded as an attributive indicator of anatase phase titanium dioxide crystallites (Fig. 6) [17]. The characteristic phase state of two types of sol samples can be considered as anatase crystalline structure due to the appearance of standard diffraction peaks with somewhat weak scattering peaks characterization. The sol particles have very broad diffraction peaks at (1 0 1) plane (2θ = 25.4◦ ), whose nature is due to their very small grain size. Both of sol samples do not appear any other diffraction peaks of new crystal phase besides anatase structure. It indicates that Ti–O atom order-structure in anatase TiO2 sol particles cannot be rearranged into rutile TiO2 structure under moderate preparation condition. As for Nd3+ –TiO2 sol, another main reason was attributed to the difference in ionic radius between neodymium and titanium ion (r(Ti4+ ) = 0.064 nm,
Fig. 4. AFM micrographs of (A) Nd3+ –TiO2 sol particles, (B) TiO2 sol particles and (C) P25 TiO2 powder particles (the x–y axis is scaled in micrometers).
Fig. 6. X-ray diffraction patterns of (a) TiO2 sol particles and (b) Nd3+ –TiO2 sol particles.
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
r(Nd3+ ) = 0.11 nm). Because the radius of Nd3+ was much larger than that of Ti4+ , without calcination treatment in high enough temperature, neodymium ions introduced by the coprecipitation–peptization method would unlikely enter into the TiO2 crystal lattice structure. Actually, during TiCl4 hydrolysis and peptization reaction, neodymium and oxygen ions could form Nd–O oxide on the superficial layer of TiO2 sol particles by chemical bonding process and Nd3+ ions mainly existed as Nd–O oxide in the Nd3+ –TiO2 mixture. Conversion from amorphous to crystalline structure usually requires high calcination temperature of at least 400 ◦ C. The above result indicates that long time peptization and digesting processing at high acidity and low temperature condition promotes the transformation of TiO2 sol particles from amorphous to crystal phase. The coprecipitation– peptization method can be promoted as a promising way to fabricate crystalline or semicrystalline TiO2 nanoparticles. 3.3. Interfacial adsorption effect The effect of adsorption–desorption equilibrium was investigated between 100 mg L−1 X-3B and 1.0 g L−1 photocatalysts in aqueous medium. X-3B solution with different photocatalysts was kept stirring for 30 min in darkness at 25 ◦ C, airproof condition. X-3B concentrations in bulk solution were measured by UV–vis spectrometer. Fig. 7 shows that the absorption intensity of X-3B at 512 and 536 nm in bulk solution decreased in comparison with original X-3B solution. The decrease of X-3B concentration due to adsorption by different photocatalyst was 30.46, 24.39 and 15.22 mg L−1 for Nd3+ –TiO2 sol, TiO2 sol and P25 TiO2 powder, respectively (see Fig. 8). Nd3+ –TiO2 sol system exhibits the superior adsorption effect on X-3B molecule, which almost has twice adsorption amount in comparison with P25 TiO2 powder.
Fig. 7. UV–vis absorption spectra of different X-3B/photocatalysts mixture after 30 min adsorption processing in darkness: (a)100 mg L−1 X-3B, (b) X-3B/P25 TiO2 suspension system, (c) X-3B/TiO2 sol system and (d) X3B/Nd3+ –TiO2 sol system (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ).
91
Fig. 8. Adsorption concentration of X-3B in terms of different photocatalysts after 30 min darkness adsorption (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ).
The heterogeneous photocatalysis was mainly occurred in molecule interfacial layers. So, the affinity and adsorption properties between reactants and photocatalyst surface play an important role on determining overall reaction rate [18,19]. Regarding the strong adsorption capability of X-3B on the surface of Nd3+ –TiO2 sol nanoparticles, the key factor was ascribed to the positive charge characteristic of Nd3+ ion banded on the TiO2 nanoparticles surface since anionic dye X-3B molecule loads negative charge. Moreover, the formation of Lewis acid–base complex between lanthanide ion and the dye substrate enhanced interfacial adsorption effect. Another important factor was attributed to the well-dispersed sol nanoparticles (10 nm for Nd3+ –TiO2 sol and 25 nm for TiO2 sol in mean size), which was very different from P25 TiO2 powder particles (45 nm for individual particle and >100 nm for particles aggregation in mean size). So, TiO2 sol particles showed better adsorption effect than P25 TiO2 powder. 3.4. Photocatalytic reaction The photocatalytic activity of Nd3+ –TiO2 sol was studied by investigating the degradation experiment of X-3B dye. UV–vis absorption spectra of X-3B solution with Nd3+ –TiO2 sol, TiO2 sol and P25 TiO2 photocatalysts were measured every 15 min following the visible light illumination. Fig. 9 shows the temporal changes of X-3B spectra with the given irradiation time intervals in the different photocatalyst/X-3B reaction system. In the studied concentration range, there is a good linear relationship between characteristic absorption intensity and X-3B concentration. Fig. 10 shows the decrease of X-3B concentration in dependence on visible light irradiation time by different photocatalysts. Experimental results show that X-3B concentration in the above different reaction system all decreased with the photocatalysis processing. The photocatalytic degradation is supposed to follow pseudo first-order reaction kinetics for the studied concentration range. The reaction kinetics can often be described in terms of the Langmuir–Hinshelwood model
92
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Fig. 10. Photocatalytic degradation of X-3B on various TiO2 photocatalysts: (a) P25 TiO2 powder, (b) TiO2 sol and (c) Nd3+ –TiO2 sol (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ; initial X3B/photocatalyst mixture through 30 min adsorption in darkness).
[20]. The curve fitting formula and calculated results were summarized in Table 1. Considering the effect of initial reaction concentration of X-3B in solution on its degradation kinetics, we modified the pseudo-first order kinetic constant (k) with another amended constant (k ), which could remove the effect of initial reactant concentration with the function of k = k(c0 )0.7 [21]. By comparison of k and r value, it shows that the overall photocatalytic degradation efficiency was observed to decrease in the following order: Nd3+ –TiO2 sol > TiO2 sol > P25 TiO2 powder, which was in good agreement with the above result of solid–liquid interfacial adsorption effect by different photocatalyst. According to the principles of semiconductor TiO2 photocatalysis, the photoactivity is mainly dependent on three factors: (a) electron–hole generation capacity; (b) electron transfer route and efficiency; (c) separation efficiency of photogenerated charge pairs. The sensitized photocatalysis mechanism is very different from UV light induced photocatalysis mechanism. The dye sensitization process involves the excitation of X-3B dye molecule with visible light and subsequent electron injection or electron transfer from excited state dye molecule to conduction band of semiconductor TiO2 . The electron generation capacity mainly depends upon the intensity of incident photos with matchable energy and absorption amount of dye molecule on photocatalysts surface. The separation of electron–cation radical and electrons transfer efficiency depend on the electron acceptor (usually TiO2 conduction band, triplet ground state O2 or other scavengers)
Fig. 9. UV–vis absorption spectra of X-3B/photocatalyst samples with different reaction time under visible light irradiation: (A) filtrate of X-3B/P25 TiO2 suspension system, (B) X-3B/ Nd3+ –TiO2 sol system and (C) X3B/TiO2 sol system (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ; initial X-3B/photocatalysts mixture was used after adsorption of 30 min in darkness).
Table 1 Photodegradation effect of X-3B solution by different photocatalyst after 120 min Photocatalyst
Model formula
R
k (min−1 )
r (%)
k
P25 TiO2 powder Nd3+ –TiO2 sol TiO2 sol
c = 84.70 exp(−0.00334t) c = 65.83 exp(−0.01499t) c = 72.61 exp(−0.00679t)
0.94641 0.99727 0.9978
0.00334 0.01499 0.00679
44.70 89.14 67.03
0.07469 0.28099 0.13632
c, X-3B concentration (mg L−1 ); t, visible light irradiation time (min); R, correlation coefficient; k, apparent rate constant (min−1 ), k , amended apparent constant and r, X-3B photodegradation ratio(%).
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Fig. 11. UV–vis absorption spectra of (a) 100 mg L−1 X-3B and (b) X-3B after 120 min direct photolysis with visible light irradiation.
and transferring route. TiO2 sol and P25 TiO2 powder crystallites referred to above photosensitization–photocatalysis processing. The difference of photoactivity between TiO2 sol and P25 TiO2 powder was ascribed to the nano-colloid size effect, adsorption effect and crystal phase structure since anatase is usually more active than rutile. The prepared TiO2 sol particles are of pure anatase crystalline structure while commercial P25 TiO2 powder is of 80% anatase and 20% rutile crystalline structure. Regarding Nd3+ –TiO2 sol system, modification ion Nd3+ has excellent electron scavenging capacity, which results in a superior transfer and separation efficiency of electron–cation radical. Considering the standard redox potentials (E0 (Nd3+ /Nd2+ ) = −0.40V, E0 (O2 /O2 − ) = 0.338 V and Ecb (TiO2 ) = −0.5V versus NHE), the presence of Nd3+ ion on nanoparticles surface may promote the following processes [22]: • The first step: Nd3+ + TiO2 (e− ) → Nd2+ + TiO2 (electron scavenging). • • The second step: Nd2+ + O2 → Nd3+ + O2 − (charge transferring).
93
As a result, photoexcitated electrons can be effectively transferred to acceptors. So, promotion of electron–cation radical separation efficiency was good for dye photodegrada• tion because less stable cation radicals (dye + ) were very susceptible to produce degradation products by molecule oxygen and superoxide anion radicals. In order to confirm photocatalysis of TiO2 sol particles, the direct photolysis of X-3B was also investigated under the same reaction condition only without photocatalyst. Fig. 11 shows the change of UV–vis absorption spectra of X-3B after direct photolysis. The result shows that only 4.25% of X-3B disappeared after 120 min photolysis. No significant decrease of the X-3B concentration was observed under visible light irradiation. It means that photolysis was a weak working factor to affect photodegradation of X-3B dye, and photocatalysis by sol photocatalyst was main reason to cause the X-3B degradation. 3.5. Photocatalysis mechanism The well-dispersed sol nanoparticles with high surface area allow high access of the reactants to the TiO2 surface in aqueous medium. Fig. 12 shows the photosensitized photocatalysis reaction mechanism of dye molecule in the Nd3+ –TiO2 sol system. In the sol photocatalysis system, physically adsorbed on the surface of the sol particles, the dye molecule was excited by absorption of a suitable visible light photon. The electrons of excited dye molecule can inject into conduction band (CB) of TiO2 . Then these electrons could be trapped by electron scavengers (usually oxygen molecule). But it was also extremely susceptible for the cation radicals and the electrons to recombine if the injected electrons accumulated in the CB of TiO2 . So, electrons trapping and electrons transfer would be two key steps to inhibit electron–cation radical recombination. The cation rad• ical (dye + ) produced by electron injection was less stable than the ground state of the compound (dye). As a result, unstable cation radical dye may directly degraded to products • or reacted with superoxide radial anion (O2 − ) to produce
Fig. 12. Sensitized photocatalysis mechanism of Nd3+ –TiO2 sol system (S0 = singlet ground state, S1 = first excited singlet, T1 = first excited triplet state, ISC = intersystem crossing).
94
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
degradation products. The photocatalysis of TiO2 sol and P25 TiO2 powder system can be interpreted with above reaction mechanism. However, in the photocatalysis of Nd3+ –TiO2 sol system, the Nd3+ species can act as an effective electron scavenger to trap the CB electrons of TiO2 , which were injected from the excited dye molecule. Nd3+ ions, as a Lewis acid, apparently was superior to the oxygen molecule (O2 ) in the capability of trapping electrons [23]. The electrons trapped in Nd3+ sites (Nd2+ ) were subsequently transferred to the surrounding adsorbed O2 by oxidation process. The presence of Nd3+ on TiO2 sol nanoparticle surface may promote following pro• cesses (Nd3+ + e → Nd2+ and Nd2+ + O2 → Nd3+ + O2 − ). More effective electron trapping and transferring would produce more cation radicals, which were so active to undergo degradation. Therefore, Nd3+ –TiO2 sol showed the superior photocatalytic activity in comparison with TiO2 sol and P25 TiO2 powder. 4. Conclusions We set up a new method in which pure TiO2 sol (or Nd3+ –TiO2 sol) particles with anatase crystalline structure were prepared at low temperature and TiO2 sol (or Nd3+ –TiO2 sol) directly acted as photocatalysts. Considering above results, it is feasible to conduct photocatalysis in hydrosol reaction system. Modification of lanthanide neodymium ion to TiO2 sol greatly improved interfacial adsorption property and photocatalytic activity due to small size effect and interfacial positive charge lanthanide ion effect of Nd3+ –TiO2 sol particles. By taking advantage of sensitiveness to the pH value, TiO2 sol photocatalyst can be applied to recycle use in photocatalytic reaction while keeping its photoactivity. So, the novel hydrosol reaction system integrated the advantages of suspension and immobilization reaction system (high photocatalytic efficiency and feasible separation of photocatalysts for recycle use) [24]. How to keep high photocatalytic activity after many times of recycle use of TiO2 sol in the present study is currently under investigation. In a sense, the effective photodegradation dye by TiO2 sol under visible light is a very exciting respect in photocatalytic area.
Acknowledgements This work was financially supported by the Hi-Tech Research and Development Program (863 Program) of China (Grant No. 2002AA302304), the National Natural Science Foundation of China (Grant No. 60121101).
References [1] F. Nerud, P. Baldrian, J. Gabriel, D. Ogbeifun, Chemosphere 44 (2001) 957. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. [3] M. Ksibi, A.S. Ben, S. Cherif, E. Elaloui, A. Houas, M. Elaloui, J. Photochem. Photobiol. A 154 (2003) 211. [4] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999) 51. [5] I. Poulios, I. Tsachpinis, J. Chem. Technol. Biotechnol. 74 (1999) 349. [6] A. Marinas, C. Guillard, J.M. Marinas, A. Fern´andez-Alba, A. Agu¨era, J.M. Herrmann, Appl. Catal. B: Environ. 34 (2001) 241. [7] K. Chiang, R. Amal, T. Tran, Adv. Environ. Res. 6 (2002) 471. [8] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [9] K.V.S. Rao, B. Lav´edrine, P. Boule, J. Photochem. Photobiol. A 154 (2003) 189. [10] L. Zang, W. Macyk, C. Lange, W.F. Maier, C. Antonius, D. Meissner, H. Kisch, Chem. Eur. J. 6 (2000) 379. [11] H. Imai, H. Hirashima, J. Am. Ceram. Soc. 82 (1999) 2301. [12] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 83 (2000) 229. [13] K.T. Ranjit, I. Willner, S.H. Bossnamm, A.M. Braun, Environ. Sci. Technol. 35 (2001) 1544. [14] J. Lin, J.C. Yu, J. Photochem. Photobiol. A: Chem. 116 (1998) 63. [15] Y.Q. Wang, H.M. Cheng, L. Zhang, Y.Z. Hao, J.M. Ma, B. Xu, W.H. Li, J. Mol. Catal. A: Chem. 151 (2000) 205. [16] L. Wang, M. Muhammed, J. Mater. Chem. 9 (1999) 2871. [17] L. Znaidi, R. S´eraphimova, J.F. Bocquet, C. Colbeau-Justin, C. Pommier, Mater. Res. Bull. 36 (2001) 811. [18] C. Hu, Y.Z. Wang, H.X. Tang, Appl. Catal. B 35 (2001) 95. [19] M.S. Chiou, H.Y. Li, Chemosphere 50 (2003) 1095. [20] C. Minero, Catal. Today 54 (1999) 205. [21] D.M. Chen, J.J. Zhong, Y.M. Wang, Fine Chem. 19 (2002) 55. [22] P. Yang, C. Lu, N.P. Hua, Y.K. Du, Mater. Lett. 57 (2002) 794. [23] G.R. Bamwenda, T. Uesigi, Y. Abe, K. Sayama, H. Arakawa, Appl. Catal. A 205 (2001) 117. [24] Y.B. Xie, C.W. Yuan, J. Mol. Catal. A: Chem. 206 (2003) 419.
Photocatalytic degradation of X-3B dye by visible light using lanthanide ion modified titanium dioxide hydrosol system Yibing Xiea,b,∗ , Chunwei Yuana , Xiangzhong Lib b
a Key Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, China Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong
Received 12 March 2004; accepted 7 October 2004
Abstract The present work was focused on photocatalytic activity of sol photocatalysts. The lanthanide neodymium ion modified titania sol was prepared by chemical coprecipitation–peptization method and its photoactivity was studied by investigating the photodegradation efficiency of active brilliant red dye X-3B in hydrosol reaction system. It was noted that pure TiO2 and Nd3+ –TiO2 sol particles had anatase crystalline structure, uniform nanoparticles distribution and spheral particle morphology, which were prepared at low temperature (70 ◦ C), ambient pressure and acidic condition (pH 1.5). This preparation method was much better than traditional high temperature calcination process to fabricate crystal TiO2 . The average size was 10 nm for Nd3+ –TiO2 and 25 nm for TiO2 sol particles. The sol photocatalysts in hydrosol system demonstrated better interfacial adsorption effect and photoactivity than commercial P25 TiO2 powder in suspension system, which was due to small particle size and well nanoparticles dispersion. Moreover, under visible light irradiation (vis, λ > 400 nm), Nd3+ –TiO2 sol showed higher photocatalytic activity than TiO2 sol, which was ascribed to the electron trapping effect of modified Nd3+ ion on TiO2 sol particles. Additionally, photosensitization-photocatalysis mechanism was discussed in the VIS/X-3B/Nd3+ –TiO2 hydrosol reaction system. © 2004 Elsevier B.V. All rights reserved. Keywords: TiO2 ; Sol photocatalyst; Visible light; Photoactivity; Lanthanide ion modification
1. Introduction Dyes wastewater released into environment mainly by dyestuff and textile industry cause severe ecological problems. These compounds are highly colored and can heavily contaminate water source. Azo dye, comprising various synthetic dyestuffs, even can be potentially reduced into carcinogenic aniline. Furthermore, they are recalcitrant to microbial decomposition due to various chemical structures. The traditional technologies are not practically feasible. The chemical oxidation by chemicals is too costly and physical adsorption by active carbon often causes secondary pollution [1]. Many attempts have been carried out to develop efficient biological methods to decolorize these effluents. But these methods ∗
Corresponding author. E-mail address: [email protected] (Y. Xie).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.10.061
have still not been very successful. Some techniques about advanced oxidation processes (AOPs), such as Fenton’s reagent oxidation (Fe2+ (or Fe3+ )–H2 O2 ), photo-Fenton oxidation (UV/Fe2+ (or Fe3+ )–H2 O2 ), ozone–hydrogen peroxide oxidation (O3 –H2 O2 ) and photo-ozone oxidation (UV/O3 ), was applied to degrade various organic dyes [2–4]. These AOPs techniques are very effective and total abatement of dyes can achieved in a short time. But high cost limits its wide application in the area of organic pollutants treatment. The heterogeneous photocatalysis appears as another promising oxidation technology leading to the total mineralization of most of organics. Semiconductor TiO2 as a photocatalyst has also been deeply investigated and can successfully degrade various organic pollutants [5–7]. TiO2 semiconductor has high bandgap (3.2 eV), which limits its wide application in visible light range of solar spectrum. So, it only absorbs in the ultraviolet range, and efficient
88
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
collection of the solar spectrum requires sensitization by a modification molecule absorbing in the visible range. Therefore, in order to realize the solar decontamination process, many efforts are contributed to develop the second-generation photocatalysts that can drive the photodegradation reaction under visible light [8]. The photosensitizer modification and transition metals doping are usually adopted methods on the base of crystal TiO2. However, many researchers have disclosed uncertain doping effects (even controversial results) about transition metal ion dopants [9]. As one of main pollutants, the dye molecule and dyestuff intermediate themselves have good absorption in visible range. So, photodegradation reaction of dye wastewater can be achieved under visible light illumination by taking good advantage of visible absorption of object molecules. Although the oxidation methods by TiO2 /visible light do not show so effective as Fenton reagent and TiO2 /UV, this method is still very attractive due to potential application of realizing solar decontamination. Another problem is how to overcome the growth of TiO2 nanocrystallites during calcining process at high temperature (>400 ◦ C) and serious aggregation of prepared nanoparticles when dispersed in aqueous solution. Regarding the titania phase state, amorphous TiO2 system was seldom studied due to its photocatalytic inactivity [10]. Many attempts have been made to prepare crystal structure TiO2 at low temperature in order to overcome the traditional high temperature calcination process. The previous related studies showed that the formation of anatase crystal structure was regarded as a unique phenomenon to the SiO2 –TiO2 system, not to pure TiO2 system. The former researching works mainly focused on the fabrication of sol–gel-derived anatase nanocomposite SiO2 –TiO2 film at low temperature, whose photocatalytic activity was not investigated [11,12]. However, pure TiO2 sol particles with anatase crystalline structure was prepared at low temperature and TiO2 sol directly acted as photocatalyst, which, to the best of our knowledge, was not reported before. In this paper, the lanthanide neodymium ion was selected as a TiO2 dopant to investigate its unique physicochemical property due to its ability to form complexes with various Lewis bases in the interaction of these functional groups with the f-orbitals of the lanthanide metal. Thus, incorporation of lanthanide ion in a TiO2 matrix could provide an effective method to concentrate the organic pollutant at the semiconductor surface [13–15]. The pure TiO2 and lanthanide metal neodymium ion (Nd3+ ) modified TiO2 sol samples were prepared at low temperature, respectively. The characteristics and properties of these sol photocatalysts were studied. The photocatalytic reaction was carried out in the novel hydrosol system, which was very different from traditional suspension or immobilization reaction system. The photoactivity was evaluated by measuring photodegradation degree of representative azo dye (reactive brilliant red dye X-3B) as a probe molecule under visible light irradiation. The possibility of the sol photocatalyst in visible region was investigated. The photocatalysis mechanism of TiO2 sol system was also discussed.
2. Materials and methods 2.1. Materials Titanium dioxide powder sample used in the experiment was commercial Degussa P25 TiO2 with 80% anatase, 20% rutile and BET area of ca. 50 m2 g−1 , which was obtained from Degussa AG Company in Germany. Titanium tetrachloride (TiCl4 ) and neodymium oxide (Nd2 O3 ) were pure reagent grade and obtained from J&K China Chemical Ltd. Ammonium hydroxide (NH4 OH), nitric acid (HNO3 ), hydrochloric acid (HCl) and other chemicals were analytical reagent grade quality and all obtained from Shanghai reagent company. The high purity water used in the experiment was double distilled and then purified with the Milli-Q system. The substrate of X-3B dye was obtained from Shanghai Dyestuff Chemical Plant and used without further purification. Fig. 1 displays the structure of reactive brilliant red dye X-3B. 2.2. Experimental procedure Nd3+ modified TiO2 sol was prepared by chemical coprecipitation–peptization method [16]. Firstly, 50 mL TiCl4 was diluted and hydrolyzed with 100 mL deionized distilled frozen water (0 ◦ C). Nd2 O3 was added to the above solution to produce transparent Nd3+ aqueous solution according to required doping amount of neodymium (Nd3+ ion dopant equivalent to 3.0 atom percent of Ti4+ in bulk solution) under vigorous stirring. In order to ensure its complete hydrolysis reaction, a diluted NH4 OH aqueous solution (10%) was added dropwise into transparent TiCl4 aqueous solution to obtained a grey precipitate, giving an ultimate suspension of pH 10. In order to remove residual NH4 + and Cl− ions, the precipitate was adequately wished with deionized water till the pH value of filtrated water was below 7.5. Then, the acquired amorphous Nd3+ –TiO2 was well dispersed into water and added with nitric acid solution (20%) in corresponding amount as a peptization catalyst and phase-transfer accelerant. The above suspension was adjusted to pH 1.5 and stirred for 4 h at room temperature, then peptized at 70 ◦ C for 24 h in airproof condition. Finally, 3.0 atom percent Nd3+ ion modified TiO2 sol was formed with uniform,
Fig. 1. Molecule structure C19 H10 O7 N6 Cl2 S2 Na2 ).
of
dye
X-3B
(chemical
formula =
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
stable, and semitransparent characteristics. The obtained sol can maintain homogenous distribution for quite a long time without sedimentation and delamination phenomena. The Nd3+ modified titania xerogel powder was prepared by aging, gelation and vacuum drying treatment of sol sample at 70 ◦ C for 8 h. The same procedure was adopted to prepare TiO2 sol. The experiment of photocatalytic reaction was conducted in a cylindrical quartz photoreactor with an effective cubage of 40 mL. Nd3+ –TiO2 and TiO2 sol were acted as photocatalyst and visible light as illuminating light source. The aqueous reactants mixture was vertically irradiated under the visible light with constant stirring speed at room temperature. Reaction system was setup by adding right-amount of sol photocatalyst (adding amount equivalent to 1.0 g L−1 TiO2 in solution) into 20 mL 100 mg L−1 X-3B solution. The pH value of X-3B/sol photocatalyst mixture was adjusted to 5.0 for hydrosol system. Prior to photocatalytic reaction, the dark adsorption experiment was firstly carried out under airproof condition. The mixture of X-3B and sol photocatalyst was sealed and continuously stirred in darkness for 30 min to establish an adsorption–desorption equilibrium. Degussa P25 TiO2 powder was also used as a standard photocatalyst to compare the catalytic activity with pure TiO2 sol and Nd3+ –TiO2 sol. 2.3. Analytic methods The particulate morphology of photocatalysts was observed on atom force microscope (AFM, Nanoscope III System, American Digital Corporation, USA) and scanning electron microscope (SEM, LEO 1530VP Field Emission Scanning Electron Microscope, LEO Electron Microscopy Inc., Germany). The phase state of samples were identified by X-ray diffractometer (XRD, XD-3A, Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu K␣) at 40 kV, 40 mA over the diffraction angle (2θ) range 10–90◦ . The cold light source acts as visible light illumination (LGY-150, 150w, halogen–tungsten lamp with UV and IR cut-off filter). Photoemission spectrum was measure by Optical Fiber Spectrometer (Model S2000, Ocean Optic Inc., USA), which mainly emits spectrum in the range of 400–800 nm and the main wavelength locates at 540 nm (see Fig. 2). Absorption spectrum of dye X-3B was also measured on UV–vis Recording Spectro-Photometer (UV-2201, Shimadazu Corporation, Japan). By comparison, the maximum emission and absorption spectrum was well matched in the range of visible light. The pH value of reaction medium was controlled by acidimeter (pHS-2, Shanghai Rex Instruments Factory). The photodegrated product was collected every 15 min. The X3B/P25 TiO2 suspension sample was filtered through two layers of Millipore 0.22 m films. The X-3B/TiO2 hydrosol mixture was centrifuged at 10 000 rpm for 15 min and then filtered by a 0.22 m Millipore filter. X-3B concentration was
89
Fig. 2. Photoemission spectrum of visible light source.
determined by measuring characteristic absorption intensity in X-3B absorption spectrum (Fig. 3).
3. Results and discussion 3.1. AFM and SEM analysis The sol particle morphology and particle size were investigated by AFM and SEM. The samples were fabricated on silica glass by means of spin-coating. AFM micrograph shows that two types of sol particles all had spheroidal shape and were uniformly spread out on the support. The average size were about 10 nm for Nd3+ –TiO2 sol particles and 25 nm for TiO2 sol particles (Fig. 4(A) and (B)). However, compositive single nanoparticle of P25 TiO2 was about 45 nm. These particles in aqueous suspension tended to aggregate and finally formed congeries with hundreds of nanometer in mean size (Fig. 4(C)). SEM micrograph shows that well-developed Nd3+ –TiO2 sol particles dispersed in colloidal system and continuously close-packed particles formed an alveolate network structure (Fig. 5). The above result means that sol particles obviously have better particles distribution in aqueous medium than P25 TiO2 powder. More-
Fig. 3. UV–vis absorption spectrum of 100 mg L−1 X-3B.
90
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Fig. 5. SEM micrograph of Nd3+ –TiO2 sol sample.
over, the difference of two sorts of sol particles indicates that Nd3+ ion modification preferably retarded the aggregation and growth of well-dispersed sol particles, which resulted in smaller grain size of Nd3+ –TiO2 sol particles. 3.2. XRD analysis The XRD patterns of Nd3+ modified TiO2 sol and bare TiO2 sol samples all show the presence of peaks (2θ = 25.4◦ , 38.0◦ , 48.0◦ , 54.6◦ ), which was regarded as an attributive indicator of anatase phase titanium dioxide crystallites (Fig. 6) [17]. The characteristic phase state of two types of sol samples can be considered as anatase crystalline structure due to the appearance of standard diffraction peaks with somewhat weak scattering peaks characterization. The sol particles have very broad diffraction peaks at (1 0 1) plane (2θ = 25.4◦ ), whose nature is due to their very small grain size. Both of sol samples do not appear any other diffraction peaks of new crystal phase besides anatase structure. It indicates that Ti–O atom order-structure in anatase TiO2 sol particles cannot be rearranged into rutile TiO2 structure under moderate preparation condition. As for Nd3+ –TiO2 sol, another main reason was attributed to the difference in ionic radius between neodymium and titanium ion (r(Ti4+ ) = 0.064 nm,
Fig. 4. AFM micrographs of (A) Nd3+ –TiO2 sol particles, (B) TiO2 sol particles and (C) P25 TiO2 powder particles (the x–y axis is scaled in micrometers).
Fig. 6. X-ray diffraction patterns of (a) TiO2 sol particles and (b) Nd3+ –TiO2 sol particles.
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
r(Nd3+ ) = 0.11 nm). Because the radius of Nd3+ was much larger than that of Ti4+ , without calcination treatment in high enough temperature, neodymium ions introduced by the coprecipitation–peptization method would unlikely enter into the TiO2 crystal lattice structure. Actually, during TiCl4 hydrolysis and peptization reaction, neodymium and oxygen ions could form Nd–O oxide on the superficial layer of TiO2 sol particles by chemical bonding process and Nd3+ ions mainly existed as Nd–O oxide in the Nd3+ –TiO2 mixture. Conversion from amorphous to crystalline structure usually requires high calcination temperature of at least 400 ◦ C. The above result indicates that long time peptization and digesting processing at high acidity and low temperature condition promotes the transformation of TiO2 sol particles from amorphous to crystal phase. The coprecipitation– peptization method can be promoted as a promising way to fabricate crystalline or semicrystalline TiO2 nanoparticles. 3.3. Interfacial adsorption effect The effect of adsorption–desorption equilibrium was investigated between 100 mg L−1 X-3B and 1.0 g L−1 photocatalysts in aqueous medium. X-3B solution with different photocatalysts was kept stirring for 30 min in darkness at 25 ◦ C, airproof condition. X-3B concentrations in bulk solution were measured by UV–vis spectrometer. Fig. 7 shows that the absorption intensity of X-3B at 512 and 536 nm in bulk solution decreased in comparison with original X-3B solution. The decrease of X-3B concentration due to adsorption by different photocatalyst was 30.46, 24.39 and 15.22 mg L−1 for Nd3+ –TiO2 sol, TiO2 sol and P25 TiO2 powder, respectively (see Fig. 8). Nd3+ –TiO2 sol system exhibits the superior adsorption effect on X-3B molecule, which almost has twice adsorption amount in comparison with P25 TiO2 powder.
Fig. 7. UV–vis absorption spectra of different X-3B/photocatalysts mixture after 30 min adsorption processing in darkness: (a)100 mg L−1 X-3B, (b) X-3B/P25 TiO2 suspension system, (c) X-3B/TiO2 sol system and (d) X3B/Nd3+ –TiO2 sol system (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ).
91
Fig. 8. Adsorption concentration of X-3B in terms of different photocatalysts after 30 min darkness adsorption (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ).
The heterogeneous photocatalysis was mainly occurred in molecule interfacial layers. So, the affinity and adsorption properties between reactants and photocatalyst surface play an important role on determining overall reaction rate [18,19]. Regarding the strong adsorption capability of X-3B on the surface of Nd3+ –TiO2 sol nanoparticles, the key factor was ascribed to the positive charge characteristic of Nd3+ ion banded on the TiO2 nanoparticles surface since anionic dye X-3B molecule loads negative charge. Moreover, the formation of Lewis acid–base complex between lanthanide ion and the dye substrate enhanced interfacial adsorption effect. Another important factor was attributed to the well-dispersed sol nanoparticles (10 nm for Nd3+ –TiO2 sol and 25 nm for TiO2 sol in mean size), which was very different from P25 TiO2 powder particles (45 nm for individual particle and >100 nm for particles aggregation in mean size). So, TiO2 sol particles showed better adsorption effect than P25 TiO2 powder. 3.4. Photocatalytic reaction The photocatalytic activity of Nd3+ –TiO2 sol was studied by investigating the degradation experiment of X-3B dye. UV–vis absorption spectra of X-3B solution with Nd3+ –TiO2 sol, TiO2 sol and P25 TiO2 photocatalysts were measured every 15 min following the visible light illumination. Fig. 9 shows the temporal changes of X-3B spectra with the given irradiation time intervals in the different photocatalyst/X-3B reaction system. In the studied concentration range, there is a good linear relationship between characteristic absorption intensity and X-3B concentration. Fig. 10 shows the decrease of X-3B concentration in dependence on visible light irradiation time by different photocatalysts. Experimental results show that X-3B concentration in the above different reaction system all decreased with the photocatalysis processing. The photocatalytic degradation is supposed to follow pseudo first-order reaction kinetics for the studied concentration range. The reaction kinetics can often be described in terms of the Langmuir–Hinshelwood model
92
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Fig. 10. Photocatalytic degradation of X-3B on various TiO2 photocatalysts: (a) P25 TiO2 powder, (b) TiO2 sol and (c) Nd3+ –TiO2 sol (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ; initial X3B/photocatalyst mixture through 30 min adsorption in darkness).
[20]. The curve fitting formula and calculated results were summarized in Table 1. Considering the effect of initial reaction concentration of X-3B in solution on its degradation kinetics, we modified the pseudo-first order kinetic constant (k) with another amended constant (k ), which could remove the effect of initial reactant concentration with the function of k = k(c0 )0.7 [21]. By comparison of k and r value, it shows that the overall photocatalytic degradation efficiency was observed to decrease in the following order: Nd3+ –TiO2 sol > TiO2 sol > P25 TiO2 powder, which was in good agreement with the above result of solid–liquid interfacial adsorption effect by different photocatalyst. According to the principles of semiconductor TiO2 photocatalysis, the photoactivity is mainly dependent on three factors: (a) electron–hole generation capacity; (b) electron transfer route and efficiency; (c) separation efficiency of photogenerated charge pairs. The sensitized photocatalysis mechanism is very different from UV light induced photocatalysis mechanism. The dye sensitization process involves the excitation of X-3B dye molecule with visible light and subsequent electron injection or electron transfer from excited state dye molecule to conduction band of semiconductor TiO2 . The electron generation capacity mainly depends upon the intensity of incident photos with matchable energy and absorption amount of dye molecule on photocatalysts surface. The separation of electron–cation radical and electrons transfer efficiency depend on the electron acceptor (usually TiO2 conduction band, triplet ground state O2 or other scavengers)
Fig. 9. UV–vis absorption spectra of X-3B/photocatalyst samples with different reaction time under visible light irradiation: (A) filtrate of X-3B/P25 TiO2 suspension system, (B) X-3B/ Nd3+ –TiO2 sol system and (C) X3B/TiO2 sol system (original X-3B concentration: 100 mg L−1 ; photocatalyst concentration: 1 g L−1 ; initial X-3B/photocatalysts mixture was used after adsorption of 30 min in darkness).
Table 1 Photodegradation effect of X-3B solution by different photocatalyst after 120 min Photocatalyst
Model formula
R
k (min−1 )
r (%)
k
P25 TiO2 powder Nd3+ –TiO2 sol TiO2 sol
c = 84.70 exp(−0.00334t) c = 65.83 exp(−0.01499t) c = 72.61 exp(−0.00679t)
0.94641 0.99727 0.9978
0.00334 0.01499 0.00679
44.70 89.14 67.03
0.07469 0.28099 0.13632
c, X-3B concentration (mg L−1 ); t, visible light irradiation time (min); R, correlation coefficient; k, apparent rate constant (min−1 ), k , amended apparent constant and r, X-3B photodegradation ratio(%).
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
Fig. 11. UV–vis absorption spectra of (a) 100 mg L−1 X-3B and (b) X-3B after 120 min direct photolysis with visible light irradiation.
and transferring route. TiO2 sol and P25 TiO2 powder crystallites referred to above photosensitization–photocatalysis processing. The difference of photoactivity between TiO2 sol and P25 TiO2 powder was ascribed to the nano-colloid size effect, adsorption effect and crystal phase structure since anatase is usually more active than rutile. The prepared TiO2 sol particles are of pure anatase crystalline structure while commercial P25 TiO2 powder is of 80% anatase and 20% rutile crystalline structure. Regarding Nd3+ –TiO2 sol system, modification ion Nd3+ has excellent electron scavenging capacity, which results in a superior transfer and separation efficiency of electron–cation radical. Considering the standard redox potentials (E0 (Nd3+ /Nd2+ ) = −0.40V, E0 (O2 /O2 − ) = 0.338 V and Ecb (TiO2 ) = −0.5V versus NHE), the presence of Nd3+ ion on nanoparticles surface may promote the following processes [22]: • The first step: Nd3+ + TiO2 (e− ) → Nd2+ + TiO2 (electron scavenging). • • The second step: Nd2+ + O2 → Nd3+ + O2 − (charge transferring).
93
As a result, photoexcitated electrons can be effectively transferred to acceptors. So, promotion of electron–cation radical separation efficiency was good for dye photodegrada• tion because less stable cation radicals (dye + ) were very susceptible to produce degradation products by molecule oxygen and superoxide anion radicals. In order to confirm photocatalysis of TiO2 sol particles, the direct photolysis of X-3B was also investigated under the same reaction condition only without photocatalyst. Fig. 11 shows the change of UV–vis absorption spectra of X-3B after direct photolysis. The result shows that only 4.25% of X-3B disappeared after 120 min photolysis. No significant decrease of the X-3B concentration was observed under visible light irradiation. It means that photolysis was a weak working factor to affect photodegradation of X-3B dye, and photocatalysis by sol photocatalyst was main reason to cause the X-3B degradation. 3.5. Photocatalysis mechanism The well-dispersed sol nanoparticles with high surface area allow high access of the reactants to the TiO2 surface in aqueous medium. Fig. 12 shows the photosensitized photocatalysis reaction mechanism of dye molecule in the Nd3+ –TiO2 sol system. In the sol photocatalysis system, physically adsorbed on the surface of the sol particles, the dye molecule was excited by absorption of a suitable visible light photon. The electrons of excited dye molecule can inject into conduction band (CB) of TiO2 . Then these electrons could be trapped by electron scavengers (usually oxygen molecule). But it was also extremely susceptible for the cation radicals and the electrons to recombine if the injected electrons accumulated in the CB of TiO2 . So, electrons trapping and electrons transfer would be two key steps to inhibit electron–cation radical recombination. The cation rad• ical (dye + ) produced by electron injection was less stable than the ground state of the compound (dye). As a result, unstable cation radical dye may directly degraded to products • or reacted with superoxide radial anion (O2 − ) to produce
Fig. 12. Sensitized photocatalysis mechanism of Nd3+ –TiO2 sol system (S0 = singlet ground state, S1 = first excited singlet, T1 = first excited triplet state, ISC = intersystem crossing).
94
Y. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 252 (2005) 87–94
degradation products. The photocatalysis of TiO2 sol and P25 TiO2 powder system can be interpreted with above reaction mechanism. However, in the photocatalysis of Nd3+ –TiO2 sol system, the Nd3+ species can act as an effective electron scavenger to trap the CB electrons of TiO2 , which were injected from the excited dye molecule. Nd3+ ions, as a Lewis acid, apparently was superior to the oxygen molecule (O2 ) in the capability of trapping electrons [23]. The electrons trapped in Nd3+ sites (Nd2+ ) were subsequently transferred to the surrounding adsorbed O2 by oxidation process. The presence of Nd3+ on TiO2 sol nanoparticle surface may promote following pro• cesses (Nd3+ + e → Nd2+ and Nd2+ + O2 → Nd3+ + O2 − ). More effective electron trapping and transferring would produce more cation radicals, which were so active to undergo degradation. Therefore, Nd3+ –TiO2 sol showed the superior photocatalytic activity in comparison with TiO2 sol and P25 TiO2 powder. 4. Conclusions We set up a new method in which pure TiO2 sol (or Nd3+ –TiO2 sol) particles with anatase crystalline structure were prepared at low temperature and TiO2 sol (or Nd3+ –TiO2 sol) directly acted as photocatalysts. Considering above results, it is feasible to conduct photocatalysis in hydrosol reaction system. Modification of lanthanide neodymium ion to TiO2 sol greatly improved interfacial adsorption property and photocatalytic activity due to small size effect and interfacial positive charge lanthanide ion effect of Nd3+ –TiO2 sol particles. By taking advantage of sensitiveness to the pH value, TiO2 sol photocatalyst can be applied to recycle use in photocatalytic reaction while keeping its photoactivity. So, the novel hydrosol reaction system integrated the advantages of suspension and immobilization reaction system (high photocatalytic efficiency and feasible separation of photocatalysts for recycle use) [24]. How to keep high photocatalytic activity after many times of recycle use of TiO2 sol in the present study is currently under investigation. In a sense, the effective photodegradation dye by TiO2 sol under visible light is a very exciting respect in photocatalytic area.
Acknowledgements This work was financially supported by the Hi-Tech Research and Development Program (863 Program) of China (Grant No. 2002AA302304), the National Natural Science Foundation of China (Grant No. 60121101).
References [1] F. Nerud, P. Baldrian, J. Gabriel, D. Ogbeifun, Chemosphere 44 (2001) 957. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. [3] M. Ksibi, A.S. Ben, S. Cherif, E. Elaloui, A. Houas, M. Elaloui, J. Photochem. Photobiol. A 154 (2003) 211. [4] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999) 51. [5] I. Poulios, I. Tsachpinis, J. Chem. Technol. Biotechnol. 74 (1999) 349. [6] A. Marinas, C. Guillard, J.M. Marinas, A. Fern´andez-Alba, A. Agu¨era, J.M. Herrmann, Appl. Catal. B: Environ. 34 (2001) 241. [7] K. Chiang, R. Amal, T. Tran, Adv. Environ. Res. 6 (2002) 471. [8] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [9] K.V.S. Rao, B. Lav´edrine, P. Boule, J. Photochem. Photobiol. A 154 (2003) 189. [10] L. Zang, W. Macyk, C. Lange, W.F. Maier, C. Antonius, D. Meissner, H. Kisch, Chem. Eur. J. 6 (2000) 379. [11] H. Imai, H. Hirashima, J. Am. Ceram. Soc. 82 (1999) 2301. [12] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 83 (2000) 229. [13] K.T. Ranjit, I. Willner, S.H. Bossnamm, A.M. Braun, Environ. Sci. Technol. 35 (2001) 1544. [14] J. Lin, J.C. Yu, J. Photochem. Photobiol. A: Chem. 116 (1998) 63. [15] Y.Q. Wang, H.M. Cheng, L. Zhang, Y.Z. Hao, J.M. Ma, B. Xu, W.H. Li, J. Mol. Catal. A: Chem. 151 (2000) 205. [16] L. Wang, M. Muhammed, J. Mater. Chem. 9 (1999) 2871. [17] L. Znaidi, R. S´eraphimova, J.F. Bocquet, C. Colbeau-Justin, C. Pommier, Mater. Res. Bull. 36 (2001) 811. [18] C. Hu, Y.Z. Wang, H.X. Tang, Appl. Catal. B 35 (2001) 95. [19] M.S. Chiou, H.Y. Li, Chemosphere 50 (2003) 1095. [20] C. Minero, Catal. Today 54 (1999) 205. [21] D.M. Chen, J.J. Zhong, Y.M. Wang, Fine Chem. 19 (2002) 55. [22] P. Yang, C. Lu, N.P. Hua, Y.K. Du, Mater. Lett. 57 (2002) 794. [23] G.R. Bamwenda, T. Uesigi, Y. Abe, K. Sayama, H. Arakawa, Appl. Catal. A 205 (2001) 117. [24] Y.B. Xie, C.W. Yuan, J. Mol. Catal. A: Chem. 206 (2003) 419.