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PARTICIPATION OF NANOCRYSTALLINE TiO2 SURFACE IN THE ELECTRON TRANSFER BETWEEN SEMICONDUCTOR SOLID AND ADSORBED COBALT(III)-#PY COMPLEX
A. S. Ganeshraja1, K. Anbalagan1 1 Department of Chemistry, Pondicherry University Kalapet, Pondicherry 605 014, India
[email protected], [email protected]
PACS 81.20.-n, 61.72.uj
Cis-[Com(tn)2(Rpy)Br]Br2, (R = 4-CN, H, 4-Bz, 4-Me, 4-Et, and 4-MeNH), in aqueous 2-propanol exhibit varying adsorption characteristics and led to surface compound formation. UV (A = 254) excitation of the nano-TiO2//cobalt(III)-(Rpy) surface compound resulted in interfacial electron transfer (IFET) reaction. The IFET has been found to be dependent upon the coordination environment of the complex, more precisely due to the Rpy ligand. In addition, the proposed mechanism of the IFET reaction includes the formation of a Co" ion implanted in nanocrystalline TiO2. This photoreduction was found to be solvent controlled. The photoefficiency of the Co^ formation was spectrally analyzed simultaneously as Con:TiO2 was isolated from the photolyte solution. The isolated solid was subjected to FTIR, DRS, PXRD, and SEM-EDX instrumental analysis. It is concluded that the removal of metal ion in the form of a complex is coordination structure dependent, hence, seems more specific in removal efficiency and in doping the anatase lattice.
Keywords: surface adsorption, interfacial electron transfer reaction, cobalt doped nano-TiO2. 1. Introduction
Interfacial electron transfer (IFET) between sensitizer molecular adsorbates and semiconductor materials has been a subject of intense research in recent years. Examples include, among others, photocatalysis [1], surface photochemistry [2,3], dye-sensitized solar cells (DSSCs) [4], organic semiconductor-based photovoltaics [5], and nanoscale optoelectronics based on a single molecule or a small group of molecules [6]. This is a fundamental process in surface chemistry, relevant to a broad range of practical applications, including effective mechanisms of solar energy conversion [7,8], photoelectro-chemistry [9], artificial photosynthesis and imaging [10]. Despite its great technological significance, IFET remains poorly understood [1,2] compared to electron transfer in homogeneous solutions [11].
At present, TiO2 is considered as the most promising photocatalyst because of its low cost, nontoxicity, excellent stability, and high efficiency [12]. However, it can be activated only through irradiation with ultraviolet (UV) light (4-5 % of solar light) because of its large band gap of 3.0 eV to 3.2 eV [13]. Thus, many attempts have been made to enhance the photocatalytic activity of TiO2 in the visible light range. Doping with transition metals (Cr, Co, V, Fe, etc.) is one of the promising approaches [14]. Co(II) is considered as one of the more promising candidates because of its effect in reducing the electron-hole recombination rate and in shifting the absorption edge into the visible light region. Co-doped TiO2 has shown high activities for degradation of acetaldehyde, 2-chlorophenol, and 2,4- dichlorophenol in aqueous solutions [15]. Transition metal-doped
TiO2 photocatalysts are active under visible light irradiation. However, these materials have certain disadvantages, such as thermal instability as well as low quantum efficiency of the photoinduced charge carriers [16,17]. To overcome these drawbacks, considerable effort has been exerted to modify the properties of transition metal-doped TiO2 using nonmetal impurities [18]. In this work, the luminescence properties of anatase TiO2 samples doped with different amounts of Co2+ when were excited using a source with energy (2.54 eV), well below the TiO2 bandgap, is reported. The effect of introducing defects through the incorporation of Co atoms in the TiO2 lattice is discussed. With this approach the electron-hole recombination, which is excitonic in nature, is not excited because laser energies greater or on the order of the bandgap are required; instead, this study is focused on the defects-related emission band observed by Sekiya et al. [19] at 1.95eV (77 K). In our sol-gel samples, this band was observed at 2.02eV at room temperature [20].
Understanding the photophysics of transition metal complexes attached to semiconductor surfaces is essential for the design of artificial systems for solar energy conversion. In particular, Cobalt(III)-pyridyl complexes have attracted a great deal of attention as a promising class of compounds with long-lived charge-separated states and rich photochemical properties. However, their electronic excitations and photoconversion mechanisms remain only partially understood. This paper builds upon our recent work [21,22] and addresses the study of TiO2 nanoparticles sensitized with cis-[Com(tn)2(Rpy)Br]Br2 (where R=4-CN, H, 4-Bz, 4-Me, 4-Et, and 4-MeNH) adsorbates. Our study includes the characterization of the electronic excitations, electron injection time scales, and IFET mechanisms [23] IFET photoefficiency and photo degradation of 2-propanol. Here, we focus on TiO2 surfaces modified by Co(III)-pyridyl complexes attached by nitrogen linkers. Emphasis is given to the characterization of the electronic excitations and injection time scales as determined by the nature of the molecular adsorbates and the attachment modes.
2. Experimental
2.1. Materials and methods
CoCl2.6H2O (99%), R-pyridine (Rpy), ferric chloride, potassium oxalate, sodium acetate, ammonium thiocyanate, nanocrystalline titanium dioxide (surface area = 200-220 m2/g and particle size = 25 nm) and DMSO-d6 (NMR solvent) were purchased from Sigma Aldrich. 1,3-diamino propane (LR), pyridine, 1,10-phenanthroline and all other chemicals were purchased from Himedia and SD. Fine Chemicals (India). All the solvents and 1,3-diamino propane were purified by distillation and water was triply distilled over alkaline KMnO4 in an all glass apparatus. Analytically pure crystals of cis-[CoIII(tn)2(Rpy)Br]Br2 adsorbates (where R = 4-CN, H, 4-Bz, 4-Me, 4-Et, and 4-MeNH) were synthesized by a modified procedure [24] and recrystallized.
2.2. Instrumentation
Cobalt(III) complexes were photolysed using a 254 nm, 6 watt low pressure mercury vapor lamp as the light source (Germicidal G4T5, 3H, model 3006) in a small quartz immersion well (model 3210, 80 mL cap. Photochemical Reactors Ltd, UK). Electronic absorption spectral studies were undertaken on a double beam spectrophotometer (Shimadzu 2450, Japan) with integrating sphere attachment (ISR-2200). UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded in absorbance mode at room temperature in the 200-1000 nm range on a Shimadzu, (UV 2450) double-beam spectrophotometer equipped
with integrating sphere attachment (ISR-2200) using BaSO4 as the reference. The instrument is interfaced with a computer for data collection and analysis. Fourier transform infrared (FTIR) spectra were recorded in the 4000-400 cm-1 range using a Thermo Nico-let 6700 FTIR spectrophotometer using KBr wafer with resolution 0.1 cm-1. 1H NMR measurements were made on a Bruker instrument; model Avance-II in 400 MHz Fourier transform-nuclear magnetic resonance with DMSO-d6 solvent. PXRDs were collected on an 800 W Philips (PANANALYTICAL, Almelo, The Netherlands) powder diffractometer equipped with an etched glass plate sample holder by rotating anode diffractometer in the 26 range 10-80 ° with step size of 0.02 ° using Cu Ka (A = 1.5406 A) radiation to determine the identity of any phase and their crystalline size. Surface morphology and chemical mapping were examined by SEM (Hitachi, S-3400N microscope), operating at 0.3-30 kV. Microanalyses (energy dispersive X-ray analysis, EDX) were performed with Thermo SuperDry II attachment of SEM.
2.3. Photocatalytic activity measurement
The photocatalytic activity of nano-TiO2 was evaluated by measuring the efficiency of the reduction of the Com(tn)2(#py)Br2+ complex in neat water/aqueous 2-propanol solutions under 254 nm light irradiation. The photoefficiency was calculated by estimating photogenerated Co° by Kitson's method [25]. Photoreduction was carried out in a reactor vessel using a 254 nm low pressure mercury vapor lamp, which were housed in a fume hood (Lab Guard), covered with black polythene sheet to prevent extraneous light. The photoreactor is a double walled quartz vessel, in which, photolyte mixture (100 mg of nano-TiO2 crystals in 80 mL of c/s-Com(tn)2(#py)Br2+ (1.68 x 10-3 M) solution and 1M NaNO3) was taken in the inner jacket and cool water in the outer jacket. Prior to irradiation, the suspension of the catalyst was achieved by ultrasonic treatment and continued magnetic stirring in the dark to attain cobalt(III) complex ion adsorption/desorption equilibrium on the catalyst. The photolyte suspension was irradiated with 254 nm light for defined periods (0, 2-16 min). About 4 mL aliquots were sampled and centrifuged to remove the semiconductor particles and then spectrally analyzed. In order to diminish experimental error, experiments were repeated at least two-three times for the same sample and the mean value was calculated. Photoefficiency of Co(II) formation in terms of percentage, PE (%) was computed using the formula: PE (%) = [(At- A,)/At] x 100 where A» and At are the absorbances of the photolysed solutions initially and at definite time interval't' respectively.
3. Results and discussion
3.1. Photocatalytic reduction of Com(tn)2CRpy)Br2+
Cobalt(III)-#pycomplex is a good UV light absorber, however, it decomposes on exposure to light over a long period of time due to ligand to metal charge transfer (LMCT) bands (A =~356 nm). Surprisingly, the addition of nano-TiO2 (anatase) provoked the complex to degrade more efficiently and a higher amount of cobalt(II) was generated. Figure 1 depicts repetitive scan spectra of CoIII(tn)2(4-Etpy)Br2+ complex observed at defined irradiation time intervals; there is a blue shift in the absorption maxima at A = 357 — 351 nm and red shift at A = 512 — 518 nm. The absence of an isosbestic point in the repetitive scan spectra indicates the decomposition of the complex during the reduction process suggesting, perturbation of the CoIII centre due to IFET. The photoefficiency of Co(II) formation by TiO2 (e^/h+J/scavenger of CoIII(tn)2(4-Etpy)Br2+ is increased with respect
to the concentration of 2-propanol and more active at higher concentrations in the pho-tolytic solutions. This implies the observed photoefficiency (Table 1) for the formation of Co11 is a summation of the individual electron transfer reactions due to (i) excited nano-TiO2: Com(tn)2(Rpy)Br2+ + nano-TiO2 + h^ (A = 254 nm) ^ Co11 + products and (ii) ligand to metal charge transfer transition in cobalt(III) complex: CoIII(tn)2(Rpy)Br2+ + h^ (A = 254 nm) ^ Co11 + products. However, the former path is predominant and requires a thorough investigation.
Wavelength (nm)
Fig. 1. Repetitive scan spectra recorded for the photocatalysed CoIII(tn)2 (4-Etpy)Br2+ complex ion in nano-TiO2 suspension in neat water at various time intervals: 0, 2, 4, 8, 12 and 16 min respectively. Complex concentration = 1.68x10"3 M, ionic strength = 1M NaNO3, pH - 7 and at 298 K
In the case of photoreduction of Com(tn)2(Rpy)Br2+ complex with nanocrystalline titania influence than that of polycrystalline titania. Nanocrystalline materials are single- or multi-phase polycrystalline solids with a grain size of a few nanometers (1 nm = 10~9 m = 10 A), typically less than 100 nm. Since the grain sizes are so small, a significant volume of the microstructure in nanocrystalline materials is composed of interfaces, mainly grain boundaries, i.e., a large volume fraction of the atoms resides in grain boundaries. Consequently, nanocrystalline materials exhibit properties that are significantly different from and often improved over, their conventional coarse-grained polycrystalline counterparts [26]. In polycrystalline wide band gap oxide semiconductors such as ZnO and TiO2, the photoconductivity (PC) has been observed to decay very slowly, over a period of many hours to several days [27,28]. This slow decay is often referred to as 'persistent PC' and different possible explanations have been proposed [29-31]. In some cases, models have been fitted to the experimental data for the determination of electronic parameters of the materials, such as the energy distribution of charge carrier traps [32,33] Pho-tocatalytic degradation studies of pollutants using either Co(II)-tetrasulfophthalocyanine grafted on TiO2 via a silane reagent or polycrystalline TiO2 samples impregnated with Cu(II)-phthalocyanine [32] were reported in the literature.
Table 1. Photoefficiency P.E (%) of formation of Co(II) upon A = 254 nm irradiation of cis Com(tn)2(Rpy)Br2+ in aqueous 2-propanol at 298 K. ionic strength 1M NaNO3, complex concentration = 1.68 x 10_3M
Rpy
PE (%) in water/PriOH(w/w)%
100/0 95/5 90/10 85/15 80/20 75/25 70/30
pKa value of Rpy
4-CN - 14±0.5 18±1.0 19±1.0 21±0.7 26±0.8 28±0.9 32±0.6 1.90
poly-TiO2 16±1.0 17±0.5 21±1.0 24±0.8 29±0.7 31±0.6 37±0.5
nano-TiO2 17±0.7 20±0.9 24±0.5 29±0.8 31±0.6 37±1.0 41±1.4
H - 15±0.8 21±0.7 29±1.0 34±0.5 38±1.4 40±0.9 42±1.2 5.25
poly-TiO2 14±0.6 24±1.0 32±0.8 35±0.8 41±0.7 44±0.9 48±1.2
nano-TiO2 18±1.0 25±0.7 34±0.6 39±1.0 45±0.5 51±0.9 55±0.5
4-Bz - 12±0.8 19±1.0 24±0.6 28±0.7 32±0.8 38±0.5 41±1.2 5.59
poly-TiO2 16±0.6 23±0.8 28±1.0 30±1.4 34±0.5 38±0.9 49±0.8
nano-TiO2 18±0.6 21±0.8 29±0.7 31±0.5 36±1.0 44±0.9 57±1.4
4-Me - 12±1.0 19±1.0 26±0.5 29±0.8 34±0.9 38±0.8 49±1.4 6.02
poly-TiO2 17±1.0 23±0.5 29±0.6 34±0.8 37±0.9 45±0.9 52±0.7
nano-TiO2 19±0.7 26±0.5 33±1.0 38±0.6 46±0.8 54±1.4 57±0.9
4-Et - 18±1.2 24±0.6 28±1.0 31±0.5 36±1.4 41±0.8 49±0.5 6.02
poly-TiO2 19±1.0 27±1.0 32±0.9 37±0.8 45±0.5 48±0.8 53±1.4
nano-TiO2 21±0.6 28±0.5 35±1.0 39±0.9 49±0.8 58±0.5 64±0.7
4-MeNH - 19±0.8 24±0.5 29±0.5 32±1.0 46±1.0 52±0.8 53±0.9 9.70
poly-TiO2 23±0.7 28±0.8 32±0.5 38±0.6 49±1.0 58±0.9 61±0.5
nano-TiO2 26±0.8 31±0.6 37±0.7 44±0.5 54±1.0 63±0.9 69±0.8
3.2. Photo-oxidation of 2-propanol to acetone by NMR
The photoreduction was systematically followed by NMR measurements which indicate the growth of acetone peak progressively appearing in signal intensity as a function of dosage of light [34]. Table 2 exhibits NMR signals due to 2-propanol appearing at 6 = 1.0 to 1.1 ppm (-CH3) and 6 = 3.8 to 3.93 ppm (-CH) before the initiation of photore-duction and after defined irradiation periods. There is a steady growth of the signal due to photo released acetone and the signal intensity increases with respect to the level of light irradiation. That is, a new NMR signal at 6 = 2.04 ppm indicates the formation of acetone and the signal strength (Table 2) increases considerably with increased irradiation times [35]. In fact, the integrated intensity of the acetone signals gradually increase, whereas the -CH and -CH3 (2-propanol) signals gradually decrease. The origination of acetone is from the oxidation of 2-propanol resulting from the scavengingof valence band holes by the alcohol: nano-TiO2 (h+) + (CH3^CHOH ^ CH3COCH3.
3.3. Mechanism of photoinduced electron transfer reaction
It is apparent from Table 1 that excited nanocrystalline TiO2 shows better catalytic activity, which critically depends upon the surface-substrate interaction. Surface affinity of molecules/ions is a competing feature of the surface, therefore, formation of poly (or) nano-TiO2//cobalt(III)-(Rpy) surface compound is inevitable. Therefore, an enhancement
Table 2. 1H NMR data for the generator of acetone upon A = 254 nm irradiation of Com(tn)2(4-Etpy)Br2+ at 298 K in DMSO-da. Photocatalyst nano-TiO2, ionic strength 1N NaNO3, complex 1.86x10"3M. 'A' and 'P', respectively, denote acetone and 2-propanol
Irradiating time (min) 2x -CH3(d) P ô ppm -CH(m) P ô ppm 2x -CH3(s) A ô ppm
0 0.98 0.99 3.72 3.73 3.75 3.61 3.78 3.79 3.81 -
8 0.98 0.99 3.72 3.73 3.75 3.76 3.78 3.79 3.81 2.02
16 0.98 0.99 3.72 3.73 3.75 3.76 3.78 3.79 3.81 2.02
45 0.98 0.10 3.71 3.73 3.75 3.76 3.77 3.79 3.80 2.02
in the photocatalytic reduction originates from (i) Rpy can modify the surface affinity of Com(tn)2(Rpy)Br2+ ion with the nano-TiO2 surface and (ii) photoexcitation can prompt the formation of microdomains in nano-TiO2 with characteristic hydrophobic/hydrophilic behavior. Therefore, accumulation of adsorbate is varied, and hence, available for reduction. In addition, charge relaxation/recombination processes of nano-TiO2 (e-, h+) pair are altered due to the formation of compact nano-TiO2//cobalt(III)-(Rpy) surface compound, (CoJ"). Such processes provide a favorable negative charge potential for reduction of the Co(III) center. Thus, the overall efficiency of heterogeneous photocatalysis is determined by the adsorbate content and population/lifetime of the charge carriers for interfacial chargetransfer processes [36,37]. Sixth ligand (Rpy) in CoIII(tn)2(Rpy)Br2+ with a hydrophobic tail imparts variation in surface adherence of the complex ion on the surface of nano-TiO2, however, the adsorption process is restricted by thermodynamic aspects. To rationalize these observations, one must invoke a mechanism that incorporates several complementary routes as given in eqs. (1-7).
Surface compound formation: nano-TiO2 + CoIII(tn)2(Rpy)Br2+ — nano-TiO2// (1)
CoIII(tn)2(Rpy)Br2+
Photoexcitation: nano-TiO2 + h^ — nano-TiO2(e-, CB) + nano-TiO2(h+, VB) (2)
Charge trapping: (e-, CB) — (e-, tr) (3)
(h+, VB) — (h+, tr) (4)
Electron-hole recombination: (e-, tr)/(e-,CB)+ (h+,tr)/(h+, VB)—(e-,tr)+heat (5) Electron transfer: nano-TiO2 (e-, CB)/ (e-, tr) + (Co^) — Col}urf + Co"g (6)
Hole transfer: nano-TiO2(h+, VB)/ nano-TiO2 (h+, tr)+(CH3)2CHOH—CH3COCH3 (7)
Where (e-, tr) and (h+, tr) represent charge trapped surface states while Co^, ColUrf, and CoJ^ represent surface compound, surface implanted species and aqueous species respectively. It is unlikely that the photoreduction of Co(III) proceeds through multielectron process, but it should occur in one electron step. The predominant reduction path of the metal centre is as given in eqs. 1-7 with a limited contribution from charge transfer population states like: a(N) — da*(Co) and a(Cl) — Co.
3.4. pKa dependent photoreduction
In this investigation, a linear dependence of photoefficiency(%) of CoIII(tn)2(Rpy) Br2+/TiO 2 suspension with respect to the Rpyligand in terms of acidity constant (pKa)
as shown in Figure 2 is observed. Linear regression analysis of PE% versus pKa yields a slope indicating that electron donating groups in Rpy (pKa > 5.25) enhances photo efficiency (%) and that electron withdrawing groups in Rpy (pKa = 1.90) reduces the photo catalytic behavior. Therefore, it can be concluded that coordination environment of the metal centre of a transition metal ion can greatly influence the photochemical character of the complex.
Fig. 2. Linear plot of photoefficiency vs. pKa value of aryl amine (R) in Com(tn)2(Rpy)Br2+; ■ absence of nano-TiO2, • presence poly-TiO2, and ▲ presence nano-TiO2 in 70/30 (w/w)% water/PriOH
It is interesting to note that the PE (%) of CoII generation is (i) linearly increasing with 2-propanol content and (ii) showing some relationship with variation in the Rpy ligand of the complex. This non-consistency can be attributed to nanoparticle (or polycrystalline) surface//cobalt(III)-Rpy affinity and subsequent interfacial electron transfer. Accumulation of cobalt(III) complex on the surface of nano-TiO2 to form a compact layer is mainly dependent upon the blocking effect of Rpy due to electron withdrawing and electron donating properties. Pellizzetti and co-workers showed a significant alteration in the distribution of aromatic intermediates on the surface of TiO2 in their study [38]. Figure 2 shows photoefficiency vs pKa of the aryl amine of the Rpy ligand of six Com(tn)2(Rpy)Br2+ complexes. It is interesting that, within this limited set of complexes at least, there is an obvious relationship between PE and the pKa value of the ligand. There is a reasonable enhancement in the PE with aromatic ring electronic nature of the Rpy ligand, which appears to asymptotically approach a limiting value.
3.5. Characterizations of cobalt-doped nano-TiO2
The isolated solids were subjected to spectral, macrostructural, microstructural, and surface morphological analyses. Therefore, the irradiated Com(tn)2(Rpy)Br2+ with nanocrystalline TiO2 were subjected to FTIR, DRS, PXRD and SEM-EDX analyses. The experimental results yield the characteristic features on the inclusion of cobalt ion in anatase lattice, which was confirmed by XRD patterns. XRD and SEM results revealed that the particle size of Co-doped nano-TiO2 is enhanced as the result of the UV-light irradiation method. Here, we focus on TiO2 surfaces modified by Com(tn)2(Rpy)Br2+
complexes attached by nitrogen linkers. Emphasis is given to the characterization of the electronic excitations and injection time scales as determined by the nature of the molecular adsorbates and the attachment modes.
3.6. FTIR, DRS and powder X-ray difraction analysis of Co/nano-TiO2
Transmittance peaks observed between 450-600 cm-1 can be assigned to the Ti-O-Ti bond as shown in Figure 3(A). Both pure and Co-doped nano-TiO2 showed a characteristic band at 511 cm-1 corresponding to a Ti-O bond of the anatase phase. Ti-O vibration tends to shift to lower energy regions with Co content in nano-TiO2. The DRS of undoped anatase [39] showed 100% reflectance, and doped samples absorb more effectively from 480-600 nm. Nano-TiO2 with low cobalt density absorbs at ~563 nm, and the \max shifts towards ~574 nm (Figure 3(B)). The red shift is attributed to the 3d Co2+ electrons into the conduction band of nano-TiO2. The absorption spectrum of TiO2 consists of a single broad intense absorption around 400 nm due to the charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d t2g orbitals of the Ti4+ cations) [40]. Pure TiO2 showed absorbance in the shorter wavelength region while Co/TiO2 and the DRS results showed a red shift in the absorption onset value in the case of Co-doped titania. The doping of various transition metal ions into TiO2 could shift its optical absorption edge from the UV into the visible light range, but no prominent change in the TiO2 band gap was observed [40].
Fig. 3. (A) Diffuse reflectance spectra, (B) FT-IR spectra and (C) PXRD pattern of the pure nano TiO2 and x%Co:nano-TiO2
Figure 3(c) shows the X-ray diffraction patterns of the undoped and cobalt-doped TiO2 samples. The nanocrystalline anatase structure was confirmed by (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) diffraction peaks [41,42]. The XRD patterns of anatase have a main peak at 29 = 25.2° corresponding to the 101 plane (JCPDS 21-1272) while the main peaks of rutile and brookite phases are at 29 = 27.4° (110 plane) and 29 = 30.8° (121 plane), respectively. Therefore, rutile and brookite phases have not been detected [43,44]. The XRD patterns didn't show any Co phase (even for 5% Co/TiO2 sample) indicating that Co ions were uniformly dispersed among the anatase crystallites. In the region of 29 = 10-80 °, the diffractive peak shapes of pure TiO2 crystal planes are quite similar to that of Co/TiO2 with different concentrations of Co. The XRD patterns indicate pure anatase phase [45] form and confirm the absence of phase transformation to other related crystalline phases like rutile and brookite. This indicates that only a small amount of the elements are substituted into the Ti4+ sites, while interstitially incorporated Co(II) cannot be discounted as shown in Figure 3(C).
Table 3. The chemical content of cobalt doped TiO2 samples, recorded for nano-TiO2/Co,x% prepared by UV sensitized photoreduction of (1.63x10~3 M) Co(tn)2(4-Mepy)Br2+ in water at various time interval of irradiation and at 27 ° according to the Energy Dispersive X-ray analysis
Irradiation Time(min) Abbreviation of Sample Content Weight % (atm. %)
Ti O Co
- nano-TiO2 59.95 (33.33) 40.05 (66.67) -
0 nano-TiO2/Co,0.26 atm. % 60.05 (33.40) 40.32 (67.12) 0.26(0.12)
2 nano-TiO2/Co, 0.53 atm. % 51.29 (26.18) 48.17 (73.60) 0.53 (0.22)
4 nano-TiO2/Co, 0.68 atm. % 35.89 (15.86) 63.43 (83.90) 0.68 (0.25)
12 nano-TiO2/Co, 0.50 atm. % 30.66 (12.93) 68.85 (86.90) 0.50 (0.17)
16 nano-TiO2/Co, 0.71 atm. % 36.97 (16.49) 62.32 (83.25) 0.71 (0.26)
3.7. Surface Morphology of cobalt doped nano-TiO2
The morphology of the undoped TiO2 sample is shown in Figure 4, revealing that the agglomeration of nanocrystals form particles or grains, which are expected to contribute more grain boundary effects. The chemical compositions of undoped and doped samples are very essential to know the exact concentration of the dopant (Co here) and the defects. The EDAX spectrum of undoped TiO2 shows the presence of Ti and O elements alone in the sample, confirming the absence of any impurities. The atomic percentage of Ti and O elements in undoped TiO2 sample is 59.95 and 40.05, respectively, but actual stoichiometric atomic percentage of Ti is 36.97 and O is 62.32, which shows oxygen deficiency in the undoped TiO2 sample. Likewise, all the doped samples show oxygen deficiency. Table 3 gives the atomic percentage of Ti, O, and Co. SEM images illustrate that the particles, which mainly belong to anatase phase, are loosely agglomerated, spherical and a few hundred nm in size (Figure 4(A)). Homogeneous and continuous surface structure with well dispersed spheres appear as the dopant density in Co/TiO2 is increased (Figure 4(B)(C)).
Fig. 4. (A) SEM images and (B), (C) EDX spectra, mapping, line spectra of pure nano-TiO2 and x%Co:nano-TiO2 particles
4. Conclusion
We have demonstrated that cis-[CoI0(tn)2(Rpy)Br]Br2 complexes are photocatalyt-ically reduced into Coj,1^. The photoreduction is found to be solvent dependent. The photoproduced acetone illustrates the scavenging effect of isopropanol. The IFET reaction mechanism is important to understand the functioning and construction of nano-TiO2 pho-tocatalysed photochemical energy conversion systems. It is concluded that the removal of metal ion, hence, seems more specific in removal efficiency and in doping the anatase lattice.
Acknowledgment
KA is thankful to the CSIR (sanction order: No. 01(2570)/12/EMR-II/3.4.2012), New Delhi for financial support through major research project. ASG is thankful to Mr. S. Thirumurugan and Mr. Kanniah Rajkumar, Department of Chemistry, Pondicherry University for their help in data measurement. The authors are thankful to CIF, Pondicherry University for instrumental facility.
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