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Photophysics and photochemistry of phenosafranine adsorbed on the surface of ZnO loaded nanoporous materials
Accepted Manuscript Photophysics and photochemistry of phenosafranine adsorbed on the surface of ZnO loaded nanoporous materials K. Senthil kumar , S. Chandramohan , P. Natarajan PII:
S0143-7208(14)00188-0
DOI:
10.1016/j.dyepig.2014.05.008
Reference:
DYPI 4380
To appear in:
Dyes and Pigments
Received Date: 4 February 2014 Revised Date:
7 May 2014
Accepted Date: 9 May 2014
Please cite this article as: Senthil kumar K, Chandramohan S, Natarajan P, Photophysics and photochemistry of phenosafranine adsorbed on the surface of ZnO loaded nanoporous materials, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.05.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Photophysics and photochemistry of phenosafranine adsorbed on the surface of ZnO loaded nanoporous materials
a
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K. Senthil kumarab*, S. Chandramohanc and P. Natarajana National center for ultrafast processes, university of madras, Chennai, India. b
Department of chemistry, National University of Singapore, Singapore
c
Department of Chemistry, Anna University, BIT Campus, Tiruchirappalli, India.
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E-mail: [email protected]
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Abstract
The photosensitizing properties of phenosafranine adsorbed on surface of the zinc oxide (ZnO) loaded nanoporous materials and colloidal zinc oxides have been investigated by the steady state, time resolved fluorescence and transient absorption spectral studies. The ZnO loaded nanoporous materials were prepared by ion-exchange method and the formation of ZnO nanoparticle inside the host materials were
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characterized by DRS, ICP-OES, XRD and BET-surface area technique. The steady state absorption and emission spectra of phenosafranine does not change when addition of ZnO colloids into the dye solution, the results suggested that excited singlet state of the dye does not participate in the charge injection process. Whereas, singlet state charge
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injection occur when the dye adsorbed on the surface of ZnO loaded nanoporous materials due to effect of compartmentalization of nanoporous materials. The time
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resolved fluorescence and picoseconds transient absorption studies of phenosafranine adsorbed on ZnO loaded host materials are investigated in detail, which would useful for making ZnO based dye sensitized solar cell.
Key words: phenosafranine, ZnO, photosensitization, transient absorption, nanoporous host materials
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1. Introduction Semiconductor nanoparticles TiO2, ZnO, etc., have been used as electron relay in dye sensitized solar cells (DSSC). Electron mobility on ZnO is much higher than TiO2, while the conduction band edge of both nanoparticles is approximately the same. Therefore,
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ZnO is a good candidate for electron carrier in dye-sensitized solar cells. However, the conversion efficiencies of solar-to-electrical energy in ZnO based dye sensitized solar cells are hitherto significantly lower than those reported for TiO2. The problems are mainly due to poor chemical stability of ZnO nanoparticle during adsorption of acidic
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nature of binding group of the dye. Encapsulation of ZnO nanoparticles into nanoporous material will be preventing dissolution of ZnO by the acid binding group as the results
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enhancement in the energy conversion efficiency.
Most of research on TiO2 deals with photocatalytic activities of TiO2 nanoparticle with nanoporous host materials used as the solid support for the semiconductor [1-5]. The photosensitization of TiO2 loaded into the nanoporous materials by organic dyes in the excited state are well documented[6-10]. Recently, ZnO nanoparticle loaded nanoporous materials are of interest in different areas of research in chemistry and physics[11]. Since
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ZnO clusters are so small and unstable, various materials such as glass, polymers and porous silicate material are used as supports or stabilizers[12-16]. Nanoporous materials provide well-defined and well-ordered nanopores to confine ZnO clusters. Thus, the size
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and configuration of ZnO clusters can be modified using different nanoporous host material. On the other hand, ZnO cluster encapsulated in the zeolites shows catalytic
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activity for some organic reactions. Can Li et al.[11,17,18] reported the photoluminance of ZnO nanoparticles encapsulated into host materials such as zeolite-Y and ZSM-5 and their photoluminescence properties in host materials. To comparison to TiO2, few reports are available on preparation and
characterization of ZnO loaded nanoporous materials. Best of our knowledge, no report is available on photosensitization of ZnO encapsulated in nanoporous host materials by organic dyes. In the present work, we have investigated the singlet state charge injection process of phenosafranine adsorbed on colloidal ZnO nanoparticle and embedded into the
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nanoporous materials by using steady state, time resolved fluorescence and transient absorption spectral studies in nano and picosecond time domain. The phenazine family of dyes has been studied widely for the conversion of solar
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energy into the electrical energy because of its strong absorption in the visible region, photochemical stability and excited state redox properties [19-22]. Phenosafranine (3,7– diamino–5–phenylphenazinium chloride) absorbs strongly in the visible region (500 to 550 nm) which is belongs to the phenazine family of dyes. Phenosafranine has been
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extensively studied as a photosensitizer in energy and electron transfer reactions in homogenous and in heterogeneous media[19-24]. Photophysical and photochemical
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behavior of the dye in homogenous solution and in covalently bound in polymer matrix are studied since few decades. These studies are important to understand the nature of the systems for the application in solar energy conversion[25-28]. The dye has also been used to probe pH induced the dynamics of polymers in aqueous solution. Kamat et al.[29] studied the photochemistry of phenosafranine surface adsorbed on the titanium dioxide
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and polymer thin film coated titanium dioxide by using diffuse reflectance laser flash photolysis technique[30]. Direct contact between the dye and TiO2 nanoparticles leads to decomposition of the dye molecules. Titanium dioxide semiconductor coated with thin polymer film suppresses the charge injection process and improves the photostability of
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the dye molecule. Similarly, nanoporous host materials provide the photostability of the dye molecules and also enhance the charge injection process. The photosensitization of
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titanium dioxide encapsulated within nanoporous host materials by surface adsorbed phenosafranine has been extensively studied[6, 8].
2. Experimental section 2.1. Materials Phenosafranine (PS+) and nanoporous host materials; zeolite–Y and ZSM–5 were obtained from Aldrich and Sud Chemie India respectively. All other chemicals were purchased from Qualigens and Merck fine chemicals.
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2.2 Sample preparation ZnO loaded nanoporous host materials (zeolite-Y and ZSM-5) were prepared by ion exchange method[17]. 1.0 g of host materials was stirred with zinc nitrate solution
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(50 ml, 1x10–3 M) for 24 hrs. The ion exchange of Na+ by Zn2+ occurred while stirring mixture of zinc nitrate and host material in aqueous solution for 24 hrs. The zinc exchanged host material was washed with excess amount of distilled water. Formation of zinc oxide nanoparticles into the nanoporous materials of zeolite–Y and ZSM-5 host were
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achieved by heating the sample at 250ºC for 6 hrs. The loading level of zinc oxide within the host material was increased by repeated ion exchange procedure followed by heat
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treatment.
Incorporation of ZnO into the MCM–41 was achieved by suspending 1.0 g MCM–41 into 50 ml of 1x10–3 M zinc acetate dissolved in water solution for 6 hrs. Similar procedure was followed for the preparation of increasing loading levels of ZnO into the nanochannels of MCM–41. The prepared ZnO loaded materials were
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characterized by XRD, BET surface area techniques and diffuse reflectance spectroscopy. The actual amount of zinc present in the host materials was determined by ICP–OES method.
The dye adsorbed on ZnO loaded the nanoporous host materials was prepared by
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stirring mixture of 50 ml of the dye in aqueous solution (5 x 10–5 M) and 1.0 g of host materials encapsulated ZnO nanoparticle for 4 hrs at room temperature. The resulting
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coloured solid was filtered and washed with an excess amount of distilled water. The concentration of the dye in the host material was maintained at ≈5 x 10–5 (mol of dye)/(g of host material). In order to minimize the error due to weighing, bulk dye solution (1 x 10-3 M in 100 ml) was prepared and required volume of the solution is used for the loading of the dye in ZSM-5, zeolite-Y and MCM-41. The prepared samples were characterized by XRD.
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2.3 Characterization BET surface measurements for nanoporous host materials; zeolite–Y, ZSM–5 and MCM–41 in presence and absence of ZnO nanoparticles were carried out using
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volumetric adsorption equipment (ASAP 2010 micrometric USA) at 77 K. Powder X–ray diffraction patterns of nanoporous materials and ZnO loaded nanoporous materials were recorded using a diffractometer with CuK∝ radiation (λ = 1.5406 Å). The scanning rate was 0.02º/min in all cases.
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Transmission electron microscopy (TEM) was carried out using a JEOL 3011 300 kV instrument with a UHR pale piece. The samples prepared by dropping the dispersion of
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MCM–41 and ZnO loaded MCM-41 in ethanol on a copper grid and dried under ambient conditions.
Zinc as ZnO present in the ZSM-5, zeolite-Y and MCM–41 host materials was determined by ICP–OES method using Perkin Elmer Optima 5300DV. The samples prepared for ICP-OES similar to procedure reported earlier[7]. Diffuse reflectance
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spectra were recorded using Agilant 8453 spectrophotometer equipped with labsphere RSH–HP–8453 reflectance accessory. The scattered light reflected by the solid surface corresponds to the intensity reflected by bare host sample surface minus that absorbed by the chromophore. Diffuse reflectance plots matching the optical spectrum in transmission
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are obtained by subtracting reflectance (R) from bare host material, converting it into a function, F(R) by Kubulka Munk expression.
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Steady state fluorescence spectra of dyes adsorbed onto nanoporous host materials
were carried out using Jobin Yuon Fluromax–4 spectrophotometer in the front face configuration at 45o.
Time resolved fluorescence decay was recorded by time correlated single photon
counting techniques exciting the sample at 430 nm laser. Data analysis was carried out by the software provided by IBH (DAS-6), which is based on deconvolution techniques
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using nonlinear least-squares method and the quality of the fit are determined with the value of χ2 < 1.2 and weighted residuals. Picosecond transient absorption experiments were carried out using Mode-Locked
rate) detailed experimental procedure reported earlier[31]. 3. Results and discussion
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3.1 Characterization of ZnO loaded nanoporous materials
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Nd:YAG laser system PY61C-10, (532 nm, 5 mj/pulse, FWHM 35 ps, 10 Hz repetition
The powder X-ray diffraction patterns of nanoporous materials with different ZnO loading are shown in Fig. S1. The X–ray diffraction pattern of MCM–41 shows several
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peaks between 1.5 to 10° suggests that the presence of well formed hexagonal in the nanoporous material pore arrays. The samples investigated exhibit strong diffraction peaks demonstrating that nanoporous materials, zeolite–Y, ZSM–5 and MCM–41 have crystal structure identical to those reported in the database. The XRD patterns of the host materials are similar in the presence and absence of zinc oxide shows that the crystallinity
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of the host material is unaffected even after the incorporation of zinc oxide nanoparticles. The results suggesting that the structural damage to the host lattice is insignificant during the ion–exchange process with the ZnO particles residing inside the nanoporous of the
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materials. However, the crystallinity of the nanoporous materials decreases marginally with increasing ZnO loading beyond certain level as reported earlier[17]. The percentage
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of decrease in crystallinity of the host material can be calculated (Table S1) by measuring the intensity of the peak at 2θ values of 2.6 º, 6.19° and 8.03º for MCM–41, zeolite–Y a0nd ZSM–5 respectively. Zeolite–Y a maximum of 4% decrease in crystallinity is observed for the samples containing 8% of zinc oxide loading. A 10% decrease is observed in the case of ZSM–5 with the encapsulation of 0.8% zinc oxide loading in the nanochannels. In ZSM-5 encapsulated ZnO, where ZnO loading is higher than 5%, the diffraction pattern of ZnO is observed at 2Ɵ = 35º. Such types of diffraction peak of ZnO particles are not observed in the case of zeolite–Y and MCM–41, even at higher ZnO
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loading. The absence of the ZnO diffraction peaks indicates that ZnO has been highly dispersed into the nanochannel and nanocavities of zeolite–Y and MCM–41. Similar observation was made in the case of ZnO loaded onto the zeolite–Y and ZSM–5 reported
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earlier[17, 32-35]. Adsorption and desorption isotherms of nitrogen on the nanoporous host materials are shown in Figure S2. BET surface area and micropore volume of nanoporous host materials with different loading of level ZnO ZnO are given in Table 1. A decrease in the
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BET surface area from 1023 to 312 cm3/g is observed in ZnO loaded MCM-41 with no considerable change in the XRD pattern indicating that ZnO nanopartilcs are distributed
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uniformly within the nanochannels of MCM–41. Similar observation is observed in zeolite-Y and ZSM-5. In the case of ZSM–5, a decrease in the BET surface area is observed when ZnO loading is upto 5%. When the ZnO loading exceeds 5% the BET surface area of ZSM–5 do not change due to forming macrocrystalline ZnO on ZSM–5 surface. Based on the results of the XRD and BET surface area, we have concluded that
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the 5 weight % loading is critical threshold of ZnO dispersion on ZSM–5. Transmission electron microscopic (TEM) image of MCM–41 is as displayed in Figure S3. indicates that MCM–41 has ordered mesoporous with uniform pore size while titanium dioxide loaded MCM–41 has less ordered mesoporous channels and also not
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able to show zinc oxide within the nanochannels. These results suggest that TEM image does not provide detailed information about ZnO nanoparticles residing inside the
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nanochannels.
Diffuse reflectance spectra of zinc oxide loaded nanoporous materials are shown
in Figure 1. Absorption band edge of ZnO encapsulated into the nanoporous host materials is blue shifted as compared to the bulk ZnO powder. The observed decrease in the band edge is due to the size quantization of nanoparticles in the confined geometries. The absorption band edge of ZnO nanoparticles encapsulated into the nanoporous host materials are given in Table S1. The absorption band edge of the ZnO nanoparticles is blue shifted by 50–100 nm when compared to that of bulk ZnO powder. The absorbance
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corresponding to ZnO increases with increase in concentration of zinc oxide without significant change in the absorption band edge, which again indicates that the particle size of the ZnO does not change at higher loading of zinc oxide in the host material. The
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band edge of zinc oxide is shifted towards red region of the spectrum above 5% of ZnO loading in ZSM–5. Both of the XRD, BET surface area and DRS results indicate that the 5 weight % loading is critical threshold of ZnO dispersion on ZSM–5.
3.2 Steady state absorption and emission spectral properties of the dye adsorbed onto the
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nanoporous host materials
Phenosafranine shows maximum absorbance in the visible region at 520 nm in
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aqueous solution which is not significantly affected in different solvents[8]. The diffuse reflectance spectra of phenosafranine adsorbed onto the nanoporous materials are shown in Figure 2. The absorption peak maximum of the dye does not change significantly while adsorbed onto the nanoporous host materials with different pore size and Si/Al ratio revealing that the dye is not sensitive to the structure and surface polarity of the host materials. The fluorescence spectra of phenosafranine (Figure 2b) are found to be
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influenced by the host materials. The photophysical properties of phenosafranine adsorbed on the surface of nanoporous host materials are given in Table 2. The fluorescence emission maximum of the dye is blue shifted when the dye adsorbed on
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zeolite–Y, ZSM–5 and silica with respect to that of the dye in aqueous solution. This observed shift in the fluorescence emission maximum is comparable to that of the dye in
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methanol:water mixture. Interestingly, the fluorescence maximum of the dye adsorbed on the external surface of host materials; zeolite–Y, ZSM–5 and silica remain unchanged even though surface polarities of the hosts are not the same. The photophysical properties of phenosafranine adsorbed onto the porous materials are well documented earlier[6,8,10]. Emission spectral maximum of phenosafranine encapsulated into MCM– 41 is comparable to that observed in aqueous solution, which reveals that the dye is present along with water molecules in the larger pores of MCM-41.
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3.3 Photophysics of phenosafranine adsorbed on the surface of ZnO loaded nanoporous host materials The fluorescence intensity of the dye in aqueous solution does not change with
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increasing concentration of colloidal ZnO nanoparticle as shown in Fig. S4. The results suggest that singlet state of dye do not involved in the charge injection processes in solution. Interesting to note that the dye adsorbed onto the nanoporous materials shows a decrease in the fluorescence intensity (Fig.3) with increasing various loading level of
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ZnO suggesting that the singlet state charge injection occurs from the dye in the excited singlet state to ZnO nanoparticles present inside the host materials. Whereas, the dye
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involved in singlet state charge injection process with similar particle size of TiO2 nanoparticles present in solution. The observed difference in the photophysics of the dye in colloidal solution is due to surface nature of semiconductor nanoparticle in aqueous solution. Once semiconductor nanoparticles are encapsulated into the host materials, the dye shows similar photophysical behavior even though surface nature of semiconductor particle is different. In general the chemical stability of ZnO is lower than that of TiO2
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which was found to be problematic in the dye adsorption process. Several reports implies that dye adsorption is the main problem in ZnO based dye sensitized solar cell[16-18]. i) Cells with higher dye loading is inefficient, whereas cells with lower dye loading show
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good conversion efficiency and also ii) the acidic nature of the dye molecule that leads to dissociation of ZnO. In this case, the host materials could solve both problems i) it
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provide large volume of space for higher dye loading and ii) provide chemical stability of ZnO nanoparticles. The photosensitization of ZnO nanoparticles encapsulated into the nanoporous silicate materials may improve the conversion efficiency of the dye molecules.
The fluorescence decay profile observed for phenosafranine in ZnO loaded nanoporous host material (Fig. S5) is fitted satisfactorily to bi-exponential function with χ2<1.5. In the case of ZnO loaded ZSM-5 and zeolite-Y, the observed fluorescence
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lifetimes of the dye are not affected notably with an increase in the loading of the ZnO into the zeolite-Y cavity as indicated in Fig. 4. Recently, Corma and co-workers[36] reported the electron transfer from the excited state of Ru(bpy)32+ adsorbed on the
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external cups of ITQ-2 zeolite to methyl viologen, MV2+ incorporated in the nanochannels. In the presence of MV2+, a considerable decrease in the emission intensity of the Ru(bpy)32+ was observed however emission lifetime essentially unaltered. The
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results are interpreted to be due to contact quenching between the excited state of Ru(bpy)32+ and MV2+ in close proximity. Excited state of Ru(bpy)32+ ion shows normal
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emission properties when the MV2+is not in the vicinity of the complex. It has also been observed that TiO2 encapsulated with MCM-41 and porous silicates also show contact quenching by the excited states of different dyes[37]. Based on these results it is suggested that the photosensitization of ZnO by surface adsorbed phenosafranine is
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occurs when the dye and ZnO (ZnO is present closer to the pore opening of host materials) are close proximity in the host material. If the ZnO are present in interior of the host materials, which are not involved in the quenching processes. In the case of the dye
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encapsulated in the nanochannels of MCM-41, a decrease in the fluorescence lifetime with increase in the loading of ZnO is observed whereas the fluorescence lifetimes are
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unaltered when the dye and titanium dioxide encapsulated into the nanochannels of MCM–41[37]. The observed difference in the behavior of the dye in MCM–41 is due to the nature of the semiconductor surface and the interaction with the nanoparticles. 3.4 Picosecond transient absorption spectral investigation of phenosafranine adsorbed on ZnO loaded nanoporous host materials.
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Picosecond transient absorption spectra of phenosafranine adsorbed onto the external surface of the nanoporous ZSM-5 host, using 532 nm laser pulse, shows broad transient absorption from 600 nm to 760 nm (Figure-5, ZSM-5_Ps). The transients observed are
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suggested to be due to the formation of the excited triplet state of the dye, trapped electron and oxidized phenosafranine radical [31, 22]. In the case of ZnO loaded host materials, transient absorption of phenosafranine dye shows two spectral maximum at ~640nm and
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750 nm (Figure-5, ZSM-5_ZnO_Ps and MCM-41_ZnO_Ps). It is suggested that in the presence of ZnO, the transient absorption with maximum at ~700nm which is due to the
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trapped electron quenched as the results splitting of two bands observed at 640nm and 750nm is suggested to be due to the phenosafranine cation radical and triplet state of the dye. Similar observation was reported [31] in the case of phenosafranine adsorbed on the Ni(bpy)32+ ion encapsulated in the supercages of the zeolite-Y host, in this case transient
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absorption spectra slightly shifted red region due to surface nature of host materials are different. Based on the picosecond transient absorption spectral studies, we conclude that the electron transfer occurs from the excited state phenosafranine to entrapped ZnO
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nanoparticles in nanoporous host materials. Phenosafranine adsorbed on ZnO loaded zeolite-Y does not observed such type of transient spectra which indicates that electron
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transfer may be very fast from the dye to ZnO in zeolite-Y host. The quenching of the excited state of the dye is unknown to occur by diffusion process in solution. However, in the solid surface, the excited state formed undergo charge transfer processes when the quencher is present in the vicinity of the excited state or mediated through lattice in presence of adsorbed water molecules. Investigations reported in the present systems are
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important to understand the excited state processes in nanoporous solids for many applications in device fabrication, sensors, and in other areas.
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4. Conclusion
Encapsulation of zinc oxide nanoparticles into the nanoporous host materials are prepared by ion exchange method and are characterized by XRD, BET, DRS and ICP-OES techniques. The results clearly showed that ZnO nanoparticles are present inside the
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nanoporous host materials and surface adsorbed ZnO nanoparticles may be negligible amount. The steady state absorption and emission spectral investigation reveal that the
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excited state processes of the dye are not much influenced by structural variation of host materials. The influence of the host materials on singlet state charge injection of the dye is demonstrated by steady state fluorescence studies. The lifetime of the excited state of the dye in the host does not change significantly in the presence of ZnO nanoparticles indicating that the quenching mechanism is due to arrested movement of the dye and ZnO nanoparticles in
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host. The picoseconds transient absorption spectral studies clearly shows that charge injection from pheneosafranine upon excitation to ZnO nanoparticle resides in the host materials. The nanoporous solid host materials provide ultrafast charge separation and also
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stability of ZnO nanoparticles.
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ACKNOWLEDGMENT:
PN is the recipient of Indian National Science Academy Senior Scientist
Fellowship. The authors acknowledge the financial support received from the Department of Science and Technology, Government of India through the Raja Ramanna Fellowship (PN). NCUFP is supported by DST-IHRPA programme.
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Supporting materials X-ray diffraction pattern of MCM-41, zeoilite-Y and ZSM-5 in the absence and presence of the ZnO nanoparticals, adsorption and desorption isotherms of nitrogen on -
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41, zeoilite-Y and ZSM-5 in the absence and presence of the ZnO nanoparticals, TEM image of MCM-41, absorption and emission spectra of phenosafranine dye with various concentration of ZnO nanoparticles in solution. Fluorescence emission decay of phenosafranine adsorbed in various loading of ZnO in host materials. Supplementary data
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associated with this article can be found in the online version, at doi:
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Figure captions 1. Diffuse reflectance spectra of ZnO nanoparticles in a) ZSM–5 zeolite i: 0.72%, ii: 0.87%, iii: 0.98%, iv: 1.15%, v: 1.32%, and vi: 5.32% b) zeolite–Y i: 5.46%, ii:
5.12%, iii: 6.90 % and iv: 8.12 %.
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6.46%, iii: 6.87%, iv: 7.14% v: 7.94% and vi: 8.30% c) MCM–41 i: 3.78 %, ii:
2. (a) Diffuse reflectance absorption and (b) emission spectra of phenosafranine adsorbed on the i) –∆–∆– silica, ii) --- MCM–41, iii) --- ZSM–5 and iv) -
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- - zeolite–Y.
3. Diffuse reflectance spectra (a, c and e) and fluorescence emission spectra (b, d and f) of phenosafranine adsorbed onto host material encapsulated with ZnO [(a
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and b) MCM–41 ( i: 0% ZnO, ii: 3.78 % ZnO, iii: 5.12 % ZnO, iv: 6.90 %
ZnO and v: 8.12 % ZnO) (c and d) ZSM–5 (i: 0% ZnO, ii: 0.87% ZnO, iii: 0.98% ZnO, iv: 1.32% ZnO, and v: 5.03% ZnO); (e and f) zeolite–Y (i: 0% ZnO, ii: 5.46% ZnO, iii: 6.87% ZnO, iv: 7.14% ZnO and v: 7.94% ZnO);]
(excitation at 520 nm).
Plot of fluorescence lifetime (a-c) and relative amplitude (d-f) of phenosafranine
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4.
versus ZnO loading; (a and d) zeolite–Y, (b and e) ZSM–5 and (c and f) MCM–41 (excitation at 470 nm and emission decay monitored at 490 nm). 5. Transient absorption spectra of phenosafraine adsorbed onto the ZSM-5 and ZnO
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loaded ZSM-5, MCM-41. (Excitation at 532 nm probe: white light delay: 133ps)
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Table 1: The adsorption characteristics of nanoporous silicate materials in the presence and absence of the ZnO nanoparticles ZnO in % (w/w)
0.382 0.320 0.317 0.313 0.311 0.167 0.191 0.175 0.197 0.69 0.52 0.403 0.394 0.392
0.00 6.46 6.87 7.94 8.30 0.00 0.72 0.87 5.03 0.00 3.78 5.12 6.90 8.12
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Zeolite–Y 845 ZnO–Y (1) 666 ZnO–Y (2) 659 ZnO–Y (3) 654 ZnO–Y (4) 650 ZSM–5 453 ZnO–ZSM(1) 358 ZnO–ZSM(2) 327 ZnO–ZSM(3) 374 MCM–41 1023 ZnO–MCM(1) 804 ZnO–MCM(2) 458 ZnO–MCM(3) 413 ZnO–MCM(4) 244 (1), (2), (3)… are sample number
Micro pore volume (cm3/g)
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BET surface area (m2/g)
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Sample
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Table 2: Photophysical characteristics of phenosafranine adsorbed on different nanoporous host materials
Host materials λabs (nm)
521
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Silica
520
EP
Ps aq a
λemi (nm)
Stokes shift ∆ν (cm ( -1)
590
2281
574
1772
MCM–41
519
582
2085
ZSM–5
523
577
1789
Zeolite–Y
528
578
1638
a – in aqueous solution
τ (ns) (% amplitude) 0.83 (100%) 0.67 (14%) 1.89 (86%) 0.34 (38%) 1.86 (62%) 0.57 (32%) 1.26 (68%) 0.39 (72%) 1.34 (28%)
τav(ns) 0.83 1.71
1.28
1.03
0.65
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Graphical abstract
Photophysics and photochemistry of phenosafranine adsorbed on the
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surface of ZnO loaded nanoporous materials
K. Senthil kumarab*, S. Chandramohanc and P. Natarajana a
National center for ultrafast processes, university of madras, Chennai, India. b
Department of chemistry, National University of Singapore, Singapore
Department of Chemistry, Anna University, BIT Campus, Tiruchirappalli, India.
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c
hυ
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e-mail: [email protected]
Ps +
e -x
Ps +
Ps +
ePs +
Ps +
Ps +
ZnO
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Ps +
Ps +
Ps +
Ps +
Ps +
Ps +
Ps +
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Ps + Ps +-Phenosaf ranine dye-Z nO in solution dye-Z nO in nanoporous host
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Supporting materials Photophysics and photochemistry of phenosafranine adsorbed on the
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surface of ZnO loaded nanoporous materials K. Senthil kumarab*, S. Chandramohanc and P. Natarajana a
National center for ultrafast processes, university of madras, Chennai, India. b
Department of chemistry, National University of Singapore, Singapore
Department of Chemistry, Anna University, BIT Campus, Tiruchirappalli, India.
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c
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E-mail: [email protected]
b
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a
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Figure S1: XRD pattern of ZnO nanoparticles in a) zeolite–Y i: 0%, ii: 6.46%, iii: 6.87%, iv: 7.94%, and v: 8.30% b) ZSM–5 zeolite i: 0%, ii: 0.72%, iii: 0.87% and iv: 5.32% c) MCM–41 i: 0%, ii: 3.78 %, iii: 5.12%, iv: 6.90 % and v: 8.12 %.
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a
b
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i i
ii iii iv
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iii
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iv
Figure S2: Adsorption and desorption isotherms of nitrogen on a) zeolite–Y i: 0%, ii: 6.46%, iii: 6.87%, and iv: 8.30% b) ZSM–5 zeolite i: 0%, ii: 0.72%, iii: 0.87% and iv: 5.32% c) MCM–41 i: 0%, ii: 3.78 %, iii: 5.12%, and iv: 8.12 %.
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ii
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i
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Absorbance
Figure S3: TEM images of i) MCM–41 and ii) ZnO loaded MCM–41
200 300 400 500 600 700 550
Wavelength (nm)
600
650
700
Wavelength (nm)
Figure S4: a) absorption and b) emission spectra of phenosafranine with various concentration of ZnO nanoparticle in aqueous solution
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Figure S5: Fluorescence decay profile phenosafranine adsorbed on a) zeolite–Y, b) ZSM– 5 and c) MCM–41 with different loading levels of ZnO nanoparticles
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ZnO in % (w/w)
– 3.81 3.81 3.81 3.64 – 4.50 4.27 3.26 – 3.81 3.81 3.81 3.81
– 6.46 6.87 7.94 8.30 – 0.7 0.87 5.03 – 3.78 5.12 6.90 8.12
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Band gap energy (eV)
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100 98 98 97 96 100 95 90 82 100 97 94 90 88
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Zeolite–Y ZnO–Y(1) ZnO –Y(2) ZnO –Y(3) ZnO –Y(4) ZSM–5 ZnO –ZSM(1) ZnO –ZSM(2) ZnO –ZSM(3) MCM–41 ZnO –MCM(1) ZnO –MCM(2) ZnO –MCM(3) ZnO –MCM(4)
Absorption band edge (nm) – 325 325 325 340 – 275 290 380 – 325 325 325 325
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Sample
Crystallinity %
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Table S1: XRD and diffuse reflectance spectral properties of ZnO encapsulated into the nanoporous silicate materials
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Excited state properties of phenosafranine adsorbed on the ZnO in nanoporous materials and in colloidal form were studied Singlet state charge injection from the dye to ZnO nanoparticles in host materials is confirmed by time resolved fluorescence spectral studies. Singlet state charge injection of phenosafranine was not observed when the dye in solution Photo-induced electron transfer between the surfaces adsorbed organic dye to encapsulated ZnO have been investigated by picoseconds transient absorption spectroscopy.
S0143-7208(14)00188-0
DOI:
10.1016/j.dyepig.2014.05.008
Reference:
DYPI 4380
To appear in:
Dyes and Pigments
Received Date: 4 February 2014 Revised Date:
7 May 2014
Accepted Date: 9 May 2014
Please cite this article as: Senthil kumar K, Chandramohan S, Natarajan P, Photophysics and photochemistry of phenosafranine adsorbed on the surface of ZnO loaded nanoporous materials, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.05.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Photophysics and photochemistry of phenosafranine adsorbed on the surface of ZnO loaded nanoporous materials
a
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K. Senthil kumarab*, S. Chandramohanc and P. Natarajana National center for ultrafast processes, university of madras, Chennai, India. b
Department of chemistry, National University of Singapore, Singapore
c
Department of Chemistry, Anna University, BIT Campus, Tiruchirappalli, India.
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E-mail: [email protected]
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Abstract
The photosensitizing properties of phenosafranine adsorbed on surface of the zinc oxide (ZnO) loaded nanoporous materials and colloidal zinc oxides have been investigated by the steady state, time resolved fluorescence and transient absorption spectral studies. The ZnO loaded nanoporous materials were prepared by ion-exchange method and the formation of ZnO nanoparticle inside the host materials were
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characterized by DRS, ICP-OES, XRD and BET-surface area technique. The steady state absorption and emission spectra of phenosafranine does not change when addition of ZnO colloids into the dye solution, the results suggested that excited singlet state of the dye does not participate in the charge injection process. Whereas, singlet state charge
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injection occur when the dye adsorbed on the surface of ZnO loaded nanoporous materials due to effect of compartmentalization of nanoporous materials. The time
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resolved fluorescence and picoseconds transient absorption studies of phenosafranine adsorbed on ZnO loaded host materials are investigated in detail, which would useful for making ZnO based dye sensitized solar cell.
Key words: phenosafranine, ZnO, photosensitization, transient absorption, nanoporous host materials
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1. Introduction Semiconductor nanoparticles TiO2, ZnO, etc., have been used as electron relay in dye sensitized solar cells (DSSC). Electron mobility on ZnO is much higher than TiO2, while the conduction band edge of both nanoparticles is approximately the same. Therefore,
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ZnO is a good candidate for electron carrier in dye-sensitized solar cells. However, the conversion efficiencies of solar-to-electrical energy in ZnO based dye sensitized solar cells are hitherto significantly lower than those reported for TiO2. The problems are mainly due to poor chemical stability of ZnO nanoparticle during adsorption of acidic
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nature of binding group of the dye. Encapsulation of ZnO nanoparticles into nanoporous material will be preventing dissolution of ZnO by the acid binding group as the results
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enhancement in the energy conversion efficiency.
Most of research on TiO2 deals with photocatalytic activities of TiO2 nanoparticle with nanoporous host materials used as the solid support for the semiconductor [1-5]. The photosensitization of TiO2 loaded into the nanoporous materials by organic dyes in the excited state are well documented[6-10]. Recently, ZnO nanoparticle loaded nanoporous materials are of interest in different areas of research in chemistry and physics[11]. Since
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ZnO clusters are so small and unstable, various materials such as glass, polymers and porous silicate material are used as supports or stabilizers[12-16]. Nanoporous materials provide well-defined and well-ordered nanopores to confine ZnO clusters. Thus, the size
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and configuration of ZnO clusters can be modified using different nanoporous host material. On the other hand, ZnO cluster encapsulated in the zeolites shows catalytic
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activity for some organic reactions. Can Li et al.[11,17,18] reported the photoluminance of ZnO nanoparticles encapsulated into host materials such as zeolite-Y and ZSM-5 and their photoluminescence properties in host materials. To comparison to TiO2, few reports are available on preparation and
characterization of ZnO loaded nanoporous materials. Best of our knowledge, no report is available on photosensitization of ZnO encapsulated in nanoporous host materials by organic dyes. In the present work, we have investigated the singlet state charge injection process of phenosafranine adsorbed on colloidal ZnO nanoparticle and embedded into the
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nanoporous materials by using steady state, time resolved fluorescence and transient absorption spectral studies in nano and picosecond time domain. The phenazine family of dyes has been studied widely for the conversion of solar
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energy into the electrical energy because of its strong absorption in the visible region, photochemical stability and excited state redox properties [19-22]. Phenosafranine (3,7– diamino–5–phenylphenazinium chloride) absorbs strongly in the visible region (500 to 550 nm) which is belongs to the phenazine family of dyes. Phenosafranine has been
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extensively studied as a photosensitizer in energy and electron transfer reactions in homogenous and in heterogeneous media[19-24]. Photophysical and photochemical
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behavior of the dye in homogenous solution and in covalently bound in polymer matrix are studied since few decades. These studies are important to understand the nature of the systems for the application in solar energy conversion[25-28]. The dye has also been used to probe pH induced the dynamics of polymers in aqueous solution. Kamat et al.[29] studied the photochemistry of phenosafranine surface adsorbed on the titanium dioxide
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and polymer thin film coated titanium dioxide by using diffuse reflectance laser flash photolysis technique[30]. Direct contact between the dye and TiO2 nanoparticles leads to decomposition of the dye molecules. Titanium dioxide semiconductor coated with thin polymer film suppresses the charge injection process and improves the photostability of
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the dye molecule. Similarly, nanoporous host materials provide the photostability of the dye molecules and also enhance the charge injection process. The photosensitization of
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titanium dioxide encapsulated within nanoporous host materials by surface adsorbed phenosafranine has been extensively studied[6, 8].
2. Experimental section 2.1. Materials Phenosafranine (PS+) and nanoporous host materials; zeolite–Y and ZSM–5 were obtained from Aldrich and Sud Chemie India respectively. All other chemicals were purchased from Qualigens and Merck fine chemicals.
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2.2 Sample preparation ZnO loaded nanoporous host materials (zeolite-Y and ZSM-5) were prepared by ion exchange method[17]. 1.0 g of host materials was stirred with zinc nitrate solution
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(50 ml, 1x10–3 M) for 24 hrs. The ion exchange of Na+ by Zn2+ occurred while stirring mixture of zinc nitrate and host material in aqueous solution for 24 hrs. The zinc exchanged host material was washed with excess amount of distilled water. Formation of zinc oxide nanoparticles into the nanoporous materials of zeolite–Y and ZSM-5 host were
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achieved by heating the sample at 250ºC for 6 hrs. The loading level of zinc oxide within the host material was increased by repeated ion exchange procedure followed by heat
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treatment.
Incorporation of ZnO into the MCM–41 was achieved by suspending 1.0 g MCM–41 into 50 ml of 1x10–3 M zinc acetate dissolved in water solution for 6 hrs. Similar procedure was followed for the preparation of increasing loading levels of ZnO into the nanochannels of MCM–41. The prepared ZnO loaded materials were
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characterized by XRD, BET surface area techniques and diffuse reflectance spectroscopy. The actual amount of zinc present in the host materials was determined by ICP–OES method.
The dye adsorbed on ZnO loaded the nanoporous host materials was prepared by
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stirring mixture of 50 ml of the dye in aqueous solution (5 x 10–5 M) and 1.0 g of host materials encapsulated ZnO nanoparticle for 4 hrs at room temperature. The resulting
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coloured solid was filtered and washed with an excess amount of distilled water. The concentration of the dye in the host material was maintained at ≈5 x 10–5 (mol of dye)/(g of host material). In order to minimize the error due to weighing, bulk dye solution (1 x 10-3 M in 100 ml) was prepared and required volume of the solution is used for the loading of the dye in ZSM-5, zeolite-Y and MCM-41. The prepared samples were characterized by XRD.
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2.3 Characterization BET surface measurements for nanoporous host materials; zeolite–Y, ZSM–5 and MCM–41 in presence and absence of ZnO nanoparticles were carried out using
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volumetric adsorption equipment (ASAP 2010 micrometric USA) at 77 K. Powder X–ray diffraction patterns of nanoporous materials and ZnO loaded nanoporous materials were recorded using a diffractometer with CuK∝ radiation (λ = 1.5406 Å). The scanning rate was 0.02º/min in all cases.
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Transmission electron microscopy (TEM) was carried out using a JEOL 3011 300 kV instrument with a UHR pale piece. The samples prepared by dropping the dispersion of
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MCM–41 and ZnO loaded MCM-41 in ethanol on a copper grid and dried under ambient conditions.
Zinc as ZnO present in the ZSM-5, zeolite-Y and MCM–41 host materials was determined by ICP–OES method using Perkin Elmer Optima 5300DV. The samples prepared for ICP-OES similar to procedure reported earlier[7]. Diffuse reflectance
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spectra were recorded using Agilant 8453 spectrophotometer equipped with labsphere RSH–HP–8453 reflectance accessory. The scattered light reflected by the solid surface corresponds to the intensity reflected by bare host sample surface minus that absorbed by the chromophore. Diffuse reflectance plots matching the optical spectrum in transmission
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are obtained by subtracting reflectance (R) from bare host material, converting it into a function, F(R) by Kubulka Munk expression.
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Steady state fluorescence spectra of dyes adsorbed onto nanoporous host materials
were carried out using Jobin Yuon Fluromax–4 spectrophotometer in the front face configuration at 45o.
Time resolved fluorescence decay was recorded by time correlated single photon
counting techniques exciting the sample at 430 nm laser. Data analysis was carried out by the software provided by IBH (DAS-6), which is based on deconvolution techniques
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using nonlinear least-squares method and the quality of the fit are determined with the value of χ2 < 1.2 and weighted residuals. Picosecond transient absorption experiments were carried out using Mode-Locked
rate) detailed experimental procedure reported earlier[31]. 3. Results and discussion
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3.1 Characterization of ZnO loaded nanoporous materials
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Nd:YAG laser system PY61C-10, (532 nm, 5 mj/pulse, FWHM 35 ps, 10 Hz repetition
The powder X-ray diffraction patterns of nanoporous materials with different ZnO loading are shown in Fig. S1. The X–ray diffraction pattern of MCM–41 shows several
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peaks between 1.5 to 10° suggests that the presence of well formed hexagonal in the nanoporous material pore arrays. The samples investigated exhibit strong diffraction peaks demonstrating that nanoporous materials, zeolite–Y, ZSM–5 and MCM–41 have crystal structure identical to those reported in the database. The XRD patterns of the host materials are similar in the presence and absence of zinc oxide shows that the crystallinity
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of the host material is unaffected even after the incorporation of zinc oxide nanoparticles. The results suggesting that the structural damage to the host lattice is insignificant during the ion–exchange process with the ZnO particles residing inside the nanoporous of the
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materials. However, the crystallinity of the nanoporous materials decreases marginally with increasing ZnO loading beyond certain level as reported earlier[17]. The percentage
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of decrease in crystallinity of the host material can be calculated (Table S1) by measuring the intensity of the peak at 2θ values of 2.6 º, 6.19° and 8.03º for MCM–41, zeolite–Y a0nd ZSM–5 respectively. Zeolite–Y a maximum of 4% decrease in crystallinity is observed for the samples containing 8% of zinc oxide loading. A 10% decrease is observed in the case of ZSM–5 with the encapsulation of 0.8% zinc oxide loading in the nanochannels. In ZSM-5 encapsulated ZnO, where ZnO loading is higher than 5%, the diffraction pattern of ZnO is observed at 2Ɵ = 35º. Such types of diffraction peak of ZnO particles are not observed in the case of zeolite–Y and MCM–41, even at higher ZnO
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loading. The absence of the ZnO diffraction peaks indicates that ZnO has been highly dispersed into the nanochannel and nanocavities of zeolite–Y and MCM–41. Similar observation was made in the case of ZnO loaded onto the zeolite–Y and ZSM–5 reported
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earlier[17, 32-35]. Adsorption and desorption isotherms of nitrogen on the nanoporous host materials are shown in Figure S2. BET surface area and micropore volume of nanoporous host materials with different loading of level ZnO ZnO are given in Table 1. A decrease in the
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BET surface area from 1023 to 312 cm3/g is observed in ZnO loaded MCM-41 with no considerable change in the XRD pattern indicating that ZnO nanopartilcs are distributed
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uniformly within the nanochannels of MCM–41. Similar observation is observed in zeolite-Y and ZSM-5. In the case of ZSM–5, a decrease in the BET surface area is observed when ZnO loading is upto 5%. When the ZnO loading exceeds 5% the BET surface area of ZSM–5 do not change due to forming macrocrystalline ZnO on ZSM–5 surface. Based on the results of the XRD and BET surface area, we have concluded that
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the 5 weight % loading is critical threshold of ZnO dispersion on ZSM–5. Transmission electron microscopic (TEM) image of MCM–41 is as displayed in Figure S3. indicates that MCM–41 has ordered mesoporous with uniform pore size while titanium dioxide loaded MCM–41 has less ordered mesoporous channels and also not
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able to show zinc oxide within the nanochannels. These results suggest that TEM image does not provide detailed information about ZnO nanoparticles residing inside the
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nanochannels.
Diffuse reflectance spectra of zinc oxide loaded nanoporous materials are shown
in Figure 1. Absorption band edge of ZnO encapsulated into the nanoporous host materials is blue shifted as compared to the bulk ZnO powder. The observed decrease in the band edge is due to the size quantization of nanoparticles in the confined geometries. The absorption band edge of ZnO nanoparticles encapsulated into the nanoporous host materials are given in Table S1. The absorption band edge of the ZnO nanoparticles is blue shifted by 50–100 nm when compared to that of bulk ZnO powder. The absorbance
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corresponding to ZnO increases with increase in concentration of zinc oxide without significant change in the absorption band edge, which again indicates that the particle size of the ZnO does not change at higher loading of zinc oxide in the host material. The
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band edge of zinc oxide is shifted towards red region of the spectrum above 5% of ZnO loading in ZSM–5. Both of the XRD, BET surface area and DRS results indicate that the 5 weight % loading is critical threshold of ZnO dispersion on ZSM–5.
3.2 Steady state absorption and emission spectral properties of the dye adsorbed onto the
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nanoporous host materials
Phenosafranine shows maximum absorbance in the visible region at 520 nm in
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aqueous solution which is not significantly affected in different solvents[8]. The diffuse reflectance spectra of phenosafranine adsorbed onto the nanoporous materials are shown in Figure 2. The absorption peak maximum of the dye does not change significantly while adsorbed onto the nanoporous host materials with different pore size and Si/Al ratio revealing that the dye is not sensitive to the structure and surface polarity of the host materials. The fluorescence spectra of phenosafranine (Figure 2b) are found to be
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influenced by the host materials. The photophysical properties of phenosafranine adsorbed on the surface of nanoporous host materials are given in Table 2. The fluorescence emission maximum of the dye is blue shifted when the dye adsorbed on
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zeolite–Y, ZSM–5 and silica with respect to that of the dye in aqueous solution. This observed shift in the fluorescence emission maximum is comparable to that of the dye in
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methanol:water mixture. Interestingly, the fluorescence maximum of the dye adsorbed on the external surface of host materials; zeolite–Y, ZSM–5 and silica remain unchanged even though surface polarities of the hosts are not the same. The photophysical properties of phenosafranine adsorbed onto the porous materials are well documented earlier[6,8,10]. Emission spectral maximum of phenosafranine encapsulated into MCM– 41 is comparable to that observed in aqueous solution, which reveals that the dye is present along with water molecules in the larger pores of MCM-41.
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3.3 Photophysics of phenosafranine adsorbed on the surface of ZnO loaded nanoporous host materials The fluorescence intensity of the dye in aqueous solution does not change with
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increasing concentration of colloidal ZnO nanoparticle as shown in Fig. S4. The results suggest that singlet state of dye do not involved in the charge injection processes in solution. Interesting to note that the dye adsorbed onto the nanoporous materials shows a decrease in the fluorescence intensity (Fig.3) with increasing various loading level of
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ZnO suggesting that the singlet state charge injection occurs from the dye in the excited singlet state to ZnO nanoparticles present inside the host materials. Whereas, the dye
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involved in singlet state charge injection process with similar particle size of TiO2 nanoparticles present in solution. The observed difference in the photophysics of the dye in colloidal solution is due to surface nature of semiconductor nanoparticle in aqueous solution. Once semiconductor nanoparticles are encapsulated into the host materials, the dye shows similar photophysical behavior even though surface nature of semiconductor particle is different. In general the chemical stability of ZnO is lower than that of TiO2
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which was found to be problematic in the dye adsorption process. Several reports implies that dye adsorption is the main problem in ZnO based dye sensitized solar cell[16-18]. i) Cells with higher dye loading is inefficient, whereas cells with lower dye loading show
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good conversion efficiency and also ii) the acidic nature of the dye molecule that leads to dissociation of ZnO. In this case, the host materials could solve both problems i) it
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provide large volume of space for higher dye loading and ii) provide chemical stability of ZnO nanoparticles. The photosensitization of ZnO nanoparticles encapsulated into the nanoporous silicate materials may improve the conversion efficiency of the dye molecules.
The fluorescence decay profile observed for phenosafranine in ZnO loaded nanoporous host material (Fig. S5) is fitted satisfactorily to bi-exponential function with χ2<1.5. In the case of ZnO loaded ZSM-5 and zeolite-Y, the observed fluorescence
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lifetimes of the dye are not affected notably with an increase in the loading of the ZnO into the zeolite-Y cavity as indicated in Fig. 4. Recently, Corma and co-workers[36] reported the electron transfer from the excited state of Ru(bpy)32+ adsorbed on the
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external cups of ITQ-2 zeolite to methyl viologen, MV2+ incorporated in the nanochannels. In the presence of MV2+, a considerable decrease in the emission intensity of the Ru(bpy)32+ was observed however emission lifetime essentially unaltered. The
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results are interpreted to be due to contact quenching between the excited state of Ru(bpy)32+ and MV2+ in close proximity. Excited state of Ru(bpy)32+ ion shows normal
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emission properties when the MV2+is not in the vicinity of the complex. It has also been observed that TiO2 encapsulated with MCM-41 and porous silicates also show contact quenching by the excited states of different dyes[37]. Based on these results it is suggested that the photosensitization of ZnO by surface adsorbed phenosafranine is
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occurs when the dye and ZnO (ZnO is present closer to the pore opening of host materials) are close proximity in the host material. If the ZnO are present in interior of the host materials, which are not involved in the quenching processes. In the case of the dye
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encapsulated in the nanochannels of MCM-41, a decrease in the fluorescence lifetime with increase in the loading of ZnO is observed whereas the fluorescence lifetimes are
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unaltered when the dye and titanium dioxide encapsulated into the nanochannels of MCM–41[37]. The observed difference in the behavior of the dye in MCM–41 is due to the nature of the semiconductor surface and the interaction with the nanoparticles. 3.4 Picosecond transient absorption spectral investigation of phenosafranine adsorbed on ZnO loaded nanoporous host materials.
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Picosecond transient absorption spectra of phenosafranine adsorbed onto the external surface of the nanoporous ZSM-5 host, using 532 nm laser pulse, shows broad transient absorption from 600 nm to 760 nm (Figure-5, ZSM-5_Ps). The transients observed are
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suggested to be due to the formation of the excited triplet state of the dye, trapped electron and oxidized phenosafranine radical [31, 22]. In the case of ZnO loaded host materials, transient absorption of phenosafranine dye shows two spectral maximum at ~640nm and
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750 nm (Figure-5, ZSM-5_ZnO_Ps and MCM-41_ZnO_Ps). It is suggested that in the presence of ZnO, the transient absorption with maximum at ~700nm which is due to the
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trapped electron quenched as the results splitting of two bands observed at 640nm and 750nm is suggested to be due to the phenosafranine cation radical and triplet state of the dye. Similar observation was reported [31] in the case of phenosafranine adsorbed on the Ni(bpy)32+ ion encapsulated in the supercages of the zeolite-Y host, in this case transient
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absorption spectra slightly shifted red region due to surface nature of host materials are different. Based on the picosecond transient absorption spectral studies, we conclude that the electron transfer occurs from the excited state phenosafranine to entrapped ZnO
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nanoparticles in nanoporous host materials. Phenosafranine adsorbed on ZnO loaded zeolite-Y does not observed such type of transient spectra which indicates that electron
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transfer may be very fast from the dye to ZnO in zeolite-Y host. The quenching of the excited state of the dye is unknown to occur by diffusion process in solution. However, in the solid surface, the excited state formed undergo charge transfer processes when the quencher is present in the vicinity of the excited state or mediated through lattice in presence of adsorbed water molecules. Investigations reported in the present systems are
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important to understand the excited state processes in nanoporous solids for many applications in device fabrication, sensors, and in other areas.
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4. Conclusion
Encapsulation of zinc oxide nanoparticles into the nanoporous host materials are prepared by ion exchange method and are characterized by XRD, BET, DRS and ICP-OES techniques. The results clearly showed that ZnO nanoparticles are present inside the
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nanoporous host materials and surface adsorbed ZnO nanoparticles may be negligible amount. The steady state absorption and emission spectral investigation reveal that the
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excited state processes of the dye are not much influenced by structural variation of host materials. The influence of the host materials on singlet state charge injection of the dye is demonstrated by steady state fluorescence studies. The lifetime of the excited state of the dye in the host does not change significantly in the presence of ZnO nanoparticles indicating that the quenching mechanism is due to arrested movement of the dye and ZnO nanoparticles in
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host. The picoseconds transient absorption spectral studies clearly shows that charge injection from pheneosafranine upon excitation to ZnO nanoparticle resides in the host materials. The nanoporous solid host materials provide ultrafast charge separation and also
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stability of ZnO nanoparticles.
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ACKNOWLEDGMENT:
PN is the recipient of Indian National Science Academy Senior Scientist
Fellowship. The authors acknowledge the financial support received from the Department of Science and Technology, Government of India through the Raja Ramanna Fellowship (PN). NCUFP is supported by DST-IHRPA programme.
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Supporting materials X-ray diffraction pattern of MCM-41, zeoilite-Y and ZSM-5 in the absence and presence of the ZnO nanoparticals, adsorption and desorption isotherms of nitrogen on -
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41, zeoilite-Y and ZSM-5 in the absence and presence of the ZnO nanoparticals, TEM image of MCM-41, absorption and emission spectra of phenosafranine dye with various concentration of ZnO nanoparticles in solution. Fluorescence emission decay of phenosafranine adsorbed in various loading of ZnO in host materials. Supplementary data
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associated with this article can be found in the online version, at doi:
5. References
[1] B. O'Regan, M. Grätzel, Nature 353, 737 (1991).
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[2] X. Chen, S.S. Mao, Chem. Rev. 107, 2891 (2007).
[3] W. Panpa, P. Sujardworakam, S. Jinawath, Appl. Catal. B: Enviro. 80, 271 (2008). [4] M. Klimenkov, S.A. Nepijko, M. Watz, J. Cryt. Growth 231, 577 (2001).
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[5] J.O. Joswig, S. Roy, P. Sarkar, M. Springborg, Chem. Phys. Lett. 365, 75 (2002). [6] S. Easwaramoorthi, P. Natarajan, Micro. Meso. Mater. 86, 185 (2005).
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Figure captions 1. Diffuse reflectance spectra of ZnO nanoparticles in a) ZSM–5 zeolite i: 0.72%, ii: 0.87%, iii: 0.98%, iv: 1.15%, v: 1.32%, and vi: 5.32% b) zeolite–Y i: 5.46%, ii:
5.12%, iii: 6.90 % and iv: 8.12 %.
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6.46%, iii: 6.87%, iv: 7.14% v: 7.94% and vi: 8.30% c) MCM–41 i: 3.78 %, ii:
2. (a) Diffuse reflectance absorption and (b) emission spectra of phenosafranine adsorbed on the i) –∆–∆– silica, ii) --- MCM–41, iii) --- ZSM–5 and iv) -
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- - zeolite–Y.
3. Diffuse reflectance spectra (a, c and e) and fluorescence emission spectra (b, d and f) of phenosafranine adsorbed onto host material encapsulated with ZnO [(a
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and b) MCM–41 ( i: 0% ZnO, ii: 3.78 % ZnO, iii: 5.12 % ZnO, iv: 6.90 %
ZnO and v: 8.12 % ZnO) (c and d) ZSM–5 (i: 0% ZnO, ii: 0.87% ZnO, iii: 0.98% ZnO, iv: 1.32% ZnO, and v: 5.03% ZnO); (e and f) zeolite–Y (i: 0% ZnO, ii: 5.46% ZnO, iii: 6.87% ZnO, iv: 7.14% ZnO and v: 7.94% ZnO);]
(excitation at 520 nm).
Plot of fluorescence lifetime (a-c) and relative amplitude (d-f) of phenosafranine
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versus ZnO loading; (a and d) zeolite–Y, (b and e) ZSM–5 and (c and f) MCM–41 (excitation at 470 nm and emission decay monitored at 490 nm). 5. Transient absorption spectra of phenosafraine adsorbed onto the ZSM-5 and ZnO
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loaded ZSM-5, MCM-41. (Excitation at 532 nm probe: white light delay: 133ps)
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Table 1: The adsorption characteristics of nanoporous silicate materials in the presence and absence of the ZnO nanoparticles ZnO in % (w/w)
0.382 0.320 0.317 0.313 0.311 0.167 0.191 0.175 0.197 0.69 0.52 0.403 0.394 0.392
0.00 6.46 6.87 7.94 8.30 0.00 0.72 0.87 5.03 0.00 3.78 5.12 6.90 8.12
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Zeolite–Y 845 ZnO–Y (1) 666 ZnO–Y (2) 659 ZnO–Y (3) 654 ZnO–Y (4) 650 ZSM–5 453 ZnO–ZSM(1) 358 ZnO–ZSM(2) 327 ZnO–ZSM(3) 374 MCM–41 1023 ZnO–MCM(1) 804 ZnO–MCM(2) 458 ZnO–MCM(3) 413 ZnO–MCM(4) 244 (1), (2), (3)… are sample number
Micro pore volume (cm3/g)
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Table 2: Photophysical characteristics of phenosafranine adsorbed on different nanoporous host materials
Host materials λabs (nm)
521
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Silica
520
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Stokes shift ∆ν (cm ( -1)
590
2281
574
1772
MCM–41
519
582
2085
ZSM–5
523
577
1789
Zeolite–Y
528
578
1638
a – in aqueous solution
τ (ns) (% amplitude) 0.83 (100%) 0.67 (14%) 1.89 (86%) 0.34 (38%) 1.86 (62%) 0.57 (32%) 1.26 (68%) 0.39 (72%) 1.34 (28%)
τav(ns) 0.83 1.71
1.28
1.03
0.65
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Graphical abstract
Photophysics and photochemistry of phenosafranine adsorbed on the
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surface of ZnO loaded nanoporous materials
K. Senthil kumarab*, S. Chandramohanc and P. Natarajana a
National center for ultrafast processes, university of madras, Chennai, India. b
Department of chemistry, National University of Singapore, Singapore
Department of Chemistry, Anna University, BIT Campus, Tiruchirappalli, India.
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e-mail: [email protected]
Ps +
e -x
Ps +
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ePs +
Ps +
Ps +
ZnO
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Ps +
Ps +
Ps +
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Ps + Ps +-Phenosaf ranine dye-Z nO in solution dye-Z nO in nanoporous host
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Supporting materials Photophysics and photochemistry of phenosafranine adsorbed on the
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surface of ZnO loaded nanoporous materials K. Senthil kumarab*, S. Chandramohanc and P. Natarajana a
National center for ultrafast processes, university of madras, Chennai, India. b
Department of chemistry, National University of Singapore, Singapore
Department of Chemistry, Anna University, BIT Campus, Tiruchirappalli, India.
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E-mail: [email protected]
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Figure S1: XRD pattern of ZnO nanoparticles in a) zeolite–Y i: 0%, ii: 6.46%, iii: 6.87%, iv: 7.94%, and v: 8.30% b) ZSM–5 zeolite i: 0%, ii: 0.72%, iii: 0.87% and iv: 5.32% c) MCM–41 i: 0%, ii: 3.78 %, iii: 5.12%, iv: 6.90 % and v: 8.12 %.
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Figure S2: Adsorption and desorption isotherms of nitrogen on a) zeolite–Y i: 0%, ii: 6.46%, iii: 6.87%, and iv: 8.30% b) ZSM–5 zeolite i: 0%, ii: 0.72%, iii: 0.87% and iv: 5.32% c) MCM–41 i: 0%, ii: 3.78 %, iii: 5.12%, and iv: 8.12 %.
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Absorbance
Figure S3: TEM images of i) MCM–41 and ii) ZnO loaded MCM–41
200 300 400 500 600 700 550
Wavelength (nm)
600
650
700
Wavelength (nm)
Figure S4: a) absorption and b) emission spectra of phenosafranine with various concentration of ZnO nanoparticle in aqueous solution
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Figure S5: Fluorescence decay profile phenosafranine adsorbed on a) zeolite–Y, b) ZSM– 5 and c) MCM–41 with different loading levels of ZnO nanoparticles
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ZnO in % (w/w)
– 3.81 3.81 3.81 3.64 – 4.50 4.27 3.26 – 3.81 3.81 3.81 3.81
– 6.46 6.87 7.94 8.30 – 0.7 0.87 5.03 – 3.78 5.12 6.90 8.12
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Band gap energy (eV)
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100 98 98 97 96 100 95 90 82 100 97 94 90 88
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Zeolite–Y ZnO–Y(1) ZnO –Y(2) ZnO –Y(3) ZnO –Y(4) ZSM–5 ZnO –ZSM(1) ZnO –ZSM(2) ZnO –ZSM(3) MCM–41 ZnO –MCM(1) ZnO –MCM(2) ZnO –MCM(3) ZnO –MCM(4)
Absorption band edge (nm) – 325 325 325 340 – 275 290 380 – 325 325 325 325
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Crystallinity %
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Table S1: XRD and diffuse reflectance spectral properties of ZnO encapsulated into the nanoporous silicate materials
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Excited state properties of phenosafranine adsorbed on the ZnO in nanoporous materials and in colloidal form were studied Singlet state charge injection from the dye to ZnO nanoparticles in host materials is confirmed by time resolved fluorescence spectral studies. Singlet state charge injection of phenosafranine was not observed when the dye in solution Photo-induced electron transfer between the surfaces adsorbed organic dye to encapsulated ZnO have been investigated by picoseconds transient absorption spectroscopy.