Polyhedron 30 (2011) 2493–2498
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Effective photocatalytic degradation of organic pollutant by ZnS nanocrystals synthesized via thermal decomposition of single-source precursor Swarup Kumar Maji a, Amit Kumar Dutta a, Divesh N. Srivastava b, Parimal Paul b,⇑, Anup Mondal a,⇑, Bibhutosh Adhikary a,⇑ a b
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, West Bengal, India Department of Analytical Sicences, Central Salt & Marine Chemicals Research Institute, Gijubhai, Badheka Marg, Bhavnagar 364002, Gujarat, India
a r t i c l e
i n f o
Article history: Received 2 May 2011 Accepted 21 June 2011 Available online 23 July 2011 Keywords: ZnS nanoparticles Single-source precursor Rose Bengal dye Photocatalytic activity
a b s t r a c t 2-Aminocyclopentene-1-dithiocarboxylate complex of zinc(II) has been synthesized and found to be an effective single-source precursor for the preparation of ZnS NCs (rod and sphere) by the use of ethylenediamine and hexadecylamine as structure directing solvents. Structural characterizations were carried out using XRD, TEM and BET measurements and the optical properties by UV–Vis and PL spectroscopic techniques. The prepared ZnS NCs show effective photocatalytic activity towards the degradation of Rose Bengal dye (RB) under light irradiation for their probable application in waste water treatment. The degradation mechanism of RB dye under light irradiation is established by terephthalic acid photoluminescence probing technique. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The photoinduced degradation of several toxic inorganic and organic compounds by photochemically active semiconductors is one of the most challenging and interesting topics of global energy and environment management . Semiconductor particles with suitable band gap and ﬂat band potential/energy levels are generally used as photocatalysts. The notable processes on photocatalysis is mainly focused on TiO2-based materials and they are most widely used because their superior properties, such as, suitable band gap energy, chemical stability, nontoxicity and high photocatalytic activity [1,2]. However, the activity of TiO2-based materials is limited mainly in the UV region because of their wide band gap. Consequently, there is considerable demand for materials which are active in the visible region, since visible light is the main component in solar light and indoor illuminations . In order to utilize the visible light and also to improve the efﬁciency of photocatalysts, attentions have been focused on designing the visible light sensitive photocatalysts. To this end, several metal oxides (e.g. CuO, ZnO, MnO2, Fe2O3, Fe3O4, Co3O4, Al2O3) and metal sulﬁdes (e.g. CdS, CuS, ZnS, MnS, Sb2S3, In2S3) have been used as catalysts for photodegradation purpose [4–6]. In recent times, transition-metal sulﬁdes, in particularly ZnS and CdS have been intensively studied because of their unique
⇑ Corresponding authors. Tel.: +91 3326684561; fax: +91 3326682916 (B. Adhikary). E-mail address: [email protected]
(B. Adhikary). 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.06.029
optical properties. ZnS, an important II–VI compound semiconductor with a large band gap, has been extensively studied not only for its application as photocatalysts but also as device material for various other purpose. ZnS nanomaterials have been used for the photocatalytic degradation of several organic pollutants such as dyes, p-nitrophenol and halogenated benzene derivatives in waste water treatment [7–11]. Vary recently, Yu et al. have made thorough studies on the mechanism of formation of surface photocatalytic behavior of ZnS-based semiconductor nanomaterials [12–14]. ZnS is also used as a key material for light-emitting diodes, cathoderay tubes, thin ﬁlm electroluminescence, reﬂector, dielectric ﬁlter, chemical/biological sensors, and window layers in photovoltaic cells [15–20]. The synthetic methods that have been generally employed to prepare ZnS nanocrystals are sol–gel process, sonochemical preparation, microwave heating, hydrothermal or solvothermal route, template method, reverse micelle, chemical vapor deposition, chemical bath deposition and thermolysis of singlesource precursor (SSP) [21–35]. In this study, we have chosen the solvothermal decomposition route of single-source precursor (SSP), because of its simplicity, potential advantages of mild condition, safety and one-pot synthetic procedure. Moreover, a materials prepared in this way have fewer defects and better stoichiometry. Mesoporous ZnS NCs have been synthesized by the solvo-thermal decomposition of a newly synthesized Zn(ACDA)2 complex without using any external templates at relatively low temperatures. As will be seen, the prepared ZnS NCs exhibit good photocatalytic activity and stability with higher degradation efﬁciency for aqueous solution of Rose Bengal dye under light irradiation.
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2. Experimental 2.1. Materials All the chemicals used for the preparation of 2-aminocyclopentene-1-dithiocarboxylic acid (ACDA) and Zn(II) complex were of analytical grade and used as received. Ethylenediamine (EN), trioctylphosphine (TOP), hexadecylamine (HDA) and Rose Bengal (RB) were purchased from Sigma–Aldrich and TiO2 (Degussa-P25) from Degussa company. Millipore water, methanol, ethanol and diethyl ether were used as solvents. 2.2. Synthesis of the precursor complex 2-Aminocyclopentene-1-dithiocarboxylic acid (ACDA) was prepared by the previously reported method . In a typical synthesis of the Zn(ACDA)2 complex, 5 ml ethanolic solution of zinc chloride (70 mg, 0.5 mmol) was added dropwise with stirring to a 10 ml ethanolic solution of ACDA (160 mg, 1 mmol). Within 5 min, a yellow compound separated out. The stirring was continued for 30 min. The yellow crystalline compound thus obtained was ﬁltered, washed with ethanol and diethyl ether and then dried in air (yield: 78%). Anal. Calc. for (C12H16N2S4Zn): C, 37.7; H, 4.22; N, 7.36. Found: C, 38.1; H, 4.25; N, 7.35%. FTIR: dNH2: 1624 cm 1, dCH2 + [email protected]
: 1459 cm 1, mC–N + [email protected]
: 1317 cm 1, [email protected]
+ mCN: 1286 m cm 1; massym CSS: 910 cm 1, msym CSS: 623 cm 1. 2.3. Synthesis of ZnS nanocrystals For ZnS nanorods, 1.0 g of the precursor complex was dissolved in 20 ml of EN in a round bottom ﬂask. The resultant clear yellow solution was heated at 110 °C for 15 min. The solution was then cooled to room temperature and about 20 ml of methanol was added to it. The as deposited white precipitate was centrifuged and washed several times with methanol for the puriﬁcation and then annealed at 250 °C under nitrogen atmosphere for 30 min. For obtaining spherical ZnS nanoparticles, 1.0 g of the precursor complex was dissolved in 5 ml of TOP and then injected into the hot solution of HDA (15 ml) at 120 °C. After 30 min, the solution was cooled to room temperature and about 20 ml of methanol was added to it. The white powder material thus precipitated was collected by the above mentioned procedure. The summarized reaction conditions and results are given in Table 1. 2.4. Physical measurements Elemental analyses (C, H and N) were performed using PerkinElmer 2400II analyzer. FTIR spectrum was recorded on KBr disks using a JASCO FTIR-460-Plus spectrophotometer. Powder XRD patterns were recorded on a Panalytical X’pert Pro MPD diffractometer using Ni-ﬁltered Cu Ka X-radiation (k = 1.540598 Å) operating at 40 kV and 30 mA. TEM images were collected by using JEOL JEM2100 microscope working at 200 kV. N2-sorption isotherms were obtained using a Quantachrome Instruments analyzer at 77 K. UV–Vis absorption spectra were recorded on a JASCO V-530
UV–Vis spectrophotometer at room temperature. Room-temperature photoluminescence (PL) measurements were carried out using a Photon Technology International ﬂuorometer. A 200 W incandescent tungsten halogen lamp was used as the light source for photocatalytic degradation processes. 2.5. Measurement of photocatalytic activity The photocatalytic activity of ZnS NCs was evaluated by the photo-degradation of aqueous solution of RB. The experiment was carried out in a round bottom ﬂask kept in a thermostatic bath at 25 °C and an incandescent tungsten halogen lamp (200 W) placed vertically on the reaction vessel at a distance of 15 cm. The catalytic experiments were carried out with 40 ml aqueous solution of RB (1.6 10 5 M) and 40 mg of the catalysts, which was magnetically stirred in dark for 45 min to reach the adsorption–desorption equilibrium. After a given interval of illumination, 3 ml of the aliquot solution was withdrawn from the solution mixture and centrifuged. The absorption spectrum of the solution was then measured in the range 400–650 nm and the peak at 540 nm was monitored to estimate the concentration of the RB solution. The photocatalytic activity of commercial Degussa TiO2-P25 was also measured as the reference. In order to test the chemical stability of ZnS NCs (prepared using HDA), it was recycled and reused for three times for the decomposition of RB under same experimental condition. After each photocatalytic test, the aqueous solution was centrifuged to collect ZnS NCs, which was then dried at 60 °C and used for the next cycle. In order to ﬁnd out whether the photodegradation of RB by ZnS occur through the generation of hydroxyl radical as is observed for TiO2. The commonly used terephthalic acid (TA) photoluminescence probing technique [12,13] was adopted. In this case, 40 ml aqueous solution of sodium terephthalate (2 10 3 M) containing 40 mg of either TiO2 (Degussa P25) or ZnS (obtained from HDA) was irradiated with light for a given period. An aliquot (3 ml) of the solution was withdrawn from the solution mixture and centrifuged and its luminescence spectrum was recorded between 350 and 600 nm using 315 nm as the excitation wavelength. 3. Results and discussion The synthetic route involved in the preparation of the complex and ZnS is outlined in Fig. 1. The precursor complex Zn(ACDA)2 was obtained by reacting ZnCl2 with ACDA in 1:2 proportion in ethanol. The purity of the product was veriﬁed by the elemental analysis and comparing with the reported IR spectral bands . 3.1. X-ray diffraction studies The powder X-ray diffraction (XRD) patterns of ZnS NCs obtained using EN and HDA are shown in Fig. 2. The diffraction patterns obtained for the nanorods (from EN) and nanospheres (from HDA) are generally in good agreement with each other and can be indexed to the pure hexagonal phase of ZnS with (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), (2 0 2) and (2 0 3) peaks (JCPDS No. 361450). It is of interest to note that while
Table 1 Summary of reaction conditions and experimental results. Amine
Average size (nm) XRD
TEM D=6 L = 50 5
EN (20 ml)
HDA (15 ml) + TOP (5 ml)
Average pore diameter (nm)
Surface area (m2 g
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the crystallite diameter, k is the wave length of X-ray i.e. 1.540598 Å, b is the value of full width at half maximum and h is the Bragg’s angle. Using this equation, average crystallite sizes of ZnS NCs were calculated and are found to be around 5.3 (nanorods) and 4.4 nm (nanospheres), respectively. 3.2. Transmission electron microscopy studies
Fig. 1. Schematic diagram of reaction sequence for the synthesis of ZnS NCs.
Fig. 2. X-ray diffraction patterns of ZnS NCs.
(1 0 0) peak is strongest for spherical ZnS NCs similar to that of the hexagonal phase, however, for ZnS nanorods the diffraction peak (0 0 2) is the strongest one. This observation is corroborated by the TEM results and seems to indicate that for the nanorods preferential growth occur along the c axis. The remaining peaks are generally broad and probably indicate relatively small dimensions of the materials. Crystal diameters for ZnS NCs were calculated using the Debye–Scherrer equation (D = 0.9k/(bcosh)), where D is
The morphological characterization of ZnS NCs was carried out using transmission electron microscopy (TEM) and is shown in Fig. 3. Rods and spherical nanoparticles are clearly seen from the images. Rod-like morphology obtained by the use of EN, suggests that EN plays the key role for the formation of rod shape, whereas, HDA plays a signiﬁcant role for the formation of spherical nanoparticles. Nanorods have average diameter of 6 nm and length of 50 nm, on the other hand, the spherical nanoparticles have average diameter of 5 nm. The average particle sizes as obtained from TEM measurements are in good agreement with the XRD results (Table 1). A set of concentric rings instead of sharp spots are evident from the selected area diffraction (SAED) patterns, which suggest the presence of small crystalline materials (Fig. 3 inset). The diffraction patterns are clearly matched with the pure phase of hexagonal ZnS (JCPDS No. 361450). From the high resolution TEM (HRTEM) images (Fig. 3 inset), lattice fringes of nanocrystals are also observed, which suggests the good crystalline nature of ZnS and the interlayer spacings of 0.307 and 0.292 nm correspond to the (0 0 2) and (1 0 1) plane of ZnS. ZnS NCs reported here have been obtained by solvothermal decomposition of a single-source dithiocarboxylate precursor [Zn(ACDA)3] using EN and HDA as solvents. The solvents used may act not only as the attacking nucleophile but also as capping agents for nanoparticles formation [37,38]. For selective growth of a face to obtain anisotropic NCs structure-directing agents are used . In our case, EN seems to serve a capping agent leaving the (0 0 2) facet to grow since similar precedence exists . On the other hand, the formation of spherical NCs in HDA seems to be controlled by thermodynamic conditions required to minimize the surface are . 3.3. N2-sorption studies Brunauer–Emmett–Teller (BET) measurements were carried out to know the porous nature and speciﬁc surface area of ZnS NCs. Typical nitrogen adsorption–desorption isotherms at 77 K and the corresponding pore size distributions are presented in Fig. 4. All the isotherms are identiﬁed as type IV isotherm, with a H2-type
Fig. 3. TEM images (inset: SAED patterns and HRTEM images) of ZnS NCs prepared using (a) EN and (b) HDA.
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Fig. 4. BET isotherms and in inset corresponding pore size distribution curves of ZnS NCs.
hysteresis, conﬁrming the mesoporous structures. The porosity of the NCs was determined from pore-size distribution curves (inset of Fig. 4) and shows the sharp distribution in the mesoporous region. The average pore diameter according to the Bopp–Jancso– Heinzinger (BJH) method of both the samples were calculated and are found to be 2.9 (from EN) and 3.7 nm (from HDA), and speciﬁc surface areas of 38.8 and 59.2 m2 g 1, respectively. Speciﬁc surface area and porosity gradually decreases as the solvent changing from HDA to EN, indicating the better surface catalytic property of ZnS NCs prepared using HDA than that of EN. 3.4. Optical properties Optical properties of ZnS NCs were investigated by the UV–Vis absorption and photoluminescence (PL) spectroscopic techniques and are shown in Figs. 5 and 6. The room temperature absorption and emission spectra were recorded by dispersing the samples in water. The nanoparticles show sharp band edge absorption at around 338 (from EN) and 321 (from HDA) nm, respectively, in the absorption spectra followed by strong excitonic peaks, exhibiting a systematic shift toward the higher energy, that is, lower wavelength (blue shift) with decreasing particle size (Fig. 5). The
Fig. 6. Room temperature photoluminescence spectra of ZnS NCs.
excitonic peak positions are 290 and 273 nm for ZnS NCs obtained from EN and HDA, and correspond to the band gap energy of 4.28 and 4.54 eV, respectively. From the band edge absorption, band gap energies of ZnS NCs were calculated using the Tauc’s relation [(ahm)1/n = A(hm Eg)], where, hm is the incident photon energy, ‘A’ is a constant and ‘n’ is the exponent the value of which is determined by the type of electronic transition causing the absorption and can take the values 1/2 or 2 depending upon whether the transition is direct or indirect . Since, ZnS is a well established direct band gap semiconductor, we can evaluate the value of Eg, from the intercept of the straight line at a = 0 from the plot of (ahm)2 versus hm, is shown in inset of Fig. 5. From the Tauc’s plot the band gap energy of nanoparticles are turn out be 3.78 (from EN) and 4.06 (from HDA) eV, respectively, which are blue shifted relative to the characteristic band gap energy of the bulk ZnS (Eg = 3.6 eV) . The higher band gap energy compared to the bulk material is due to the well known quantum conﬁnement effect, which becomes signiﬁcant when semiconductor particle size is smaller than or comparable to the Bohr excitonic radius . The effect of particle size (R) on the extent of blue shift (DE) can be appreciated from the relationship DE = El Eg = (⁄2p2/2lR2) (1.8e2/4peR), where, El is the lowest transition level, Eg is the original band gap energy, ⁄ is the reduced Planck’s constant, R is the radius of the nanocrystal, l is the effective mass of electron, e is the permittivity of the material and e is the electronic charge. Since DE is inversely proportional to R, the reduction of crystallite size lead to increased blue shift. ZnS is a well known photoluminescent material and therefore, the room temperature PL spectra of the ZnS NCs were measured with an excitation wavelength (kexc) of 275 nm in water and are shown in Fig. 6. The emission spectra show very strong band at ca. 336 nm for ZnS NCs obtained using EN, while at ca. 322 nm for the sample obtained using HDA. The observed strong band is due to the band to band transition of ZnS, whereas the observed blue shift is due to the reduction of particle size and is consistent with the absorption spectral studies. In addition, a relatively weak violet emission observed at about 450 nm in the PL spectra, may be attributed to the presence of sulfur vacancies in the lattice, as has been established earlier .
3.5. Photocatalytic activity of ZnS NCs
Fig. 5. UV–Vis absorption and in the inset corresponding (ahm)2 vs. hm plot.
To establish the active photocatalytic performance of ZnS NCs, we have carried out the degradation of RB subjecting in to light irradiation as followed by spectrophotometric monitoring. The
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photocatalytic degradation process of RB in aqueous solution has previously been described by other researchers [45–47]. Fig. 7a shows the continuous decrease in concentration of aqueous solution of RB in presence of ZnS NCs (from HDA) with light irradiation. After irradiation for 225 min, the spectrum does not show any characteristic absorption, which suggests the complete decolorization/degradation of RB. Similar measurements carried out by ZnS NCs obtained using EN show relatively slower catalytic response (see later). The high photocatalytic activity of ZnS NCs became evident when a comparative experiment was carried out with TiO2 (Degussa-P25). The comparative studies were made using the RB solution (1.6 10 5 M) in the following way: (i) without catalyst in dark, (ii) ZnS NCs (obtained using HDA) (40 mg) in dark, (iii) without catalyst in light, (iv) TiO2 (40 mg) in light. (v) ZnS NCs (obtained using EN) (40 mg) in light and (vi) ZnS NCs (obtained using HDA) (40 mg) in light. The experimental results are expressed by the change in relative concentration of RB with irradiation time and are shown in Fig. 7b. No notable changes in relative concentration of RB is not observed under condition i, ii and iii (Fig. 7b, line i, ii and iii). A signiﬁcant change is observed for condition iv (Fig. 7b, curve iv), i.e. using TiO2 as the catalyst. Finally, remarkable changes are observed for conditions v and vi, showing excellent photocatalytic activities by the nanorods (Fig. 7b, curve v) and spherical (Fig. 7b, curve vi) ZnS NCs. In presence of TiO2 decomposition of RB of around 40% is achieved, whereas for ZnS NCs 93% (nanorods) and 99% (spherical) decomposition of RB occur for the same irradi-
ation time. Clearly ZnS NCs synthesized in our laboratory show much batter catalytic activity than commercially available TiO2. The greater catalytic activity of spherical ZnS can be attributed to its smaller size and higher surface to volume ratio. The decomposition process is modeled as a pseudo-ﬁrst-order reaction with the kinetics expressed by the equation ln(Co/Ct) = kt, where Co represents the initial concentration, Ct denotes the concentration at a given reaction time ‘‘t’’, and k is the reaction rate constant. From the linear extrapolations (Fig. 7b inset), the reaction rate constants were calculated and are found to be 2.17 10 2 min 1 (from HDA) and 1.24 10 2 min 1 (from EN), respectively. Furthermore, the stability of ZnS NCs was also examined by pursuing the degradation process with the same ZnS sample (prepared using HDA) for three successive reactions. After these successful reuse of ZnS slight decrease in activity (96%) (Fig. 7c) was observed. Therefore, it is also established that our prepared ZnS NCs are stable enough for degradation reactions at normal conditions. It is well established that photodegradation catalyzed by TiO2 involves the generation of hydroxyl radical (OH) in protic solvent. The question whether a similar mechanism is followed by ZnS NCs has been examined by caring out terephthalic acid (TA) photoluminescence probing measurement [12,13]. In the case of TiO2, nonluminescent terephthalic acid is converted to strongly luminescing 2-hydroxylterephthalic acid (HTA) on exposure to light due to the generation of hydroxyl radicals (Fig. 8a). With ZnS NCs no signiﬁcant luminescence due to 2-hydroxylterephthalic acid could be
Fig. 7. (a) Absorbance spectra of RB solution in presence of ZnS NCs (sample S2), (b) photodegradation of RB under different conditions, inset: ln(Co/Ct) vs. t plot for ZnS samples (condition v and vi), and (c) cyclic runs in the photocatalytic degradation of RB in presence of ZnS NCs obtained using HDA.
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Fig. 8. Photoluminescence spectral changes of terephthalic acid solution in presence of (a) TiO2 and (b) ZnS (obtained using HDA) photocatalysts under light irradiation.
observed even on long exposure to light, indicating the lack of hydroxyl radical formation (Fig. 8b). For ZnS the mechanism reported for photocatalysis namely by the direct participation of the photogenerated holes [12,13] seems to be operative. 4. Conclusions In conclusion, we have synthesized a new zinc(II) complex for the preparation of porous ZnS NCs, via solvo-thermal decomposition route. Nanorods (diameter 6 nm and length 50 nm) and spherical nanoparticles (5 nm) are deposited using EN and HDA as decomposing solvents. A considerable amount of blue shift in band gap energy is evident from the optical measurements. Finally, the ZnS NCs show enhanced photocatalytic activity and good stability for RB degradation. Considering their unique features of high surface-to-volume ratios, rich photocatalytic and luminescent properties it can be suggested that these ZnS NCs ﬁnd many interesting applications in semiconductor photocatalysis, environmental remediation and nanodevices. Acknowledgements Authors are very much thankful to Prof. K. Nag, Department of Inorganic Chemistry, IACS, Kolkata, India, for helpful discussion. S.K. Maji is indebted to UGC, India, for his Rajiv Gandhi National Fellowship [F. No. 16-1292(SC)/2009, SA-III] and A.K. Dutta is also indebted to UGC, India, for his SRF [F. No. 10-2(5)/2007(i)-E.U.II]. We are also acknowledging MHRD (India) and UGC-SAP (India) for providing instrumental facilities to the Department of Chemistry, BESUS, India. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2011.06.029. References  M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69.  A.L. Linsebigler, G. Lu Jr., J.T. Yates, Chem. Rev. 95 (1995) 735.  W. Zhao, C. Chen, X. Li, J. Zhao, H. Hidaka, N. Serpone, J. Phys. Chem. B 106 (2002) 5022.  Y. He, D. Li, G. Xiao, W. Chen, Y. Chen, M. Sun, H. Huang, X. Fu, J. Phys. Chem. C 113 (2009) 5254.
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