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Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach
Accepted Manuscript Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach F. Dehghan, M. Molaei, F. Amirian, M. Karimipour, A.R. Bahador PII:
S0254-0584(17)30940-9
DOI:
10.1016/j.matchemphys.2017.11.061
Reference:
MAC 20181
To appear in:
Materials Chemistry and Physics
Please cite this article as: F. Dehghan, M. Molaei, F. Amirian, M. Karimipour, A.R. Bahador, Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.11.061 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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•ZnSe/ZnS QDs were prepared by a photochemical approach • TGA acted as both of the capping
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agent and S2- source for ZnS shell growth • Optical properties and photocatalytic activity were improved after ZnS shell growth
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Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new
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approach
F. Dehghan, M. Molaei*, F. Amirian, M. Karimipour and A. R. Bahador
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Department of Physics, Faculty of Science, Vali-e-Asr University, Rafsanjan *Corresponding author email: [email protected]
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ABSTRACT:
In this work ZnSe and ZnSe/ZnS QDs were synthesized via a rapid, room temperature photochemical approach. Zn(Ac)2, Na2SeO3 and TGA were used as precursors. As an UV-sensitive material, TGA acted simultaneously both
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as the capping agent and S2- source for ZnS shell growth. Crystal structures and optical properties of the QDs were characterized by means of X-ray
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diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), Field
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emission scanning electron microscope (FESEM), Photoluminescence spectroscopy (PL), Energy dispersive X-ray analysis (EDAX), UV-Visible (UV-Vis) spectroscopy. Band-gap of the ZnSe QDs was obtained about 3.45 eV. PL spectra deconvolution obtained three peaks which are related to the emission of band edge, surface states and deep levels. After ZnS shell growth, surface trap states emission was quenched significantly and band
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edge emission was increased considerably. ZnSe QDs indicated a good activity for Methylene Orange (MO) photo-degradation that it was improved
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after ZnS shell growth significantly. KEYWORDS
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1. INTRODUCTION:
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ZnSe/ZnS, Core-shell, Optical properties, Photocatalytic activity.
II-VI quantum dots (QDs) are interesting material possessing individual physical and chemical properties [1,2]. These materials have been
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developed for different applications, such as LEDs, screen devices [3-7], solar cells [8,9], lasers [10-12], photocatalytic activity [13-17], biological probes [18-20], medical imaging [21-23], sensors and other possible
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applications[24-26]. ZnSe QDs are a class of the II-VI semiconductor with a
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little toxicity in comparison with cadmium compounds which are more suitable for different applications [27-30]. ZnSe QDs indicated excellent luminescence and photocatalytic properties [16, 31, 32]. Shelling of the QDs by an inorganic semiconductor is a conventional way for improving their optical, photocatalytic properties and decreasing toxicity [33-35]. Different methods have been reported to synthesis ZnSe and its core-shell structures 2
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[16, 31-41]. Recently we introduced an interesting UV-assisted approach for the synthesis of ZnSe QDs [31]. Subsequently in this work we have
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presented a novel approach for growth of the ZnSe/ZnS core-shell QDs. In this approach TGA as an UV-sensitive material acted simultaneously as both the capping agent and S2- source for ZnS shell growth. To the best of our
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knowledge this approach has not been reported by the others yet.
Synthesized QDs were characterized by means of different analysis tools
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and they indicated good photocatalyst activity for MO photo-degradation.
2. EXPERIMENTAL
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2.1. Materials
Zinc acetate dihydrate (Zn(CH3COO)2.2H2O), Sodium selenite
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(Na2SeO3), Thioglycolic acid (TGA), Sodium hydroxide (NaOH) and
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Methylene Orange (MO) were purchased from the Merck Chemical Company. All chemicals were of the analytical grade and were used without further purification.
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2.2. Synthesis of ZnSe QDs ZnSe QDs were synthesized via a photochemical approach [31]. In a
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typical synthesis, 0.1314 g of Zn(Ac)2 was dissolved in 50 ml D.I water and 0.102 g of Na2SeO3 was dissolved in 30 ml D.I water. 0.1 ml of TGA was
added to Zn(Ac)2 solution and pH of the solution was adjusted to 9 value by
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adding proper amount of NaOH. Subsequently Na2SeO3 solution was added
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to the Zn(Ac)2 prepared solution. The final solution was placed under a high-pressure mercury lamp and was irradiated for 15 min. The ZnSe QDs formation is as below reaction:
Zn(Ac)2 + TGA + NaOH + Na2SeO3 + UV illumination
ZnSe QDs
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The method is a room temperature approach and the reaction time is too and the reported approach is more faster and simpler than the other previously
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reported methods [41- 44 ].
2.3. Growth of ZnS shell. Comparing with the previously reported approaches, the synthesis
method contains two main novel aspects. In most of the previously reported works, after synthesis of the ZnSe QDs, synthesized QDs were extracted from the solution and ZnS shell was grown on the extracted ZnSe QDs 4
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[45- 48], however in this approach ZnS shell was grown on the as prepared and non- extracted QDs, therefore this method can be nominated as one-pot
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method. Moreover, TGA is an UV-sensitive material which it will be decomposed under UV-illumination [49], and releases S2- for the ZnS shell
growth. Actually TGA acts simultaneously as both of the capping agent and
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S2- source [50] for the ZnS shell growth. TGA decomposition process and
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ZnSe/ZnS QDs formation has beeb indicated in Fig. 1 schematicaly. Typically, 10 ml of as prepared ZnSe QDs was used as core. 0.066 g of Zn(Ac)2 was dissolved in 30 ml D.I water and then 0.05 ml TGA was added to that. The pH value of the solution was adjusted to 9 by adding
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proper amount of NaOH. 10 ml of ZnSe prepared solution was added slowly to the solution and the final solution was irradiated by high pressure,
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mercury lamp.
2.5. Characterization
XRD data were recorded using an advanced d8 Burker system.
Energy dispersive X‐ray spectra (EDAX) were taken using a JEOL JSM 6390 LV scanning electron microscope. The infrared absorption data were obtained using a FT‐IR spectrometer. UV–Vis and PL spectra were recorded 5
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with an Avantes spectrometer (AvaSpec‐2048 TEC). FE-SEM image was recorded by a Zeiss ΣIGMA VP-FE-SEM field emission scanning electron
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microscope.
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3. RESULTS AND DISCUSSION
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3.1. XRD
Fig.2 demonstrates XRD patterns of the ZnSe and ZnSe/ZnS prepared QDs. The three main peaks at 28.02° , 47.3° and 55° are related to the diffraction from (111), (220) and (311) planes of cubic Zinc blend phase of
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ZnSe which is in consistent with (JCPDS Card No. 80–0021) standard cards. The broad diffraction XRD peaks confirm the nano-aspect of the synthesized particles. The crystallite size of the particles can be calculated using the full
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width at half-maximum (FWHM) of the first main XRD peak at 28.02° and
D=
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Scherrer's formula: λ
βθ
(1)
In this equation, D is the crystal size, k is a constant which is about
0.94, λ is the x-ray wavelength (0.154 nm), β is the FWHM of the diffraction
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peak and θ is the diffraction angle [51]. The crystallite size of QDs is estimated to be about 2.7 nm.
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XRD pattern of the core-shell ZnSe/ZnS QDs is same as the ZnSe pattern without any extra peaks, but only the diffraction peaks were shifted to the higher angels. This behavior can be explained using Bragg equation: (2)
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2dsinθ=nλ
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In this equation, d is the interplanar space, θ is the diffraction angle, n is a positive integer and λ is the x-ray wavelength 0.154 nm. Actually, growth of the ZnS shell is decreased the lattice parameters of the ZnSe QDs and therefore it is expected the shift of the diffraction peaks to the higher angels due to shrinkage of core structure
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3.2. FT-IR
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[34,51].
Fig. 3 presents FT-IR spectra of TGA and TGA-capped QDs. One
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role of the TGA is to cap QDs. There is an improve peak in TGA spectrum at 2560 cm-1 related to the S-H bond [52, 53]. The absence of this peak in TGA-capped ZnSe QDs spectra proves decomposition of the S-H bond and formation of Zn-S bond [54]. In both spectra, there is a broad peak between 3000 cm-1 to 3500 cm-1 related to the O-H vibration of H2O molecule in the
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TGA capped QDs spectra and also there is a peak at above 1590 cm-1
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correspond to the COO- asymmetric vibration [55, 56].
3.3. FE-SEM
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FE-SEM analysis is a powerful tool to investigating the morphology of the nanostructures. Fig. 4 is a typical FE-SEM image of the ZnSe
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synthesized QDs, clearly the particles have spherical morphology and the grain sizes are estimated about 40 nm with a uniform size distribution.
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3.4. EDAX:
An EDAX analysis of ZnSe and ZnSe/ZnS core-shell synthesized
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QDs have been demonstrated in Fig. 5. Zn and Se orbital energies can be observed clearly in ZnSe pattern (a), in addition to Zn and Se elements an
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extra S signal is observable in ZnSe/ZnS plot (Fig. 5 (b)).The obtained results are matched with the chemical composition of the synthesized particles.
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3.5. Optical properties The band-gap of the ZnSe is about 2.72 eV in bulk case, decrease of
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the particles size results in the increase of the band-gap and indicating quantum confinement effects. Fig. 6 shows absorption and PL spectra of the
ZnSe core QDs. The band edge of the ZnSe QDs is estimated about 3.45 eV
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which is wider than that of its bulk counterpart. Increase of the band gap
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indicates the small size of the particles in the nano range and appearing quantum confinement effects. The particles size can be Calculated from this value of the band gap using effective mass approximation (EMA): 1.78 % ħ 1 1 ∗ ! ! − 0.248)* − + 3 2 ∗ ∗ &
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− =
The particles size was obtained about 2.12 nm.
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PL spectra consists of three main peaks at 385, 445 and 488 nm. The bandgap of the NPs is obtained about 3.45 eV (360 nm), therefore the first
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emission peak can be attributed to the near band edge recombination of electron-hole pairs, the second peak at 445 nm, related to the surface defects [31] and the third at 488 nm related to the recombination of the donoracceptor pairs from deep traps [37]. All of the possible recombination pathways for ZnSe and ZnSe/ZnS are illustrated in the Fig.7 briefly. For ZnSe there are three possible recombination pathways, i) recombination of 9
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the band edge electron-hole pairs, ii) recombination of the electron-hole trapped in the surface defects and iii) recombination of the electron-hole in
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deep levels. Growth of an inorganic shell on the QDs surface is a powerful
approach for covering surface trap states and dangling bonds and
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consequently decrease of the surface trap states density. Decrease of the trap
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states results in increasing the electron-hole band edge recombination and amplifying of the band edge emission in the PL spectra. ZnS has a higher and lower conduction and valance bands, respectively, compare to the ones belonging to ZnSe that it results in the formation of a type I core-shell
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structure (Fig.7 (b)). PL spectra of the ZnSe/ZnS core-shell QDs is depicted in Fig.8. Comparing to the ZnSe core emission, the trap states emission was
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quenched considerably and on the other hand the band edge emission increased significantly (FWHM is about 50 nm). Fig. 8 is an absorption and
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PL analysis of the ZnSe/ZnS core-shell structure, as it was expected, ZnS shell growth should cover the surface trap states which it will decrease the non-radiative and subsequently increase of the recombination of the electron-hole pairs from the other radiative mechanisms [37]. The PL intensity of the band-edge emission and deep levels were increased about 7.24 and 2.305 times receptivity comparing to those for ZnSe QDs. 10
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Quenching of the surface trap states emission (445 nm) and considerably increasing the band edge and deep level emissions, confirm the successfully
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growth of the ZnS shell around ZnSe cores and formation of ZnSe/ZnS coreshell structure. Fig. 9-inset indicates the micrograph emission of the ZnSe
and ZnSe/ZnS core-shell QDs under a 254 nm UV-lamp excitation. The
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considerably increase of the PL intensity is observable.
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In this obtained approach, the ZnS shell thickness is tunable by UVillumination time in the ZnS shell growth step. Fig.10 indicates the PL spectra of ZnSe/ZnS QDs (a) and PL intensity of the band edge and deep levels (b) with different ZnS shell thicknesses. Clearly band edge emission is
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increased and shows an optimum value for the 55 min UV-illumination, but deep levels emission is ascendant. Actually the shell thickness has an
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optimum value, because there is a competitive process. On one hand, the shell can cover the surface traps and increases the band edge emission and
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on the other hand it inherits itself non-radiative trap states inside the shell thus it can quench the emission of the core. Thus, there would be an optimum shell thickness for the maximum of band edge emission and the growth time of the shell should be optimized in this manner.
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3.6. Photocatalytic performance: QDs possess high surface to volume therefore they are photosensitive
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materials and proper to apply as photocatalytic for removing pollutant. Metyle Orange (MO) was selected as a pollutant and ZnSe and ZnSe/ZnS QDs have been applied as a photocatalyst to remove MO from an aqueous
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solution. QDs were extracted from the solution by centrifuging at 6000 rpm
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for 5 min. Three samples were prepared. The first sample was 0.1 g/l MO solution without QDs, the second was 0.1 g/l MO solution containing 5 mg of ZnSe QDs and the third was0.1 g/l MO solution containing 5 mg of ZnSe/ZnS QDs. The prepared solutions were placed under a UV-lamp and
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were illuminated for different interval times.
Fig. 11 indicates absorption spectra of the samples containing ZnSe
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and ZnSe/ZnS QDs during UV-illumination. MO has an important peak at about 465 nm which it was monitored for investigating the MO degradation.
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By increasing UV-illumination the absorption peak intensity was decreased indicating photo-degradation of the MO. Change of the solution colour (Inset of fig 12) also indicates and confirms the photo degradation of MO by QDs.
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Fig. 12 (a) is absorption spectra demonstrating photo-degradation of MO in absence of QDs (a), in presence of ZnSe (b) and ZnSe/ZnS core-shell
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(c) QDs after 90 min. Clearly for the first sample the intensity of the MO peaks has not been decreased by UV-illumination however for both of the solution containing QDs, MO peak at 465 nm was decreased which confirms
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the photocatalytic effect of ZnSe and ZnSe/ZnS. Fig. 12 (b) demonstrates comparison of photocatalytic activity of ZnSe and ZnSe/ZnS QDs. After 90
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min about 89% and 94% of MO was degraded for ZnSe and ZnSe/ZnS, respectively, therefore the photocatalytic activity of the ZnSe/ZnS core-shell QDs is better than that for ZnSe QDs. Fig. 13 is a schematic demonstrating
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the operating mechanism of ZnSe and ZnSe/ZnS as a photocatalyst. Clearly ZnS shell enhanced the photocatalytic activity of ZnSe cores, actually, based on the proposed mechanism in Fig. 13, compare to the ZnSe, ZnS has a
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higher and lower conduction and valance bands, respectively, which it will
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results in the decrease of redox potential and consequently enhancing the photocatalytic activity. The obtained results are better than that for ZnSe photocatalyst activity by the others [57-59].
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4. CONCLUSION ZnSe and ZnSe/ZnS core-shell QDs were synthesized using a fast
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photochemical approach. TGA acted simultaneously as both the capping agent and S source in the ZnS shell growth step. ZnSe QDs exhibited an
emission consists of three peaks related to the, band edge, surface tarp states
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and deep-levels. After ZnS shell growth, because of the covering surface
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defects, the band edge and deep-levels emissions increased considerably and trap states emission was quenched. Both of the core sizes and shell thickness were tunable only with UV-illumination extension. Synthesized QDs indicated strong photocatalytic activity. The photocatalytic performance of
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ZnSe and ZnSe/ZnS QDs were investigated by Methyl Orange (MO) as pollution. The activity of the ZnSe/ZnS QDs for photo-degradation of MO
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was stronger than that for ZnSe QDs.
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properties”, Mater. Sci. Semi. Process. 27 (2017) 865-872. [58] H. Cao, Y. Xiao, S. Zhang, “ The synthesis and photocatalytic activity of ZnSe microspheres”, Nano. Technol. 22 (2011) 015604.
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[59] F. Cao, W. Shi, L. Zhao, Sh. Song, J. Yang, Y. Lei, H. Zhang, “ Hydrothermal synthesis and high photocatalytic activity of 3D
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Wurtzite ZnSe Hierarchical nanostructures”, J. Phys. Chem. C 112
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(2008) 17095- 17101.
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Figure captions Fig. 1. TGA decomposition process (a) and synthesis procedure of the ZnSe/ZnS
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synthesized QDs (b). Fig. 2. XRD patterns of ZnSe and ZnSe/ZnS QDs.
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Fig. 3. FT‐IR spectra of TGA and TGA‐capped ZnSe QDs. Fig. 4. FE-SEM image of ZnSe QDs.
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Fig. 5. Elemental composition analysis of ZnSe (a) and ZnSe/ZnS (b) QDs.
Fig. 6. Absorption and emission spectra of the ZnSe QDs (Inset: micrograph emission of the ZnSe QDs).
ZnSe/ZnS core-shell (b).
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Fig. 7. Energy level diagram and possible recombination path ways of the ZnSe (a) and
Fig. 8. Absorption and PL spectra of the ZnSe/ZnS core-shell QDs without S sours
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(Inset: micrograph emission of the ZnSe/ZnS core-shell QDs).
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Fig. 9. PL spectra of ZnSe and ZnSe/ZnS core–shell NCs. Fig. 10. Time evolution of PL spectra and PL intensity of the band edge and deep level emissions (b) of the ZnSe/ZnS core-shell QDs . Fig. 11. Time evolution of absorption spectra of photocatalytic decomposition of Methylene Orange (MO) by (a) ZnSe and ZnSe/ZnS core–shell QDs (b) (Inset: change of the solution colour during the catalytic degradation process).
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Fig. 12. Absorption comparison of photocatalyst treatment of Methylene Orange (a) and the comparison of time evolution of MO concentration (b) by ZnSe and ZnSe/ZnS QDs.
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he catalytic degradation process). Fig. 13. The suggested mechanism for photo-degradation of Methylene Orange by ZnSe
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and ZnSe/ZnS core–shell QDs.
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Fig.1. TGA decomposition process (a) and synthesis procedure of the ZnSe/ZnS synthesized QDs (b).
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Fig. 2. XRD patterns of ZnSe and ZnSe/ZnS QDs.
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Fig.3. FT‐IR spectra of TGA and TGA‐capped ZnSe QDs
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Fig. 4. FE-SEM image of ZnSe QDs.
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Fig.5. Elemental composition analysis of ZnSe (a) and ZnSe/ZnS (b) QDs.
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Fig.6. Absorption and emission spectra of the ZnSe QDs (Inset: micrograph emission of the ZnSe QDs).
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Fig.7. Energy level diagram and possible recombination pathways of the ZnSe (a) and ZnSe/ZnS core-shell (b).
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Fig.8. Absorption and PL spectra of the ZnSe/ZnS core-shell QDs without S sours (Inset: micrograph emission of the ZnSe/ZnS core-shell QDs).
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Fig.9. PL spectra of ZnSe and ZnSe/ZnS core–shell QDs.
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Fig. 10. Time evolution of PL spectra and PL intensity of the band edge and deep level emissions (b) of the ZnSe/ZnS core-shell QDs .
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Fig. 11. Time evolution of absorption spectra of photocatalytic decomposition of Methylene Orange (MO) by (a) ZnSe and ZnSe/ZnS core–shell QDs (b) (Inset: change of the solution colour during the catalytic degradation process).
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Fig. 12. Absorption comparison of photocatalyst treatment of MO (a) and the comparison of time evolution of MO concentration (b) by ZnSe and ZnSe/ZnS QDs.
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Fig.13. The suggested mechanism for photo-degradation of MO by ZnSe and ZnSe/ZnS core– shell QDs.
S0254-0584(17)30940-9
DOI:
10.1016/j.matchemphys.2017.11.061
Reference:
MAC 20181
To appear in:
Materials Chemistry and Physics
Please cite this article as: F. Dehghan, M. Molaei, F. Amirian, M. Karimipour, A.R. Bahador, Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.11.061 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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•ZnSe/ZnS QDs were prepared by a photochemical approach • TGA acted as both of the capping
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agent and S2- source for ZnS shell growth • Optical properties and photocatalytic activity were improved after ZnS shell growth
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Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new
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approach
F. Dehghan, M. Molaei*, F. Amirian, M. Karimipour and A. R. Bahador
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Department of Physics, Faculty of Science, Vali-e-Asr University, Rafsanjan *Corresponding author email: [email protected]
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ABSTRACT:
In this work ZnSe and ZnSe/ZnS QDs were synthesized via a rapid, room temperature photochemical approach. Zn(Ac)2, Na2SeO3 and TGA were used as precursors. As an UV-sensitive material, TGA acted simultaneously both
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as the capping agent and S2- source for ZnS shell growth. Crystal structures and optical properties of the QDs were characterized by means of X-ray
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diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), Field
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emission scanning electron microscope (FESEM), Photoluminescence spectroscopy (PL), Energy dispersive X-ray analysis (EDAX), UV-Visible (UV-Vis) spectroscopy. Band-gap of the ZnSe QDs was obtained about 3.45 eV. PL spectra deconvolution obtained three peaks which are related to the emission of band edge, surface states and deep levels. After ZnS shell growth, surface trap states emission was quenched significantly and band
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edge emission was increased considerably. ZnSe QDs indicated a good activity for Methylene Orange (MO) photo-degradation that it was improved
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after ZnS shell growth significantly. KEYWORDS
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1. INTRODUCTION:
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ZnSe/ZnS, Core-shell, Optical properties, Photocatalytic activity.
II-VI quantum dots (QDs) are interesting material possessing individual physical and chemical properties [1,2]. These materials have been
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developed for different applications, such as LEDs, screen devices [3-7], solar cells [8,9], lasers [10-12], photocatalytic activity [13-17], biological probes [18-20], medical imaging [21-23], sensors and other possible
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applications[24-26]. ZnSe QDs are a class of the II-VI semiconductor with a
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little toxicity in comparison with cadmium compounds which are more suitable for different applications [27-30]. ZnSe QDs indicated excellent luminescence and photocatalytic properties [16, 31, 32]. Shelling of the QDs by an inorganic semiconductor is a conventional way for improving their optical, photocatalytic properties and decreasing toxicity [33-35]. Different methods have been reported to synthesis ZnSe and its core-shell structures 2
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[16, 31-41]. Recently we introduced an interesting UV-assisted approach for the synthesis of ZnSe QDs [31]. Subsequently in this work we have
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presented a novel approach for growth of the ZnSe/ZnS core-shell QDs. In this approach TGA as an UV-sensitive material acted simultaneously as both the capping agent and S2- source for ZnS shell growth. To the best of our
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knowledge this approach has not been reported by the others yet.
Synthesized QDs were characterized by means of different analysis tools
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and they indicated good photocatalyst activity for MO photo-degradation.
2. EXPERIMENTAL
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2.1. Materials
Zinc acetate dihydrate (Zn(CH3COO)2.2H2O), Sodium selenite
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(Na2SeO3), Thioglycolic acid (TGA), Sodium hydroxide (NaOH) and
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Methylene Orange (MO) were purchased from the Merck Chemical Company. All chemicals were of the analytical grade and were used without further purification.
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2.2. Synthesis of ZnSe QDs ZnSe QDs were synthesized via a photochemical approach [31]. In a
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typical synthesis, 0.1314 g of Zn(Ac)2 was dissolved in 50 ml D.I water and 0.102 g of Na2SeO3 was dissolved in 30 ml D.I water. 0.1 ml of TGA was
added to Zn(Ac)2 solution and pH of the solution was adjusted to 9 value by
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adding proper amount of NaOH. Subsequently Na2SeO3 solution was added
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to the Zn(Ac)2 prepared solution. The final solution was placed under a high-pressure mercury lamp and was irradiated for 15 min. The ZnSe QDs formation is as below reaction:
Zn(Ac)2 + TGA + NaOH + Na2SeO3 + UV illumination
ZnSe QDs
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The method is a room temperature approach and the reaction time is too and the reported approach is more faster and simpler than the other previously
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reported methods [41- 44 ].
2.3. Growth of ZnS shell. Comparing with the previously reported approaches, the synthesis
method contains two main novel aspects. In most of the previously reported works, after synthesis of the ZnSe QDs, synthesized QDs were extracted from the solution and ZnS shell was grown on the extracted ZnSe QDs 4
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[45- 48], however in this approach ZnS shell was grown on the as prepared and non- extracted QDs, therefore this method can be nominated as one-pot
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method. Moreover, TGA is an UV-sensitive material which it will be decomposed under UV-illumination [49], and releases S2- for the ZnS shell
growth. Actually TGA acts simultaneously as both of the capping agent and
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S2- source [50] for the ZnS shell growth. TGA decomposition process and
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ZnSe/ZnS QDs formation has beeb indicated in Fig. 1 schematicaly. Typically, 10 ml of as prepared ZnSe QDs was used as core. 0.066 g of Zn(Ac)2 was dissolved in 30 ml D.I water and then 0.05 ml TGA was added to that. The pH value of the solution was adjusted to 9 by adding
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proper amount of NaOH. 10 ml of ZnSe prepared solution was added slowly to the solution and the final solution was irradiated by high pressure,
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mercury lamp.
2.5. Characterization
XRD data were recorded using an advanced d8 Burker system.
Energy dispersive X‐ray spectra (EDAX) were taken using a JEOL JSM 6390 LV scanning electron microscope. The infrared absorption data were obtained using a FT‐IR spectrometer. UV–Vis and PL spectra were recorded 5
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with an Avantes spectrometer (AvaSpec‐2048 TEC). FE-SEM image was recorded by a Zeiss ΣIGMA VP-FE-SEM field emission scanning electron
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microscope.
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3. RESULTS AND DISCUSSION
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3.1. XRD
Fig.2 demonstrates XRD patterns of the ZnSe and ZnSe/ZnS prepared QDs. The three main peaks at 28.02° , 47.3° and 55° are related to the diffraction from (111), (220) and (311) planes of cubic Zinc blend phase of
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ZnSe which is in consistent with (JCPDS Card No. 80–0021) standard cards. The broad diffraction XRD peaks confirm the nano-aspect of the synthesized particles. The crystallite size of the particles can be calculated using the full
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width at half-maximum (FWHM) of the first main XRD peak at 28.02° and
D=
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Scherrer's formula: λ
βθ
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In this equation, D is the crystal size, k is a constant which is about
0.94, λ is the x-ray wavelength (0.154 nm), β is the FWHM of the diffraction
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peak and θ is the diffraction angle [51]. The crystallite size of QDs is estimated to be about 2.7 nm.
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XRD pattern of the core-shell ZnSe/ZnS QDs is same as the ZnSe pattern without any extra peaks, but only the diffraction peaks were shifted to the higher angels. This behavior can be explained using Bragg equation: (2)
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2dsinθ=nλ
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In this equation, d is the interplanar space, θ is the diffraction angle, n is a positive integer and λ is the x-ray wavelength 0.154 nm. Actually, growth of the ZnS shell is decreased the lattice parameters of the ZnSe QDs and therefore it is expected the shift of the diffraction peaks to the higher angels due to shrinkage of core structure
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3.2. FT-IR
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[34,51].
Fig. 3 presents FT-IR spectra of TGA and TGA-capped QDs. One
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role of the TGA is to cap QDs. There is an improve peak in TGA spectrum at 2560 cm-1 related to the S-H bond [52, 53]. The absence of this peak in TGA-capped ZnSe QDs spectra proves decomposition of the S-H bond and formation of Zn-S bond [54]. In both spectra, there is a broad peak between 3000 cm-1 to 3500 cm-1 related to the O-H vibration of H2O molecule in the
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TGA capped QDs spectra and also there is a peak at above 1590 cm-1
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correspond to the COO- asymmetric vibration [55, 56].
3.3. FE-SEM
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FE-SEM analysis is a powerful tool to investigating the morphology of the nanostructures. Fig. 4 is a typical FE-SEM image of the ZnSe
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synthesized QDs, clearly the particles have spherical morphology and the grain sizes are estimated about 40 nm with a uniform size distribution.
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3.4. EDAX:
An EDAX analysis of ZnSe and ZnSe/ZnS core-shell synthesized
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QDs have been demonstrated in Fig. 5. Zn and Se orbital energies can be observed clearly in ZnSe pattern (a), in addition to Zn and Se elements an
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extra S signal is observable in ZnSe/ZnS plot (Fig. 5 (b)).The obtained results are matched with the chemical composition of the synthesized particles.
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3.5. Optical properties The band-gap of the ZnSe is about 2.72 eV in bulk case, decrease of
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the particles size results in the increase of the band-gap and indicating quantum confinement effects. Fig. 6 shows absorption and PL spectra of the
ZnSe core QDs. The band edge of the ZnSe QDs is estimated about 3.45 eV
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which is wider than that of its bulk counterpart. Increase of the band gap
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indicates the small size of the particles in the nano range and appearing quantum confinement effects. The particles size can be Calculated from this value of the band gap using effective mass approximation (EMA): 1.78 % ħ 1 1 ∗ ! ! − 0.248)* − + 3 2 ∗ ∗ &
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− =
The particles size was obtained about 2.12 nm.
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PL spectra consists of three main peaks at 385, 445 and 488 nm. The bandgap of the NPs is obtained about 3.45 eV (360 nm), therefore the first
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emission peak can be attributed to the near band edge recombination of electron-hole pairs, the second peak at 445 nm, related to the surface defects [31] and the third at 488 nm related to the recombination of the donoracceptor pairs from deep traps [37]. All of the possible recombination pathways for ZnSe and ZnSe/ZnS are illustrated in the Fig.7 briefly. For ZnSe there are three possible recombination pathways, i) recombination of 9
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the band edge electron-hole pairs, ii) recombination of the electron-hole trapped in the surface defects and iii) recombination of the electron-hole in
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deep levels. Growth of an inorganic shell on the QDs surface is a powerful
approach for covering surface trap states and dangling bonds and
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consequently decrease of the surface trap states density. Decrease of the trap
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states results in increasing the electron-hole band edge recombination and amplifying of the band edge emission in the PL spectra. ZnS has a higher and lower conduction and valance bands, respectively, compare to the ones belonging to ZnSe that it results in the formation of a type I core-shell
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structure (Fig.7 (b)). PL spectra of the ZnSe/ZnS core-shell QDs is depicted in Fig.8. Comparing to the ZnSe core emission, the trap states emission was
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quenched considerably and on the other hand the band edge emission increased significantly (FWHM is about 50 nm). Fig. 8 is an absorption and
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PL analysis of the ZnSe/ZnS core-shell structure, as it was expected, ZnS shell growth should cover the surface trap states which it will decrease the non-radiative and subsequently increase of the recombination of the electron-hole pairs from the other radiative mechanisms [37]. The PL intensity of the band-edge emission and deep levels were increased about 7.24 and 2.305 times receptivity comparing to those for ZnSe QDs. 10
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Quenching of the surface trap states emission (445 nm) and considerably increasing the band edge and deep level emissions, confirm the successfully
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growth of the ZnS shell around ZnSe cores and formation of ZnSe/ZnS coreshell structure. Fig. 9-inset indicates the micrograph emission of the ZnSe
and ZnSe/ZnS core-shell QDs under a 254 nm UV-lamp excitation. The
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considerably increase of the PL intensity is observable.
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In this obtained approach, the ZnS shell thickness is tunable by UVillumination time in the ZnS shell growth step. Fig.10 indicates the PL spectra of ZnSe/ZnS QDs (a) and PL intensity of the band edge and deep levels (b) with different ZnS shell thicknesses. Clearly band edge emission is
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increased and shows an optimum value for the 55 min UV-illumination, but deep levels emission is ascendant. Actually the shell thickness has an
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optimum value, because there is a competitive process. On one hand, the shell can cover the surface traps and increases the band edge emission and
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on the other hand it inherits itself non-radiative trap states inside the shell thus it can quench the emission of the core. Thus, there would be an optimum shell thickness for the maximum of band edge emission and the growth time of the shell should be optimized in this manner.
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3.6. Photocatalytic performance: QDs possess high surface to volume therefore they are photosensitive
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materials and proper to apply as photocatalytic for removing pollutant. Metyle Orange (MO) was selected as a pollutant and ZnSe and ZnSe/ZnS QDs have been applied as a photocatalyst to remove MO from an aqueous
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solution. QDs were extracted from the solution by centrifuging at 6000 rpm
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for 5 min. Three samples were prepared. The first sample was 0.1 g/l MO solution without QDs, the second was 0.1 g/l MO solution containing 5 mg of ZnSe QDs and the third was0.1 g/l MO solution containing 5 mg of ZnSe/ZnS QDs. The prepared solutions were placed under a UV-lamp and
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were illuminated for different interval times.
Fig. 11 indicates absorption spectra of the samples containing ZnSe
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and ZnSe/ZnS QDs during UV-illumination. MO has an important peak at about 465 nm which it was monitored for investigating the MO degradation.
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By increasing UV-illumination the absorption peak intensity was decreased indicating photo-degradation of the MO. Change of the solution colour (Inset of fig 12) also indicates and confirms the photo degradation of MO by QDs.
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Fig. 12 (a) is absorption spectra demonstrating photo-degradation of MO in absence of QDs (a), in presence of ZnSe (b) and ZnSe/ZnS core-shell
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(c) QDs after 90 min. Clearly for the first sample the intensity of the MO peaks has not been decreased by UV-illumination however for both of the solution containing QDs, MO peak at 465 nm was decreased which confirms
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the photocatalytic effect of ZnSe and ZnSe/ZnS. Fig. 12 (b) demonstrates comparison of photocatalytic activity of ZnSe and ZnSe/ZnS QDs. After 90
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min about 89% and 94% of MO was degraded for ZnSe and ZnSe/ZnS, respectively, therefore the photocatalytic activity of the ZnSe/ZnS core-shell QDs is better than that for ZnSe QDs. Fig. 13 is a schematic demonstrating
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the operating mechanism of ZnSe and ZnSe/ZnS as a photocatalyst. Clearly ZnS shell enhanced the photocatalytic activity of ZnSe cores, actually, based on the proposed mechanism in Fig. 13, compare to the ZnSe, ZnS has a
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higher and lower conduction and valance bands, respectively, which it will
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results in the decrease of redox potential and consequently enhancing the photocatalytic activity. The obtained results are better than that for ZnSe photocatalyst activity by the others [57-59].
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4. CONCLUSION ZnSe and ZnSe/ZnS core-shell QDs were synthesized using a fast
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photochemical approach. TGA acted simultaneously as both the capping agent and S source in the ZnS shell growth step. ZnSe QDs exhibited an
emission consists of three peaks related to the, band edge, surface tarp states
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and deep-levels. After ZnS shell growth, because of the covering surface
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defects, the band edge and deep-levels emissions increased considerably and trap states emission was quenched. Both of the core sizes and shell thickness were tunable only with UV-illumination extension. Synthesized QDs indicated strong photocatalytic activity. The photocatalytic performance of
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ZnSe and ZnSe/ZnS QDs were investigated by Methyl Orange (MO) as pollution. The activity of the ZnSe/ZnS QDs for photo-degradation of MO
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was stronger than that for ZnSe QDs.
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Figure captions Fig. 1. TGA decomposition process (a) and synthesis procedure of the ZnSe/ZnS
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synthesized QDs (b). Fig. 2. XRD patterns of ZnSe and ZnSe/ZnS QDs.
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Fig. 3. FT‐IR spectra of TGA and TGA‐capped ZnSe QDs. Fig. 4. FE-SEM image of ZnSe QDs.
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Fig. 5. Elemental composition analysis of ZnSe (a) and ZnSe/ZnS (b) QDs.
Fig. 6. Absorption and emission spectra of the ZnSe QDs (Inset: micrograph emission of the ZnSe QDs).
ZnSe/ZnS core-shell (b).
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Fig. 7. Energy level diagram and possible recombination path ways of the ZnSe (a) and
Fig. 8. Absorption and PL spectra of the ZnSe/ZnS core-shell QDs without S sours
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(Inset: micrograph emission of the ZnSe/ZnS core-shell QDs).
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Fig. 9. PL spectra of ZnSe and ZnSe/ZnS core–shell NCs. Fig. 10. Time evolution of PL spectra and PL intensity of the band edge and deep level emissions (b) of the ZnSe/ZnS core-shell QDs . Fig. 11. Time evolution of absorption spectra of photocatalytic decomposition of Methylene Orange (MO) by (a) ZnSe and ZnSe/ZnS core–shell QDs (b) (Inset: change of the solution colour during the catalytic degradation process).
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Fig. 12. Absorption comparison of photocatalyst treatment of Methylene Orange (a) and the comparison of time evolution of MO concentration (b) by ZnSe and ZnSe/ZnS QDs.
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he catalytic degradation process). Fig. 13. The suggested mechanism for photo-degradation of Methylene Orange by ZnSe
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Fig.1. TGA decomposition process (a) and synthesis procedure of the ZnSe/ZnS synthesized QDs (b).
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Fig. 2. XRD patterns of ZnSe and ZnSe/ZnS QDs.
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Fig.3. FT‐IR spectra of TGA and TGA‐capped ZnSe QDs
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Fig. 4. FE-SEM image of ZnSe QDs.
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Fig.5. Elemental composition analysis of ZnSe (a) and ZnSe/ZnS (b) QDs.
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Fig.6. Absorption and emission spectra of the ZnSe QDs (Inset: micrograph emission of the ZnSe QDs).
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Fig.7. Energy level diagram and possible recombination pathways of the ZnSe (a) and ZnSe/ZnS core-shell (b).
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Fig.8. Absorption and PL spectra of the ZnSe/ZnS core-shell QDs without S sours (Inset: micrograph emission of the ZnSe/ZnS core-shell QDs).
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Fig.9. PL spectra of ZnSe and ZnSe/ZnS core–shell QDs.
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Fig. 10. Time evolution of PL spectra and PL intensity of the band edge and deep level emissions (b) of the ZnSe/ZnS core-shell QDs .
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Fig. 11. Time evolution of absorption spectra of photocatalytic decomposition of Methylene Orange (MO) by (a) ZnSe and ZnSe/ZnS core–shell QDs (b) (Inset: change of the solution colour during the catalytic degradation process).
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Fig. 12. Absorption comparison of photocatalyst treatment of MO (a) and the comparison of time evolution of MO concentration (b) by ZnSe and ZnSe/ZnS QDs.
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Fig.13. The suggested mechanism for photo-degradation of MO by ZnSe and ZnSe/ZnS core– shell QDs.