Investigation on structural properties of Al-substituted

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Mar 31, 2017 - Materials. Sodium Sulfide (Na2SБ9H2O, >95%) was purchased from Ajax. Fine Chem, Co, LTD, Thailand Zinc chloride (ZnCl2Б7H2O, 99.99%).

Results in Physics 7 (2017) 1245–1251

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Investigation on structural properties of Al-substituted ZnS particle prepared from wet chemical synthetic route Yingyot Infahsaeng a,⇑, Sarute Ummartyotin b a b

Division of Physics, Faculty of Science and Technology, Thammasat University, Pathum Thani 12120, Thailand Materials and Textile Technology, Faculty of Science and Technology, Thammasat University, Pathum Thani 12120, Thailand

a r t i c l e

i n f o

Article history: Received 13 December 2016 Received in revised form 26 March 2017 Accepted 26 March 2017 Available online 31 March 2017 Keywords: ZnS Al-substituted ZnS Photoluminescence Morphology

a b s t r a c t ZnS, a wide energy band gap semiconductor, is the potential candidates as a buffer layer for solar cells application. Here, ZnS and Al-substituted ZnS were prepared by simply chemical synthetic route with various high concentration of Al dopant from 0 at% to 40 at%. The structures of ZnS and Al-substituted ZnS powder are all in cubic zinc blende phase. Interestingly, the crystallite size slightly decreases with increasing of Al concentration. A presence of Al content is related with the absence of Zn atom indicate that aluminum is partially substituted into ZnS structure. However, the crystalline structure and morphology of Al-substituted ZnS are not dramatically affected by the aluminum dopant concentration. The band gap energy of the bulk ZnS is approximately at 3.62 eV and slightly increase with increasing of Al dopant. The photoluminescence of Al-substituted ZnS were slightly red-shift and broaden from that of bare ZnS. Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction In recent years, with the growth of economic and worldwide population, the development on energy and its resources have been extensively researched in order to respond on the increasing demand on human life. The role of energy was necessary in many human activities starting from fundamental to welfare. It was remarkable to note that the source of energy can be discovered in many strategies such as electricity, wind, water as well as nuclear power. However, the utilization of these energy sources will be limited in many particular policies. On the other hand, with the need on energy source with respect to human activities, one of the most effective energy sources was referred to solar based energy. From the fundamental point of view, solar energy was definitely referred to radiant light and heat from sun that is harnessed using a range of ever-evolving technologies such as solar heat, photovoltaics, solar thermal energy, solar architecture as well as artificial photosynthesis. The development on solar based energy was therefore versatile in many activities of human life. In order to use solar based energy with higher efficiency, numerous investigations were focused on the development of wide band gap semiconductors such as ZnO, TiO2, CdS, or CdTe. They are extensively studied and utilized for solar cells application. It was ⇑ Corresponding author. E-mail address: [email protected] (Y. Infahsaeng).

remarkable to note that semiconductor material play an important role on for solar cell technology. For example, a high power conversion efficiency of 12.3% for dye sensitized solar cells (DSCs) based TiO2 was obtained [1]. In addition, a high crystallinity of ZnO nanorods were applied to DSCs leading to a high conversion efficiency[2,3]. Although, the efficiencies of CdS/CdTe solar cell as high as 16.5% have been achieved, Cd is not an environmentally friendly element [4]. Apparently, the quantum dot (QD) sensitizer and photoanode are the promising material for DSCs. This is due to the quantum confinement and mini band effect [5]. It is well known that the existence of charge recombination limits the photocurrent of DSCs and thereby reduces the cell efficiency [6]. To retard the back transfer of electrons, the ZnO/ZnS core/shell nano-structures were introduced as the photoanode of DSCs [7–9]. Moreover, ZnS nanostructure have been reported its application for many types of novel solar cell such as dye-sensitized solar cells (DSCs), thin film Cu(In,Ga)Se2) solar cells (CIGS), organicinorganic hybrid solar cells, or Perovskite solar cells [10]. Usually, a buffer or transport layer in solar cells is the main function of ZnS nanoparticles utilization. In additional, the band gap energy of ZnS with a range of 3.6–3.9 eV is rather high compare with other semiconductors leading to a highly suggestion for ultra-violet application. In the past time, ZnS semiconductors with the bulk and nanostructure such as sphere, rod, wire, and tube have been successfully synthesized and properties characterized. Interestingly, many results reported that the morphology and optical

http://dx.doi.org/10.1016/j.rinp.2017.03.034 2211-3797/Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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properties of ZnS nanoparticles including theirs functions can be controlled and added by spin state arrangement of metal dopant [11]. Recently, Mn-doped, Cu-coped, Mn-Cu-codoped ZnS powders have been successfully synthesized by simply chemical method and theirs effect of metal doping on physical and luminescence properties have been investigated [12]. Yunfei et.al. have been applied ZnO/ZnS:Mn2+ photoanode to the QD–sensitized solar cells [13] which revealed that the back transfer of photoelectrons was suppressed and therefore the conversion efficiency exhibited 3.6 folds increments. In addition, optical absorbance and electron transport of QDSSCs can be enhanced by Mn2+-doped ZnS passivation layer which is due to the improvement of the ZnS surface by Mn ions [14]. As a result, metal-doped ZnS acts as a promising passivation layer in novel solar cells that can reduce the recombination process and increase the conversion efficiency. Among of metal dopant, aluminum-doped ZnS is significantly interested due to its low electrical resistivity [15]. Recently, the increasing of aluminum concentration exhibited the grain size increment and film roughness decreasing [16]. However, most Al-substituted ZnS was prepared by physical method such as thermal evaporation which is rather high cost method. Many attempts have developed the low temperature and simple chemical synthesis methods which exhibited the low band gap energy and low electrical resistivity at low aluminum dopant [17–19]. In this article, we wish to present on the preparation of ZnS from conventional synthetic route. Aluminum particle was therefore inserted to ZnS structure. Then the physical and optical properties will be investigated and correlated with the aluminum concentration. Materials and methods

Characterizations FTIR was performed by using a mid-IR spectroscopy, all FTIR absorption spectra were recorded over 4500–400 cm1 wavenumbers region at a resolution of 1 cm1 with 1024 scans. A straight line between two lowest points in the respective spectra region was chosen as a baseline. The crystal structure and structural properties of ZnS and Al-substituted ZnS powder were carried out by Xray diffraction (XRD, model D8-discover, Bruker) system using Cu Ka radiation (k = 1.542 Å). Diffraction patterns were recorded over a range of 25–70°. The trace element of samples was determined by X-ray fluorescence (A Phillips 1404 XRF Wavelength Disperse Spectrometer). It equipped with an array of five analyzing crystals and fitted with a Rh X-ray tube target was used. A vacuum was used as the medium of analyses to avoid interaction of X-rays with air particle. The powders were investigated by scanning electron microscopy or SEM (JOEL JSM-6301F). The machine was operated at an acceleration voltage of 20 keV at a working distance of 10 mm to identify the morphological properties of powders. Before investigation, the samples were sputter-coated with Au to enhance the electrical conductivity. The structural micrographs were obtained by Transmission electron microscopy or TEM (JOEL JEM2100 Plus) at 200 kV operation. Ultraviolet–visible (UV–Vis) absorption measurements of the bulk samples were performed by UV–Vis spectrophotometers (Shimadzu, UV-2600) in the wavelength range of 200–900 nm. Photoluminescence (PL) spectra of the bulk samples were measured via spectrometer (Edinburgh Instrument, FLS980) with an excitation wavelength of 300 nm.

Results and discussion

Materials Sodium Sulfide (Na2S9H2O, >95%) was purchased from Ajax Fine Chem, Co, LTD, Thailand Zinc chloride (ZnCl27H2O, 99.99%) and Aluminum Hydroxide (Al(OH)3, >95%) were purchased from Sigma Aldrich, Co, LTD, Thailand. Analytical grade of methanol was purchased from RCI Labscan, Co, LTD, Thailand. Distilled water and analytical grade of methanol were used as solvent. All the chemical reagents were used as received without any purification. Methods

The FTIR measurements were carried out to confirm the formation of ZnS powder and identify any absorbed species on the crystallite surface. Fig. 1 shows the FTIR transmission spectra of ZnS and Al-substituted ZnS for a various dopant concentrations. The broad absorption peak around 3499 cm1 correspond to the AOH group of H2O stretching, indicating the existence of water on the sample powder. The band around 1612 cm1 is due to [email protected] groups stretching mode [19]. The peaks appearing at 1117, 934, 612 and 493 cm1 are due to Zn-S vibration mode which correspond to the previous report [20,21].

The ZnS powder was prepared by one pot synthesis method as follow: 0.4 mol of ZnCl27H2O was prepared in 400 ml of distilled water. At the same time, 0.4 mol of Na2S9H2O was also dissolved 400 ml of distilled water. Both mixtures were stirred for 30 min at 55 °C. Subsequently, Na2S9H2O solution was poured into ZnCl27H2O solution and stirred for 60 min at 55 °C. The stoichiometric of chemical reaction is 

Na2 S  9H2 O þ ZnCl2  7H2 O ! ZnS precursor þ Naþ þ Cl

For Al-substituted ZnS powder, ZnCl2 was blended with Al(OH)3 with ZnCl2:Al(OH)2 molar ratio of 9:1, 8:2, 7:3, and 6:4. The blend was dissolved into 400 ml of distilled water and stirred for 30 min at 55 °C. The ZnCl2:Al(OH)27H2O and Na2S9H2O solution were mixed and stirred to conducted the continually reaction for 60 min at 55 °C. The stoichiometric of chemical reaction is

Na2 S  9H2 O þ ZnCl2  7H2 O þ AlðOHÞ3 

! Al-substituted ZnS precursor þ Naþ þ Cl

Afterward, methanol was employed to use in order to remove impurities. The ZnS and Al-substituted ZnS powder were obtained through the filter and kept in oven at 80–100 °C overnight. The powder sample was grinded through 36 mm filter.

Fig. 1. FTIR spectra of (a) ZnS and Al-substituted ZnS for (b) 10%, (c) 20%, (d) 30% and (e) 40% aluminum concentration.

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As the aluminum was doped into ZnS, the peaks of Alsubstituted ZnS powder are observed at 529 cm1 and 1144 cm1 band which are shifted from the peaks of ZnS at 493 cm1 and 1117 cm1, respectively. The observed peaks may belong to Al-S vibration mode. Similarly, the band shift from 612 cm1 to 669 cm1 for 10% Al and to 728 cm1 for higher Al concentration may due to the aluminum doping effect. Note that the increasing of aluminum dopant, the broad and strong IR bands are observed. Note that the FTIR result of Al-substituted ZnS at 50% dopant exhibited the non-pattern peaks, indicating that high aluminum concentration of 50% is the dopant limitation of Al-substituted ZnS. By comparing the XRD patterns of ZnS and Al-substituted ZnS powder as shown in Fig. 2, the XRD patterns do not show other phases and the significant difference in the shape and peak positions for the doping concentration range 0–40 at%. Three diffraction peaks are observed in all samples, which correspond to the lattice planes of (1 1 1), (2 2 0), and (3 1 1). Although the Al2S3 phase have been reported at high dopant percentage in film sample [15], this extra phase is not observed in this present XRD patterns of bulk sample. This implies that the phase segregation of other species at higher aluminum concentration may not be formed by one pot synthesis route. These peaks matched with the cubic zinc blended structure (JCPDS No. 05-0566), verifying the purity of the synthesized ZnS and Al-substituted ZnS. The XRD result was strongly associated with FTIR experiment. The (1 1 1) peak became more broadened and smaller relative intensity peak when ZnS was doped with higher aluminum dopant. This observation suggests that the crystallinity of ZnS was decreased as a result of doping. According to Debye-Scherer formula, crystallite size was calculated by the equation:

D ¼ kk=b cos h where D is the mean crystalline size, k is constant (shape factor approximately 0.9 for ZnS), k is the X-ray wavelength (1.5406 Å for Cu-Ka), b is the full width at half maximum (FWHM) of the diffraction peak and h is the Bragg angle. The crystallite size of ZnS was 21.5 Å and slightly decrease to the range of 19–20 Å when the aluminum dopant concentration increases as shown in Table 1. In additional, the lattice parameters of all sample are faintly decreased from 5.39 Å to 5.36 Å as the aluminum concentration increase which correspond to the previous reported[15,22,23]. The decreasing in crystalline size and lattice parameter confirm that the substitution of Al3+ (radii of aluminum ions = 0.54 Å) into the ZnS matrix (radii of zinc ions = 0.74 Å) was successful substitution.

Table 1 Crystallite size and lattice parameter of ZnS and Al-substituted ZnS. Sample

Relative Intensity of peak (1 1 1)

Crystallite size, Å

Lattice parameter, Å

ZnS Al:ZnS-10% Al:ZnS-20% Al:ZnS-30% Al:ZnS-40%

100 83.60 88.04 73.48 70.52

21.5 20.3 19.6 19.3 19.0

5.394 5.383 5.379 5.372 5.368

Note that the relative intensity of peak (1 1 1) of Al-substituted ZnS with 20% aluminum concentration is slightly high, which may cause by impurity of sample or setup alignment during the XRD measurement. As shown in Table 2, the element contents of ZnS and Alsubstituted ZnS were presented. The major element of all samples are Zn and S, while the existence of minor element of Al is increased as the Al dopant concentration increase. Note that the element of S is insignificantly changed. This result can confirm the presence of aluminum in Al-substituted ZnS samples at various Al dopant concentrations. However, the content percentage of aluminum does not perfectly agree with the amount percentage of Al dopant. This aluminum loss may be an incomplete synthetic process during the Al-substituted ZnS synthesis and may be the concentration limitation. The morphologies and compositions of ZnS and Al-substituted ZnS samples were characterized by SEM and EDX, respectively. Fig. 3(a) represent the SEM image of pure ZnS powder, whereas Fig. 3(b)–(e) represent the SEM images of Al-substituted ZnS samples. All images exhibit that the porosity of all samples is approximately the same and blocky particles shape can be observed due to the formation of agglomeration among particles. Although the actual particle size cannot directly be determined, the SEM images may indicate that the grain size of all samples has a smallest diameter of hundred nanometers and the morphologies are insignificantly difference. An X-ray energy dispersive spectroscopy (EDX) spectrum obtained from ZnS (the inset in Fig. 3(a)) confirm that the samples consist of Zn and S. Moreover, the EDX spectrum from Al-substituted ZnS (the inset in Fig. 3(b)–(e)) exhibit an additional amount of Al atom with mainly a stoichiometric ZnS composition. The observed Al peak indicates that the amount of Al atom increases as the Al dopant concentration increases which is consistent with the amount in preparation step and XRF result. These results indicate that the successive substitution of aluminum into ZnS structure was successfully prepared. A TEM images of the samples, as shown in Fig. 4, shown that the ZnS and Al-substituted ZnS consists of an irregular shape nanoparticles which agglomerate to form a cluster of particles. The nanoparticle sizes for all samples are range from 1.7 to 4.9 nm and the average sizes of these nanoparticles are 2.76, 2.74, 2.48, 2.54, and 2.64 nm for ZnS and Al-substituted ZnS with 10%, 20%, 30%, and 40% concentration, respectively. The insets of all TEM images shown the representative nanoparticles for each sample which clearly shown the irregular shape. The insets in Fig. 4(a) show the planar spacing of about 0.358 nm which corresponds to the spacing for (1 1 1) planes of ZnS.

Table 2 XRF quantitative analysis of ZnS and Al-substituted ZnS samples.

Fig. 2. XRD pattern of (a) ZnS and Al-substituted ZnS powder for (b) 10%, (c) 20%, (d) 30% and (e) 40% aluminum concentration.

Sample

Zn

S

Al

ZnS Al:ZnS-10% Al:ZnS-20% Al:ZnS-30% Al:ZnS-40%

77.82 71.91 68.85 67.76 64.17

22.18 19.89 21.06 18.88 19.61

– 8.21 10.10 13.37 16.22

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Fig. 3. SEM images of ZnS (a) and Al-substituted ZnS samples for (b) 10%, (c) 20%, (d) 30% and (e) 40% aluminum concentration. The insets show the EDX spectrum of each correlated samples.

For the optical properties of undoped-ZnS and Al-substituted ZnS powder with different Al-dopant concentration, the optical absorbance spectrum of all samples is depicted in Fig. 5. All absorbance spectrum show that the samples are transparent in the visible region and absorb light in the ultra-violet region with strong absorption peak at ca. 300 nm. Interestingly, doping of ZnS with Al atom does not show any significant bulk absorption change at all Al dopant concentration. Note that the change of transmittance characteristics in the visible region have been reported in doping ZnS films [15,18]. Moreover, the spectrum edge of all samples is gradual broad which imply that the size distribution of ZnS and

Al-substituted ZnS powder are rather similar with broad distribution size. Although crystalline size observed by XRD slightly decrease, but the agglomeration of powder during the one pot synthesis may enhance the particle size distribution which can be observed by UV–Vis spectrum. The Tauc’s plot of the square of the photon energy absorption coefficients (ahm)2 against a function of energy (hm) is shown in the inset of Fig. 5. The optical band gap of the samples can be evaluated from the equation:

ðahmÞ

1=n

¼ Aðhm  Eg Þ

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Fig. 4. TEM images of ZnS (a) and Al-substituted ZnS samples for (b) 10%, (c) 20%, (d) 30% and (e) 40% aluminum concentration. The insets are TEM images of representative particles.

Fig. 5. UV–Vis absorption spectra for the ZnS and Al-substituted ZnS samples. The inset shows the photon energy absorption coefficients as a function of energy.

where A is a constant related to the effective mass, a is the absorption coefficient, Eg is the energy band gap, hm is the photon energy, and n is ½ for direct band gap semiconductor. Using an extrapolation of the linear portion of the curve, the band gap (Eg) can be estimated at the point (ahm)2 = 0. As a result from absorption, the energy band gap of bulk ZnS and Al-substituted ZnS with 10% Al concentration is approximately at 3.62 eV which is nearly similar to the values reported [19,24,25]. By doping with Al concentration higher than at 20%, the band gap of bulk Al-substituted ZnS is slightly increased to ca. 3.63 eV. Previously, Prathap P. et al. reported the decreasing of band gap due to aluminum doping prepared by close-spaced evaporation (CSE) technique [15]. The forma-

tion of shallow levels or impurity levels in the band gap region was assigned as the main contribution of band gap lowering. However, Liao J. et al. reported that the band gap of film prepared by chemical bath deposition would be increased with Al concentration [22]. According to the electron density of states results studied by Imai Y. et al. [26], the bottom of the conduction band and the top of the valence band are decided by Zn state and S state, respectively. Thus, from Liao J.’s work, the increasing of band gap was assigned to the decreasing of overlapping degree of electron cloud formed by Zn and S in the Zn-S bound which lead to the increasing of Zn states energy and band gap. In the present study, the little increasing of band gap due to Al doping have been observed and may be assigned as the Zn-S bonding energy lowering and the decreasing of the state density of Zn which shift the bottom of the conduction band to a higher energy level. However, it seems that the band gap is not dramatically increased and limited at ca. 3.63 eV. This limitation may due to the saturation doping at high doping levels which also can be observed in the XRF and EDX results or due to the compensation of the increasing of the top of the valence band which dominated by S atom. Moreover, the previous work reported that the linear decrease of band gap with the dopant concentration could be compensated by improvement in the crystalline quality [15]. In this present study, the increasing of the top of the valence band with the Al concentration increasing may be effected by the low crystalline quality which is consistence with the broad XRD spectrum. Thus, the absorption analysis shows that the energy gap is not dramatically changed because of the compensation between the increasing of the bottom of the conduction band due to Zn-S bonding energy lowering and the increasing of the top of the valence band due to Al-S bonding and low crystalline quality. Fig. 6 shows the room temperature photoluminescence (PL) spectra of bulk ZnS and Al-substituted ZnS samples when excited with the radiation of wavelength 300 nm. The peak of bulk ZnS observed at 438 nm which corresponds to a photon energy of

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Acknowledgements The authors would like to thank department of physics, Mahidol University for XRD facility and Vidyasirimedhi Institute of Science and Technology for photoluminescence facility. The authors also thank Prapan Manajaroensub and On-anong Suwanmanee for their help. The authors gratefully acknowledge the financial support provided by the Thailand Toray Science Foundation (TTSF), Thailand Contract No. 19-Phy01-2015 and Faculty of Science and Technology, Thammasat University, Thailand Contract No. (2) 9/2559.

References

Fig. 6. Photoluminescence spectrum for the ZnS and Al-substituted ZnS samples. The inset shows the photoluminescence intensity at maximum peak for the ZnS and Al-substituted ZnS samples.

2.83 eV. As the Al dopant concentration increase up to >10%, the PL peak is slightly shifted to a longer wavelength and the Full-Width-Half Maxima (FWHM) is slightly expanded. This broaden FWHM indicate that the density of state (DOS) of either conduction or valence band may be extended. However, the broaden FWHM of PL may be effect by the increasing of grain size distribution due to the agglomeration during the sample kept. The red-shift of spectra can be assigned as the lower energy transition due to the formation of shallow level or impurity level. As been discussed from previous work, the S atom could affect on the top of valence band. When Al dopant concentration increase, Al-S bound can be formed and introduce a shallow energy level, especially, close to valence band edge. Consequently, the energy transition of high Al-substituted ZnS is decreased from that of bulk ZnS. Obviously, the lowering of energy transition is more pronounced than that the increasing of band gap, suggesting that the aluminum may cause the effect on the valence band edge rather than the conduction band edge. Thus, the red-shift and FWHM broaden of photoluminescence spectra of Al-substituted ZnS are dominated by the substitution of aluminum into ZnS structure. Note that the photoluminescence intensity at maximum peak of 10% aluminum substituted ZnS is higher than that of ZnS. However, the PL intensity at Al concentration dopant higher than of 10% is decreased. Conclusion ZnS and Al-substituted ZnS powder were successfully synthesized by simply chemical synthetic method. The chemical bonding and crystal structure were investigated by FTIR and XRD, respectively. The XRF analysis verify the existence of Al-substituted ZnS. The crystallite size of Al-substituted ZnS is slightly decreased as the increasing of high Al dopant concentration. Thus, the aluminum can be successively substituted into ZnS lattice. SEM and TEM reveal that their nanoparticle was agglomerated and blocky shape formed. The absorption spectra can exhibit the energy band gap of ca. 3.62 eV and 3.63 eV for ZnS and high Al-substituted ZnS, respectively. The photoluminescence of Al-substituted ZnS were slightly red-shift from that of bare ZnS, suggesting that the shallow energy level may be formed and the valence band of ZnS may be dominated by Al substitution.

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