DSpace at VNU: The photoluminescence enhancement of Mn2+ ions and the crystal field in ZnS:Mn nanoparticles covered by polyvinyl alcohol

DSpace at VNU: The photoluminescence enhancement of Mn2+ ions and the crystal field in ZnS:Mn nanoparticles covered by p...

The photoluminescence enhancement of Mn2+ ions

and the crystal field in ZnS:Mn nanoparticles covered

by polyvinyl alcohol

Dang Van Thai1•Pham Van Ben1•Tran Minh Thi2•

Nguyen Van Truong1•Hoa Huu Thu3

Received: 13 November 2015 / Accepted: 7 June 2016

 Springer Science+Business Media New York 2016

Abstract The polyvinyl alcohol (PVA)-capped ZnS:Mn nanoparticles with Mn content of

8 mol% and different PVA mass (denoted as ZnS:Mn/PVA) are synthesized by co-pre-cipitation method, in which PVA solution is mixed from the beginning with the initial solutions used to synthesize ZnS:Mn nanoparticles The microstructures, morphology and average crystalline size of ZnS:Mn/PVA nanoparticles were investigated by X-ray diffraction patterns, high resolution transmission electron microscopy, thermal gravimetric analysis (TGA) and differential thermal gravimetric analysis and Fourier transform infrared absorption spectra The role of PVA to the photoluminescence (PL) of Mn2?ions

in these nanoparticles at 300 K were studied by the PL and photoluminescence excitation (PLE) spectra The investigated results show that the PVA covering for ZnS:Mn nanoparticles almost is not change the microstructure, morphology, the crystal field and the peak positions in their PL and PLE spectra, but the maximum intensity of peaks increased with PVA mass from 0.2 to 1.0 g The clear peak positions in PLE spectra show that the energy levels of Mn2?ions were splitted into multiple levels in the ZnS:Mn crystal field, that its strength Dqwas caculated Furthermore, the effect of PVA on the PL enhancement

of Mn2?ions in ZnS:Mn/PVA nanoparticles also was explained

Keywords Co-precipitation method Nanoparticles  PL spectra  PLE spectra  Crystal field strength

& Tran Minh Thi

tranminhthi@hnue.edu.vn

1

Faculty of Physics, Hanoi University of Science, VNU, Hanoi, Vietnam

2 Faculty of Physics, Hanoi National University of Education, Hanoi, Vietnam

3

Faculty of Chemistry, Hanoi University of Science, VNU, Hanoi, Vietnam

DOI 10.1007/s11082-016-0622-y

1 Introduction

Recently, Mn doped ZnS nanoparticles (denoted by ZnS:Mn) is widely used in photonic, biological markers, photocatalytic applications and many other applications because they are wide band gap semiconductors with stable and strong PL intensity in the yellow-orange region and high luminescent efficiency (Bhargava et al.1994; Bhargava1996; Pouretedal

et al.2009; Chitkara et al.2011; Sajan et al.2015)

To increase the application capacity from biological sensors to optical displays, the synthesis and characterization of semiconductor nanocrystals of various sizes and com-positions and with different capping agents remains an active area of current research ZnS:Mn nanoparticles were often capped by surface active substances such as C2H4O2S thyoglycolic acid, (C6H9NO)npolyvinyl pyrrolidone (PVP) and [CH2CH(OH)]npolyvinyl alcohol (PVA) 3-mercaptopropionic acid (MPA) (Chitkara et al 2011; Onwudiwe et al

2014; Murugadoss2010; Murugadoss et al.2010; Thi et al.2013; Hirankumar et al.2005; Kareem et al.2012; Zhou et al.2015) Meanwhile, ZnS:Mn nanoparticles isolated with the environment, un-aggregation, thus the particle size reduced In addition, the PL intensity increased due to excitation energy transfer from the surfactant to ZnS:Mn nanoparticles (Onwudiwe et al.2014; Murugadoss et al.2010; Zhou et al.2015)

Typical of polymers, PVA acts as a ligand and forms a bond with the metal ions by donor, acceptor interactions leading to the formation of a coordination sphere In the polymer chain, the N and O atoms have lone pairs of electrons which could be used in the formation of the bond (Onwudiwe et al 2014) In our earlier studies, we have reported the synthesis and the energy transition process in ZnS:Mn/PVP nanoparticles (Thi et al.2013) PVP controls the growth of the particles by forming passivation layers around the particle core via coordination bond formation, in which PVP part acts as the head group, while the polyvinyl alcohol (PVA) part acts as the tail group (Onwudiwe

et al.2014) Furthermore, PVA with the energy band gap of 5.4 eV (Hirankumar et al

2005) have the semi-crystalline nature of organic material (Wang et al 2014), that is composed mainly of 1,3-diol linkage [–CH2–CH(OH)–CH2–CH(OH)–] but a few percent

of 1,2-diol [–CH2–CH(OH)–CH(OH)–CH2–] occurred, thus the covered formation and their properties created by PVA around the ZnS:Mn particles are different than the PVP coating However, PVA has received a significant amount of interest in both academic and industrial research for a long time due to its biocompatibility and degradability by certain bacteria PVA has been widely used in the form of hydrogels in the biotechnology area, the topics of intense research due to their size-related electronic, magnetic and optical properties (quantum size effect) and their wide applications from optoelectronics

to biology (Murugadoss et al.2010; Hammad et al.2015) Furthermore, there are not any papers that completely investigated the optical properties and calculated of the crystal field strength of ZnS:Mn/PVA nanoparticles

The capping of ZnS:Mn nanoparticles can be done by one of two methods: (1) dis-persing of ZnS:Mn nanoparticles into surfactant solution (Kareem et al.2012) or (2) from the beginning, the surfactant solution and the initial solutions of nanoparticles simulta-neous are mixed each other (Chitkara et al 2011) By second method, the ZnS:Mn nanoparticles formed in surfactant solution matrix, thus ZnS:Mn nanoparticles are un-aggregation to each other and its particle size decreased, the quantum confinement effect increased

The paper report the synthesis of ZnS:Mn/PVA nanoparticles by second method, in which PVA solution is mixed from the beginning with the initial solutions used to

synthesize ZnS:Mn nanoparticles The microstructures, morphology of ZnS:Mn/PVA nanoparticles, the PL enhancement of Mn2?ions by different PVA mass, the absorption and radiation transitions in the ZnS:Mn crystals and the crystal field strength Dq are investigated and explained

2 Experimental

In order to investigate the role and influence of PVA to the properties of ZnS:Mn nanoparticles, the ZnS:Mn powder mass of 0.5 g is kept constant for all ZnS:Mn/PVA nanopowder samples, while the PVA mass changed from 0 to 1.5 g By second method, ZnS:Mn/PVA nanopowder samples have been synthesized from the initial solutions Zn(CH3COO)20.1 M (A); Mn(CH3COO)20.1 M (B); Na2S 0.1 M (C), which were calcu-lated to create the nominal ZnS:Mn powder mass of 0.5 g with Mn content of 8 mol%, while PVA mass changed with values: 0; 0.2; 0.4; 0.6; 0.8; 1.0; 1.2 and 1.5 g The process of this method was performed as following: the solutions A and B were mixed each other and stirred for 30 min to receive the solution D The different PVA mass of 0.2; 0.4; 0.6; 0.8; 1.0; 1.2; 1.5 g in turn were stirred in distilled water of 50 mL for 3 h at 80C, to obtained the solutions E, respectively The solutions D and E were mixed and stirred each other for 1 h to receive the solutions F The solution C was putted in the solutions F drop by drop and stirred for 1 h to create precipitation The reaction equation occurred as follow:

Zn CHð 3COOÞ2þ Mn CHð 3COOÞ2þ 2Na2S þ PVA ! ZnSMnS½ð Þ  PVA

# þ 4CH3COONa

[(ZnSMnS)-PVA] precipitations (8 samples of ZnS:Mn/PVA nanoparticles with nominal

mZnS:Mnof 0.5 g and different PVA mass: mPVA= 0; 0.2; 0.4; 0.6; 0.8; 1.0; 1.2 and 1.5 g) were separated by centrifugation with speed of 2500 rpm and filtered, washed several times

by distilled water The ZnS:Mn/PVA nanoparticles obtained by drying at 80C for 10 h and finely grind The crystal structure and average crystalline size of these nanoparticles were investigated by X-ray diffraction patterns (XRD) recorded on XD8 Advance Bukerding using CuKa radiation (k = 1.5406 A˚ , 2h = 10–70) The morphology of ZnS:Mn/PVA nanoparticles with different PVA mass were also demonstrated by HRTEM image on the high resolution transmission electron microscopy JEM-2100 The PL and PLE spectra of nanoparticles at 300 K were recorded on MS-257 Oriel, FL3-22 spectrometers, respectively, using 325 nm excitation radiations of He–Cd laser and XFOR-450 xenon lamp Thermal gravimetric analysis (TGA) and differential thermal gravimetric analysis (DTG) were per-formed using Setaram instrumentation The samples were placed in the specimen holder, in which the heating rate was set at 10C/min and the measurements were carried out in argon gas ambient from 30 to 700C Fourier Transform Infrared (FT-IR) absorption spectra of the nanoparticles also at 300 K were recorded on spectrometer Nicolet 6700 FT-IR

3 Results and discussions

3.1 Crystal structure and morphology of ZnS:Mn/PVA nanoparticles

XRD pattern was used to characterize the original PVA powders (Fig.1a) The XRD pattern of the original PVA powders shows a strong diffraction peak centered at 20 and a

weak diffusion diffraction peak centered at 41, indicating its semi-crystalline nature But these diffraction peaks of PVA in XRD pattern of ZnS:Mn/PVA nanoparticles decrease strongly and become smooth range (Fig.1b), while the peaks of (111), (220) and (311) belong to ZnS:Mn crystal phase This phenomenon is suggesting a decrease in the degree

of crystallinity of PVA derivatives in ZnS:Mn/PVA nanocomposite The breakage of PVA crystal region should be attributed to the decrease of the intermolecular hydrogen bonding between PVA chains (Murugadoss et al.2010; Wang et al.2014) due to the embedment of ZnS:Mn nanoparticles in PVA matrix

Figure2 present XRD patterns of ZnS:Mn/PVA (CMn= 8 mol%) nanoparticles with different PVA mass (0.0, 0.4, 0.8, 1.0, 1.5 g) These patterns include (111), (220) and (311) diffraction peak, in which intensity of (111) peak is greatest This XRD patterns also showed that these nanoparticles are single phase with T2d F43m symmetry cubic struc-ture The strong and sharp diffraction peaks suggest that the obtained products are well crystallized with ZnS:Mn crystal phase, without any strange phase is observed It has to be noticed that for XRD patterns of ZnS:Mn/PVA nanoparticles, the width at haft maximum

of diffraction peaks are larger than those observed for ZnS:Mn crystallites (Fig.2a) and suggesting that the particle size becomes slightly smaller Further, there is no significant change in the position of the peaks It shows that the PVA capping agent unchanged the phase of ZnS:Mn nanoparticles

The lattice constant and average crystalline sizes were determined from XRD patterns and using Debye-Sherrer formula:

D¼ 0:9k

where, k(nm) is the wavelength of CuKa radiation; b(rad) is the full width at haft maxi-mum and h (rad) is the diffraction angle The results showed that the average crystalline size of uncapped ZnS:Mn nanoparticles is about of 3.6 nm, but this size decreased to 2.6–2.7 nm when PVA mass increased from 0.2 to 1.5 g These size decrease are explained due to PVA solution mixed from the outset with the initial solutions used to synthesize ZnS:Mn/PVA nanoparticles, thus the formation of ZnS:Mn nanoparticles happened in PVA matrix simultaneously with the breakage of the crystal region of PVA Meanwhile, PVC capping prevented the aggregation and growth of ZnS:Mn nanoparticles

Fig 1 XRD patterns of PVA

(a) and ZnS:Mn/PVA

nanoparticles (m PVA = 1.0 g) (b)

20 30 40 50 60 70

2θ (degree)

(111)

(220)

(311)

h 1.5g

f 1.0g

e 0.8g

c 0.4g

a 0.0g

a

c e f h

Fig 2 XRD patterns of

ZnS:Mn/PVA nanoparticles with

different PVA mass:

a m PVA = 0 g; c m PVA = 0.4 g;

e m PVA = 0.8 g;

f m PVA = 1.0 g; h m PVA = 1.5 g

Fig 3 HRTEM images of ZnS:Mn nanoparticles (a–c) and ZnS:Mn/PVA nanoparticles with m PVA = 1 g (d–f)

However, the lattice constant of ZnS:Mn/PVA nanoparticles with different PVA mass is almost unchanged with a = 5.373 A˚ , this value is approximately to standard lattice con-stant of ZnS (JCPDS Card No 05-0566, a = b = 5.406 A˚´ ) From the lattice constant a, the lattice spacing of ZnS:Mn/PVA nanoparticles with cubic structure has been determined about of 0.31 nm

Figure3shows HRTEM images of the ZnS:Mn and ZnS:Mn/PVA nanoparticles with

mPVA= 1 g HRTEM image of ZnS:Mn nanoparticles includes a some of crystal planes (Fig.3a) distributed in different directions, that proves the polycrystalline structure of ZnS:Mn nanoparticles The HRTEM image in Fig.3b and the corresponding fast Fourier transform image (FFT) in Fig.3c (Wang et al.2006) show that the lattice spacing was estimated to be around 0.31 nm This value agrees with the (111) lattice spacing of ZnS, ZnS:Mn crystals (about of 0.30–0.31 nm) (Han et al.2014; Son et al.2007), which cal-culated from XRD patterns The above results are also correct for ZnS:Mn/PVA nanoparticles (Fig.3d–f) Thus, the covering of ZnS:Mn nanoparticles by PVA almost did not affect their microstructure and morphology

3.2 Analyses of TGA, DTG and FT-IR spectra

The PVA capping of ZnS:Mn nanoparticles was proved by TGA, DTG and FT-IR spectra Figures4, 5 show TGA and DTG spectra of PVA and ZnS:Mn/PVA nanoparticles at a heating rate of 10C/min and in the range from room temperature to 750 C A derivative weight loss curve can be used to tell the point at which weight loss is most apparent TGA and DTG curves of PVA and ZnS:Mn/PVA revealed three main weight loss regions In the TGA curve of PVA (Fig.4), the initial weight loss for pure PVA about of -6 wt% occurredat the temperature region from 30 to 190C, in which there is the endothermic peak at 140C in the DTG curve, due to the evaporation of the trapped water from PVA It was also observed that the major weight losses about of -44 wt% have occurred in the second range from 250 to 370C This is due to the degradation of O–H chains of PVA in this range with sharp endothermic peak of 335C in the DTG curve (Ahad et al.2012; Alkan and Benlikaya2009) In the third temperature range from 370 to 730C with the sharp endothermic peak of 420C in the DTG curve, the weight loss about of -49.3 wt%

-120 -100 -80 -60 -40 -20 0 20

Temperature ( o C)

-44.7%

-49.3%

TGA and DTG of PVA in Argon

TGA

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.2

-6%

DTG

335 o C

420 o C

140 o C Fig 4 TGA and DTG spectra of

PVA

may correspond to the decomposition of CC, CO, CH backbones of PVA in the TGA curve At the temperature upper 730C PVA completely decomposed

For the TGA curve of ZnS:Mn/PVA (Fig.5), the weight loss about of -7 % wt% due to the evaporation of trapped water occurred at the temperature region from 30 to 190C, in which there is the sharp endothermic peak at 102C in the DTG curve In the second temperature region from 180 to 310C, the weight loss of ZnS:Mn/PVA is about of -6.4 wt%, meanwhile this weight loss is about of -11.6 wt% in the third temperature region from 310 to 750C However, due to the decrease of the intermolecular hydrogen bonding between PVA chains by the embedment of ZnS:Mn nanoparticles in PVA matrix, the endothermic peaks (248 and 379C) occurred at lower temperatures than that in com-parison with pure PVA sample (in DTG curves of Figs.4,5) The degradation of O–H chains, the backbones of CH, CO, and CC of PVA occurred from 180 up to 750C, after that the mass of ZnS:Mn remained about of 77 wt%

In FT-IR spectrum of PVA (Fig.6a), the typical peaks of this spectrum were assigned to the stretching vibrations of groups: OH at 3450 cm-1; CH/CH2 at 2954 cm-1; C–O at

1108 cm-1; and the bending vibration of water absorbed by PVA at 1638 cm-1, in which

OH group has largest absorptance (Venyaminov et al.1997; Ilcin et al.2010; Wong et al

2009) For ZnS:Mn/PVA nanoparticles, on basic, its FT-IR spectrum also shows charac-teristic peaks of PVA Besides that, the additional appearance of some peaks in this spectrum, that were assigned to the stretching and bending vibrations of CH group at

1544 cm-1; 1419 cm-1; the stretching vibration of oxygen at 1006 cm-1; Zn–S at 620,

472 cm-1(Fig.6b) (Baishya and Sarkar2011) However, the vibration of OH group is shifted towards the shorter wavenumber at 3410 cm-1 This result shows the bonds between OH group of PVA and ZnS:Mn nanoparticles The appearances of peaks of CH group at 1544 and 1419 cm-1 in FT-IR spectra of ZnS:Mn/PVA nanoparticles are the result of coordinate bonding between PVA and Zn (Thottoli and Achuthanunni2013) The analyzed results of the XRD patterns, TGA, DTG, FT-IR spectra of ZnS:Mn/PVA nanoparticles proved that the PVA capping agent unchanged the phase of ZnS:Mn nanoparticles The embedment of ZnS:Mn nanoparticles into PVA matrix caused the decrease of the intermolecular hydrogen bonding between PVA chains, simultaneous their

-30 -20 -10 0 10

-7%

Temperature ( o C)

-5.4%

-11.6%

DTG

TGA

102 o C

248oC

379 o C TGA and DTG of ZnS:Mn/PVA in Argon

-0.4 -0.3 -0.2 -0.1 0.0 0.1

Fig 5 TGA and DTG of

ZnS:Mn/PVA nanoparticles

particle size has reduced due to un-aggregation of ZnS:Mn nanoparticles The schema of PVA-capped ZnS:Mn nanoparticles is presented in Fig.7

3.3 The photoluminescence enhancement of Mn21ions in ZnS:Mn/PVA nanoparticles

Figure8 shows the PL spectra of ZnS:Mn/PVA nanoparticles with different PVA mass excited by 325 nm radiation of He–Cd laser The PL spectrum of ZnS:Mn nanoparticles appeared the blue band at about of 440 nm with little intensity and the yellow-orange band

at about of 603 nm with greater intensity (Fig.8a)

The blue band is attributed to Zn, S vacancies and their interstitial atoms (Denzler et al

1998), while the yellow-orange band assigned to the radiation of Mn2? ions [4T1(4G) ?6A1(6S)] in ZnS crystals (Bhargava et al 1994) For the ZnS:Mn/PVA

Fig 6 FT-IR spectra of PVA

(a) and ZnS:Mn/PVA

nanoparticles (m PVA = 1 g) (b)

Fig 7 The schema of

PVA-capped ZnS:Mn nanoparticles

nanoparticles, the intensity of blue band changes little, but the intensity of yellow-orange band increased with the increasing of PVA mass from 0.2 g and reached the maximum at PVA mass of 1 g (Fig.8b–f), then it decrease with the increasing of PVA mass to 1.5 g (Fig.8g, h) The dependence of yellow-orange band intensity on PVA mass is present in inset of Fig.8 However, the peak positions of blue and yellow-orange bands almost unchanged with the change of PVA mass (Fig.8)

The PL spectra of ZnS:Mn/PVA nanoparticles with PVA mass of 1 g by different excitation power densities of 325 nm radiation were presented in Fig.9 It is clear that the intensity of yellow-orange band is increased with the increase of excitation power densities from 0.05 to 0.33 W/cm2, but its peak position still unchanged The dependence of PL intensity on excitation power density is given by the equation IPL¼ A  In

ep, where IPLis PL

0.0 4.0x103 8.0x103 1.2x104 1.6x104 2.0x104

Mass of PVA(g)

Wavelength (nm)

b 0.2 g

c 0.4 g

d 0.6 g

e 0.8 g

f 1.0 g

g 1.2 g

h 1.5 g

a b

c d

f

g

h

440

e

Fig 8 PL spectra of ZnS:Mn/

PVA nanoparticles with different

PVA mass a m PVA = 0 g;

b m PVA = 0.2 g;

c m PVA = 0.4 g;

d m PVA = 0.6 g;

e m PVA = 0.8 g;

f m PVA = 1.0 g; g m PVA = 1.2 g

and h m PVA = 1.5 g

0.0 4.0x103 8.0x103 1.2x104 1.6x104

8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8

Wavelength (nm)

603

a 0.05 W/cm 2

b 0.08 W/cm2

c 0.12 W/cm2

d 0.15 W/cm 2

e 0.19 W/cm2

f 0.24 W/cm2

g 0.26 W/cm2

h 0.31 W/cm2

i 0.33 W/cm2

a b c d e f g h i

440

Fig 9 PL spectra of ZnS:Mn/PVA (m PVA = 1 g) with different excitation powder densities

intensity, Iep is excitation power density, A is a constant and n is a coefficient, that depended on radiation nature By the linearization of this equation (inset of Fig.9), the calculated result for ZnS:Mn/PVA nanoparticles received the value n & 0.9, which is appropriate to the result of other authors (Chen et al.2004)

3.4 The photoluminescence excitation spectra and crystal field strength

In order to absorption transitions of Mn2? ions in ZnS:Mn/PVA nanoparticles and cal-culation of crystal field strength, PLE spectra investigated using excitation radiation of xenon lamp Figure10 show PLE spectra monitored at the yellow-orange PL band of ZnS:Mn/PVA nanoparticles with different PVA mass For ZnS:Mn nanoparticles, this spectrum appeared the peak about of 341 nm with great intensity and the other peaks at about of 395, 430, 468 and 492 nm with smaller intensity, in which the peaks of 468 and

492 nm appeared very clearly (Fig.10a)

The peak of 341 nm (3.633 eV) is attributed to the near edge absorption transition of ZnS crystals because the photon energy corresponding to this transition is very near the width of its band gap (Cadis et al 2010) The lines of 395, 430, 468 and 492 nm are assigned to the absorption transitions of the 3d5 electrons from6A1(6S) ground state to

4E(4D),4T2(4D),4A1(4G)–4E(4G) and4T2(4G) excited states of Mn2?ions in ZnS crystals (called Mn2?absorption band), respectively (Chen et al.2001) These results prove that

Mn2?ions were replaced on some positions of the Zn2?ions in ZnS crystal lattice This replacement ability is rather high because radii of Mn2?ion (0.89 A˚ ) very close to that of

Zn2?ion (0.88 A˚ )

For ZnS:Mn/PVA nanoparticles with PVA mass from 0.2 to 1.5 g, the intensity of near edge gap absorption band and the intensity of Mn2? absorption band increased and achieved maximum at PVA mass of 1 g (Fig.10b–e), then that decrease with the increasing of PVA mass to 1.5 g (Fig.10f, g) This result also is similar as PL spectra of ZnS:Mn/PVA nanoparticles (Fig.8) However, the peak position of the near gap

0.0 2.0x10 6

4.0x106 6.0x106 8.0x106 1.0x10 7

1.2x10 7

468 492

430

a b

c d e f g

Excitation wavelength (nm)

Excitation wavelength (nm)

345

468 492 395

a 0.0g

b 0.2g

c 0.4g

d 0.6g

e 1.0g

f 1.2g

g 1.5g

341

430

a b

c

f g

Fig 10 The PLE spectra monitored at the yellow-orange PL band of ZnS:Mn/PVA nanoparticles with different PVA mass: a m PVA = 0 g; b m PVA = 0.2 g; c m PVA = 0.4 g; d m PVA = 0.6 g; e m PVA = 0.8 g;

f m PVA = 1.0 g; g m PVA = 1.2 g and h m PVA = 1.5 g