Optical and photocatalytic properties of rare earth metal-modified ZnO quantum dots

Accepted Manuscript Optical and photocatalytic properties of rare earth metal-modified ZnO quantum dots Jakub Sowik, Magdalena Miodyńska, Beata Bajorowicz, Alicja Mikołajczyk, Wojciech Lisowski, Tomasz Klimczuk, Daniel Kaczor, Adriana Zaleska Medynska, Anna Malankowska PII: DOI: Reference:

S0169-4332(18)32520-0 https://doi.org/10.1016/j.apsusc.2018.09.104 APSUSC 40408

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

23 July 2018 10 September 2018 11 September 2018

Please cite this article as: J. Sowik, M. Miodyńska, B. Bajorowicz, A. Mikołajczyk, W. Lisowski, T. Klimczuk, D. Kaczor, A. Zaleska Medynska, A. Malankowska, Optical and photocatalytic properties of rare earth metal-modified ZnO quantum dots, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.104

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Optical and photocatalytic properties of rare earth metal-modified ZnO quantum dots Jakub Sowika, Magdalena Miodyńskaa, Beata Bajorowicza, Alicja Mikołajczyka, Wojciech Lisowskib, Tomasz Klimczukc, Daniel Kaczord, Adriana Zaleska Medynskaa, Anna Malankowskaa*

a

Department of Environmental Technology, Faculty of Chemistry, University of Gdansk, Wita

Stwosza 63, 80-308 Gdansk, Poland, b

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224

Warsaw, Poland, c

Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gdansk

University of Technology, G. Narutowicza 11/12, 80-233, Gdansk, Poland, d

Faculty of Chemistry, Nicolaus Copernicus University, 7 Gagarina, 87-100 Torun, Poland.

*

Corresponding author:

Tel.: (+48 58) 523 52 23, e-mail address: [email protected]

ABSTRACT

A series of novel ZnO quantum dots modified with rare earth metals was successfully prepared by a simple sol-gel approach. The effects of types (Eu, Er, Tb, Yb, Ho, La) and amounts (from 0.09 to 0.45 mmol) of lanthanides on the optical properties, structural characterization and photocatalytic activity of ZnO/RE QDs were systematically investigated. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) with energy dispersive X-ray analysis (EDX) and UV-Vis diffuse reflectance spectroscopy were used to characterize surface properties, while photoluminescence (PL) emission spectroscopy and UV-Vis- driven degradation of phenol in aqueous phase were applied to understand optical and photocatalytic properties. The experiments demonstrated that generally modification of ZnO QDs by lanthanides resulted in increase of photoactivity while photoluminesce decrease at the same moment. The highest photocatalytic activity among all obtained nanomaterials and the lowest photoluminescence quantum yield among ZnO/Er photocatalysts were observed for ZnO QDs modified with 0.09 mmol of erbium. Increased photocatalytic activity has been shown in successive samples: 1

ZnO/0.09_Er> ZnO/0.18_Eu> ZnO/0.09_La> ZnO/0.27_Er> ZnO/0.18_Tb> pristine ZnO QDs. The experimental results were supported by calculations based on density functional theory (DFT) which revealed atomic and electronic structure of Er/La modified ZnO QDs.

1. INTRODUCTION Quantum dots (QDs) structures contain from 200 to 10,000 atoms and due to high surface-tovolume ratio QDs show unique features, since surface atoms affect catalytic, electronic, optical, magnetic properties of nanomaterials [1, 2]. QDs possess unique properties, such as long time of radiance emission, high quantum yield (QY), narrow emission and broad absorption spectra [1, 3, 4]. The absorption range of QDs can be easily tuned by adjusting their size due to their unique size- and shape- dependent optical properties [5]. Up to date, the most of the commonly used QDs in photocatalysis are made of cadmium eg. CdS [6, 7], CdSe [8-13], CdTe[14, 15]. These kind of QDs showed good photo-stability narrow band gap and high quantum yield, but their high toxicity and potential pollution hazard as the released Cd+ from the surface of QDs might limit their applications [16]. ZnO QDs in contrast to for e.g. CdS QDs are non-toxic as well as environment friendly, comparatively cheap and chemically stable [17]. Unfortunately, ZnO QDs have also drawback, they can be excited only by UV light [18, 19]. This ZnO shortcoming can be improved by modifications[20] eg. doping or loading [21-25]. Recently, optical properties of Co [17], Mg [26], Cd [27, 28], Sn [29, 30], Cu [31, 32], Ag [30], P [33] doped ZnO QDs have been studied. The transition metal doped materials has poor thermostability and can generated recombination centers of electrons and holes [34]. Currently, rare earth (RE) metals with unique optical properties and electronic structure have been used as dopants [35]. Trivalent lanthanide ions with ladder-like energy levels embedded in an inorganic host lattice can emit UV or visible irradiation through the sequential absorption of multiple near-infrared photons[36]. This phenomenon, which can result in the conversion of light from the NIR and vis range to the UV wavelengths, could be used to excite wide band semiconductor, eg. ZnO [37]. The luminescence properties of lanthanide ions results from the f–f electronic transitions within their partially filled 4f orbitals. These orbitals are sterically shielded from the surrounding microenvironment by the filled 5s and 5p orbitals, meaning that there are almost no perturbations of these transitions [38, 39]. Moreover, modification with RE3+ prevents the electron-hole recombination process[40]. Recently, RE3+ doped wide band gap semiconductor nanoparticles in heterogeneous photocatalysis, were investigated[41]. To 2

our best knowledge, there has not been a study on the photocatalytic applications of ZnO/RE quantum dots. The current research mainly focuses on the study of photoluminescence properties of ZnO/RE. Liu et al. prepared Gd-doped ZnO QDs with 4 nm size as dual modal fluorescence and magnetic resonance imaging nanoprobes [42]. The Gd doped ZnO QDs exhibited significantly enhanced yellow emission due to the Gd doping. They also demonstrated that HeLa cells could be successfully imaged in short time and stay high viability even at high dose of QDs [42]. Li et al. confirmed the energy transfer between Pr 3+ ions and ZnO QDs by the enhancement of the PL emission peaks at 578, 630, and 752 nm in doped Pr3+ ZnO QDs [43]. Hong et al. revealed that the most of Pr3+ ions distribute on the surface of ZnO QDs while a few of them penetrate into the ZnO lattice to substitute Zn2+ which causes the lattice distortion and the change of the crystal size [44]. Sun et al. demonstrated that quantum yield (QY) increased from 30.5% for pristine ZnO QDs to 77.9% for La-doped ZnO QDs. They prepared anti-counterfeiting inks by incorporating La-doped ZnO QDs into the transparent oil [45]. Considering the points in the above discussion, the main purpose of this study was to obtain ZnO QDs loaded with rare earth metals with enhanced optical and photocatalytic properties. The effects of types (Eu, Er, Tb, Yb, Ho, La) and amounts (0.09- 0.45 mmol) of lanthanides on the optical properties, chemical characteristics as well as on the photocatalytic activity of the ZnO/RE QDs under UV−Vis, were systematically investigated. Moreover, theoretically based investigation confirmed experimentally obtained results described in presented study. 2. EXPERIMENTAL SECTION 2.1. Materials and instruments Zinc acetate dihydrate (Zn (Ac)2·2H2O, >98% ACROS ORGANICS) was used as precursor for the preparation of ZnO QDs. The Eu(NO3)3·5H2O (99.9%), Ho(NO3)3·5H2O (99.9%), Tb(NO3)3·5H2O (99.9%), Yb(NO3)3·5H2O (99.9%), Er(NO3)3·5H2O (99.9%), La(NO3)3·6H2O (99.99%) were purchased from Sigma-Aldrich and were used as precursors of rare earth metals. 3-aminopropyltriethoxysilane (APTES) (>98% Sigma Aldrich), KOH (pure p.a., POCH S.A.) and ethanol (> 99,8% and 96% POCH S.A.) were used for synthesis of QDs. Powder X-ray diffraction (PXRD, Philips/PANalytical X'Pert Pro MPD diffractometer, (Cu Kα radiation λ = 1.5418 Å) was used to determine the phase composition of ZnO/RE QDs. A profile analysis (LeBail method) was performed by using HighScore Plus software. X-ray photoelectron spectroscopic (XPS) measurements were performed using the PHI 5000 VersaProbe (ULVACPHI) spectrometer with monochromatic Al Kα radiation (hν=1486.6 eV). The X-ray source, 3

operating 25 W and 15 kV, was focused on 100 µm spot size, and the analyzed area was defined as a 250 µm x 250 µm. The high-resolution (HR) XPS spectra were collected with the hemi-spherical analyzer at the pass energy of 23.5 eV, the energy step size of 0.05-0.1 eV and the photoelectron take off angle 45° with respect to the surface plane. The binding energy (BE) scale of all detected spectra was referenced by setting the BE of the aliphatic carbon peak (C-C) signal to 284.8 eV. The size and elemental composition of the obtained QDs was investigated with transmission electron microscopy (TEM-EDX, FEI Europe, model TecnaiF20 X-Twin). FT-IR spectra were carried out on a Bruker model IF S66 FTIR spectrometer using KBr discs. The DRS UV-Vis absorption spectra were recorded on Evolution 220, Thermo Scientific spectrometer. The photoluminescence (PL) emission spectra were measured using a PerkinElmer Luminescence Spectrometer LS 50B. The samples were excited with 340 nm wavelength light and the emission was measured between 300–800 nm. The quantum yield (QY) was measured using a solution of Rhodamine 6G in ethanol (QY=95%) as a reference material and was calculated according to the following equation:

where: QY - is the fotoluminescence quantum yield, A - the value of the absorbance at the excitation wavelength, F - the fluorescence intensity (the area of the fluorescence spectrum), n the refractive index of the solvents used. The subscripts ref and sample refer to the reference (Rhodamine 6G) and the QDs, respectively.

2.2. Preparation of RE-modified ZnO QDs ZnO/RE QDs have been obtained by sol-gel method. 1.86 mmol of zinc acetate dihydrate (Zn(Ac)2·2H2O) and appropriate amount of rare earth metals precursors (0.09-0.45 mmol) was dissolved in 10 ml ethanol (99,8%) in a flask under vigorous stirring and refluxed for 90 min at 68°C (sample A). Next, the solution was cooled down to room temperature. KOH was dissolved in 5 ml of ethanol and kept in an ultrasonic bath for 40 min (sample B). The obtained KOH solution was added dropwise to the solution A and was stirred for 60 minutes. After that, 0.5 ml of deionized water and 0.34 mmol of 3-aminopropyltriethoxysilane (dissolved in 5 ml of ethanol) were added to the obtained solution and was stirring for 120 minutes. APTES was used as a capping agent to avoid aggregation as well as prevent further growth. QDs powder was centrifuged, washed with toluene and then washed using ethanol (96%). The final QDs

4

powders were re-dispersed in ethanol for further characterization. Nonmodified ZnO QDs was obtained using the same method but without the rare earth metal precursors. 2.3. Measurement of photocatalytic activity under UV-Vis irradiation The photoactivity of obtained samples was measured in phenol aqueous phase as model contaminant under UV-Vis irradiation (the source of irradiation was Xe lamp (Oriel, 1000 W). 12.5 mg of photocatalyst was dispersed 25 mL of phenol solution (C 0=20 mg/L) in cylindrical quartz reactor – without irradiation for the first half hour in order to achieve adsorption balance between solid particles and phenol molecules. Then the blank sample was taken and the measuring system was illuminated for one hour. Control samples with phenol solution were taken after every 20 minutes and passed through syringe filters with pore size around 0.2 µm. The photocatalytic system was also aerated (5 L/h), stirred (600 rpm) and cooled to 10°C during measurements. The phenol concentration in samples was determined by colorimetric method after coupling with diazo-р-nitroaniline with using UV–vis spectrophotometer (λmax=480 nm). Photolysis of phenol in the absence photocatalysts in the same conditions was also carried out in order for proving photoactivity of certain obtained quantum dots. 2.4.Theoretical modeling We have performed plane-wave calculations using density-functional theory (DFT) implemented into Vienna ab initio Simulation Package (VASP) [46, 47]. The exchangecorrelation potential has been approximated by the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional [48-51]. All structures have been developed within supercell geometry using the periodic boundary conditions (PBC). The PBE computed optimal lattice parameters for bulk ZnO were a=b=3.25Å, c=5.23Å. The ZnO surface was modeled with six O-Zn-O layers and 15 Å vacuum thickness between slabs among (101) direction. All the atoms in the system are allowed to relax except the two bottom O-Zn-O layer that is kept fixed to mimic the bulk structure. To investigate the metal adsorption on pristine ZnO, we employed a c(4x2) surface supercell with 96 atoms in order to avoid interaction between clusters and defects in neighboring cells. The geometry of these supercells allows having equidistant separation between image clusters and defects. The Brillouin zone was integrated using a  centered Monkhorst-Pack grid with 0.032 Å-1 separation 0.03 Å-1 separation (equivalent to a k-points mesh of 4 × 4 × 1 for supercell) and a cut–off energy of 700 V, the total energy was converged to less than 1meV/atom. All the structures considered in this study were fully relaxed until all the forces were <0.02 eV Å -1 [48, 52, 53]. The pseudopotential and core electrons for O-1s2-2p4, Zn-4s2-3d10 and Er 5p65s24f126s2 were described by projector augmented wave potential (PAW) [53-55]. The surface and subsurface 5

vacancy defects in ZnO system were generated then modified by La-/Er-atoms and then modeled with use of 4 × 4 × 1 Er-/La-ZnO supercell (96 atoms). 3. RESULTS AND DISCUSSION 3.1. Morphology and distribution of rare earth metal Based on the microscopic analysis we can assume that the ZnO/RE QDs were mostly spherical in shape with agglomeration (Fig. 1). ZnO/RE QDs with average diameters in the range of 49 nm were formed. We have not seen correlation between type/amount of RE modifier and size of QDs. The ZnO/RE quantum dots were made by hydrolysis of Zn(Ac)2·2H2O in the presence of KOH in ethanol and at this stage of process preliminary formed ZnO cores are coated by a Zn(OH)2 shell. Addition of APTES into the reaction system resulted in partial hydrolysis of alkoxyl groups and one group would react with the –OH group on the ZnO surface to assemble a capping layer containing unreacted –NH groups on the surface, which prevents agglomeration and destabilization [56]. Additionally, RE ions are deposited on the surface of the ZnO or in the crystal lattice which hinders the movement of the grain boundary and limits the growth of crystal grains[45]. Hydrolysis reaction leads to the formation of nuclei or basic units, but the condensation reaction leads to particle growth[57]. APTES was cross-linked on the surface was used as a capping agent to avoid aggregation as well as to control the particle growth. Sun et al. [45] showed that the average particle size of ZnO QDs doped with La obtained by sol-gel method becomes slightly smaller as the La doping amount increases. It could be related to the formation of La–O–Zn bonds on the surface of La doped-ZnO. The EDX analysis confirmed the presence of RE metal in all studied samples, eg. Eu content in ZnO/Eu_0.18 QDs was in the range 4.34- 5.34 % wt. It has been shown that the rare earth metals were well dispersed on the surface of the ZnO quantum dots. Furthermore, for ZnO/Tb_0.18 QDs the terbium was heterogeneously distributed over the ZnO QDs surface with places with a high concentration of deposits and others relatively empty. Additionally, EDX analysis showed that the quantum dots are also composed of O, C, K and Si elements (not presented here). The existence of the Si signal is due to APTES, the K signal is due to KOH and C can be ascribed to APTES and CH3COO-.

6

Figure 1. TEM images and EDX mapping of (a) ZnO/Eu_0.18, (b) ZnO/Tb_0.18, (c) ZnO/Yb_0.18, (d) ZnO/Ho_0.18, (e) ZnO/Er_0.45, (f) ZnO/La_0.45.

7

Table 1. Preparation method, optical properties and photocatalytic activity of ZnO and ZnO/RE QDs Lp.

Sample label

Type of RE

Type of precursor

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

ZnO ZnO/Eu_0.18 ZnO/Ho_0.18 ZnO/Tb_0.18 ZnO/Yb_0.18 ZnO/Er_0.09 ZnO/Er_0.18 ZnO/Er_0.27 ZnO/Er_0.36 ZnO/Er_0.45 ZnO/La_0.09 ZnO/La_0.18 ZnO/La_0.27 ZnO/La_0.36 ZnO/La_0.45

no dopant europium holmium terbium ytterbium erbium erbium erbium erbium erbium lanthanum lanthanum lanthanum lanthanum lanthanum

Eu(NO3)3·5H2O Ho(NO3)3·5H2O Tb(NO3)3·5H2O Yb(NO3)3·5H2O Er(NO3)3·5H2O Er(NO3)3·5H2O Er(NO3)3·5H2O Er(NO3)3·5H2O Er(NO3)3·5H2O La(NO3)3·6H2O La(NO3)3·6H2O La(NO3)3·6H2O La(NO3)3·6H2O La(NO3)3·6H2O

Amount of precursor of RE (mmol) 0 0.18 0.18 0.18 0.18 0.09 0.18 0.27 0.36 0.45 0.09 0.18 0.27 0.36 0.45

Intensity of PL

Wavelength for max. PL (nm)

138 110 114 107 137 121 165 162 221 205 87 170 239 207 239

545 545 544 545 544 545 541 544 545 549 553 548 539 539 545

Excitation wavelength (nm)

Quantum yield (QY) [%]

Efficiency of phenol degradation after 60 min. (%)

Reaction rate constant (µmol*dm-3*min-1)

340

48.7 37.2 39.4 35.2 48.2 42.8 54.4 56.0 75.4 72.1 31.0 53.2 80.5 71.3 80.9

77.2 86.7 76.5 79.3 60.7 89.9 68.3 82.0 69.8 69.8 82.3 78.5 75.8 77.3 62.1

0.0246 0.0337 0.0241 0.0260 0.0157 0.0366 0.0191 0.0285 0.0197 0.0200 0.0290 0.0253 0.0229 0.0246 0.0162

8

3.2. Optical properties Fig. 2 a, b, c displays the room temperature PL spectra of ZnO quantum dots modified with various types and amounts of rare earth metals, and pristine ZnO QDs as a reference at an excitation wavelength of 340 nm. The PL spectra of all obtained samples exhibited green emission bands centered at about 545 nm, which originate from surface defects and oxygen vacancies of QDs [58, 59] (Table 1). Very small peak at about 380 nm present in all spectra can be ascribed to the emission from conduction band to valence band of ZnO [60]. A comparison of PL spectra of Eu, Ho, Tb, Yb, Er and La-modified ZnO nanocrystals with the same concentration of RE modifiers (Fig. 2a) indicated that incorporation of lanthanum and europium into ZnO enhanced PL intensity whereas loading of holmium, terbium and yterbium caused lower PL intensity compared to unmodified ZnO sample. This tendency is in accordance with the PL quantum yield calculated for ZnO/RE samples (Table 1). Fig. 2b shows PL spectra of ZnO/La samples with different amounts of lanthanum (from 0.09 to 0.45 mmol). The ZnO/La_0.09 exhibited much lower PL intensity as compared to pristine ZnO. However, modification of ZnO by 0.18 or more mmol of La caused increaed in PL emission intensity. The highest PL quantum yield: 80.9% and 80.5% was achieved for ZnO/La_0.45 and ZnO/La_0.27 samples, respectively and these were the highest QY values among all obtained rare earth modified ZnO samples. The large difference of ion radius between Zn2+ and RE ions leads to the formation of stress in the lattice, and the interaction between RE ions and ZnO QDs can weaken Zn–O bonds[61]. Therefore, the incorporation of La ions results in a higher concentration of defects, which increases the effective luminescence centers and thus makes the quantum yield significantly improved. Sun et al. [45] reported that QY obtained for ZnO/La QDs first increased and then decreased with increasing amount of lanthanium. They observed the highest QY (77.9%) for sample containing 10% molar ratio of La [45]. The excessive RE ions might be deposited on the surface of the doped samples, which lowers the concentration of surface defects, thus resulting in a decrease of the fluorescence intensity[42, 45]. Fig. 2c shows PL spectra of ZnO/Er samples with different amounts of erbium (from 0.09 to 0.45 mmol). Similarly, to ZnO/La samples, modification of ZnO with small amount of rare earth metal (0.09 mmol of Er) influenced lower PL intensity whereas loading of 0.18, 0.27, 0.36 or 0.45 mmol of Er enhanced ZnO photoluminescence properties. The highest quantum yield (75.4%) among ZnO/Er QDs was achieved for sample containing 0.36 mmol of erbium. In general, obtained results 9

both for ZnO/La and ZnO/Er samples suggested that modification of ZnO with amount higher than 0.09 mmol of rare earth metal influenced higher PL quantum yield than that of pristine ZnO QDs.

(a) PL Intensity (a.u.)

ZnO ZnO/Eu_0.18 ZnO/Ho_0.18 ZnO/Tb_0.18 ZnO/Yb_0.18 ZnO/Er_0.18 ZnO/La_0.18

300

400

500

600

700

800

Wavelength (nm)

(b)

(c)

300

400

500

600

Wavelength (nm)

700

ZnO ZnO/Er_0.09 ZnO/Er_0.18 ZnO/Er_0.27 ZnO/Er_0.36 ZnO/Er_0.45

PL Intensity (a.u.)

PL Intensity (a.u.)

ZnO ZnO/La_0.09 ZnO/La_0.18 ZnO/La_0.27 ZnO/La_0.36 ZnO/La_0.45

800

300

400

500

600

700

800

Wavelength (nm)

Figure 2. Effect of (a) type of rare earth metal (Eu, Ho, Tb, Yb, Er and La), (b) amount of

lanthanum (0.09-0.45 mmol of La), (c) amount of erbium (0.09-0.45 mmol of Er) on the PL properties of ZnO quantum dots.

DRS UV-Vis absorption spectra of the synthesized ZnO/RE quantum dots are presented in Figure 3 a, b, c. In general, all RE-modified ZnO quantum dots revealed maximum absorption peak at about 340 nm. Fig. 3a shows the spectra of ZnO modified with various types of RE (Eu, Ho, Tb, Yb, Er and La). For La-modified ZnO QDs a small blue shift can be observed 10

whereas ZnO/Yb and ZnO/Tb are red-shifted to the longer wavelengths compared to pristine ZnO QDs. In the case of ZnO/La samples (Fig. 3b), loading of 0.09 mmol of lanthanum significantly extended the absorption properties of ZnO/La into the visible region. However, ZnO/La samples containing higher amounts of lanthanum (from 0.18 to 0.45 mmol) exhibited similar absorption profiles as unmodified ZnO (Fig. 3b). As can be seen from Fig.3c, in the case of ZnO/Er samples containing 0.09 and 0.27 mmol of erbium bathochromic effect can be observed whereas absorption spectra of ZnO/Er_0.18 and ZnO/Er_0.36 samples are shifted to shorter wavelength compared to pristine ZnO. It indicated that there was no correlation between amount of RE modifier and absorption properties of ZnO/RE QDs.

Absorbance (a.u.)

(a)

0

ZnO ZnO/Ho_0.18 ZnO/Tb_0.18 ZnO/Yb_0.18 ZnO/Er_0.18 ZnO/La_0.18

300

400

500

600

700

Wavelength (nm)

(b)

300

Absorbance (a.u.)

Absorbance (a.u.)

(c) ZnO ZnO/La_0.09 ZnO/La_0.18 ZnO/La_0.27 ZnO/La_0.36 ZnO/La_0.45

400

0

500

Wavelength (nm)

600

700

ZnO ZnO/Er_0.09 ZnO/Er_0.18 ZnO/Er_0.27 ZnO/Er_0.36 ZnO/Er_0.45

300

400

500

600

700

Wavelength (nm)

Figure 3. DRS UV-Vis absorption spectra of (a) ZnO/RE QDs (RE=Eu, Ho, Tb, Yb, Er or

La), (b) ZnO/La with different amounts of lanthanum (0.09-0.45 mmol), (c) ZnO/Er with different amounts of erbium (0.09-0.45 mmol).

11

3.3.XRD analysis The powder X-ray diffraction (PXRD) patterns are presented in Fig. 4 a-l. There are two phases detected: ZnO and KNO3 impurity originating from the reaction of RE(III) nitrate pentahydrate and KOH. The LeBail method was used to analyse PXRD data and estimated lattice parameters for ZnO (hex. P 63 m c, s.g. #186) and KNO3 (orth. P n m a, s.g. #62) are gathered in Table 2. Fig. 4a presents the PXRD pattern for ZnO / 0.09 mmol La. Presence of ZnO is confirmed by broad reflections at 2 around 32 and 36 degrees. A reference pattern for unmodified ZnO is shown in panel (h). As the La concentration increases, a relative amount of KNO 3 increases and hence ZnO xrd reflections become smaller – see panels (b - d). Similar situation is observed for ZnO/Er series presented in (e - g). There is no significant change of the ZnO lattice parameters in both the ZnO/La and ZnO/Er series. Fig. 4i-l present patterns for ZnO / RE, where RE = La (f), Eu (j), Tb (k), Ho (l). Although the same RE concentration (0.18 mmol) was used for the synthesis process, a relative amount of ZnO is the highest for ZnO/Eu sample, and the lowest for ZnO/Tb sample. The estimated a lattice parameter for the samples in the ZnO/RE series does not change and is close to a = 3.2473(1) Å obtained for the unmodified ZnO. The c lattice parameter is the largest for ZnO/La sample (c = 5.198(3) Å) and the smallest for ZnO/Tb (c = 5.179(4) Å). A clear trend of the c value is expected for the lanthanide bearing compounds (lathanide contraction) and

Intensity (arb. u.)

confirms successful modification of the ZnO QD.

ZnO / 0.27 mmol Er

(f)

ZnO / 0.36 mmol Er

ZnO / 0.18 mmol Tb

(g)

ZnO / 0.45 mmol Er

ZnO / 0.18 mmol Ho

(h)

ZnO

ZnO / 0.18 mmol La

(j)

ZnO / 0.18 mmol Eu

ZnO / 0.36 mmol La

(k)

ZnO / 0.45 mmol La

(l)

ZnO / 0.09 mmol La

(b)

ZnO / 0.18 mmol La

(c)

(d)

20

(e)

(i)

(a)

40

60

2 (deg)

80

20

40

60

2 (deg)

80

20

40

60

2 (deg)

12

80

Figure 4. Powder X-ray diffraction patterns and LeBail analysis for (a)-(d) ZnO/La QDs, (e)(g) ZnO/Er QDs and (f)-(l) ZnO/RE QDs. A PXRD pattern for unmodified ZnO is shown in panel (h). The black circles represent data points, a solid red line is a LeBail profile fit and the vertical bars show positions of the expected Bragg peaks for ZnO and KNO 3, respectively. Table 2. The lattice parameters for ZnO and KNO 3 Sample label ZnO/La_0.09 ZnO/La_0.18 ZnO/La_0.36 ZnO/La_0.45 ZnO/Er_0.27 ZnO/Er_0.36 ZnO/Er_0.45 ZnO/Eu_0.18 ZnO/Tb_0.18 ZnO/Ho_0.18 ZnO

ZnO a (Å) 3.2496(3) 3.251(1) 3.249(4) 3.27(1) 3.258(2) 3.260(4) 3.253(3) 3.252(1) 3.251(2) 3.250(2) 3.2471(1)

c (Å) 5.1970(7) 5.198(3) 5.18(2) 5.16(3) 5.149(9) 5.16(2) 5.128(4) 5.195(1) 5.183(5) 5.179(4) 5.2048(4)

a (Å) 6.4259(2) 6.4294(1) 6.4344(1) 6.4314(1) 6.4294(3) 6.4268(2) 6.4302(1) 6.4293(1) 6.4262(1) 6.4344(1) ---

KNO3 b (Å) 5.4086(2) 5.4113(1) 5.4156(1) 5.4134(1) 5.4134(2) 5.4096(2) 5.4121(1) 5.4114(1) 5.4109(1) 5.4160(1) ---

c (Å) 9.1554(3) 9.1596(2) 9.1662(2) 9.1622(2) 9.1628(3) 9.1570(2) 9.1604(2) 9.1595(2) 9.1587(2) 9.1664(2) ---

3.4. XPS analysis The elemental composition (in at. %) in the surface layer of pristine ZnO and RE (Eu, Ho, Tb, Yb, Er and La) doped ZnO QDs was evaluated by XPS. The samples with the same nominal concentration of RE modifiers (0.18 mmol) were analyzed. The HR XPS spectra of Zn 2p and all modified RE elements are presented in Fig. 5a and Fig. 5b, respectively. The Zn 2p spectrum shows the peaks at BE of 1021.3 and 1044.4 eV, which are described to the core level Zn 2p3/2 and Zn 2p1/2 signals, respectively [62]. The presented data confirm the effective modification of the RE and reveal their different surface concentration (Table 3). The Eu 3d spectrum exhibits two Eu 3d5/2 and Eu 3d3/2 doublet components. The first one, representing by Eu 3d5/2 signal at 1134.8 eV, is assigned to Eu3+ species[63-67] whereas the second component (signals at 1124.8 eV and 1156.1 eV) is interpreted as Eu2+ species [64, 65] or as the shake-down satellites of both Eu 3d5/2 and Eu 3d3/2 peaks [66, 67]. The Ho 4d, Tb 3d5/2, Yb 4d, Er 4d and La 3d5/2 peak positions located at 161.3 eV, 1239.6 eV, 186.6 eV, 169.0 eV and 835.0 eV, respectively (Fig. 5b), indicate the formation of Ho3+ [68-70], Tb3+ [71, 72], Yb3+ [73, 74], Er3+ [68, 73] and La3+ [61] species, respectively. The Yb 4d signal at 181.7 eV evidences also the small contribution of Yb0 in addition to the Yb3+ [62]. The separation distance between both La 3d doublet peaks and their satellite signals is found to be 3.6 eV (Fig. 5b), what suggest that the La(OH) 3 is a dominant form of La compounds formed in the surface region of La doped ZnO QDs [61].

13

The surface contents of Zn and RE atoms is evidently affected by contaminations originated from preparation processing like carbon, nitrogen, potassium and silicon (Table 3). However, the RE/Zn atomic ratio values indicate the different surface distribution of RE atoms in the ZnO matrix (see column RE/Zn in Table 3). In Fig. 5c we show the Zn 3d and the valence band (VB) XPS spectra taken for all REmodified ZnO QDs. The different BE shift is observed in the Zn 3d and the valence band spectra. This chemical shift can be assigned to RE dopants interaction with the ZnO lattice and also can be result of coexistence of surface contamination remaining after synthesis (KNO3).

Figure 5. HR XPS spectra of (a) Zn 2p and (b) all RE elements, (c) Zn3d and the valence band (VB) XPS spectra for ZnO/RE_0.18 QDs.

14

Table 3. The elemental composition (in at. %) in the surface layer of pristine ZnO and RE (Eu, Ho, Tb, Yb, Er and La) modified ZnO QDs. Sample ZnO ZnO/Eu_0.18 ZnO/Ho_0.18 ZnO/Tb_0.18 ZnO/Yb_0.18 ZnO/Er_0.18 ZnO/La_0.18

Zn (at %) 48.22 30.77 25.70 30.11 9.48 26.70 37.89

O (at %) 45.22 47.40 47.10 45.41 50.58 39.21 45.54

Eu 0.82 -

Lanthanide compounds (at %) Ho Tb Yb Er La 0.54 1.20 0.26 0.58 0.70

Contaminants (N, C, K, Si) 6.57 21.00 26.68 23.27 39.68 33.51 15.87

RE/Zn 0.0266 0.0210 0.0399 0.0274 0.0217 0.0185

3.5.FTIR analysis The FTIR analysis shows that the obtained samples were correctly coated with APTES ((3aminopropyl)triethoxysilane). This is evidenced by presence of two very weak minimums in the range 1050 – 1000 cm-1 are derived from siloxanes [75]. In addition, the band at 825.34 cm-1 may indicate the presence of a bond carbon – silicon. It proves success of coverage ZnO quantum dots with APTES. In addition, hydroxyl (-OH) and amine or amide (NHx) groups were presented in the analyzed samples, as evidenced by the band at the wave number 3421.97 cm- 1 [76]. The bands at approximately 2800 – 2700 cm-1 were due to the presence of > NH2+ or ≡ NH+. The absorption peak at 1765cm−1 could be attributed to the carbonyl (>C=O) groups, and peak at 1385 cm-1 to amide groups. On the analyzed spectra is visible contaminations such as water vapor (a series of very weak peaks in the range from 3775 cm-1 to 3550 cm-1) and CO2 (series of pulses in the range of 2600 – 2380 cm-1 and 700 – 650 cm-1) both were present in the chamber of spectrophotometer. The analyzed samples were formed into tablets made of KBr. Presence of KBr is proven by two bands in the range 3000– 2850 cm- 1.

(b)

4500

Transmittance

Transmittance

(a)

ZnO/Er_0.09 ZnO/Er_0.18 ZnO/Er_0.27 ZnO/Er_0.36 ZnO/Er_0.45 ZnO

4000

3500

3000

2500

2000

1500

1000

500

4500

ZnO/La_0.09 ZnO/La_0.18 ZnO/La_0.27 ZnO/La_0.36 ZnO/La_0.45 ZnO

4000

3500

3000

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2000

1500

1000

500

-1 Wavenumber (cm )

-1 Wavenumber (cm ) 0.0

Figure 6. FTIR spectra of (a) ZnO/Er, (b) ZnO/La samples. 15

3.6.

Photocatalytic activity

The photocatalytic efficiency of all as-prepared samples was examined in the decomposition of phenol solution under visible light as well as in the entire spectrum of UV-Vis irradiation emitted by the Xe lamp. None of the photocatalysts obtained showed activity in the range of low-energy visible irradiation. In turn, in the UV-Vis range, the activity of pristine ZnO and increased ZnO activity were demonstrated when modification with certain amounts of various rare-earth metals. Fig. 7 presents a graph of photocatalytic degradation of phenol in the presence of both pristine ZnO and ZnO/RE photocatalysts. It is clearly visible that the reaction kinetics in the presence of these photocatalysts are similar. The degradation efficiency results for these and other samples obtained are summarized in Table 1.

Figure 7. Photocatalytic degradation of phenol solution in the presence of ZnO/RE quantum dots under UV-Vis irradiation.

To understand the effect of dopant type and amount, ZnO QDs doped with various mole amounts of erbium and lanthanum ions and ZnO QDs doped with 0.18 mmol of europium, terbium, ytterbium or holmium ions have been synthesized. The sample which exhibited the highest activity is ZnO/0.09_Er and the reaction efficiency is almost 90%. The modification of ZnO by doping with erbium ions is quite often observed in the literature probably because of the facile of occupying substitutional Zn sites by this type of elements. Moreover, the 16

incorporation of erbium ions into the ZnO crystal lattice makes it possible to reduce the width of the band gap of this semiconductor and to make it excited by lower energy irradiation. However, an increase in the amount of dopant showed a decrease in ZnO photoactivity. Furthermore, the luminescence spectra of ZnO/Er (Figure 2c) showed that only one sample with Er, ZnO/0.09_Er exhibits lower intensity than pristine ZnO QDs. This trend is sometimes combined with photoactivity because the energy absorbed by the photocatalyst is more effectively used in the photocatalytic process and not immediately lost in the way of emitting luminescence. That effect is probably responsible with increased activity of ZnO/0.09_Er phototocatalyst. Erbium forms the most frequently trivalent Er3+ ions and the more precursor will be used during the synthesis, the more ions present. Those that are located on the surface of ZnO particles can be reduced to Er 0 during the electron accumulation in the ZnO conduction band over excitation. Therefore, increasing the amount of erbium is associated with the concentration of excitons and an increase in the intensity of luminescence is observed. According to the theory combining luminescence with photocatalytic activity, this is not a favorable effect. A similar mechanism of creating metallic clusters Yb0 formed on the ZnO surface by electron reduction can be observed for Yb3+. The similar relationship between photoactivity and intensity of luminescence as in erbium dopant case can be observed in the lanthanum doping. Only the smallest amount of metal ions 0.09 mmol reduces the intensity of luminescence and increases photocatalytic activity to refer to pristine ZnO QDs. Furthermore, doping ZnO by lanthanum ions is only theoretical because in earlier literature was confirmed that the diameter size of its are too huge to fit in the crystal structure of ZnO in exchange for an oxygen or zinc atom and doping may occur to a small extent on the surface [60, 77]. The XPS analysis confirmed also the presence of La(OH) 3 as main surface La compound, which probably constituted clusters on QDs surface. La(OH)3 is formed at the stage of synthesis, La3+ ions derived from the dissolved lanthanum nitrate found in the reaction medium combine with hydroxyl ions derived from dissolved KOH. As a result, the lanthanum ions, unable to be incorporated into the ZnO crystal structure, are precipitated in the form of a hydroxide. In this way, a by-product of the synthesis of KNO3 (potassium cations from hydroxide and nitrate anions from the lanthanide precursor) is also formed. Moreover, the XRD analysis demonstrated strong correlation between the decreasing signal on pattern from ZnO along with the increase of the La doping in the sample. In fact, these are the main reasons why the greater amount of lanthanum in the sample caused decreased of their activity. A similar relationship between the luminescence spectra and photocatalytic activity can be observed for the ZnO/0.09_Er, ZnO/0.18_Tb and ZnO/0.18_Eu samples. PL 17

signals and their intensity are closely related to its photocatalytic activity, because PL signals of semiconductor materials result from the recombination of photo-induced charge carriers. Generally, we observed that modification of ZnO QDs by lanthanides resulted in increase of photoactivity while photoluminesce decrease at the same moment. The metal basically acts as electron sink and thereby results in the suppression of electron-hole recombination. In this case, fewer electrons would come back to the valence band and would emit lesser energy in the form of light. Subsequently, the PL intensity is lower, the separation rate of photo-induced charges is higher the and the photocatalytic activity is also higher [78]. Based on the available scientific literature and research, a probable photocatalytic mechanism has been created and showed in Fig. 8. Modification with rare earth metals makes additional electronic states that allow additional electron transitions from the VB ZnO. Under the influence of irradiation, the electron-hole pairs are generated in VB and CB, respectively. Photoexcited electrons in CB ZnO reduce RE3+ ions to RE2+ form. The electron is trapped in RE2+ and then is transferred to the O2 molecules promoting •O2 formation. Also, holes are involved

in the creation of reactive oxygen species because in the reaction with oxygen molecules or hydroxyl anions they form hydroxyl radicals. These reactive oxygen species take part in decomposition process of pollutants.

Figure 8. Proposed mechanism of photocatalytic reactions in the presence of ZnO_Er QDs under UV-Vis irradiation. In literature can be found information about application ZnO/RE in photocatalysis for reaction of contaminants decomposition but the main particles size of doped ZnO was much bigger than quantum dots. However, the part of another researches where prepared and characterized 18

ZnO nanoparticles with different RE suggest nearby similar results in activity and using metal as dopant [77, 79, 80]. Summarizing, modification of photocatalysts with lanthanide ions improves the absorption properties of the semiconductor surface, facilitates its band excitation and prevents the recombination of electron-hole pairs, simultaneously.

3.7.Theoretical study The atomic structures and the electronic energy bands together with the partial density of states (PDOS) for pristine ZnO are presented in Fig. 9. While the Fig. 10 displays the most stable atomic structures and electronic configuration of surface modified Er-/La-ZnO(001) system with zinc vacancy (VZnsur1) site in ZnO(001) surface (Figure 9). The obtained results are in agreement with XPS analysis (see part 3.4). In the Er-ZnO(001) configuration, the the La-ZnO(001) configuration, the closest atoms distans (dLa-Otop) is 2.4 Å (Fig. 10a), while in Er-ZnO(001) the Er atom is the closest to the top most O atoms, where the minimum distance dEr-Otop is 2.1 Å (Fig. 10a), respectively. The theoretical study shown as Fig. 10a, b confirm surface modification of rare-earth metals at the ZnO(001) surface (Fig. 10a, b). The computed adsorption energies (>-0.2 eV) of RE metals at ZnO(001) surface as well as atoms distance (dRE-Otop> 2.1 Å) are suggesting that the considered rare-metals are only physisorbed onto pristine ZnO(001) surface. The computed electronic structures of pristine ZnO(001) and Er/La-ZnO(001) structures are displayed in Fig. 9a, 10a and 10b, respectively. The first computed PBE electronic structure of La-ZnO(001) configuration (Fig. 10a) is displayed in Figure 10c. The system has a semiconductor ground state with Zn-3d and O-2p states populating the upper valence band (Figure 10c). The main state (near the Fermi level EF) in the lower part of the conduction band is occupied mainly by La-3d/4f states, with small contribution of Zn-3d states. The outcome of our calculations indicates also that the most active systems Er-ZnO system (~90% of phenol removal under UV, Table 1) has a semiconductor ground state with Er-3d/4f states populating the band gap region of defective ZnO(001) surface (Figure 10d). It can be expected that improved photocatalytic properties of the most active sample ZnO/0.09_Er may be relate to new Er3+-3d/4f electronic states (Fig. 10d) in the band gap region of ZnO, that improve absorption properties of the semiconductor surface, facilitates its band excitation and prevents the recombination of electron-hole pairs, simultaneously.

19

(a)

(b)

(c)

Figure 9. (a) Side view of O-Zn-O layer ZnO(001) slab, (b) the surface Zn and O sites are pointed as well as the possible oxygen and zinc vacancy sites on the surface (V sur) and subO

surface VOsub, (c) PBE computed partial density of states for pristine ZnO(001).

20

(a)

(b)

(c)

(d)

Figure 10. PBE computed atomic and electronic structure of adsorbed Er and La onto pristine ZnO (001) surface. (a) La-ZnO atomic structure, (b) Er-ZnO atomic structure, (c) partial density of states for La-ZnO structure, (d) partial density of states for Er-ZnO structure. Please notice that for simplify picture only first layer of O-Zn-O is presented on fig. a, b. 4. CONCLUSION In summary, ZnO QDs modified with Eu, Er, Tb, Yb, Ho or La with enhanced UV-Vis light photocatalytic activity were synthesized via sol-gel method. The obtained results demonstrated that optical, structural and photocatalytic properties of ZnO/RE were dependent on the type and amount of rare earth metal. In the case of ZnO/La and ZnO/Er samples modification of ZnO with high amount of rare earth metal (0,18-0,45 mmol) caused higher PL intensity compared to pristine QDs. The highest PL quantum yield (about 81%) among all ZnO/RE samples was exhibited by ZnO nanocrystals containing 0.45 mmol of lanthanum. Furthermore, ZnO/Er sample containing 0.09 mmol of erbium exhibited the best 21

photocatalytic performance – the activity reached almost 90% of phenol removal under UVVis light irradiation and it was in good agreement with much lower PL intensity this sample compared to pristine ZnO QDs. The presence of KNO3 impurity, detected by XRD and XPS analysis, could result in lower photoactivity of ZnO modified with holmium and ytterbium comparing to pristine quantum dots. Moreover, the obtained experimental results clearly correspond with results obtained by theoretical simulations. Combined for the first time DFT studies with experimental results for Er and La-ZnO photocatalyst indicate that increased photocatalytic activity may be related to additional electronic RE states in band gap region of ZnO that improve absorption properties of the semiconductor surface, facilitates its band excitation and consequently prevents the recombination of electron-hole pairs.

ACKNOWLEDGMENTS This research was financially supported by Polish National Science Centre (Grant No. NCN 2016/23/B/ST8/03336 entitled “Mechanism of quantum dots excitation in photocatalytic reaction” and supported by the Foundation for Polish Science (FNP).

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Highlights     

Eu, Er, Tb, Yb, Ho, La (0.09-0.45 mmol) modified ZnO QDs obtained by sol-gel approach Modification of ZnO QDs by RE resulted in increase of photoactivity and decrease of photoluminesce ZnO/Er_0.09 showed the highest photocatalytic activity in phenol degradation (90%) The photocatalytic properties of ZnO/0.09_Er can be related to new Er3+-3d/4f electronic states ZnO/La_0.45 showed the highest quantum yield (81%)

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