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Original article
06 2022
:15;
103808
doi:
10.1016/j.arabjc.2022.103808

Effect of amines on the peroxo-titanates and photoactivity of annealed TiO2

, , , , , , , , , , , , ,
a Institute of Inorganic Chemistry of the Czech Academy of Sciences, 250 68 Husinec-Řež, Czech Republic
b University of Ostrava, Department of Chemistry, 30. dubna 22, 701 03 Ostrava, Czech Republic
c Institute of Chemical Technology, Prague, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic
d Faculty of Science, J. E. Purkyne University, Pasteurova 1, 400 96 Ústí nad Labem, Czech Republic
e Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 812 37 Bratislava, Slovak Republic
f Jaroslav Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova 2155/3, 182 23, Praha 8, Czech Republic

⁎Corresponding author. motlochova@iic.cas.cz (Monika Motlochová)

Received: , Accepted: ,
© The Author(s) 2022
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The aim of this study was to explore the influence of various n-propylamines (mono, di, and tri) and tert-butylamine on the properties of prepared peroxo-titanates and annealed TiO2. Detailed structural characterization (SEM, TEM with EDX, XRD) confirmed the 2D-foil morphology of TiO2 nanocrystals and the CHNS analysis together with XPS showed presence of carbon (under 1 wt%). The annealed TiO2 showed excellent photocatalytic activity and up to four times higher decomposition rate constant upon UV irradiation than the P25. Favourable growth of TiO2 crystals was observed especially when propylamine and tert-butylamine were used (as precursors for the peroxo-titanates). Increased photocatalytic efficiency of the highly crystalline nano-TiO2 was confirmed due to high-reactive anatase facets and the morphology composed of microsheets formed by interconnected nanocrystals. The photoinduced electron transfer was confirmed via EPR spectroscopy. This synthesis’ procedure offers a novel preparation method of highly photoactive C-doped TiO2 achieved relatively easily after annealing of lyophilized amino-peroxo-titanate.

Keywords

Peroxo-titanates
Amines
Photoactivity
C-doped TiO2
Photocatalyst
1

1 Introduction

Photocatalytic degradation of organic pollutants and bacteria by various photocatalysts in both liquid and gas phases is a well-known phenomenon. Except the materials commonly known for photocatalytic efficiency, many approaches of preparation of new photocatalytic systems as composites were recently described (Chen, Tang et al. 2021, Wang, Xie et al. 2021, Yu, Wu et al. 2022). TiO2 is currently one of the most widely used photocatalysts in environmental sciences due to its non-toxicity, low cost and availability. Many factors, such as the particle size and shape, allotropic modification, doping and impurities are crucial for its photoactivity (Ibhadon and Fitzpatrick 2013, Lee and Park 2013, Fan, Meng et al. 2016). The ability to control the particle size down to tens of nanometers leads to a reduction in visible light scattering and an increase in catalytic activity (Sasaki, Nakano et al. 1997). As reported in the literature, one possible way of obtaining photoactive titania nanoparticles is annealing of peroxo-titania precursor (Bessekhouad, Robert et al. 2003, Erjavec, Kaplan et al. 2015, Krivtsov, Ilkaeva et al. 2015, Savinkina, Obolenskaya et al. 2018). Our group has recently described a preparation procedure (Šubrt, Pulišová et al. 2014, Pližingrová, Volfová et al. 2015) that provides highly photocatalytically active TiO2 via a peroxo-titanic intermediate. The described synthetic procedure is based on the reaction of hydrogen peroxide with suspensions of thoroughly washed precipitates, obtained by neutralisation of an aqueous solution of titanium sulfate with an aqueous solution of ammonia, resulting in transparent yellow colloids. The colloidal solution prepared this way is, however, a two-phase system in which, in addition to the aqueous phase, a liquid hydrated component of peroxo-poly-titanic acid is also present (Šubrt, Pulišová et al. 2014, Pližingrová, Volfová et al. 2015, Šubrt, Pližingrová et al. 2017). The freeze-drying of this colloidal liquid provides a foam material consisting of thin films of peroxo-poly-titanic acid which contain small but not negligible amounts of chemically bound ammonia (Palkovská, Slovák et al. 2017). The freeze-drying leads to such a morphology of thin leaves that the growth of nanoparticles is significantly reduced during heating. Annealing of the foam prepared this way results in well-formed anatase nanocrystals with high temperature resistance to rutile transformation (800–900 °C). Together with small crystallite size (∼tens of nanometers), the resulting product has a higher efficiency compared to the commercial photocatalyst P25. Based on these results, the effect of doping with metallic or non-metallic elements from inorganic precursors was studied (Pližingrová, Klementová et al. 2017, Barbieriková, Pližingrová et al. 2018, Svora, Ecorchard et al. 2020, Volfová, Pližingrová et al. 2020). We expect that the use of organic amines instead of inorganics could be advantageous for several reasons. Amines appear to be suitable precipitation agents as they are commonly available basic substances and can also be a source of nitrogen and carbon atoms. The non-metal elements remaining after annealing in TiO2 can act as dopants and shift absorption edge into the visible part of the light (Spadavecchia, Cappelletti et al. 2010, Diker, Varlikli et al. 2011). Surface-modification or incorporation of TiO2 surface with amines could simplify the control over the final titania properties, i.e. particle size, shape and assembly (Chemseddine and Moritz 1999, Sugimoto, Zhou et al. 2003, Ryu, Lee et al. 2007, Colón, Hidalgo et al. 2008, Spadavecchia, Cappelletti et al. 2010, Diker, Varlikli et al. 2011). The amine modification of TiO2 thus allows covalent bonding e.g. with polymer, and guarantee their permanent mobilization and prevent agglomeration (Malekshahinezhad, Ahmadi-khaneghah et al. 2020).

The thermal behaviour of the amino-peroxo-titanates has been described in detail in our previous work (Komárková, Motlochová et al. 2021). This work and its results show a way to tailor the TiO2 properties via a suitable combination of the annealing temperature and chosen amine.

The aim of this study is to study the effect of bulk modification of different n-propylamines (propylamine, dipropylamine and tripropylamine) and tert-butylamine used as precipitating agents in the preparation of peroxo-titanates on the properties of the resulting lyophilized systems and also their effect on the photocatalytic activity of TiO2 prepared by their annealing at different temperatures.

2

2 Materials and methods

2.1

2.1 Chemicals

Titanium(IV) oxysulfate (TiOSO4··H2O, min. 29% Ti as TiO2, Sigma-Aldrich) served as a titanium precursor and hydrogen peroxide (H2O2, aqueous solution, p.a., 30%, Penta) as a dissolving agent. As precipitating agents, propylamine (MPA, ≥ 99%), dipropylamine (DPA, 99%), tripropylamine (TPA, ≥ 98%), tert-butylamine (TBA, 98%) were used, all from Sigma-Aldrich. 4-chlorophenol (4-CP, Across Organics, 99 %) was used as a model compound for photocatalytic test and commercially used TiO2 (Evonik P25) as a standard photocatalyst.

2.2

2.2 Synthesis procedure

Unlike the previously used approach when ammonia was used as a precipitant, in this instance, amines were used (as described below) and the synthesis was also conducted on a larger scale (three times) than previously attempted (Šubrt, Pulišová et al. 2014, Pližingrová, Volfová et al. 2015, Komárková, Motlochová et al. 2021). Titanium oxysulfate (14.4 g) was dissolved in 450 ml of demineralized water (DW) at 35 °C and this solution was cooled down to 0 °C. Selected amine was added to form a precipitate. Amine was added until achieving pH > 8 (propylamine 0.143 mol, dipropylamine 0.143 mol, tert-butylamine 0.153 mol, tripropylamine 0.273 mol). The suspension was stirred for 30 min and then filtered off. The filter cake was re-suspended in 1200 ml DW and then filtered twice. After the second filtration, the solid was resuspended in 1050 ml DW and 60 ml H2O2 was slowly added under constant stirring followed by the dissolution of precipitate. The solution was left to stand for approximately 24 h and then immersed into liquid nitrogen and dried by lyophilization.

The following sample-annealing procedure in a muffle furnace was applied: heating rate 1 °C/min until 250 °C, then 2 °C/min until 500 °C and 3 °C/min until 650 °C with isotherms of 1 h. Finally, all samples were heated with heating rate 3 °C/min until achieving the required temperature (750 °C, 800 °C and 900 °C) and maintained for 1 h.

Samples were named as amine_RT (e.g. MPA_RT for peroxo-titanate prepared from propylamine) or amine annealing temperature (e.g. MPA_900 for TiO2 prepared after annealing of peroxo-titanate prepared from propylamine after annealing at 900 °C for 1 h). Abbreviations were used for designation of used amines: propylamine (MPA), dipropylamine (DPA), tert-butylamine (TBA) and tripropylamine (TPA).

2.3

2.3 Characterization methods

2.3.1

2.3.1 Elemental analysis (EA)

Elemental analyses (C, N, H) were performed on a Thermo Scientific FlashSmartTM 2000 Elemental Analyser using around 1 mg of sample with 10 mg of V2O5 catalyst. A sample was burned in a vertical reactor in the dynamic mode (at 950 °C) in a He flow (140 ml/min) with oxygen added (21 ml) at the moment of sample introduction. After separation on a multiseparation column, the resulting gases (N2, CO2 and H2O) were measured with thermal conductivity detector and concentration of elements were calculated from corresponding peak areas.

2.3.2

2.3.2 Adsorption of nitrogen at −196 °C

A two-day vacuum outgas at 35 °C was used before measurement. Surface area was determined by the Brauner-Emmett-Teller (BET) method using a Quantachrome Nova 4200e instrument. Nitrogen adsorption was carried out at −196 °C (77 K).

2.3.3

2.3.3 Raman spectroscopy

The Raman spectra were collected using a DXR Raman microscope (Thermo Scientific); 256 two-second scans were accumulated with 532 nm laser (2 mW) using 50 μm slit under 10 × objective of Olympus microscope in full range.

2.3.4

2.3.4 Fourier-transformation infrared spectroscopy (FTIR)

The Fourier transform infrared spectra of the samples were recorded by Nicolet Nexus 670 FTIR spectrometer on the diamond ATR crystal in the region 4000 – 400 cm−1.

2.4

2.4 X-Ray photoelectron spectroscopy (XPS)

The surface composition of the samples and the chemical states of the elements were inspected by X-ray photoelectron spectroscopy (XPS) using a SPECS PHOIBOS 100 hemispherical analyser with a 5–channel detector and an achromatic SPECS XR50 X-ray source equipped with an Al and Mg dual anode. No flood gun was used. The measurements were performed with Al anode at Epass 40 eV for survey spectra and Epass 10 eV for high resolution spectra in Fixed Analyzer Transmission mode, Medium Area settings with entrance slit size 7 × 15 mm2 and exit slit was open, iris slit was set to 35 mm. The acquired data were processed in CasaXPS software with Shirley background profile and built-in RSFs used for calculations of the composition from the high–resolution spectra. Spectra were taken over Ti 2p, O 1s, C 1s and N 1s region. The binding energy calibration was made on C–C component of C 1s high-resolution peak aligned to 284.5 eV.

2.5

2.5 Scanning electron microscopy (SEM)

High resolution scanning electron microscopy (HRSEM) images were obtained with a FEI Nova NanoSEM 450 microscope equipped with an Everhart − Thornley secondary electrons detector and a through-lens detector. Measurements were made in high vacuum with an acceleration voltage of 10 kV. Suspension of samples in deionized water were pipetted on the silicon chip substrates and dried at room temperature.

2.6

2.6 Transmission electron microscopy (TEM)

The TEM analysis was measured with JEOL JEM 3010 microscope operated at 300 kV (LaB6 cathode, point resolution 1.7 Å) with an Oxford Instruments Energy Dispersive X-ray (EDX) detector attached. EDX analyses were acquired and treated in the INCA software package. Electron diffraction patterns were evaluated using the Process Diffraction software package. A drop of very dilute suspension was placed on a holey-carbon-coated copper grid after the suspension was treated in ultrasound for 5 min and dried by evaporation at ambient temperature.

2.7

2.7 Atomic force microscopy (AFM)

Morphology of samples was investigated with Bruker Icon Dimension microscope using ScanAsyst-air contact mode. Diluted ethanol suspension was pipetted on the smooth synthetic mica support.

2.8

2.8 Electron paramagnetic resonance (EPR)

EPR experiments were performed by X-band EMX micro cw-EPR spectrometer (Bruker, Germany) with X-band premium bridge and at 100 kHz field modulation in the high sensitivity cylindrical cavity ER 4119HS using thin-walled quartz EPR tubes (Bruker). For the low temperature measurements, the EPR tube with sample was placed into cold finger-shaped liquid nitrogen Dewar flask filled with liquid nitrogen and placed inside an EPR resonator. To prevent superheating of liquid nitrogen and undesirable nitrogen bubbling in the resonator part, liquid nitrogen was bubbled by dried gaseous helium 4.8 (Linde) above the resonator. This also leads to a small decrease (3–4 K) in the boiling temperature of liquid nitrogen below 77 K (Minkoff, Scherber et al. 1957). Powder TiO2 samples (annealed at 900 °C) were placed into argon (Messer) filled quartz sample tube, then closed by cap. The LED light source KL 1600LED (T = 5600 K; Schott, Germany) was applied to provide the visible light irradiation (>420 nm), characterized with the illuminance of 160 klx measured by a digital lux meter (Metra Blansko, Czech Republic) (Barbieriková, Pližingrová et al. 2018). Exposure at λmax = 400 nm (LED@400 nm) was realized by a home-made power LED irradiation source. The g-values were determined using internal frequency and magnetic field meter with correction performed daily using BDPA standard.

2.9

2.9 UV–VIS spectrometry (UV–VIS)

The UV–VIS spectra were measured from 200 to 800 nm using Perkin Elmer Lambda 750 spectrophotometer with a spherical integrator (100 mm in diameter). The spectra were recorded in a diffuse reflectance mode and transformed to absorption spectra via the Kubelka-Munk function.

2.10

2.10 X-Ray diffraction (XRD)

XRD measurements were carried out using a PANalytical X’Pert PRO diffractometer equipped with a conventional X-ray tube (Cu Kα radiation, 40 kV, 30 mA, line focus) in transmission mode. An elliptic focusing mirror, a divergence slit of 0.5°, an antiscatter slit of 0.5°, and a Soller slit of 0.02 rad were used in the primary beam. A fast-linear position sensitive detector PIXcel with an antiscatter shield and a Soller slit of 0.02 rad were used in the diffracted beam. All patterns were collected in the 2θ range of 4–80° (step of 0.013° and 400 s/step). The samples were ground in an agate mortar in a suspension with cyclohexane. The suspension was then placed on top of mylar foil. After the evaporation of solvent, a thin layer of sample was covered with the second mylar foil. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, Netherlands, version 4.8.0), DiffracPlus software package (Bruker AXS, Germany, version 8.0) and ICDD PDF-4 + database. The quantitative phase analysis was performed with the DiffracPlus Topas (Bruker AXS, Germany, version 4.2) with structural models based on ICSD database. This program allows to estimate the weight fractions of crystalline phases by means of Rietveld refinement procedure. The estimation of the size of crystallites was performed on the basis of the Scherrer formula as implemented within the DiffracPlus Topas software.

2.11

2.11 Photocatalytic experiments

A homemade reactor was used to conduct the photocatalytic experiments. The photoreactor is equipped with 10 fluorescent black lamps (λmax = 365 nm, 8 W, Sylvania, flux density 6.24 mW cm─2 measured by UVa probe #28949; ILT 1400-A Photometr). 50 mg of photocatalysts were added to 50 ml of 4-chlorophenol (4-CP) solutions (c0 = 10─4 mol/l) in Pyrex beaker and stirred throughout whole experiment. Aliquots (0.5 ml) of irradiated suspensions were taken at given time intervals (5, 10, 20, 30, 45, 60, 90 and 120 min) and centrifuged. Decrease in concentration of 4-CP solutions was evaluated with HPLC (Agilent Technology 1200 Series; column LiChroCART® 125–4 LiChrosphere 100 RP-18 (5 µm), mobile phase 64:36 water/methanol, flow rate 1 ml/min, detection wavelength 254 nm). For comparison, a parallel test was performed with a benchmark Evonik P25 photocatalyst.

3

3 Results and discussion

Chemical, spectroscopic and microscopic analyses were performed only for non-annealed amino-peroxo-titanates and the samples annealed at 900 °C which were the ones with the most photocatalytically active TiO2 (except TPA_900). Photocatalytic activity for 4-chlorophenol was evaluated for all annealing temperatures (750 °C, 800 °C and 900 °C).

3.1

3.1 Elemental analysis (EA)

The elemental analysis was used to evaluate the content of organic fragment in non-annealed and annealed samples at 900 °C. Results of CHN elemental analysis can be seen in Table 1. The quantity of organic fraction was in units of wt. % in non-annealed amino-peroxo-titanates. Established content of carbon varied around 3 wt%. The largest quantity of tripropylamine used during the synthesis was related to the higher amount of carbon in TPA_RT (7 wt%). The nitrogen content achieved experimentally was close to the theoretical value as calculated from C:N ratio in amine. No presence of nitrogen was confirmed in DPA_RT (or was below the detection limit for this method). The hydrogen content (higher than expected) could have been caused by moisture in the samples as well as hydrogen from peroxide. During annealing, the oxidation of organics mainly to CO2 and H2O proceed in agreement with (Komárková, Motlochová et al. 2021). Residual amount of carbon 0.6–0.7 wt% was found by elemental analysis in samples annealed at 900 °C in agreement with XPS results discussed below. Hydrogen and nitrogen were not recorded by elemental analysis in annealed samples at 900 °C.

Table 1 Elemental composition of non-annealed amino-peroxo-titanates and samples annealed at 900 °C, values for N and H given in brackets were calculated as theoretical amount recalculated from experimentally quantified content of carbon in amine.
N (wt. %) C (wt. %) H (wt. %)
MPA_RT 0.9 (theor. 1.2) 3.1 1.8 (theor. 0.8)
DPA_RT 0 (theor. 0.5) 2.5 1.7 (theor. 0.5)
TBA_RT 0.7 (theor. 0.9) 3.2 1.9 (theor. 0.6)
TPA_RT 0.8 (theor. 0.9) 7.0 2.4 (theor. 1.6)
MPA_900 0 0.7 0.1
DPA_900 0 0.7 0
TBA_900 0 0.7 0
TPA_900 0 0.6 0

3.2

3.2 Surface area

The specific surface area (SBET) characteristic for all the samples were determined by adsorbed volume of N2 at ─196 °C. Results are recorded in Table S1 in Supplementary material. Non-annealed samples showed relatively low surface area, between units up to 96 m2/g in order TPA_RT < DPA_RT < MPA_RT < TBA_RT. The same trend was achieved also in comparison of annealed samples at the same temperature. TBA_RT showed increased microporosity. Distinctive decreasing values of SBET were found (in agreement with literature data) driven by the annealing accompanied by the transformation of amorphous materials to crystalline (Ayers and Hunt 1998, Šubrt, Pulišová et al. 2014, Barbieriková, Pližingrová et al. 2018, Motlochová, Slovák et al. 2019).

3.3

3.3 Raman spectroscopy

The Raman spectra of all non-annealed samples, containing broad bands typical for this type of amorphous samples, are shown in Fig. 1. The stretching modes ν(Ti–O2) confirmed the presence of Ti(IV)O22─ species in the gel by vibrations at 520 cm─1 and 680 cm─1 (Bobrova, Zhigun et al. 1968; Tengvall, Vikinge et al. 1993). The bands presented at 276 cm─1 and 386 cm─1 could be attributed to the photon mode Eg symmetry for Ti─O─Ti species (Dutta, Gallagher et al. 1993; Tengvall, Vikinge et al. 1993). All samples showed band around 901 cm─1 for stretching vibration of free or complexed ─O─O─ bond of peroxo group (Tengvall, Vikinge et al. 1993, Ayers and Hunt 1998). Due to the relatively low content of organic phase as determined by elemental analysis, the intensities of vibration for C─H bonds were also featureless. Weak intensity band around 1448 cm─1, as can be seen in case of MPA_RT and TPA_RT, can be related to C─H asymmetrical bending or/and scissoring mode (K. Kipkemboi, C. Kiprono et al. 2003, Stuart 2004). Only the Raman spectrum of TPA_RT with higher weight representation of amine as well as higher number of its aliphatic chains for amine, shows asymmetric C─H vibrations for CH3 group at 2985 cm─1, and for CH2 group at 2938 cm─1 and the overlap of their symmetric vibrations around 2874 cm─1. Nevertheless, two typical bands representing stretching symmetrical and asymmetric NH2 vibration above 3350 cm─1 typical for amines were not seen as described in the literature (Gamer and Wolff 1973, Friedman and Schwartz 1982, Durig, Beshir et al. 1989). Raman spectra for samples annealed at 900 °C are shown in Supplementary material in Fig. S1. The spectra of MPA_900, DPA 900 and TBA_900 show typical signals for anatase (significantly prevailing phase from XRD results) at 139 (Eg), 191 (Eg), 391 (B1g), 512 (B1g/A1g), and 635 (Eg) cm─1. The sample TPA_900 show just typical rutile bands at 139, 231 (second order Raman scattering), 444 (A1g) and 607 (Eg) cm─1 (Gao, Fjellvåg et al. 2009).

Raman spectra of the powdered non-annealed samples.
Fig. 1
Raman spectra of the powdered non-annealed samples.

3.4

3.4 FTIR spectroscopy

Non-annealed amino-peroxo-titanates showed a broad band around 3600–3000 cm─1 and 1627 cm─1 attributed to stretching and deforming vibration of O─H from adsorbed water or Ti─O─H on the photocatalyst surface (Ryu, Lee et al. 2007) as can be seen in Fig. 2. This vibration was confirmed in similar position alike in peroxo-titanates using ammonia (1627 cm─1) or alkali metals (1635 cm─1) as precipitation agents (Šubrt, Pulišová et al. 2014, Svora, Ecorchard et al. 2020). The band noticed at 901 cm─1 was assigned to stretching O─Ti─O vibration. Ti─O and/or Ti─O─Ti stretching vibrations were indicated as bands around 679 cm─1 and 485 cm─1 (Selvamurugan, Hirankumar et al. 2016; Mahalingam, Selvakumar et al. 2017, Svora, Ecorchard et al. 2020). The stretching vibrations for C─H bond of methyl or methylene bonds at 2977 and 2881 cm─1 were weakly visible together with the associated deformation modes around 1450 cm─1 mainly for TPA_RT (with shoulders and overlap), less intense for DPA_RT and MPA_RT (K. Kipkemboi, C. Kiprono et al. 2003). Weak band at 1480 cm─1 can be attributed to methylene scissoring C─H vibration (Zhu, He et al. 2005, Chen, Chen et al. 2008).

FTIR spectra of the powdered non-annealed samples.
Fig. 2
FTIR spectra of the powdered non-annealed samples.

The most different spectrum of non-annealed samples was found for TBA_RT. The out-of-plane skeletal vibration of C3C─N was found at 1214 cm─1 (K. Kipkemboi, C. Kiprono et al. 2003). TBA_RT showed the band related to Ti─OH mode (Parvanova 2006) or CH3 deformation mode presented at 1381 cm─1, 1407 cm─1, and at 1297 cm─1 likely belongs to CC3 skeletal stretching vibration (K. Kipkemboi, C. Kiprono et al. 2003, Svora, Ecorchard et al. 2020). MPA_RT and TBA_RT samples have shown a deformation peak band at 1514 cm─1 which was associated to a NH2 deformation (Durig, Beshir et al. 1989, Bacsik, Ahlsten et al. 2011), which is hidden in shoulder in case of DPA_RT.

Infrared spectra of annealed samples (900 °C) are shown in Fig. S2 in Supplementary material. The vibration band around 3300 cm─1 could be attributed to physically adsorbed water together with band at 1645 cm─1 attributed to O─H bending mode, mainly visible in the most photoactive samples MPA_900 and TBA_900. Absorption band at 731 cm─1 can be associated with ν (Ti─O) vibrations in TiO2 nanoparticles (Verma, Gangwar et al. 2017). Stretching vibration of TPA_900 of Ti─O and/or Ti─O─Ti at 483 cm─1 could be similar to non-annealed samples found in TPA_RT.

3.5

3.5 XPS

XPS study is useful tool for investigation of various oxidation states of elements of photocatalysts (Meng, Wang et al. 2013, Wang, Meng et al. 2013). The chemical states and presentation of elements (Ti, O, C, N) in non-annealed precursors and samples annealed at 900 °C were examined. Analysis of Ti and O bonds was performed with Ti 2p peak with model used for TiOx species. The data showing ratio between stoichiometric TiO2 and other substoichiomertic TiOx, and evaluated atomic % of each element were summarized in Table 2. A reduction of carbon species is evident together with significant improvement in TiO2 stoichiometry in annealed samples.

Table 2 Results of XPS analyses.
TiO2 stechio-metric [%] TiOx substoichio-metric [%] C (C 1 s) [at.%] N (N 1 s) [at.%] Ti (Ti 2p) [at.%] O (O 1 s) [at.%]
MPA_RT 97 3 15.4 2.2 19.7 62.7
DPA_RT 91 9 11.4 1.1 19.0 68.5
TBA_RT 96 4 14.9 1.8 19.0 64.3
TPA_RT 90 10 22.7 1.8 17.1 58.4
MPA_900 99 1 6.0 0.4 27.8 65.7
DPA_900 99 1 7.4 0.1 26.5 66.0
TBA_900 98 2 8.4 0.4 26.7 64.5
TPA_900 97 3 5.9 0.1 25.5 68.4

XPS spectrum of element O 1s can be seen in Fig. 3. Annealed samples showed spectra consisting of the dominant peak (typical for TiO2) attributed to the prevailing O─Ti signal (BE at approx. 529 eV) and, the presence of minor Ti─OH surface hydroxyl groups of TiO2 explains the peak at 531 eV (Ahmad and Bhattacharya 2009, Zou, Gao et al. 2010). The shift in peaks to higher values (about 1 eV) of BE was observed for non-annealed samples. An additional third signal at approx. 532.5 eV is caused by the presence of peroxo groups (Zou, Gao et al. 2010, Li, Yu et al. 2013, Seo et al., 2018) in agreement with literature according to which a 3-component signal of O 1s XPS spectrum is being typical for peroxo-titanates. Loss of signal of peroxide group and depression in intensity of peak from hydroxyl groups can be explained by annealing process, coupled with dehydration of samples and decomposition of peroxide.

O1s XPS spectra of non-annealed (red line) and annealed samples at 900 °C (black line).
Fig. 3
O1s XPS spectra of non-annealed (red line) and annealed samples at 900 °C (black line).

The positions of Ti 2p peaks at values of BE approx. 464 eV and 458 eV are typical for TiO2 structure, which can be defined as the main component for all the samples as can be seen in Fig. S3 in Supplementary material (Ahmad and Bhattacharya 2009, Nomura, Fukahori et al. 2020). Higher quantities (around 10 %) of substoichiometric TiOx species were observable mainly in TPA_RT and DPA_RT at 456.9 eV, resulting in asymmetry and broadening of mentioned peaks indicating imperfections of TiO2 amorphous structure on the surface. Crystallization of amorphous amino-peroxo-titanate to anatase and later to rutile structure occurred during annealing, which is associated with the BE shifting to smaller values for Ti 2p peaks similarly as for O 1s discussed above.

The carbon peaks C1s showed three components in both sample sets (RT and annealed at 900 °C) as showed in Fig. 4. The main C─C/C─H component was at 284.5 eV. Less distinctive structures, found in XPS spectra at 286.6 eV can be attributed to C─N or C─O, both their group which signals are overlapping in XPS (Zorn, Liu et al. 2014). Third component, quantified similarly in units % for C 1s, can be associated with N─C=O bond (BE = 288 eV) (Zorn, Liu et al. 2014). Higher representation of carbon (compared to the theoretical calculation from value of atomic ratio C:N of amine) in samples can be explained by contamination of their surface. As can be seen, after the annealing, the amounts of both carbon and nitrogen dropped significantly. Half-width of the peak for C─C bond (284.5 eV) decreased in all the annealed samples which suggests some changes.

C 1s XPS spectra of non-annealed (red line) and annealed samples at 900 °C (black line).
Fig. 4
C 1s XPS spectra of non-annealed (red line) and annealed samples at 900 °C (black line).

The XPS N1s signals composed of two overlapping peaks are suggesting 2 states of nitrogen in RT samples with BE at 401.6 and 399.6 eV. According to the literature, the position at 399.6 eV usually indicates NH2 (Graf, Yegen et al. 2009, Li, Yu et al. 2013) and the traces remained in samples annealed at 900 °C, as can be seen in Table 2 and Fig. 5. The second peak (which is lost after annealing) likely belongs to C─N bond or to O─Ti─N bond (Wu and Ju 2014).

N 1s XPS spectra of non-annealed (red line) and annealed samples at 900 °C (black line).
Fig. 5
N 1s XPS spectra of non-annealed (red line) and annealed samples at 900 °C (black line).

3.6

3.6 SEM

SEM images of non-annealed samples were taken to evaluate their morphology (Fig. 6a-d). Samples discussed in this work showed a typical structure of 2D irregularly shaped thin sheets in agreement with the previously reported works of our group (Šubrt, Pulišová et al. 2014, Pližingrová, Volfová et al. 2015, Svora, Ecorchard et al. 2020). Thinness of individual sheets confirmed the fact, that the electron beam got through, and the next layer lying under the microsheet can be seen (e.g. Fig. 6a or 6c). The 2D microsheets’ diameters achieved different sizes in both directions from 100 µm to the fragments in micrometer units. Formation of leaves’ fragments can be explained by the cracking of sheets together with a presence of holes which were visible for example in Fig. 6a (Nagabhushana, Ashoka et al. 2013). Surface of sheets was rather rough than smooth. Smaller particles (separated or agglomerated forming “cobwebs” or chains) appeared locally, as shown detail in Fig. 6b or 6c. Structure of TPA_RT (Fig. 6d) seemed to be rougher and more compact compared to the rest of non-annealed samples, however, thinness of sheets was preserved.

SEM micrographs of non-annealed samples a) MPA_RT, b) DPA_RT, c) TBA_RT and d) TPA_RT.
Fig. 6
SEM micrographs of non-annealed samples a) MPA_RT, b) DPA_RT, c) TBA_RT and d) TPA_RT.

The structure of thin leaves of amino-peroxo-titanates achieved by the lyophilization procedure significantly restricts growth of TiO2 crystals which remained in nanometric scale even after annealing at high temperatures (as 900 °C). This morphology offers high number of suitable photocatalytic centres. After annealing at 900 °C, the leaf-like morphology of all the samples was preserved, as can be seen in the Fig. 7, which shows the annealed samples under higher magnification (150 000x). The samples MPA_900, DPA_900 and TBA_900 (Fig. 7a-7c) were composed of smaller, morphologically diverse connected particles with varying dimensions ranging from 230 nm to as small as 40 nm, copying the shape of the original leaves (or chains). The particles creating the leaves of the sample TPA_900 (in which rutile phase significantly prevails), were also diverse in shape and size, but in this case the „plate-like “particles are generally larger (with dimensions in units of µm) and created a rather smooth surface.

SEM micrographs of the annealed samples at 900 °C a) MPA_900, b) DPA_900, c) TBA_900 and d) TPA_900.
Fig. 7
SEM micrographs of the annealed samples at 900 °C a) MPA_900, b) DPA_900, c) TBA_900 and d) TPA_900.

TEM

TEM images (Fig. 8) of non-annealed samples confirmed the observation made via SEM. Besides the prevailing amorphous character, also nuclei of nanocrystals with size 1–2 nm in case of TPA_RT, TBA_RT and DPA_RT were found in the high-resolution images. Nanocrystals with higher size (4–5 nm) were registered in sample MPA_RT. SAED (Selected Area Electron Diffraction) patterns together with dark field images were attached to the supplementary materials (Fig. S4). Holes with diameter in tens or units of nanometres were seen in individual sheets (Fig. 8a or 8d), especially in TPA_RT. The surface morphology can resemble wool felt structure. Tiny nanoparticles, both apart or interconnected into the fibber (distinguished by darker colour in TEM figures), were present in all non-annealed samples. Low magnification TEM analysis of samples annealed at 900 °C can be seen in the Fig. 9.

TEM images of non-annealed samples a) MPA_RT, b) DPA_RT, c) TBA_RT and d) TPA_RT.
Fig. 8
TEM images of non-annealed samples a) MPA_RT, b) DPA_RT, c) TBA_RT and d) TPA_RT.
TEM images of annealed samples at 900 °C a) MPA_900, b) DPA_900, c) TBA_900 and d) TPA_900.
Fig. 9
TEM images of annealed samples at 900 °C a) MPA_900, b) DPA_900, c) TBA_900 and d) TPA_900.

Flat-like structure persisted despite the high annealing temperature (900 °C). A significant change in the sheets was observed as they were formed by interconnected crystals rather than their previous compact nature. The shape as well as the size of crystals showed meaningful variations. Diameter of particles moved from 30 to 100 nm, and bigger particles formed “nodal points” which were bound through chains of smaller crystals. Particles showed various shapes usually with rounded edges. Sample TPA_900 (Fig. 9d) showed a quiet different morphology linked to its rutile nature. The detailed TEM characterization and evaluation (SAED patterns, HRTEM micrographs and lattice fringes) of samples annealed at 900 °C is attached as supplementary material in Figs. S5 and S6.

3.7

3.7 AFM

AFM analyses (Fig. 10) were conducted to investigate the structural features of sheets. The thickness of the sheets was experimentally determined at 2–3 nm in non-annealed samples. This value slightly raised to 4 nm after annealing. Thickness 1–3 nm of various peroxo-titanate system was revealed in several studies (Sasaki, Ebina et al. 2001, Hong 2002, Manga, Zhou et al. 2009, Xiang, Wu et al. 2011). The fact that the TiO2 flakes with thickness about tens nanometers could be prepared by lyophilization of colloidal suspension was described in detail in literature (Sasaki and Watanabe 1998). Sanaki et. al reported that layered protonic titanate was exfoliated to the single sheets (thickness less than 1 nm) due to the presence of aqueous solution containing bulky quaternary ammoniums ions.

AFM images of non-annealed samples: a) MPA_RT, b) DPA_RT, c) TBA_RT and d) TPA_RT, and of annealed samples at 900 °C: e) MPA_900, f) DPA_900, g) TBA_900 and h) TPA_900.
Fig. 10
AFM images of non-annealed samples: a) MPA_RT, b) DPA_RT, c) TBA_RT and d) TPA_RT, and of annealed samples at 900 °C: e) MPA_900, f) DPA_900, g) TBA_900 and h) TPA_900.

3.8

3.8 EPR spectroscopy

States of elements (such as reduced or neutral metal ions, or oxygen vacancies) are crucial e.g. for electron transfer or sufficient separation of charge carriers and significantly influence (photo)catalytic or magnetic material’s performance (Meng and Sun 2009, Meng, Lu et al. 2010, Wang and Meng 2013, Meng, Fan et al. 2017, Xie, Wang et al. 2021). Electron paramagnetic resonance (EPR) spectroscopy provides a useful tool for the detection and identification of photoinduced single charge carriers (electrons and holes) formed in the crystalline TiO2 nanostructures as well as the monitoring of paramagnetic sites in crystalline titania matrix prior to exposure (Brezová, Barbierikova et al. 2013, Pližingrová, Klementová et al. 2017, Barbieriková, Pližingrová et al. 2018, Polliotto, Livraghi et al. 2018, Barbieriková, Dvoranová et al. 2021). The high-temperature treatment (annealing at 900 °C), as well as doping, may lead to the generation of various point crystal defects in titania matrix, which may be detected via EPR as paramagnetic species ─ Ti(III) and oxygen vacancies. Prior to the exposure, the powdered annealed samples MPA_900, DPA_900 and TBA_900 (with the dominant anatase phase) were measured at 77 K and several low-intensity signals in the region under 2.000 (“electron trapping site”) were detected (Fig. 11a-c, black lines). These signals can be attributed to the anatase matrix collection of subsurface and surface reduced Ti(III) centers (Polliotto, Livraghi et al. 2018, Barbieriková, Dvoranová et al. 2021). The sharp asymmetric signal (at gef = 2.002) dominates in the EPR spectra, this signal was also detected at room temperature (data not shown). The region of EPR spectra with g > 2.000, so-called “hole trapping site” is typical for the detection of oxygen paramagnetic species (O - and O2•–/•O2H) evidently formed via photogenerated charge carriers (Polliotto, Livraghi et al. 2018, Fan et al. (2016a), Fan et al. (2016b), Li et al. (2013), Meng et al. (2017), Meng et al. (2013), Minnekhanov et al. (2012), Pližingrová et al. (2017), Wang et al. (2013), Wang et al. (2021), Xie et al. (2021), Yu et al. (2022), Yu et al. (2003), Barbieriková, Dvoranová et al. 2021), however, the presence of paramagnetic O2− around 2.002 is too low and signal may be assessed as hole-type centers (Pližingrová, Klementová et al. 2017). Upon in situ exposure of powdered MPA_900, DPA_900 and TBA_900 with LED@400 nm at 77 K only slight changes of EPR spectra were observed (Fig. 11a-c, green lines). The similar situation was detected upon > LED 400 nm (data not shown). The low photoactivity correlates with the absorption region of studied samples which is shifted more to the UVA region (Fig. S7). Nevertheless, the small changes in spectra in the “electron trapping site” resulted from the photoelectron transfer between Ti(IV) and Ti(III) ions (Polliotto, Livraghi et al. 2018). Changes in the “hole trapping site” represent the EPR signals characteristic for the superoxide radical anion or hydridodioxygen O2•–/HO2• radicals formed via the reaction of photogenerated electrons with molecular oxygen adsorbed on the photocatalyst’ surface, as well as for to the paramagnetic O•− produced by the reaction of photogenerated holes with oxygen ions O2− (Pližingrová, Klementová et al. 2017). It should be mentioned here that due to the presence of residual nitrogen from synthesis, formation of monomeric nitrogen centers (Nb•) cannot be excluded (Barbieriková, Pližingrová et al. 2018). For the carbonaceous materials, the symmetric singlet with giso ∼ 2.003 is typical (Minnekhanov, Deygen et al. 2012), nevertheless, was not observable in spectra measured for studied samples due to its overlap with signals originating from titania matrix.

X-band EPR spectra of annealed samples: a) MPA_900, b) DPA_900, c) TBA_900 and d) TPA_900 measured at 77 K.
Fig. 11
X-band EPR spectra of annealed samples: a) MPA_900, b) DPA_900, c) TBA_900 and d) TPA_900 measured at 77 K.

The signals detected for TPA_900 sample, containing only rutile modification upon annealing sample was completely different. The response to light exposure was quite notable and was caused by the interfacial photoinduced electron-transfer processes while the concentration of paramagnetic centres in the spectrum of TPA_900 was quite small.

Prior to exposure, only negligible signals were detected (Fig. 11d, black line), the EPR lines of anatase and rutile are sensitive to structural parameters (crystal field strength and related distortions) and exhibit different profile (Brezová, Barbierikova et al. 2013, Polliotto, Livraghi et al. 2018). The exposure upon LED@400 nm rendered a strong response (Fig. 11d, green line). The broad low-intensity signal with giso ∼ 1.97 may be attributed to the Ti(III) centers and asymmetric signal in “hole trapping site” represents superposition of the superoxide radical anion and/or hydridodioxygen radical O2•–/HO2•, along with signal of holes trapped at the lattice oxygen atoms located in the subsurface layer (Brezová, Barbierikova et al. 2013, Polliotto, Livraghi et al. 2018,). Similar signals were also monitored upon visible light exposure, but the intensity was significantly lower, such behavior of rutile crystalline is in accordance with the absorption properties which are shifted bathochromically when compared the anatase nanopowders.

Based on the EPR experiments the limited response of prepared compounds on light exposure was observed, however the photoinduced electron transfer was detected, which is crucial for their application as potentially applicable photocatalysts.

3.9

3.9 XRD analysis

As the attributes of TiO2 nanocrystals are crucial to photoactivity, XRD analysis was performed to evaluate them. XRD analysis of non-annealed samples confirmed their amorphism, as well as HRTEM, which, additionally, found emerging crystaĺs nuclei. After the annealing of peroxo-titanate materials, pure anatase or two-phase anatase–rutile systems crystallized, in agreement with peroxo-titanates precipitated with ammonia (Šubrt, Pulišová et al. 2014, Pližingrová, Volfová et al. 2015, Palkovská, Slovák et al. 2016). Anatase phase lattice parameters (a, c), anatase representation (in %) and crystallite size were established pursuant to XRD data which are shown in Tab. 3 and corresponding XRD patterns into Figs. S8-S10 in the supplementary material. As anatase allotropic modification is the most photocatalytically active, complementary information about rutile phase were given in the supplementary material in Table S2.

The TiO2 composition and parameters were affected both by the annealing temperature as well as the choice of amine. Higher annealing temperature led to transformation of anatase to rutile phase, first rutile fractions were recorded while annealing at 900 °C, except for samples based on tripropylamine in which rutile phase was observed even at 750 °C. Higher annealing temperature also caused an increase in particle size of anatase (except sample TPA_900 in which was only 1% of anatase) as can be seen in Fig. 12. Anatase lattice parameter a increased with increasing crystallite size. Trend in behaviour of lattice parameter c was observed only in samples based on dipropylamine, as c value increased, then a/c ratio dropped with increasing temperature. Annealed samples based on TBA showed an opposite trend when growth in annealing temperature was accompanied by growth in a/c ratio (Fig. 12). As can be seen in Fig. 12, rows of annealed samples prepared from the same amines showed close a/c value suggesting that the amine influenced the way of TiO2 growth. Ratio between a/c associated with crystal shape played an important role in photocatalytic activity of samples as discussed below.

Ratio of lattice parameters a/c of anatase and its crystallite size of annealed samples.
Fig. 12
Ratio of lattice parameters a/c of anatase and its crystallite size of annealed samples.

3.10

3.10 Photocatalytical decomposition of 4-CP

The photocatalytic activity of the prepared amine modified samples were studied following the decomposition of 4-chlorophenol (4-CP) upon UV irradiation. The progress of degradation was evaluated via the pseudo first-order kinetic model, as expressed by equation: k = l n ( c 0 c t ) t , c0 reveals initial 4-CP concentration (mol/l) and t is reaction time (min). The calculated values of rate constants (k) and graphs are given in Table 3 and Fig. 13.

Table 3 Anatase representation (in %), its lattice parameters (a, c) and their ratio (a/c), crystallite size (D), and calculated rate constants (k) of kinetic of degradation of 4-CP for all annealed samples.
% a (Å) c (Å) a/c D (nm) k (min−1)
MPA_750 100 3.78392 9.51774 0.397565 34 17.8·10─3
MPA_800 100 3.78412 9.51875 0.397544 43 24.1·10─3
MPA_900 97 3.78412 9.51744 0.397599 67 35.4·10─3
DPA_750 100 3.78339 9.52195 0.397334 38 9.4·10─3
DPA_800 100 3.78391 9.52348 0.397325 51 13.8·10─3
DPA_900 94 3.78409 9.52430 0.397309 83 19.4·10─3
TBA_750 100 3.78415 9.51505 0.397701 26 19.8·10─3
TBA_800 100 3.78466 9.51598 0.397717 32 22.7·10─3
TBA_900 97 3.78492 9.51475 0.397796 54 37.0·10─3
TPA_750 90 3.78340 9.52153 0.397352 45 3.7·10─3
TPA_800 44 3.78343 9.52186 0.397342 57 3.0·10─3
TPA_900 1 3.78403 9.51829 0.397553 47 1.3·10─3
P25 ca. 78* 7.7·10─3

* values taken from (Ohtani, Prieto-Mahaney et al. 2010), determined content of rutile varied at 14–18%, the rest is completed by amorphous phase.

Kinetic of degradation of 4-chlorophenol studies (lines show theoretic process according to the pseudo-first kinetic order) and calculated rate constants for annealed samples.
Fig. 13
Kinetic of degradation of 4-chlorophenol studies (lines show theoretic process according to the pseudo-first kinetic order) and calculated rate constants for annealed samples.

Photocatalytic activity depends on many inseparable factors acting at the same time. Phase composition and particle size (affected by the annealing temperature) as well as quantity of available surface centres as well as BET surface rank among the most important parameters (Akpan and Hameed 2009, Hamidi and Aslani 2019). Among the observation, a substantial trend was noticed (except for tripropylamine) that the higher the annealing temperature, the higher the kinetic constant.

The set of samples prepared with TPA showed the lowest photocatalytic activity regardless of the annealing temperature (Fig. 13). The explanation for that can be found in the combination of the following factors: firstly, the already lowest SBET value in non-annealed state (which decrease even further during heating) and secondly, increased quantity of rutile phase (less photoactive). Apparently, the higher quantity of organic component in TPA_RT (Table 1) manifested in lower transformation temperature when compared to the samples based on other amines in which the rutile phase was observed from 900 °C upwards. The effect of presence of organic dopants or impurities on anatase–rutile transformation temperature is not unambiguous (Hanaor and Sorrell 2011). Unlike the other samples, the set of samples with tripropylamine showed an opposite trend, TPA_750 showing a very low photoactivity which further decreased with additional annealing up to a point close to inactivity.

All annealed samples based on TBA, MPA and DPA showed a very similar phase composition as can be seen in Table 3. Despite this fact, rate constants were lower in samples prepared from DPA likely driven by a slightly bigger particle size, decreased SBET and a/c value of anatase crystals. Nevertheless, the trend of higher rate constant with higher annealing temperature was preserved.

The results of annealed TiO2 materials based on the primary amines (TBA and MPA) showed the most promising results and a very perspective photocatalytic performance at various temperatures. Features (particle size, ratio of phases and BET surface) of these samples were nearly identical. Catalytic properties of anatase depend on its exposed external surface, as well as with the increasing percentage of high-reactive centres available to heterogenous reaction (Liu, Yang et al. 2009, Wen, Jiang et al. 2011). Annealed TiO2 samples based on TBA and MPA showed the highest a/c ratio, which could be advantageous in terms of higher portion of available high-reactive facets as well as the shape of crystals. Only anatase phase was registered when annealing at 750 and 800 °C. The higher annealing temperature for the pure anatase phase was beneficial and resulted in better crystallinity, as also proved in other publications (Yu, Y***u et al. 2003, Mathews, Morales et al. 2009, Pližingrová, Volfová et al. 2015). Only about 3 % of rutile was registered in TBA_900 and MPA_900. It is a well-known fact that the presence of rutile phase can be advantageous in photocatalysts as it leads to a more efficient separation of charge carriers (Ibhadon and Fitzpatrick 2013). The 3 % rutile presence is especially noticeable when compared to the benchmark commercial photocatalyst which contains approx. 14–17 % of rutile (Ohtani, Prieto-Mahaney et al. 2010). The most active TBA_900 showed about four times higher rate constant. All samples of titania prepared by annealing of amino-peroxo-titanates reported faster degradation than Evonik P25 (with the exception of samples annealed from TPA_RT sample).

4

4 Conclusion

A series of various carbon doped 2D-titanium dioxide microsheets were prepared from the lyophilized colloids of peroxo titanic acid by precipitation of n-propylamine (mono, di, and tri) and tert-butylamine and annealed in the temperature range 750 – 900 °C. Thickness of units of nanometers of initial amorphous nanosheets remains preserved even after annealing. In the most cases, higher annealing temperature positively influences photocatalytic performance as with higher temperature rose the crystallinity of anatase and advantageously particle size remained in nanometric scale (up to 83 nm). The most of organics was burnt during heating, but the elemental analysis and XPS confirmed doping of annealed samples of C (under 1 wt%). The EPR spectroscopy brought valuable information confirming the presence of paramagnetic sites in the annealed samples which is characteristic for the anatase and rutile structures of Ti(III). The choice of amine had an impact of the way of growth of TiO2 crystal. Favourable procedure showed samples precipitated with primary amines (TBA and MPA) which displayed the highest photocatalytic efficiency, even four times higher photodegradation rate compared to P25 benchmark upon UV radiation. The key to the high photoactivity lays in the combination of high anatase crystallinity with the relatively low crystallite size, sufficient availability of photoactive centres (given by the a/c value), electron transfer and low representation of rutile while maintaining leaflet structure.

Acknowledgment

The authors thank to Dr. Dmytro Bavol, Pavla Kurhajcová and Monika Maříková for EA, FTIR and Raman measurements.

The authors acknowledge the project 8X20001supported by the Ministry of Education, Youth and Sports of the Czech Republic and Southeast Asia – Europe Joint Funding project SEAEUROPEJFS19ST-076.

This work was supported by the Slovak Research and Development Agency under the contract No. DS-FR-19-0001 (Multilateral Scientific and Technological Cooperation in the Danube Region). D. Dvoranová thank Ministry of Education, Science, Research and Sport of the Slovak Republic for funding within the scheme “Excellent research teams”.

This work was supported by Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project no. LM2018124 and by the Ministry of Education, Youth and Sports of the Czech Republic and The European Union - European Structural and Investments Funds within the project Pro-NanoEnviCz II (Project No. CZ.02.1.01/0.0/0.0/18_046/0015586).

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2022.103808.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

Supplementary data 1


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