Introduction

Zinc oxide (ZnO)-based oxides have n-type semiconducting properties, wide bandgap, and excellent chemical and thermal stability, which makes them potentially useful in solar cells, light-emitting diodes, and thin-film transistor liquid–crystal displays [1–3]. Zinc oxide is a semiconductor with a wide band gap (3.3 eV), which can absorb UV light with the wavelength equal or less than 385 nm. Zinc oxide is used in many applications such as optical coatings, solar cells, electrical devices, and in gas sensors. It is desirable that zinc oxide absorbs not only UV but also visible light. This can be achieved by implanting transition metal ions such as manganese [4–6]. Different methods have been applied to prepare zinc oxide and doped zinc oxide nanoparticles and can be categorized into either chemical or physical methods. The chemical methods for preparation of zinc oxide and doped zinc oxide nanoparticles are thermal hydrolysis techniques and sol–gel method while the physical method are spray pyrolysis and vapor condensation [7]. Band gap of zinc oxide can be narrowed or split into several sub-gaps by implanting transition metal ions to absorb visible light [8]. Doping of transition metals (like manganese) increases the surface defects [9, 10] and it affects the optical and electronic properties and can shift the optical absorption towards the visible region. Differential thermal gravimetry (DTG) and thermogravimetry (TG) are useful to study the modes of thermal decompositions of precursors [11–14]. The objective of this work is to synthesis manganese-doped zinc oxide using new manganese precursors by thermal decomposition. In this paper, for the first time, we examined preparation and characterization of manganese zinc oxide nanocrystals using thermal decomposition of new manganese precursors using sol–gel process. Among all of synthesis methods, the sol–gel process used in this study have the advantages of good homogeneity, ease of composition control, low processing temperature, large area coatings, low equipment cost, and good optical properties.

Experimental

Materials and methods

All solvents and chemicals were purchased from Sigma-Aldrich and used as received, except for the amines which were distilled under reduced pressure before use. UV–Vis spectra were recorded on a Varian Cary 500 Scan spectrophotometer. Infrared spectra (KBr pellets) were obtained on a Bruker FT-IR (Tensor 27) spectrophotometer. Thermal gravimetric differential thermal analysis (TG–DTA) measurements were carried using a Mettler TA4000 system. The phase structure of undoped and manganese-doped zinc oxide was identified by a Brucker D8-advance X-ray diffractometer with Cu Kα radiation. The surface morphology and chemical composition of undoped and manganese-doped zinc oxide were studied using scanning electron microscope (SEM) (Philips XL-30 operated at 10 kV of acceleration voltage). The Brunauer-Emmett-Teller (BET) method, using nitrogen adsorption at liquid nitrogen temperature 77 K, was employed to measure total surface area of the photocatalysts.

Synthesis and structural determination of manganese complex

Synthesis of [Mn(L)(H2O)]2(PF6)2·TZ, TZ = (5-phenyl-1,3,5-triazine-2,6-diamine) (1) as precursor is as followings: To a stirring solution of Mn(CH3COO)2·2H2O (0.0662 g, 0.500 mmol) in ethanol (25 mL) was added an equimolar of N,N′-Bis(2-hydroxy-5-methoxybenzylidene)-propane-1,2-diamine (0.5 mmol) and 2 mmol of 5-phenyl-1,3,5-triazine-2,6-diamine and the mixture was stirred for 3 h. The pink solution turned dark brown immediately upon the formation of Mn(III) complex, and then 0.500 mmol of NH4PF6 was added. A dark brown microcrystalline solid was produced by slow evaporation of ethanol at room temperature. The product was then recrystallized from methanol–propanol (2:1 v/v) and dark brown crystals suitable for X-ray crystallography were obtained. Similar procedure was used for preparation of [Mn(L)(H2O)]2(PF6)2 (2) using manganese acetate and ligand. Dark brown crystals of complexes 1 and 2 were mounted with a cryoloop and flash-cooled by cold nitrogen stream. All measurements were made at 193(2) K on a Rigaku RAXIS RAPID imaging plate area detector with graphite monochromated Mo Kα radiation. Absorption corrections were applied by the numerical method [15]. The structure was solved by the direct method using SIR2004 [16] and refined on F 2 with all independent reflections by full-matrix least-square method using SHELXL97 program [17]. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced at the positions calculated theoretically and treated with riding models.

Synthesis and characterization of nanocrystalline zinc manganese oxide

Undoped zinc oxide sol was prepared using zinc acetate dehydrate (99.99 %), 2-propanol and monoethanolamine (≥99.0 %) as sol stabilizer. ZnO sol was heated to 60 °C continuously stirred for 1 h to achieve a transparent ZnO sol and then aged for 1 day. Two new dinuclear manganese(III) complexes precursors were used as manganese precursor. It was dissolved in ethanol and acetonitrile as complexing agent which was then added to ZnO sol. The sols were dried at 275 °C for 10 min to evaporate the solvent and remove organic residuals and then were annealed at 350–550 °C for 1 h.

Results and discussion

Synthesis and thermal investigation

Two Mn(III) complexes have been prepared by the reaction of an equimolar of N,N′-Bis(2-hydroxy-5-methoxybenzylidene)-propane-1,2-diamine and manganese(II) acetate in the presence of the appropriate axial ligands (water) in aerobic conditions. The air oxidation was continued for a period of 3 h, which resulted in the dark brown color solution. Dark brown crystals of these complexes were obtained in good yield (70–80 %). TG–DTA analysis was conducted to clarify the thermal stability of the two complexes. TG–DTA of 1 and 2 are shown in Figs. 1 and 2, respectively. TG–DTG results in Table 1 indicate that complex 1 shows a major weight loss at 269 °C and complex and 2 shows a major weight loss at 314 °C.

Table 1 TG results of manganese complexes
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Fig. 1
figure 1

TG–DTG [Mn(L)(H2O)]2(PF6)2·TZ, TZ = (5-phenyl-1,3,5-triazine-2,6-diamine) (1)

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Fig. 2
figure 2

TG–DTG of [Mn(L)(H2O)]2(PF6)2 (2)

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X-ray structural, X-ray diffraction, and SEM analysis

X-ray structural analysis shows the presence of dimanganese core in the complexes with the bond distance of Mn–O (axial) and 2.46; Mn–O (equatorial), 1.89 Å [18]. The electronic spectra of complexes 1 and 2 in methanol and ethanol have two maxima at 238 and 345 nm for 1 and 237 and 360 nm for 2, respectively. The absorptions at 232 and 238 nm can be related to the spin allowed π–π* azomethane intraligand transition [19–21]. X-ray diffraction (XRD) diffractogram of the pure zinc oxide and manganese zinc oxide was analyzed to study crystalline aspects. Figure 3 shows the XRD patterns of manganese-doped zinc oxide nanocrystals. The results showed that the samples are highly crystalline with sharp and intense peaks. The XRD peaks indicate the formation of crystalline phase with wurtzite structure of undoped zinc oxide. The XRD patterns of Mn-doped ZnO are similar to pure ZnO. The crystalline size of pure and Mn-doped ZnO can be calculated using Scherrar’s formula. The diffraction peaks of the Mn-doped zinc oxide showed a slight shift toward smaller diffraction angle when compared with undoped zinc oxide, indicating that the lattice constants of the manganese-doped zinc oxide changed slightly due to the larger ionic radius of Mn2+ (0.83 Å) in comparison with that of Zn2+ (0.74 Å). This result shows that manganese-doped zinc oxide has the same wurtzite zinc oxide phase. SEM of undoped and manganese-doped zinc oxide nanocrystals at different stage of annealing (350, 450 and 550 °C) depicting the topography are shown in Fig. 4a, b, c, d, respectively. The surface area of the manganese-doped zinc oxide (Fig. 5) showed the surface area increased when the annealing temperature increases up to 450 °C. However, the surface area was shown to decrease at annealing temperature of 550 °C. Among the four samples, undoped-zinc oxide has the lowest BET surface area (28 m2g−1), while manganese-doped zinc oxide annealed at 450 °C has the highest BET surface area.

Fig. 3
figure 3

XRD pattern of manganese-doped zinc oxide

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Fig. 4
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a SEM image of undoped zinc oxide. b SEM image of manganese-doped zinc oxide annealed at 350 °C. c SEM image of manganese-doped zinc oxide annealed at 450 °C. d SEM image of manganese-doped zinc oxide annealed at 550 °C

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Fig. 5
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Effect of annealing temperature on the specific surface area (BET) of manganese-doped zinc oxide

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Conclusions

Two new homo binuclear manganese(III) complexes were prepared and characterized as new precursors for manganese-doped zinc oxide. Hexagonal wurtzite-structured manganese-doped zinc oxide nanocrystals were prepared by the thermal decomposition method. Manganese ions were successfully incorporated into lattice sites in the zinc oxide lattice. TG–DTA results indicate that the major weight lose is occured above 250 °C.