1 Introduction

Zinc oxide nanoparticles (NPs) possess a broad range of useful properties (semiconducting, magnetic, optical, luminescence, chemical), are non-toxic and relatively stable chemically and thermally. Due to these properties, they play essential part in the development of new materials for electronics and optoelectronics [1–4], photovoltaics [5–7], photocatalysis [8], chemical sensing [9, 10], biomedical visualization and marking [11, 12], as well as in the development of the preparations with antiseptic, bactericidal, and cytotoxic action in medical nanotechnologies [13–15].

A large number of methods to obtain ZnO NPs are known, including chemical methods working in solutions [8, 16]. Among them are widespread syntheses, which include the hydrolysis of Zn2+ salts in an alcohol-containing medium based on ethanol (EtOH) [12,17–20], isopropanol (i-PrOH) [21–23], methanol (MeOH) [17, 24], benzyl alcohol (BnOH) [25], 2-methoxyethanol (2-ME) [17]. In many cases, hydrolysis is carried out in the presence of strong bases (LiOH, NaOH, KOH, (CH3)4NOH), however, additional purification of ZnO from the compounds containing alkaline metal cations is necessary. To stabilize the NPs, provide a narrow size distribution or render definite physicochemical properties to the surface of NPs, functionalized organic compounds are introduced into the reaction mixture during the synthesis. These compounds act as protective ligands, for example octanethiol, dodecanethiol, octanedodecylsulfonic acid, dopamine (Electronic Supplementary Material, Table S1).

In this paper, we investigated solutions of acrylamide (AAm) in 96% ethanol and water as new reaction media for the synthesis of ZnO nanoparticles and a “NPs/polymer” type composite in the form of a hydrogel. AAm molecules have vinyl, carbonyl, and amide functional fragments. Due to the electron-donor properties of the amide group, AAm can play the role of a protective ligand, as is observed for Au NPs [26], or act as an organic base that accepts protons arising during the reactions. Due to the vinyl group, AAm can polymerize into polyacrylamide (PAAm) [27], used to produce micro-patterned gel structures for many applications, particularly in biotechnology area [28]. Taking into account solubility in ethanol-AAm is soluble, PAAm is insoluble (both compounds are soluble in water)-syntheses of NPs and composite gel by hydrolysis of zinc acetate were carried out in 96% ethanol and water, respectively. To synthesize the composite gel, the hydrolysis of zinc acetate was carried out simultaneously with the radical polymerization of AAm initiated by (NH4)2S2O8. We expected that AAm, which is taken for synthesis in excess, will promote the hydrolysis of the zinc salt by binding the emerging protons: Zn2++ H2O + 2 CH2CHC(O)NH2 → ZnO + 2 CH2CHC(O)NH3 +. The amide groups of polyacrylamide should also have similar basic properties.

Data are available on acrylamide products with micro-sized ZnO particles, which improve the physico-chemical characteristics of the drilling fluid for effective oil extraction [29]; no other data on the ZnO NPs associated with AAm or PAAm was found.

2 Experimental

2.1 Materials

Zinc acetate dihydrate (Kh. Ch.–chemically pure grade, Chimmed, Russia), acrylamide for electrophoresis (>99.0%, Os. Ch.–specially pure grade, Medigen, Russia), ammonium persulfate (reagent grade, 98%, Sigma-Aldrich), and rectified 96% ethanol were used without further purification.

2.2 Synthesis of ZnO NPs in ethanol

A mixture of solid substances: 0.232 g Zn(CH3COO)2·2H2O (1.1 mmol) and 0.180 g AAm (2.5 mmol) was placed in a broad vial; 25 ml of 96% ethanol was added, and mixing was carried out. The resulting turbid solution was homogenized for 5 min with ultrasound, then heated for 4 h inside a drying chamber at 55 °C in the closed vial. After cooling to room temperature, a transparent colorless solution and voluminous white precipitate were obtained. The vial with solution and precipitate was sonicated for 5 min. About 1 ml of suspension was taken from the vial for TEM examination (sample I). After suspension settling, again a transparent solution (colloid, judging from its ability to scatter laser beam) and a voluminous white precipitate were obtained. About 1 ml of the transparent supernatant was taken for TEM (sample II). The precipitate was centrifuged, separated from the liquid phase, washed with ethanol, and dried at a temperature of 130 °C (sample I-130) then a part of the substance was additionally heated at 300 °C for 1 h (sample I-300). The samples were stored in a desiccator above P2O5.

In connection with the synthesis in alcoholic media, we noted one interesting experimental fact, although not directly related to this work. In one of the preliminary experiments on the synthesis of ZnO NPs by the reaction of zinc acetate with KOH, the solvent was not ethanol but isopropanol which was stored for a long time in the laboratory, and which evidently had accumulated a significant amount of peroxide compounds due to oxidation in the air. As a result, the product of synthesis turned out to be the NPs of ZnO2 about 4–5 nm in size. The experiment was repeated, and the result was reproduced in full.

2.3 Synthesis of ZnO NPs/PAAm composite in water

Twenty five milliliter of water was added to a mixture of 0.221 g of Zn(CH3COO)2·2H2O (1.0 mmol) and 1.25 g of AAm (18 mmol) in a broad vial. The sonication was carried out for 5 min. A transparent solution was obtained. Then ~6 mg of (NH4)2S2O8 (0.03 mmol) was added as an initiator of the radical polymerization of AAm [27, 28]. Heating for 4 h inside the drying chamber at 55 °C in the closed vial was carried out. A transparent colorless product in the form of hydrogel was obtained. It was scattering the laser beam and characterized by high viscosity. About 1 ml of the product was sampled for TEM (sample III), a fraction was left for the examination by dynamic light scattering (DLS), while the rest fraction was transferred into a separate test tube, and ethanol was added at a volume ratio of 1:1. Flaky white precipitate was immediately formed. The mixture in the test tube was sonicated; about 1 ml of the resulting suspension was taken for TEM (sample IV). The composite hydrogel synthesized in this way was stable: its appearance did not change during observations for 9 months. The synthesis of PAAm hydrogel was carried out in parallel with the synthesis of the composite under the same conditions but without Zn(CH3COO)2·2H2O.

3 Methods

The average hydrodynamic diameter of the particles in hydrogel III was measured by DLS at an angle of 90° in a quartz cell with a cross section of 1 cm2 at room temperature with the help of a 90Plus spectrometer (Brookhaven Inst). To remove dust, liquid samples were many times passed through a FPV-403-030 membrane (Jet Biofil) with pore diameter 0.45 μm.

XRD examination of NPs samples I-130 and I-300 was carried out with a Shimadzu XRD-7000 diffractometer (CuKα radiation, Ni filter, Bragg–Brentano arrangement, scintillation detector, room temperature) within the angle range 2θ from 5° to 60°. Scanning step was 0.03°, the time of accumulation in a point was 1 s. The samples for measurements were prepared by drying the suspension in heptane at the polished side of the quartz cell at room temperature. Indexing of diffraction patterns was carried out relying on the PDF database, release 2010. Crystallite size was calculated using the Scherrer equation and three individual peaks of ZnO at 31.8° (100), 36.3° (101), and 56.6° 2θ (110) taking into account the half-width of the peaks of reference Si.

The morphology, size and composition of the particles in the suspension in ethanol (sample I) and in supernatant (sample II), as well as in hydrogel (sample III) and precipitated composite (sample IV) were studied by means of TEM, HRTEM, and SAED with a JEM-2010 instrument (JEOL Ltd.) with the accelerating voltage of 200 kV. For measurement, a drop of the dispersion of NPs sample treated with ultrasound was dried on a thin perforated carbon film substrate fixed on a copper grid.

Examination of NPs by means of SEM and EDX was carried out with a JSM-6700F instrument (JEOL) equipped with an energy-dispersive X-ray analyzer EX-23000BU. Accelerating voltage was 5 keV, the working distance was 3 mm, the detector of secondary electrons was used to record the images. The samples were prepared by evaporating the drops of ethanol suspensions of NPs precipitate dried at 130 °C (sample I-130) and heated at 380 °C (sample I-380). Evaporation was performed on silicon substrates. Then a conducting layer of Au-Pd alloy was deposited on the samples.

The Raman spectra of the NPs powder dried at 130 °C (sample I-130) were recorded with a LabRAM HR spectrometer (Horiba) in the back-scattering geometry using the 633 nm line of a He–Ne laser and a Raman microscope. The spectral resolution was 2.5 cm−1.

The IR spectra of powder samples prepared by pressing as tablets with KBr were recorded with Scimitar FTS 2000 instrument within the range 375–4000 cm−1 and Vertex 80 within the range 100–600 cm−1.

The spectra of photoluminescence (PL) and excitation of luminescence (EL) for powders of NPs (sample I-130) and ААм, as well as for the hydrogels of ZnO NPs/PAAm (sample III) and PAAm were recorded with a Fluorolog-3 spectrofluorimeter (Horiba Jobin Yvon) with a cooled module of photon registration PC177CE-010 equipped with a photo-electronic multiplier R2658.

Analyses of the NPs sample I-130 on Zn content (atomic absorption after the acid dissolution of the sample) and C, H, N (microanalysis) were carried out at the Analytical Laboratory of Nikolaev Institute of Inorganic Chemistry.

4 Results and discussion

4.1 Characterization of ZnO NPs from ethanol

As stated above, the synthesis products in the ethanol solution of AAm were a voluminous white precipitate suspended in the lower layer of the liquid phase and a transparent, colorless solution in the upper layer that scattered the laser beam, therefore it contained colloidal particles.

The TEM data for the samples of products prepared as films on substrates by the evaporation of suspension drops (sample I) or supernatant (sample II) containing excess solid AAm along with NPs are shown in Fig. 1. As expected, the particles in the sedimentation precipitate were somewhat larger than the particles that were not settled from the supernatant and had a size of about 3–5 nm. Judging from the SAED electron diffraction patterns, both samples of NPs were polycrystalline and related to the hexagonal phase of ZnO. The SAED pattern of sample I (Fig. 1a, inset) showed the hexagonal arrangement of arched reflections with substantial azimuthal broadening ±∆ ≈ 20 degrees. The circular kind of diffraction (without arched reflections) for sample II (Fig. 1c, inset) is the evidence of the stronger disordering of NPs after their aggregation in the form of a film. At the same time, for NPs 3–5 nm in size, HRTEM images (Fig. 1d) reveal periodicity characteristic of interplanar spacings d 100 = 0.28 nm of the nanocrystalline ZnO phase [30].

Fig. 1
figure 1

a TEM and b HRTEM images of ZnO NPs of sample I; c TEM and d HRTEM images of ZnO NPs of sample II; the insets show the SAED data

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The results of XRD studies for the NPs precipitate, isolated, washed and dried at 130 °C (sample I-130) and 300 °C (sample I-300), confirmed that the product contained one crystal phase of ZnO (Fig. 2). Crystallite sizes estimated on the basis of XRD data were somewhat different for different 2θ and increased with an increase in temperature (Table 1). These estimations gave large NP size than the size of NPs observed in TEM images shown in Fig. 1, thus, isolation from solution and drying of NPs precipitate was accompanied by a slight increase in the linear dimensions of NPs in different directions.

Fig. 2
figure 2

XRD patterns of samples I-130 (lower line) and I-300 (upper line)

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Table 1 Crystallite sizes, evaluated from XRD data
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SEM images of the samples I-130 and I-300 showed that they were composed of oblong aggregates of NPs, with cross section of the order of 101 nm and the length ~102 nm. The appearance of the aggregates did not change after drying temperature was increased from 130 to 300 °C (Fig. 3). The EDX spectrum of the sample I-130 deposited on a silicon substrate contained intense lines of Zn and O from particles, the line of Si from the substrate, and a low-intensity line of C that could be attributed to AAm and the products of its reactions or to the initially introduced CH3COO− ions. Besides, lines of nitrogen were not observed in an explicit form. A similar situation was for sample I-300.

Fig. 3
figure 3

SEM images of samples a I-130 and b I-380; c EDX analysis of sample I-130

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Raman and IR spectra of sample I-130 shown in Fig. 4 differed from the known spectra of AAm and PAAm, in particular, the bands of vibrations related to the amido group were absent [31, 32]. At the same time, they were concordant with the combinations of the known spectra of ZnO [33, 34] and CH3COO− in the salts with Zn2+, Pb2+, Li+ [35]. This was also evidenced by the experimental spectra of reference substances, that were reagents such as ZnO powder for phosphors and crystalline salt of chemically pure Zn(CH3COO)2·2H2O. In the Raman spectra of sample I-130, the peaks at 98, 336, and 436 cm−1 are related to ZnO, while the peak at 220 cm−1 and all the peaks within the range 600–3200 cm−1 are related to CH3COO− (Table 2). Similarly, comparing the IR spectra with the spectra of reference substances, we state that absorption related to ZnO was observed in the IR spectrum of sample I-130 within the range 300–600 cm−1, while the bands in the region 600–3600 cm−1 were due to the vibrations in CH3COO− (Fig. 4b, Table S2).

Fig. 4
figure 4

a Raman spectrum of sample I-130; b IR spectra of sample I-130 (1) and reference substances ZnO (2) and Zn(CH3COO)2·2H2O (3)

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Table 2 Wavenumbers (cm−1) of peaks in the Raman spectrum of sample I-130 of ZnO NPs and the known data for ZnO [33], [34] and CH3COO− in Zn(CH3COO)2·2H2O [35] and Pb(CH3COO)2·3H2O [35]
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4.2 Chemical composition of ZnO NPs

The chemical analysis confirmed the absence of nitrogen in sample I-130, therefore AAm was absent. The detected Zn, C, and H content was in good agreement with the calculated content of these elements in the particle having conditional composition {ZnO} m {Zn(CH3COO)2} n or ZnO1/(1+n/m)(CH3COO)2(n/m)/(1+n/m) calculated per one Zn atom for the ratio of n/m = 0.292, which was optimized by the least squares method (Table 3). Agreement was also observed for the formula mass M* and for the H/C atomic ratio.

Table 3 Characteristics of the NPs composition, determined in the analysis of sample I-130 and calculated for ZnO1/(1+n/m)(CH3COO)2(n/m)/(1+n/m) at n/m = 0.292
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According to the notion about the structure of NPs, Zn2+, and CH3COO− ions should most probably belong to the surface layer. The adsorption of CH3COO− ions on the ZnO NPs surface has already been reported [23]. Therefore, the particles can be represented as {(ZnO) m (Zn2+) n }(CH3COO−)2n , where n surface cations Zn2+ make the charge of the particle which is compensated by 2n anions CH3COO−. From the viewpoint of composition, this stoichiometry is formally equivalent to the above-indicated stoichiometry {ZnO} m {Zn(CH3COO)2} n . As for AAm, it probably has an insufficient affinity for ZnO to play the role of a protective ligand, as was the case with Au NP [26]. Thus, in the synthesis of NPs in ethanol, AAm acts only as organic base which binds H+ ions formed during the hydrolysis of Zn(CH3COO)2·2H2O.

4.3 Modeling of ZnO NPs by “equivalent spheres” method

The absolute values of m and n were estimated using the equivalent spheres approach on the basis of experimentally determined stoichiometric ratio n/m = 0.292. Within this approach, the molecules of a crystalline substance are represented as spheres with the volume V which is equivalent to the V cell/z ratio, where V cell and z are the unit cell volume of the crystal and the number of formula units per cell. Taking into account available crystallographic data [30, 36, 37] the equivalent volumes of molecules and the corresponding radii were estimated as V 1 = 0.02388 nm3, r 1 = 0.179 nm for ZnO and V 2 = 0.1656 nm3, r 2 = 0.341 nm for Zn(CH3COO)2. Similar values were also obtained in the calculations of these parameters from the known densities of ZnO (5.61 g/cm3) and Zn(CH3COO)2 (1.84 g/cm3). The problem of the estimation of m and n values was formally reduced to the determination of radius R for a sphere with volume V c inside which it would be possible to accommodate m balls with radius r 1, and on its surface it would be possible to place n balls with radius r 2 in the form of a monolayer (Scheme S1). The relationships of R with the volume parameters of the nanoparticle is shown by the equations: V NP = (4/3)π(R + 2r 2)3–volume of the {ZnO}m{Zn(CH3COO)2}n particle, V c = (4/3)πR 3–volume of the {ZnO}m spherical fragment, V s = V NP−V c = (4/3)π{(R + 2r 2)3−R 3}–volume of the {Zn(CH3COO)2}n surface layer. Then, m = V c/V 1, n = V s/V 2 and n/m = {(1 + 2r 2/R)3−1}(V 1/V 2). For n/m = 0.292, we obtained R ≈ 1.53 nm, as well as V c ≈ 14.9 nm3, m ≈ 624, V s ≈ 30.2 nm3, n ≈ 182. The volume of the last spherical layer of ZnO molecules in direct contact with the outer layer of Zn(CH3COO)2 molecules was V l = (4/3)π{R 3−(R−2r 1)3}≈8.2 nm3 and contained m l = V l/V 1≈344 molecules. Therefore, the actual ratio was n/m l ≈ 0.53, that is, there was about 1 Zn(CH3COO)2 molecule per two ZnO molecules on the surface of NP. This result correlates with the fact that the equivalent spherical radius of ZnO molecule is about twice smaller than the corresponding value for Zn(CH3COO)2–r 1/r 2≈0.52. The size of the equivalent spherical particle was estimated as d NP* = 2(R + 2r 2) ≈ 4.4 nm, which agreed with TEM results.

It is interesting to note that a [Zn4O(CH3COO)6] cluster was reported [38], which can be formally represented as {ZnO}{Zn(CH3COO)2}3 and considered as a minor term of the same series of particles as those obtained by us {ZnO}m{Zn(CH3COO)2}n, but with m = 1 and n = 3.

4.4 PL data for ZnO NPs powder

The normalized EL and PL spectra of the NPs powder (sample I-130) are shown in Fig. 5. In the PL spectrum, under excitation at λ ex = 340 nm, two bands with emission maxima at λ em = 384 and 577 nm were observed. Under excitation at λ ex = 390 nm, the PL spectrum was a single broad band with the maximum at λ em = 590 nm. As it is known, the PL band in the UV region is caused by the recombination of free excitons, and the yellow emission in the visible region is associated with oxygen vacancies in the structure of ZnO NPs [39–41].

Fig. 5
figure 5

Normalized luminescence spectra of ZnO NPs in the powdered sample I-130: EL at λ em = 590 nm (1), PL at λ ex = 340 nm (2) and PL at λ ex = 390 nm (3)

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4.5 Characterization of ZnO NPs/PAAm composite from water

Though AAm turned out to be useless as a protective ligand, Zn(CH3COO)2 hydrolysis and AAm polymerization proceeding concurrently in the aqueous medium resulted in obtaining a composite material which was ZnO NPs distributed in the PAAm matrix. The synthesis product looked like a viscous transparent colorless hydrogel.

An attempt to estimate the size of polymer particles by means of DLS directly in the hydrogel (sample III) showed unreliability in the measurement data because of the polymodal distribution and a large value of the average hydrodynamic diameter at a level of 104–105 nm. After 1000-fold dilution with deionized water and multiple filtration through a membrane with pore size 0.45 μm, monomodal distribution was obtained, with the average hydrodynamic diameter of PAAm particles and standard deviation for 60 measurements equal to 448 ± 62 nm. Thus, the basis of the gel were polymer particles of PAAm, ≥400 nm in size.

TEM images of sample III clearly exhibit a polymeric matrix with ingrained NPs crystals (Fig. 6a,b). The crystallite sizes in different projections were estimated at 1–3 nm. The SAED image in the inset in Fig. 6a contains diffusely broadened circular reflections 100, 002, 101, 110, 103 etc., which points to the chaotic orientation of NPs crystallites in the polymeric matrix. Several nanocrystals about 2 nm in size with the most pronounced structure are marked in Fig. 6b by hexagonal frames corresponding to their shapes.

Fig. 6
figure 6

a TEM and b HRTEM images of ZnO NPs in sample III, several nanocrystals with the most pronounced structure are marked by hexagonal frames corresponding to their shapes; c TEM and d HRTEM images of ZnO NPs in sample IV, one of the ZnO nanocrystallites is marked with a white circle; the insets show the SAED data

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To precipitate the solid ZnO NPs/PAAm composite, the hydrogel was destroyed by adding ethanol in which PAAm is insoluble. The deposited composite (sample IV) contained NPs in more concentrated form, which was evidenced by the dense arrangement of NPs on the substrate, observed in TEM images and confluence of the structural pictures from separate particles in HRTEM images (Figs. 6c, d). It should be noted that NPs appeared in the TEM images of sample IV (Fig. 6c) gradually during the destruction of the polymeric matrix under the action of the electron beam. The size of ZnO crystallites was about 5 nm. The SAED picture presented in the inset in Fig. 6c points to the formation of the crystallographic texture with the axis of the zone [001] for diffraction reflections. In this situation, reflections 100 and 110, belonging to this zone, have the azimuthal broadening ±∆ ≈ 5 degrees. Thus, it may be concluded that the crystallites, initially disoriented chaotically, then with an increase in their size to 5 nm form a better oriented ZnO particles layer with the С axis going perpendicularly to the substrate. The image of one of these ZnO nanocrystallites is marked in Fig. 6d. The HRTEM and SAED data showed that NPs in the samples III and IV belong to the hexagonal phase of ZnO.

The UV-Vis-spectrum of the ZnO NPs/PAAm composite hydrogel, recorded relatively to the PAAm hydrogel, had the absorption band related to ZnO with the maximum at λ max = 308 nm (Fig. S1). This band was shifted to the shorter wavelength region in comparison with the exciton band of the bulk ZnO at ~370 nm, which is characteristic of nanoparticles [20].

The PL spectra of the hydrogels of the ZnO NPs/PAAm composite (sample III) and PAAm under excitation at λ eх = 330 nm are shown in Fig. 7. In both cases, the spectra were broad bands with the emission maxima at λ em = 428 and 449 nm for the composite and for PAAm, respectively. The presence of ZnO NPs in PAAm matrix caused a shift of the spectrum into the shorter wavelengths region by about 21 nm.

Fig. 7
figure 7

Normalized PL spectra of hydrogels of the ZnO NPs/PAAm composite (1) and PAAm (2), λ ex = 330 nm

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5 Conclusions

The study allowed us to develop simple one-step syntheses of ZnO NPs and ZnO NPs/PAAm hybrid composite. Synthesis procedures do not require special conditions and equipment. The reagents are cheap and available crystalline substances Zn(CH3COO)2·2H2O and AAm. The specific features of our synthesis of ZnO NPs by the known reaction of zinc acetate hydrolysis are the use of AAm as the organic base that accepts protons, and the conventional (not absolute) 96% ethanol as the reaction medium. The product of synthesis in ethanol were the crystalline particles of hexagonal ZnO of ~3–5 nm in size, exhibiting yellow photoluminescence with emission maximum at λ em = 590 nm. Based on the results of chemical analysis, physico-chemical studies and modeling, it was concluded that the surface layer of ZnO nanocrystals consisted of Zn2+ and CH3COO− ions. The average composition of the NPs was estimated as {ZnO}624{Zn(CH3COO)2}182.

The ZnO NPs/PAAm composite was obtained for the first time. The original feature of the synthesis is that the hydrolysis of Zn(CH3COO)2 was carried out in the aqueous medium, concurrently with the radical polymerization of AAm initiated by (NH4)2S2O8. The product of synthesis was the hydrogel of PAAm with nanocrystals of hexagonal ZnO of ~1–3 nm in size distributed over the hydrogel. The composite hydrogel was colorless, transparent, stable and exhibited photoluminescence with emission maximum at λ em = 431 nm. The solid ZnO NPs/PAAm composite was precipitated by breaking the hydrogel by the addition of ethanol, in which PAAM is insoluble.