Controlled Chemical Transformation and Crystallization Design for the Formation of Multifunctional Cu-Doped ZnO/ZnAl2O4 Composites

Photoactive Cu-doped ZnO–ZnAl2O4 ceramic nanocomposites and coatings were prepared by polymer-salt method. The luminescent spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction methods were used for the study the structure, morphology and materials properties. The nanocomposites consist of small hexagonal ZnO and cubic ZnAl2O4 nanocrystals having size about 10 nm. The study of luminescence properties shows that prepared nanocomposites can be used as light down-converters that transfer short-wave ultraviolet (UV-C) radiation into long-wave ultraviolet (UV-A) and visible spectral range. These nanocomposites can be very attractive in photovoltaic applications as spectral down-converters. Also obtained nanocomposites demonstrate the ability to generate chemically active singlet oxygen under UV-A radiation and blue light. The experiments show that Cu-doped ZnO–ZnAl2O4 materials demonstrate antibacterial activity against gram-positive bacteria.

The highly dispersive materials, consisting of small nanoparticles with high surface area, show higher photocatalytic properties and antibacterial activity * Andrey Aleksandrovich Shelemanov shelemanov@mail.ru compared with macroscopic ones [27]. The decrease of the size of photoactive particles and the optimization of materials morphology are used for the enhancement of their photocatalytic and bactericidal efficiency [22,23,[28][29][30][31]. It is known [22,23,32] that the sizes of crystals in two-component oxide composites are smaller than in one-component analogs obtained by the same method under similar technological conditions. In [17,22,23] this approach was used for the fabrication of highly dispersive photoactive materials ZnO-Al 2 O 3 [17], ZnO-SnO 2 [22] and ZnO-MgO-Ag [23]. Different methods have been applied for the preparation of Cu-containing ZnO nanomaterials: glucothermal method [7], co-precipitation [11,15], polymer-salt [16], etc. Polymer-salt method based on the application of initial solutions containing metals salts and soluble organic polymers is widely used for the preparation of different materials [9, 10, 17-19, 22, 23]. The temperature-time schemes of technological processes used in [17,22,23] provide the simultaneous formation of different crystals (ZnO + γAl 2 O 3 [17], ZnO + SnO 2 [22] and ZnO + MgO [23]) without their chemical interaction. The simultaneous formation of different crystals prohibits their growth and the aggregation and provides the formation of the material structure consisting of small particles with high specific surface area.
The aim of this work was to synthesis of Cu-doped ZnO-ZnAl 2 O 4 nanocomposites which can be used as luminescent spectral down-converters and bactericidal materials by the polymer-salt method and to study their structure, luminescent and bactericidal properties, and the ability to generate singlet oxygen.

Materials and Methods
The polymer-salt method which is applied for the synthesis of different nanoparticles [9,10,[17][18][19][20] was used in this study. The aqueous solutions of Zn(NO 3 ) 2 , Al(NO 3 ) 3 and CuSO 4 were used as raw materials for the nanocomposites synthesis. The solution of polyvinylpyrrolidone (PVP) (K30; M w = 25,000-35,000) in propanol-2 was added to the mixture of aqueous solutions of metal salts. Obtained mixtures were stirred for 30 min at room temperature. After drying obtained polymer-salt composites were calcined in air atmosphere at 680 °C for 2 h. Chemical compositions of initial solutions and obtained composites are given in Table 1.
The diffractometer Rigaku Ultima IV was used for X-ray diffraction (XRD) analysis of prepared materials. The diffraction patterns were scanned from 20° to 100° (2θ). The crystallite size was calculated using the Scherrer's equation: where d is the average grain size of the crystallites, λ the incident wavelength, θ the Bragg angle (radians) and β is the full width at half maximum (FWHM) in radians.
The photoluminescence measurements were carried out on the Perkin Elmer LS-50B fluorescence spectrophotometer in the spectral range 200-650 nm.
To study the antibacterial activity of the oxide composites, the method based on the diffusion into agar and described in [44] was used. The test used representative of the gram-positive bacteria Staphylococcus aureus ATCC 209P. The bactericidal activity was assessed by measuring the size of the inhibited zone. The experiments were carried out in natural light.  Table 2.
The structures of prepared composites consist of small nanocrystals. The average sizes of ZnO crystals are 8-25 nm that is less than the size of similar crystals in ZnO-MgO (32-35 nm, [45]) and ZnO-MgO-Ag (22-25 nm, [23]) powder composites previously formed by the similar polymer-salt technique at 550 °C. In addition, the size of ZnO crystals is close to that was observed in ZnO-ZnAl 2 O 4 composites obtained by the sol-gel method at a temperature of 600 °C (19-22 nm) [16]. In [23,45] MgO particles played the barrier role by spatially separating ZnO crystals and preventing their aggregation and the growth. Based on the presented experimental results, it can be concluded that limiting the growth of the formed ZnO crystals by chemical transformation of some of them into another crystalline matrix (ZnAl 2 O 4 ) at the stage of crystallization of the material is also effective.
The lattice constants of hexagonal ZnO crystals mostly range from 3.2475 to 3.2501 Å for the a-parameter and from 5.2042 to 5.2075 Å for the c-parameter [3]. Cu 2+ ions slightly smaller than Zn 2+ (ionic radii 0.57 and 0.60 Å, correspondingly) and Cu 2+ easily replace Zn 2+ in crystal structure that leads to contraction of crystal cell [46]. Therefore, the absence of any peaks of Cu compounds (Fig. 1) and lower values of lattice parameters a and c of formed ZnO crystals compare with the literature data [3] ( Table 2) may indicate the incorporation of copper ions into their structure. This corresponds to the data reported in [16] that ZnO-CuO materials with copper content lower than 15% are one-phase wurtzite-like Cu x Zn 1−x O.
SEM analysis showed that the composites consist of nanoparticles with a size of < 20 nm (Fig. 2a) that can facilitate their effective contact with the environment and impart high photocatalytic and bactericidal characteristics to the materials. The morphology of the resulting composites is similar to that demonstrated in [9] for ZnAl 2 O 4 xerogels. Observed nanoscale morphology of composites fully agrees to the data of their crystal structure obtained from XRD analysis.

Photoluminescence
Nu m e ro u s e m i s s i o n p e a k s a r e o b s e r ve d i n photoluminescence spectra of prepared powders (Fig. 2). These peaks are located at 343, 399, 423, 440, 461, 487 and 533 nm. Usually, two main emission bands are observed in luminescence spectra of ZnO-based materials: excitonic peak (NBE) in near UV region and wide emission band in visible spectral range that is related to different defects of ZnO crystal structure [48].
In the photoluminescence spectra of ZnO thin films obtained by radio frequency (RF) magnetron sputtering, many emission peaks were observed associated with the recombination of photogenerated holes with various structural defects, for example, ionized charge states of intrinsic defects, oxygen vacancies, zinc interstices, and zinc vacancies [2]. These luminescence bands were in ultraviolet (λ max = 399 nm), violet (λ max = 417, 438, 453 nm), blue (λ max = 467 nm) and green spectral ranges.
ZnAl 2 O 4 crystals are considered as potential ultraviolet emitting phosphor for the medical sterilization lamps [49]. These crystals emit short-wave ultraviolet (UV-C) light under vacuum UV (λ ex. < 200 nm) irradiation with high effectiveness [49]. Three emission bands with maximums at 400, 411 and 444 nm were observed in luminescence spectra (λ ex. = 325 nm) of ZnAl 2 O 4 nanoparticles prepared by polymer-salt method [9]. These emission peaks can be ascribed to intra band gap defects such as oxygen vacancies [9,10]. The luminescence peaks located at 428 and 561 nm attributed to structural defects were observed in ZnAl2O4/ ZnO composites obtained by citrate sol-gel method [16].
The peak located at ~ 340 nm (Fig. 3) can be assigned to the emission of ZnAl 2 O 4 crystals. It is worth noticing that this emission is observed under UV irradiation (λ ex. = 240 nm). So, obtained materials play the role of down-converters absorbing UV-C radiation and emitting the light of long-wave ultraviolet (UV-A) spectral range. Such down-converters can increase the efficiency of solar panels based on the ground and in space.
It is known that UV radiation can be divided into three parts: UV-A (320-400 nm), middle-wave ultraviolet (UV-B) (280-320 nm), and UV-C (200-280 nm). Because UV-C radiation has higher photon energy than the binding energy of carbon-carbon bonds it can destroy different hazardous materials and bacteria and shows a function as sterilization. However, UV-C is very hazardous to organisms [50,51]. UV-A radiation is friendly to living organisms and is effective in producing tannins and vitamin D [49]. Also, it was reported in [52] that the growth of vegetables plants exposed to UV-A radiation was greater than that of plants exposed to no UV radiation. This effect of UV-A radiation has associated with an increase in chlorophyll content in vegetables and increased photosynthetic activity.
It is worth noticing that some overlapping of the bands in luminescence excitation spectra (curve 5 (Fig. 3a); curve 4 ( Fig. 3b)) and emission peaks in luminescence spectra (curves 1 and 2 (Fig. 3a); curve 1 ( Fig. 3b) that is observed in the spectral range 330-360 nm. Taking in account this fact and close nanoparticles package in the material structure (Fig. 2) it is possible assuming the possibility of the energy transfer between different centers or to the light reabsorption in composites with the following emission in the visible spectral range. Figures 4 and 5 show the photoluminescence spectra of prepared powders in near infrared (NIR) spectral range. The luminescent band with λ max. = 1270 nm which is characteristic for singlet oxygen was observed in photoluminescence spectra of all samples (Figs. 4, 5). The comparison of Figs. 4 and 5 shows that the luminescent band intensities are higher at the irradiation of blue light (λ ex = 405 nm) (Fig. 5). This is related to the higher power density of blue LED compare with UV LED (see "Material and Methods" section).

Antibacterial Activity
Experiments demonstrated antibacterial activity composites against gram-positive bacteria. Figure 6 shows the photo of composite 1 disposed inside agar with gram-positive bacteria Staphylococcus aureus ATCC 209P. This photo demonstrates the dark zone surrounding the sample which zone is free from bacteria. The comparison of the thickness of these zones with our previous results [22,23] shows that prepared nanocomposites demonstrate comparable bactericidal properties against gram-positive with Ag-containing ZnO-based composites.
The mechanisms of antibacterial effect of ZnO-containing nanomaterials include the generation of reactive oxygen species [23,24,53,54], the destruction of bacterial cell integrity [55], diffusion antimicrobial Zn 2+ ion into the bacterial cell [56]. The obtained experimental results of singlet oxygen photogeneration (Fig. 4) suggest that the generation of reactive oxygen species (ROS) can plays the key role in the antibacterial effect of prepared materials.

Conclusion
Cu-doped ZnO/ZnAl 2 O 3 nanocomposites were prepared by polymer-salt synthesis at 680 °C. The nanocomposites consist of small hexagonal ZnO and cubic ZnAl 2 O 4 nanocrystals having size about 10 nm. The limitation of forming ZnO crystals growth by the chemical conversion of their part into another crystal matrix during material crystallization stage is effective for the synthesis of highly dispersive photoactive materials. Cu ions were embedded into the crystal structure of Zn-containing crystals. Obtained