Introduction

Due to their considerable performances in electronics, photonics, solar cells, and environmental applications such as air and water purification, the development of nano-sized metal-oxide semiconductors has attracted increasing attention [1,2,3,4,5,6,7]. Since the observation of the photosensitization effect by Fujishima et al. in 1971 [1], titanium dioxide (TiO2) semiconductors have been extensively studied because of their good biological and chemical stability, low cost, nontoxicity, and remarkable optical decomposition ability [8]. Titanium dioxide can be found in four forms of distinct polymorphs including anatase, rutile, brookite, and monoclinic phases [9]. TiO2 nanostructures have been potential candidates for applications as pigments, optical filters, antireflection coatings, chemical sensors, sterilization materials, and catalysts [10]. A wide variety of techniques like the co-precipitation method [10], hydrothermal treatment [11], and sol-gel method [12] have been utilized for the synthesis of TiO2 nanostructures. For example, Nagaraj et al. proposed a novel photon-induced method (PIM) to produce oxygen-rich TiO2 with modified optical band-gap and high stability [13]. Abisharani et al. described the green synthesis of TiO2 nanoparticles from titanium try chloride (TiCl3) solution using the extract of Cucurbita pepo seeds [14]. In a study, nonaqueous reactions between titanium (IV) chloride and alcohols (benzyl alcohol or n-butanol) were employed for the synthesis of anatase TiO2 particles, while aqueous media by acidic hydrolysis of titanium (IV) chloride were used for the preparation of rutile TiO2 particles [15]. Ren et al. also formed a heterojunction between anatase TiO2 nanosheets and anatase TiO2 nanoparticles by a vapor-induced hydrothermal method followed by photothermocatalytic treatment to reach a significant improvement in photocatalytic activity [16].

It is believed that some key limiting factors, including the wide ban-gap, the fast photo-generated electron-hole recombination, and the low quantum efficiency, are confining practical applications of TiO2 nanostructures to photocatalysis [17, 18]. In recent years, many researchers have focused on improving the photocatalytic performance of TiO2 nanostructures by various methods such as doping with noble metals, non-metals, transition metals, and dyes [19]. It has been explored that coupling with metal-oxide semiconductors can be a suitable strategy for promoting photocatalytic performances of TiO2 nanostructures [20]. In this way, Bandara et al. fabricated a new bilayer TiO2/MgO nanoporous photocatalyst to minimize the charge recombination and achieve an enhanced photocatalytic activity [21]. Toloman et al. synthesized interface modified SnO2-TiO2 composite nanoparticles by a two-stage chemical precipitation process to investigate photocatalytic activity [22]. Hou et al. described the performance of BiVO4@TiO2 core-shell hybrid mesoporous nanofibers towards efficient visible light–driven photocatalytic hydrogen production. Gaurav et al. prepared ZnO:TiO2 nanocomposites using the sol-gel method for photocatalyst application in the visible light [23]. The hybrid WO3/TiO2 photocatalysts were fabricated by Tahir et al. via the hydrothermal method for the degradation of methylene blue dye under visible light irradiation [24]. El-Sayed et al. also presented an investigation analysis on irradiated MgO-TiO2 binary oxide synthesized by the sol-gel method [25].

Among various semiconductors, Ag2O is a noble metal-oxide semiconductor with a band-gap ranging from 0.49 to 3.1 eV relevant to the quantum size effect [26, 27]. It has been reported that coupling n-type TiO2 semiconductor with p-type Ag2O leads to the formation of n-p heterojunctions resulting in separated photo-generated electron-hole pairs and inhibited charge carrier recombination [8]. To prepare nitrogen-doped TiO2 thin films composed of ultrafine Ag2O semiconductor nanoparticles with enhanced visible light absorption, Fang et al. utilized ion beam–assisted deposition [28]. Sadanandam et al. synthesized highly stabilized Ag2O-loaded nanoTiO2 using a hydrothermal-impregnation two-stage method for the photocatalytic production of hydrogen from glycerol:water mixtures under solar light irradiation [19]. Endo-Kimura et al. constructed Ag2O/TiO2 heterojunctions to examine the photocatalytic and antimicrobial properties for bacteria (Escherichia coli) and fungi (Candida albicans and Penicillium chrysogenum) [29].

It has been demonstrated that the structural benefits of binary heterostructured photocatalysts can be promoted through fabricating ternary graphene-based photocatalytic systems due to the high electron mobility (~ 15.000 m2/V s) of charge carriers in graphene. Graphene has been reported to be a two-dimensional structure with excellent chemical stability, optical absorption, and mechanical strength resulting in improved charge separation and facilitated the formation of superoxide radicals [8, 30]. In this respect, Saleh et al. presented the fabrication of ternary Ag2O/TiO2/nanographene platelet composites using a microwave-assisted method for the degradation of organic dyes in aqueous solution [8]. Hou et al. demonstrated ultrasonic impregnation–assisted in situ photoreduction deposition synthesis of Ag/TiO2/RGO ternary composites with a synergistic enhanced photocatalytic activity [30]. Liu et al. introduced integrating Z-scheme heterojunction into a novel Ag2O@rGO@reduced TiO2 photocatalyst with broadened light absorption and accelerated charge separation co-mediated highly efficient UV/visible/NIR light photocatalysis [31]. Herein, the synthesis of ternary Ag2O/GO/TiO2 composite nanoparticles has been reported by a two-stage route including wet chemical and sol-gel techniques and the characterization of Ag2O/GO/TiO2 nanoparticles using X-ray diffraction (XRD), Fourier transfer infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), and UV-Vis diffuse reflectance spectroscopy (DRS). These methods for the synthesis of this nanocomposite material were simple and versatile and the Ag2O/GO/TiO2 nanoparticles were stable in the synthesis processes. The existence of more available surface areas compared to that with simple structures resulted in more improved performances for potential applications such as photocatalysis. Also, these Ag2O/GO/TiO2 nanoparticles can be a useful catalyst for the synthesis of new heterocyclic organic compounds, drug delivery application, and drug discovery synthesis and also for removal degradation of organic dyes and antibiotics from wastewater.

Experimental section

Materials

Graphene oxide (GO) was purchased from Nano Altin Carbon. Other reagents and materials were obtained from Merck. All chemicals were at an analytical grade and utilized without further purification.

Synthesis of Ag2O nanoparticles

A wet chemical technique was utilized to synthesize Ag2O nanoparticles according to the literature [32]. In a typical synthesis process, 80 mL of a 0.005 M silver nitrate (AgNO3) aqueous solution was heated at 60 °C. After that, 20 mL of a 0.025 M sodium hydroxide aqueous solution was added drop by drop to the prepared AgNO3 solution under continuous magnetic stirring at 60 °C for 2 h. After cooling to room temperature, the formed precipitate was collected by a centrifuge with a speed of 3000 rpm, washed with ethanol several times, and dried at a constant temperature of 40 °C for 24 h.

Synthesis of Ag2O/GO/TiO2 composite nanoparticles

Ag2O/GO/TiO2 composite nanoparticles were synthesized through the sol-gel method according to a process reported by Xiao et al. [33] as follows: Firstly, 5 g of cetyltrimethylammonium bromide (CTAB) as the precursor of TiO2 was added into 30 mL of ethanol and kept under continuous stirring. Secondly, 25 mL of a butyl titanate solution that was separately dissolved in 50 mL of absolute ethanol, added into the obtained CTAB solution at a rate of one drop every 3 s. Thirdly, a solution containing 7 mg of as-synthesized Ag2O nanoparticles in 5 mL of absolute ethanol and another solution containing 20 mg of as-purchased graphene oxide in 10 mL of ethanol were prepared and after 1 h were slowly added to the above solution. The resultant mixture was stirred for 2 h to reach a titanium dioxide gel. The obtained product was finally dried at 65 °C for 12 h and calcined at 450 °C for 2 h.

Instrumental techniques

The crystalline phase of the as-synthesized sample was identified by X-ray diffraction (XRD) measurements by the means of a Ultima IV Multipurpose X-ray diffractometer equipped with Cu Kα1 (λ = 0.15406 nm) radiation. Fourier transform infrared (FT-IR) spectrum was obtained using a Perkin Elmer BX-II spectrophotometer. Surface morphology was determined by a field emission scanning electron microscope (FESEM, Zeiss SIGMA VP-500) equipped with side detectors including energy-dispersive X-ray spectroscopy (EDS) and high-resolution elemental mapping to examine elemental compositions. Ultraviolet-visible diffuse reflectance spectra (DRS) were recorded on a scan UV-Vis spectrophotometer (Avaspec-2048-TEC). The band-gap energy was estimated according to the Tauc method. The morphological features of the sample were investigated with a Zeiss (EM10C, Germany) transmission electron microscope (TEM) operating at 100 kV.

Results and discussions

Structural studies

Phase compositions of as-synthesized nanoparticles were identified using the X-ray diffraction (XRD) technique. Figure 1 displays the XRD pattern of Ag2O/GO/TiO2 composite nanoparticles. The XRD pattern of the composite nanoparticles exhibits four distinct peaks at 25.76°, 48.49°, 54.39°, and 55.53° corresponding to (101), (200), (105), and (211) crystal planes of anatase TiO2 crystalline structure, respectively (JCPDS No. 00-021-1272). Two diffraction peaks are also found at 62.98° and 69.29°, which can be assigned to (214) and (301) planes of the TiO2 rutile phase, respectively (JCPDS card No. 00-021-1276). The formation of Ag2O structures is confirmed by arising characteristic peaks at 38.30°, 70.76°, and 75.55° relevant to (200), (222), and (311) Bragg planes, respectively (JCPDS No. 01-072-2108). As shown in Fig. 1, the presence of graphene sheets is justified by appearing a characteristic peak at 13.45° that can be indexed to carbon structures as presented in JCPDS Card No. 01-089-8491. It is well known that the existence of GO in semiconductor composite structures compared to other carbon materials can improve the transitional performance of charge carriers relevant to high-temperature thermal effects [33].

Fig. 1
figure 1

XRD pattern of Ag2O/GO/TiO2 composite nanoparticles

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The surface chemical composition of the as-synthesized sample was investigated by FT-IR spectroscopy. Figure 2 shows the FT-IR pattern of Ag2O/GO/TiO2 composite nanoparticles in the range of 450–4000 cm−1. It can be seen in Fig. 2 that, due to Ti-O stretching in TiO2 lattice, the absorption band appears at 716.64 cm−1 [34]. The characteristic band at 2331.95 cm−1 is ascribed to the stretching modes of carboxyl (C=O) groups [14]. The band observed at 2828.11 cm−1 also corresponds to the C-H stretching frequency [34].

Fig. 2
figure 2

FT-IR pattern of Ag2O/GO/TiO2 composite nanoparticles

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The surface morphology of the as-synthesized sample was observed using FESEM micrographs. FESEM images of Ag2O/GO/TiO2 composite nanoparticles in two different magnifications along with the corresponding histograms of particle size have been illustrated in Fig. 3. The FESEM image shown in Fig. 3a indicates a relatively uniform distribution from spherical-like particles with an average diameter of about 300 nm (Fig. 3c). It can be seen in Fig. 3b that the obtained spherical structures consist of numerous small nanoparticles with an average size of about 35 nm (Fig. 3d). Such a formed architecture revealed more available surface areas compared to that with simple spherical structures resulting in more improved performances for potential applications such as photocatalysis.

Fig. 3
figure 3

a, b Typical FESEM images and c, d the corresponding histograms of particle size for Ag2O/GO/TiO2 composite nanoparticles

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FESEM-EDS mapping was carried out to verify the surface element dispersion states of the as-synthesized sample. Figure 4 presents the results obtained from the FESEM-EDS mapping of Ag2O/GO/TiO2 composite nanoparticles. Figure 4b–e demonstrate the presence and uniform distribution of Ti, O, C, and Ag elements in the selected surface area of the as-synthesized sample (Fig. 4a), offering visual evidence for the successful formation of Ag2O/GO/TiO2 composite nanoparticles. EDS spectrum recorded for Ag2O/GO/TiO2 sample is also plotted as an inset in Fig. 4a, indicating the existence of Ti, O, C, and Ag with atom percentages of 50.3%, 23.1%, 6.3%, and 20.3% in Ag2O/GO/TiO2 composite structure, respectively.

Fig. 4
figure 4

a FESEM image; inset: corresponding EDS spectrum. b–e EDS mappings of Ag2O/GO/TiO2 composite nanoparticles

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The morphological features of the sample were investigated with a Zeiss (EM10C, Germany) transmission electron microscope (TEM) operating at 100 kV. These images were prepared as follows: The dilute aqueous solution of the sample was sonicated for 15 min. Then, a portion of the sample (20 μL) was dropped onto holey carbon film on copper grid 300 mesh (EMS, USA) and dried thoroughly at room temperature.

For the closer look of synthesized microstructures, the transmission electron microscope (TEM) was used. Transmission electron microscope image of Ag2O/GO/TiO2 is shown in Fig. 5. TEM image of Ag2O/GO/TiO2 is observed with 50 nm to 300 nm magnification. According to the figure, it can be said that the particles have irregular geometric shapes.

Fig. 5
figure 5

a DRS spectrum and b the corresponding Tauc plot for Ag2O/GO/TiO2 composite nanoparticles

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Optical studies

Optical characteristics of the as-synthesized sample were studied using UV-Vis diffuse reflectance spectra (DRS). Figure 5a demonstrates a UV-Vis absorption spectrum of Ag2O/GO/TiO2 composite nanoparticles in a wavelength range of 200–800 nm. It can be seen in Fig. 5a that Ag2O/GO/TiO2 composite nanoparticles exhibit a strong light absorption ability in the UV region with a steep edge towards the visible region. The absorption edge of Ag2O/GO/TiO2 composite nanoparticles can be precisely identified by estimating optical band-gap energy. The information on the band-gap energy (Eg) can be obtained using the Kubelka-Munk theory as follows [35, 36]:

$$ (ahv)=A\Big( hv-{E_g}^{1/2} $$
(1)

where α presents the absorption coefficient, h is Planck’s constant, ν is the light frequency, and A is the proportionality constant. According to the Tauc method, the numerical value of the band-gap energy is calculated by plotting (ahv)2 against photon energy (hν) and then obtaining the intercept of the tangent to hν-axis. As given in Fig. 5b, the band-gap energy for Ag2O/GO/TiO2 composite nanoparticles is estimated to be about 3.2 eV. The optical absorption edge (λmax) for Ag2O/GO/TiO2 composite nanoparticles can be also calculated as [37]:

$$ {\lambda}_{max}(nm)={E}_g(ev)/1240 $$
(2)

Therefore, an absorption edge of about 387 nm is calculated for as-synthesized Ag2O/GO/TiO2 composite nanoparticles. The obtained optical results indicate a UV-light-active material that can be used in possible applications benefiting from the advantages of the presence of UV light photons.

Conclusion

Ag2O/GO/TiO2 composite nanoparticles were successfully synthesized via a two-step process. The formation and composition of composite structures were investigated by the XRD, FT-IR, and FESEM methods. The structural studies demonstrated the fabrication of spherical structures with an average diameter of about 300 nm composed of nanoparticles with an average size of about 35 nm. The optical studies were also performed by UV-Vis diffuse reflectance spectroscopy and indicated an absorption edge in the UV region with a band-gap energy of about 3.2 eV. In conclusion, we found the versatile and simple method for the synthesis of new Ag2O/GO/TiO2 composite nanoparticles with the doped nanostructured Ag2O and TiO2 on the graphene surface that could the best candidate for future work in the wastewater treatment and synthesis of new organometallic compounds.