Introduction

The waste of the major industries as a whole consists of dyes that are organic in nature but non-biodegradable and toxic (Amini et al. 2011). These non-degradable dyes are the basic source of water pollution (Ong et al. 2014). There are many techniques available for water purification and sanitization that are widely used all over the world like filtration with membrane (Chidambaram et al. 2015) and oxidation in electrochemical process (Pirilä et al. 2015). Among all these techniques, the simplest and economically feasible technique is photocatalytic process (Azat and Gaukhar 2018; Konsowa et al. 2010). In this process, the catalyst is used in the presence of light to degrade the contaminants of water (Saravanakumar et al. (2016)). Many semiconductor including TiO2, ZnO and Ce2O etc. were used for photocatalytic process. (Wei et al. 2018; Kwong et al. 2007; Ahmad et al. 2013; Alammar and Mudring 2009; Xie et al. 2014; Akbari-Fakhrabadi et al. 2015; Khalid et al. 2019a). Among these, the Ce2O has gained a lot of interest due to its low cost and good optical and photocatalytic properties (Amanulla et al. 2018; Wang et al. 2011a; Ranjith et al. 2018). It is highly stable chemically like TiO2 and has band gap between 2.8–3.2 eV (Krishna Chandar and Jayavel 2013; Rohini et al. 2017). Therefore, it has the ability to absorb ultraviolet light efficiently. However, its absorbance of visible light is very low due to wider band gap (Ansari and Khan 2014). Recently, some researchers have focused on nanostructured ceria to improve its optical and photocatalytic properties. Different studies demonstrated that nanostructured ceria is more effective than bulk ceria in terms of its photocatalytic performance (Ghori and Veziroglu 2018). Moreover, it was also observed that doping of noble metals and their combination have showed good photoelectric response of nanostructured ceria under visible light irradiations (Durmus et al. 2019). The analysis of the earlier reported research revealed that the doping or combination of noble metal oxides has significantly enhanced degradation rate of the pollutants present in the contaminated water (Mathew et al. 2020; Li et al. 2020). The intrinsic properties of Ce2O-based materials could be further modified by fabricating cerium oxide particles on nanoscale level as described earlier (Sayyed et al. 2016; Munoz-Batista et al. 2015). The synthesis methods also play vital role in modifying the properties of nanostructured materials. However, some methods are complex and use toxic chemicals. Therefore, there are only few preparation methods which can provide controlled morphology and good properties (Saravanan et al. 2013). The synthesis of Ce2O nanoparticles at higher temperature using conservative solid-state reaction pathway results in poor chemical activity and high impurity concentration with bulky particle size. Hydrothermal technique for the fabrication of nanoparticles in the form of aqueous solutions potentially provides an easy route that allows controlled morphology and the desired properties of the synthesized Ce2O nanoparticles.

In addition to above, the coupling of pure Ce2O with less band gap semiconductor (Ag2O) is also an active technique to boost its efficiency as photocatalyst (Wen et al. 2018a). Ag2O has 1.2 eV band gap energy and is also p type semiconductor due to which it can absorb visible light effectively (Yang et al. 2016; Chu et al. 2016; Wang et al. 2011b; Yu et al. 2016). However, alone Ag2O photocatalyst has lower performance due to poor stability and short life time of electron–hole pairs. Therefore, formation of Ag2O-based p–n heterojunction is very effective technique to enhance its photocatalytic performance as well as photogenerated charge carrier’s life time (Khalid et al. 2019b; Wang et al. 2012; Ivanova et al. 2018; Wen et al. 2018b). Recently, Wen et al. (Wen et al. 2018b) have prepared novel Ag2O/Ce2O heterojunction photocatalyst by thermal decomposition process and applied for the degradation enrofloxacin. They observed that the formed heterojunction between Ag2O and Ce2O is good strategy to enhance charge carrier serration as well as good photocatalytic activity. However, complete understanding of Ag2O/Ce2O-based p–n heterojunction is still lacking and needs further study.

Here, we have applied simple hydrothermal process for the synthesis of Ag2O/Ce2O-based p–n heterostructure by varying the concentrations of Ag2O. The loading of Ag2O into Ag2O/Ce2O nanocomposite had reduced Ce2O band gap and inhibited the electron–hole recombination. Moreover, the prepared Ag2O/Ce2O-based nanocomposites showed excellent photocatalytic efficiency for discoloring of methyl orange under visible photo-illumination.

Experimental

Synthesis of pure Ce2O and Ag2O/Ce2O nanocomposite

Pure ceria and Ag2O coupled Ag2O/Ce2O nanocomposites were synthesized via simple hydrothermal technique. Firstly, 2 g cerium nitrate (Ce (NO3)3) and required amount of silver nitrate was mixed in 10 ml deionized water to prepare the required solution under magnetic stirring of 15 min. In the second step, 13 g of sodium hydroxide NaOH was mixed into 70 ml deionized water to make another solution. For obtaining homogeneous solution, both above mentioned solutions were mixed under continuous magnetic stirring of 30 min. The mixture immediately after stirring was put into autoclave (Teflon-lined stainless-steel) having 100 mL volume, sealed tightly and thermally treated in temperature-controlled oven at 180 °C for 6 h. After hydrothermal treatment, the solution was thoroughly washed using deionized water for six time and dried at 100 °C for 10 h in an oven. Then obtained powder was calcined at 550 °C for 3 h in a muffle-furnace. The loading concentration of Ag2O into Ag2O/Ce2O nanocomposite was controlled to be 0, 2, 4 and 6 wt%.

Characterization

Crystal size and structure of all prepared samples were investigated by XRD (JCPDS Card No:65-6811) having CuKα source with λ = 0.1541 nm. Scanning electron microscopy (SEM) was carried out on JEOL JSM-6330F to analyze the morphology of synthesized nanocomposites. The UV–visible spectrometer (Shimadzu UV–visible 1800) was utilized to study the optical absorption properties of photocatalysts. XPS analysis was carried out for chemical composition by Thermo ESCALAB 250 with Al Kα X-ray. The JASCO FP-8200 fluorescence-spectrophotometer was used with excitation wavelength of 350 nm for photoluminescence (PL) properties.

Photocatalytic performance testing

The all prepared photocatalyst were used to discolor the methyl orange dye dissolved in aqueous solution using irradiation of visible-light (λ ≥ 420 nm). The solution for photocatalytic reaction was prepared via mixing 10 mg photocatalyst into the 100 ml aqueous solution that contains the 10 mg/ methyl orange dye. Before exposure of visible-light, the solution was kept in dark for 30 min under magnetic stirring to obtain equilibrium of absorption & desorption. Then the mixture was irradiated with visible light to proceed the photocatalytic process. After each interval of 30 min, sample was collected to determine the dye degraded concentration using UV–visible spectrometer.

Results and discussion

XRD analysis

The structural characteristics of the prepared Ce2O, Ag2O and Ag2O/Ce2O nanocomposite were examined through the XRD pattern analysis as displayed in Fig. 1. The pure Ce2O XRD patterns indicated the sharp intensity peaks at angle (2θ) of 28.20°, 33.32°, 47.11°,55.48°, 59.19°, 69.21°,76.71° and 79.06° with the planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) respectively. It was noted that the all peaks belong to cubic ceria phase according to JCPDS Card No: 43-1002. On the other hand, the sharp peaks of pure Ag2O were obtained at 2θ values of 26.74°,32.5°, 38.5°, 54.9°, 65.71° and 68.91° can be indexed to (1 1 0), (1 1 1), (2 0 0), (2 2 0), (3 3 1) and (2 2 2) confirmation the face centered cubic structure of Ag2O (JCPDS, Card No.65-6811). XRD patterns of Ag2O coupled Ce2O (Ag2O/Ce2O) nanocomposites demonstrate that the there is no major difference between pure Ce2O and Ag2O/Ce2O composite samples. However, intensity of peaks was decreased after increasing the ratio of Ag2O. Secondly, Ce2O peak at 33.32° 2θ angle was shifted toward lower angle. It could be attributed to the reason that the main observed peak of Ag2O at 32.5° is close to 33.32° peak of Ce2O which might be overlapped in nanocomposite samples. The results show that Ag2O/Ce2O nanocomposites were formed and incorporation of silver oxide did not severely affect the crystalline structure of cerium oxide. Moreover, Scherer’s equation was utilized to determine the crystallite size of all synthesized nanostructures using XRD patterns by the following formula (Alammar and Mudring 2009): wherever k is a factor of shape, λ is X-rays wavelength, β represents the full width of half maximum and \(\uptheta \) represents angle of peak respectively.

$$ {\text{D}} = \frac{{k{\uplambda }}}{{{\upbeta }\cos {\uptheta }}} $$
(1)
Fig. 1
figure 1

XRD crystal structures of Ce2O, Ag2O and Ag2O/Ce2O nanocomposites

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The calculated average crystallite sizes were 12 nm, 20 nm, 11 nm, 10.2 nm, 9.8 nm for Ce2O, Ag2O, 2Ag2O/Ce2O, 4Ag2O/Ce2O and 6Ag2O/Ce2O, respectively. This shows that Ag2O loading into Ag2O/Ce2O nanocomposite has decreased the crystallite size of samples which again confirms the formation development of Ag2O/Ce2O nanostructure.

SEM analysis

Surface morphology of 4% Ag2O/Ce2O nanostructure was inspected with scanning electron microscopy. Figure 2 demonstrates the SEM images of nanocomposite at varying resolutions. The images clearly reveal nanorods like morphology of composite sample. These nanorods have average length of ~ 2 µm and diameter ~ 0.15 µm. This novel morphology prepared through hydrothermal process confirms that our prepared sample has greater surface area, surface active sights and thus will act as excellent photocatalytic material.

Fig. 2
figure 2

SEM image of the Ag2O/Ce2O composite at different concentrations of temperature and time

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XPS Analysis

The chemical states of 4Ag2O/Ce2O nanostructure were examined via XPS and spectra are displayed in Fig. 3. In Ce 3d core level spectrum (Fig. 3a), four peaks are detected, the first two peaks at 906 eV and 902.2 are due to Ce 3d3/2 and the other two peaks at 888.6 eV and 885.4 eV are due to Ce 3d5/2. These peaks clearly indicate Ce4+ chemical state of cerium in Ag2O/Ce2O nanocomposite (Wen et al. 2018b; Xu and Wang 2012). The Fig. 3(b) displays XPS highly resolved Ag 3d spectrum, it has two peaks at binding energies of 374.5 and 368.5 eV which could be ascribed to Ag 3d3/2 and Ag 3d5/2 of Ag + (Wen et al. 2018b, 2017). In O 1 s core spectrum two peaks are present (Fig. 3c), the first peak at 532 eV is due to adsorbed oxygen and H2O and the second appearing at 530.0 eV can be attributed to Ag–O and Ce–O bonds (Wen et al. 2018b).

Fig. 3
figure 3

XPS core level spectra (a) Ce 3d, b Ag 3d, c O 1 s of 4%Ag2O/Ce2O nanocomposite

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UV–visible analysis

The optical absorption properties of pure Ce2O and 4Ag2O/Ce2O nanocomposite was studied using UV–visible spectroscopy. As displayed in Fig. 4a, pure Ce2O exhibits absorption edge at 421 nm wavelength in visible-light region. After the loading of Ag2O into Ag2O/Ce2O composite, absorption edge is shifted towards higher wavelength at about 445 nm. The energy band gap (Eg) of both samples were determined using Tauc plot equation (Khalid et al. 2019b); where \(\mathrm{h }\nu\) denotes photon energy of incident light, \(\alpha \) is known as absorption coefficient, A is known as proportionality constant and Eg is energy band gap. Band gap energy was founded by plotting graph between \(\left(\alpha \mathrm{h }\nu\right)\)2 versus \(\left(\mathrm{h }\nu\right)\) using spectral data of Fig. 4a and results are displayed in Fig. 4b:

$$ (\alpha {\text{h}}\nu )^{2} \; = \;{\text{A}}\;{\text{(h}}\nu \; - \;{\text{E}}_{{\text{g}}} {)}. $$
(2)
Fig. 4
figure 4

a UV–visible absorption spectra (b) Tauc plot of Ce2O and 4% Ag2O/Ce2O nanocomposite

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Thus, band gap (Eg) of pure Ce2O and 4Ag2O/Ce2O nanocomposite was found to be 2.94 nm and 2.79 nm, respectively. These finding confirms that Ag2O incorporation has the ability to reduce the band gap of cerium oxide.

PL analysis

The photoluminescence (PL)spectroscopy has gained greater attention in photocatalysis to study the surface processes in which photoexcited electrons and holes participate (Wen et al. 2018b; Swain et al. 2017). Moreover, PL emission represents the photocreated charge carrier’s recombination. The higher PL emission intensity represents greater electron–hole recombination. Therefore, in order to explore the effect of Ag2O loading onto Ag2O/Ce2O nanocomposites, PL spectra were measured for pure Ag2O, Ce2O and Ag2O/Ce2O nanocomposites and results are displayed in Fig. 5. The spectra demonstrate that the bare Ag2O and Ce2O exhibited higher PL emission intensity indicating greater recombination of electrons and holes. However, with the incorporation of Ag2O into Ag2O/Ce2O nanocomposite the PL emission intensity was decreased. It could be observed that among all samples, 4Ag2O/Ce2O nanocomposite showed lowest PL emission intensity confirming the inhibition of electrons and holes recombination. Therefore, it is expected that this nanocomposite with optimum loading of Ag2O (4%) will perform excellently for discoloration of dye during photocatalysis.

Fig. 5
figure 5

PL spectra of different samples

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Photocatalytic degradation activity

The photocatalytic efficiencies of as-prepared photocatalysts were inspected by degradation of methyl orange (MO) dye under visible photo-irradiation. The dye (MO) adsorption on catalysts surface in the dark for 30 min and photocatalytic performance results are displayed in Fig. 6. It was seen that 4Ag2O/Ce2O nanocomposite showed maximum adsorption than all other samples. It could be attributed to nanorods like morphology of nanocomposite. Furthermore, photocatalytic activity results show that nanocomposites had degraded the dye more effectively than pure Ce2O and Ag2O photocatalysts. Interestingly, it can be seen that with increasing ratio of Ag2O into Ag2O/Ce2O nanocomposite upto 4%, the photocatalytic performance was increased. This shows that loading ratio of Ag2O has significant role to improve photocatalytic efficiency of photocatalyst. It can be ascribed to the fact that optimum loading ratio may develop more active surface sites on photocatalysts which would increase its efficiency. However, further increase in Ag2O incorporation (6%) had decreased the performance of nanocomposite due to aggregation of Ag2O particles onto the photocatalyst surface which resulted into lower efficiency of photocatalyst.

Fig. 6
figure 6

a UV–visible absorption spectra of methyl orange (MO) dye degradation using 4% Ag2O/Ce2O nanocomposite, b Photocatalytic degradation performance of different photocatalysts for MO under visible light irradiation (λ ≥ 420 nm)

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Photocatalytic degradation mechanism

The relative band positions of both semiconductors were found to understand the improved photocatalytic activity of Ag2O/Ce2O nanocomposite. It is generally known that band-edge potential levels play an important role in determining the transfer of photocreated electrons and holes in heterostructure nanocomposites. Therefore, valence band (VB) tops for both semiconductors were calculated according to following equation (Swain et al. 2017); where X and Eg are the electronegativity and semiconductor band gap energy and E0 represents free electron energy on hydrogen scale (~ 4.5).

$$ {\text{E}}_{{{\text{CB}}}} = {\text{E}}_{{{\text{VB}}}} - {\text{E}}_{{\text{g}}} $$
(3)

The values of electronegativity (X) for Ce2O and Ag2O were calculated to be 5.578 and 5.29 respectively. The energy band gap value of Ce2O was used 2.94 eV from Fig. 4 and band gap energy of Ag2O was used 1.3 eV according to previous reports (Akel et al. 2018). The calculated valence band tops for Ce2O and Ag2O were 2.58 and 1.44 respectively.

The conduction band (CB) bottoms of Ce2O and Ag2O were determined from following equation (Swain et al. 2017);

$$ {\text{E}}_{{{\text{CB}}}} \;{ = }\;{\text{E}}_{{{\text{VB}}}} - {\text{E}}_{{\text{g}}} $$
(4)

The calculated conduction band bottoms of Ce2O and Ag2O were  − 0.36 and + 0.14 respectively. Based on above calculations, a photocatalysis mechanism of Ag2O/Ce2O heterostructure is proposed in Fig. 7. Upon exposure of visible light, both semiconductors Ce2O and Ag2O create holes in valence band and electrons in conduction band. Due to more positive potential of valence band of Ce2O (2.58) than that of Ag2O (1.44), the photocreated holes in valence band of Ce2O could transfer to valence band Ag2O. At the same time, electrons will be transferred from conduction band of Ce2O toward conduction band of Ag2O because Ce2O has more negative conduction band potential of conduction band as compare to conduction band potential of Ag2O.

Fig. 7
figure 7

Schematic diagram of proposed reaction mechanism for MO dye degradation using % Ag2O/Ce2O nanocomposite

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Conclusion

In the present research, a facile hydrothermal technique is adopted for the synthesis of Ag2O/Ce2O nanorod-based photocatalysts. The obtained Ag2O/Ce2O nanocomposites exhibited superior photocatalytic performance for methyl orange dye degradation under visible photoillumination. The improved photocatalytic efficiency was ascribed to novel morphology, reduced band gap of nanocomposite and inhibited recombination of electrons and holes due to formed heterojunction between Ag2O and Ce2O. This study could offer a new methodology to fabricate a novel p–n heterojunction photocatalyst for advanced photocatalysis in energy and environmental field.