Introduction

Over the former few years, water contaminants have industrial wastewater become the greatest challenging ecological concerns and thus aroused abundant courtesy in the progress of modern society (Gour and Jain 2019). Wastewater usually covers a huge amount of organic pollutants (such as reactive dyes, pesticides and antibiotics) which is adverse things on aquatic ecologies equilibrium and human healthiness. The textile, paper-making, cosmetics, food industries and dye houses have used countless organic dyes which are the prime causes for the contamination of environmental wastewater due to its toxicity and non-biodegradability is a vital environmental issue (Dhmees et al. 2019). Various traditional systems, such as electrochemical oxidation, membrane filtration, adsorption, chlorination, reverse osmosis, and photocatalysis, have been agreed to treat the harmful dyes covering wastewater. Amid these skills, semiconductors (SCs) based photocatalysis has drawn increasing interest because it offers a capable substitute strategy to eradicate the dye-containing wastewater since of its high efficacy, green reaction route and moderate reaction settings (Rohini et al. 2017) for solving the recent severe problems of environmental pollution and energy shortages. Unfortunately, the utmost of these physical, chemical and biological systems could custom to secondary impurities simply through degradation manners (Venkatasubramanian et al. 2008). The advanced oxidation practices (AOPs) of semiconductor photocatalysts (PCs) were broadly considered by deprivation of various noxious organic toxins in environment remediation and antimicrobial action (Girase et al. 2011; Depan and Misra 2014; Zhang et al. 2019b; Shanmugam et al. 2020). The photodegradation and mineralization of the dyes by nano-configuration semiconductor in visible-light treated progress has engrossed great concern in modern years. Various metal oxides (such as ZnO, TiO2, Fe2O3, CeO2, and WO3) have been repeatedly employed as support in heterogeneous catalysis of organic pollutants degradation in the wastewater and antimicrobial activity (Rana et al. 2006; Rawat et al. 2007a; Ke et al. 2008; Sunkara and Misra 2008; Xu and Wang 2012; Li et al. 2019).

As a significant rare earth metal oxide chains, cerium oxide (CeO2), a wide bandgap SCs which fascinates light in the nearby UV and slightly visible region. Likewise, the CeO2 has eco-friendly photocatalytic material, has encouraged abundant concern of the researchers owing to its favorable applications, for instance, plentiful oxygen sensors, luminescent things besides admirable chemical constancy, high catalytic action and cost-effective nature (Li et al. 2019; Qi et al. 2019). This material was also functional for solar water splitting (Ce4+/Ce3+) into hydrogen creation and concerned increasing attention used for deletion of organic pollutants from wastewater owed to its strong light captivation (Priyadharsan et al. 2017a). Though, the key weakness of CeO2 is the absence of visible-light consumption since the large bandgap (2.92 eV) and weak separation efficacy of photoexcited carriers hinders have its widespread request in photocatalysis. Hence it is probable to outspread the visible-light captivation skill of CeO2 either by doping of metals/nonmetals or fashioning a heterojunction between CeO2 and another narrow bandgap SCs to create VLD PCs (Cano-Franco and Álvarez-Láinez 2019a). Unfortunately, the UV region signifies a little amount (3–5%) of photon flux whereas the visible region attains 45% of daylight. So that, it has required to progress the photocatalytic efficacy by doping or coupling the CeO2 with narrow bandgap materials which could diminish the recombination of the photogenerated electron/hole pair rate and extend their lifetime important to the excellent light absorption from UV to the visible region for the elimination of organic pollutants (Ma et al. 2019). As a member of the tungstate family, lead tungstate (PbWO4) is a scientific importance inorganic scintillating semiconductor, which has vast potential applications like humidity identical sensors, solid-state lasers ground, optical fibers, and catalysts (Pourmasoud et al. 2017; Rajendran et al. 2019). PbWO4 is utmost smart for high-energy physics uses since of its great density (8.3 g/cm3), short decay time (< 10 ns for a large part of light output), high-irradiation destruction resistance, exciting excitonic luminescence, thermo-luminescence, encouraged Raman scattering manners (Yue et al. 2016).

In this present effort, we report novel CeO2/PbWO4 heterojunction PCs that were effectively fabricated via a simplistic hydrothermal method. The as-obtained nanocomposites (NCs) were categorized by numerous analytical tools such as XRD, FTIR, HR-SEM, HRTEM, PL and UV-DRS spectroscopy were employed to study the crystalline formation, phase configuration, morphology, and optical possessions parallel with CeO2 and PbWO4 samples. The as-organized CeO2/PbWO4 NCs were used as a UV and/or visible-light focused photocatalyst towards the photodegradation of MB dye. Coupled with PbWO4 on the superficial of CeO2 NPs are projected to increase the surface area of composite providing further response sites owing to lower bandgap and so improve its photocatalytic action. Eventually, a reasonable mechanism and photo-stability of the catalyst are also anticipated in detail.

Experimental part

Materials

Lead nitrate hexahydrate (Pb(NO3)3·6H2O; 99%), Sodium tungstate dihydrate (Na2WO4.2H2O, 99%), Ethylene glycol (99%) were procured from Himedia Ltd. Potassium chloride (Merck, 99%), Cerium nitrate, (Ce(NO3)3·6H2O; 98%) were obtained since SRL Chem. Limited. Sodium hydroxide (NaOH), isopropanol (IPA), Di-sodium Ethylene Diamine Tetra Acetic Acid (EDTA-2Na), benzoquinone (BQ), and absolute ethanol (CH3CH2OH) were acquired from SDFCL Chemical Reagent Co., Pvt. Ltd. Methylene blue (MB; C16H18ClN3S) dye from SD Fine and was used as received. Deionized water (D.I) was used for the preparation of all solutions. All the chemicals were analytical reagent (A.R) grade also have auxiliary purified before use.

Preparation of CeO2/PbWO4 photocatalyst

In a typical synthesis, 0.03 mol of Ce(NO3)3·6H2O was ultrasonically dissolved in 100 mL of D.I water, then 10 mL NH4OH was gradually dropped directly into the above reaction mixture for pH extended at ~ 12 under constant stirring for 30 min. Lastly, the composed precipitants (Cui et al. 2019) were kept dry at 60 °C for 8 h and extra calcined at 400 °C for 1.5 h to attain CeO2 NMs. In this research, the CeO2 blended PbWO4 NMs, via 0.03 mol of Pb(NO3)3·6H2O and 0.03 mol of Na2WO4.2H2O solution was added by 50 ml of D.I water. And 1 mol (50 mL) of NH4OH solution was additional in the pioneer solution, although the pH value was touched at ~ 11–12. Afterwards, being stirred for 3 h, the 0.1 g of as-obtained CeO2 NMs was auxiliary added in the upstairs produces and formerly stirred for 2 h. Then, the reaction mixer was relocated to heat-treat by 160 °C aimed at 24 h in a 250 mL Teflon-lined stainless steel autoclave. Lastly, the autoclave was then cooled to room temperature, hence the as-attained CeO2/PbWO4 precipitate was centrifugated and rinsed carefully with ethanol/D.I. water further dry at 70 °C for 8 h. Besides, the CeO2/PbWO4 NCs was attained (Jeyakanthan et al. 2018). Advising to this outline, the pristine PbWO4 NPs was also attained via without accumulation of CeO2 NMs.

Characterization of the obtained samples

The crystal organization and phase of as-obtained nanocomposite was estimated by X-ray diffraction (Rigaku Miniflex-II; X-ray diffractometer) over CuKα radiation in the 2θ range from 10 to 80°. FTIR revisions were done by Perkin Elmer RX-1 FTIR spectrophotometer. Surface morphologies and microstructure of as given NMs were scrutinized via high-resolution (HR-SEM; HITACHI S-3000 H) scanning electron microscope and high-resolution transmission electron microscopy (HR-TEM; JEM-2011; JEOL-Japan) instruments. Energy-Dispersive X-ray spectroscopy (EDX-attached with HRSEM) was used to evaluate the elemental compositions of the NCs. To measure the optical assets of the attained samples were considered by a UV–Vis DRS spectrophotometer (UV2550 model, Shimadzu—Japan). Photoluminescence (PL) spectrophotometry was performed with a (Perkin-Elmer-LS 100) to determine the electron–hole recombination rate at an excitation series of ~ 342 nm. The optical absorption of dye degradation samples was performed via a UV–Vis spectrophotometer (UV; Perkin-Elmer Lambda-19). The absorption spectra in the photodegradation rate process of MB dye solutions also measured with a UV–Vis (Perkin Elmer-Lambda 35) spectrometer.

Description of the photocatalytic activity of MB dye

The photodegradation performing of as-attained samples (50 mg) was measured via deprivation of MB dye (20 ppm; 100 mL solution; 10 mg/L) under visible-light exposure (300 W Xe lamp by λ > 420 nm cutoff filter in a Pyrex photocatalytic vessel). Preceding to exposure, the suspensions stayed constant magnetically stirred for around 30 min in the dark to certify that the dyes might extend the absorption–desorption balance on the photocatalyst superficial and dyes (Ali Baig et al. 2020). At certain time pauses of 20 min irradiation, 2.5 mL of aliquots were collected. The degraded resolutions were explored by UV–Vis absorption peaks of corresponding MB dye (wavelength at ~ 663.5 nm) (Zeleke and Kuo 2019). The photodegradation efficacy was extent as resulting formula, Efficiency (%) = (C0-Ct)/C0*100, wherever C0 and Ct exist the absorbance rate of dyes solution earlier and afterwards destruction. Finally, through the degradation manner, photocatalyst was separated from the reaction blend and dried to succeed in the reusability trials (Zhang et al. 2017).

Active species trapping experiments

To detect that the reactive species, certain caused/trapping mechanism during the photocatalytic manner, 1 mM of EDTA-2Na IPA and BQ and were added as scavengers of holes (h+) hydroxyl radicals (% •OH−) and superoxide radical (% O2•−) exclusively surveyed by the photocatalytic valuations, hence to catch the detection of dynamic reactive species (Cardillo et al. 2016).

Results and discussion

Crystal structure investigation

To explore the crystal constructions and phase composition of the as-obtained samples, XRD evaluations were carried out. As exposed in Fig. 1a, the key diffraction peaks are indexed to the (111), (200), (220), (311), (222), (400) and (311) crystal planes, which was in moral contract with the typical cubic crystalline phase structure pattern of CeO2 (JCPDS card No: 89-8436) (Syed Khadar et al. 2019). For PbWO4, the diffraction peaks (Fig. 1b) and consistent planes of (112), (004), (200), (204), (220), (116) and (312), respectively, could be well-indexed to the pure tetragonal stolzite phase (JCPDS card No. 19-0708) (Xiong et al. 2015). In Fig. 1c, designating a good crystallinity of CeO2/PbWO4 NCs revealed the association with both the distinctive diffraction peaks of CeO2 and PbWO4 crystalline phases. The solid and sharp diffraction peaks proposed that the CeO2/PbWO4 nanocomposite was fine crystalline in nature. No characteristic impurity peaks from other crystalline forms were detected which proves that the CeO2/PbWO4 NCs has high purity. The average crystallite size (D) of as-prepared NCs was intended by the full width half maximum (FWHM) using Scherer’s equation (Ramos-Corella et al. 2019), The average crystalline sizes for the pristine CeO2, PbWO4 and CeO2/PbWO4 nanocomposite was found to be 24, 28 and 21.5 nm, individually. The intensity variance and peak extending of the CeO2/PbWO4 NCs are ascribed to a substantial reduction of crystallite sizes (Table 1).

Fig. 1
figure 1

XRD pattern of as obtained nanomaterials

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Table 1 Comparison of visible-light assisted MB dye photodegradation rate (%) over earlier reported nanocomposite materials
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FT-IR analysis

To decide the proper functional groups in final catalysts samples, FT-IR spectra of as-obtained CeO2, PbWO4 and CeO2/PbWO4 NCs were perceived in the series of 400–4000 cm−1 and shown in Fig. 2. An exact extensive band of 3100–3650 cm–1 is recognized to the typical surface hydroxyl (O–H) stretching mode (Saravanakumar et al. 2019). The bending vibration group of actually adsorbed water (H2O) molecules are besides observed at 1635–1670 cm–1 band. An increase in the number of surfaces -OH groups could expand the photocatalytic action. The existence of sharp dominated absorption peaks on 570–730 cm–1 which is linked to the metal-O (Ce–O, W–O, Pb–O) bonds/stretching vibration are confirmed that the prepared NCs (Syed Khadar et al. 2019). Usually, spinel oxide and metal–oxygen broadening frequencies are perceived in the peak range of 650–850 cm–1. No auxiliary absorption group was sensed in the experimental FTIR spectrum (Rana et al. 2005b).

Fig. 2
figure 2

FT-IR spectra of as-prepared samples

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Morphology and microstructure analysis

To acquire detailed evidence about exterior morphology and microstructure of the CeO2, PbWO4 and CeO2/PbWO4 NCs were inspected by HRSEM and HRTEM. Figure 3a the HRSEM images of pristine CeO2 demonstrates spherical shaped aggregates morphology. Figure 3b for PbWO4 shows a uniform ball-shaped structure was obtained. Also the Fig. 3c the CeO2/PbWO4 NCs displays the high agglomeration of CeO2 NPs rendered with non-uniform spherical fashioned aggregates (Ramasamy Raja et al. 2019). The elemental composition and purity analysis of CeO2/PbWO4 NCs have been determined from EDX extents. The EDX inquiry authorizes the existence of all essential elements such as Ce, W, Pb, and O individually from CeO2/PbWO4 NCs as described in Fig. 3d, also the weight % are detected and inserted in Fig. 3d (Lan et al. 2018). Moreover, the EDX elemental mapping analysis characterizes that the distribution of W, Ce, O and Pb elements separately and as illustrated in Fig. 4. It is well evident that PbWO4 and CeO2 are evenly circulated in the CeO2/PbWO4 NCs. Furthermore, the EDX spectra and corresponding elemental mapping outcomes of as-obtained CeO2/PbWO4 NCs which are very pure and no other impurities are found. The HRTEM analysis of CeO2/PbWO4 NCs is publicized in Fig. 5a–b. The HRTEM micrograph indicates that the number of CeO2 fine NPs has indeed deposited compactly on the PbWO4 surface structure and also homogeneous dispersion nature were forming the nano-sized composite (Aboutaleb and El-Salamony 2019). Moreover, the surface has several irregular small granules with additional agglomeration and the shape is more or less spherical. The intimate contact amid PbWO4 and CeO2 facilitates the separation of the photoexcited carriers, which favours the enhancement of photocatalytic concert (Hezam et al. 2017).

Fig. 3
figure 3

HRSEM micrographs of as-synthesized a CeO2 b PbWO4 c CeO2/PbWO4 NCs and d EDX spectrum of the CeO2/PbWO4 NCs

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Fig. 4
figure 4

EDX elemental mapping analysis of CeO2/PbWO4 nanocomposite

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Fig. 5
figure 5

HRTEM images of as-obtained CeO2/PbWO4 NCs

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Optical properties

The optical properties and energy bandgap of the as-obtained samples have very important to determine the photocatalytic behaviors (Liang et al. 2017) were studied by UV–Vis DRS as exhibited in Fig. 6. The CeO2, PbWO4 and CeO2/PbWO4 nanomaterials (NMs) were shown in strong absorption ability in the wavelength range of 200–800 nm. Related with pristine CeO2 and PbWO4, the CeO2/PbWO4 NCs consume broader absorption competence (Rana et al. 2005a; Cano-Franco and Álvarez-Láinez 2019b), which is superior visible-light harvesting capacity and redshift (~ 290–418) of absorption edge implying that the CeO2/PbWO4 NCs owns admirable visible-light dynamic photocatalytic action (David et al. 2018; Wang et al. 2019). The bandgap energy was determined by fitting the absorption facts since the direct transition equation, (αhν) = A(hν-Eg)n, where h refers the Planck’s constant, α stands for absorption coefficient, Eg stands for bandgap (eV), ν has shorted in the frequency of vibration, A is the relatively constant. Also, n refers could have values ½ and 2 contingent on the kind of inter-band conversion, i.e., direct and indirect allowed transition, individually. The bandgap energy (Eg) values are optically deduced from the Tauc plots and the graph plotted by (αhν)2 versus the photon energy (hν). The Eg values of CeO2, PbWO4 and CeO2/PbWO4 heterojunction NCs are nearly 2.92, 3.52 and 2.68 eV, separately were exposed in insert of Fig. 6. However, the CeO2 combined PbWO4 NCs could reduce the bandgap energy of CeO2/PbWO4 NCs which is owed to an energy transition since the visible region triggered by an active band of PbWO4 effectively deposited on the CeO2 surface (Liu et al. 2019). In the combined effect of light, absorption would have an obvious effect while catalytic action towards the degradation of dyes qualified to absorption of quite visible-light to probably make more charge carriers (Velusamy and Lakshmi 2017).

Fig. 6
figure 6

a UV–Vis DRS absorption spectra b Tauc’s plots of the CeO2, PbWO4 and CeO2/PbWO4 NCs

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Photoluminescence analysis

The photoluminescence (PL) system is also an effective approach to assess the separation ability of photoexcited electron–hole (e−-h+) pairs since it directly related to photocatalytic efficacy. The PL spectrum of the as-synthesized pristine CeO2, PbWO4 and CeO2/PbWO4 heterojunction NCs were investigated as shown in Fig. 7. The PL emission intensity of CeO2/PbWO4 nanocomposite was lesser than that of pristine CeO2 and PbWO4, signifying that the coupling of PbWO4 NPs might reduce the fluorescence from the CeO2 NPs while the recombination rate of (e−-h+) pairs is seriously reserved and extend the lifetime of charge carriers (Jeyakanthan et al. 2018). It is well established that coupling of PbWO4 NPs onto the superficial of CeO2 donated for reduced recombination rate of photoexcited charges related to CeO2. This reduction might be accredited to (i) a greater amount of nominal defects and (ii) effectual charge separation of CeO2/PbWO4 NCs (Koli and Kim 2019). The detected stronger characteristic PL near-band-edge emissions ranges of ~ 440–510 nm (448, 487 and 503 nm) for the visible region (apparently excitation peak at ~ 342 nm). Normally, the effective charge separation and inhibited (e−-h+) recombination rate by coupling of CeO2/PbWO4 NCs was auspicious for enhancing the photocatalytic efficacy of CeO2 NPs (Lu et al. 2020).

Fig. 7
figure 7

Photoluminescence emission spectra of as-prepared samples

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Photocatalytic activity

The photocatalytic performance was verified against the deprivation of MB aqueous dye in the existence of as-synthesized CeO2, PbWO4 and CeO2/PbWO4 PCs under visible-light exposure. It is seen from Fig. 8a, the characteristic UV–Vis absorption peak of MB dye solution at ~ 663.5 nm has constantly reduced by CeO2/PbWO4 photocatalyst and hence the supreme degradation efficacy almost 94% was degraded within 140 min. The photocatalytic proficiency of pristine CeO2 and PbWO4 for MB dye degradation was 42% and 58% in identical exposure time separately. The C/C0 (where C0 = absorption intensity of the initial dye solution and C = main absorption peak intensity of dye) has schemed vs. the wavelength as exposed in Fig. 8b. Related to the blank photodegradation testing of MB dye showed almost no obvious degradation in presence of a catalyst under the dark condition, along with an absence of catalyst and without catalyst in the light source, hence the curves could be neglected (Wen et al. 2018). Mostly, the MB organic dyes could be photodegraded by three conceivable reactions containing photolysis, photosensitization, and photocatalysis also. Largely, degradation efficacy was improved via accumulation of CeO2/PbWO4 photocatalyst effort might be the outcomes from (i) synergetic influence of the two metal oxides, (ii) leads to the decreases of bandgap energy, (iii) hindrance of the recombination rate of photoexcited (e−-h+) pairs separation amid CeO2 and PbWO4 heterojunction, (iv) accessibility of surface reactive sites, (v) development of light absorption ability of photoexcited charges generated in the visible-light (Rawat et al. 2007b). The primary absorbance of the peak disappeared entirely after 140 min of visible-light exposure in the CeO2/PbWO4 photocatalyst which specifies the cleavage of conjugated chromosphere structure of MB dye and exchange into small aromatic intermediary (Rožić et al. 2019).

Fig. 8
figure 8

a UV–Vis absorption spectra of MB dye solution using CeO2/PbWO4 NCs under visible-light exposure b Photodegradation rate of prepared photocatalysts

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Kinetics of MB dye photodegradation

The photocatalytic reaction kinetics of MB dye in visible-light, overall the photocatalysts, was considered by a consistent pseudo-first-order kinetic equation of lnC0/Ct = kt using CeO2, PbWO4 and CeO2/PbWO4 PCs as shown in Fig. 9a. Here, k refers to a reaction rate constant, C0 and Ct are the initial and residual concentrations of MB aqueous dye solution at the agreeing time, and t (min) was initiate to be linear regression (Yu et al. 2013; Xu et al. 2020). The obvious rate constants for CeO2, PbWO4 and CeO2/PbWO4 PCs were calculated as 0.0004, 0.0082, and 0.0164 min−1. The k value has increased in the order of CeO2 < PbWO4 < CeO2/PbWO4. It has decided that CeO2/PbWO4 PCs has a greater rate constant and it has 4.2 and 2.1 fold enrichment linked to that of pristine CeO2 and PbWO4 PCs, separately. Also, the CeO2/PbWO4 NCs owns expressively high photocatalytic dye degradation effectiveness as compared with related metal oxides and reported in other nanocomposites (Kumar et al. 2013; Wei et al. 2014; Xian et al. 2015; Li et al. 2016; AlShehri et al. 2017; Reddy Yadav et al. 2017).

Fig. 9
figure 9

a Photodegradation linear plot ln (C0/Ct) versus time of as-prepared photocatalytic samples b Reusability performance of CeO2/PbWO4 photocatalyst

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Reusability of the photocatalyst

To further explore the stability and reusability were of great consequence factor for its practical application, and the Fig. 9(b) has exhibits the recycling investigates of chief CeO2/PbWO4 PCs was repeatedly used (Shanmugam et al. 2019). For both cycles, the CeO2/PbWO4 PCs were recycled via centrifuging, washing and drying for the next cycling runs. For the CeO2/PbWO4 PCs, the MB dye photodegradation still existing good stability with only about 5% decreases from the initial activity (94%) during the 4th recycle. The activity loss has which largely might be owed to the photo-corrosion of the catalyst by light in CeO2/PbWO4 photocatalyst (Yu et al. 2015). As well, the XRD and FTIR analysis of CeO2/PbWO4 PCs were surveyed further to prove the stability of before and after the photocatalytic replies were carried out. As shown in Fig. 10a, b the XRD and FTIR outcomes might designate that no notable changes are witnessed after 4th recycles, representing the structural stability of the CeO2/PbWO4 PCs has measured (Saravanakumar et al. 2016). These consequences propose that the CeO2/PbWO4 PCs used as effective superior photo-stability, and suitable recyclable PCs might also be reused constantly for wastewater treatment under visible-light contact (Vignesh et al. 2019).

Fig. 10
figure 10

a XRD pattern b FTIR spectra of before and after catalytic reacted photocatalyst

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Reactive species study

As well known, to validate the radicals of nanocomposites in the photodegradation progression, the trapping tests of reactive species are executed by utilizing light drive. Photoexcited (e−-h+) pairs thereby form numerous reactive species for instance (% O2•−) and hydroxyl radical (% •OH−), hence these reactive species could source in the decomposition of the organic toxins (Negi et al. 2019). Figure 11 as could be seen that photodegradation rate of as-obtained PCs has faintly reductions resultant from the adding of EDTA-2Na, hence validating that h+ are not a major reactive species in this concerned systems. Likewise, the degradation rate is slightly declined when the addition of IPA, hence the % •OH− is minor/deliberate part of the allied photocatalytic system. This is because the (VB) valence band potential of CeO2 and PbWO4 NPs have conjugated into the % OH/•OH− also being % •OH− cannot be fashioned. Though, the addition of BQ initiated significantly suppress the photocatalytic movement from 91 to 26% as presented in the tentative results. Based on the trapping investigation effects, it’s resolved that % O2•− radicals are the dominant reactive species (O2 + e− = % O2•−) accountable for the decomposition of CeO2/PbWO4 photocatalytic scheme (Ravishankar et al. 2015).

Fig. 11
figure 11

Effect of scavengers on the MB dye photodegradation process

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Possible photocatalytic mechanism

To obtain better consideration on the CeO2/PbWO4 PCs heterojunction, their valence band (VB) and conduction band (CB) edge potentials were intended via Mulliken electronegativity theory from the Eqs. (1) and (2)

$${E}_{VB}=\mathrm{ X }- {E}_{e} + 0.5 {E}_{g}$$
(1)
$$ E_{{{\text{CB}}}} = E_{{{\text{VB}}}} {-} E_{g} $$
(2)

wherever χ states the absolute electronegativity of certain semiconductors (χ is 5.56, 6.15 eV for CeO2 and PbWO4, singly). And the EVB, ECB, Eg and Ee are VB potential, CB potential, estimated optical bandgap of CeO2 and PbWO4, and energy of free electrons vs. hydrogen scale (4.5 eV), separately (Channei et al. 2019). Agreeing to the beyond equations, the EVB and ECB band edge potential values were assessed to be + 2.52 and − 0.11 eV for CeO2 and as − 0.40 and + 3.41 eV for PbWO4, separately. From the above-mentioned results, the probable photocatalytic mechanism (Fig. 12) comprises by visible-light which leads to the conception of a photoexcited electron (e−) flowing towards CB of CeO2 and thereby the VB of PbWO4 acts as a drop for the hole (h+) (Tomova et al. 2015). Thus, the e− and h+ could be proficiently divided; the e−-h+ pairs are essentially gathered in the CB of CeO2 and the VB of PbWO4, individually. It is important to note that Ce element has two valence state (Ce4+ and Ce3+), and Ce4+/Ce3+ combine owns with multiple roles in endorsing the separation of photoexcited e−-h+ pairs of CeO2/PbWO4 PCs, by which the photocatalytic performance is enriched (Misra et al. 2012; Priyadharsan et al. 2017b; Wang and Tian 2020). Essentially, Ce3+ could effortlessly trap the O2•− making chemical adsorption of oxygen on the exterior of CeO2 centered on the Ce3+  + O2 = Ce4+—% O2•−, which stimulates photocatalytic oxidation response. Instead of Ce3+, which comprises oxygen defects could also absorb visible-light to harvest the photoexcited electron (e−) (Ali Baig et al. 2021). All these gathered electrons (e−) and Ce3+ own a strong reduction capacity was augmented easily and speedily decrease the absorbed O2 on CeO2 to cause active superoxide (% O2•−) radicals (Cano-Franco and Álvarez-Láinez 2019). This is because the VB band edge potential of CeO2 (+ 2.52 eV vs. NHE) develops more positive than PbWO4 (-0.11 eV vs. NHE), which is beneficial for MB dye deprivation (Fukumura et al. 2017). While electron in the CB supports an O2 molecule to form an O2•− radicals and formerly O2•− will respond with surface water molecules (H2O) to yield the •OH− radical. Lastly, the reactive radicals respond with MB dye molecules decayed to the intermediates or degradation yields over the highly oxidizing species (O2•−, •OH− etc.). The appropriate photocatalytic reaction progression (Reddy Yadav et al. 2016; Zhang et al. 2019a) could be also agreed upon by the succeeding Eqs. (3–7)

$$ {\text{CeO}}_{2} /{\text{PbWO}}_{4} + {\text{Dye}} + {\text{Light}} \to {\text{CeO}}_{2} /{\text{PbWO}}_{4} + {\text{Dye}} + \left( {e^{ - } \left( {{\text{CB}}} \right) + h^{ + } \left( {{\text{VB}}} \right)} \right) $$
(3)
Fig. 12
figure 12

Possible mechanism for the photoexcited electron–hole separation of the degradation of MB dye by CeO2/PbWO4 photocatalytic interface under visible-light

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$$\mathrm{Dye} +{e}^{-}\left(\mathrm{CB}\right)\to \mathrm{ Reduction Products}$$
(4)
$$\mathrm{Dye} +{h}^{+}(\mathrm{VB})\to \mathrm{ Oxidative Products}$$
(5)
$$\mathrm{Dye}+ \mathrm{OH}\to {\mathrm{OH}}^{.-} +\mathrm{Degradation} \mathrm{Products}$$
(6)
$$\mathrm{Dye} +{O}_{2}\to {O}_{2}^{.-}+\mathrm{ Degradation Products}$$
(7)

Conclusions

To sum up, CeO2/PbWO4 heterojunction photocatalyst was effectively formed via a facile and effectual hydrothermal scheme. The CeO2/PbWO4 nanocomposite was characterized by a range of techniques to study its phase purity, surface morphology, chemical composition and optical belongings were examined and debated in detail. In specific, CeO2/PbWO4 photocatalyst displayed the utmost photodegradation efficacy for 94% of MB aqueous dye within 140 min in visible-light treatment which is greater than other as-obtained samples. The degradation rate constant was closely 4.2 and 2.1 fold greater than those of pristine CeO2 and PbWO4 PCs singly. Also, •OH radicals were enabled in the key reactive species in the photodegradation route and the plausible degradation pathways have also been projected. Moreover, the optimized CeO2/PbWO4 photocatalyst presented an outstanding photocatalytic steadiness also reusability upto four consecutive cycles. The developed photocatalytic activities were recognized to the pairing of CeO2 and PbWO4 NPs were initiated by superior charge separation/transfer, reduced bandgap, and strong visible-light absorption proficiency thus suppressing the recombination of photoexcited charges. This study promotes the novel CeO2/PbWO4 NCs with outstanding visible-light photocatalytic action and admirable stability was estimated to inspire potential environmental applications in may near future.