Introduction

Fingerprints (FPs) have provided a vital source of forensic evidence for human identification and individualization. The patterns on the FPs are individual to each person and remain same throughout lifespan. Normally most of FPs seen in the daily life is latent and invisible to naked eyes; as a result, some certain methods are necessary to recognize personal identification in forensic science. Till date numerous methods (chemical, optical) have been established to recognize LFPs. However, the recognition was restricted only level 1 and level 2 ridge details due to the poor image qualities (Saif et al. 2015; Sharma et al. 2014; Wang et al. 2015a, b, c).

Further, the level 1 and 2 features are easily forged, by the generation and use of artificial skin imprints. However, the analysis of level 3 features required some efficient nano powders with uniform particle size and morphology (Basavaraj et al. 2017; Dhanalakshmi et al. 2017; Venkatachalaiah et al. 2017). Hence, there is an urgent need to progress a simple and efficient route in developing LFPs with superior sensitivity, little background, extraordinary efficiency, lesser toxicity and stress-free detection of LFPs, which is the essential requirement for forensic investigators. Further, most of the available literature for the detection of LFPs is due to excited ultraviolet (UV) radiation; rare earth doped fluorescent nanopowders can enhance FPs contrast. Conversely, UV light has many drawbacks including high background interference owing to the significant auto-fluorescence from the substrates, photo damage to the skin and eyes of the operators, and the possibility of severe irradiation-induced damage. Therefore, NPs are highly essential for the recognition of level 3 ridge details under normal light in the field of forensic science (Darshan et al. 2016; Suresh et al. 2017).

In recent years, the change in physiochemical properties of bulk material to corresponding nano regime has created numerous interest for material scientists. Due to its versatile applications in gas sensors, fuel cells, solar cells and very good photocatalytic activity, Cerium oxide nanoparticles have attracted much attention in recent years (Choi et al. 2006; Lehnen et al. 2014; Liu et al. 2013; Sathyamurthy et al. 2005; Sun et al. 2005). Its diverse applications is due to the efficiency of transfer of elevated oxygen from reduced to oxidized state (Lin et al. 2013; Zhou and Rahaman 1997). Till date, various reports of CeO2 synthesis routes are available in the literatures. In the recent years, bio-synthesis route using plant extracts is found to be environmental friendly, novel and inexpensive. Further, it does not require high pressure and high temperature (Derakhshandeh and Soleimannejad 2016; Huang et al. 2005; Maensiri et al. 2014; Maensiri et al. 2007; Wang et al. 2010). With no 4f electron CeO2 would be a good host material for photoluminescent materials (Jamshidi et al. 2013; Weber 1973). Ling Li et al. reported violet/blue emission of cerium composited with silicon for its good compatibility with the silicon-based materials used in the fabrication of optoelectronics devices (Li et al. 2015; Arumugam et al. 2015; Ketzial and Nesaraj 2011; Malleshappa et al. 2015; Lee et al. 2014; Lopez and Mendoza 2013).

Till date, limited reports are available for the synthesis of CeO2 NPs by solution combustion method using Tamarindus indica fruit extract as fuel. Further, the optimized product is utilized for visualization of LFPs on both porous and non-porous surfaces. Finally, the photocatalytic degradation is examined using MB as a dye. The detailed investigation on the mechanism related to the photocatalytic degradation is discussed.

Experimental

Fruit extract

Tamarindus indica fruit is collected from the local market of Tumkur and dried for about 5 days under the sunlight. 100 g of T. indica fruit was soaked in water for ~ 16 h. The obtained solution is kept in reflex for ~ 5 h and then washed several times with distilled water and filtered. Finally, the obtained T. indica fruit extract was stored in refrigerator until further use.

Synthesis

A simple solution combustion route is used to prepare pure CeO2 NPs using T. indica fruit extract as fuel. Initially, Cerium (III) nitrate [(Ce(NO3)3·6H2O): 0.5 g] is taken in a petri dish and 5 ml of T. indica fuel is added (1 g of extract is dissolved in 250 ml of water and stirred for 10 min). The precursor solution is stirred thoroughly using magnetic stirrer (~ 5–10 min) till homogeneous mixture is obtained. The resultant reaction mixture is placed in a preheated muffle furnace maintained at a temperature of ~ 450 ± 10 °C. The mixture undergoes dehydration with liberation of large amount of gases. Finally, a product is left behind in the petri dish. The portion of obtained product is calcined at ~ 800 °C for 3 h. The same experimental procedure is repeated for different concentrations of T. indica extract (5, 10, 15, 20 and 30 ml). The prepared as-formed and calcined CeO2 NPs were used for further characterization. Figure 1 shows the schematic representation of the steps involved for the extraction of T. indica fuel and synthesis of CeO2 NPs.

Fig. 1
figure 1

Schematic representation to show various steps involved in extraction of fuel and synthesis of CeO2 NPs by solution combustion route

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Characterization

The powder X-ray diffraction (PXRD) measurements were performed on the Shimadzu made X-ray diffractometer (Shimadzu 7000) with graphite monochromatized Cu–Kα radiation (λ = 0.15406 nm) to study the phase and purity of the prepared samples. The surface morphology is studied by Hitachi made table top scanning electron microscope (SEM) (Hitachi-TM 3000). Transmission electron microscope (TEM) (Hitachi H-8100, Kevex sigma TM Quasar, USA) is used to estimate the crystallite size of the material. Spectrophotometer (Lambda-35, Perkin Elmer) is used to study the diffuse reflectance of the prepared material. Spectroflourimeter equipped with Fluorolog-3 (Jobin–Yvon) is utilized to measure the photoluminescence (PL) properties. The Fourier transform infrared (FTIR) spectrum is recorded in Bruker alpha made FTIR instrument. The Jobin–YVON Horiba LABRAM-HR-Visible micro Raman system is used for Raman studies with He–Ne laser (~ 632.8 nm) as the source.

Visualization of LFPs using CeO2 NPs as labeling agents

The LFPs are collected from healthy donor on various surfaces including porous and non-porous surfaces. The hands of the donor are washed thoroughly with soap and dried before impression. Subsequently, the washed fingers are gently pressed against various porous and non-porous surfaces to acquire LFPs. Thereafter, the obtained LFPs are stained by optimized CeO2 (15 ml) NP using soft feature brush with smooth powder brushing method. Finally, developed FPs are visualized under normal light using a Nikon D3100/AF-S Nikkor 50 mm f/2.8G ED lens digital camera.

Results and discussion

In order to inspect its suitability for recognizing latent fingerprints (LFPs), optimized CeO2 NPs were stained on the LFPs developed on various non-porous substrates, namely stainless steel, glass, mobile screen and aluminium foil. In the present studies, all FPs were extracted from the same donor. Figure 2 display the LFPs images visualized by staining CeO2 NPs illuminated under normal white light. A complete ridge details including type 1, 2 and 3 patterns with good adhesion, well-defined ridge flow and ridge orientation field is observed. Further, the FPs were well defined in terms of finger ridge details which due to their nano-sized CeO2. FP types have been described into three groups; (i) level 1 features are described by fingerprint ridge flow and general morphological information, (ii) level 2 features provide pattern matching followed detection of individual fingerprint ridges, (iii) level 3 features is defined as all attributes of a ridge, including shape, width, pores and curvature. Further, the stability of the prepared powders are assessed by aging the FPs up to 21 days and displayed comparable brightness (Fig. 3). The brightness of the CeO2 NPs-stained LFPs specifies that the NPs are physically and chemically steady in ambient conditions. Figure 4 displays detailed ridge patterns (type 1, 2 and 3) developed on glass which consists of clear and distinct ridges of FPs can be undoubtedly detected under normal visible light. The level 2 details, such as the ridge bifurcation, island, scar, eye, bridge, ridge end, termination, lake and sweat pores, were clearly distinguished. Further, overlapped LFPs can be recognized on aluminium foil surfaces and shown in Fig. 5. The images clearly evident that the recognition of various ridge features includes level 1, level 2 and also level 3 identification. Further, LFPs developed on glass were compared with commercially available (Fe2O3 and TiO2) powders and shown in Fig. 6. According to the detailed results obtained in the present study, we can observe the FP types specifically ridge flow, sweat pore, terminates, bifurcation and lack corresponding to the levels 1–3. Therefore, it is authorized that the probability for the recognition of the foremost ridge details of FPs in different surfaces was well established by optimized CeO2 NPs. The high-quality FP images developed on non-porous surfaces display the possible usage of CeO2 NPs fabricated via green synthesis route for forensic application.

Fig. 2
figure 2

a–f LFPs visualized by staining calcined CeO2 (15 ml) NPs on various non-porous surfaces and (a *, b *, b 1*, b 2*, c *, d *, e * and f 1*, f 2*) shows the enlarged views of FPs which exhibits various ridge prints

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Fig. 3
figure 3

LFPs aged on the surface of glass for various periods of time, stained by calcined CeO2 (15 ml) NPs under normal light a 1 day, b 1 week, c 2 weeks and d 3 weeks

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Fig. 4
figure 4

Various ridge levels of FP patterns visualized by staining optimized calcined CeO2 (15 ml) NP on glass surface under normal light

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Fig. 5
figure 5

Overlapped and various ridge details of FP patterns visualized using calcined CeO2 (15 ml) NP on aluminium foil surface

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Fig. 6
figure 6

Comparison of FP images visualized by staining Fe2O3, TiO2 and calcined CeO2 (15 ml) NP on glass surface under visible light

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Figure 7a, b shows the PXRD patterns of as-formed and calcined (800 °C) CeO2 NPs. All the diffraction patterns are well indexed to JCPDS card No. 81-0792 (Malleshappa et al. 2014). The observed XRD peaks are well matched with the cubic planes (111), (200), (220), (311), (222), (400) and (311). No secondary or impurity peaks were present in the samples, which confirms the single cubic phase of CeO2. However a small lower angle (28.51°) peak shift in the plane (111) is observed due to compressive stress. The average crystallite size (D) is estimated using Scherrer’s relation:

Fig. 7
figure 7

PXRD patterns and W–H plots of CeO2 NPs (a, c) as-formed and (b, d) calcined at 800 °C

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$$D = \frac{k\lambda }{\beta \cos \theta },$$
(1)

where λ is the wavelength of the X-ray radiation (1.54 Å), K is the constant (0.9), β is the full-width at half-maximum (FWHM), and θ is the diffracting angle. The estimated values of D are tabulated in Table 1. The lattice strain present in the product is estimated using the Williamson and Hall (W–H) plots using the following relation:

Table 1 Estimated average crystallite size, strain, and energy gap (E g) values of the as-formed and calcined CeO2 NP
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$$\beta \cos \theta \; = \;\frac{k\lambda }{D}\; + \;4\varepsilon \sin \theta ,$$
(2)

where λ is the wavelength of X-rays (1.54 Å), β is the full width at half maximum (in radians), θ is the diffraction angle, k is the shape factor (0.9), D is the crystallite size, and ε is the micro-strain. The plots of βcosθ v/s 4sinθ are shown in Fig. 7c, d. The slope of line gives the strain (ε) and intercept (k λ/D) on Y axis gives crystallite size. The estimated values of D and lattice strain present in the samples are listed in Table 1.

One of the drawbacks of the diffraction patterns is accurate determination of crystal structure because of the overlapping of the diffraction peaks. Therefore, Rietveld refinement analysis is used to create the virtual separation of the overlapped peaks (Fig. 8). The Rietveld refinement method is used to calculate the entire powder pattern of the sample with the different refinable parameters and to minimize the difference between the observed and calculated pattern by least square method. The lattice parameters are evaluated by performing Rietveld refinement using FULLPROF suit program (Konysheva et al. 2010; Malleshappa et al. 2016; Shivaram et al. 2014; Xiaoyuan et al. 2001).

Fig. 8
figure 8

Rietveld refinement of a as-formed and b calcined CeO2 NPs (800 °C) (Inset: Packing diagram)

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Pseudo-Voigt function has been used to fit the several parameters like one scale factor, one zero shifting, six background, three cell parameters, five shape and width of the peaks, one global thermal factor and two asymmetric factors. The refined parameters namely occupancy, atomic functional positions for CeO2 NPs are studied. The obtained results are in good agreement with the theoritically calculated ones. The space group used for the particular pattern is Fm-3m (No. 225). The refined parameters are tabulated in the Table 2. The obtained R p, R wp, R Braggs, and χ 2 values indicate the good fit of the profile.

Table 2 Structural parameters of CeO2 NPs estimated using Rietveld refinement
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Figures 9a, b shows the FTIR spectra of the as-formed and calcined CeO2 NPs prepared at different concentrations of T. indica extract (5–30 ml). From the figure, it is observed that the principal vibrational modes of CeO2 NPs were in good agreement with the reported literature. In the calcined sample, a diffuse band observed at 2918 cm−1 is due to the stretching vibration of the physically associated water (O–H) molecule. Another bending vibrational band of physically adsorbed water molecules is observed around 1619 cm−1. A weak broad peak observed at 314 cm−1 which is assigned to the Ce–O stretching band (Binet et al. 1994; Khan et al. 2013; Sahu and Rao 2000). The peak at 1350 cm−1 is a characteristic vibration mode of CeO2 (Ketzial and Nesaraj 2011; Yan and Zhu 2008).

Fig. 9
figure 9

FTIR spectra of a as-formed and b calcined CeO2 (5–30 ml) NPs

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The diffuse reflectance (DR) spectra of the as-formed and calcined samples of CeO2 NPs are shown in Fig. 10a, c. The spectra are recorded in the range of 200–1100 nm wavelength at room temperature. It can be seen from the Fig. 10a, c that a strong absorption peak is observed in the range of 350–400 nm.

Fig. 10
figure 10

a, c Diffuse reflectance spectra and b, d energy band gap plots of as-formed and calcined CeO2 NPs

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To determine the energy band gap, Kubelka–Munk function is used (Yuan et al. 2016). The Kubelka–Munk function F (R ∞) and band gap energy (\(h\nu\)) is estimated by utilizing the following equations:

$${\text{F}}\left( {R_{\infty } } \right)\; = \;\frac{{\left( {1 - R_{\infty } } \right)^{2} }}{{2R_{\infty } }},$$
(3)
$$h\nu = \frac{1240}{\lambda },$$
(4)

where R ∞ is the reflection coefficient of the sample and λ is the absorption wavelength. The energy band gap values were evaluated and are summarized in Table 1. The energy band gap (E g) values of as-formed CeO2 NPs are found to be in the range of ~ 3.24–3.69 eV and that for calcined sample it was found to be ~ 2.86–3.20 eV, which is much lesser than the as-formed CeO2 due to oxygen vacancies present in the sample (Fig. 10b, d), therefore, which gives the evidence for the quantum confinement of calcined CeO2 NPs. The decrease in Eg also indicate the red shift in the calcined sample (Table 1) (Li et al. 2015; Jorge et al. 2012; Periyat et al. 2011).

Figure 11a, c shows the Raman spectra of as—formed and calcined CeO2 NPs and its enlarged portions are shown in Fig. 11b, d. The spectra exhibit four characteristic modes at ~ 254, 460, 546 and 612 cm−1 (Balakrishnan et al. 2013). The mode at ~ 254 and 460 cm−1 are attributed to twice degenerated 2TA and first order F2g, respectively. An F2g mode confirms the fluorite cubic structure of the prepared samples and also signifies the symmetrical stretching vibration of the CeO8 vibrational unit. A broad and asymmetric mode at ~ 460 cm−1 is red shifted as compared to bulk form owing to oxygen defects, phonon confinement, size distribution, deviation in phonon relaxation and strain present in the sample (Xu et al. 2017; Lopez et al. 2015). The other modes at ~ 546 and 612 cm−1 are due to oxygen vacancies (intrinsic and extrinsic). The relative intensities of Raman modes at ~ 460, 546 and 612 cm−1 are calculated based on the relation (Arumugam et al. 2015):

$$\frac{{{\text{Oxygen}}\,\,{\text{vacancies}}}}{{{\text{F}}_{{ 2 {\text{g}}}} }}\; = \;\frac{{\left( {{\text{Area}}_{{R_{{{\text{Ce}} - {\text{O}}}} }} \, } \right)}}{{{\text{Area}}\,{\text{F}}_{{ 2 {\text{g}}}} }}.$$
(5)
Fig. 11
figure 11

a, c Raman spectra and b, d enlarged portion of Raman spectra of as-formed and calcined CeO2 NPs

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The rate of oxygen vacancies is estimated and given in Table 3. The probable reaction mechanism for intrinsic oxygen vacancies are:

$${\text{Ce}}_{\text{Ce}} \, + \,2{\text{O}}_{\text{o}} \to \,{\text{V}}^{\prime \prime \prime \prime } + \,2{\text{V}}_{\text{o}}^{ \cdot \cdot } \, + \,{\text{CeO}}_{2} ,$$
(6)
$${\text{Ce}}_{\text{Ce}} \, \to \,{\text{V}}_{\text{Ce}}^{\prime \prime \prime \prime } \, + \,{\text{Ce}}_{\text{i}}^{ \cdot \cdot \cdot \cdot } ,$$
(7)
$${\text{O}}_{\text{o}} \, \to \,{\text{V}}_{\text{o}}^{ \cdot \cdot } \, + \,{\text{O}}_{\text{i}}^{\prime } .$$
(8)
Table 3 Estimated Oxygen vacancies and crystallite size of CeO2 NPs
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Besides, the average crystallite size of the CeO2 powder scale down to nano range leads to more oxygen loss (Jiang et al. 2012).

Figure 12 shows the SEM micrographs of as-formed and calcined CeO2 NPs. It can be observed from the Fig. 12a–c that the particles are porous in nature. Particles are mostly containing voids with different sizes due to the evolution of large amount of gases during the combustion. The porosity and size of the voids increase with increase in temperature as observed in Fig. 12d–f. TEM, HRTEM images and SAED patterns of as-formed CeO2 NPs are shown Fig. 13. The particles with agglomeration are observed in Fig. 13a. The estimated crystallite size varies between ~ 5 and 20 nm which is in good agreement with the PXRD results. The interplanar distance (d) was found to be around ~ 0.32–0.34 nm (Fig. 13c). The polycrystalline nature of the sample was further confirmed by SAED patterns. Figure 14 shows the TEM, HRTEM images and SAED patterns of calcined CeO2 NPs. It is evident that in calcined samples the crystallinity has been increased as shown in Fig. 14a, b. The interplanar spacing d value is found to be ~ 0.33 nm.

Fig. 12
figure 12

SEM micrographs of a–c as-formed and d–f calcined (800 °C) CeO2 (5, 15 and 30 ml) NPs

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Fig. 13
figure 13

a TEM, b enlarged view, c HRTEM image and d SAED pattern of as-formed CeO2 (15 ml) NPs

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Fig. 14
figure 14

a, b TEM, c HRTEM image and d SAED pattern of calcined CeO2 (15 ml) NPs

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Photoluminescence excitation spectrum of as—formed CeO2 NPs upon 530 nm emission wavelength as shown in inset of Fig. 15a. The spectrum exhibit a sharp peak at ~ 386 nm. Figure 15a shows the PL emission spectra of as—formed CeO2 NPs prepared with various fuel concentration under λExc = 386 nm. The spectra exhibits peaks at ~ 362, 391 and 441 nm may be attributed to surface defects, charge transition from the 4f band to the valence band of CeO2 and oxygen vacancies (Li et al. 2008; Dang et al. 2010; Luo et al. 2015). The emission spectra of calcined CeO2 NPs excited under 386 nm wavelength as shown in Fig. 15b, e. The spectra exhibits characteristic peaks at ~ 485, 528 and 542 nm are believed to be due to surface defects and oxygen vacancies  (Li et al. 2011; Arunkumar et al. 2014; Morshed et al. 1997; Maensiri et al. 2007).

Fig. 15
figure 15

PL emission spectra of a as-formed and b calcined CeO2 NPs excited at 386 nm wavelength (Inset: excitation spectrum recorded at 530 nm emission)

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The Commission Internationale de L’Eclairage (CIE) diagram of as—formed and calcined CeO2 NPs is shown in Fig. 16a. In general, the emission color of any phosphor can be represented by the (x, y) chromaticity co-ordinates (Sayed et al. 2011). The CIE co-ordinates are estimated using PL emission spectra and the values are listed in a Table 4. It is noticed that the CIE co-ordinates of as-formed and calcined samples were located in the blue region. The correlated color temperature (CCT) is one of the important parameters to know the color appearance of the light emitted by a source, relating it is color with respect to a reference light source when it is heated up to a specific temperature, in Kelvin (K) (Guo et al. 2012). The CCT is calculated and shown in Fig. 16b by the method as reported earlier (Shrivastava et al. 2015). The obtained values are summarized in Table 4.

Fig. 16
figure 16

a CIE and b CCT diagrams of as-formed and calcined CeO2 (5–30 ml) NPs

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Table 4 Photometric values of as-formed and calcined CeO2 NPs
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Photocatalytic studies are carried out using 150 × 75 mm batch reactor at room temperature. Methylene blue is used as dye for photo degradation of CeO2 NPs as catalyst. The particle size of as-formed CeO2 NPs prepared with T. indica fuel (15 ml) was very small (5.2 nm) compared to other concentrations of the fuel; due to this reason, CeO2 NPs with 15 ml samples were considered for photocatalytic activity. When solution of CeO2 NPs with dye was exposed to UV light, electron transfer from valence to conduction band by absorbing UV light. The e-–h+ pairs offers efficient path for oxidation and reduction reactions to degradation of MB dye. The generated electrons react with molecular oxygen and produce hydrogen-peroxide molecules in the aqueous medium and expressed by the following reactions. Conversely, the generated holes produce hydroxyl radicals from the water. These hydroxyl radicals act as an influential oxidizing agent to degradation of pollutant MB. The effect of pH concentration on the photocatalytic activity is also studied in detail. It is clear from the literature that photocatalytic activity depends on the pH value of solution. Here, four different pH concentrations effect on photocatalytic activity was studied. Surface property of CeO2 and dislocation of dye molecule varies with the different pH value. In basic media, there is an increase in hydroxyl radicals which leads to the increase in photocatalytic activity (Arul et al. 2015; Khan et al. 2014; Lei et al. 2015).

Figure 17a shows the effect of catalytic load on the catalytic activity of CeO2 NPs. To study the effect of catalytic load 5 ppm in 100 ml of water is maintained constant with varying catalytic load. The catalytic load increased up to 150 mg; the percentage of degradation was also increased and then remains constant for 200 mg catalytic load owing to the formation of turbidity in the solution which reduces the penetration of light through the solution. The following reactions explain the splitting of water molecule and dye degradation into organic acid and by-products.

Fig. 17
figure 17

Effect of a catalytic load and b pH value on photocatalytic activity of calcined CeO2 (15 ml) NPs

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$$h_{\text{vb}}^{ + } + {\text{OH}} \to {\text{OH}}^{ \cdot } ,$$
(9)
$${\text{e}}_{\text{CB}}^{ - } + {\text{O}} \to {\text{O}}_{2}^{ \cdot - } ,$$
(10)
$$\left( {{\text{OH}}^{ \cdot } , {\text{O}}_{2}^{ \cdot - } } \right) + {\text{MB}}\,{\text{dye}} \to {\text{Degradation of dye,}}$$
(11)
$${\text{H}}_{ 2} {\text{O}} + {\text{e}}^{ - } \to {\text{H}} + {\text{OH}}\;{ + }\;{\text{e}}^{ - } ,$$
(12)
$${\text{H}}_{ 2} {\text{O}}\; + \;h\nu \; \to \;{\text{H}} + {\text{OH,}}$$
(13)
$${\text{OH}} + {\text{OH}} \to {\text{H}}_{ 2} {\text{O}}_{2} ,$$
(14)
$${\text{H}}\;{ + }\;{\text{H}} \to {\text{H}}_{2} ,$$
(15)
$${\text{H}}_{ 2} {\text{O}}_{2} + h\nu \to {\text{OH}} + {\text{OH,}}$$
(16)
$${\text{OH}} + {\text{MB}} - {\text{OH}} + {\text{OH}} \to {\text{MB}} - \left( {\text{OH}} \right)_{2} + {\text{OH}} \to {\text{MB}} - \left( {\text{OH}} \right)_{3} ,$$
(17)
$${\text{MB}} - \left( {\text{OH}} \right)_{3} \to a\text{mines} ,\;{\text{organics acids}},\,{\text{etc}} \to {\text{Co}}_{2} + {\text{H}}_{ 2} {\text{O}} .$$
(18)

Figure 17b shows the effect of pH on catalytic activity of CeO2 NPs. To study the effect of pH, 5 ppm dye concentration was maintained constant in 100 ml of water with 100 mg of catalytic load by varying pH concentrations. For pH 9, CeO2 NPs have shown good photocatalytic activity while for pH 3 it exhibit poor activity which confirms that in basic medium activity it was more when compared with the acidic medium (Magesh et al. 2009; Wetchakun et al. 2012). Figure 18 shows the schematic representation of the mechanism involved in photocatalytic degradation of the MB dye using CeO2 NPs as photocatalyst.

Fig. 18
figure 18

Schematic representation to show the mechanism of photocatalytic degradation of MB using CeO2 NPs

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Figure 19 shows the cyclic voltagrams (CVs) of as-formed CeO2 NPs modified carbon paste electrode recorded in 0.5 mM dopamine concentration in PBS buffer of pH 7. The bare carbon paste electrode shows only a small oxidation peak centered at 0.2 V. Whereas, CeO2 NPs modified carbon paste electrode shows well-defined oxidation peak at 0.2 V and also corresponding reduction peak around 0.13 V (Fig. 19c). These results confirm that presence of CeO2 NPs on electrode assists in catalytic oxidation of dopamine. With increase in dopamine concentration, the anodic peak current also increases as shown in Fig. 20. This confirms that the redox peak corresponds to surface confined species (Li et al. 2015). The ratio of anodic and cathodic peak current was approximately equal to one, which indicates that the electrochemical reaction is highly reversible (Wei et al. 2006). The electrochemical sensing results obtained for modified electrodes CeO2 NPs reveals the highly stable and can be used to fabricate good electrochemical sensing devices.

Fig. 19
figure 19

CV plots of a bare carbon paste electrode in PBS buffer of pH = 7, b a + 0.5 mM dopamine, c calcined CeO2 NPs modified carbon paste electrode in PBS buffer of pH 7, d c + 0.5 mM dopamine

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Fig. 20
figure 20

CV plots of calcined CeO2 NPs modified carbon paste electrode in PBS buffer of pH = 7 with dopamine concentrations ranging from 0.5 to 0.25 mM

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Conclusions

A simple green combustion route is used to prepare CeO2 NPs using T. indica fruit extract as fuel. The structural, optical and catalytic properties of as-formed as well as calcined CeO2 NPs are studied in detail. A cubic phase of prepared CeO2 NPs is confirmed by PXRD patterns. The crystallite size, energy band gap and PL properties are highly dependent on concentration of fuel. The prepared CeO2 NPs are highly pores and irregular shape with large number of surface voids. The particle size was estimated from TEM and found to be in the range of 5–40 nm which is in good agreement with PXRD results. From the CIE chromaticity diagrams, a blue color emission from the prepared sample is noticed. Further, modified electrodes by CeO2 NPs exhibit enhanced electrochemical sensing properties and, hence, can be used to fabricate the electrochemical sensing devices. The CeO2 NPs also showed good photocatalytic activity with respect to variation in pH and catalytic load. The above results clearly confirmed that the product obtained in green combustion route is quite useful for multifunctional applications.