1 Introduction

The increased human dependency on portable electronic gadgets resulted in the high demand for the energy sources, such as batteries and other storage devices. Thus, the demand for low powered and self-powered devices make their mark worldwide. The replacement of conventional devices along with powering sources by self-powered may address the energy crises for a large extent. In view of this, the triboelectric nanogenerators (TENGs) play a crucial role which harvests naturally occurring energy as a power source. The advantages of TENGs are not only the low cost production but also simple and economic fabrication of the devices. The triboelectric device works on the principle of conjunction of triboelectrification and electrostatic induction [1]. The recent reports on active materials for TENGs have mainly focused to enhance the performance by morphology tuning, fabrication methods, and selecting the best materials [2,3,4,5,6,7]. The recent reports focused mainly on triboactive materials like zinc oxide (ZnO), polyvinylidene fluoride (PVDF), and polyimide aerogel [8, 9]. There were less or no reports on TENGs using La2O3 as active material. Hence, the study is focused mainly on TENGs based on La2O3.

Lanthanum has been widely explored in its compound states such as oxides, hydroxides as well as phosphate forms. Numerous techniques for synthesizing the lanthanum oxide nanoparticles have been proposed, such as thermal decomposition, homogeneous precipitation, solvothermal, hydrothermal, and other chemical routes [10]. From the literature, it is shown that there are many reports on microwave-assisted hydrothermal synthesis and surfactant-assisted La2O3 nanorods, nanoneedles, and nanorod bundles [10, 11]. Solution phase synthesis favors the agglomeration and spherical particle formation because of its high surface energy. To reduce the agglomeration of the 1D lanthanum oxide nanoparticles, suitable capping agents like surfactants, polymers, or templates were employed [12].

Lanthanum oxide is a ceramic material and is brittle in nature. Lanthanum and its compounds have been widely used in optical, electrical, magnetic materials, and most importantly in impurity extraction, such as arsenic (As (III)) [13, 14]. Recent literature on the La2O3 materials have revealed that it could be used in solid fuel cells, as a dielectric material in parallel plate capacitors and high-temperature superconductors [15,16,17]. Lanthanum oxide has found great attention in the area of piezoelectric materials, thermoelectric materials, automobile exhaust-gas convectors, optoelectronic devices, sensors, catalysis, and solid electrolyte [14, 18].

The exploration of such lanthanum oxide architectures have opened an area of interest for innovative ceramic–metal oxide nanoparticles with tuneable material properties like electronic, magnetic, and catalytic properties [19,20,21]. Further, high tribo-polarity, surface area, and dielectric constant are essential requirements for an active triboelectric material. In view of this, lanthanum oxide being a high permittivity dielectric finds a suitable place in the triboelectric series, thus, it has been utilized in the study; also, polarized nature of the lanthanum oxide favors its application in the area of self-powered devices [22,23,24,25]. Finally, the piezoelectric nature of the La2O3 material favors its application in the area of self-powered devices.

In this work, a single-step hydrothermal synthesis of La2O3 nanocrystals is presented. The processing is simple, cost effective, and demands a minimal thermal budget. The La2O3 nanocrystals synthesized with this technique processes uniform morphology and size distribution. The synthesized La2O3 nanocrystals are characterized to study its morphological and structural properties. The synthesized nanocrystals are grounded with suitable capping agent to form paste for the screen printing technique without modifying its base properties. The La2O3 film is fabricated using synthesized nanocrystals and suitable reagents by the screen printing technique. The prepared film is then examined to study the triboelectric properties.

2 Materials and methods

2.1 Hydrothermal synthesis of La2O3 nanocrystals

A known weight of (1.4 g) cetyltrimethylammonium bromide (CTAB) is added to 100 ml of de-ionized (DI) water. The solution is stirred vigorously for a few minutes. Then, 3 g of lanthanum chloride (LaCl3) salt is added to the solution and stirred continuously using magnetic bar. 3 ml of 25% ammonia (NH3) solution is added drop-wise to the solution to maintain a basic pH ranging between 9 and 10. The resultant solution is stirred for 12 h and made in to colloidal dispersion with a translucent appearance. The prepared solution is poured into 200 ml stainless steel autoclave and kept in an oven at 100 °C for 48 h. The products obtained post heat treatment is then rinsed with DI water and ethanol several times to annihilate the residuals. Finally, the La2O3 nanocrystals are subjected for drying in an oven at 80 °C for 24 h. The dried product-La2O3 is stored in an air-tight bottle to avoid moisture contamination.

2.2 Screen printing of La2O3 film

45 weight percent (45 wt%) of the as-synthesized La2O3 nanocrystals are mixed with 5 wt% of ethylcellulose binder and 50 wt% of terpinol solvent. The mixture is then grounded rigorously in Mortar-pestle for 30 min to get high viscous agglomeration-free La2O3 screen printable paste. The mask for screen printing is created using a screen with #120 mesh. The snap-off distance of 5–10 mm is maintained to facilitate the quick release of the screen. The copper adhesive tape cleaned with isopropyl alcohol, acetone, and DI water is used as a substrate for screen printing. Here, the copper adhesive tape is flexible, which helps in device fabrication and testing, and also acts as an electrode. The films are then dried under infrared (IR) radiation for three to four hours to ensure complete evaporation of the solvent.

2.3 Characterization

The X-ray diffractometry (XRD, make-JEOL-JPZ-8 with a copper target (CuKα = 1.54 Å)) is used for the examination of phase purification of the La2O3 nanocrystals. The chemical nature of La2O3 nanocrystals is studied by using X-ray photoelectron spectroscopy (XPS, Kratos Analytical, UK, monochromatic AlKα ~ 1486.6 eV as X-ray source and XPS; PHI5000VersaProbeII). Prior to the XPS measurements, the sample is treated under argon gas to eliminate the surface impurities. The calibrations of all the XPS data are performed with the standard reference carbon 1s (C 1s) peak at 284.7 eV, and ± 0.2 eV of accuracy is maintained for measuring the binding energies. The deconvolution of oxygen 1s (O 1s) and lanthanum 3d (La 3d) is performed after the subtraction of the background using Shirley function. Further, the addition of synthetic peaks is performed using Gaussian–Lorentzian peak function with area resolution of 1 eV. The nature and surface morphologies of the La2O3 nanocrystals are examined by using field emission scanning electron microscopy (FESEM, make-JEOL-JSM-6380LA, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL-JEM-2100, Tokyo Japan). A very minute amount of La2O3 nanocrystals is dispersed in ethanol and a drop of prepared colloidal solution is poured on carbon-coated copper grid. Finally, the grid is dried under bulb (60 W). The grid containing La2O3 nanocrystals is then subjected for TEM examination. The Fourier transform infrared (FTIR, make-JASCO-4200 spectrometer, in KBr mode) is used to study the quality and the formation of La2O3 nanocrystals. The Brunauer–Emmett–Teller (BET) apparatus is used for the calculation of specific surface area of La2O3 nanocrystals following the standard protocols at 77 K. Prior to the BET measurements, the La2O3 nanocrystals are degassed in the presence of flowing N2 at 300 °C for 12 h [26].

To evaluate the triboelectric performance, the screen-printed film is tested in an in-house built motorized fixture as shown in Fig. 1 and Fig. S1. The dimension of the screen-printed La2O3-TENG device is (2.5 cm × 2.5 cm). Teflon is used as a counter surface for testing the La2O3-TENG device, since it is fluorine rich, leading to high electronegativity. The thickness of the Teflon used is 0.25 mm. Before the measurement of triboelectric response, the La2O3 screen-printed film thickness is measured to be 10 μm with average surface roughness of 0.25 μm (Fig. S2). While testing, the fixture is operated at around 15 Hz with applied maximum load of approximately 300 g at the heterojunction. The electrical parameters (i.e., voltage and current) are logged using an oscilloscope (Tektronix DPO 2014B) and Keithley parameter analyzer (4200 s), respectively.

Fig. 1
figure 1

a Image, b schematic representation of a motorized fixture for testing, and c working principle of contact-separation mode of triboelectric response of La2O3 film

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3 Results and discussion

3.1 Surface morphology of La2O3 nanocrystals

The micrographs of La2O3, captured from a scanning electron microscope (Fig. 2a–b) depict the rod-like morphology. The synthesized La2O3 nanocrystals are uniform in size and shape, showing its homogeneous formation during the synthesis. The micrographs at different locations present the uniform morphology with dimensions varying in few hundreds of nanometers.

Fig. 2
figure 2

Morphology of La2O3 nanocrystals captured using, a, b SEM, c TEM, d corresponding magnified high-resolution micrograph and e selected area electron diffraction (SAED) ring depicting the polycrystalline nature of La2O3 nanocrystals

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The diameter of the La2O3 nanocrystals is in the range of 5 to 30 nm and length is 100 to 300 nm (Fig. 2a, b). Figure 2c shows the TEM micrograph of the La2O3 nanocrystals, corresponding high-resolution image (Fig. 2d), and SAED ring pattern (Fig. 2e). The TEM analysis shows the crystalline structure of La2O3. The interplanar spacing is 0.334 nm (from Fig. 2d). The ring pattern with intense spot is shown in Fig. 2e, the La2O3 nanocrystals showed the intense diffraction spots suggesting the particles formed.

3.2 Crystal structure of La2O3 using XRD

The XRD of hydrothermally synthesized La2O3 nanocrystals is shown in Fig. 3a. The synthesized La2O3 nanocrystals are of high purity and pattern indexed with hexagonal phase (space group P-3m1, cell constant a = 3.9397 Å, b = 3.9397 Å, and c = 6.1359 Å, ICDD No. 83-1344) [27] (Fig. 3b). The spacing of crystal planes “d” calculated from the Bragg’s equation (2d sin θ = nλ) is found to be 0.3347 nm. The magnitude of “d” is in good agreement with the d-spacing from TEM fringe pattern. The average crystallite size of La2O3 nanocrystals estimated from the Scherrer equation is found to be 36 nm [19].

Fig. 3
figure 3

a The XRD spectrum and b crystal structure of La2O3 (green-lanthanum and red-oxygen) of hydrothermally synthesized La2O3 nanocrystals

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The sharp diffraction peaks at respective Bragg angles indicate that the high crystallinity is achieved at considerably low temperatures. Thus, both morphological and structural analyses conclude the quality of the synthesized nanocrystals. Also, the broad peaks with large FWHM depict the nanocrystalline nature, which is in good agreement with high-resolution TEM studies presented in Fig. 2c–d.

3.3 Chemical composition of La2O3 nanocrystals using XPS

Further, the La2O3 sample is subjected for XPS study to examine the composition. All the binding energy data of La2O3 sample obtained from the XPS analysis are corrected according to the standard referencing C 1s peak (284.7 eV). From Fig. 4a, the XPS survey spectrum shows only the presence of two metal elements, lanthanum and oxygen. The survey also shows that there is no presence of other metal elements on the surface of La2O3 sample. The presence of minor C 1s peak (Fig. 4a) is due to the surface-adsorbed carbon atoms/molecules during the hydrothermal synthesis. The binding energy at 833.6 and 850.1 eV are indexed to the presence of La 3d5/2 and La 3d3/2, respectively, as shown in Fig. 4b. The binding energy peak at 529.5 eV, in Fig. 4c, is indexed to the O2− in the La2O3 crystal. It is also seen that the O 1s profile is asymmetric indicating the presence of two oxygen species in the nearby region.

Fig. 4
figure 4

XPS survey spectra of the La2O3 showing both a low- and high-resolution scans, b La 3d region, and c O 1 s region

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3.4 BET surface area analysis

Brunauer–Emmett–Teller (BET) nitrogen gas adsorption–desorption measurements are used to find out the specific surface area of the La2O3 nanocrystals. The isotherm shows that the particles are porous (Fig. 5). The specific surface area of La2O3 from the BET apparatus is measured to be 72.33 m2/g. The value is predominant compared to already published literatures [28,29,30]. Table 1 represents some of the already published surface area values of the La2O3 nanocrystals.

Fig. 5
figure 5

Nitrogen (N2) adsorption/desorption isotherms for La2O3 nanocrystals

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Table. 1 Specific surface area of La2O3 nanocrystals published by researchers
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3.5 Identification of chemical bonding by FTIR

The FTIR spectrum is recorded to show the functional groups of the La2O3 nanocrystals (as shown in Fig. 6). The stretching vibration of O–H bond at 3427 cm−1 and the bending vibration of H–O–H absorption peak at 1631 cm−1 are due to the presence of moisture in La2O3 sample [31]. The absorption bond at 3608 cm−1 is assigned to the presence of bond tension in hydroxyl groups of lanthanum oxide. Further, the bands at 1483 cm−1 and 1440 cm−1 are attributed to asymmetric stretching mode of the C–O bond [27]. The absorption bands at 858 and 657 cm−1 are assigned to bending out of plane vibrations and La–O stretching vibration, respectively [32].

Fig. 6
figure 6

FTIR analysis of La2O3 nanocrystals

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3.6 Output characteristics of triboelectric nanogenerators

To evaluate the maximum power generated by the device, TENG device is connected to an electrical load (resistor)s [33]. The obtained voltage is as shown in Fig. 7. The resistance value is swept from 0 to 50 MΩ. Respective voltage and current produced by the La2O3-TENG device are plotted against the external load resistance. The product of the same (i.e., voltage and current) gives the power value as is found to be maximum at the point where current and voltage intersect each other at 30 MΩ.

Fig. 7
figure 7

The characteristic electrical output of the La2O3 triboelectric device showing its dependency of voltage and current output on external load resistance

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The current amplitude reduces with growing external load resistance owing to resistive loss, during which the voltage increases. Oscilloscope is used to record the voltage and current generated by La2O3-TENG device. The performance of the device is tested by tapping the TENG using the motorized fixture (Fig. 1). The phenomenon of chemisorptions on the surface of teflon and La2O3 film surface of molecular oxygen species results in resistivity changes of triboelectric material [34, 35]. When Teflon and La2O3 nanorods film come into contact, spontaneous polarization occurs [36]. This results in the dipole moments on Teflon film and La2O3 surface and thus voltage generates. The open-circuit voltage and short circuit produced by the La2O3-TENG device is 120 V and 23.7 μA. The device yields a maximum power of 2.85 mW at an external load resistance of 30 MΩ (Fig. 7). The corresponding power density of the La2O3-TENG device is calculated to be 7.125 W/m2.

The self-powered (TENG) vibration sensors help in predicting the maintenance of industrial machineries. Recent literatures suggest that the materials with high dielectric constant and highly negative triboelectric polarity are selected for the TENG devices. The recent reports suggest that the La2O3 is proven to be a material with high dielectric permittivity and PTFE is having a high triboelectric negative polarity (Fig. S4). The La2O3-based TENG device here is tested for the power density and longer life cycle of the materials. It is proved from the testing that the fabricated device is capable of generating enough power to drive the self-powered vibration sensors. The power output of the La2O3-TENG devices can be improved by following many modifications and engineering of the materials. The power output can be enhanced by physical, chemical, and hybrid modifications of La2O3 film [17]. These methods enhance the power output of the La2O3-based TENG devices. The lanthanum oxide nanoparticles as a triboactive material along with the PTFE materials have proven to be competitive enough to be used in the self-powered vibration sensors.

4 Conclusions

The synthesis of high surface area (~ 72.33 m2/g) La2O3 nanocrystals using the hydrothermal technique is presented with its direct utilization in the form of screen-printed film in TENGs. Further, TEM-SAED pattern of La2O3 nanocrystals showed high intense diffraction spots conclude that the particles were crystallized. FTIR analysis showed the presence of La–O bond. XPS analysis showed the chemical nature of the nanocrystals. The film of La2O3 was investigated for its triboelectric behavior and the results depict that the peak output power density could reach up to 7.125 W/m2 at load resistor of 30 MΩ. Thus, these results depict that La2O3 film TENG device could be used for the self-powered devices and many improvements could be done to improve the power density to use it in various energy harvesting applications.