3.1 Structural and morphological studies
Structural and morphological studies like XRD, BET, SEM and TEM for the prepared samples have been analyzed and discussed in our previous report [20]. The mapping of NiTe NRs revealed the distribution of each Ni and Te inside the composite (Fig. 1b, c and d), with corresponding FESEM image (Fig. 1a).
3.2 XPS spectra of NiTe NRs
Figure 2a shows the XPS of NiTe NRs. The binding energies of C1s and O1s are ascribed to the peak values of 284.9 and 532.0 eV, respectively. At 573.2 and 576.1 eV, Te's binding energy 3d5/2 levels are observed and it confirms the presence of Te2− and Te4+ [21]. Because Te is quickly oxidised in air, the creation of Te is a possibility (IV) [22]. Ni 2p is best suited with two spin-orbits and a satellite using Gaussian fit. The peak fitted at the binding energy of 856.1 eV in Ni 2p3/2 and 874.5 eV in Ni 2p1/2 is assigned to Ni2+ in the Ni 2p spectrum, as illustrated in Fig. 2c [23]. The satellite peaks appeared at 861.7 eV and the energy difference between the two peaks is 18.4 eV [21].
3.3 Magnetic Properties NiTe NRs
Figure 3a presents the magnetization versus applied field (M-H) curves recorded at temperature dependence VSM for NiTe sample in the field range of -10000 to + 10000 Oe under various isothermal magnetization temperatures (100 K to 300 K). The saturation magnetization (Ms) and coercivity (Hc) values of NiTe NRs measured at room temperature are 0.0008 emu g− 1 and 158 Oe. The magnetic responses of NiTe expose the paramagnetic behavior at high field and soft ferromagnetism at low magnetic field. NiTex nanorods exhibit between diamagnetism and paramagnetism based on the experimental study have been reported by Lie et al. [24].
Magnetization temperature (M-T) curves for zero-field-cooled (ZFC) and field-cooled (ZC) observations with temperature collected under a 500 Oe applied magnetic field. The solution was cooled to 2 K in the absence of a magnetic field, then a 500 Oe external field is introduced, and the behavior of the material is observed while heating from 2 K and 300 K. (Fig. 3b). The behavior is observed in the case of NiTe samples that demonstrate the paramagnetic nature. In both ZFC and FC curves, the magnetization increases with decreasing temperature. According to the literature the temperature dependence of magnetization illustrates the transition temperature at about 20 K for NiTe shows that the Curie temperature of the system is much higher than 400 K [25]. For the prepared NiTe sample the magnetic transformation occurs at 32 K. In the present work, there is no magnetic transition in the prepared samples due to the existence of paramagnetic nature. Room temperature ferromagnetism and superparamagnetic blocking behavior have been observed in NiTe [26].
3.4 Nonlinear properties of NiTe NRs
Third-order nonlinear behaviour was observed using the Z-scan approach. Z-scan is the most common method for identifying third-order nonlinear properties such nonlinear index of refraction and absorption coefficient since it is the simplest and uses the most accurate single beam approach. The polarisation of the material by the intense laser light falling on it causes nonlinear response in NiTe NRs. The material's third-order NLO performance is a critical physical trait for a variety of applications, involving optical switching and limiting. One of the most fundamental methods to determine third-order nonlinearity in a medium is the Z-scan method. An open-aperture Z-scan approach with a nanosecond pulse with a wavelength (λ) was used to calculate the nonlinear absorption coefficient (β) in the present work.
A NiTe dispersion media was placed between both the source and the detector to estimate the transparency for that specific area (z). The convex lens focused a laser source with varied energies of 100 µJ, 150 µJ, and 200 µJ on the specimen, and the sample was shifted to both sides of the focus. In Fig. 4a, the observations are plotted, and it can be shown that the material has reverse saturation absorption (RSA). When the materials are optically pumped at 532 nm, the electrons are stimulated to trapping levels of impurity or crystal defects with long lives (2.3 eV). Because intermediate trapping levels have longer durations, the likelihood of resonance two-photon transition increases.
Through numerically fitting the open aperture Z-scan curve with typical nonlinear equations, the origin of the nonlinearity was discovered. The experimental data rely on the two-photon absorption (2PA) equation
Where αo, I, Is and βeff are the linear absorption coefficient, input laser intensity, saturation intensity and two-photon absorption coefficient respectively. The corresponding propagation equation is given [10],
Genuine TPA in metal tellurides is uncommon, whereas nonlinearity is linked to free carrier absorption (FCA). There is a presence of TPA with FCA in the produced samples, which has a noticeable absorbance at 532 nm. Figure 4 has been used to calculate the optical limiting value (b). Nonlinear scattering, in addition to nonlinear absorption, plays a significant role in the optical limiting mechanism at ns laser excitation [27]. Cheng et al. revealed that the optical limiting property of graphene-PbS nanohybrids under 4 ns laser excitations is probably a combination of FCA and nonlinear scattering [28]. In this scenario, the repetition rate is 0.25 seconds, which allows bubbles to quickly develop and generate nonlinear scattering. The repetition rate in our case (9ns) is 0.11 sec, which is insufficient for forming nonlinear scattering centres. As a result, the 2PA induced free FCA for the prepared material is the source of nonlinearity. The value of and optical limiting thresholds is calculated from all of these computations and is displayed in Table 1. The positive value of indicates that this NiTe NRs absorbs all of the light at the focus and has effective two-photon absorption (2PA), making it useful in optical limiters.
Table 1
Comparison of nonlinear optical parameter of NiTe with other metal tellurides.
Samples
|
Energy (µJ)
|
β (m/W) × 10− 10
|
OL (J/cm2)
|
References
|
Te
Ag2Te3
|
-
-
|
0.38
1.5
|
-
-
|
[29]
|
PbTe
|
60
|
-
|
5
|
[30]
|
CdTe
|
100
150
200
|
12
5.5
6.8
|
1.60
3.65
2.84
|
[10]
|
NiTe
|
100
150
200
|
5.2
6.2
6.6
|
3.13
2.56
2.44
|
Present work
|
TPA cross-section (σTPA) is calculated using the relation [31],
The ground state absorption cross-section (σGS) is estimated,
The excited state lifetime (τ) is evaluated,
Where c denotes light speed, T0 denotes linear normalised transmittance and N denotes the total number density of active molecules (5.0181 ⋅ 1017 cm− 3) respectively. The calculated values of σTPA, σGS and τ for energies 100, 150 and 200 µJ are 3.0053, 3.5833 and 3.8145 ⋅ 10− 44 cm4 s; 1.9324, 2.0226 and 2.1129 ⋅ 10− 18 cm2; 12.23, 13.38 and 14.98 ns. A high value of τ demonstrates the attribution of ESA to the TPA. In the excited substates, the charge carriers went through many electronic transitions, resulting in a prolonged lifetime before returning to the ground states. These findings showed that CdTe had a stable nonlinear optical response and that it might be used in optical data storage applications.
3.5 Electrocatalysis of H2O2 reduction
Figure 5a displays the CV recorded for unmodified and NiTe NRs modified GCE measured at a scan rate of 50 mVs–1 in the potential range of 0 to − 0.8 V. In the supporting electrolyte comprised 50 µM H2O2, an enhanced electrocatalytic ability (24 folds higher) and fast electron transfer is found for NiTe NRs/GCE than unmodified GCE. Moreover, the over potential observed at NiTe NRs/GCE is lower (540 mV) than that of unmodified GCEs. The NiTe NRs improved electrocatalytic activity can be attributed to a strong synergetic effect and adequate tailoring. Furthermore, by a variety of interactions, the edge planes of NiTe NRs can provide additional sites for electrocatalysis. The influence of scan rate on H2O2 reduction is investigated by using different scan rates ranging from 10 to 100 mVs− 1, with oxidation increasing as the scan rate increases (Fig. 5b). The cathodic peak current increases linearly as H2O2 concentration rises. The linearity of the figure between cathodic peak current and the square root of the scan rate indicates that the reduction process is diffusion regulated (Fig. 5c). The CV achieved at NiTe NRs/GCE in 0.1 M PB (pH 7.0) having varied amounts of H2O2 ranging from 50 to 350 M is shown in Fig. 5d.
3.6 Amperometric Determination of H2O2
Figure 6a shows the amperometric response of a NiTe NRs modified electrode (rotation speed = 1200 RPM) to 50s intervals of H2O2 injections into PB (pH 7.0) (Eapp= − 0.56 V). The reactions were well-defined and rapid, and the steady-state current was attained in less than 5 s. The linear range is 35 nM–1950.12 µM (Fig. 6b) and limit of detection (LOD) is 6 nM. The sensitivity is calculated as 6.35 µAµM–1 cm–2. The previously reported H2O2 sensor and the LOD, pH and linear range are summarized in Table 2. The electrode exceptional sensing performance is demonstrated by a low limit of detection at the nanomolar level. The sensor properties are higher than the NiTe NRs integrated electrodes that are currently available.
Table 2
Comparison of analytical parameters of NiTe NRs modified electrode.
Electrode
|
LOD(µM)
|
Linear range(µM)
|
pH of PB solution
|
Ref
|
CoS/RGO
|
0.042
|
0.12–2542.4
|
7
|
[5]
|
CuS
|
1.1
|
10-1900
|
7.4
|
[8]
|
MoS2/GR-MWCNT
|
0.83
|
5-145
|
7
|
[32]
|
Te NPs
|
0.3
|
0.67–8.04
|
7
|
[33]
|
Pt/TeO2
|
0.6
|
0.2-16000
|
7
|
[34]
|
Te NWs
|
0.001
|
0.5-30000
|
7
|
[35]
|
NiCoTe NRs
|
0.02
|
0.002–1835
|
7
|
[36]
|
NiTe NRs/GCE
|
0.006
|
0.035-1950.12
|
7
|
Present work
|
The interference experiment was used to test the sensor's selectivity in the presence of possible interfering substances. Figure 6c depicts the electrode's amperometric response to (a) 5 µM of H2O2, (b) 5 µM of dopamine, (c) 5 µM of uric acid, (d) 5 µM of ascorbic acid, (e) 5 µM of glucose and (f) 5 µM of folic acid, the electrode quickly responds to H2O2. However, because it is insensitive to other species, the electrode recognises and detects H2O2 in a pool of many other biological analytes.
3.7 EIS spectra for NiTe NRs
The EIS recorded at GCE (a) NiTe NRs and (b) in 0.1 M KCl adding 5 mM Fe(CN)63−/4− is shown in Fig. 8. The experimental data (inset to Fig. 8) was fitted using the Randles equivalent circuit model, in which Rs, Rct, Cdl, and Zw denote the electrolyte resistance, charge transfer resistance, double layer capacitance, and Warburg impedance, respectively. Semicircles indicate the parallel combination Rct and Cdl at electrode surface arising from electrode impedance in EIS measurements, which are depicted as Nyquist plots. The curve size (diameter of the semicircle part) obtained for GCE (621.2 Ω) is larger than that of NiTe NRs/GCE (12.4 Ω). NiTe NRs/GCE has a considerably lower Rct value than GCE, implying a reduced diffusion resistance and charge-transfer resistance.
3.8 Durability, Reproducibility and Repeatability
The sensor a of the NiTe NRs/GCE electrode material was measured every week in order to assess the sensor longevity and determine the electrode's storage stability. The electrode was maintained at 4°C while not in use. Even after 9 weeks of continuous usage, the electrode preserved 96.7% of its initial current, indicating excellent durability. Measurements at five separate electrodes in PB (pH 7.0) towards 10 M H2O2 yielded an RSD of 3.52% for the repeatability trials. Five repeated measurements at one modified electrode were used to calculate the repeatability; the RSD was 3.53%. As a result, the sensor's repeatability and reproducibility are within acceptable limits.
Table 3
Determination of H2O2 in human blood and rat brain Serum samples
Real Samples
|
Added (nM)
|
Found (nM)
|
Recovery (%)
|
*RSD (%)
|
Human blood Serum
|
100
|
101.48
|
101.48
|
2.81
|
200
|
196.9
|
98.45
|
3.54
|
Rat brain
Serum
|
100
|
97.6
|
97.6
|
3.83
|
200
|
196.4
|
98.2
|
2.96
|
* Related standard deviation (RSD) of 3 independent experiments |
3.9 Real-Time Applicability
The fabricated sensor practical feasibility has been proved in human blood serum and rat brain serum. Before the analysis, the genuine materials were diluted in PBS (pH 7). For the optimal experimental circumstances, NiTe NRs were used in the amperometric tests. Table 3 lists the results of the recovery tests. The sensor's average recovery value is obtained to be 97.6-101.4%. The results show that the suggested NiTe NRs/GCE has a lot of potential for real-time H2O2 sensing.