Introduction

Nanotechnology is a new and fast-arising field that involves the fabrication, treatment, and application of structures, systems, and devices by controlling size and shape at the nanometer scale. Nanoparticles have attracted significant research interest in recent years due to their specific chemical, physical, and surface properties associated with their size, which can vary from those of individual atoms or bulk materials [1,2,3,4]. One of the several applications for materials with nanostructures is gas sensor, and numerous studies have been conducted on metal oxide semiconducting nanostructure–based gas sensors. It has been proven that decreasing the size of the crystals results in a high surface-to-volume ratio, which considerably improves sensor performance. This is because metal oxide grains that are nearly depleted in carriers display larger conductance changes when exposed to target gases due to more carriers being activated from their trapped states to the conduction band [5,6,7,8]. Among metal oxide semiconductor materials, CdO nanoparticles are particularly interesting due to their outstanding characteristics, such as narrow bandgap, high optical transparency in the visible region, simple crystal structures, high carrier mobilities, and high electrical conductivity [9,10,11,12,13]. These characteristics make them applicable in photoelectric devices, liquid crystal displays, semiconductor lasers, and gas sensors. However, CdO has been further improved in terms of gas-sensing ability by doping it with other metals such as Pt, Al, In, Zn, Au, Ag, and Pd [14,15,16,17,18,19]. Silver, in particular, has been used in modification processes due to its low cost, ease of oxidation, strong electrical conductivity with quick electron transfer, ease of preparation, and ability to drastically alter the bandgap in the structure; it has been used in modification processes [20,21,22]. Various methods, including sol–gel [17], spray pyrolysis [18], co-precipitation [13], and hydrothermal [8], have been used to create pure and doped CdO nanoparticles. Among them, pulsed laser ablation in liquid is the most straightforward, economical, exact, and environmentally friendly technology used to create nanomaterials of various sizes and shapes by adjusting the laser ablation settings and types of liquid media [4, 23,24,25,26,27,28]. In this study, we present the fabrication of gas sensors based on Ag-doped CdO nanoparticles produced in liquid by pulsed laser ablation. We also describe the structural, morphological, and optical characteristics of pure and Ag-doped CdO nanoparticles and investigate their gas-sensing capabilities.

Experimental Work

The following steps were used to create samples of both pure and Ag-doped CdO NPs: to create pure CdO NPs, a cadmium target (Cd, 99.99%, Sigma-Aldrich, Saint Louis, USA) was placed at the bottom of a small container filled with 3 ml of deionized water (DIW). The Q-switch Nd:YAG laser source (1064 nm, 1 Hz, 9 ns) was used for the ablation process, producing 300 mJ of laser energy over 300 pulses. To create an Ag-doped CdO nanocomposite, a silver target (Ag, 99.9%) was added to a solution containing CdO NPs, and the mixture was ablated using a constant laser energy of 300 mJ and various pulse counts (20, 30, 40, and 50).

Different experimental measures were used to characterize the resulting nanostructure materials. Fourier-transform infrared (FTIR) spectra were captured in the 4000 to 400 cm−1 regions using a Shimadzu (8000 Series, Shimadzu, IRAffnity-1, Japan) spectrometer. X-ray analyses were performed using a PW 1730 diffractometer and Cu K radiation (30 kV X 10 mA). The optical absorptance characteristics of the NPs were evaluated using a UV–visible double-beam spectrophotometer (Schimatzu,1800). The morphology was examined using field-emission scanning electron microscopy (FESEM, Czech Republic, Tescan, Mira III).

The sensing device was fabricated by drop-casting a nanocomposite solution onto a silicon wafer. The resistance in air (Ra) and the resistance in test gases H2S and NO2 (Rg) were measured to calculate the sensitivity (S). The time required to reach 90% of the response signal was known as the response time, or Tres. The recovery times, Trec, were measured in terms of the time required to restore 90% of the initial baseline signal. The experiments were conducted using 3 V bias voltages, several operating temperatures, and a 25-ppm gas concentration.

Result and Discussion

The FTIR spectra of pure and Ag-doped CdO NPs with various doping ratios (0.33%, 0.60%, and 0.71%) are presented in Fig. 1. The wide band at approximately 3400 cm−1 for all samples is caused by the O–H stretching bands of the hydroxyl assembly of CdO NPs and the water molecules chemically associated with them. The peaks at 1380 cm−1 are associated with the wagging band of the pure sample, while new peaks arise at 1130 cm−1 in doped samples, designated as a metal–oxygen bond on Ag-CdO NP samples, confirming the successful doping of Ag in CdO NPs. The peaks at 660 cm−1 in all samples are due to the Cd–O stretching vibration [3, 11, 29].

Fig. 1
figure 1

FTIR spectra of pure and Ag-doped CdO NPs

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Figure 2 shows the XRD pattern of pure and Ag-doped CdO NPs at different concentrations. The cubic CdO phase (311), (111), (200), (422), and (440) orientations at 2ϴ of 30.55°, 36.5°, 38.5°, 43.95°, and 50.05°, respectively, are observed in the pure CdO NPs (JCPDS: 05–0640), and additionally peak with an index of (111) planes appeared at 2ϴ of 33.25° for the sample doped with 0.71% Ag [22]. The cubic phase of CdO can be observed in the diffraction peaks of both pure and Ag-doped samples, and their intensities increase as the doping concentration rises, and no additional impurity peaks are observed.

Fig. 2
figure 2

XRD pattern of pure and Ag-doped CdO NPs

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Figure 3 shows the optical absorbance of pure and Ag-doped CdO NPs with various doping ratios (0.33%, 0.60%, and 0.71%). The absorbance was measured as a function of wavelength, and it is evident that the intensity of the absorbance increased with increasing amounts of Ag doping [30]. This is due to the formation of increased levels of impurities inside the energy gap, which increased the absorption, with a peak at around 220 nm that slightly shifted to red wavelength due to the n-π* transition of electrons. The optical band gap Eg can be calculated by Tauc’s formula: \(ahv=A{\left(hv-Eg\right)}^{n}\), where α is the absorption coefficient, h is Planck’s constant, ν is the photon frequency, A is constant, and n is the transition constant [20]. Figure 4 shows the optical band gaps, which were found to be 2.9 eV for pure CdO and between 1.6 and 3 eV for Ag-doped CdO. The presence of two optical band gaps may be due to the formation of nanoparticles with different sizes and shapes [31]. Furthermore, the variations in the optical band gap in nanomaterials can be explained based on the quantum size effect [2]. Thus, the increase in sensitivity with a decrease in particle size can be attributed to the surface-to-volume ratio. The smaller the diameter, the higher the surface-to-volume ratio, and therefore, the greater the adsorption of gas [32].

Fig. 3
figure 3

absorbance spectra of CdO NPs and doped with Ag at different ratios

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Fig. 4
figure 4

The optical band gaps of pure and Ag-doped CdO NPs

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A FESEM micrograph of pure CdO NPs and Ag-doped CdO NPs with varying doping concentrations is shown in Fig. 5. In Fig. 5a, pure CdO NPs are found agglomerated in bundles that resemble tangled nanowires due to the van der Waals force that occurs between the wires [20]. Figure 5b shows the FESEM morphology of 0.6% Ag-doped CdO NPs, where larger nanorod aggregates of CdO NPs and erratic Ag particles with a size shift from (21.7 nm) to (52.7 nm) are observed. The FESEM morphology of 0.71% Ag-doped CdO NPs is shown in Fig. 5c, where additional nanorods can be seen interacting and interconnecting as web architectures of CdO NPs and irregular Ag particles with size changes from (18.55 nm) to (66.89 nm); therefore, sensitivity increases as particle size decreases [33]. The presence of Cd, O, and Ag elements was verified by the EDS elemental composition of pure CdO NPS and Ag-doped CdO NPs (Fig. 6). While pollution may be the cause of the presence of the C element, it is also evident that the intensity of the elements significantly changes when the doping ratio varies.

Fig. 5
figure 5

FESEM micrograph of pure and Ag-doped CdO NPs: a pure CdO, b 0.6%, and c 0.71%

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Fig. 6
figure 6

EDS spectrum of pure and Ag-doped CdO NPs at various doping ratios: a pure CdO NPs, b 0.60%, and c 0.71%

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Pure and Ag-doped CdO nanoparticles were imaged using an AFM across an area of (1.0 × 1.0) \(\mu {m}^{2}\) in Fig. 7. These images demonstrate that the Ag doping concentration had a significant impact on the surface morphologies of the films. It is evident that as the Ag doping level increased, the films’ surface roughness decreased. The sensing response of pure and doped CdO nanoparticles is enhanced when the surface roughness is reduced owing to improved adsorption and interaction between the gas molecules and the sensing material. Additionally, a smoother surface may reduce the scattering of incident light and increase the transmission of light through the sensing material, leading to improved signal-to-noise ratios in optical sensing applications [34].

Fig. 7
figure 7

AFM images of pure and Ag-doped CdO NPs

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Figures 8 and 9 illustrate the sensitivity for pure CdO NPs and doping with various concentrations of Ag at H2S and NO2 gases as a function of operating temperature in the range of 100–300 °C. The testing for gas sensitivity started at 100 °C and went up to 300 °C in 100 °C increments. The data show that the sensitivity increases with increasing operating temperature. The statistics show that the sensitivity of all samples increases with increasing operating temperature. This is connected to a rise in the rate of surface reactivity of the target gas. The relationship between sensitivity and temperature for semiconductor metal oxide gas sensors resembles a volcano because maximal peak values are seen at specific temperatures known as the optimum temperature and then decline as the temperature rises [35, 36]. According to Fig. 8, pure CdO NPs are substantially less sensitive to H2S than CdO NPs that have been doped with Ag. The created oxygen vacancies can act as a trapping site for O2 molecules when a semiconductor is exposed to air, providing electrons as a result of their local electronic charge, and the free O2 molecules transform into peroxide or superoxide ions. The H2S sensor also has an impressive detection range of around S = 190 at 300 °C. The much lower O2 dissociation adsorption energy on the Ag-doped surface compared to the undoped CdO surface further shows that Ag is a far better oxygen dissociation catalyst than CdO. Almost 5.1 times larger than the reaction of the pure CdO NP sensor, the composite NP sensor’s response to H2S is (190) (37.7). As shown in Fig. 9, the great sensitivity for NO2 detection was demonstrated at 200 °C for a sample doped with 0.71% Ag. Additionally, Ag-doped CdO NWs have higher sensitivity than pure CdO NWs. This study demonstrates how silver doping and silver ion precipitation can increase sensitivity. Moreover, the response of the composite NP sensor to NO2 (46) is 1.8 times higher than that of the pure CdO NP sensor (24.9). This result is consistent with reference [37] and shows that reducing gas (H2S) is more sensitive to Ag/CdO composite NP sensors than oxidizing gas (NO2).

Fig. 8
figure 8

Sensitivity of pure and Ag-doped CdO NPs to H2S gas at various doping ratios

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Fig. 9
figure 9

Sensitivity of pure and Ag-doped CdO NPs to NO2 gas at different doping ratios

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For NO2 (Fig. 10a) and H2S (Fig. 10b) gases at various operating temperatures, Fig. 10 illustrates the relationship between the response time and the recovery time for pure and Ag-doped CdO NPs. According to Fig. 10a, samples containing 0.6% of Ag have a short response-recovery time, which is likely due to the catalytic activity of metallic Ag [38]. Additionally, the 0.60 Ag-CdO sensor in Fig. 10b has the fastest reaction time. Ag may effectively speed up the measurement of the gas and the adsorption of oxygen ions. Ag is easily transformed into Ag2O in the presence of air, and CdO NPs lose electrons, resulting in a deeper electron depletion area. CdO/Ag effectively lowers the optimal operating temperature, increases the gas response value, and accelerates response and recovery times [39].

Fig. 10
figure 10

Response and recovery time to NO2 (a) and H2S (b) gases for pure and Ag-doped CdO NPs

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Conclusions

In a green and ecologically friendly process, pure and Ag-doped CdO nanoparticles are created using Nd:YAG pulsed laser ablation in liquid. The presence of CdO nanoparticles in Ag is confirmed by the FTIR results, while XRD analysis reveals a crystalline material with a cubic structure. Additionally, FESEM analysis shows that the synthesized nanoparticles form nanorod-shaped aggregates, and EDS analysis confirms the presence of Cd, O, and Ag. The optical absorbance and surface morphology of CdO NPs are affected by the Ag concentration, and the doped structure displays high sensitivity and a quick response/recovery time. As a result, this method represents an inexpensive and efficient way to create metal oxide nanostructures without the need for post-synthesis heating.