CdO:Ag thin films with enhanced visible light photocatalytic activity against metanil yellow
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The photodegradation quality of Ag-doped CdO (CdO:Ag) thin films deposited using perfume atomizer has been reported in this paper. Ag concentration in CdO is varied as 0, 1, 2 and 3 wt%, respectively. XRD studies reveal cubic crystal structure for all the CdO:Ag thin films ruling out the presence of any secondary phases and decreased crystallite size was observed with Ag doping. CdO:Ag films surfaces are composed of cauliflower shaped grains and EDX spectra ascertains that Ag has been successfully incorporated into the CdO lattice. Optical band gap tends to increase with increase in Ag concentration. Electrical resistivity showed a decreasing trend with increase in Ag concentration. Photocatalytic tests against metanil yellow confirmed that the Ag-doped CdO films exhibit better degradation efficiency than the undoped film and an efficiency of 84.44% was realized for the 2 wt% Ag-doped CdO catalyst. The recycle tests confirmed the reusable nature of the CdO:Ag catalysts.
KeywordsDoping Cubic structure Photocatalytic activity Band gap
Mathematics Subject Classification74K35
Cadmium oxide (CdO) in thin film form finds applications in solar cells, smart windows, optoelectronic devices, gas sensors, anti-reflecting coatings, etc. due to its dual combination of good optical and electrical properties . The high electrical conductivity, surface morphology and large surface to volume ratio possessed make CdO suitable for sensing gases such as LPG, ethanol, ammonia gas, etc. . CdO is photoactive due to its n-type conductivity and due to its narrow direct band gap of 2.3 eV . The high optical absorption and charge carrier mobility makes CdO suitable for dye degradation . However, the low resistivity, stability, band gap and toxicity restrict CdO in many technological applications. As the conductivity of CdO is dominated by the oxygen vacancies and cadmium interstitials , tuning them through dopants could enhance its resistivity and stability . In photocatalytic applications, CdO exhibits low degradation efficiency due to its shortened band gap which cause the photogenerated electrons and holes to recombine rapidly. Also, the release of cadmium ions is a major concern from the environmental point of view due to its natural toxicity. These two drawbacks can be removed by doping CdO with suitable metallic ions. The non-stoichiometric structure of CdO with cadmium interstitials provide a good environment for dopants to diffuse in its lattice and hence enhanced resistivity, stability, band gap could be realized and the release of Cd2+ ions are very much suppressed.
Literature results showed that dopants with smaller ionic radii than Cd2+ such as Mg, Mn, Al, and Sn enhanced the structural, optical and electrical properties of CdO [7, 8, 9, 10]. Beside these, metal ions such as Ba  and Sr  with ionic radii greater than Cd2+ also influenced the physical properties of CdO. In this work, a transition metal ion with ionic radius greater than that of Cd2+ is used as dopant to enhance the physical properties of CdO. Silver (Ag+) is a noble metal which has ionic radius of 1.22 Å greater than that of Cd2+ (0. 97 Å) and exhibit high electrical conductivity facilitating fast electron transfer and low work function which favours the formation of good band alignment and the Ag-doped catalysts showed enhanced photocatalytic activity due to the surface plasmon resonance phenomenon under visible light . Also, Ag-doped metal oxides exhibit strong absorption under visible light . In view of these features, Ag-doping has been performed on CdO to enhance its resistivity, stability, band gap and to suppress the release of Cd2+ ions. In the present work, Ag-doped CdO (CdO:Ag) thin films were deposited using perfume atomizer and the influence of Ag doping on some properties of CdO was investigated.
2 Experimental details
Pure and Ag-doped CdO (CdO:Ag) thin films were deposited using perfume atomizer with 0, 1, 2, and 3 wt% Ag doping concentrations. Aqueous solution (50 ml in volume) containing 0.1 M cadmium acetate [Cd(CH3COO)2] is used as the precursor solution to deposit pure CdO thin films. To this solution, 1, 2 and 3 wt% silver nitrate [Ag NO3] of the weight of cadmium acetate was added to deposit CdO:Ag thin films. The respective solutions when sprayed over hot glass substrates kept at 400 °C using perfume atomizer, pure and Ag-doped CdO thin films were obtained. XRD patterns, SEM images, transmittance, PL spectra and electrical properties were analyzed using X’Pert PRO-Analytical diffractometer, HITACHI S-3000H scanning electron microscope, LAMBDA-35 UV–Vis-NIR double beam spectrophotometer, Varian Cary Eclipse fluorescence spectrophotometer and four point probe setup, respectively. Photodegradation ability of the CdO:Ag thin films against metanil yellow (MY) dye was judged by recording absorption spectra at λ = 433 nm at regular time intervals during the photocatalytic tests.
3 Results and discussion
3.1 XRD studies
Crystallite size, strain, lattice parameter, band gap, electrical resistivity values and elemental composition of the CdO:Ag thin films
Ag doping concentration (wt %)
Crystallite size [D (nm)]
Strain (ε × 10−3)
Lattice constant ‘a’ (Å)
Band gap [Eg (eV)]
Resistivity [ρ (Ω-cm)]
Elemental composition (at.%)
0.71 × 10−2
0.98 × 10−3
0.16 × 10−3
0.34 × 10−3
3.2 Surface morphology and elemental analysis
3.3 Optical studies
3.4 PL studies
3.5 Electrical studies
The electrical resistivity values of the CdO:Ag thin films deposited with 0, 1, 2 and 3 wt% Ag doping concentrations measured using a four point setup are presented in Table 1. The electrical resistivity value of 0.71 × 10−2 Ω cm observed for pure CdO exactly matched the earlier reported value . With Ag doping up to the 2 wt% electrical resistivity seems to decrease drastically above this concentration it slightly increases. For each Ag+ ion replacing Cd2+ ion, one electron is released in the CdO lattice which in turn increases the carrier concentration, thereby reducing the resistivity of the Ag-doped CdO thin films . However for the 3 wt% Ag doping, the resistivity seems to increase slightly due to the occupancy of Ag+ ions in the interstitial places in the CdO lattice, thereby causing lattice distortion which decreases the electrical mobility. Also, the increase of ionized impurities scattering and electron–electron scattering increases the resistivity of the 3 wt% Ag-doped CdO thin films .
3.6 Photocatalytic activity
CdO:Ag thin films with 0, 1, 2 and 3 wt% Ag concentration were deposited using perfume atomizer. The CdO:Ag thin films with 1, 2 and 3 wt% Ag doping concentrations showed better degradation efficiencies against metanil yellow dye, than the undoped film. The enhanced degradation efficiencies observed for the doped catalyst might be due to the synergetic and surface-plasmon resonance effects of Ag. The 2 wt% Ag-doped CdO catalyst exhibited a maximum degradation efficiency of 84.44% after 75 min light irradiation due to its reduced crystallite size, large surface area and increased band gap values which was well acknowledged from the XRD, SEM and optical studies. Thus, the CdO:Ag catalysts seem to be very effective in degrading organic dyes.
The authors are thankful to the Gandhigram Rural University, Dindugal, Tamilnadu for the SEM and EDX analyses.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All the authors declare that the article is original and all are aware of its contents and they approve for its submission. The authors also confirm that the described work has not been published before; it is not under consideration for publication anywhere else and publication has been approved by all co-authors and the responsible authorities of the institute where the work has been carried out. No conflict of interest exists in the article. The authors also declare that no research has been performed on human participants or animals. The authors declare that if the article is accepted it will not be published elsewhere in the same form in any language without the written consent of the publisher.
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