Synthesis and Fabrication of In2O3: CdO Nanoparticles for NO2 Gas Sensor

The physical and morphological characteristics of porous silicon (PS) synthesized via gas sensor was assessed by electrochemical etching for a Si wafer in diluted HF acid in water (1:4) at different etching times and different currents. The morphology for PS wafers by AFM show that the average pore diameter varies from 48.63 to 72.54 nm with increasing etching time from 5 to 15min and from 72.54 to 51.37nm with increasing current from 10 to 30 mA. From the study, it was found that the gas sensitivity of In2O3: CdO semiconductor, against NO2 gas, directly correlated to the nanoparticles size, and its sensitivity increases with increasing operating temperature.


Introduction:
The possibility of changing some of semiconductor materials properties (such as electrical and structural properties) by doping, making a mixture of them or making then in nanosized shapes has made them very important in many an application of general rules to particular cases such as in solar cells (1,2), optoelectronics (3)(4)(5), spintronic (6), piezoelectric and gas sensors (7)(8) .There have also been studies showing that when semiconductors are exposed to some gases, lead to change their electrical properties. This phenomenon is used in the application of gas sensing (9)(10)(11). The sensitivity of semiconductor against target gas can be improved by doping or making composite (12)(13)(14)(15) or by reducing their particles to nanosized (16,17) or by increasing operating temperature. Porous silicon (PSi) having distinctive physical and chemical properties which differ from single crystal Si properties. Some of these properties are significant in gas sensor technology field. In the PSi some part silicon crystals etched away by the electrochemical process make the silicon surface contains many pores. As a result, numerous and unusual physical phenomena can be observed (12,13).
Cadmium oxide is an n-type semiconductor, crystal structure (FCC) with direct optical energy gap (4).
Its electrical accessibility is rise, also the ratio of light energy falling on a body to transmitted through it, in the visible region makes it helpful for different applications, photodiodes, gas sensors, etc. (5). Indium oxide is a wide band gap n-type semiconductor with direct band gaps of 3.75 eV, with a cubic bixbyite structure. It is a best material used as in transparent conducting oxide (18). Operating temperature is an essential parameter for gas sensitivity. Also, the crystallinity is an important thing that alter semiconductor electronic properties which is the basic for gas sensitivity. Gas sensors depend on CdO semiconductors against different gases have been widely reported (8). Porous silicon (PS) for a crystalline silicon, having within it a branch of nano-sized pores, it has a direct wide energy gap and high resistivity. The most important advantage of PS is the layers and its great and interacting internal surface. That the internal surface would play a significant role in the specific properties of PS layers enabling this material to be entirely different from the bulk (9). Electrochemical etching is a more applied technicality for the synthesis of PS (10).
In this study, the morphology, nanoparticle size of PS and properties gas sensing using a static gas sensing system were examined. Nanostructured of In 2 O 3 : CdO films have been tested that several gases. These were observed to be more selective than NO 2 at different operating temperatures. There was a robust connection between the surface and the bulk in semiconductors, illustrate why these materials are used as gas sensors in addition to many other applications. The practical form of appearance of semiconductor gas sensors is different for a thin film with a comparable thickness of the space charge layer to the total layer thickness (simple, or with the doped surface) (14,15). The sensing mechanism is based on changes in the film's resistance, which is controlled by gas species. The sensitivity (S) of the film is defined as the ratio of the film resistance in air (R air ) and gas (R gas ). Mixed semiconductor was used in many researches to detect different gases at a relatively low operating temperature (16 -18). Must be used synthetic approaches or used subsequent analytical size selected methods to controlled the morphologies and dimensions of Silicon nanoparticles (19). This study investigates the effect of morphological characteristics of porous silicon on In 2 O 3 : CdO nanoparticles as a gas sensor.

Material and Method:
In 2 O 3 : CdO thin films for In 2 O 3 doped with 0.09 wt.% CdO were synthesised using a technique pulsed laser deposition technique using Nd: YAG laser. The thin films were grown in a vacuum chamber with a background pressure of ~1 x 10 -3 mbar. The Nd: YAG laser has a wavelength of λ = 1064 nm with 6 Hz frequency. Morphologically characteristics of the porous silicon (PSi) structure were fabricated using an electrochemical etching process in HF acid. In 2 O 3 : CdO/Psi was produced via the deposition of In 2 O 3 : CdO. Electrochemical etching formed the PS. F‾ ions are previously obtained available in sufficient amounts at the surface which agrees with (20).
Heater consists of a hot plate and a K-type thermocouple located inside the chamber to control the operating temperature of the sensor, and a bias voltage between the two sides of the electrode is applied. A switch on the rotary pump clears the test chamber for approximately 1 mbar, and the gas sensor is set to the desired operating temperature.
where in the gas is tested under a different volumetric concentration of gas to air ratio, Resistance variation is measured using a computer connect to a digital multimeter. Initially, the biasing air flow resistance records by the digital multimeter records. After turning on the multimeter for several seconds, the testing gas (NO 2 ) shows a low variation of resistance. After switching off the device, the test gas is used to record the recovery time to give the real sensitivity corresponding to Ali Ahmad Yousif, (21). The sensitivity is calculated by applying Equation (1), as given in (22): where (Ra) and (Rg) represent the electrical resistance of the sensor in air, and the presence of gas respectively and (N) is the gas concentration. This may also be calculated from conductance as in Equation (2), (23): σ a σg is the film conductivity in gas, while σ a is represented the film conductivity in air under 300°C. A layer of porous silicon is doped with In 2 O 3, with a CdO ratio concentration of 9 wt.% of wafer silicon (P-type) and with an orientation of 100, resistively 0.01 -0.02 and a thickness of 525 ± 25 µA. The PS samples were prepared by the electrochemical etching (ECE) method in hydrofluoric (HF) acid 17 %, and the concentration and the etching cell are made from Teflon as this does not act in such a way as to have an effect on another with HF acid, and a rubber O-ring is used before the up part of the cell. The electrochemical cell used has two electrode configurations with a platinum (stainless steel) and electrode as a cathode, and a silicon wafer as an anode. Platinum is one of the least reactive metals. It has remarkable resistance to corrosion, even at high temperatures, and is therefore considered a noble metal Consequently, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the alluvial sands of various rivers, Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewelry. Being a heavy metal, it leads to health problems upon exposure to its salts; but due to its corrosion resistance, metallic platinum has not been linked to adverse health effects (24). A digital millimetre device was connected to the silicon, and the other side was connected to the power supply. After cutting the silicon samples into 2.25 × 2.25 cm 2 pieces, washes the wafer is with acetone and methanol to eject any oxide layer. The samples were put in 48 % concentration of acid in a mixing ratio 4:1 HF.
Using chemical ethanol with a purity of 99.9 % and a concentration of 17 %, different etching times of 5, 10, 15 min, at a constant current of 20 mA, and with various currents of 10, 20, 30 mA was applied. Applying a constant etching time of 30 min for the development time, plus the addition of the mixing motor, helped the electrical conduction process. This led to the acceleration of the emptying of bubbled hydrogen (H 2 ) as a catalyst and alcohol ethanol that was used for the evacuation process. The porous silicon samples were next washed in ethanol after their removal from the solution. The samples were left to dry at ambient temperature for several minutes and then stored in containers put within including methanol to stop the formation of an oxide layer on the prepared samples.

Results and Discussion:
Atomic force microscopy (AFM): Atomic force microscope (AFM) was used to obtain microscopic facts provided of the surface structure and fraction the t/1.mn3v2`opographies of the surface comfort. Figure 1 and Figure 2 show the (AFM) of the surface morphology of the oxidised PS layer. This focused completely on the size measurable in nanometres characteristics of the pores silicon films when the etching time increased. Fraction of the pores coagulated to form big structures corresponding to (25), showing AFM images of the porous silicon for both cases. Distributed nanocrystalline silicon pillars randomly and space on top of the all surface could be observed. Pore morphology resulted when the current flowed in the electrochemical cell. The dissociation reaction was localised on the distinct side of the silicon surface, thus initiating the etching of an array of the pores in the silicon wafer. At the high etching time, a highly branched, randomly directed and highly interconnected mesh-work of the pores was obtained. AFM parameters (average diameter and roughness) for these samples are shown in Table 1. Several affecting parameters were examined such as the current density and etching time. Morphological property (AFM) parameters were examined for porous silicon wafer at a constant current of 20 mA for different etching times of 5, 10, 15 min respectively. An average diameter of porosity and that the PS layer thickness increased with increasing etching time. Furthermore, the average diameter pore range was observed between 48.63 -72.54 nm, and with various currents of 10, 20, 30 mA the average diameter of porosity and thickness of the PS layer showed decrease with increasing the current density at constant etching time of 30 min. An average diameter pore range between 72.54 -51.37 nm. The pore width was classified as mesoporous type, as shown in Table 2, which corresponds to the results found by Patil et.al. (22). and this in agreement with the results found by Biswas et.al. (26).    Gas sensor:

Principle.
The increase or decrease of the semiconductor sample resistance, when exposed to target gas, depends on the type of majority carriers and the nature of gas molecules (whether oxidising or reducing) at the surrounding. The resistance of the thin film was increase when the materials were n type, oxidising gases acceptor while reducing gases (donor) decrease with correspondingly for ptype materials (27). The transfer of electrons during the adsorption and desorption of gas molecules because of the chemo resistance of the metal oxides which is responsible for the width of the depleted region formed on the crystallites. The offer of the space charge region acts as a potential hurdle in the conduction system operation between the grains, succeed in persuading changes over time in the Fermi level. due to the depletion of electrons, the band bends upward When the oxidising gas interacted with the n-type material and due to the accumulation of electrons for the reducing gas, the band bends downwards (28).
The operating temperature effects sample sensitivity through its effect on the chemical dynamics between gas-solid and thus limits the sensor properties such as sensitivity, selectivity, stability, response and recovery times (29). Also, the humidity level has a much higher influence on the sensor response (30), due to the variation in electronic and ionic conducting properties of the semiconducting metal oxides when adsorbed water vapour on its surface.

Oxidation mechanisms for oxidising gas molecules Oxygen (O 2 )
Oxygen is one of the most important oxidizing gases, which quickly adsorbs the surfaces of metal oxides. This adsorption can be enhanced by increasing operating temperature, using vaccination and reducing grains size (25). O 2 can accept one electron, when the temperature below of 200 °C, and it can accept two electrons from the metal oxide surface when the temperature above 200 °C, (31,32). When The adsorbed oxygen molecules/atoms are interacting with other gas molecules, they are desorbed quickly (NO 2 ) Nitrogen Dioxide NO 2 is a potent oxidising factor and has a robust electrophilic advantage (33), which enables the molecule to become rapidly adsorbed on the metal oxide surface. Nitrogen dioxide ability interact together a surface in the appearance and non-attendance from O 2 (34). The oxidation of NO 2 leads to the reduction of conduction electrons in the conduction band. Nitrogen oxides are among the six most common air pollutants. Nitrogen gas plays an important role in the production of ozone. For the accuracy of the NO 2 gas sensor has an important role in monitoring emissions processes (35).
To determination the operating temperature of the sensor is presented next. Significant impediment of metal oxide gas sensors is the elevated temperature in demand for sensor operation (30 -300 °C). For this reason, the effect of the operation temperature of thin films sensitivity is intended with the purpose of optimising the operating temperature to the less than average height from the top possible value and that agree with (16). The sensitiveness semiconductor sensor is specified by the interaction design among the target gas and the sensor surface. The higher surface area of the materials, the robust interaction between the adsorbed gases and the sensor surface, i.e. the higher sensitivity to target gas. The consequence display that the rising in the operating temperature improve the film sensitivity, Such is referring to the increasing rate of surface reactivity of the target gas. The extreme top amount is observed in a confirmed temperature, (optimal temperature) which thereafter decrease, where under other conditions would increase. This expound wherefore the connexion between temperature and sensitivity simulate a hill track for the metal oxide gas sensors (19) and observing the growing and decreases in the sensitivity thereby indicator the adsorption and desorption a fact or situation that is observed to exist of the gases. Figure 3 shows that the value of the resistance increases, when the films are exposed to Nitrogen Dioxide gas, (Gas open), then the resistance value decreases at the shutdown of the (Gas OFF). The cause for this behaviour can be refer, Nitrogen dioxide gas submit an ionic rebuttal with adsorption of oxygen, where the electron is taken away from the semiconductor and causes reduce the conductivity of the materials, thereby increasing the resistance, and that agree with (32). The sensitiveness was seen to excess with the operative temperature up to 100 °C, probably impute to the most favourable conditions rate of oxidation of Nitrogen dioxide (16). The sensitiveness of the In 2 O 3 : CdO to Nitrogen dioxide was to be 156 % at 200 °C on the silicon substrate which then decreases with additional rise in the operating temperature at an etching time of 30 min. This is agreeing with the result found by Yousif et.all. (21) for, In 2 O 3 : CdO, films exposed to NO 2 gas which is found that the maximum sensitivity was 142.1 % at concentration of 1 % In 2 O 3 .

Response Time and Recovery Time
This section describes the relationship between the response time and the recovery time with operating temperature at different etching times, for In 2 O 3 doped with 9 % CdO deposited on a porous silicon wafer for 3 % NO 2 mixed with air at a bias voltage of 6 V.
The response \ recovery time is shown as a function of the operating temperature for In 2 O 3: CdO and reveals the decrease of response \ recovery time with increasing operating temperature. Table 2 shows that the 30-min etching time sample exhibits a fast response speed of 50 s and recovery time of 75 s at an RT operation temperature. This revealed that a 30-min etching time is an optimal time to achieve a fast response sensor showing the decrease of response and recovery time with increasing operating temperature (21).

Conclusion:
In 2 O 3 : CdO nanomaterials were prepared using pulsed laser deposition on silicon (100) and porous silicon substrate samples prepared by the electrochemical etching (ECE) method in HF acid at room temperature. The nanoscale improved the properties of the material. The sensitivity S % increased with increasing operating temperature T. The topmost sensitivity for NO 2 gas was 156 %, with an etching time of 30 s and operating temperature of 200 °C. The faster response speed was 50 s and 75 s at RT for NO 2 gas which was observed at the etching time of 30 s. Therefore, the conclusion from this study is that it is possible to examine gas sensing properties of different gases like H 2 S, CO 2, and volatile solutions like methanol and acetone.