Electrical Characterization of CdS Nanoparticles for Humidity Sensing

Article pubs.acs.org/IECR

Electrical Characterization of CdS Nanoparticles for Humidity Sensing Applications Ramazan Demir,*,† Salih Okur,‡ and Mavişe Şeker§ †

Department of Physics, Faculty of Science and Art, Canakkale Onsekiz Mart University, Canakkale, 17100, Turkey Department of Metallurgy, Faculty of Engineering and Architecture, Izmir Katip Celebi University, Cigli, Izmir, 35620, Turkey § Department of Physics, Faculty of Science and Art, Ondokuz Mayis University, Samsun, 55139, Turkey ‡

ABSTRACT: Resistive type relative humidity sensors based on CdS nanoparticles were synthesized using the chemical bath deposition (CBD) method. Using the drop-casting method, CdS nanoparticles were deposited between thermally evaporated gold electrodes separated by 17 μm with the electrodes having channel widths of 1400 μm and supported on a glass substrate with a thickness of 120 nm. The CdS nanoparticles were exposed to moisture to measure the change in electrical resistance. The resistance of the CdS sensor element changed by 3 orders of magnitude as the relative humidity of the test cell was varied between 17 and 85%. The experimental results demonstrate that CdS nanoparticles are very sensitive to changes in relative humidity and can be used as a sensing element for CdS-based humidity sensor applications.

1. INTRODUCTION Fabrication of nanoscale materials for chemical and biological sensing has proven to be promising due to their highly tunable size- and shape-dependent chemical and physical properties. Especially their high surface areas and large pore volumes per unit mass improve the sensitivity and response times of sensors.1,2 Cadmium sulfide (CdS) is one of the essential technological materials used as a II−VI compound semiconductor due to its large direct band gap, optical absorption, and good stability.3,4Various methods have been used to produce CdS nanoparticles such as chemical vapor deposition,5 sol−gel chemical solution growth,6 ultrasonic spray pyrolysis,7 the screen-printing−sintering technique8 and chemical bath deposition (CBD).9−12 Among these deposition methods, the CBD technique is one of the least expensive and simplest methods, which makes it very attractive for obtaining reproducible and uniform CdS nanoparticles. There are many potential electronic and photonic applications of CdS nanoparticles such as solar cells,13−16 photosensors,17 DNA sensors,18 and gas sensors.19−22 Miremadi et al. showed that the photoconductive surface of CdS can be used as a analytical gas sensor for the detection of oxygen23 and the analysis of hazardous gases in the environment.24 Gao and Wang found that the SnO2 nanobelt/ CdS nanoparticle core/shell heterostructured sensor had high sensitivity to 100 ppm for ethanol vapors in air at 400 °C.25 The authors suggested that the CdS nanoparticles served as additional electron sources, greatly improving electron conduction in the SnO2 nanobelts. They also found similar significant changes in the conductance of CdS-coated ZnO nanorod composite used for ethanol sensing, compared to its uncoated counterpart.26The working principle of metal oxide or ceramic types of sensors is based on variation in electrical parameters such as resistance,27 impedance,28 and capacitance.29−31 The signal-to-noise ratio of sensing devices depends on the amount of physisorbed molecules (i.e., absorbed molecules on the sensor element32,33). Our recent quartz © 2012 American Chemical Society

microbalance study indicated that CdS nanoparticles synthesized by the chemical bath deposition method have a great affinity to humidity at room temperature.12 We suggest that the presence of moisture is an important parameter for degradation of device parameters, e.g., conversion efficiency and lifetime of CdS solar cells.34,35 In this work, CdS nanoparticles were fabricated using the CBD method and their electrical response to relative humidity changes between 17 and 85% relative humidity (RH) was measured to show the effect of moisture on a resistive type sensor. Our results show that CdS nanoparticles can be used to fabricate a sensing element for CdS-based humidity sensor applications.

2. EXPERIMENTAL SECTION CdS nanoparticles were synthesized by the CBD technique as given in our previous work.12 CdS nanoparticles were dispersed in ethanol solvent, and then 5 μL of CdS−ethanol solutions was drop-cast between the gold electrodes as shown in Figure 1. Gold electrodes (120 nm thick, placed 17 μm apart, and with each electrode having a width of 1400 μm) were thermally evaporated on the glass substrates using a shadow mask as shown in Figure 1. A Dektak profilometer from Veeco was used to measure both the gold electrode thickness and the drop-cast CdS nanoparticle sensing film. Figure 2 shows the experimental setup to measure the electrical response of CdS nanoparticle films under various relative humidity conditions between 17 and 85% RH. The CdS nanoparticles between the gold electrodes were exposed to varying relative humidity conditions between 17 and 85% inside a 100 cm3 test cell by controlling the ratio of wet/ dry air flow between 0 and 100% in 10 steps using a MKS flow meter control system at flows ranging from 0 to1000 sccm. The Received: Revised: Accepted: Published: 3309

July 13, 2011 January 25, 2012 January 25, 2012 January 25, 2012 dx.doi.org/10.1021/ie201509a | Ind. Eng. Chem. Res. 2012, 51, 3309−3313

Industrial & Engineering Chemistry Research

Article

The variation of resistance is presented by a red continuous line as shown on the left sides of the plots in Figure 3. The blue continuous lines show the measured relative humidities, and the black dashed lines represent the wet/dry air flow ratios on the right sides of the plots shown in Figure 3. Figure 3b and Figure 3d are zoomed-out counterparts of Figure 3a and Figure 3c, respectively, and demonstrate three types of adsorption and desorption cycles that can be used to determine the electrical response of the CdS nanoparticle sensing element between the gold electrodes as a function of relative humidity changes. Furthermore, the CdS nanoparticle coated electrode measured the response of resistance changes because of adsorption and desorption of moisture molecules. The corresponding RH values in the test cell were synchronously recorded with a Sensirion sensor. The Sensirion humidity sensor showed 17% RH when 1000 sccm dry air was sent through the test cell. On the other hand, it showed 85% RH when 1000 sccm wet air, which was acquired by passing dry air through a half-filled water bubbler kept at a constant temperature of 30 °C, was sent through the test cell. To see the maximum response of the desiccation and moistening processes, only dry and wet air were sent alternately at 200-s intervals as shown in Figure 3b. The minimum resistance of the CdS nanoparticles was 1.53 × 108 Ω when 100% (by flow) wet air at 1000 sccm was sent at 85% RH (that is, no dry air was allowed to flow through MFC-1). On the other hand, the maximum resistance of CdS nanoparticles was obtained as 8.27 × 1010 Ω, when pure dry air with 1000 sccm was sent at 17% RH. The response time of the CdS nanoparticles was measured to be 1 min (60 s) by taking the time required for the resistance to drop to 10% of the maximum change in the resistance. The recovery time of CdS nanoparticles was determined to be 0.5 min by taking the time required for the resistance to come to 90% of the maximum change in the resistance. Figure 3c shows the long-term response of the CdS nanoparticle electrodes for 1 h. The wet/

Figure 1. Schematic diagram of two terminal electrical measurements for drop-cast CdS nanoparticles.

measurements were taken synchronously with a Keithley 2420 source meter and a commercial Sensirion humidity sensor. The Sensirion sensor has EI-1050 selectable digital relative humidity and temperature sensors with a response time of 4 s. The humidity sensor was connected to a PC using a Labview program to collect data via a USB port controlled by a U12 ADC system combined with a single chip sensor module (SHT11) (Sensirion, Switzerland).

3. RESULTS AND DISCUSSION Figure 3 shows the resistance changes and relative humidity as a function of time during the desiccation and moistening processes over a 10-h period. The ratios of wet/dry air flow through the test cell for various relative humidity conditions as a function of moisture concentration are shown in Figure 3. Wet and dry air flows were used alternately to obtain the recovery and response characteristics of the CdS sensing element for humidity sensing. The resistance of the CdS thin film decreased due to the increased amount of physisorbed or capillary-condensed water molecules as a consequence of increasing relative humidity as shown in Figure 3.

Figure 2. Experimental setup to measure the electrical response of CdS nanoparticle films under various relative humidity conditions between 17 and 85% RH. 3310

dx.doi.org/10.1021/ie201509a | Ind. Eng. Chem. Res. 2012, 51, 3309−3313


Article

Figure 3. Change in resistance of CdS nanoparticle sensing element at various relative humidity (RH) values between 17 and 85%.

dry air ratio was increased in 100 sccm steps for equal time intervals of 200 s. The resistance of the CdS nanoparticles dropped when the RH was between 30 and 85%. At low concentration values (e.g., below 40%), the slope of the conductance response of CdS decreased. Figure 3d shows the short-term response of CdS. The wet/ dry air ratio was increased in 100 sccm steps with 200 s per step. The electrical response of the CdS nanoparticles recovered to the minimum RH value when only dry air at 1000 sccm was flowed between the electrodes. Figure 4 shows the changes in resistance (left) and conductance (right) of the CdS nanoparticles as a function of relative humidity (between 17 and 85% RH). The sensitivity of the CdS nanoparticle based sensor was determined via the relationship [(dR/d(RH))/R0](100%), where R is the resistance and dR/d(RH) is the slope of R vs RH.36 The average sensitivities were found to be 46 and 48% for long-term and short-term responses, respectively. The sensitivity is proportional to dR/d(RH). The slope of the conductance increases with increasing RH. The resistance of the CdS nanoparticle sensing element decreases exponentially with increasing RH due to the increasing adsorption rates of CdS nanostructures. At low RH values, the large surface area of the first layer of the CdS nanoparticles with active OH groups will be filled with water molecules, which can be explained using the Langmuir model.12 However, the resistance change shows a nonlinear (decreasing

Figure 4. Change in the resistance (left) and conductance (right) of CdS nanoparticles as a function of relative humidity (between 17 and 85% RH).

exponential) behavior as a function of increasing RH as a result of the possible condensation at higher relative humidity levels above 60%. On the other hand, the adsorption of water vapor on the CdS nanostructured surface is governed by a multilayer adsorption process, such that at higher RH values the decreasing rate in the resistance of the CdS nanoparticle sensing element can be explained by considering the increase in the ionic conduction mechanism of hydroxyl groups due to the decomposition of adsorbed water molecules under the influence of a relatively high electric field (close to 1 kV/cm) 3311



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under various relative humidity conditions between 17 and 85% RH. Charlesworth et al. investigated the relationship between mass and conductance changes for a poly(pyrrole) (PPy) film and found that the fractional change in resistance varies linearly with fractional mass uptake when the mass change is below about 5%.37 They assumed that the PPy film behaved like a uniform sheet; therefore, the uptake of mass during exposure to vapors is described by Fick’s equation for diffusion:

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 00902862180018. Fax: 00902862180533. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.D. thanks Muhsin Zor for his valuable suggestions during synthesis of CdS nanoparticles. The authors thank Dr. Ritchie Eanes for checking the manuscript for grammatical errors. This research was partially supported by DPT (State Planning Organization of Turkey) under Project No. DPT2003K120390, Tubitak (Turkish Scientific Association) under Project No. TBAG109T240, and IYTE research Project No. 2010IYTE25.

∞

⎛1 ⎞ M (t ) 1 = 1 − ⎜ 2⎟ ∑ 2 ⎝π ⎠ M (∞ ) n = 0 (2n + 1) ⎡ −2D(2n + 4)2 π 2 ⎤ ⎥ exp⎢ ⎢⎣ ⎥⎦ 4L2

Article

(1)

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where M(t) is the mass taken up by the film at time t, M(∞) is the equilibrium mass uptake by the film, L is the film thickness, and D is the diffusion coefficient. However, the study fails to consider that the sensing element here consists of sphericalshape CdS nanoparticles with large porosity, so that it may not behave like a uniform sheet. Horzum et al.38 has recently shown an exponential relationship between adsorbed mass and resistance when the former is simultaneously measured using a quartz crystal microbalance (QCM) under a constant dc bias voltage during the adsorption of water vapor on ZnO nanofibers that have been fabricated using an electrospinning process. They found that the QCM resonance frequency changes linearly with varying relative humidity between 10 and 90%, which is directly related to mass uptake (the mass change (Δm) on the surface of the quartz crystal can be calculated using the Sauerbrey linear frequency change relationship39). CdS nanoparticles studied here show similar behavior: the change in the frequency during the QCM measurements in our previous work12 showed a linear relationship to relative humidity for RH values between 17 and 80%. However, it shows larger slope due to the condensation of water molecules at RH values above 80%. Similarly, there is an exponential relationship between the change in the resistance of a CdS nanoparticle film (that is, CdS sensing element) and mass uptake due to adsorption of water molecules when the relative humidity is between 17 and 85% for electrodes spaced 17 μm apart. The existing literature on this subject fails to resolve the contradiction between the linear and exponential dependencies of changing the resistance of nanostructured materials as a function of increased relative humidity. The present study was designed to determine the effect of varying humidity conditions on the electrical properties of devices made of CdS nanoparticles. These findings show that CdS nanoparticles are very sensitive to humidity changes.

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4. CONCLUSION The electrical response of CdS nanoparticles when exposed to relative humidities between 17 and 85% has been measured. The resistance of the CdS nanoparticles gave reproducible responses to humidity changes. The conductance measurements showed a nonlinear (exponential) increasing behavior with increasing RH due to possible condensation under a multilayer adsorption process of water vapor on the CdS nanostructured surface with large porosity. These findings show that the electrical response of CdS nanoparticles is very sensitive to humidity changes and can be further investigated as a humidity sensor. 3312



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Electrical Characterization of CdS Nanoparticles for Humidity Sensing

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