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porous silicon hybrid structure
Accepted Manuscript Title: Electrochemical synthesis and the gas sensing properties of the Cu2O nanofilms/porous silicon hybrid structure Author: Dali Yan Shenyu Li Ming Hu Shiyu Liu Yun Zhu Meng Cao PII: DOI: Reference:
S0925-4005(15)30387-7 http://dx.doi.org/doi:10.1016/j.snb.2015.09.080 SNB 19065
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4-6-2015 16-8-2015 15-9-2015
Please cite this article as: D. Yan, S. Li, M. Hu, S. Liu, Y. Zhu, M. Cao, Electrochemical synthesis and the gas sensing properties of the Cu2O nanofilms/porous silicon hybrid structure, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.09.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical synthesis and the gas sensing properties of the Cu2O nanofilms/porous silicon hybrid structure
Dali [email protected], Shenyu Lib, Ming [email protected], Shiyu Liua, Yun Zhua, Meng Caoa a
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College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China b School of Marine Science and Engineering, Hebei University of Technology, Tianjin 300130, China c School of Electronics and Information Engineering, Tianjin University, Tianjin 300072, China
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Abstract A novel composite of Cu2O nanofilms/porous silicon hybrid structure has been successfully synthesized using porous silicon as growth substrate by electrochemical synthesis. Orderly porous silicon (PS) substrate with the aperture about 1.5µm and hole depth about 10µm was prepared by electrochemical etching of a p-type monocrystalline silicon wafer in a double-tank cell. The Cu2O nanofilms have been grown onto PS substrates by electrochemical deposition with different electrodeposition time. The obtained Cu2O nanofilms/PS products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The gas sensing properties of Cu2O nanofilms/PS composites to NO2 were studied by the gas sensing test system. The result indicates that the electrodeposition time has a significant impact on the microstructure and gas sensing properties of Cu2O nanofilms/PS composites. Due to the high specific surface area and special microstructure, the Cu2O nanofilms/PS gas sensor with the eletrodeposition time of 30 min showed good gas sensing properties to NO2 with a high gas response, fast response-recovery characteristic, excellent repeatability and good selectivity at a working temperature of 175°C. At the working temperature, the gas sensor has a gas response of about 4.5 to 1 ppm NO2. The related gas sensing mechanism will be discussed. Keywords Cuprous oxide; Nanofilms; Porous silicon; Electrochemical deposition; Gas sensor
1. Introduction
Nitrogen dioxide (NO2) is one of the most toxic gases in the atmosphere which results from
combustion and automotive emission[1]. According to the Italian regulation, the air-quality standard for NO2 of attention level in the ambient air is 100 ppb for long-term exposure[2]. Therefore, the development of detector for NO2 has attracted intensive attention. The NO2 gas sensors with some characteristics such as a high gas response, short response-recovery time, low power consumption and low cost are demanded, but not yet marketed. Porous silicon (PS) is an important porous medium which is sensitive to many gases such as 1
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NOx, NH3, H2S, SOx, ethanol and acetone[3-4]. With the features of high specific surface area and high surface chemical activity, sensors made from porous silicon can be operated at relatively low temperatures, even at room temperature[5]. What’s more, PS can easily prepared by chemical etching of silicon in HF solution, and the silicon-based PS is compatible with silicon IC technology[5-6]. However, the poor gas-sensitive thermal stability limits its commercial
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applications of porous silicon[7].
Cuprous oxide (Cu2O) is a brick-red, p-type semiconductor with a direct band gap of
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2.17eV[8]. Due to the feature of low toxicity, abundance in natural source, low cost, and good
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electrical properties, it has been wildly employed in many fields, such as gas sensors, catalysts, solar energy conversion, and biosensors[9]. As one of the competitive candidates for applications in gas sensors, Cu2O thin film has already been widely used for detecting gases such as NO2 [10],
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H2S[11], and ethanol[9, 12]. So far, many approaches have been developed for preparation of Cu2O thin film, including sputtering deposition[13], hydrothermal[14], sol-gel[15] and
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electrochemical deposition[9, 16-18]. The electrochemical deposition of cuprous oxide nanofilms onto different substrate has been a hot area of research in recent years. All kinds of materials such
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as Au[19], InP[20, 21], Cu[22], TFO[23] and monocrystalline silicon[8] have been reported to
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selected as substrates to electrochemical deposition of cuprous oxide nanofilms. Ahmad Sabirin Zoolfakar et. al.[24, 25] reported an ethanol vapour sensing devices based on nanostructured CuO and Cu2O thin films, which are synthesized on the quartz substrates by RF sputtering under different conditions. Both nanostructured CuO and Cu2O gas sensors are able to detect ethanol vapour as low as several ppm .The sensors showed high sensitivity and repeatability, as well as fast response and recovery towards ethanol vapour. Shishiyanu[10] prepared the cuprous oxide thin film on the glass substrate by chemical deposition and tested the gas sensing properties of the samples. The result indicates that the samples have a high gas response and short response time to NO2. However, like other metal oxides, the high working temperature (200-350°C) brings about much inconvenience such as high power consumption and device integration, thus, limits the application of the Cu2O-based gas sensor. There were already plenty of researches on Cu2O film, however, to the authors’ knowledge, the preparation of Cu2O/PS composites via the electrodeposition of cuprous oxide onto porous silicon substrate was still blank, and their gas sensing properties were still unclear. The deposition 2
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of cuprous oxide onto the porous silicon substrate by other method is also rarely reported[26]. Because of the typical p-p heterojunction structure of the Cu2O/PS composites, the gas sensor based on low-dimensional nanomaterials/porous silicon composites may be beneficial in enhancing gas response for their synergetic enhancement or heterojunction effects. One typical example is that a gas sensor based on p-porous silicon/p-TeO2 nanowires showed excellent
that the p-p heterojunction effect played an important role in composite sensor.
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NO2-sensing properties at room temperature [27]. The gas sensing mechanism analysis is revealed
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In this work, electrochemical deposition of cuprous oxide nanofilms onto porous silicon and
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their gas sensing properties to NO2 were reported. The Cu2O/PS structure may have great potential in the field of low concentration NO2 detection.
2. Exprimental
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The process flow for the preparation of Cu2O nanofilms/PS gas sensor was shown in Fig. 1. The detail information will be given herein below.
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2.1 Porous silicon formation
The porous silicon substrates were produced by electrochemical etching in a double-tank cell (step 1 in Fig. 1). The schematic diagram of the electrochemical etching setup is shown in Fig. 2.
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In brief, the specimen size (silicon wafer) was 24mm×9mm after cutting from a p-type
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<100>-oriented monocrystalline silicon wafer with a resistivity of 10-15Ω·cm. The electrolyte (volume ratio HF: DMF=1:2) was added into an etching tank. After cleanup, the silicon slice was hold by clamps and inserted into the middle of the tank which was separated into two half-cells. Two platinum (Pt) electrodes were immersed in the electrolyte of the two half-cells, respectively. The etching process was carried out using galvanostatic etching technique with a constant current density of 100mA/cm2 for 8min in the dark at the room temperature. The PS layer with the dimension 16mm×4mm was formed on the polished surface of silicon slice which was directly faced to the cathode.
2.2 The preparation of the extraction electrode and seal In order to realize electrodeposition on the semiconductor substrate such as silicon and porous silicon, ohmic contact should be formed in the electrode contact region. Before the Cu2O deposition, a layer of 100-nanometer-thick Pt films was sputtered on the edge of the PS substrate to form the ohmic contact (step 2 in Fig. 1). A DPS-III high vacuum facing-target magnetron sputtering system with a circular Pt target was used for the Pt films deposition. The electrode was 3
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extracted from the Pt films by copper wire and sealed with silica gel layer for waterproof. The edge and back of Si substrate which had not been etched was sealed by nail polish to restrict the deposition area (step 3 in Fig. 1). Then the silicon oxide layer on the surface was removed and the Hydrogen terminated surface was produced by dipping the PS in 50% HF solution for 1min. Finally, the products were rinsed thoroughly with deionized water and used for electrodeposition
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immediately.
2.3 The electrochemical deposition of cuprous oxide
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The electrochemical deposition of Cu2O thin films using potentiostatic method was carried out at 60°C in an aqueous solution of 0.4 M of copper sulfate (CuSO4) and 3M of lactic acid (step
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4 in Fig. 1). The pH value of electrolyte was adjusted to 10 by adding 5M potassium hydroxide solution. The electrochemical cell used for the electrodepositing Cu2O thin films was consisted of
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a three-electrode system, a heating magnetic stirrer and a potentiostat. The PS substrate was used as working electrode, a Pt net as counter electrode, and a saturated calomel electrode (SCE) as the
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reference electrode, respectively. The deposition potential was controlled at -600mV versus SCE. The schematic diagram of electrochemical deposition of Cu2O onto PS is show in Fig. 3.
proposed as follows[17]:
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(1)2Cu2++2e-→2Cu+ (2)2Cu++2OH-→2CuOH (3)2CuOH→Cu2O+H2O
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The mechanisms for Cu2O growth by the cathodic electrochemical deposition theory is
The cupric ion (Cu2+) in the aqueous solution was reduced to cuprous ion (Cu+) by a single
electron process (Eq. (1)). Cuprous ion reacts with hydroxyl ions to form cuprous oxide which deposited at the surface of the cathode (Eq. (2) and Eq. (3)). The deposition time had a great influence on the morphology of product. The PS substrates
were electrodeposited in the same electrolyte for 10 min, 30 min, and 60 min respectively. The obtained Cu2O/PS samples were named S1, S2 and S3, respectively. After deposition, the samples were rinsed with deionized water, followed by air dry.
2.4 Characterization The structures of the Cu2O/PS composite were characterized by X-ray diffraction pattern (BRUKER D8-Focus) with CuKα radiation λ=0.1541nm. For determination of the morphology of specimens, the products were characterized by field emission scanning electron microscope 4
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(FESEM, FEI Nanosem 430; Hitachi S-4800), transmission electron microscope (FETEM, TECNAIG2F-20) and selected area electron diffraction (SAED).
2.5 Sensor preparation and measurement In order to measure the resistance of the samples, two Pt electrodes of dimensions 3mm×3mm were deposited on the top of samples (PS, S1, S2 and S3) by magnetron sputtering
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(DPS-III system) using a square mask (step 5 in Fig. 1).
The gas sensing properties were measured in a static gas sensing testing system. The
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schematic diagram of the gas sensing test system is shown in Fig. 4.The system consisted of a 30
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L sealed glass test chamber with a movable cover, a flat heating plate with a PID digital temperature controller, a professional digital multimeter (UNI-T UT70D) connected with a data acquisition system. Ambient humidity may have a large impact to the gas sensing properties of
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PS[28], so the environment humidity was kept to about 40% by a dehumidifier. The tested sample was connected with the multimeter by two test probes and placed on the heating plate fixed into
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the test chamber. In test, the precalculated amount of target gas for the desired concentration was injected into the test chamber by a syringe and diffused quickly under the effect of the blowing
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mini-fan fixed at the top of the chamber. The multimeter which connected to the PC was kept
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monitoring the resistance variation. When the resistance is stable for a period of time in the target gas atmospheres, the cover of the chamber was open and the gas concentration decrease quickly, and then the resistance of the sensors recovered. The operating temperature of the samples can be changed by adjusting the temperature controller.
3. Results and discussion 3.1 Structural characterization
Fig. 5 is the XRD spectra of potentiostatic deposition Cu2O nanofilms onto PS for 10min
(sample S1), 30min (sample S2) and 60min (sample S3), respectively. The XRD pattern of sample S1 shows the highest peak corresponding to the Si (200). Besides the Si peak, the relatively weak peak of cuprous oxide phase is observed in the 2θ range of 25-65° which can be assigned to the (111), (200) and (220) planes. The weak peaks may be due to a thin Cu2O nanocrystalline layer on the top surface. In the XRD patterns of sample S2 and sample S3, all the sharp peaks can be indexed well to the cubic Cu2O phase (JCPDS Card no. 05-0667). The strongest diffraction peak in the XRD patterns of S2 and S3 all appears at 2θ = 36° corresponding to the (111) plane,
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exhibiting a preferential growth in the [111] direction. The top view of the SEM for the samples PS, S1, S2 and S3 are shown in Fig. 6(a-d), respectively. The cross-sectional SEM images of the samples S1, S2 and S3 are given in Fig. 7(a-c), respectively. It indicates that the as-fabricated PS substrate possesses an orderly and uniform distribution of pores with the aperture about 1.5µm and hole depth about 10µm. The
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porosity was evaluated to be about 40% by a gravimetric assessment. It is classified as macro-PS according to the nomenclature of IUPAC.
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After the Cu2O films were electrodeposited onto the PS substrates, the morphology of Cu2O
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nanofilms/PS samples was modulated dramatically by the electrodeposition time, as shown in Fig. 6(b) (c) (d) and Fig. 7 (a) (b) (c). When the electrodeposition time was 10 min (sample S1), a thin Cu2O nanocrystalline layer consisted of many nanoparticles was deposited on the top surface of
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the PS, as presented in Fig. 6(b). Besides, some Cu2O nanoparticles with dozens of nanometers in diameter were scattered on the pore wall of porous silicon, as presented in Fig. 7(a). The holes of
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PS were clearly visible. As the electrodeposition time was increased to 30 min (sample S2), the Cu2O nanocrystalline layer was augmented markedly to form large pieces of films as shown in Fig.
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6(c). The Cu2O nanoparticles tended to have equal distribution on the pore wall of PS, as shown in
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Fig. 7(b). Many holes were still visible on the surface, which implied high porosity and large specific surface area. These voids provided plenty of adsorption sites and diffusion channels for gas molecules, so a high gas response can be expected. When the electrodeposition time was further increased to 60 min (sample S3), as shown in Fig. 6(d), the Cu2O nanocrystalline layer was thick enough to form agglomerate on the surface of PS. The Cu2O nanoparticles on the pore wall of samples S3 have accumulated and grown large enough so that they almost covered the walls of the holes of porous silicon, as shown in Fig. 7(c). Visible voids on the surface of PS were reduced dramatically, decreasing the specific surface area and the diffusion channels for gas molecules. This morphology may release its negative impact on the sensing performance. Fig. 7(d) shows a TEM image of the sample S2, which identify the Cu2O nanoparticles with diameters of dozens of nanometers. The SAED image of sample S2 is shown in Fig. 7(d), giving evidence that the Cu2O particles was nanocrystalline structure.
3.2 NO2-sensing properties In order to determine the best operating temperature of the Cu2O nanofilms/PS composite 6
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sensors to NO2 gas, all the sensors towards 1 ppm NO2 were measured at different operating temperatures ranging from 25°C (RT) to 275°C. The resistance of Cu2O nanofilms/PS sensors will decrease when exposed to the strong oxidizing gas such as NO2. So the gas response (S) to NO2 was defined as S= Ra/Rg, where Ra and
response (S) to reducing gases (NH3 or H2S) is defined as S= Rg/Ra.
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Rg denote the resistance of the sensor in the clean air and NO2 gas, respectively. Similarly, the gas
Fig. 8 illustrates the relationship between the gas response of the Cu2O nanofilms/PS sensors
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and operating temperature. Here, each sensor was tested at least three times at each operating
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temperature to ensure the reliability of test data. As shown in the Fig. 8, just as the typical Cu2O gas sensor[10], when the temperature increasing, the gas response of all the three sensors increased firstly, achieved the maximum at about 175°C, and then decreased. It means that the
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three sensors have the same working temperature (~175°C) as a contrast to the working temperature of PS sensor which was room temperature (RT)(~25°C)[5, 29]. Moreover, in the
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whole temperature range from 25°C (RT) to 275°C, the gas response of sample S2 were higher than the sample S3 and S1, the gas response of sample S3 were higher than the sample S1. At the
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working temperature, the gas sensor S2 has a maximum gas response of about 4.5 to 1 ppm NO2.
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It means the electrodeposition time plays a vital role in determining the NO2-sensing properties of the Cu2O nanofilms/PS sensors.
The dynamic response of the sensors (sample S1, S2 and S3) in various NO2 gas
concentrations (100ppb-1ppm) at the working temperature (~175°C) is shown in Fig. 9. For comparing, the dynamic response of the PS sensor to NO2 at its working temperature (~25°C) was also shown in Fig. 9. The measured resistances of all the samples decreased upon exposure to NO2 gas. It means all the samples exhibit a typical p-type semiconductor behavior towards oxidizing gases. The Cu2O nanofilms/PS gas sensors (sample S1, S2 and S3) exhibit a higher gas response to NO2 and better response-recovery characteristics than that of PS. The relationship between NO2 concentrations and gas response of the sample S1, S2 and S3 at the working temperature is illustrated in the Fig. 10. As shown in the figure, the gas response of the three samples increases with the increasing of the NO2 concentration in the range of 0.1ppm to 1ppm. But as the NO2 concentration increasing, the gas response becomes more or less saturated. In the whole concentration range, the gas response of S1, S3 and S2 increase successively under 7
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the same NO2 concentration. It means that the gas response of Cu2O nanofilms/PS sensors increase with the increasing of electrodeposition time firstly, attain a maximum value, and then decrease with the further increase in electrodeposition time. The time spent by the sensor on achieving 90% of the total resistance change is defined as the response time in the case of adsorption or as the recovery time in the case of desorption. The
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response and recovery times of the Cu2O nanofilms/PS gas sensor (sample S2) as a function of
NO2 concentrations are shown in Fig. 11. The response times and the recovery times all decreased
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with increasing the concentrations of NO2. The response times varied from 152s to 100s, while the
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recovery times varied from 145s to 95s as the sensor was exposed to NO2 from 100ppb to 1000 ppb. The Cu2O nanofilms/PS gas sensor (sample S2) exhibits a fast response-recovery characteristic.
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The reasons for the sensor gas sensing behaviors were taken into consideration. The gas sensing mechanism of the Cu2O nanofilms/PS sensor can be analyzed using the surface-controlled
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models[30]. According to the structure of the gas sensor shown in Fig. 12, the measuring resistance of Cu2O nanofilms/PS may be mainly affected by four different types of charge
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depleted regions, including the PS surface, the Cu2O nanofilms surface, Cu2O nanograins
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homojunction, and the p-pCu2O-PS heterojunction. As we all know, the Cu2O and the PS substrate all belong to the p-type surface resistance
control semiconductor. Holes are the dominant charge carriers on the surface of Cu2O and PS. When the Cu2O/PS sensor is exposed to the air, some oxygen molecules in the air will be adsorbed on the surface of Cu2O and PS through diffusion and extract electrons from the conduction band −
of Cu2O and PS. The oxygen molecules then transform into chemisorbed oxygen species ( O 2 −
and O ) (Eqs. (4)(5)(6)). As a result, the electron-depleted space-charge layers in the surface of Cu2O and PS are formed. When the Cu2O/PS sensor is exposed to the strong oxidizing gas NO2, the NO2 molecules may be directly adsorbed onto the surfaces of Cu2O and PS by trapping −
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electrons and react with the surface adsorbed species O 2 and O (Eqs. (7)(8)(9)). The reaction will decrease further the thickness of the space charge layer of the Cu2O and PS surface, but increasing the conducting channel width of the holes in the Cu2O and PS surface.
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(4)O2(gas)↔O2 (ads) (5)O2(gas)+e-↔O2-(ads) (6)O2-(ads)+e-↔2O-(ads)
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(7)NO2(gas)+e-↔NO2-(ads) (8)NO2(gas)+O2-(ads)+2e-↔NO2-(ads)+2O-(ads)
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(9)NO2(gas)+O-(ads)+NO+(ads)+2O-(ads)
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As mentioned in the introduction, the interface resistance of the Cu2O nanograins homojunction and the p-pCu2O-PS heterojunction may have great effect to the improvement of
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gas sensing performance. When the Cu2O nanofilms/PS gas sensors were exposed to the NO2 gas, the NO2 molecular adsorbed on the surface of the depletion layers formed in the Cu2O nanograins
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homojunction and p-pCu2O-PS heterojunction and changed the height of the related barriers. Because of the amplification effect of the heterojunction and the homojunction[31], the interface
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greatly upon exposure to NO2.
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resistance of the Cu2O nanograins homojunction and the p-pCu2O-PS heterojunction changed
In addition, the fact that sample S2 has a higher gas response than the sample S3 and S1 can
be explained as follow. The gas sensing properties of metal oxide semiconductor (MOS) sensors can be greatly influenced by microstructure parameters such as film thickness, grain size, grain network, surface geometry, film texture, crystal face and porosity[32]. Combined with the SEM images of Cu2O nanofilms/PS composite (Fig. 6), it can be seen the sample S2 with electrodeposition time of 30 min provided the most active sites and exhibited the strongest response at each NO2 concentration. However, when the electrodeposition time was increased to 60 min, the agglomerate of cuprous oxide particles reduced the amount of active sites dramatically on the surface and decreased the sensor response of the sensing material. Compared with the porous silicon (PS), the sample S3 and S1, Cu2O nanofilms/PS gas sensor (sample S2) also shows better response/recovery performance. The response/recovery performance of gas sensor is often affected by some factors such as the thickness of sensing layer and the gas diffusion speed. We think that the suitable thickness Cu2O nanofilms may ensure the 9
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sample dense array of channel. It provides direct diffusion channels and large surface accessibility for the mass transportation of gas molecules, lead to the good response/recovery performance. Lastly, the repeatability is also an important consideration for the practical application of gas sensor. Fig. 13 shows the dynamic response of Cu2O nanofilms/PS gas sensor (sample S2) to five representative cyclic exposures of 1 ppm NO2 gas. Notably, the repeatability test indicates the
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sensor has a similar response value and response-recovery time, which suggests an excellent repeatability and stability.
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The gas selectivity is also an important parameter for evaluating the gas sensor. Fig. 14
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shows the Cu2O nanofilms/PS gas sensors responses to various gases, including 1 ppm NO2, 10 ppm NH3, H2S and 100 ppm methanal, ethanol as well as acetone at working temperature, respectively. Apparently, the sensor exhibits a good selectivity to NO2 gas.
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Conclusions
In this work, we demonstrate successfully NO2 sensing devices based on Cu2O nanofilms/PS
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hybrid structure, which are synthesized by electrochemical deposition Cu2O nanofilms onto PS from the aqueous solution based on copper sulfate and lactic acid with the electrodeposition time
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10 min, 30 min, and 60 min respectively. SEM images, TEM images and the XRD spectrum
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confirmed the formation of cubic Cu2O nanoparticles on the PS. The SEM images and gas sensing tests indicated that the microstructure and gas sensing properties of Cu2O nanofilms/PS composites were highly dependent on the electrodeposition time. With the increasing of deposition time, the Cu2O film thickness of the samples S1, S2 and S3 increases clearly, but the sensitivity of the samples S1, S2 and S3 to NO2 increase firstly and then decrease. At the working temperature of 175°C, the gas sensor with the eletrodeposition time of 30 min has a gas response of about 4.5 and the response/recovery time of 100/95s to 1 ppm NO2. The result of the gas-sensing mechanism analysis indicates the high specific surface area, special microstructure, the synergistic effect of the heterojunction may be responsible for the excellent gas-sensing properties of the Cu2O nanofilms/PS hybrid structure.
Acknowledgments Project supported by the National Natural Science Foundation of China (Nos. 61271070, and 11104203) and the Tianjin Normal University Doctoral Foundation of China (Nos. 52XB1416)
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[23] P.E. De Jongh, D. Vanmaekelbergh and J.J. Kelly, Cu2O: electrodeposition and characterization, Chem Mater 12 (1999) 3512-3517.
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[24] A. S. Zoolfakar, M. Z. Ahmad, R. A. Rani, J. Z. Ou, S. Balendhran, S. Zhuiykov, K. Latham, W. Wlodarski and K. Kalantar-zadeh, Nanostructured copper oxides as ethanol vapour sensors, Sens. Actuators B: Chem. 185 (2013) 620-627.
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[25] A.S. Zoolfakar, R.A. Rani, A.J. Morfa, A.P. O‘Mullane and K. Kalantar-zadeh, Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications, Journal of Materials Chemistry C 2 (2014) 5247-5270.
[26] P. Kumar and P. Huber, Quenching of reducing properties of mesoporous silicon and its use as
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template for metal/semiconductor deposition, J Electrochem Soc 3 (2010) D172-D176. [27] Y. Wu, M. Hu, Y. Qin, X. Wei, S. Ma, and D. Yan, Enhanced response characteristics of p-porous 181-188.
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silicon (substrate)/p-TeO2 (nanowires) sensor for NO2 detection, Sens. Actuators B: Chem. 195 (2014)
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[28] J. Mareš, J. Krištofik and E. Hulicius, Influence of humidity on transport in porous silicon, Thin Solid Films 1 (1995) 272-275.
[29] S. Ma, M. Hu, P. Zeng, M. Li, W. Yan and Y. Qin, Synthesis and low-temperature gas sensing properties of tungsten oxide nanowires/porous silicon composite, Sens. Actuators B: Chem. (2014) 341-349.
[30] J. Mizsei, How can sensitive and selective semiconductor gas sensors be made? Sens. Actuators B: Chem. 2 (1995) 173-176.
[31] P. Su and T. Pan, Fabrication of a room-temperature NO2 gas sensor based on WO3 films and WO3/MWCNT nanocomposite films by combining polyol process with metal organic decomposition method, Mater Chem Phys 3 (2011) 351-357.
[32] G. Korotcenkov, The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors, Materials Science and Engineering: R: Reports 1 (2008) 1-39. Fig. 1 Schematic diagram of the process flow for the preparation of Cu2O nanofilms/PS gas sensor Fig. 2 The schematic diagram of the electrochemical etching setup Fig. 3 The schematic diagram of electrochemical deposition of Cu2O onto PS Fig. 4 The schematic diagram of the gas sensing test system Fig. 5 XRD patterns of the Cu2O nanofilms/PS samples: (a) S1; (b) S2; (c) S3; (vertical lines: JCPDS no. 05-0667)
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Fig. 6 The surface SEM images of the samples: (a) PS; (b) S1; (c) S2; (d) S3; the inset in (a), (b), (c) and (d) are the high magnification SEM images of the corresponding products Fig. 7 The cross-sectional SEM images of the samples: (a) S1; (b) S2; (c) S3; Figure (d) is the TEM images of the sample S2; the inset in (d) is the SAED image of sample S2 Fig. 8 Relationship between operating temperature and gas response to 1 ppm NO2 for Cu2O nanofilms/PS gas sensors (sample S1, S2 and S3) Fig. 9 Dynamic response of the sensors (sample PS, S1, S2 and S3) to various concentration of NO2 at the working temperature
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Fig. 10 Relationship between the gas response and NO2 concentration for sample S1, S2 and S3 at the working temperature
Fig. 11 Response time and recovery time curves of the sensor S2 toward various NO2 concentrations at
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the working temperature
Fig. 12 Schematic diagram of the resistive-type Cu2O nanofilms/PS gas sensor
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Fig. 13 the dynamic response of Cu2O nanofilms/PS gas sensor (sample S2) to five representative cyclic exposures of 1 ppm NO2 gas
Fig. 14 Gas response of the Cu2O nanofilms/PS sensors (sample S2) to various gases at working
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temperature
Fig.1
Schematic diagram of the process flow for the preparation of Cu2O nanofilms/PS gas sensor
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Fig.2 The schematic diagram of the electrochemical etching setup
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Fig.3 The schematic diagram of electrochemical deposition of Cu2O onto PS
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Fig.4 The schematic diagram of the gas sensing test system
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(c)
S3
30
35
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(200)
Cu2O(JCPDS:05-0667)
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S1
(111)
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Si(200)
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(110)
Intensity(a.u.)
(b)
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Fig.5 XRD patterns of the Cu2O nanofilms/PS samples: (a) S1; (b) S2; (c) S3; (vertical lines: JCPDS no.
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05-0667)
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Fig.6 The surface SEM images of the samples: (a) PS; (b) S1; (c) S2; (d) S3; the inset in (a), (b), (c) and
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(d) are the high magnification SEM images of the corresponding products
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Fig.7 The cross-sectional SEM images of the samples: (a) S1; (b) S2; (c) S3; Figure (d) is the TEM images of the sample S2; the inset in (d) is the SAED image of sample S2
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5.0
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225 250 275
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Fig.8 Relationship between operating temperature and gas response to 1 ppm NO2 for Cu2O
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nanofilms/PS gas sensors (sample S1, S2 and S3)
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1000ppb 400ppb
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Fig.9 Dynamic response of the sensors (sample PS, S1, S2 and S3) to various concentration of NO2 at
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5.0 4.5
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180
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response time
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Fig.12 Schematic diagram of the resistive-type Cu2O nanofilms/PS gas sensor
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4.5
1ppm NO2
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exposures of 1 ppm NO2 gas
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100ppm acetone
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Gases
Fig.14 Gas response of the Cu2O nanofilms/PS sensors (sample S2) to various gases at working
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Biographies: Dali Yan received his Ph.D. in microelectronics and solid-state electronics from Tianjin University in 2014. He is currently an experimentalist in college of physics and materials Science in Tianjin Normal University. His research interest is in the areas of novel nanomaterials for gas sensing applications. Shenyu Li received her M.S. in inorganic chemistry from the National University of Singapore in 2010. She is now a doctoral student at Hebei University of Technology. Her current research interest is in the area of solution chemistry.
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Ming Hu received her M.S. in microelectronics and solid-state electronics from Tianjin University in 1991. She is now a full professor in Department of Electronics Science and Technology in Tianjin University. Her research interests include MEMS, gas sensor, and sensitive materials and functional thin film devices.
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Shiyu Liu received his Ph.D. in information functional materials from Beijing University of Aeronautics and
Astronautics in 2010. He is currently an associated professor in college of physics and materials Science in Tianjin
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Normal University. His research interest is in the areas of computational condensed matter physics.
Yun Zhu received her Ph.D. in condensed matter physics from Chinese Academy of Sciences. She is currently an associated professor in college of physics and materials Science in Tianjin Normal University. Her research interest
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is in the areas of semiconductor functional thin film devices.
Meng Cao received his M.S. in condensed matter physics from Tianjin Normal University in 2008. He is currently an experimentalist in college of physics and materials Science in Tianjin Normal University. His research interest
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is in the areas of nano-film materials.
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S0925-4005(15)30387-7 http://dx.doi.org/doi:10.1016/j.snb.2015.09.080 SNB 19065
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4-6-2015 16-8-2015 15-9-2015
Please cite this article as: D. Yan, S. Li, M. Hu, S. Liu, Y. Zhu, M. Cao, Electrochemical synthesis and the gas sensing properties of the Cu2O nanofilms/porous silicon hybrid structure, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.09.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical synthesis and the gas sensing properties of the Cu2O nanofilms/porous silicon hybrid structure
Dali [email protected], Shenyu Lib, Ming [email protected], Shiyu Liua, Yun Zhua, Meng Caoa a
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College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China b School of Marine Science and Engineering, Hebei University of Technology, Tianjin 300130, China c School of Electronics and Information Engineering, Tianjin University, Tianjin 300072, China
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Abstract A novel composite of Cu2O nanofilms/porous silicon hybrid structure has been successfully synthesized using porous silicon as growth substrate by electrochemical synthesis. Orderly porous silicon (PS) substrate with the aperture about 1.5µm and hole depth about 10µm was prepared by electrochemical etching of a p-type monocrystalline silicon wafer in a double-tank cell. The Cu2O nanofilms have been grown onto PS substrates by electrochemical deposition with different electrodeposition time. The obtained Cu2O nanofilms/PS products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The gas sensing properties of Cu2O nanofilms/PS composites to NO2 were studied by the gas sensing test system. The result indicates that the electrodeposition time has a significant impact on the microstructure and gas sensing properties of Cu2O nanofilms/PS composites. Due to the high specific surface area and special microstructure, the Cu2O nanofilms/PS gas sensor with the eletrodeposition time of 30 min showed good gas sensing properties to NO2 with a high gas response, fast response-recovery characteristic, excellent repeatability and good selectivity at a working temperature of 175°C. At the working temperature, the gas sensor has a gas response of about 4.5 to 1 ppm NO2. The related gas sensing mechanism will be discussed. Keywords Cuprous oxide; Nanofilms; Porous silicon; Electrochemical deposition; Gas sensor
1. Introduction
Nitrogen dioxide (NO2) is one of the most toxic gases in the atmosphere which results from
combustion and automotive emission[1]. According to the Italian regulation, the air-quality standard for NO2 of attention level in the ambient air is 100 ppb for long-term exposure[2]. Therefore, the development of detector for NO2 has attracted intensive attention. The NO2 gas sensors with some characteristics such as a high gas response, short response-recovery time, low power consumption and low cost are demanded, but not yet marketed. Porous silicon (PS) is an important porous medium which is sensitive to many gases such as 1
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NOx, NH3, H2S, SOx, ethanol and acetone[3-4]. With the features of high specific surface area and high surface chemical activity, sensors made from porous silicon can be operated at relatively low temperatures, even at room temperature[5]. What’s more, PS can easily prepared by chemical etching of silicon in HF solution, and the silicon-based PS is compatible with silicon IC technology[5-6]. However, the poor gas-sensitive thermal stability limits its commercial
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applications of porous silicon[7].
Cuprous oxide (Cu2O) is a brick-red, p-type semiconductor with a direct band gap of
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2.17eV[8]. Due to the feature of low toxicity, abundance in natural source, low cost, and good
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electrical properties, it has been wildly employed in many fields, such as gas sensors, catalysts, solar energy conversion, and biosensors[9]. As one of the competitive candidates for applications in gas sensors, Cu2O thin film has already been widely used for detecting gases such as NO2 [10],
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H2S[11], and ethanol[9, 12]. So far, many approaches have been developed for preparation of Cu2O thin film, including sputtering deposition[13], hydrothermal[14], sol-gel[15] and
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electrochemical deposition[9, 16-18]. The electrochemical deposition of cuprous oxide nanofilms onto different substrate has been a hot area of research in recent years. All kinds of materials such
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as Au[19], InP[20, 21], Cu[22], TFO[23] and monocrystalline silicon[8] have been reported to
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selected as substrates to electrochemical deposition of cuprous oxide nanofilms. Ahmad Sabirin Zoolfakar et. al.[24, 25] reported an ethanol vapour sensing devices based on nanostructured CuO and Cu2O thin films, which are synthesized on the quartz substrates by RF sputtering under different conditions. Both nanostructured CuO and Cu2O gas sensors are able to detect ethanol vapour as low as several ppm .The sensors showed high sensitivity and repeatability, as well as fast response and recovery towards ethanol vapour. Shishiyanu[10] prepared the cuprous oxide thin film on the glass substrate by chemical deposition and tested the gas sensing properties of the samples. The result indicates that the samples have a high gas response and short response time to NO2. However, like other metal oxides, the high working temperature (200-350°C) brings about much inconvenience such as high power consumption and device integration, thus, limits the application of the Cu2O-based gas sensor. There were already plenty of researches on Cu2O film, however, to the authors’ knowledge, the preparation of Cu2O/PS composites via the electrodeposition of cuprous oxide onto porous silicon substrate was still blank, and their gas sensing properties were still unclear. The deposition 2
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of cuprous oxide onto the porous silicon substrate by other method is also rarely reported[26]. Because of the typical p-p heterojunction structure of the Cu2O/PS composites, the gas sensor based on low-dimensional nanomaterials/porous silicon composites may be beneficial in enhancing gas response for their synergetic enhancement or heterojunction effects. One typical example is that a gas sensor based on p-porous silicon/p-TeO2 nanowires showed excellent
that the p-p heterojunction effect played an important role in composite sensor.
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NO2-sensing properties at room temperature [27]. The gas sensing mechanism analysis is revealed
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In this work, electrochemical deposition of cuprous oxide nanofilms onto porous silicon and
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their gas sensing properties to NO2 were reported. The Cu2O/PS structure may have great potential in the field of low concentration NO2 detection.
2. Exprimental
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The process flow for the preparation of Cu2O nanofilms/PS gas sensor was shown in Fig. 1. The detail information will be given herein below.
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2.1 Porous silicon formation
The porous silicon substrates were produced by electrochemical etching in a double-tank cell (step 1 in Fig. 1). The schematic diagram of the electrochemical etching setup is shown in Fig. 2.
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In brief, the specimen size (silicon wafer) was 24mm×9mm after cutting from a p-type
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<100>-oriented monocrystalline silicon wafer with a resistivity of 10-15Ω·cm. The electrolyte (volume ratio HF: DMF=1:2) was added into an etching tank. After cleanup, the silicon slice was hold by clamps and inserted into the middle of the tank which was separated into two half-cells. Two platinum (Pt) electrodes were immersed in the electrolyte of the two half-cells, respectively. The etching process was carried out using galvanostatic etching technique with a constant current density of 100mA/cm2 for 8min in the dark at the room temperature. The PS layer with the dimension 16mm×4mm was formed on the polished surface of silicon slice which was directly faced to the cathode.
2.2 The preparation of the extraction electrode and seal In order to realize electrodeposition on the semiconductor substrate such as silicon and porous silicon, ohmic contact should be formed in the electrode contact region. Before the Cu2O deposition, a layer of 100-nanometer-thick Pt films was sputtered on the edge of the PS substrate to form the ohmic contact (step 2 in Fig. 1). A DPS-III high vacuum facing-target magnetron sputtering system with a circular Pt target was used for the Pt films deposition. The electrode was 3
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extracted from the Pt films by copper wire and sealed with silica gel layer for waterproof. The edge and back of Si substrate which had not been etched was sealed by nail polish to restrict the deposition area (step 3 in Fig. 1). Then the silicon oxide layer on the surface was removed and the Hydrogen terminated surface was produced by dipping the PS in 50% HF solution for 1min. Finally, the products were rinsed thoroughly with deionized water and used for electrodeposition
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immediately.
2.3 The electrochemical deposition of cuprous oxide
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The electrochemical deposition of Cu2O thin films using potentiostatic method was carried out at 60°C in an aqueous solution of 0.4 M of copper sulfate (CuSO4) and 3M of lactic acid (step
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4 in Fig. 1). The pH value of electrolyte was adjusted to 10 by adding 5M potassium hydroxide solution. The electrochemical cell used for the electrodepositing Cu2O thin films was consisted of
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a three-electrode system, a heating magnetic stirrer and a potentiostat. The PS substrate was used as working electrode, a Pt net as counter electrode, and a saturated calomel electrode (SCE) as the
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reference electrode, respectively. The deposition potential was controlled at -600mV versus SCE. The schematic diagram of electrochemical deposition of Cu2O onto PS is show in Fig. 3.
proposed as follows[17]:
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(1)2Cu2++2e-→2Cu+ (2)2Cu++2OH-→2CuOH (3)2CuOH→Cu2O+H2O
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The mechanisms for Cu2O growth by the cathodic electrochemical deposition theory is
The cupric ion (Cu2+) in the aqueous solution was reduced to cuprous ion (Cu+) by a single
electron process (Eq. (1)). Cuprous ion reacts with hydroxyl ions to form cuprous oxide which deposited at the surface of the cathode (Eq. (2) and Eq. (3)). The deposition time had a great influence on the morphology of product. The PS substrates
were electrodeposited in the same electrolyte for 10 min, 30 min, and 60 min respectively. The obtained Cu2O/PS samples were named S1, S2 and S3, respectively. After deposition, the samples were rinsed with deionized water, followed by air dry.
2.4 Characterization The structures of the Cu2O/PS composite were characterized by X-ray diffraction pattern (BRUKER D8-Focus) with CuKα radiation λ=0.1541nm. For determination of the morphology of specimens, the products were characterized by field emission scanning electron microscope 4
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(FESEM, FEI Nanosem 430; Hitachi S-4800), transmission electron microscope (FETEM, TECNAIG2F-20) and selected area electron diffraction (SAED).
2.5 Sensor preparation and measurement In order to measure the resistance of the samples, two Pt electrodes of dimensions 3mm×3mm were deposited on the top of samples (PS, S1, S2 and S3) by magnetron sputtering
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(DPS-III system) using a square mask (step 5 in Fig. 1).
The gas sensing properties were measured in a static gas sensing testing system. The
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schematic diagram of the gas sensing test system is shown in Fig. 4.The system consisted of a 30
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L sealed glass test chamber with a movable cover, a flat heating plate with a PID digital temperature controller, a professional digital multimeter (UNI-T UT70D) connected with a data acquisition system. Ambient humidity may have a large impact to the gas sensing properties of
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PS[28], so the environment humidity was kept to about 40% by a dehumidifier. The tested sample was connected with the multimeter by two test probes and placed on the heating plate fixed into
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the test chamber. In test, the precalculated amount of target gas for the desired concentration was injected into the test chamber by a syringe and diffused quickly under the effect of the blowing
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mini-fan fixed at the top of the chamber. The multimeter which connected to the PC was kept
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monitoring the resistance variation. When the resistance is stable for a period of time in the target gas atmospheres, the cover of the chamber was open and the gas concentration decrease quickly, and then the resistance of the sensors recovered. The operating temperature of the samples can be changed by adjusting the temperature controller.
3. Results and discussion 3.1 Structural characterization
Fig. 5 is the XRD spectra of potentiostatic deposition Cu2O nanofilms onto PS for 10min
(sample S1), 30min (sample S2) and 60min (sample S3), respectively. The XRD pattern of sample S1 shows the highest peak corresponding to the Si (200). Besides the Si peak, the relatively weak peak of cuprous oxide phase is observed in the 2θ range of 25-65° which can be assigned to the (111), (200) and (220) planes. The weak peaks may be due to a thin Cu2O nanocrystalline layer on the top surface. In the XRD patterns of sample S2 and sample S3, all the sharp peaks can be indexed well to the cubic Cu2O phase (JCPDS Card no. 05-0667). The strongest diffraction peak in the XRD patterns of S2 and S3 all appears at 2θ = 36° corresponding to the (111) plane,
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exhibiting a preferential growth in the [111] direction. The top view of the SEM for the samples PS, S1, S2 and S3 are shown in Fig. 6(a-d), respectively. The cross-sectional SEM images of the samples S1, S2 and S3 are given in Fig. 7(a-c), respectively. It indicates that the as-fabricated PS substrate possesses an orderly and uniform distribution of pores with the aperture about 1.5µm and hole depth about 10µm. The
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porosity was evaluated to be about 40% by a gravimetric assessment. It is classified as macro-PS according to the nomenclature of IUPAC.
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After the Cu2O films were electrodeposited onto the PS substrates, the morphology of Cu2O
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nanofilms/PS samples was modulated dramatically by the electrodeposition time, as shown in Fig. 6(b) (c) (d) and Fig. 7 (a) (b) (c). When the electrodeposition time was 10 min (sample S1), a thin Cu2O nanocrystalline layer consisted of many nanoparticles was deposited on the top surface of
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the PS, as presented in Fig. 6(b). Besides, some Cu2O nanoparticles with dozens of nanometers in diameter were scattered on the pore wall of porous silicon, as presented in Fig. 7(a). The holes of
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PS were clearly visible. As the electrodeposition time was increased to 30 min (sample S2), the Cu2O nanocrystalline layer was augmented markedly to form large pieces of films as shown in Fig.
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6(c). The Cu2O nanoparticles tended to have equal distribution on the pore wall of PS, as shown in
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Fig. 7(b). Many holes were still visible on the surface, which implied high porosity and large specific surface area. These voids provided plenty of adsorption sites and diffusion channels for gas molecules, so a high gas response can be expected. When the electrodeposition time was further increased to 60 min (sample S3), as shown in Fig. 6(d), the Cu2O nanocrystalline layer was thick enough to form agglomerate on the surface of PS. The Cu2O nanoparticles on the pore wall of samples S3 have accumulated and grown large enough so that they almost covered the walls of the holes of porous silicon, as shown in Fig. 7(c). Visible voids on the surface of PS were reduced dramatically, decreasing the specific surface area and the diffusion channels for gas molecules. This morphology may release its negative impact on the sensing performance. Fig. 7(d) shows a TEM image of the sample S2, which identify the Cu2O nanoparticles with diameters of dozens of nanometers. The SAED image of sample S2 is shown in Fig. 7(d), giving evidence that the Cu2O particles was nanocrystalline structure.
3.2 NO2-sensing properties In order to determine the best operating temperature of the Cu2O nanofilms/PS composite 6
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sensors to NO2 gas, all the sensors towards 1 ppm NO2 were measured at different operating temperatures ranging from 25°C (RT) to 275°C. The resistance of Cu2O nanofilms/PS sensors will decrease when exposed to the strong oxidizing gas such as NO2. So the gas response (S) to NO2 was defined as S= Ra/Rg, where Ra and
response (S) to reducing gases (NH3 or H2S) is defined as S= Rg/Ra.
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Rg denote the resistance of the sensor in the clean air and NO2 gas, respectively. Similarly, the gas
Fig. 8 illustrates the relationship between the gas response of the Cu2O nanofilms/PS sensors
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and operating temperature. Here, each sensor was tested at least three times at each operating
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temperature to ensure the reliability of test data. As shown in the Fig. 8, just as the typical Cu2O gas sensor[10], when the temperature increasing, the gas response of all the three sensors increased firstly, achieved the maximum at about 175°C, and then decreased. It means that the
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three sensors have the same working temperature (~175°C) as a contrast to the working temperature of PS sensor which was room temperature (RT)(~25°C)[5, 29]. Moreover, in the
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whole temperature range from 25°C (RT) to 275°C, the gas response of sample S2 were higher than the sample S3 and S1, the gas response of sample S3 were higher than the sample S1. At the
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working temperature, the gas sensor S2 has a maximum gas response of about 4.5 to 1 ppm NO2.
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It means the electrodeposition time plays a vital role in determining the NO2-sensing properties of the Cu2O nanofilms/PS sensors.
The dynamic response of the sensors (sample S1, S2 and S3) in various NO2 gas
concentrations (100ppb-1ppm) at the working temperature (~175°C) is shown in Fig. 9. For comparing, the dynamic response of the PS sensor to NO2 at its working temperature (~25°C) was also shown in Fig. 9. The measured resistances of all the samples decreased upon exposure to NO2 gas. It means all the samples exhibit a typical p-type semiconductor behavior towards oxidizing gases. The Cu2O nanofilms/PS gas sensors (sample S1, S2 and S3) exhibit a higher gas response to NO2 and better response-recovery characteristics than that of PS. The relationship between NO2 concentrations and gas response of the sample S1, S2 and S3 at the working temperature is illustrated in the Fig. 10. As shown in the figure, the gas response of the three samples increases with the increasing of the NO2 concentration in the range of 0.1ppm to 1ppm. But as the NO2 concentration increasing, the gas response becomes more or less saturated. In the whole concentration range, the gas response of S1, S3 and S2 increase successively under 7
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the same NO2 concentration. It means that the gas response of Cu2O nanofilms/PS sensors increase with the increasing of electrodeposition time firstly, attain a maximum value, and then decrease with the further increase in electrodeposition time. The time spent by the sensor on achieving 90% of the total resistance change is defined as the response time in the case of adsorption or as the recovery time in the case of desorption. The
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response and recovery times of the Cu2O nanofilms/PS gas sensor (sample S2) as a function of
NO2 concentrations are shown in Fig. 11. The response times and the recovery times all decreased
cr
with increasing the concentrations of NO2. The response times varied from 152s to 100s, while the
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recovery times varied from 145s to 95s as the sensor was exposed to NO2 from 100ppb to 1000 ppb. The Cu2O nanofilms/PS gas sensor (sample S2) exhibits a fast response-recovery characteristic.
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The reasons for the sensor gas sensing behaviors were taken into consideration. The gas sensing mechanism of the Cu2O nanofilms/PS sensor can be analyzed using the surface-controlled
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models[30]. According to the structure of the gas sensor shown in Fig. 12, the measuring resistance of Cu2O nanofilms/PS may be mainly affected by four different types of charge
d
depleted regions, including the PS surface, the Cu2O nanofilms surface, Cu2O nanograins
Ac ce pt e
homojunction, and the p-pCu2O-PS heterojunction. As we all know, the Cu2O and the PS substrate all belong to the p-type surface resistance
control semiconductor. Holes are the dominant charge carriers on the surface of Cu2O and PS. When the Cu2O/PS sensor is exposed to the air, some oxygen molecules in the air will be adsorbed on the surface of Cu2O and PS through diffusion and extract electrons from the conduction band −
of Cu2O and PS. The oxygen molecules then transform into chemisorbed oxygen species ( O 2 −
and O ) (Eqs. (4)(5)(6)). As a result, the electron-depleted space-charge layers in the surface of Cu2O and PS are formed. When the Cu2O/PS sensor is exposed to the strong oxidizing gas NO2, the NO2 molecules may be directly adsorbed onto the surfaces of Cu2O and PS by trapping −
−
electrons and react with the surface adsorbed species O 2 and O (Eqs. (7)(8)(9)). The reaction will decrease further the thickness of the space charge layer of the Cu2O and PS surface, but increasing the conducting channel width of the holes in the Cu2O and PS surface.
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(4)O2(gas)↔O2 (ads) (5)O2(gas)+e-↔O2-(ads) (6)O2-(ads)+e-↔2O-(ads)
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(7)NO2(gas)+e-↔NO2-(ads) (8)NO2(gas)+O2-(ads)+2e-↔NO2-(ads)+2O-(ads)
cr
(9)NO2(gas)+O-(ads)+NO+(ads)+2O-(ads)
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As mentioned in the introduction, the interface resistance of the Cu2O nanograins homojunction and the p-pCu2O-PS heterojunction may have great effect to the improvement of
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gas sensing performance. When the Cu2O nanofilms/PS gas sensors were exposed to the NO2 gas, the NO2 molecular adsorbed on the surface of the depletion layers formed in the Cu2O nanograins
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homojunction and p-pCu2O-PS heterojunction and changed the height of the related barriers. Because of the amplification effect of the heterojunction and the homojunction[31], the interface
Ac ce pt e
greatly upon exposure to NO2.
d
resistance of the Cu2O nanograins homojunction and the p-pCu2O-PS heterojunction changed
In addition, the fact that sample S2 has a higher gas response than the sample S3 and S1 can
be explained as follow. The gas sensing properties of metal oxide semiconductor (MOS) sensors can be greatly influenced by microstructure parameters such as film thickness, grain size, grain network, surface geometry, film texture, crystal face and porosity[32]. Combined with the SEM images of Cu2O nanofilms/PS composite (Fig. 6), it can be seen the sample S2 with electrodeposition time of 30 min provided the most active sites and exhibited the strongest response at each NO2 concentration. However, when the electrodeposition time was increased to 60 min, the agglomerate of cuprous oxide particles reduced the amount of active sites dramatically on the surface and decreased the sensor response of the sensing material. Compared with the porous silicon (PS), the sample S3 and S1, Cu2O nanofilms/PS gas sensor (sample S2) also shows better response/recovery performance. The response/recovery performance of gas sensor is often affected by some factors such as the thickness of sensing layer and the gas diffusion speed. We think that the suitable thickness Cu2O nanofilms may ensure the 9
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sample dense array of channel. It provides direct diffusion channels and large surface accessibility for the mass transportation of gas molecules, lead to the good response/recovery performance. Lastly, the repeatability is also an important consideration for the practical application of gas sensor. Fig. 13 shows the dynamic response of Cu2O nanofilms/PS gas sensor (sample S2) to five representative cyclic exposures of 1 ppm NO2 gas. Notably, the repeatability test indicates the
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sensor has a similar response value and response-recovery time, which suggests an excellent repeatability and stability.
cr
The gas selectivity is also an important parameter for evaluating the gas sensor. Fig. 14
us
shows the Cu2O nanofilms/PS gas sensors responses to various gases, including 1 ppm NO2, 10 ppm NH3, H2S and 100 ppm methanal, ethanol as well as acetone at working temperature, respectively. Apparently, the sensor exhibits a good selectivity to NO2 gas.
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Conclusions
In this work, we demonstrate successfully NO2 sensing devices based on Cu2O nanofilms/PS
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hybrid structure, which are synthesized by electrochemical deposition Cu2O nanofilms onto PS from the aqueous solution based on copper sulfate and lactic acid with the electrodeposition time
d
10 min, 30 min, and 60 min respectively. SEM images, TEM images and the XRD spectrum
Ac ce pt e
confirmed the formation of cubic Cu2O nanoparticles on the PS. The SEM images and gas sensing tests indicated that the microstructure and gas sensing properties of Cu2O nanofilms/PS composites were highly dependent on the electrodeposition time. With the increasing of deposition time, the Cu2O film thickness of the samples S1, S2 and S3 increases clearly, but the sensitivity of the samples S1, S2 and S3 to NO2 increase firstly and then decrease. At the working temperature of 175°C, the gas sensor with the eletrodeposition time of 30 min has a gas response of about 4.5 and the response/recovery time of 100/95s to 1 ppm NO2. The result of the gas-sensing mechanism analysis indicates the high specific surface area, special microstructure, the synergistic effect of the heterojunction may be responsible for the excellent gas-sensing properties of the Cu2O nanofilms/PS hybrid structure.
Acknowledgments Project supported by the National Natural Science Foundation of China (Nos. 61271070, and 11104203) and the Tianjin Normal University Doctoral Foundation of China (Nos. 52XB1416)
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[18] Y.L. Liu, Y.C. Liu, R. Mu, H. Yang, C.L. Shao, J.Y. Zhang, Y.M. Lu, D.Z. Shen and X.W. Fan, The structural and optical properties of Cu2O films electrodeposited on different substrates, Semicond Sci Tech 1 (2005) 44. [19] E.W. Bohannan, M.G. Shumsky and J.A. Switzer, Epitaxial electrodeposition of copper (I) oxide on single-crystal gold (100), Chem Mater 9 (1999) 2289-2291. [20] R. Liu, F. Oba, E.W. Bohannan, F. Ernst and J.A. Switzer, Shape control in epitaxial electrodeposition: Cu2O nanocubes on InP (001), Chem Mater 26 (2003) 4882-4885. [21] R. Liu, E.A. Kulp, F. Oba, E.W. Bohannan, F. Ernst and J.A. Switzer, Epitaxial electrodeposition
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[26] P. Kumar and P. Huber, Quenching of reducing properties of mesoporous silicon and its use as
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[28] J. Mareš, J. Krištofik and E. Hulicius, Influence of humidity on transport in porous silicon, Thin Solid Films 1 (1995) 272-275.
[29] S. Ma, M. Hu, P. Zeng, M. Li, W. Yan and Y. Qin, Synthesis and low-temperature gas sensing properties of tungsten oxide nanowires/porous silicon composite, Sens. Actuators B: Chem. (2014) 341-349.
[30] J. Mizsei, How can sensitive and selective semiconductor gas sensors be made? Sens. Actuators B: Chem. 2 (1995) 173-176.
[31] P. Su and T. Pan, Fabrication of a room-temperature NO2 gas sensor based on WO3 films and WO3/MWCNT nanocomposite films by combining polyol process with metal organic decomposition method, Mater Chem Phys 3 (2011) 351-357.
[32] G. Korotcenkov, The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors, Materials Science and Engineering: R: Reports 1 (2008) 1-39. Fig. 1 Schematic diagram of the process flow for the preparation of Cu2O nanofilms/PS gas sensor Fig. 2 The schematic diagram of the electrochemical etching setup Fig. 3 The schematic diagram of electrochemical deposition of Cu2O onto PS Fig. 4 The schematic diagram of the gas sensing test system Fig. 5 XRD patterns of the Cu2O nanofilms/PS samples: (a) S1; (b) S2; (c) S3; (vertical lines: JCPDS no. 05-0667)
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Fig. 6 The surface SEM images of the samples: (a) PS; (b) S1; (c) S2; (d) S3; the inset in (a), (b), (c) and (d) are the high magnification SEM images of the corresponding products Fig. 7 The cross-sectional SEM images of the samples: (a) S1; (b) S2; (c) S3; Figure (d) is the TEM images of the sample S2; the inset in (d) is the SAED image of sample S2 Fig. 8 Relationship between operating temperature and gas response to 1 ppm NO2 for Cu2O nanofilms/PS gas sensors (sample S1, S2 and S3) Fig. 9 Dynamic response of the sensors (sample PS, S1, S2 and S3) to various concentration of NO2 at the working temperature
ip t
Fig. 10 Relationship between the gas response and NO2 concentration for sample S1, S2 and S3 at the working temperature
Fig. 11 Response time and recovery time curves of the sensor S2 toward various NO2 concentrations at
cr
the working temperature
Fig. 12 Schematic diagram of the resistive-type Cu2O nanofilms/PS gas sensor
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Fig. 13 the dynamic response of Cu2O nanofilms/PS gas sensor (sample S2) to five representative cyclic exposures of 1 ppm NO2 gas
Fig. 14 Gas response of the Cu2O nanofilms/PS sensors (sample S2) to various gases at working
Ac ce pt e
d
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an
temperature
Fig.1
Schematic diagram of the process flow for the preparation of Cu2O nanofilms/PS gas sensor
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Ac ce pt e
d
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Fig.2 The schematic diagram of the electrochemical etching setup
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Ac ce pt e
d
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Fig.3 The schematic diagram of electrochemical deposition of Cu2O onto PS
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Fig.4 The schematic diagram of the gas sensing test system
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(c)
S3
30
35
ip t (220)
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(200)
Cu2O(JCPDS:05-0667)
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25
S1
(111)
(a)
cr
Si(200)
S2
(110)
Intensity(a.u.)
(b)
40
45
50
55
60
65
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2θ(°)
Fig.5 XRD patterns of the Cu2O nanofilms/PS samples: (a) S1; (b) S2; (c) S3; (vertical lines: JCPDS no.
Ac ce pt e
d
05-0667)
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Fig.6 The surface SEM images of the samples: (a) PS; (b) S1; (c) S2; (d) S3; the inset in (a), (b), (c) and
Ac ce pt e
(d) are the high magnification SEM images of the corresponding products
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Fig.7 The cross-sectional SEM images of the samples: (a) S1; (b) S2; (c) S3; Figure (d) is the TEM images of the sample S2; the inset in (d) is the SAED image of sample S2
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5.0
S1 S2 S3
4.5
3.5
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3.0 2.5
cr
2.0 1.5 1.0
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Gas Response(Ra/Rg)
4.0
0.5 0.0 25
50
75
100 125
150 175 200
an
0
Operating temperature(
225 250 275
300
)
Fig.8 Relationship between operating temperature and gas response to 1 ppm NO2 for Cu2O
Ac ce pt e
d
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nanofilms/PS gas sensors (sample S1, S2 and S3)
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4
1000ppb 400ppb
3 2
200ppb
100ppb
S3
4
0
500
1000
1500
400ppb 100ppb 200ppb
2000
2500
500
1000
2
100ppb 200ppb
1500
2000
2500
1000ppb
400ppb
0
1.2
500
100ppb
1000
1500
400ppb
200ppb
0.9 0
500
us
1
1.5
1000
1500
3000
3500
cr
0
3500
S2
2 3
3000
1000ppb
ip t
6
2000
2500
1000ppb
an
Gas Response(Ra/Rg)
1
2000
2500
S1
3000
3500
PS
3000
3500
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Time(s)
d
Fig.9 Dynamic response of the sensors (sample PS, S1, S2 and S3) to various concentration of NO2 at
Ac ce pt e
the working temperature
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5.0 4.5
S1 S2 S3
3.5
ip t
3.0
cr
2.5 2.0 1.5 1.0 200
400
600
800
an
0
us
Gas Response(Ra/Rg)
4.0
1000
1200
NO2 Concentration(ppb) Fig.10 Relationship between the gas response and NO2 concentration for sample S1, S2 and S3 at the
Ac ce pt e
d
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working temperature
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180
180
response time
160
recovery time 140
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140
Recovery ti me(s)
120
120
cr
Response time(s)
160
100
80 200
400
600
800
an
0
us
100
80
1000
NO2 concentration(ppb)
Fig.11 Response time and recovery time curves of the sensor S2 toward various NO2 concentrations at
Ac ce pt e
d
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the working temperature
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ip t cr us an
Ac ce pt e
d
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Fig.12 Schematic diagram of the resistive-type Cu2O nanofilms/PS gas sensor
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4.5
1ppm NO2
3.5
ip t
3.0
cr
2.5 2.0
us
Gas Response(Ra/Rg)
4.0
1.0 -500
0
500
1000
an
1.5
1500
2000
2500
3000
3500
4000
Time(s)
Fig.13 the dynamic response of Cu2O nanofilms/PS gas sensor (sample S2) to five representative cyclic
Ac ce pt e
d
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exposures of 1 ppm NO2 gas
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100ppm acetone
cr
100ppm ethano l
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an
1
100ppm methanal
2
10p pm H2S
3
10ppm NH 3
4
ip t
S2 5
1ppm NO2
Gas Response (Ra /Rg or Rg/Ra )
6
0
M
Gases
Fig.14 Gas response of the Cu2O nanofilms/PS sensors (sample S2) to various gases at working
Ac ce pt e
d
temperature
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Biographies: Dali Yan received his Ph.D. in microelectronics and solid-state electronics from Tianjin University in 2014. He is currently an experimentalist in college of physics and materials Science in Tianjin Normal University. His research interest is in the areas of novel nanomaterials for gas sensing applications. Shenyu Li received her M.S. in inorganic chemistry from the National University of Singapore in 2010. She is now a doctoral student at Hebei University of Technology. Her current research interest is in the area of solution chemistry.
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Ming Hu received her M.S. in microelectronics and solid-state electronics from Tianjin University in 1991. She is now a full professor in Department of Electronics Science and Technology in Tianjin University. Her research interests include MEMS, gas sensor, and sensitive materials and functional thin film devices.
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Shiyu Liu received his Ph.D. in information functional materials from Beijing University of Aeronautics and
Astronautics in 2010. He is currently an associated professor in college of physics and materials Science in Tianjin
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Normal University. His research interest is in the areas of computational condensed matter physics.
Yun Zhu received her Ph.D. in condensed matter physics from Chinese Academy of Sciences. She is currently an associated professor in college of physics and materials Science in Tianjin Normal University. Her research interest
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is in the areas of semiconductor functional thin film devices.
Meng Cao received his M.S. in condensed matter physics from Tianjin Normal University in 2008. He is currently an experimentalist in college of physics and materials Science in Tianjin Normal University. His research interest
Ac ce pt e
d
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is in the areas of nano-film materials.
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