Amperometric Formaldehyde Sensor Based on a Pd Nanocrystal Modified C/Co2P Electrode

Ultrafine Pd nanocrystals were grown on the cobalt phosphide (Co2P) decorated Vulcan XC-72 carbon (C/Co2P), which is realized by first implementing the corresponding metal precursor and then the further chemical reduction process. The as-synthesized C/Co2P/Pd composite was further constructed to form a gas permeable electrode. This electrode can be applied for formaldehyde (HCHO) detection. The results demonstrate that the Co2P nanocrystal can significantly improve the sensing performance of the C/Co2P/Pd electrode for catalytic oxidation of HCHO, which is considered to be attributed to the effective electron transfer from Co2P to Pd in the C/Co2P/Pd composites. Furthermore, the assembled C/Co2P/Pd sensor exhibits high sensitivity of 617 nA/ppm and good selectivity toward various interfering gases such asNO2, NO, SO2, CO2, andCO. It also shows the excellent linear response that the correlation coefficient is 0.994 in the concentration range of 1–10 ppm. Therefore, the proposed cost-effective C/Co2P/Pd nanocomposite, which owns advantages such as high activity and good stability, has the potential to be applied as an effective electrocatalyst for amperometric HCHO detection.


Introduction
Formaldehyde, which is a colorless and toxic volatile organic compound (VOC), is an essential material that is applied in many aspects including the chemical factories and the residence decorating field.It is also well known that the formaldehyde can significantly increase the probability to get cancer for people [1].Therefore, the detection of formaldehyde gas is significantly important.Until now, numerous technologies have been employed, such as spectrophotometry [2], gas chromatography (GC) [3], high-performance liquid chromatography [4], ion chromatography [5], polarography [6], and integrated sensor [7].Among the above-mentioned techniques, the gas sensors have been widely reported by many research groups due to the advantages of high sensitivity, fast response, high stability, small size, and cost-effective features.In addition, these small packaged sensors can be mass fabricated easily in order to satisfy the large demand in many industrial fields.Generally, there are two types of formaldehyde gas sensor: (1) semiconductors formaldehyde gas sensor based on metal oxide such as SnO 2 [8,9], ZnO [10], In 2 O 3 [11,12], and NiO [13] and (2) amperometric formaldehyde gas sensor made from noble metal nanomaterial, such as platinum or gold [14,15].In contrast with semiconductor gas sensor, the amperometric formaldehyde gas sensors have numerous advantages such as lower power consumption, excellent linear response, high sensitivity, and reasonable selectivity at room temperature.
Due to their high catalytic activity and excellent chemical stability, Pt-based catalysts are generally used in amperometric gas sensor.However, the scarcity and high cost of platinum greatly hinder its application.Much effort has been devoted to developing the platinum-free electrocatalysts, among which palladium-containing catalysts have been proven to be an effective candidate [16,17].The C/Pd catalyst is a good choice, due to its low cost and competitive intrinsic electrocatalytic activity [18].Furthermore, it is reported that the addition of another element, such as Fe, Co, Ni, N, or P, can enhance the catalytic activity of the C/Pd catalyst [19][20][21][22].Unfortunately, the catalytic performance decreases rapidly by the dissolution or instability of the addition elements.It is well known that the transition metal phosphide nanoparticles such as Ni 2 P, CoP, and FeP can significantly improve the catalytic activity and stability of catalyst [23][24][25].But only a few works were reported about metal phosphide nanoparticles decorated Pd/C hybrids.
In this work, an amperometric formaldehyde gas sensor was fabricated by using the C/Co 2 P/Pd catalyst as active electrode in H 2 SO 4 electrolyte.The obtained C/Co 2 P/Pd sensor exhibits better sensing performance for HCHO detection than that of the C/Co 2 P and C/Pd sensor.

Preparation of Co 2 P Modified
Pd/C Composites.Pd modified C/Co 2 P composite was synthesized by solid phase reaction and ethylene glycol reduction according to a reported method [18,23,26,27] and the corresponding schematic illustration is shown in Figure 1.Typically, 0.5 g Vulcan XC-72 was added to 20 mL aqueous solution containing 0.664 g CoCl 2 .Then the mixed solution was stirred overnight followed by drying at 120 ∘ C for 5 h.The resulting Vulcan XC-72 containing CoCl 2 and 1.46 g NaH 2 PO 2 ⋅H 2 O was mechanically mixed in a quartz boat at room temperature.The precursor was directly heated to 250 ∘ C and was kept for 1 h under N 2 flowing condition.Then it was cooled to room temperature.The resulting C/Co 2 P composite was passivated in a 1.0 mol% O 2 /N 2 mixture for 3 h.The obtained sample was washed with deionized water and was dried at 80 ∘ C for 24 h.
The 41.6 mg PdCl 2 and 100 mg C/Co 2 P composites were dispersed into ethylene glycol (EG, 60 ml) followed by sonicating for 1 h under ambient condition.Then the solution was further stirred for 5 h.Its pH value was adjusted to 10 by 1 M NaOH solution.Then the mixture was placed in water bath and was heated at 70 ∘ C for 3 h with vigorous stirring.After reaction, its pH value was adjusted to 3 by adding hydrochloric acid and then incubated overnight.The resulting product was collected by vacuum filtration and was washed with hot deionized water and ethanol, and the resulting C/Co 2 P/Pd composites were dried at 80 ∘ C for 12 h.For comparison, the C/Pd composite was prepared under the same condition.

Preparation of Modified Electrodes.
The glass carbon electrodes (GCE,  = 3 mm) surfaces were carefully polished to a mirror using alumina slurries with different diameter (1.0, 0.3, and 0.05 mm) and then thoroughly cleaned ultrasonically with ethanol and deionized water.Subsequently, 5 L of homogeneous C/Co 2 P, C/Pd, and C/Co 2 P/Pd dispersion (5 mg mL −1 ) was dropped onto the surface of GCEs, respectively.After being dried at room temperature for 24 h, three modified GCEs were obtained and denoted as C/Co 2 P/GCE, C/Pd/GCE, and C/Co 2 P/Pd/GCE.Each working of C/Pd and C/Co 2 P/Pd electrodes contained ca. 15 g/cm 2 of Pd.

Fabrication and Measurement of Gas
Sensor.Amperometric HCHO sensor was fabricated by the method depicted in Figure 2. Circular working electrode (diameter: 1 cm, area: 0.785 cm 2 ) was prepared as follows: 2.0 mg of catalyst was dispersed in ethanol (0.25 mL) containing 1 L of 0.5 wt% Nafion ethanol solution to form homogenous suspension by sonication.The mixture was sprayed onto the porous PTFE membrane (PM 28Y, Porex) and was dried at room temperature.The Pd density of the C/Co 2 P/Pd electrode was 0.667 mg/cm 2 .The platinum wire counter electrode and Ag/AgCl (in 3 M KCl) reference electrode were then inserted into the sensor housing.The electrolyte is 0.5 M H 2 SO 4 .The distances among the three electrodes are all equal to 6 cm.

Characterizations. Transmission electron microscope
(TEM), high-resolution TEM (HRTEM) (TEM, JOEL TEM-2010), scanning electron microscope (SEM, Phenom ProX), and energy-dispersive X-ray (EDX, Phenom ProX) were employed to inspect the morphologies and composition of samples.X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB-MKII X-ray photoelectron spectrometer (VG Co.) with Al Ka X-ray radiation as the Xray source for excitation.Cyclic voltammetric (CV) and the gas sensing properties were performed by a CHI660C electrochemical workstation (CHI).A conventional three-electrode cell comprises a modified GCE or prepared membrane as working electrode, a platinum wire as the auxiliary electrode, and Ag/AgCl (in 3 M KCl) as reference electrode.All the gas sensors were polarized in 0.5 M H 2 SO 4 solution for 24 h at 0.75 V prior to amperometric measurements.Amperometric responses were measured to various gases of HCHO, NO 2 , NO, SO 2 , CO 2 , and CO at the applied potential of 0.75 V (versus Ag/AgCl).The standard gases of HCHO, NO 2 , NO, SO 2 , CO 2 , and CO were purchased from Dalian Special Gases Co., Ltd.The detection gas was mixed by using a mass flow controller which was linked to one N 2 chamber and a HCHO standard chamber.Different gas concentration can be obtained by modulating the mass flow controller digitally.including reversible H upd region (−0.2V-0.1 V) and Pd oxidation/reduction regions.Interestingly, compared with curve B, peak current of curve C is significantly increased, which is attributed to the strong electronic interaction between Co 2 P and Pd. Figure 6(b) is the CV curve of 5 mM formaldehyde in 0.5 M H 2 SO 4 solution at the C/Co 2 P/Pd/GCE.In the anodic sweep from 0.2 to 0.9 V, the oxide peak appears at 0.75 V attributed to the oxidation of CO ads intermediate to CO 2 on Pd surface during the formaldehyde oxidation.In the successive cathodic sweep, the current peak observed at 0.38 V was due to the reduction of palladium oxide [31].

Electrochemical Properties of the C/Co 2 P/Pd Composites Electrodes.
In order to compare the catalytic activity of the C/Co 2 P/ GCE, C/Pd/GCE, and C/Co 2 P/Pd/GCE, CVs of 50 mM formaldehyde in 0.5 M H 2 SO 4 solution at the three electrodes were measured, which are shown in Figure 7(a).It is found that the C/Co 2 P/Pd/GCE exhibits the relative negative oxidation potential and large current response for a given formaldehyde concentration, indicating that the C/Co 2 P/Pd/GCE has much higher catalytic activity toward the oxidation of formaldehyde than C/Co 2 P/GCE and C/Pd/GCE.Figure 8(a) shows the CVs with different scan rates at the C/Co 2 P/Pd/GCE in the same solution.The result reveals that as the scan rate increased, the forward oxidation peak currents increased and their peak potentials shifted slightly toward positive directions.In the case of backward potential sweep, increasing in the sweeping rate can cause the peak potential to shift negatively and peak currents increased.The peak current toward formaldehyde oxidation as function of the square root of the scan rate (V 1/2 ) in the range of 10 to 150 mV s −1 is shown in Figure 8(b).A good linear relationship with correlation coefficient of 0.9985 between current response and square root of the scan rate (V 1/2 ) indicates a diffusion-controlled electron transfer process.which indicates that the reaction on the C/Co 2 P/Pd sensor was limited by the diffusion rate of HCHO gas [32].On the basis of Fick's diffusion law, the limiting current  lim was shown by the following equation [33]:

Properties of the
where the current is directly proportional to the gaseous concentration.It further indicates that the response is under the diffusion control at steady state.Figure 9(c) is the enlarged parts of Figure 8(a).It shows that when the C/Co 2 P/Pd sensor is exposed to HCHO gas, its current increases quickly, and it will quickly recover in the absence of HCHO.The response and recovery time are 3.8 s and 23.1 s, respectively, which are much shorter than those of the C/Pd and C/Co 2 P sensor.The sensitivity of the C/Co 2 P/Pd sensor is 617 nA/ppm.The background noise level is about 50 nA, which is confirmed by a previous report [34].The limit of detection (LOD) of the C/Co 2 P/Pd sensor is about 0.25 ppm (S/N = 3).The reproducibility of the HCHO gas sensor was evaluated in 10 ppm HCHO/N 2 mixture gas at six sensors prepared under the same condition.The RSD ( = 10) of the peak current is 3.25%.To evaluate the operational reproducibility of the C/Co 2 P/Pd sensor, - curves were determined by 10 successive measurements in 10 ppm formaldehyde flow.The obtained RSD is 2.56%, which indicates that the C/Co 2 P/Pd sensor can be used for routine analysis of HCHO in clinical use.The stability of the electrochemical sensor was also studied.After continuous 30 times' measurements, the current response almost does not decrease.The amperometric response with long time test for the C/Co 2 P/Pd sensor was studied under continuous 10 ppm formaldehyde flow.No obvious decrease is observed in 400 s, indicating the good stability of the C/Co 2 P/Pd sensor (Figure S2).
In order to investigate the selectivity of the C/Co 2 P/Pd gas sensor, the amperometric measurements were performed under various testing gases such as HCHO, NO 2 , NO, SO 2 , CO 2 , and CO. Figure 9(d) shows the current response of the C/Co 2 P/Pd sensor upon exposure to 10 ppm HCHO, 10 ppm NO 2 , 10 ppm NO, 10 ppm SO 2 , 5000 ppm CO 2 , and 50 ppm CO.The detection selectivity, defined as the relative ratio of the response current of 10 ppm HCHO to that of 10 ppm NO 2 , 10 ppm NO, 10 ppm SO 2 , 5000 ppm CO 2 , and 50 ppm CO, was 9.3, 10.0, 33.5, 13.2, and 40.5, respectively.The as-prepared C/Co 2 P/Pd sensor exhibits excellent selectivity for HCHO detection [33].

Conclusions
The C/Co 2 P/Pd hybrids were successfully synthesized through a two-step method and further applied for fabricating a sensor for detecting HCHO.The results show that the C/Co 2 P/Pd based sensor has high sensitivity of 617 nA/ppm and its LOD can be determined as 0.25 ppm.Meanwhile, it also owns a good linear response in the concentration range of 1-10 ppm.The good performance of C/Co 2 P/Pd gas sensor can be analytically attributed to the fast electron transfer from Co 2 P to Pd nanocrystal.The proposed work provides an alternative way for developing the cost-effective catalyst for fabricating HCHO gas sensor.

Figure 6 (Figure 5 :
Figure 5: SEM image (a); element mapping images (b)-(f) for C, Pd, O, Co, and P in the C/Co 2 P/Pd composites and EDX spectrum of the C/Co 2 P/Pd composites.
Figure 7(b) shows CV responses of the C/Co 2 P/Pd/GCE toward formaldehyde with different concentrations.It is clearly shown that the peak current of formaldehyde increases with the rise of formaldehyde concentration from 0 to 50 mM.

Figure 8 :
Figure 8: (a) CVs of the C/Co 2 P/Pd/GCE in 5 mM formaldehyde of 0.5 M H 2 SO 4 solution with different scan rates (10, 20, 40, 60, 80, 100, 120, and 150 mV/s).(b) The relation of the oxidation current of formaldehyde with square root of scan rate.

Figure 9 :
Figure 9: (a) Amperometric current responses of the C/Pd (black line), C/Co 2 P composite (blue line), and C/Co 2 P/Pd (red line) composites based gas sensors to 3 ppm formaldehyde.(b) Typical dynamic current response curves of C/Co 2 P/Pd sensor to formaldehyde ranging from 1 ppm to 10 ppm (inset: calibration curve).(c) Enlarged part of the current responses to 3 ppm HCHO gas of the C/Pd (red line), C/Co 2 P composite (green line), and C/Co 2 P/Pd (black line) electrode based gas sensors.(d) Bar plot of current response of the C/Co 2 P/Pd composites based formaldehyde sensor to different interfering gases under exposure to 10 ppm formaldehyde, 10 ppm NO, 10 ppm NO 2 , 10 ppm SO 2 , 5000 ppm CO 2 , and 50 ppm CO.Applied potential to working electrode was +0.75 V (versus Ag/AgCl).
[23] P composite (blue line), and C/Co 2 P/Pd (red line) sensor.It clearly shows that the C/Co 2 P/Pd sensor exhibits a higher current response than that of the C/Pd and C/Co 2 P sensor.The enhanced activity of C/Co 2 P/Pd may be attributed to the electron transfer from Co 2 P to Pd, which was demonstrated by XPS (FigureS1in Supplementary Material available online at https://doi.org/10.1155/2017/2346895)andpreviouswork[23].Typical dynamic current response for the C/Co 2 P/Pd sensor was conducted under different formaldehyde concentrations (Figure9(b)).It is seen that the peak Amperometric Gas Sensor. Figure 9(a) presents the amperometric current response of the C/Pd (black line), C/