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Article

Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide

by
Chuxuan Pan
1,2,3,4,
Yuanyuan Zheng
1,2,3,4,
Jing Yang
1,
Dongyang Lou
1,2,3,4,
Jian Li
1,2,3,4,
Yujing Sun
1,* and
Wei Liu
1,2,3,4,*
1
School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510006, China
2
The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University, Guangzhou 510006, China
3
Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Sun Yat-sen University, Guangzhou 510006, China
4
State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510006, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(5), 528; https://doi.org/10.3390/catal12050528
Submission received: 5 April 2022 / Revised: 4 May 2022 / Accepted: 5 May 2022 / Published: 8 May 2022
(This article belongs to the Special Issue Metal-Based Aerogels and Porous Composites as Efficient Catalysts: Synthesis and Catalytic Performance)
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Abstract

:
Hydrogen peroxide (H2O2) plays an indispensable role in the biological, medical, and chemical fields. The development of an effective H2O2 detecting method is of great importance. In the present work, a series of PtxPdy bimetallic aerogels and Pt, Pd monometallic aerogels were controllably synthesized by one-step gelation method. Their morphologies and compositions were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and so forth. These aerogels were used as nonenzyme electrocatalysts for the detection of H2O2. The cyclic voltammetric and amperometric results demonstrated that the performance of the metal aerogels showed volcano-type behavior, with the Pt50Pd50 aerogel sitting on top. The Pt50Pd50 aerogel-based electrochemical sensor exhibited excellent comprehensive performance, with a low overpotential of −0.023 V vs. Ag/AgCl, a broad linear range from 5.1 to 3190 μM (R2 = 0.9980), and a high sensitivity of 0.19 mA mM−1 cm−2, in combination with good anti-interference ability and stability. A comprehensive study indicated that the superior sensing performance of the Pt50Pd50 aerogel is closely related to its optimized d-band center and larger cumulative pore volume. This work first applied Pt–Pd bimetallic aerogels into the detection of H2O2 and shows the promising application of noble metal aerogels in the electrochemical sensing area.

Graphical Abstract

1. Introduction

Hydrogen peroxide (H2O2), as one of the most important reactive oxygen species (ROS) in the metabolism of organisms, is considered to be a biomarker due to its vital function in aging and disease [1,2]. H2O2 is not only the intermediate or final product in wide fields, such as wastewater treatment, bleaching, mining, metal processing, and other chemical engineering fields [3,4,5], which may cause diseases and explosion due to its oxidative and corrosive properties [6]. Therefore, developing effective and accurate methods to detect H2O2 is of great importance [7].
Up to now, various analytical techniques have been used to detect H2O2, such as spectrophotometry, electrochemistry, and chromatography [8,9]. Among various analytical techniques for H2O2 detection, the electrochemical method is considered to be the most outstanding one due to its great advantages, such as simplicity, low cost, rapid response, high sensitivity, and good selectivity [10]. The core materials of electrochemical sensors are electrocatalysts, among which bimetallic nanocatalysts, such as Pd–Cu, Au–Pd, Pt–Pd, Au–Pt, and Pt-Ni, with various morphologies or supported on various supporting materials have been investigated extensively for sensing H2O2 [11,12,13,14,15,16,17,18], because of their prominent electronic and catalytic properties over their monometallic counterparts resulting from the synergetic effect [19]. Despite great efforts made, it is still a great challenge for such materials to meet simultaneously the requirements of electrochemical sensors for practical application from the points of view of low detection limit, broad linear range, high sensitivity, good selectivity, good reproducibility, and good stability.
Noble metal aerogels (NMAs) own unique three-dimensional (3D) self-supported solid networks with interconnected macropores or mesopores filled with air [20]. Combining the structural features of aerogels and physicochemical properties of noble metals, the NMAs simultaneously combine numerous active sites, rapid electron transfer paths, and fast mass transport channels, which render them highly effective electrocatalysts [21]. Although NMAs have distinguished themselves in electrocatalysis [22,23,24,25,26,27,28,29,30,31,32] (e.g., methanol/ethanol oxidation reaction (M/EOR), carbon dioxide reduction reaction (CO2RR), oxygen reduction reaction (ORR), formic acid oxidation, glucose oxidation), using them as electrochemical nanocatalysts for H2O2 detection is still in its infancy, and a lot more treasures need researchers to unlock.
Here, electrochemical sensors for direct detection of H2O2 were fabricated by Pt, Pd monometallic, and PtxPdy bimetallic aerogels (Pt20Pd80, Pt50Pd50, and Pt80Pd20), which were synthesized via a simple one-step gelation method. For the first time, Pt, Pd, and PtxPdy aerogels were used as electrocatalyst to detect H2O2. The electrochemical performance of the Pt, Pd, and PtxPdy aerogels showed volcano-type behavior, with the Pt50Pd50 aerogel sitting on top. The Pt50Pd50 aerogel-based electrochemical sensor exhibited excellent comprehensive performance for the detection of H2O2, with a low overpotential of −0.023 V vs. Ag/AgCl, a broad linear range from 5.1 to 3190 μM (R2= 0.9980), and a high sensitivity of 0.19 mA mM−1 cm−2, in combination with good stability and good anti-interference ability.

2. Results

2.1. Physicochemical Properties of PtxPdy Bimetallic Aerogels and Pt, Pd Monometallic Aerogels

The PtxPdy bimetallic aerogels and the Pt and Pd monometallic aerogels were prepared through a simple one-step gelation method, followed by supercritical drying, as shown in Scheme 1. The morphologies of the catalysts used in the H2O2 sensor fabrication was characterized by transmission electron microscopy (TEM). As shown in Figure 1, the PtxPdy aerogels with different constituents and the Pt and Pd monometallic aerogels all have semblable porous three-dimensional network structures with interconnected metal nanowires. The average diameters of the ligaments are 4.4 ± 1.3, 4.4 ± 1.2, 4.9 ± 1.3, 4.6 ± 1.6, and 7.4 ± 2.0 nm for the Pt aerogel, Pt80Pd20 aerogel, Pt50Pd50 aerogel, Pt20Pd80 aerogel, and Pd aerogel, respectively. The corresponding statistical histogram is shown in Figure S1.
X-ray diffraction (XRD) was used to reveal the crystalline structures of the aerogels, and the results are shown in Figure 2a; the diffraction peaks at about 39°, 46°, 67°, 81°, and 86° corresponded with the (111), (200), (220), (311), and (222) planes of the face-centered cubic (fcc) structure of Pt and Pd (JCDPS card No.: 04-0802 and No.: 46-1043), respectively. As the metal Pt and Pd have the same crystal structure and a small lattice mismatch (Pt: a = 3.93 Å; Pd: a = 3.89 Å), the diffraction peaks of the two metals are overlapped. However, as observed from Figure 2a, the diffraction peaks shift slightly to the right with the increasing proportion of Pd in the PtxPdy bimetallic aerogels. This can be ascribed to the smaller lattice constant of Pd, which is incorporated into the lattice of Pt, leading to the decreased cell parameters and crystal plane spacing, which may introduce compressive strain [33]. It has been previously reported that the appropriate strain existing in Pt-based alloy systems can optimize the Pt d-band center and therefore enhance the catalytic activity [34].
Energy dispersive spectroscopy (EDS) (Figure 2b) and X-ray photoelectron spectroscopy (XPS) (Figure 3a–f) were used to reveal the composition and surface electronic properties of bimetallic aerogels, respectively. The results are summarized in Table 1. It can be seen that the compositions of bimetallic aerogels are in good agreement with the feed ratio.
The high-resolution Pd 3d and Pt 4f XPS spectra are shown in Figure 3a–f. Obviously, Pt 4f spectra can be deconvoluted into four peaks: Pt 4f7/2 and Pt 4f5/2 at 70.9 and 74.2 eV can be ascribed to Pt0; meanwhile, Pt 4f7/2 and Pt 4f5/2 peaks at 72.1 and 75.4 eV correspond to PtⅡ. Analogously, Pd 3d spectra are deconvoluted into four peaks: the peaks at 335.2 and 340.5 eV can be ascribed to Pd 3d5/2 and Pd 3d3/2 of Pd0, respectively, while the peaks at 335.8 and 341.1 eV arose from Pd 3d5/2 and Pd 3d3/2 of PdⅡ, respectively [30]. The peak neighboring to the Pd 3d5/2 appearing at 331 eV is Pt 4d3/2 (Figure 3c,e) [35].
The surface compositions of PtxPdy and Pt, Pd aerogels were also calculated semiquantitatively (Table 1). On the surface of PtxPdy aerogels, about 20–30% Pt element is PtⅡ, while 16–38% Pd element is PdⅡ. The proportion of Pt in the surface region is higher than the result obtained from EDS, which may be caused by the faster reduction rate of the Pd metal salt precursor [35].
Surface valance band spectra collected by XPS were used to calculate the d-band center of the Pt, Pd monometallic aerogels and PtxPdy bimetallic aerogels. It can be seen from Figure 3g that the calculated d-band center for Pt, Pt80Pd20, Pt50Pd50, Pt20Pd80, and Pd aerogels gradually shifts down from 3.27 to 2.47 eV with the increase in Pd content.

2.2. Electrocatalytic Reduction of H2O2 by the Metal Aerogel Catalyst Modified Glassy Carbon Electrodes

The performance of PtxPdy aerogels on glassy carbon electrodes (GCEs) toward the electrocatalytic reduction reaction of H2O2 (H2O2RR) was investigated in detail by cyclic voltammetry (CV). Figure 4a shows the electrocatalytic current response of the Pt50Pd50 sensor in the absence and presence of H2O2 between 0 and 2.0 mM within −0.64−0.96 V vs. Ag/AgCl. It is evident that excellent electrocatalytic response can be observed, and electrochemical reduction current at about −0.15 V gradually increased with the increasing concentration of H2O2.
Figure 4b shows the electrocatalytic responses of all the as-prepared catalysts at a concentration of 1 mM H2O2, and a summary of the results is shown in Table 2. It should be noted that the CV background signals in 0.1 M PBS were subtracted. Pd, Pt20Pd80, and Pt50Pd50 aerogels all display very low overpotentials at about 0.004, −0.014, and −0.023 V, respectively. However, the Pt50Pd50 aerogel displays the highest peak current, about 27 μA, while Pt20Pd80, Pt80Pd20, Pt, and Pd aerogels display 20, 23, 22, and 17 μA, respectively. The H2O2 reduction catalytic performances of the as-prepared aerogels show volcano-type behavior, and the superiority of the Pt50Pd50 aerogel catalyst can be ascribed to the modulated electronic structure and bimetallic synergistic effect with an optimized d-band center.
Figure S3 shows the effect of scan rates on current. The reduction peak currents were proportional to the square root of the scanning rate, and the linear regression equation is Y = 0.98 × X−10.02 (R2 = 0.994). These results indicate that the electroreduction process is a diffusion-controlled process [15].

2.3. Effect of pH in H2O2 Detection by the Pt50Pd50 Aerogel Catalyst Modified Glassy Carbon Electrode

In electrochemical process, pH may have an important impact on the electrochemical reaction. Consequently, in our study, an experiment was conducted to evaluate this effect in H2O2 detection using the prepared Pt50Pd50 aerogel modified glassy carbon electrode. As part of this study, cyclic voltammetry responses of the Pt50Pd50 aerogel in PBS solutions with different pH comprising 1 mM concentration of H2O2 at a constant scan rate of 50 mV s−1 was recorded, and the result is shown in Figure 5. As we can see, the response currents display “Volcanic type” behavior, while the peak potentials have no obvious rule. The Pt50Pd50 aerogel in PBS solutions with 7.4 pH showed the highest response current and lowest peak potential. Additionally, the atmosphere of pH 7.4 provides a possibility for biological applications. Hence, we selected pH 7.4 PBS as electrolyte for further sensing performance evaluation.

2.4. Amperometric Sensing of H2O2 by PtxPdy Bimetallic Aerogels and Pt, Pd Monometallic Aerogels

Figure 6 displays the current-time (i-t) responses of Pt50Pd50 aerogel and other aerogels to the successive addition of different concentrations of H2O2 into N2-saturated 0.1 M PBS solution under continuous stirring at a working potential of −0.05 V. All sensors exhibit steplike linear responses, and the responses are fast and the sensors could achieve 95% of the steady-state current within about 5 s. Compared with other catalysts, the Pt50Pd50 aerogel display the highest sensitivity of 0.190 mA mM−1 cm−2 (Figure 6b) and the lowest limit of detection (LOD) of 2.21 μM on a signal-to-noise (S/N) ratio of 3, while the sensitivities of the Pt80Pd20, Pt20Pd80, Pt, and Pd aerogels are 0.160, 0.114, 0.143, and 0.047 mA mM−1 cm−2, respectively (Figure 6c)). Their LOD ranges from 2.73 to 21.3 μM. The linear ranges of all the aerogel catalysts cover three or four orders of magnitude with a high correlation coefficient (≥0.998).
Figure 7 shows the relationship between the atom ratios of Pt and Pd in the PtxPdy bimetallic aerogels and their electrochemical sensing performances, including the sensitivity, LOD, and potential of reduction peak (Epc), as well as the physical properties, including the d-band center, BET surface area, and cumulative pore volume. Compared with the other as-prepared aerogel sensors, the Pt50Pd50 bimetallic aerogel-based sensor is the optimal one, showing higher sensitivity, broader linear range, and lower detection limit in H2O2 detection. The excellent electrocatalytic response of the Pt50Pd50 aerogel is closely related to the synergy bimetallic effect. Based on the d-band center energy theory (εd) [36], milder binding energy to the reaction intermedia is beneficial for the release of products and adsorption of reactants, thus leading to better catalytic performance. The appropriate downshifted d-band center of Pt50Pd50 aerogels caused by alloying effect and electronic effect might be one reason for the higher sensing performance of the Pt50Pd50 aerogel than other PtxPdy aerogels [27,37]. As for the influences of BET surface area, pore size distribution, and cumulative pore volumes, it is found that there are no direct influences of BET surface area and pore size distribution [30] on sensing performance; however, the changing tendency of cumulative pore volume with Pt/Pd ratio is the same as the PtxPdy aerogels’ sensitivity and contrary to the onset potential. The biggest pore volume may also be one of the reasons for the better sensitivity and lower onset potential of the Pt50Pd50 aerogel than the other PtxPdy aerogels. Meanwhile, the advanced structure of NMAs, including the high hierarchically porous 3D network, interconnected nanowires, faster electron and mass transport, more abundant active sites, and excellent structural stability, should be also considered.
A comparison between the as-prepared electrochemical sensors based on PtxPdy bimetallic aerogels and Pt, Pd monometallic aerogels with other Pt-containing and Pd-containing monometallic or bimetallic ones reported in the literatures for the determination of H2O2 is shown in Table 2.
Compared with the reported Ag wire aerogel sensor [38], our PtxPdy bimetallic aerogels and Pt, Pd monometallic aerogel catalysts here have a much wider linear range, a much lower working potential, and a much faster reaction rate, which may be ascribed to the more active composition of Pt and Pd bimetal with better catalytic activity and their thinner size. When compared with other reported bimetallic catalysts containing Pt and Pd elements, the Pt50Pd50 aerogel has lower overpotential and a generally wider linear range. In the previous reports, the Pt and Pd containing catalysts are mostly combined with various supporting materials, such as graphene, carbon nanotubes, and MoS2 [12,39,40]. Using support materials is to make use of the large specific surface area of their conductive network structure to anchor the metal catalyst particles or clusters [15,41]. However, most of the support materials have no catalytic properties, leading to slower kinetics and higher working potential. Our metal aerogel catalysts own a self-supporting metallic porous conductive network structure, which can provide a large specific surface area and a large amount of catalytic active sites, fast electron transfer and mass transport, and correspondingly a much lower overpotential and a generally wider linear range. Even though loadings of metal aerogels on electrodes are very low, the sensors can maintain generally very good sensitivity.

2.5. Selectivity, Reproducibility, Repeatability, and Stability of the As-Prepared Sensors

Selectivity is an important factor for nonenzymatic sensors. The possible interferences with H2O2, such as glucose (GLU), saccharose (SUC), and ascorbic acid (AA), in Pt50Pd50 aerogel were investigated. As shown in Figure 8a, after injecting 2 μM H2O2 into 20 mL N2-saturated 0.1 M PBS aqueous solution, 2 mM of the above interfering materials was also injected successively. There is no significant change and influence on the current curves and the subsequent response for another 2 μM H2O2 injected later.
Five different Pt50Pd50-modified electrodes prepared under the same conditions were used to test the reproducibility of this sensor by measuring the current responses to a concentration of 1 mM H2O2. Obviously, Figure 8b shows an excellent reproducibility of the aerogel sensor with the relative standard deviation (RSD) of 3.26%.
Moreover, the stability of the Pt50Pd50 aerogel sensor was also evaluated. As shown in Figure 8c, after 6 days of storage at room temperature and air atmosphere, the response current of the sensor could be kept at least at 90%. Additionally, the repeatability of the Pt50Pd50 sensors was evaluated by testing responses to continuous addition of 2 mM (50 μL) H2O2 into 20 mL N2-saturated PBS. As can be seen in Figure S2, the sensor had good repeatability, and the RSD of the response current was 6.3%. These results indicate that the Pt50Pd50 aerogel sensor for detecting H2O2 has great selectivity, reproducibility, repeatability, and stability.

3. Materials and Methods

3.1. Reagents and Chemicals

Chloroplatinic acid hexahydrate (H2PtCl6·6H2O), potassium chloropalladite (K2PdCl4, 99.95%), sodium borohydride (NaBH4, 98%), disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O,99%), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), and L-ascorbic acid (AA, ≥99%) were purchased from Aladdin. D- (+) glucose (GLU) and sucrose (SUR) were obtained from Macklin. Gamma alumina powder (0.3 μm) was received from Gaoss Union. Hydrogen peroxide (30%) was bought from Guangzhou Chemical Reagent Factory. Milli-Q water (18.2 MΩ cm−1) was used in the aqueous solutions through all the experiments. The phosphate buffer solution (PBS, pH = 7.4) was prepared by mixing NaH2PO4·2H2O (0.1 M) and Na2HPO4·12H2O (0.1 M) aqueous solution.

3.2. Synthesis of PtxPdy Bimetallic Aerogels and Pt, Pd Monometallic Aerogels

NMAs were synthesized based on the reported one-step gelation method [30] with slight modification. For the bimetallic aerogels, taking the Pt50Pd50 aerogel as an example, typically, 13.0 mg K2PdCl4 (final reacting concentration of 0.1 mM) and 227.7 μL of 0.176 M H2PtCl6·6H2O (final reacting concentration of 0.1 mM) were added to 395 mL deionized water and stirred for 10 min. Then, 5 mL of fresh 40 mM NaBH4 solution (final reacting concentration of 0.5 mM) was injected rapidly into the previous mixed metallic salt aqueous solution under a disturbing ambient, and then the color of the solution changed rapidly from bright yellow to gray black. The solution was kept being stirred for another 30 min, then settled still for about 6–12 h; a black Pd50Pt50 hydrogel would deposit at the bottom of the container. The ratio of metal elements in hydrogels can be controlled by changing the proportion of metal salts.
The Pt aerogel was prepared in this way: a 0.2 mM Pt metal salt precursor was added into 395 mL deionized water and stirred for 10 min. Then, 5 mL of fresh 80 mM NaBH4 solution was injected into the metallic salt solution, and the mixed solution was stirred for 30 min. Finally, the solution was settled down for gelation for about 2 days. The Pd hydrogel was prepared in the same way, while the obtained mixed solution was settled for about 12 h.
The obtained hydrogels were exchanged with water for 6–8 times, then exchanged with absolute ethanol for at least 9 times to gain an anhydrous atmosphere. The resulting wet gels were transferred into a critical point dryer, and liquid CO2 was used as medium for the supercritical drying. Finally, the dry aerogels were taken.

3.3. Apparatus

XRD was performed on a Rigaku SmartLab X-ray diffractometer operated at 40 kV and 40 mA with Cu-Kα radiation (λ = 1.5405 Å). The data were collected in the 2θ ranges of 10–90° with a step size of 10°. TEM and EDS were carried out on a Bruker Nano GmbH (Germany) Quantax transmission electron microscope operated at 120 kV. XPS was carried out on a Thermo Fisher Scientific instrument with a mono Al Kα source (hν = 1486.6 eV). Charging effect-induced peak shifts were corrected using the binding energy (BE) of the C1s peak of the substrate (284.8 eV). To estimate the d-band center of the aerogels, the following equation was used [27]:
ε d = ∫   N ( ε ) ε d ε ∫   N ( ε ) d ε
where N(ε) is the intensity of the surface valance band spectra after Shirley background correction. The surface valance band spectra were collected by XPS.
The nitrogen physisorption isotherm of the Pt aerogel was measured at 77 K on a Quantachrome Autosorb iQ2 instrument via the same process as reported in the literature [30]. About 50 mg of the as-prepared aerogel sample was transferred into the measuring cell and degassed overnight at 323 K under vacuum before measurement. The specific surface area was calculated by using the multipoint Brunauer–Emmett–Teller (BET) equation (0.05 < P/P0 < 0.3), and the pore size distribution was determined from the isotherm using the Barrett–Joyner–Halenda (BJH) theory from the desorption branch. Pore volume was obtained for pores smaller than about 140 nm. BET surface areas and cumulative pore volumes of other PtxPdy aerogels were cited from the literature [30].

3.4. Electrochemical Performance Tests

The electrochemical performance of the prepared catalysts was studied by using a CHI760E electrochemical station (Chenhua Instruments Co., Ltd., Shanghai, China) in a standard three-electrode electrolytic system including a Pt foil (1 cm × 1 cm) counter electrode, a Ag/AgCl (3.0 M KCl) reference electrode, and a glassy carbon working electrode (GCE, 3 mm in diameter). All the electrochemical experiments used 20 mL 0.1 M PBS as electrolyte.
Typically, all the electrochemical catalytic inks were prepared by the following steps: First, 1.0 mg catalyst aerogels were dispersed in 1.0 mL water. Then the mixture was ultrasonicated for 45–90 min to create a stable homogeneous black solution. After that, the solution was further diluted nearly 117.6 times (42.5 μL ink was diluted with 4957.5 μL water). Finally, the ink was stored in the refrigerator (4 °C) for 12 h. Before modification, the GCEs were polished with a 0.3 μm alumina slurry. Then, the GCEs were successively ultrasonicated in deionized water, absolute ethanol, and deionized water. Finally, they were blown dry by nitrogen stream to create a mirrorlike surface. Modified working electrodes were prepared by simply dropping coating 4 μL ink, which had been ultrasonicated again for 20 min onto the cleaned GCE surface, followed by drying up in air.
Before every electrochemical test, PBS electrolyte was bubbled with N2 flow for about 30 min to get N2-saturated electrolyte. The modified working electrode was scanned from −0.64 to 0.96 V (vs. Ag/AgCl, scan speed: 50 mV s−1) in N2-saturated PBS until stable cyclic voltammetry (CV) response was obtained.

4. Conclusions

In conclusion, a series of PtxPdy bimetallic aerogel and Pt, Pd monometallic aerogel-based electrochemical sensors for the direct determination of H2O2 were developed. The catalytic performance of PtxPdy aerogels shows volcano-type behavior with the increase in Pd. The “volcano top”: Pt50Pd50 aerogel showed the highest sensitivity of 0.19 mA mM−1 cm−2, a broad linear range of 5.1–3190 μM, and a lower detection limit of 2.21 μM. Compared with the sensors based on the composite of Pt–Pd and supporting materials, the Pt50Pd50 aerogel sensor showed much lower overpotential and a generally wider linear range. The superiority of the Pt50Pd50 aerogel sensor can be ascribed to the synergetic bimetallic effect aroused by the introduction of the second metal, which tailors the electronic properties of the catalyst and abundant catalytic sites, fast electron transfer, and superior mass transportability provided by the self-supported 3D porous network structure of the metal aerogel. This work demonstrates the merit of bimetallic effects and the advanced porous structure of metal aerogels, frees the nanocatalysts of substrate materials, and extends the application field of noble metal aerogels.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12050528/s1, Figure S1: Histograms of TEM ligaments sizes of (a) Pd, (b) Pt, (c) Pt20Pd80, (d) Pt50Pd50, (e) Pt80Pd20 aerogels; Figure S2: Responses of Pt50Pd50 aerogel sensor of continuous addition of 2 mM (50 μL) H2O2 into 20 mL N2-satuated PBS; Figure S3: The effect of different scan rates (30 mV s−1, 50 mV s−1, 70 mV s−1, 100 mV s−1, 200 mV s−1) on the cathodic current of H2O2 on Pt20Pd80 aerogel. The inset picture was the linear regression of reduction currents and square root of the scan rate, the linear regression equation is Y = −0.98X − 10.02, R2 = 0.994.

Author Contributions

Conceptualization, W.L.; methodology, C.P. and Y.Z.; validation, J.Y., D.L. and J.L.; formal analysis, C.P.; investigation, C.P. and Y.S.; resources, W.L.; data curation, C.P.; writing—original draft preparation, C.P., Y.S. and W.L.; writing—review and editing, W.L.; visualization, C.P; supervision, W.L.; project administration, W.L.; funding acquisition, W.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by financial support from the National Natural Science Foundation of China, grant number 52173296; the Natural Science Foundation of Guangdong Province (China), grant number 2022A1515011826, and 100 Top Talents Program—Sun Yat-sen University (China).

Data Availability Statement

Data are contained within the article.

Acknowledgments

Financial support from the National Natural Science Foundation of China (52173296), the Natural Science Foundation of Guangdong Province (China) (2022A1515011826), and the 100 Top Talents Program—Sun Yat-sen University (China) is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00528 sch001 550
Scheme 1. Schematic illustration of the synthesis of PtxPdy aerogels for electrochemical sensing of H2O2.
Scheme 1. Schematic illustration of the synthesis of PtxPdy aerogels for electrochemical sensing of H2O2.
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Catalysts 12 00528 g001 550
Figure 1. TEM images of (a,b) Pt50Pd50, (c) Pt20Pd80, (d) Pt80Pd20, (e) Pd, and (f) Pt aerogels.
Figure 1. TEM images of (a,b) Pt50Pd50, (c) Pt20Pd80, (d) Pt80Pd20, (e) Pd, and (f) Pt aerogels.
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Catalysts 12 00528 g002 550
Figure 2. (a) XRD spectra of PtxPdy bimetallic and Pt, Pd monometallic aerogels. (b) TEM–EDS of Pt80Pd20, Pt50Pd50, and Pt20Pd80 aerogels.
Figure 2. (a) XRD spectra of PtxPdy bimetallic and Pt, Pd monometallic aerogels. (b) TEM–EDS of Pt80Pd20, Pt50Pd50, and Pt20Pd80 aerogels.
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Catalysts 12 00528 g003 550
Figure 3. XPS spectra of (a,b) Pt20Pd80, (c,d) Pt50Pd50, and (e,f) Pt80Pd20 aerogels; (g) surface valance band spectra of the Pt, Pd monometallic aerogels and PtxPdy bimetallic aerogels. All the spectra were corrected using a Shirley background. The yellow bars indicate the location of the d-band center.
Figure 3. XPS spectra of (a,b) Pt20Pd80, (c,d) Pt50Pd50, and (e,f) Pt80Pd20 aerogels; (g) surface valance band spectra of the Pt, Pd monometallic aerogels and PtxPdy bimetallic aerogels. All the spectra were corrected using a Shirley background. The yellow bars indicate the location of the d-band center.
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Catalysts 12 00528 g004 550
Figure 4. (a) CV curves of Pt50Pd50 aerogel modified GCE at concentrations of 0, 0.02, 0.06, 0.1, 0.3, 0.5, 0.7, 1.0, and 2.0 mM H2O2 in N2-saturated 0.1 M phosphate buffer saline (PBS). (b) CV curves of bimetallic aerogels and monometallic aerogel modified GCEs at a concentration of 1.0 mM H2O2 in N2-saturated 0.1 M PBS. Potential range: −0.64-0.96 V vs. Ag/AgCl. Scan rate: 50 mV s−1.
Figure 4. (a) CV curves of Pt50Pd50 aerogel modified GCE at concentrations of 0, 0.02, 0.06, 0.1, 0.3, 0.5, 0.7, 1.0, and 2.0 mM H2O2 in N2-saturated 0.1 M phosphate buffer saline (PBS). (b) CV curves of bimetallic aerogels and monometallic aerogel modified GCEs at a concentration of 1.0 mM H2O2 in N2-saturated 0.1 M PBS. Potential range: −0.64-0.96 V vs. Ag/AgCl. Scan rate: 50 mV s−1.
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Figure 5. (a) CV peak currents of Pt50Pd50 in PBS solutions with different pH comprising 1 mM concentration of H2O2. (b) Peak potentials of Pt50Pd50 in PBS solutions with different pH comprising 1 mM concentration of H2O2.
Figure 5. (a) CV peak currents of Pt50Pd50 in PBS solutions with different pH comprising 1 mM concentration of H2O2. (b) Peak potentials of Pt50Pd50 in PBS solutions with different pH comprising 1 mM concentration of H2O2.
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Catalysts 12 00528 g006 550
Figure 6. (a) Amperometric responses of the aerogel modified electrodes with successive addition of 2 × 10−5 M (50 μL), 0.002 M (50 μL), 0.02 M (50 μL), 0.6 M (17 μL), and 1 M (10 μL) into 20 mL N2-saturated 0.1 M PBS aqueous solution at −0.05 V. (b) (b1) Amperometric responses of the Pt50Pd50 aerogel modified electrodes (b2,b3). Enlarged part of amperometric response of Pt50Pd50 aerogel modified electrode. (b4) Calibration curve of the Pt50Pd50 for H2O2RR in N2-saturated PBS aqueous solution. (c) (c1–c4) Calibration curve of Pt20Pd80, Pt80Pd20, Pt, and Pd for H2O2RR in N2-saturated PBS aqueous solution.
Figure 6. (a) Amperometric responses of the aerogel modified electrodes with successive addition of 2 × 10−5 M (50 μL), 0.002 M (50 μL), 0.02 M (50 μL), 0.6 M (17 μL), and 1 M (10 μL) into 20 mL N2-saturated 0.1 M PBS aqueous solution at −0.05 V. (b) (b1) Amperometric responses of the Pt50Pd50 aerogel modified electrodes (b2,b3). Enlarged part of amperometric response of Pt50Pd50 aerogel modified electrode. (b4) Calibration curve of the Pt50Pd50 for H2O2RR in N2-saturated PBS aqueous solution. (c) (c1–c4) Calibration curve of Pt20Pd80, Pt80Pd20, Pt, and Pd for H2O2RR in N2-saturated PBS aqueous solution.
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Catalysts 12 00528 g007 550
Figure 7. The relationship between the ratio of Pt and Pd in the as-prepared PtxPdy aerogels and their sensitivity (mA mM−1 cm−2), LOD (μM), Epc (V), BET surface area (m2 g−1), BJH pore volume (cm−3 g−1), and d-band center (eV).
Figure 7. The relationship between the ratio of Pt and Pd in the as-prepared PtxPdy aerogels and their sensitivity (mA mM−1 cm−2), LOD (μM), Epc (V), BET surface area (m2 g−1), BJH pore volume (cm−3 g−1), and d-band center (eV).
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Catalysts 12 00528 g008 550
Figure 8. (a) The amperometric response of the Pt50Pd50 aerogel modified GCE with respective additions of 2 μM H2O2, 2 mM glucose, 2 mM saccharose, 2 mM ascorbic acid, and 2 μM H2O2 in N2-saturated 0.1 M 20 mL PBS at −0.05 V. (b) The current responses of five different Pt50Pd50 aerogel modified GCEs synthesized under the same conditions at a concentration of 1 mM H2O2. The inset picture is the current responses calculated in the form of bar charts. (c) An example of stability test of one Pt50Pd50 modified GCE over 6 days in the air at room temperature. The inset picture is the current responses of three Pt50Pd50 modified GCEs over 6 days (in the air at room temperature) calculated in the form of bar charts.
Figure 8. (a) The amperometric response of the Pt50Pd50 aerogel modified GCE with respective additions of 2 μM H2O2, 2 mM glucose, 2 mM saccharose, 2 mM ascorbic acid, and 2 μM H2O2 in N2-saturated 0.1 M 20 mL PBS at −0.05 V. (b) The current responses of five different Pt50Pd50 aerogel modified GCEs synthesized under the same conditions at a concentration of 1 mM H2O2. The inset picture is the current responses calculated in the form of bar charts. (c) An example of stability test of one Pt50Pd50 modified GCE over 6 days in the air at room temperature. The inset picture is the current responses of three Pt50Pd50 modified GCEs over 6 days (in the air at room temperature) calculated in the form of bar charts.
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Table
Table 1. Molar ratio of Pt/Pd and metal/metal oxides in the PtxPdy aerogels revealed from EDS and XPS and location of the d-band center of all the aerogels.
Table 1. Molar ratio of Pt/Pd and metal/metal oxides in the PtxPdy aerogels revealed from EDS and XPS and location of the d-band center of all the aerogels.
AerogelsExpected
Pt/Pd
(at%)		EDS
Pt/Pd
(at%)		XPS (at%)D-Band Center
Location
(eV)
Pt/PdPt/PtOPd/PdO
Pd0/100---57.3/42.72.47
Pt20Pd8020/8020.4/79.724.9/75.180.2/19.862.0/38.02.74
Pt50Pd5050/5050.6/49.459.6/40.475.2/24.883.7/16.33.02
Pt80Pd2080/2083.2/16.885.7/14.370.2/29.864.3/35.73.11
Pt100/0--83.3/16.7-3.27
Table
Table 2. Summary of the performances of the nonenzymatic H2O2 electrochemical sensors based on aerogel materials and Pt and Pd elements containing bimetallic materials.
Table 2. Summary of the performances of the nonenzymatic H2O2 electrochemical sensors based on aerogel materials and Pt and Pd elements containing bimetallic materials.
ElectrodeCV
Epc
(V)		Eapplied
(V vs. Ag/AgCl)		Linear Range
(μM)		LOD
(μM)		Sensitivity
(mA mM−1
cm−2)		tresponse
(s)		Substrate
Materials		Ref
Pt50Pd50 aerogel−0.023−0.055.1–31902.210.193-This work
Pt80Pd20 aerogel−0.115−0.055.1–42002.730.162-This work
Pt20Pd80 aerogel−0.014−0.0510–37006.730.1143-This work
Pt aerogel−0.136−0.055.2–27004.210.143≤5 s-This work
Pd aerogel0.004−0.0519–557621.30.047≤5 s-This work
Ag wire aerogel~−0.14−0.460–8002.10.42~50 s-[38]
Pt–Pd/CFMEnot obvious−0.45–39200.4810.4≤5 scarbon fiber[42]
Pt–Pd NPs/GO-CFMs~−0.2−0.21–350.30.98 μA mM−1-graphene oxide sheathed carbon fiber[43]
Pt–Pb/graphene~−0.19−0.20.002–25160.0021.82, 0.91, 4.052graphene[12]
Pt (750 s) Pd/MoS2not obvious−0.3510–803.47.64-MoS2[39]
Pt48Pd52−Fe3O4 NPs−0.456−0.250.02–0.1
0.1–2.0
2.0–14,000		0.005-2Fe3O4[44]
PdPt NCs@SGN~0.13−0.061–3000.314,968.753SnO2/graphene nanosheets[41]
Pt–PdCu NCs~0.050.051.5–11601.50.562-none[45]
[PB/WV-Pt@Pd]6<−0.5−0.30.4–26500.10.0551 PW9V3O403−[46]
Pd core–Pt NDs-rGO~0.020.0185–5000.0270.672≤5 sPDDA–rGO[47]
Pt–Pd/IL-rGOP(II)−0.350.2 *0.1–37.60.01 ≤5 sIL–graphene paper[48] *
* Means the references in which Epc is shown here in the table for comparison, while in the reference the aerometric detection of H2O2 was performed utilizing the oxidation of H2O2. All potentials listed here are referenced to or converted to be referenced to Ag/AgCl (3 M KCl). CFME: carbon fiber microelectrode; MWCNTs: multiwalled carbon nanotubes; CGE: carbon glass electrode; NPs: nanoparticles; GO: graphene oxide; CFMs: carbon fiber microelectrodes; rGO: reduced graphene oxide nanosheets; CNTs: carbon nanotubes; SPGFE: screen-printed gold nanofilm electrode; BNC: bimetallic nanoclusters; NCs: nanocages; SGN: SnO2/graphene nanosheets; NCs: nanocages; rGOP: reduced graphene-based nanohybrid paper, Prussian blue; WV: PW9V3O403−; NDs: nanodendrites; PDDA: poly(diallyldimethylammonium chloride); G: graphene; IL: ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate).
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Pan, C.; Zheng, Y.; Yang, J.; Lou, D.; Li, J.; Sun, Y.; Liu, W. Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide. Catalysts 2022, 12, 528. https://doi.org/10.3390/catal12050528

AMA Style

Pan C, Zheng Y, Yang J, Lou D, Li J, Sun Y, Liu W. Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide. Catalysts. 2022; 12(5):528. https://doi.org/10.3390/catal12050528

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Pan, Chuxuan, Yuanyuan Zheng, Jing Yang, Dongyang Lou, Jian Li, Yujing Sun, and Wei Liu. 2022. "Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide" Catalysts 12, no. 5: 528. https://doi.org/10.3390/catal12050528

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