Pt decorated Cu₂O nanocomposite to detect ppb level Ozone concentration

2.1. Material and equipment

The crystalline phase of the synthesized materials was characterized by X-ray diffraction (XRD) using a SHIMADZU XRD-6000X diffractometer with Cu Kα radiation (λ = 0.15404 nm). The measurements were conducted in the 2θ range of 10° to 80° with a step size of 0.1° and a scan speed of 2°/min, operating at 35 kV and 35 mA. Surface morphology was examined by scanning electron microscopy (SEM) using a JEOL IT-100 instrument at accelerating voltages of 0.5–20 kV and magnifications up to 300,000×. Elemental composition was determined via energy-dispersive X-ray spectroscopy (EDS) using an OXFORD AZtecOne system, which offers a resolution of <129 eV for Mn Kα, <67 eV for F Kα, and <57 eV for C Kα, with detection capability for elements from beryllium (Z = 4) onwards. Microstructural analysis was performed by transmission electron microscopy (TEM) using a JEM-2010 instrument equipped with a LaB₆ electron gun and operating at accelerating voltages of 120–200 kV.

2.2. Preparation of cuprous oxide Cu₂O and Pt/Cu₂O composite materials with different proportions

Copper(II) acetate (Cu(CH₃COO)₂) was dissolved in 50 mL of ultrapure water under constant stirring. To this solution, 10 mL of sodium hydroxide (NaOH, 12.5 M) was added dropwise under continuous agitation. After stirring for 15 min, a deep blue suspension formed. Subsequently, L-ascorbic acid (C₆H₈O₆) was introduced, and the mixture was stirred for an additional 15 min, during which the color gradually changed from dark blue to orange-red. The resulting precipitate was collected by centrifugation and washed three times each with deionized water and absolute ethanol (20 min per wash cycle). Finally, the product was dried in an oven at 80°C for 8 h.

The Pt/Cu₂O composites were synthesized via a solution-based deposition method. First, Cu₂O powder was dispersed in 30 mL of ultrapure water by ultrasonication for 20 min. Sodium citrate was then added as a stabilizer, and the mixture was agitated for an additional 20 min. Subsequently, a calculated volume of chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O) solution, corresponding to the target Pt loading (e.g., 0.5, 1, 1.5, 2 wt%), was introduced. The mixture was shaken for 30 min to allow for Pt deposition onto the Cu₂O surface. The resulting product was isolated by centrifugation and washed sequentially with ultrapure water and anhydrous ethanol. This wash cycle was repeated several times to remove residual reagents. Finally, the collected solid was dried in an oven at 80°C for 8 h to yield the Pt/Cu₂O composite powders with varying Pt loadings.

2.3. Substrate pretreatment and fabrication of gas sensing system

Prior to use, the substrates underwent a thorough cleaning procedure to eliminate surface impurities that could compromise sensing performance. Each substrate was sonicated in ethanol for 15 min, rinsed copiously with ultrapure water, and then dried in an oven at 80°C. To fabricate the sensing element, a uniform paste was prepared by mixing the synthesized active material with a 10 wt% polyvinyl alcohol (PVA) solution in a weight ratio of 1:1.5. This viscous mixture was then carefully coated onto the gold electrode region of the pre-cleaned substrate. The coated substrates were subsequently dried in an oven at 200°C for 2 h, yielding the final sensing elements (Figure 1a).

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Figure 1.

(a) Flow diagram of the preparation of sensing elements (b) Flow diagram of the sensing system.

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The fabricated sensing element was mounted within a sealed reaction chamber. A constant bias voltage was applied via an external power supply. A controlled gas atmosphere was established by introducing a background of synthetic air, followed by a defined concentration of ozone generated by an external ozone source. The chamber was sealed, and the gas mixture was allowed to equilibrate. During measurements, the chamber’s internal temperature, relative humidity (RH), and ozone concentration were continuously monitored using integrated sensors. A mass flow controller (MFC) regulated the inlet gas streams to maintain the target ozone concentration. The electrical response (e.g., resistance change) of the sensing element was recorded in real-time using a data acquisition system. A schematic of the complete sensing setup is presented in Figure 1(b).

2.4. Gas sensor principle

All sensing measurements were performed at room temperature (25 ± 2°C) and ambient humidity (55 ± 10 RH%). The sensor response was evaluated by periodically exposing the device to alternating atmospheres of baseline synthetic air and a target concentration of ozone. A constant operating voltage (Vₛ) was applied, and the resulting voltage drop across the sensor (Vₘ) was monitored in real-time using a data logger. The measured voltage (Vₘ) was used to calculate the sensor’s electrical resistance (R) during each exposure cycle. Definition of Sensing Parameters, the sensor response (S) was quantified as the resistance ratio S=R_air/R_gas, where R_air​ is the stable baseline resistance in air and R_gas​ is the steady-state resistance in ozone. For this p-type semiconductor material, the resistance decreases upon exposure to the oxidizing ozone gas. The dynamic performance was characterized by two key metrics, response time (T₉₀) is the time required for the resistance to decrease to 90% of its total change from the baseline to the steady-state value in ozone (R_gas​) upon gas introduction. Recovery time (Tᵣ₉₀) is the time required for the resistance to return to 90% of its original baseline value (R_air​) after the ozone flow is stopped and the atmosphere is restored to pure air.

3.1. Characterization of sensing materials

X-ray diffraction (XRD) analysis confirmed the crystalline phases present in the synthesized materials. The pattern for pure Cu₂O (Figure 2) exhibits characteristic diffraction peaks at 2θ = 29.16°, 35.89°, 41.81°, 61.05°, 73.24°, and 77.08°, which are indexed to the (110), (111), (200), (220), (311), and (222) lattice planes of cubic Cu₂O, respectively [12,13]. These peaks align well with the standard reference pattern (JCPDS-78-2076). The absence of any peaks attributable to CuO confirms the phase purity of the synthesized Cu₂O. For the 1 wt% Pt/Cu₂O composite, the XRD pattern retains all the primary reflections of the Cu₂O phase. Additionally, three distinct new peaks are observed at 2θ = 37.69°, 44.90°, and 64.43°. These correspond to the (111), (200), and (220) planes of face-centered cubic (fcc) platinum (JCPDS 65-2868), confirming the successful incorporation of Pt nanoparticles into the Cu₂O matrix.

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Figure 2.

XRD patterns of Cu₂O and composite material 1% Pt/Cu₂O.

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The morphology of the synthesized materials was examined by SEM. As shown in Figures 3(a,b), the pure Cu₂O sample consists of well-defined, uniformly distributed cubic crystals. In contrast, the 1 wt% Pt/Cu₂O composite (Figures 3c,d) exhibits a distinctly coarser surface texture, with fine, needle-like nanostructures attached to the Cu₂O cubes. Consistent with previous reports, these nanostructures are attributed to Pt nanoparticles adhered to the Cu₂O surface. This Pt surface decoration is pivotal for enhancing sensing performance. By creating active interfacial sites, it effectively suppresses the recombination of photogenerated electron-hole pairs in Cu₂O, thereby facilitating more efficient charge transfer during ozone adsorption and improving the material’s sensitivity. The elemental composition of the composite was further verified by energy-dispersive X-ray spectroscopy (EDS). The quantitative analysis (Figure 4a) confirms the presence of Cu (74.6 wt%), O (24.1 wt%), and Pt (1.3 wt%), with no detectable impurities. Elemental mapping (Figure 4b) demonstrates the homogeneous distribution of Pt across the Cu₂O particles, indicating successful and uniform dispersion of the noble metal within the composite matrix.

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Figure 3.

(a) Large range, (b) Local SEM image of Cu₂O (c) Large range, (d) Local SEM image of 1% Pt/Cu₂O.

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Figure 4.

(a) EDX spectrum, (b) Element distribution diagram of 1% Pt/Cu₂O.

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High-resolution transmission electron microscopy (HR-TEM) was employed to investigate the microstructure of the 1 wt% Pt/Cu₂O composite. As shown in Figure 5(a), the image reveals numerous needle-like nanoparticles distributed around the cubic Cu₂O particles, consistent with the morphology observed by SEM. The crystal structure was further analyzed via lattice fringe imaging. Figures 5(b,c) show HR-TEM images where distinct lattice fringes are resolved. The measured interplanar spacings of approximately 0.30 nm, 0.25 nm, and 0.24 nm (red boxes in Figure 5 (b,c)) correspond to specific crystallographic planes, confirming the presence of both Cu₂O and Pt phases. Using image analysis software, the average size of the Pt nanoparticles was determined to be approximately 4.52 nm. HR-TEM was employed to investigate the microstructure of the 1 wt% Pt/Cu₂O composite. The HR-TEM image (Figure 5a) confirms the fine needle-like morphology of the surface nanostructures initially observed by SEM, showing their distribution around the cubic Cu₂O particles. This agreement between SEM and TEM observations further validates the presence of distinct Pt nanoparticle assemblies on the Cu₂O support.

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Figure 5.

(a) TEM image, (b and c) distinct lattice fringes of 1% Pt/Cu₂O.

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UV-Vis Spectrophotometer analysis given in Figure 6(a) shows absorption band of Cu₂O falls approximately between 400-600 nm, while the absorption band of the Pt/Cu₂O is blue shifted to 400-485 nm, indicating that the incorporation of Pt affects the optical properties of the material. Using Tauc’s plot, the differences between the optical materials was calculated using Eq (1).:

$A=\frac{K{\left(hv-Eg\right)}^{\frac{1}{n}}}{hv}$

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Figure 6.

(a) UV-visible spectra of Cu₂O and Pt/Cu₂O, (b) Energy gap diagram of Cu₂O and Pt/Cu₂O.

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Since Cu₂O is a direct energy gap, the part of n in the formula is equal to 2, and the energy gap of Cu₂O prepared in this experiment is about 2.26, while that of Pt/Cu₂O composites is 2.05 (Figure 6b).

3.2. Gas sensing results

3.2.1. Comparative ozone sensing performance

Ozone sensing measurements were performed at room temperature with a constant ozone concentration of 1 ppm and a RH of approximately 50%. In a typical cycle, ozone was introduced into the chamber for 5 min, followed by a recovery period in air. The sensor’s resistance was monitored throughout, and each test was repeated three times to ensure reproducibility. Figure 7(a) presents the dynamic resistance curve for pristine Cu₂O. Upon ozone exposure, the resistance gradually decreases, confirming the adsorption and interaction of ozone with the material surface. Upon switching back to air, the resistance begins to recover. The corresponding sensor response (S=Rair/Rgas) was 1.114 at 1 ppm ozone, with a response time (T₉₀) of 364 s and a recovery time (Tᵣ₉₀) of 595 s.

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Figure 7.

(a) Sensing resistance diagram of ozone concentration of pristine Cu₂O, (b) 1% Pt/Cu₂O sensing resistance diagram of ozone concentration at 1 ppm.

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For the 1 wt% Pt/Cu₂O composite, the same testing protocol revealed a substantially larger resistance drop upon ozone exposure compared to pure Cu₂O (Figure 7b), indicating a significantly enhanced response. The sensor response reached 12.206 at 1 ppm ozone—an order-of-magnitude improvement. However, this enhanced sensitivity came with extended response kinetics, with a response time (T₉₀) of 499 s and a recovery time (Tᵣ₉₀) of 2210 s. To address these kinetics, future work will focus on heterogeneous structural or porous structure engineering [13]. This synergistic approach is designed to leverage photocarriers to accelerate surface redox reactions and the critical desorption step during recovery, while enhanced porosity facilitates gas diffusion. This strategy is expected to significantly improve response and recovery dynamics without a substantial increase in power consumption.

Triplicate measurements were performed on Pt/Cu₂O composites with varying Pt contents under the same test conditions at a fixed ozone concentration of 1 ppm. The incorporation of Pt enhances electron transfer through the induction effect, facilitating more efficient ozone adsorption on the surface of the sensing material. As shown in Figure 8, the 1% Pt/Cu₂O composite exhibited the highest sensing signal amongst the tested ratios. The sensing properties for all Pt/Cu₂O proportions are summarized in Table 1. These results indicate that both insufficient and excessive Pt doping lead to suboptimal performance, and an optimal Pt content exists that maximizes the sensing response.

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Figure 8.

Sensing signal diagram of different ratios of Pt/Cu₂O at 1 ppm ozone concentration.

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Table 1. Sensor response (S), response and recovery time (T₉₀/Tr₉₀) of each material at 1 ppm ozone gas.

Sensing material	Sensor response	T90/Tr90
Cu2O	1.12	364/595 s
0.5% Pt/Cu2O	3.01	336/1974 s
1.0% Pt/Cu2O	12.21	499/2210 s
1.5% Pt/Cu2O	5.11	321/1924 s
2.0% Pt/Cu2O	4.86	371/2875 s

To evaluate the sensing performance of the optimized 1% Pt/Cu₂O composite, the sensor was tested at various ozone concentrations: 1000, 500, 300, 150, and 70 ppb. The corresponding sensing signals were 12.206, 4.429, 3.833, 1.889, and 1.559, respectively, as shown in Figure 9(a). The results indicate that the sensing signal increases with increasing ozone concentration. Figure 9(b) shows that the correlation coefficient (R²) at 1000 ppb is 0.9897, demonstrating good linearity. The detection limit (LOD) was calculated according to the standard formula, where a lower LOD corresponds to better ozone sensing performance. The LOD of the 1% Pt/Cu₂O composite was determined to be 32.75 ppb, highlighting its high sensitivity for low-concentration ozone detection.

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Figure 9.

(a) Sensing signal diagram of 1% Pt/Cu₂O at various concentrations, (b) 1% Pt/Cu₂O sensing signal calibration line.

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3.2.2. Long-term stability and humidity tolerance

The long-term operational stability of a sensor is critical for practical deployment. To evaluate this, the 1 wt% Pt/Cu₂O composite sensor was tested at five-day intervals over a 30-day period. As shown in Figure 10(a), the sensor exhibited consistent performance for the first 25 days. By day 30, the response signal had declined from an initial 4.43 to 3.62, reflecting a ∼18% reduction yet indicating overall robust stability. This gradual decline can be attributed to the accumulation of strongly adsorbed oxygen species (e.g., O⁻, O²⁻), which are by-products of ozone decomposition (O₃ → O₂ + O*). These species progressively occupy active surface sites, reducing the number available for subsequent ozone sensing reactions. To further improve long-term stability in future work, structural engineering approaches such as creating core–shell or heterojunction architectures [13] could be employed. Such designs can enhance stability by isolating the catalytic function from the primary conduction pathway or by providing a more robust and resilient material framework.

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Figure 10.

(a) Repeatability test of 1% Pt/Cu₂O composite material at ozone concentration of 500 ppb, (b) 1% Pt/Cu₂O 50-70% relative humidity test at 1 ppm ozone concentration.

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The effect of RH on sensor performance was systematically evaluated. 1 wt% Pt/Cu₂O sensor was exposed to a constant ozone concentration (1 ppm) at controlled RH levels of 50%, 60%, and 70% (Figure 10b). The results indicate an inverse relationship between RH and sensor response: higher humidity significantly attenuates the signal, while lower humidity yields an optimal response. This behavior is attributed to the competitive adsorption of water molecules on the active surface sites. At elevated RH, water vapor occupies adsorption sites and forms a surface layer, hindering the access and interaction of ozone molecules with the sensing material and consequently diminishing the electronic response. Future work to enhance practical applicability will focus on mitigating humidity interference. Promising strategies include the integration of a micro-heater for periodic desorption or the application of a selective hydrophobic membrane over the sensing layer to minimize competitive water adsorption while preserving high sensitivity to ozone.

Selectivity is a critical performance metric for the practical deployment of gas sensors. The selectivity of the 1 wt% Pt/Cu₂O sensor was evaluated by measuring its response to 1 ppm ozone (O₃) and various interfering gases at relevant concentrations (Table 2). The sensor exhibited a strong and preferential response of 12.2 to ozone, which is markedly higher than its responses to 1 ppm NO₂, 10 ppm H₂, 10 ppm NH₃, and 10 ppm NO. This pronounced contrast confirms the composite’s excellent selectivity for ozone detection.

Table 2. Interference effect of various gases on 1% Au/Cu₂O.

Gases	Concentration of gas	Sensor response
O3	1 ppm	12.2
NO2	1 ppm	3.2
H2	10 ppm	1.5
NH3	10 ppm	2.2
CO	10 ppm	2.3

As summarized in Table 3, the 1 wt% Pt/Cu₂O composite exhibits competitive performance relative to other higher than room-temperature ozone sensors reported in the literature [9-11]. Operating without external heating, it provides significant advantages in energy efficiency and simplifies device integration. The composite demonstrates a strong response (S = 12.21) to 1.0 ppm ozone and achieves a low detection limit (LOD) of 32.8 ppb. These attributes—high sensitivity, low power operation, and a ppb detection capability—collectively confirm the potential of this nanocomposite for fabricating practical, high-performance ozone sensors with promising commercial viability.

Table 3. The comparison of trace amount of ozone sensing properties of previously sensing materials.

Author/reference	Sensing material	O3 (ppm)	Working temperature T (°C)	Sensor response	T90/Tr90 (s)	Limit of detection (LOD)
Ziegler, D., et al [9]	W-doped In2O3	0.5	100°C	464	267/199	--
Zhu, Z., et al [10]	Au@TiO₂	0.5	25°C	3.27	5/24	--
de Lima, B. S., et al [11]	rGO-ZnO	0.1	300°C	49.6	85/78	0.1 ppb
Current Work	1% Pt/Cu2O	1.0	25°C	12.21	499/2210	32.8 ppb

--: No measurement.

3.3. Sensing mechanism

The proposed ozone sensing mechanism for p-type Cu₂O is illustrated in Figure 11 and described by the following reaction pathway [13,28-30]. Upon exposure to air, oxygen molecules adsorb onto the Cu₂O surface, forming physisorbed O₂ (Eq. 2). These molecules capture electrons from the material, generating chemisorbed superoxide ions (O₂⁻) and oxidizing surface Cu⁺ sites to Cu²⁺ (Eq. 3) [13]. This process establishes a stable hole accumulation layer and a baseline resistance. When ozone (O₃)—a stronger oxidizing agent—is introduced, it interacts directly with surface Cu⁺ ions, extracting electrons to form adsorbed ozonide ions (O₃⁻) and additional Cu²⁺ (Eq. 4)). As a p-type semiconductor, this electron depletion results in a significant increase in hole concentration, sharply decreasing the electrical resistance. The adsorbed O₃⁻ is unstable and rapidly decomposes into adsorbed atomic oxygen (O⁻) and gaseous O₂ (Eq. 5). During recovery in clean air, the adsorbed O⁻ species react with adjacent Cu²⁺ ions, reducing them back to Cu⁺ and forming adsorbed neutral oxygen atoms (O) (Eq. 6) [13]. This recombination decreases the hole concentration, causing the resistance to recover. Finally, two adjacent adsorbed oxygen atoms recombine and desorb as molecular oxygen (O₂), restoring the sensor surface to its initial state (Eq. 7).

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Figure 11.

Reaction mechanism diagram.

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The incorporation of Pt nanoparticles enhances the sensing mechanism through two primary catalytic functions [24-26]. First, the Pt/Cu₂O interface promotes the adsorption and subsequent activation of atmospheric oxygen, facilitating its ionization into reactive superoxide species (O₂⁻(ads)) as described by Equations (3) and (4). This process increases the density of active surface sites and elevates the baseline hole concentration compared to pure Cu₂O [25]. Second, Pt nanoparticles serve as efficient catalysts for ozone reactions. They facilitate the adsorption and dissociation of O₃ molecules on the surface (Equation 4) and catalyze the reaction between incoming O₃ and pre-adsorbed O₂⁻(ads), efficiently generating atomic oxygen species (O⁻(ads)) and releasing O₂ (Equation 4). This dual role significantly accelerates the surface redox kinetics, leading to the enhanced sensor response observed in the Pt/Cu₂O composite.