Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and

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Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and Chemoresistive Gas Sensors Marco Sturaro,† Enrico Della Gaspera,*,§ Niccolò Michieli,‡ Carlo Cantalini,∥ Seyed Mahmoud Emamjomeh,∥ Massimo Guglielmi,† and Alessandro Martucci*,† †

Dipartimento di Ingegneria Industriale and ‡Dipartimento di Fisica e Astronomia, Università di Padova, Padova, Italy School of Science, RMIT University, Melbourne, VIC, Australia ∥ Dipartimento di Ingegneria Industriale, Università di L’Aquila, L’Aquila, Italy §

S Supporting Information *

ABSTRACT: Highly doped wide band gap metal oxide nanocrystals have recently been proposed as building blocks for applications as transparent electrodes, electrochromics, plasmonics, and optoelectronics in general. Here we demonstrate the application of gallium-doped zinc oxide (GZO) nanocrystals as novel plasmonic and chemiresistive sensors for the detection of hazardous gases including hydrogen (H2) and nitrogen dioxide (NO2). GZO nanocrystals with a tunable surface plasmon resonance in the near-infrared are obtained using a colloidal heat-up synthesis. Thanks to the strong sensitivity of the plasmon resonances to chemical and electrical changes occurring at the surface of the nanocrystals, such optical features can be used to detect the presence of toxic gases. By monitoring the changes in the dopant-induced plasmon resonance in the near-infrared, we demonstrate that GZO thin films prepared depositing an assembly of highly doped GZO colloids are able to optically detect both oxidizing and reducing gases at mild (200 °C) were found to drastically decrease the plasmonic absorption due to decrease in oxygen vacancies and therefore led to baseline instability. After exposure to hydrogen (a reducing gas), the absorbance in the NIR range increases, while upon exposure to NO2 (an oxidizing gas) a decrease in optical absorption in the NIR is observed (Figure 3a,b). To highlight this difference, Figure 3c shows the optical absorbance change parameter, which is defined as the difference between absorbance during gas exposure and absorbance in air (OAC = AbsGas − AbsAir = ΔAbs). This behavior is consistent with IR plasmonic resonance shifts due to electron density variations and band gap shifts in n-type semiconductor-like ZnO. In the case of hydrogen, electrons are injected in the metal oxide according to reaction 2, causing an increase in the concentration of free carriers which generates a consequent blue-shift of the LSPR peak and an increase of the optical band gap. In fact, the frequency of the LSPR is proportional to the square root of the free electron concentration according to the formula57,58 ωLSPR 2 =

Ne 2 meε0(ε∞ + 2εm)

(4)

Therefore, an increase in electrons within ZnO following the oxidation of hydrogen will cause an increase in the LSPR frequency. Conversely, the interaction with oxidizing gases like NO2 causes the removal of electrons according to reaction 3, with the related red-shift of the plasmon peak and the reduction of the optical band gap. These LSPR shifts, which are driven by different gases, represent a great opportunity to improve sensor selectivity. Tests could be performed exploiting the change in absorbance both in the infrared range and in the band gap region. 30444

DOI: 10.1021/acsami.6b09467 ACS Appl. Mater. Interfaces 2016, 8, 30440−30448

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ACS Applied Materials & Interfaces

Figure 5. Normalized resistance changes to 400 ppb NO2 exposure (green shaded box) for ZnO, GZO10, and GZO20 at different operating temperatures under dark (a−d) and purple−blue light irradiation (e−h) conditions.

Table 1. Comparison of RR, RP, τads, and τdes to 400 ppb NO2 in Dark Conditions and PB Light Illumination at Different OT (25−100 °C)a RR = RG/RA

τads (min)

τdes (min)

OT (°C)

sample

dark

light

dark

light

dark

light

dark

light

25

ZnO GZO10 GZO20 ZnO GZO10 GZO20 ZnO GZO10 GZO20 ZnO GZO10 GZO20

1 3.52 47.73 1 1 5.80 1.18 4.29 15.78 1.46 6.17 10.81

2.94 2.44 10.44 2.65 5.31 27.50 2.03 8.81 20.82 1.58 8.74 24.41

0 0 19.28 0 15.77 69.62 0 87.47 95.28 54.59 96.24 97.52

90 80.66 93.80 88.15 88.74 98.01 99.00 99.33 99.06 89.34 98.25 99.23

/ 32 105 / / 82 / 19 73 / 68 99

4 30 77 4 16 67 4 15 49 4 80 100

/ / / / / / / / / / 69 86

/ / / / / 119 55 115 105 35 105 105

50

75

100

a

RP = [ΔD/ΔA ] × 100 [%]

The notation “/” indicates that no adsorption or desorption equilibrium conditions have been achieved within the time scale of the experiment.

the sensing cycle (see Figure 5b for its graphical representation). Regarding these points, we first clarify that given the time scale of a sensing cycle, gas adsorption/desorption times can be eventually numerically quantified only if the equilibrium conditions are achieved. It may happen that equilibrium conditions are achieved (i.e., τads/des can be quantified), but the sensor resistance fails to regain its original baseline value. This occurs in the case of irreversible adsorption, when gas molecules are bound so strongly to the surface that they do not desorb, regardless of the time scale of a sensing cycle. In some cases, no equilibrium conditions are achieved during desorption so that neither baseline recovery is achieved nor τads/des times can be quantified. Figure 5 shows the normalized change of the electrical resistance of the ZnO, GZO10, and GZO20 samples to 400 ppb NO2 by increasing the OT from 25 to 100 °C under dark or under PB light illumination. By analyzing the electrical response at different OT and under dark conditions (Figure 5a−d), we observed that gallium doping enhances relative responses (RRs), improves the

(250 ppm) gases. The resistance change during hydrogen exposure was always lower with respect to NO2 exposure, for all the conditions we investigated, as shown in Figures S5 and S6. This behavior is in line with the smaller gas sensing response to H2 as respect to NO2 observed during optical sensing tests (see Figure 4). For this reason, we focus the next discussion on NO2 sensing. The following figures of merit have been introduced to characterize and compare “gas response properties” of the GZO samples: (i) baseline resistance (BLR): the resistance in dry air at equilibrium before any gas exposure; (ii) relative response (RR): the ratio (RG/RA) to 400 ppb NO2, where RA and RG represent the resistance in dry air and in gas, respectively; (iii) adsorption/desorption time (τads/des): the time required to reach 90% of the full response at equilibrium, during both gas adsorption and desorption; (iv) recovery percentage (RP): a measure of the sensor ability to recover its baseline, calculated as the percentage ratio (ΔD/ΔA) × 100, where ΔD and ΔA are the variations of the electrical resistance during gas desorption and absorption, respectively, calculated within the time scale of 30445


Research Article


with higher doping levels always show lower baseline resistances. This is a consequence of both the aliovalent ntype Ga3+ doping and oxygen desorption as described in reaction 5. Regarding the larger RRs of the PB irradiated GZO20 with respect to the others, a possible explanation is that the gallium content increases the concentration of surface oxygen vacancies (V•• O ), which according reaction 5 become available to react with NO2 (reaction 3). The increase of free adsorption sites (V•• O ) induced by light irradiation could also explain why the GZO10 seems to saturate its response under dark, whereas under PB light illumination, steadily increases its resistance with increasing the NO2 concentration as reported in Figure 6b. Analyzing also the RR vs NO2 concentration plot presented in Figure 6d, we can conclude that the GZO20 yields higher RRs, higher sensitivity, and better recovery of the baseline, identifying it as the best sample for sub-ppm of NO2 monitoring at mild temperatures (75 °C).

recovery percentages of the baseline (RPs), and slightly increases the response times. Numerical figures of these parameters are reported in Table 1. RRs at room temperature are the highest with a maximum of RR = 47.7 for the GZO20, but with associated poor recovery of the baseline during desorption, as represented by the blue plot of Figure 5a. No gas response was recorded for undoped ZnO at room temperature. We found that at 75 °C OT and 20% gallium doping a reasonable compromise in terms of RR, RP, and response times can be achieved, as shown in Figure 5 and Table 1. Irradiation with PB light, as shown in Figure 5e−h, further enhances RRs and remarkably increase the RPs with respect to dark conditions. Given the positive effect of PB light irradiation, reasonable gas sensing features are maintained even at 50 °C OT, yielding for the 20% GZO, RR and RP values of 27.5 and 98.0%, respectively. Moreover, undoped ZnO films activated with PB light show gas response even at room temperature (Figure 5e) as compared to the undoped ZnO in dark (Figure 5a). Finally, the dynamic gas responses at 75 °C OT and at NO2 concentrations ranging from 100 to 400 ppb are compared in Figure 6 in the dark and under PB light. For all the investigated

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CONCLUSIONS In conclusion, we have demonstrated plasmonic gas sensing using the infrared LSPR peak of degenerately doped semiconductors, specifically Ga-doped ZnO. Zinc oxide nanocrystals doped with varying amounts of gallium were synthesized using colloidal methods and used to deposit thin-film coatings that have been used for plasmonic and chemoresistive sensing. Aliovalent dopants in ZnO induce the formation of a plasmonic resonance in the near-infrared, which, analogously to LSPR in metal nanoparticles, is affected by the presence of both reducing and oxidizing gases. We elucidated the role of Ga dopant, oxygen vacancies, and overall sensing mechanism combining the optical gas sensing data with electrical measurements. Moreover, by exposing the GZO film to blue light during the sensing tests, we achieved room temperature sensitivity to sub-ppm of NO2 concentrations. These very thin GZO films are optically transparent in the visible range and can be used as electrical and optical sensors for the detection of hazardous gases at low operating temperatures, paving the way to the fabrication of highly efficient, multifunctional “invisible” sensors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09467. Picture of the colloidal solution is reported in Figure S1; absorption spectra of films are reported in Figure S2 and S3; SEM pictures of film cross section are reported in Figure S4; additional conductometric gas sensing experiments are reported in Figures S5 and S6; the mean crystallite diameters evaluated from XRD and the EDX compositional analysis are reported in Table S1 (PDF)

Figure 6. Dynamic gas responses to NO2 at 75 °C under dark and PB light irradiation of (a) ZnO, (b) GZO10, and (c) GZO20. (d) Relative response (RR = RG/RA) as a function of NO2 for the different samples measured under dark or under PB light irradiation.

samples, by exposing all the films to PB light, baseline resistances (horizontal dashed lines in Figure 6) always decrease with respect to the dark. This behavior could be explained,65,66 considering the photodesorption of surface oxygen (O2−ads) induced by the impinging photons, according to reaction 5: •• − (O2−−V •• O )ADS + hν ⇌ O2(g) + V O + eCB

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

(5)

Notes

The authors declare no competing financial interest.

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Considering that the binding energy of chemically adsorbed oxygen atoms on defective SnO2 is around 1.67 eV,67 PB light with associated photon energy of 2.88 eV is suitable to activate the desorption mechanism of reaction 5, therefore explaining the decrease of the baseline resistance. Moreover, GZO films

ACKNOWLEDGMENTS E.D.G. thanks RMIT University for the funding provided through the Vice-Chancellor’s Research Fellowships scheme. 30446


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Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and

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