- Email: [email protected]
Pd2+ doped ZnO nanostructures: Structural, luminescence and gas sensing properties
Materials Letters 160 (2015) 200–205
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Pd2 þ doped ZnO nanostructures: Structural, luminescence and gas sensing properties Gugu H. Mhlongo a,n, David E. Motaung a, Hendrik C. Swart b a b
DST/CSIR-National Centre for Nano-structured Materials, PO Box 395, 0001 South Africa Department of Physics, University of Free State, Bloemfontein 9300, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 27 May 2015 Received in revised form 23 July 2015 Accepted 27 July 2015 Available online 30 July 2015
We report on synthesis of Pd-doping of ZnO-nanostructures and its role on the gas sensing properties of NH3 and H2S. The complementary investigation aimed at understanding, explaining and confirming the role of Pd-doping on the surface morphology, photoluminescence (PL) and gas sensing behaviour exhibited at low gas concentrations were conducted. X-ray diffraction (XRD) analysis revealed a singlephase wurtzite structure of ZnO. X-ray photo-electron spectroscopy (XPS) results showed that partial Pd ions aggregate on the surface of ZnO while the rest are doped into the ZnO lattice. Surface morphology analysis revealed the increase in surface to volume ratio with the incorporation of the Pd-dopant into ZnO. The dominant violet-blue emission observed in the PL spectra of undoped and Pd-doped ZnO indicated the presence of Zni shallow donors resulting to improved sensing response. & 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO Palladium Gas sensors
1. Introduction
2. Experimental
ZnO has been explored for detection of various oxidizing and reducing gases such as O2, O3, and H2, CO, hydrocarbons, respectively [1]. To overcome drawbacks imposed by using bulk ZnO gas sensors including long-term instability, sensitivity towards ambient humidity and poor selectivity, addition of certain dopants has been validated as an effective way to improve gas sensing properties [2,3]. Thus far, researchers have been focused on fabrication of nanostructured ZnO due to its large surface-to-volume ratio obtained at the nanoscale level which allows quick diffusion of gas molecules, consequently leading to faster response–recovery time [1]. Introduction of any dopant in nanocrystalline ZnO results in a complex distribution of impurities between the surface and volume of the grains and this affects the microstructure, surface reactivity, electrical and consequently gas sensor properties of sensitive material. Previous studies reported enhanced sensitivity of Pd functionalized and sensitized ZnO towards CO and LPG, respectively [4,5]. Herein, undoped and Pd-doped ZnO-nanostructures with different Pd concentrations were prepared using a sol–gel method. Microstructural characterization and surface defects studies in relation with the sensor signal toward reducing gases H2S and NH3 are investigated.
Undoped ZnO was prepared by dissolving zinc acetate in boiling ethanol and the resulting solution was cooled in ice and then combined with the ethanol solution of sodium hydroxide. For preparation of doped ZnO with different Pd (0.5 and 0.75 mol%) concentrations, the ethanol solution of palladium (II) chloride was added to the hydrolyzed Zn2 þ solution prepared according to the above experimental procedure. The resulting solution was kept at room temperature for 24 h and the unwanted impurities were removed by centrifuging and washed repeatedly with de-ionized water. The resulting precipitates were annealed at 350 °C for 2 h. The products were then characterized by XRD, Nanolog photoluminescence spectrometer (λExc. 320 nm), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and XPS. Sensing measurements were carried out by dispersing ZnO nanostructures in ethanol, drop-coated onto the alumina substrates and then tested for response to different concentrations of NH3 and H2S gases at 200 °C using a Kinesistec testing station.
n
Corresponding author. E-mail address: [email protected] (G.H. Mhlongo).
http://dx.doi.org/10.1016/j.matlet.2015.07.139 0167-577X/& 2015 Elsevier B.V. All rights reserved.
3. Results and discussions The XRD patterns of undoped and Pd-doped ZnO presented in Fig. 1(a) revealed that all the ZnO products possess a hexagonal structure. No additional diffraction peaks corresponding to metallic Pd or secondary phases of Pd were detected. A shift in the (101) diffraction peak towards higher 2θ angle (see inset, Fig. 1), which may be due to substitution of Zn2 þ ions with Pd2 þ ions into
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
201
Fig.1. (a) XRD patterns and (b) Raman spectra of undoped and Pd-doped ZnO. The inset corresponds to (101) diffraction peak.
ZnO lattice was observed. Raman spectra obtained from undoped and Pd-doped ZnO are shown in Fig. 1(b). The predominant 437 cm 1 peak is the high-frequency E2 mode (E2H mode) characteristic of the wurtzite structure while the other peaks at 199, 330, 538, 574 and 692 cm 1 correspond to the acoustic overtone (2E2L), multiphonon, A1(LO), E1(LO) and TA þLO modes, respectively [6,7]. A new peak at 638 cm 1 detected only in Pd-doped ZnO can be ascribed to Pd-doping in the ZnO lattice [6]. Fig. 2 shows the SEM, and TEM images of undoped and Pddoped ZnO. SEM and TEM images show that the morphology of the ZnO-nanostructures is still maintained even after Pd incorporation. The particle size distribution (see insets, Fig. 2) indicated that the average diameter of undoped ZnO is 35 nm. When adding 0.5 and 0.75 mol% of Pd ions into ZnO lattice, a decrease in the average particle size from 35 nm to 23 nm was
observed which can be explained by Nae-Lih Wu's theory [8] wherein the motion of the crystallites is restricted due to the interaction between the host and dopant crystallites [3]. Clear lattice fringes with the d-spacing of 0.234 and 0.281 nm corresponding to the (101) and (100) planes of wurtzite ZnO were observed for 0.5 mol% Pd-doped ZnO and undoped ZnO, respectively. The corresponding HRTEM image of 0.75 mol% Pd-doped ZnO revealed the presence of Pd ions on the ZnO surface with the size of 3 to 5 nm in diameter. The lattice fringes of these Pd-nanostructures with a d-spacing of ∼0.224 nm corresponding to (111) plane of Pd were clearly visible. This demonstrates that some of the Pd ions formed small clusters on the surface of ZnO while the rest were incorporated into the ZnO lattice which correlates well with Raman results. To gain more information about chemical states of metal ions
202
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
Fig.2. SEM, TEM and HRTEM images of undoped and Pd-doped ZnO. The insets show the particle size distribution of the ZnO-nanostructures.
on the surfaces of ZnO-nanostructures, XPS analyses were performed. The XPS spectra of O 1s and 3d core levels of undoped and Pd-doped ZnO were fitted by multiple Gaussians (Fig. 3(a–e)). As observed from Fig.3(a–c), the O 1s core level spectra of undoped, 0.5 and 0.75 mol% Pd-doped ZnO are partitioned into three components denoted as O1, O2, and O3. The O1 component at 529.7 eV is associated with O2 ions in the ZnO wurtzite structure [9]. The O2 component positioned at 530.7 eV is related to O2 ions in the oxygen deficient regions within the ZnO matrix and/or Zn–OH groups [9]. The O3 peak at 532.3 eV is usually associated with the specific chemisorbed oxygen on the surface of ZnO including CO3, adsorbed O2, or adsorbed H2O [9]. At least two kinds of Pd species exist in the near surface region of Pd-doped ZnO samples (Fig.3(d,e)). The fitted double peaks of Pd 3d5/2 at 335.2 and 336.0 eV can be individually identified as Pd0 and Pd2 þ [10]. The coexistence of Pd0 and Pd2 þ confirms that part of Pd is loaded on the ZnO surface with the rest doped into ZnO lattice which is in good agreement with TEM and Raman results. Fig. 4(a) shows the PL spectra of undoped and Pd-doped ZnO. The PL spectra of both undoped and Pd-doped ZnO exhibited a
broad strong violet-blue emission in the range of 380–480 nm with a low intensity contribution centred at 550 nm. By contrast, the PL intensity first decreased with 0.5 mol% Pd concentration then increased when further increased to 0.75 mol%. In order to understand the origination of these emission peaks, the spectral region 350–750 nm was de-convoluted using Gaussian fit. The sub-peak at 381 nm which is directly related to formation of excitons is related to band-edge absorption of ZnO [9]. The emission peak at 391 nm can be ascribed to the shallow donor zinc interstitial (Zni) or Zni related complex defect [11,12]. The blue emission peaks located at 401, 416, and 433 nm correspond to electronic transitions related to neutral, single charged, and double charged Zni donor levels to the valence band, respectively [13]. The weak green emission at 552 nm is due to double charged oxygen vacancies (VO þ þ ) [14]. Fig. 4(b,c) depicts the representative response–recovery curves of the undoped, 0.5 and 0.75 mol% Pd-doped ZnO sensors to NH3 and H2S at 200 °C. The results show that both NH3 and H2S sensing properties are strongly influenced by Pd-doping concentration. When the sensors were exposed to H2S (or NH3) the
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
203
Fig.3. The XPS of the (a–c) O 1s core, (d,e) 3d core levels acquired from Pd-doped ZnO.
corresponding response value increased fast and reached almost its equilibrium value then decreased rapidly to its baseline once disengaged from the H2S or NH3 atmosphere, indicating satisfactory reversibility. When increasing the doping concentration (0.5– 0.75 mol%), the sensor response increased considerably denoting its dependence on Pd-doping. Pd is known to play an important role in enhancing the response and response rate of a metal-oxide sensor as a catalyst, which increases the reaction rate by activating the dissociation of molecular oxygen, resulting in atomic products that diffuse onto the metal oxide support [6]. The observed high response for Pd-doped ZnO in the current study can be attributed to the effective catalytic activity of the Pd whereby the analyte gas molecules are activated by the noble metal additive and the activated fragments of the gas then spill-over to the ZnO surface to react with adsorbed oxygen. Also, reduced particles size and enhanced specific surface area due to Pd incorporation into ZnO lattice is expected to promote sensor response owing to availability of more active sites for the surface reaction. This makes Pd catalytic effect of NH3 or H2S oxidation to become highly effective at the low operating temperature of 200 °C [15]. Fig. 4(d,e) shows the variation of sensor response with different H2S and NH3 concentrations. The higher response of ZnO towards H2S gas compared to NH3is probably due to high reactivity of H2S and the smaller bonding energy of H–SH in H2S (381 KJ/mol) when compared to that of H–NH2in NH3(435 kJ/mol) [16]. The response values were shown to increase with increasing gases concentrations showing the dependence of response in gas concentration. The obtained low response values at low gas concentrations may be due to low coverage of gas molecules on the sensor surface which results in lower surface reaction. With increasing gas
Table 1 Response (ta)-recovery times (tr) of undoped and Pd-doped ZnO. ZnO ta (s) NH3 (ppm) 30 169 75 370 150 359 225 326 300 244 H2S (ppm) 12 32 30 129 60 144 90 88 120 165
tr (s)
ZnO: 0.5 % ta (s) tr (s)
ZnO: 0.75 % ta (s) tr (s)
960 974 673 565 813
198 201 310 223 294
334 440 281 391 347
275 261 321 324 184
431 446 281 398 350
7 51 40 50 62
24 102 144 77 153
7 30 40 42 33
160 190 237 124 208
54 67 79 131 67
concentration, the surface reaction also increases due to a larger surface coverage thus increasing sensor response [5]. The response-time (defined as the time required for reaching 90% of the equilibrium value of the resistance after gas exposure) and recovery-time (defined as the time required for the resistance to return to 10% below the original resistance in air after the test gas is released) are summarized in Table 1.
4. Conclusion Undoped and Pd-doped ZnO-nanostructures with different concentrations were successfully prepared using a sol–gel method.
204
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
Fig.4. (a,b) PL spectra-schematic diagram, (c,d) response-recovery curves, (e,f) variation of sensors response of undoped and Pd-doped ZnO.
Strong violet-blue emissions associated with Zni were observed from both undoped and Pd-doped ZnO-nanostructures. Pd-doped ZnO exhibited improved gas sensing performance as compared with the undoped ZnO-nanostructures.
Acknowledgements This work was supported by the Department of Science and Technology, Council for Scientific and Industrial Research (HGER39X and HGER27S). Dr. Ella Linganiso and Ms. Randzu Rikhotso are acknowledged for Sensing and TEM measurements.
References [1] A. Wei, L. Pan, W. Huang, Mater. Sci. Eng. B 176 (2011) 1409–1421. [2] Y.C. Liang, W.-K. Liao, X.-S. Deng, J. Alloys Compd. 599 (2014) 87–92. [3] Y.-F. Sun, S.-B. Liu, F.-L. Meng, J.-Y. Liu, Z. Jin, L.-T. Kong, J.-H. Liu, Sensors 12 (2012) 2610–2631. [4] S.-W. Choi, S.S. Kim, Sens. Actuators B 168 (2012) 8–13. [5] V.R. Shinde, T.P. Gujar, C.D. Lokhande, Sens. Actuators B 123 (2007) 701–706. [6] L.-L. Xing, C.-H. Ma, Z.-H. Chen, Y.-J. Chen, X.-Y. Xue, Nanotechnology 22 (2011) 215501. [7] J. Yang, X. Liu, L. Yang, Y. Wang, Y. Zhang, J. Lang, J. Alloys Compd. 477 (2009) 632–635. [8] C.N. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 3 (1991) 147–155. [9] G.H. Mhlongo, D.E. Motaung, S.S. Nkosi, H.C. Swart, G.F. Malgas, K.T. Hillie, B. W. Mwakikunga, Appl. Surf. Sci. 293 (2014) 62–70. [10] X. Li, H. Yang, Appl. Clay Sci. 100 (2014) 43–49.
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
[11] N. Han, P. Hu, A. Zuo, D. Zhang, Y. Tian, Y. Chen, Sens. Actuators B 145 (2010) 114–119. [12] P.V. Ramakrishna, D.B.R.K. Murthy, D.L. Sastry, K. Samatha, Spectrochim. Acta Part A 129 (2014) 274–279. [13] S.C. Singh, R. Gopal, Appl. Surf. Sci. 258 (2012) 2211–2218.
205
[14] D.E. Motaung, P.R. Makgwane, S.S. Ray, Mater. Lett. 139 (2015) 475–479. [15] C. Liewhiran, N. Tamaekong, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Sens. Actuators B 176 (2013) 893–905. [16] Z. Liu, T. Fan, D. Zhang, X. Gong, J. Xu, Sens. Actuators B 136 (2009) 49–509.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Pd2 þ doped ZnO nanostructures: Structural, luminescence and gas sensing properties Gugu H. Mhlongo a,n, David E. Motaung a, Hendrik C. Swart b a b
DST/CSIR-National Centre for Nano-structured Materials, PO Box 395, 0001 South Africa Department of Physics, University of Free State, Bloemfontein 9300, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 27 May 2015 Received in revised form 23 July 2015 Accepted 27 July 2015 Available online 30 July 2015
We report on synthesis of Pd-doping of ZnO-nanostructures and its role on the gas sensing properties of NH3 and H2S. The complementary investigation aimed at understanding, explaining and confirming the role of Pd-doping on the surface morphology, photoluminescence (PL) and gas sensing behaviour exhibited at low gas concentrations were conducted. X-ray diffraction (XRD) analysis revealed a singlephase wurtzite structure of ZnO. X-ray photo-electron spectroscopy (XPS) results showed that partial Pd ions aggregate on the surface of ZnO while the rest are doped into the ZnO lattice. Surface morphology analysis revealed the increase in surface to volume ratio with the incorporation of the Pd-dopant into ZnO. The dominant violet-blue emission observed in the PL spectra of undoped and Pd-doped ZnO indicated the presence of Zni shallow donors resulting to improved sensing response. & 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO Palladium Gas sensors
1. Introduction
2. Experimental
ZnO has been explored for detection of various oxidizing and reducing gases such as O2, O3, and H2, CO, hydrocarbons, respectively [1]. To overcome drawbacks imposed by using bulk ZnO gas sensors including long-term instability, sensitivity towards ambient humidity and poor selectivity, addition of certain dopants has been validated as an effective way to improve gas sensing properties [2,3]. Thus far, researchers have been focused on fabrication of nanostructured ZnO due to its large surface-to-volume ratio obtained at the nanoscale level which allows quick diffusion of gas molecules, consequently leading to faster response–recovery time [1]. Introduction of any dopant in nanocrystalline ZnO results in a complex distribution of impurities between the surface and volume of the grains and this affects the microstructure, surface reactivity, electrical and consequently gas sensor properties of sensitive material. Previous studies reported enhanced sensitivity of Pd functionalized and sensitized ZnO towards CO and LPG, respectively [4,5]. Herein, undoped and Pd-doped ZnO-nanostructures with different Pd concentrations were prepared using a sol–gel method. Microstructural characterization and surface defects studies in relation with the sensor signal toward reducing gases H2S and NH3 are investigated.
Undoped ZnO was prepared by dissolving zinc acetate in boiling ethanol and the resulting solution was cooled in ice and then combined with the ethanol solution of sodium hydroxide. For preparation of doped ZnO with different Pd (0.5 and 0.75 mol%) concentrations, the ethanol solution of palladium (II) chloride was added to the hydrolyzed Zn2 þ solution prepared according to the above experimental procedure. The resulting solution was kept at room temperature for 24 h and the unwanted impurities were removed by centrifuging and washed repeatedly with de-ionized water. The resulting precipitates were annealed at 350 °C for 2 h. The products were then characterized by XRD, Nanolog photoluminescence spectrometer (λExc. 320 nm), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and XPS. Sensing measurements were carried out by dispersing ZnO nanostructures in ethanol, drop-coated onto the alumina substrates and then tested for response to different concentrations of NH3 and H2S gases at 200 °C using a Kinesistec testing station.
n
Corresponding author. E-mail address: [email protected] (G.H. Mhlongo).
http://dx.doi.org/10.1016/j.matlet.2015.07.139 0167-577X/& 2015 Elsevier B.V. All rights reserved.
3. Results and discussions The XRD patterns of undoped and Pd-doped ZnO presented in Fig. 1(a) revealed that all the ZnO products possess a hexagonal structure. No additional diffraction peaks corresponding to metallic Pd or secondary phases of Pd were detected. A shift in the (101) diffraction peak towards higher 2θ angle (see inset, Fig. 1), which may be due to substitution of Zn2 þ ions with Pd2 þ ions into
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
201
Fig.1. (a) XRD patterns and (b) Raman spectra of undoped and Pd-doped ZnO. The inset corresponds to (101) diffraction peak.
ZnO lattice was observed. Raman spectra obtained from undoped and Pd-doped ZnO are shown in Fig. 1(b). The predominant 437 cm 1 peak is the high-frequency E2 mode (E2H mode) characteristic of the wurtzite structure while the other peaks at 199, 330, 538, 574 and 692 cm 1 correspond to the acoustic overtone (2E2L), multiphonon, A1(LO), E1(LO) and TA þLO modes, respectively [6,7]. A new peak at 638 cm 1 detected only in Pd-doped ZnO can be ascribed to Pd-doping in the ZnO lattice [6]. Fig. 2 shows the SEM, and TEM images of undoped and Pddoped ZnO. SEM and TEM images show that the morphology of the ZnO-nanostructures is still maintained even after Pd incorporation. The particle size distribution (see insets, Fig. 2) indicated that the average diameter of undoped ZnO is 35 nm. When adding 0.5 and 0.75 mol% of Pd ions into ZnO lattice, a decrease in the average particle size from 35 nm to 23 nm was
observed which can be explained by Nae-Lih Wu's theory [8] wherein the motion of the crystallites is restricted due to the interaction between the host and dopant crystallites [3]. Clear lattice fringes with the d-spacing of 0.234 and 0.281 nm corresponding to the (101) and (100) planes of wurtzite ZnO were observed for 0.5 mol% Pd-doped ZnO and undoped ZnO, respectively. The corresponding HRTEM image of 0.75 mol% Pd-doped ZnO revealed the presence of Pd ions on the ZnO surface with the size of 3 to 5 nm in diameter. The lattice fringes of these Pd-nanostructures with a d-spacing of ∼0.224 nm corresponding to (111) plane of Pd were clearly visible. This demonstrates that some of the Pd ions formed small clusters on the surface of ZnO while the rest were incorporated into the ZnO lattice which correlates well with Raman results. To gain more information about chemical states of metal ions
202
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
Fig.2. SEM, TEM and HRTEM images of undoped and Pd-doped ZnO. The insets show the particle size distribution of the ZnO-nanostructures.
on the surfaces of ZnO-nanostructures, XPS analyses were performed. The XPS spectra of O 1s and 3d core levels of undoped and Pd-doped ZnO were fitted by multiple Gaussians (Fig. 3(a–e)). As observed from Fig.3(a–c), the O 1s core level spectra of undoped, 0.5 and 0.75 mol% Pd-doped ZnO are partitioned into three components denoted as O1, O2, and O3. The O1 component at 529.7 eV is associated with O2 ions in the ZnO wurtzite structure [9]. The O2 component positioned at 530.7 eV is related to O2 ions in the oxygen deficient regions within the ZnO matrix and/or Zn–OH groups [9]. The O3 peak at 532.3 eV is usually associated with the specific chemisorbed oxygen on the surface of ZnO including CO3, adsorbed O2, or adsorbed H2O [9]. At least two kinds of Pd species exist in the near surface region of Pd-doped ZnO samples (Fig.3(d,e)). The fitted double peaks of Pd 3d5/2 at 335.2 and 336.0 eV can be individually identified as Pd0 and Pd2 þ [10]. The coexistence of Pd0 and Pd2 þ confirms that part of Pd is loaded on the ZnO surface with the rest doped into ZnO lattice which is in good agreement with TEM and Raman results. Fig. 4(a) shows the PL spectra of undoped and Pd-doped ZnO. The PL spectra of both undoped and Pd-doped ZnO exhibited a
broad strong violet-blue emission in the range of 380–480 nm with a low intensity contribution centred at 550 nm. By contrast, the PL intensity first decreased with 0.5 mol% Pd concentration then increased when further increased to 0.75 mol%. In order to understand the origination of these emission peaks, the spectral region 350–750 nm was de-convoluted using Gaussian fit. The sub-peak at 381 nm which is directly related to formation of excitons is related to band-edge absorption of ZnO [9]. The emission peak at 391 nm can be ascribed to the shallow donor zinc interstitial (Zni) or Zni related complex defect [11,12]. The blue emission peaks located at 401, 416, and 433 nm correspond to electronic transitions related to neutral, single charged, and double charged Zni donor levels to the valence band, respectively [13]. The weak green emission at 552 nm is due to double charged oxygen vacancies (VO þ þ ) [14]. Fig. 4(b,c) depicts the representative response–recovery curves of the undoped, 0.5 and 0.75 mol% Pd-doped ZnO sensors to NH3 and H2S at 200 °C. The results show that both NH3 and H2S sensing properties are strongly influenced by Pd-doping concentration. When the sensors were exposed to H2S (or NH3) the
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
203
Fig.3. The XPS of the (a–c) O 1s core, (d,e) 3d core levels acquired from Pd-doped ZnO.
corresponding response value increased fast and reached almost its equilibrium value then decreased rapidly to its baseline once disengaged from the H2S or NH3 atmosphere, indicating satisfactory reversibility. When increasing the doping concentration (0.5– 0.75 mol%), the sensor response increased considerably denoting its dependence on Pd-doping. Pd is known to play an important role in enhancing the response and response rate of a metal-oxide sensor as a catalyst, which increases the reaction rate by activating the dissociation of molecular oxygen, resulting in atomic products that diffuse onto the metal oxide support [6]. The observed high response for Pd-doped ZnO in the current study can be attributed to the effective catalytic activity of the Pd whereby the analyte gas molecules are activated by the noble metal additive and the activated fragments of the gas then spill-over to the ZnO surface to react with adsorbed oxygen. Also, reduced particles size and enhanced specific surface area due to Pd incorporation into ZnO lattice is expected to promote sensor response owing to availability of more active sites for the surface reaction. This makes Pd catalytic effect of NH3 or H2S oxidation to become highly effective at the low operating temperature of 200 °C [15]. Fig. 4(d,e) shows the variation of sensor response with different H2S and NH3 concentrations. The higher response of ZnO towards H2S gas compared to NH3is probably due to high reactivity of H2S and the smaller bonding energy of H–SH in H2S (381 KJ/mol) when compared to that of H–NH2in NH3(435 kJ/mol) [16]. The response values were shown to increase with increasing gases concentrations showing the dependence of response in gas concentration. The obtained low response values at low gas concentrations may be due to low coverage of gas molecules on the sensor surface which results in lower surface reaction. With increasing gas
Table 1 Response (ta)-recovery times (tr) of undoped and Pd-doped ZnO. ZnO ta (s) NH3 (ppm) 30 169 75 370 150 359 225 326 300 244 H2S (ppm) 12 32 30 129 60 144 90 88 120 165
tr (s)
ZnO: 0.5 % ta (s) tr (s)
ZnO: 0.75 % ta (s) tr (s)
960 974 673 565 813
198 201 310 223 294
334 440 281 391 347
275 261 321 324 184
431 446 281 398 350
7 51 40 50 62
24 102 144 77 153
7 30 40 42 33
160 190 237 124 208
54 67 79 131 67
concentration, the surface reaction also increases due to a larger surface coverage thus increasing sensor response [5]. The response-time (defined as the time required for reaching 90% of the equilibrium value of the resistance after gas exposure) and recovery-time (defined as the time required for the resistance to return to 10% below the original resistance in air after the test gas is released) are summarized in Table 1.
4. Conclusion Undoped and Pd-doped ZnO-nanostructures with different concentrations were successfully prepared using a sol–gel method.
204
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
Fig.4. (a,b) PL spectra-schematic diagram, (c,d) response-recovery curves, (e,f) variation of sensors response of undoped and Pd-doped ZnO.
Strong violet-blue emissions associated with Zni were observed from both undoped and Pd-doped ZnO-nanostructures. Pd-doped ZnO exhibited improved gas sensing performance as compared with the undoped ZnO-nanostructures.
Acknowledgements This work was supported by the Department of Science and Technology, Council for Scientific and Industrial Research (HGER39X and HGER27S). Dr. Ella Linganiso and Ms. Randzu Rikhotso are acknowledged for Sensing and TEM measurements.
References [1] A. Wei, L. Pan, W. Huang, Mater. Sci. Eng. B 176 (2011) 1409–1421. [2] Y.C. Liang, W.-K. Liao, X.-S. Deng, J. Alloys Compd. 599 (2014) 87–92. [3] Y.-F. Sun, S.-B. Liu, F.-L. Meng, J.-Y. Liu, Z. Jin, L.-T. Kong, J.-H. Liu, Sensors 12 (2012) 2610–2631. [4] S.-W. Choi, S.S. Kim, Sens. Actuators B 168 (2012) 8–13. [5] V.R. Shinde, T.P. Gujar, C.D. Lokhande, Sens. Actuators B 123 (2007) 701–706. [6] L.-L. Xing, C.-H. Ma, Z.-H. Chen, Y.-J. Chen, X.-Y. Xue, Nanotechnology 22 (2011) 215501. [7] J. Yang, X. Liu, L. Yang, Y. Wang, Y. Zhang, J. Lang, J. Alloys Compd. 477 (2009) 632–635. [8] C.N. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 3 (1991) 147–155. [9] G.H. Mhlongo, D.E. Motaung, S.S. Nkosi, H.C. Swart, G.F. Malgas, K.T. Hillie, B. W. Mwakikunga, Appl. Surf. Sci. 293 (2014) 62–70. [10] X. Li, H. Yang, Appl. Clay Sci. 100 (2014) 43–49.
G.H. Mhlongo et al. / Materials Letters 160 (2015) 200–205
[11] N. Han, P. Hu, A. Zuo, D. Zhang, Y. Tian, Y. Chen, Sens. Actuators B 145 (2010) 114–119. [12] P.V. Ramakrishna, D.B.R.K. Murthy, D.L. Sastry, K. Samatha, Spectrochim. Acta Part A 129 (2014) 274–279. [13] S.C. Singh, R. Gopal, Appl. Surf. Sci. 258 (2012) 2211–2218.
205
[14] D.E. Motaung, P.R. Makgwane, S.S. Ray, Mater. Lett. 139 (2015) 475–479. [15] C. Liewhiran, N. Tamaekong, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Sens. Actuators B 176 (2013) 893–905. [16] Z. Liu, T. Fan, D. Zhang, X. Gong, J. Xu, Sens. Actuators B 136 (2009) 49–509.