Ag–Ni and Al–Ni nanoparticles for resistive response of humidity and photocatalytic degradation of Methyl Orange dye

Materials Chemistry and Physics 244 (2020) 122748

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Ag–Ni and Al–Ni nanoparticles for resistive response of humidity and photocatalytic degradation of Methyl Orange dye Kausar Shaheen a, b, c, Hongli Suo a, *, Zarbad Shah d, **, Lintha Khush d, Tofail Arshad d, Sher Bahadar Khan e, f, Mohsin Siddique d, Lin Ma a, Min Liu a, Jin Cui a, Yao Tang Ji a, Yi Wang a a

The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology, Beijing, 100124, China Department of Physics, University of Peshawar, Peshawar, 25120, Khyber Pakhtunkhwa, Pakistan Department of Physics, Jinnah College for Women, University of Peshawar, Peshawar, 25120, Khyber Pakhtunkhwa, Pakistan d Department of Chemistry, Bacha Khan University Charsadda, Charsadda-24420, Khyber Pakhtunkhwa, Pakistan e Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, 21589, P.O. Box 80203, Saudi Arabia f Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah, 21589, Saudi Arabia b c

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Ag–Ni & Al–Ni NPs. � Humidity/MO degradation. � Low hysteresis. � Recyclability.

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanoparticles Humidity sensing Photocatalytic degradation Stability

In the present work, Ag–Ni and Al–Ni nanoparticles (NPs) were synthesized via co-precipitation method and their structural, resistive response of humidity and photocatalytic degradation properties evaluated. The as prepared NPs were characterized by XRD, FTIR, TGA, DRS, BET, XPS, SEM and TEM. XRD patterns indicated well defined peaks for Ag, Al and Ni elements. SEM and TEM micrographs displayed nano size heterogeneous microstructures both for Ag–Ni and Al–Ni NPs along with some agglomeration. The average particle size calculated by XRD, SEM and TEM was ranged from 42 to 95 nm. FTIR and XPS investigated about various functional groups and elemental contents within the synthesized samples. TGA showed total weight loss of 14.93 and 39.29% whereas, surface area, pore volume and pore size were 18.63 m2/g, 0.2846 mL/g, 1.3277 nm and 8.52 m2/g, 0.0556 mL/g, 1.3678 nm for Ag–Ni and Al–Ni NPs respectively. Low band gap energies ~1.30 (for Ag–Ni) and 1.51eV (for Al–Ni) were calculated via DRS. Resistance was decreased from (879–240MΩ for Ag–Ni and 745 to 169MΩ for Al–Ni NPs) with increased humidity level (from 10 to 95%) due to surface adsorption of water by fabricated

* Corresponding author. The Key Laboratory of Advanced Functional Materials, Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, China. ** Corresponding author. Department of Chemistry, Bacha Khan University Charsadda, Charsadda-24420, Khyber Pakhtunkhwa, Pakistan. E-mail addresses: [email protected] (H. Suo), [email protected], [email protected] (Z. Shah). https://doi.org/10.1016/j.matchemphys.2020.122748 Received 10 September 2019; Received in revised form 23 December 2019; Accepted 29 January 2020 Available online 30 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.

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samples. Response time ~60s, 59s and recovery time ~39s, 20s with low hysteresis was measured for Ag–Ni and Al–Ni NPs respectively. Furthermore, photocatalytic degradation ability of the synthesized samples was also investigated against Methyl Orange (MO) dye. Ag–Ni and Al–Ni NPs showed 91 and 75% degradation during total time of 80 and 120 min respectively. Degradation ability of the samples was also evaluated for various factors such as catalyst loading, pH and different scavengers. Fast response/recovery time, low hysteresis, better catalysis and excellent stability indicated for both the samples categorize them well suited for resistive humidity sensing and photocatalytic degradation of MO dye from industrial effluents.

1. Introduction

efficient photo material for decomposition of hazardous dyes. It can also be utilized as an electro catalyst for water splitting technique to generate H2. Some other very important applications such as energy storage, solar cells, humidity sensing techniques and electronic devices are also involving Ni as an important and favorable component based on its antiferromagnetic mode, high stability and various other attractive electrical properties [28–33]. Similarly, silver (Ag) is also considered as well suited for photoelectro-catalytic and other electronic applications due to its smaller size, attractive morphology, and larger surface to volume ratio [34]. The higher stability, greater hardness and strength, low dielectric loss, and efficient degradation ability categorize Al2O3 with all the phases (α-Al2O3, γ-Al2O3, η-Al2O3 and θ-Al2O3) as an effective component for various industrial applications. Wider band gap energy and suitable charge separation can contribute a lot towards multiple characteristics and applications of Al2O3 [35,36]. Herein, a cost effective and motivating approach was followed for the preparation of simple but efficient NPs with improved physi­ ochemical properties. The synthesized Ag–Ni and Al–Ni NPs showed effective results for resistive response of humidity and photocatalytic degradation of MO dye. These NPs were selected due to low cost, effective excitation/absorption capability for light, efficient degradation performance and active response/recovery time for any sensing device. Ni NPs have shown remarkable novelty and expansion for the last three decades and are successfully used for disinfection of water, energy and environment under solar, visible and ultraviolet irradiations. Ag NPs are also extensively utilized in various industries such as agriculture, food, textile, medicine, and electronics whereas, Al based NPs are the basic constituent in sensing technology, automotive energy and petroleum industries. The synthesized NPs have been explored as the most favor­ able candidates for the suggested applications owing to its costeffectiveness, simplicity and efficiency. The present research could open a gateway to various similar studies for other heterogeneous nanocomposites prepared through the same simple and easy synthesis route with advanced applications. The low catalytic ability for Al–Ni NPs (probably due to low surface area, i.e. 8.52 m2/g), can be further enhanced by modifying the processing parameters. The loss of photo­ catalysts during separation stages via centrifugation as well as filtration and the nonlinear response/recovery time for both the NPs can also be considered for future recommendations.

Extensive research is on the way towards metal oxides NPs (MONs) due to their potential use in wide range of applications such as storage devices, sensors, catalysis, and optoelectronics. Air quality including ambient temperature and humidity can greatly effect living organisms, industrial manufacturing techniques, and agri­ cultural usage of materials [1]. Relative humidity (RH) and temperature are the parameters with prime concern in various optoelectronic in­ dustries. An accurate and frequent determination of these parameters is very essential [2,3]. Weather telemetry organizations and various other industries require reliable, portable and cost effective humidity sensors for optimization of manufacturing parameters, plants protection, floods prediction, processing and preservation of food [4,5]. Humidity sensors are categorized as resistive, capacitive, and hydrometric based on sensing mechanism and their performance can be evaluated by sensor’s manufacturing scheme and nature of the material used during fabrica­ tion process. Low cost synthesis, greater chemical and physical stability, wide range for sensing, active and linear response and low hysteresis are necessary and requisite features for the best and accurate sensing device. Nanomaterials involving metal oxides, semiconductors, ceramics and polymers have received considerable attention towards the develop­ ment of humidity sensors with debatable qualities [6]. However, developing a novel material that possesses greater sensitivity for full range of humidity is still a big challenge [7–10]. One of the other major problems, modern world is facing these days is the contaminated water. Water contamination has emerged due to release of effluents (containing heavy metals and other toxic organic compounds) from different industries such as leather, paper, textile, batteries manufacturing and cosmetics [11–13]. Various abnormalities are attributed towards the accumulated contaminants in the tissues of living organisms through food chain. Dyes released by different in­ dustries make a foam like layer on water surface, which effect oxygen availability for aquatic flora and fauna [14,15]. Dyes removal or degradation (which is not possible through conventional methods) is very essential for clean and safe water environment. Presently, various methods are reported for detoxification and removal of such hazardous dyes, among which heterogeneous photocatalysis is the most effective and novel method [16,17]. Heterogeneous photocatalysts generate an electron-hole (e , hþ) pair, when light of suitable wavelength hits its surface. The (e , hþ) pair reduces and oxidizes dyes/organic com­ pounds, generating reactive oxygen (O2 ) species and hydroxyl (�OH) radicals, which convert pollutants into eco-friendly compounds such as CO2 and H2O [18–21]. Various active properties are usually lacked by any individual entity due to absence of suitable phases, which can be developed and improved by combining various components (metal and nonmetals) together [22,23]. Mixed nano compositions are always found with improved developed phase coupling and hence enhanced properties. Various synthesis routes such as co-precipitation method, hydrothermal process, sol-gel technique, wet chemical method, micro-emulsion, colloidal solution and solid state reaction are reported for the preparation of heterogeneous nanocomposites [24]. Co-precipitation method is considered as one of the favorable synthesis techniques due to low cost, easy processing steps and large scale pro­ duction [25–27]. Among the transition metals nickel (Ni) is considered as one of the

2. Experimental details 2.1. Materials Chemicals such as AgNO3 (purity �99.9%) Ni(NO3)2 (purity �99.9%), Al(NO3)3 (purity �99.9%), NaOH (purity �99.5%) and silver paste (C2080415P2, Conductive ink) were purchased from SigmaAldrich, while MO dye (�98%) was purchased from BDH. All these chemicals were of analytical grade and used without any further purification. 2.2. Synthesis of Ag–Ni and Al–Ni NPs Co-precipitation method was followed in order to synthesize Ag–Ni and Al–Ni NPs. Equimolar ratios of AgNO3 (16.987 g/mol) and Ni(NO3)2 (18.270 g/mol) were mixed to prepare Ag–Ni NPs whereas, Al–Ni NPs 2

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Fig. 1. Schematic details for the synthesis of Ag–Ni and Al–Ni NPs.

resistance with respect to varying humidity. Schematic representation about humidity sensing measurements is shown in Fig. 2 [37]. 2.4. Photocatalytic degradation experiments Photocatalytic experiments were carried out in 250 mL beaker with magnetic bar for stirring. The synthesized Ag–Ni and Al–Ni NPs each of 85 mg/85 mL and MO dye solution with concentration ~0.03 M was utilized during experiments. Initially, solution mixture along with NPs was investigated for adsorption/desorption equilibrium in darkness for 30 min. Afterwards, the reaction was shifted under solar light for degradation. Decrease in concentration for MO dye was recorded by UV–Vis spectrophotometer by separating 5 mL of dye solution after every 10 min. Following equation was used to calculate degradation percentage for MO dye: �� �� Ao At � 100 (1) Degradationð%Þ ¼ Ao

Fig. 2. Schematic representation for humidity sensing measurements.

where, initial concentration of MO dye is Ao and At is the concentration at any specific time interval after degradation.

were synthesized by mixing Al(NO3)3 (21.299 g/mol) and Ni(NO3)2 (18.270 g/mol) together. The solutions were prepared with double distilled water and were stirred constantly at room temperature. Solu­ tion of NaOH (0.1 M) was added dropwise at constant stirring such as precipitates were appeared at pH value of 9. The solutions with pre­ cipitate were further stirred on a hot plate at a temperature of 60 � C for 24h. A mixture of ethanol and distilled water (60%:40%) was used to wash the precipitate 5 to 6 times in a centrifuge machine. The washed precipitate was then kept in oven at 60 � C overnight for drying. The dried powders were calcined at 400 � C (heating/cooling rate of 5 � C/ min) for 2h. The calcined powder were stored in vials for future use. Schematic details for the synthesis of Ag–Ni and Al–Ni NPs is given in Fig. 1.

2.5. Instruments used for characterization X-Ray diffractometer (XRD, Bruker-D8) with CuKα radiation source and 2θ ranged from 5 to 80� was utilized in order to evaluate the crys­ tallinity and phase structure for resultant NPs. Various functional groups and elemental compositions of the synthesized NPs were examined using Fourier transform infrared (FTIR) spectroscopy with wave number ranged from 400 to 4000 cm 1 and X-ray photoelectron spectroscopy, XPS (250Xi-UK) with AlKα as the excitation source. Thermogravimetric analysis (TGA-Perkin-Elmer Pyris1) at temperature ranged from 30 to 1000 � C with air atmosphere provided us with real content and thermal stability. Band gap energies were calculated via UV–vis spectropho­ tometer (UV2800-Wincom, China). Specific surface area, pore volume and pore size were measured at 77K using Bestech Instrument Tech­ nology (JW-BK300 Beijing). Microstructure and morphology was investigated via scanning electron microscope (SEM QUANTA FEG-450) operating at 15.0 kV and transmission electron microscope (TEM, JEOLJEM2100F).

2.3. Humidity sensing measurements The calcined powders were pressed into 2–3 mm high and 10 mm thick pellets at 100 MPa via hydraulic press. The two faces of pellets were coated with conducting silver paste to fabricate it into electrodes. This complete set-up was kept in controlled humidity chamber. Water vapors were injected inside humidity chamber at a steady rate to change its level from 10 to 95%. Digital hygrometer (RH-101) and LCR meter (4284-A) were utilized to indicate humidity level and variations in 3

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Fig. 3. XRD patterns for (a). Ag–Ni and (b). Al–Ni NPs. (c). FTIR and (d, e). TGA curves for Ag–Ni and Al–Ni NPs.

3. Results and discussion

could be attributed towards NiO cubic phase for both Ag–Ni and Al–Ni NPs. Whereas, peaks at 38.12� (110), 44.32� (111), and 58.23� (012) represented Ni and peaks at 31.02� (002) and 65.84� (004) were found due to Ni2O3. The crystallite size calculated from XRD measurements by using Scherer’s equation (i.e. Eq. (2)) was found to be 42.32 and 51.46 nm for Ag–Ni and Al–Ni NPs respectively.

3.1. XRD, FTIR and TGA XRD patterns were utilized in order to evaluate the crystal structure of the synthesized NPs as displayed in Fig. 3a, b. Crystalline phase for Ag NPs with FCC structure (PDF#04–0783, 01-087-0719) were attributed towards the peaks at 32.80� (110), 38.00� (111), 44.20� (200), 62.72� (220) and 64.70� (311) for Ag–Ni NPs. Similarly, for Al–Ni NPs, peaks at 18.78� (101), 25.03� (121), 35.32� (001), 37.98� (111), 44.32� (200), and 65.58� (220) were representing aluminum phase for Al2O3 (PDF#46–1215). Peaks at 42.12� (200), 62.28� (220), 74.49� (311)

x ¼ 0:89λ=βcosθ

(2)

where x is the average particle size, λ represents X-Rays wavelength, β is FWHM (full width at half maximum) and θ is the Bragg’s angle. In order to elaborate, the details about various functional groups for

Fig. 4. (a). DRS curves and Nitrogen adsorption/desorption isotherms for (b). Ag–Ni and (c). Al–Ni NPs. 4

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synthesized NPs, FTIR spectrum was utilized as shown in Fig. 3c. Bands ranged from 3375 to 3475 cm 1 and 1643 cm 1 were due to OHstretching and OH-bending vibrations respectively, indicating the presence of water vapors [23,24]. Vibrations due to NO3 1 stretching were indicated at wavenumber ~1340–1400 cm 1. The C–O stretching and M-O-M, M ¼ O absorption was shown at 1092 cm 1 and 599 cm 1 respectively [38]. TGA curves were obtained in order to know about the thermal

Table 1 Textural data for Ag–Ni and Al–Ni NPs. Nanomaterials Ag–Ni Al–Ni

Surface area 2

18.63 m /g 8.52 m2/g

Pore volume

Pore size

0.2846 mL/g 0.0556 mL/g

1.3277 nm 1.3678 nm

Fig. 5. (a). Low resolution XPS spectrum, Survey scan, and high-resolution spectra for (b). Ag3d/Ag–Ni (c). Al2p/Al–Ni (d, e). Ni2p/Ag–Ni, Al–Ni (f, g). O1s/Ag–Ni, Al–Ni NPs. 5

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3.2. DRS and BET

Table 2 Spectrum used for detailed scans. Peaks

Nanoparticles & Peak B.E (eV) Ag–Ni

Al–Ni – 73.68/68.64

Ni2p

373.72/367.71 – 876.30/861.36/ 856.94/854.99 –

O1s O1s

530.94 –

Ag3d Al2p Ni2p

873.93/862.27/ 857.98/856.22 – 530.93

%Area

Step Size

40.55/59.45 79.96/20.04 31.14/34.74/ 18.40/15.72 27.58/45.08/ 32.31/27.34 100 100

1.00eV 1.00eV 1.00eV

Band gap energies as an important factor in photocatalytic ability for Ag–Ni and Al–Ni NPs were calculated ~1.30 and 1.51eV respectively, as shown in Fig. 4a. Decreased grain boundary density could be responsible for decreased band gaps of the synthesized NPs [25,26]. A proper energy difference associated with band gap is very necessary for effective photocatalytic ability of the NPs, and this energy difference was measured through DRS [39]. Surface features of the as prepared samples were analyzed via BET adsorption/desorption isotherms as displayed in Fig. 4b, c. The textural data for Ag–Ni and Al–Ni NPs is summarized in Table 1.

1.00eV 1.00eV 1.00eV

stability and real content of the synthesized samples as shown in Fig. 3d, e. Total weight loss was calculated as 14.93 and 39.29% for Ag–Ni and Al–Ni NPs respectively. Ag–Ni NPs were found more stable as compared to Al–Ni NPs, as can be seen from the corresponding mass loss curves.

3.3. XPS Variations in binding energies, associated with core levels, chemical shifts, multiple splitting and shake-up satellite structures, accompanied by photoelectron spectrum were investigated via XPS instrument [40]. Co-existence of constituent elements inside the fabricated NPs were

Fig. 6. SEM images for (a). Ag–Ni and (b). Al–Ni NPs.

Fig. 7. TEM micrographs for (a, b). Ag–Ni and (c, d). Al–Ni NPs. 6

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Fig. 8. (a, d). HRTEM micrographs and (b, c, e, f). SAED patterns for Ag–Ni and Al–Ni NPs.

confirmed by the presence of Ag3d, Al2p, Ni2p and O1s peaks as shown in Fig. 5a. Low-resolution scans with step size of 1eV were collected over whole energy range of (0–1361)eV, and detailed spectra are summarized in Table 2. Peaks for Ag3d5/2 indicating Ag, Ag2O and AgO were found at binding energies ~367.71 and 373.72eV. Lower energy peak repre­ sented Ag3d3/2 and high energy peak was due to Ag3d5/2 as shown in Fig. 5b. Peaks at binding energies of 73.68 and 68.64eV for Al–Ni NPs referred to Al2p within Al2O3 and Al-hydroxide as shown in Fig. 5c. Ni2p spectrum exhibited four peaks at 876.30, 861.36, 856.94, 854.99 (for Ag–Ni) and 873.93, 862.27, 857.98, 856.22eV (for Al–Ni), which were due to Ni(0)2p3/2 and Ni(0)2p1/2 as shown in Fig. 5(d and e). The Ni(0) 2p3/2 peak at binding energy lower than those of typical Ni(0) NPs was due to increased charge density around Ni0 and decreased binding en­ ergy. In general, binding energy of Ni2p is sensitive to the surrounding chemical environment. High-resolution spectrum for oxygen is shown in Fig. 5(f and g) for Ag–Ni and Al–Ni NPs respectively. Each sample contained one peak at 530.94eV for Ag–Ni and 530.93eV for Al–Ni NPs. Low energy peak could be assigned as surface –O while, highest energy peak could be generally associated with adsorbed oxygen species [41].

in Fig. 9a, c, which is also in very good agreement with average size estimated from XRD measurements. EDS mapping obtained via TEM represented the distributed patterns for Ag, Ni and Al accordingly, as shown in Fig. 9 (b, d). EDS spectrum confirmed the presence of Ag, Ni and Al and the absence of impurities in the prepared samples. 4. Humidity sensing performance 4.1. Resistive response and sensing ability of Ag–Ni and Al–Ni NPs Resistance measured for Ag–Ni and Al–Ni NPs as a function of RH ranged from (10–95%) is shown in Fig. 10a. Conductive behavior of NPs was enhanced with water adsorption. Hence decrease in resistance was observed with rise in humidity level. The proton hoping of chemisorbed –OH groups at lower RH values were responsible for electrical sensing response of the device [30–32]. Sensing ability of NPs was greatly influenced due to active participation of cations and higher amount of charge density. Value for resistance was decreased from 879 to 240MΩ and 745 to 169MΩ for Ag–Ni and Al–Ni NPs respectively. Increased sensitivity of NPs with increased RH% (as depicted in Fig. 10b) was calculated by utilizing an equation as follow;

3.4. SEM images and TEM, HRTEM micrographs

S¼

SEM and TEM images for Ag–Ni and Al–Ni NPs revealed grains with different morphology and sizes as shown in Fig. (6, 7) respectively. Particles were found with smooth surfaces and appeared to be aggre­ gated into small particles for Ag–Ni but for Al–Ni, particles were grown bigger in size. HRTEM images and SAED patterns were shown in Fig. 8 (a–f) provided us with crystallographic information about the synthe­ sized NPs. Various diffraction rings associated with corresponding planes such as (200), (111), (220), (010) and (311) confirmed the Ni phase for Ag–Ni and Al–Ni NPs were shown in Fig. 8 (b, e). Planes indicated as (111), (200), (220) and (311) revealed FCC crystalline phase for Ag (Fig. 8c) whereas planes as (111), (220), and (200) were due to Al as shown in Fig. 8f. HRTEM micrographs and SAED diffraction patterns are well consistent with XRD results. Particle size calculated via SEM was ranged from 72 to 95 nm, whereas, TEM (after fitting the histogram to Gaussian distribution) revealed an average crystallite size as 46. 52 and 55.46 nm for Ag–Ni and Al–Ni NPs respectively as depicted

ΔR ΔRH

(3)

4.2. Response and recovery behavior In order to evaluate qualitative performance of any sensing material, recovery and response times are the necessary factors to be determined [31]. During humidification process, the time interval to obtain 90% of final value for resistance, is termed as response time, whereas, the time to attain 10% of final resistance is known as recovery time [32]. RH value of 10 and 95% was maintained in two separate chambers, in order to indicate the response and recovery times of Ag–Ni and Al–Ni NPs accurately. Ag–Ni sample was recorded with 60s of response time, when it was moved from chamber at 10% of RH to the chamber at 95% of RH, whereas, recovery time was 39s when samples were shifted back from 95% RH level to 10% RH. Similarly, 59s of response time and 20s of recovery time were calculated for Al–Ni sample under the same exper­ imental conditions. The switching time was adjusted as 1s in each case. 7

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Fig. 9. (a, c). TEM particle size histogram and (b, d). EDS mapping for Ag–Ni and Al–Ni NPs.

8

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Fig. 10. (a). Resistance vs. RH (b). Sensitivity vs. RH (c). Response/recovery time, (d, e). Hysteresis curves and (f, g). Stability check for Ag–Ni and Al–Ni NPs respectively.

process due to absorbing surface and pore geometry is considered as one of the drawbacks for an efficient sensor known as hysteresis [42–46]. Hysteresis behavior for Ag–Ni and Al–Ni NPs at room temperature is shown in Fig. 10d, e. For absorption phenomenon, humidity level of the chamber was gradually increased from 10 to 95%, whereas for desorp­ tion curves, humidity parameter was retraced back from 95 to 10%. Higher absorption heat as compared to desorption heat indicated delayed mechanism of bonding energy between water vapors and sensing particles. Furthermore, different time rates could be attributed towards the slight lowering of resistance during desorption as compared to absorption [48,49].

Table 3 Comparison between the present work and previously published data. Nanomaterials

Response time

Recovery time

Ref.

SnO2 γ-Fe2O3 α- Fe2O3 ZnO Fe2O3/Si Cu–Fe ZnO–ZrO2 Ag–Ni Al–Ni

120–170s 150s 60s 36s 20s 60s 53s 60s 59s

32s 450s 350s 530s 80s 90s 69s 39s 20s

42 43 44 45 46 [47] 48 This work This work

4.4. Stability

The characteristic response and recovery curves for Ag–Ni and Al–Ni NPs are shown in Fig. 10c. This present work can be compared to pre­ vious reported literature as summarized in Table 3.

The factor of stability is very considerable for practical utilization of any humidity sensing device [46]. Therefore, variation of resistance with passage of time for Ag–Ni and Al–Ni NPs is depicted in Fig. 10f, g. Stability tests for both the samples were carried out after every 10 days for 2 months at RH level of 11, 34, 75, and 95% at a temperature of ~300K and frequency of 100Hz. Resistance variation of less than 2%

4.3. Hysteresis Formation of water clusters during absorption and desorption 9

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crystallites within sensing material causing dissociation of water mole­ cules into OH and Hþ ions. With increase in humidity, water molecule gets adsorbed to neighboring hydroxyl groups through hydrogen bond and this forms first physisorbed layer which remains fixed due to hydrogen bonding [42–45]. Subsequently more water molecules get adsorbed on the first physisorbed layer forming a second physisorbed layer which is mobile and hence electrolytic as well as protonic con­ duction is started within adsorbed layers. With further increase in RH values, multi physisorbed layers get stacked up and behave like bulk liquid water. During this stage H3Oþ releases a proton to neighboring water molecules, which accept it while releasing another proton and so forth, causing Grotthuss chain reaction [51,52]. 5. Photocatalytic performance of Ag–Ni and Al–Ni NPs

Fig. 11. Mechanism chain reaction).

for

humidity

sensing

measurements

Ag–Ni and Al–Ni NPs were analyzed for photocatalytic activity of MO dye under solar light exposure. Initially, no degradation was observed when the dye was exposed to solar light without catalysts. Also, for initial 30 min, reaction was carried out in dark in order to investigate the adsorption/desorption effect of MO dye. Afterwards, reaction was shifted to solar light along with synthesized photocatalysts under constant stirring. After every 10 min, 5 mL of solution was separated continuously through pipette and was investigated through UV–Vis spectrophotometer. A small decrease in MO dye concentration and hence low efficiency of catalysts was recorded in dark during first half hour. However, with further increase in irradiation time, decom­ position of MO dye was increased as shown in Fig. 12a, b. Absorption peaks for MO dye degradation were recorded at a wavelength of 464 nm. Decrease in absorption intensity with passage of time and decreasing intensity of UV–Vis light was observed for MO dye. The degradation results including decrease in concentration and linear correlation existing between ln(Ct/Co) and time are shown in Fig. 12c-e. Ag–Ni and Al–Ni NPs displayed 91 and 75% degradation for MO dye with total time recorded as 80 and 120 min respectively. Prominent increase in degradation percent was recorded with increase in irradiation time. The photocatalytic reaction rate and equilibrium amongst oxidant and reductant can be determined through kinetic re­ actions, which is mathematically represented as;

(Grotthuss

was indicated by both the NPs, revealing good stability of sensor apart from being good room temperature humidity sensing device. 4.5. Mechanism for humidity sensing The mechanism for increase in conduction due to adsorption of water vapors by NPs is elaborated by Grotthuss mechanism, involving three sequential steps such as chemisorption, physisorption and capillary condensation as depicted in Fig. 11 [50]. Initially during chemisorption stage, at low RH, water molecules get chemically adsorbed at neck of the

Fig. 12. UV–Vis spectrum for MO dye with (a). Ag–Ni and (b). Al–Ni NPs (c). Decrease in concentration vs. time, Linear correlation between ln(Ct/Co) and reaction time for (d). Ag–Ni (e). Al–Ni NPs. 10

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Table 4 Degradation results for Ag–Ni and Al–Ni NPs against MO dye. Nanoparticles Ag–Ni Al–Ni

Rate constant 2

1.92 � 10 min 6.50 � 10 3min

1 1

In

R2

Deg.

Time

0.94 0.92

91% 75%

80min 120min

Dye

Time

Degradation

Reference

TiO2-Chitosan/Glass CdS TiO2 Ni/TiO2 Al2O3⋅Fe2O3 Fe2O4/ZnO/ZF Ag–Ni Al–Ni

MO MO MO MO MO MO MO MO

240min 480min 180min 300min 140min 60 min 80min 120min

87% 30% 80% 40% 45% 52% 91% 75%

10 11 13 14 [53] [54] Present work Present work

kt

(4)

Table 4 summarizes degradation results along with reaction rate and correlation constant for Ag–Ni and Al–Ni NPs against MO dye. Table 5 summarizes comparison between the present work and previously re­ ported data.

Table 5 Comparison between the present work and previously reported data. Nanoparticles

Ct ¼ Co

5.1. Different parameters effecting photocatalytic degradation Additional photocatalytic degradation experiments were carried out in order to evaluate the influence of various factors such as different catalyst loading, varying pH, and various types of scavengers. Leaching was also investigated at different pH values such as 3, 6, 8 and 10 but no leaching was observed through atomic absorption. Furthermore, sta­ bility test was also studied for the synthesized NPs. 5.1.1. Different catalyst loading More active sites are provided by greater amount of catalyst, but beyond certain limit the degradation rate is decreased even with in­ crease in catalytic amount. This decrease may be explained on the basis

Fig. 13. Effect of (a). Catalyst loading (b). pH values (c). Different scavengers (d). Reusability (e). XRD and (f). FTIR for Ag–Ni and Al–Ni NPs after degradation of MO dye. 11

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5.2. Mechanism of photocatalytic degradation

Table 6 Recyclability results for Ag–Ni and Al–Ni NPs. Catalyst/Pollutant

1st Recyclability

2nd Recyclability

3rd Recyclability

Ag–Ni/MO AL-Ni/MO

88.75% 78.94%

82.84% 72.98%

74.73% 65.69%

Solar light make electrons excited from valence band to conduction band for Ag–Ni and Al–Ni NPs as expressed in Eq. (5). Excitation of electrons from valence band to conduction band generate electron-hole pair [8–12]. Ag and Al prevent the recombination process. Solar light (hv) þ Catalyst → hþ þ e

(5)

Hydroxyl free radicals ( OH) are produced due to reaction of water molecule with hole (hþ), hydrogen ion (Hþ) and hydrogen peroxides (H2O2). The H2O2 is fragmented into two �OH, which degrade MO dye. Electrons (e ) in conduction band generate super oxides free radicals (O�2 ). Due to high reactive nature of oxygen species (ROS) they are combined and make H2O2 molecule. The H2O2 can form �OH free rad­ icals which degrade MO dye as shown in Eq. 11 and 12. The complete mechanism for MO dye degradation is depicted in Fig. 14. �

(Ag–Ni or Al–Ni nanoparticles) hþ þ H2O → Hþ þ Ag–Ni or Al–Ni nano­ particles (�OH) (6) (Ag–Ni or Al–Ni nanoparticles) hþ þ �OH → Ag–Ni or Al–Ni nanoparticles (�OH) (7)

Fig. 14. Mechanism for MO dye degradation.

2 hþ þ 2 H2O → 2 Hþ þ H2O2

(8)

H2O2 → 2 �OH

(9)

(Ag–Ni or Al–Ni nanoparticles) e þ O2 → (Ag–Ni or Al–Ni nanoparticles) O�(10) 2

of shielded photon absorption ability due to agglomeration and satu­ ration of catalyst particles [20–23]. So, increase in catalyst quantity is beneficial but only to a specified point [22]. Reaction rate was increased with increasing amount for Ag–Ni and Al–Ni NPs from 20 mg/85 mL to 80 mg/85 mL, as shown in Fig. 13a.

O2�- þ 2 �OH þ Hþ → O2 þ H2O2, H2O2→ 2 �OH -

�

O2� or HO þ MO → H2O þ CO2

(11) (12)

6. Conclusion

5.1.2. pH effect Degradation ability of Ag–Ni and Al–Ni NPs was greatly influenced with varying values of pH. Because pH of catalyst can influence surface charge and hence the photocatalytic ability [10–12]. Decomposition of MO dye was thoroughly investigated with Ag–Ni and Al–Ni NPs at different pH values of 3, 6, 9 and 12 as shown in Fig. 13b. Highest catalytic activity was indicated by Ag–Ni NPs at a pH value of 9, whereas, Al–Ni NPs were revealed best at pH value of 12.

We have successfully synthesized Ag–Ni and Al–Ni NPs via coprecipitation method. The synthesized NPs were characterized by XRD, FTIR, TGA, DRS, BET, XPS, SEM and TEM. Well defined crystal structure was revealed by XRD patterns. SEM and TEM images indicated nano size heterogeneous microstructures for both Ag–Ni and Al–Ni samples with average particle size ranged from 42 to 95 nm. Band gap energies were measured as 1.30 and 1.51eV for Ag–Ni and Al–Ni NPs respectively. The humidity performances of the as-prepared nano­ structures were tested systematically. Increase in conductivity, hence decrease in resistance has been observed with increase in humidity concentration. Active response/recovery time, low hysteresis behaviour with excellent stability were noticed for both the samples. Furthermore, Ag–Ni and Al–Ni NPs degraded MO dye up to 91 and 75% with total reaction time of 80 and 120 min respectively. Degradation efficiency of the NPs were thoroughly investigated for various factors such as catalyst loadings, pH, and different scavengers. Stability check and reusability of the NPs were also satisfactory. The detailed investigations about Ag–Ni and Al–Ni NPs confirmed that these NPs could be used as potential candidate for practical use in humidity sensing and efficient catalysts for water purification.

5.1.3. Scavenging effect Ethanol and H2O2 were utilized in order to investigate scavenging effect for Ag–Ni and Al–Ni NPs. Rate of degradation reaction for MO dye was a bit higher in presence of ethanol scavenger as compared to H2O2 as shown in Fig. 13c. 5.1.4. Reusability and stability check Ag–Ni and Al–Ni NPs were used over three successive cycles in order to evaluate their regeneration and stability. Reusability experiments were carried out by using 85 mg/85 mL of NPs with 0.03 M MO dye concentration. The photocatalysts were recovered and washed with enough acetone and water mixture for three consecutive cycles. As shown in Fig. 13d and Table 6, degradation efficiency for both the NPs were found to be more or less decreased indicating the accumulation of pollutant particles upon the active sites of photocatalysts [11,25]. Also decreased percent degradation could be attributed towards the reduced (almost 90%) recycled amount of photocatalysts during each cycle. Additionally, stability for Ag–Ni and Al–Ni NPs was also tested by investigating XRD and FTIR patterns after degrading MO dye. As shown in Fig. 13(e and f), no apparent change was indicated in XRD peaks and FTIR. The results indicated that the synthesized NPs hve great stability and potential to be applied as an efficient photocatalyst.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors are highly grateful to National Natural Science 12

Materials Chemistry and Physics 244 (2020) 122748

K. Shaheen et al.

Foundation of China (51571002), Beijing Natural Science Foundation (2172008), Program of Beijing City and Beijing University of Technol­ ogy, Evaluation Research for the Performance of Tapes (GH201809CG005), General Program of Science and Technology, Devel­ opment Project of Beijing Municipal Education Commission of China (No. KM201810005010), Project of Advanced Discipline (No. PXM2019-014204-500031), the Department of Chemistry Bacha Khan University Charsadda, Khyber Pakhtunkhwa Pakistan, and the Center of Excellence for Advanced Materials Research King Abdulaziz University Jeddah Saudi Arabia for their joint research collaboration.

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