Synthesis, characterization, optical, and sensing investigations for Fe-BDC doped with 10 wt.% of activated food waste biochar

ABSTRACT

Traditional methods of synthesizing nanomaterials via physical or chemical methods are typically expensive and result in toxic byproducts that adversely affect the environment. In this study, we propose synthesis, characterization, optical and sensing properties of hybrid nanocomposites consisting of Fe-1,4-benzene-dicarboxylates (Fe-BDC) as a porous metal-organic framework (MOF) doped with 10 wt. % of Biochar (BC) derived from food waste (Fe-BDC@BC). The bio-based MOF was characterized by using SEM, TGA, FTIR, and XRD. The structural investigation showed that BC improved the porosity, thermal stability, and crystallinity of Fe-BDC, resulting in a particle size of around 50 nm. Optical investigation showed that the energy gap of the bio-doped MOF decreased by about 6%. A sensitive layer of Fe-BDC@BCs was created on an ITO substrate using the spin coating method to study humidity sensing. The dynamic response of the FE-BDC@BC nanocomposite sensor to the change in humidity from 10% to 95% RH. Fe-BDC@BC demonstrated a sensitive response to humidity of about 96% and quick reaction and recovery times of 10 s and 50 s, respectively, Due to their ease of synthesis, and excellent optical and humidity-sensing response, these bio-based MOFs could be used for the fabrication of optoelectronic and sensing devices.

KEYWORDS:

• Biomaterials
• energy gap
• refractive index
• sensor

1. Introduction

The primary concern for multiple research groups is the replacement of traditional chemical materials with green materials that could have a crucial role in reducing environmental pollution and promoting sustainability [1]. Green materials usually have lower carbon footprints, utilize renewable resources, and can be recycled or biodegraded. Furthermore, the use of green materials typically has positive effects on human health as they are generally less toxic and harmful compared to traditional chemical materials [2]. One solution to minimize pollution is converting food residues such as animal bones, date kernels, and mango kernels into biochar [3]. Biochar has undergone extensive research in effectively removing harmful substances like organic compounds and heavy metals from the environment [4]. Carbonaceous material derived from biomass demonstrates exceptional activity and stability due to its porous three-dimensional/multilayer structure and the presence of nitrogen (N), phosphorus (P), or sulfur (S) through self-doping [5]. The activated biochar derived from animal bones can be utilized as a valuable carbon-rich material with various functions in industries such as agriculture, environmental remediation, and water treatment systems, among others [6]. Smith et al. [7] demonstrated the use of activated biochar as a sensing platform for detecting heavy metals in water. The biochar-based sensor exhibited high sensitivity and selectivity, highlighting its potential for environmental monitoring. In another study, Syed et al. [8] investigated the biofabrication of Pd/TiO doped with biomass from rice husk activated biochar and studied its electrocatalytic and biomedical properties.

Metal-organic frameworks (MOFs) offer brand-new materials with an active porous surface that can be used for a variety of purposes such as catalysis, water purification, energy storage, and sensing [9]. 1,4-benzene-dicarboxylates is one of the most significant MOFs because they are highly porous, chemically stable, and have modular decompositions [10]. Wang et al. [11] reported the utilization of BDC MOF as a fluorescence probe for the detection of metal ions. The unique properties of BDC MOF allowed for selective and sensitive detection due to the interaction between the metal ions and the MOF framework. In addition, Liu et al. [12] explored the use of BDC MOF as a luminescent material for sensing volatile organic compounds (VOCs). The emissions from BDC MOF exhibited changes in response to various VOCs, enabling the detection and identification of specific gases. Herrera et al. investigated the effects of adding nickel and cobalt to MOF-525 on the charge transfer rate [13]. They discovered that the characteristics of the metal center are one of the numerous factors that influence the speed at which charge is transferred. Their results emphasize the significance of the properties of the redox active site in enhancing charge transfer in electrodes based on MOFs. Furthermore, Rahmanifar et al. developed and researched MOFs that contained different types of metals and their effective in MOFs applications [14]. Finally, Panda et al. successfully improved the electrical and visible light absorption of MOFs by incorporating Zn and made it ideally suited for optoelectronic and sensing applications [15]. These studies demonstrate the potential of BDC MOF as a versatile material for optical and sensing applications, paving the way for further research and development in this field.

Regulating moisture is a crucial factor, and humidity and soil moisture measures are recognized as universal criteria in food processing, food storage, and environmental studies [16,17]. Fe-BDC@BC has shown significant potential in various applications related to food processing, food storage, and environmental studies.

It can be utilized as a food preservative due to its antimicrobial properties. Fe-BDC@BC can help inhibit the growth of spoilage-causing microorganisms, extending the shelf life of food products and reducing the need for synthetic preservative, Fe-BDC doped biochar has excellent adsorption properties and can be used to remove various toxic compounds, such as heavy metals, pesticides, and organic pollutants, from food matrices during processing. This application can help enhance food safety. Furthermore, it can act as an ethylene adsorbent, reducing spoilage and extending the freshness and shelf life of stored produce. Fe-BDC@BC provides an additional advantage by releasing iron into the soil, which can promote plant growth and enhance soil health.

Several studies have demonstrated its ability to absorb and release moisture in response to changes in relative humidity. For instance, researchers have utilized activated biochar-coated electrodes to develop a capacitive humidity sensor with high sensitivity and excellent stability [18]. In another study, activated biochar nanocomposites were successfully employed in resistive humidity sensors, showing good linearity and repeatability over a wide humidity range [19]. Overall, these findings highlight the potential of activated biochar for humidity sensing applications, offering cost-effective and eco-friendly alternatives in this field [20,21]. Our research has the potential to contribute to the development of a cost-effective and efficient optical and humidity sensor. By incorporating Fe-BDC (iron-benzene-1,4-dicarboxylate) into biochar, you are exploring a novel approach that could enhance the sensor’s performance. Biochar, with its porous structure and large surface area, can adsorb gases and molecules effectively, making it an excellent candidate for sensor materials. Additionally, the inclusion of Fe-BDC could enhance the biochar’s sensing properties due to its ability to interact with various analytes, such as water molecules or specific gases. Overall, our research has the potential to advance sensor technology by providing a more affordable and reliable solution for optical and humidity sensing applications.

To improve the structural, optical, and sensing characteristics of Metal-Organic Frameworks (MOFs) like Fe-BDC, a sustainable material, activated biochar derived from food waste was incorporated into Fe-BDC. The crystalline and morphological properties of the Fe-BDC/BC composite were analyzed using various techniques including UV/Visible, SEM, TGA, FTIR, and XRD. Subsequently, the optical and humidity sensing properties of the composite were examined. Experimental protocol for our thesis is presented in Scheme 2. Overall, the incorporation of biochar from food waste in Fe-BDC MOFs demonstrated a promising material that is environmentally friendly, cost-effective, and efficient in various applications such as optoelectronics, energy storage, and sensing in the industry.

2. Methodology

2.1. Preparation of fe-BDC@BC

To prepare the biochar from food waste, chicken bones and mango kernels have been collected and cleaned them using methanol and distilled water. After drying the samples in an oven for 24 hours, we combusted them for 5 hours in a tube furnace set to a temperature of 700°C to obtain a carbon-based powder. To remove impurities from the black powder produced, the sample has been washed with a solution mixture of nitric acid and sulfuric acid having a volume ratio of 3:1 at a temperature of 80°C for 5 hours. Then, the sample was dried overnight at a temperature of 60°C to obtain Biochar. The biochar fabrication has been explained in detail in [2]. For this investigation, we used FeCl₃.6 H₂O, BDC, and ethanol, which were all analytical grade and received from Sigma Aldrich without further purification. We dissolved 2 gm of BDC in 50 ml of FeCl₃.6 H₂O by using N,N-dimethylformamide (DMF) to form Fe-BDC. We then added 2.5955 g (13 mmol) of FeCl₃.6 H₂O at a 1:1 molar ratio and thoroughly stirred to dissolve. By heating to near dryness in a microwave oven, cooling, and adding 10 mL of ethanol, the Fe-BDC MOF formation has been improved. Finally, the product was dried at 70°C. Scheme 1 shows T the chemical structure formation of the assembled Fe-BDC MOFs. To dope the BC with Fe-BDC, we weighed 0.9 g of Fe-BDC and 0.1 g of BC individually using a sensitive electronic balance. Then, we ground them using a ball mill for 30 minutes to make a homogeneously mixed powder. The resulting mixture of powder has been located in a microwave oven and heated to near dryness, cooling, and adding 10 mL of ethanol, repeat the step of microwave for more than three times and finally, we obtained Fe-BDC@BC powder.

Scheme 1. The chemical structure formation of the assembled Fe-BDC MOFs.

2.2. Instrumentation

The Fe-BDC@BC was examined for its structural and morphological features using X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Fourier Transmission Infrared (FTIR), and Thermal Gravimetric Analyzer (TGA). The crystal structure of the bio-based MOF was characterized using an X-ray Diffractometer (Rigaku RINT 2200, Japan) with Cu Kα radiation (λ = 1.540598 Å) in the range of 2Ɵ = 4° to 80°. The surface morphology of the deposited Fe-BDC@BC thin films was studied using SEM Model No. JEOL-SEM JFC-1100E, China, at a magnification of 20,000× while operating at 25 kV. During SEM analysis, a double-coated carbon tape was used to disperse the electron beam charge and heat buildup on the analyzed sample. Then, in a vacuum, an ion-sputtering coating method was used to apply a thin layer of gold. Thermogravimetry (TGA) was carried out using a Shimadzu, Japan, TGA-50 thermal analyzer within the range 50–500°C. The chemical structures of thin films were examined using FTIR (Shimadzu Japan, FT-IR 8400S Spectro-photometer using the Kbr pellet method) in the 400–4000 cm⁻¹ spectral region.

2.3. Fabrication of sensing device and sensor characterizations

The method for synthesizing Fe-BDC@BC nanocomposite as previously described was used to create 0.5 gm of the material. To ensure the distribution of the composite, our material will be ultrasonicated for 30 minutes. Next, the ITO-coated glass substrates were cleaned in ultrasonic baths and rinsed with deionized water, acetone, and isopropyl alcohol (IPA). The substrates were then dried for 2 hours at 100°C in an oven. The cleaned ITO-coated substrates were treated with nitrogen and oxygen plasma (1 Torr for 10 minutes) to increase their hydrophilicity. The prepared ITO glass slides were then spin-coated with the Fe-BDC@BC nanocomposite to form a thin film, which was subsequently dried in a dynamic vacuum for two hours at 100°C to eliminate any leftover water content. A sensing device, as schematically represented in Scheme 2, was made by first etching the gold electrodes into the film, then utilizing the thermal deposition technique, depositing silver contact electrodes.

Scheme 2. Fabrication of sensing device and experimental technique used for humidity sensing.

As previously reported [16], the Fe-BDC@BC humidity sensing at ambient temperature in our humidity sensor setup. By employing a Keithley (6487 pico ammeter/voltmeter system), the resistance variations for various %RH are recorded. Using the relation, the sensing film’s sensitivity to various %RHs is estimated.

(1) $\mathrm{Sensitivity}($(1)

where RH and RA stand for the sensor films’ resistances to dry air and moisture, respectively. In the presence of humidity, the reaction time is determined as the time it takes to reach a 90% change in baseline resistance, and the recovery time is the time it takes to reach equilibrium in the absence of humidity.

3. Results and discussion

3.1. Sem

Scanning Electron Microscopy (SEM) analysis is typically used to investigate the morphology of particles on a material’s surface. This analytical technique reveals the surface characteristics of the biobased-MOF. Figure 1 shows the SEM images for our investigated composites. Both images exhibit a higher density of particles at the surface due to the type of material being investigated. The SEM images demonstrate regular particle distribution with high homogeneity, indicating the success of the Fe-BDC@BC synthesis. Based on the SEM image, Fe-BDC is identified as a rod and bar structure as in [22]. Additionally, Fe-BDC@BC contains clear surface voids, indicating an increase in porosity within Fe-BDC due to BC doping. This enhancement could improve the sensing and storage characteristics of our nanocomposites.

Figure 1. SEM images for Fe-BDC (a) and Fe-BDC@BC (b).

3.2. Xrd

The XRD technique was used to determine the crystalline structure and average particle size of Fe-BDC@BC, which is commonly used to analyze nanoparticle structural characteristics [23]. The XRD patterns for our nanocomposites, Fe-BDC and Fe-BDC@BC, are shown in Figure 2, respectively. X-rays are diffracted at an angle variation of 2Θ between 0 and 90 degrees on lattice planes. The crystalline nature of Fe-BDC and Fe-BDC@BC is examined by the existing lattice planes, which are represented in Table 1. According to the table, Fe-BDC is represented by three main peaks at 17.37°, 28.01°, and 38.10° related to indices (210), (221), and (440), respectively. The XRD results for Fe-BDC are consistent with (JCPDS no. 09–0432) [24,25]. On the other hand, BC is represented by 25.6°, 30.01°, 34.01°, and 37.01° related to indices (333), (442), (002), and (111).

Figure 2. XRD spectra for Fe-BDC and Fe-BDC@BC.

Table 1. Shows the XRD peaks for Fe-BDC and Fe=BDC@BC.

2 ϴ Fe-BDC	2 ϴ Fe-BDC@BC	planes	crystallinity degree
17.37°	17.3°	(2 1 0)	78%
-	25.6°	(333)	72%
28.01°	28.00°	(2 2 1)	74%
-	30.01°	(442)	63%
-	34.01°	(002)	59%
-	37.10°	(111)	61%
38.10°	38.10°	(440)	51%

The degree of crystallinity of the peak pattern could be measured using [26]:

(2) $X_c\left(\mathrm{\%}\right)=\frac{A_c}{A_c+A_a}\times 100$(2)

where A_c and A_a stand for the crystalline region and the amorphous region, respectively. The degree of crystallinity for each XRD peak has been listed in Table 1. By applying Scherer’s equation [27] to all planes demonstrates above, the average crystallite size of Fe-BDC and Fe-BDC@BC is 61.12, and 67 nm, respectively.

3.3. Thermogravimetric analysis

The TGA analysis was used to calculate the weight loss of the nanocomposites with varying temperature. The mass degradation of the Fe-BDC@BC is introduced in Figure 3 through main four steps: mass losses of 0.77 g (36.7% loss), 0.74 g (35.2% loss), and 0.4 g (19.1% loss) and 0.11 gm (5.2%) that occurred at 0 to 100°C, 100 to 180°C, 250 to 400°C and 460 to 630°C, respectively. The degradation can be explained based on three main steps. First, the adsorbed water molecules are removed from the surface of Fe-BDC@BC. Second, there is a partial thermal degradation of the organic volatile traces. Lastly, the Fe-BDC@BC decomposes, resulting in a residual mass of 0.10 g (4.7%). Therefore, we can conclude that the synthesized biomass MOF has excellent thermal stability.

Figure 3. TGA for Fe-BDC@BC.

3.4. FT-IR analysis

The main purpose of FTIR spectroscopy is to examine the nature of chemical bonding within the nanocomposite. IR spectra could be used to precisely identify the interactions between its constituent parts. The FTIR spectra of Fe-BDC and Fe-BDC@BC in the wavenumber range between 400 to 4000 cm⁻¹ are shown in Figure 4. The peak that appears at 3610 cm⁻¹, verifies the presence of phenol in food waste extract, denotes O-H stretching vibration bonds [28]. The C=O expanding vibration of the carboxylic and keto groups is related to the rise at 1810 cm ¹. While the peak around 850 cm⁻¹, are related to the existence of the -CO bonds such as H-CO and C-O-C.

Figure 4. FTIR spectra for Fe-BDC and Fe-BDC@BC.

3.5. Optical properties measurements

Optical properties are essential in investigating novel synthesized materials. The transmittance (T) and reflectance (R) in the wavelength range up to 1100 nm is studied for bio-based thin films. The T and R spectra are shown in Figure 5 as a function of wavelength (in nm) for the investigated composites. As it can be seen in 325–1100 nm wavelength range, Fe – BDC@BC has an optical transmission approximately 10% higher than Fe-BDC. This could be related to the increase in porosity allowing light to transmit much higher.

Figure 5. The transmission and reflection of Fe-BDC and Fe-BDC@BC.

According to the WDD model, the energy band gap could be calculated by using [29]:

(3) ${\left(\mathit{\alpha h\nu }\right)}^{0.5}=\mathit{A}\left(\mathit{h\upsilon }-E_g^{opt}\right)$(3)

where α is the absorption coefficient and it is calculated $\mathit{\alpha }=\frac{1}{x}\mathit{ln}\left(\frac{1-\mathit{R}}{T}\right)$, A is a constant and the abscissa extrapolation results in the energy E_g with the matching prohibited bandwidth. Figure 6 shows the relation between (αhν)^0.5 and hν for Fe-BDC and Fe-BDC@BC samples. As we can see from Figure 6, the energy gap for Fe-BDC and Fe-BDC@BC are 3.93 and 3.71 eV, respectively [28]. The addition of BC act to decrease the energy gap of sample by about 0.22 eV. This could be related to the increase in the density that happens to the valence and conduction bands from BC addition.

Figure 6. The energy gap calculation for Fe-BDC and Fe-BDC@BC.

The refractive index, n, of our fabricated thin film, is determined from [30]

(4) $\mathit{n}=\frac{\left(1+\mathit{R}\right)+\sqrt{4R-{\left(1-\mathit{R}\right)}^2\mathit{k}}}{\left(1-\mathit{R}\right)}$(4)

Where, n is an essential parameter in optoelectronic devices. Fig. 7 shows the refractive index variation with the photon energy. The refractive index of Fe-BDC@BC has a high value than Fe-BDC by about 10% (between 380 nm to 1100 nm). This could be related to the porosity increase in the biobased compound.

Figure 7. The refractive index variation for Fe-BDC and Fe-BDC@BC with wavelength.

3.6. Humidity sensing measurements

As shown in Scheme 2, the glass panels that make up the humidity sensor chamber have side lengths of 250 mm by 250 mm and a thickness of 5 mm. The chamber’s airtightness is achieved via rubber beading. The sampler holder is positioned in the chamber’s center, and the inlet on the left side of the chamber distributes humidity evenly throughout. Hygrometers and Keithley source meters are utilized to detect changes in the composite films’ resistance. A vacuum pump is attached to the right side of the chamber to de-humidify the space in preparation for the subsequent sample measurement. Figure 8A depicts the resistance change of investigated bio-MOF (Fe-BDC@BC) under a constant frequency of 100 Hz. It can be seen in the figure that the resistance’s magnitude decreases with the relative humidity of the two investigated samples. This may be related to the decrease in polarization that happens within the materials because the molecules could not follow the electric field direction. The resistance change of Fe-BDC@BC is much greater than Fe-BDC; this could be related to the existence of biochar bonds and elements previously discussed in the FTIR investigation. These results show that our bio-MOF has better humidity-sensing abilities. Figure 8B shows the resistance change for Fe-BDC@BC at desorption and adsorption of the vapor. The results at desorption and adsorption are nearly equal with a slight difference of 8%. We can conclude that our bio-MOF has high reverse stability and efficiency as a humidity sensor [31].

Figure 8. (A) resistance change with %RH for Fe-BDC and Fe-BDC@BC (B) resistance change with %RH for Fe-BDC@BC during absorption and desorption.

The sensing response of the humidity sensor as determined using eqn (1)(1) $\mathrm{Sensitivity}($(1) in the humidity range 0 to 95% RH for both Fe-BDC and Fe-BDC@BC nanocomposites is depicted in Figure 9A. The humidity sensing range is corresponding to previously published papers [32]. The sensitivity of Fe-BDC shows a maximum of 38% at 95%RH where as the Fe-BDC@BC nanocomposite exhibit a sensitivity of 91% @ 95%RH. The improved sensitivity of Fe-BDC@BC nanocomposite could be ascertained to the highly porous structure that facilitates the adsorption of water molecules at the sample surface [33,34]. This film surface adsorption process will create more protons (H₃O⁺→H₂O+ H⁺) [35]. The higher number of protons generated on the surface of the film increases the proton between the molecules of water and hence reduces the resistance of the film at higher %RH. Also, according to the Grotthuss chain reactions H₂O + H₃O+→H₃O⁺ + H₂O. The conductivity of the sensor rises and the resistance reduces when water molecules move into the intermediary layers between the Fe-BDC@BC film [36]. Fe-BDC@BC composite sensor exhibits superior humidity-detecting behavior compared to bare Fe-BDC because BC promotes increased conductivity by producing conducting islands and makes charge carrier hopping easy. Figure 9B shows the variation of the sensitivity of Fe-BDC@BC nanocomposite at 95% RH for a period of 60 days. The Fe-BDC@BC nanocomposite sensor shows extremely high stability with marginal change in sensitivity values for a period of 60 days, indicating that the sensor used in the present investigation is highly stable. Toward the development of effective humidity sensing devices, the response/recovery times of a humidity sensing device play a very important role in technological applications. A good sensing device needs to show a short response/recovery time to be used as an ultra-fast sensor [37]. Figure 10 depicts the dynamic response of the FE-BDC@BC nanocomposite sensor to the change in humidity from 10% to 95%RH. The Fe-BDC@BC nanocomposite shows very fast response and recovery characteristics with response times of 31 s and 47 s respectively. The Fe-BDC@BC nanocomposite was found to be a good humidity sensing material, extremely effective when used at room temperature, and has great stability, making it highly viable for humidity sensor device manufacturing. Table 2 compares the optical and humidity results of Fe-BDC@BC with similar published materials.

Figure 9. (A) the sensing response with %RH for Fe-BDC and Fe-BDC@BC (B) stability of Fe-BDC@BC nanocomposite @95%RH for 60 days.

Figure 10. The dynamic response of Fe-BDC@BC sensor (to evaluate response/recovery time).

Table 2. Comparison between the optical and humidity results between Fe-BDC@BC and similar published materials.

Material	Energy gap, eV	Humidity sensitivity, %	Responds time, Sec	Ref.
Fe-BDC@BC	3.7	95	10	-
MOF−derived ZnCo2O4/polypyrrole	5	95	8	[38]
MOF−derived SnO2/chitosan	2.95	97	11	[39]

4. Conclusion

In this work, we fabricate bio-based MOF thin films, which could be employed as a potential material for optical and humidity sensing applications. SEM, XRD, FTIR, and TGA methods were used to characterize the produced bio-based MOF thin film nanocomposites. The surface morphology has been investigated by SEM images before and after adding BC. BC is strongly dispersed in the Fe-BDC matrix as observed by the nanocomposite film’s FTIR spectra. The optical properties assured that the refractive index increased by about 10% because of BC addition. The TGA thermogram of the thin films shows that Fe-BDC@BC is a thermally stable material. The humidity sensing response of Fe-BDC@BC organic thin films is extremely high. The Fe-BDC@BC demonstrates a sensitivity of 91% in the humidity range of 5–95%RH. Due to ease and low-cost biosynthesis, and improved optical and humidity sensing performance, this Fe-BDC@BC nanocomposite can be potentially used in the fabrication of optical and humidity sensing devices in technological applications.

Authors contributions

All authors contributed equally throughout the whole manuscript.

Statements and declarations

The authors declare that there is no conflict of interest in the manuscript

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The Authors confirm that the datasets generated during and analyzed during the current study are available from the corresponding author upon reasonable request.

Additional information

Funding

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number (0117-1443-S)