Pectins as Emulsifying Agent on the Preparation, Characterization, and Photocatalysis of Nano-LaCrO₃

Abstract

The use of environmentally friendly chemicals as the emulsifying agent in the preparation of the advanced materials is a focus and is very interesting to do. Although the focus is important, the advanced material that is made remains a top priority regarding characterization and its activity. One of the chemicals for making advanced and environmentally friendly materials such as LaCrO3 perovskite is pectin functioning as the emulsifying agent. In general, perovskite compounds are materials with very wide applications such as fuel cells, electronic equipment, sensors, magnetic materials, photoluminescent materials, thermic catalysts, and photocatalysts. Furthermore, the use of various solvents to produce perovskite compounds with the aim of getting good applications has been done a lot such as water, alcohols, or other organic solvents, respectively, in mixing precursors directly, precipitation and coprecipitation, microwave, auto-combustion, sol-gel, and hydrothermal methods. In this chapter, preparation of LaCrO3 using pectin as an emulsifying agent will be discussed in advance together with characterization and application of LaCrO3 in the photocatalytic reaction of dyes and cellulose conversion.

1. Introduction

1.1 General introduction of pectin

Pectin is a natural organic compound that has unique structure and characteristics. The uniqueness of the pectin structure is seen from its constituents consisting of three components [1, 2, 3, 4], namely, homogalacturonan (HG), rhamnogalacturonan-I (RGI), and rhamnogalacturonan-II (RGII). In principle, the main structure is RG. Furthermore, the functional groups possessed by pectin are ferulic acid, methoxy, acetyl esters, and esters. Interactions between the functional groups that are held provide uniqueness to the characteristics of pectin through hydrogen bonds, hydrophobic interactions, polyelectrolyte behavior, specific ion interactions, and even covalent bonds [5, 6] as external cation binding matrices prepared in the preparation of the advanced material.

Schematically the binding and distribution of cations dissolved in the pectin solution can be illustrated as in Figure 1.

Figure 1.

Illustration of binding of cations by pectin during the dissolution process.

In general, pectin can bind various cations with oxidation numbers of +1, +2, and +3 through various functional groups that belong to one or more pectin molecules, so that the cations bound are ready to react to produce a compound that will be well-distributed and the size of the particles produced can reach the nanoscale.

1.2 General introduction of LaCrO₃

Perovskite compounds, ABO₃ (where A = cation of alkali, alkaline earth, or lanthanide metal and B = cation of transition metal), have unique chemical and physical properties such as oxidative, magnetic, conductive, refractive, luminescent, and catalytic. With such interesting and valuable characteristics, these compounds have been utilized tremendously in electronic devices as a tuner of the dielectric/ferroelectric responses [7] and an overcomer inefficiency on photovoltaics and other optoelectronic devices [8], sensors as a hydrazine detector [9] and ozone sensing property [10], magnetisms as a huge magnetoresistance [11] and a magnetoelectric response [12], photoluminescences as a highly photoluminescent thin film [13] and a light-emitting material [14], catalysts as CO2/H2 converter into alcohol [15] and pollutant decomposer [16], solid oxide fuel cells as a self-anode in the next power generator [17] and a good performance cathode [18], and photocatalysts as a photo-oxidator of benzylic alcohol [19] and a decomposer of dyes [20].

Perovskite structural material (ABO₃) can be synthesized by mixing the oxide of lanthanide or third main group elements with the oxide of the transition elements. The cations can fit into both the A and B sites of the perovskite structure. In principle, the ABO₃ structure should obey the formulae of Goldschmidt’s tolerance factor [21], $\mathrm{t}=0.71\left({\mathrm{r}}_{\mathrm{A}}+{\mathrm{r}}_{\mathrm{O}}\right)/\left({\mathrm{r}}_{\mathrm{B}}+{\mathrm{r}}_{\mathrm{O}}\right)$ . This t-value led to the formation of crystalline structures such as cubic [22], orthorhombic [23], and hexagonal forms [24].

One of the perovskite materials, lanthanum chromites (LaCrO₃), has been extensively examined due to its applicability as interconnector for solid oxide fuel cell [25], excellent chemical stabilizer [26], good electrical properties at high temperature [27], total oxidation catalyst [28], partial oxidation catalyst [29], oxidative dehydrogenation catalyst [30], and photocatalyst [31]. Nowadays, various kinds of methods have been utilized to prepare the perovskite compounds such as hydrothermal [32, 33], precipitation [34, 35], coprecipitation [36, 37], auto-combustion [38, 39], and sol-gel [40, 41, 42]. Among these various kinds of the preparation methods, sol-gel holds particular importance, since it offers many advantages over the others. In the sol-gel technique, a homogeneous product is affected by all steps of preparation such as selection and dissolution of raw material, homogeneous mixing, and pH and temperature adjustments. In addition, these steps led to the opportunity to gain the nanomaterial. Therefore, unique physical and chemical properties of nanomaterial will be more feasible to obtain compared to their micro- and macro-size counterparts in a huge range of updated technologies [43, 44, 45].

Since the transformation of raw materials from the dissolved state into a solid state is also crucial in the sol-gel method, gelation, as well as solidification, determines the particle size of the product. To gain the nanomaterial, the agglomeration should be avoided by controlling carefully the process of both gelation and solidification as well as that of thermal treatment.

1.3 Typical property of photocatalyst

In heterogeneous photocatalytic processes, the semiconductors used are chalcogenide-type semiconductor materials (oxides, TiO₂, ZnO, ZrO, and CeO₂, or sulfides, ZnS and CdS). Semiconductors can be used as photocatalyst because they have a void energy region called band-gap energy, which lies between the conduction band boundary (LUMO) and the valence band (HOMO) that does not provide energy for promoting recombination of electrons and holes produced by a photoactivation in these semiconductors.

This semiconductor will function as a catalyst if it is illuminated with photons that have energy that is equal to or more than the energy bandgap (E_g) of the semiconductor used (hv ≥ E_g). Induction by these rays will excite the electrons (from the valence to the conduction band) in semiconductor materials [46]. As a result of the photon illumination, the formation of electron pairs (e⁻) and holes (h⁺) which are separated into free photoelectrons in the conduction band and photo hole in the valence band is ready to trigger the reaction as shown in Figure 2.

Figure 2.

Band-gap energy diagram in the photocatalytic process ([47].

The reaction that occurs in this event is:

$$\text{Semiconductor}+\mathrm{h\upsilon }\to \left({{\mathrm{e}}_{\mathrm{CB}}}^{-}+{{\mathrm{h}}_{\mathrm{VB}}}^{+}\right)$$

There are several possibilities that occur in electron-hole pairs, namely:

1. Some pairs recombine in particles (volume recombination).

2. Electron-hole pairs recombine on the surface (surface recombination) or in bulk particles in just a few nanoseconds (energy is lost as heat).

   The electron-hole pair recombination reaction can be written as follows:

   $$\text{Semiconductor}\left({{\mathrm{e}}^{-}}_{\mathrm{CB}}+{{\mathrm{h}}^{+}}_{\mathrm{VB}}\right)\to \text{semiconductor}+\text{heat}$$

3. Each electron pair can react with donor species (D) and acceptor (A), which are adsorbed on the particle surface. In other words, the electrons in the conduction band that reach the surface will reduce the substrate (A) or solvent on the surface of the particles, while the holes in the valence band will oxidize the substrate (D) either directly or indirectly through hydroxyl radical formation. This phenomenon follows the reaction equation as follows:

$$\mathrm{h\upsilon }+\text{semiconductor}\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

$${\mathrm{A}}_{\left(\mathrm{ads}\right)}+{\mathrm{e}}^{-}\to {{\mathrm{A}}^{-}}_{\left(\mathrm{ads}\right)}$$

$${\mathrm{D}}_{\left(\mathrm{ads}\right)}+{\mathrm{h}}^{+}\to {{\mathrm{D}}^{+}}_{\left(\mathrm{ads}\right)}$$

Some possible reactions that can occur with radical ions formed (A⁻ and D⁺) include:

1. A⁻ and D⁺ react between fellow radical ions or react with adsorbates (species adsorbed to the surface).

2. A⁻ and D⁺ combine by transferring the electron back to form an excited state from one of the reactants or releasing heat.

3. A⁻ and D⁺ diffuse from the surface of the semiconductor and participate in chemical reactions that occur in the solution medium.

In general, the process of the occurrence of a photocatalytic reaction based on the energy-gap concept indicates the difference in HOMO energy (the top band of valence contains electrons) and LUMO (the lowest band of conduction without electrons) that must be passed. In other words, the promotion of electrons from the top band of valence to the lowest band of conduction requires minimum energy equivalent to its band-gap energies. If the energy owned is zero or larger than 4 eV, owned are zero or large (>4 eV), then each is a metal or insulator, whereas semiconductor has energy between these values. Furthermore, the band-gap energy is classified as direct and indirect. Direct means that the minimum energy from the lowest band of conduction is just above the maximum energy of the valence band at the same momentum of crystals. If the condition is not so, it is called indirect band-gap energy. The range of band-gap energy possessed by a material will determine the type of energy that will be used so that a photocatalytic reaction can occur. The type of energy can be used for the reaction as shown in Figure 3.

Figure 3.

Schematic radiation energy of electromagnetic wavelength.

In principle, all light electromagnetic waves can be used as an energy source for a chemical reaction process. So far, light that can be used as a trigger for chemical reactions through electron transfer from the HOMO to the LUMO level in the degradation and/or breaking of a compound bond into an environmentally friendly product is visible and ultraviolet.

Visible radiation is often used to degrade toxic compounds of dyes since the waste of dyes which is channeled directly into a river or sea body will have a negative effect on aquatic biota. In general, visible light radiation has a wavelength range of 400—800 nm. In other words, the energy needed to break the chemical bonds of a compound is low. Generally, these dyes are compounds that have chromophore groups, such as methine, nitro, azo, anthraquinone, triarylmethane, and phthalocyanine groups. In fact, dyes used in industry can be either natural compounds or syntheses.

Ultraviolet radiation has a high ability to breakdown the bond and cause decomposition because of its high energy compared to infrared radiation and visible light [48]. Sources of ultraviolet radiation can be obtained from sunlight or artificial light. Ultraviolet (UV) radiation of the sun is electromagnetic energy with wavelengths between 200 and 400 nm and has more energy than visible light. Based on its wavelength, solar UV radiation is divided into:

1. UVA with a wavelength of 320—400 nm is a high wavelength and emits radiation of constant magnitude throughout the year. This radiation can cause premature aging of the skin.

2. UVB with a wavelength of 280–320 nm is a shorter wavelength and is more intense than UVA. UVB is more strongly absorbed by several biomolecular pollutants.

3. UVC with a wavelength of 200—280 nm is the most intensive and dangerous UV radiation and has the potential to cause damage to organisms.

Therefore, to choose a type of UV light in photocatalytic reaction depends on the type of bond and the energy required. In general, the type of bond with its energy can be seen in Table 1.

Num	Type of bond	Bond energy
		(kJ/mol)	(eV)
1	H▬H	436	11.58
2	C▬H	413	10.97
3	O▬H	366	9.72
4	C▬O	360	9.56
5	C▬C	348	9.25
6	C〓C	614	16.31
7	C〓O	745	19.79
8	C〓N	615	16.33
9	C☰C	839	22.29
10	N☰N	941	24.99

Table 1.

Some typical bonds and bond energies.

2. Method of preparation

Preparation of LaCrO₃ catalyst material was carried out using the sol-gel method with pectin as an emulsifying agent. The preparation was conducted by dissolving specified mass of La(NO₃)₃.9H₂O and Cr(NO₃)₃.6H₂O, respectively, in 100 mL of pectin solution (4 g pectin). The overall procedure was described in the previous article [49].

3. Characterizations

Before the material made was applied, the photocatalyst was characterized to determine the physical and chemical properties associated through X-ray diffraction analysis, electron transmission microscopy, distribution of particle distribution, energy-gap, and functional groups related to the structural formation and the acidity of Brønsted-Lowry and Lewis.

3.1 Analysis of X-ray diffraction

X-ray diffraction can be utilized to identify the phase formed and the relative percentages of different phases of the materials obtained. Then, the real structural parameters like particle size, lattice parameters (a, b, and c), lattice volume, and theoretical density can be calculated from their diffractogram using Rietveld calculation [50].

X-ray diffraction experiments are carried out by the procedure as described in the previous article [49]. To know the crystallite size, the representative peak of a diffractogram can be elucidated by using the Scherrer method of calculation [51]. The results of the diffractogram, determination of the size of the crystalline phase, and the Rietveld calculation are presented in Figure 4.

Figure 4.

Diffractogram of LaCrO₃ (a) experimental results and (b) the result of the Rietveld calculation using LaCrO₃ calcined at 700°C.

In general, the results of LaCrO₃ prepared using pectin provide a single crystalline phase, nanosize, and other parameter values shown in Table 2.

Num	Parameter	LaCrO3 calcined at			Ref
		600°C	700°C	800°C
1	Crystalline phase	Cubic	Orthorhombic	Orthorhombic	[50]
2	Crystalline size (nm)	24.84	24.12	27.09	[48]
3	hkl plane	110	112	121	[50]

Table 2.

Parameters of LaCrO₃ prepared in various calcination temperatures.

3.2 Transmission electron microscope analysis

TEM can be used to study the morphology and surface characteristics of the perovskite nanomaterials. To evaluate the surface morphology, the samples were characterized using TEM. The analysis was conducted on polished and thermally etched samples with different magnifications. TEM results of LaCrO₃ material are presented in Figure 5.

Figure 5.

TEM micrographs of LaCrO₃ prepared using pectin as the emulsifying agent: calcined at (a) 600, (b) 700, and (c) 800°C.

It seems that there is still a relatively large region of agglomeration in the crystalline phase formed in each of these preparations. Nevertheless, the particle sizes obtained using TEM in this study are significantly smaller than that of LaCrO₃ prepared using sol-gel method reported by another research group [52]. It was found that the particle sizes of the sample calcined at 600, 700, and 800°C are 34.6, 30, and 28 nm, respectively.

3.3 Particle size distribution analysis

Analysis of the particle size distribution of the solid sample was examined by the technique of dynamic light scattering (DLS). The measurement of the sample using this instrument can be determined by either wet or dry method. If a measurement is using the wet method, the sample is prepared using alcohol dispersant such as methanol, ethanol, or propanol. However, if the measurement is using a dry method on preparing the sample, air dispersant could be utilized. More information can be obtained in the manual book [53]. The more important in preparing sample is to prevent the irreversible change to the particle (dissolution, milling, or aggregation) happened.

From Figure 6, it can be implied that there are two or three regions of the particle size which are quantum dot, nano-, and micron sizes. Overall, it can be said that nanosize of the particles 21.9, 86.4, and 89.11% referred to LaCrO₃ calcined at 600, 700, and 800°C, respectively [49].

Figure 6.

The particle size distribution of the LaCrO₃ calcined at (a) 600, (b) 700, and (c) 800°C.

The results also can be implied that the more nanosize particle obtained, the higher the temperature of calcination applied. In other words, the temperature of calcination plays a role to determine the nanosize of the particle. In other studies, the particle size of LaCrO₃ prepared using hydrothermal method was determined by PSA method. The result proved that the average size of particle is in the range of micron [54] and 57 nm [55].

3.4 Analysis of diffuse reflectance UV-Vis spectroscopy

In order to know the band-gap energy of the LaCrO₃ prepared in a different calcined temperature, the analysis was run using diffuse reflectance UV-Vis spectroscopy as shown in Figure 7.

Figure 7.

Reflectance (A) and absorption (B) features of LaCrO₃ calcined at 600, 700, and 800°C, respectively.

To determine the bandgap of a powder sample using the diffuse reflectance spectrophotometer is a common technique [56]. So, in this study the band-gap energy is calculated using a Kubelka-Munk method [57] based on the equation below:

$$\mathrm{\alpha }\left(\mathrm{h}\mathrm{\nu }\right)\approx \mathrm{\beta }{\left(\mathrm{h}\mathrm{\nu }-{\mathrm{E}}_{\mathrm{op}}\right)}^{\mathrm{n}}$$

where β is a constant and n is an index related to the possible type of electron transition. The value of n could be 1/2, 2, 3/2, and 3. Those values are corresponding to the nature of electron transition. In principle, there are two kinds of electron transition, which are direct and indirect. If the n value is 1/2 or 2, it means allowed direct or indirect electron transition happened. But if the n value is 3/2 or 3, it means forbidden direct or indirect electron transition occurred [58].

The results of the band-gap energy from LaCrO₃ calcined at 600, 700, and 800°C, respectively, are 2.62, 2.89, and 2.98 eV. The magnitude of those band-gap energies is suitable for photocatalytic reactions using UV and visible light irradiation.

3.5 Fourier transform infrared analysis

In principle, FTIR analysis in the material field is used to assess the functional groups and what bonds are formed in the material prepared in relation to the expected compound. In preparing samples for their analysis, the procedures performed are standard and can be referenced in various libraries [49, 59, 60].

The chemical bonding and chemical structure of the prepared perovskites can be identified. The FTIR spectra can give structural confirmation supporting XRD analysis. Infrared spectra of LaCrO₃ material are presented in Figure 8.

Figure 8.

Infrared spectra of LaCrO₃ calcined at (a) 600°C, (b) 700°C, and (c) 800°C [48].

From Figure 8, it can be implied that perovskite LaCrO₃ is actually formed and can be assessed based on the type of bond vibration which can be referred to in detail in the following literature [a]. The La▬O▬La and La▬O▬Cr bonds through information on bending vibrations are increasingly apparent as the calcination temperature increases. In other words, the structure of LaCrO₃ which is formed along with the increase in the calcination temperature is getting closer and can be assessed through the diffractogram data.

The results also reflect that the existence of Brønsted-Lowry and Lewis acid sites is indicated by the presence of absorption bands at wave number 1400 and 1630 cm⁻¹, respectively. In detail, the acidity characteristics of the LaCrO₃ calcined at the various temperatures were described in the previous article [49].

4. Applications

The advanced material LaCrO₃ prepared using pectin emulsifier and then tested on dye degradation and cellulose conversion was described below.

4.1 Photocatalysis of dye

The photocatalytic activity test on LaCrO₃ nanocatalysts was carried out on the methanyl yellow compound by mixing as much as 0.08 g of LaCrO₃ nanocatalyst into 300 mL methanyl yellow with a concentration of 100 ppm into a beaker and then homogenized. After that the mixture was pipetted as much as 20 mL with various time variations (0, 10, 20, 30, 40, and 50 min) which had been irradiated by a UV lamp with a distance of 30 cm. After that, UV-Vis spectrophotometry was tested to see the absorbance rate of methanyl yellow.

Then in the photocatalytic reaction for visible light, as much as 0.08 g of LaCrO₃ catalyst was put into 300 mL of 100 ppm methanyl yellow in a beaker. Then homogenized it by stirring, while the sample is placed that the mixture is placed under the sun in the range of time 11 am to 1 pm. Then the sample was pipetted as much as 20 mL with various time variations (0, 10, 20, 30, 40, and 50 min) and tested with UV-Vis spectrophotometry to see the absorbance rate of methanyl yellow. Photodegradation under UV light irradiation is shown in Figure 9.

Figure 9.

Dye photodegradation results under visible light irradiation (a) and ultraviolet (b) for 0–50 min.

4.2 Photocatalysis of cellulose

The application of LaCrO₃ photocatalyst to cellulose conversion was carried out on a laboratory scale using procedural as follows:

The LaCrO₃ catalyst was then used in the photocatalytic test process for the conversion of nanocellulose to sugar alcohol. As much as 0.5 g of nanocellulose in 100 mL of distilled water are distilled for 30 min. The nanocellulose solution that has been ultrasonified is transferred into a three-neck flask, then stirred with a stirrer, and then added LaCrO₃ nanocatalyst as much as 0.1 g. Furthermore, hydrogen gas is flowed and irradiated with UV light with a variation of time 15, 30, 45, and 60 min. The reaction results were analyzed by high-performance liquid chromatography (HPLC). The results of photocatalytic cellulose conversion using LaCrO₃ under UV light are shown in Figure 10.

Figure 10.

Photocatalytic cellulose conversion results using LaCrO₃ under ultraviolet light irradiation during 1 h exposure.

Quantitatively, the results obtained are compared with the standard results for each xylitol sorbitol solution, and the concentration of mannitol is known. Then the quantitative results of the experiment, even though it is still relatively small, are, respectively, 190, 180, and 120 ppm at 1 h exposure with UV radiation [61].

5. Conclusions

Preparation of advanced materials using the sol-gel method with pectin emulsifier is able to make the active catalyst degrade methanyl yellow dyes and convert cellulose into glucose reducing sugars and sugar alcohols such as xylitol, sorbitol, and mannitol. The particle size and crystalline produced by the preparation method can reach nanosize with a range of 24-50 nm. Furthermore, the band-gap energy results state that the LaCrO₃ advanced material is in the range of 2.89–3.0 eV.

Acknowledgments

The author would like to acknowledge both the financial support from Indonesian Government through the Directorate Research, Ministry of Research and Higher Education on the contract number 384/UN26.21/PN/2018 in doing the Research and the Research Institution and Community services of the University of Lampung for funding this book chapter.