Chiang Mai Journal of Science

Effect of Monoethanolamine on the Structural, Optical, and Photocatalytic Properties of Sol–Gel Derived CuSnO3 for Methyl Orange Degradation

1. INTRODUCTION

Synthetic dyes are widely employed in the textile, paint, leather, paper, pharmaceutical, and food industries because of their strong color intensity, versatility, and stability under processing conditions. However, their large-scale production and widespread use have led to a substantial increase in the discharge of dye-laden wastewater into aquatic environments [1]. Once released, many synthetic dyes persist in water bodies because of their high chemical stability, complex aromatic structures, and resistance to biodegradation, making their removal from wastewater a persistent environmental challenge.

Methyl Orange (MO) is a commonly used synthetic azo dye that exhibits carcinogenic, mutagenic, and toxic properties. Exposure to MO-contaminated wastewater may cause irritation of the skin and eyes, as well as respiratory and digestive disorders. In aquatic systems, MO can reduce water quality by decreasing light penetration and dissolved oxygen concentration, thereby disrupting photosynthetic activity and the survival of aquatic organisms [2]. Photocatalysis is an advanced oxidation process in which semiconductor materials absorb light and generate electron–hole pairs that produce highly reactive species such as hydroxyl radicals and superoxide ions [3]. These species can oxidize complex dye molecules into less harmful intermediates or fully mineralized products [4].

Among semiconductor photocatalysts, ZnO has attracted considerable attention because of its chemical stability and UV photocatalytic activity; however, its wide band gap (~3.37 eV) restricts light absorption and rapid recombination of electron–hole pairs reduces efficiency [5]. A comprehensive review on ZnO/TiO₂-based photocatalysts emphasized that precursor chemistry, crystal size, and surface engineering are decisive for photodegradation performance [6]. These limitations have encouraged the investigation of alternative semiconductors.

CuSnO₃ (CSO) is an oxide semiconductor with a reported band gap in the range of 2.0–2.5 eV, composed of earth-abundant, non-toxic, and thermally stable elements [7]. Earlier studies have established important benchmarks for CuSnO₃. Singh et al. reported the structural and optical characterization of CuSnO₃ prepared by a ceramic route, providing fundamental physicochemical reference data [8]. Abolhasani et al. demonstrated that sonochemically synthesized CuSnO₃ nanoparticles are effective photocatalysts for methylene blue degradation and environmental remediation [9]. These pioneering works confirm the potential of CuSnO₃ while highlighting that systematic control of morphology, crystallinity, and synthesis conditions in sol–gel systems remains insufficiently explored.

Among synthesis routes, the sol–gel method offers distinct advantages over competing approaches such as hydrothermal synthesis and ball milling: it operates at relatively low temperatures, provides homogeneous mixing at the molecular level, enables precise control over composition and particle morphology, and avoids the high-pressure equipment required by hydrothermal routes [10]. Ball milling can introduce contamination and produces broad particle-size distributions. These considerations justify the selection of sol–gel synthesis for the present work.

Monoethanolamine (MEA) is a promising additive for sol–gel synthesis that functions as a complexing agent, regulates nucleation and crystal growth, and suppresses particle agglomeration [11]. Gómez-Núñez et al. showed that ethanolamine coordinates metal ions in sol–gel ZnO inks, stabilizing the precursor network and delaying premature precipitation [12]. Tumbul confirmed that MEA content directly controls the crystallinity of ZnO thin films, establishing MEA as a structural control parameter rather than merely a solvent aid [13]. These mechanistic insights support using MEA in the present CuSnO₃ synthesis.

Florence reported that Citrus limetta-mediated ZnO nanopellets enhance photocatalytic dye degradation, highlighting the potential of morphology-controlled oxide nanomaterials [14]. Phatai et al. demonstrated that ultrasonic-assisted sol–gel synthesis tunes phase formation of zinc–silver doped hydroxyapatite [15]. Xu et al. reported that Cu₂O(ClO₄)₂/TiO₂ dandelion-shaped nanospheres exhibit combined photocatalytic functions, emphasizing the importance of structural design [16].

The novelty of the present work lies in the systematic use of MEA not merely as a stabilizing solvent but as a controllable chelating and structure-directing agent in the sol–gel synthesis of CuSnO₃. By varying the MEA-to-metal precursor molar ratio (1:1, 1:2, and 2:1), this study establishes for the first time a direct quantitative correlation between additive concentration, crystallinity, particle morphology, optical band gap, and methyl orange photocatalytic degradation efficiency — providing mechanistic insight into MEA's role during precursor complexation, nucleation, and crystal growth.

The study aims to synthesize CuSnO₃ by the sol–gel method and examine the effect of MEA concentration on the structural, morphological, optical, and photocatalytic properties of the resulting materials. Samples were characterized by XRD, SEM, UV–DRS, X-ray fluorescence (XRF), and FTIR spectroscopy.

2. MATERIALS AND METHODS

2.1 Materials
All chemicals were of analytical grade and obtained from Merck. Tin(II) chloride (SnCl₂) and copper(II) nitrate (Cu(NO₃)₂) were used as metal precursors for CuSnO₃ synthesis. Monoethanolamine (MEA) was employed as the chelating and structure-directing additive. Isopropanol and glacial acetic acid served as solvents. All reagents were used as received without further purification [17].

2.2 Synthesis Procedure
CuSnO₃ (CSO) was prepared through a sol–gel powder synthesis route. In the standard synthesis, 0.7584 g of SnCl₂ and 0.7504 g of Cu(NO₃)₂ were dissolved in isopropanol to form a clear precursor solution. The mixture was ultrasonicated for 1 h, followed by magnetic stirring at 50 °C for 12 h to promote hydrolysis and condensation. The resulting gel was dried at 50 °C for 12 h to yield a xerogel precursor [18].

Based on TGA of the xerogel, three calcination temperatures were selected: 400 °C, 600 °C, and 800 °C; each sample was calcined for 2 h under atmospheric conditions. MEA was introduced at molar ratios of 1:1, 1:2, and 2:1 relative to the total metal precursors. After MEA addition, solutions were ultrasonicated for 1 h and stirred at 50 °C for 12 h. Gels were calcined at the optimum temperature of 600 °C for 2 h [19]. A reference sample was prepared without additive under identical conditions.

2.3 Photocatalytic Experiments
Photocatalytic experiments were conducted using MO as the model pollutant under the following conditions: initial MO concentration, 10 mg/L; catalyst loading, 50 mg per 100 mL; solution pH, ~6.8 (neutral); reaction volume, 100 mL. Prior to irradiation, the suspension was stirred in the dark for 30 min to establish adsorption–desorption equilibrium. Irradiation was performed using a UV lamp (365 nm, 8 W; light intensity ~1.2 mW/cm²). Aliquots of 3 mL were withdrawn at predetermined intervals, filtered through 0.45 μm membrane filters, and analyzed by UV–Vis spectrophotometry at λ_max = 464 nm. Degradation efficiency was calculated as:

where C₀ is the initial MO concentration and C is the concentration at irradiation time t. Pseudo-first-order kinetics was applied: ln(C/C₀) = −k_appt, where k_app is the apparent pseudo-first-order rate constant (min⁻¹). Control experiments included: (1) photolysis alone (irradiation without catalyst); (2) dark adsorption (catalyst without light); and (3) a blank test (neither catalyst nor light). Each experiment was conducted in triplicate and results are reported as mean values.

2.4 Characterization
Phase composition and crystallinity were analyzed by XRD with peak indexing referenced against JCPDS card No. 00-021-1250 for CuSnO₃. Surface morphology was examined by SEM. Optical properties were investigated by UV–DRS; the Kubelka–Munk function and Tauc plots (direct-allowed transition, n = 1/2, i.e., (αhν)² vs. hν) were used to estimate optical band gap energy [20]. Elemental composition was examined by XRF. Functional groups and metal–oxygen bonding were identified by FTIR, with particular attention to the fingerprint region below 600 cm⁻¹.

3. RESULTS AND DISCUSSION

3.1 Formation of CuSnO₃ through the Sol–Gel Route
The sol–gel approach provided an effective route for synthesizing CuSnO₃ with good compositional homogeneity. Stirring at 50 °C for 12 h promoted hydrolysis and condensation, gradually transforming the solution into a stable sol and then a gel [21]. After drying at 50 °C for 12 h, a solid xerogel with a distinct turquoise color was obtained, characteristic of copper-containing oxide precursors [22]. The uniform precursor distribution established at this stage directly affects phase development and crystallinity after heat treatment.

3.2 Phase Identification and Crystallinity
The XRD patterns (Figure 1) confirm the formation of the CuSnO₃ cubic spinel phase in all samples, consistent with JCPDS card No. 00-021-1250. All major diffraction peaks are indexed accordingly. Minor peaks at approximately 35.5° and 38.7° 2θ observed in the unmodified sample are consistent with trace CuO (JCPDS 45-0937); these peaks diminish in MEA-modified samples, indicating that MEA promotes phase purity. No distinct SnO₂ peaks were identified, confirming effective incorporation of Sn into the spinel lattice.

3.3 Particle Morphology
The SEM micrographs (Figure 2) reveal the influence of MEA on particle morphology. The unmodified sample (Figure 2a) exhibited irregularly shaped particles with pronounced agglomeration and large grain sizes, typical of sol–gel-derived oxides synthesized without chelating control [23].

The 1:2 and 2:1 samples showed less optimal morphologies with partial agglomeration. Both insufficient and excessive MEA disturb the nucleation–growth balance: too little additive cannot prevent aggregation, while too much may leave residual organics or cause coordination crowding that interferes with crystallization.

3.4 Thermal Behavior and Calcination Temperature Selection
The TGA profile (Figure 3) indicates continuous mass loss over 30–800 °C. The first mass-loss region (30–200 °C) is attributed to removal of physically adsorbed water and residual solvent. A more pronounced mass-loss event (200–400 °C) corresponds to decomposition of organic residues; the endothermic feature near 250 °C confirms combustion of these residues [24]. Beyond 400 °C, the TGA curve became nearly stable, indicating that the Cu–Sn–O framework had reached thermal stability.

3.5 Elemental Composition and Functional Group Analysis
The elemental composition and FTIR fingerprint data are summarized in Table 1. Cu and Sn are the dominant elements across all samples, confirming the formation of CuSnO₃. Minor P, Cl, and Ca are attributed to residual precursor species at low concentrations unlikely to alter material behavior significantly.

3.6 Optical Properties and Band Gap Analysis
The UV–DRS Tauc plots (Figure 4) show strong absorption in the 300–400 nm region for all samples. Band gap values were estimated using the Kubelka–Munk transformation under the direct-allowed transition assumption (n = 1/2): (αhν)² vs. hν, with linear extrapolation to the x-axis [26].

3.7 Photocatalytic Degradation of Methyl Orange
The photocatalytic activity of the CuSnO₃ samples was assessed by MO degradation under UV irradiation (365 nm). Control experiments confirmed that appreciable MO removal did not occur via photolysis alone (<5%), dark adsorption (<8%), or in a blank test (<1%), confirming that the observed removal is dominated by photocatalytic activity.

3.8 Integrated Discussion of Structure–Property–Performance Relationships
MEA's amino (–NH₂) and hydroxyl (–OH) groups coordinate with Cu²⁺ and Sn⁴⁺ ions during sol–gel precursor formation, moderating hydrolysis and condensation rates and distributing cations more evenly throughout the precursor gel [12, 13]. This yields a more ordered crystal structure and a more uniform octahedral particle morphology after calcination, as confirmed by XRD peak sharpening and SEM. The FTIR peak shifts (541.82 → 523.23 cm⁻¹) and XRF elemental distribution data confirm that MEA actively modifies local coordination chemistry and stoichiometric distribution of Cu and Sn in the final oxide.

When MEA concentration deviates from the optimum 1:1 ratio, the coordination effect becomes either insufficient (1:2 dilution) or excessive (2:1 excess). In the latter case, residual organic species, coordination crowding, and surface blockage interfere with crystallization and inhibit active-site accessibility. Although the 1:1 MEA sample exhibits the highest band gap (3.13 eV), it also shows the highest photocatalytic activity — confirming that photocatalytic efficiency in this system is governed by the combined effects of crystallinity, morphology, and surface quality rather than by simple band gap narrowing.

4. CONCLUSIONS

CuSnO₃ was successfully synthesized by the sol–gel method, and monoethanolamine was shown to be an effective chelating and structure-directing additive. The 1:1 MEA-to-metal precursor molar ratio produced the most favorable outcome: highest crystallinity (XRD), well-defined octahedral morphology (SEM), and the highest photocatalytic performance — 78.56% MO degradation with k_app = 0.0124 min⁻¹, approximately 2.1-fold higher than the unmodified sample. Pseudo-first-order kinetics was confirmed for all samples. XRD confirmed the CuSnO₃ cubic spinel phase, and FTIR confirmed the metal–oxygen framework. These results demonstrate that MEA concentration is a key parameter for controlling the physicochemical properties of CuSnO₃, and that the 1:1 MEA-modified CuSnO₃ is a promising photocatalyst for wastewater dye remediation.

AUTHOR CONTRIBUTIONS

Hary Sanjaya: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing – Original Draft, Visualization. Hermansyah Aziz: Methodology, Supervision, Formal analysis, Writing – Review & Editing. Yeni Stiadi: Resources, Validation, Writing – Review & Editing. Syukri Arief: Conceptualization, Supervision, Project administration, Writing – Review & Editing. Septian Budiman: Formal analysis, Validation, Visualization, Writing – Review & Editing. Yohandri: Supervision, Methodology, Resources, Writing – Review & Editing.

CONFLICT OF INTEREST STATEMENT

The authors declare that they have no conflict of interest.

DECLARATION OF USE OF GENERATIVE AI

Generative AI tools were used solely to assist in the design of graphical elements (icons and visual components) in the graphical abstract for presentation purposes only. AI assistance was strictly limited to this graphical task and was not used in any aspect of scientific content, including data analysis, interpretation, manuscript writing, or reference management. All scientific content is the exclusive work of the authors, who take full responsibility for the accuracy and integrity of this manuscript.

REFERENCES

[1] Sarkar A., Das B., Selvaraj M. and Basumatary S., Development of hybrid MMT/g-C₃N₄/Ca–Ni–Al layered triple hydroxide composite for highly effective removal of cationic and anionic dyes. Journal of the Indian Chemical Society, 2025; 102(12): 102267. DOI 10.1016/j.jics.2025.102267.

[2] You J., Zhang L., He L. and Zhang B., Photocatalytic degradation of methyl orange on ZnO–TiO₂/SO₄²⁻ heterojunction composites. Optical Materials, 2022; 131: 112737. DOI 10.1016/j.optmat.2022.112737.

[3] Basumatary F., Sarkar A., Mushahary N., Das B., Saikia P., Selvaraj M., et al., Graphitic carbon nitride composites as versatile materials for adsorption and photocatalytic degradation of pollutants from wastewater. Process Safety and Environmental Protection, 2024; 191(Part B): 2416–2468. DOI 10.1016/j.psep.2024.09.077.

[4] Abdeta A.B., Wu Q., Kuo D.-H., Li P., Zhang H., Zhang J., et al., Activated carbon-supported AgMoOS bimetallic oxysulfide as a catalyst for photocatalytic hydrogen evolution and pollutants reduction. Journal of Alloys and Compounds, 2022; 913: 165287. DOI 10.1016/j.jallcom.2022.165287.

[5] Wang J., Li Z. and Gu Z., A comprehensive review of template-synthesized multi-component nanowires: From interfacial design to sensing and actuation applications. Sensors and Actuators Reports, 2021; 3: 100029. DOI 10.1016/j.snr.2021.100029.

[6] Ghamarpoor R., Fallah A. and Jamshidi M., A review of synthesis methods, modifications, and mechanisms of ZnO/TiO₂-based photocatalysts for photodegradation of contaminants. ACS Omega, 2024; 9(24): 25457–25492. DOI 10.1021/acsomega.3c08717.

[7] He H., Liao A., Guo W., Luo W., Zhou Y. and Zou Z., State-of-the-art progress in the use of ternary metal oxides as photoelectrode materials for water splitting and organic synthesis. Nano Today, 2019; 28: 100763. DOI 10.1016/j.nantod.2019.100763.

[8] Kim B.N., Seo G.K., Hwang S.W., Yu H., Ahn B., Seo H., et al., Photophysical properties and photoelectrochemical performances of sol–gel derived copper stannate (CuSnO₃) amorphous semiconductor for solar water splitting application. Ceramics International, 2018; 44(2): 1843–1849. DOI 10.1016/j.ceramint.2017.10.119.

[9] Abolhasani M., Boroujerdnia M. and Cheraghizade M., Sonochemical synthesis of CuSnO₃ NPs as a photocatalyst for efficient degradation of MB and environmental remediation. Physica B: Condensed Matter, 2025; 717: 417837. DOI 10.1016/j.physb.2025.417837.

[10] Chang C., Rad S., Gan L., Li Z., Dai J. and Shahab A., Review of the sol–gel method in preparing nano TiO₂ for advanced oxidation process. Nanotechnology Reviews, 2023; 12(1): 20230150. DOI 10.1515/ntrev-2023-0150.

[11] Liu N., Ouyang D., Cai Y. and Li Y., Heterostructured Fe₂O₃/BiVO₄ nano-photocatalyst for the reduction of nitroarenes into amines. Ceramics International, 2020; 46(15): 24534–24543. DOI 10.1016/j.ceramint.2020.06.240.

[12] Gómez-Núñez A., Alonso-Gil S., López C., Roura P. and Vilà A., Role of ethanolamine on the stability of a sol–gel ZnO ink. The Journal of Physical Chemistry C, 2017; 121(42): 23839–23846. DOI 10.1021/acs.jpcc.7b09935.

[13] Tumbul A., Effect of monoethanolamine content on the crystallinity of ZnO thin films. Süleyman Demirel University Faculty of Arts and Science Journal of Science, 2019; 14(1): 155–164. DOI 10.29233/sdufeffd.513412.

[14] Florence S., Enhanced photocatalytic degradation of selected contaminant dyes from textile sewage discharge using Citrus limetta mediated ZnO nanopellets. Chiang Mai Journal of Science, 2024; 51(6): e2024106. DOI 10.12982/CMJS.2024.106.

[15] Phatai P., Kamonwannasit S., Siri-udom S., Futalan C.M., Khemthong P., Youngjan S., et al., Increased antibacterial activity of zinc-silver doped hydroxyapatite synthesized via ultrasonic-sol-gel technique. Chiang Mai Journal of Science, 2024; 51(3): e2024044. DOI 10.12982/CMJS.2024.044.

[16] Xu X., Yang Y., Miao Y., Liu K., Chen Y. and Yue S., Cu₂O(ClO₄)₂/TiO₂ dandelion-shaped nanospheres with photocatalytic antibacterial activity. Chiang Mai Journal of Science, 2022; 49(2): 434-445. DOI 10.12982/CMJS.2022.021.

[17] Hermawan A., Asakura Y., Inada M. and Yin S., A facile method for preparation of uniformly decorated-spherical SnO₂ by CuO nanoparticles for highly responsive toluene detection at high temperature. Journal of Materials Science & Technology, 2020; 51: 119–129. DOI 10.1016/j.jmst.2020.02.041.

[18] Rahimi A. and Kazeminezhad I., Amorphous copper stannate/reduced graphene oxide composite as an efficient photoanode for photoelectrochemical water splitting. Optical Materials, 2025; 165: 117107. DOI 10.1016/j.optmat.2025.117107.

[19] Ma J., Zhang Z., Mentbayeva A., Yuan G., Wang B., Wang H., et al., Enhanced electrochemical performance of hollow heterostructured carbon encapsulated zinc metastanate microcube composite for lithium-ion and sodium-ion batteries. Electrochimica Acta, 2019; 312: 31–44. DOI 10.1016/j.electacta.2019.04.167.

[20] Raina P., Nike T., Kaushal D., Chauhan V., Shandilya P. and Kumar M., A review on stannate perovskites-based heterojunctions and their applications in the development of sustainable technologies and materials. Materials Science in Semiconductor Processing, 2025; 188: 109224. DOI 10.1016/j.mssp.2024.109224.

[21] Danks A.E., Hall S.R. and Schnepp Z., The evolution of ‘sol–gel’ chemistry as a technique for materials synthesis. Materials Horizons, 2016; 3(2): 91–112. DOI 10.1039/C5MH00260E.

[22] Stambouli I., Zerouali M., Daïra R., Bouras D., El-Hiti G.A., Grigorian S., et al., Enhancement of tin-doping on the structural and optical properties of copper oxide thin films for optoelectronic applications. Ceramics International, 2025; 51(13): 17689–17703. DOI 10.1016/j.ceramint.2025.01.540.

[23] Sun H., Boke Abdeta A., Kuo D.-H., Wu Q., Guo Y., Ahmed Zelekew O., et al., Activated carbon supported CuSnOS catalyst with an efficient catalytic reduction of pollutants under dark condition. Journal of Molecular Liquids, 2021; 334: 116079. DOI 10.1016/j.molliq.2021.116079.

[24] Meng Y., Ding J., Liu Y., Hu G., Feng Y., Wu Y., et al., Advancements in amorphous oxides for electrocatalytic carbon dioxide reduction. Materials Today Catalysis, 2024; 7: 100065. DOI 10.1016/j.mtcata.2024.100065.

[25] Phromma S., Wutikhun T., Kasamechonchung P., Eksangsri T. and Sapcharoenkun C., Effect of calcination temperature on photocatalytic activity of synthesized TiO₂ nanoparticles via wet ball milling sol-gel method. Applied Sciences, 2020; 10(3): 993. DOI 10.3390/app10030993.

[26] Rehman B., Khan S.A., Nasar A., Shahjahan., Alimuddin. and Ahamad Z., Synergistic Fe-Sn-Ni ternary metal oxide nanocomposite for Congo red adsorption: Synthesis, characterization, and Box-Behnken optimization. Next Materials, 2026; 10: 101502. DOI 10.1016/j.nxmate.2025.101502.

[27] Lv J., Gong W., Huang K., Zhu J., Meng F., Song X., et al., Effect of annealing temperature on photocatalytic activity of ZnO thin films prepared by sol–gel method. Superlattices and Microstructures, 2011; 50(2): 98–106. DOI 10.1016/j.spmi.2011.05.003.

[28] Liu Y., Xu X., Sun X., Li W. and Duan J., Recent research progress of photoelectrocatalysis technology in the removal of organic pollutants from water. Journal of Water Process Engineering, 2025; 75: 107911. DOI 10.1016/j.jwpe.2025.107911.

[29] Chen X., Shang Y., Li K., Lv Y. and Pan X., Photothermal activation in amorphous Cu-Fc MOF nanoarchitectures for ultrafast catalytic reduction of high concentration organic pollutants under near-infrared irradiation. Applied Surface Science, 2025; 706: 163546. DOI 10.1016/j.apsusc.2025.163546.

[30] Zhang P., Wang H., Lai Y., Xu Y., Chen L., Wu Q., et al., Synergistic Co/S co-doped CeO₂ sulfur-oxide catalyst for efficient catalytic reduction of toxic organics and heavy metal pollutants under dark conditions. Journal of Water Process Engineering, 2024; 58: 104820. DOI 10.1016/j.jwpe.2024.104820.

Related Articles

Adsorption of Methyl Orange from Aqueous Solution Using Chitosan/ZSM-5 Composite
DOI: 10.12982/CMJS.2026.085.

Mencui Ning and Runhu Zhang*

Vol.53 No.5 (September 2026)
Research Article View: 361 Download: 116

Preparation of Dye-sensitized POM and Application in Treatment of Cotton Fabric Dyeing Wastewater
DOI: 10.12982/CMJS.2026.058.

Yan Zhang #, Qian Liu #, Huai-yi Zhang, Chang Liu, Jun Lu* and Yue-tao Liu*

Vol.53 No.3 (May 2026)
Research Article View: 480 Download: 67