X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES 359–366 TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES DVOSTOPENJSKA SINTEZA IN MEHANIZEM TVORBE LACUOS NANOAGLOMERATOV Xing Li * , Qiang Li, Zhenning Ma, Yan Sun Shenyang Jianzhu University, Department of Science, Shenyang, 110168, P.R. China Prejem rokopisa – received: 2023-05-14; sprejem za objavo – accepted for publication: 2023-06-21 doi:10.17222/mit.2023.879 We have developed a two-step synthesis method to obtain pure LaCuOS nano-sized agglomerates using La(NO 3) 3·6H 2O, CuSO 4·5H 2O and NH 3·H 2O as the starting materials. The result shows that the precursor can be converted into La 2O 2SO 4, La 2(SO 4) 3 and CuO phases at 800 °C for2hi na i r ,which was then converted into a pure LaCuOS phase by a reduction at 800°Cfor5hinaflo wing argon and hydrogen atmosphere. The as prepared LaCuOS nano-aggregates have poor dispersion and a wide size distribution range (50–100 nm). Keywords: lanthanide copper oxychalcogenide, two-step synthesis, co-precipitation, reduction Avtorji opisujejo razvoj metode dvostopenjske sinteze za izdelavo ~istega LaCuOSv obliki nanoaglomeratov. Kot izhodi{~ne materiale so uporabili La(NO 3) 3·6H 2O, CuSO 4·5H 2Oi nN H 3·H 2O. Rezultati analiz so pokazali, da se ta prekurzor oziroma izhodi{~ne materiale lahko pretvori v fazo La 2O 2SO 4,L a 2(SO 4) 3 in CuO pri 800 o C po 2 urah segrevanja na zraku. Pretvorbo v ~isto LaCuOS fazo pa so potem izvedli z redukcijo pri 800 o C po 5 urah segrevanja v reduktivni me{anici argona in vodika. Izdelani nanoagregati LaCuOS so imeli dokaj obse`no porazdelitev velikosti delcev med 50 nm in 100 nm. Klju~ne besede:bakrov oksihalogenidni lantanid, dvostopenjska sinteza, koprecipitacija, redukcija 1 INTRODUCTION Quaternary-layered oxychalcogenides with the gen- eral formula LnCuOQ (Ln=lanthanides and Q=S, Se), re- cently termed the "1111 "structure, are known to ex- hibit interesting structures and exciting physical properties, such as ionic conductivity, transparency, cou- pled with semiconductivity, and medium-temperature su- perconductivity. 1,2 In particular, it is a crucial and well-known challenge to develop a wide-gap p-type semiconductor, which was already applied to ultravio- let-green light-emitting diodes (LEDs) utilizing pn junc- tions, full-color LEDs or white LEDs. 3,4 Interest in rare-earth Cu-based oxychalcogenides, typical layered oxysulfide LaCuOS with a band gap of 3.1 eV , is known to be one of the few transparent, p-type semiconductors and exhibits efficient blue photoluminescence and large third-order optical nonlinearity due to exciton at room temperature. 5 LaCuOS is a mixed-anion material com- posed of divalent oxygen and sulfur anions. The impor- tant feature of its crystal structure is that LaCuOS crys- tallizes in a layered structure, and Cu + ions are coordinated by sulfur anions exclusively. LaCuOS is a tetragonal system (space group: P4/nmm), and its crystal structure is composed of a (La 2 O 2 ) 2+ oxide layer and (Cu 2 S 2 ) 2– sulfide layer stacked alternately along the c-axis. The ionic LaO layers confine the Cu–S bonds in the two-dimensional CuS layers and preserve the trans- parent quality of LaCuOS. 6–8 This naturally layered crys- tal structure brings interesting electrical and optical properties to these materials. These features suggest that LaCuOS is a promising material and has received con- siderable attention, owing to the potential optoelectronic application as the active or contact layers of light-emit- ting devices in ultraviolet and/or blue region, an efficient transparent anode for OLEDs and other photocathode, as well as for use as transparent p-type electrodes. 9–16 Several different routes have been proposed to pre- pare different forms of lanthanide copper oxychalco- genide compounds, including oxidization, 16 vacuum solid-state reaction (VSSR), 17 sulfurization, 18 flux, 19–20 heteroepitaxial growth, 21–23 sputtering, 24 and so on. For example, Palazzi prepared the layered oxysulfide LaCuOS by the oxidization of LaCuS 2 for the first time in 1981. Yosuke Goto also prepared the LaCuOS by VSSR with La, Cu, S and La 2 O 3 as the starting materials. Thus, it is difficult to obtain the product with nano size and good morphology, and at the same time, it raise costs and high-risk safe concerns during the synthesis pro - cess. 17 Moreover, VSSR is a complicated preparation process, which needs high temperatures and a long reac- tion time, and is very energy-intensive. In 2017, with the use of Cu 2 O, La 2 O 3 and S starting materials, high-quality LaCuOS was synthesized by a solid state-reaction fol- lowed by sulfurization. 18 Although Lian et al. briefly re- ported about the synthesis of LaCuOS nanopowder by a precipitation combined with reduction route, 25 its forma- tion mechanism has not yet been examined in detail. Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 359 UDK 691.73:54.057:669.333.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(4)359(2023) *Corresponding author's e-mail: syjz_lixing1979@163.com Based on these facts, it is necessary to systematically ex- plore the synthesis and formation mechanism of LaCuOS. In this study we have developed a two-step synthesis for preparing pure LaCuOS nano-sized ag- glomerates and this method has the characteristics of safe, simple, efficient, economical, ease of mass produc- tion and is environment friendly. Moreover, we also re- ported the results of Fourier transform infrared spectra, X-ray diffraction, thermogravimetric analysis, morphol- ogies and energy-dispersive spectroscopy. 2 EXPERIMENTAL PROCEDURES 2.1 Materials and synthesis La(NO 3 ) 3 ·6H 2 O (AR), CuSO 4 ·5H 2 O (AR) and NH 3 ·H 2 O (AR) reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China and used as the start- ing materials without further purification. Firstly, 0.1-M La(NO 3 ) 3 and CuSO 4 solutions were obtained by dissolv- ing a stoichiometric amount of La(NO 3 ) 3 ·6H 2 O and CuSO 4 ·5H 2 O in deionized water, respectively. The mother liquor was obtained by mixing the above La(NO 3 ) 3 and CuSO 4 solutions according to a molar ratio of 1:1:1 for La 3+ :Cu 2+ :SO 4 2– . The 3-M NH 3 ·H 2 O solution was prepared by dissolving stronger ammonia water into deionized water and used as the precipitant. Secondly, the precursor was prepared by dropping the precipitant solution at a rate of 2 mL·min –1 into the mother liquor under vigorous stirring at room temperature. During the co-precipitation process, the pH value of the reaction system was continuously monitored with the ZDJ-4B au- tomatic potentiometric titrator. After dropping, the re- sulting light-blue precipitate was aged for 2 h, and then separated and washed repeatedly with deionized water to obtain the precursor. The precursor was then dried at 80 °C for 12 h and calcined at different temperatures for 2 h in air. Thirdly, the calcined powder was pressed into 16-mm-diameter disks by bidirectional uniaxial compac- tion under 100 MPa. Finally, the powder compact was placed in a tubular furnace and reduced at specific tem- peratures (450 °C and 800 °C) for5hi nt h eh ybrid at- mosphere of flowing hydrogen and argon (90 % Ar + 1 0%H 2 ), followed by furnace cooling to room tempera- ture. 2.2 Characterization The titration curve was recorded with a Shanghai ZDJ-4B automatic potentiometric titrator. Fourier-trans- form infrared spectra (FTIR) were obtained in the region 4000–400 cm –1 using an Agilent Cary 660 FTIR spectro- photometer and the KBr method. The phase structures were identified by X-ray diffractometer (XRD, D8 Ad- vance) operating at 40 kV and 30 mA with Cu-K (0.15406 nm) radiation. Thermogravimetry (TG), deriva- tive thermogravimetry (DTG) and differential thermal analysis (DTA) were performed using simultaneous dif- ferential thermal analysis and thermo-gravimetry (SDT 2960). The particle morphologies and energy-dispersive spectroscopy (EDS) pattern were obtained on a JEOL-2010 TEM at an acceleration of 200 kV . 3 RESULTS AND DISCUSSION 3.1 Selection of pH value in co precipitation process To determine the appropriate ammonia volume and completely precipitate all the La 3+ and Cu 2+ ions, the ef- fect of titrated ammonia volume on the pH value in the La(NO 3 ) 3 -NH 3 ·H 2 O, CuSO 4 -NH 3 ·H 2 O and La(NO 3 ) 3 - CuSO 4 -NH 3 ·H 2 O systems were investigated, respectively, as shown in Figure 1. From Figure 1a, with the increase of ammonia to 0.08 mL, pH value of La(NO 3 ) 3 solu- tion increases dramatically from an initial 6.15 to 8.27, and at the same time the white precipitate was formed. When the titration amount of ammonia water continues to increase, the pH value of La(NO 3 ) 3 -NH 3 ·H 2 O system increases slowly and shows an approximate plateau around 8.30–9.00, which is attributed to La 3+ ions react- ing with OH – groups completely. The increase of the concentration of OH – generated by ammonia hydrolysis leads to an increase in pH value with further increasing the volume amount of ammonia water. From Figure 1b, the pH value increases from an initial value of 5.13 in the CuSO 4 -NH 3 ·H 2 O system and the blue precipitate was ob- served in this clear blue solution, corresponding to the following chemical reaction: 2CuSO 4 + 2NH 3 ·H 2 O=C u 2 (OH) 2 SO 4 + (NH 4 ) 2 SO 4 (1) A titration jump occurred when the volume of ammo- nia reached 2.72 mL with a pH value of 6.00, indicat- ing that the titration endpoint (2.72, 6.00) had been reached. To continue to increase the amount of ammonia water is unfavorable to the precipitate and excessive am- monia water will react with the precipitate, which causes the precipitation to become a dark-blue solution. The corresponding complex reaction can be expressed as fol- lows: CuSO 4 + 4NH 3 ·H 2 O=[Cu(NH 3 ) 4 ]SO 4 +4 H 2 O (2) As can be seen from Figure 1c, when the NH 3 ·H 2 O volume reaches 0.38 mL, the pH value increases from an initial value of 4.58 to 5.22 in the La(NO 3 ) 3 -CuSO 4 - NH 3 ·H 2 O system. Then pH value increases slowly from 5.22 to 5.91 when the NH 3 ·H 2 O volume reaches 2.95 mL, which is the Cu 2+ ion precipitation interval according to the result of Figure 1b, corresponding to equation (1). With continuously increasing NH 3 ·H 2 O dosage to 5.33 mL, pH amounts to 8.33 at the same time, La 3+ ion precipitation occurs, javascript:; which is attributed to La 3+ ions reacting with the OH – and SO 4 2– groups. The reaction concerning the La 3+ ion precipita- tion can be expressed as follows: 2La 3+ +S O 4 2– + 4OH – + xH 2 O=L a 2 (OH) 4 SO 4 ·xH 2 O (3) X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES 360 Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 In the chemical formula (3), the SO 4 2– and the OH – groups derive from the hydrolysis of CuSO 4 and NH 3 ·H 2 O, respectively. Here, the SO 4 2– , the OH – groups are hard bases and the La 3+ ion is a hard acid, thus the SO 4 2– and OH - groups prefer to coordinate with La 3+ ions more easily to form La 2 (OH) 4 SO 4 according to the HSAB (hard-soft acid-base) principle. 26 Moreover, the yield of blue precipitation shows the increment tendency with increasing pH value. However, a minor pH value change can be observed on the pH value curve when the NH 3 ·H 2 O volume is greater than 8.87 mL. In this work, 8.87 mL of NH 3 ·H 2 O is considered the best ammonia volume and the appropriate pH value is 9.00 in La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O co-precipitation system. 3.2 FTIR analyses of three precursors To qualitatively determine the functional group com- position of three precursors synthesized by three differ- ent systems, i.e., La(NO 3 ) 3 -NH 3 ·H 2 O, CuSO 4 -NH 3 ·H 2 O and La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O, FTIR analyses were car- ried out, as displayed in Figure 2. The FTIR spectros- copy confirms the presence of the OH – and NO 3 – groups in the synthesized precursor based on La(NO 3 ) 3 - NH 3 ·H 2 O system (Figure 2a). The vibration peaks at 3520 cm –1 and 1640 cm –1 are originated from the O–H stretching vibration and the H–O–H bending vibration of hydroxy and adsorbed water molecule in the precursor, respectively. The sharp peak located at 1380 cm –1 is as- signed to the antisymmetric stretching vibration mode of the nitrate group (NO 3 – ). Moreover, the weak peak around 540 cm –1 can be attributed to the La-O bond ab- sorption. In the CuSO 4 -NH 3 ·H 2 O system, evidence of the presence of sulfate (SO 4 2– ) and hydroxyl (OH – ) groups in the precursor can be obtained from Figure 2b. The ob- served fundamental vibrations of the sulfate groups (SO 4 2– ) are located at 1125 (v 3 ) and 600 cm –1 (v 4 ), respec- tively. Besides the O-H stretching vibration peaks at 3570 cm –1 and 3390 cm –1 they are also observed in Fig- ure 2b. The peak at 875 cm –1 is attributed to multiple frequency absorption of the OH – groups. The above-mentioned results indicate that the precursor is composed of basic copper sulfate. The FTIR spectrum of the synthesized precursor based on the La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O system is labeled in Fig- ure 2c. The broad absorption band centered at 3455 cm –1 and the weak peak centered at 1640 cm –1 are attributed to the stretching and bending vibration peaks of the OH – groups, respectively. Moreover, the peak around 980 cm –1 could be assigned to the strong H-bonding in Cu-O-H. 27 The peaks at 1125 cm –1 (v 3 ) and 600 cm –1 (v 4 ) are regarded as the stretching modes of the SO 4 2– groups. The result is consistent with that of Lian et al. In addi- tion, two weak absorption peaks centered at 1500 cm –1 X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 361 Figure 2: FTIR spectra of three precursors: a) La(NO 3 ) 3 -NH 3 ·H 2 O, b) CuSO 4 -NH 3 ·H 2 O, c) La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O Figure 1: Titration curves for : a) La(NO 3 ) 3 -NH 3 ·H 2 O, b) CuSO 4 - NH 3 ·H 2 O, c) La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O systems and 1420 cm –1 were unexpectedly observed in the pre- cursor, which can be assigned to the characteristic asym- metrical split stretching of the CO 3 2– groups. The trace amount of CO 3 2– on the surface of the precursor may be caused by the adsorbed water and carbon dioxide from the ambient atmosphere. 3.3 Thermal analyses of three precursors To understand the decomposition behavior and deter- mine the optimal calcination temperature for the three precursors, DTA-TG-DTG curves of these precursors were conducted from room temperature to 1000 °C, and these results are shown in Figure 3. Figure 3a depicts DTA-TG-DTG curve for the synthesized precursor based on the La(NO 3 ) 3 -NH 3 ·H 2 O system. The TG curve con- tains three main weight loss steps, and the total weight loss of the precursor is 35.62 w/%. The first temperature range of the weight loss is from room temperature to 300 °C and a weak endothermic peak centered at 50 °C appears on the DTA curve, which is caused by the removal of the crystal water in the precursor. The second weight loss, accompanied by a sharp peak at 343 °C and a weak peak at 522 °C on the DTG curve, is mainly related to the gradual dehydroxylation between 300 and 600 °C. Besides, a sharp peak at 349 °C and a weak peak at 527 °C appear on the DTA curve, respec- tively, which indicate that the dehydroxylation process is an endothermic reaction and is carried out step by step. The third weight loss starting at 600 °C is caused by the removal of the nitrate group in the precursor with an endothermic peak at 718 °C and DTG maximum value at around 720 °C. Moreover, as shown in Figure 3a, little weight changes can be observed at temperatures greater than 800 °C on the TG curve, which indicates that at higher than 800 °C the thermal decomposition is basi- cally finished. The DTA-TG-DTG curve of the synthe- sized precursor based on CuSO 4 -NH 3 ·H 2 O system is also shown in Figure 3b. The TG curve shows a continuous weight loss between room temperature and 1000 °C with an overall weight loss of approximately 32.84 w/%. The total weight loss mainly consists of the two following steps in the whole temperature range, as seen from the DTG curve. The first weight loss in the temperature range from room temperature to 450 °C is about 16.20 w/%, which seems to be related mostly to the re- moval of adsorbed and hydroxy water from the precur- sor. This weight loss corresponds to a weak endothermic peak at around 234 °C in the DTA curve, and DTG max- ima at about 229 °C in the DTG curve. Moreover, the TG curve shows an approximate plateau between 450 and 650 °C, accompanied by a upward peak at 552 °C on the DTA curve, suggesting that the crystallization pro- cess of Cu 2 (OH) 2 SO 4 phase is an exothermic reaction. The second weight loss starting at 600 °C is caused by the complete desulfurization reaction of the Cu 2 (OH) 2 SO 4 phase with an obvious endothermic peak at 749 °C and DTG maximum value at around 743 °C, which corresponds to the following chemical equation (4). CuSO 4 ·CuO = 2CuO + 2SO 2 ↑ +O 2 ↑ (4) Based on the above Figure 3a and 3b, we continued to analyze the thermal decomposition process of the syn- thesized precursor based on the La(NO 3 ) 3 -CuSO 4 - NH 3 ·H 2 O system. As shown in Figure 3c, the TG curve shows a continuous weight loss between room tempera- ture and 1000 °C with an overall weight loss of approxi- mately 29.53 w/%. According to the DTG curve, the weight loss can be divided into four stages. The first two processes before 400 °C are about 18.70 w/%, which corresponds to the removal of physically adsorbed and crystal water from the precursor. This weight loss corre- sponds to an endothermic peak at around 113 °C in the DTA curve, accompanied by a broad peak at 111 °C and a shoulder weak peak at 293 °C on the DTG curve. The third weight loss between 400 °C and 600 °C is about 10.20 w/%, which is attributed to the complete dehydroxylation of the precursor. A weak peak appears at 582 °C in DTG curve. At the same time, an obvious exothermic peak around 597 °C in the DTA curve, which may be due to the fact that the exothermic heat of crys- tallization is greater than the endothermic in dehydro- xylation process. The fourth weight loss is at tempera- tures greater than 800 °C, with an obvious endothermic peak at 916 °C and DTG maximum value at around 914 °C, is associated with decomposition of the sulfate X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES 362 Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 Figure 3: DTA-TG-DTG curves of three precursors: a) La(NO 3 ) 3 - NH 3 ·H 2 O, b) CuSO 4 -NH 3 ·H 2 O, c) La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O group in the precursor. Therefore, to obtain pure LaCuOS phase, 800 °C is recommended in this study. This thermal analysis result is consistent with that re- ported by Lian et al. 25 3.4 Structural transformation of three precursors dur- ing calcining Figure 4 shows the XRD patterns of three precursors and their corresponding calcination products at different temperatures. For the La(NO 3 ) 3 -NH 3 ·H 2 O system, it can be seen from left figure in Figure 4 that the structure of the precursor changed during the calcination. The XRD analysis shows that the precursor and its calcination product at 200 °C are crystalline in structure with essen- tial diffraction peaks. Unfortunately, there is no relevant crystal structure information in standard JCPD card data- base and their crystal structure is yet to be identified. Af- ter the precursor calcined at 400 °C, XRD pattern shows that the obtained diffraction pattern is consistent with the LaONO 3 data reported in JCPD cards No. 00-031-0665 and 00-028-0513. When the precursor was calcined at 600 °C, XRD analysis shows that the calcined product is composed of the La 5 O 7 NO 3 major phase (JCPDS card No. 00-038-0891) and a small amount of La 2 O 3 phase (JCPDS card No.00-005-0602), indicating that the LaONO 3 phase has been completely converted to La 5 O 7 NO 3 , and a small amount of La 5 O 7 NO 3 phase has begun to decompose into the La 2 O 3 phase. Upon further increasing calcining temperature to 800 °C and 1000 °C, all the diffraction peaks of the calcination products can be indexed to the standard JCPDS card (No.00-005- 0602) of the La 2 O 3 phase. These results are also consis- tent with those obtained using the DTA-TG-DTG analy- sis. For the CuSO 4 -NH 3 ·H 2 O system, as shown in the middle figure in Figure 4, the precursor and its calcined product at 200 °C also have an unknown crystalline structure. However, after the precursor calcined at 400 °C, the XRD pattern shows that the diffraction peaks become a weak signal and gradually broaden, which cor- responds to the destruction of the crystalline structure due to the removal of crystal water and hydroxyl groups. Similarly, when the precursor was calcined at 600 °C, the diffraction peaks of the CuO phase (JCPD card No. 00-010-1268) begin to appear in the XRD pattern and the diffraction peaks of CuSO 4 ·CuO still exist in the XRD pattern, which indicates that the CuSO 4 ·CuO does not completely transform to CuO at 600 °C. A further in- crease in the calcination temperature from 800 °C to 1000 °C leads to the generation of pure CuO phase. Finally, we discussed the phase-formation process of the synthesized precursor based on the La(NO 3 ) 3 -CuSO 4 - NH 3 ·H 2 O system at different temperatures, as shown in the right figure in Figure 4. When the calcination tem- perature is less than or equal to 400 °C, the precursor has an amorphous structure without obvious diffraction peaks in the XRD pattern. When the precursor was cal- cined at 600 °C and 800 °C, the calcined product is com- posed of La 2 O 2 SO 4 (JCPD card No. 00-016-0501), La 2 (SO 4 ) 3 (JCPD card No. 00-045-0904) and CuO (JCPD card No. 00-010-1268) phases according to the X-ray diffraction pattern. It is particularly important to note that the La 2 (SO 4 ) 3 phase plays a crucial role in the formation of the LaCuOS target product. This will be discussed in the following section. With a further in- creasing of the calcination temperature to 1000 °C, the calcined product is only composed of La 2 O 2 SO 4 and X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 363 Figure 4: XRD patterns of three precursors and their corresponding calcination products at different temperatures. Left: La(NO 3 ) 3 -NH 3 ·H 2 O Middle: CuSO 4 -NH 3 ·H 2 O Right: La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O CuO phases owing to the complete decomposition of La 2 (SO 4 ) 3 to La 2 O 2 SO 4 . 3.5 Transformation of the calcination product during reducing To understand the formation mechanism of the LaCuOS phase and identify the appropriate reduction temperature for the 800 °C calcination product based on the La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O system, DTA-TG-DTG was conducted from room temperature to 850 °C in ar- gon and hydrogen atmosphere and the result is shown in Figure 5. The TG curve shows a continuous weight loss between room temperature and 850 °C with an overall weight loss of approximately 22.85 w/%. Little weight change can be observed at temperatures less than 420 °C on the TG curve, which indicates that the re- duction reaction of the calcination product has not started before 420 °C. Within the temperature range of 420 °C to 500 °C, the TG curve has changed signifi- cantly and the weight loss is about 5.67 w/%. Mean- while, a sharp peak at 436 °C on the DTG curve and an upward peak centered at 438 °C on the DTA curve ap- pear, suggesting that the reduction reaction is an exother- mic process. The above reactions are mainly related to the reduction of CuO and La 2 (SO 4 ) 3 in the argon and hy- drogen atmosphere, which corresponds to the following chemical equations (5) and (6). 2CuO + H 2 =C u 2 O+H 2 O (5) La 2 (SO 4 ) 3 +2 H 2 =L a 2 O 2 SO 4 + 2SO 2 +2 H 2 O (6) This result is also consistent with that obtained by XRD analysis in the following section. Moreover, when the reduction temperature is greater than 500 °C, a larger weight-loss process occurs according to the TG curve and the prominent peak at 688 °C and shoulder peak at 635 °C appear on DTG curve. A possible formation mechanism of LaCuOS is proposed according to relevant literature 28–29 and the following XRD analysis. The chemical reactions in the formation of LaCuOS can be expressed as follows: 2H 2 +S O 2 =2 H 2 O+S ( 7 ) Cu 2 O+H 2 =2 C u+H 2 O (8) La 2 O 2 SO 4 +4 H 2 =L a 2 O 2 S+4 H 2 O↑ (9) S+2 C u+L a 2 O 2 S = 2LaCuOS (10) To further verify the above thermal analysis result and investigate the phase evolution of the products at dif- ferent temperature upon reduction, XRD analysis was conducted for two typical samples and the results are shown in Figure 6. When the 800 °C calcination product was reduced at 500 °C, the reduced product is composed of La 2 O 2 SO 4 (JCPD card No. 01-085-1534) and Cu 2 O (JCPD card No. 01-077-0199) phases according to X-ray diffraction pattern (Figure 6a). The disappearance of La 2 (SO 4 ) 3 in the reduced product indicates that it has been reduced to La 2 O 2 SO 4 . Compared with Figure 6a, when the 1000 °C calcination product was reduced at 500 °C, the reduction product is also composed of La 2 O 2 SO 4 and Cu 2 O. However, the XRD pattern shows the enhanced diffraction-peak intensity due to the high crystallinity of the 1000 °C calcination product. From X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES 364 Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 Figure 6: XRD patterns of the calcination products at different tem- perature under reducing atmosphere Figure 5: TG-DTG-DTA curves of the calcination product (800 °C, 2h) synthesized by the La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O system Figure 6c, for the 800 °C calcination product, a further increasing reduction temperature of 800 °C results in the formation of pure LaCuOS phase owing to complete re- action of La 2 O 2 S, copper and sulfur according to the chemical equation (10). The phase evolution is consis- tent with the thermal analysis before (Figure 5). From Figure 6d, when the 1000 °C calcination product was re- duced at 800 °C, all the diffraction peaks recorded from the reduction product can be indexed to hexagonal La 2 O 2 S (JCPD No. 01-027-0263) and copper (JCPD No. 01-085-1326) mixture. Thus, the selection of calcination products plays a decisive role in the formation of the tar- get product, i.e., LaCuOS. The existence of La 2 (SO 4 ) 3 in the calcined product is the key factor to obtain the final target product. Therefore, the calcination temperature of the synthesized precursor based on La(NO 3 ) 3 -CuSO 4 - NH 3 ·H 2 O system should not exceed 800 °C in this study. 3.6 TEM morphologies and EDS analysis Figure 7 shows TEM morphologies of a) the precur- sor, b) the calcination product (800 °C, 2h), c) the reduc- tion product and d) EDS of the reduction product. It can be seen from Figure 7a that the precursor has a nearly spherical shape, a poor dispersion, a wide size distribu- tion (20–50 nm). Moreover, it can be seen from Fig- ure 7b that the calcination product inherits the morphol- ogy of the precursor, but it aggregates to some degree and becomes a bigger particle size (50–100 nm) due to the high-temperature calcination. Furthermore, shown in Figure 7c, for the reduction product, LaCuOS, the dispersivity becomes worse and the agglomeration phe- nomenon is more obvious compared with the calcination product (Figure 7b). Besides, the EDS analysis was con- ducted to gain the composition of the reduction product and the result is shown in Figure 7d, which demon- strates that the nano-sized agglomerates consist of lan- thanum (La), copper (Cu), oxygen (O), and sulphur (S). The elemental ratios of La, Cu, O and S contained in the LaCuOS are quantified as 21.40:22.33:22.14:34.13. Al- though the ratio is not 1:1:1:1 due to the error of the EDS measurement method itself, it can be confirmed that the product is a quaternary compound containing La, Cu, O and S. 4 CONCLUSIONS In summary, LaCuOS nano-sized agglomerates were produced via a two-step synthesis of a co-precipitation and a reduction step, which use commercially available La(NO 3 ) 3 ·6H 2 O, CuSO 4 ·5H 2 O and NH 3 ⋅H 2 O as the start- ing materials. The present study shows that the appropri- ate pH value is 9.00 in the La(NO 3 ) 3 -CuSO 4 -NH 3 ·H 2 O co-precipitation system. Moreover, the selection of calci- nation temperature plays a decisive role in the formation of LaCuOS and the calcination temperature of the syn- thesized precursor based on the La(NO 3 ) 3 -CuSO 4 - NH 3 ·H 2 O system should not exceed 800 °C in order to obtain pure LaCuOS phase in this study. The as-prepared LaCuOS nano-aggregates have poor dispersion and a wide size distribution (50–100 nm). The two-step syn- thesis reported in this paper is an economical, convenient and environmentally friendly method to synthesize pure LaCuOS nano-sized agglomerates. In addition, the two-step synthesis method is also helpful for the prepara- tion of other lanthanide copper oxychalcogenide. Acknowledgements This work was financially supported by the Provin- cial Education Department of Liaoning (No. lnqn202021). 5 REFERENCES 1 D. O. Charkin, A. V . Urmanov, S. M. Kazakov, J. Alloys Compd, 516 (2012), 134 2 S. Lardhi, A. Curutchet, L. Cavallo, M. Harb, T. L. Bahers, Phys. Chem. Chem. Phys., 19 (2017), 12321 3 H. Matsushita, H. Takashima, A. Katsui, Phys. Stat. Sol., 8 (2006)3 , 2888 4 J. Llanos, O. Pena, J. Solid State Chem., 178 (2005)4 ,9 5 7 5 J. J. Ma, Q. Y . Liu, P. F. Liu, P. Zhang, B. Sanyal, T. Ouyang, B. T. Wang, Phys. Chem. Chem. Phys., 24 (2022), 21261 6 H. Kamioka, H. Hiramatsu, H. Ohta, M. Hirano, App. Phys. Lett., 84 (2004)6 ,8 7 9 7 K. Ueda, K. Takafuji, H. Hiramatsu, H. Ohta, T. Kamiya, M. Hirano, H. Hosono, Chem. Mater., 15 (2003) 19, 3692 8 H. Kamioka, H. Hiramatsu, M. Hirano, K. Ueda, T. Kamiya, H. Hosono, Opt. Lett., 29 (2004) 14, 1659 9 K. Ueda, K. Takafuji, H. Hosono, J. Solid State Chem., 170 (2003)1 , 182 X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366 365 Figure 7: TEM morphologies of: a) the precursor, b) the calcination product (800 °C, 2h), c) the reduction product, d) EDS of the reduc- tion product 10 H. Sato, S. Nishimoto, K. Tsuji, K. Takase, H. Nakao, Y . Takahashi, T. Takano, K. Sekizawa, H. Negishi, S. Negishi, M. Nakatake, H. Namatame, M. Taniguchi, J. Alloys Compd, 408–412 (2006), 746 11 D. O. Scanlon, J. Buckeridge, C. R. A. Catlow, G. W. Watson, J. Ma- ter. Chem. C, 2 (2014) 17, 3429 12 K. Takase, K. Sato, O. Shoji, Y . Takahashi, Y . Takano, K. Sekizawa, Y . Kuroiwa, M. Goto, App. Phys. Lett., 90 (2007) 16, 161916 13 K. Ueda, S. Inoue, H. Hosono, N. Sarukura, M. Hirano, App. Phys. Lett., 78 (2001) 16, 2333 14 A. Renaud, L. Cario, Y . Pellegrin, E. Blart, M. Boujtita, F. Odobel, S. Jobic, RSC Adv., 5 (2015) 74, 60148 15 H. Yanagi , M. Kikuchi, K. Kim, H. Hiramatsu, T. Kamiya, M. Hirano, H. Hosono, Org. Electron. 9 (2008)5 ,8 9 0 16 M. Palazzi, C. R. Acad. Sci. Paris, 292 (1981), 789 17 Y . Goto, M. Tanaki, Y . Okusa, T. Shibuya, K. Yasuoka, M. Matoba, Y . Kamihara, App. Phys. Lett., 105 (2014) 2, 022104 18 N. D. Zhang H. Gong, Cera. Int., 43 (2017) 8, 6295 19 Y . Nakachi, K. Ueda, J. Cryst. Growth, 311 (2008)1 ,1 1 4 20 Y . Takano, C. Ogawa, Y . Miyahara, H. Ozaki, K. Sekizawa, J. Alloys Compd, 249 (1997) 1–2, 221 21 H. Hiramatsu, K. Ueda, H. Ohta, M. Orita, M. Hirano, H. Hosono, App. Phys. Lett., 81 (2002)4 ,5 9 8 22 H. Hiramatsu, K. Ueda, H. Ohta, M. Hirano, T. Kamiya, H. Hosono, Thin Solid Films, 445 (2003)2 ,3 0 4 23 H. Hiramatsu, K. Ueda, K. Takafuji, H. Ohta, M. Hirano, T. kamiya, H. hosono, Appl. Phys. A, 79 (2004), 1517 24 N. D. Zhang, D. W. Shi, X. X. Liu, A. Annadi, B. S. Tang,T. J. Huang, H. Gong, Appl. Mater. Today, 13 (2018), 15 25 J. B. Lian, N. N. Li, H. L. Wang, Y . Su, G. M. Zhang, F. Liu, Cera. Int., 42 (2016) 9, 11473 26 L. H. Lee, Prog. Colloid Polym. Sci., 82 (1990), 337 27 K. Huang, J. J. Wang, D. F. Wu, S. Lin, RSC Adv., 5 (2015) 11, 8455 28 E. I. Sal’nikova, D. I. Kaliev, P. O. Andreev, Russ. J. Phys. Chem. A, 85 (2011) 12, 2121 29 D. L. Murdock, G. A. Atwood, Ind. Eng. Chem., Process Des. De- velop., 13 (1974)3 ,2 5 4 X. LI et al.: TWO-STEP SYNTHESIS AND FORMATION MECHANISM FOR LACUOS NANO-SIZED AGGLOMERATES 366 Materiali in tehnologije / Materials and technology 57 (2023) 4, 359–366