Damage-Free Removal of Residual Carbon in a Dielectric Barrier

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Damage-free removal of residual carbon in a dielectric barrier discharge (DBD) plasma for carbothermal-synthesized materials Zhiyong Mao, Jingjing Chen, Guanghao Li, Dajian Wang, Zhihao Yuan, and Bradley D. Fahlman Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02842 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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Chemistry of Materials

Damage-free removal of residual carbon in a dielectric barrier discharge (DBD) plasma for carbothermal-synthesized materials Zhiyong Mao,*,†,⊥ Jingjing Chen,‡,⊥ Guanghao Li,† Dajian Wang,*,‡ Zhihao Yuan,‡ and Bradley D. Fahlman§ †

School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, PR. China.

‡

Tianjin Key Laboratory for Photoelectronic Materials and Devices, Tianjin University of Technology, Tianjin 300384, PR. China. §

Department of Chemistry & Biochemistry and Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, MI USA 48859. ABSTRACT: In this work, we demonstrate damage-free removal of residual carbon in a dielectric barrier discharge (DBD) plasma for carbothermal-synthesized materials, such as CaAlSiN3:Eu2+ phosphors, SnSb alloy anode materials and TiN ceramic powders. The efficiency of residual carbon removal and the damaging effects of the plasma for treated materials are investigated in detail, exampling with carbothermal-synthesized CaAlSiN3:Eu2+ phosphors. Results show that the residual carbon in carbothermal-synthesized CaAlSiN3:Eu2+ phosphors could be removed effectively within a DBD plasma generator, resulting in the significant improvement of luminescent properties. The damage-free character of this DBD plasma decarburization process to phosphors is revealed, showing amazing superiority over the traditional hightemperature decarburization route. These results offer an attractive strategy for the removal of residual carbon for various carbothermal-synthesized materials with finely controlled compositions.

Introduction The carbothermal reduction method is widely used in the synthesis of non-oxide materials for the development of new materials 1-4. Some examples include, the industrial carbothermal reduction of iron and various non-ferrous metals, carbothermal synthesis of AlN and other hightemperature ceramics, and the carbothermal preparation of nitride-hosted phosphors (M2Si5N8:Eu2+, MAlSiN3:Eu2+, M=Ca, Sr, Ba). In particular, nitride-based phosphors applications for solid-state lighting technology have been rapidly developed due to their excellent luminescent properties and outstanding thermal stability4-7. Compared to the commonly used solid-state reactions 8,9 and gasreduction-nitridation process 10,11 for the synthesis of nitride phosphors, the carbothermal reduction method is attractive for industrial production and experimental research. This is primarily due to its advantages in low-cost raw materials and simple equipments 12-15. Considering the reducing atmosphere provided by the Boudouard reaction (C(s) + CO2(g) ⇄ 2 CO(g)) and the related reaction thermodynamics and kinetics, an excess of a carbon-based reducing agent is a prerequisite for efficient carbothermal reduction reactions. The presence of residual carbon in as-prepared materials is mostly unavoidable, which may seriously impair the performance of target materials. For example, residual carbon may degrade the oxidation resistance of ceramics and impair the luminescent properties of phosphors. The hightemperature oxidation method, which involves calcina-

tion in air at a temperature as high as 600-800 ℃, is commonly used to remove residual carbon 16. Although a high efficiency of carbon removal was recognized for this method, oxidative structural damage and the resultant degradation of properties was pronounced for non-oxide materials. Other methods such as, chemical oxidation decarburization and flotation decarburization routes are inadvisable due to their environmental hazards and less effective removal, respectively. Thus, an efficient, damage-free, energy-saving and environmentally-friendly decarburization technique is urgently needed for carbothermal-synthesized materials. Plasma, the fourth state of matter, is a quasi-neutral discharge gas composed of electrons, ions, atoms, molecules, radicals, and other species. For a low-temperature plasma, the electronic temperature is much higher than its ionic temperature 17. This means that a large number of strongly oxidizing free radicals, excited-state atoms/molecules, and other active particles are produced by high-energy inelastic electron collisions, which cause excitation, dissociation, or ionization of gas molecules. On the other hand, the gaseous system temperature is very low, even close to room temperature. Kogelschatz summarized that low-temperature plasmas have been widely utilized in the degradation of organic pollutants and purification of waste gas/water 18-20. Low-temperature plasma was also employed for the oxidative combustion of carbonaceous materials by taking advantage of its strong oxidizing characteristics. For example, Yao et al.

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CaAlSiN3:Eu2+/C sample was placed within the quartz reactor chamber. A high voltage generator (CTP-2000K, Corona Laboratory, Nanjing, China) supplying a voltage from 0 kV to 30 kV with a sinusoidal waveform at a frequency of about 22 kHz, was used to generate the plasma. During the carbon removal, the average voltage and input power (forward power) were tuned to be 10 kV and 300 W, respectively. The plasma was initiated at room temperature and atmospheric pressure without external heating or cooling, using air as the plasma-generating gas. The duration of the plasma treatment was varied from 5 min to 25 min. For comparison, the residual carbon was also removed by traditional high-temperature oxidation method in air by exposing the materials at 650 ℃ and 750 ℃ for about 30 min.

reported the removal of diesel particulate matter (dominant component is carbon) with a removal ratio above 70 % by using dielectric barrier discharge (DBD), which is a common way to generate a low-temperature plasma 21-23; Huang et al. showed that amorphous carbon could be easily removed by using a radio frequency glow-discharge plasma 24; Liu et al. demonstrated the 'micro-combustion' of a carbon template with efficient carbon removal, initiated by active oxygen species within a DBD plasma 25. However, to the authors’ knowledge, there is no precedent for the removal of residual carbon from carbothermal-synthesized materials by using a low-temperature plasma. In particular, the afore mentioned reports seldom paid attention to the damaging effects of the plasma itself to the materials, which is a crucial issue in the removal of residual carbon from materials. Herein, we report the use of a DBD low-temperature plasma for the decarburization of carbothermalsynthesized phosphors, as demonstrated in detail by the removal of residual carbon from carbothermalsynthesized CaAlSiN3:Eu2+ phosphors. The removal efficiency of residual carbon by DBD plasma and the resultant influences on luminescent properties of the phosphors are investigated. The damaging effect of the plasma decarburization technology to the phosphor is also compared with the traditional high-temperature decarburization route.

Figure 1 Schematic of the DBD plasma generation setup and the quartz reactor used in the experiment.

Experimental Preparation of CaAlSiN3:Eu2+ nitride phosphors by carbothermal reduction method. CaAlSiN3:Eu2+ redemitting nitride phosphors were prepared by a carbothermal reduction method similar to our previous work 15 . The raw materials of CaH2 (99.5%, Aladdin), AlN (99%, Ube, Honshu, Japan), Si3N4 (99%, Ube, Honshu, Japan), Eu2O3 (99.99%, Aladdin), and graphite powder (99%, Aladdin) were first weighed according to the nominal composition ratio of Ca0.90AlSiN3:0.10Eu2+ followed by adding the graphite powder in a mole ratio of C/O = 1.20. These starting materials were thoroughly ground in an agate mortar and then heated in a horizontal tube furnace at 1550 ℃ for 6 h under a flow of N2/H2 (8%) at 400 mL/min. Finally, the fired sample was ground finely for the follow-up measurements and removal of carbon residues. The obtained phosphor sample was denoted as CaAlSiN3:Eu2+/C in view of the existence of residual carbon for this carbothermal-synthesized phosphor.

Samples characterization. The phase and crystallinity 0f phosphors samples were measured by X-ray power diffraction (XRD) on a diffractometer (Rigaku D/max2500/pc, Japan). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra as well as the absolute quantum efficiency (QE) were measured on a fluorescence spectrometer (Hitachi F-4600, Japan) equipped with an integrating sphere (Orien KOJI, QY-2000, China). The diffuse reflection spectra (DRS) were recorded by a UV-Visible spectrometer (TU-1901, China) with a 60-mm diameter integration sphere. SEM images were obtained on a JSM-6700F field-emission scanning electron microscope. TEM images and the SAED pattern, as well as elemental analyses, were performed on the copper TEM grid with holey carbon film by a JEM-2100F transmission electron microscope equipped with an X-ray EDS. The fluorescence microscope photographs were taken using a fluorescence microscope (DFM-40C，China).

Removal of residual carbon by DBD plasma treatment. Dielectric-barrier discharge (DBD) plasma was employed to remove the residual carbon from the above carbothermal-synthesized CaAlSiN3:Eu2+/C phosphor. A schematic of the DBD plasma setup used in our experiment was shown in Figure 1. Two steel electrodes with a diameter of 50 mm were connected to a high voltage generator with a fixed gap distance of ca. 15 mm. A quartz reactor chamber with a diameter of 90 mm and a wall thickness of 2.5 mm was placed between the two electrodes. The discharge gap between the inner surfaces of the quartz reactor is about 8 mm. The as-prepared

Spark phenomenon could be observed during the removal of residual carbon within the DBD plasma system. The presence of a spark clearly indicates the oxidative combustion of residual carbon initiated by the strong oxidizing plasma. Figure 2 shows the XRD patterns of the carbothermal-synthesized CaAlSiN3:Eu2+/C phosphor sample and DBD plasma treated samples with different durations from 5 - 25 min. For the carbothermalsynthesized CaAlSiN3:Eu2+/C phosphor sample, a dominant CaAlSiN3 phase was indexed. In addition, a small

Results and discussion

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amount of AlN impurity phase was also detected. The coexistence of AlN impurity phase is common in the preparation of CaAlSiN3:Eu2+ phosphors by atmospheric pressure synthetic routes. A distinct diffraction peak of residual carbon at around 2θ = 26.5° was recorded due to an excess of the graphitic carbon reducing agent. After carbon removal, we found that the diffraction intensity of residual carbon decreased gradually with increasing plasma duration, and nearly disappeared after treatment for 25 min. However, the diffraction peaks for CaAlSiN3 phase and AlN impurity remained nearly unchanged. These observations show that the residual carbon in CaAlSiN3:Eu2+/C sample could be removed effectively by our proposed DBD plasma treatment.

2+

Figure 3 SEM images of CaAlSiN3:Eu /C sample (a) without and (b) with DBD plasma treatment for 25 min.; (c) TEM 2+ image of the CaAlSiN3:Eu /C sample without DBD plasma treatment; (d) EDS elemental analysis for point A and point B marked in Figure 3(c); (e) SAED pattern for the transparent sheet observed in Figure 3(c); (f) TEM image of the 2+ CaAlSiN3:Eu /C sample with DBD plasma treatment for 25 min.

Figure 2 XRD patterns of carbothermal-synthesized 2+ CaAlSiN3:Eu /C phosphor sample and DBD plasma treated samples from varying durations (5 - 25 min).

Figure 3(a) and (b) depict SEM images of CaAlSiN3:Eu2+/C phosphor samples without and with DBD treatment for 25 min, respectively. The carbothermal-prepared CaAlSiN3:Eu2+ phosphor particles are in a size range of about 2-5 µm. Some thin plates (indicated by red circles) interspersed with CaAlSiN3:Eu2+ particles were found to exist in non-treated samples, while few could be found in DBD-treated samples. This indicates that the observed plates were likely due to the presence of residual carbon in the phosphor, which could be effectively removed by the DBD plasma treatment. TEM image together with EDS elemental analysis for the CaAlSiN3:Eu2+/C phosphor sample were further measured to confirm this conclusion, as shown in Figure 3(c) - (d). Thick particles and transparent sheets were observed in the TEM image for CaAlSiN3:Eu2+/C phosphors (Figure 3(c)). Figure 3(d) presents the EDS elemental analysis results for point A and point B marked in Figure 3(c), which illustrates that the thick particle and transparent sheet could be assigned to the CaAlSiN3:Eu2+ phosphor and residual carbon, respectively. SAED of the transparent sheet was indexed as the intrinsic diffraction pattern of graphite (Figure 3(e)). On the other hand, there were no transparent graphite sheet that could be observed in the TEM image for DBD-treated sample (Figure 3(f)), implying an effective removal of the residual carbon achieved by DBD plasma treatment.

The diffuse reflection spectra (DRS) of the CaAlSiN3:Eu2+/C phosphor sample and the DBD plasma treated samples with varying durations are provided in Figure 4(a). For comparison, the DRS of a commercial CaAlSiN3:Eu2+ sample free of carbon is given as a reference. The absorption band from 300 nm to 650 nm could be assigned to the transition absorption of Eu2+ ions doped in CaAlSiN3 lattice for the reference sample. The light in the wavelength range of 650 nm - 850 nm could not be absorbed by CaAlSiN3:Eu2+ phosphor, showing a high reflectance of 97 %. For our carbothermal-synthesized CaAlSiN3:Eu2+/C phosphor sample, the DRS showed a similar absorption profile, but the absorption intensity was significantly stronger than that of the commercial CaAlSiN3:Eu2+ reference throughout the entire wavelength range. Apparently, the higher absorption intensity for the as-synthesized CaAlSiN3:Eu2+/C phosphor sample was ascribed to the existence of residual carbon, owing to the strong light absorption character of carbon. For the DBD plasma treated samples, the absorption intensity was found to decrease with the increased duration of plasma treatment. This decreased absorption was ascribed to the decrease of residual carbon content in samples, resulting from the DBD plasma treatment.



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SEM, TEM and DRS verifications. Accordingly, the absolute quantum efficiency for the prepared CaAlSiN3:Eu2+/C sample was enhanced from 8.18 % to 17.83 % after a DBD plasma treatment of 25 min. In comparison, the absolute (internal) quantum efficiency for the commercial CaAlSiN3:Eu2+ reference phosphor was measured to be 80 %. The relative lower emission intensity (quantum efficiency) for our carbothermal-synthesized CaAlSiN3:Eu2+/C phosphor is likely due to the coexistence of an AlN impurity phase and residual carbon. Further improvement of the luminescence efficiency is required for its practical applications by optimizing the preparation conditions to suppress the occurrence of AlN impurities, and by adopting efficient decarburization technique to remove the residual carbon. The above-demonstrated results revealed that the residual carbon in carbothermalsynthesized CaAlSiN3:Eu2+/C phosphors could be removed effectively by the DBD plasma decarburization technique proposed in this paper, resulting in the improvement of luminescence. As discussed above, the light absorption (reflectance) intensity, especially for the light isolated from the Eu2+absorption in the range 650 nm to 850 nm, was strongly dependent on the carbon content in the samples. Herein, we propose an evaluation method to quantify the residual carbon content of phosphors and the carbon removal efficiency of DBD treatment by setting the reflectance at 700 nm as a baseline. First, the reflectance at 700 nm was measured for standards with a known carbon content. These standards composed uniform mixturex of graphite and commercial CaAlSiN3:Eu2+ phosphor in various ratios of graphite/CaAlSiN3:Eu2+ from 0 - 60 wt. %. Figure 5 shows the relationship between reflectance and graphite content in a CaAlSiN3:Eu2+ phosphor. Then, the residual carbon content for as-prepared CaAlSiN3:Eu2+/C phosphor sample and the DBD plasma treated samples could be quantified based on this standard curve through accessing the reflectance at 700 nm. According to the reflectance data, the residual carbon content for our-prepared CaAlSiN3:Eu2+/C phosphor sample and the 25 min DBD plasma treated sample was evaluated to be 17.1 % and 7.8 %, respectively. This corresponds to a residual carbon removal efficiency of 54 %. Photographs for the CaAlSiN3:Eu2+/C phosphor sample and 25 min DBD plasma treated sample are presented as an inset in Figure 5. It was found that a distinct color change from dark brown to orange red was observed, due to the combustion loss of residual carbon upon DBD plasma treatment.

2+

Figure 4(a) DRS; (b) PL/PLE spectra for CaAlSiN3:Eu /C phosphor sample and the DBD plasma treated samples with different durations (5-25 min). The DRS and PL/PLE spectra 2+ for commercial CaAlSiN3:Eu phosphor free of carbon are provided as a reference.

Figure 4(b) depicts the PL and PLE spectra for CaAlSiN3:Eu2+/C phosphor sample and DBD plasma treated samples with varying treatment durations. Similar PL and PLE spectra shapes but with lower intensities were recorded for the as-prepared CaAlSiN3:Eu2+/C sample, as compared to the commercial CaAlSiN3:Eu2+ reference. A broad emission band centered at 665 nm was detected for the as-prepared CaAlSiN3:Eu2+/C phosphors sample. This broad emission originates from the 5d→4f transition of the Eu2+ dopant in the CaAlSiN3 lattice. An enhancement of the emission intensity was observed for the DBD plasma treated samples with an increase in duration of the plasma treatment. It is well known that the degradation pathway of residual carbon on luminescent properties is due to the reabsorption effect; namely, the emitting light of the phosphor is reabsorbed by residual carbon. The improvement of luminescence implies a decrease in the reabsorption effect for DBD plasma treated samples. In other words, the effective removal of residual carbon resulted in the enhancement of luminescence for the CaAlSiN3:Eu2+/C phosphor after DBD plasma treatment, which is in accord with those results deduced from XRD,

The above results indicate that the residual carbon in carbothermal-synthesized CaAlSiN3:Eu2+/C phosphors could be removed effectively by DBD plasma, resulting in a significant improvement of its luminescent properties. Carbon removal efficiency and damaging effects are the two issues of concern for decarburization technology. Hence, we turn to provide an insight into the damaging character of plasma decarburization through comparing this DBD plasma process with the traditional hightemperature oxidation route. Figure 6(a) shows the XRD diffraction patterns for CaAlSiN3:Eu2+/C phosphor sam-


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ples without and with DBD treatment for 25 min. We also provide a comparison with high-temperature oxidation treatment for 30 min at 650 ℃ and 750 ℃. For the DBD plasma-treated samples, the diffraction peak of residual carbon decreases dramatically and eventually disappears completely. In contrast, the CaAlSiN3 phase exhibited no obvious changes, as compared to the non-treated sample. For samples treated by high-temperature oxidation at 650 ℃, the diffraction intensities of both residual carbon and CaAlSiN3 phase were shown to decrease. The persistent diffraction peak of residual carbon meant that the removal of residual carbon at 650 ℃ was inferior, needing a higher oxidation temperature. It was found that the diffraction peak of residual carbon could be quenched completely when a higher oxidation temperature at 750 ℃ was performed. However, the diffraction peaks of the CaAlSiN3 phase were further lowered for samples treated at 750 ℃. The decreasing diffraction intensity of the CaAlSiN3 phase observed in the high-temperature oxidation treated samples indicates the reduction of crystallinity, which will likely degrade the luminescence of the phosphor sample.

Figure 6 (a) XRD patterns; and (b) PL spectra (λex=460 nm) 2+ for CaAlSiN3:Eu /C phosphor samples without and with DBD plasma treatment for 25 min., as well as with hightemperature oxidation treatment for 30 min. at 650 ℃ and 750 ℃, respectively.

Fluorescence microscope photographs (Figure 7(a)) for non-treated and 750 ℃ treated samples were further measured to illustrate the serious damaging effect of the high-temperature decarburization route. Intense redemitting particles were observed for the CaAlSiN3:Eu2+/C sample under an excitation of blue light. The absence of red emission for the 750 ℃ treated particles from an analogous excitation indicated that the phosphor had been damaged at high temperature. Similar destructive damage of high-temperature decarburization technology was observed by researchers for Sr2Si5N8:Eu2+ phosphor treated at 600 ℃ for 2 h 26. A decrease in crystallinity induced by the high temperature oxidative treatment was the dominant reason proposed for the luminescence degradation of the nitride-based phosphors.

Figure 5 Relationship between reflectance at 700 nm and 2+ graphite content in CaAlSiN3:Eu phosphor. Inset was the 2+ photographs for CaAlSiN3:Eu /C phosphor sample and the 25 min DBD plasma treated sample.

A comparison of the influence of DBD plasma decarburization and high-temperature oxidation decarburization routes on phosphor luminescence is presented in Figure 6(b). After DBD plasma treatment, the inherent 665 nm red emission band for the CaAlSiN3:Eu2+ phosphor under an excitation of 460 nm blue light could be enhanced significantly owing to the effective removal of residual carbon from CaAlSiN3:Eu2+/C. In comparison, the emission band intensity weakened significantly for the hightemperature oxidation treated samples. This was most pronounced for the sample treated at 750 ℃, which showed a dramatic loss of its luminescence character.

In addition, the oxidation of activator from Eu2+ to Eu3+ was also responsible for the degradation of luminesce, as demonstrated in Figure 7(b). Intrinsic sharp emission lines originating from 5D0→7FJ transitions of Eu3+ instead of the broad emission band originating from the 5d→4f transition of Eu2+ were recorded for the 750 ℃ treated



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sample when using a higher-energy UV excitation light (250 nm), which matches the excitation energy of Eu3+ ions. The appearance of Eu3+ detected from PL spectra implies the oxidation of Eu2+ ions during the hightemperature oxidative treatment. Thus, the damaging effects of a high-temperature decarburization technology are mainly reflected in two aspects: one is the decrease of crystallinity, and another is the oxidation of activator from Eu2+ to Eu3+. Although high carbon removal efficiency is recognized for high-temperature decarburization technology, the serious damaging effects on luminescent properties severely limits its practical applications. Alternatively, the DBD plasma decarburization technology we presented herein effectively avoids these damage effects, due to a much lower system temperature.

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bothermal-synthesized SnSb alloy anode materials and TiN ceramic powders could be removed effectively by DBD plasma treatment. As compared to the traditional high temperature oxidative route, free-damage or lowerdamage character was identified for our proposed DBD plasma decarburization technique.

Conclusions In summary, a DBD plasma is employed to effectively remove the residual carbon present in carbothermalsynthesized CaAlSiN3:Eu2+/C phosphors, SnSb alloy anode materials and TiN ceramic powders with high removal efficiency and free/lower-damage to the materials. The residual carbon of carbothermal-synthesized CaAlSiN3:Eu2+/C phosphor was removed effectively with a removal efficiency of 54 % through DBD plasma treatment of 25 min, resulting in the significant improvement of luminescence. Damaging effects of decreasing crystallinity and oxidation of activator, which are easily introduced in traditional high-temperature oxidation route for phosphors, are shown to be substantially avoided using our plasma decarburization technology. Hence, our technology shows promise for industrial-scale applications by carefully designing plasma fluidized bed reactors to remove residual carbon for various carbothermalsynthesized particulate materials, or to treat other carbonaceous advanced functional materials.

ASSOCIATED CONTENT Supporting Information. Removal of residual carbon from carbothermal-synthesized SnSb alloy anode materials and TiN ceramic powder by DBD plasma. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Corresponding authors: E-mail address: Prof. Mao, [email protected]. E-mail address: Prof. Wang, [email protected].

Author Contributions ⊥

Zhiyong Mao and Jingjing Chen contributed equally to this work and should be considered co-first authors. The manuscript was written and revised by Zhiyong Mao and Jingjing Chen through contributions of all authors.

Notes The authors declare no competing financial interest.

Figure 7(a) Fluorescence microscope photographs; (b) PL 2+ spectra (λex=250 nm) for CaAlSiN3:Eu /C phosphor samples without and with high-temperature oxidation treatment at 750 ℃, respectively.

ACKNOWLEDGMENT We gratefully acknowledge the financial support by the National Natural Science Foundation of China (nos. 50872091 and 51102265) and Program of Discipline Leader of Colleges and Universities (Tianjin, China) and “Foreign Experts” Thousand Talents Program (Tianjin, China).

In order to demonstrate the generality of our proposed DBD plasma decarburization technique to be functional for other materials, the removal of residual carbon from carbothermal-synthesized SnSb alloy anode materials and TiN ceramic powders was also performed (see Supporting Information). From the XRD results (Figures S1 and S2), we could observe that the residual carbon in both of car-


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Damage-Free Removal of Residual Carbon in a Dielectric Barrier

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