Size Control of Ti4O7 Nanoparticles by Carbothermal Reduction Using a Multimode Microwave Furnace

Utilization of Ti4O7 in applications such as catalyst support calls for control over the size of the Ti4O7 nanoparticles. This can be achieved using a simple process such as carbothermal reduction. In this study, various sizes of Ti4O7 nanoparticles (25, 60, and 125 nm) were synthesized by carbothermal reduction using a multimode microwave apparatus. It was possible to produce Ti4O7 nanoparticles as small as 25 nm by precisely controlling the temperature, heating process, and holding time of the sample while taking advantage of the characteristics of microwave heating such as rapid and volumetric heating. The results show that microwave carbothermal reduction is advantageous in controlling the size of the Ti4O7 nanoparticles.

For all these applications, it is necessary to synthesize single-phase Ti 4 O 7 nanomaterials with a high specific surface area [4].In previous works, Magnéli phase nanoparticles were synthesized using different methods, such as calcination of titanium ethoxide and polyethylene glycol solution [17], thermal plasma treatment of H 2 TiO 3 under Ar/H 2 [18], sol-gel and calcination [19], sol-gel and vacuum-carbothermic processes [20], pulsed UV laser irradiation [21], and thermal-induced plasma processes [22].In addition, Zhang et al. prepared fiber-like Ti 4 O 7 by heating intermediate H 2 Ti 3 O 7 at 1050 • C under a hydrogen atmosphere [23], but the fibers sintered and became submicron chains.Size control of Ti 4 O 7 nanoparticles is still a challenge because nanoparticles are prone to heavy sintering, e.g., sintering of TiO 2 nanoparticles begins at approximately 700 • C [24].
In addition to controlling the size of the nanoparticles, it is also necessary to employ simple, practical techniques for the synthesis.In our previous works, Ti 4 O 7 nanoparticles were prepared using microwave irradiation by a carbothermal reduction process, where the device used was a single-mode furnace [25,26].To scale up the microwave carbothermal process for industry, the process has to be adapted to multimode furnaces.It is more difficult to control the sample temperature in a multimode-type furnace than in a single-mode microwave furnace because the electromagnetic field distribution tends to be no-uniform in the former case.In order to prepare nanoparticles by the carbothermal reduction process, accurate temperature control is required, especially with rapid temperature increase.
Taking all these factors into account, we synthesized Ti 4 O 7 nanoparticles in various sizes (25,60, and 125 nm) using a carbothermal reduction method with a multimode-type microwave apparatus.Our experimental set-up included a self-made susceptor (using carbon powder with high microwave absorbing power) and proportional-integral-derivative (PID) temperature control.

Materials and Methods
Figure 1 shows the sample setting.TiO 2 samples with three particle sizes were used as pristine samples: Sample 1 (TTO-51(A), particle size about 25 nm, Ishihara Sangyo Kaisha, Ltd.), Sample 2 (particle size about 60 nm, 99.5%, IoLiTec-Ionic Liquids Technologies GmbH, Heilbronn, Deutschland), and Sample 3 (particle size about 110 nm, EM Japan Co., Ltd., Tokyo, Japan).Sample 1 was washed with 5M NaOH to eliminate Al(OH) 3 , which is used as a protective agent.Pristine TiO 2 was dispersed in water with dissolved polyvinylpyrrolidone (PVP, molecular weight: 40,000, Wako Pure Chemical Industries, Ltd., Osaka, Japan) by sonication.The ratio of weight between TiO 2 and PVP is 1.21 g:5.59 g.The solution was dried to obtain PVP-coated TiO 2 nanoparticles.PVP decomposes and becomes a carbon source during microwave heating.The PVP-coated TiO 2 nanoparticles (0.15 g of Sample 1, 0.3 g of Samples 2 and 0.3 g of Sample 3) were filled in a quartz tube.After sealing with silica wool, 0.8 g of titanium sponge (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was added as an oxygen absorber.This quartz tube was set in a quartz container filled with a carbon susceptor.The pressure inside the quartz tube was controlled with a rotary pump so that the base pressure of the system was about 10 Pa.A microwave irradiation furnace from µReactor Ex (Shikoku Instrumentation Co., Ltd., Kagawa, Japan) was used.The process temperature was measured with a thermocouple, which was placed between the susceptor and the quartz tube.Microwave-irradiated samples were analyzed by X-ray diffraction (XRD, RINT-2200/PC, Rigaku Co., Tokyo, Japan) and a field emission scanning electron microscope (FE-SEM, S4800, Hitachi High-Technologies Co., Tokyo, Japan).In the analysis of particle size distribution, we traced each surface of particles in FE-SEM images, and used ImageJ software to calculate the number average particle diameter from the area of traced particles.
nanoparticles by the carbothermal reduction process, accurate temperature control is required, especially with rapid temperature increase.
Taking all these factors into account, we synthesized Ti4O7 nanoparticles in various sizes (25, 60, and 125 nm) using a carbothermal reduction method with a multimode-type microwave apparatus.Our experimental set-up included a self-made susceptor (using carbon powder with high microwave absorbing power) and proportional-integral-derivative (PID) temperature control.

Materials and Methods
Figure 1 shows the sample setting.TiO2 samples with three particle sizes were used as pristine samples: Sample 1 (TTO-51(A), particle size about 25 nm, Ishihara Sangyo Kaisha, Ltd.), Sample 2 (particle size about 60 nm, 99.5%, IoLiTec-Ionic Liquids Technologies GmbH, Heilbronn, Deutschland), and Sample 3 (particle size about 110 nm, EM Japan Co., Ltd., Tokyo, Japan).Sample 1 was washed with 5M NaOH to eliminate Al(OH)3, which is used as a protective agent.Pristine TiO2 was dispersed in water with dissolved polyvinylpyrrolidone (PVP, molecular weight: 40000, Wako Pure Chemical Industries, Ltd., Osaka, Japan) by sonication.The ratio of weight between TiO2 and PVP is 1.21 g: 5.59 g.The solution was dried to obtain PVP-coated TiO2 nanoparticles.PVP decomposes and becomes a carbon source during microwave heating.The PVP-coated TiO2 nanoparticles (0.15 g of Sample 1, 0.3 g of Samples 2 and 0.3 g of Sample 3) were filled in a quartz tube.After sealing with silica wool, 0.8 g of titanium sponge (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was added as an oxygen absorber.This quartz tube was set in a quartz container filled with a carbon susceptor.The pressure inside the quartz tube was controlled with a rotary pump so that the base pressure of the system was about 10 Pa.A microwave irradiation furnace from μReactor Ex (Shikoku Instrumentation Co., Ltd., Kagawa, Japan) was used.The process temperature was measured with a thermocouple, which was placed between the susceptor and the quartz tube.Microwave-irradiated samples were analyzed by X-ray diffraction (XRD, RINT-2200/PC, Rigaku Co., Tokyo, Japan) and a field emission scanning electron microscope (FE-SEM, S4800, Hitachi High-Technologies Co. Tokyo, Japan).In the analysis of particle size distribution, we traced each surface of particles in FE-SEM images, and used ImageJ software to calculate the number average particle diameter from the area of traced particles.

Results and Discussion
To achieve a reduction of the pristine TiO 2 to Ti 4 O 7 and retaining of the nanomorphology of pristineTiO 2 at the same time, various experiments were conducted in different heating regimes to Crystals 2018, 8, 444 3 of 8 decide the optimal experiment condition.The experimental results of various heating regime to synthesize Ti 4 O 7 nanoparticles from Sample 1 was summarized as Table 1.In 25 nm pristine TiO 2 case, reduction reaction was too fast to obtain Ti 4 O 7 when the heating regime was same to the previous paper (No. 7) [25].To control the reduction-reaction speed in high temperature region, rate of heating, holding temperature and holding time was changed to fast, low and short, respectively.Below 900 • C, sample was not reduced.At 925 • C, Ti 4 O 7 phase was obtained, and when the holding time was 13 min, single phase Ti 4 O 7 was obtained.Figure 2 shows the profiles of microwave power and process temperature during microwave processing when a reduction of the pristine TiO 2 to Ti 4 O 7 and retaining of the nanomorphology of pristineTiO 2 was achieved at the same time.To achieve a reduction of the pristine TiO2 to Ti4O7 and retaining of the nanomorphology of pristineTiO2 at the same time, various experiments were conducted in different heating regimes to decide the optimal experiment condition.The experimental results of various heating regime to synthesize Ti4O7 nanoparticles from Sample 1 was summarized as Table 1.In 25 nm pristine TiO2 case, reduction reaction was too fast to obtain Ti4O7 when the heating regime was same to the previous paper (No. 7) [25].To control the reduction-reaction speed in high temperature region, rate of heating, holding temperature and holding time was changed to fast, low and short, respectively.Below 900 °C, sample was not reduced.At 925 °C, Ti4O7 phase was obtained, and when the holding time was 13 minutes, single phase Ti4O7 was obtained.2 shows the profiles of microwave power and process temperature during microwave processing when a reduction of the pristine TiO2 to Ti4O7 and retaining of the nanomorphology of pristineTiO2 was achieved at the same time.2 shows the optimal condition of the rate of heating, holding temperature, holding time, and average microwave power during temperature holding for each process.For Sample 1, the holding temperature was lower and the holding time was shorter than the other samples as a smaller particle size results in a more effective reduction reaction.In addition, to obtain Ti4O7 nanoparticles, it is necessary to increase the temperature rapidly and shorten the holding time at high temperatures Table 2 shows the optimal condition of the rate of heating, holding temperature, holding time, and average microwave power during temperature holding for each process.For Sample 1, the holding temperature was lower and the holding time was shorter than the other samples as a smaller particle size results in a more effective reduction reaction.In addition, to obtain Ti 4 O 7 nanoparticles, it is necessary to increase the temperature rapidly and shorten the holding time at high temperatures (above 700 • C) in order to prevent grain growth.The process temperature was 975 • C for Sample 2. This temperature is different compared to a previous study [25] and is due to the temperature measurement method.In the previous study, the thermocouple was located in the sample powder.However, in this experiment, the thermocouple was placed between the susceptor and the quartz tube used to hold the sample powder.Thus, the process temperature measured in this experiment is different from the sample temperature: it is possible that the true sample temperature is lower.We can consider that the temperature of the susceptor is the main factor in maintaining the temperature of the system because the carbon volume of the susceptor is much larger than that of PVP.Regardless of the particle size, the average microwave power values during the holding process were almost the same.(above 700 °C) in order to prevent grain growth.The process temperature was 975 °C for Sample 2. This temperature is different compared to a previous study [25] and is due to the temperature measurement method.In the previous study, the thermocouple was located in the sample powder.However, in this experiment, the thermocouple was placed between the susceptor and the quartz tube used to hold the sample powder.Thus, the process temperature measured in this experiment is different from the sample temperature: it is possible that the true sample temperature is lower.We can consider that the temperature of the susceptor is the main factor in maintaining the temperature of the system because the carbon volume of the susceptor is much larger than that of PVP.Regardless of the particle size, the average microwave power values during the holding process were almost the same.
Figure 3 shows XRD patterns of pristine TiO2 and the samples after the microwave process.All pristine TiO2 samples were in the rutile phase.The right-side figure shows XRD patterns of synthesized samples.In this figure, Ti4O7_1, Ti4O7_2 and Ti4O7_3 refer to synthesized samples after microwave processing from Sample 1, Sample 2 and Sample 3, respectively.The synthesized samples were single-phase Ti4O7 without any other Ti-O phase.Crystallite diameters of the pristine TiO2 and the synthesized Ti4O7 were analyzed from XRD patterns using Scherrer equation, and the results were summarized in Table 3.All crystallite diameters were smaller than the primary particle diameter confirmed by FE-SEM: thus the pristine TiO2 and the synthesized Ti4O7 was polycrystalline.The crystallite diameter of pristine TiO2 was almost the same value as synthesized Ti4O7.31.0 (0.9) nm 29.5 nm Figure 4 shows FE-SEM images of pristine TiO2 and synthesized Ti4O7 nanoparticles acquired in SE mode.Although some coarse particles were observed in pristine TiO2, the particle diameter was uniform.From SE-images of Ti4O7_1 and Ti4O7_2, there seems to be grain growth, but this is because of residual carbon around the Ti4O7 nanoparticles. Figure 5 shows FE-SEM images of the samples acquired in TE mode, where Ti4O7 particles can be clearly observed, since electrons go through the carbon layer.The TE-images for Ti4O7_1 and Ti4O7_2 also show the difference between particles and  31.0 (0.9) nm 29.5 nm Figure 4 shows FE-SEM images of pristine TiO 2 and synthesized Ti 4 O 7 nanoparticles acquired in SE mode.Although some coarse particles were observed in pristine TiO 2 , the particle diameter was Crystals 2018, 8, 444 5 of 8 uniform.From SE-images of Ti 4 O 7 _1 and Ti 4 O 7 _2, there seems to be grain growth, but this is because of residual carbon around the Ti 4 O 7 nanoparticles.Figure 5 shows FE-SEM images of the samples acquired in TE mode, where Ti 4 O 7 particles can be clearly observed, since electrons go through the carbon layer.The TE-images for Ti 4 O 7 _1 and Ti 4 O 7 _2 also show the difference between particles and residual carbon when compared with SE-images.For Ti 4 O 7 _3 nanoparticles, the carbon coating was relatively thinner, making it easier to observe and determine the diameter of the nanoparticles.residual carbon when compared with SE-images.For Ti4O7_3 nanoparticles, the carbon coating was relatively thinner, making it easier to observe and determine the diameter of the nanoparticles.Figure 6 shows histograms of particle sizes for pristine TiO2 and synthesized Ti4O7.Table 4 shows the average particle size (Ave.diameter), standard deviation (S.D.), maximum particle diameter (Max.), minimum particle diameter (Min.), sample number (n), and standard error (S.E.).The average nanoparticle diameter of Sample 1, Sample 2, and Sample 3 was 25.8, 54.6, and 108.0 nm, respectively.The average nanoparticle diameter of Ti4O7_1 was 24.7 nm, which is within the error margin of the average diameter of Sample 1.On the other hand, the average nanoparticle diameter of Ti4O7_2 was 60.4 nm; indicating a 3.3 nm grain growth.For Ti4O7_3, the average nanoparticle diameter was 6.3 nm larger compared to the average nanoparticle diameter of Sample 3.However, since the maximum particle size of Sample 3 was larger than the measured sizes of residual carbon when compared with SE-images.For Ti4O7_3 nanoparticles, the carbon coating was relatively thinner, making it easier to observe and determine the diameter of the nanoparticles.Figure 6 shows histograms of particle sizes for pristine TiO2 and synthesized Ti4O7.Table 4 shows the average particle size (Ave.diameter), standard deviation (S.D.), maximum particle diameter (Max.), minimum particle diameter (Min.), sample number (n), and standard error (S.E.).The average nanoparticle diameter of Sample 1, Sample 2, and Sample 3 was 25.8, 54.6, and 108.0 nm, respectively.The average nanoparticle diameter of Ti4O7_1 was 24.7 nm, which is within the error margin of the average diameter of Sample 1.On the other hand, the average nanoparticle diameter of Ti4O7_2 was 60.4 nm; indicating a 3.3 nm grain growth.For Ti4O7_3, the average nanoparticle diameter was 6.3 nm larger compared to the average nanoparticle diameter of Sample 3.However, since the maximum particle size of Sample 3 was larger than the measured sizes of Ti4O7_3 particles, it can be deduced that there was no significant grain growth.Figure 6 shows histograms of particle sizes for pristine TiO 2 and synthesized Ti 4 O 7 .Table 4 shows the average particle size (Ave.diameter), standard deviation (S.D.), maximum particle diameter (Max.), minimum particle diameter (Min.), sample number (n), and standard error (S.E.).The average nanoparticle diameter of Sample 1, Sample 2, and Sample 3 was 25.8, 54.6, and 108.0 nm, respectively.The average nanoparticle diameter of Ti 4 O 7 _1 was 24.7 nm, which is within the error margin of the Crystals 2018, 8, 444 6 of 8 average diameter of Sample 1.On the other hand, the average nanoparticle diameter of Ti 4 O 7 _2 was 60.4 nm; indicating a 3.3 nm grain growth.For Ti 4 O 7 _3, the average nanoparticle diameter was 6.3 nm larger compared to the average nanoparticle diameter of Sample 3.However, since the maximum particle size of Sample 3 was larger than the measured sizes of Ti 4 O 7 _3 particles, it can be deduced that there was no significant grain growth.3 From these results, even when a simple apparatus such as a multimode microwave irradiation furnace is used, it is possible to produce Ti4O7 nanoparticles with an average size as low as 25 nm.This can be done using a carbon thermal reduction method by precise control of the heating rate, holding temperature, and holding time.

Conclusions
Ti4O7 nanoparticles of various sizes (25, 60, 125 nm) were prepared in a multimode microwave furnace.In general, the synthesized Ti4O7 nanoparticles retained the size of the pristine TiO2 nanoparticles.The process time and temperature varied depending on the size of nanoparticles because the reduction reaction was slower when larger pristine nanoparticles were used.It is possible to precisely control the temperature, heating process, and holding time of the sample while taking advantage of the characteristics of microwave heating such as rapid and volume heating.This microwave carbothermal reduction method is thus highly effective in controlling the size of the synthesized Ti4O7 particles.

No.
Ave From these results, even when a simple apparatus such as a multimode microwave irradiation furnace is used, it is possible to produce Ti 4 O 7 nanoparticles with an average size as low as 25 nm.This can be done using a carbon thermal reduction method by precise control of the heating rate, holding temperature, and holding time.

Conclusions
Ti 4 O 7 nanoparticles of various sizes (25, 60, 125 nm) were prepared in a multimode microwave furnace.In general, the synthesized Ti 4 O 7 nanoparticles retained the size of the pristine TiO 2 nanoparticles.The process time and temperature varied depending on the size of nanoparticles because the reduction reaction was slower when larger pristine nanoparticles were used.It is possible to precisely control the temperature, heating process, and holding time of the sample while taking advantage of the characteristics of microwave heating such as rapid and volume heating.This microwave carbothermal reduction method is thus highly effective in controlling the size of the synthesized Ti 4 O 7 particles.

Figure 1 .
Figure 1.Schematic view of the experimental setup.

Figure 1 .
Figure 1.Schematic view of the experimental setup.

Figure 2 .
Figure 2. Temperature and microwave power profiles in microwave processing.

Figure 2 .
Figure 2. Temperature and microwave power profiles in microwave processing.

Figure 3
Figure 3 shows XRD patterns of pristine TiO 2 and the samples after the microwave process.All pristine TiO 2 samples were in the rutile phase.The right-side figure shows XRD patterns of synthesized samples.In this figure, Ti 4 O 7 _1, Ti 4 O 7 _2 and Ti 4 O 7 _3 refer to synthesized samples after microwave processing from Sample 1, Sample 2 and Sample 3, respectively.The synthesized samples were single-phase Ti 4 O 7 without any other Ti-O phase.

Figure 3 .
Figure 3. XRD patterns of (a) pristine TiO2 (b) the samples after synthesis by microwave process.Ti4O7_1, Ti4O7_2 and Ti4O7_3 refer to particles after synthesis from Sample 1, Sample 2, and Sample 3, respectively.

3.
XRD patterns of (a) pristine TiO 2 (b) the samples after synthesis by microwave process.Ti 4 O 7 _1, Ti 4 O 7 _2 and Ti 4 O 7 _3 refer to particles after synthesis from Sample 1, Sample 2, and Sample 3, respectively.Crystallite diameters of the pristine TiO 2 and the synthesized Ti 4 O 7 were analyzed from XRD patterns using Scherrer equation, and the results were summarized in Table3.All crystallite diameters were smaller than the primary particle diameter confirmed by FE-SEM: thus the pristine TiO 2 and the synthesized Ti 4 O 7 was polycrystalline.The crystallite diameter of pristine TiO 2 was almost the same value as synthesized Ti 4 O 7 .

Figure 4 .
Figure 4. FE-SEM (field emission scanning electron microscope) images of pristine TiO2 and synthesized Ti4O7 nanoparticles acquired in secondary electron (SE) mode.

Figure 5 .
Figure 5. FE-SEM images of Ti4O7_1 and Ti4O7_2 acquired in SE and transmission electron (TE) mode.

Figure 4 .
Figure 4. FE-SEM (field emission scanning electron microscope) images of pristine TiO 2 and synthesized Ti 4 O 7 nanoparticles acquired in secondary electron (SE) mode.

Figure 4 .
Figure 4. FE-SEM (field emission scanning electron microscope) images of pristine TiO2 and synthesized Ti4O7 nanoparticles acquired in secondary electron (SE) mode.

Figure 5 .
Figure 5. FE-SEM images of Ti4O7_1 and Ti4O7_2 acquired in SE and transmission electron (TE) mode.

Figure 5 .
Figure 5. FE-SEM images of Ti 4 O 7 _1 and Ti 4 O 7 _2 acquired in SE and transmission electron (TE) mode.

Table 1 .
Experimental results of various heating regime to synthesize Ti 4 O 7 nanoparticles from Sample 1.

Table 1 .
Experimental results of various heating regime to synthesize Ti4O7 nanoparticles from Sample 1.

Table 2 .
Experimental conditions in microwave processing.

Table 2 .
Experimental conditions in microwave processing.

Table 3 .
Crystallite diameter of pristine TiO2 and synthesized Ti4O7 by microwave; MW: microwave