Titanium Oxide Nanoparticle Photocatalysts
Since the early work of Asahi et al. [ 24 ] on TiON, it has been extensively studied to extend its photocatalytic activity into the visible-light region [ 24– 36 ].
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In these previous works, TiON was fabricated into both powder and thin fi lm.
Compared with TiON thin fi lms, TiON powder photocatalysts off er the advantages of high surface area, low cost, and suitability for large-scale production. Among various synthesis methods for preparing TiON powders, sol–gel based processes [ 28– 29 , 32 ] seem to have the most potential. Fully developed, these processes can provide a reliable production route of TiON powders at relatively low cost through careful control of initial precursor composition and ratio.
Recently, we reported a systematic study of the precursor ratio eff ect on the structure, composition, and optical properties of sol–gel derived TiON nanoparticles [ 54 ]. In that study, tetramethylammonium hydroxide (TMA) and titanium tetraisopropoxide (TTIP) were used as precursors to synthesize TiON nanoparticle photocatalyst. The preparation of TiON precursor was done at room temperature in a sol–gel process as follows. First, TMA was dissolved in Ethyl alcohol (EtOH) at a mol ratio of 1:10. The solution was stirred magnetically for 5 minutes, and then TTIP was added into the solution with various TMA/TTIP mol ratios at 1:3, 1:5, and 1:10, respectively.
For each TMA/TTIP mol ratio, the mixture was loosely covered and stirred continuously until a homogenous gel was formed. The hydrolysis of the precursor was initiated by exposure to moisture in air. The gel was aged in air for several days to allow further hydrolysis and drying. Then, the xerogel was crushed into fi ne powders and calcined at various temperatures in air for 3 hours to obtain the nanoparticle photocatalysts. For comparison, TiO 2 precursor was prepared by the same sol–gel process as mentioned earlier, but without the addition of TMA. Calcination of the TiO 2 xerogel was conducted at 400 o C in air for 3 hours.
X-ray diff raction (XRD) patterns of obtained powders are shown in Fig. 2.3 . After being calcinated in air at 400 o C for 3 hours, TiO 2 powders are well crystallized into the anatase-type crystal structure. At the initial TMA/TTIP mol ratio of 1:10, the TiON powder also shows the anatase-type structure, but the XRD peaks are broad and the peak heights are weaker than those of TiO 2 powders, indicating only partial crystallization. For TiON powders made at higher initial TMA/TTIP mol ratios, no discernable refl ection peaks could be identifi ed, implying little or no crystallization. It is clear that the introduction of N into the TiO 2 structure (the addition of TMA) disrupts the crystallization of these sol–gel powders. For TiON nanoparticle powders calcinated in air at higher temperatures, an anatase-type structure was obtained for all powders within the temperature range investigated (from 430 to 500 o C). As an example, Fig. 2.3(b) presents the XRD patterns of TiON powders calcinated in air at 500 o C. With the increase of the calcination temperature, the XRD peak intensity increases, indicating improved crystallization. For each calcination temperature, powders made at lower initial TMA/TTIP ratios have stronger peak intensity and sharper peaks, which confi rm that the addition of TMA has the eff ect of disrupting the crystallization of sol–gel TiON nanoparticle powders.
As a result, a higher calcination temperature would be needed to achieve
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complete crystallization. Figure 2.4 shows scanning electron microscope (SEM) and transmission electron microscope (TEM) images of a highly crystallized TiON sample, which consist of nanosized particles with nonuniform shapes.
The composition of the powder was examined by X-ray photoelectron spectroscopy (XPS). Figure 2.5(a) summarizes the N/Ti atomic ratio of these powders. It is clear that with the increase of calcination temperature, the
Figure 2.4 (a) Scanning electron microscope (SEM) and (b) transmission electron microscope (TEM) images of nitrogen-doped titanium oxide powders with initial tetramethylammonium hydroxide/titanium tetraisopropoxide ratios at 1:5 and obtained by calcinating xerogels in air for 3 hours at 500 o C. Adapted from [ 54 ].
(a) (b)
Figure 2.3 X-ray diff raction patterns of nitrogen-doped titanium oxide (TiON)/titanium oxide powders [TiO 2 : brown line, TiON with various initial tetramethylammonium hydroxide/titanium tetraisopropoxide ratios at 1:3 (blue line), 1:5 (red line), and 1:10 (black line)] obtained by calcinating xerogels in air for 3 hours at (a) 400 o C, and (b) 500 o C, respectively. Adapted from [ 54 ].
(a) (b)
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N/Ti atomic ratio decreases for all these powders, whereas a higher initial TMA/TTIP ratio in the TiON precursors results in a faster decline of N/Ti ratio. This is further illustrated in Fig. 2.5(b) , where the residual N is lower at higher initial TMA/TTIP ratios. Therefore, a high initial TMA/TTIP ratio in the TiON precursor does not necessarily result in a high N/Ti atomic ratio in the TiON powders because the rate of N loss is faster in the calcination process. To obtain a higher N/Ti atomic ratio, the initial TMA/TTIP ratio should be controlled carefully in combination with calcination temperature.
The optical absorption of TiON nanoparticle powders was characterized by the diff use refl ectance measurements. The optical absorbance is approximated from the refl ectance data by the Kubelka–Munk function [ 55 ], as given by Equation 2.1:
− 2 (1 )
( ) = ,
2 F R R
R (2.1)
where R is the diff use refl ectance. Figure 2.6(a) shows the light absorbance of TiON powders obtained by calcinating xerogels in air at 500 o C for 3 hours, compared with a commercial Degussa P25 powder. P25 shows the characteristic spectrum with the fundamental absorbance stopping edge at approximately 400 nm. TiON powders, however, show a clear shift into the visible-light range (> 400 nm). With the increase of the N content, more visible-light absorbance is observed. Figure 2.6(b) shows the Tauc Plot [ 55 ] (( F ( R )* hv ) n vs. hv ) constructed from Fig. 2.6(a) to determine the semiconductor band gap. The band gap of the Degussa P25 powder is approximately 3.20 eV, whereas TiON
Figure 2.5 (a) N/Ti atomic ratio, and (b) the residue percentage of N content within the total tetramethylammonium hydroxide (TMA) addition in obtained nitrogen-doped titanium oxide (TiON) powders with various initial TMA/titanium tetraisopropoxide ratios at 1:3 (), 1:5 (), and 1:10 (), respectively. (The lines merely guide the eye.) Adapted from [ 54 ].
(a) (b)
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powders show a smaller band gap, and thus are well suited for the visible-light activation in photocatalysis. With the increase of the N content (caused by the various initial TMA/TTIP ratios), the band gap of TiON powder decreases steadily from approximately 3.05 to 2.90 eV, in agreement with the band gap narrowing in TiON fi lms [ 56 ].
From such a systematic study, TMA as a nitrogen source is found to retard the crystallization of sol–gel TiON powders so that a higher calcination temperature is required for the crystallization of TiON powders, especially at high initial TMA/TTIP ratios. The increase of the calcination temperature promotes the crystallization level, but reduces N concentration in the TiON powders. The rate of N loss depends on the initial TMA/TTIP ratio, with a higher TMA/TTIP ratio resulting in a faster N loss. These sol–gel TiON nanoparticle photocatalysts have shown visible-light absorbance. For a strong visible-light absorbance, the initial TMA/TTIP ratio and calcination temperature must be controlled together to achieve a high N/Ti atomic ratio and a complete crystallization of TiON nanoparticles. Under our experimental conditions, the best initial TMA/TTIP ratio was determined to be 1:5 at a calcination temperature of 500 o C.