Detailed Spectroscopic and Structural Analysis of TiO 2 / WO 3 Composite Semiconductors

Faculty of Chemistry and Chemical Engineering, Babeş-Bolyai University, Arany János 11, Cluj-Napoca 400028, Romania Nanostructured Materials and Bio-Nano-Interfaces Center, Institute for Interdisciplinary Research on Bio-Nano-Sciences, Treboniu Laurian 42, Cluj-Napoca 400271, Romania Faculty of Physics, Babeş-Bolyai University, Mihail Kogălniceanu 1, Cluj-Napoca 400084, Romania Institute of Environmental Science and Technology, Tisza Lajos krt. 103, Szeged 6720, Hungary


Introduction
e study of semiconductors remained in the last years a systematically investigated research topic.
e implementation of nanomaterials in the industry had a major role in the blooming research of nanomaterials.One of these nanomaterials is tungsten trioxide (WO 3 ), a transition metal oxide with large applicability spectra that is commonly used in paints as pigment [1], in solar cells for electricity production [2,3], and in coatings for heat production from absorbing solar energy [4], such as humidity, moisture, and gas sensors [5][6][7].is oxide is also an important component in "smart windows" due to its electrochromic properties [8].Moreover, WO 3 is used as a catalytic and photocatalytic purifier for air and water [9,10].
Tungsten trioxide has an interesting peculiarity; in certain cases, it can act as a charge separator [22].Due to this feature, it is a viable component for binary composite systems, in which another metal oxide is used as an electron donor, generally TiO 2 [23] or ZnO [24], but NiO [25] was also used.e final goal of these composite systems is either to apply them as a sensor or as a photocatalyst, or even both simultaneously.Photocatalytic efficiency of WO 3 semiconductor can be enhanced if noble metals are added, WO 3 /Au, WO 3 /Ag, or WO 3 /Pt composite systems being related to show an improved photocatalytic efficiency towards the removal of organic pollutants in comparison with commercial TiO 2 (Evonik Aeroxide P25) [26][27][28].e photocatalytic activity of ternary composites based on WO 3 , commercial TiO 2 , and noble metals (WO 3 /TiO 2 /noble metals) was also intensively studied [29][30][31].
e most commonly used methods for the preparation of WO 3 /P25 composites are the mechanical mixing or the adjustment of the semiconductors' surface charge, and in both cases, the composites photocatalytic efficiency was improved as compared to that exhibited by P25 [32,33].
In this study, tungsten trioxide microcrystals were synthesized via hydrothermal crystallization, and their spectroscopic and structural features were investigated.Various weight percentage composites based on the synthesized WO 3 and commercial TiO 2 (Evonik Aeroxide P25) were prepared by mechanical mixing method, and the photocatalytic activity of these binary composite systems was assessed.M) hydrochloric acid (HCl) was added to the solution which was stirred for 15 minutes at room temperature.A yellow suspension was obtained after the hydrothermal crystallization, which was carried out at 180 °C for 4 hours.After the autoclave cooled down at room temperature, the product was centrifuged (3 × 15 minutes, 1600 rpm) and washed with deionized water in order to remove the impurities remained in the product.e product was dried at 70 °C for 6 hours and annealed at 500 °C for 30 minutes (heating rate 5 °C •min −1 ) [34].

Experimental
e WO 3 -AMT abbreviation was further used to identify the WO 3 crystals synthesized from ammonium metatungstate hydrate.

Characterization Methods.
e assessment of the crystalline structure of the composite components was carried out by the means of X-Ray Diffraction (XRD) measurements.e XRD diffractograms were recorded on a Shimadzu 6000 diffractometer (Shimadzu Corporation, Kyoto, Japan), by using Cu-Kα irradiation, (λ �1.5406 Å).
e crystalline phases of the semiconductors were evaluated and the crystallites' average size was calculated by using the Scherrer equation [35], whereas the anatase/rutile ratios in P25 were evaluated by the well-known Banfield approach [36].Diffuse reflectance spectroscopy (DRS) measurements were performed by using the JASCO-V650 spectrophotometer (λ � 250 -800 nm) equipped with ILV-724 integration sphere.
e band-gap energy of the composites system was determined using the following equation [37][38][39]: where (E) is the band-gap energy, h is Plank constant, c is the speed of light � 3.0 × 10 8 m•sec −1 , and λ is the cut-off wavelength.
A JASCO 4100 (Jasco, Tokyo, Japan) spectrometer was used to record the IR spectra of the composites, at room temperature, in the spectral range of 400-4000 cm −1 , with a spectral resolution of 4 cm −1 .e samples were prepared in the form of KBr pellets.
e SEM micrographs were recorded by using an FEI Quanta 3D FEG scanning electron microscope operating at an accelerating voltage of 25 kV.e WO 3 nanomaterials were covered with Au to amplify the secondary electron signal, while the morphological peculiarities of the semiconductor were uncovered.
e investigation of photocatalytic performance was carried out in the presence of 2 × 60 W fluorescence UV lamps with λ ≈ 365 nm emission maximum, under vigorous stirring (C suspension � 1 g•L −1 ; V suspension � 75 mL; C oxalic acid � 3 mM).e photocatalytic degradation was followed for 3 hours using high-performance liquid chromatography (HPLC).e measurements were carried out by using Merck-Hitachi type D-7000 chromatograph equipped with an L-4250 UV-Vis detector.e volume of the loop was 20 μL and the chromatography column was installed with Grom Resin ZH-type load.e eluent was 0.06% H 2 SO 4 aqueous solution, and the applied flow rate was 0.8 mL•min −1 .e key parameters investigated here were the conversion (X) and the reaction rate.

Crystalline Structure and Particle Size of the Semiconductors.
e first step in the investigation series was to check the quality of the composite components.From the XRD patterns (Figure 1), the crystalline phase and the mean primary particle size of the synthesized semiconductors were established.In the case of WO 3 , only the monoclinic crystalline phase was detected, as it can be seen from the diffractogram.However, based on our previous work [33], one can infer that this synthesis procedure gives rise to hierarchical structures made up from fine micrometric needle crystals (30-50 nm wide and 3-4 µm long) that form a star-like shaped structure (therefore the Scherrer equation was not used).e particle size of the WO 3 stars was between 3 and 4 µm (as described in Section 3.3).Regarding the commercial TiO 2 , both anatase and rutile crystalline phases were observed, the ratio between anatase and rutile was 2 Journal of Spectroscopy estimated (89 : 11), and the primary calculated particle size (25-40 nm) was very close to the values reported in the literature.

Optical Properties of the Prepared Composite System.
As the composite structure contains both oxides, it was crucial to investigate the optical properties of these materials (Figure 2).e band-gap energy values were determined by using the light absorption threshold method, as mentioned in Section 2.4.In the case of the WO 3 -AMT semiconductor, the light absorption threshold was found to be around 550 nm and the calculated band-gap energy was of ≈2. 25 eV, but it should be kept in mind that the band-gap energy value of the commercial TiO 2 is ≈3.2 eV [33].Concerning the composites, the light absorption thresholds and the band-gap energy values were as follows: 394 nm, ≈3.14 eV (99-1 wt.% P25-WO 3 ); 414 nm, ≈2.99 eV (90-10 wt.% P25-WO 3 ); 449 nm, ≈2.76 eV (76-24 wt.% P25-WO 3 ); 447 nm, ≈2.77 eV (67-33 wt.% P25-WO 3 ); and 451 nm, ≈2.74 eV (50-50 wt.% P25-WO 3 ).e lowest band-gap energy was found for the 50-50 wt.% P25-WO 3 composite.One observes that the WO 3 amount has a significant effect on the band-gap energy value, and a very interesting fact is that even 1% of monoclinic tungsten trioxide can influence it, by slightly reducing this value by 0.06 eV.By adding 10% WO 3 to the composite composition, the band-gap energy was found to further decrease by 0.21 eV.By increasing the amount of WO 3 to 24%, the band-gap energy was lowered by 0.44 eV.

Morphological Features of the Synthesized Semiconductor.
SEM measurements revealed that the morphology of the WO 3 (WO 3 -AMT) microcrystals synthesized from ammonium metatungstate hydrate was of star-like type (Figure 3).e diameter of the stars was between 3 and 4 µm, each star being constructed from microfibers of 3-4 µm length.More importantly, it was found that all the microstars showed the same structure and morphology (i.e., high monodispersity), which can reinforce all the conclusions derived from the study.

FT-IR Characterization of the Prepared Composites
System.By analyzing the IR spectra (Figure 4) of the obtained composites, the specific signals of TiO 2 were detected without any special changing trends, excepting the alteration of some signals proportionally with the composite components' ratio.e main spectral feature associated with titania was the large band between 400 and 700 cm −1 , which can be attributed to the stretching vibrations of Ti-O-Ti and Ti-O bonds.In the case of WO 3 , several specific spectral characteristics were observed, such as the ones between 600 and 1000 cm −1 (the most intense one being located at 931 cm −1 ), which were assigned to different W-O-W stretching modes.e small but distinct band at 1035 cm −1 was given by the stretching vibration of the W � O bonds [40].
ese signals involving tungsten bond vibrations were also dependent on the WO 3 concentration.e band at 1390 cm −1 was interestingly found to be given by NH 4 + ions [41].At the first view, this is rather surprising; but actually, it can be considered an expected appearance having in view that WO 3 was obtained by using ammonium metatungstate.e only bands that differently changed self-dependent on the WO 3 content were those directly related to the surface hydrophilicity, namely, those at 1630 and 3427 cm −1 assigned to OH vibrations.ese bands exhibit a relatively high intensity for the samples with ≥24 wt.% WO 3 and a slow decrease of it for smaller WO 3 content.is result points out the high water affinity of 90-10 wt.% P25-WO 3 and 76-24 wt.% P25-WO 3 , which could have on impact on the photoactivity of these materials.

Photocatalytic Activity.
e evaluation of the photocatalytic performance was carried out by analyzing the oxalic acid degradation curves, which provide qualitative and quantitative information (Figure 5).e photocatalytic performance was quantitatively described by using the conversion values (X).
No photocatalytic activity was observed when bare WO 3 and 50-50 wt.% TiO 2 /WO 3 were used as photocatalysts in 3 mM oxalic acid solution.is photocatalytic inefficiency of bare WO 3 could be due to the WO 3 particles dimension, which is relatively high (3-4 µm).In the case of 50-50 wt.% TiO 2 /WO 3 composite, the reason could be the screening effect of the WO 3 crystals on the TiO 2 particles so that the system had a deficiency being activated under UV light irradiation.In this case, the generation of charge carriers was decreased, and consequently, the photocatalytic activity was low in the composites with high WO 3 content.Only 28% of oxalic acid was removed using the 67-33 wt.% TiO 2 /WO 3 composite system in contrast to 76-24 wt.% TiO 2 /WO 3 system, where 99% conversion was achieved.68% conversion was obtained in the case of 90-10 wt.% TiO 2 /WO 3 composites and 95% conversion rate was observed for the 99-1 wt.% TiO 2 /WO 3 composites.e reference catalyst (commercial TiO 2 ) degraded 73.3 wt.% of oxalic acid.
e most efficient composite for oxalic acid degradation was the 76-24 wt.% TiO 2 /WO 3 system because the recombination process was inhibited successfully so that the separation of the charge carriers was the most efficient in the case of this sample.
e first five points were taken into consideration for the calculation of the initial reaction rates.
e concentration changes of oxalic acid (at 0, 15, 30, 45, and 60 min) were plotted versus time to determine the initial reaction rate (r i ) values.e linearization of these two parameters and its slope gave the initial reaction rate values.
All the activity-related parameters clearly show that it must be a specific parameter responsible for the high photoactivity.e band-gap energy values of the composites can be eliminated as the main reason because it is a parameter that varied concomitantly with the WO 3 content.As no structural and morphological changes occurred during the composite preparation, other approaches should be exploited.Firstly, an analog case can be involved, in which a similar phenomenon was explained [29].As the amount of WO 3 increases, so does the charge separation efficiency in the composites.However, after a specific concentration of WO 3 , this was detrimental, because the WO 3 itself is not photoactive.is means that increasing too much the ratio of a charge separator (without self-activity), a lowering of the overall photoactivity occurs.However, this approach may be not sufficient alone.
e intensity of the IR bands at 1630 cm −1 and 3427 cm −1 showed nearly the same trend as the photoactivity. is means that the photocatalytic degradation is in direct relationship with the hydrophilicity of the photocatalyst (a fact well-known for TiO 2 [42]), which was confirmed here for the first time in case of TiO 2 -WO 3 composites.

Conclusions
In the herein presented study, WO 3 -TiO 2 composites with different TiO 2 /WO 3 ratios (1 wt.% of WO 3 to 50 wt.%)were obtained by using commercial titania (Evonik Aeroxide P25) and hydrothermally crystallized WO 3 .e morphology of the synthesized hierarchical WO 3 semiconductors was starlike shaped with a diameter between 3 and 4 µm, and WO 3 's determined crystal phase was monoclinic.e present study proves that WO 3 microcrystals of relatively large dimension, without photoactivity, can improve the photocatalytic efficiency of the commercial TiO 2 , acting as a charge separator.e band-gap energy values of the composites were found to be dependent on the WO 3 content as well, but no correlation was established with the photoactivity.e 76-24 wt.% TiO 2 /WO 3 composite system has shown the highest photocatalytic activity, reaching a conversion rate of 99%.Also, this sample and the one with 10 wt.% of WO 3 exhibited the most intense water affinity as revealed by the IR bands assigned to water vibrations, showing a clear correlation between these structural entities and photoactivity.
e obtained results from this study also suggest that these composites system could be used as efficient photocatalysts for other pollutants removal (methyl orange and salicylic acid), gas sensors, and sensors for detection of organic pollutants containing the carboxylic functional group or could be even used for ternary WO 3 /TiO 2 /noble metal composites.

Figure 1 :Figure 3 :Figure 4 :
Figure 2: e reflectance spectra of the prepared WO 3 /TiO 2 composites system and the band-gap energy dependence on the WO 3 content (inset figure).

Table 1 :
Summary of the photocatalytic properties for the various composite system and reference catalysts.