Preparation and Photocatalytic CO Oxidation Performance Study of Au/Oxygen-Deﬁcient (Anatase/B-Phase) TiO 2 Heterojunction Microspheres

: Multi-heterojunctions are more promising than single heterojunctions in photocatalysis due to the availability of more interfaces between each component. However, photocatalytic activity is highly dependent on the contact mode of individual components. In this work, we assembled gold nanoparticles/oxygen-deﬁcient (anatase/B-phase) TiO 2 multi-heterojunction microspheres using spray pyrolysis and focused on their contact mode-governed photocatalytic activity. The results reveal that using oxygen-deﬁcient (anatase/B-phase) TiO 2 microspheres as building blocks could signiﬁcantly enhance the absorption of visible light and the photocatalytic activity of a gold–TiO 2 system toward the photocatalytic carbon monoxide oxidation. Furthermore, loading gold nanoparticles onto B-phase TiO 2 could facilitate a more prominent enhancement of activity than that of pure B-phase TiO 2 , beneﬁting from the two-electron reduction of oxygen at the interface of TiO 2 /Au. Meanwhile, the high crystallinity of B-phase TiO 2 microspheres allows for a gold loading amount of 1 wt% in the gold/(anatase/B-phase) TiO 2 system, which is 1.67 times more active than pure B-phase TiO 2 microspheres, in the photocatalytic oxidation of carbon monoxide to generate carbon dioxide.


Introduction
Photocatalytic oxidation reactions have been investigated extensively over the past few years for the treatment of contaminated water sources and air.The above-mentioned reactions are considered a promising approach in pollution control [1][2][3][4][5][6][7].Titanium dioxide, a prominent photocatalyst, has aroused major attention in the research and application of photocatalytic oxidation reactions in air pollution control or green energy production [8][9][10][11][12][13][14][15].Carbon monoxide (CO) has been confirmed as one of the major air pollutants and a byproduct of numerous oxidation reactions.CO is difficult to remove completely since it is stable at ambient temperature and often released into the atmosphere without treatment [16].At the same time, carbon monoxide is also an intermediate product of many multi-carbon products [17].Photocatalytic oxidation has been reported as a potential method to contribute to the control of CO pollution.Thus, several studies have placed a focus on increasing the photocatalytic activity of CO oxidation using titanium dioxide as the main photocatalyst [18][19][20][21].For instance, Alexander V Vorontsov, et al. [22].deposited platinum nanoparticles on the surface of titanium dioxide using a photoreduction deposition method.The platinum loading amount was 3% relative to the mass of titanium dioxide.The platinum-modified titanium dioxide exhibited significantly higher activity in photocatalytic CO oxidation than pure titanium dioxide.The factors for the photocatalytic activity of platinum-loaded titanium dioxide (e.g., the type of titanium dioxide and the oxygen content in the reaction) were systematically studied.Bosc F, et al. [23] synthesized a large surface area rutile-phase titanium dioxide (200 m 2 /g) and loaded it with precious metal platinum.The above-described large surface area titanium dioxide can allow the platinum nanoparticles to be uniformly dispersed on the surface.The resulting visible-light-absorbing platinum/titanium dioxide composite photocatalyst exhibits excellent activity in visible light-driven CO oxidation.The platinum loading on the high surface area titanium dioxide photocatalyst exhibits better photocatalytic stability (stable for over three days) than platinum-loaded P25-type titanium dioxide (stable for less than an hour).Moreover, Professor Li, et al. [24].conducted a series of studies on platinum-loaded titanium dioxide photocatalysts.As indicated by their results, after annealing at 300 • C, the surface of the platinum-modified titanium dioxide forms an interaction with the titanium dioxide substrate, such that the interface resistance can be effectively reduced, and the photogenerated charge carriers can be separated, leading to improved photocatalytic activity in CO oxidation.As indicated by the above studies, surface modification of titanium dioxide with noble metals has been commonly used to increase the photocatalytic activity of CO oxidation since the noble metal loading can effectively increase the separation efficiency of photogenerated charge carriers in titanium dioxide [25] and adsorb CO [26,27].Accordingly, it is reasonable to consider whether surface modification of titanium dioxide with gold nanoparticles could also enhance the photocatalytic activity of CO oxidation.
In fact, the use of gold-loaded titanium dioxide as a catalyst for CO oxidation has been reported in existing research [28][29][30][31][32][33][34][35][36] though most of the relevant research has placed a major focus on thermal catalysis.Nevertheless, the above-mentioned works have confirmed that gold-loaded titanium dioxide can indeed enhance the activity of CO oxidation, as gold as a noble metal has the capability to effectively increase the separation efficiency of photogenerated charge carriers in titanium dioxide.Additionally, gold nanoparticles can adsorb CO [36].Furthermore, we chose oxygen-deficient titanium dioxide as the semiconductor material to support the noble metal, as oxygen-deficient titanium dioxide exhibits better light response in the visible region in comparison with pure titanium dioxide.Numerous studies have been conducted on oxygen-deficient titanium dioxide, and the relevant results have confirmed that this material is capable of completely oxidizing substances (e.g., benzoic acid, isopropanol, and methyl orange) into CO 2 and H 2 O in the visible light region [37][38][39][40][41][42][43][44][45][46].
Thus, in this study, gold nanoparticles were modified onto the surface of visiblelight-absorbing B-phase titanium dioxide using a spray pyrolysis method.On that basis, a photocatalyst retaining the advantages of visible-light absorption and efficient separation of photogenerated electron-hole pairs can be prepared.The use of the above-described intentionally synthesized composite photocatalyst for CO oxidation can achieve high photocatalytic activity.This work aims to build a surface modified TiO 2 (B) photocatalyst with excellent photocatalytic activity.Compared with dip coating, hydrothermal and doctor-blading methods, the spray pyrolysis method can quickly, evenly and high-quality load the Au modified layer on the surface of TiO 2 (B), so that the Au and TiO 2 (B) can get a good contact [47][48][49].In the following sections, the morphology, structure characterization, and photocatalytic activity of this gold-loaded B-phase titanium dioxide will be elucidated.
The morphology of the samples was analyzed through scanning electron microscopy (SEM), and the composition of the samples was determined through transmission electron microscopy (TEM).The SEM analysis was conducted under an FEI Quanta250 scanning electron microscope (FEI Company, Hillsboro, Oregon, USA), and the TEM analysis was conducted under a JEOL JEM-2100 transmission electron microscope (JEOL, Beijing, China) with an accelerating voltage of 200 kV.The crystal structure and composition of the samples were analyzed through X-ray diffraction (XRD) with a Rigaku D/max-2500 X-ray diffractometer (Rigaku, Tokyo, Japan).Moreover, the photoluminescence (PL) spectra were obtained using a Jobin-Yvon HR800 Raman spectrometer (HORIBA Jobin Yvon, Paris, France) with a 325 nm laser and Ar ion as the excitation light source.Furthermore, the UVvisible absorption spectra of the samples were obtained using a Lambda 900 UV-vis-NIR spectrophotometer (Avantes China, Beijing, China).

Synthesis of B-Phase Titanium Dioxide
Experimental procedure: measure out 45 mL of ultrapure water and place in a 250 mL plastic beaker, seal, and freeze in a refrigerator to form ice. Slowly add 0.8 mL of titanium tetrachloride to the ice, then add 5 mL of ammonia water.A certain amount of white flocculent material will appear in the beaker.Add 60 mL of ultrapure water to the beaker and stir for nearly 5 min to completely hydrolyze the titanium tetrachloride under alkaline conditions.After complete hydrolysis, uniform white flocculent material will appear in the beaker.Add 10 mL of hydrogen peroxide solution to the beaker and stir for approximately 10 min.The solution will become clear and transparent with a light-yellow color.After the above phenomena occur, weigh 0.5 g of acetic acid on a balance and add it to the solution.Continue to stir for nearly 5 min and set aside.
The prepared light-yellow transparent solution should then be placed in a hot airdrying oven and heated at 80 • C for 390 min.After naturally cooling to ambient temperature, a yellow viscous gel-like material is obtained.
Add ultrapure water to the obtained yellow gel-like material to reach the 50 mL mark on the beaker and stir for nearly 5 min to obtain a uniform and stable yellow solution.A dropper can be adopted to add the concentrated sulfuric acid dropwise to the solution.The color of the solution will slightly deepen with the addition of the sulfuric acid.Continuously measure the pH of the solution with a pH meter to maintain the pH of the solution at around 1. Continue stirring for five min to mix well.
Transfer the solution abov4 into a 50 mL high-temperature reaction hydrothermal autoclave and place it in a blast drying oven.Heat it at a constant temperature of 160 °C for 50 min, and let it cool naturally to ambient temperature before removing it.This yields the required B-phase titanium dioxide solution.

Preparation of Gold-Loaded B-Phase Titania Porous Microspheres
Initially, a solution containing 1 g of B-phase titania was dispersed in a beaker containing 1 L of ultrapure water and stirred for 10 min using a magnetic stirrer.Then, using a pipette, a solution of chloroauric acid (concentration: 1 g/20 mL) containing gold nanoparticles with a mass ratio relative to B-phase titania of x (x = 0.1%, 1%, 5%) was added dropwise into the beaker (42 µL, 420 µL, and 1050 µL, respectively), and the mixture was stirred for an additional 30 min.The resulting mixture of chloroauric acid and B-phase titania was then placed in a designated position in a spray-drying apparatus, and under conditions of a spray temperature of 150 • C and an injection rate of 450 mL/h, three types of B-phase titania powder samples containing different proportions of gold were spray-dried and collected for further use.Two additional samples of B-phase titania powder loaded with 1% gold by weight were prepared based on the same spray-drying method but at different temperatures (200 • C and 250 • C, respectively).In addition, the pure B-phase titania solution was also spray-dried at 150 • C to prepare powder samples.The six resulting powder samples were all subjected to heat treatment at 370 • C for one hour.The above-described samples were then termed as follows: TiO 2 B-150 (oxygen-deficient B-phase titania, prepared through spray drying at 150 • C), B-Au0.1%-150(B-phase titania loaded with 0.1% gold by weight, prepared by spray drying at 150 • C), B-Au1%-150 (B-phase titania loaded with 1% gold by weight, prepared by spray drying at 150 • C), B-Au5%-150 (B-phase titania loaded with 5% gold by weight, prepared by spray drying at 150 • C), B-Au1%-200 (B-phase titania loaded with 1% gold by weight, prepared by spray drying at 200 • C), and B-Au1%-250 (B-phase titania loaded with 1% gold by weight, prepared by spray drying at 250 • C).

Photocatalysis Experiment Section
The photocatalytic performance of the gold-loaded B-phase titanium dioxide photocatalyst was determined by evaluating the ability of the test sample to generate carbon dioxide from carbon monoxide under visible light.The specific testing method is elucidated as follows.A total of 0.05 g of the prepared gold/B-phase titanium dioxide porous microsphere sample powder was weighed and then placed on a sample slot with an area of 0.25 cm 2 .Subsequently, the sample slot was placed on the bottom of a flat round reactor made of Pyrex glass, covered with a quartz glass plate, and then sealed.Pure air (20% oxygen and 80% nitrogen) was introduced into the container through an inlet on one side to eliminate the effect of carbon dioxide originally present in the atmosphere on the photocatalytic degradation experiment.After 10 min of ventilation, the inlet and outlet pipes on both sides were sealed with hemostatic forceps.Afterward, 0.25 mL of carbon monoxide standard gas was injected into the reactor using a gas-tight syringe produced by Agilent to achieve the carbon monoxide concentration of 250 ppm inside the reactor.The illumination was started from the top of the reactor after a dark environment was maintained for 15 min (to eliminate the dark adsorption process of the photocatalyst).A 150 w xenon lamp box (Hayashi UV410) served as the light source, and the light intensity was adjusted to 150 mW/cm 2 (Thorlabs PMD100 photometer).A cutoff filter with a wavelength of λ > 420 nm was introduced into the lamp box to endow the outgoing light with a wavelength between 420 nm and 700 nm, and then the sample was illuminated from above.The degradation rate of carbon monoxide and the generation rate of carbon dioxide were detected through gas chromatography (Shimadzu GC-2014C, Japan) equipped with a flame ionization detector (FID) and a 5A molecular sieve column and Porapak-Q chromatography column in series.The volume changes of carbon monoxide and carbon dioxide in the reactor were detected based on the comparison result of the standard curves formulated using carbon monoxide standard gas and carbon dioxide standard gas.Furthermore, the rate of photocatalytic oxidation of carbon monoxide to carbon dioxide for the different gold/B-phase titanium dioxide samples was determined.

Scanning Electron Microscopy
First, the morphology and structure of the B-phase titanium dioxide samples with different quality ratios of gold loading were analyzed and examined after being prepared through spray deposition at different temperatures.The morphology of the samples was photographed and then analyzed through scanning electron microscopy and transmission electron microscopy.Figure 1 illustrates the scanning electron micrographs of six different samples.Figure 1a represents a pure oxygen-deficient B-phase titanium dioxide sample prepared through spray pyrolysis at 150 • C. The sample exhibited a relatively regular spherical shape with a rough surface due to the spray pyrolysis preparation process.The diameter of the spheres was approximately 1 µm.Figure 1b-d represent samples that were prepared under the same conditions as Figure 1a, except for the addition of gold in the precursor solution at mass ratios of 0.1%, 1%, and 5% relative Catalysts 2023, 13, 1078 5 of 15 to the B-phase titanium dioxide, respectively.The three samples exhibited a similar morphology of the porous microspheres, with sizes of approximately 1 µm.Images Figure 1e,f represent that were prepared with the same ratio of gold addition (1%),but under different spray pyrolysis temperature,200°C and 250°C, respectively.Brightly, granular objects were observed on the surfaces of the larger sphere in Figure 1c,d.This was caused by the aggregation of gold nanoparticles during the spray pyrolysis process at high temperature.Furthermore, the number of gold particles in Figure 1d was significantly higher than that in Figure 1c because the B-phase titanium dioxide in Figure 1d contained 5 wt% gold, which was more than the amount in Figure 1c.Additionally, although the morphology of the three samples prepared with different gold loadings at different temperatures was similar, the probability of forming large gold particles on the surface of the titanium dioxide increased with the increasing spray temperature.Accordingly, we found that excessive gold loading and a high spray temperature could lead to the agglomeration of gold nanoparticles into larger particles, which contradicts our initial intention of preparing composite materials with uniformly modified gold on the surface of B-phase titanium dioxide.
the precursor solution at mass ratios of 0.1%, 1%, and 5% relative to the B-phase titan dioxide, respectively.The three samples exhibited a similar morphology of the po microspheres, with sizes of approximately 1 μm.Images Figure 1e,f represent that w prepared with the same ratio of gold addition (1%),but under different spray pyro temperature,200℃ and 250℃, respectively.Brightly, granular objects were observed the surfaces of the larger sphere in Figure 1c,d.This was caused by the aggregation of g nanoparticles during the spray pyrolysis process at high temperature.Furthermore number of gold particles in Figure 1d was significantly higher than that in Figure 1c cause the B-phase titanium dioxide in Figure 1d contained 5 wt% gold, which was m than the amount in Figure 1c.Additionally, although the morphology of the three sam prepared with different gold loadings at different temperatures was similar, the prob ity of forming large gold particles on the surface of the titanium dioxide increased w the increasing spray temperature.Accordingly, we found that excessive gold loading a high spray temperature could lead to the agglomeration of gold nanoparticles into la particles, which contradicts our initial intention of preparing composite materials w uniformly modified gold on the surface of B-phase titanium dioxide.

Transmission Electron Microscopy
Figure 2 presents the transmission electron microscopy (TEM) images of B-phas tanium dioxide (TiO2) loaded with gold at different weight ratios that was prep through spray pyrolysis at different temperatures.The first five images illustrate B-p TiO2 loaded with 0.1%wt gold prepared under spray pyrolysis conditions at 150℃ (Fig 2a), B-phase TiO2 loaded with 1%wt gold prepared at 150 °C (Figure 2b), B-phase loaded with 5%wt gold prepared at 150 °C (Figure 2c), B-phase TiO2 loaded with 1 gold prepared at 200 °C (Figure 2d), as well as B-phase TiO2 loaded with 1%wt gold pared at 250 °C (Figure 2e).After the spray pyrolysis process was completed, the

Transmission Electron Microscopy
Figure 2 presents the transmission electron microscopy (TEM) images of B-phase titanium dioxide (TiO 2 ) loaded with gold at different weight ratios that was prepared through spray pyrolysis at different temperatures.The first five images illustrate B-phase TiO 2 loaded with 0.1%wt gold prepared under spray pyrolysis conditions at 150°C (Figure 2a), B-phase TiO 2 loaded with 1%wt gold prepared at 150 • C (Figure 2b), B-phase TiO 2 loaded with 5%wt gold prepared at 150 • C (Figure 2c), B-phase TiO 2 loaded with 1%wt gold prepared at 200 • C (Figure 2d), as well as B-phase TiO 2 loaded with 1%wt gold prepared at 250 • C (Figure 2e).After the spray pyrolysis process was completed, the five abovementioned samples were subjected to TEM analysis.As depicted in the TEM images, the gold nanoparticles were significantly aggregated in the images of samples c, d, and e. Notably, the aggregated gold nanoparticles in sample c were the largest, possibly due to the higher gold loading in this sample, such that there was more aggregation during the spray pyrolysis process.In samples d and e, the gold nanoparticles were more significantly aggregated in the sample prepared at 250 • C than those in the samples prepared at 200 • C. The possible reason for this result is that at higher temperatures, gold nanoparticles are more prone to aggregation.In all three TEM images showing severe aggregation of the gold nanoparticles, there was no uniform distribution of the gold particles on the surface of B-phase TiO 2 .In image a, although there was no aggregation of gold nanoparticles, there was also no evidence of contact between the gold particles and B-phase TiO 2 .The possible reason for this result is that the gold loading is too low, thus causing the gold nanoparticles to be trapped inside the porous microspheres of B-phase TiO 2 .In contrast, image b shows no aggregation of gold nanoparticles, and image f indicates that the gold nanoparticles were well dispersed on the surface of B-phase TiO 2 .The peak at d = 6.21Å represents the (001) plane of B-phase TiO 2 (monoclinic phase), while the peak at d = 2.33 Å represents the (111) plane of the gold nanoparticles, suggesting good contact between the gold nanoparticles and B-phase TiO 2 .As depicted in the six TEM images above, the gold nanoparticles exhibited aggregation at high temperatures.Thus, gold can be well dispersed on the surface of B-phase TiO 2 only under the conditions of spray pyrolysis at 150 • C with 1%wt gold loading.
ER REVIEW 6 of 14 prepared at 200 °C.The possible reason for this result is that at higher temperatures, gold nanoparticles are more prone to aggregation.In all three TEM images showing severe aggregation of the gold nanoparticles, there was no uniform distribution of the gold particles on the surface of B-phase TiO2.In image a, although there was no aggregation of gold nanoparticles, there was also no evidence of contact between the gold particles and B-phase TiO2.The possible reason for this result is that the gold loading is too low, thus causing the gold nanoparticles to be trapped inside the porous microspheres of B-phase TiO2.In contrast, image b shows no aggregation of gold nanoparticles, and image f indicates that the gold nanoparticles were well dispersed on the surface of B-phase TiO2.The peak at d = 6.21Å represents the (001) plane of B-phase TiO2 (monoclinic phase), while the peak at d = 2.33 Å represents the (111) plane of the gold nanoparticles, suggesting good contact between the gold nanoparticles and B-phase TiO2.As depicted in the six TEM images above, the gold nanoparticles exhibited aggregation at high temperatures.Thus, gold can be well dispersed on the surface of B-phase TiO2 only under the conditions of spray pyrolysis at 150 °C with 1%wt gold loading.

X-ray Diffraction Spectra
The crystal structure of B-phase titanium dioxide supported by gold was analyzed in-depth by X-ray diffraction spectra (XRD), which involved the identification of the crystal phases and the properties of phase composition.As depicted in Figure 3, the sample represented by curve "a" was prepared using spray pyrolysis at 150 °C and treated with

X-ray Diffraction Spectra
The crystal structure of B-phase titanium dioxide supported by gold was analyzed in-depth by X-ray diffraction spectra (XRD), which involved the identification of the crystal phases and the properties of phase composition.As depicted in Figure 3, the sample represented by curve "a" was prepared using spray pyrolysis at 150 • C and treated with annealing at 370 • C for an hour, exhibiting a relatively standard XRD pattern of B-phase titanium dioxide with characteristic peaks at positions 24.9 • , 28.6 • , 43.5 • , and 48.5 • , all belonging to B-phase titanium dioxide (JCPDS 46-1237).Next, we compared the curves "a", "d", "e", and "f", which were obtained by spraying at the same temperature, but with different amounts of gold loading.As indicated by the result, when 0.1% gold was added to B-phase titanium dioxide, the XRD pattern of the B-phase titanium dioxide remained unchanged.However, with the increase of the gold loading, the peak intensity of the gold XRD characteristic peaks (38.3 • , 44.5 • , 64.7 • ) was also increased.The result above suggests that with an increase in the amount of chloroauric acid added to the precursor solution, the proportion of gold in the final B-phase titanium dioxide-gold porous microspheres was also increased.Subsequently, curves "a", "b", and "c" were compared.The result indicates that when the sample was prepared by spray pyrolysis at 200 • C and 250 • C and then annealed, the resulting B-phase titanium dioxide exhibited a certain degree of phase transformation, with the four characteristic peaks at 25.3 • , 48.1 • , 53.9 • , and 55.1 • representing the anatase phase of titanium dioxide.This finding revealed that the B-phase titanium dioxide obtained by spray pyrolysis exhibits better dispersibility, such that it can be more easily transformed.The comparison of curves "b", "c", and "e" indicated that the samples prepared by spraying at 200 • C and 250 • C exhibited a slight transformation from the B-phase to the anatase phase in comparison with those prepared at 150 • C. Furthermore, although the amount of chloroauric acid added to the precursor solution for the three samples before preparation was the same, the peak intensity of the gold characteristic peak in the samples prepared at 200 • C and 250 • C was stronger than that of the sample prepared at 150 • C. The possible reason for this result is that at higher temperatures, the gold particles tend to aggregate more, thus making it easier for them to crystallize during the subsequent annealing process and resulting in better crystalline gold particles.The XRD spectra suggests that different spray pyrolysis temperatures exerted significant effects on the gold particles and the structure of B-phase titanium dioxide in the gold-loaded samples.
Catalysts 2023, 13, x FOR PEER REVIEW 7 of the proportion of gold in the final B-phase titanium dioxide-gold porous microsphe was also increased.Subsequently, curves "a", "b", and "c" were compared.The res indicates that when the sample was prepared by spray pyrolysis at 200 °C and 250 °C a then annealed, the resulting B-phase titanium dioxide exhibited a certain degree of ph transformation, with the four characteristic peaks at 25.3°, 48.1°, 53.9°, and 55.1° rep senting the anatase phase of titanium dioxide.This finding revealed that the B-phase ti nium dioxide obtained by spray pyrolysis exhibits better dispersibility, such that it can more easily transformed.The comparison of curves "b", "c", and "e" indicated that samples prepared by spraying at 200 °C and 250 °C exhibited a slight transformation fr the B-phase to the anatase phase in comparison with those prepared at 150 °C.Furth more, although the amount of chloroauric acid added to the precursor solution for three samples before preparation was the same, the peak intensity of the gold charact istic peak in the samples prepared at 200 °C and 250 °C was stronger than that of sample prepared at 150 °C.The possible reason for this result is that at higher tempe tures, the gold particles tend to aggregate more, thus making it easier for them to cryst lize during the subsequent annealing process and resulting in better crystalline gold p ticles.The XRD spectra suggests that different spray pyrolysis temperatures exerted s nificant effects on the gold particles and the structure of B-phase titanium dioxide in gold-loaded samples.

UV-Visible Diffuse Reflectance Spectra
In this portion of the study, several tests were performed on the optical properties of gold-loaded B-phase titanium dioxide.Several samples were first examined using UV-Vis diffuse reflectance spectra (Figure 4).As indicated by the UV diffuse reflectance spectra, Figure 4a showed a wide absorption range since its surface exhibited many oxygen defects, leading to defect absorption [50][51][52].Accordingly, the bandgap of B-phase titanium dioxide was narrowed from 3.13 eV [53] to 2.30 eV.The resonance absorption peak of gold near 530 nm was observed with the introduction of gold nanoparticles [54,55].Furthermore, with the increase in the amount of gold loading, the defect absorption of the B-phase titanium dioxide was reduced.The possible reason for this result is the surface modification of B-phase titanium dioxide by chloroauric acid, which also made up for the surface defects of B-phase titanium dioxide.As indicated by the comparison of the absorption spectra curves of gold-loaded B-phase titanium dioxide with the same mass ratio prepared by spraying at three different temperatures, the samples prepared through spraying at 200 • C and 250 • C showed significantly decreased visible light absorption in comparison with the three ratios of gold-loaded B-phase titanium dioxide samples synthesized through spraying at 150 • C. The possible reason for the result above is that B-phase titanium dioxide is subjected to some phase transitions due to the spray pyrolysis at higher temperatures, resulting in a certain proportion of anatase-phase titanium dioxide, consistent with the previous XRD test results.Pure anatase-phase titanium dioxide exhibited a wider bandgap than pure B-phase titanium dioxide, such that the samples contained a mixture of an anatase phase weaker in visible light absorption than the pure B-phase samples [56].
atalysts 2023, 13, x FOR PEER REVIEW defects of B-phase titanium dioxide.As indicated by the comparison of spectra curves of gold-loaded B-phase titanium dioxide with the same pared by spraying at three different temperatures, the samples prepared th at 200 °C and 250 °C showed significantly decreased visible light absorpt son with the three ratios of gold-loaded B-phase titanium dioxide samp through spraying at 150 °C.The possible reason for the result above is th nium dioxide is subjected to some phase transitions due to the spray pyr temperatures, resulting in a certain proportion of anatase-phase titanium sistent with the previous XRD test results.Pure anatase-phase titanium di a wider bandgap than pure B-phase titanium dioxide, such that the samp mixture of an anatase phase weaker in visible light absorption than the pur ples [56].

Photoluminescence Spectroscopy
As depicted in Figure 5, photoluminescence (PL) spectroscopy was performed on the prepared samples.PL spectroscopy takes on critical significance in testing the recombination process of free carriers, such that it is effective in evaluating the efficiency of carrier capture, transfer, and transport in semiconducting materials.On that basis, insight can be gained into the ultimate fate of photogenerated carriers in the materials.
Fermi level of gold, which is lower than the conduction band positi can serve as an electron-rich center to transfer photo-generated electr gold nanoparticles after the gold nanoparticles are loaded onto the TiO2.On that basis, photo-generated electron-hole pairs can be effecti the recombination process can be inhibited, such that the luminesce photoluminescence spectrum can be reduced [23].Accordingly, the plains why B-phase TiO2 prepared by spraying at 150 °C with 1%wt go significantly suppress the recombination of photo-generated electrongold nanoparticles can be most effectively dispersed on the surface good contact with B-phase TiO2, and without aggregation.

Visible Light Photocatalytic Oxidation of Carbon Monoxide
Next, the visible light photocatalytic activity of our prepared g titanium dioxide during the oxidation of carbon monoxide to carbon ined (Figure 6).The design of this composite photocatalyst aimed to ta visible light absorption capability of the B-phase titanium dioxide, as gold to adsorb carbon monoxide, for the visible light photocatalytic monoxide.Figure 6 presents the data for carbon dioxide production lytic oxidation of carbon monoxide under visible light for a wide var porous microsphere samples of B-phase titanium dioxide.As depicte pure B-phase titanium dioxide produced nearly 30 microliters of ca consuming approximately 30 microliters of carbon monoxide over 1 B-phase titanium dioxide samples exhibited better photocatalytic ac phase titanium dioxide during the oxidation of carbon monoxide to reason for the above-described result is that the gold nanoparticles In the PL test results, a distinct emission peak in the range of 700-1000 nm esd was first observed with the introduction of gold, which was absent in the pure phase B-phase titanium dioxide.The intensity of the emission peak esd increased with the amount of gold loading.Furthermore, for samples with the same mass ratio of gold loading on the surface of B-phase titanium dioxide, the PL peak of gold esd decreased with the decrease of the spray temperature.The possible reason for this result is the less pronounced plasma resonance absorption with a smaller gold particle size, resulting in weaker PL intensity [57].The above-described finding is consistent with the previous scanning electron microscopy and transmission electron microscopy images, suggesting that the samples prepared under low-temperature spray conditions had smaller gold particles that were less likely to aggregate.
In addition, luminescence was observed in the range of 400 nm to 700 nm for the pure B-phase TiO 2 , with the peak centered around 520 nm [58,59].When gold nanoparticles were decorated on the surface of the B-phase TiO 2 , the luminescence intensity was decreased due to the B-phase TiO 2 .Moreover, under similar spray conditions, the luminescence in B-phase TiO 2 was optimally suppressed when 1%wt of gold nanoparticles were loaded.When the loading of gold nanoparticles was too low (0.1%wt) or too high (5%wt), the suppression of luminescence in B-phase TiO 2 was slightly less effective.As discussed above, the intensity of the photoluminescence can reveal the recombination process of photo-generated electron-hole pairs in the sample.The stronger the luminescence peak, the greater the probability of recombination of photo-generated carriers.The loading of gold on the surface of B-phase TiO 2 led to the decreased luminescence intensity of B-phase TiO 2 , which can also be considered a reduction of the probability of recombination of photo-generated carriers in B-phase TiO 2 .The possible reason for this result is that the Fermi level of gold, which is lower than the conduction band position of B-phase TiO 2 , can serve as an electron-rich center to transfer photo-generated electrons to the surface of gold nanoparticles after the gold nanoparticles are loaded onto the surface of B-phase TiO 2 .On that basis, photo-generated electron-hole pairs can be effectively separated, and the recombination process can be inhibited, such that the luminescence intensity of the photoluminescence spectrum can be reduced [23].Accordingly, the analysis above explains why B-phase TiO 2 prepared by spraying at 150 • C with 1%wt gold loading can most significantly suppress the recombination of photo-generated electron-hole pairs, since the gold nanoparticles can be most effectively dispersed on the surface of B-phase TiO 2 , in good contact with B-phase TiO 2 , and without aggregation.

Visible Light Photocatalytic Oxidation of Carbon Monoxide
Next, the visible light photocatalytic activity of our prepared gold-loaded B-phase titanium dioxide during the oxidation of carbon monoxide to carbon dioxide was examined (Figure 6).The design of this composite photocatalyst aimed to take advantage of the visible light absorption capability of the B-phase titanium dioxide, as well as the ability of gold to adsorb carbon monoxide, for the visible light photocatalytic oxidation of carbon monoxide.Figure 6 presents the data for carbon dioxide production from the photocatalytic oxidation of carbon monoxide under visible light for a wide variety of gold-loaded porous microsphere samples of B-phase titanium dioxide.As depicted in Figure 6, 1 g of pure Bphase titanium dioxide produced nearly 30 microliters of carbon dioxide, while consuming approximately 30 microliters of carbon monoxide over 1 h.The gold-loaded B-phase titanium dioxide samples exhibited better photocatalytic activity than pure B-phase titanium dioxide during the oxidation of carbon monoxide to carbon dioxide.The reason for the above-described result is that the gold nanoparticles on the surface of B-phase titanium dioxide are capable of effectively transferring photogenerated charge carriers and inhibiting electron-hole recombination, such that the electrons can be transferred more efficiently to carbon monoxide.Among the five gold-loaded B-phase titanium dioxide samples, the one loaded with 1 wt% gold and prepared by spray pyrolysis at 150 • C exhibited the highest visible light photocatalytic activity in the oxidation of carbon monoxide to carbon dioxide.One gram of this sample produced around 50 microliters of carbon dioxide, while oxidizing nearly 50 microliters of carbon monoxide over one hour of the photocatalytic reaction.The samples loaded with 0.1 wt% and 5 wt% gold prepared under the same conditions exhibited slightly lower photocatalytic activity in the oxidation of carbon monoxide to carbon dioxide, with each gram of the sample oxidizing 32 and 34 microliters of carbon monoxide, respectively, and generating the same volume of carbon dioxide.The possible reason for the result above is that the insufficient gold loading (0.1%) does not effectively transfer photogenerated charge carriers and reduce recombination, whereas too much gold loading (5%) causes aggregation of gold nanoparticles during high-temperature spray pyrolysis, preventing them from being uniformly dispersed on the surface of B-phase titanium dioxide.
reduce the probability of recombination.Furthermore, the photocatalytic oxidat ity of carbon monoxide (CO) of the five samples was compared, excluding th phase TiO2, and the result suggested that the proportion of gold loading on the B-phase TiO2 was more important than the temperature factor in terms of the p lytic oxidation activity of CO.As indicated by the comparison of the photocatalytic activity of the B-phase titanium dioxide (TiO 2 ) samples loaded with 1 wt% gold (Au) prepared by spraying at three different temperatures, the sample prepared at 150 • C still exhibited the optimal photocatalytic activity.The samples prepared at 200 • C and 250 • C exhibited slightly lower activity, whereas their activity levels were similar to each other.Furthermore, we measured the specific surface area of the TiO 2 (B) microspheres prepared by ultrasonic spray pyrolysis at 150 • C, 200 • C, and 250 • C, which were 19.5 m 2 /g, 18.8 m 2 /g, and 16.2 m 2 /g, respectively.This proves that the improvement of photocatalytic performance of the Au-TiO 2 (B) heterojunction microsphere is not related to the specific surface area.Therefore, we can consider that at higher temperatures, the aggregation of gold nanoparticles becomes more pronounced, resulting in the formation of large-sized gold particles that are not conducive to the uniform decoration of the B-phase TiO 2 surface with gold nanoparticles.Consequently, the gold nanoparticles cannot effectively transfer the photogenerated carriers and reduce the probability of recombination.Furthermore, the photocatalytic oxidation activity of carbon monoxide (CO) of the five samples was compared, excluding the pure B-phase TiO 2 , and the result suggested that the proportion of gold loading on the surface of B-phase TiO 2 was more important than the temperature factor in terms of the photocatalytic oxidation activity of CO.

Mechanism Analysis
The following mechanism diagram was generated in accordance with the abovementioned data (Figure 7).First, B-phase titanium dioxide with visible light absorption was synthesized (the bandgap of traditional B-phase titanium dioxide of 3.13 eV) [60].Subsequently, the gold nanoparticles were modified using the spray pyrolysis method onto the surface of the B-phase titanium dioxide.Moreover, the B-phase titanium dioxide was transformed into a mixture of locally ordered visible-light-absorbing anatase and B-phase titanium dioxide.The gold loading led to the increased separation efficiency of photogenerated electrons and holes in the anatase/B-phase titanium dioxide interface, such that the probability of electron-hole recombination on the surface of anatase/B-phase titanium dioxide was reduced.As a result, the efficiency of photo-generated electron transfer to carbon monoxide was increased, such that the activity of photocatalytic carbon monoxide oxidation to carbon dioxide could be increased.
was transformed into a mixture of locally ordered visible-light-absorbing anat phase titanium dioxide.The gold loading led to the increased separation ef photo-generated electrons and holes in the anatase/B-phase titanium dioxide such that the probability of electron-hole recombination on the surface of anata titanium dioxide was reduced.As a result, the efficiency of photo-generate transfer to carbon monoxide was increased, such that the activity of photocataly monoxide oxidation to carbon dioxide could be increased.

Conclusions
In this study, B-phase titanium dioxide with visible light absorption was fi sized and then assembled using the spray pyrolysis technique into self-doped phase titanium dioxide and titanium dioxide/gold nanoparticles heterojuncti microspheres.Due to the oxygen defects of the self-doped anatase/B-phase titan ide and the plasma of gold, the visible light absorption range of titanium di significantly widened.Moreover, electron-hole separation was facilitated by t junction of self-doped anatase/B-phase titanium dioxide and titanium dioxide/ particles.Due to the advantages of the visible light absorption of B-phase titaniu and the effective promotion of titanium dioxide-photogenerated carrier separ gold nanoparticles, the composite photocatalyst that was synthesized in this st ited better activity than pure B-phase titanium dioxide in the visible light catal tion of carbon monoxide to carbon dioxide.B-phase titanium dioxide with 1 loaded, which was prepared by spray pyrolysis at 150 °C, was confirmed as t with the optimal photocatalytic activity.Each gram of this sample oxidized 50 bon monoxide and produced an equal volume of carbon dioxide in 1 h, whic times the activity of pure B-phase titanium dioxide in the photocatalytic oxidat bon monoxide to carbon dioxide.In addition, as revealed by the result of this amount of gold nanoparticles loaded on the surface of the B-phase titanium d the temperature of the spray-prepared samples can notably affect the modificat on the surface of B-phase titanium dioxide.At high temperatures or under high ing, gold nanoparticles will aggregate on the surface of B-phase titanium dioxid ing the uniform dispersion and good contact of gold nanoparticles on the su

Conclusions
In this study, B-phase titanium dioxide with visible light absorption was first synthesized and then assembled using the spray pyrolysis technique into self-doped anatase/Bphase titanium dioxide and titanium dioxide/gold nanoparticles heterojunction porous microspheres.Due to the oxygen defects of the self-doped anatase/B-phase titanium dioxide and the plasma of gold, the visible light absorption range of titanium dioxide was significantly widened.Moreover, electron-hole separation was facilitated by the heterojunction of self-doped anatase/B-phase titanium dioxide and titanium dioxide/gold nanoparticles.Due to the advantages of the visible light absorption of B-phase titanium dioxide and the effective promotion of titanium dioxide-photogenerated carrier separation with gold nanoparticles, the composite photocatalyst that was synthesized in this study exhibited better activity than pure B-phase titanium dioxide in the visible light catalytic oxidation of carbon monoxide to carbon dioxide.B-phase titanium dioxide with 1 wt% gold loaded, which was prepared by spray pyrolysis at 150 • C, was confirmed as the sample with the optimal photocatalytic activity.Each gram of this sample oxidized 50 µL of carbon monoxide and produced an equal volume of carbon dioxide in 1 h, which was 1.67 times the activity of pure B-phase titanium dioxide in the photocatalytic oxidation of carbon monoxide to carbon dioxide.In addition, as revealed by the result of this study, the amount of gold nanoparticles loaded on the surface of the B-phase titanium dioxide and the temperature of the spray-prepared samples can notably affect the modification of gold on the surface of B-phase titanium dioxide.At high temperatures or under high gold loading, gold nanoparticles will aggregate on the surface of B-phase titanium dioxide, hindering the uniform dispersion and good contact of gold nanoparticles on the surface of B-phase titanium dioxide.Consequently, the activity of gold-loaded B-phase titanium dioxide in the photocatalytic oxidation of carbon monoxide can be reduced.In this study, the visible-light-absorbing gold-loaded B-phase titanium dioxide was prepared through spray pyrolysis with good activity in the visible light photocatalytic oxidation of carbon monoxide, which can provide good inspiration for researchers who want to prepare photocatalysts with high activity in the visible light photocatalytic oxidation of carbon monoxide.

Figure 1 .
Figure 1.SEM images of pure oxygen-deficient B-phase titanium dioxide (a) and B-phase titan dioxide loaded with different mass ratios of gold prepared at different temperatures (b-f).

Figure 1 .
Figure 1.SEM images of pure oxygen-deficient B-phase titanium dioxide (a) and B-phase titanium dioxide loaded with different mass ratios of gold prepared at different temperatures (b-f).

Figure 2 .
Figure 2. TEM images of oxygen-deficient B-phase titanium dioxide loaded with different mass ratios of gold prepared at different temperatures (a-e) and high-resolution images of B-phase titanium dioxide loaded with 1% wt gold prepared by spray pyrolysis at 150 °C (f).

AuFigure 2 .
Figure 2. TEM images of oxygen-deficient B-phase titanium dioxide loaded with different mass ratios of gold prepared at different temperatures (a-e) and high-resolution images of B-phase titanium dioxide loaded with 1% wt gold prepared by spray pyrolysis at 150 • C (f).

Figure 3 .Figure 3 .
Figure 3. XRD patterns of pure oxygen-deficient B-phase titanium dioxide and B-phase titani dioxide loaded with different mass ratios of gold prepared at different temperatures.

Figure 4 .Figure 4 .
Figure 4. UV-vis DRS spectra of pure oxygen-deficient B-phase titanium dioxide nium dioxide loaded with different mass ratios of gold prepared at different temp

Figure 5 .
Figure 5. PL spectra of pure oxygen-deficient B-phase titanium dioxide and B ide loaded with different mass ratios of gold prepared at different temperatu

Figure 5 .
Figure 5. PL spectra of pure oxygen-deficient B-phase titanium dioxide and B-phase titanium dioxide loaded with different mass ratios of gold prepared at different temperatures.

Figure 6 .Figure 6 .
Figure 6.The photocatalytic oxidation rate of carbon monoxide to carbon dioxide by pu deficient B-phase titanium dioxide and B-phase titanium dioxide loaded with different m of gold prepared at different temperatures.

Figure 7 .
Figure 7. Mechanism diagram of gold-loaded oxygen-deficient B-phase titanium dioxid light photocatalytic oxidation of carbon monoxide.

Figure 7 .
Figure 7. Mechanism diagram of gold-loaded oxygen-deficient B-phase titanium dioxide in visible light photocatalytic oxidation of carbon monoxide.