1 Introduction

The increasing concentration of CO2 in the atmosphere was considered by Cao et al. [1] as the main cause of global warming; thus, efficient methods for CO2 removal are needed. One possible method is CO2 capture and storage, which stores the CO2 underground. Between 2012 and 2020, Japan's first large-scale demonstration project was completed and reported by Sawada et al. [2], which included CO2 separation/capture, injection, storage, and monitoring. Another method is direct air capture, in which low CO2 concentrations in the air are captured and stored using a polymer membrane explored by Fujikawa et al. [3]. In recent years, biological methods that cultivate microalgae to capture CO2 and produce chemical substances such as pigments [4] and biofuels [5] in cells have received considerable attention. A simpler method for reducing CO2 concentration in the air is to use photocatalysts to convert CO2 into other substances such as carbon monoxide (CO), methane (CH4), formic acid, methanol, and ethanol. TiO2 is one of the most promising materials for this purpose; it has been extensively studied [6, 7, 8, 9, 10] because it is inexpensive, abundant, and highly stable. However, the intrinsic conversion efficiency of CO2 to reduction products on the surface of TiO2 is low. The main reasons for the low conversion efficiency are: (1) its band gap is large (~ 3.2 eV), and only light in the ultraviolet range can be absorbed; and (2) the lifetime of the electrons produced from the photoexcitation of TiO2 is short. The efficiency can be enhanced by loading TiO2 with metal ions [11, 12] or supporting metal nanoparticles on the surface of TiO2 to increase light absorption in the visible range (via the surface plasmon effect of the metal nanoparticles) [13, 14, 15, 16, 17] and enhance the electron transfer from the TiO2 to the metal.

Two types of methods are mainly used for the photocatalytic reduction of CO2, namely, liquid phase reaction method, in which the photocatalyst is dispersed in a solution and CO2 is bubbled into the solution [18, 19], and gas phase reaction method [20], in which the photocatalyst is applied onto a substrate and the gaseous CO2 is in direct contact with the photocatalyst. The liquid phase methods are simple and easy to handle, but the probability of contact between the bubbled CO2 and the surface of the photocatalyst is low. In contrast, in the gaseous phase methods, the CO2 can easily reach the surface of the photocatalyst; however, when the reduction product is a liquid (such as formic acid, methanol, or ethanol), the surface is eventually blocked unless the product is removed.

Au is a strong candidate that supports the metal on the surface of TiO2 and enhances its photocatalytic effect [21]. In addition, Au nanoparticles (AuNPs) function as "charge-carrier traps" and have the effect of suppressing the recombination of electron–hole pairs. To further enhance this effect, it is necessary to increase the surface area of AuNPs by synthesizing a large number of smaller AuNPs and supporting them on the photocatalyst surface. Maruo et al. reported that the size of Au nanoparticles was controlled by changing the pH of the solution when the Au nanoparticles were supported on the TiO2 surface [22]. In this study, we synthesized a photocatalyst by supporting AuNPs on a mixture of TiO2 and CaCO3; CaCO3 was chosen because it acts as a carbon source and can facilitate CO2 reduction on the TiO2 surface. Materials with different ratios of CaCO3 to TiO2 were synthesized, and the surface characteristics of the obtained photocatalysts were studied. For CO2 adsorbed on CaCO3 to contribute toward the photocatalytic reaction, CaCO3 and TiO2 should necessarily be in mutual contact. Therefore, a vapor phase method was used wherein these materials were supported on a substrate to perform a photocatalytic reaction. The effects of the particle size and amount of AuNPs on the photocatalytic performance for the reduction of CO2 to CO and CH4 were investigated. The experimental section describes the methods of photocatalyst fabrication, photocatalyst characterization, and photocatalyst reaction testing. Furthermore, Sect. 3 discusses the details of the photocatalytic characterization, including the crystal structure, particle and surface elemental analysis, and photocatalytic ability. The conclusions are presented in Sect. 4.

2 Experimental

2.1 Preparation of photocatalysts

Anatase TiO2 (> 98.5 %, Kanto Chemical Co., Inc.) was purchased and used without further purification. To remove any organic contaminants from the surface, the TiO2 powder was placed in an alumina crucible in the center of a muffle oven (MMF-1, AS ONE Corp.), and calcined at 200 °C for 4 h.

The synthesis of the photocatalysts consisting of AuNPs deposited on TiO2 and CaCO3 was performed as follows. Hydrogen tetrachloroaurate (III) tetrahydrate (0.1 g, Kanto Chemical Co., Inc.) was dissolved in 58.5 mL of deionized water. The pH of the solution was then adjusted to 9 using a 0.1 mol L−1 NaOH solution. The calcined TiO2 (2.0 g) and CaCO3 (> 99.5 %, Kanto Chemical Co., Inc.) were then added to the solution, and the mixture was stirred with a magnetic stirrer for 4 h. Three different samples were prepared using 0.4, 0.8, and 4.0 g of CaCO3; the percentages of CaCO3 with respect to TiO2 were 20, 40, and 200 wt%, respectively. A sample without CaCO3 was also prepared.

Initially, the mixtures were cloudy owing to the suspended TiO2 and CaCO3. The solids were filtered by suction and placed in a drying oven at 40 °C for 24 h. The white powder was then calcined at 40 °C for 2 h. After calcination, the mixtures turned dark purple, and a solid precipitate was formed. This suggested that Au particles deposited on the TiO2 or CaCO3 and that the surface plasmon resonance phenomenon resulted in the absorption of visible light. The synthesized photocatalysts were denominated as AuNP/TiO2/CaCO3 (X g), where X corresponded to the amount of CaCO3 added. Before the photocatalytic performance tests, the samples were prepared as follows: Deionized water (3 mL) was added to 0.193 g of a calcined photocatalyst powder. The mixture was then stirred and poured on a silica filter paper (20 mm × 20 mm) (GB-100R, Advantec MFS Inc.). The powder was then allowed to dry in a nitrogen atmosphere at 40 °C for 24 h. The dried sample was then placed in a muffle oven and calcined at 200 °C for 2 h.

2.2 Characterization of the synthesized photocatalyst

The X-ray diffraction (XRD) patterns of the samples were obtained using an Ultima IV device with Cu-Kα radiation (Rigaku Corporation). The analysis was performed in the range of 20° to 90° (2θ) with a pace of 0.02°. Here, θ is the Bragg angle.

An ultraviolet–visible (UV–Vis) spectrometer with an integrating unit (V770, JASCO, Co.) was used to analyze the samples. The band gap was estimated using the Kubelka–Munk equation (1).

$$\frac{K}{S}=\frac{{(1-R)}^{2}}{2R}$$
(1)

here K is the absorption coefficient, S is the scattering coefficient, and R is the reflectance of the photocatalyst. The samples were also characterized via scanning electron microscopy (SEM; Phenom ProX, Thermo Fisher Scientific K.K.), energy dispersive X-ray spectroscopy (EDS; Phenom ProX, Thermo Fisher Scientific K.K.), and transmission electron microscopy (TEM; JEM-2100F, JEOL Ltd.) The sample without CaCO3 was analyzed using an X-ray Photoelectron Spectroscopy (XPS 5700 system, Physical Electronics, Inc.), and the sample with 0.4 g of CaCO3 was analyzed using an XPS device (Nexsa G2, Thermo Fischer Scientific K.K.).

2.3 Photocatalytic reaction tests

The photocatalytic reaction tests were performed according to the following procedure. As shown in Fig. 1, each photocatalyst sample deposited on a silica filter paper was placed in the center of a cylindrical cell (with a diameter of 3 cm, length of 10 cm, and volume of 60 mL) sealed with BaF2 windows on both sides. CO2 from a cylinder (99.99 %, Suzuki Shokan Co. Ltd.) was used for the tests. The CO2 flow rate was first adjusted using a mass flow controller. The gas was then bubbled through water to reach a relative humidity of 50% and finally introduced in the sample cell. After the gas filled the cell, the two valves were closed. A xenon lamp (OPM2-302X, Ushio, Inc.) was used for light irradiation of the sample through one of the BaF2 windows. The light irradiation intensity on the photocatalyst surface was measured using a photometer (UV-340C, CUSTOM Corporation); the value was 4200 μW cm−2 in the range of 250–380 nm. The concentrations of the produced CO and CH4 were measured every hour from the start of the irradiation via Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo Fisher). For this, the cell was placed in the spectrometer. The two BaF2 windows were originally installed to fit through the optical path of the spectrometer. The concentrations of CO and CH4 were estimated using calibration curves obtained in advance at 2165 cm−1 and 3104 cm−1, respectively. The CO calibration curve was obtained using a standard gas containing a CO concentration of 100 ppm in nitrogen gas and a gas obtained by diluting this standard gas with nitrogen gas. The CH4 calibration curve was obtained using a standard gas containing a CH4 concentration of 1 % in nitrogen gas and a gas obtained by diluting this standard gas with nitrogen gas. For each test, the photocatalyst was irradiated with light for a total of 5 h. Each test was repeated at least twice, and the results were obtained as the average values.

Fig. 1
figure 1

Schematic diagram of the photocatalytic reaction testing equipment

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3 Results and discussion

3.1 XRD analysis

The XRD patterns of the prepared photocatalysts are shown in Fig. 2. The patterns in Fig. 2a are the complete scans in the range from 20° to 70°, and Fig. 2b shows a magnification in the range of 43.5°–46.0°. Diffraction peaks corresponding to anatase TiO2 (ICDD PDF 01-071-1166) and Au (ICDD PDF 01-071-4615) were observed in the patterns of all the samples. Additionally, for the photocatalysts containing CaCO3 [AuNP/TiO2/CaCO3 (X g); X = 0.4, 0.8, and 4.0], the diffraction peaks corresponding to calcite (CaCO3, ICDD PDF 01-071-3699) were detected. The intensity of the CaCO3 peaks increased as the percentage of CaCO3 with respect to TiO2 increased.

Fig. 2
figure 2

X-ray diffraction patterns of the prepared AuNP/TiO2/CaCO3 (X g) catalysts, X = 0 g: black, 0.4 g: red, 0.8 g: blue, 4.0 g: Green

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Using the width at half maximum of the peaks, Williamson-Hall plots were constructed according to the following Eq. (2) to determine the crystallite sizes and lattice strain values of TiO2 and CaCO3.

$$\beta {\text{cos}}\theta = 4\varepsilon {\text{sin}}\theta + \frac{{K\lambda }}{D}$$
(2)

where β is the full width at half maximum, K is the Scherrer constant, and λ is the X-ray wavelength. The value of K used was 0.9 [23, 24]. The crystallite size (D) of each sample was obtained from the Y-intercept of the corresponding plot, and the lattice strain (ε) was obtained from the slope. Figs. 3 and 4 show the Williamson-Hall plots of TiO2 and CaCO3, respectively, for the samples with different CaCO3 contents. The results are summarized in Table 1. The crystallite size and lattice strain of TiO2 ranged from 43 to 53 nm and from 1.1 × 10−3 to 1.5 × 10−3, respectively, and those of CaCO3 ranged from 53 to 73 nm and from 1.2 × 10−3 to 1.5 × 10−3, respectively. In good agreement with one of the observations by Julien et al. [25], it was found that the crystallite size of TiO2 used in this study was about the same as that of the size of pure TiO2. No correlation was found between the amount of CaCO3 and the crystallite sizes and lattice strains of TiO2 and CaCO3.

Fig. 3
figure 3

Williamson-Hall plots of TiO2 in the synthesized AuNP/TiO2/CaCO3 (X g) catalysts; X = a 0, b 0.4, c 0.8, and d 4.0 g

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Fig. 4
figure 4

Williamson-Hall Plots of CaCO3 in the synthesized AuNP/TiO2/CaCO3 (X g) photocatalysts; X = a 0.4, b 0.8, and c 4.0 g

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Table 1 Crystallite sizes and lattice strain estimated by Williamson–Hall plot of catalysts AuNP/TiO2/CaCO3 (X g)
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3.2 Microscopic observation and elemental analysis

A SEM and EDS analysis of the AuNP/TiO2/CaCO3 (4.0 g) sample was performed; the elemental mapping image was superimposed on the SEM image (Fig. 5). The detected elements O, C, Ca, and Ti correspond to blue, red, yellow and green colors, respectively. The EDS spectrum is shown in Fig. 6. In the superimposed image (Fig. 5), Ca was detected on large cubic particles and Ti was detected on fine particles. Thus, the cubic particles were composed of CaCO3, and the fine particles were composed of TiO2. Au was not detected in the EDS analysis [22], and the presence of Au particles could not be confirmed owing to the SEM resolution in this image. High-resolution SEM images are shown in Fig. 7a–d. White particles detected in all images were identified as AuNPs because the probability of secondary electron emission of Au is larger than those of Ti, Ca, C, and O. From the SEM images and the EDS and XRD results, it was deduced that the particles with large sizes of several hundred nanometers were composed of TiO2 and the particles with small sizes of approximately 10 nm were composed of Au. The number of AuNPs per 1 μm2 in the SEM images was determined for each sample. The areas were calculated using the lengths of the scale bars. The results are listed in Table 2.

Fig. 5
figure 5

Energy dispersive X-ray spectroscopy elemental mapping of the AuNP/TiO2/CaCO3 (4.0 g) catalyst superimposed on the corresponding scanning electron microscopy image. The following elements were found: blue: O; red: C; yellow: Ca; green: Ti

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Fig. 6
figure 6

Energy dispersive X-ray spectroscopy spectrum of the AuNP/TiO2/CaCO3 (4.0 g) catalyst

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Fig. 7
figure 7

Scanning electron microscopy images of the synthesized AuNP/TiO2/CaCO3 (X g) catalysts; X = a 0, b 0.4, c 0.8, and d 4.0 g

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Table 2 Average particle size and number per unit area of AuNP in AuNP/TiO2/CaCO3 (X g) sample
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TEM images of the calcined photocatalysts are shown in Fig. 8a–d. Large particles with sizes of several hundred nanometers and small particles with sizes of approximately 10 nm can be seen; they were considered to correspond to TiO2 and Au, respectively. The AuNPs supported on the TiO2 surfaces were measured using the scale bars. The average AuNP size in each AuNP/TiO2/CaCO3 (X g) sample (X = 0.4, 0.8, and 4.0 g) was calculated from measurements of up to 10 randomly picked particles in each TEM image. The average AuNP size in the AuNP/TiO2/CaCO3 (0 g) sample was calculated from the measurements of four particles because the number of supported particles in the corresponding TEM image was smaller than 10. The average particle sizes are summarized in Table 2.

Fig. 8
figure 8

Transmission electron microscopy images of the synthesized AuNP/TiO2/CaCO3 (X g) catalysts; X = a 0, b 0.4, c 0.8, and d 4.0 g

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The AuNP size was found to first decrease and then increase as the amount of CaCO3 added increased. The minimum particle size was found for a CaCO3 amount of 0.8 g. Additionally, the number of supported AuNPs first increased and then decreased as the amount of CaCO3 increased, reaching a maximum for a CaCO3 amount of 0.8 g. More precisely, the number of Au particles determined for the sample with a CaCO3 amount of 0.8 g was one order of magnitude larger than those determined for the other samples.

3.3 UV–Vis spectroscopy analysis

To investigate the effect of AuNP sizes on the appearance of the photocatalysts, UV-vis reflectance measurements were conducted, and the Kubelka-Munk conversion (Fig. 9) was performed [Eq. (1)].

Fig. 9
figure 9

Kubelka–Munk conversion curve of the catalyst reflectance measurements

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The photocatalysts exhibited light absorption in the visible range owing to the surface plasmon resonance of the AuNPs supported on a mixture of TiO2 and CaCO3. As shown in Fig. 10, there is a positive correlation between the maximum absorption wavelength in the visible light range and the AuNP size estimated from the TEM images; the absorption peak shifts toward higher wavelengths as the AuNP size increases. This is consistent with results reported in the literature by Kochuveedu et al. [26].

Fig. 10
figure 10

Maximum absorption wavelength in the visible light range as a function of the Au nanoparticle size estimated from transmission electron microscopy images

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From Fig. 9, the full width at half maximum (FWHM) in the visible light range was determined. The FWHM values in the visible light region were 137 nm, 98 nm, 90 nm, and 92 nm for the samples with CaCO3 amounts of 0, 0.4, 0.8, and 4.0 g, respectively; these values showed a positive correlation with the standard deviations of the AuNP sizes estimated from the TEM images in Fig. 8 (4, 3, 0.8, and 3 nm, respectively). This could be because a large variation in the AuNP size leads to a large variation in the absorption wavelength of the surface plasmon, and thus, to a wide FWHM. The AuNPs in the sample with a CaCO3 amount of 0.8 g had a smaller size variation than those in the other samples.

To compare the TiO2 crystalline phases in the synthesized photocatalysts, the band gaps were calculated from the inflection points of the Kubelka-Munk curves in Fig. 9. As shown Fig. 11, the curve of the AuNP/TiO2/CaCO3 (0 g) photocatalyst was linearly fitted (yellow dashed line) between 345 and 365 nm and between 400 and 440 nm, and the absorption wavelength at the intersection (red circle) was determined. The relationship between the bandgap (Eg) and the absorption wavelength at the intersection (λ) is depicted by Eq. (3).

Fig. 11
figure 11

Bandgap determination by fitting the Kubelka–Munk conversion curve spectra of the AuNP/TiO2/CaCO3 (0 g) catalyst

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$$\mathrm{Eg}=\frac{1240}{\lambda }$$
(3)

For the synthesized photocatalysts with 0, 0.4, 0.8, and 4.0 g of CaCO3, the band gaps were 3.32, 3.33, 3.33, and 3.32 eV, respectively. No significant effect of the amount of CaCO3 on the band gap of TiO2 was observed. It can be deduced that there are no differences between the TiO2 crystalline phases in the different photocatalysts, which is in agreement with the XRD results, in which only the anatase phase of TiO2 was detected.

3.4 XPS analysis

The XPS spectra for the photocatalysts with CaCO3 amounts of 0 and 0.4 g are shown in Fig. 12a–e. The black solid lines correspond to the spectra of the different elements for the sample with 0.4 g of CaCO3 after subtracting the background intensity for peak separation. The blue, orange, yellow, and green lines correspond to the peak deconvolution results. The black dashed lines correspond to the spectra of the different elements for the sample without CaCO3. The binding energy of the most prominent Ti 2p peak in was 459.5 eV for the photocatalyst with 0.4 g of CaCO3 (Fig. 12a). This value is in good agreement with the reported values for Ti4+ in TiO2 [27, 28]. In contrast, the peaks corresponding to Ti3+ [28] were not detected. In the spectra for the sample without CaCO3 (black dashed line) the peak corresponding to Ti4+ is located at 458.6 eV, which is in agreement with the results published by Maruo et al. [22]. Thus, in the sample with 0.4 g CaCO3, the peak was shifted 0.9 eV toward higher binding energies. In the spectral range of Ca 2p (Fig. 12b), two peaks were observed at 351.9 eV and 348.3 eV for the photocatalyst with 0.4 g of CaCO3. These peaks correspond to the binding energies of calcite and can be attributed to Ca 2p1/2 and Ca 2p3/2, respectively [29]. For the photocatalyst with 0.4 g of CaCO3 (black solid line), four peaks were observed in the spectral range of C 1s (Fig. 12c) at 285.7, 290.2, 287.3, and 283.9 eV (in descending order of peak height). The main peak was shifted 0.3 eV toward higher binding energies with respect to that of the photocatalyst without CaCO3 (Fig. 12c, black dashed line). Maruo et al. reported that the peak near 285 eV corresponded to C originating from organic matter in the air and that Au favored the adsorption of organic matter [22]. They found that the amount of organic matter adsorbed by Au particles supported on TiO2 was 20 times higher than that adsorbed by TiO2 alone. According to reports in the literature, the peaks at 285.7, 290.2, 287.3, and 283.9 eV can be attributed to C–O [30], C 1s in CaCO3 [31], C=O [30], and C–H [30]. For the photocatalyst with 0.4 g of CaCO3 (black solid line), three peaks were observed in the spectral region of O 1s at 530.7, 532.4, and 531.5 eV (in descending order of peak height). The peak at 530.7 eV is attributed to O2− in TiO2, based on the values reported by Kruse et al. [28] for TiO2 on which Au was supported. For the sample without CaCO3 (Fig. 12d black dashed line), the O2− peak derived from TiO2 was located at 530.0 eV, which is in agreement with the results reported by Maruo et al. [22]. Thus, for the sample with 0.4 g CaCO3, the peak was shifted 0.7 eV toward higher binding energies. The peaks at 532.4 and 531.5 eV are attributed to O− (OH−) in TiO2 observed by Kruse et al. [28] and O in CaCO3 observed by Baer et al. [31], respectively. For the photocatalyst with 0.4 g of CaCO3 (solid black line), the peaks corresponding to Au 4f were located at 84.1 and 87.8 eV (Fig. 12e), which are similar values to those of the sample without CaCO3 (Fig. 12e, dashed line) and in good agreement with the values reported in the literature [23, 31].

Fig. 12
figure 12

X-ray photoelectron spectroscopy spectra for the AuNP/TiO2/CaCO3 (0.4 g) and AuNP/TiO2/CaCO3 (0 g) catalysts. a Ti 2p; b Ca 2p; c C 1s; d O 1s; e Au 4f

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3.5 Photocatalytic performance tests

Figure 13 shows the plot of amounts of CO and CH4 as functions of the UV irradiation time for the AuNP/TiO2/CaCO3 (4.0 g) photocatalyst, as an example. The slopes of the curves changed during the first 2 h of irradiation and then stabilized. Thus, the concentration change rates of the different samples were compared using the slopes of the linear regression curves over a time period of 3 to 5 h, during which the slopes remained stable. In order to quantify the observed increase in the amount of production with irradiation time for CO and CH4 as presented in Fig. 13, it is realistic to adopt the slope linear regression through the data points presented by Isaac Isaac et al. [32], adopted and found to be a useful technique for data exploration. It is seen that the amount of production for CO increases irradiation time at the rate of 5.5×10−3 (μmol/h) while the amount of production for CH4 increases with irradiation time at the higher rate of 9.0×10−3 (μmol/h).

Fig. 13
figure 13

Concentrations of CO and CH4 as functions of the UV irradiation time for the AuNP/TiO2/CaCO3 (4.0 g) catalyst

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Figure 14 shows the relationships between the AuNP size and the amounts of CH4 and CO produced per unit time, standardized by the area of photocatalyst. The photocatalysts with CaCO3 exhibited smaller AuNP sizes and larger amounts of CH4 and CO produced per unit time than those of the photocatalysts without CaCO3. The amounts of CO and CH4 produced per unit time increased by factors of 2.9–4.0 and 1.3–2.1, respectively, owing to the addition of CaCO3. The samples with 0.4 and 0.8 g of CaCO3 had different AuNP sizes (11 and 7 nm, respectively) but produced similar amounts of CH4 and CO. The CO production reaction involves 2 electrons, and the CH4 production reaction involves 8 electrons. It can be considered that the numbers of electrons involved in the reactions was not modified within this AuNP size range. The samples with 0.4 g and 4.0 g of CaCO3 exhibited similar AuNP sizes; however, the sample with 4.0 g of CaCO3 produced a smaller amount of CH4 and a larger amount of CO than those produced by the sample with 0.4 g of CaCO3. Because the amount of CaCO3 was increased by a factor of 10 (from 0.4 to 4 g), the light irradiation area of TiO2 was decreased. This suggests that, for the sample with 0.4 g of CaCO3, a reaction involving 2 electrons is more likely to occur than the one involving 8 electrons.

Fig. 14
figure 14

Relationships between the Au particle size and the amounts of a CH4 and b CO produced per unit time, standardized by unit area

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Figure 15 shows the relationship between the Au surface area per 1 nm2 photocatalyst area and the amounts of CH4 and CO produced per unit time. The addition of CaCO3 reduced the AuNP size and increased the number of AuNPs (Table 2), resulting in an increase in the Au surface area. The increased amounts of CH4 and CO produced by the photocatalysts with CaCO3 were attributed to the increased reaction area. However, no correlation was found between the Au surface area and the amounts of CH4 and CO produced for the catalysts with 0.4 to 4.0 g of CaCO3.

Fig. 15
figure 15

Relationships between the Au surface area per 1 nm2 photocatalyst area and the amounts of a CH4 and b CO produced per unit time

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4 Conclusion

In this study, we presented a simple and practical one-pot synthesis for synthesizing photocatalysts that undergo CO2 reduction reactions. Photocatalysts consisting of AuNPs supported on a mixture of TiO2 and CaCO3 were synthesized. The percentage of CaCO3 with respect to TiO2 was varied between 0 and 200 wt%. The crystalline phase of TiO2 (anatase) was not modified, and no correlation was found between the amount of CaCO3 added and the values of crystallite size and lattice strain of TiO2 and CaCO3. The addition of CaCO3 reduced the average AuNP size and the variation in particle size. In particular, the sample with 40 wt% of CaCO3 showed a smaller particle size than the other samples, and a large amount of AuNPs with a low size variation was obtained. With the addition of CaCO3, the amounts of CO and CH4 produced per unit time increased by 2.9–4.0 and 1.3–2.1 times, respectively. In future, studies on the support position of AuNPs on photocatalysts for each synthesis condition and its relationship with the photocatalytic performance may be planned.