Introduction

Photoelectrochemical cells (PECs) were first demonstrated in 1972 and have attracted considerable attention recently [1]. In direct methanol fuel cells, methanol can be photocatalytically oxidized by semiconductors under solar illumination; this reaction is especially well-known for TiO2 semiconductors [1]. Metallic nanoparticles (NPs), such as Ag [2], Au [3], Pt [4], Pd [5], Ru [6], and Fe [7], can enhance the photocatalytic activity of TiO2 by inhibiting electron–hole recombination and prolonging the photoexcitation of electrons. The decorated metallic NPs can also enhance absorption and utilize the visible light much efficiently. Although TiO2 is chemically stable, environmentally friendly, abundant, and cheap, it can only be excited by ultraviolet light, which limits its applications in the visible light region [1]. Therefore, numerous other composited materials have been proposed to enhance photocatalytic activity.

Nanomaterials have large surface area-to-volume ratios and tend to enhance chemical reactivity. The wavelength or energy absorbed by nanosized metal materials is affected by the shapes and the sizes of nanostructures [8, 9]. Compared with one-dimensional nanomaterials, geometrically complex nanostructures have higher reactivity levels and specific reaction rates because they have larger specific surface areas and lower densities. Porous morphology enhances mechanical robustness, maximizes harvesting of light, and promotes electrolyte diffusion [10]. It has been shown in a previous work that Au dendritic nanoflowers (Au DNFs) absorbed visible light more effectively than Au NPs [11].

Transition metal nitrides, specifically titanium nitride (TiN), have attracted considerable attention in connection with novel plasmonic materials [12, 13]. TiN exhibits optical properties analogous to noble metals and has become an alternative for practical plasmonic applications in the visible and infrared regions [14]. TiN is extremely hard and possesses remarkable electrical and thermal conductivity, chemical stability, and catalytic activity [15, 16]. Having an extremely high conductivity, namely 4.5 × 106 S/m, nanostructured TiN performs well as an electrode material for applications in electrochemical energy storage [16]. Used as a catalyst, Pt-TiN demonstrated better performance in electrochemical oxidation of methanol than did Pt-TiO2 [17].

The objective of this study was to investigate new materials for applications in PECs. Au DNFs with substantial specific surface areas and strong localized surface plasmon resonance (LSPR) for wide wavelengths [11] were combined with TiN with high electrical conductivity and broadband plasmonic properties in this study. A novel L-DNFs-TiN composite material was synthesized, and its performance in methanol oxidation reactions was tested.

Materials and methods

Sample preparation and characterization

Following a published method [18], the TiN thin film was deposited on silicon wafers by using a reactive high-power impulse magnetron sputtering system (HiPIMS, SPIK 2000A, Melec GmbH) in this study. In brief, the deposition chamber was pumped down to 8 × 10−6 Torr by a cryopump. The titanium sputtering target was a 4-in. disc. Ar-Ti HiPIMS discharge was operated at a pulsed on/off cycle of 45/955 μs. An average power supply of 300 W was used in this sputtering process. The flow rates of argon and reactive nitrogen were fixed at 30 and 1.5 sccm, respectively.

A n-type silicon wafer (2 cm × 2 cm) was used as a substrate for the deposition of Au DNFs. Au DNFs were synthesized with a fluoride-assisted galvanic replacement reaction (FAGRR) [11]. In brief, a silicon wafer with and without TiN layer coating was dipped in a mixture of 10 mM chloroauric acid and buffered oxide etchant solution composed of 7% HF and 34% NH4F for 3 min. Table 1 shows the conditions of solutions and reaction times for Au NPs and Au DNFs.

Table 1 Condition of reactants and reaction time for different Au samples
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The sample morphology and element analysis were conducted by scanning electron microscope (SEM; JSM-7800F, JEOL, Japan) and energy-dispersive X-ray spectroscopy (EDS), respectively. Crystalline properties were observed by X-ray diffraction (XRD; D2200, RIGAKU, Japan). The optical reflection was determined by ultraviolet-visible spectroscopy (U-3900/3900H, Hitachi, Japan).

Electrochemical measurement

The electrochemical properties of the prepared Au samples were measured by a potentiostat (Jiehan 5600, Jiehan Technology, Taiwan) in a quartz three-electrode rectangular tank (10 cm × 10 cm × 10 cm) containing 500 mL of a mixture of 0.5 M NaOH and 1 M CH3OH. A tested sample with a defined 1-cm2 projection area was utilized as the working electrode. A Pt-coated titanium mesh and a saturated calomel electrode were used as counter and reference electrodes, respectively. The scan range was from − 0.2 to 0.6 V [19], and a 30-mV/s scan rate was applied in cyclic voltammetry (CV). The effects of irradiation on the chronoamperometry (CA) of methanol oxidation were tested with a solar simulator (SADHUDESIGN; class B; 150 W; λ > 400 nm).

Results and discussion

Morphology and elements

Figure 1 presents SEM images of Au NPs, for thin (250 nm) and thick (800 nm) Au DNFs on silicon wafers. The Au NPs were isolated and distributed evenly (Fig. 1a) to form a uniform Au NP film (Fig. 1b) on the silicon surface. The S-DNFs-Si (Fig. 1c, d) exhibited a three-dimensional structure and aggregation of Au NPs. This result was due to the prolonged synthesis reaction time for Au, which ranged from 2.5 to 3 min. The L-DNFs-Si (Fig. 1e, f) exhibited leaf shapes consisting of multiple levels of branches. The morphology of the sample L-DNFs-Si was growth with much more and longer Au DNFs than that of the sample S-DNFs-Si because a fourfold greater amount of HAuCl4 was applied in the Au synthesis of the L-DNFs-Si. The EDS spectrum revealed clear signals of Au and Si without any other by-products such as Cl or F, indicating that FAGRR deposited Au cleanly on the Si substrate [11].

Fig. 1
figure 1

SEM examinations of Au. a Top view and b side view of Au NPs/Si; c top view and d side view of S-DNFs-Si; e top view and f side view of the L-DNFs-Si; g EDS spectrum of L-DNFs-Si

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Figure 2 presents SEM images and an EDS spectrum of the L-DNFs-TiN. The morphology of the L-DNFs-TiN was somewhat different from that of the L-DNFs-Si. Numerous dense NPs aggregated for the L-DNFs-TiN. The average deposited thicknesses of Au NPs/Si, the S-DNFs-Si, the L-DNFs-Si, and the L-DNFs-TiN were 50, 250, 800, and 650 nm, respectively. The deposited thickness of the L-DNFs-Si was greater than that of the L-DNFs-TiN because of the higher electron density generated in the FAGRR process to deposit Au. In the redox reactions of gold (III) chloride complex ions, for the same amount of fluoride, the Si reaction (Si + 6F− → SiF62− + 4e−) [20] generated a fourfold greater number of electrons than the TiN reaction did (TiN + 3HF2− → TiF62− + NH3 + e−) [21].

Fig. 2
figure 2

SEM examinations of L-DNFs-TiN. a Top view. b Side view. c EDS spectrum

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Crystalline properties

Figure 3 shows the XRD patterns of Au DNFs and TiN. Four Au peaks, namely Au(111), Au(200), Au(220), and Au(311), were evident in all Au-containing samples. This result reflects the sharp cubic gold data and agrees with JCPDS 04-0784. Au(111) was the highest among the four Au peaks, indicating that the growth of Au has a strong preferential (111) orientation [22, 23]. The intensity of the Au peaks among diffrent samples was in the following order: L-DNFs-Si, S-DNFs-Si, and Au NPs. In the L-DNFs-TiN sample, the growth of TiN(111), TiN(200), TiN(220), and TiN(311) was found. The growth of TiN was determined to have a preferential (111) orientation, which agrees with the TiN information in JCPDS38-1420.

Fig. 3
figure 3

XRD patterns of Au NPs/Si, S-DNFs-Si, L-DNFs-Si, and L-DNFs-TiN

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Reflection

Figure 4 shows the optical reflection results of tested samples irradiated with light at 350–900 nm, close to the major spectral irradiance of sunlight. The polished Si wafer had a typical reflectance of approximately 40%, and the reflectance of Au NPs deposited on the Si wafer was greatly decreased in the 350–550 nm range. Compared with Au NPs, the geometric complexity of Au DNFs can engender multiple scattering of incident light and enhance light trapping [24]. Therefore, considerable antireflection behavior can be observed in Au DNFs, especially for thick samples. Relative to the reflectance result for the S-DNFs-Si, L-DNFs-Si exhibited superior antireflection properties in the wavelength range of 500–850 nm. The reflection characteristics of the L-DNFs-Si and L-DNFs-TiN were similar. However, the tested sample film thickness of the L-DNFs-TiN was lower than that of the L-DNFs-Si, indicating that TiN has enhanced resonant plasmon characteristics in the visible region [14, 25, 26]. A low reflection means a high absorption in this study. The layer having higher photocatalytic activities is the one having higher absorption. Thus, the improved photocatalytic activity may result from an improved light trapping mechanism.

Fig. 4
figure 4

Reflection spectra of different samples

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Compared with the gold nanoparticles, the gold dendritic nanoflowers not only largely increase the deposited mass of gold on same substrate area, but also greatly increase the surface area as photocatalysts (Fig. 1). The largely increased surface area by the complex crystal structure not only brings in large amount of photocatalytic reaction simultaneously but also enhances the utilization efficiency through generation of localized surface plasmon resonance. The generation of localized surface plasmon resonance results in enhanced absorption of external illuminated light and wide-band decreased optical reflection.

Electrochemical measurements

Figure 5 depicts the effects of different Au catalysts on the electrochemical properties of a methanol solution. The peak anodic (oxidation) current could not be determined for Au NPs/Si and S-DNFs-Si (Fig. 5a), but there was typical methanol polarization curves for L-DNFs-Si and L-DNFs-TiN (Fig. 5b). A comparison between CV plots for samples indicated that the thick Au DNFs on TiN/Si or Si were obviously active toward the methanol oxidation reaction process (Fig. 5b), but Au NPs/Si and the thin Au DNFs were not (Fig. 5a). Due to poor electron conductivity between the isolated Au NPs/Si and thin Au DNFs on the silicon wafer, Au NPs/Si and the S-DNFs-Si exhibited no signals of methanol oxidation [11].

Fig. 5
figure 5

Cyclic voltammograms in 1 M CH3OH + 0.5 M NaOH for various working electrodes: a Au NPs/Si and S-DNFs-Si and b L-DNFs-Si and L-DNFs-TiN. A saturated calomel electrode served as the reference electrode

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Figure 6 illustrates the effects of light illumination on the electrochemical performance levels of the L-DNFs-Si and L-DNFs-TiN. As revealed by the photoresponse of the L-DNFs-Si, the oxidation peak current (Ip) increased from 0.71 to 0.83 mA cm−2, whereas the peak voltage (Vp) decreases from 0.25 to 0.22 V in the presence of light illumination. Regarding the L-DNFs-TiN, Ip increased from 1.12 to 1.52 mA cm−2, whereas Vp decreased from 0.22 to 0.19 V. These results provide direct evidence of photoenhanced methanol oxidation reactions with L-DNFs-Si and L-DNFs-TiN as photocatalysts. The L-DNFs-TiN had higher Ip and lower Vp values than the L-DNFs-Si did.

Fig. 6
figure 6

Cyclic voltammograms in 1 M CH3OH + 0.5 M NaOH for L-DNFs-Si and L-DNFs-TiN working electrodes with controlled light sources. A saturated calomel electrode served as the reference electrode

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Figures 7 presents CA plots with 0.1 V polarization potential under dark and light conditions for the L-DNFs-Si and L-DNFs-TiN, respectively. Photocatalytical methanol oxidation reactions can be also verified in this figure. Corresponding to the non-symmetrical result of the reduction-oxidation curve in Fig. 6, the increasing resistance of tested systems results in the decay of current density in CA plots. Regardless of light, L-DNFs-TiN has a higher current density than L-DNFs-Si. Compared with the dark condition, visible light was observed to have a current density enhancement in both the L-DNFs-Si and L-DNFs-TiN, agreeing with the current enhancement observed in Fig. 6. This demonstrates that the absorption of visible light enhances the photocatalytical methanol oxidation reactions, coinciding with the reflection data in Fig. 4. Table 2 further investigates current enhancement of L-DNFs-Si and L-DNFs-TiN under light treatment. Results indicate that there is a higher current enhancement in L-DNFs-TiN than L-DNFs-Si. The current enhancement of L-DNFs-Si decreases substantially with time, but little decrease for L-DNFs-TiN. The extra TiN layer reduced the absorption of light energy by Si substrate and has enhanced resonant plasmon characteristic for light. Furthermore, the ratio of current enhancement with light illumination by L-DNFs-TiN substrate retains longer time than that of Au DNFs/Si substrate. Therefore, an additional TiN layer provides superior utilization of incident light illumination compared to that of only L-DNFs-Si composite.

Fig. 7
figure 7

Chronoamperometric test of L-DNFs-Si and L-DNFs-TiN under different light conditions

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Table 2 Comparison of current density and enhancement of L-DNFs-Si and L-DNFs-TiN under light treatment
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Figure 8 shows repeatability performance in continuous cyclic on/off light illumination testing over 200 s. The test conditions were the same as those presented in Fig. 7 for the L-DNFs-TiN. As revealed by the results, a steady performance of photocatalytical reaction could be observed after the second on/off test. The rising time and falling time of the response in current density are within 1 s. The generation of localized surface plasmon under light illumination is very fast; therefore, its effect turns on/off quickly. The almost instant response of current density with on/off light illumination switching indicates the practical utility of the L-DNFs-TiN composite in PECs.

Fig. 8
figure 8

Chronoamperometric on/off light testing of L-DNFs-TiN

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In this study, 0.5 M NaOH was added to a methanol solution to enhance the oxidation reaction. A methanol oxidation reaction is more favorable with alkaline electrolytes than with neutral or acidic aqueous solutions. OH− ions scavenge holes and thus reduce electron–hole recombination and accelerate the oxidation reaction. Alkaline electrolytes can improve the oxygen reduction reaction [27].

Conclusions

This study proposes a novel L-DNFs-TiN composite for PECs. L-DNFs-TiN and L-DNFs-Si composites were determined to have oxidation current signals in methanol oxidation reactions with alkaline electrolytes. Regardless of light illumination, the L-DNFs-TiN exhibited better current density than the L-DNFs-Si. The system exhibited more current enhancement under light illumination than it exhibited under a dark condition, thus providing direct evidence of a photoenhanced methanol oxidation reaction. An additional absorbance of 400 nm in optical reflection appeared for TiN because of its unique semiconductor electron band transition mode. The cyclic on/off light illumination test presented a repeatability performance in CA, indicating that the synthesized L-DNFs-TiN composite has practical applications.