Fig. 1. Schematic illustration of the SERS detection of uric acid based on HfTe2 nanosheets.

Fig. 2. Synthesis and characterization of HfTe2 nanosheets. (a) Schematic representation of the preparation process of HfTe2 nanosheets. (b)–(c) TEM images of multilayer and few-layered hafnium ditelluride nanomaterials. The inset shows the HRTEM image and SAED pattern, respectively. (d) SEM image of HfTe2 nanosheets. (e) HAADF image and corresponding elemental mapping (Te and Hf) of HfTe2 nanosheets. (f) EDX pattern of HfTe2 nanosheets. (g) AFM image with the corresponding size distribution (inset) and (h) height analysis of HfTe2 nanosheets. (i) Raman spectra of bulk and few-layered HfTe2.

Fig. 3. Atomic structure of monolayer HfTe2 nanosheet. (a) Left and (b) top views of lattice structures. (c) Unit cell structure of the nanostructures. Green spheres, tellurium atoms; and yellow spheres, hafnium atoms. (d) XRD pattern of HfTe2 nanosheets.

Fig. 4. (a) XPS analysis of HfTe2 nanosheets. The high resolution XPS spectra of Hf 4f in (b) bulk and (c) few-layered HfTe2 sheets. (d) UV-Vis-NIR absorbance spectrum. (e) Typical optical absorption curve of HfTe2 nanosheets, where Eg is estimated to be 4.93 eV. (f) Valence band spectrum of HfTe2 nanosheets, where Evb is calculated to be −4.98 eV.

Fig. 5. (a) Raw Raman spectra of Rh6G (10−3  M) dye on Si wafer with or without HfTe2 substrate. (b) SERS spectra of different concentrations of Rh6G on HfTe2 substrate. (c) Intensity values of typical Raman peaks in (b). Fitting curve of the inset is the SERS intensity-concentration plot for 1313   cm−1 (R2) band of Rh6G. (d) EF values of five typical Raman peaks of Rh6G at different concentration levels.

Fig. 6. (a) Cluster of SERS spectra of Rh6G samples randomly collected at 20 sites on the HfTe2 substrate. (b)–(d) SERS intensities of three typical peaks at 1514  cm−1, 1365  cm−1, and 1313  cm−1 in the 20 SERS spectra.

Fig. 7. (a) SERS image of Rh6G molecules (1514  cm−1) on HfTe2 nanosheets. (b) Contour map of 170 SERS spectra collected from the Raman mapping. (c) Average spectrum (blue line) of the 170 spectral data and the reconstructed spectrum (green line) along the green diagonal line in (b).

Fig. 8. Schematic diagram of the photo-induced charge transfer process between HfTe2 and Rh6G under 785 nm laser excitation.

Fig. 9. Mean SERS spectra of pure uric acid, urea, and their mixture on HfTe2 nanosheets.

Fig. 10. (a) SERS detection of UA at different concentrations on HfTe2 nanosheets. Curves a to j: SERS spectra of UA at 1000, 600, 500, 400, 350, 300, 250, 200, 150, and 100 μM, respectively. Curve k: normal Raman spectrum of UA at 1 mM. (b) Fitting curve of SERS intensity-logarithmic concentration for 1039  cm−1 band of UA. (c) SERS spectra of UA (0.1–0.5 mM) on HfTe2 nanosheets in the presence of 4 mM urea. (d) Calibration curve by plotting the peak intensity ratio (I1039/I1012) as a function of UA concentration. Enlargements of the typical Raman peaks (urea 1012  cm−1, uric acid 1039  cm−1) are shown as insets.

Table 1. Calculated Maximum EFs of Typical SERS Peaks of Different Dyes on ${\mathrm{HfTe}}_2$ Substrate