2.2.1. Mixed WO3 and S with H2

The SEM images of the as-received WO3 precursor have been displayed in Figure S4; and the average particles size was about 60 nm, estimated using Derby-Scherrer Equation. The XRD pattern shown in Figure S5 matches very well with the monoclinic WO3 (JCPDS No. 43-1035).

**Figure S4.** SEM images of the as-received WO3 nanoparticles (~60 nm in size).

**Figure S5.** XRD pattern of the as-received WO3 precursor.

2.2.1. WO3 and S reaction under H2

The XRD patterns of Sample S2 from both the inlet and the hot zones are shown in Figure S6. Only a few WS2 layers may have formed, as the majority was WO*x*, along with S peaks (labelled with circle), although the hot zone samples exhibited a more complete oxide-to-sulphide conversion.

**Figure S6.** XRD patterns of Sample S2. (**a**) from the inlet zone and (**b**) from the central hot zone.

2.2.2. APT as Precursor and H2S as Reaction Gas—Production of WO*x* Nanoparticles from APT

WO*x* nanoparticles, as an important precursor for IF-WS2 nanoparticles, have been produced by the decomposition of APT at a high temperature furnace. At high temperature, the following reaction took place: (NH4)10[H2W12O42] 4H2O ĺ 12 WO3 + 10NH3 + 11H2O.

In fact, the APT started to decompose into WO3 at 500 °C. The WO3 then started to melt and vaporise in the central zone where temperature reached 1200 °C and 1250 °C respectively, and the WO*x* vapour would be brought downstream by the Ar flow and deposit at the inner wall of the quartz tube.

After optimization of the parameters, very fine nanoparticles with high batch yield were produced at 1350 °C, with long quartz tube (1.5 m) and high gas flow rate (6 L/min). The results were shown in Figure S7a–c. Both spherical and polyhedral particles are presented in all images, with uniform size less than 100 nm, mostly around 50 nm. XRD pattern in Figure S7d matches very well with monoclinic WO3 (JCPDS No. 43-1035), with the strongest peaks appear at 23–25 degree.

**Figure S7.** SEM images (**a**–**c**), and XRD pattern (**d**) of tungsten oxide nanoparticle (1350°C, 6 L/min, 30 min, 1.5 m).

**Figure S8.** TEM images of WO3 from decomposition of APT (1350, 1.5 m, 6 L/min, 30 min).

Figure S8 shows the TEM images of WO3 nanoparticles from the decomposition of APT. The majority of nanoparticles are of a polyhedral shape, around 50 nm.
